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A multi-mode sonar transmitter

1. AA 1 P e giug xonbngrneue 3 i NN i i i i i 1 l AA j X Y j J A A 1 i l i y 15 use an array of identical relatively high Q transducers The scanning of the direction of maximum sensitivity being achieved by signal processing within the receiver electronics A swept beam transmitter scans its narrow sonar beam across the sec tor by injecting a set of closely spaced frequencies into the water along the array face Each transducer transmits a frequency differing from its neighbor ing element by the sweep rate The technique demands a low Q transducer to ensure adequate bandwidth to accomodate the frequency span determined by the sweep rate The transducer element bandwidth therefore defines the maximum sweep rate This swept sector application is restricted by the length of the available buffer stack As each channel differs in frequency the block contents are unique to specific parts of the swept sector i e repeating a block from the stack would re scan that segment The maximum length of a swept pulse cannot exceed 8 milliseconds with the hardware of the data buf fers currently configured as 16 x 256 samples Clocked at 500 kHz However this
2. E e Li hifi tetas mo m 5 e 36kHz 40 kHz 45 kHz 56 kHz units were racked together fig 3 4 2 and wired initially for the 240 volt single phase supply at Foremark reservoir The mains supply was taken via a 32 amp circuit distribution breaker and a 30 milliamp earth leakage RCCB The current rating of the breaker was close to the calculated maximum load and the system tripped safely if the programmed output significantly ex ceeded the 1 10 mark space specification at full power For later trials on 36 board Maytime the system was reconfigured to use a three phase star supp ly cable fed from the shore The on board power distribution incorporated 10 amp phase circuit breakers When transmitting at full power and using long pulses the cable impedance of the three phase supply causes a significant dip in the mains supply voltage Fig 3 4 2 The complete Mk 2 Transmitter Assembly Heas AL nx Seas cs Se obn ra 0 3 i T 37 3 5 Transducers and Arrays The conventional sandwich transducer construction is based on a stack of PZT4 piezo ceramic disks prestressed between a piston head and tail mass a construction usually referred to as a Tonpiltz design For commercial and military power transducer applications in the range between 1 kHz and 50 kHz this design is the most commonly used device Such elements are also used in matched receiver arrays but alternative cheaper
3. Large arrays for testing have to be ferried using a small boat to a point below this hoist and attached together with the training gear For stability at depth it was also found necessary to add a large paddle structure above the pan amp tlt to absorb the torsional Fig 5 3 2 Foremark Draw Tower outer working platform with free standing array support and hydrophone pole m qu n JN T s stress during plotting operations The supporting cable is fitted with tubing collars as used on the Staines reservoir facility and when back tensioned these create a stiff assembly An additional complication of this external ex posed support is its vulnerability to movement in high winds especially when the reservoir water level is lowered as in fig5 3 2 The support system suffered some damage from surface ice movement during the extreme winter of 1985 Torsional stability was a problem without the stabiliser as the deployed depth to the array was more than double that available at Staines A very adequate mains supply 60 amp single phase was provided inside the tower and security of the electronic equipment left stored inside the tower was good Working conditions in the winter months leave something to be desired as the structure is impossible to heat Hydrophones for the beam plotting experiments were deployed from a stiff fibre glass pole constructed from two sailboard masts This 10 metre support deployed horizontally above the water
4. Predicted response of a 15 element array using real dimensions Piston diam 38 mm Centre freq 39 kHz 40 mm inter element pitch Calculated for seawater Note The shifted diffraction secondary at endfire RMPLITUDE Fig 2 0 4 Computed plot of a 15 element 1 pitch array Phase Steered 0 5 radians off axis to demonstrate the equal amplitude diffraction lobes DEGREES 1 amp 3 9p 15 ELEHENT RRRRY RIPPLEFIRED RCROSS SECTOR The plot also includes an overlay of the peak responses produced by a set of steered data files demonstrating the Ripple fire response 15 B DEGREES 415 38 using real elements with directivity can be shown to be the point source dif fraction pattern multiplied by that of the individual element the peak amplitude response as the array steers off axis will be progressively reduced fig 2 0 4 This theoretical plot includes an overlay of 16 phase steered beams whose peak amplitudes reflect the effect of the individual elements directivity The pattern generated by these peak responses simulates the effect of a sector scanning ripplefire transmission To implement these theoretical predictions the necessary phase related drive waveforms are calculated digitally and stored in parallel sets of auxiliary RAM buffers fig 2 0 5 These RAM memory buffers co exist as read only data banks outside the micro computer s memory map and are ac cessed simultaneously by a pro
5. creen 1 8 2 for Ki1 1 to 20 do begin if odd L then LO else HI CHAR end screen 21 gt 3writeCBds2 sscreen 249 8 3 1 2 Block Full block for K 197 to 127 do 239 to 255 do begin if odd L then LO else HI CHAR end screen C1 Bd j endi end end 111 xii procedure PAUSE begin REPEAT UNTIL keyboard end procedure TITLE begin CLS screen 1 16 write lt DISPLAY TX PHASES from DISC Ui 8 ADGIS 5 837 gt screen idywrite mammomummmmuuxz re m m UE SI ams Desin muma end Begin C MAIN PROGRAM s TITLE Hiv 6000 1 7 1009 repeat screen C1i2 10 swrite lt New Filename If Files s then CLS else begin LoadmcCBad File PLOT PAUSE end until Files x End III xiii 3jiscreen 28 105 readlntFile 2
6. out B 48 screenci 11 writelnt FINISHED at end of TX block If wrltelnt ELIISUUEINSDICITXDIDUSSNEINGUSE EISILDIZIERUDIDZEHESLIIL EERE i writeln writetc New tFJile 7 7 Rlestart Q1ui t 721 screen 23 0 readin P if Pz F then CLS if Pz F then DATAIN if Pa Q then DOS else goto RESTART21 End 111 xi Program DISPLAY f j Author amp D Goodson v 62 15 9 86 2 Reads selected disk files and then displays 2 start and ends of 16 channels to check phases Requires H Graf 4 Char Gen Chip amp 16 tine mod Uses mem 48008 to 6800 2 var A B REAL Bd L H 1 J K X 2 s INTEGER File iString Sl 3 S iStringL 25517 1 iStringL 12 Eof Bad Pleasedelay Boolean RE Reat I Sectorcount Loop Integer Procedure Loadmc Uar Bad Bocleanj FileiStringft83 External 2048 Procedure CLS Code 3E C F 1 procedure PARAM begin write Enter Start Address 3060 screen 21 923 readin H2j4if then lt 228808 3 write Enter Increment 180 gt screen 21 6 3 readinC12jif 1 then I1 7 4160 end procedure CHAR begin I 160 2 160 to 145 writeCchr CI 22 end procedure HI begin ZiwememLH 4CJOos 1800 K 123 MOD 14 end procedure LO begin ZiztruncimemtHeC CJO x 1002 K 13 14 end procedure PLOT begin Bd118 CLS screeni t for 20 to 7 do begin for Limi to 2 do begin BdizBd 1 j
7. 2 Test Signal Channel Alignment Signal received on the hydrophone after channel adjustments to match the uaa received amplitudes are i 7 offi complete 1 The Levels are set to a calibrated power level by CE 1 ET I 1 pit ai at 7 first adjusting channel 16 TIER ean using a dummy load Fig 2 8 3 Test Signal 36 50 kHz Frequency Multi Burst Generated by ripple firing the complete data stack Each block contains a different frequency Note This photo was taken during a high power test The onset of cavitation is detectable in the negative part of the waveform 235 tion of a sector scanning transmission bearing coded by frequency is a con cept still to be fully investigated A possible approach to a receiver design based on the concept of transmitter scanning is discussed in chapter 7 24 A MULTI MODE SONAR TRANSMITTER Chapter 3 HARDWARE 3 0 Hardware introduction The Multi mode system comprises A microcomputer based signal generator which can simultaneously out put 16 analogue channels The analogue drive signals are fed to a set of linear power amplifiers 15 x 130 watts in the Mk 1 design and 16 x 1 Kilowatt in Mk II These amplified signals drive a multi channel array via an umbilical cable The Mk II array comprises 256 wide band elements ar ranged in 16 vertical staves of 16 to permit phase steering in azimuth The 16 elements in each stave are cabled in sub groups of 4 eleme
8. Loughborough England August 1989 Department of Electronic and Electrical Engineering Loughborough University of Technology C by Anthony David Goodson 1989 TETIS CO a RE Pago deu ass Loughborough University of Technology Library n oS OM Mn e Ciass 4 Ace No OSD TLL a eT WAS 1 7313 Acknowledgements I would like to express my thanks to the following First to Professor Roy Griffiths who in guiding the Sonar Research Group activity has provided an unfailing source of ideas Funding for this work was provided by the Ministry of Defence Procurement Executive through the Admiralty Research Establishment Ted dington Contract No DIERII9 4 2170 0104 To Dr J Cook of ARE for arranging access to the trials facilities on the Admiralty Acoustic Ranges and for his support during the discussion of ideas and the evaluation of the results Derek White and Tony Dicker of ARE for arranging the logistic support and instrumentation recordings of the sea trials The practical assistance from the ARE Technical Support Group at Lochgoil while conducting these was particularly appreciated The Severn Trent Water Authority for permission to use the South Der byshire reservoirs at Melbourne and Foremark Mike Large and the Area District Controllers Dai Davies Allan Bird and Douglas Ball The Water Bailiffs at Foremark Peter Harrison and Colin Lawrenso
9. could be produced in signal propagating in the water Cable lengths and mounting limitations precluded deep deployment of this array but at 7 metres depth at Foremark and at full power 2 kilowatts low frequency echoes could be clearly heard reflected from the shoreline at 600 metres range using a square wave modulated carrier transmission 3 6 Wide Band Transducer Elements The Mk 2 system was required to generated much higher powers for parametric signal experiments An investigation into suitable Low Q ele ments was undertaken with samples provided by the Institute for Oceanographic Sciences and by Universal Sonar Ltd The sample element from 1 0 5 was constructed from titanium with the piston head precisely trepanned from the solid mounting block leaving it supported by a thin diaphram fig 3 6 1 The PZT4 ceramic rings and the tail mass are as sembled to the piston using a rolled thread titanium tie bar The element is carefully preloaded by measuring the extension of the tiebar as the assembly 39 is tightened The resulting element performs extremely well the admittance plot of the sample yielding a very smooth low Q circle However 1 0 5 ad vice suggested that to achieve the maximum bandwidth performance neces sitated by our application the design should be restricted to a 1 2 lambda diameter piston It was also noted that the compliant diaphram support techni que absorbed a significant proportion of the inter element spac
10. ee E E 9 am 5 finalogue Supply for D R conver tere 7 FeR See SE x Sub Resembly PCB A D Coodaon MULTI MODE 12 6 89 SONAR TRANSMITTER 32k RAM BUFFER 2 x 4 blt Channels Yeralon 2 Bosed on Microcode Ltd PCB FTX 19249 adg Ea I X of XM z 2S 2 X 0 Q Qc C5 L a rc Ll 0 Lu a i Lit O a a C C5 a o a T zZ O e lu t MULT I MODE SONRR TRRNSMITTER PROJECT TITLE 1 KH MOSFET POWER AMPLIFIER 18 7 89 FTX 87279 adg A D Coodaon 606 7 FTX i B n t 7n n n Printed Circuit Mask for the piggyback board required to be mounted on each channel buffer memory The card provides direct access to the internal memory data bus and latches 8 bit samples on command for conversion via two 4 bit D A converters The analogue signals are low pass filtered and accessed via two Suhner minia ture coaxial connectors on the card front edge I vi 6 ep 4 ver 4 a as OO ee ease sie RENE oe HI SLEW DRIVER SONAR GROUP LUTEE aa o oro s t RAN eras Se Li mma VY XI rn Eo S ee VH I M est
11. gt egin case N of 8 bsel 1 1 bsels 2 21 bsel 124 3 bsel 1x8 41 0sel1714 St bselt 32 6 bsel 64 71 bsel i 28 end out B 01 DELAY 0ut A beel gt sout 8 86 DELAY creen 5 0 2 gt One Nti Selected gt d4screent25 6 end I viii fier ts TRANSFER Copies Data from base memory to selected page egin TXN s TXN 100 for N 0 to 7 do begin BLKs N 180 SELECT Card N out CB 6 2 Re enable bridge for 1 86 to FF do begin memf 6000 14 TXN3 s meml G000 I4 BLK2 endj writet N 1 Loaded 2 TXN end out 8 81 gt DELAY jyout A EFF outCB 0 end Procedure POSITION begin Hx2 emid FILE 3 1 3 hexno i2Hx2 HEXCON 3 TXN zh end procedure COPY begin screen 11 42 writeC File FILE Loading CharSi mid FIle 3 1 3 hexnor Char3 HEXCON TXN h ecreen i 6 writeln Block transfers from base memory 8000 writeln to output RAM pages from A00 offset 2 writelnt in increments of 180 bytes block 23 TRANSFER end 5 procedure READFAIL begin screent8 14 write DISC ACCESS FAILED NO FILE PAUSE CLS end procedure SELFREQ Select Mod frequency tabe REENTER4 begin REENTER4 screen 2 142 writeln write Enter Modulation frequency 8 2 3 4 KHz screen 35 0 s readt FF if ord FF gt 36 then goto REENTER4 if ord FF 31 then goto REENTER4 if ord FF lt 38 then FF
12. 1 Beam plotting arrangements at Staines a I M Fig 6 0 2 Mk 1 Array Beam Plot 200 degree scan 1 i E gt s3 308 i i 1 188 88 68 46 20 8 28 48 68 8t 106 Degrees Fig 6 0 3 Mk 1 Array Beam plot 39 kHz 40 degree sector ed8 308 i Beamwidth 0 H 0 i F 3 6 D 1 j eq 1 r 0 a 0 1 0 a 0 1 0 0 0 L 3 0 0 0 0 0 a H 0 0 28 16 12 8 4 8 4 8 12 16 28 Degrees 72 and a test signal transmission mode developed which ripples through each channel sequentially as shown by the signals in chapter 2 fig 2 8 1 2 Some difficulties were experienced as a result of the size of nearfield when attempting to plot the far field performance of the array Using a 15 element array of nominally one lambda pitch at 40 kHz implies that the far field is not fully formed until the measurement point is placed at least 9 metres from the array face ie if the start of the far field is taken to be approximately d A In freshwater using the Mk 1 array s actual inter element dimensions 15 elements each with a diameter of approximately pitched at 40 millimetre spacing gives an aperture of 0 6 metres The length of the near field therefore approximates to 0 6 x 0 6 0365 9 9 metres at 40 kHz The Mk 2 arr
13. 1988 A Synthetic Aperture Sonar LUTEE PhD Thesis p223 Jensen and Wirth 1975 Pascal User manual and Report Pub Springer Verlag N Y 2nd ed Marsh 1985 Pascldos Pub Kenilworth Computers Ltd TurboPascal 4 0 Pub Borland International 4585 Scotts Valley Drive Scotts Valley California U S A Hill 1986 An Automatic Beam Plotting System Proc Lo Acoust Salford v8 3 p207 212 Goodson et al 1989 Subaqueous Terrain Modelling An aid to Scanned Sonar Interpretation Symposium S U T Aberdeen Pub J Soc for Underwater Technology v15 1 p22 24 Gida 1988 Synthetic Aperture Sonar LUTEE PhD thesis 96 4 6 5 6 6 8 Chapter 7 1 0 Brown P PvDF Plate Hydrophones Data sheet and calibration chart Central Research Laboratory EMI Ltd Hayes Middx LUTEE internal reports various 1986 7 evaluation of Plessey SL560 561 1612 etc L S I Colour Sound Spectrograph supplied by Loughborough Sound Images Ltd Epinal Way Loughborough Leicestershire LE11 OQE Hemsley 1988 Examination of Polyurethane Coating from Underwater Cables Loughborough Consultants Ltd MP ARE 752 Consultants Report commissioned by ARE T 97 A MULTI MODE SONAR TRANSMITTER Appendix I 11 111 iv V vi vii Vili Appendix II i ii iii Appendix III 11 V xii APPENDICES Drawings and Circuit Diagrams Bridge Address and Data Buffers Port switched Bridge B Programmable R
14. 47 screen 1 6 j write Selected Modulation Frequency FF kHz end Procedure SEQUENCE begin Ki CLSsscreen 1 i6 write 43 KHz 1 38 degrees sector or B 6 degree sscreen 35 8 5 readin SF SELFREG REPEAT tf SF A then FRQa else FRGb Loadmc Bad FILE if Bad then READFAIL else COPY screen 1 25 juriteln FILE Loaded 23 Ki K 1 UNTIL K 15 SF A then GET3 else GET49 endi 111 ix Procedure DATAIN label REENTER2 begin write Load Data from Disc Y N or 23111 mem 2 screen 28 0 5 jreadintP CLS MODE IF then SEQUENCE Fill memory with Block Sequence begin IF Pae Y then begin REENTERZ1 begin writeln writetC Enter Filename 5 2 202 5 721 screenc28 0 readin FILE jwri tein FILE 9Cconcat mc FILE end 3rd char in filename TXN no LoadmcCBad FILE 3 Fois Blank as Bearing is unknown MODE1 FILE 4Display Filename just loaded if Bad then READFAIL e se COPY POSITION Hx31 5 yhexnotzHx 3jHEXCON c 121 6h j GMSB 1c TXN out B 04 5 i DELAY sOUTCA AMSB gt DELAY Reset MSB to new file gt writelnj write Load More Data from Disc a a a EYN Led IF Pz Y then goto REENTER2 end end end Procedure TXSEQUENCE begin write lt 7 Number of Transmissions 255 max f 1 screen 41 0 jreadln TXNS end Procedure LOADPARAM begin CLoa
15. NBbegin NBend SAMPLE X i BLOCK BLKS CHANNEL YBYTE Bd L H J K 2 ADDR CYCLES FREQ LAMBDA FI F D 0MEGA Focus PhaseSTR A 8 Y B Q File S Si Eof 8ad Pleasedel ay RE I Sectorcount Loop Function Function Function Function procedure CLS CODE 3E 0C 4F7 procedure PAUSE var Kiinteger tinteger 1 integer sinteger ireal trealy IARRAYCO 25Slof integer 1 tringL 11 1Stringt 8 Stringt 2551 iString 11 1Boolean Real tInteger ItoSCI Integer2 StringlS sExternal 0051 Stol S Stringl S 11ntegerjExternal 2054 7 RtoSCRiReal jL P Integer sStringl 583 External 0057 StoR S Stringl6 sReal jExternal 2656 begin REPEAT Krekeyboard UNTIL k lt gt jend procedure CLMEM begin for 1 26 to 888 do mem 4809941 7 end procedure DECHEX begin case NBbegln of 1 1Bi1 1 21 81m 2 j 3 8 3 4 43B1m 4 5 8 5 1 61B1m 71B19 7 1 B8 B im 8 91B15 9 181B1 46 11 B1 87 12 8 8 131B19 D 14 8 E 151B1 9 F end end procedure BYTE begin 8 ound OX 1 6 C SAMPLE YI SAMPLE 198 2 ADDR 8600 SAMPLE CHANNEL 2 1 28 BLOCK NBbegI n 2848 memLADDR1 YBYTE Gur i teln ADDR end YBYTE SAMPLED 2 IM ii procedure ELEMENT begin for SAMPLE to 256 do begin Xi1 roundC 7 sginCOMEGA SAMPLE Focus tPhaseSTR22 if odd C
16. RACK Version 1 SMALL PSU DUAL LINEAR AMPLIFIERS 1 2 KW Modules SONAR RESEARCH GROUP LOUGHBOROUGH UNIVERSITY 0509 263171 x 5071 uogi dur j3eAo pp F x 204401010 410182100781 eup T g 214 A prototype 2 x 1 kilowatt amplifier with integral power supply was con structed in the laboratory to assess the performance of these modules at sonar frequencies between 10 kHz and 100 kHz fig 3 3 1 This unit was a very useful testbed and revealed some deficiencies in the circuitry Some thermal drift in the quiescent operating point was detected and some high amplitude very low frequency instability appeared when the amplifiers were con figured to drive an impedance matching transformer as recommended by the manufacturer When tested with a dummy load the circuit modules were un able to achieve their rated maximum power at 40 kHz The 1 kilowatt out put available at 20 kHz fell to less than 800 watts at 40 kHz Alternative driver circuitry was supplied which just achieved the 1 kilowatt rating for the 100 millisec maximum pulse length specification fig 3 3 2 The low fre quency instability noted was eliminated in this experimental amplifier by ac coupling the bridge output connection with a physically large 22 microfarad high voltage paper capacitor Experiments demonstrated that the sustainable one kilowatt output was limited in duration primarily by the reactance of the mains input transformer when required to generate
17. can occur under trials conditions An examination of the characteristics of the newer complementary power MOSFET devices demonstrated that comparatively simple amplifier designs can offer high power and a wide band frequency response from a low component count circuit The basic specification for the anticipated sonar applications suggested that the linear power amplifier would be required to perform well at frequencies from a few hundred Hz up to 80 kHz Several promising cheap amplifier Hi Fi modules based on complementary MOS FET devices were available commercially and the investigation started using a 6O watt module purchased from a local Leicestershire manufacturer Con figuring a pair of these amplifier circuits in a directly coupled pushpull bridge mode generated satisfactory power levels in excess of 130 watts at 4OKHz f The high impedance and reactive load presented by the transducer was matched using a toroidal wound output transformer the output impedance of which was designed to appear resistive at the operating frequency The compact nature of this solution encouraged some modifications to improve the slew rate of these devices and a batch of upgraded Hi Slew units were obtained which were expected to achieve 15O watts at 5OkHz without stress 29 The units were manufactured as a low cost domestic product and required some re engineering for this sonar application Initially the amplifier pairs were found to run excess
18. context of parametric transmissions it has already been noted that at long ranges a very high source level can be achieved at 80 kHz generated as the non linear distortion of the signal enhances the second har monic of the transmission frequency The signal is sustained while within the length of the NLA virtual endfire array and this pump effect partially of fsets the normal absorption loss Further experiments are planned but the ef fect has obvious uses especially if rippled or swept across a sector which is synchronously scanned by a more conventional within pulse modulation scan receiver The multi mode transmissions permit both rapid interrogation of a sector or the selection of a specific single bearings Data for several types of transmission with differing characteristics can also be stacked for instant ac cess The flexibility gained by these techniques should be especially ad vantageous for target detection and classification applications Applications of the Multi Mode Transmitter to fisheries research need to be explored Sonar estimates of fish shoal bio mass are traditionally based on echo sounding sections made through a selected fish shoal using a single channel precision beam echosounder with precision signal processing to assess the target strengths of resolved individual fish This effect is par ticularly pronounced in the case of fish with swim bladders which rarely ap proximate spherical shapes and hence have sign
19. framestore The data was writ ten to disk and displayed simultaneously on the TV monitor as it was 69 received The disk images could be processed off line to generate colour amplitude coded hardcopy At the Lochgoil facility on board Maytime a Hewlett Packard Dynamic Analyser was available This equipment facilitates waveform cap ture and the analysis of spectral components for short data samples The 8 bit resolution system when set for a 0 100 kHz analysis band can capture and display a 4 millisecond window of data The images generated are dis played on the built in monitor and can be hardcopied directly to a plotter The last few trials depended on this machine for much of the data analysis made at the time and it will be even more important in the future NLA ex periments Portable recording equipment of adequate bandwith for 40 kHz has not been available although this is a desirable facility A Nagra IV recorder offering a maximum frequency response of 32 kHz at 38 cm s has been used to obtain examples of the non linear acoustic effects Recordings of the Multi mode sonar transmissions made by ARE T staff using an Ampex PR2200 at Lochgoil have been duplicated for replay on a older FR1300 machine at Loughborough These instrumentation re corders are both capable of recording the 40 kHz carrier and 80 kHz har monic responses generated by the multimode transmissions as the necessary wideband response is possible using the direct r
20. gross overload even one which can trip the supply breakers In pursuit of simple operator interaction some sacrifice in flexibility has resulted in the issued versions of the software The concept pursued in the later versions of the control software resulted in a set of dedicated disks each designed to carry out specific sonar operations Separate control disks are now used to select operations under the headings Test Signals Frequency Chirps Within Pulse Sweep Signals Focus Steer Steer Ripple Ping Sector The program STEERd incorporates most of the features typifying a control program This software is analysed in more detail in this chapter and its listing is included in Appendix III The data preparation programs have evolved as a family of similar structures and the program RIPPLE60 is included as an example of these Several supporting programs also written in Pascal relate to the modification of the prepared sets of file data e g to add amplitude modula tion for NLA experiments A utility program DISPLAYf was also written to 50 extract the first and last few samples from each of the 16 parallel data chan nels in each file and display them graphically for comparison as exampled in chapter 2 A variety of such software tools were written as required while developing and debugging the data files as were programs to model and graph plot the predicted array beam patterns 4 3 Program Example Data Syn
21. has worked well in practice supporting the hydrophones at the edge of the far field of the array The Civil Engineering Department at the University conducted a preliminary subsurface survey of Foremark reservoir in cooperation with the Sonar Research Group and the resulting data has made computerised modell Fig 5 3 3 Underwater Terrain Model of Foremark reservoir computed from a limited echosounder survey Fromi200 1950 47 to 1700 900 30 FOREMARK RESERVOIR Msd Water View from the dam wall looking towards the Spit 1987 Loughborough University Hydrographic Survey ing of the underwater terrain possible f1g 5 3 3 These model views should be useful when planning target deployment in future trials The modelling work was extended to a hydrographic data base supplied for Lochgoil and is proving to be a very useful aid when interpreting the long range reverberant echoes 65 5 4 Maytime Lochgoil This is an Admiralty Research Establishment facility constructed from an enclosed barge which is semi permanently moored in relatively sheltered water 60 metres deep fig 5 4 1 The site is equipped with hydraulic cranes to assist in array deployment and although several hundred metres from the shore it is connected to the mains with both single and three phase feeders A professional boat crew is normally available to assist in the deployment of hydrophones and targets as required and technicians of the Technical Support Group are ava
22. hydrophone con structions are also possible when the high power transmission characteristic is not required The arrays designed for this project utilised sandwich ele ments with centre frequencies near 40 kHz The initial experiments utilised an array made of 39 kHz Hi Q transducers These were standard echosounder elements with a Q of about 7 and a piston diameter of 38 millimeters The array consisted of 15 of these devices pitched 4O millimeters apart ie approximately lambda in seawater for 4O kHz The admittance circle plots measured from these demonstrated that they were not ideally matched Comparing the diameters of the admittance circles demonstrated a spread of 3045 but only two elements were particular ly badly matched and they were fortunately not positioned near the array centre The construction of this array followed a proven LUTEE technique The transducer elements pocketed in a rigid Urethane foam pressure release material within a nylatron outer case The front face was sealed after assemb ly with an Adiprene window fig 3 5 1 The electrical connections were passed through an internal bulkhead in the nylotron case to a tag strip junc tion with the cables This terminal block was sealed with a back plate and a flexible conduit moulded to it to protect the wiring from damage in the field Fig 3 5 1 Array Transducer Fig 3 5 2 15 element Mk 1 Array construction detail 38 The nylatron case in addition had
23. increment 8 2 degrees 008 1 0 0 H H 1 3dB FULL ARRAY 7 3d8 NEM HALF ARRAY 7 44 12 18 24 38 NS 12 18 4 38 Fig 3 8 7 Foremark Reservoir Lake Bed 400 metre sector 120 degree mechanical scan at 43 kHz Array tilted down 6 degrees 3 8 Array Testing A variety of modes have been tested successfully using the new array The array stave matching was measured in the water using the multiburst test signals fig 3 8 1 2 3 and the plotted beams formed by each half array and by the combined full array are well matched and very close to the simu lated patterns fig 3 8 4 5 6 Moving the underwater training gear in small angular steps under com puter control permitted mechanical sonar scans of the reservoir to be produced Using 43 kHz transmissions a sequence of images which include lake bed detail of the original valley floor fig 3 8 7 8 and the shore line out to 700 metres range have been plotted and displayed These results were ob tained using the 16th stave of the array as a matching hydrophone to feed a single channel Hybrid receiver A Compare with fig 5 3 3 The full size array was deployed at a depth of 13 metres in Foremark reservoir The array was driven with 10 5 kilowatts which was determined to 45 Fig 3 8 8 Foremark Reservoir Lake Bed 800 metre sector 120 degree mechanical scan Array tilt is zero be the maximum power that c
24. option of amplitude modulation at 0 2 3 4 5 or 6 kHz for steered NLA experiments The program selects and automatically transfers datafiles from disk into buffer memories for a 30 or 60 degree sector and with modulation as required 0 2 Unmodulated transmission The program defaults to an EPROM buffer set of test signals for system checks on in itialisation menu option is presented when the default settings are to be changed to enable the mode and timings to be selected i e Single bearing Steered transmissions or a Ripplefire of all these bearings across the sector in one contiguous pulse or Ping on each bearing sequentially stepping across the sector The menu then sets the transmitted pulse length the pause period between transmissions And then prompts to start transmitting A single keypress initiates the transmission sequence which continues until a second key is pressed The option of continuing transmitting modifying parameters or of loading a new data set from disk is displayed at this time Flow Diagram STEERd Source text listed in Appendix III Initial Command JU Calls boot sector on disk Loads and runs the Object code file mcSTEERd Operation Links to Pascal machine code disk routines Display 1st menu Choice Load data files Y N Defaults to system test signals stored in EROM for system checks Mode set to RIPPLE Block address set to CFFF EPROM Test routines Yes Display 2nd Menu Select steered
25. p 15 dB ref 1 V 80 kHz 35 mV p p 29 dB 4 kHz 3 2 mV p p 50 dB 8 kHz 0 45 mV p p 67 dB Corrected for 55 dB post filter amplification Filter insertion loss at 40 kHz 7 dB RMS conversion from pk pk 9 dB Combined effect 2 dB 40 kHz Signal 17 dB ref 1 Volt D140 Hydrophone sensitivity ARE P 90 dB ref 1V Pascal Pressure at hydrophone 73 dB ref 1 Pascal Spherical Spreading 0 250m 20 log 250 48 dB Cylindrical Spreading 250 375m 101og 375 250 2 1 8 dB Absorption loss 1 25 dB km 6 0 55 dB 40 kHz Total loss 48 1 8 0 5 50 dB Referred to 1 metre gives 123 dB ref 1 Pascal 40 kHz Source Level 243 dB ref 1 1 Absorption loss 80 kHz 3 7 dB km 1 5 dB 80 kHz Source Level 230 dB ref 1 Pascal At 4 kHz and at 8 kHz the absorption loss is assumed to be negligable 4 kHz Source level 208 dB ref 1 pPascal 8 kHz Source Level 191 dB ref 1 Pascal 82 6 6 Reference Target detection Foremark A standard 18dB target Fig 6 6 1 Midwater reference target sphere was deployed at 300 22222727 38 metres range in Foremark reser i voir at approximately mid water The water depth at this range was approximately 18 metres as can be seen from the echosounder chart in fig 6 6 1 The reservoir was artificially low at this time due to reconstruction work on the dam The Mk 2 array driven with 10 kilowatts of electrical power was configured to use only 15 of th
26. permits 500 microsecond insonification of each point target in the sector and at 40 kHz has been found satisfactory Shorter pulses are easi ly defined and a one millisecond sweep takes two blocks of the stack The block contents displayed in figure 2 3 2 demonstrate that the signals in ad jacent channels start 180 degrees out of phase i e the beam is steered from the sector edge and progresses until the channels are all in phase at the end of the first block at this point the beam will be formed on the centre axis and as it continues through the second block fig 2 3 3 the phase differences progressively increase steering the beam towards the other edge of the sector A hydrophone placed on the centre axis and insonified by this signal mode will detect the array beam pattern as its response fig 2 3 4 2 4 Focussing The distance from the array at which the beam pattern is fully formed ie its far field pattern becomes inconveniently large with increasing array size and higher frequencies Using a positive focus acoustic lens in front of the array can effectively reduce the length of the nearfield Examples of this technique are not commonly found in sonar systems although the con verse effect has been employed to synthesise a curved radiator from a flat array by defocussing the array with an acoustic lens However with a mul tichannel system similar effects can also be achieved by re phasing the data 16 Fig 2 4 I 9 Metr
27. pulses longer than 100 millisecs Using a variac to increase the supply rail voltages from 70 volts to 80 gave a significant power increase fig 3 3 3 and an undistorted 1100 watts could be obtained in a matched load for a full 100 millisecond transmis sion Alternatively short drive pulses of less than 10 milliseconds increased the available output power to 2 kilowatts or if driven by the maximum pulse length synthesisable 2 seconds a CW transmission of better than 900 watts could be sustained The detailed specification was then discussed with the manufacturer who was requested to quote for batch manufacture followed by an order for one unit A pre production unit supplied for evaluation was returned as problems affecting performance and long term reliability were identified with the driver stage of the circuit To solve these problems speedi ly this circuit assembly was removed and redesigned at Loughborough An improved drive circuit card was then made in the laboratory and when this was installed in the amplifier the unit performed very satisfactorily The new circuit Appendix I and PCB photo mask was then issued to the manufac turer for incorporation in the construction of the production amplifiers The performance advantages gained from this re designed drive circuit included the elimination of a dc feedback loop and the upper frequency response was increased to over 100 kHz The low frequency coupling capacitors were al 33 Fi
28. simulated the combined loading reactance of the transducer and its cabling A further problem arose when the transducer manufacturer was unfortunately unable to duplicate the reactive parameters measured in the prototype ele ments due to variations in his supply of piezo ceramic The transducer units finally supplied differed significantly with much increased capacitance from the approved samples Accepting these transducers created a new loading specification for the power amplifiers and the delivered amplifier units all showed evidence of distress when run for long periods with the new reactive load This design of amplifier prefers to see excess inductance rather than capacitance in a reactive load The output transformers were redesigned for these new parameters and unfortunately the original cores now proved inade quate Larger toroidal cores were purchased and the amplifier manufacturer employed to wind a replacement set of transformers Once these replace ments were fitted the amplifier system again exceeded the full power specification of 1 kilowatt channel for 100 millisecs when tested using a redesigned dummy load fig 3 3 4 As the transducer array was supplied as two identical matched half arrays each amplifier was also required to drive either a half or full transducer stave 8 or 16 elements This power splitting was achieved using a matched bi filar dual secondary winding on the trans former The calculated transformer match
29. te FNAME 4 Savemc Bad 8008 8866 6 FNAME screen 15 30 if Bad then write FNAME Failed else write FNAME Written ends 111 iii BEGIN MAIN PROGRAM s CLSjscreen 4 16 swrite 69 DEGREE SECTOR DATA v2 3 ADG 16 6 87 screen 4 1 jwriteln lt mummenm YR nis CLMEM Setting memL 8000 gt 8860 to 77 screen 8 3 wri te Required Cycles BlocK 7 REENTER1 1 screen 35 3 readCCYCLES jur i teln 1 FREQ r CYCLES 5E5 256 screen 10 5 juriteC Freq FREQ 108012 13 KHz C Bkz CYCLES1212 3 LAMBDA 1 1 590 FREOQ screencC 8 72 write lt Focal distance metres readin Fozif Fe then goto REENTERI D F LAMBDA OMEGA Pi xCYCLES 128 NB8begin 2x8 REPEAT NBbegin izNBbegin t 1 NBendim NBbegin for BLOCKi NBbegin to NBend do begin screent18 1125 writet Calculating Bearing No BLOCK12 writeln DIRECTION end BLKS NBend NBbegi nti DECHEX screen 1 1421 writeC Uriting File 21 s amp creen I 142 AUTOFILE until NBbegin 13 END III iv STEERG Program uses disc sides 8 amp 1 POF exible Transmitter 5 N LiA Control Program H Written by 1 A D Goodson Last Mod 7 Modes SCAN SECTOR RIPPLE STEER 2 Selects 36 or 15 degrees STEERED DATA gt Uses Unmodulated or NLA modulated datafiles 2 6 kHz GREE HEE I Sa NOTE Text Code Size exceeds 8090 data bo
30. tested before the remaining half was assembled The expected handling advantages of this split design have been realised This dual array can be split and either half used alone at lower power should the need arise fig 3 7 1 4 Fig 3 7 1 The Mk 2 Dual Array being deployed at Foremark Reservoir Each half comprises 16 staves of 8 elements i e 16 x 16 in total a ae as e Wo B 1 NE BERE sU i eb ME ES n 1 0 s 4 i gt J 1 y 1 y ot te 3 E E E A Te xd 1 OT CO v Se bs 8 p ae 7 l 1 T i 4 a x k Re 3 er a d 3 1 EXPE Fg LT S o TREATY V CADET mm 2 I ae atin 42 Fig 3 8 1 36 50 kHz Multiburst Test Signal EE Full array fr 42 kHz 1 7 volts pk pk ES Scales m 0 5 volts div 1 millisec div Fig 3 8 2 Multiburst Test Signal First half array 0 2 volts div 42 kHz 0 94 volts pk pk Fig 3 8 3 Multiburst Test Signal Second half array 0 2 volts div 42 kHz 0 78 volts pk pk Eel 1 43 Fig 3 8 4 Freshwater Beam Plot at 42 kHz Full array Linear Plot Beam Width 3 4 Fig 3 8 5 Freshwater Beam Plot at 42 kHz First half array Linear Plot Beam Width 5 Fig 3 8 6 Freshwater Beam Plot at 42 kHz Second half array Linear plot Beam Width 3 5 Scan width 68 degrees Sweep
31. transmitter assembly Transducer Array construction detail viii 3 5 2 3 6 1 3 6 2 3 7 1 3 8 1 3 8 2 3 8 3 3 8 4 3 8 5 3 8 6 3 8 7 3 8 8 5 1 1 5 2 1 5 3 1 3 3 2 5 3 3 5 4 1 5 4 2 3 4 3 6 0 1 6 0 2 6 0 3 6 0 4 6 0 5 6 1 1 6 1 2 6 1 3 6 1 4 6 1 5 6 1 6 6 2 1 6 2 2 6 2 3 15 element Mk 1 Array I O S Transducer element construction Universal Sonar Ltd Mk 2 Wideband transducer Mk 2 Dual array being deployed at Foremark reservoir Multiburst test signal Full 256 element array Multiburst test signal First 128 element half array Multiburst test signal Second 128 element half array Beam plot of the full array at 42 kHz Beam plot of the first half array Beam plot of the second half array Foremark Reservoir Lakebed Mechanical scan 6 degrees tilt Foremark Reservoir Lakebed 800 metre sector LUTEE Acoustic test tank Trials Facility at Staines Reservoir Hydrophone support deployed Foremark Reservoir and Draw Tower Foremark Outer working platform with array support etc Foremark Underwater Computed Terrain Model Maytime Acoustic Range Facility in Lochgoil Lochgoil Computed Terrain Model viewed from transducer position Lochgoil Modelled with 50 metres of water removed Beamplotting arrangements at Staines Mk 1 Array Beam Plot 200 degree scan Mk 1 Array Beam Plot 40 degree scan Beam Plot at 2 metres range Inside the near field Beam Plot at 2 metres range
32. 1 2 1 2 2 1 3 2 2 1 2 3 1 2 3 2 2 3 3 2 3 4 2 4 1 2 4 2 2 4 3 2 7 1 2 1 2 2 7 3 2 8 1 2 8 2 2 8 3 3 2 1 3 3 1 3 3 2 3 3 3 3 3 4 3 4 1 3 4 2 3 5 1 LIST OF FIGURES Beam Steering Diagram 15 element point source array 1 lambda pitch Predicted response of array based on actual dimensions Predicted response of array in phase steered increments Block diagram of RAM buffers and Host computer memory map RAM Buffer contents unsteered 0 RAM Buffer contents steered 2 RAM Buffer contents steered 4 Ripple fire timing skew across the sector Swept pulse across sector Buffer contents 1 millisecond sweep Ist part Buffer contents 1 millisecond sweep 2nd part Hydrophone response to a sweep signal Buffer contents 9 metre focus Buffer contents 2 metre focus Buffer contents 1 metre focus Three contiguous sweep transmissions 1 5 1 milisecs Hydrophone response on 0 axis Hydrophone response on 249 axis Test signal channel alignment misaligned Test signal channel alignment adjusted Test signal 36 50 kHz multiburst Mk 1 transmitter assembly The laboratory prototype 2 x 1 kilowatt Amplifier Prototype amplifier full power output 70 volt supply rails Prototype amplifier output with raised supply rails 80 volts Production Design Amplifier at full power Frequency response of the complete Mk 2 high power system including the power amplifiers and transducer The Mk 2
33. 2 An Introduction to Radar Systems Pub Mcgraw Hill 2nd Ed p280 282 Kock 1973 Radar Sonar and Holography an Introduction Pub Acad Press London Warnes 1982 The use of anti phased zones in an acoustic Fresnel Lens for a scanning sonar transmitter Ultrasonics 7 184 188 Hansen 1986 Linear Arrays p714 773 in The Handbook of Antenna Design Ed s Rudge Milne Olver and Knight Pub P Perigrinus for IERE Rusby 1970 Onset of Sound Wave Distortion and Cavitation in Water and Sea Water Pub J Sound Vib 13 3 257 267 Clay and Medwin 1977 The Cavitation limit p170 175 in Acoustic Oceanography Principles and Applications Pub Wiley Clay and Medwin 1977 Ihe virtual endfire array p162 170 in Acoustic Oceanography Principles and Applications Pub Wiley 94 8 6 9 6 10 7 11 7 Chapter 3 1 1 2 1 4 2 5 2 6 2 7 3 Westervelt 1963 Parametric Acoustic Array Pub J Acoust Soc Am Vol 35 535 537 Merklinger 1975 Improved Efficiency in the Parametric Transmitting Array Pub J Acoust Soc Am Vol 58 No 4 Ross 1973 Coded Signal Design for a Transmitter Scanned Sonar Pub Journ Sound amp Vib 29 2 p227 255 Hill Bahl Carey Smith et al 1987 An I amp Q Sonar Receiver Using Digital Signal Processing Techniques Int Rep LUTEE Nascom II Manual 1980 Pub Lucas Logic Ltd The 80 bus standard 1980 Pu
34. A MULTI MODE TRANSMITTER Chapter 7 CONCLUSIONS amp DISCUSSION 7 0 Objectives and Conclusions The study was initiated with the aim of demonstrating that microprocessor and digital logic techniques could increase the flexibility and effectiveness of a sonar transmitter In the simplest practical sonar transmit ter 1 6 the dedicated single frequency echosounder the economic advantages of introducing digital synthesis are marginal However for applications where greater waveform precision or frequency changes are envisaged the use of a microprocessor permits a single hardware circuit design to be tailored by software to suit changing applications The investigation of the phased array concept applied to a sonar trans mitter has lead to the successful development of reliable hardware and ap propriate software for signal synthesis and control The associated development of high power MOSFET transistor linear amplifiers suitable for sonar applications has resulted in a commercially avail able 1 kW linear amplifier which meets most of the forseeable requirements in the frequency band from a few hundred hertz to 100 kHz The transducer requirements for wideband high power operation in a staved phased array entailed investigation of suitable elements and resulted in a customised Tonpiltz 40kHz design The manufacturer regards this element as a commercially important development and now offers it for a variety of demanding high power wide band ap
35. An analogue receiver based on this concept is planned Exploiting non linear signals where a low frequency parametric product is developed in the water offers even greater bandwidth possibilities although the efficiency is low as is the achievable source level In addition to very wide band performance such NLA transmissions have the unique ad vantage of very high directivity without sidelobes This property of sharply defining the beam width of a low frequency signal has clear advantages in highly reverberant conditions If the low frequency products exhibit similar directivity to the pump primary can be steered and can be encoded easily then a low frequency long range transmitter scanning sonar may become a practical possibility In the absence of bearing encoded transmissions the reception of such low frequency signals will require a very large receiving aperture to discriminate target bearing angles Bearing coded transmissions at both primary carrier frequencies and at NLA secondary parametric fre quencies are therefore seen as the key to an integrated transmitter scanned sonar TIranscan concept It is hoped that the techniques described in this study indicate practi cal alternative approaches to some of the problems of sonar signal generation 92 A MULTI MODE SONAR TRANSMITTER REFERENCES Chapter 1 Lo Voglis and Cook 1966 Underwater applications of an advanced acoustic scanning system Ultrasonics 4 2 0 Duck
36. Focussed data Theoretical Beam Pattern of a 15 element 1 lambda pitch array Theoretical pattern with Dolph Chebychev shading 20 1 Mk 1 Array plot Unshaded 38 kHz Mk 1 Array plot Dolph Chebychev shaded Theoretical pattern with array phase errors added Theoretical pattern with phase errors and Dolph Chebychev shading Beam Plot Mk 2 Array 8 steered data log plot Beam Plot Mk 2 Array 15 steered data Beam Plot Mk 2 Array 28 steered data ix 6 3 1 6 3 2 6 3 3 6 3 4 6 4 1 6 4 2 6 6 1 6 6 2 6 6 3 6 6 4 Beam Plot Ripplefire Transmission Hydrophone response to a 4 x 1 ms Sweep Pattern Hydrophone response to a 7 ms Sweep pattern 7 ms Sweep Hydrophone placed 30 off axis NLA transmission spectrum Foremark NLA beamwidth Mid water 18 dB reference target Reference Target echo 0 degrees tilt Reference Target multiple echoes Reference Target Bottom reverberation 2 degrees tilt A MULTI MODE SONAR TRANSMITTER Chapter 1 INTRODUCTION 1 0 Sonar Transmitter Evolution Techniques for Sound Navigation and Ranging are usually referred to by the acronym SONAR a name originated by the American Navy during the Second World War In recent years the term has acquired a generalised usage referring to all forms of acoustic target location including passive detection systems and is occasionally used when referring to underwater communica tions In this study the unqualified term SONAR wil
37. Goodson and Griffiths 1984 Seavision A New Sector Scanning Sonar Proc IEE Colloquium London Underwater Navigation c3 p1 4 3 0 Skolnik 1962 Introduction to Radar Systems Pub Macgraw Hill 2nd Ed 8 278 342 40 Janes Weapons Systems 1987 Marine Radar UK USA etc Pub Janes Pub Inc N York 5 1 Goodson Griffiths Gida and Cook 1987 A High Power Flexible Sonar Transmitter Proc Conf IERE Edinburgh v72 101 112 6 2 Pratt 1978 High Power Transmitters For Sector Scanning Sonar Systems Proc Conf I o Acoust Lowestoft 7 2 Vyas 1980 High Definition Deep Sea Sonar Int Rep LUTEE IIT Delhi Phase II 8 2 Bi polar Sonar Transmitters of this family designed and built at Loughborough University were supplied to ARE T MAFF DAFS The Rijkswaterstaat in Holland and the Belgian Hydrographic Service Most of these transmitters are still part of operational equipment 9 5 Goodson Griffiths Wood and Cook 1986 A Flexible Sonar Transmitter Proc I o Acoust Salford v8 3 p197 206 93 10 3 ip Chapter 2 1 0 2 4 3 4 4 5 5 6 6 6 7 6 Cook Goodson and Griffiths 1988 A Low Frequency Sector Scanner 1 Using NLA in Underwater Data Processing Ed Chan Pub Kluwer p47 53 Cook Goodson and Griffiths 77 1989 A Sector Scanning Sonar Using Transmitter Scanning Proc Under Sea Defense Technology London p718 724 Skolnik 198
38. HANNEL then YC SAMPLE xX else BYTE end end procedure DIRECTION begin for CHANNEL 1 to 16 do begin screen 18 132 writet Calcula amp ting Channel No x CHANNEL 12 writeinj Focus Pi sqr CHANNEL 8 lt 0 PhaseSTR BLOCK amp CHANNEL 8 P i 73 Pi 77 4 deg ELEMENT CPi 14 2 deg end Pi 28 1 deg end procedure SET77 Originally used to avoid hardware hangup begin for Ti to 8 do mem G0900 1 4 1908 1 77 end 3 procedure PARAM begin Hi 80900 1 2 100 endj procedure CHAR begin Tim 168 23 160 to 1452 write chrCI22j endj procedure HI begin Zi ememLH CCJO 100 K 1J MOD 1 6 1 procedure LO begin ZietruncimemLH CJ 1000 4K 112 146 2 ends procedure PLOT begin Bd 0 CLS scre entd 146 4 for to 7 do begin for 1 to 2 do begin Bd Bdt1 screen 1 8 for K 1 to 28 do begin if odd L then LO else HI CHARj end screen 21 0 2 swrite lt Bd 2 s screent 24 4 1 2 Block Full block for Kir 187 to 127 dod 235 to 255 do begin if oddCL then LO else HIsCHARjend screen 1 Bd2j end endj end procedure DISPLAYm Used to display start and end phases begin CLS PARAM PLOT PAUSE ends procedure Savemc Uar BadiBooleanjS F Er Integer FiStringt 83 jExternal Caqe procedure AUTOFILE var FNistringlL lg Foistring 21 FNAMEtistringt 81 begin FNiEB TtoSCBD4 FoiuRtoSCF 2 02 FNAME concat mc FN S8m 9 wr i
39. Loughborough University Loughborough University Institutional Repository A multi mode sonar transmitter This item was submitted to Loughborough University s Institutional Repository by the an author Additional Information e A Masters Thesis Submitted in partial fulfilment of the requirements for the award of Master of Philosophy of Loughborough University Metadata Record https dspace lboro ac uk 2134 18581 Publisher Anthony David Goodson Rights This work is made available according to the conditions of the Creative Commons Attribution NonCommercial NoDerivatives 4 0 International CC BY NC ND 4 0 licence Full details of this licence are available at https creativecommons org licenses by nc nd 4 0 Please cite the published version LOUGHBOROUGH UNIVERSITY OF TECHNOLOGY LIBRARY AUTHOR FILING TITLE T eO A XA p ee ee m CA ee dem D wm GER ee e HH oe M m ee ee NP c m ACCESSION COPY NO EXE TEM O33123152 000 oa VOL NO CLASS MARK g 199 LoA coPy T5 DEC 1955 2 JUL 1993 2 22 MAR 1998 6 6 5 1499 9 NOVAG5 24 MAR 1999 26 APR 1999 00537270274 di iium MIN A MULTI MODE SONAR TRANSMITTER by Anthony David Goodson A Masters Thesis submitted in partial fulfilment of the requirements for the award of Master of Philosophy of the University of Technology
40. S S MS OS S4 S SN PO we We ama D hA Um n OSE unb UB ap P Procedure GET39 begin SF n A case Hx2 of B 3 Fqim zcm F 1 Fq1 14 E t Fq1 12 D Fase i C Fqim 8 B s Fqiu 6 A 1 Fqim 4 Fq 2 8 Fqiw e 7 Fqi 2 6 1 Fasm 4 5 Fqiz 4 s 8 3 Fqiz 10 2 Fa 12 1 Fq 14 end end xs SN BARS SN 0S NS NOM A we Wo we UA we app MO waa MP M SE mn Procedure ADDR label REENTER3 var HEXADDR STRINGI 42 begin writeln write Enter Start Address in Hex I 1 34 REENTER31 K 1701 amp crceen C40 0 readCHEXADDR Hx31 mi dCHEXADDR 1 121i f Hx3 gt F then Ksmtyif Hx3 lt 9 then K 1 Hx21 mi dC HEXADDR 2 193 Hx11 mi dCHEXADDR 3 1 Hx6 2mi dC HEXADDR 4 15 CAL Cmsb CALCI sb if K 1 then goto REENTERS out B 04 DELAY 6 68158 sDELAY Start Address MSB s out B 82 sDELAYsout A ALSB DEL Y Start Address LSB writeing uriteln writelnt Start Address of Transmission Data Hx3 Hx2 Hx1 Hx98 if SFm A then GET38 else GETSO if left MODE 2 ST then MODEtaconcatt STEER Fq ends II vii Procedure RIPSTEER begin Default settings1 Prescaler 500 KHz SAM pre 68 sets count down to 589 KHz bits 0 3 prescaler ra
41. The control software for plotting is menu driven and permits the selec tion of a sector angle to be panned through and the increment size between each sample point Additionally the zero position can be set after manually determining the best position The program starts by rapidly offsetting the array position to half the required sector angle and then with predetermined delays for sampling steps through the whole sector whilst recording 10 bit data samples at each step On completion of the scan the array is rapidly panned back to the centre zero position Simultaneous with the action of sam pling the data a screen plot graphing the amplitude received versus the an gular position is displayed This data can be subsequently labeled and stored onto a floppy disk and a hard copy plot can be generated on either a printer or plotter When redisplaying the data either linear or log plots options can be selected The control software has evolved in several generations The first ver sion written very quickly for the initial Staines reservoir trial contained a subtle bug which incrementally produced a zero offset after each plot The data files recording these plots are therefore difficult to relate to precise steered angles At the time additional accuracy checks were made using the mechanical azimuth scales on the array support and these gave good con fidence that the phase steering modes were consistant and correlatable Later versions of this soft
42. ade instantaneously by simply altering the read start address A further advantage gained from stacking sets of data files is created if the data access is continued throughout the length of the extended stack In this way a sequence of functions can be transmitted contiguously i e generating a Ripple fire mode The final arrangement utilised 28 8 buffer memory cards based on 32 Kb of static CMOS memory Two data channels 4 bit resolution are multiplexed into each card To recover the data a control card with latched pre programmed parameters clocks out the digital buffer contents into individual D A con verters which are mounted as small piggyback pcb s on each buffer memory card An alternative and larger 64 Kb x 8 bit resolution buffer has been designed and tested in anticipation of modes requiring higher resolution and longer unique pulse structures For most of the trials work so far 4 bit data signals have been employed successfully to define waveforms using a sam pling rate of 500 kHz Note Circuit diagrams are included in Appendix I 3 2 MOSFET Power Amplifiers Bi polar power amplifier designs for a multi channel system were rejected in favour of a power MOSFET approach primarily as a doubts were felt about achieving closely matched linearity at high power with simple cir cuits From past experience it was also realised that sophisticated protection circuitry was required to withstand all the forms of reactive mismatch that
43. angerous acoustic mismatch is created at the array face sig nificantly increasing the mechanical and thermal stress on the transducer As source levels increase and as the transducer approaches its cavitation limit signal distortions will occur Close to the transducer face the presence of asymmetrical distortion in the signal waveform will indicate the onset of cavitation For a parametric sonar a second non linear effect is normally ex ploited During the passage of the compression cycle of the signal waveform the density of the water is effectively increased and during the subsequent rarefaction decreases Since the velocity of sound depends primarily on the density of the medium different parts of the waveform will tend to propagate at different speeds If the pressure differential is large enough and sustained for sufficient range the sinusoidal transmission will degrade towards a saw tooth shape and if achieved will initiate a shock wave at some specific range t significantly greater ranges the more rapid attenuation of the high frequency components in the waveform starts to return the wave shape towards a sinusoid This non linear behaviour encourages the transfer of ener gy into the harmonics of the signal Low frequency pressure waves can be derived either by mixing two coaxially propagating beams of differing fre quency which beat together as they propagate in the water or from an amplitude modulated transmitted carrier The ap
44. ay also has the same aperture 16 staves pitched at 0 0375 also equals 0 6 metres In either case it is evident that the University test tank dimensions are too small to attempt to establish the array s farfield beamwidth However as noted in Chapter 2 4 the introduction of an acoustic lens between the array face and the measurement point can significantly alter the fresnel zone dimen sion Choosing a lens with suitable positive focus can effectively shorten distance at which the farfield pattern can be determined A simple emulation of this lens effect can be achieved by advancing the phase of the outer ele ments in the array to correct for the increased path lengths so that all the ele ment contribute in phase at the hydrophone distance The concept has been tested successfully with computed focus points at various ranges within the near field A very small focus correction 9 metres was applied to the sig nals measured at Foremark to ensure that the limited length of the pole sup ported the hydrophone in the farfield In the Test Tank focussed data for 2 metre and 1 metres range has been tested successfully Fig 6 0 4 demonstrates a beam plot made using unfocussed data at 2 metres range showing that the array directivity is seriously impaired within the nearfield of the array Repeating the experiment at 2 metres using a focussed data transmission fig 6 0 5 demonstrates that a well formed beam pattern now exists which matches the beam
45. b Gemini Microcomputers Ltd This is based on a SU 77 way edge connector and defines all the signals required for expansion of a Z8O microprocessor CPM is a trade mark of Digital Research Inc Control Program and Monitor is a disc based operating system designed to be implemented on 8080 8085 and Z8O micro processor systems It creates a machine independent system by routing its I O communications through reconfigurable lookup tables BLS PASCAL written by APS of Denmark initially released as Blue label software and later by Lucas Logic Ltd as NasPas Crimson Elektrik Leicester Ltd Now based at Stoke on Trent LUTEE internal report 1986 The 100 watt Crimson MOSFET Power Amplifier for the Flexible Transmitter Formerly Pantechnic now Tunewell Transformers Ltd 115a Myddleton Road Wood Green London N22 4NG 95 8 5 9 5 10 6 11 7 12 8 Chapter 4 11 2 1 3 1 Chapter 5 1 2 4 3 5 Decarpigny Debuss Hamonic 1987 Bossut Tierce Morrel Boucher and Tocquet Finite Element Analysis of Low Frequency Sonar Transducers Conf Proc Lo A Birmingham v9 2 p42 51 Wood et al 1986 Investigation of the prototype 15 element array Int Rep LUTEE Wood et al 1986 Wideband elements for the flexible transmitter Int Rep LUTEE Wood Wood Goodson amp Griffiths 1987 A High Power Transducer Array Conf Proc Lo Acoust Birmingham v9 2 p88 100 Gida
46. can synthesis waveforms at a sampling rate not exceeding 190 kHz using a stored data look up table If the synthesised signal is to be used in a tuned narrow band application then the number of samples per waveform cycle can be reduced towards the Nyquist limit and the maximum signal frequency could approach 60 kHz using 3 samples cycle If more than 256 samples are required to be output then additional timing factors enter the output loop and the maximum speed of transfer will be impaired However the output data rate achievable from the parallel port is acceptably fast for single channel operation and some early tests of D A converters fil ters and power amplifiers were made at 40 kHz in this way with signals syn thesised from approximately 5 samples per cycle As a machine code module can be incorporated as an in line or code procedure within a Pascal program or called as an external routine the full speed advantage of the as sembled code can be achieved while retaining the flexibility of this high level language The single channel in direct synthesis technique described offers some advantages over earlier hardwired transmitter designs and one hardware circuit design can be easily adjusted to operate over a very wide frequency range by simple software modification However the economic advantages may not be obvious if the system requires a computer terminal to communi cate to the operator This synthesis approach is less prac
47. character set created for this purpose to create a graphic display on the internal VDU without the neces sity for a graphic display processor card The program prompts for the number of the disk drive stonng the data and a filename The data is read into a buffer and demultiplexed for dis play as 16 horizontal waveforms The LSB s of the block are displayed on the left and the MSB s on the right of the screen As the data is normally out put by decrementing the buffer address the transmission starts at the right of this display DISPLAYf has been very useful aid to check the computed data as the phase relationships at the start and end of each block are very easy to compare on the screen See examples in chapter 2 59 A MULTI MODE SONAR TRANSMITTER Chapter 5 DATA ACQUISITION 5 0 Acoustic Measurement and Trials Range Facilities Practical sonar equipment calibration ideally requires anechoic condi tions for reverberation free measurements Acoustic interference from exter nal noise sources within the spectrum of interest should also be zero However such conditions are quite difficult to achieve in practice At the higher sonar frequencies small enclosed water tanks preferably with sound absorbent treatment are commonly used for measurement work In such tanks the reverberations from the water surface tank bottom and sides severe ly restrict the transmitted pulse lengths that can be used For lower frequency sonar testing there is li
48. circulating small fixed blocks of memory containing in teger numbers of carrier waveform cycles This compromise using a block length of 256 samples clocked at 500 kHz has been found a very effective technique For normal sonar operations the required types of emission are always predetermined and can be calculated in advance To alter the type of transmis sion requires the contents the output memory buffers to be replaced by alter native pre computed datafiles called from disk As data transfer operations re quire serial access to each buffer in turn there is time penalty enforced when changing the transmission mode of about 50 seconds when using modern 5 25 80 track floppy disk drives The use of a RAM disk has been inves tigated as this offers the fastest method of changing mode however although this is a practical solution most of the available boards are volatile and re quire re loading at power on Winchester technology would seem to be a preferrable solution for faster data access This has not been implemented as the availability and cost of the Lucas hard disk options has been a deter rent so far Double Density 5 25 80 track floppy disk drives have been used for data storage in this equipment and the inconvenient mode changing delays minimised by creating enlarged data buffers to contain a number of different function data files stacked ready for immediate use Access to the different functions stored in the stack can be m
49. coustic Test Tank The test tank at Loughborough is an unlined concrete construction ap proximately 9 x 6 x 2 metres in depth Heavy duty rails assists the accurate support of equipment and an overhead gantry crane aids deployment and recovery fig 5 1 1 Acoustically this tank is highly reverberant and as the maximum water depth is less than 2 metres the first multipath echoes arrive at a hydrophone with very short delays over the direct signal At 40 kHz the achievable separation of 1 2 milliseconds makes it just possible to gate sig nals for beam width measurement The second problem at 40 kHz relates to the nearfield of the array Taking the maximum length of the nearfield zone to be defined by A we have A 15 x 0 04 A 0365 for 40 kHz in fresh water ie 9 8 metres The dimensions of this test tank are therefore very restrictive for large arrays However the ability of the multi mode transmitter to focus the transmission by adding an appropriate progressive phase ad vance to the outer elements of the array can reduce the Fresnel zone ef fects Successful beam plots have been obtained within the tank using this technique to establish the far field beamwidth at short range Fig 5 1 1 LUTEE Acoustic Test Tank x A i 61 5 2 King George VI Reservoir Trials The first open water tests of the multi mode transmitter equipment took place on the King George VI reservoir at Staines The facilities avail able at this sit
50. creen 19 0 2 writelnt Dat amp Start Address Hx3 Hx2 Hx1 Hx0 screen 10 0 If pre 175 then DIR Up else DIRs Down 3 screen 10 0 5 writeintreps Modulation Blocks Searing screen 10 82 writein Pause Length pause 5 millisecs gt screenc19 0 writelnt Pulse Length pulse1d11 millisecs 2 screen 19 08 writelnt Operational Mode MODE end Procedure DEFAULT begin out B 015 s DELAY outCA FF DELAY out B 2 DELAY out A ALSB DELAY out B 045 DELAY out A AMSB DELAY out B 98 DELAY out CA pre DELAY out B 28 DELAY out A reps DELAY out B 40 DELAY cutcA ped DEL6Y out B AG gt DELAY out A TXNS DELAY 0009 0001 Call RAM cards selected 8008 89810 LSB address set 0000 90190 MSB address set 0000 10890 U D Prescate 0010 0889 BlocK repeats 9100 00008 Pause Length 10180 1 No 1Tx Pulses PRESET end III vi Procedure GETS begin 6 Default loads 50 Deg Sector case Hx2 of NOTE angies set by data Faot 5 0 8 Fos E Farm 4 O Fqi 260 C s Farm 16 B Fqiw 12 Als Fara 8 Fasm 4 9 13 Fas 8 1 Farm 4 7 1 Fqim 8 6 Fq1x 12 3 1 Farm t 74 Far 20 3 Fas 24 1 2 Fq1 28 1 end end a W
51. d steers smoothly across the sector during the pulse fig 2 3 1 The full energy from the defined narrow beam isonifies every part of the sector in turn This mode mirrors the function of a sector scanning receiver although there are some important differences to be recognised A scanning recelver array receives target echoes generated from a pulsed single frequency narrow fractional bandwidth transmission and can 13 Fig 2 2 1 Ripple Fire Mode Contiguous Pulses propagate across the sector with an incremental delay determined by the pulse lengths RIPPLE FIRE TRANSMISSION Fig 2 3 1 Sweep Sector Transmission a within pulse scan of sector SWEPT PULSE 14 Fig 2 3 2 One millisecond sweep First half millisecond data block Signals start 180 out of phase on the right at buffer address 5 157 and progress to become in phase by the end of the block A100 i e The steered beam is now pointing on axis Fig 2 3 3 One millisecond Sweep Second half millisecond data block The 16 data channels continue the progressive phase change until the resulting beam Is steered to the far edge of sector Buffer AO0FF A000 Note Each channel transmits a different frequency Fig 2 3 4 Hydrophone Response A hydrophone deployed on axis and in the far field responds to the beam pattern of the array when swept by the electronically Steered transmission hb
52. data and recording the hydrophone output Sets of disk files containing steered data in 4 degree steps were prepared off line Each file was then used to generate a sonar transmission aimed at the specified angle and its beam pattern plotted Log Plots of steered transmissions using the data files for 8 16 and 28 off axis are included in figures 6 2 1 2 3 demonstrate the effect Further data files were computed for 2 degree and for 1 degree steered increments i e steering within 30 degree or 15 degree sectors Later versions of the control programs prompt for a sector angle and automatically load the selected steered data sets Fig 6 2 1 Mk 2 array 43 kHz Transmission Log Scale Beam Plot Steered Data g phase shift Fig 6 2 2 Mk array Log Scale Beam Plot Steered Data 169 off axis Fig 6 2 3 Mk 2 array Log Scale Beam Plot Steered Data 28 off axis 4 i i i r i i i i i i i f i 1 i 1 1 i J t Log i i i Plot A 1 i 1 i 58 46 38 i6 31 020 34 498 56 Degrees d s i i i i i 1 i i i i 3848 P Leg H i i i Plot 1 i i 56 46 30 26 16 8 18 28 38 48 8 Degrees dB 3808 gt t Log i i Plot i d 6848 56 48 38 24 16 a 16 28 38 48 58 Degrees 77 6 3 Sector Scanning Ripplefire and Sweep Modes Two methods of scanning a sector have been developed The first referred to here as Ripplefire utili
53. dme Bad PARAM 70 parameter file from disc and display end Procedure ChPOSN begin if teft MODE 2 SE then Psn 43 if left MODE 2 RI then Psniz39 if left lt MODE 2 ST then Pen 41 end Procedure NEWPARAM label REENTERS begin CLS TITLE screen 0 4 ChPOSN wri tet Ripple Steered Ping Sector R S P1 screen Psn 8 read P ywritelnsRIPSTEER wr i telnj write Select Start Address CY NI 3 screen 43 0 readln P IF Pz Y then ADDR ELSE begin out B 02 j DELAY outCA 6LSB DELAY out B 94 DELAY out A 6MSB jDELAY jend wr i teln writet Select Modulation Blocks Pulse Y N1 5 screen 43 0 readin P IF P Y then PASS ELSE begin outCB 20 DELAY outtA reps DELAY endjwriteln 111 x writed Select Pulse Repeat Rate a LY NI7 screen 43 9 f readin P gt IF Pm Y then PLENGTH ELSE begin 8 40 DELAY out A pc DELAY jend uri te1n C writetc Select Number of Transmissions gt tY N1 screen 43 0 jreadInCP jur i te1n IF Pe Y then TXSEQUENCE ELSE begin out B A0 DELAY ou tCA TXNS DELAY jend wr i teln REENTER t PRESET end Procedure Dosfetch Procedure DfcallsExternal F66 Begin init mem F 6 to 0D 0 08 1 4 21 12 4F 4622 21 0 CD 90 209 198 806 8 6 C7 4 50 441 53 43 4C 444 4F 53 Df cal end
54. e 16 channels 4 The 16th stave of elements was 7 used as a matched hydrophone 1 and the array mechanically panned onto the target The received echoes were preamplified and envelope detected using an operational amplifier precision detector circuit and then low pass filtered before being displayed on the oscilloscope Fig 6 6 2 is a polaroid of the display from a single 5 millisecond 43 kHz transmission made with the array horizontal the ripple in the pulse envelope suggesting the arrival of a multipath signal Fig 6 6 4 results from a multiple transmis sion which confirms a variable multipath component presumed to be via the surface which is modifying the envelope after the initial edge Fig 6 6 5 is a single transmission taken with the array tilted down 2 degrees The reverbera tion from the lake bed has increased significantly The vertical beam width of the full Mk 2 array is better than 4 degrees so at 300 metres range in 18 metres of water with the array untilted the beamwidth just ensonifies the full water column i e cylindrical spreading is assumed from about 260 metres 83 Fig 6 6 2 Reference Target 18 dB Sphere Deployed at 300 metres Tilt O degrees single 5 ms transmission Array horizontal Fig 6 6 3 18 dB Reference target at 300 metres Multiple transmission Array horizontal Fig 6 6 4 18 dB reference target at 300 metres Single 5 ms transmission Array tilted down 2 degrees 84
55. e Focussed Data Data buffer display of contents unsteered with a small focussing correction applied progressively towards the ends of the array Fig 2 4 2 2 Metre Focussed Data Data buffer display of contents with a 2 metre focus This focussing effect reduces the length of the near field and aids plotting of the far field directivity Useful when working at short ranges in a test tank environment Fig 2 4 3 1 Metre Focussed Data Data buffer display of contents with data to focus at one metre Note This technique can very significantly increase the signal intensity at the focus which can aid investigation of cavitation and non linear effects 1 i i r i DU AUC y l y j W i if 1 u J 0 1 2 3 4 5 6 7 8 9 AP 020 NI 1 t 1 17 signals to form a focus at any required range in the near field The reduction of the near field allows accurate plotting of farfield beam patterns within the confinement of a test tank and as the intensity at the focus is increased cavitation and non linear effects can be investigated without overstressing the array transducers Comparative beam patterns plotted at 2 metres range i e well inside the conventional nearfield are shown in chapter 6 The phase ad vances required to focus the array are
56. e LUTEE system devised for precision beam plotting utilises a remotely controlled stepper motor driven waterproof training gear with azimuth and elevation axis Two versions of which have been acquired from the manufacturer Molynx Ltd The first type is constructed using a cast aluminium waterproof housing and this is used in both fresh and salt water Some corrosion problems occurred agravated by surface damage which necessitated overhaul but the units have functioned reliably for some con siderable some time The more powerful unit commissioned primarily for this project is shown in chapter 3 Fig 3 7 1 supporting the Mk 2 array during deployment at Foremark This second type is constructed from stainless steel and has a rated operating depth in excess of 500 metres Functionally the un derwater units differ only in their final gear box drive ratio An internal view of the housing and gearbox is shown below r Fit e 0 a m TB 11 1 The devices can be controlled manually by joystick or by axis push buttons or the controls can be interfaced to a microcomputer for programmed motions In practice while beam plotting only the horizontal panning axis is driven by the computer The synchronous acquisition of the signals received by the remote hydrophone has to be arranged by additional sampling electronics controlled by the same program and taking its delayed timing from the transmitter trigger pulse 2 Beamplotting Software
57. e essential if multipath signals are to be ignored during the plot The analogue sample triggered by the range gate delay logic feeds a peak detector and this outputs a a latched DC level between samples derived from the signal amplitude The output of this peak detector is fed to a A D converter within the microcomputer BBC plus One significant modification is needed in the BBC microcomputer to take full advantage of this system this involves replacing the internal A D converter s reference voltage with one of greater precision The simple reference used as standard is vulnerable to power rail noise and it is not nor mally possible to achieve the full 10 bit accuracy without a circuit change The modification involves a pre regulator applied to the 12 volt rail to generate a quiet 5 volt source This is then used with a bandgap precision reference to supply the A D converter with an effectively noise free voltage for comparison The result works well and the full 1O bit resolution can be achieved JI ill APPENDIX HI Program Listings IIT ii Pascal Listing RIPPLE60 III v Pascal Listing STEERd III xii Pascal Listing DISPLAYf 111 i Program RIPPLES Computes data for 60 degree sector ripple Writes set of unmodulated files mcXSOm a Last update ADG 15 6 87 Calculates focussed and steered data one bearing at a time Assumes Hardware Modification to clamp O P SET77 removed label REENTER1j var
58. e included a raft with mains electricity which is permanently moored some 200 metres from the side Heavy equipment requires the use of a crane to facilitate transfer from the delivery vehicle to the boat moored against the steep sloping reservoir wall The raft is fitted with a small jib with block and tackle to assist unloading The water depth is normally main tained at 14 metres and the available range depending on the chosen direc tion can extend to over 1000 metres Array testing is facilitated by a central ly mounted electric hoist from which the transducer and training gear can be lowered into the water The raft s middle floor boards are lifted during deployment and recovery and refitted around the support while working During deployment the hoist wire is fitted with aluminium slotted tubes which interlock When the hoist is tensioned these sections tighten into a rigid torsion tube between the array and the rotating head gear fig 5 2 1 In the initial experiments hydrophones were deployed some 30 metres from the raft from a convenient buoyed mooring However it proved dif ficult to maintain a sonar beam pointing accurately at the hydrophone as any wind caused the raft to execute slow angular oscillations about its position For later trials this problem was largely overcome by the use of a 10 metre carbon fibre pole as a horizontal hydrophone support This technique ensured that any angular shift in the raft position was tracked by t
59. e is defined by the difference frequency be tween adjoining elements Fast sweeps imply large difference frequencies and hence the element bandwidth rapidly becomes a limiting factor with increas 78 ing array size For a lambda pitch array a difference frequency of 1 kHz is needed between each transducer to sweep the beam across the sector in 1 mil lisecond A 15 element array requires elements with bandwidths adequate to accomodate frequencies 7 kHz about the centre frequency This 15 kHz bandwidth centred on 40 kHz necesitates wideband transducers with a Q in the order of three The Mk 1 array constructed from narrow band ele ments generates rather distorted beam patterns when driven at sweep rates faster than 2 5 millisecs The Mk 2 staved array which utilised wideband ele ments has been shown to produce acceptable beam patterns with a variety of sweep rates down to 1 millisecond fig 6 3 2 3 4 Fig 6 3 2 Mk 2 Sweep Pattern Hydrophone response to a multiple 4x 1 ms Sweep transmission 0 5 ms div Fig 6 3 3 Mk 2 Array Sweep Hydrophone response to a 7 millisecond sweep 1 ms div Fig 6 3 4 Mk 2 Array Sweep Hydrophone response to a 7 millisecond sweep Hydrophone placed at 30 degrees away from the centre axis to show the equal ambiguity lobes 1 ms div 79 These sector scanning modes were investigated using the Mk 2 array which was used to generate scans of a 60 degree sector at the 40 kHz pri
60. e signal waveforms at high source levels in the water which can be steered as required to interrogate a target It is an ticipated that this will prove to be a useful tool for studies of acoustic propagation Used for target evaluation the flexible modes of operation 86 should assist classification studies The equipment has not been configured for mobile operation and clearly to operate at full power without cavitation implies that the array will be deployed at some reasonable depth If the sys tem is required to be hull mounted on a surface vessel the maximum power will be restricted as approximately 20 metres of static water pressure is needed to avoid cavitation effects at full power 7 2 General Discussion The multi mode transmitter project has demonstrated the feasibility of applying micro processor controlled signal synthesis to a phased array sonar The component parts of the system are reproduceable and two complete sys tems the 2kW Mk 1 and the 16kW Mk 2 were constructed The system design has been influenced by the investigative requirement and the result is primarily the intended flexible research tool However many of the in dividual modes could now be exploited in isolation using simpler control cir cuitry and with signal synthesis data fixed in PROM memory The multi mode transmitter is a complete self contained transmission system and the support software permits signal synthesis modifications to be made in a trials envir
61. each channel together with an arrangment to repeat this data as necessary to estab lish the required pulse length In practice oversampling the waveform will Fig 2 0 5 Block Diagram of the Auxiliary Memory Buffers and the Host Microcomputer Memory Map 8 Auxiliary Ram Buffers 32 Kilobytes Each Page Addressed 8000 FFFF OF RAM FFFF TOP ROM 8 Kilobyte T P A RAM TEST Overwritten during SIGNALS buffer loading C000 T P A RAM Overwritten During Auxiliary Buffer Loading 16 ANALOGUE OUTPUTS TO DRIVE POWER AMPLIFIERS FOR THE STAVED ARRAY vs 1000 MONITO 0C80 VIDE R STACK RAM quss EUN RP ae ae eus 10 reduce the filtering requirements and at 40 KHz it has proved convenient to sample at 500 KHz A block of data containing 256 samples 100 Hexadecimal has proved easy to manipulate and can contain several com plete cycles of the carrier data To establish a transmit pulse of given dura tion this block of data needs to be recirculated which implies that the data pulse length will be incremented block length steps 0 512 millisecs incre ments The sample block must contain an integer number of carrier cycles if a repeating phase shift glitch in the transmission is not to occur at the block boundary As the length of the data block in circulation can be ad justed to any number of samples 1 255 the block technique does not restrict precise freq
62. ead Address Generator 32 k x 4 bit Channel Buffer Memory and D A converters 1 kW linear MOSFET amplifier including Hi Slew Driver PCB mask for Channel Buffer D A piggyback card PCB mask for improved drive card for 1 kW amplifier Pre release drawing of 256 element Mk 2 Array LUTEE Beam Plotter Computer controlled beam plotter hardware Beamplotting Software Sampling Electronics Software Control Program Listing Pascal listing of program RIPPLE60 Pascal listing of program STEERd Pascal listing of program DISPLAYf 88 8 2 8 T 0 Ld 4 9 WEB Im s Uk G 0 i CN CON RIBBON CASLE LINK TO BRIDGE A LUTEE SONAR amp SIGNAL PROCESSING RESEARCH GROUP DRN BY R D Coodaon PROJECT HULTI MODE em 20 TITLE SONAR TRANSMITTER REF NO FTX 18849RDG 8 BUS EXTN CONTROL BRIOGE A n fpi ih E loin 7 IRE AW S A HE T it Hii HL 0 debo up can Dow a IT 8 5131111 NT a TSR ae ns BRI M fum T ae AI Ita Protect hommel 1 NASSAAAN a a GE MANA 8 eee ee Bowe SEES SES SEE 3 22 558 7 rgz322322232522222752 p Analogue o p f A Cord Enob ied EI ma 111111 inh ERE EFT ul HL E EH 2 Adjustment NE EN Es
63. ecord modes However most of the data recordings concentrated on the low frequency NLA transmissions using the FM record mode to obtain the best signal to noise ratio for later analysis The acquisition of a very high resolution speech spectrograph has proved an invaluable tool for the analysis of the recorded NLA signals The equipment is extremely flexible and produces colour coded hard copy The spectrograph can analyse signals into 25 Hz bands if required however the in ternal anti aliassing filters are set for an upper limit of 8 kHz Much higher frequency data than this can only be examined by using tape speed transposi tion techniques but the spectrograph is of most use at the lower frequencies 70 A MULTI MODE TRANSMITTER Chapter 6 TRIALS RESULTS 6 0 Initial System Tests Beamforming and Focussing The Mk 1 low power system comprising a 15 element line array 130 watt channel amplifiers and the multi mode signal synthesiser was taken to Staines reservoir for the initial open water tests The first beam plots made with the LUT computer controlled pan and tilt demonstrated that the system functioned acceptably close to theory The unsteered main lobe approximated 4 degrees in beamwidth the sidelobes were a little irregular but the beam pattern with endfire lobe positions ap peared as predicted by the inter element spacing and operating frequency i e at 75 off axis rather than 90 Chap 2 These first beam plots suff
64. er from some underwater acoustic interference believed to be sonar transmissions originating from an adjoining raft The hydrophone used for these plots was hung from the end of a 10 metre long supporting pole deployed from the side of the raft moored on Staines reservoir The 15 element array was supported from the centre of the raft and the distance between the array and the hydrophone was 15 metres Both the transducer and the hydrophone were deployed midwater at 7 metres depth fig 6 0 1 The hydrophone response plotted while the array was mechanically scanned through an angle of 200 degrees is shown in fig 6 0 2 The transmitted data in this example is the same in all channels and therefore unsteered The endfire response is acceptably low and can be compared with the computer model of this array shown in chapter 2 fig 2 0 2 Individual channel power adjustment was achieved by preselecting resistors values to generate equal amplitude responses from each stave at the hydrophone This method of adjustment proved to be an unwieldy time consuming operation Optimum channel matching was not achieved for the first trials as is evidenced by the asymmetry visible in the first order sidelobe responses fig 6 0 3 The beam width of the main lobe was measured several times and averaged close to the expected 4 degrees Subsequently the channel match ing adjustments were simplified by the addition of a plug in attenuator card 71 Fig 6 0
65. er with the drive computer and this transportable assembly is usually referred to as the Mk 1 Transmitter fig 3 2 1 The equipment was transported to various reservoir trials sites and used in this configuration for all the initial low power experiments The maximum power available from this system some 2 kilowatts was able to generate significant non linearity in the water using the 15 element prototype array and when the transmissions were modulated some parametric products were detectable The success of this prototype lead to the Mk 2 specification requiring 1 kilowatt channels to produce an en gineered 16 kilowatt system to assist in parametric performance investiga tions 3 3 High Power Linear Amplifiers Alternative MOSFET power amplifier modules with a claimed 1 kilowatt RMS rating were purchased from small North London company 30 Fig 3 2 1 The Mk 1 Transmitter Assembly ee N 8 E a 5 an ne x oo ses oe esee oeousssosescusa pe X zw er ro G amp G we ee the seo ees ons cesavese sd 3T E 8 TAS mga dv C a A UNIT JEE vmm mite 31 43 MAINS I P EP t Ei 18888 uF 4108 V 1 16880 uF 4108 V B wu YY S YNI N O Y OW EET 9 C 392 l A a PROTOTYPE
66. esa LJ ae JULI e e vt al 0 0 bd a by tetera ee bret ODOOCDOOOOCON 0 0 nu E ON E ROC P LOO DRIVER o ee a e e enero ete e s LJ LI 9 9 9 4 9 9 9 eee 2 4 gt m 9 9 LLLA LLLA ve E AJ L 5 5 5 SLEW PR ADG S 85v 3 in the 1 kW MOSFET amplifier Each amplifiers uses two of these mounted above the MOSFET block on socket pins The drive cards are iden buffer The track link between pin 2 and 3 needs to be broken to insert an tical with one configured for high gain i p and other as a low gain inverter optional external gain control potentiometer Printed Circuit Mask for the improved High Slew version of the drive circuit used NA 1 SONAR RESEARCH GROUP LOUGHBOROUGH UNIVERSITY 506 ARRAY MOUNT CEXISTING 172 TEE SLOTS on 254 centres 00000 99 EETA EE ATE mE A r O 1 ami EE Ee PE Aii d 1888 1588 S IIIT XLI LLL Z SAS xl RUNE g p ASA LL JEEP aur DUAL 16 x 8 ARRAY PRERELEASE DRAWING FLEXIBLE SONAR PFE NO ADG84 1 1 TRANSMITTER DATE 19 1 85 APPENDIX II LUTEE Beam Plotter 1 Pan and Tilt Hardware 2 Control Software f ii 3 Sampling Electronics 111 1 The LUT beam plotting hardware Th
67. etc The transmitter logic was developed around a relatively cheap Z80 based microcomputer which was available for commercial OEM applications from Lucas Logic Ltd This particular microcomputer whilst a relatively old design offers a number of features that are not usually available together in more modern alternatives The standard facilities on the main board include Parallel PIO and serial UART communications a video display drive and full access to the Control Address and Data buses The microprocessor utilises a 4 Megahertz clock Expansion of the system is simplified by a standard 8O bus backplane which is supported by a number of manufac turers From a construction point of view the 8O Bus is compatible with the common SU 203 mm square development cards which can to carry large amounts of circuitry The standard operating system is well supported with software languages which include a graphics extended BASIC 280 As semblers and a compact implementation of Pascal The Pascal language had already been exploited in previous projects and found to compile efficient and fast Z8O code which when linked with a runtime package produces stand alone executeable EXE or COM type files CPM 2 2 and CPMplus are also available as alternative disk operating systems The original Lucas Nascom monitor Nas Sys3 and the associated Nas Dos2 1 disk operating system has been used as the speed of the disk access routines was cons
68. fast multiplication data manipulation and transfer e g TMS320c25 or MC56000 Electrically Programmable Read Only Memory Fast Fourier Transform Low power Schottky Logic e g 74LS00 High Speed CMOS logic e g 74HCT244 Loughborough University of Technology Electronic amp Electrical Engineering Department at LUT Metal Oxide Silicon Field Effect Transistor Non Linear Acoustics Parametric Sonar Original Equipment Manufacturer Pascal ISO unit of pressure micro Pascal vi PIO Parallel In Out Microprocessor peripheral communication port Dual 8 bit parallel data Q Quality factor of a resonant network Usually expressed as the ratio of Centre Frequency to Bandwidth ROV Remotely Operated Vehicle Small submersible vehicle carrying instrumentation TV cameras etc used for inspection work especially where divers would be at risk Usually controlled from the surface by an umbilical cable RISC Reduced Instruction Set Computer RAM Random Access Memory ROM Read Only Memory TTL Transistor Transistor Logic Family UART Universal Asynchronous Receiver Transmitter Serial data communication port device Greek symbol useage lambda one wavelength mu micro 10 Pi 3 141592654 or computed from 4 Atn 1 Theta Angle of propagating wavefront to array face 2nf Angular frequency lt sp TAF Psi Inter element phase shift in array vii 2 0 1 2 0 2 2 0 3 2 0 4 2 0 5 2 1
69. fficiency but frequently generate coarsely quantised waveforms which necessitate narrow band filtering in the final coupling stage if unwanted modes of vibration are not to be excited within the transducer Linear power amplification was selected for this project primarily to maintain the system bandwidth as the possible use of the system with widely different frequency transducer arrays was considered likely Advantages were also forseen for a system which could exploit wideband transmissions with defined amplitude characteristics These ideas required initial investigations into power amplification techniques using MOSFETS and led to the commis sioning of some useful Kilowatt rated sonar amplifier modules 3 1 2 Signal Waveform Synthesis During the late 1970 s investigations into suitable bi polar designs for high power sonar transducer drivers explored and exploited digital logic tech niques to simplify the control frequency generation and synthesis of the drive waveform This research at Loughborough 6 7 Jed to the develop ment of a family of efficient switched mode power transmitters operating at frequencies up to 300 kHz and at peak power levels approaching 20 kilowatts The gated pulse train of these transmitters were typically adjustable between 50 200 microsecond pulses with repetition rates between 0 25 and 1 second The waveforms were very coarsely synthesised 2 bit resolution sine waves three amplitude leve
70. fied by the use of EPROM based test signals see 2 8 Signals received by a hydrophone in the far field for each channel are adjusted until equal in amplitude Alterna tive attenuator cards can be preset for specific power levels or used to apply fixed weightings to the channel signals and these cards can be interchanged as required The experimental investigation of Dolph Chebychev shading described in chapter 6 used channel amplitude weightings introduced by this attenuator card 18 2 6 High Power Non linear Operation An anticipated requirement of the Mk 2 equipment was a need to achieve very high source levels in the water to facilitate the investigation of non linear acoustic effects Transducer power output is limited operationally by the static water pressure and by the area of transducer face coupling the signals into the water A limit to the maximum pressure differential between positive and negative pressure peaks in the wave form exists at the cavita tion limit when the negative signal pressure drops below the static pressure If attempts are made to drive the array beyond this limit dissolved gases will start to emerge from solution and a rectified diffusion effect 6 can take place causing rapid growth in bubble size In extremis the gas bubbles form as streamers on the face of the array and the dispersive effect of the bub bles will destroy much of the acoustic beam forming Under these conditions a potentially d
71. for full load proved quite success ful when driving the half transducer Approximately 3 4 power can be coupled into the half transducer load 3 4 Power Amplifier Performance Summary The frequency response of the basic 1 KW amplifier module tested without an output transformer extends from 1kHz to 1OOkHz at the 3dB 35 points and is essentially flat throughout this range The output transformer was designed for resonance at 4O kHz at full load to match a transducer ele ment stave and cable capacitance This assumes that the transformer output is connected to the transducer array using 4 of the coaxial cores sub stave con nections in a 40 metre long pair of multi core cables These polyurethane jacketed umbilicals one feeding each half array each contain 45 miniature coaxial cables and were manufactured by De Regt of Holland to an Admiral ty specification The overall frequency response of the system including the transducer was measured in the water and was evaluated using the frequency multiburst test signal mode These tests indicated a useful 3dB bandwidth extending between 37 kHz and 50 kHz centred on 42 kHz The anomalous dip in the response at 39 AOKHz is believed to be a characteristic of the transducer array construction fig 3 4 1 The eight dual power amplifier Fig 3 4 1 Frequency response of the complete Mk 2 high power system including the power amplifiers and transducer 0 dB 7 dB 3 E a evt 0 EC
72. g 3 3 2 The Prototype Amplifier x 9 2 Maximum O P p Dinala XOU NM a 70v rail sagging EEEEE Sas EEE SS EEE b 100 millisec pulse measured across 41 ohm load 955 watts Note onset of distortion The power rails are sagging under load which severely limits the pulselength at full power Fig 3 3 3 The Prototype Amplifier Maximum O P a 80v supply rail b 100 millisec pulse measured across 35 ohm load 1125 watts Note No visible distortion at the full rated power pulse length Fig 3 3 4 Final Amplifier for the Mk 2 System Maximum output power into reactive dummy load 100 milliseconds 40 kHz Transmission 1250 watts into 125 ohm load 34 tered to roll off the low frequency response below 1000 Hz The new PCB layout reorganised the copper track thickness and pad sizes and incorporated a ground plane This design permitted a directly coupled transformer output and eliminated the bulky capacitor introduced in the original experimental unit Eight production versions of these dual amplifier units 16 kilowatts total were eventually delivered complete with internal toroidal output match ing transformers These amplifiers were all thoroughly tested on arrival to en sure that they met the design specification The units were accepted after completing all the specification checks which included a sustained soak test at full power for several hours using a dummy load The dummy load used
73. generator The writing of control and signal synthesis software The design evaluation and commissioning of suitable linear power amplifiers Investigations into suitable transducers and phased array design lead ing to the manufacture of suitable matched wideband multi channel staved transducer arrays Finally a series of trials were made in a variety of open water condi tions to evaluate the systems performance and investigate the multiple modes of operation that have been developed The system has successfully demonstrated that transmitter beam steer ing is both practical and flexible The techniques implemented permit sector interrogation by within pulse type sweeps by Ripple fire and by transmit ting steered Pings sequentially on prededermined bearings Each mode al lows considerable flexibility in the generated waveform shape and frequency The Multi Mode capability of this approach was conceived primari ly as a research tool but many of the modes can be isolated and exploited in dedicated applications 11 A MULTI MODE SONAR TRANSMITTER by A D Goodson CONTENTS Acknowledgements Abstract Contents GLOSSARY Lists of Symbols List of Figures Chapter1 INTRODUCTION 1 0 1 1 1 2 1 3 Sonar Transmitter Evolution Transmitter Driver Technologies Signal Waveform Synthesis Design Objectives Chapter 2 MODES OF OPERATION 2 0 2 1 2 2 2 3 2 4 2 5 2 6 2 7 2 8 Introducing the Mu
74. grammable read control card at transmission time The read control card functioning from control parameters stored in latches clocks out the data samples into parallel sets of D A converters and the resulting phase locked analogue signals are amplified and used to drive the transducer staves The host micro computer accepts parameters from the operator console and programs the control card to implement them During the file transfer process data from disk is first loaded into a scratch pad area at 8000 8FFF in the microcomputer memory This temporary area is re quired while demultiplexing the block file into 16 channels Each buffer memory card is sequentially enabled in parallel with main memory while the copy process takes place There are some areas of contention in the com puter memory map which need protecting specifically the dynamic stack created during program execution by the Pascal language This stack exists below 510000 and as a result access to COO00 CFFF in the auxilary buffers is restricted These buffer addresses are utilised by mapping ROM test signal data which are therefore available at all times The remaining accessible 20 kilobytes of each buffer permits data for a large number of alternative modes to be called from disk and stored ready for immediate transmission 2 1 Steered Beams The data required to generate CW pulses on any bearing is minimal Only a single cycle needs to be defined appropriately phase shifted in
75. h an aluminium piston with a 0 95 lambda face and this demonstrated a single smooth admittance circle in dicating a Q of about 4 After extensive tests this was the transducer design chosen for use in the 256 element phased array fig 3 6 2 Fig 3 6 2 Universal Sonar Ltd Mk 2 Wideband Transducer yv AEN p ar 4 ad AK d PA 3 7 High Power 256 Element Array As the design of a 16 stave x 16 element array based on these ele ments proceeded it became apparent that the total mass of the assembly was likely to make the array difficult to deploy without specialised lifting facilities The element design had produced devices with 0 95 lambda AOkHz diameter pistons and a matrix of these at lambda pitch was en visaged The side effect of diffraction secondaries was accepted in order to achieve a 4 degree beam The endfire propagation from a lambda pitch array was also an accepted disadvantage However in modelling the possible effects of a split dual 128 element array it was noted that the introduction of a deliberate discontinuity of half lambda at the array junction could be beneficial in that the endfire component in the vertical unscanned plane could be nulled significantly A final proposal for two identical 128 ele ment arrays was therefore pursued with an assembled aperture of 16 lambda horizontally Steerable and 16 5 lambda vertically Appendix VI A half array was commissioned first and thoroughly
76. he hydrophone The use of Staines reservoir has the drawback of distance from the University which is slightly compensated by the proximity of ARE T A more serious disadvantage relates to shared useage The reservoir also sup ports three separate trials facilities and significant time can be lost while avoiding acoustic interference to or from other operators 5 3 Foremark Reservoir South Derbyshire This large capacity reservoir 11x10 Litres completed in 1977 func tions as a pumped storage reservoir for Leicestershire s water supply The original valley contours were widened during the construction of the dam and the final water depth exceeds 30 metres when full Working from the draw off tower a deep water path extends for over 1 kilometer with a depth exceed ing 20 metres The very wide valley has required a long curved dam wall 62 Fig 5 2 1 Trials facility at Staines Reservoir Hydrophone support pole deployed So o TEN y 63 and a very large sector of water is available to work into Equipment can be transported by vehicle to the narrow roadway on the dam wall and trans ferred to the tower using trolleys to man handle the equipment across a foot bridge fig 5 3 1 Transducer deployment is more difficult at this site as the working platform can be several metres above the water depending on the level A free standing array support designed by ARE T overhangs the water from the work platform fig 5 3 2
77. he triple pulse Fig 2 7 2 Multiple Sweep Pattern Hydrophone response on the centre axis 0 Fig 2 7 3 Multiple Sweep Pattern Hydrophone deployed off axis at 24 21 SWEPT PULSES of DIFFERENT SWEEP RATES signal transmission constructed in this example from three sector sweeps generated contiguously A combination transmission comprising 1 5 and 1 millisec sweeps across the sector are transmitted Total pulse duration 7 mil lisecs during which each potential target has been scanned 3 times The triple echo response from a target exhibits a temporal displacement of the middle pulse directly related to target bearing A matching receiver to exploit this has not been attempted It seems probable that the technique will involve receiver complexity approaching that of a conventional scanning receiver and is unlikely to be competitive 2 8 Test Signals The multi mode system comprising 16 parallel channels can tolerate the loss of one or more channels without serious performance degradation in some applications Such soft fail characteristics are desirable but the effects may not be immediately apparent from target echo examination To align the system correctly and to assist in system fault diagnosis a number of test modes have been developed For setting up and matching channel outputs identical unsteered data is used in each channel drive Each of the 16 stacked locations in the buffer contains the drive fo
78. his program utilises 43 kHz data files but does not offer the NLA modulation files Disc AREO3 This disc duplicates all the operations described for AREOI but uses datafiles computed for 15 degrees and 30 degree sectors Disc 4 This disc contains a more dedicated version of the control program STEERf with restricted parameters The program uses data files computed to produce a centre frequency of 39 75 kHz The non linear second harmonic of this signal generated in the water matches the operating frequency of an avail able within pulse sector scanning receiver and the multi mode transmitter has been demonstrated to insonify the sector very effectively at long range The program was modified and the new frequency data computed in order 57 to experiment with a synchronised scanning transmit receive combination The control program slightly shortens the transmitted block length by fixing the MSB address to ensure the transmission of an integer number of carrier cycles at this frequency Disc AREOS This disc contains the Multi burst frequency chirps Each block con tains a different carrier frequency and when ripple fired mode the result is a frequency chirp Two sets of data are stored on this disc offering a frequency range between 36 and 50 kHz or 10 to 80 kHz The first band has been used to demonstrate the frequency response of the system Disc DataPrep This disc collates the Pascal source texts of several data prepa
79. ical 15 element response with phase errors and Dolph Chebychev weightings UPIPP oer m PD m m dm dr P A art this array is rather asymmetrical fig 6 1 3 and a careful investigation of the phase responses of each element demonstrated significant variations in the mechanically defined phase of the piston transducers The theoretical array responses were recomputed to include the measured phase deviations and the modified predictions are much closer to the practical plots The phase cor rected but unshaded predicted pattern fig 6 1 5 and the corrected and shaded version fig 6 1 6 demonstrate that the Dolph Chebychev shading concept of fers useful advantages if the inter element phase errors are very are small or can be compensated for The use of 4 bit resolution data in these preliminary experiments limited the precision available to define the necessary amplitude and phase and further work on shading techniques was deferred until an 8 bit system could be employed 6 2 Beam Steering Beam steering is implemented by computing the requisite phase delays for each transducer channel Beam plotting the resulting array response to establish the positions of the peak and sidelobes is achieved by 76 Ateneo ui ae nes ue ama cuni Ard MAE m mechanically turning the array while transmitting the phase shifted
80. ificant directivity which varies with swim attitude High sonar frequencies are necessary for multi tar get discrimination but an argument can also be made for the use of much 90 lower sonar frequencies where the target directivity becomes less variable The use of a non linear transmission may conceivable permit both the low and high frequency characteristics of such bio mass to be assessed simul taneously A multi beam or swept transmission could sample a much greater volume of the shoal and thus increase the data acquisitton rate Extending phased array steering into both azimuth and elevation planes opens the way towards stabilised beam forming and three dimensional scanning Such a system would require increased channel complexity but the techniques required to implement it can be seen to be practical The existing Mk 2 array was configured to allow access to the vertical axis in substaves of 4 elements If phase steering techniques are applied to the vertical axis it can be seen that steering is limited to a maximum angle of 7 degrees 4 X pitch before the grating lobes create ambiguities This limited beam shifting might prove useful as a correction factor for vehicle orientation instabilities especially if the array is to be installed on an ROV or as part of deep towed assembly Chirp or coded pulse transmissions improve target detection thresholds very significantly if suitable matched pulse compression or correla tion reception techn
81. ilable on board to organise the facilities Acoustically the maximum range visible from Maytime is limited by the geography of the glaciated valley to approximately 4 kilometers of deep water The Royal Navy Hydrographer at Taunton made available a recent detailed echosounder survey and this data has been digitised on the University s mainframe computer and used as a data base for some interest ing 3 D perspective modelling of the Lochgoil subsurface terrain These models have assisted in the interpretation of echo returns from the geographic features which characterise the reverberation on this range fig 5 4 2 3 Currently the Mk 2 multi mode transmitter equipment is Fig 5 4 1 Maytime Acoustic trials facility in Lochgoil 66 Fig 5 4 2 Computed Terrain Model Perspective view of Lochgoil as seen from the transducer position at 30 metres depth from 1070 8350 77 te 1705 4550 7 VIEW from MAYTIME towards RAFTS 60 degree Passpeciive Viewing point 23 metres below ihe water surface Marker incicates water suriace andcente of Can Buoys between rahs Azimuth Centre Lina Fig 5 4 3 Lochgoil Modelled from above the end of the loch The sea level has been removed to a depth of 50 metres from 787 7125 300 to 1705 4550 77 67 deployed in Lochgoil to assist continuing research into the parametric non linear modes of sonar emission 5 5 Sonar Receivers The multi mode transmitter was conceived as a stand al
82. illisec to 10 seconds Preset parameters programmed into control latches Accept displayed parameters Y N Ready to transmit Press any key to transmit TRANSMISSION SEQUENCE STARTED Press any key to stop keypress terminates transmission Options Change sector Restart Quit to operating system 56 4 5 Summary of Software used for the Lochgoil trials Disc 11 This disc contains the STEERd example of the control program which offers the basic modes of operation STEER RIPPLE PING SECTOR The program selects data files from disk which can steer within a sector either 28 degrees or 14 degrees producing unmodulated or modulated signals at 2 3 4 5 or 6 kHz using a 43 kHz carrier ie Any of the modulation frequen cies in 15 discrete bearing steps of either 4 degrees or 2 degrees The control program and all supporting data files are held on one double sided double density disk and a BOOT sector permits the program to be initiated by a simple keyboard entry by the operator JU is an in built operating system command to load and execute a Bootstrap program stored on sector 1 track 0 of a diskette in drive 0 Disc AREO2 This contains a very similar host program to AREOI reorganised to offer a selection of within pulse SWEEP modes at rates from 1 millisec to 7 milliseconds period In addition the program can also load and run the steered unmodulated data files with the modes of operation RIPPLE STEER T
83. illustrated by the display of the buffer contents fig 2 4 1 2 3 2 5 Sidelobe Shading Techniques for array shading primarily to achieve reductions in sidelobe amplitude at the expense of the primary beam width are well estab lished theoretically e g Dolph Chebychev and have been applied commer cially in some radio communication antenna Acoustic applications of this method published in the open literature appear to have been limited to a few specialised examples of fixed beam forming applied in fishery research Sidelobe suppression even at the expense of beam width ought to prove ad vantageous in reverberant conditions but the advantages gained may not com pensate for the loss of source level incurred The application of sidelobe reduction techniques to swept beams may prove to be important if bearing coded transmissions are employed for a transmitter scanned sector sonar In practice the success of Dolph Chebychev shading has proved to be very de pendent upon the mechanical phasing of the transducer piston heads i e on the manufacturing tolerances maintained during the array assembly A channel matching facility has been included in the multimode trans mitter which permits fine amplitude adjustment of the synthesised drive sig nals to compensate for gain variations and transducer efficiency variations in each channel The amplitude adjustments are made using a plug in passive at tenuator card and the matching technique is simpli
84. ing The initial estimated cost of a 256 element array based on this technology was competi tive However the need to achieve a large transducer surface area to efficient ly couple the maximum possible acoustic power for parametric experiments implied a need for much greater numbers of 1 2 lambda size elements and the economic choice was forced towards a Universal Sonar one lambda design Fig 3 6 1 I O S Transducer Titanium element construction The first samples provided by Universal Sonar proved unsuitable as the specified low Q response was apparently achieved by merging two or more close spaced resonances This technique produces circle plots which dis play several deep re entrant dips in the admittance plot indicating points of rapid phase shift between the resonances During the sweep modes of opera tion such phase changes are unacceptable as they create undesirable distor tions in the beam pattern In the sweep sector modes the individual transducer staves all transmit different frequencies A second disadvantage of this element design proved to be a self destruct mode at high power caused by the very poor thermal conductivity through the piston face The pis ton head of this transducer included a glass epoxy component which delaminated with the heat dissapated during a very short high power test made during the examination of cavitation characteristics later example 40 from Universal Sonar was manufactured wit
85. int The Directivity Index is 34 dB The RMS source level at 41 kHz was at 245 dB reference 1 microPascal at 1 metre measured with 16 Kilowatts power input to the array 47 A MULTI MODE SONAR TRANSMITTER Chapter 4 SOFTWARE 4 0 Software Introduction The microcomputer chosen has a large library of commercial and public domain software packages most of which are targeted on the CPM disk operating system environment The deliberate choice of the rather basic Lucas NasDos2 1 environment was made primarily for its fast floppy disk file handling capability This speed advantage is gained as direct result of simple disk file handling primitives which do not try to emulate a virtual machine for transportability as occurs under CPM However although the support software available is more restricted in this environment several good language implementations and Z80 assemblers are available The majority of the software for this project has been written in Pascal 4 1 Languages Z8O Assembler BASIC Pascal A number of small machine code routines were written in Z8O as sembler during the early development phase of the projects Some of these programs were test routines used to obtain timing comparisons and most of these have been superceded A few small Z8O code segments have been retained and are called as external routines from the Pascal control programs These pre assembled code segments can be conveniently trapped in the prog
86. ions to the syntax was neces sary for successful recompilation As Turbo Pascal is now available running under MS DOS it becomes a better choice if the system is to be transported in the future to run for example on an IBM PC 49 4 2 System Software The system software evolved into two basic suites of programs The first group is a dedicated set of data file preparation programs This generates sets of data files onto disk as required for each specific signal mode The second group consists of operational programs which first call and load ap propriate data files and then offer the operator a choice of parameter options to generate specific sonar emissions A large number of programs were written as the project evolved many of which are quite similar varying only in the accessibility to par ticular functions One difficulty experienced which requires further refine ment is the level of user friendliness that is required by an unskilled operator of the system The early software retained a greater level of flexibility but required care to be exercised by the operator as error trapping to avoid illegal transmission parameters was incomplete The program ex ample STEERd examined in 4 3 does not eliminate all the conceivable entry errors when setting parameters However as installed the Mk 2 transmit ter hardware is tolerant of this form of abuse and the system as a whole is designed to be robust to the point of surviving a programmed
87. iques are used in the receiver The multi mode transmitter simplifies the synthesis of such signals and provides a suitable test facility for the exploration of such signal processing techniques when used with steered signals The ability of the multi mode transmitter to steer signals rapidly into adjoining parts of a sector by ripple fire or sweep techniques is demonstrably an efficient insonification technique However the echoes returning from these adjacent bearing require some method of bearing discrimination The use of a within pulse sector scan receiver locked to these transmissions is clearly an optimum solution although the most complex and expensive The wide bandwidth of the Mk 2 transducer 11 kHz opens the way to bearing coding of the transmissions The simplest option being a discrete frequency transmission for each bearing Decorrelating the bearing information from the echoes becomes simply a matter of spectral analysis but this needs to be accomplished in real time Fast Fourier Transforms can be used to analyse a signal for its frequency components and software routines to implement FFT s on the new generation of Digital Signal Processing ic s e g 6000 TMS320c25 or the Inmos Transputer need to be investigated The 91 speed of such routines will define the achievable range resolution Simple parallel analogue or digital filtering techniques are practical and will not suf fer the same speed restrictions
88. is a cost effective alternative to the hard wired logic of earlier transmitters To exploit the possibilities of this approach fully more operator inter action is required which necessitates a keyboard and visual display unit The consequent increased equipment cost has fortunately been matched by the rapid reduction in costs of the small mass produced single board micro com puter 1 3 Design Objectives Traditional sonar transmitter designs are dedicated pieces of hardware with relatively rigid parameters A simple transducer replacement often requir ing internal modifications ie a change of output filter and the crystal reference In modern sonar equipment the generation of the transmit waveform has already moved away from gated analogue techniques towards the digital synthesis domain Computer logic elements have simplified the pulse timing and analogue output signals derived from D A conversion are now common place often deriving the signal parameters from a PROM look up table The direct computation of the waveform using a microprocessor is a logical development and the synthesis of multiple phased related signals has been examined Exploiting microprocessor techniques to replace hard wired logic permits a signal synthesiser design to add considerable flexibility through software offering both standardisation and simplification of the electronic hardware This project was initiated after an investigation into microcomputer sig
89. istently quicker than when operating under CPM An initial study of digital frequency synthesis using a micro computer demonstrated that in realtime the computation of sinewave samples to generate an analogue carrier would restrict the system to the very low audio frequencies The enforced low sampling rate definition of the waveform necessitates high Q filtering and this bandwidth restriction reduces the flexibility still further Speed comparisons were made between sine wave syn thesis routines written in the available high level languages These gave benchmark timings of 0 035 0 044 and 0 030 seconds per calculated sine sample when using interpreted 8K BASIC Microsoft Extended Crystal BASIC and the compiled BLS Pascal respectively The un impressive im provement when using compiled Pascal is misleading as in this case the com putation used double precision arithmetic A second test utilised precomputed sine values reduced to single byte precision and stored in memory as a 26 lookup table Using a procedure written in Pascal to simply transfer data from memory to the parallel ouput port increased the data sample rate to ap proximately 6 kilosamples second Replacing the Pascal procedure with a Z80 assembler routine using the block move commands OTIR or OTDR was more realistic and the output data rate improved to 190 kilosamples second A micro computer based on a Z80A microprocessor running at 4 MHz with no wait states
90. ivative of Standard Pascal written by ApS of Copenhagen specifically for the Lucas Nascom microcomputer This compact version of Pascal whilst omitting some features of the ISO standard E is well suited to programs designed to run on the compiling machine BLS Pascal v 2 consists of a 12 k development package comprising Compiler Editor Runtime routines extended by a further 4 k set of disk operating routines Pascldos 2 The advantages of this version includes one pass compilation direct to Z8O object code which when linked to a small runtime package produces fast stand alone code that is quite compact The structured modular nature of the language suits the development of large programs especially where continuous program development and updating is required The self documenting nature of the language is also seen as an ad vantage Turbo Pascal 3 is a more recent version of the Pascal language with similar advantages to the BLS Pascal This is available to run on Z8O based microprocessors under the CPM operating system However the use of the CPM operating system imposed some speed disadvantages when compared with the BLS Nascom implementation and whilst Turbo Pascal remains a more transportable version of the language with useful additional extensions to Standard Pascal the original BLS version was retained for this project It was established that text file transfer was possible between the BLS and Turbo versions and that only minor modificat
91. ively hot with a full power but low duty cycle pulse mark space ratio of 1 10 The thermal stress induced several MOSFET failures despite the presence of a thermal trip installed to sense heat sink temperatures Investigation of this effect revealed a design limitation in the in ternal phase splitter This circuit was found to be slew limiting at higher fre quencies and at 40 kHz the phase split shifted significantly from the essen tial 180 degrees A DC component was produced by this tracking failure during high power operation and the phase splitter circuit was modified to eliminate the effect The units were used successfully for the early trials in this form but other less serious but significant thermally related effects remained until a thorough investigation of the complementary power FET devices demonstrated asymmetrical thermal tracking characteristics A conven tionally connected configuration AC coupled was immune from the problem and ran quite cool at the desired mark space power level However when the amplifier pairs were configured in a bridge mode the primary winding of the output transformer permitted significant dc currents to flow as the mis matched FET devices increased in temperature A modification using suitable electrolytic capacitors placed in series with the transformer primary produced an effective cure and full power can now be obtained for long periods without thermal distress These amplifiers were racked togeth
92. j Begin Ctt MAIN PROGRAM 9x Dosfetch Use in Merged code version only 2 Default Parameters O P Bd Selectw CARDSEL e FFiHx31m C 43 HxZi1e F Hx 115 F 3Hx0 F 1 Start Address ALSBiIw FF 6MSBiv CF Prescale u d preiw 68 S MiIv 5688 Char Positiones Psn3 39 RIPPLE ModgblockKs Tx reps 01 Pause length pc sz Z2S ppause 17180 No of Tx s TXNS STEFF Steered Mode MODE Channel Test 3 Select Title SFiv S TITLE writeln writelnt Channel Test Mode Data from COQ08 CFFF 31 writeln writelnj INITPORT DATAIN Loads all 8 boards with precomputed data RESTART21 TITLE writeln Selected Parameters are 2 wnritet Ripple Steer Ping Sector decs pre DECHEX writeln write Start Address dectis MSB DECHEX wr i teln dec ALSB OECHEX writeln write Modulation Blocks Txn decr reps3DECHEX writeln write Period between Txn dect pey DECHEX3 writeln write Operational Mode MODE 33 AAA 0 9 ro 08 a CS se we 4e writelnj PRESET screen 1 1125 writet Are different parameters required Y N screen 43 3 readIn CP 2 IF P lt gt Y then DEFAULT else NEWPARAM Select ALL RAM cards 2 out B 281 s DELAY out A EFF DELAY swriteln screen 11 3 wri tet Ready to Transmit 9 C 9 a s 31 PAUSE screen 37 0 wr i teln lt RUNNING out B 80 gt jout B 08 PAUSE
93. l be restricted to mean active echolocation systems All sonar echolocation systems comprise A transmit signal synthesiser to define the waveform duration and frequency of the transmission j A transmit transducer assembly to couple the drive signal into the water and create the required acoustic pressure waves A receive transducer to convert the incident echo energy back into electrical signals A receiver amplifier and signal processing system to detect and con vert target echo signals into a form which can be presented to the operator for interpretation Most modern systems present the echo information using cathode ray tube CRT displays although paper printouts are still in common use for echosounders and side scan sonars In many systems a single transducer array is used for both transmis sion and reception and this is switched between the signal processing electronics as required Most commercial sonar systems use a single channel transmitter and the associated receiver either exploits the vessels forward movement as a scanning axis Side scan or if the transducer is rotatable may scan mechani cally by transmitting on adjacent bearings progressively across a sector The more sophisticated within pulse sector scanning systems use a multichannel receiving transducer and employ modulation techniques to scan a sector For each time resolvable range cell as the transmit pulse propagates the receiver direction of maximum se
94. limited The technique evolved to minimise stability effects utilised a rip plefire transmission computed for a 15 degree sector This ensured that the hydrophone deployed from a moored boat remained within the overlapping beams to accurately record the peak amplitude The buffered hydrophone sig nal was fed to a selectable set of passive narrow band filters with known in sertion losses in order to measure the relative amplitude components of the signal at the spot frequencies 40 80 4 and 8 kHz 6 4 1 The dual trace os 81 cilloscope powered by a battery inverter in the boat was used to simul taneously display the filtered primary and one of the selected non linear fre quencies Polaroid photographs were taken from the oscilloscope display and the peak amplitudes for each filtered component logged The results from one of these trials analysed in 6 3 indicated a primary source level at 40 kHz of 243 dB ref 1 uP Taking the reduced power used to drive the array in Foremark 10 5 kW into account this result seems to be in reasonable agreement with the later experiments at Lochgoil using the full 16 kW trans mission from which a 245 dB source level was realised 6 5 Source Level Results Foremark trial 13 5 87 Measuring hydrophone deployed at mid water from a boat at 375 metres range Vertical beam width 4 degrees Water depth 17 5m Temperature 6 Signal insonifies the full water column after 250 metres range 40 kHz 180 mV p
95. ls which necessitated sharply tuned coupling transformers These transmitters were designed primarily for use with scan ning sonar receivers and were successfully applied at frequencies down to 40 kHz and many of the units constructed are still in use 5 Dedicated transmit ter designs where the frequency is crystal derived to match a specific transducer and which utilise highly tuned output transformers to filter the coarsely stepped drive waveforms are inflexible devices when an operational choice of arrays is needed or when broad band signals for correlation applica tions are required These design limitations resulted in suggestions that a modern microprocessor controlled frequency synthesis approach should offer a more flexible alternative This project was initiated with an investigation of available frequency synthesiser techniques which might simplify the generation of precise signal frequencies to suit alternative transducer arrays The results of this investiga tion indicated that synthesis using a simple 8 bit microcomputer could be both cost effective and flexible If a relatively large number of digital samples are computed for each waveform cycle then the need for high Q band limiting filtering to smooth the resulting analogue signal is removed and the phase and amplitude of the signal can be defined accurately and repeatably The use of a dedicated microprocessor running a control program reading waveform codes stored in memory
96. lti Mode Concept Steered beams Ripple fire Swept within pulse transmissions Focussing Sidelobe Shading High Power Non Linear Operation Bearing Coded Transmissions Test Signals Chapter 3 HARDWARE 3 0 3 1 Hardware Introduction Micro Computer iii jj iii viii WN e 13 13 16 18 19 20 22 25 25 3 2 3 3 3 4 335 3 6 3 7 3 8 3 9 MOSFET Power Amplifiers High Power Linear Amplifiers Power Amplifier Performance Summary Transducers and Arrays Wide Band Transducer Elements High Power 256 Element Array Mk 2 Array Tests Mk 2 Array Parameter Summary Chapter 4 SOFTWARE 4 0 4 1 4 2 4 3 44 4 5 Software Introduction Languages Z8O Assembler Basic Pascal System Software Program Example Data Program Example Data Handling and Control Summary of Software used for Lochgoil Trials Chapter 5 DATA ACQUISITION 5 0 5 1 2 2 3 3 5 4 5 5 5 6 a Acoustic Measurement and Trials Range Facilities Loughborough University s Acoustic Test Tank King George VI Reservoir Staines Foremark Reservoir Derbyshire Maytime Lochgoil Argyle Sonar Receivers Hydrophones and Preamplifiers Data Logging and Signal Analysis Chapter 6 TRIALS RESULTS 6 0 6 1 6 2 6 3 6 4 6 5 6 6 Initial System Tests Beam forming and Focussing Beam Forming and Shading Beam Steering i Sector Scanning Ripplefire a
97. mary and at the non linear product frequencies The Ripplefire mode per mits long pulses to be developed which can be frequency coded to assist bear ing discrimination It was noted during these experiments that the NLA en hanced 2nd harmonic of the swept signal at 80 kHz proved to be a particular ly effective insonification source when exploited in conjunction with a con ventional within pulse scanning receiver 6 4 Source Levels and Non Linear Acoustics The Mk 1 low power system demonstrated that detectable non linear signals could be obtained from a 2 kilowatt transmission Experiments at Foremark with the Mk 2 equipment running at 10 5 kW confirmed this Directly recorded signals at 800 metres range were made using a Nagra IV portable recorder and the data replayed through a spectrograph At this range the 40 kHz square wave modulated transmission Fig 6 4 1 N L A transmission spectrum Analysed from data recorded at 800 metres down range s E a Ae i 9 ZZ ain i he 2 ES 8923 I 8 4 LI Foremark Reservoir NLA Trial 50 ms Pulse 4 kHz modulation Analysis Bandwidth 300 Hz Vertical Scale 0 32 kHz Horizontal Scale 0 750 ms 0 250 500 750 was expected to generate a significant low frequency component in the water at the modulation frequency The recorded signals were carefully kept well below the tape distortion levels and the hydrophone resonse was filtered to reject co
98. mponents above 30 kHz The spectrogram fig 6 4 1 demonstrates the presence of the low frequency 4 kHz component and its harmonics at 8 12 16kHz etc 80 Fig 6 4 2 Foremark experimental determination of the 4 kHz Non linear beam width r eee eee ee MEASURED 4 KHz NLA BEAM WIDTH 2 8 degrees Excitation beamwidth at 43 KHz 3 5 degrees AKPLITUDE 08 Nagra Tape recorded data Measured at 00 metres range Chanel 1 Recorded Amplitudes 1 bal hydrophone Channel 2 Radio Taliback Voice Log Transmit Azimuth Angles The Non linear beam width was also measured at this time by record ing both the hydrophone signal and via a radio link the simultaneous voice log calling the steered angle as the array was rotated in small increments The replayed data was then measured and plotted manually fig 6 4 2 this experiment indicated that a 4 kHz non linear beam with a 3 dB width of ap proximately 3 degrees was formed from a 43 kHz primary at 800 metres range To assess both primary and non linear source levels obtainable in Foremark the transmissions made from the tower were intercepted at ranges greater than 350 metres using a hydrophone deployed from a boat The received signals were displayed on an oscilloscope and polaroid photographs recorded the traces This experiment was repeated on four occasions but severe weather conditions caused the boat and hydrophone to move excessive ly and the accuracy achieved was
99. n To my colleagues at Loughborough University who have been in volved in various aspects of this work at different times and particularly to those who shared in the winter measurement trials including Dorothy Anne Lees and Keith Lowe Avtar Gida Bill Wood Phil Hill and David Payne Technicians Gary Wagg et al who helped deploy and recover the equipment at Foremark Ian Sinclair and Dr Bryan Woodward for diving support during inves tigation and redeployment of targets in Foremark Roger Mayo and colleagues in the Civil Engineering Department for survey work at Foremark and for the subsequent sub surface computer ter rain modelling Anthony David Goodson August 1989 A MULTI MODE SONAR TRANSMITTER Abstract This project was initiated to evaluate appropriate microprocessor and digital logic techniques that could increase the flexibility and effectiveness of a sonar transmitter The study led to a multi channel signal synthesis concept designed to exploit phased array steering techniques Two versions of the equipment have now been built and evaluated Mk I is a relatively low power 15 channel system with 2 kilowatts total electrical power using a 40 kHz 15 A x1 line array This system proved the practicability of the basic con cept and its success led to the 16 kilowatt Mk 2 high power version which drives a 16 A x 16 X wideband transducer array The study included The design and construction of a multi channel signal
100. n outtcA FF gt j10ut cA 00 Sets port4 into mode3 all bits set as outputs outCcB FF jout cB 198 Sets portS into mode3 with bit 4 as input 2 others output 2 end Procedure DELAY begin now effectively removed end MI v Procedure C LCmsb begin hexno Hx3 HEXCON c z kh hexnoi wHx2 HEXCON AMSBishtcs Hx2 selects load address end Procedure CALCI sb begin hexnoi Hx1 HEXCON ciz1 hj 5 HEXCON amp 6LSBiIxh cj end Procedure TITLE begin if SFm S then titlei 30 50 degree Sector if SFu then SFs B if SFm amp then titleiz 30 degree Sector if SFm B then 1 8 60 degree Sector CLS screent8 14 2 writeint Fix Tx Steered NLA title d screen CQ 12 wP teln mummmaramimiwmasumEEnSRNIZEIIIN MININIEXZEILIUINENEIINEIKNENEIT UNE BEEN UE ELE endy Procedure CALCpulse begin pulse s ALSB 2E 3 0 5133 If mid CFILE S 128 3 then begin pulser 0 34 13Hx01 586 I 6 end if mid FILE 5 1 gt 9 S then begin puls amp etw8 488 Hx1 12 C sHxG C sALSBreeCCj end pulseispulsesreps if left HODE 2 RI then pulseizpulsex15 end Procedure PRESET begin TITLE CALCpulse writeln screen i2 9 3 writeln Preset output conditions creen 12 82 wr l tein wmumamuis zi s
101. nal synthesis demonstrated that computation of the signal waveform could be a very flexible tool Establishing the feasibility of extend ing such micro computer signal generation to control and exploit a multichan nel transducer array as a steerable phased array sonar is therefore the main objective It was recognised at the start that a single 8 bit micro processor s computational speed would limit the direct synthesis concept to very low fre quency multi channel applications While a multiple processor approach is considered feasible the technique exploited separates the waveform synthesis computation from the transmission in order to achieve realistic operating fre quencies with a minimum of complexity The synthesised multichannel waveforms pre calculated off line are stored for fast simultaneous access in parallel digital RAM buffers This technique has now evolved into a self contained microcomputer controlled multi mode sonar transmitter driving a multichannel transducer array the combination of which permits a large number of operational modes to be evaluated Two practical systems have been developed Mk 1 a rela tively low power 2 kilowatt 15 channel configuration and Mk 2 a much higher power 16 kilowatt system intended to extend the techniques into non linear sonar research The further development of this high power transmitter into the non linear operational modes is on going research and some initial encouragi
102. nd Sweep Modes Source Levels and Non Linear Acoustics Source Level Results Reference target detection Foremark iv 29 30 35 38 39 41 45 47 48 48 50 51 54 27 60 61 62 62 66 68 68 69 71 74 76 78 80 82 83 Chapter 7 CONCLUSIONS amp DISCUSSION 7 0 7 1 12 73 7 4 7 5 III Objectives and Conclusions The Advantages of a Multi Mode System General Discussion System Reliability Improving Hydrophone Data Acquisition Future developments REFERENCES List of References APPENDICES Drawings and Circuit diagrams The LUTEE Beam Plotter Program Software Listings 85 86 87 88 89 90 93 GLOSSARY 0 1 LIST of SYMBOLS and ABBREVIATIONS A D ALS ARE ARE T ARE P ARE LG Bt kbt CMOS CPM D A DSP EPROM FFT LS HCT LUT LUTEE MOSFET NLA OEM Jd Analogue to Digital conversion Advanced Low power Schottky Logic e g 74ALSOO Admiralty Research Establishment ARE Teddington ARE Portland ARE Lochgoil Byte 8 bit word kilo 10 kilobyte 1024 bytes or 400 Hexadecimal bytes Complementary Metal Oxide Silicon Field effect transistors and logic elements 8080 and 780 micro computer disk operating system Digital Research Trademark CP M deciBel Digital to Analogue conversion Digital Signal Processor A specialised microprocessor with an architecture optimised for very
103. ng results have been published in recent conference papers 10 xii A MULTI MODE SONAR TRANSMITTER Chapter 2 MODES of OPERATION 2 0 Introducing the multi mode concept A sonar transmitter capable of producing several simultaneous phase related signals can be used to drive a multi element transducer array to inject phase related acoustic signals into the water from different positions dis tributed along the array The interaction of the resultant individual wave fronts can be exploited to generate a coherent response which can be steered in a desired direction If all the transducer channels are driven with identical signal data then the array will form its main directivity lobe along the axis perpendicular to the array face This broadside response typifies most transducer array applications where a single signal source drives all the array elements in parallel If the array is subdivided into individual elements or staves then the application of a uniform progressive phase shift to each ele ments driving signal will cause the principal lobe to be formed and propagated off axis The amount of angular shift obtained being a function of the array element spacing and the relative phasing of the drive signals Fig 2 0 1 Wave Front V 0 For beam steering 1 consider an array of N equally spaced elements Fig 2 0 1 The spacing between each element is d and the signals at each element are assumed to be of e
104. nsitivity is scanned across the sector to determine the bearing of target echoes within the cell This process is repeated continuously and generates a range bearing raster display of the full sector for each transmit inter pulse period Military and commercial sys tems utilising within pulse scanning include Plessey s Type 193 mine hunt ing sonar and the Marconi Hydrosearch equipment The circuit complexity of such receivers is very high and it is only in recent years that the use of digi tal synthesis has successfully simplified this technique To date the concept of phase steering the transmitted beam pattern does not seem to have at tracted much support probably a direct economic consequence of increased circuit and array complexity The advantages of such techniques in the electromagnetic spectrum are well established and are exploited in the elevation scanning mode of a number of modern phased array radars e g Marconi Martello Plessey AR 320 and ITT s Gilfillian etc 4 Theoretical ly it is feasible to exploit similar phase steering techniques in a sonar trans mitter system In water the low propagation velocity of the acoustic wave provides a significant time window between transmissions a parameter not easily exploited in radar The steady increases in computational speeds of digital signal processors seems likely to encourage the development of adap tive techniques in which echo responses can be u
105. nts although only a single drive signal is generated for each complete stave In the future this array design should permit limited phase steering in the elevation plane if the additional drive signal circuit complexity can be accepted The power to drive the system is normally derived from a 3 phase 50 Hz supply but is star connected to feed the individual units at 240 volts This arrangement permits development testing and maintenance of individual sections using a single phase supply however when driven at full power the distributed load is less demanding of the trials site feeder cables 3 1 Micro Computer A 280 based microcomputer is used to precompute transmission data for a given mode of operation which is first stored on disk or in EPROM This data is recalled demultiplexed and stored in auxiliary paged memory output buffers These buffer memory pages of battery backed CMOS static RAM are then read simultaneously and the contents translated by 16 simple D A converters to produce phase related analogue output signals These syn thesised analogue signals feed 16 linear power amplifiers to drive the transducer array The simultaneous output of the signal data is controlled by a programmable logic control card the parameters of which are set for a given transmission by operator entered instructions to the control program 25 Flexible control exists over frequency pulse length pause period pulse se quencing mode modulation
106. oadband 6 MHz FM video link plus an optional auxiliary FM voice channel both with excellent signal noise ratio Operating at low power 4 watts RF this has been tested over water to 4 kilometers and should function over much greater line of sight ranges without problems For the initial tests the received sig nals were fed directly into a 100 kHz spectrum analyser with very satisfac tory results Encoding the hydrophone signals digitally prior to transmission is envisaged as the best technique for the future as the telemetry bandwidth could accomodate 16 bit encoding at data rates suitable for signals containing 89 significant components up to 100 kHz The study of the non linear signal dis tortion and the propagation characteristics of the parametric products result ing from high power modulated transmissions will require a series of measurements at increasing ranges The near field of such virtual end fire ar rays may extend for several hundred metres and the radio telemetry approach should provide consistent measuring characteristics regardless of range 7 5 Future Developments The flexibility of the multi mode concept should ensure that it will remain a useful research tool for testing custom designed signals which can be synthesised quickly and projected for evaluation In considering future applications the use of the system as a parametric source of low frequency sound with high directivity appears very likely In the
107. on of this computer controlled beam plotter has now replaced an earlier analogue system for most transducer beam plotting applications at Loughborough 7 4 Improving Hydrophone Data Acquisition The recovery of hydrophone signals for analysis purposes has for most trials been achieved with direct cabling The source level experiments in Foremark reservoir pointed out the problems of attempting direct measure ment using equipment in a small boat Recording signals for later analysis re quires a wide band instrumentation recorder and although the Nagra IV S re corder has been useful in acquiring low frequency signals a much wider band instrument is needed to satisfactorily record the carrier second harmonic at 80 kHz The available Ampex FR1300 recorder is unsuitable for such mobile experiments and it is now recognised that feedback to the transmitting point is essential if the transmitted signals are to be optimised on the hydrophone position The anticipated work in Lochgoil will require measurements made at significantly increased ranges and whilst the Maytime research facility has demonstrated the capability of hydrophone cable runs deployed along the sur face to ranges greater than 500 metres this technique has clear limitations An investigation of broadband data radio telemetry has led to the construc tion of an experimental microwave system operating in the 1 3 GHz amateur band This battery powered equipment offers the capability of a br
108. one research tool with which to experiment with phased array techniques The acoustic measurements of performance have in the main been made using standard commercial hydrophones For certain measurements wideband low noise pre amplifers were constructed as were several passive bandpass filters The mechanically scanned images made at Foremark reservoir utilised the dis play equipment developed for synthetic aperture experiments 3 This equip ment required some modification and the addition of a preamplifier bandpass filter and detector stage No TVG or RCG signal processing was used in the reservoir measurements and although these facilities have now been developed for the scanning receiver they are not detailed here The high power trials in Lochgoil benefitted from access to the wide range of commer cial receiver and signal processing equipment installed on board To fully ex ploit the multimode transmitter s capability as a scanning sonar system will require a matching Transcan receiver to decode and display the bearing in formation from a single input channel Equipment for such a receiver is being developed and the basic idea for this concept is outlined in chapter 7 5 6 Hydrophones and Preamplifiers A variety of hydrophones have been used during this research however a 20 mm diameter ball hydrophone Universal Sonar Ltd was used as the reference hydrophone for most of the reservoir trials This hydrophone was calibrated at R D V C
109. onment if necessary without requiring separate com puter facilities Software can also be developed conveniently off line using a similar microcomputer without access to the transmitter hardware and data and control files transferred by floppy disk The Mk 1 low power system is retained as a test facility for use both in the University s tank at Lough borough and at Foremark reservoir for testing newly developed software The Mk 2 16 kilowatt version is now based at the ARE LG acoustic range where the long deep water paths available permit the investigation of low fre quency non linear parametric signals The next phase of this work is planned to exploit non linear acoustic effects and to develop suitable matching receiver techniques The original expectations of flexible operation have been implemented successfully and new applications continue to evolve To ex pand the system beyond its present capability specifically if long complex waveforms are to be synthesised will require some significant hardware modifications The use of the compact 256 sample data blocks when linked to the 16 adjoining blocks limits the maximum pulse length for unique data to approximately 8 milliseconds at the 500 kHz clock rate Using a slower clock rate can increase this pulse length but at the expense of the waveform 87 resolution However as the waveform amplitude is defined by only 4 bits this resolution is already marginal for some applications Direct syn
110. ost of these designs are detailed in the LUTEE internal reports they are not re examined here s 5 7 Data Logging and Signal Analysis Equipment A variety of wide band oscilloscopes have been used to display the hydrophone signals for amplitude measurement The close range measure ment technique employed permits a rapid transmission rate and waveform ex amination is eased as a result Echo signals from longer ranges can only be examined at slower rates and a Gould digital storage oscilloscope was ac quired capture the transient data The storage parameters of this equipment 2048 samples at a 200 kHz maximum sampling rate are unfortunately rather close to the 40 kHz transmissions and problems of aliassing frequently oc curred with some time base setting To avoid problems of this nature Polaroid photographs of the wideband oscilloscope displays have been used for much of the transient data capture Digitisation of the received signals for computer processing and storage has been employed in the beam plotting equipment Appendix II The gated hydrophone responses are converted to 10 bit digital samples in a BBC microcomputer The data being available as an on screen graphical display of the beamplot or as a data file for hardcopy and subsequent analysis The mechanical scanned sonar images of Foremark reservoir were ob tained by a modified Synthetic Aperture Receiver and although digitised to 8 bit accuracy were stored for display in a 6 bit
111. ould be coupled into the water without cavita tion at this depth Very significant waveform non linearity was visible on the test hydrophone deployed at 9 metres range and when using square wave modulated transmissions very strong low frequency echoes from the far shore line were clearly audible Finally when redeployed at Lochgoil in Scot land the 256 element array was deployed at 33 metres depth limited by the maximum cable length The full 16 Kilowatts power has been employed and is coupled effectively to the water with the increased static pressure The ini tial Lochgoil experiments re measured all the parameters established in the Foremark reservoir lower power fresh water trials These first trials in deep water in a less confined range environment demonstrated that the source level and array beam forming performance in seawater at full power were close to the predictions made from the earlier freshwater limited power tests provided the effects of sound velocity on the array directivity and increased absorption were included 46 3 9 Mk II Array Parameters Summary Sea Water Measurements made at Lochgoil The array centre frequency was 42 kHz 3 dB points were measured at 36 5 kHz and 48 kHz i e the array bandwidth was 11 5 kHz a Q of 3 6 The array beamwidth at 42 kHz unsteered azimuth was 4 1 degrees The array beam pattern could be accurately phase steered through 30 degrees The equal amplitude ambiguity po
112. patterns measured during the reservoir trials in the true far field The technique has an additional advantage of generating very high 73 3 Scan width 28 degrees Sweep increment 8 1 degrees Fig 6 0 4 Beam Plot of the Mk 1 Array 346 j Geamidth r13 8 Deg at 2 metres range Inside the near field Caen Plot 18 8 6 4 2 8 2 4 6 8 18 Degrees Scan width 28 degrees Sweep increment 8 1 degrees 6808 yen in nt ne Fig 6 0 5 E Beam Plot Mk 1 Array 348 Seamwidth 73 9 Deg at 2 metres using focussed data f Linear i Plot j 6 4 2 a 2 4 8 18 Degrees source levels at the focus permitting cavitation levels to be reached without excess stress at the transducer face 6 1 Beam Forming and Shading The theoretical beam pattern predicted for a 15 element 1 lambda pitch array within the 30 degree sector when all the elements contribute equally is shown in fig 6 1 1l A variety of shading techniques exist in the literature which are designed to reduce the amplitude of the unwanted sidelobes One of these based on the Dolph Chebychev polynomial is of par ticular interest as it can be applied by amplitude weightings distributed across the array aperture without needing phase reversals The technique permits all the sidelobes to be reduced to a specified amplitude and this reduction ratio is gained at the expense of the primary beam width and source level The theoretical
113. pearance in the water of a parametric beam formed within the original main lobe approximates a vir tual endfire array 5 This endfire array absorbs energy from the pump primary excitation beam untill range attenuation of the originating carrier 19 reduces the pressure differential below that which generates significant non linearities Whilst the transfer efficiency is quite low the technique permits wide band low frequency signals to be formed into very narrow beams which are effectively sidelobe free if the full length of the virtual array exists before the measurement point Additionally the source array can be physical ly small compared with a comparable primary array for the required low fre quency beam width Some preliminary experiments demonstrating parametric effects are detailed in chapter 6 2 7 Bearing Coded Transmissions Sweeping or Rippling the transmission across a sector still requires a method of establishing the return echo bearing to produce a complete target range bearing sector identification Synchronising a sector scanning receiver to the transmission is one solution Combining both transmitter and receiver scanning provides excellent sector insonification which significantly enhances the useable detection range largely as a result of the transmitter directivity A signal processing technique which only requires a single hydrophone or vertical receive array is a very attractive concept especially fo
114. plications The microprocessor controlled multi channel synthesis of signal data has been demonstrated to be a powerful tool generating a very wide range of signal types for projection into the water by the phased array A variety of transmission modes have been tested and all of the original concepts have been successfully demonstrated i e within pulse sweeps ripplefire and electronically steered pings Additional concepts of focussing and sidelobe shading Dolph Chebychev have also been tested and the measured signal parameters in the water have matched the theoretical predictions in almost all of the experiments The beam forming and steering characteristics established using the Mk 2 array have been remarkable close to theory 85 7 1 The Advantages of a Multi Mode System Phased array transmitter techniques have been demonstrated to give a variety of advantages The apparent complexity and cost of a multichannel system is an economic disincentive but outweighed by the resulting increased operational facilities The inertia free steering of the beam direction is clearly superior to mechanical steering techniques where rapid steered response or swept sector characteristics are required The high directivity ensures that all targets are insonified efficiently with the maximum possible intensity as the beam is steered across the sector This mode of insonification also ensures optimum target detection in conditions of high reverbe
115. ppendix III Notes on the operation of RIPPLE60 Enter the number of cycles of carrier to be stored in a 256 sample block Note non integer number required to produce a specific frequency implies that the block length must be abreviated from the basic 256 samples to ensure that an integer number of cycles fit precisely to permit the data to be re circulated The program outputs the resulting carrier frequency in kHz to the screen before continuing with data calculation The data is generated as odd even channel pairs and the program mul tiplexes these two 4 bit data streams into a single 8 bit file which it saves to disc as a block The 8 multiplexed blocks stored on the disk are file named automatically The loading address for transmission is encoded within the file name by the 3rd character i e mc8str9 is a data file focussed at 9 metres to be loaded into the 8th position in the transmission stack This middle posi tion would normally be loaded with the zero phase shifted data to transmit on the centre axis 52 Flow diagram RIPPLE60 START JE RIPPLEGO MENU Enter No of Carrier Cucies Blk FN 16 Display Frequency Calculate next bearing 16 channels multiplex Odd Even Construct Filename Save Data on Disk FN FN 1 53 Micro Computer 4 4 Program Example Data Handling and Control STEERd This is a transmitter control program for the Ripple Steer Ping steered modes with the
116. qual amplitude If the same phase is applied to all elements the relative phase difference between adjacent elements is zero and the position of the main beam will be broadside to the array at an angle 0 The main beam will point in a direction other than broadside if the relative phase difference between the elements V is other than zero The direction of the main beam is at an angle 99 when the phase difference is yw 2 n d A sin 0 The phase at each element is therefore Vc py where u 0 1 2 N 1 and We is any constant phase applied to all elements The normalised radiation pattern of the array when the phase difference between adjacent elements is Y is given by Go sin N z d A sin 6 sin 80 NZ sin 1 d A sin 6 sin 00 The maximum of the radiation pattern occurs when sin 0 sin 09 The Mk 1 multi mode system was constructed with the exploration of phased array techniques as one of the prime objectives This equipment ini tially used a single line array comprising 15 sandwich transducers equally pitched at 40 mm spacing with a centre frequency of 39 KHz The use of a one lambda pitch array can be shown to produce not only the required broad side response but an additional significant endfire mode Utilising these array dimensions computer modelling techniques were employed to establish the theoretical directivity pattern and to predict the modified response produced by the circula
117. r one channel only By observing the transmission on a hydrophone as the data buffers are ripplefired a pulse constructed from the sequential output each of the 16 channels can be examined For practical pur poses the observing conditions remain constant and direct amplitude com parisons can be made fig 2 8 1 A missing or mismatched channel is im mediately obvious and the appropriate matching adjustment simplified fig 2 8 2 By first pre setting the power level in one channel using a dummy load to create a reference channel the remainder can all be matched to establish the total power transmitted A multi burst frequency test is constructed by storing different carrier frequencies in each stack location fig 2 8 3 All amplifiers contribute and the transmissions beamform on axis The multiburst mode generates a chirp of discrete frequencies and can be used to measure the complete system response This mode also has a potential application as a wideband pulse source for examining spectral response of a target A multi frequency chirp used with a matching correlator could aid the evaluation of pulse compres sion methods of enhancing signal to noise performance The possible exploita 22 Fig 2 8 1 Test Signal Channel Alignment Each amplifier channel is transmitted sequentially to permit the relative amplitudes to be compared within one transmission Mismatched or missing channels are immediately detectable Fig 2 8
118. r the reception of low frequency parametric signal echoes For transmitter scanning alone to identify target bearings the transmission must be encoded Sophisticated orthogonal codes combinations for transmission have been proposed by Ross However a simple practical set of orthogonal codes can use frequency modulation provided that adequate bandwidth can be exploited in the transmission to permit separation of the bearing information on receive to be achieved by narrow band filtering The design of a totally digital receiver utilising a TMS320 10 Digital Signal Processor to perform real time digital filtering has been the subject of parallel LUTEE research work A number of alternative bearing coding methods are possible with the increased band widths available from NLA transmissions e g The use of phase rever sal techniques or non ambiguous pseudo random sequences of frequency blocks Quite complex code sequences can be implemented using the Multi mode Transmitter However suitable multichannel correlation techniques will need developing before they can be exploited by a practical receiver A unusual method of encoding bearing information into the transmis sion at the carrier frequency which illustrates the adaptability of the transmit ter is shown in fig s 2 7 1 2 3 This mode is derived from a multiple sweep 20 Fig 2 7 1 Multiple Sweep Coding Echoes display bearing information as a centre pulse position displacement within t
119. r transducer piston heads fig s 2 0 2 amp 3 The com puted patterns demonstrate that the unwanted endfire component is reduced significantly in practice by a shading factor defined by the transducer ele ments finite dimensions 0 95 A diameter In freshwater assuming c 1460 m s the model predicts that the endfire grating lobe will be shifted from 90 degrees towards 75 degrees off axis when all the elements are fed with the same phase 1 04 A inter element pitch The effect was observed to occur during the reservoir trials Computer modelling also demonstrates that a lambda pitch array can be phase steered through an angle of one radian At deflected angles of plus or minus a half radian from the centre axis the dif fraction secondary lobe will be equal in amplitude to the steered main lobe and this defines the useable scanned sector This grating lobe can be shown to be generated from the phase shifted endfire response At this maximum steered angle the symmetrical dual lobes will generate ambiguous echo responses from targets at either edge of the sector Since the array response Fig 2 0 2 a A 15 element point source array with a 1 A inter element pitch 15 ELEMENTS 1 LAMBDA PITCH ARRAY 1 b Directivity pattern of a circular piston transducer 0 95 A diameter ui a Las n T a c Response of a 15 element transducer modified by the individual element responses Fig 2 0 3
120. ra tion programs including RIPPLE60 Prepares Ripple Steer data as described above SWEEPg3 This is a similar program which requests a sweep time and computes a set of data files which contain continuously changing bearing data These files when rippled together transmit a smooth within pulse sweep signal at a pre determined scanning rate The maximum pulse length sweep time is limited to less than 8 milliseconds by the current configura tion of the hardware CREATEcp Generates unsteered data files for a set of carrier fre quencies for a chirp or multiburst test signal MODIFILE This is a general utility program which interacts direct ly with the steered data files created on the disk by the data preparation programs e g STEERg3 The program processes each file in turn to produce square wave modulated signals for NLA signal generation The modified data is then re filed back onto the disk with a new constructed filename ie mc8S4mod where the 5th character refers to the modulation frequency The modulated data files can be constructed for 2 3 4 5 or 6 kHz and as these are super sets of the original steered unmodulated data they also generate steered transmissions across the sector Data required for NLA experimental transmis sions DISPLAYf This program accesses a named data file stored on a floppy disc and displays for comparison the first and last 2O samples in each 58 data block The display exploits an alternative
121. ram text file as code modules ready for insertion during compila tion using the Pascal External and Code function calls A graphics extended version of Interpreted BASIC was used to com pute some of the some of the array simulations The BBC microcomputer proving a useful tool where a graphic display of output was required BBC BASIC and routines written in the BBC inline 6502 Assembler were employed to control the pan and tilt beam plotting hardware As the plotter program evolved its size outgrew the limited memory of the machine and necessitated breaking it into several smaller programs which are chained as required from a menu The Extended Crystal BASIC available on the Nas com II offers similar graphics advantages but as with all interpreted lan guages the program size in memory becomes a problem with very large 48 programs Speed of computation became a secondary consideration after the design approach changed from on line synthesis to pre computed data Achieving the maximum speed of data transfer from the floppy disk was regarded as important as the data file transfer time creates a significant delay when changing between modes Both Interpreted BASIC and the Z 80 As sembler language were seen to be restrictive in this application The available Pascal language subset was preferred for the system control and data prepara tion programs The version of Pascal selected as the high level language for this project is a der
122. ration i e in shallow water or under ice particular ly when synchronised with a matched scanning receiver Transmissions from a multichannel system incorporate a fail soft characteristic and single channel failures are rarely catastrophic in effect The system has proved to be reliable in use and has been operated in environmen tal extremes i e with air temperatures below freezing and above 33 Celcius The use of a number of small power amplifiers to drive individual staves of the transducer array enables ideal channel amplitude matching to be achieved resulting in optimised transmitted beam patterns Very even in sonification of a swept sector can be achieved with less energy wasted in sidelobes outside the designated sector The technique of precomputing and storing the waveform for transmis sion can be applied over a very wide range of frequencies by optimising the sampling clock rate The maximum practical frequency that can be generated depends on appropriate D A conversion techniques and requires fast RAM memory devices in the data buffers The existing signal synthesis hardware can generate sinusoidal signals to over 200 kHz with very minor circuit chan ges For more dedicated applications especially if fast bi polar PROMs are used as buffers signal synthesis for phased array applications could be ex tended to over 1 MHz The multi mode transmitter in its present form has been developed as a research tool for generating precis
123. response for a 20 1 sidelobe ratio is shown in fig 6 1 2 Using the Mk 1 system with its 15 element array an experimental in vestigation of a Dolph Chebychev shaded array was undertaken with rather disappointing results fig 6 1 4 However the unshaded beam pattern from 74 Fig 6 1 1 Theoretical Beam Pattern of a 15 element 1 A array Fig 6 1 2 Theoretical Beam Pattern of the Dolph Chebychev 20 1 Shaded Response Fig 6 1 3 Unshaded Beam Pattern Plotted using the Mk 1 array 38 kHz Fig 6 1 4 Modified Beam Pattern obtained using Dolph Chebychev shading parameters 38 kHz VH 20 8 28 ry mmm ie cM ee es i gt H 1 t 1 r i Beanwidth i 3 7 Deg Linear i i Plot I l TST i earns eg Se 28 16 12 4 8 4 8 12 16 28 Degrees A Seanvidth 4 6 beg f Linear i Plot 848 3d8 EITE PERRA an ERE n 4 ILS AER on 2 P AA P G 28 I6 142 8 4 8 4 8 12 16 28 Degrees 75 Fig 6 1 5 Theoretical 15 element 1 response modelled with Mk 1 array phase errors added 28 e Fig 6 1 6 Theoret
124. resses from the RAM scratch pad versions they can be utilised without the mode change delays involved in disk data transfer Currently this ROM technique is employed to hold the channel comparison test signals which are required for alignment and fault diagnostics 11 Fig 2 1 1 UNSTEERED DATA Tu Beam forms on the 1 ae array axis T a e B NP Display of channel Lo aae phasing created by oo 9 ae program DISPLAYf 5g eee First and last stored ee cycles in the 256 byte 8 i buffer M Note Data is normally ee output from right to left ee rg 14 _ fy m 3 e _ Fig 2 1 2 STEERED DATA Beam forms 2 off the centre axis JJ y qid Fig 2 1 3 STEERED DATA Beam forms 4 off the centre axis 12 The test program DISPLAYf accesses the precomputed stored data to enable a visual display of the 16 relative phases using the first and last few cycles in each block of the output memory buffer figs 2 1 1 2 3 Note the program displays the data transmission starting on the right FF and proceeding to the left 00 These three examples demonstrate the progres sive phase shifts needed to phase steer the sonar beam off axis in 2 degree in crements i e these are from a data set restricted to
125. rystal ARE Portland and has an acceptably flat response to well above 80 kHz Other devices included lower frequency ball hydrophones D1 4O D1 7O 101 80 Universal Sonar a precision PVdF plate hydrophone from EMI 4 and an experimental PVdF Vibetec device Raychem Ltd Use of a single stave of the Mk 2 array as a directional receive hydrophone also gave very good results in the reservoir trials The hydrophones used in the Lochgoil experiments included B amp K hydrophones BK8100 several experimental F E A active hydrophones a vertically deployed line array and a vertical 1O x 2 array of ORE 3 KHz low frequency elements The later is now being used in the parametric scanned sonar experi 68 elements The later is now being used in the parametric scanned sonar experi ments as it provides some useable receive directivity in the vertical plane for the low frequencies of interest For most of the initial trials the signals recovered from the hydrophones were at very high amplitudes and no buffering or amplification was required for oscilloscope display or for beam plotting Low noise battery powered preamplifiers were required in the reser voir trials when examining echoes from target spheres at 300 metres and also when plotting the far shoreline at 1 kilometer Several unsophisticated preamplifiers were constructed for these trials with the design emphasis placed on low noise and low distortion performance As the performance of m
126. sector to be scanned 30 60 degrees 54 START JU Boot a aL Flow Diagram of STEERd Exec PROGRAM mcSTEERd Link to DOS Ruto Load Files into Buffers 16 RESTARTe Preset Menu Change Preset Parameters Yes C NEHPRRRM Mode R S P Start Addr No of 8 Pause Time READY to TX Press any Ke Press any Key to Stop New Flile estart uit CR L RJ t DEFAULT Setup Cord B pd c CQ DATA IN Menu Set Sector Anale 59 30 Mod UnMod Fred No 0 55 i AA pg fi 1 0 ut j N END Set System Defaults PROGRAM mcSTEERF RIPPLE STE P ER ING 68 38 SECTOR Micro Computer Controller amp n A D By LLL q 16 Buffers 16 A Ds Outputs Ampiifi Array TO ers amp Select modulation frequency O to 6 kHz O Unmodulated 41 kHz data steered across selected sector 2 6 kHz square wave modulated data for steered NLA Operation Compute file name and load sequentially from disk 8 files to each set loaded in 33 seconds Display preset parameters Choice Change parameters Y N N Ready to transmit Pause for key press Y Display Parameter menu Change Mode Ripple Steer Ping sector Block Address Hexadecimal Start of Block address Pulse length Number of block repeats Pause period Select from menu 1 m
127. sed to define the subsequent transmission The design of a sonar transmitter with controlled inertia free beam steering and flexible waveform synthesis is a step towards this objec tive This project implements an azimuth steered phased array sonar trans mitter based on micro computer techniques and evaluates the system perfor mance in a variety of operational modes 1 1 Transmitter Driver Technologies Many technologies have been employed to generate high acoustic source levels in the water to insonify potential targets ranging from explosive charges electro magnetic boomer s capacitive discharge sparkers to the use of compressed air driven mechanisms However for the majority of sonar applications systems which drive a piezo electric or magneto strictive transducers have remained the most adaptable Electrical signal synthesis techniques for these transducers have evolved from the long lived tone wheel interrupters used in the World War II type 144 ASDIC to the electronic oscillator and power amplifier which displaced them by the mid 1950 s Thermionic valve power amplifier designs must now be regarded as obsolete The bi polar transistor which finally replaced the valve for power amplification applications now competes with a range of power MOSFET devices which already tbreaten to supercede them for many applications Most modern sonar transmitters are built using transistor switching techni ques which offer excellent power e
128. ses a stacked set of the steered data described in 6 2 The name Ripplefire was taken from naval gunnery ter minology as the similarity to that operation illustrates how sequential trans missions on each bearing join to appear as one contiguous pulse The sector is scanned in discrete bearing increments as a beam plot of the combination transmission demonstrates fig 6 3 1 The pulse length transmitted on each bearing can be programmed to be of any length however the total pulse length is the sum of all the in dividual bearing components and a timing slew across the sector is un avoidable The technique can be altered to introduce the listening period be tween each bearing transmission simulating a ping and listen mode which Fig 6 3 1 Linear Beam plot of a Ripplefire transmission n BRE Dansa avaa vene tadaa Linear 1 i Plot ae tea 9692944060 v 4940 4997499 esl as laeso coco aon 24 18 12 6 8 amp 12 18 24 38 Degrees a walks across the sector The bearing data can also be stacked in any order for transmission should it be desired to randomise the scanning sequence The second method generates a smooth sector scan by continuously changing the phases between each channel i e each channel is driven with a different frequency the sweep rat
129. steer across a 30 degree sector The beam width formed by the array is 4 degrees so this data set will over illuminate the sector Examples of actual beam patterns plotted from these phase steered transmissions are included in chapter 6 2 4 Ripple fire This application uses the full set of steered bearing data blocks with each bearing selected sequentially and transmitted as part of a contiguous bearing pulse The sonar beam forms at one end of the sector and ripples across the sector in discrete steps The technique is efficient in insonifying the sector as the full intensity of the narrow formed beam falls on each part in turn The bearing steps within the transmission must be a compromise be tween overlapping patterns to avoid missing targets and the total pulse dura tion An unavoidable timing skew across the sector occurs as a result of the contiguous transmission fig 2 2 1 An example of the composite beampat tern that results from a ripple fire transmission is plotted in chapter 6 The technique offers some operational advantages in reverberant conditions as once a target is localised the insonified sector can be narrowed by switching to alternative sector data held in adjacent parts of the buffer Ripple sector data for 60 30 and 15 degree sectors have been generated and used zi Swept Within Pulse Transmissions The swept beam is a simple extension of the ripple fire mode The beam forms at one edge of the sector an
130. the power amplifiers but fortunately no failures This unexpected problem was apparently caused by fatigue failures in the outer polyurethane jacket of the cables after the array had been deployed in the sea for several months at 30 metres depth A con sultants report on the cable failure indicated a manufacturing defect possibly aggravated by the high static pressures involved The heavy duty pan and tilt training gear suffered a similar sheath failure fortunately the low voltages involved have enabled this unit to continue to function but the cable and penetrator need replacing The training gear tilt axis flange shaft mounting was found to work loose causing some undesirable backlash after a few days operational use The design of the flange fixing was apparently at fault and simple retightening together with the application of screw locking compounds did not provide a long term cure A position indicating potentiometer also 88 failed within this unit and to cure both problems quickly required the pan and tilt head to be returned for a short period to the manufacturer Apart from some early computer interface problems which required modifications to the stepper motor control circuits the pan and tilt assembly has proved to be reliable The integration of the precision training gear with a microcom puter has produced a beam plotting facility that has been most valuable in as sessing the performance of the multi mode transmitter A second versi
131. thesis RIPPLE60 This program generates the data required to synthesise sonar signals at a specified carrier frequency and prepares a set of 16 data blocks each defining a phase steered increment across the sector The program source text can be modified easily before compilation if a narrower sector is required to be scanned The stacked set of data for all 16 bearings are stored with constructed filenames onto a floppy disk Each file contains the data for one 256 byte block in all 8 RAM card buffers i e the data required to steer a transmission onto a single bearing The third character in the filename encodes the posi tion of the block within the stack In the transmitter the set of files are called for in turn by a matching control program STEERd and the filename en sures that the data is automatically loaded at the correct stack address on each output buffer memory card The data is computed as 4 bit resolution 500 kHz sampled data to fill a 256 byte sample buffer Channel paired 4 bit signals are multiplexed as an 8 bit buffer block for efficient file storage and speed of data transfer This program prepares data off line quite slowly and although its operation could be speeded by the use of a look up sine table this was not found to be necessary The program requires no operator intervention once started and will generate a complete data set on the floppy disk automatically 51 Flow Diagram RIPPLEOO0 Source Text listed in A
132. thesis in real time of each channel s data could eliminate this problem and given the steady improvement in speed and computing power of each new generation of microprocessors it is anticipated that practical systems using DSP devices and RISC architectures will make this possible An interim solution which im proves the existing multi mode transmitter has been designed which increases the size of the memory buffers This uses 8 bit resolution to define the waveform amplitude and can exploit much longer buffer lengths This solu tion has the side effect of dramatically slowing the data loading time and the data files will need to be held on a hard disk to minimise the loading time penalties A single 8 bit version of an enlarged buffer memory has already been built which increases the channel data capacity from 32 x 4 bits to 64 kbytes and the initial tests with this prototype card driving all the amplifiers in parallel was successfully used to generate a sequence of amplitude modu lated raised cosine pulses 7 3 System Reliability The final version of the high power system has proved to be quite reli able the only significant down time was caused by a failure in the polyurethane sheathing of both transducer umbilical cables This resulted in the cables flooding with seawater and the immediate loss of one channel which shorted out The cable capacitance of the remaining wires increased dramatically which in turn caused some overload to
133. tical when required to generate signals for a multi channel operation as the necessary multiplex ing divides the data transfer rate by the number of channels Multichannel synthesis using this approach is therefore limited to very low frequencies un less the pre computed data samples can be accessed in parallel Dedicating a microprocessor to control the signal generation for each channel under the control of a host processor is one possible solution to this n alternative ap proach has been used in this equipment which overcomes the speed problem successfully by extracting the lookup tables from the computer memory and creating an auxiliary RAM memory buffer for each channel Simultaneous 27 parallel access to these buffers is made under the control of a hardwired con troller which is programable The synthesis and transfer of the computed waveform data can now be removed from the transmission loop timing and as a result the speed restriction is imposed entirely by memory speed of ac cess and the associated D A settling times With this approach data rates of several Megahertz become possible without requiring exotic components High data rates may require larger memory buffers as the size is dictated by the length of the transmission and the signal sampling rate required to achieve wideband performance with good phase resolution If the required multichannel signals differ only in phase then significant savings in memory size can result by re
134. tio 1 bit 4 not used bit 5 STEER RIPPLE SSP bit 6 RIPPLE STEER1I SSP if Pm P then begin pre 8 MODE SECTOR SCAN end if P R then begin 8104 s MODE RIPPLE FIRE end If Pm S then begin pre 48 MODE concat STEERED Fq end outCB 885 DELAY out lt A pre DELAY end Procedure PASS Circulate Modulation Block begin writelnj write Block repeats Tx Pulse 1 2595 6 9 screen 4i readin reps tf reps 255 then reps 255 If reps lt i then repss 1 DELAY outCA reps DELAY 29 8 end Procedure WAIT CPLENGTH delay looKup table 3 begin CASE pause OF i pcir 30 Sipcim 35 18 1pciw S4 SO pcirz 0D 180 lt 24281 200 pcim 265 300tpcixc 2E 400 pc12 27 60 ipcim 2D 60 pcir 2B 1000 1pc iw 18 2009 165 3000 pc 1 48001 pc 159 9S080 pc im 1D 56086 pc i 1B 106080 1 pc 17 1C OTHERS tpc ends end Procedure PLENGTH Pause between transmissions begin writelnty 0 writeln Pause times available are wri teln 1 5 10 50 108 2060 300 400 500 4600 msec writelnt and 1 2 3 4 9 6 10 seconds writeln wri tet Enter Pause Length in milliSecs 91 REPEAT screen 40 0 1 read pause j WAIT UNTIL pc 0 writeln OutCB 40 DELAY j ou t A pc i DELAY end ore SELECT Page select to load output RAM card
135. ttle alternative to the use of relatively deep open water conditions Still water of reasonable depth can be found in lakes and reser voirs weather permitting To utilise such places as laboratories for acoustic measurement work also requires a sheltered work room equipped with an ade quate electricity supply and a stable transducer support designed to assist in the safe deployment of equipment in the water Most of these demanding re quirements were met for the initial low power trials by the ARE T facility on the King George VI reservoir near Staines The need to establish a test facility close to the University has led to very successful cooperation with the Severn Trent Water Authority who permit us access to the draw off tower on their reservoir at Foremark in South Derbyshire The full power testing of the Mk 2 system required much greater ran ges and deeper water than possible at Foremark and as a result the equip ment was redeployed in a Scottish sea loch on board an ARE LG floating test facility known as Maytime This facility is well equipped for acoustic measurement work and staffed by a technical support group All of these acoustic range facilities have required some specialised array support and han dling equipment The basic parameters of beam width steered angles sidelobe amplitude etc have been measured with the aid of a computerised beam plotting system developed for this project 1 60 5 1 Loughborough University A
136. two T slots milled on one side to assist in mounting the array to an underwater training gear fig 3 5 2 Early trials with this array were encouraging The test mode signals enable the direct comparison of the individual transmitted channel amplitudes received on a test hydrophone If the receiving hydrophone is deployed on axis and in the far field of the array then after trimming the transmitted gains for equal received amplitudes the resulting combined signal beam pat tern forms a good match to the computed simulation This amplitude match ing cannot compensate for the significant phasing errors that were inherent in this array s manufacture But the beam patterns especially in view of the known poor transducer matching are quite acceptable These high Q ele ments were found to have an inadequate bandwidth for the very rapid swept signal modes The element performance was not found to be a limit ing factor for the phase delay steered transmissions and steered and rip pled modes were successfully demonstrated as were slower rate 5 mil lisecond sweeps During the first trials at Staines reservoir this array was used to demonstrate that basic phased array concepts were practical and that steering and sweeping of the directivity lobe through the 60 degree sector was possible During later trials with the array driven with the maximum available power it was established that significant waveform non linearities
137. uency generation provided that the frequency of the waveform data is the same for each channel Beam steering becomes possible by rotat ing the waveform data within the block by an appropriate number of clock cycles to generate the progressive phase shifts required to beam steer in a specific direction Long pulses are constructed from up to 256 repeated block lengths 131 millisecs The system was originally intended to synthesise a maximum pulse length of 100 milliseconds This economical use of buffer memory permits the stacking of 16 sets of bearing data in a 1000 Hex stack The discrete bearings stored in in dividual blocks within this stack can then be re addressed instantly ena bling very flexible beam switching to interrogate selected parts of a 60 de gree sector For pulses longer than 131 milliseconds the contents of the stack ed memory can be transmitted contiguously i e as each block of data com pletes its programmed recirculation the transmission continues using the next block in the stack until all the blocks have been utilised This technique provides for either a very long pulse of 2 seconds maximum duration 16 x 131 millisecs or for unique data that cannot be recirculated a maximum pulse length of 8 milliseconds Specific data for regular usage e g the steered channel data can op tionally be stored in EPROMs for dedicated applications and as these fixed buffers can be located at different memory add
138. undary CAI EH E E E HE E PE HE E E E E E E TTI RATES label RESTART2 const A 4 Be5 cA 46 cB 7 Var pulse IREAL pre SAM c h k bsel Psn pc reps t INTEGER N I K TXN BLK pause s INTEGER CARDSEL 6LSB 0158 TXNS dec s INTEGER P U D Hx3 Hx2 Hx1 Hx hexno S STRINGL1 Char3 FF SF STRINGI 11 DIR 1S8TRINGL 41 MODE STRINGI 131 ti tle 1STRINGE481 FILE Fq tSTRING 8 3 Bad 1 BOOLEAN Procedure Loadmc Var Bad BooleanjFILEsString 8 External C04B Procedure Savemc Var BiBoolean S F EtInteger FiStringf 8 Externa CO4E Procedure CLS Code 3 C F7 3 Procedure DOS CODE C3 08 6D8 CJump to Operating System Procedure Drive Irinteger gt sExternal C9048 Procedure PAUSE begin repeat KiwKeyboard until K458 end Procedure HEXCON begin hszord hexnod 48 if ho then hi h 7 end Procedure CONHEX const HEXSTRING 81234546789 BCDEF begin hizh ijhexnot mi dCHEXSTRING h 1 ends Procedure DECHEX T begin call with value in dec 2 hr edec DIV 16 CONHEX writeChexno j hizdec MOD 16 CONHEX wreiteChexno end Procedure FRGa begin Drive did h s K CONHEX FILE i concat amp mc hexno S FF mod gt end 30 degree sector data FRQb Constructs required Filename for bearing and frequency Drive CONHEX j 3 h FILE i concat mc hexno 5 FF MOD gt end 60 degree sector data Procedure INITPORT begi
139. ware were used successfully with both pan and tilt systems and only very recently has a re written version taken over Jt was noted during a laboratory test of the Mk 2 heavy duty pan and tilt using a laser that an error in scaling of 08 existed in the positioning accuracy This problem existed as the software written in BBC assembler code used in teger arithmetic and the low ratio gear box in this second unit included a non integer division factor Additionally the original disk storage technique 11 ii retained only the plotted screen positions and not the actual amplitude data This technique seriously limited the re displayed dynamic range when a logarithmic plot was required and introduced an undesirable artificial quantisa tion noise floor below 45 dB A re written program in Basic has eliminated both problems although fewer data files can now be stored on a single floppy disk Only the stepper motor control signals have been retained in assembler language for efficiency and these routines were changed to assist the equip ment to decelerate as it approaches each sampling point 3 Sampling Electronics The analogue data acquisition required additional circuitry before the digital samples can be obtained The interface box between the sampling hydrophone and the computer contains adjustable gating delays triggered from the sonar transmitter These delays permit an variable width range gate to be applied to the hydrophone signal and ar

A multi-mode sonar transmitter

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