WBK18 - Measurement Computing
Contents
1. contact the factory as this indicates a problem may exist with your WBK18 module Each WBK18 has one user replaceable fuse designated as F900 F900 Input Power Fuse 4 0 A MINI ATO This 4 amp fuse is located near the rear panel s SDIN POWER IN connector If this fuse has blown the WBK18 module will not power up Factory Part Number FU 8 4 Littelfuse Part Number 297 004 Littelfuse Body Color Code Pink You will need the following e Phillips Screwdriver e 3AG Fuse Puller or needle nose pliers e Grounding wrist strap and associated anti static pad e Replacement fuse F900 Needle nose pliers can be used to pull MINI ATO fuses but should not be used to insert fuses WBK18 Dynamic Signal Input Module 926896 WBK18 pg 21 1 Ifyou have not already done so turn OFF the power to and UNPLUG the WBK18 module and all connected equipment Remove all signal I O lines from the unit 2 Using a Phillips screwdriver remove the four Cover Plate Screws from the left and right sides of the unit The following figure points out the screw locations 3 Remove the Cover Plate not shown Cover Plate Screw Locations Cover Plate Screw Locations F900 F900 Fuse Location in a WBK18 Module 4 Locate the F900 fuse It 1s located near the rear panel edge close to the DINS POWER IN connector 5 While wearing a grounded wrist strap remove and replace the bad fuse Ensure that the new fuse is fully seated 6 Replac2. Bypass this parameter is fixed at 200000 Hz Reference Note Application information regarding the proper setting of LPF Mode and LPF Cutoff is provided in the sections entitled Maximizing Alias Protection page 9 and Using the 2 Pole Filter and Bypass page 11 HPF Cutoff This is used to select input coupling Click on a cell in the HPF Cutoff column to make the cell active and then change its setting Options for HPF Cutoff are a 0 1 Hz selects the 0 1 Hz 1 pole high pass filter b 10 Hz selects the 10 Hz 2 pole high pass filter c DC selects DC coupling Note that when the Range is 25V the HPF Cutoff is fixed at DC Range This is used to select the input range Click on a cell in the Range column to make the cell active and then change its setting Options for Range are 25V 5V 2 5V 1V 500mV 250mV 100mV 50mV and 25mV Note that when the Range is 25V the HPF Cutoff is fixed at DC WBK18 Dynamic Signal Input Module 926896 WBK18 pg 7 Source Level Source Level refers to the transducer bias current and is not to be confused with the excitation source that is discussed on pages 4 and 14 The current Source Level column of WaveView s main window is used to turn the transducer bias current on at a current value of 4 mA or to turn the transducer bias current off Click on a cell in the Source Level column to make the cell active and then change its setting to 4 mA or to off according
3. The current source features an operating compliance of 24V and is short circuit and overvoltage protected Operating compliance refers to the highest voltage that can be applied without change of the current source value In the absence of a transducer the current source will output an open circuit voltage of 30V For applications that do not require bias the current source can be disconnected from the input via software control on a per channel basis When the current source is enabled the input voltage is continuously monitored with level detection circuitry Recognition of a voltage greater than 25V transducer open or less than 1V transducer short triggers a transducer fault condition for the affected channel This error is communicated to the user via a front panel LED and is also available through a software status request at the end of an acquisition When recognized an error is latched until the commencement of a new acquisition Consequently even intermittent faults are detected and communicated Detection of a fault does not however alter the acquisition process or its data Input Coupling Each WBK18 channel offers the selection of three coupling modes 0 1 Hz 10 Hz or DC The 0 1 Hz path is a 1 pole high pass filter with a 3dB point at 0 1 Hz The 10 Hz path is a 2 pole high pass filter with a 3dB point at 10 Hz The DC path provides a direct signal connection Programmable Gain Amplifier PGA The WBK18 provides progra
4. hardware settings All WBK18 configurations are controlled by software Reference Note Setup information pertaining to power expansion control and expansion signal connections is contained in the chapter System Setup and Power Options in the WaveBook User s Manual It is possible to connect a WaveBook to the host computer s parallel port with either a 2 foot CA 35 2 or 6 foot CA 35 6 communication cable To minimize the amount of noise that is introduced to the WBK18 use of the 2 foot CA 35 2 cable with the WaveBook is recommended The WBK18 module can be powered by an AC power adapter or directly from any 10 to 30 VDC source such as a 12 V car battery For portable or field applications the WBK18 and the WaveBook can be powered by the DBK30A rechargeable battery module Reference Note For details regarding power refer to the chapter System Setup and Power Options in the WaveBook User s Manual As described in this referenced chapter it is possible to power the WBK18 from the POWER OUT DINS connector on the WaveBook and to power other WBK expansion modules from the POWER OUT DINS connector on the WBK18 The following notes apply to those types of power connections Tables for determining amp load are provided in the WaveBook User s Manual chapter entitled System Setup and Power Options The following factors are very important e Calculate system amp load prior to creating a system daisy chain Although WaveBo
5. pg 13 WBK18 pg 14 Excitation Source In order to configure the excitation source click on the Configure Sine Wave Output button in the Module Configuration window This brings up a secondary window in which the excitation source is configured Do not confuse excitation source with the Source Level column in WaveView s main window Continuous Mode Continuous mode refers to a continuously running sine wave of the selected amplitude and frequency To configure select a frequency and amplitude and then click Start The selected output will begin and continue running until the lt Stop gt button is clicked or WaveView is terminated Note You can save output sine wave configuration files and open pre saved files as discussed in the upcoming section How to Save or Open Output Sine Wave Configuration Files page 17 Sweep Mode Sweep mode refers to a constant amplitude sine wave that is being swept in frequency from a selected start frequency to a selected stop frequency over a selected sweep time duration The frequency sweep characteristic can be chosen to be linear or logarithmic and is distributed among 1280 discrete steps The transitions between steps are continuous in phase and in amplitude To configure make selections for the available parameters and then click the lt Start gt button Once a sweep is completed the waveform returns to its start frequency and is Swept again and again until stopped by the user
6. to the application If a channel is connected to a transducer that requires a current source set the source level to 4 mA otherwise set the source level to off TEDS Transducer Electronic Data Sheet TEDS is a purchased option that enables WBK18 modules to access calibration information from TEDS compatible sensors The WBK18 automatically scales the readings and sets the range according to the information stored on the sensor This is done independently for each TEDS associated channel providing that Yes appears in the TEDS Data cell right hand column following figure 38 WaveView WAVEVIEW CFG Bel x File Edit View System wer E E 19 p PAN Eh Pa 0 1 Channel Configuration z 0 MoN Use TEDS Data sE Label Readings Units Range ae al Mone LPF Cutoff a ee ae Wee CtrLo CtrHi Dig counts 0 7 WaveBook 516 Off CHO1 Yy 10 to 10 No 0 Bypass 20000 00 D 2 WaveBook 516 Off CHO2 Yy 10 to 10 No 0 Bypass 20000 00 0 3 WaveBook 516 Off CHOS Yy 10 to 10 No 0 Bypass 20000 00 0 4 WaveBook 516 Off CHO4 y 10 to 10 No 0 Bypass 20000 00 0 5 WaveBook 516 Off CHO5 Y 10 to 10 No 0 Bypass 20000 00 0 6 WaveBook 516 Off CHO6 Yy 10 to 10 No 0 Bypass 20000 00 0 7 WaveBook 516 Off CHO y 10 to 10 No 0 Bypass 20000 00 0 8 WaveBook 516 Off CHOS Yy 10 to 10 No 0 Bypass 20000 00 St weKi8 On CHO3 0 g 51 04 to 51 04 No 0 2 Pole 200 00 DC Off HA wBK18 On CH10 0 toS No 0
7. 50 ms Example of Waveforms Created in the Custom Mode WBK18 pg 16 926896 WBK18 Dynamic Signal Input Module How to Save or Open Output Sine Wave Configuration Files Within all modes there is the ability to save and recall waveform configurations which is very beneficial for complex custom configurations Within WaveView this feature is accessed through File in the WBK18 Output Sine Wave Configuration Window To Save a Configuration 1 Click on File in the upper left corner of the screen 2 Click on the option labeled Save Waveform Configuration As 3 Name the file Note that it will have a dds extension 4 Save the file to the desired drive To Open a Configuration 1 Click on File in the upper left corner of the screen 2 Click on the option labeled Open Waveform Configuration 3 Open the desired file WBK18 Dynamic Signal Input Module 926896 WBK18 pg 17 Using Accelerometers Overview A low impedance piezoelectric accelerometer consists of a piezoelectric crystal and an electronic amplifier When stretched or compressed the two crystal surfaces develop a charge variation that is related to the amount of stress shock or vibration on the crystal The amplifier outputs a corresponding signal and transforms the sensor s high impedance to a lower output impedance of a few hundred ohms Note that in addition to acceleration these sensors can also measure pressure and force The ci
8. 8 Pole 5000 00 DC Off No 3 wBK18 Off CH11 g 25 52 to 25 52 No 0 Bypass 200000 00 DC Off Yes 4 weK18 Off CH12 y 5to5 No 0 Bypass 200000 00 DC Off 55 wBK18 Off CH13 g 5 1 to 5 1 No 0 Bypass 200000 00 DC Off Hee wBK18 Off CH14 y 5to5 No 0 Bypass 200000 00 DC Off 27 weK18 Off CH15 g 1 02 to 1 02 No 0 Bypass 200000 00 DC Off Yes 8 weK18 Off CH16 y 5to5 No 0 Bypass 200000 00 DC Off Li WaveView Configuration Window If no WBK18 channels are connected to a TEDS sensor the entire TEDS Data column including the heading will be grayed out When a channel Template Type IEEE 1451 4 template Template ID Accelerometer Manufacturer PCB does have a TEDS sensor it will have an associated cell You can click on the cell to access the Use TEDS Data pull down list which offers 3 Model Number 333 M O7 choices No Yes and Show Serial Number 16026 Sensitivity 97 9655 mV gq e No instructs WaveView not to use TEDS When TEDS is not used Fee ieee ae e UA RES the associated channel will use the default range and units of volts Soe PENE V Meas Pos ID O Phase Inv 0 Deg User Data 0 e Yes instructs WaveView to use TEDS The channel s ranges will be automatically scaled according to the TEDS sensor s calibration data and the units will appear as g e Show accesses an Accelerometer TEDS Information box for the Stor associated channel An example is provided to the right The cha
9. LPF Low Pass Filter is in the Bypass mode the bandwidth of the system depends on the coupling mode selected For the 0 1 Hz and 10 Hz selections the bandwidth is approximately 190 kHz For the DC selection the bandwidth is approximately 130 kHz For the 25 V range the bandwidth is approximately 120 kHz Reference Note Application information regarding the 2 pole and Bypass settings of the filter is provided in the section entitled Using the 2 Pole Filter and Bypass page 11 Overrange Detection Each WBK18 channel is equipped with overrange detection circuitry Use of this feature insures that all data collected during an acquisition did not exceed a user specified level set as a percentage of range In its most common use with the level set to 100 the user is notified if the input signal exceeded the input full scale range even momentarily This protection is critical for overrange signals result in clipped data that significantly corrupts FFT analysis This error is communicated to the user via a front panel LED and is also available through a software status request at the end of an acquisition When recognized an error is latched until the commencement of a new acquisition Consequently even intermittent faults are detected and communicated However an overrange event does not stop the acquisition process or change the data providing the user with full control over the disposition of data An extension of the overrange capability
10. WBK18 Dynamic Signal Conditioning Module Description 1 Software Setup 7 Current Source with Transducer Fault Detection 2 General 7 Input Coupling 2 Maximizing Alias Protection 9 Programmable Gain Amplifier PGA 2 Using the 2 Pole Filter and Bypass 11 Low Pass Anti Aliasing Filter 3 Module Configuration 13 Overrange Detection 3 Using Accelerometers 18 Simultaneous Sample and Hold 3 OVi 18 TEDS Support 4 Accelerometer Specification Excitation Source 4 Parameters 18 LEDs 4 Electrical Grounding 20 Hardware Setup 5 Cable Driving 20 Configuration 5 Fuse Replacement 22 Power 5 S ficati 24 Assembly 6 pecifications Input Connections 6 Description The WBK18 is a dynamic analog signal input module for the WaveBook data acquisition system The WBK18 provides a complete system to interface to piezoelectric transducers that include accelerometers microphones force pressure transducers and others WBK18 CHANNEL DYNAMIC 8 SIGNAL CONDITIONING MODULE WBK18 Front and Rear Panel Views Each WBK18 channel features a 4 mA current source for transducer biasing hardware detection of a transducer fault AC 0 1 Hz or 10 Hz or DC coupling a programmable gain amplifier range selection hardware overrange detection an anti aliasing low pass filter a simultaneous sam
11. ccelerometer s basic sensitivity or the data unless the thermal shift in the operation bias level results in clipping Where drastic thermal shifts are expected use 12 V bias models The effect s severity is related to the mass of the accelerometer In 100 mV g industrial units the effect is usually negligible Using rubber thermal boots can reduce the effect significantly WBK18 Dynamic Signal Input Module 926896 WBK18 pg 19 Overload Recovery Recovery time from clipping due to over ranging is typically less than 1 ms Recoveries from quasi static overloads that generate high DC bias shifts are controlled by the accelerometer input RC time constant that is fixed during manufacture Connector This parameter specifies the connector type and size 4 48 6 40 10 32 coaxial etc and the location on the sensor that is top or side usually on the hex base Where there is no connector on the sensor an integral cable is specified with the length and the connector that is integral 6 ft to 10 32 Electrical Grounding Case Grounded Design In case grounded designs the common lead on the internal impedance matching electronics 1s tied to the accelerometer case The accelerometer base stud assembly forms the signal common and electrically connects to the shell of the output connector Case grounded accelerometers are connected electrically to any conductive surface on which they are mounted When these units are used take care to avoid err
12. ce and overrange detection parameters that can be chosen from the Module Configuration window The Module Configuration window can be accessed from the View pull down menu or by use of the first toolbar button located just below the File pull down menu Reference Note For detailed WaveView information refer to the WaveView Document Module 15 x EE 4X 201 104 4 amp amp 0 1 Channel Configuration Current Source Level A men merr ft a TE M E e e E uto ero LPF HPF zi Module Type Ea Label Readings A Range er Clifset Made T LPF ont HE 1 WBK18 On CHOS 1 489 Y Fto 0 Bypass 200000 00 Dc 2 WwBK18 On CH10 5 ow to5 vE 0 Bypass 200000 00 DC Arn 3 WBK18 On CH11 I W to No 0 Bypass 200000 00 DC 4m 4 WaveView Configuration Window In the WaveView Configuration main window see figure the following columns are applicable to the WBK18 LPF Mode Click on a cell in the LPF Mode column to make the cell active and then change its setting Options for WBK18 s LPF Mode are a 8 Pole selects the 8 pole low pass filter b 2 Pole selects the 2 pole low pass filter c Bypass bypasses the low pass filter LPF Cutoff Click on a cell in the LPF Cutoff column to make the cell active and then change its setting Options for LPF Cutoff in Hz are 10 20 50 100 200 500 1000 2000 5000 10000 20000 and 50000 Note that when the LPF Mode is
13. could involve its integration into a process monitor application whereby the fault condition is used to monitor the stability of a previously characterized dynamic signal The overrange level is programmable from 1 to 100 of range on a per WBK 8 basis Overrange detection can be enabled or disabled on a per channel basis Simultaneous Sample and Hold All WBK18 channels are sampled simultaneously after which the WaveBook measures each output until all channels are digitized The time skew between sampling on all channels is 100ns regardless of the number of WBK 18s connected to the WaveBook This maximizes channel to channel phase matching When using WaveBook with an SSH channel enabled the per channel sample rates are reduced The rate reduction is the same as that which would occur if another channel were added The per channel rate with SSH enabled is 1 MHz n where n is the number of active channels WBK18 Dynamic Signal Input Module 926896 WBK18 pg 3 TEDS Support TEDS Transducer Electronic Data Sheet is a purchased option The TEDS feature enables a WBK18 module to access the calibration information that is stored within TEDS compatible sensors The WBK18 can read calibration information from sensors and automatically scale the readings from each sensor using the acquired calibration information TEDS support is a software option that can easily be added after the initial purchase of the WBK18 Reference Note Infor
14. e alias rejection it provides 06 039 The 8 pole filter provides excellent filter response However no filter is perfect meaning that some signal attenuation occurs for frequencies just 08 0 68 below Fc and maximum attenuation is not exhibited for frequencies just above Fc For reference the typical response of the 8 pole filter 1s provided in the table at the right and the graphs that follow WBK18 Dynamic Signal Input Module 926896 WBK18 pg 9 WBK18 pg 10 Gain of 8 pole Filter Mode 0 0 5 1 1 5 2 2 5 3 3 5 4 0 20 3 40 amp 60 80 100 FIN FC Gain of 8 pole Filter Mode 0 0 2 0 4 0 6 0 8 1 1 2 0 2 _ 4 6 c f 8 10 12 14 FIN FC Zoom in to Filter CHAF requency Region As described above aliasing results from the relationship between input frequency and sampling frequency Configuring the filter correctly serves to attenuate undesired frequencies However thought must also be given to the sampling rate of the A D converter In general for alias considerations the sampling rate should be set as high as possible given the number of active channels being used Recall that the maximum sampling rate is 1 MHz n 1 where n is the number of active channels Because the sampling rate determines the frequency at which aliasing occurs it determines the input signal bandwidth for a given level of alias rejection T
15. e the Cover Plate and secure it to the chassis with the 4 screws that were removed in step 2 7 Return the WBK18 module to normal service Should any problems be noted consult the factory WBK18 pg 22 926896 WBK18 Dynamic Signal Input Module Specif ications WBK18 Specifications are subject to change without notice General System Connectors BNC connector mates with expansion signal input on the WaveBook female DB15 connector mates with expansion control signal on the WaveBook Input Power Range 10 to 30 VDC Input Power Fuse F900 4A MINI ATO user replaceable refer to page 21 for details Power Consumption 12V 1 6A 15V 1 3A 24V 0 8A Environment Operating 0 to 50 C 0 to 95 RH non condensing Storage 0 to 70 C Vibration MIL STD 810E category 1 Dimensions 280 mm W x 216 mm D x 42 mm H 11 x 8 5 x 1 65 Weight 1 3 kg 2 9 Ibs Analog Inputs Channels 8 Signal Connection 1 BNC per channel Input Impedance 200k Ohm single ended Input Coupling AC DC software programmable per channel High Pass Filter 0 1 Hz or 10 Hz software programmable per channel Input Ranges 25V DC coupled only 5V 2 5V 1V 500mV 250mV 100mV 50mV 25mV software programmable per channel Overrange Detection Programmable from 1 to 100 of range Overrange Indication Front panel LED per channel software status Low Pass Filter software programmable per channel Type 8 pole Butter
16. ed on following page WBK18 pg 24 926896 WBK18 Dynamic Signal Input Module Typical Performance Characteristics WBK 18 Frequency Spectrums typical Each spectrum was taken from a WaveBook 516A that was sampling at 20kSamples sec 4096 point FFT B H Window 3 Vpp 1000Hz Sine wave input 5V range 10Hz High pass filter 10 kHz 8 pole Low pass filter 0 2000 4000 6000 8000 g 0000 dB normalized 8 Vpp 1000Hz Sine wave input 5V range 10Hz High pass filter 10 kHz 2 pole Low pass filter 2000 4000 6000 8000 10000 Hertz Geel ip Ce mage ahi oe T E ee een Bees ER a s7 EAT PES SRATI AOR TU RTTA KEN 2 forded bey eae N OD x D hanen aa ty ty dB normalized do 120 bisa c s 8 Vpp 1000Hz Sine wave input 5V range 10Hz High pass filter 200 kHz Low pass filter Bypass mode 2000 4000 6000 8000 10000 Hertz testy l l gt e N O O eee es tnai ere ze oes Avy oe gt bot gt F Rin IR y RE iae on ciel as ERCT 5 R EW E ET 2190 ore aah Ce ou Any r iex chin eres bilgi eddies Bal re dB normalized Go WBK18 Dynamic Signal Input Module 926896 WBK18 pg 25 Note WBK18 pg 26 926896 WBK18 Dynamic Signal Input Module
17. es This filter provides excellent passband accuracy at the expense of a more gradual roll off characteristic as detailed below Gain of 2 pole Filter Mode 0 10 20 30 40 50 60 70 80 90 100 Gain dB FIN FC Gain of 2 pole Filter Mode 0 0 5 1 1 5 2 2 5 3 3 5 4 4 5 5 0 5 S 10 15 20 25 FIN FC Zoom in to Filter Cutoff Frequency Region WBK18 Dynamic Signal Input Module 926896 WBK18 pg 11 Bypass Mode Bypass mode also provides excellent passband accuracy and provides minimal signal attenuation up to very high frequencies for those applications that need to measure signals of frequency exceeding 20 kHz For reference the typical roll off characteristic of Bypass mode 1s the following Gain of Bypass Mode 0 25 50 15 100 125 150 175 200 225 250 0 5 1 1 5 2 2 5 3 3 5 4 4 5 Gain dB FIN kHz Gain of Bypass Mode O Oo 10 15 20 25 30 35 40 45 50 0 05 0 1 0 15 0 2 Gain dB 0 25 0 3 FIN kHz Zoom tin to Filter Cutoff Frequency Region WBK18 pg 12 926896 WBK18 Dynamic Signal Input Module DC Accuracy As previously mentioned the 2 pole and Bypass filter modes are most commonly used in applications where DC signal information is important The AC response of each of these modes is described above The DC accuracy e
18. he cable capacitance and the ratio of the maximum peak signal voltage to the current available from the constant current source Frequency Response to 5 of eA ECM Ceo SET Maximum Output Signal Amplitude i Sass 1000 5 5 kHz 1 1 kHz Where f Maximum frequency in Hz K K 3 45 x10 Kis the scale factor to convert Farads to picoFarads and Amperes to milliAmperes and a factor to allow cable capacitance to charge to 95 of the final charge p d 4 C Cable capacitance in picoFarads TU V Maximum peak measured voltage from sensor in volts Icc Constant current from current source in mA Ib Current required to bias the internal electronics typically 1 mA Icc Ib 926896 WBK18 Dynamic Signal Input Module Fuse Replacement for WBK18 CAUTION Turn OFF the power to and UNPLUG the WBK18 module and all connected equipment Remove all signal I O lines from the unit Electric shock or damage to equipment can result even under low voltage conditions Take ESD precautions to include using a grounded wrist strap Use care to avoid touching board surfaces and onboard components Ensure boards do not come into contact with foreign elements such as oils water and industrial particulate You should only replace a fuse if your device shows no sign of damage If your device appears damaged such as evidenced by a smoked component contact the factory as soon as possible If the replacement fuse blows
19. his relationship is shown in the following example Example of channels 4 Alias rejection 70dB min The sampling rate Fs is chosen to be the maximum of 1 MHz 4 1 200 kHz The alias frequency is Fs 2 or 100 kHz Referring to the attenuation table to achieve a minimum of 70dB of alias rejection there must be at least a 2 8 ratio between Fyw and Fe For the Fy value of maximum value of Fc of 100 kHz 2 8 35 7 kHz The largest available 100 kHz this translates into a Fc value that satisfies this condition is 20 kHz The input signal bandwidth for this case is then 20 kHz 926896 WBK18 Dynamic Signal Input Module Using the 2 pole Filter and Bypass 2 pole Filter Mode For applications where the signal of interest is entirely AC in nature the low pass filter mode of 8 pole is the best choice Vibration signals coupled through an ICP accelerometer are an example of this However for applications where the DC component of the input signal is also of importance 2 pole mode may provide better results Proximity sensor measurements are an example of this This is because the 8 pole switched capacitor filter while providing excellent attenuation characteristics does not process DC signals with a high level of accuracy For accelerometer applications this is not a limitation as there is no information in the DC component of the input signal The 2 pole filter in contrast is formed by two 1 pole RC filters in seri
20. ions are applied Care should be taken at higher frequencies because mounting conditions greatly affect the frequency range see Mounting Effects in upcoming text Dynamic Range The dynamic measurement range is the ratio of the maximum signal for a given distortion level to the minimum detectable signal for a given signal to noise ratio The dynamic range is determined by several factors such as sensitivity bias voltage level power supply voltage and noise floor Bias Level Under normal operation a bias voltage appears from the output signal lead to ground There are two basic MOSFET configurations commonly used One exhibits a 7 8 V bias and the second a 9 12 V bias Operation of the two circuits is identical except for the available signal swing Thermal Shock Temperature Transients Piezoelectric accelerometers exhibit a transient output that is a function of a temperature s rate of change This thermal shock is usually expressed in g C and is related to e Non uniform mechanical stresses set up in the accelerometer structure e A pyroelectric effect in piezoelectric materials an electrical charge is produced by the temperature gradient across the crystal This quasi static effect produces a low frequency voltage input to the MOSFET amplifier This voltage 1s usually well below the low frequency corner but the effect can reduce the peak clipping level and cause loss of data This effect does not affect the a
21. itude minimum g that can be measured There are two main sources of noise as follows e Noise from the crystal and microcircuit inside the accelerometer Some types of crystals such as quartz are inherently noisier than others A good noise floor is 10 to 20 uV e Noise from electrical activity on the mounting surface Since the signal from the accelerometer is a voltage 60 Hz or other voltages ground loop etc can interfere with the signal The best protection is to electrically isolate the accelerometer WBK18 pg 18 926896 WBK18 Dynamic Signal Input Module Sensitivity The sensitivity of an accelerometer is defined as its output voltage per unit input of motion The unit of motion used is g One g is equal to the gravitational acceleration at the Earth s surface which is 32 2 ft sec sec or 981 cm sec sec The output is usually specified in millivolts per g mV g Sensitivity 1s usually specified under defined conditions such as frequency testing levels and temperature An example 100 mV g at a frequency of 100 Hz level 1 g at 72 F Note that although a sensor may have a typical sensitivity of 100 mV g its actual sensitivity could range from 95 to 105 mV g when checked under stated conditions Manufacturers usually provide sensor calibration values Transverse Sensitivity An accelerometer is designed to have one major axis of sensitivity usually perpendicular to the base and co li
22. mation regarding TEDS in relation to the WaveView software application begins on page 8 Excitation Source The WBK18 s AC excitation source is a sine wave based voltage source that is programmable in frequency from 1 Hz to 5 kHz and in discrete amplitudes from 100 mVp p to 10 Vp p Continuous sine sweep sine and custom sine modes are available It can be used as a test source for the input channels or as excitation for other system elements such as the amplifier for a shaker table All of its parameters are software controlled and its output is conveniently provided on a front panel BNC connection Detailed information on the excitation source and its operation can be found in the Software Setup section Do not confuse excitation source with the Source Level column in WaveView s main window as source level refers to transducer bias current see Source Level page 8 and not to the excitation source LEDs The right hand side of the WBK18 front panel includes 19 indicator LEDs There are eight Transducer Fault LEDs 1 for each channel eight Overrange LEDs 1 for each channel an Active LED Ready LED and a Power LED The indicators have the following meanings Transducer Fault When lit a Transducer Fault LED indicates that the transducer for the associated channel has 1 LED per Channel either an open circuit or a short circuit In addition to LED indication transducer fault information is available through a software status reque
23. mmable gains of 1 2 5 10 20 50 100 and 200 These correspond to bipolar input ranges of SV 2 5V 1V 500mV 250mV 100mV 50mV and 25mV Additionally there is a 25V DC coupled only range that is suitable for proximity sensor measurements Range selection is programmable on a per channel basis WBK18 pg 2 926896 WBK18 Dynamic Signal Input Module Low Pass Anti Aliasing Filter Each channel features a programmable low pass filter to provide alias protection and to allow for the removal of undesired frequencies from the measured response This filter is configurable within 3 modes 8 pole 2 pole and Bypass The 8 pole Butterworth switched capacitor filter offers the greatest alias protection and is most commonly used in vibration measurements Its 3dB cutoff frequency is settable from 10 Hz to 50 kHz in a 1 2 5 progression Application information regarding the proper setting of this filter is provided in Maximizing Alias Protection 2 pole mode utilizes two RC filters in series Its 3dB cutoff frequency is settable as well from 10 Hz to 50 kHz in a 1 2 5 progression It provides tighter amplitude and offset accuracy than 8 pole mode at the expense of reduced alias protection due to its more gradual roll off It is most commonly used in proximity sensor measurements where the DC component of the input signal is critical Additionally the filter can be bypassed altogether resulting in a signal bandwidth of up to 200 kHz When the
24. near with its major cylindrical axis The output caused by the motion perpendicular to the sensing axis is called transverse sensitivity This value varies with angle and frequency and typically is less than 5 of the basic sensitivity Base Strain Sensitivity An accelerometer s base strain sensitivity 1s the output caused by a deformation of the base due to bending in the mounting structure In measurements on large structures with low natural frequencies significant bending may occur Units with low base strain sensitivity should be selected Inserting a washer smaller in diameter than the accelerometer base under the base reduces contact surface area and can substantially reduce the effects of base strain Note that this technique lowers the usable upper frequency range Acoustic Sensitivity High level acoustic noise can induce outputs unrelated to vibration input In general the effect diminishes as the accelerometer mass increases Use of a light foam rubber boot may reduce this effect Frequency Response An accelerometer s frequency response is the ratio of the sensitivity measured at frequency f to the basic sensitivity measured at 100 Hz This response is usually obtained at a constant acceleration level typically 1 gor 10 g Convention defines the usable range of an accelerometer as the frequency band in which the sensitivity remains within 5 of the basic sensitivity Measurements can be made outside these limits if correct
25. nnel in this case channel 9 is identified in the title bar When the Accelerometer TEDS Information Box information box is closed the TEDS Data cell will indicate its previous status of Yes or No If you add remove or relocate a TEDS sensor at the WBK18 channel inputs the TEDS information and WaveView s range and units will not reflect the change until either a the WaveBook is reselected as a device or b WaveView is closed and then reopened WBK18 pg 8 926896 WBK18 Dynamic Signal Input Module Maximizing Alias Protection What is Aliasing Aliasing is a phenomenon of sampled data systems wherein a high frequency signal is misrepresented as a low frequency signal when the A D converter sampling rate being used is too slow This misrepresentation can result in severe data corruption and incorrect FFT results Aliasing 1s a well documented data acquisition effect and interested users are encouraged to research detailed information that 1s available on line from companies such as Analog Devices and Texas Instruments This text aims to not supplant those resources but to provide most users with sufficient knowledge to avoid most alias problems through proper filter and sampling rate configuration For a given sampling rate Fs input signals of frequency up to Fs 2 will be processed correctly However input signals above Fs 2 are subject to aliasing For example a sampling rate of 100 kHz can process signals up to 50 kHz witho
26. ok device connectors and CA 115 power cables have 5 amp limits TR 40Us are limited to 2 2 amps If necessary use auxiliary or high current power supplies e If using an AC power adapter for the system power use separate adapters for the WaveBook and the WBK18 e The WBK18 has a5 amp current limit TR 40U power supplies are limited to 2 2 amps Power consumption calculations must be done to ensure that a particular daisy chain scheme does not exceed either of these current limits WBK18 Dynamic Signal Input Module 926896 WBK18 pg 5 To ensure that the software recognizes all system components when powering up an Ethernet connected WaveBook system it is important that the WaveBook S516E or WBK25 is powered last and that the most remote system components are powered first Other power up sequences will result in software s failure to recognize all components e First power on the WBK expansion modules e Second power on WaveBooks or WBK modules that are connected to the expansion ports of the WaveBook 516E or WBK25 e Finally power on the WaveBook 516E and or WBK25 devices An exception to this power up scheme is to power on the entire system at once Assembly The WBK18 has the same footprint as the WaveBook and other modules allowing for convenient mounting A fastener panel allows multiple units to be stacked vertically Screw on handles are available for portable applications For more assembly information see chap
27. or WaveView is terminated Note You can save output sine wave configuration files and open pre saved files as discussed in the upcoming section How to Save or Open Output Sine Wave Configuration Files page 17 926896 Wbk18 Output Sine Wave Configuration File i gt Output a continuous Sine Wave at a given amplitude and frequency Frequency in Hz 100 00 Amplitude in Volts pp 2o Continuous Tab Selected Wbkig Output Sine Wave Configuration File Sweep Time O Start Freg End Freg Output a continuous swept sine wave that changes from the Start frequency to the End frequency over the Sweep Time period Megs Start Freq End Freq ee 10 00 Hz s000 00 Hz fe aaa Sweep Time Amplitude 0 1 Sec fioo Vop Start Stop OF Sweep Tab Selected WBK18 Dynamic Signal Input Module Sweep Mode continued Cs T D g T 55 150 ms Example of a Linear Sweep Waveform 35 950 ms Example of a Log Sweep Waveform WBK18 pg 15 926896 WBK18 Dynamic Signal Input Module Custom Mode In this mode the user can create customized waveforms built out of specified sine wave based components For example a waveform could be 1V 1 kHz for 2 seconds followed by 5V 100 Hz for 5 seconds and then 2V 500 Hz for 1 second Up to 1280 distinct points can be entered providing the capability to create a virtually unlimited waveform set Within this mode
28. ors due to ground noise lsolated Base Design To prevent ground noise error many accelerometers have base isolated design The outer case base of the accelerometer is isolated electrically off ground by means of an isolation stud insert The proprietary material used to form the isolation provides strength and stiffness to preserve high frequency performance Cable Driving WBK18 pg 20 Operation over long cables is a concern with all types of sensors Concerns involve cost frequency response noise ground loops and distortion caused by insufficient current available to drive the cable capacitance The cost of long cables can be reduced by coupling a short 1 m adapter cable from the accelerometer to a long low cost cable like RG 58U or RG 62U with BNC connectors Since cable failure tends to occur at the accelerometer connection where the vibration is the greatest only the short adapter cable would need replacement Capacitive loading in long cables acts like a low pass second order filter and can attenuate or amplify high frequency signals depending on the output impedance of the accelerometer electronics Generally this is not a problem with low frequency vibration 10 Hz to 2000 Hz For measurements above 2000 Hz and cables longer than 100 ft the possibility of high frequency amplification or attenuation should be considered The maximum frequency that can be transmitted over a given length of cable is a function of both t
29. ple and hold SSH amplifier support for optional TEDS Transducer Electronic Data Sheet if purchased All of these parameters are independently controlled in software on a per channel basis except for overrange detection level which is set on a per module basis WBK18 Dynamic Signal Input Module 926896 WBK18 pg 1 The WBK18 module includes a built in programmable voltage excitation source This source can be used to stimulate dynamic systems for transfer function measurements and also serves as a test signal for the input channels Multiplexed CH 8 Analog Output to WaveBook Expansion CH 2 Signal BNC iki REE ARE RREN D AER EENES LEERE AEEA AE EESEL R ELLE OEA AELE ENL IEEE MUX v V One of eight channels Transducer Bias Current Source Filter Output i l i l i 1 1 1 Bypass C2 Sample CH 1 i lN and Hold 7 1 l i 1 i 1 i l BNC 3 eae F Programmable i Coupling Filter l l Cay Over range i Detection N PSANI EEEE E E REE CCEE EOS TEDS Microprocessor Communication and l Control Logic BNC 080 Excitation wae Filter and Waveform oa From Source Amplifier Generation Sy WaveBook Control Bus hag WBK18 Block Diagram Current Source with Transducer Fault Detection The WBK18 module provides constant current of 4mA to bias ICP transducers The bias current is sourced through the center conductor of the input channel BNC connector and returns to the WBK18 by the outer conductor
30. rcuit requires only two wires coax or twisted pair to transmit both power and signal At low impedance the system is insensitive to external or triboelectric cable noise Cable length does not affect sensitivity The following figure shows a simple sensor WBK18 connection The voltage developed across R is applied to the gate of the MOSFET The MOSFET is powered from a constant current source of 4 mA and 30 volts Sensor to WBK18 ARR NR i Goasila Cabjg geamanenesneanesnegoeanesonngnoa AN gt NN l l ny MOSFET C Amplifier i i Input A Constant i i Bias atk Voltage me i R l 30 VDC Power l I i i GND Cy Macon a cat pet i a ts tt watt ese isn one l ee a Sensor WBK18 The MOSFET circuit will bias at approximately 12 V in the quiet state As the system is excited voltage is developed across the crystal and applied to the gate of the MOSFET This voltage will cause linear variation in the impedance of the MOSFET and a proportional change in bias voltage This voltage change will be coupled to the WBK18 input amplifier through the capacitor C The value of R and the internal capacitance of the piezoelectric crystal control the low frequency corner Units weighing only a few grams can provide high level outputs up to 1 V g with response to frequencies below 1 Hz Accelerometer Specification Parameters Noise in Accelerometers The noise floor or resolution specifies the lowest discernible ampl
31. st at the end of an acquisition Transducer fault errors are latched until the commencement of a new acquisition Consequently even intermittent faults are detected and communicated Detection of a transducer fault does not stop an acquisition or alter data For related information refer to Current Source with Transducer Fault Detection on page 2 Overrange When lit an Overrange LED indicates that the associated channel s input signal has exceeded 1 LED per Channel the input full scale range which was programmed for that specific WBK18 module Even a momentary exceeding of the range will cause the LED to light This indication is critical for overrange signals result in clipped data that significantly corrupts FFT analysis In addition to LED indication the overrange condition is available through a software status request at the end of an acquisition Overrange errors are latched until the commencement of a new acquisition Consequently even intermittent faults are detected and communicated Overrange events do not stop an acquisition or alter data For related information refer to Overrange Detection on page 3 Lights when data is being converted Lights when the WBK18 has established communication via its Expansion Control In connector Lights when power to the unit is turned on and present WBK18 pg 4 926896 WBK18 Dynamic Signal Input Module Hardware Setup Configuration Power The WBK18 requires no physical
32. ter 3 of the WaveBook User s Manual p n 489 0901 Input Connections All input connections are made into the front panel BNCs in which the BNC center conductor is the signal HI and the BNC shell is the signal LO The BNC shell is common among all eight input channels and is not isolated from earth ground Consequently the shell is not meant to be driven with respect to earth ground An additional consideration exists regarding the setup of the input transducer If the transducer case is effectively earth grounded through its connection to a device under test there exists the possibility for added measurement noise due to the ground loop that is created This issue is minimized by electrically isolating the transducer from the device CAUTION The BNC shell is not to be driven with respect to earth ground Attempting to do so could result in equipment damage Tr Additional measurement noise may be present when using earth grounded transducers For best results electrically isolate the input transducers from earth ground WBK18 pg 6 926896 WBK18 Dynamic Signal Input Module Software Setup General Depending on your application you will need to set several software parameters Proper settings will allow WaveView to organize data to meet your requirements Some items of importance to the WBK18 are the low pass and high pass filter options that can be selected from the WaveView Configuration main window and the excitation sour
33. there is also an amplitude selection of 0 mV This corresponds to an active zero voltage and is used to create off time in a voltage waveform Programming of this mode is done by creating editing a list of output amplitude frequency points After the desired list is entered the waveform output is begun by clicking Start Once the waveform sequence completes it returns to the first entry in the list and cycles through again and again until stopped by the user Note You can save output sine wave configuration files and open pre saved files as discussed in the following section How to Save or Open Output Sine Wave Configuration Files Wbk18 Output Sine Wave Configuration File fe at 3 Same dt for all bursts max 1280 bursts Output one or more Sine Wave Bursts for the specified time duration fdt at a given amplitude and frequency The burst repeat rate is determined by the cycle time setting Cycle time and burst duration time are specified in seconds with a range of 50 ps to 85 89 seconds ct 100 0 mSec dt 10 0 mSec x Burst configuration Add Edit Amp 10 0 Vpp er Freq 1000 00 Hz Reset Delete L 1000 00 Hz 500 00 Hz 2 500 00 Hz 2 3 999695 a eS eS eS a Se i a a a aes ca aa aa tol aa lt a at ue EE y ca a a a I ee ee 4 99992 Wrdiv l l l SS Ee SSS Se ee ne ee ee eS ES Ss SS SS ee ee Te es Ue Ses ce 0 000 ps 43 9
34. ut aliasing An input signal of 90 kHz however will be aliased Specifically it will appear in the sampled data as a signal of frequency Fs Fyy which in this case is 100 kHz 90 kHz 10 kHz Aliasing and its prevention should be a consideration in all sampled data systems This is especially important in mechanical vibration measurements because most mechanical systems exhibit a resonance apart from their fundamental frequency That is there may be signal energy present that has the potential to be aliased that is unknown to the user And the worst part of aliasing is that its effects are indistinguishable from real input signals That is in the given example it is not apparent to the user whether the 10 kHz energy is real or an alias Aliasing Protection using the WBK18 The WBK18 provides alias rejection via its 8 pole filter This filter has an Gain dB extremely steep roll off characteristic very closely achieving an ideal brick wall response It consequently passes frequencies of interest without 0 15 significant attenuation but significantly attenuates frequencies just above Its attenuation is so high that most alias frequency energy is reduced to a level below the noise floor of the measurement system However it must be configured correctly to achieve these results In general the cutoff frequency Fc of the filter should be set as close to but above the highest input frequency of interest This will maximize th
35. worth with simultaneous sample and hold SSH Cutoff Frequency Fc 10 Hz to 50 kHz in 1 2 5 progression Alias Rejection 75dB min Channel to Channel Phase Matching 1 typ 2 max Unit to Unit Phase Matching 1 typ 2 max SSH latency 100ns max Amplitude Accuracy 0 5dB Fin lt F 2 Total Harmonic Distortion 70dB typ ICP Bias Source 4mA 24V compliance on off software programmable per channel ICP Fault Detection Thresholds lt 1V short gt 25V open ICP Fault Indication Front panel LED per channel software status Conditions for Low Pass Filter Phase Matching 8 pole LPF mode 0 1 Hz or DC HPF mode 1 Hz lt Fin lt F 2 200 Hz lt F lt 20 kHz All WBK18 units connected to same WaveBook Condition for Amplitude Accuracy For Fin lt 20 kHz WBK18 Dynamic Signal Input Module 926896 WBK18 pg 23 Excitation Source Channels 1 Signal Connection BNC Frequency Range 1 Hz to 5 kHz Frequency Resolution 0 01 Hz Amplitude Settings p p 10V 5V 2V 1V 500mV 200mV 100mV OmV Waveform Modes Continuous sine Sweep sine Custom sine Output Impedance 50 Ohm Accuracy 0 1dB DC Accuracy Excluding Noise Applies to 2 Pole and Bypass Filter Modes Accuracy at 0 to 50 C 32 to 122 F Range igea Maximum Offset Range Typical Maximum _ These numbers are valid for 1 year after calibration and over the entire operating temperature range of the unit Specifications are continu
36. xcluding noise for both modes 1s Accuracy at 0 to 50 C 32 to 122 F Range of Reading Range Typical 0 5 1 15mV 1V 2 5V 0 15 2mV 0 15 900uV These numbers are valid for 1 year after calibration and over the entire operating temperature range of the unit Module Configuration Overrange Detection The configuration of overrange detection is done through the Module Configuration me ik Module Configuration window The Module Configuration window can be accessed from the View pull down menu or by System Inventory Module Chassis Option use of the first toolbar button located just below the File pull A cca ate WBK1 34 down menu In this window the user enables disables overrange detection on a per channel basis A checkmark next to the channel number indicates that overrange detection is enabled for that channel The overrange detection level is also set in this screen It is set as a percentage of range that applies to all channels of the WBK18 unit being configured After channel enable and level selections are made the Apply button must be clicked to accept the changes Module Settings Channel Over range detection Percent Module Configuration Window When configuring overrange detection after channel enable and level selections are made the lt Apply gt button must be clicked to accept the changes WBK18 Dynamic Signal Input Module 926896 WBK18
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