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航海仪器英文讲义

1. eeeeeee eere eene nnn 99 LESNCDLI I E 102 3 9 SUIMMARY e 103 3 10 Revision QUESTIONS 5 csecesceccsesssscecsesesciceccsscoucwocsssevscsacessocseseccdsersercessesss 104 3 6 Principles of speed measurement using the Doppler effect 90 ep Q Chapter 4 The Ship s Magnetic COMpaSS ccccccssssssssceeececccecssssccesceesccecessessseeees 105 CNN irse lindo pc inau Nisi 105 LAETI Eee 114 4 3 Theory of magnetic compass adjustment eeeeeeeeeeeee 117 44 GSS ANY Me YM 125 c2 4 5 REVISION questio essendi etd rii cio denisei vin ngon utri eR ODE RR 125 2 Referentes accorso toann caie ea E DIDI ICON Ee aS 126 Chapter 1 The Ship s Gyrocompass c2 1 1 Introduction Of all the navigation instruments in use today the master compass is the oldest and probably the one t st navigators feel happiest with However even the humble compass has not escaped the of microelectronics Although modern gyrocompasses are computerized the principles upon hi ey work remain unchanged e 1 2 wre principles At the heart of a gyrocompass assembly is a modern gyroscope consisting of a perfectly balanced wheel arrange i etrically at high speed about an axis or axle The wheel or rotor spins about its own axis and uspending the mass in a precisely designed gimb
2. Certain materials such as Rochelle salt and quartz exhibit pressure electric ffects when they are subjected to mechanical stress This phenomenon is particularly outstand ng in the element lead zirconate titanate a material widely used for the construction of the se electrostrictive transducers Such a material is termed ferro electric becaus of i ilarity to ferromagnetic materials e The ceramic material contains random electric domains which when subjected to mechamigal s ill line up to produce a potential difference p d across the two plate ends of the mate section Alternatively if a voltage is applied across the plate ends of the ceramic crystal section its length wa be varied Figure 2 5 illustrates these phenomena 53 Stress a Stress f m ted in the forward section of a large merchant vessel where pressure stress can be intolerable The fra e crystal also imposes limits on the transmitter power that may be applied because mechanica stress is dire The power restraints thus established make the electrostrictive transducer u stressed by a voltage applied across its ends thus the thicker the crystal slice the needed to stress it The electrostrictive transducer is only fitted on large merchant vessels when the power trarismitte is low and the frequency is high a combination of factors present in Doppler speed logging sy s Such a transducer is manufactured by mounting two crystal slic
3. at any given latitude It is therefore present even when the ship is stationary As has sly begn stated a gyrocompass will always settle close to the meridian with an error in tilt Tomai the eyro pointing north it must be precessed at an angular rate varying with latitude At the equator th eartly linear speed of rotation is about 900 knots and rotation from west to east causes a fixed poinf to effectively move at 900 cos latitude knots in an easterly direction For any latitude A the rate o spin is 15 h This may be resolved into two components one about the true vertical at a give as illustrated in Figure 1 21 latitude csinA and the other about the north south earth surface horizontal at a given latitude z0 20 Gyro horizontal Direction of axis earth rotation asp olar 1 w 15 perhour axis C SOS 4 90 p aS UIN Gyro N31 x vertical axi l die i xis Put sin 4 a True norizontal for P Equator 4 0 wea ee B Figure 1 21 Apparent movement o eproduced courtesy S G Brown Ltd The component of the earth s rotation about the no izontal may be resolved further into two components mutually at right angles to each other The first ent is displaced a to the east of the meridian producing a rate of spin cos sin a whilst produce a rate of spin wcosdcos a Correction for latitude error requires that a torque be applied t t an angular ra
4. depth below the keel is related to the time taken for the acoustic wave to travel to the ocean oor and return Put simply if the delay is 1 s and the wave travels at 1500 ms then the depth is 500 759m Pulsed ystems lik length or duratiggf determines the resolution of the equipment A short pulse length will identify se used in maritime RADAR are used in an echo sounder The pulse objects close If all other parameters remain constant the pulse repetition frequency PRF r of pulses per minute determines the maximum range that can be indicated The width of the transmi mes wider as it travels away from the transducer It should not be excessively narrow or te 4S away from or miss the returned echo Modern echo sounding equipment is co trolled and therefore is able to produce a host of other data besides a depth indicati 2 10 Revision questions 1 Why do deep sounding echo sounders operate with a low trafi mi equency 2 For a given ocean depth how is it possible for returned echoes tO vary Th strengt 3 If a vessel sails from salt water into fresh water the depth indicated by an ech error Why is this and what is the magnitude of the error 4 Noise can degrade an echo sounder display How does narrowing the trans system noise and at what cost 5 Why are electrostrictive transducers used in maritime applications in preference toy ezoelgStric e resonators 6 Why do marine echo sounding s
5. 9 State the difference between the permanent magnetism and the induced magnetism 125 References L Tetley and D Calcutt Electronic Navigation Systems 3rd edition Butterworth Heinemann gt Linacre House Jordan Hill Oxford 2001 Handbook of Magnetic Compass Adjustment National Geospatial Intelligence Agency Bethesad D 2004 esolution A 424 XI Performance Standards for Gyrocompasses solution A 382 X Magnetic Compasses Carriage and Performance Standards IMQff esolution MSC 96 72 Recommendation on Performance Standards for Devices to a IM Indigate Speed and Distance utioMSC 74 69 Recommendation on Performance Standards for Echo so g Equip IMO Chapter V of S Convention 126
6. The sensing element follow up system is a transformer with an E shaped laminated core and a single primary winding c and two secondary windings connected as shown in Figure 1 25 With the E shaped pri two secondaries is such that they_will eancel and the total voltage produced across R1 is the supply voltage only This is the stable n during which no rotation of the azimuth servo rotor occurs If core in its central position the phase of the e m f s induced in the there is misalignment in any directionfbe e phantom and the vertical rings the two e m f s induced in the two secondaries will be unbalafiteds the voltage across R1 will increase or decrease accordingly I Follow up transformer l Primar Y Secondary Mounted on phantom element Mounted on vertical ring movement o um iu J phantom ring Figure 1 25 The Sperry compass azimuth follow up circuit This error signal is pre amplified and used to drive a complementary push pull power son S producing the necessary signal level to cause the azimuth servo to rotate in the required direction to re align the rings and thus cancel the error signal Negative feedback from T2 secondary to the preamplifier ensures stable operation of the system Another method of azimuth follow up control was introduced in the Sperry SR220 gyrocompass Figure 1 26 25 Difference between aandb tilt est axis At the north and south ends of the
7. n excessive list will affect athwartship speed A Janus configuration t is prevalent in areas with strong tides or ocean currents In the error may be introduced Ocean eddy currents Whils problem is more likely to be found i ters with big tidal changes or in river mouths Sea state Following seas may result ge tB speed indication in the fore aft and or port starboard line depending upon the vector offfie approaching sea relative to the ship s axis Temperature profile The temperature of t ffects the velocity of the propagated acoustic wave see Figure 2 2 in Chapter 2 Te 3 7 The Furono Doppler Sonar DS 5 ste Another respected manufacturer of marine equipment Furuno produces a Doppler gOnar system the DS 30 based on the principles of Doppler speed measurement Whilst the syStem pri s are the same as with other speed logs in this category Furuno have made good use of datagpro essiftg circuitry and a full colour 10 inch wide LCD display to present a considerable amount f infogmation to a navigator The display modes or shown in Figure 3 24 Q ye 4 99 SPEED MODE Tracking mode Echo monitor Ship s speed and course from ext nav sensor Transverse speed at bow Under keel clearance measured by ext Longitudinal E Keel Clearance by D6 20 echo sounder speed 8 2 kt 2m Transverse v Total distance run Tota Distance with laser gyro LJ kt FT speed at stern
8. v f f fiche v If an observer moves at velocity v towards a stationary sound source the number of cycles reaching the receiver per second is increased thus the apparent received frequency is increased The received frequency is f5 fi v and 1 A ffc therefore fit file fil v c fc v e e ver now moves away from the stationary transmitter the apparent received frequency is f fie ie If as in ler speed log both the observer and the sound source transmitter and receiver are moving towards a reflect urface the received frequency is c CHV tv v 4 c v The Doppler frequency shift i fa fer hi or fo fo v e fa x v The velocity of radio waves c is always far in excess of and fherefore the expression above can be simplified to 2f fa where fa Doppler frequency shift in cycles per second v relative sp in the difection of the transmitted wave f transmitted frequency and c velocity of propagation O the rafio wave e 3 6 Principles of speed measurement using the Doppl Z The phenomenon of Doppler frequency shift is often used to measure the speed of a moving Lo carrying a transmitter Modern speed logs use this principle to measure the vessel s speed with respect to the seabed with an accuracy approaching 0 1 Q If a sonar beam is transmitted ahead of a vessel the reflected energy wave will have suffer
9. 88 3 5 The Doppler principle In the early 19th century Christian Doppler observed that the colour emitted by a star in relative movement across the sky appeared to change Because light waves form part of the frequency spectrum it was later concluded that the received wavelength must be changing and therefore the apparent received frequency must also change This phenomenon is widely used in electronics for measuring velocity 15 b shows that the wavelength X is compressed in time when received from a transmitter g towards a receiver 1 and expanded Figure 3 15c in time from a transmitter moving away onsider a transmitter radiating a frequency fi The velocity of propagation of radiowaves in free ig9300X 10 ms and in seawater it is much slower at approximately 1500 ms After a Figure 3 15 Expansion and compression of wavelength In Figure 3 15 b the transmitter has moved towards an observer by a distance d Mis is Py travelled during the time of generating one cycle T IL T I f and d vxT v f Q Therefore the apparent wavelength is e X 1 2 ovf fri c X i e 1 vif cf ft v 9 cf c v L For a moving transmitter that is approaching a receiver the received frequency is apparently increased and the frequency is The reverse is true of a transmission from a transmitter moving away from an observer when the e 89 wavelength will be stretched and the frequency decreased A2 A
10. Em n Q 92 t Ahead transmission only Speed error in 96 Janus configuration transmission 1 2 8 4 5 6 7 8 9 1011 12 13 14 15 16 o Trim angle in degrees Figure S of speed error caused by variations of the vessel s trim The additioR of a second sducer assembly set at right angles to the first one enables dual axis speed to be indicated Fig 4 element transducer array Fore Port Figure 3 19 Dual axis speed is measured by transmitting sona narrow beams towards the sea bed 3 6 1 Vessel motion during turn manoeuvres A precise indication of athwartships speed is particularly important on largefyessels wh e bow and stern sections may be drifting at different rates during docking or turning mameeuvres Speed vectors during a starboard turn 2 e the logdtio A dual axis Doppler speed log measures longitudinal and transverse speed at n ef the transducers If transducers are mounted in the bow and stern of a vessel the rate o an computed and displayed This facility is obviously invaluable to the navigator during diffieult manoeuvres Q Q 93 X X3 300m V 8Knots Viy 4 Knots w 35 min Figure 3 20 Speed vectors during a sta board turn with no current Reproduced courtesy of Krupp Atlas Elektronik Figure 3 20 shows the speed vecto Sw and stern transducer data when a ship is turning to starboard without the effect of water curre
11. Figure 3 8 Relationship between the vessel s e output from the sensors The following points should be noted The a c supply to the solenoid produces inductive pick e coil and the wires that carry the signal This in turn produces a zero error that mu mpensated for by backing off the zero setting of the indicator on calibration Theinduced e m f is very small for reasonable amplitudes of energizi uV per knot The induced e m f and hence the speed indication will vary with the conductivity ater The device measures the speed of the water flowing past the hull of the ship ThiS floyg an due to the non linearity of a hull design Ocean currents may introduce errors Pitching and rolling will affect the relationship between the water speed and the hull Error dieto this effect may be compensated for by reducing the sensitivity of the receiver This is achieved using a CR timing circuit with a long time constant to damp out the oscillatory effect Accuracy is typically 0 1 of the range in use in a fore and aft direction and approximately 2 e Q thwartships 81 Energizing current Output to amplifier Sensors for pickup of induced e m f Vy Streamlined housing aaao of movement ige 3 9 Constructional details of an electromagnetic log sensor Figure a typical sensor cutaway revealing the solenoid and the pick up electrodes A speed irs Stem is de ed in Fi
12. blanking pulse generator to syn Hronize the digital and processing circuits The transmit timin t sets the pulse length to trigger the 24 kHz oscillator transmission Quse generator also initiates the swept gain circuit ion the swept gain control circuit holds the gain of the input tuned amplifier low At cesSatio tr ansion the hold is removed permitting the receiver gain to gradually increase at a rate e n inverse fourth power law This type of inverse gain control is necessary because echoes that ar d soon after transmission ceases are of large amplitude and are likely to overload the receiver The echo amplitude gradually decreases as the returned amp cho d fay period increases Thus the swept gain control circuit causes the average amplitude of the echoeS disffa be the same over the whole period between transmission pulses However high intensity e hoes returned large reflective objects will produce a rapid change in signal amplitude and will cause a largexfStgnal to be coupled to the logarithmic amplifier causing a more substantial indication to be made o The logarithmic amplifier and detector stages produce a d c output the ampl logarithmically proportional to the strength of the echo signal paper is tightly drawn over the grounded roller guides by a constant speed paper drive mot Pa marking is achieved by applying a high voltage a c signal to the stylus which is drawn at 90 tof fhe paper movement
13. damping prec more evident as the north end of the gyro spin axis appraagvies sridian Note also that the minor axis of the ellipse is decreas nt bb As the north end of the gyro spin axis passes west of the meridian earth rotation causes a downward tilt of the gyro This movement is now in sympathy with the downward precession that is due to torque Ty Torque TH however corttinues t precess the gyro westwards so that the gyro spin axis passes through the earth s horizontal at H at which point torques TH and Ty cease Note that the major axis of the ellipse is considerably decreased 12 S J SQ Earth rotation continues to tilt the gyro downwards and once the i TH gyro spin axis is below the earth s horizontal the gravity control again exerts torques TH and Ty but this time in an opposite direction Torque T now causes precession eastwards whilst toraue T causes precession upwards and thus damps the downward tilt that is due to earth rotation The resultant movement of the gyro causes the north o end of the gyro spin axis to pass through the meridian at a point that reduces even more the minor axis of the ellipse X o gyro spin axis passes east of the meridian pward tilt of the gyro This movement is ward precession that is due to torque cT ind As the n e earth rotat A cau Ty now in sympa Ty The north ehd the ellipse zu 5 SS SK The damping effect of the precession due to t
14. in GPS calculations is WGS84 In some areas of the world electronic chart coverage is by raster charts scanned paper charts alone The datum of many raster charts is not WGS84 When GPS shows a compass course it is not showing the ship s heading it is showing the track of the vessel where she has been in relation to her current position With the vessel stationary GPS will not provide any directional information 105 K A gure 1 Failure to observe ENA errors was a major factor in the grounding of this U S warship ost amp lectronic compasses GPS and gyro compasses are two exceptions are effected by magnetic tio inc y are also reliant on a power supply Electronic compasses used for marine navigation e G oppass comprising 2 or preferably 3 antennas aligned symmetrically fore and aft will show We ship s he in either true or magnetic form and is normally accurate to within one degree on heading As with all satellite derived data it is vulnerable to signal error re updated from GPS It can take many hours for a gyro compass to operate correctly fro time it is switched on or switched back on after a power outage Laser and Atomic Compasses still in early days of develo or CO ial marine use but may be commonplace in the not too distant future In Summary State of the Art Technology can be a great asset to the mode properly As we all know it sometimes doesn t and then things
15. It can readily be seen that the change is slight and is normally only compengated for in apparatus fitted on survey vessels Seasonal changes affect the level of the thermocline and t is small annual velocity variation However this can usually be ignored O e 2 2 3 Noise Noise present in the ocean adversely affects the performance of sonar equipment Water ye main causes The steady ambient noise caused by natural phenomena Variable noise caused by the movement of shipping and the scattering of one s own transmitted signal reverberation O gt Ambient noise Figure 2 4 shows that the amplitude of the ambient noise remains constant as range increases whereas Jy both the echo amplitude and the level of reverberation noise decrease linearly with range Because of beam spreading scattering of the signal increases and reverberation noise amplitude falls more slowly than the echo signal amplitude 51 Ambient noise possesses different characteristics at different frequencies and varies with natural conditions such as rainstorms Rain hitting the surface of the sea can cause a 10 fold increase in the noise level at the low frequency approx 10 kHz end of the spectrum Low frequency noise is also increased particularly in shallow water by storms or heavy surf Biological sounds produced by some forms of aquatic life are also detectable but only by the more sensitive types of equipment The steady amplitude of ambient noise
16. can very ear shaped User error due to inadequate training fatigue and information overload can als cofitribute innacuracies and misinterpretation of data Over reliance on electronic navigat ids to complacency and sometimes to disaster In recent years there have been numerous well documented occasions and many not so yell documented on which a sudden unexpected loss of power or the undetected inaccuracy of elect fiic instruments has rapidly developed into a serious crisis Q Very often the ability and readiness to switch to old fashioned manual navigation including the use of a reliable magnetic compass and looking out of the window has made the difference between continuing the voyage safely and a major marine incident 106 Figure 3 Compass installation on the Monkey Island on vessel s centre li e Ideally the compass should be installed on the vessel s centre line so that deviating ntagneti fo are mostly symetrical around the compass On certain vessels such as aircraft carriers so ish offset vessels and some modern container ships with a narrow superstructure section the compasS is and this can create interesting challenges for compass adjusters On small vessels the compass is usually located in front of the helm position Care should be taken QA ensure the compass is installed far enough away from structural members equipment and instruments such as radios speakers engine rev counters tachom
17. constant but as the earth L rotates in an anticlockwise direction viewed from the North Pole beneath it the gyro appears to rotate clockwise at a rate of one revolution for each sidereal day see Figure 1 6 7 I i i Js Direction of 5 Direction of S5 i amp 7 apparent j earth s rotation azimuth drift Direction of earth s rotation s a Drift of the N end of the spin axis is to Tilt of the N end of the spin axis is upwards the E in the northern hemisphere and if the N end is to the east of the meridian to the W in the southern hemisphere and downwards if it is to the W of the meridian Earth rotation E de Figure 1 6 a Effect of earth rotation on the gyro Re esy of Sperry Ltd b View from the South Pole The earth rotates once every 24 h carrying th it Gyroscopic inertia causes the gyro to maintain its plane of rotation with respect to the celestial ref re oint However in relation to the surface of the earth the gyro will tilt The reciprocal effect will occur at the South Pole This phenomenon is known yro drift Drift of the north end of the spin axis 1s to the east in the northern hemisphere and e southern hemisphere There will be no vertical or tilting movement of the spin axis i tilt occurs if the mechanism is placed with its spin axis horizontal to the equator The spin axis wi ilized fn line with a star point because of inertia As the earth rotates the eastern end of the s pPpears to tilt
18. deviation westerly 360 omy He a Figure 20 Effects of asymm ttical tal E induced magnetism The quadrantal deviations will not vary with lati ecause the horizontal induction varies tion in certain other asymmetrical arrangements of horizontal soft iron creates a constant A deviati n curve The magnetic A and E errors are of smaller magnitude than the other errors but when eritoufite re generally found together addition to this since they both result from asymmetrical arrangements of hor l soft iro magnetic A error there are constant A deviations resulting from 1 physi isalignments of the compass pelorus or gyro 2 errors in calculating the sun s azimuth or taking bearings The nature magnitude and polarity of all these induced effects are dependent upon 1sBbsition metal the symmetry or asymmetry of the ship the location of the binnacle the strefigth e h s magnetic field and the angle of dip Certain heeling errors in addition to those resulting from permanent magnetism are created bythe presence of both horizontal and vertical soft iron which experience changing induction as the shi S in the earth s magnetic field This part of the heeling error will naturally change in magnitude wit changes of magnetic latitude of the ship Oscillation effects accompanying roll are maximum on north and south headings just as with the permanent magnetic heeling errors 4 3 4 Adjustments and correctors P Since som
19. ea velocity If a transmitter TX and receiver RX are both stationary the received signal will be same frequency as that transmitted However if either the TX or the RX move during transmissi then fhe received frequency will be shifted If the TX and or RX move to reduce the distance between the wavelength is compressed and the received frequency is increased The opposite effect occurs if the TX and or RX move apart A Doppler speed logging system a transmits a frequency typically 100 kHz towards the ocean floor and calculates the vessel s speed P from the frequency shift detected b measures both W T and G T speed Q c produces a speed indication the accuracy of which is subject to all the environmental problems affecting the propagation of an acoustic wave in salt water 103 d uses a Janus transducer arrangement to virtually eliminate the effects of the vessel pitching in heavy weather e may use more than one transducer arrangement One at the bow and another at the stern to show vessel movement during turn manoeuvres 3 10 Revision questions A speed indication is only of value if measured against another parameter What is the speed indig tion produced by a pressure tube speed log referenced to is the approximate velocity of propagated acoustic energy in seawater 3 puessure tube speed logging system why is the complex system of cones required in the m age 9 4 Wh es indication produced by an e
20. from greater epth stem shown in Figure 2 12 is essentially a paper recorder and two LCD displays showing start pth And seabed depth As before transmission is initiated at the instant the stylus marks the zero line ive paper by a trigger sensor coupled to the control integrated circuits Depending upon the he Suse length modulates the output from the transmit oscillator which is power voltage to drive th pri determined by the time del hus the depth is marked on the sensitive paper at some point etween transmission and reception and the distance the stylus has travelled over the paper 64 Figure 2 12 Furuno FE 606 echo sounding system Reproduced courtesy of Furuno Electric Co 65 2 7 Amicrocomputer echo sounding system As you would expect the use of computing technology has eliminated much of the basic circuitry and in most cases the mechanical paper display system of modern echo sounders Current systems are much more versatile than their predecessors The use of a computer enables precise control and processing of the echo sounding signal Circuitry has now reached the point where it is virtually all contained on a few chips However the most obvious changes that users will be aware of in modern systems are the isplay and user interface again there are many manufacturers and suppliers of echo sounders or as they are often now ish finders The Furuno navigational ech
21. guideline only In practice sufficient transmitted power will overcome these losses Salinity pressure and the velocity of the acoustic wave ce depth sounder operates by precisely calculating the time taken for a pulse of energy to travel to oor and return any variation in the velocity of the acoustic wave from the accepted calibfated seed of Roo ms will produce an error in the indicated depth The speed of acoustic waves vaffes with temperature pressure and salinity Figure 2 2 illustrates the speed variation caused by changes in the 5 salinity 3 496 normal 1 5 salinity Velocity in ms gt M o rs S 1425 the salinity of seawate Ocean water salinity is approximately 3 4 but it does vary e ively thro ut the world As ibnored except when the vessel transfers from seawater to fresh water when the indicated depth wilfbe approximately 3 greater than the actual depth The variation of speed with pressure or depth is aic graph in Figure 2 3 50 Velocity in ms 1475 1485 1495 1505 1515 1525 1470 1480 1490 1500 1510 1520 E Sub surface layer ENS MEMEMMEME i thermocline I l Echo sounding l 500 i equipment E calibration velocity Approx 2 STOT a Main thermocline in e S Depth Deep Isothermal layer l i I l i 1500 2000 Figure 2 3 Variation of the velocity of acoustic waves with
22. has a zero phase shift from T1 through T2 to the demodulator Demodulation is carried out Figure 3 11 Simplified diagram of an e m log Oy Q 84 by TRI TR2 that are switched in turn from an a c reference voltage derived from a toroidal transformer monitoring the energizing current of the transducer By driving TR1 TR2 synchronously the phase relationship of the voltage detected by the electrodes determines the polarity of the demodulated signal 0 and 180 phasing produce a positive or negative component 90 and 270 produce no output and hence a complete rejection of such phase quadrature signals The demodulated signal is applied to the Miller Integrator IC3 which in turn drives the current generator Speed repeaters are current driven from this source Operation of the loop ith Rt vessel movement there will be a zero signal at the input to IC1 and consequently there will be n nal at the multiplier chip input No feedback signal is developed at the input to IC1 As the vessel oveg ahead the small signal applied to IC1 is processed in the electronic unit to produce a current the speed repeaters and the multiplier There now exists an output from the multiplier prop tiong to the speed repeater current and the reference voltage produced by the toroidal toring the transducer energizing current The a c from the multiplier is fed back to IC1 in seri S with and 1 out of phase with the small signal secondary of T1 This a c si
23. it has freedom to move about the horizontal and vertical axes However if the gyrocompass is to be mounted on a ship the base phantom ring needs to be capable of rotating through 360 without introducing torque about the vertical axis Freedom about the vertical axis is particularly difficult to achieve without introducing torque to the P system The most common way of permitting vertical axis freedom is to mount the gyro in a vertical ring with ball bearings on the top and base plates Obviously the weight of the unit must be borne on the lower bearing which can create considerable friction and introduce torque A number of methods have been developed to eliminate torque about the vertical axis These include the use of high tensile 17 amp torsion wires and buoyancy chambers as described for each compass later in this chapter 1 7 Compass errors The accuracy of a gyrocompass is of paramount importance particularly under manoeuvring situations where the compass is interfaced with collision avoidance radar An error either existing or produced between the actual compass reading and that presented to the radar could produce potentially atastrophic results Assuming that the compass has been correctly installed and aligned the static s errors briefly listed below should have been eliminated They are however worthy of a brief ic errors error n be amor existing between the indicated heading and the vessel s lubb
24. magnetic fields often found around a console mounted compass 108 Suffice to say all fastenings used to install the compass should be of non ferrous non magnetic material e g bronze or marine grade stainless steel 4 1 3 Variation deviation and compass correction MAGNETIC VARIATION or DECLINATION is the difference between True North and Magnetic North It is due to The earth s magnetic field which travels from South to North not travelling in a straight line In some locations variation can be in excess of 30 degrees In some locations it is zero The Magnetic North and South Poles being located considerable distances from the Geographic h and South Poles respectively The Magnetic North Pole is over 1 000 miles from the eographic North Pole and this distance is currently increasing by about 40 miles a year e pass is said to be pointing magnetic north when it is perfectly aligned with the earth s eti d along the magnetic meridian Therefore the direction of magnetic north will vary SEA egites and in excess of 30 degrees to east or west of true north depending on the 1 ati COMPASSWEVIATI compass is pointin the difference between magnetic north and the direction in which the otli variation and deviation are measured in degrees east or west SS LEAST to the magnetic heading to give the true heading and e Remember ERROR EA Similarly easterly variation westerly variation must be subtra CAUS
25. measur ift using water pressure When a tube with an opening at its CK aered in water a pressure proportional to the depth to which the tube is submerged wi oped in the tube If the tube is held stationary the pressure remains constant and is termed stati p water whilst keeping the depth to which it is submerged cofjstanta second pressure called dynamic pressure is developed The total pressure in the tube calle static and dynamic pressures To ensure that the dynamic pressure reading and thus speed is te the efi must be eliminated This is achieved by installing a second tube close to the fifin sucha way that the static pressure produced in it is identical to that created in the Pitot tub increase due to movement through the water see Figure 3 1 72 Mechanical linkage Pressure seal See Pressure chambers Ship s hull Direction of movement LI H d tube intakes of a pressure tube speed logging system In a practical installation tubg B the Pitot tube extends below the vessel s hull to a depth d whereas ta b is flush with the hull With the vessel stationary the static japhragm and tube B to its underside almost cancel The the vessel moves through the water in the dire ffon of water is forced into tube B producing a combined pressure in the lower half of the chamber ggal to both the static and dynamic pressures The difference in pressure between upper
26. min from 58 to 44 m and P the instantaneous depth also shown as a large numerical display is 47 5 m Other operation detail is as L shown in the diagram What is not indicated on the display is the change of pulse length and period as selected by range 67 amp Table 2 2 Echo sounder range vs pulse length vs PRF Depth metres Pulse length ms PRF pulses per minute 5 10 and 20 0 25 750 40 0 38 375 100 1 00 150 200 2 00 75 400 and 800 3 60 42 s shan in Table 2 2 the pulse length is increased with the depth range to effectively allow more p to be contained in the transmitted pulse whilst the pulse period frequency is reduced to permit ngeg gaps in the transmission period allowing greater depths to be indicated In addition to the tigation mode Furuno FE 700 users are provided with a number of options adequately 68 OS DATA Mode 1 di VA E E UL EM Figure 2 14 Different display modes demonstrating the flexibility of a microcomputer controlled echo sounder Reproduced courtesy of Furuno Electric Co There are four display mode areas OS DATA mode Indicates own ship position GPS derived course time and a digital display of water depth DBS mode Provides a draft adjusted depth mode for referencing with maritime charts 69 LOGBOOK mode As the name suggests provides a facility for manually logging depths over a given period HISTORY mode Provides a mixture of contour and strata d
27. name suggests this is caused by the surfa in the ocean Marine life prevalent at depths between 200 and 750 m is the main ca se type of interference Bottom reverberation This depends upon the nature of the seabed Solid seabeds such as ha rock will produce greater scattering of the beam than silt or sandy seabeds Beam scattering caused by a solid seabed 1s particularly troublesome in fish finding systems because targets close to the seabed can be lost in the scatter lt P Q 52 Echo level Reverberation level Echo amplitude in dB Range in km Figure ison of steady state noise reverberation noise and signal amplitude A transducer is a convert WRF energy when applied to a transducer assembly will cause the unit to oscillate at its natural re p requency If the transmitting face of the unit is placed in contact caus acoustic waves to be transmitted in the water Any reflected acoustic energy will cause action at the transducer If the reflected energy comes into contact with the transducer face al nt Scillations will again be produced These oscillations will in turn cause a minute ele fromotuye force e m f to be created which is then processed by the receiver to produce the necessa a fi Three types of transducer construction are availa magnetostrictive Both the electrostrictive and the piez piezoelectric ceramic materials and the two should not be co 2 3 1 Electrostrictive transducers
28. only compl x arrangement of pressure tubes and chambers mounted in the engine room of a ship a ito e protruding through the hull c produ non linear sMdication of speed which must be converted to a linear indication to be of any value This is achi ither mechanically or electrically in the system e non linear characteristics of the vessel s hull and by the vessel pitching and rolling e possesses mechanical nst quire regular maintenance An electromagnetic speed log a measures W T speed only b produces a linear speed indications c operates by inducing a magnetic rl owing past the hull and detecting a minute change in the field d produces a varying speed indication as the co tivi e Indication may be affected by the vessel pitching e seawater changes Speed logs that use a frequency or phase shift betwe itted and the received radio wave generally use a frequency in the range 100 500 kHz They a d transmission format A log using the acoustic correlation technique for speed calculati a can operate in either W T or G T mode G T speed is also measured with res to a water mass b measures a time delay between transmitted and received pulses c produces a speed indication the accuracy of which is subject to all t i ental problems affecting the propagation of an acoustic wave into salt water See Chapter 2 Doppler frequency shift is a natural phenomenon that has been used for many years
29. rod of soft iron in a plane parallel t s horizontal magnetic field H will have a red end toward the south geographic pole This same ro tal plane but at right angles to the horizontal earth s field would have no magnetism inducedUn i cause its alignment in the magnetic field is such that there will be no tendency toward linear magngfiz and the rod is of negligible cross section Should the rod be aligned in some horizontal dif ction betwee create maximum and zero induction it would be induced by an amount that is ction of the angle of construction is equivalent to combinations of such rods will vary with the intensit heading and heel of the ship 4 3 1 Magnetic adjustment 4 3 Theory of magnetic compass adjustment Q gt The magnetic compass when used on a steel ship must be so corrected for the ship s magnetic conditions that its operation approximates that of a nonmagnetic ship Ship s magnetic conditions create deviations of the magnetic compass as well as sectors of sluggishness and unsteadiness Deviation is defined as deflection of the card needles to the right or left of the magnetic meridian Adjustment of 117 the compass is the arranging of magnetic and soft iron correctors about the binnacle so that their effects are equal and opposite to the effects of the magnetic material in the ship thus reducing the deviations and eliminating the sectors of sluggishness and unsteadiness The magnetic conditions in a ship whic
30. speed motor has been used in order that a distance indication may be produced that i inde ent of the non linear characteristic of the system The motor is started by contact 5 as previo scribed The main shaft 7 whose angle of rotation is directly proportional to the speed of th fitted with a screw spindle 12 The rotation of the shaft causes a lateral displacement of the fricti A ero speed the friction wheel rests against the apex of the distance cone 14 whilst at maximu ee wheel has been displaced along the cone to the rim The distance indicator 11 is driven Wom co tat speed motor 10 via the cone The nearer to the rim of the cone the friction wheel rides the greffer will be the distance indication Revolutions of the distance shaft 15 are transmitted to the remote system 16 and 17 Operation of the SAL 24E The SAL 24E utilizes the same system of tubes pressure indicator via the servo transmission n phragm to convert pressure variations due to speed to electrical pulses suitable to drive the e 1C circuits the mechanical arrangement of the SAL 24 log The distance integration me 1sm with servo cone and counter has been fully replaced with electronic circuitry tension on the spring assembly and producing an output from the differential transformet This out t is applied to the USER board shown in Figure 3 6 where it is processed to provide the drive fo the speed servo control winding v
31. the diaphragm to move upwards pushing the pressure rod 2 76 and moving the lever 3 to the right on pivot 4 The upper end of the lever 3 moves the electric start contact 5 to the right to connect power to a reversible motor 6 The motor now turns causing the main shaft 7 to move a spiral cam 8 clockwise This action tilts the lever 9 also pivoted on 4 to the left The deflection stretches the main spring producing a downward pressure on the diaphragm via lever 3 causing it to cease rising at an intermediate position This is achieved when equilibrium has been established between the dynamic pressure acting on the lower side of the diaphragm and the counter pressure from the spring on the upper side At this point the motor 6 stops and thus holds the spiral cam 8 in a fixed position indicating speed his thod of pressure compensation provides accurate indications of speed independent of d to produce a linear indication of speed from the non linear characteristics of the system atta to the spiral cam is a second gearing mechanism 19 that transfers the movement of the spee r to R three phase speed transmission system 20 An identical servo receiver 22 is fitted in he rexf te speed repeater unit fitted on the ship s bridge and thus remote speed indication has been achie Distance recording i ed by using a constant speed motor 10 which drives the distance counter 11 via friction g aringy T t
32. upwards Tilt of the north end of the spin axis is upwards if the north end is to meridian and downwards if it is to the west of the meridian The gyro will appear to eXecute one the gyro is directly over the equator The relationship between drift and tilt can be shown graphically complete revolution about the horizontal axis for each sidereal day No drift in azimuth occurs Affen Q see Figure 1 7 4 Drift 15 sin lat East degrees per hour N Pole Equator Equator Tilt 15 cos lat degrees per hour West 15h Figure 1 7 The graphical relationship between drift and tilt fe af gyro drift will be maximum at the poles and zero at the equator whilst gyro tilt is the reciprocal gf this At any intermediate latitude the gyro will suffer from both drift and tilt with the magnitude Of each error proportional to the sine and cosine of the latitude respectively When a gyro is placegfexagtly with its spin axis parallel to the spin axis of the earth at any latitude the i i relative to the earth There is no tilt or azimuth movement and the gyro may be considered e Meridian stabilized As the earth rotates the gyro will experience a movement under the influefiige o t and azimuth motion The rate of tilt motion is given as o latitude degrees per hour n azimuth drift 1s azimuth dri de degrees per hour 1 2 3 Movement over the earth s surfa e The free gyroscope as detailed so far is o ic
33. ES OF DEVIATION AII vessels nugs rous magnetic fields Some of these fields are permanently built into the structure of th amp vess l an so are caused by the type of cargo carried electronic instruments position of machine d e uipment etc Figure 6 Some cargoes may affect the magnetic compass more than others These magnetic fields can combine to cause the compass needle to point away or deviate fm magnetism is influenced by the earth s own The vessel s soft iron magnetism changes with the magnetic north The amount of deviation can vary considerably from heading to heading as the v S Q orientation and location of the vessel and is known as induced magnetism Hard iron magnetism remains constant is built into the vessel and is known as permanent magnetism The aim of the compass adjuster is to nullify the effect of the unwanted magnetic fields by placing compensating magnets and soft iron correctors adjacent to the compass These create equal but opposing magnetic fields thus eliminating the deviating fields around the compass enabling it to align correctly Each axis vertical longitudinal and athwartships is treated seperately 109 4 1 4 Swinging the compass Swinging the compass or swinging the ship as the operation is sometimes called typically involves taking the vessel to a suitable location and with the vessel steady on each of the eight primary compass points comparing the difference between existing com
34. Figure 3 24 Furuno Doppler Sonar DS 30 display modes Reproduced courtesy of Furuno Electric Co O The system uses a triple beam 440 kHz pulsed transmission and from the received Doppler shifted signal calculates longitudinal thwartship speeds and depth beneath the keel at the bow d In addition a Laser Gyro may be fitted on the stern to provide a further data input of transverse speed and rate of turn information see Figures 3 21 and 3 25 Position data from a GPS receiver may also be input to the CPU 100 There are three principle modes of data display The Speed Mode showing all the normal speed depth distance indications The Berthing Mode which with the additional inputs from a laser gyro at the stern shows a vessel s movements during low speed manoeuvres see Figure 3 25 The Nav Data Mode with a display reminiscent of an integrated navigation system Figure 3 25 Triple beam transducer configuration of the Fufuno D ppler Sonar Log Note the forces acting on the vessel during a starboard turn under the influenc lof 4 cr ss current from the port side Reproduced courtesy of Furuno Electric Co Berthing Mode display The display diagram key shows the following A Intersection of perpendicular from ship s ref point to marker line B Yellow arrowhead showing wind direction C Blue arrowhead showing current direction D Echo monitor E Tracking mode F Heading input from gyro G Rate of turn
35. Other buttons functions are self evident 1 9 2 System description Figure 1 31 shows to the left of the CPU assembly the gyro and to the right of the CPU the Display and Control Panel and ou ta lines all its control function lines 30 peyoymsun Buipeeu zezsu peyoums Buipeeu zezrsu Asse ipeuouws Buipeeu zezSu SICV Bojyeue uim Jo ayey Sav ipeuowwsun g y49 deis E JeAUp deis UO Asse peyoms ier L yo deis SIS reuondo pejoeloo Asse oijou s x oJuou s Lam a cg Jeppny L J8PPNY puondo Jep1 9el ezgu esinog xew JOP ai OONO eiqneduioo uosd3 jaued jouoo Aejdsiq Asse fido Asse de eHeAuco WNdd 007 poo _9a 90 Vc SL Asse slay eersy veisi jpeeds 19 SQV cecsH VE AG VS AvcC Jojoedoeo ues ejqeu3 43H ZHoor ASLL EU Uu piei pexy eiqeu3 amp y Asse L IPSUM X n I W HIL a E o 2q og Q vc to a e Jd OY OWA OEgc SLL o p E O E q Oo a 5 c OVA SIL 5 o o E Buu 9 feo Jeeuw 5 e1eudsoi ey o o moq o Sees 5i snojnpueg g requibm 3 g yo a woueyd Jeeb v unuzv 9 am eyed 5 yoddns c 2 JO OUW Jeyiusue y o unulzv oJuou gG z 4 7 eyep BuipeeH 5 gt O Y e o 3 D LL The gyrosphere is supported by a phantom yoke and suspended below the main support plate A 1 speed synchro transmitter is mounted to
36. T py e evic Stepper systems are transmissiorKd y the bearing on the master compass to remote repeaters e 1 15 Revision questions Describe what you understand by the term gyroscopic inerti 2 What do you understand by the term precession when applied ta gyr gompass 3 Why is a free gyroscope of no use for navigation purposes 4 How is earth s gravity used to turn a controlled gyroscope into a north seeka 5 How is a north seeking gyroscope made to settle on the meridian and indi 6 When first switched on a gyrocompass has a long settling period in some cas 75 min Why is this e 7 Explain the terms gyro tilt and gyro drift 8 How is a gyrocompass stabilized in azimuth 9 What is rolling error and how may its effects be minimized 10 Why do gyrocompass units incorporate some form of latitude correction adjustment 11 What effect does an alteration of a ship s course have on a gyrocompass 12 What are static errors in a gyrocompass system 13 When would you use the slew rate control on a gyrocompass unit P 14 Why is temperature compensation critical in a gyrocompass 15 What is a compass follow up system 16 What is a compass repeater system 47 amp Chapter 2 The Ship s Echosounder 2 1 Introduction Sonar sound navigation and ranging is the acronym identifying those systems that rely for their n on the transmission and reception of acoustic energy in water The term is widely used to all modern s
37. a puke from the data store The new depth is now displayed on the indicator and the countets a setpat the start of the next transmission pulse C4 With any echo sounder it is necessary that the clock pulse rate be directly related to depth f e shallow 100 m range is selected a high frequency is used which is reduced by a factor of 10 wh deep range 1000 m is selected Modern echo sounders rely for their operation on the TT e microprocessor and digital circuitry but the system principles remain the same It is the display of information that is the outward sign of the advance in technology e 2 6 A digitized echo sounding system L The Furuno Electric Co Ltd one of the world s big manufacturers of marine equipment produces an 63 amp echo sounder the FE606 in which many of the functions have been digitized Transmission frequency is either 50 or 200 kHz depending upon navigation requirements A choice of 50 kHz provides greater depth indication and a wider beamwidth reducing the chance that the vessel may run away from an echo see Figure 2 10 The pulse length increases with depth range from 0 4 ms on the shallow ranges to 2 0 ms on the maximum range This enables better target discrimination on the lower ranges and ensures that sufficient pulse power is available on the higher ranges Pulse repetition rate sounding rate is reduced as range increases to ensure adequate time between pulses for echoes to be returned
38. abe pth exceeds a predetermined figure 20 m is typical from a water mass below the keel Pro ise Pin CW operation particularly in deep water when the transmitted beam is caused to scatter by ing number of particles in the water Energy due to scattering will be returned to the transd dition 9 the energy returned from the water mass due to the increasing effects of scattering The speed indi is now very erratic and may fall to zero CW systems are rarely used for this reason Pulse mode operation To overcome the problems of the CW system a pulse mode oper to that described previously for depth sounding where a high y pulse is receiver off The returned acoustic energy is received by the same transduc switched to the receive mode In addition to overcoming the signal loss pr in the CW system the pulse mode system has the big advantage that the number of transducers is required O e Comparison of the pulse and the CW systems Pulse systems are able to operate in the ground reference mode at depths up to 300 mi d di n ed This is virtually identical em caus scattering upon the carrier frequency used and in the water track mode in any depth of water whefeas t CW systems are limited to depths of less than 60 m However CW systems are superior in v shallow water where the pulse system is limited by the pulse repetition frequency PRF of the Q operating cycle The pulse system requires only one transducer two for the J
39. across the surface of the paper on top of the left hand roller The paper is mar y burning the surface with a high voltage charge produced through the paper between the stylus an ground Depending upon the size of the returned echo the marking voltage is between 440 and 1100 V and is produced from a print voltage oscillator running at 2 kHz Oscillator amplifier output is a constant amplitude signal the threshold level of which is raised by the d c produced by a detected echo signal Thus a high intensity echo signal causes the marking voltage to be raised above the threshold level by a greater amount than would be caused by a detected small echo signal For accurate depth marking it is essential that the stylus tracking speed is absolutely precise The stylus 62 is moved along the paper by a belt controlled by the stylus d c motor Speed accuracy is maintained by a complex feedback loop and tacho generator circuit Digital circuits The digital display section contains the necessary logic to drive the integral three digit depth display the alarm circuit and the remote indicators Pulse repetition frequency PRF of the clock oscillator is pre set so that the time taken for the three digit counter to count from 000 to 999 is exactly the same as that taken by the paper stylus to travel from zero to the maximum reading for the range in use The counter output is therefore directly related to depth herxthe chart recorder is switched off the digital
40. adjuster Effective correction or compensation of the marine compass for any deviation error found duri compass swing requires an understanding of the earth s and ship s magnetic fields and an ability Q differentiate between the permanent magnetism of the ship s hard iron and the induced magnetism o the ship s soft iron It is necessary to recognise the effect the various magnetic fields have on the ship s compass and to have a practical knowledge of the workings of the marine compass and its correctors Simply reducing or eliminating compass deviation on a vessel in one location can actually make it worse when the vessel travels to another location particularly when substantial changes in latitude are involved Whilst amateur or DIY compass adjusting is not a completely outrageous concept on pleasure craft it 113 has been known to transform a relatively simple problem into a fairly complex one particularly on steel vessels Most licensed compass adjusters are highly skilled technicians professional seafarers and qualified navigators who have undertaken rigorous and comprehensive training to meetnational and international standards National marine agencies specify that commercial vessels have their compass adjusted only by a person qualified and authorised to do so International standards for magnetic compasses and compass adjusting are governed by the International Organization for Standardization ISO and the jonal Maritime Organizati
41. al use for navigation since its rotor axis is h s surface The stabilized gyroscopic a correction for the earth s rotary motion Movement in latitude along a meridian of longitu es rotation about an axis through the centre of the earth at right angles to its spin axis Moveme h chanism in any direction is simply a combination of the latitudinal and longitudinal motions faster the scope moves the greater the rate of angular movement of the rotor axle attributable to these fact 1 3 The controlled gyroscope e It has been stated that a free gyroscope suffers an apparent movement in both azimut oa e own rotor axis depending upon its latitudinal location When fitted to a vessel the latitude is consequently the extent of movement in azimuth and tilt is also known It is possible therefi calculate the necessary force required to produce a reciprocal action to correct the effect of C movement A force can be applied to the gyro that will cause both azimuth and tilt precession to occur in opposition to the unwanted force caused by the gyro s position on the earth The amplitude of the reciprocal force must be exactly that of the force producing the unwanted movement otherwise over or P under correction will occur If the negative feedback is correctly applied the gyro will no longer seek a celestial point but will be terrestrially stabilized and will assume a fixed attitude If the gyro is drifting in azimuth at N degrees per hour i
42. als assembly the unit is free to move in two plan each at right angles to the plane of spin There are therefore three axes in which the gyroscope is free to illUgtrated in Figure 1 1 oe e thespin axis the horizontal axis the vertical axis Jp Balanced Horizontal Gimbal syste pivot h d i M e Sxis v Mounting platform verticli axis Figure 1 1 A free gyroscope Reproduced courtesy of S G Brown Ltd Q In a free gyroscope none of the three freedoms is restricted in any way Such a gyroscope is almost the constrained and the spring restrained are now rarely seen In order to understand the basic operation of a free gyroscope reference must be made to some of the first principles of physics A free gyroscope possesses certain inherent properties one of which is inertia a phenomenon that can be directly related to one of the basic laws of motion documented by Sir universally used in the construction of marine gyrocompass mechanisms Two other types of gyroscope lt P Q 1 Isaac Newton Newton s first law of motion states that a body will remain in its state of rest or uniform motion in a straight line unless a force is applied to change that state Therefore a spinning mass will remain in its plane of rotation unless acted upon by an external force Consequently the spinning mass offers opposition to an external force This is called gyroscopic inertia A gyroscope rotor maintains the direc
43. also occurs but this is detected by the azimuth pick up coils The azimuth servomotor now drives the secondary gimbal to rotate the tank in azimuth to seek cancellation of the error signal Since the azimuth secondary gimbal maintains a fixed position relative to the gyro spin axis in azimuth a direct heading indication is produced on the compass card mounted on this gimbal Control of the sensitive element in tilt is done in a similar way Therefore signals injected into the tilt and azimuth servo loops having a sign and amplitude that produce the required precessional directions d rates will achieve total control of the gyrocompass It 47a relatively simple task to control the gyroball further by the introduction of additional signals cauge each of the feedback loops is essentially an electrical loop One such signal is produced by the action the gyroscopic unit must detect movement about the east west horizontal axis Th lum unit is therefore mounted to the west side of the tank level with the centre line It is an Sect system consisting of an E shaped laminated transformer core fixed to the case with pendaflim bob freely suspended by two flexible copper strips from the top of the assembly The transfo igure jl 37 has series opposing wound coils on the outer E sections lulum bob centres on the middle arm of the E core and is Sili luid windings Suspension Can strip Limit Ibar magnet scr
44. and lower w forces the diaphragm upwards thus more the diaphragm will move and the greater will be the d Unfortunately the dynamic pressure developed in tube B by t ovement through the water is proportional to the square of the vessel s speed Pitot s Law sta t this press is proportional to the square of the ship s speed v multiplied by the coefficient K p Kxv e where the constant K is derived from the vessel s tonnage shape of hull speed of the shipgand length of the protruding part of the Pitot tube distance d As shown in Figure 3 2 the speed indication produced is not linear It is necessary therefi o eliminate the non linear characteristics of the system and produce a linear speed indication This is achieved mechanically by the use of precisely engineered cones or electronically using r6 capacitive resistive time constant circuitry P 73 amp o Pressure N m x 10 3 2 MO EDN o 0 3 0 3 6 9 12 15 18 21 24 27 30 33 36 39 Speed knots igure 3 2 Graph indicating the non linear increase in pressure due to speed ssure tfe speed logging system Figure 3 sh a typical installation of the Pitot system on board a vessel with a double bottom The Pitot tube 1SX ncased in a cock arrangement with valve control to enable the tube to be withdrawn without shipping wa en the vessel goes alongside The static pressure opening is controlled by static pressures are transferred via air collectors and st
45. anus configuration whereas separate elements are needed for CW operation P CW systems are limited by noise due to air bubbles from the vessel s own propeller particularly L when going astern Q Pulse system accuracy although slightly inferior to the CW system is constant for all operating 98 depths of water whereas the accuracy of the CW system is better in shallow water but rapidly reduces as depth increases 3 6 4 Environmental factors affecting the accuracy of speed logs Unfortunately environmental factors can introduce errors and or produce sporadic indications in any system that relies for its operation on the transmission and reception of acoustic waves in salt water Water clarity In exceptional cases the purity of the seawater may lead to insufficient scattering of the acoustic energy and prevent an adequate signal return It is not likely to be a significant factor because most seawater holds the suspended particles and micro organisms that adequately scatter coustic beam Aeration Aerated water bubbles beneath the transducer face may reflect acoustic energy of ficient strength to be interpreted erroneously as sea bottom returns producing inaccurate depth indi ns and reduced speed accuracy Proper siting of the transducer away from bow thrusters A wit reduce this error factor Ve t c rity and list speed indication a change in the vessel s trim from the calibrated normal will affect fore aft
46. beam ner As Figure 2 9 shows the main beam is central to the transducer face and shorter sidelobes are Also produced The beamwidth must not be excessively narrow otherwise echoes may be Q 1 Q particularly in heavy weather when the vessel is rolling Figure 2 9 Transmission beam showing the sidelobes 59 2 1o A low PRF combined with a fast ship speed can in some cases lead to the vessel running away from an echo that could well be missed In general beamwidths measured at the half power points 3 dB used for depth sounding apparatus are between 15 and 25 To obtain this relatively narrow beamwidth the transducer needs to be constructed with a size equal to many wavelengths of the frequency in use This fact dictates that the transducer will be physically large for the lower acoustic frequencies used in depth sounding In order to reduce the transducer size and keep a narrow beamwidth it is possible to increase the transmission frequency However the resulting signal attenuation negates this change and in practice a ise must once again be reached between frequency transducer size and beamwidth Figure mp 2 hows typical beamwidths for a low frequency 50 kHz sounder and that of a frequency four es eater Figure 2 10 Typical beamwidths for echo sounders tra t w and high frequencies Reproduced courtesy Furuno Elect td 2 5 A generic echo sounding system Compared with other systems echo sounder circuitr
47. ccuracy of a gyrocompass can be s slyf ffected by violent movement of the vessel particularly heavy rolling caused by severe stormsXind noeuvring A carefully calibrated error ill be present due to misalignment of the tank and gyro spin axis during such conditions an is to the tilt amplifier to control the tilt gimbals The system will provide partial and adequate comperisati ors that arise due to violent rolling conditions The correction system is more than adequate ittings on Mgitchant Navy vessels that are rarely subjected to rapid manoeuvres 1 10 5 Slew rate The purpose of the slew rate control VR27 see Figure 1 36 is to rapidly leWel an during the start up procedure The potentiometer VR27 is connected across the 24 e tappgd secondary winding of a transformer and is therefore able to produce an output of opposi phage and varying amplitude The signal voltage level set by VR27 may be applied to the input of gfther azimuth or tilt amplifiers separately by the use of push buttons The buttons are interconnected in f a way that the signal cannot be applied to both amplifiers at the same time If the output of VR27 is firstly applied to the tilt servo amplifier by pressing the azimuth slew button the gyro will precess towards the meridian If the tilt slew button is now pressed the gyro will O gt levelled by applying the output of VR27 to the azimuth servomotor The slew rate control VR27 adjusts the rate at which the gyr
48. ce the necessary anti tilt precession for the gyro e made suitable for use as a navigation instrument Figure 1 10 shows the c ow ibed by the north end of the damped gyrocompass which will settle in the meridian An alte ore commonly used method of applying anti tilt damping is shown in Figure 1 13 Damping WF force C Force inward Motion inward Control force Damping force Total force a b ind Figure 1 13 a Effect of control force plus damping force b An alternative method pplyi offset damping Reproduced courtesy of Sperry Ltd Damping gyroscopic precession by the use of weights provides a readily adjustable system for ap damping The period of gyro damping is directly related to the size of the damping force and thus the weight If the weight is increased the damping percentage will be increased The effect of alternative damping application is illustrated in Figure 1 14 lt gt Q 15 Damping Damping or force anti tilt precession Control precession Damping motion Control 1 force B Force inward Motion inward 27 Figure 1 14 The effects of alternative damping application therefore that damping a maximum at the equator However the damping period will always remain constan ximately 86 min for some gyros despite the change of amplitude of west Of the gyro axle All gyrocompasses therefore require time to settle Figure 1 15 shows a typical s rv sfor a gy
49. component log the ship can be navigated so that heading steered plus drift angle 95 measured by the log results exactly in the intended chart course see Figure 3 22 Elektronik The transverse speed a eMis computed from the transverse speed of the bow the ship s rate of turn and the ship s length aSfoll oO V Vii oL A o where Vq2 stern transverse speed V41 bow transv pee rate of turn angular velocity and L distance between bow and stern points of measurement 3 6 2 Choice of frequency transducer As with depth sounding the size of the transducer can be kept within reasonabi fimits by using a high frequency This is particularly important in the situation where many eleme ounted in the same assembly Unfortunately as has already been discussed attenuation lo ponentially with the transmission frequency The choice of frequency is therefore a com between acceptable transducer size and the power requirements of the acoustic wave in ordeKto the signal losses due to the transmission media Frequencies used in speed logging system v id and are usually in the range 100 kHz to 1 MHz The factor with the greatest effect on speed accuracy is the velocity of the acoustic wave in sea T Propagation velocity is affected by both the salinity and the temperature of the seawater through e e the wave travels However velocity error due to these two factors can be virtually eliminated by mounting salinity and tem
50. controlled precessions are produced Referring to Figure 1 36 to precess the gyroball in azimuth only an external signal is injected into the tilt amplifier The null signal condition of the pick up coils is now unbalanced and an output is produced and fed back to drive the tilt servomotor This in turn drives the tilt secondary gimbal system to a position in which the tilt pick up coil misalignment voltage is equal and opposite to the external voltage applied to the amplifier mm lt z ge 22 ee E BS E z 254 2 if 7 e zt 5 lt LES Fi 2i 8 f A EAN ET 55 3 i o Bee cii 388 Ez s Samat Ty E iP x H B x5 9 E j 1 sg y X a 8 DL EY v S 1 E i HT E ix Rn usi d E an us Bgm d 7 D i HET a gt en co ED 2s ii is o zi t j 70 mo ME Eid N 33 ero wd ax eat E io MER E a i T E LE SPEEO UNIT DRIVEN BY SHIP S LOG TEMPERATURE COMPENSATOR gt Figure 1 36 Compass circuits schematic Reproduced courtesy of S G Brown Ltd The tilt servo feedback loop is now nulled but with the tank and gyroball out of alignment in a tilt mode A twist is thus produced of the horizontal torsion wires creating a torque about the horizontal 37 TEMPERATURE 5 eC p 1 axis of the gyroball and causing it to precess in azimuth As azimuth precession occurs azimuth misalignment of the tank gyroball
51. d 2 Wi OX ae eM WX fiiit in Support of the Subject of Shipborne Navigational Aids includi rocompass Echosounder Speedl ran Magnetic Compass Zo Shanghai Maritime University O Merchant Marine College 4 gt Contents 1 1 Tel idee VT Ce wins icivice ees cus swscca ccuscudcuvsevscedcucce denncescedccacewceessescesddadesd vecedcedccaccecs 1 2 Chapter 1 The Ship s Gyrocompass e eese eene eene eene eee eene enne nnn nnne nnne 1 1 14 Summary 1 15 Revision questions 5 Chapter 2 The Ship s Echosounder 2 1 Introduction 2 3 TYANSAUCEIS icisinscvecssaivscoscessssceecensssee Nr vos 2 4 Depth sounding principles 2 5 A generic echo sounding system 2 6 A digitized echo sounding system 2 7 A microcomputer echo sounding system 2 8 GIOSS ANY enc N 2 9 SUMMAT m ERR 2 10 Revision questions sessssssssosssssssssessossssssseseseosssssossssssesse Chapter 3 The Ship s Speed Log 3 1 Introduction 5 oe eere erre toe eere eve Eu Ve ro EN uu Ea resur EF ropa ve EE PEE VE vana Meese 3 2 Speed measurement using water pressure 3 3 Speed measurement using electromagnetic induction 3 4 Speed measurement using acoustic correlation techniques 85 3 5 The Doppler principle oseriocie sorti pix puc Vis eu xke exin s Dua oco s coi Rv EU CM VP Cad ER REG 89 3 7 The Furono Doppler Sonar DS 50 System
52. d bearings are protected from excessive s ads Sensitivity to shifts of the gyrosphere s centre of mass relative to the sengifjve axis 1s eliminated The effects of accelerations are minimized because the gyrosphere s ce the centre of buoyancy are coincident The system s applications software compensates for the effects of the ship s varying s local e latitude in addition to providing accurate follow up data maintaining yoke alignment With gyrosphere during turn manoeuvres L 1 9 1 Control panel Lo All command information is input via the control panel which also displays various data and syst Q ye 3 indications and alarms see Figure 1 30 29 9 10 16 15 14 13 12 11 rol panel Reproduced courtesy of Litton Marine Systems The Mode switch nu y fixed when using a single system the Active indicator lights and a figure 1 appear in winde Other Mode indicators include STBY showing when the gyrocompass is in a dual co on awd not supplying outputs Settle lights during compass start up Primary lights to show e primary compass of a dual system and Sec when it is the secondary unit Number 7 indicates the Heading display accu to yit 1 10th of a degree Other displays are number 14 speed display to the nearest knot ber 5 latitude to the nearest degree and 16 the data oll buttons 17 18 and 19 control this display used to display menu options and fault Mess display
53. d gear train and the bearing stepper transmitter 1 8 2 The movable element With the exception of the phantom ring the movable element is called the sensitive element Figure 1 24 At the heart of the unit is the gyro rotor freely spinning at approximately 12 000 rpm The rotor in diameter and 60 mm thick and forms along with the stator windings a three phase ransformer seconda Damping weight Follow up transformer _ Te Suspension primary ii wire assembly low up X i A EIE LAE en L E d Spirit level Z d les p 4 fic He i o o a ENN m q HE Jj l N 9 NG KO o o Liquid ballistic Sim S mido zeli Vertical ring dm or e assembly Figure 1 24 The compass sensitivealement A sensitive spirit level graduated to represent 2 min of af isafOunted on the north side of the rotor case This unit indicates the tilt of the sensitive element A dampi t is attached to the west side of the rotor case in order that oscillation of the gyro axis can be d and thus le the compass to point north The rotor case is suspended along the vertical axis inside the vertical ri suspension wire 7 This is a bunch of six thin stainless steel wires that are e absolutely free oa from torsion Their function is to support the weight of the gyro and thus remove t rom ge support bearings 2 1 8 3 Tilt stabilization liquid ballistic L To enable the compass to develop a north seeking action
54. deviation history if available may be made prior to sailing Other adjustments if made with the vessel alongside will be largely based on guess work and cannot be relied upon until the compass has been fully swung The compass adjuster must be on board the vessel for the compass swing A few compass adjusters will claim that because of their expertise there is no need for them to go to the trouble of swinging the ship Large discrepancies between actual deviation and that predicted by the adjuster sometimes as much as 30 degrees have been observed on compasses which have been expertly adjusted without swinging the compass A valid deviation card cannot be issued until the mpass has been properly swung 4 The period the compass shall be swung ver a period of time or after certain events the vessel s magnetic fields may change altering the al deviation of the compass In some circumstances the changes can be quite dramatic New steel ey a compass adjusted when first commissioned It is not unusual for a one or two year old Wess record deviation of 30 to 40 degrees as the residual magnetic fields created during the building prdgess gradual Sea going vessels ar ed to observe and record compass deviation daily whilst on passage These for safe navigation but also to assist the compass adjuster in making an accurate analysis e causes of deviation should the compass require adjustment d organisations including the US Coastguard
55. discrimination This factor is particularly important in fish fe apparatus where very short duration pulses typically 0 25 or 0 5 ms are used Q Echo discrimination D is D V xl in metres P Q where V the velocity of acoustic waves and pulse length For a 0 5 ms pulse length 58 D 1500 x0 5 x10 0 75m For a 2 ms pulse length D 1500 x2 10 3m Obffously a short pulse length is superior where objects to be displayed are close together in the water ort pulse lengths tend to be used in fish finding systems length also improves the quality of the returned echo because reverberation noise will be atjon Roise is directly proportional to the signal strength therefore reducing the pulse length r amp g nal strength which in turn reduces noise Unfortunately reducing the signal strength in this way feduces the energy transmitted thereby limiting the maximum depth from which satisfactory echoes c ceived Obviously a compromise has to be made Most depth sounders are For a given pulse length the PI E eti tivaly determines the maximum range that can be indicated It is a measure of the time interval b S Phen transmission has ceased and the receiver is awaiting the returned echo delayed for a pre determined period after transmission overcomes the over range indication e 2 4 2 Transmission beamwidth Acoustic energy is radiated vertically downwards from the transducer in the form of a
56. ducer active elements are kept to a minimum by the use of a high frequency and a wide lobe angle A wide lobe angle beamwidth is used because echo target discrimination is not important in the speed log operation and has the advantage that the vessel is unlikely to run away returned echo place in the correlation block mpfed output of each channel eed and distance counters An analogue voltage the gradient of which i drive the potentiometer servo type speed indicators Apulse frequency proportional to speed The freque into the digital counter by a 1 8 s gate pulse A positive negative voltage level to set the ahead astern indication or the B tr indication 2000 pulses per nautical mile to drive the stepping motor in the digital The depth unit provides the following outputs to drive the depth indicators w unding facility is used Ananalogue voltage with a gradient of 0 01 Vm to drive the analogue depth indicat Pulses of 2msm which are used to gate a 5 kHz standard frequency into the de indicator A positive negative voltage level to cause the indicator to display normal operatio or overrange Q When correctly installed and calibrated a speed accuracy of 0 1 knot is to be expected Distance accuracy is quoted as 0 2 The SAL ICCOR speed log can be made to measure the vessel s e Q transverse speed with the addition of a second transducer set at 90 to the first
57. e T has been determined the speed of the vessel v can be accurately calculated 86 di 5 gt B8 z o 2 25 meus 228 28 998 se 358 cog Da5 oS nas Does cov PS cas s Lae LUS od amp Xo as E w Correlation i E frequency 5 kHz Clock unit e e E Standard to speed SE 2 g 2 0 aa cog cs 32 O lt a Gating pulses proportional to speed 0 1V knot proportional to depth 0 01Vm 1 Voltage Voltage Frequency proportional rative unit Administ Carrier frequency 150 Depth unit Speed unit Depth signi unit Sampling Amplifier E E E o Power Figure 3 14 System diagram of the SAL ACCOR acoustic correlation speed log Lb Reproduced courtesy of SAL Junger Marine Q It should be noted that the calculated time delay 7 is that between the two transducer echoes and not that between transmission and reception Temperature and salinity the variables of sound velocity in seawater will not affect the calculation Each variable has the same influence on each received echo P channel Consequently the variables will cancel L It is also possible to use the time delay T between transmission and reception to calculate depth In this case the depth d in metres is 87 amp d TxC 2 where C the velocity of sonic energy in seawater 1500 ms Dimensions of the trans
58. e 22 illustrates a point about compass operation Not only is an uncorrected co large deviations but there will be sectors in which the compass may sluggishly turn ship and other sectors in which the compass is too unsteady to use These performances may be appr d visualizing a ship with deviations as shown in Figure 22 as it swings from west through north toward er east Throughout this easterly swing the compass deviation is growing more easterly and wh ss pubject AG te steering in this sector the compass card sluggishly tries to follow the ship Similarly there is a unsteady sector from east through south to west These sluggish and unsteady conditions are always characterized by the positive and negative slopes in a deviation curve These conditions may also be associated with the maximum and minimum directive force acting on the compass It will be observed P that the maximum deviation occurs at the point of average directive force and that the zero deviations L Q occur at the points of maximum and minimum directive force 123 East Deg Dev 90 oint of Point of Point of Maximum Maximum Maximum Sluggish Deviation Unsteadiness ness Heading Degrees West g g Figure 22 Uncompensated deviation curve compass errors is generally achieved by applying correctors so as to reduce the cMupass for all headings of the ship Correction could be achieved however by d is more generally used becaus
59. e 3 17 a shows this angle Using trigonometry cos_ Adjacent Hypotenuse raat Py Adjacent C cos gt Given a propagation angle of 60 cos 0 5 L 91 fd 2vftcos C vft C Adjacent Adjacent cos Hypotenuse Hypotenuse c Speed adjacent c cos 6 Figure 3 17 a Derivation of long ed using trigonometry b The effect of pitching on a Janus transducer configuration e It follows that if the angle changes the ed ulated will be in error because the angle of propagation has been applied to the speed G lcu mts in this way If the vessel is not in correct trim or pitching in heavy weather the longitu meters will change and the speed indicated will after the Roman god who reputedly possessed two faces a past Figure 3 17 b shows the Janus assembly The Doppler frequency shift formula now becomes 2vft fd um cos cos cos 60 cos 60 1 therefore the transmission angle can effectively be ignore e As Figure 3 17 b shows in heavy weather one angle increases as the other decfeasesfeffeetively cancelling the effects of pitching on the speed indication Figure 3 18 shows the advantage of having a Janus configuration over a single transducer arrangement It can be seen that a 3 change of trim on a vessel in a forward pointing Doppler system will pro a 5 velocity error With a Janus configuration transducer system the error is reduced to 0 2 but is not
60. e illustrated in Figure 18 showing the ship on various compass headings The other heading fectg may be similarly studied a lation curve is one of the curves in Figure 15 b It will be noted that these D deviations are 9 th ntercardinal headings and zero on the cardinal headings North heading Northeast heading East heading Southeast heading y s by compass by compass by compass E Dev No deviation Maximum deviation No de easterly ximum deviation terly D Deviations East Deg Dev West Compass Heading Degrees Figure 18 Effects of symmetrical horizontal D induced magnetism Asymmetrical arrangements of horizontal soft iron may exist about the compass in a patte similar to one of those in Figure 19 Lb Q Figure 19 Asymmetrical arrangements of horizontal soft iron The deviations resulting from the earth s field induction of these asymmetrical arrangements of horizontal soft iron are illustrated in Figure 20 showing the ship on different compass headings The 121 other heading effects may be similarly studied Such an deviation curve is one of the curves in Figure 15 b It will be observed that these E deviations are maximum on cardinal headings and zero on the intercardinal headings North heading Northeast heading East heading Southeast heading by compass by compass by compass by compass E Dev W Dev Compass Compass L Needle yo deviation Maximum deviation No
61. e it utilizes the compass itself to indicate results rather than sopfe additional instrument for measuring the intensity of magnetic fields Occasionally the overcome the earth s iy force H This condition will not only create sluggish and unsteady sectors but may even free ss to one reading or to one quadrant regardless of the heading of the ship Should the compa beg n the polarity of the magnetism which must be attracting the compass needles is indicated cota may be effected simply by the application of permanent magnet correctors in suitable gto neutralize this magnetism Whenever such adjustments are made it would be well to shi iffa on a heading such that the unfreezing of the compass needles will be immediately evident example a ship whose compass is frozen to a north reading would require fore and aft B corre amp tor with the red ends forward in order to ade on an east heading such an was freed so as to indicate an east heading Listed below are several reasons for correcting the errors of the etic compass 1 It is easier to use a magnetic compass if the deviations are small 2 Although a common matter what the deviations are as long as they are known this is in error sluggishness and unsteadiness accompany large deviations and consequ ethe compass operationally unsatisfactory This is the result of unequal directive forces on the ee the ship swings in heading 3 Furthermore even though the deviations are
62. e magnetic effects remain constant for all magnetic latitudes and others vary with changes of L Q magnetic latitude each individual effect should be corrected independently Further it is apparent that the best method of adjustment is to use 1 permanent magnet correctors to create equal and opposite 122 vectors of permanent magnetic fields at the compass and 2 soft iron correctors to assume induced magnetism the effect of which will be equal and opposite to the induced effects of the ship for all magnetic latitude and heading conditions The compass binnacle provides for the support of the compass and such correctors Study of the binnacle in Figure 21 will reveal that such correctors are present in the form of 1 Vertical permanent heeling magnet in the central vertical tube 2 Fore and aft B permanent magnets in their trays 3 Athwartship C permanent magnets in their trays 4 Vertical soft iron Flinders bar in its external tube 5 Soft iron spheres The heeling magnet is the only corrector that corrects for both permanent and induced effects and nseggently must be readjusted occasionally with radical changes in latitude of the ship It must be nos d however that any movement of the heeling magnet will require readjustment of other rrecdors Degaussing Compensating Flinders Bar Sphere e p j Fore and aft B Magnet Trays Figure 21Binnacle with compass and correctors 4 3 5 Compass operation Figur
63. e off s as the ship is in the same magnetic latitude its vertical induced pole swinging about the c Ww ramos the same effect on the compass as a permanent pole swinging about the compass Figure 16 I es the vertical induced poles in the structures of a ship Generally this semicircular deviation waffbe a B Se curve as shown in Figure 16 b since most ships are symmetrical about the centerlifie_an e their compasses mounted on the centerline The magnitude of these deviations will change force and the ship s vertical induction both change wit Vertical Induced Magnetic B Deviations East Resultant of Vertical Vertical I Ns A Deviations from Unsymmetrical Induced Components Mimi ram West Horizontal Soft Iron North Latitudes Ship s Compass Heading Degrees Figure 16 a Ship s vertical induced magnetism Figure 16 b Induced magnetic deviation effects because they reverse polarity in each of the four quadrants The masses of horizontal soft iron that are subject to induced magnetization create m C deviations as indicated in Figure 16 b The D and E deviation curves are called quadrantal curves P Symmetrical arrangements of horizontal soft iron may exist about the compass in any one of the patterns illustrated in Figure 17 120 2 Figure 17 Symmetrical arrangements of horizontal soft iron he dgyiation resulting from the earth s field induction of these symmetrical arrangements of horizontal sofffiton ar
64. e south the signal is again maximum but is of opposite phase to the northerly signal This will cause an opposite tilt of the gyroball to be produced With the ship sailing due east the synchronous transmitter SG1 is in a position which will produce a zero signal across RV24 and no correction signal is applied to the azimuth amplifier irrespective of the speed setting of RV24 Any intermediate setting of SG1 will produce a corresponding correction signal to be developed across RV24 1 10 2 Latitude correction The latitude correction circuit provides a signal proportional to the sine of the vessel s latitude to use Xbe gyroball to precess in azimuth at a rate equal and opposite to the apparent drift caused by the n of the earth This signal will be zero at the equator and maximum at the poles It must also be Both the vertical an ontal torsion wires may twist with a change in ambient temperature A corrective signal 1S pr of the tilt and azimuth temperature compensation circuits to counteract any precession of tlf gyroball caused by a change in temperature The corrective signals are produced in the compensation circuits and connected to the tilt and azimuth amplifiers in such a way that both signal amplitude and ill ganse torques to be produced which are equal and opposite to those produced by twisting of the i i ne effect of ambient temperature on the torsion wires is therefore cancelled 1 10 4 Error decoupling circuit e The a
65. ed a frequency shift see Figure 3 16 the amount of which depends upon P the transmitted frequency the velocity of the sonar energy wave the velocity of the transmitter the ship The frequency shift in hertz of the returned wave is 90 amp fd fi fr where ft the transmitted wave frequency and fr the received wave frequency The Doppler shift formula for a reflected wave is given as fd 2vft c where v the velocity of the ship and c the velocity of the sonar wave 1500 ms in seawater Obviously there can be no objects directly ahead of a vessel from which the acoustic wave may be reflected The wave is therefore transmitted towards the seabed not vertically as with echo sounding but ahead at an angle of 60 to the horizontal This angle has been found to be the optimum angle of cid with the seabed which will reflect a signal of sufficient strength to be received by the 5 The shape of the seabed has no effect on the frequency shift Provided that the seabed is not rfe smooth some energy will be reflected Beam X refraction Water mass a o Transmitt acoustic wave 1 Wavelength after refraction Wavelength before refraction Figure 3 16 Illustration of the change of wavelength that occurStw oustic wave crosses a water mass The angle between the horizontal plane and the transmission must now be ap to the basic Doppler formula fd 2vficos 0 C in hertz Oo Figur
66. eeking As the pendulum swings towards the centre of gravity a downward for e tK wheel axle which causes horizontal precession to down occur This gravitational force actin on the spinner axle causes the compass to precess horizontally and maintain the axle pointi war no amp h actio The two main ways of achieving precession e to gravity are to make the gyro spin axis e either bottom or top heavy Bottom heavy cont d Wise rotating gyro spinner are used by some manufacturers whereas others favour a top he ith an anticlockwise rotating spinner Figure 1 8 a illustrates this phenomenon With bottom heavy control tilting upwards of the south en ownward force on the other end which for this direction of spinner rotation produces a prec sion Of the north end to the west In a top heavy control system tilting upwards of the north end of the gyro produ downwardforce on the south end to causes a westerly precession of the north end The result fo ent will be the same 1 4 1 Bottom heavy control Figure 1 8 b illustrates the principle of precession caused by gravity acting on ott ighffa spin axis of a gyroscope The pendulous weight will always seek the centre of gravity in omg will exert a torque about the gyro horizontal axis Because of the earth s rotation and gyro Wgidity Ahe pendulum will cause the gravity control to move away from the centre of gravity The spi rotating clockwise when viewed from the south end a
67. end into the water or be fi flush with the hull As the vessel moves the seawater the conductor flowing through the agngBc field has a small e m f induced into it This minute e m f the amplitude of which is Magnetic Flemings right hand rule Magnetic e m f field H induced in ctor otion V Wn a tiny right hand rule shows that the generated e m f is at right angles to the magnetic current flowing in the conductor produces an indication of the e m f on the meter If the energizing current for the solenoid is d c the induced e m f is B v where _ the induced magnetic field the length of the conductor and v the velocity of the conductor B is approximately equal to 4 Q H the magnetic field strength Therefore e m f H v assuming no circuit losses 80 To reduce the effects of electrolysis and make amplification of the induced e m f simpler a c is used to generate the magnetic field The magnetic field strength H now becomes Hmsinwt and the induced e m f is Hmlvsin t If the strength of the magnetic field and the length of the conductor both remain constant then e m f velocity gure 3 8 illustrates that the changes of e m f brought about by changes in velocity produce a linear s a linear indication of the vessel s speed The e m f thus produced is very small but if me larger by increasing the energizing current or the number of turns of wire on the solefigid
68. ep data line shows a change in heading Scheduled maintenance and troubleshooting he ster compass is completely sealed and requires no internal maintenance As with all confputerbased equipment the Sperry MK 37 VT gyrocompass system possesses a built in test system ITEJ to enable health checks and first line trouble shooting to be carried out Figure 1 33 shows the is chart for the Sperry MK 37 VT system In addition to the health check automatically carri oui S m various indicators on the control panel warn of a system error or malfunction Referri extensive information contained in the service manual it is possible to locate and in some cases emedy a fau The MK 37 VT gyrocorp is malfunctionigg Are fault codes Check associated wiring Replace DC DC converter assy Is problem solved present on display unit Yes No See Table 5 4 for suggested corrective action Replace AC DC power supply assy problem solved Is LED status indicator on CPU assembly blinking Yes Check ship s power P and associated wiring Figure 1 33 Sperry MK 37 VT digital gyrocompass trouble analysis chart Reproduced courtesy of Litton Marine Systems So far this description has only considered gyrocompass equipment using a top heavy control 33 mechanism Many manufacturers prefer to use a bottom heavy control system One of the traditional manufacturers S G Brown Ltd provides some f
69. er and the receiver must Ss adequate sensitivity to overcome all of the losses in the transmission medium seawater and determines the specifications of the equipment to be fitted on a merchant vessel ous Wave pulse system The pulse system 1n whigrapid short high intensity pulses are transmitted and received by a single transducer The epth is calgulated by measuring the time delay between transmission and reception The latter system is preferred in t ri am licaliohs Both the pulse length duration and the pulse repetition frequency PRF are impo considering the function of the echo sounding apparatus e Continuous wave system This system is rarely used in commercial echo sovndin tions Because it requires independent transmitters and receivers and two transducer assem nsive Also because the transmitter is firing continually noise is a particular problem Civi time echo sounders therefore use a pulsed system Pulsed system In this system the transmitter fires for a defined period of time and is then gWitched off The pulse travels to the ocean floor and is reflected back to be received by the sa ich is now switched to a receive mode The duration of the transmitter pulse and the frequency PRF are particularly important parameters in this system g with the display method used enables objects close together in the water or close to the seabed to be4fecor separately It is called target or echo
70. er line ane xisting bet the indicated lubber line and the fore and aft line of the vessel Both of these errors accurately eliminated by critically aligning the compass with the ship s An error existing betwee indi amp ffted heading on the master compass and the heading produced by any remote repeater is a trans r Transmission errors are kept to a minimum by the use of multispeed pulse transmission Oo Variable errors Variable compass errors can effectively b ssi to tib groups Dynamic errors that are caused by the angitlar moff n of the vessel during heavy weather and manoeuvring Speed latitude errors that are caused by movem f th el across the earth s surface The magnitude of each error can be reduced to some ext Ss in the following text 1 7 2 Dynamic errors Rolling error The gyrocompass is made to settle on the meridian under the influence of weights Thus it will also be caused to shift due to other forces acting upon those weights When a v swung like a pendulum causing a twisting motion that tends to move the pla towards the plane of the swing For a simple explanation of the error conside es caused in both the north and south reservoirs by a vessel rolling If the ship is stea ing north r south no redistribution of mercury occurs due to roll and there will be no error see Fig Q 18 t i ure 1 17 A ship steaming due north or south produces no roll error But wit
71. er than it used to be particularly when interfaced with A LS radar and electronic chart disp s aggECDIS It is however worth taking the following into consideration GPS is currently the only fully o ion NSSg It is owned and controlled by the U S Department of Defence and its use by c ercidbshipping is incidental to its primary military purpose GNSS signals are vulnerable to loss and erg entional and unintentional Malicious jamming of GNSS is a very real threat GPS si terminated or corrupted by the US military for security purposes Commercial GPS operates on a single frequency only Mili receivers operate on a dual frequency system which is more reliable and less vulnera O error c by atmospheric conditions GNSS signals are extremely vulnerable to solar activity such as solar flares ThefSun is currently entering a phase of intense solar flare activity which is due to last for sev ars i no Some areas of the world particularly in the higher latitudes have problemat coverage Other signal errors such as multipath effect occur locally when the signal to eA reflected off nearby objects such as superstructure masts and funnels Entering the wrong antenna height into the receiver can cause large errors the difference between a large vessel down on her marks and in ballast is significant Entering the wrong datum can put the vessel s position miles from where it really is Datum used
72. ers Side view Front rear view Showing vertical torsion wires Showing horizontal and vertical and helixes torsion wires and helixes Figure 1 34 Arrangement of the gyroball Reproduced courtesy of S G Br The gyroball is centred within the tank by means of two vertical and two horizontal loryyires forming virtually friction free pivots The torsion wires permit small controlling torques to ppl in both the vertical and the horizontal axes to cause precessions of the axes in both tilt and azimut In addition the torsion wires are used to route electrical supplies to the motor The gyroball asse IS totally immersed in a viscous fluid called halocarbon wax the specific gravity of which gives the CQ neutral buoyancy at normal operating temperatures so that no mass acts on the torsion wires The tank containing the gyroball sensitive element is further suspended in a secondary gimbal system as shown in Figure 1 35 to permit free movement of the spin axis This axis is now termed the P free swing axis which under normal operating conditions is horizontal and in line with the local L meridian The secondary gimbal system also permits movement about the east west axis Each of the movable axes in the secondary gimbal system can be controlled by a servomotor which in turn 35 amp provides both tilt and azimuth control of the gyroball via a network of feedback amplifiers Compass card Sensitive element p I 1 PA A
73. es in a sandwich of two stainless steel cylinders The whole unit is pre stressed by inserting a stainless steel bolt through the centre of the active unit as shown in Figure 2 6 lt P Q 54 Connecting cables Compression bolt Two ceramic crystal slices Stainless steel _ cylinders RI H c Transmitting o face Flexible seal igure 2 struction details of a ceramic electrostrictive transducer If a voltage is applie the ends of the unit it will be made to vary in length The bolt is insulated from the crystal sli VC collar and the whole cylindrical section is made waterproof by means of a flexible Seal olt tightens against a compression spring permitting the crystal slices to vary in length under thevinfluence ef the RF energy whilst still remaining mechanically stressed This method of construction i fo on the electrostrictive transducers used in the Merchant Navy For smaller vessels wher te sies are not so severe the simpler piezoelectric resonator is used 2 3 2 Piezoelectric resonator This type of transducer makes use of the flexible qualities of a crystal slice If the ceramic crystal slice is mounted so that it 1s able to flex at its natural re produced The action is again reciprocal If the cerami amp cry is mounted at its corners only and is caused to flex by an external force a small p d will be across the ends of the element This phenomenon is widely used in industry for prod
74. ession Rule 1 Direction of resulting precession about vertical axis esi Spin axis Y meses Horizontal axis Direction of axis S of rotation X Direction of applied torque about horizontal axis Direction of rotation of wheel Vertical axis b Precession Rule 2 Figure 1 5 a Resulting precession P occurs at 90 in the direction of spin fro applied force F This ry Ltd b lied torque direction of precession is the same as that of the applied force Reproduced The direction of axis rotation will attempt to align itself with the direction of the Reproduced courtesy of Sperry Ltd e 1 2 2 The free gyroscope in a terrestrial plane Now consider the case of a free gyroscope perfectly mounted in gimbals to permit fre dom movement on the XX and YY axes In this description the effect of gravity is initially ignored It should be noted that the earth rotates from west to east at a rate of 15 h and completes one revollition in a sidereal day which is equivalent to 23 h 56 min 4 s The effect of the earth s rotation beneath the gyroscope causes an apparent movement of the mechanism This is because the spin axis of the gyroscope is fixed by inertia to a celestial reference star point and not to a terrestrial reference point If the free gyro is sitting at the North Pole with its spin axis horizontal to the earth s surface an P apparent clockwise movement of the gyro occurs The spin axis remains
75. eters etc which can produce strong magnetic lt P fields A few inches one way or the other can sometimes be the difference between major and minor L 107 amp deviation c ig f 4 Magnetic compass amp electronic instruments in close proximity It shoul amp be i led so it is easily readable from the helm and also accessible for adjusting A great many mod m vessels p larly luxury motor yachts have not been designed with this in mind On one particular sleek illion dollar super yacht it was found that in order to access the integral correctors of the flush fiffin either the console would need to be partially demolished or the raked wheelhouse windscree uld have to be removed Ideally the compass shouldbe sited sobthat bearings of objects and other vessels may be taken This is not always practicable particu smaller vessels in which case other means of taking bearings should be provided It should n en Mhat the compass is a valuable tool in collision avoidance manufacturers produce compasses which can be mounted in this fashion This has an obvious Figure 5 Overhead mounted compass Q Some vessels have their compass installed in an overhead deckhead mounted position A number of advantage in being easy to read close to eye level In an upside down type such as that pictured above it also means that air bubbles in the compass liquid are not such a problem It is also away from a lot of the deviating
76. ews Pendulum bob Electrical connections Initially the bob will centre in the middle of the E core but if the gyro tank tilts the bob will offset causing the normally equalized magnetic field to be unbalanced and produce a stronger field on the outer arm towards which it is offset The result is that a tilt signal of correct sense and amplitude is produced This signal is fed to the tilt and azimuth amplifiers as required 38 Figure 1 37 The pendulum assembly and its electrical connections Q Reproduced courtesy S G Brown Ltd P The output signal of the pendulum unit is also used to enable the gyro to settle in the meridian and become north settling A small carefully calibrated portion of the output signal is applied to the azimuth amplifier to cause azimuth misalignment of the gyro tank and hence a twist of the vertical torsion wires The result is a tilt of the sensitive element the direction of which depends on whether the gyro spin axis is north or south end up with respect to the horizontal The amplitude of the pendulum signal fed to the azimuth amplifier will determine the settling period of the gyro which for this compass is 40 min Loop feedback versatility is again made use of by applying signals in order to achieve the necessary rrections for latitude and speed errors The injected signals result in the required precessional rates in th for latitude correction and in tilt for speed correction peed co
77. f The received energy returns to the same tr Reverberation noise Noise that decreases as range increases Sonar Sound navigation and ranging Velocity Speed of acoustic waves in seawater 1505 ms or approximated to 1900 ms 2 9 Summary Sonar stands for sound navigation and ranging Sound travels relatively slowly in seawater at 1505 ms This is approximated to 1500 ms fo convenience The velocity is not a constant it varies with the salinity of seawater Ocean salinity is Q approximately 3 4 Transmitted signal amplitude is attenuated by saltwater and the ocean floor from which it is reflected Noise caused by sea creatures and ocean activity is a major problem affecting sonar equipment L The temperature of the seawater affects the velocity of the acoustic wave and consequently affects Q the accuracy of the displayed data Temperature sensors are contained in the transducer housing to 70 produce corrective data Transducers are effectively the antennas of sonar systems They transmit and receive the acoustic energy There are two main types of transducer in use magnetostrictive and electrostrictive Magnetostrictive transducers are large and heavy and tend to be used only on large vessels Low frequencies are often used in deep sounding systems typically in the range 10 100 kHz c2 Electrostrictive transducers are lighter and often used in speed logging systems and on smaller craft e
78. fected by latitude and will produce an angle of tilt in the settled gyro Hence latitude course speed error is sometimes referred to as LCS error 1 7 3 Use of vectors in calculating errors With reference to Figure 1 22 rface velocity 900 cos lat ure 1 22 Use of vectors in calculating errors V xships speed in knots In triangle abc Error in degrees angle bac 0 tan Obviously the ship s speed is very much less than the earth s surface velocit V cos course tan 9 900 cos latitude e The angle 9 may be approximately expressed in degrees by multiplying both side of the e a factor of 60 Now V cos course 15 cos latitude Q 1 8 Top heavy control master compass d 1 23 1s a good example of an early top heavy controlled system The master compass consists of two 22 amp approximate error in degrees Produced before the move towards fully sealed gyro elements the Sperry SR120 gyrocompass Figure main assemblies the stationary element and the movable element s DOW EE d mr O0 q Figure 1 23 A south elevation sectional view of a Sperry m pass Key 1 Stepper transmitter 2 Support ball bearings 3 Ballistic pots 4 Rotor encased 5 Rotor das amping weight 7 Suspension wire 8 Cover 9 Compass card 10 Slip rings 11 Maf support frame Ag Phantom ring support assembly cutaway 13 Follow up primary transformer 14 Follow
79. ful IR d in deviations or the strength of deviating fields For example a ship as shown in Figure 16 6n an st magnetic heading will subject its compass to a combination of magnetic effects namely the S horizontal field H and the deviating field B at right angles to the field H The compass needle wil align itself in the resultant field which is represented by the vector sum of H and B as shown A similar analysis on the ship in Figure 16 will reveal that the resulting directive force at the compass would be maximum on a north heading and minimum on a south heading the deviations being zero for both conditions The magnitude of the deviation caused by the permanent B magnetic field will vary with L different values of H hence deviations resulting from permanent magnetic fields will vary with the Q magnetic latitude of the ship 119 Resultant Field in Magnitude Directive Force and Direction Deviation Earth s Field gt A East Magnetic Heading Deviating Field B Compass Needle Figure 16 General force diagram agftism and its effects on the compass sm varies with the strength of the surrounding field the mass of metal and the e field Since the intensity of the earth s magnetic field varies over the earth s surface the ind ic agnetism in a ship will vary with latitude heading and heel of the ship sultant vertical induced magnetism if not directed through the with heading of the ship Th
80. gnal rises slowly and eventuall the time constant of the demodulator is equal to the signal p d developed ignal applied to IC1 falls to zero and therefore the demodulator output remains at a consta gure Any further change in speed results in an imbalance in the secondary of T1 producing resultant amp c signal to ICI As a result the demodulator output increases or decreases faster or slower eed suntil the balance condition is restored The speed repeaters will indicate the appropriate chang of gspeed Distance integration The speed current is passed through a resiStive onthe distance integration board in order that a proportional voltage may be produced for i ion The output of this board is a pulse train the rate of which is proportional to the indicated spe d board which holds the necessary logic to give the fol pulses per nautical mile and 1 pulse per nautical mile 3 4 Speed measurement using acoustic co atio niques Unlike the previously described speed log which measure the vessel s speedWwi ct to water only the SAL ICCOR log measures the speed with respect to the seabed or to a S Spen er mass The log derives the vessel s speed by the use of signal acoustic correlation Simply is agyay of e combining the properties of sonic waves in seawater with a correlation technique Spee as e is achieved by bottom tracking to a maximum depth of 200 m If the bottom echo becomes Weak or the depth exceeds 200 m the
81. gure 3 10 i d br d f N Sy k Data transmission output Mechanical SPLIT P to t nmote saison linkage S pm bio E M 2 M m m oun amplifier a P input co incidence LA networks x NLS d hy i 5 N Servo N e reference Sy input N e m f 1 n N x Z N s N N E o M el e m senso Figure 3 10 An e m speed log translating system Description of the speed translating system The small signal speed voltage from the sensor e m f 1 1s applied to a differential trans is compared to a reference voltage e m f 2 produced from a potentiometer across the input atc a The potential difference produced across the reference resistor provides the energizing current fi solenoid in the sensor If the signal voltage e m f 1 differs from the reference voltage e m f 2 an error signal voltage 6 e m f i P is produced This error voltage is applied to the speed signal amplifier where it is amplified to produce L sufficient power to drive the servo motor The servo will in turn produce a speed reading via a 82 amp mechanical linkage on the indicator Also coupled to the servo shaft is the slider of the speed potentiometer that turns in the direction to reduce the error voltage _ e m f When this error voltage drops to zero the servo ceases to turn The speed indicator is stationary until the next error voltage 6 e m f is produced Each time an error voltage is created the servo turns to cancel the error and thu
82. h affect a magnetic compass are permanent magnetism and induced magnetism 4 3 2 Permanent magnetism and its effects on the compass The total permanent magnetic field effect at the compass may be broken into three components mutually 90 apart as shown in Figure 14 a The effect of the vertical permanent component is the tendency to tilt the compass card and in the event of rolling or pitching of the ship to create oscillating eflections of the card Oscillation effects that accompany roll are maximum on north and south comipass headings and those that accompany pitch are maximum on east and west compass headings e hgfizontal B and C components of permanent magnetism cause varying deviations of the compass yings in heading on an even keel Plotting these deviations against compass heading will d sinc curves as shown in Figure 14 b These deviation curves are called es because they reverse direction in 180 East Athwartship Permanent Magnetic C Deviations and aft B Component Athwartship C Component Deg Q Dev V Fore and aft Permanent Total Permanent Magnetic Magnetic R Deviations Vertical Heeling Field Across Compass We Component Ship s Compass Heading Degrees Figure 14 a Components of permanent magnetic field Figu ermanent magnetic deviation effects The permanent magnetic semicircular deviations can be ill str a series of simple sketches representing a ship on successive compass headings as
83. ha shap steaming due east or west maximum lateral acceleration occurs in the north south direction c ing precessi6m of the compass However rolls to port and starboard are equal producing equivalent easterly a erly precession The resulting mean error is therefore zero as illustrated in Figure 1 18 Excess mercury North heavy causing easterly precession South heavy ing westerly precession east west course are equal and If the ship is on an intercardinal course the force exerted By ury or pendulum must be resolved into north south and east west components see Figure 1 Anticlockwise couple Anticlockwise couple Figure 1 19 For a vessel on an intercardinal course rolling produces an anticlockwise torque P The result of the combined forces is that precession of the compass occurs under the influence of an L effective anticlockwise torque Damping the pendulum system can dramatically reduce rolling error In a top heavy gyrocompass this is achieved by restricting the flow of mercury between the two pots The 19 amp damping delay introduced needs to be shorter than the damping period of the compass and much greater than the period of roll of the vessel Both of these conditions are easily achieved Electrically controlled compasses are roll damped by the use of a viscous fluid damping the gravity pendulum Such a fluid is identified by a manufacturer s code and a viscosity number F
84. horizontal axis are e follow up pick off transformers With no tilt present the sphere centre line will be horizontal and central causing distance a to be equal to distance b producing equal amplitude outputs from the fo ormers which will cancel Assuming the gyrocompass is tilted up and to the east of the meridi ire will take up the position shown in Figure 1 26 The sphere has moved closer to the sotth sidefof t amber producing a difference in the distances a and b The two pick off secondary coils wi ode outputs that are no longer in balance Difference signals thus produced are directly ort nal to both azimuth and tilt error Each pick off transformer is formed by a primary coil mount osphere and secondary pick off coils a c supply used for the gyrowheel rotor which cou depending upon the relationship between the two coils Figure 1 27 shows that the secondary coils are wound in such that one output signals is produced by relative movement of the gyrosphere X a si corresponding to the distance of the sphere from each secondary coil a signal correspondin 0 a signal corresponding to horizontal movement 26 X pick off distance Vertical axis error signal Primary coil Mounted on gyrosphere Hor tal axise ror 9 signal ignal pick off coils In the complete follow up system shown 128 f horizontal servomechanism mounted on the west side of the horizontal ring permits th ment to follow
85. ia a 24 V switching amplifier The servo now turns and rotates theeam assembly via gearing and the drive shaft An increase in speed is now shown on the speed pointer XQ the cam rotates it forces the balance arm to the left and tightens the spring until the pushrod arm and the diaphragm bellows are balanced The cam is carefully designed so that the spring force is proportional to the square of the rotation angle and thus the non linearity of the pressure system is P counteracted The speed potentiometer turns together with the speed pointer to provide an input to the L UDIS board This input produces a variety of outputs enabling the system to be interfaced with other electronic equipment 77 amp ere os UDCK 10 pulses per nautical mile 100 pulses per nautical mile UDIS 200 pulses per nautical mile LI 20000 pulses per nautical mile Differentia transformer Speed Speed VIRO pointer gear Pushrod adjustment Pushrod PEPEES prup assembly Figure 3 5 Pressure mechanical assembly of the SAL CQ pressbre speed log Reproduced courtesy of SAL Jungner Marine 78 d c supply board Main transformer f 5V IEEE 8V A at ov 3 si eS regulator 8V Status 200 pulses per nautical mile 10 100 pulses per nautical mile 48V switching amplifier reference 24V switching amplifier control f p pe 7324 H2 a EE OM 8V crysta D 0 5 Hz k oscilla
86. ill assume regions of concentrated magnetism called poles Any such magnet will have at least two poles of unlike polarity Magnetic lines of force flux connect one pole of such a magnet with the other pole as indicated in Figure 11 The number of such lines per unit area represents the intensity of the magnetic field in that area If two such magnetic bars or magnets are placed side by side the like poles will repel each other and the unlike poles will attract each other a N Pa N Pd WS CN A77 N Lines of Force Fd peu NN Flux Fig ne amp ef magnetic force about a magnet t a8 induced A bar having permanent magnetism will will lose its magnetism when removed fro ag tite field Whether or not a bar will retain its magnetism on removal from the magnetizing depend on the strength of that field the degree 4 2 3 Terrestrial magnetism The accepted theory of terrestrial magnetism considers the ea of magnetic force that connect its two magnetic poles These magnetic poles are n coincidental with the geographic poles of the earth Since the north seeking e conventionally called a red pole north pole or positive pole it must therefo a pole of opposite polarity or to a blue pole south pole or negative pole The magnetic pole near the north geographic pole is therefore a blue pole south pole egative polg and the magnetic pole near the south geographic pole is a red pole north pole or positive Figure 12 i
87. in Figures and 15 b Bhe ships illustrated in Figures 15 a and 15 b are pictured on cardinal compass headings rather th fon cardinal magnetic component magnetic fields are either in line with or perpendicular to the compass only P cardinal compass headings 2 Such a presentation illustrates the fact that the com tends to float in a fixed position in line with the magnetic meridian Deviations of the card to right o east or west of the magnetic meridian result from the movement of the ship and its magnetic fields about the compass card 118 amp e 1 North heading by East heading by South heading by West heading by compass compass compass compass Le W Dev E Dev 4 Compass ij o T Compass Fore and aft B Needle gU Magnetic Field eViation Maximum deviation No deviation Maximum deviation easterly westerly ange in di change in di ctive forc re tive force only Figtike 1 rams for fore and aft permanent B magnetic field North heading b ast heading by South heading by West heading by compass mpass compass compass M Dev Athwartship C Permanent Magnetic Field Maximum deviation No deviation Maximum deviation No deviatj easterly westerly Figure 15 b Force diagrams for athwartship permanent C magnetic fi e Inasmuch as a compass deviation is caused by the existence of a force at the com ass that is superimposed upon the normal earth s directive force H a vector analysis is help
88. ine examples of bottom heavy gyroscopic control 1 10 A bottom heavy control gyrocompass Modern bottom heavy controlled gyrocompasses tend to be sealed gyroscopic units with full computer control and electronic interfacing For the purpose of system description this early gyrocompass is a ood example of bottom heavy control used to settle and stabilize a compass roscopic element called the sensitive element is contained within a pair of thin walled m hemispheres joined as shown in Figure 1 34 to form the gyroball At the heart of this ball eegphase induction motor the rotor of which protrudes through the central bobbin assembly but is ab e bec use of the high quality support bearings At each end of the rotor shaft a heavy rimm o spilither is attached to provide the necessary angular momentum for gyroscopic action to be establishg Rotational speed of the induction motor is approximately 12 000 rpm 34 Sprung cruciform Eus Tank section of Silver helix Middle section _ or bobbin Primary gimbal E Thin walled hemisphere TT E pE orend bell n JU i Goss MIERNIE Bearing housing M uH a se Lo Il A ET hil LET Eos Pi Electro magnet Eeo oree EE Su d H TUN Electric motor q il HE il Vertical torsion wire ma TT LED e E a M Un f i E m aii r windings rr a Gyro spinn
89. ing platform Gyroscope A perfectly balanced wheel that is Ble to at high speed symmetrically about an axis Gyroscopic inertia A gyroscope rotor maintains te dire f its plane of rotation unless an external force of sufficient amplitude to overcome inertia is a Latitude error A constant value error the magnitude o at any given latitude Linear momentum The product of mass and velocity Manoeuvring error An error caused by a vessel s rapid changes of speed and North seeking gyro One which is partly controlled and as a consequence will not settle Further control is required to convert this type of gyro into a North settling gyro One which is fully controlled and will settle to point north Precession Movement at 90 from the applied force If a force is applied to a spinn oto ovii one end of its axle the gyroscope is displaced at an angle of 90 from the applied force Rolling error As the name suggests this error is caused by a vessel rolling The error cancefg when fhe ship is steaming north or south and is maximum when following an east west course Settling time The period taken for a gyrocompass to settle on the meridian from startup Slew rate control A control setting an electrical input to rapidly level and orientate the gyro during start up Stepper systems A step motor compass repeater circuit P Synch systems A synchronous motor compass repeater circuit Tilt By virtual of precession the earth s rotation cause
90. ion of a typical magnetostrictive transducer unit lt P The transmitting face is at the base of the diagram 56 n i Permanent Transducer winding magnets PE a C N mI T T IRI EN Annealed nickel Nickel element retaining laminations bars oss section of a magnetostrictive transducer Reproduced courtesy of Marconi Marine ncy and above 100 kHz the efficiency of magnetostrictive transducers falls to b low t ormal 40 Above this frequency electrostrictive transducers are normally used 2 3 4 Transducer siting c The decision of where to mount th uc uf not be made in haste It is vital that the active face th wat of the transducer is in contact with h it should also be mounted well away from areas close to turbulence that will cause noise Areas lose to propellers or water outlets must be avoided Aeration is undoubtedly the biggest problem unf red when transducers are wrongly installed Air bubbles in the water for whatever reason will pass clo transducer face and act as a reflector of the acoustic energy As a vessel cuts through the water severe turbulence is cr ated Water containing huge quantities of air bubbles is forced under and along the hull The bow wave is a amp ra s forced above the surface of the sea along the hull The wave falls back into the sea at appro ly one thi the length of the vessel from the bow A transducer mounted aft of the posit
91. ion to the west ceases Q ye 4 Once the gyro spin axis is below the earth s horizontal the gravity control exerts a torque in the opposite direction and precession to the east results Because the north end of the gyro spin axis is still west of the meridian it continues to tilt downwards spin axis passes east of the meridian je to earth rotation becomes an upward tilt The gravit Continues to precess the gyro towards the east i more passing through the earth s horizontal The gyro es to precess towards the west again still tilting upwar ually pass through point P The ellipse about North is thus and continues Figure 1 9 Behaviour of the gravity controlled undamped Reproduced courtesy of S G Brown Ltd This action causes the north end of the spin axis of a gravity controlled un and from the earth s horizontal The term north seeking is given to the undamped gravity controlled gyro mechanism because the northeast end of the spin axis describes an ellipse around the Nort e but never settles Obviously such a gyro is not suitable for use as a precise north reference compass C 1 4 2 The north settling gyro The ellipse described by the previous gyro mechanism possesses a constant ratio of the major and minor axes Clearly therefore if the extent of one axis can be reduced the length of the other axis will P be reduced in proportion Under these conditions the gyro spin axis will event
92. irement on all sea going commercial vessels Cp Q 110 F e 8 Usi Deviation be determi objects such as a m peak or lighthouse are considered most accurate In certain circumstances such as poor visibi i ith other navigation instruments such as a gyro or GPS compass are sometimes made Using other navigation instfuments tobfind deviation is only satisfactory if the absolute accuracy of these instruments has first be ed yer any known error is factored into the calculations Most professionals prefer something tangi uc a fixed landmark with a known position and bearing to work with GPS compasses are normally accurate to withi egree g so with the vessel on a steady heading but are often useless on a swinging vessel All navigation mstruments whether portable or fixed including GPS compasses should themselves be checked Yor e time they are used for calibrating a magnetic compass It should be noted that the compass cannot be adjusted to any degree of verifiable accuracy with the vessel alongside Deviation must be observed and required adjustments made with the ship s head steady on numerous headings This requires the vessel to be in open water clear of other vessels and away from magnetic interferences such as cranes steel piles reinforced concrete jetties etc Figure 9 Signal flags OSCAR over QUEBEC Denotes swinging the ship lt P Q 111 Some preliminary adjustments based on a detailed analysis of compass
93. is where the bow wave re enters the sea would suffer badly from the problems of aeration Mountj this point even in the bulbous bow would be ideal It should be remembe irregularities in it will cause air bubbles to be produced leading to aeration of the transducer 2 4 Depth sounding principles irregularities are impossible to predict as they are not a feature of the vessel s design Jo In its simplest form the depth sounder is purely a timing and display system that makes use of a transmitter and a receiver to measure the depth of water beneath a vessel Acoustic energy is transmitted perpendicularly from the transducer to the seabed Some of the transmitted energy is reflected and will be received by the transducer as an echo It has been previously stated that the velocity of sound waves in seawater is accepted to be 1500 ms Knowledge of this fact and the ability 57 to measure precisely the time delay between transmission and reception provides an accurate indication of the water depth velocity x time Distance travelled where velocity 1500 ms in salt water time time taken for the return journey in seconds and distance depth beneath the transducer in metres Thus if the time taken for the return journey is 1 s the depth of water beneath the transducer is 750 m If the time 1s 0 1 s the depth is 75 m and so on he tiamsmitter and transducer must be capable of delivering sufficient pow
94. isplays The contour display can be shifted back over the past 24 h whilst the strata display right hand side of display shows sounding data over the last 5 min 2 8 Glossary ise Noise that remains constant as range increases owing lists abbreviations acronyms and definitions of specific terms used in this chapter a Aerated water bubbles clinging to the transducer face cause errors in the system n B ng TI transmitted pulse of energy spreads as it travels away from the transducer The use o eb will cause noise problems in the receiver and a narrow beam may lead to an echo being mis S the vessel steams away from the area Chart recorder A se paper recording system which when the surface is scratched by a stylus marks the contou Continuous wave sy An echo sounding system e transducers and transmits and receives energy at the same time Electrostrictive transducer A Fans er design based on piezoelectric technology It is used when a s in Speed logging equipment or fish finding sounders Magnetostrictive transducer A design base magpetic induction A large heavy transducer capable of transmitting high power Used in deep di ems Pulse duration length The period of the tran mitte se when the transmitter is active Pulse repetition frequency PRF The num Similar to RADAR Pulse wave system A system that like RADAR transmi transmitted per minute by the system energy from a transducer which is then switched of
95. known if they ate they will be subject to appreciable change with heel and latitude changes of the ship Subsequent chapters will deal with the methods of bringing a ship to the desired heading an e methods of isolating deviation effects and of minimizing interaction effects between correctors fice properly adjusted the magnetic compass deviations should remain constant until there is some in the magnetic condition of the vessel resulting from magnetic treatment shock from gunfire vibration repair or structural changes Frequently the movement of nearby guns doors gyro repeaters or cargo affects the compass greatly 124 Q 4 4 Glossary Magnetism Any piece of metal on becoming magnetized that is acquiring the property of attracting small particles of iron or steel will assume regions of concentrated magnetism called poles Permanent Magnetism A bar having permanent magnetism will retain its magnetism when it is removed from the magnetizing field Induced Magnetism A bar having induced magnetism will lose its magnetism when removed from acnetizing field with the geographic poles of the earth g Shffs heading is the angle expressed in degrees clockwise from north of the ship s e angle between the magnetic meridian and the true meridian If the northerly part of th and if this part is to the le meridian lies to the right of the true meridian the variation is easterly the t eridian the variatio
96. lectromagnetic log referenced to 5 How the non lig arity of a ship s hull affect the speed indication produced by an electromagnetic spee 6 Does the amo e water affect the speed indication produced by an acoustic correlation speed log 7 Why do all Doppler spe gsu anus configuration transducer assembly 8 How does aeration cause C ed indicated by a Doppler log 9 Using the Vx and Vy speed comp hen ced y a Doppler speed log how is it possible to predict a vessel s drift rate 10 Why are pulsed transmission systems in enco a continuous wave mode of operation 11 Why are water temperature sensors include in the yansducer assembly of a Doppler speed logging system 12 How may the distance run be calculated in a spee gin m 104 Chapter 4 The Ship s Magnetic Compass 4 1 Introduction 4 1 1 The magnetic compass in the age of electronic navigation spite the modern tendency to rely heavily on Electronic Navigational Aids ENA the magnetic s remains a primary navigation instrument on any vessel and continues to operate ndep ndently in the not uncommon event of an electrical failure or electronics malfunction rs should be ayare that ENA have limitations and have been known to provide erroneous info Rehigble and accessible alternatives for back up and cross reference should always be readily avail There is little doubt tha al igation Satellite Systems GNSS such as GPS help to make modern sea travel generally i
97. llustrates the earth and its surrounding magnetic field The flux lines enter the of t earth at different angles to the horizontal at different magnetic latitudes This angle is called the an of magnetic dip 0 and increases from zero at the magnetic equator to 90 at the magnetic poles total magnetic field is generally considered as having two components namely H the horizontal component and Z the vertical component These components change as the angle 0 changes such that H is maximum at the magnetic equator and decreases in the direction of either pole Z is zero at the lt P magnetic equator and increases in the direction of either pole 115 i ff F i North Geographic SSS Vi pole Blue l Magnetic South V Red 7 ut Magnetic Geographie r Z PN Poe Pole Fd ae NT QM NS NU aedi gt p TUI SU NN WM n A PF RY NN ped 7M TS e X ss 6 NS AN u AS M RET ye ws panung Ole oat A ship while in the process of being constructed will acquire magnetism of a permanent nature under the extensive hammering it receives in the earth s magnetic field After launching the ship will lose some of this original magnetism as a result of vibration pounding etc in varying magnetic fields and will eventually reach a more or less stable magnetic condition This magnetism which remains is the permanent magnetism of the ship 116 gs Lb 180 Figure 13 Compass r
98. logue compass repeaters are simply mechanized compass cards driven either by a stepper motor or a synchro bearing transmission system Digital heading displays can also be produced by 42 digitizing the stepper grey code waveform before applying it to a suitable decoding system This section deals with the most popular bearing transmission systems 1 12 1 Stepper systems Figure 1 39 shows a mechanical switching stepper system which because its robustness is still found on many merchant ships for bearing transmission to remote repeaters The rotor of the transmitter is c2 geared to the azimuth ring gearing of the master compass The transmitter is a multi contact rotary switch that completes the circuit for current to flow through the appropriate repeater motor coils The transmitter rotor has two rotating arms spaced at 165 to each other Each rotor arm makes contact with ppesssegments arranged in four groups of three with each segment being wired to its corresponding number in the other three groups e ratio of transmitter rotor to azimuth gear is 180 1 Therefore 180 rev 360 e l rev 2 L 12 seg 2 1 seg 2 12 or 10 min of arc The rotating arms e 2 steps per revolution Because of the 180 1 gear reduction each step therefore correspo ee or 10 min of arc on the compass card 43 Transmitter compass Master Repeater compass 70V supply E Note B In some cases a vernier repea
99. measured by laser gyro H Readout of speed and direction of water current I Readout of wind speed and direction input from wind sensors J Under keel clearance measured by an external echo sounder K Range and bearing true to marker line L Marker line M Ship s speed transverse longitudinal and transverse at stern with laser gyro 101 N Grid scale and presentation mode O Ship s predicted motion Nav Data Mode display The display diagram key for this mode shows the following 1 Ship s speed and course 2 Echo monitor 3 Tracking mode and echo level indicator 4 Date and time 11 Ship s transverse speeg af bow longitudinal speed and transverse speed at stern with laser gyro 12 Drift angle deviat3 n f course over ground from ship s course 13 Course heading 3 8 Glossary Aeration The formation of bubbles ofthe tra cer face causing errors in the system ALPHA Atlas Low A flush fitting t c i Frequency Phased beam Array transducer Beamwidth The width of the transmitted aco travels away from a transducer g 1ffultiple elements to create the transmitted p ave The beam spreads the further it BITE Built in test circuitry A self test or manually oper stic system CW mode Continuous wave transmission Both the transmi er are active the whole time Requires two transducers Distance integrator The section of a speed log that produces an indication of distance travelled fr
100. n an anticlockwise direction an upward force Q sufficient to cause clockwise precession at a rate of N degrees per hour must be applied vertically to 6 the appropriate end of the rotor axle The result will be that the gyro drift is cancelled and the instrument points to a fixed point on earth Gyro tilt movement can also be cancelled in a similar way by applying an equal and opposite force horizontally to the appropriate end of the rotor axle Although the gyro is now stabilized to a terrestrial point it is not suitable for use as a navigating compass for the following reasons It is not north seeking Since the recognized compass datum is north this factor is the prime reason why such a gyro is not of use for navigation tis liable to be unstable and will drift if the applied reciprocal forces are not precise omplex system of different reciprocal forces needs to be applied due to continual changes in atitude cause of precessional forces acting upon it through the friction of the gimbal bearings the me ism is liable to drift This effect is not constant and is therefore difficult to compensate for 1 4 orth seeking gyro idian seeking maintaining the spin axis parallel to the earth s spin axis by the use of a pe ng under the influence of earth gravity The pendulum causes a force to act upon the gyro asse cau it to precess Precession the second fundamental property of a gyroscope enables the instru ome north s
101. n is westerly Deviation A ship s magnetic e will generally cause the compass needle to deflect from the needle Pints east of the magnetic meridian the deviation is easterly if it points west of the magnetic meridian eviation is westerly Dip An angel between the total force of netigffic dles Md the horizontal force of theirs Soft iron A kind of iron that is able to prod ce t magnetic meridian If the north e uced magnetism under the influence of the terrestrial magnetism Hard iron A kind of iron that is able to produce the perm agnetism under the influence of the terrestrial magnetism Soft sphere A kind of soft iron installed on the magnet o compensate for deviation produced by one of the induced magnetisms Flinder s bar A kind of soft iron installed on the magnetic compass to coifpensate for deviation produced by one of the induced magnetisms 4 5 Revision questions 2 e Explain the reasons that the magnetic compass fitted on the steel ships suffers from the d 10n 2 State the relationship between dip H and deviation 3 Describe the common components of the deviation 4 State the situations that require the compass shall be swung Q 5 Describe the basic deviation correctors installed in the magnetic compass onboard 6 What kinds of signal flags shall be hoisted when swinging the compass P 7 What is the relationship between true north and magnetic compass north 8 How to determine the variation
102. nd therefore precession caused by the gravitational force exerted on the spin axis will cause the northeast end of the spin axis to move to the east when it is below the horizontal A reciprocal action will occur causing the northeast end of the spin e axis to precess towards the west when above the horizontal Force Precession Precession A Li d 4 T F Ww E EARTH S HORIZONTAL Ls SPE G ORM C PRECESSES WEST db principal and top heavy control b Principle of Its d 8S4 from the earth up when to the east of POINTING NOR Figure 1 8 a Methods of gravity c gravity control Reproduced courtesy The spin axis will always appear to tilt wi the meridian and its north end towards the e d en to the west of the meridian see Figure 1 9 S N Riz The north end of the gyro spin axis is initially pointing at P which is above the earth s horizontal and to the east of the meridian The gravity control therefore exerts a torque T about the gyro s horizontal axis Because of the direction of the torque relative to the direction of rotation of the spinner V He As the north end of the gyro spin axis passes west of the meridi n t upward tilt due to earth rotation becomes a downward tilt The gravi control however continues to precess the gyro westwards Event ually the north end of the gyro spin axis passes through the earth s horizontal at H at which point precess
103. o precesses and not the extent of precession which is a function of time P It is essential that this control is centred before either slew button is pressed otherwise a violent kick of L the gyro ball will occur in one direction making compass alignment more difficult to achieve The selector switch S1 must be in the free slew position during this operation 41 amp 1 11 Starting a gyrocompass As has been previously stated from start up a gyrocompass needs time to settle on the meridian The time taken depends upon the make model and the geographic location of the compass but in general it is between one and several hours The duration also depends upon whether the gyro wheel is already rotating or not If the compass has been switched off it will take much longer to bring the compass into use Inputting the ship s heading to reduce the initial error factor can reduce the time period As an xam the following section considers the start up procedure for the Sperry MK37 VT Digital G ompass po o d terngme if the equipment is operating within specified parameters The CPU also initializes the sy arf communication channels During this procedure the gyro wheel is checked for movemeitt If 188 stationary the system ops for a cold start if it is rotating a hot start is programmed During a col start if no r up and prior to entering the settle mode the system performs the automatic bite procedure ding data 1s in
104. o sounder FE 700 is typical of many Depending upon the system is able to operate with a 200 kHz transmission frequency giving shall amp depth performance or 50 kHz for deep water sounding data is displayed on a 6 5 inch high brightness TFT colour LCD display which provides th navigator wi history of soundings over a period of 15 min much as the older paper recording systems di igure 2 13 66 Display mode Gain setting Range setting Auto Alarm Range scale mode setting ALARM 15m SS DEDECRENSSIES IND ee 1 e Depth alarm line 7 4 2 8 o 10 OGBOO Depth HISTORY OS DATA x DBS HELP NAV MENU 47 5 POW BELOW TRANSDUCER 4 a t Li K Screen Depth unit Co anel Explanation of depth Below transducer or below surface Figure 2 13 Furuno FE 700 LCD TFT data display Navigation M de Reproduced courtesy of Furuno Electric Co Depths associated time and position are all stored in 24 h memory and can be played back atany tj This is a useful function if there is any dispute following an accident The main depth display em a cross sectional profile of the ocean over the past 15 min At the top of the display in Figure 2 13 the Q solid zero line marks the ocean surface or transducer level whichever is selected At 15 m down a second line marks the depth at which the alarm has been set The undulating line showing the ocean floor depth is shown varying over 15
105. om speed and time data Doppler principle A well documented natural phenomenon enabling veloc Iculated from a frequency shift detected between transmission and reception of a radio signal E M log An electronic logging system relying on the induction of electromagnetic fiergy diy s awafer to produce an indication of velocity G T Ground tracking or ground referenced speed NMEA National Marine Electronic Association Interfacing standards Pitot log An electromechanical speed logging system using changing water pressure to un Gy velocity Pulse mode Acoustic energy is transmitted in the form of pulses similar to an echo sounding device or e RADAR Transducer The transmitter receiver part of a logging system that 1s in contact with the water Similar to an antenna in a communications system L Translating system The electronic section of a logging system that produces the speed indication from 102 amp a variety of data W T Water tracking or water referenced speed 3 9 Summary To be accurate speed must be calculated with reference to a known datum At sea speed is measured with reference to the ocean floor ground tracking G T or water flowing past the hull water tracking W T raditionally maritime speed logging devices use water pressure electromagnetic induction or the ission of low frequency radio waves as mediums for indicating velocity pressure speed log occasionally called a Pitot log W T speed
106. on IMO SOLAS 74 Convention units and cannot be refilled If the compass is refillable and is at repairing might be made before refilling Often particularly in the rc asing a new compass is found to be the most economical case of small cheaper com option Finding the correct liquid fluid for the required ingredients can be determined it may be possible tof6btain suitable liquid from local sources at a much cheaper rate To check compatibility draw some existing liquid out of the bow Mitha syringe Ix with the new liquid It will often be immediately obvious if it is not compatible The following are some of the main types of compass liquid ingredients Ethyl alcohol ethanol distilled water Isopropanol rubbing alcohol distilled water 2 e Kerosene paraffin oil Silicon oil Mineral oil 4 2 Magnetism gt 4 2 1 The magnetic compass P The principle of the present day magnetic compass is in no way different from that of the compass used L by the ancients It consists of a magnetized needle or array of needles pivoted so that rotation is in a horizontal plane The superiority of the present day compass results from a better knowledge of the laws of magnetism which govern the behavior of the compass and from greater precision in 114 Q construction 4 2 2 Magnetism Any piece of metal on becoming magnetized that is acquiring the property of attracting small particles of iron or steel w
107. or example in the code number 200 20 200 refers to the manufacturer and 20 the viscosity A higher second number indicates a more viscous silicon fluid One viscous fluid should never be substituted for another bearing a different code number Additionally since roll error is caused by lateral acceleration mounting the gyrocompass low in the vessel and as close as possible to the centre of roll will reduce this error still rth Maffoeuvring ballistic error is effor occurs whenever the ship is subject to rapid changes of speed or heading Because of its lo re the compass gravity control moves away from the centre of gravity whenever the vess S oft or alters course Torque s produced about the horizontal and vertical axis by manoeuV ing gise the gyro mechanism to precess in both azimuth and tilt If the ship is steaming due north and fapidly reduci peed mercury will continue to flow into the north pot or the gravity pendulum continues g making the gyro spin axis north heavy and thus causing a precession in azimuth In Figure 1 20 the decelerati ship increases speed the co essel causes easterly precession of the compass Alternatively if the as ceses to the west A Shi 9 Resultant down a deceleration error Figure 1 20 Resultant easterly error caused by the vessel slowfff s down Latitude damping error Latitude error is a constant error the magnitude of which is directly proportidgal to gfe h s rotation
108. orque Ty continues with a the horizontal and vertical excursions of the gyro spin axis being progressively reduced until the gyro finally settles in the meridian and lt P Q horizontal to the earth s surface Figure 1 10 Behaviour of the gravity controlled gyro damped Reproduced courtesy of S G Brown Ltd 1 4 3 Top heavy control 13 Whereas the previous compass relies on a bottom weighted spin axis and a clockwise spinning rotor to produce a north settling action other manufacturers design their gyrocompasses to be effectively top weighted and use an anticlockwise spinning rotor But adding a weight to the top of the rotor casing produces a number of undesirable effects These effects become pronounced when a ship is subjected to severe movement in heavy weather To counteract unwanted effects an apparent top weighting of the compass is achieved by the use of a mercury fluid ballistic contained in two reservoirs or ballistic pots As shown in Figure 1 11 each ballistic pot partly filled with mercury is mounted at the north and ides of the rotor on the spin axis A small bore tube connects the bases of each pot together in such a way that when the gyro tilts the fluid will also tilt and cause a displacement of is action produces a torque about the horizontal axis with a resulting precession in azimuth Small bore tube Point of attachment will cause a torque about the horizontal axis This in turn causes
109. ose showing variation and annual change Q 4 2 4 Ship s magnetism The fact that a ship has permanent magnetism does not mean that it cannot also acquire induced magnetism when placed in a magnetic field such as the earth s field The amount of magnetism induced in any given piece of soft iron is dependent upon the field intensity the alignment of the soft iron in that field and the physical properties and dimensions of the iron This induced magnetism may add to or subtract from the permanent magnetism already present in the ship depending on how the ship is aligned in the magnetic field The softer the iron the more readily it will be induced by the earth s magnetic field and the more readily it will give up its magnetism when removed from that field netism in the various structures of a ship which tends to change as a result of cruising permanent instant A ship then has axo manent subpermanent and induced magnetism since its metal structures are of varyi of hardness Thus the apparent permanent magnetic condition of the ship is subject to change rming excessive shocks welding vibration etc and the induced magnetism of the ship will va th gstrength of the earth s magnetic field at different magnetic latitudes and with the alignment o Ip i at Meld 4 2 5 Resultant induced magnetism from e gnetic field The above discussion of induced magneti CIT tid magnetism leads to the following facts A long thin
110. pass headings or bearings with what we know the actual magnetic headings or bearings should be the difference being the deviation igure 7 Compass Card During the process any magne ated by the ship s structure equipment etc which cause the e efflninated by creating equal but opposite magnetic fields using compensating correctors These p inside the compass binnacle or adjacent to the compass thw Magnets are aligned fore and aft and compass to deviate are reduced ips to create horizontal magnetic fields to compensate for the permanent horizontal compone e ship s magnetism Soft iron correcting spheres or plates and the F pensate for the induced magnetism caused by the effect the earth s magnetic field has o magnetism Heeling error magnets compensate for the vertical compolte hip s magnetism The timing and logistics of this operation are often governed by tle tide the weather and other vessels in the vicinity The time it takes to swing and adjust the compass is also infl ed by the condition and accessibility of the compass and correctors the manoeuvrability of helmsman and the complexity of and reasons for the deviating magnetic fields involy d On successful completion of compass swing a table recording any remaining residual devration and a statement as to the good working order of the compass will be issued A currefit deviafion cam certificate of adjustment is a legal requ
111. perature sensors in the transducer array Data from both sensors are lt P processed to provide corrective information for the system Alternatively the Krupp Atlas Alpha beam 1 transducer system effectively counteracts the effects of salinity and temperature by the use of a phased ALPHA transducer array The necessity of a tilted beam normally dictates that the transducer protrudes below the keel and therefore may suffer damage It is possible to produce the required angle of propagation by the use of a number of flush fitting transducers The Krupp Atlas Alpha Atlas Low Frequency Phased Array multiple transducer Janus assembly uses 4 18 72 flush fitting elements in each of the fore and aft positions In theory any number of elements may be used but the spacing of the elements must not c2 exceed certain limits in order to keep unwanted side lobes down to an acceptable level 2 Figure 3 23 a is a cut away bow section of a vessel fitted with an Alpha transducer array For clarity only a three element assembly is shown If the three elements are fed with in phase signal voltages the eam Xfermed would be perpendicular However if the signal voltages to each element are phase deled in this case by 120 the main lobe is propagated at an angle which under these conditions is uto In this case the elements are fed with three sine waves each shifted clockwise by 120 For nu 0 figuration the same elements are fed al
112. precessio the spin axis will move in azimuth towards the meridian 2 Tilting of Earth s horizontal plane 3 Precession in azimuth caused by apparent tilt of gyro 4 In the meridian with maximum tilt S Note Time interval between 1 and 4 21 Ys minutes Bons l P f 1 Initial position L 14 Figure 1 12 Precession of a controlled gyroscope at the equator The right hand side of the gyro spin axis now moves towards the north and is referred to as the north end of the spin axis Without the application of additional forces this type of gyro is northseeking only and will not settle in the meridian The north end of the spin axis will therefore describe an ellipse as shown in Figure 1 9 As the extent of the swings in azimuth and the degree of tilt are dependent upon each other the gyro can be made to settle by the addition of an offset control force SJA practical gyrocompass anti lt precession allowing the unit to settle in the meridian This is achieved by pagent tilting of the gyroscope can be reduced by producing an offset controlling force which in ay the vertical axis to cause precession about the horizontal axis This is achieved ste Th effec creat in this gy m by tting the mercury ballistic controlling force slightly to the east of the vertical The point of attachment must be precise so that damping action causes the gyro to settle exactly in the m y ively small force is required to produ
113. processing section and the transmitter are triggered froiffthe processor trigger pulse generator circuit Both the transmit and receive sections work in the me Ayay as previously described A low logic pulse from the trigger pulse standardizing circuit the logic functions The d c output from the receiver detector is coupled via a data pulse uit to Re interface system Unfortunately in any echo sounder it is likely that unwanted eived due to ship noise aeration or other factors False echo ould be di ed as false depth indications on the chart and would be easily recognized To prevent this happening echoes are stored in a data store on id echoes will produce a reading on the display ave indicated the same depth for two consecutive sounding cycles The data store therefore consists o sta amp e counter which holds each echo for one sounding cycle and compares it with the next echo be is Rolayed on the digital display The display circuit consists of three at are clocked from the clock oscillator circuit Oscillator clock pulses are initiated by theSyst igger e the instant of transmission The first nine pulses are counted by the lowest order de significant figure LSF element The next clock fed back to stop the clock Each time transmission takes place the counters are reset to ze ore being abled This is not S 4f an echo is received during the counting process the output 1s stopped and the output latches t
114. produced by these and other factors affects the signal to noise ratio of the received signal and can in some cases lead to a loss of the returned echo Signal to noise ratio can be improved by transmitting more power This may be done by increasing the pulse repetition increasing the amplitude or duration of the pulse Unfortunately such an increase which Its amplitude is directly ortional to the transmitted signal Its amplitude is inver proportibnal to the distance from the target Its frequency is the same transmitted signal The signal to noise ratio cannot be improv m osos transmitter power because reverberation noise is directly proportional to the power 3 smitted wave Also it cannot be attenuated by improving receiver selectivity because the Noise uf frequency as the transmitted wave Furthermore reverberation noise increases ause of increasing beamwidth The area covered by the wavefront progressively increases i er area from which back scattering will occur This means that reverberation noise does not litude as rapidly as the transmitted ceed the signal noise amplitude as shown in Figure 2 4 and the echo will be lost The amplitude t echo and reverberation noise decreases linearly with range However because of beam spread ack sca reverberation noise amplitude falls more slowly than the echo signal amplitu scattering sources produce reverberation noise Surface reverberation As the
115. put to the system when requested the gyrocompass selects Automatic After an initial period ch the bite 1s active the following sequence is initiated and the settle indicator lamp will be lit Two bleeps prompt the o frato 0 the gyro switches to an auto heading input If heading data is not entered within 5 min ill be offset based on this data It will be slewed from the meridian either clockwise nti ise 9 Assuming heading data has been input Thegyrowheel is brought up to speed wi 14 The yoke is slewed back and forth to level tf amp balli Is action takes about 4 min Again assuming heading data has been input th gyro will settle within 1 h and the settle indicator lamp goes out If no heading data was e within 5 h Other inputs to the gyrocompass are as follows e compass will automatically settle Heading in the range 0 to 359 If the entered heading is in error by morgfffian 20 from the true heading the compass takes 5 h to settle Initialize and Synchronize Step Repeaters An operator selects a repeat amp and wh n requested uses check this procedure because there must be no alignment errors in a repeater systen Speed Input Using the left or right arrow keys an operator inputs a speed in the range 0470 knots Latitude Input Using the arrow keys an operator inputs latitude in degrees north or south of the e p 1 12 Compass repeaters Remote ana
116. rainer 74 Switch ENGINE ROOM 1 Cofferdam J iltank e Dynamic Intake Static Intake Figure 3 3 A shipboard installation Reproduced courtesy of SAL Jungner Marine Lb m Figure 3 4 shows the basic speed and distance translating system of a Pitot tube log The di includes two repeating systems for speed and distance data transmission to remote indicators on the ship s bridge This system was superseded by the SAL24E which replaced some of the mac apparatus with electronics The original log has been included here because it is still in use on many 75 amp vessels and is a fine example of a pressure type speed logging system NX gt D a 1 pressure chamber 12 screw spindle 2 pressure rod 13 friction wheel 3 lever 14 distance cone e 4 pivot 15 distance shaft 5 electric start contact 16 servo transmission system 6 reversible motor 17 servo transmission system 7 main shaft 18 gear wheels 8 spiral cam 19 gear wheels 9 lever 20 speed servo transmitter 10 11 Reproduced ourtesy of SAL Jungner Marine constant speed motor 21 remote speed indicator Q distance counter 22 servo receiver Figure 3 4 The mechanical speed translating system of the SAL 24 pressure tube log P Description of operation L An increase in the vessel s speed will cause an increase in the dynamic pressure beneath the diaphragm in the pressure chamber 1 This causes
117. ries with the consistency of the ocean floor 2 e Noise Either inherent noise or that produced by one s own transmisSio usas the signal to noise ratio to degrade and thus weak echo signals may be lost in noise Two additional factors should be considered L Frequency of transmission This will vary with the system i e depth sounding or Doppler Speed log Q Angle of incidence of the propagated beam The closer the angle to vertical the greater will be the energy reflected by the seabed 2 2 1 Attenuation and choice of frequency P The frequency of the acoustic energy transmitted in a sonar system is of prime importance To achieve L a narrow directive beam of energy the radiating transducer is normally large in relation to the wavelength of the signal Therefore in order to produce a reasonably sized transducer emitting a 48 amp t narrow beam a high transmission frequency needs to be used The high frequency will also improve the signal to noise ratio in the system because ambient noise occurs at the lower end of the frequency spectrum Unfortunately the higher the frequency used the greater will be the attenuation as shown in Figure 2 1 10000 1000 100 10 1 0 1 0 01 0 001 bsorption loss in dB 1 10 100 1000 10000 Frequency in kHz by plotting absorption loss against frequency Salinity of the seawater is 3 4 at 15 C The choice of transmissio Juencybus therefore a compromi
118. ro possessing a damping period of greater than 80 min The time taken for one oscill om A to A3 is termed the natural period of the compass Arc degrees o Figure 1 15 The settling curve of a typical gyro compass with a io e 1 5 1 The amount of tilt remaining on a settled gyro The settling curve traced by the north end of the gyrospin axis illustrated in Figure 1 10kassufftes tiat the gyrocompass is situated at the equator and will therefore not be affected by gyro tilt Nt is EL o likely that a vessel will be at some north south latitude and consequently drift must be take account Q It has been stated that for a gyrocompass in northern latitudes the gyrospin axis will drift to the east of the meridian and tilt upwards For any fixed latitude the easterly drift is constant Westerly precession however is directly proportional to the angle of tilt of the rotor axle from the horizontal which itself is P dependent upon the deviation between it and the meridian At some point the easterly deviation of the L north end of the spin axis produces an angle of tilt causing a rate of westerly precession that is equal and opposite to the easterly drift The north end although pointing to the east of the meridian is now 16 amp stabilized in azimuth As the north end moves easterly away from the meridian both the rate of change of the tilt angle and the angle itself are increasing The increasing angle of tilt produces an increasing ra
119. roduces a Master compass n t Compass transmission circuit Repeater compass Figure 1 41 A synchro bearing transmission system 45 The error signal present when the rotors are not synchronized is directly proportional to the error angle y existing between the horizontal and the plane of the rotor This error signal is amplified to the level required to drive a servo to turn the compass card Also mechanically coupled to the servo shaft is the receiver rotor that turns to cancel the error signal as part of a mechanical negative feedback arrangement The receiver rotor will always therefore line up at 90 with the transmitter rotor to produce the synchronous state 1 13 Glossary gular momentum In the case of a gyrowheel this is the product of its linear momentum and the Contaffiers of viscous liquid to add damping to a gyrocompass equipment Automatic or manually commanding equipment test circuits isplay of compass information in which the movement caused by earth rotation is controlled uth of a gyroscope due to earth rotation Dynamic errors Error the angular motion of the vessel during heavy weather or manoeuvring Flux gate The electrical sensin a magnetic compass ixe by inertia to some celestial reference point and not Free gyroscope A gyroscope wit to a terrestrial point Not suitable as a Eyroco s Follow up A system enabling control of yro it i fitted on board a mov
120. rrection is proportional to the ship s speed and the cosine of the ship s course is coupled back to the lifer to cause the gyroball to tilt in opposition to the apparent tilt caused by the northerl or erly component of the ship s speed The signal will therefore be maximum in amplitude Wien the cour due north or south but will be of opposite sense If the course is due east or west no correctio essary The system uses a 1 1 ratio azimuth synchronous transmitter SG1 which is mechanically imuth servomotor gearing and a balanced star connected resistor network as shown in 39 s3 SG 1 R14 s2 c MR e ues 980 RV24 To azimuth R15 Maximum amplifier signal Se voltage 1 a Ship sailing north To azimuth amplifier b Ship saili azimuth i RV24 lifi Maximum ampli er os signal voltage e Arrows denote instantaneous current flow Figure 1 38 Signal output of synchro SG1 for different headings Reproduced courtesy S G Brown Ltd Alternatively an external signal derived from the ship s speed log may be used In Figure 1 38 the error for a ship sailing due north is maximum and therefore the feedback signal produced across RV24 by the currents flowing through SG1 S1 and S2 coils will be maximum A portion of this signal dependent upon the speed setting of RV24 is fed to the azimuth amplifier to produce a tilt of the 40 c Ship sailing south Lb Note Q gyroball For a course du
121. rticles where K is the radius of gyration the angular velocity o The angular momentum is now proportional to 0MK If one or more of these factors is changed the rotor s gyroscopic inertia will be affected In order to maintain momentum a rotor is made to have a large mass the majority of which is concentrated at its outer edge Normally the rotor will also possess a large radius and will be spinning very fast To spin freely the rotor must be perfectly balanced its gravity will be at the intersection of the three axes and its mounting bearings must be as t point A on the rotor circumference is pushed downwards into ning clockwise producing gyroscopic inertia restricting the effective to phe paper Applied Resultant path force of pointA Momentum of pointA Spindle Y move towards the paper Point A will therefore move along a path that is the vector s origiggl path and pap The plane of rotation of the rotor has therefore moved about the H axis although the applied force Was moves deeper into the paper point C undergoes a reciprocal action and moves away to the V axis The angular rate of precession is directly proportional to the applied force 1S inversely proportional to the angular momentum of the rotor Figure 1 5 illustrates the rule of gyroscopic precession Vertical axis Direction of rotation of wheel ae M Horizontal axis gt i wen Spin axis d T o re xd L Prec
122. s balances the system 3 1 practical electromagnetic speed logging system otential developed across the transducer electrodes is proportional to magnetic field strength and mseguently the energizing current and the flow velocity in the volume of water influenced by the acnetic field strength is in no way stabilized against any changes in the ship s main voltage et buby effectively comparing the energizing current with the voltage at the electrodes s a meas re of the ship s speed The input ffansformer T own in Figure 3 11 possesses a very high inductance and a step down ratio of 5 1 This res n input impedance as seen by the pick up electrodes approaching 20 M Q which when compared ance presented by salt water can be considered an open circuit Hence changes in salin O effect on the measured voltage and the resulting speed indication A switched resistor chain R t th amp gain of the overall amplifier in conjunction with resistor chain R6 R10 which controls the a o feedback signal e 83 Zeke e SESEGE pE agases 52g 2 ES SSSSS a D E x c 0 oo tm os fy op 075 mao 23 og o8 age amp multiplier IC3 de T2 i N _ d Demodulator by wm r l l l l l I L Reproduced courtesy of Thomas Walker and Son Ltd The output of IC1 is coupled via IC2 which because of capacitive feedback not shown ensures that the circuit
123. s the spin axis to tilt upwards to an angle Q dependent upon its position in latitude Transmission error An error existing between the master compass and any repeaters 46 1 14 Summary There are three axes in which a gyroscope is free to move the spin axis the horizontal axis and the vertical axis In a free gyroscope none of the three axes is restricted A free gyroscope is subject to the laws of physics the most important of which when considering gyrocompass technology is inertia cession is the term used to describe the movement of the axle of a gyroscope under the fluence of an external force Movement of the axle will be at 90 to the applied force t is the amount by which the axle tilts because of the gyroscope s position in latitude zi drift is the amount by which the axle drifts due to the earth s rotation led gyfoscope is one with its freedoms restricted A h sg King gyroscope is a controlled gyro that never settles pointing north A no ettling gyr pe is a damped controlled gyro that does settle on the meridian Bottom and top controls are methods used for settling a north seeking gyroscope A gyrocompags fi a ship is affected by dynamic errors They are rolling error manoeuvring err d course error and latitude or damping error All these errors are predictable and contr le When starting from cold ses require time to settle on the meridian A settling time period of 75 min is typical G
124. se between transducer size freedom sguencies between 15 and 60 kHz are typical for depth indicate great depths with low attenuation ight craft use depth sounders that transmit in the band 200 400 kHz This enables compact ictive 9 ceramic transducers to be used on a boat where space is limited Speed logs use frequ the range 300 kHz to 1 MHz depending upon their design and are not strictly sonar devices in th ion of the sense Beam spreading Transmission beam diverging or spreading is independentof fi ameters such as frequency but depends upon distance between the transducer and the seab ter the depth the more the beam spreads resulting in a drop in returned energy Temperature Water temperature also affects absorption As temperature decreases attenuation decreages gt The effect of temperature change is small and in most cases can be ignored although ern gonarequipment is usually fitted with a temperature sensor to provide corrective data to the processor 2 e Consistency of the seabed bedyand The reflective property of the seabed changes with its consistency The main types of attenuation which they cause are listed in Table 2 1 The measurements were made with an o sounder transmitting 24 kHz from a magnetostrictive transducer Q 49 Table 2 1 Sea bed consistency and attenuation Consistency Attenuation dB v3 Os o UA Soft mud Mud sand Sand mud Sand Stone rock These figures are typical and are quoted as a
125. ssiwsued dnyaid 2 1s4nq esjnd poued yous E ses nd seonpouJg indino sjes Mon E 815008 NO 10je jioso Burun ZHZ asind ndino ajqesouow y y i dwe g oso yeya abeyoa Jaded Wud 1012919p a AMUUXO1d 320 q RAW As esind 186Buj sn Ais 10j0uJ ueb 2 9 up saded oyde E d o e ABZ es nd yndyno yap XOld Figure 2 11 A block schematic diagram of the Seahorse echo sounder Reproduced courtesy of Marconi Marine 61 The system used a transmission frequency of 24 kHz and two ranges either manually or automatically selected to allow depths down to 1000 m to be recorded The shallow range was 100m and operated with a short pulse length of 200 us whereas the 1000 m range uses a pulse length of 2 ms Display accuracy for the chart recorder is typically 0 5 producing indications with an accuracy of 40 5 m on the 100 m range and 5 m on the deepest range 2 5 1 Description Receiver and chart recorder When chart recording has been selected transmission is initiated by a pulse from a proximity detector hiclisyiggers the chart pulse generator circuit introducing a slight delay pre set on each range to eng re that transmission occurs at the instant the stylus marks zero on the recording paper This system ey pulse or that from the trigger pulse generator circuit when the chart is switched off has three 88 on e cy e pese timing circuit to
126. stipulate that the magnetic compass is to be swung and ally Prudent mariners and vessel operators will always ensure that the compass is regular Soy adjusted Merchant vessels are often subject to costly i y Port State Control authorities should they fail to maintain a record of deviation observati ns offie mts is found to have deviation in excess of 5 degrees Any vessel operating under state survey is required its magnetic compass examined and intervals In addition to regular routine checking of the compass for lation and adjustment for survey compliance all sea going vessels should have their compass ifispg te ng and adjusted and a new deviation card issued when any of the following apply After periods of lay up When a new compass is installed When deviation exceeds 5 degrees On a new vessel or in a new area of operation e After trauma such as lightning strike grounding fire etc When compass performance is unsatisfactory or unreliable Cp After alterations and additions to vessel s structure and equipment When a record of compass deviation has not been maintained After repairs involving welding cutting grinding etc which may affect the compass When electrical or magnetic equipment close to the compass is added removed or altered When compass deviation does not appear to correspond with that shown on deviation card lt P 4 1 6 A Compass card tilts The earth s magnetic field travels from the Magne
127. system automatically switches to water mass tracking and will recor vessel s speed with respect to a water mass approximately 12 m below the keel The transducer transmits pulses of energy at a frequency of 150 kHz from two active piezoceramic elements that are arranged in the fore and aft line of the vessel see Figure 3 12 Each element e Q transmits in a wide lobe perpendicular to the seabed As with an echo sounder the transducer elements are switched to the receive mode after transmission has taken place 85 3 78mm Figure 3 12 Pi ceramic transducer for the SAL acoustic correlation speed log The seabed or water mass flected signals possess a time delay T dependent upon the contour of the seabed as shown in Figure 3 tligyeceived echo is uniquely a function of the instantaneous position of each sensor element pl ip off The echo signal therefore in one channel will be identical to that in the other channel but willg ss time delay as shown l Channel 1 i Channel 2 Figure 3 13 Illustration of the time delay 7 between each channel ech n e The time delay T in seconds can be presented as T 0 5xsv where s the distance between the receiving elements and v the ship s velocity In the SAL ACCOR log see Figure 3 14 the speed is accurately estimated by a correlation technique e Q The distance between the transducer elements s is precisely fixed therefore when the tim
128. t he rudder is put hard over the transverse speed indication vector Vy can point either to i icine rudder has been moved or to the other ship s movement is completely determinable The bo the direction of the ship s turning circle Under the influence of the 4 knot current shown in Figur g ver V3y points to port The transverse speed development along the ship s length is represen a dotted lj V3y The intersection of this line with the longitudinal axis produces a poi to that shown in Figure 3 20 It is obvious therefore that an accurate indication of transverse various points along the vessel enables the navigator to predict the movement of his ship 94 Example X1 X3 300 m V 8 Knots 2 m ZU 1 Ke o Qa o oO a Figure 3 21 Speed vectors for a starboard turn under the influenc a four knot c courtesy of Krupp Atlas Elektronik Speed components with the rudder amidships Dual axis Doppler logs are able to measure accurately the ship s speed in a components i e the velocity vector it is possible to optimize the vessel s course by com drift angle x arc tanV Vx In the water tracking mode this is the leeward angle caused by wind which is the angle between the d Q true course heading and the course made good CMG through the water In the bottom tracking mode it is the angle due to wind and tidestream between the heading and the CMG over the ground With the help of a two
129. te varying with latitude to cancel the error This will be an external corre that can be either mechanical or electronic For mechanical correction a weight on the gyro necessary torque The weight or mechanical latitude rider is adjustable thus enabli ions to be made for varying latitudes Another method of mechanical correction is to move the lubber amoypt equal to the error Latitude correction in a bottom weighted compass is achieved by n of a signal proportional to the sine of the vessel s latitude causing the gyro ball to precess 18 azyuj th rate equal and opposite to the apparent drift caused by earth rotation JA Speed and course error If a vessel makes good a northerly or southerly course the north end of the gyro spin axis C apparently tilt up or down since the curvature of the earth causes the ship to effectively tilt bows up or down with respect to space Consider a ship steaming due north The north end of the spin axis tilts upwards causing a westerly precession of the compass which will finally settle on the meridian with P some error in the angle the magnitude of which is determined by the speed of the ship On a cardinal L course due east or west the ship will display a tilt in the east west plane of the gyro and no tilting of the gyro axle occurs hence no speed error is produced The error varies therefore with the cosine of the 21 ship s course Speed course gyrocompass error magnitude must also be af
130. te of downward damping tilt until a point is reached where the upward and downward rates of tilt cancel The north end of the axle is above the horizontal although the rotor axle is stabilized Figure 1 16 shows that the gyrocompass has settled at point 0 to the east of the meridian and is tilted up F Rate of westerly precession of meridian caused b able the gyro caused by the north movement of the Earth s end having tilted upwards G Rate of upward tilt of north H Rate of downward precession end of spin axis due to gyro of north end of spin axis due setting to east of meridian to damping anti tilt precession Figure 1 16 A curve showing error to theeast4 nd tl caffed by latitude on a settled gyrocompass X is the angle away from the meri a ig the angle with the horizon tilt Reproduced co Ltd latitude An increase in latitude causes an increase in both erly deviation from the meridian and the angle of tilt above the horizontal It is necessary therefo a error as the discrepancy is called to be corrected in a gyrocompass As latitude increases the effect of the earth s rotation becomes progressive ss and consequently tilting of the rotor axle becomes less It follows therefore that the rate of i ssion needed to cancel the rate of tilt will also be less e 1 6 Follow up systems 2 A stationary gravity controlled gyrocompass will adequately settle close to the horizontal and n the meridian provided that
131. ter motor may be used which dispenses with intermediate gearing Step motor connected coils 1 and under the influence of the magnetic field produced the rotorkt p position shown As the switch moves to position 3 its make before break action causes curr nt to flow through both coils 1 and 3 and the rotor moves to a position midway between the coils due east AW st The next movement of the switch energizes coil 3 only causing the rotor to line up with this coil In this way the rotor 1s caused to rotate one revolution in 12 steps The construction details of a step motor are given in Figure 1 40 lt P Q 44 Figure 1 40 Construction details of a step motor r system such as this may also be used as part of a direct digital control d d c system in inary code or gray code for its operation The gray code is easily produced using shaft or disc TS ed to the compass azimuth gearing ec 1 1 A sync is device t ro syste ms t uses the basic principle of a single phase transformer with magnetic coupling b tween a rotati rimary rotor and a number of secondaries stators For the purpose of this description three through 360 within th coil is energized by c ndaries are located at 120 intervals on the stator The rotor may be rotated or assembly holding the three secondary windings The primary rotor coil couples with the three secondaries t three circuits Each current flow p
132. ternately clockwise and counter clockwise The Alp also Mercomes the external factors that influence the velocity of acoustic waves in salt water afis thafS able to counteract the unwanted effects of salinity and temperature change Fo Aft AM o x ismission a ctiong n o gitudinal axis epit Transducer element b Figure 3 23 a Principle of the alpha transducer array b A 72 element alpha transdu rr e The standard Doppler formula from which velocity is calculated comprises a number offfaratmeters two of which are variable Ideally the vessel s speed v should be the only unknowr fa in formula but unfortunately the velocity of acoustic waves C is also a variable Since speed accugacy depends upon the accuracy of acoustic wave velocity in salt water it is advantageous to elimina from the formula C E 97 With the Alpha system the angle of propagation 60 is a function of the velocity of acoustic waves because of the geometry and mode of activating the multiple elements see Figure 3 23 a The angle Y of propagation is 3a is a fixed Parameter a cos h 3a C 3a ft erejg the transducer element spacing and is therefore a fixed parameter C ft one acoustic wavelength in salt water le eqM tions in this section are now combined the Doppler frequency shift is fd 2w3a erefore v is now the only variable Two modes of operation are possible reflected either from the se
133. the support plate close to the azimuth motor and is geared to rotate the compass dial The phantom yoke supports the east west gimbal assembly through 31 horizontal axis bearings To permit unrestricted movement electrical connections between the support plate and the phantom yoke are made by slip rings The east west gimbal assembly supports the Vy vertical ring and horizontal axis bearings See Figure 1 32 PHANTOM YOKE AIR TUBE SPHERE TANK PENDULOUS BOWL LIQUID TUBE GYRO WHEEL Figure stem of the Sperry MK 37 VT gyrocompass d esy of Litton Marine Systems The gyrosphere The gyrosphere is 6 5 inches in diameter 3 about the vertical axis within the vertical ring which in turn is pivoted about the horizontal is 1 th east west gimbal assembly At operating temperature the specific gravity of the spher kis th same as the liquid ballistic fluid in which it is immersed Since the sphere is in neutral buoyancyxuit e load on the vertical bearings Power to drive the gyro wheel is connected to the gyrosphefe fr vertical ring through three spiral hairsprings with a fourth providing a ground connection The liquid ballistic assembly also known as the control element b ga is the component that makes the gyrosphere north seeking consists of two interconnected bras partially Small bore tubing connects the tanks and restricts the free flow of fluid betwe Because the time for fluid
134. tic South Pole to the Magnetic North Pole For the sake of mathamatical convenience it is divided into two major components vertical and horizontal The 112 closer to the poles the stronger the vertical component and the weaker the horizontal component At the magnetic equator the horizontal component is at its strongest and the vertical component is zero earth As the compass 1 is ritegral with the card of a marine compass the upwards or downwards magnetic force can affec counter weight attached to enable A boat compass specifically designed for Nofe misphere use will have a weight positioned to counter the downward magnetic force thi passis brought to the Southern Hemisphere the combination of the weight and the upwards netigff rce will create an exagerated tilt on the card Obviously the same thing will happen to a South e compass when it goes to the north Rebalancing the compass for the opposite hemisphe ismantling the compass and moving the weight to the opposite side of the card and is not sidered economically viable For a yacht travelling between the higher latitudes of one o the other carrying two interchangable compasses one balanced for each hemisphere migi be advisable Other reasons for compass card tilt Heeling error magnets require adjustment Damaged card float chamber Damaged jewel pivot Card dislodged from pivot Low liquid fluid level 4 1 7 A professional compass
135. tion of its plane of rotation unless an external force of sufficient amplitude to overcome inertia is applied to alter that direction In addition a rapidly spinning free gyroscope will maintain its position in free space irrespective of any movement of its supporting gimbals see Figure 1 2 Figure 1 2 The gyro Also from the laws of phy of its mass and velocity mv to think in terms of angular mome cyli r of mass m Figure 1 3 A spinning rotor possessing a solid mas The angular momentum of a particle spinning about an axis is the product gf its linear entum and the perpendicular distance of the particle from the axle angular momentum mv r e where r rotor radius The velocity of the spinning rotor must be converted to angular velocity by dividing lin tangential velocity v by the radius r The angular momentum for any particle spinning about an Axis is now mor Q For a spinning rotor of constant mass where all the rotating particles are the same and are concentrated at the outer edge of the rotor the angular momentum is the product of the moment of inertia and the angular velocity angular momentum J L where J 0 5 mr It can now be stated that gyroscopic inertia depends upon the momentum of the spinning rotor The j Q momentum of such a rotor depends upon three main factors the total mass M of the rotor for all particles the radius r summed as the constant K for all the pa
136. to flow from one tank to the other is long comp Mf S roll period roll acceleration errors are minimized Follow up control An azimuth pick off signal proportional to the azimuth movement of the vertical rin an E core sensor unit and coupled back to the servo control circuit and then to the mounted on the support plate When an error signal is detected the azimuth motor drives the azindlith gear to cancel the signal Heading data from the synchronous transmitter is coupled to the synchro to digital converter N ASSY where it is converted to a 14 bit word before being applied to the CPU The synchro heading data 115V a c 400 Hz reference 90 V line to line format is uncorrected for ship s speed error and latitude error Corrections for these errors are performed by the CPU using the data connected by the P analogue digital isolated serial board ADIS from an RS 232 or RS 422 interface Interface data L1 Compass interfacing with external peripheral units is done using NMEA 0183 format along RS 232 32 amp and RS 422 lines CPU assembly The heart of the electronic control and processing system the CPU is a CMOS architecture arrangement communicating with the Display and Control Panel and producing the required outputs for peripheral equipment Two step driver boards allow for eight remote heading repeaters to be connected Output on each channel is a 24 V d c line a ground line and three data lines D1 D2 and D3 Each three st
137. tor eS FE 5 p T7 0 5 Hz 8V N 0000 pulses per Gradient PT nautical mile Speed gt and 20000 pulses per potentiometer a iat nauyeal mile cir 0 knot From Dipat O dummy m box A Figure 3 6 The electronics unit Reproduced courtesy of SAL Jungner Marine Lb The accuracy of the Pitot type speed log when correctly installed and calibrated is typically better OQ 0 75 of the range in use Electromagnetic speed logs continue to be popular for measuring the movement of a vessel through water This type of log uses Michael Faraday s well documented principle of measuring the flow of a 79 3 3 Speed measurement using electromagnetic induction 4 Q fluid past a sensor by means of electromagnetic induction The operation relies upon the principle that any conductor which is moved across a magnetic field will have induced into it a small electromotive force e m f Alternatively the e m f will also be induced if the conductor remains stationary and the magnetic field is moved with respect to it Assuming that the magnetic field remains constant the amplitude of the induced e m f will be directly proportional to the speed of movement In a practical installation a constant e m f is developed in a conductor seawater flowing past the sensor and a minute current proportional to the relative velocity is induced in a collector The agn tic field created in the seawater is produced by a solenoid which may ext
138. two ballistic pots 3 are mounted tohe north and south sides of the vertical ring Each pot possesses two reservoirs containing the high d liquid Daifloil Each north south pair of pots is connected by top and bottom pipes providing a tota liquid air sealed system that operates to create the effect of top heaviness Because the vertical ring and the rotor case are coupled to each other the ring follows the tilt of the gyro spin axis Liquid in the ballistic system when tilted will generate a torque which is proportional P to the angle of the tilt The torque thus produced causes a precession in azimuth and starts the L northseeking action of the compass Q 1 8 4 Azimuth stabilization phantom ring assembly 24 Gyro freedom of the north south axis is enabled by the phantom ring and gearing This ring is a vertical circle which supports the north south sides of the horizontal ring on the spin axis by means of high precision ball bearings A small oil damper 6 is mounted on the south side of the sensitive element to provide gyro stabilization during the ship s pitching and rolling The compass card is mounted on the top of the upper phantom ring stem shaft and the lower stem shaft is connected to the support ball bearings enabling rotation of the north south axis The azimuth gearing located at the lower end of the phantom ring provides freedom about this axis under a torque from the servomotor and feedback system
139. ually settle both on the L meridian and horizontally If the gyro axis is influenced by a second force exerting a damping torque Q about the vertical axis so as to cause the spin axis to move towards the horizontal it is obvious from 10 Figure 1 10 that the minor axis of the ellipse will be reduced As the north end of the spin axis moves to the west of the meridian the earth s rotation will cause a downward tilt of the axis This effect and the torque Tv will cause the gyro axis to meet the earth s horizontal at point H which is a considerable reduction in the ellipse major axis As Figure 1 10 clearly shows this action continues until the gyro settles in the meridian and to the surface of the earth point N 11 The north end of the gyro spin axis is initially pointing at P which is above the earth s horizontal and to the east of the meridian The gravity control therefore exerts a torque TH about the gyro s horizontal axis and at the same time is arranged to exert a torque about the gyro s vertical axis Because of the direction of rotation of the spinner HS ko Rizo Ni the gyro to precess towards the west Because the ind gyro js east of the meridian earth rotation results in an ofthe gyro but this movement is damped by the downwi g SSion that is due to torque Ty The resultant path traced by th axis is therefore Pb and not Pb as would be the case witho ping torque Ty Note that the downward
140. ucing s ch Afin electronic cigarette lighters and fundamental crystal oscillator units for digital watches Ho a ceramic this way is subject to the same mechanical laws as have previously bee frequency of oscillation the thinner the slice needs to be and the greater external stress or overdriving For these reasons piezoelectric resonators are d atsea 2 3 3 Magnetostrictive transducers l e Figure 2 7 shows a bar of ferromagnetic material around which is wound a coil If the ba eld rigid and a large current is passed through the coil the resulting magnetic field produced wil y to change in length This slight change may be an increase or a decrease depending upon t e as used for construction For maximum change of length for a given input signal annealed nick been found to be the optimum material and consequently this is used extensively in the construction Q J 55 amp marine transducers Annealed nickel Length J a l Length 6 OQ magnetic field is produced around the coil A minute alternating current is caused to inthe chi and a small e m f is generated This is then amplified and processed by the receiver as the re d echo To limit the effects of magnetic hysteresis and eddy current losses common in low frequency transformer construction the annealed nickel bar is made of laminated strips bonded together with an insulating material Figure 2 8 illustrates the construct
141. up ndary transformer 15 Follow up amplifier 16 Latitude corrector 17 Spring shock absorber as 1 8 1 The stationary element frame 11 Figure 1 23 holds the slip rings lubber line and the scale illumination circu st main shaft connected to the phantom ring 12 protrudes through the supporting frame o hold a compass card that is visible from above A high quality ball bearing race supports the movable element on the base of the main support frame OQ order that movement in azimuth can be achieved The base of the whole assembly consists of upper and lower base plates that are connected at their centre by a shaft Rotation of the upper plate in relation to the lower plate enables mechanical latitude correction to be made The latitude corrector 16 is e provided with upper and lower latitude scales graduated in 10 units up to 70 north or south latitude L either side of zero Latitude correction is achieved by mechanically rotating the movable element relative to the stationary element thus producing a shift in azimuth The fixed scale of the latitude 23 a adjuster 16 is secured to the stationary element with a second scale fixed to the movable element To set the correction value which should be within 5 of the ship s latitude is simply a matter of aligning the ship s latitude on the lower scale with the same indication on the upper scale of the vernier scale Also supported by the base plate are the azimuth servomotor an
142. up the gyrosphere about the horizontal axis This servo operates from the diffe oduced by the secondary pick off coils which is processed to provide the amplitude required t he sensitive element assembly in azimuth by rotating the phantom yoke assembly in the i eeded to cancel the error signal In this way the azimuth follow up circuit keeps the gyrosphere ani sitive element chamber in alignment as the gyro precesses 27 Card Power adapter i i er anamer A gt Repeater N S Erates E SIE i 3 Servo L 8mp motor E Azimuth Sensitive follow up element amp assembly sphere Horizontal follow up amp he Sperry SR220 follow up system In common with all other maritime equipmefif the itional gyrocompass is now controlled by a o 1 9 A digital contro COR gyrocompass system ion on the traditional principles already d The Sperry MK 37 VT Digital es available The system has three microcomputer Whilst such a system still relie its described most of the control functions are com main units the sealed master gyrocompass assembly the ele i nd the control panel The master compass a shock mounted fluid filled binnacle u provides uncerrected data to the electronics units which processes the information and outputs it as corrected ing and rate of turn previous gyrocompasses 28 The weight of the gyrosphere is removed from the sensitive afis ings The gyrosphere an
143. y is relatively simple Most man fac f ddp sounding systems now opt for microprocessor control and digital displays but it wasit alwys so Many mariners preferred the paper recording echo sounder because the display was clear S sy to and provided a history of soundings Marconi Marine s Seahorse echo sounder Figure 2 11 was typical of the standard pu echo sounder Built in the period before microprocessor control it is used here to describe the description it is easy to see that an echo sounding system is simply a timing device relatively simply circuitry needed to produce an accurate read out of depth beneath the keel From the ep Q 60 suue v abueu2 1018 9uaB s nd a661 40SS3901g eBue pue lt uueje ony Jaya ots ndeg uonejano oq28 Buioepie1ur und A greed eouepioutoo aul 34O Heyo ojeue jew andino eiols sjeq ew lesind uuopun q 2 8 3 J041u02 UNN Buizipsepueys 6 andino uoneziuosyouds 1966113 WAS S T d Aejdsip jeybiq FETA t lt NO Heyo JOVE 9SO synon Aejdsiq Joyesauabii asind 496614 yeu unos 49019 anao BuuebBu 1e3 61q Buissaoo1d pue 7 7 X Aejdsip e1i6iQ eed 96614 uie1sAs eubis jeyibic H 7 E p pal iig pueog 1eA 928H ley asing 0u23 T 401e12u8B Bunjuejg i ala 1 101818 1 YA asynd 11 es nd Er soyeoipul Jayiuusues f d O 19d ononpa1 3 uol
144. ystems that propagate acoustic or electromagnetic energy into seawater to e a vessel s speed or the depth of water under the keel This book is not concerned with those ar teghniques that are used for locating submerged objects either fish or submarines A erchant Navy is interested only in the depth of the water beneath the vessel an indication of fhe speed of his ship and the distance run See Chapter 3 for a description of speed logging equipment Th t section of this chapter deals with the characteristics and problems that arise from the ne gate energy in seawater 2 2 The chara of sound in seawater Before considering the problems of tr of the environment must be understood Sy s rely on the accurate measurement of reflected frequency or in the case of depth sounde ise measurement of time and both these parameters are affected by the often unpredictable oceaa e nment These effects can be summarized as follows Attenuation A variable factor related to the tr er the frequency of transmission salinity of the seawater and the reflective consistenc cean floor Salinity of seawater A variable factor affecting both t o f the acoustic wave and its attenuation Velocity of sound in salt water This is another variable parameter precisely 1505 ms at 15 C and atmospheric pressure but most egljo soundi e calibrated at 1500 ms Reflective surface of the seabed The amplitu reflected energy va
145. ystems use pulsed transmission and not a continuous wavemnode operation 7 Many echo sounders offer the ability to vary the transmission pulse duration Why is this 8 How are the pulse repetition frequency PRF and the maximum depth indicated by an echo sounding system related 9 Why is the siting of an echo sounder transducer important P 10 What do you understand by the term target discrimination 11 What effect may a narrow transmission beamwidth have on returned echoes if a ship is rolling in Q heavy seas 71 Chapter 3 The Ship s Speed Log 3 1 Introduction Speed measurement has always been of the utmost importance to the navigator The accuracy of a dead kofig position plotted after a long passage without star sights being taken is dependent upon a owledge of the vessels heading and speed o f value the speed of any object must be measured relative to some other point At sea speed easbred relative to either the seabed ground reference speed or to the water flowing past the hull efergnce speed Both of these types of speed measurement are possible and both have their place in m navigation This chapter deals wit One of these th electromagnetic log is ethods of speed logging that are in general use on board modern vessels tube log is old but it still gives a satisfactory performance Another the se on smaller vessels and the popular Doppler speed log is to be found everywhere 3 2 Speed
146. zimuth gimbal 0 Tilt servo motor ond Y i OA g Viscous v na Va q Ce damp Split pinion Synchro drive 36 1 synchro drive Synchro transmitter fine Synchro transmitter coarse muth servo motor iscous p Figure 1 35 Schematics showing the arrangement of the secondary An electromagnetic pick up system initiates the signal feedback system maintaining tank If there is no twist in the two pairs of torsion wires and no spurious torques are pri ut spin axis no precession of the gyroball occurs and there will be no movement of the cornffol servomotors The gyro spin axis is in line with a magnet mounted in each hemisphere of the gyr Pick up coils are mounted on the north south ends of the containment tank and are arranged so N when the gyro ball is in alignment with the tank no output from the coils is produced If any misalignment occurs output voltages are produced that are proportional to the displacement in both tilt and azimuth These small e m f s are amplified and fed back as control voltages to re align the axis by P precession caused by moving the secondary gimbal system The tiny voltages are used to drive the L secondary gimbal servomotors in a direction to cancel the sensor pick up voltages and so maintain the correct alignment of the gyroball within the tank 36 amp With a means of tank gyroball alignment thus established

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