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Chapter 2

Using the Module

NI PXIe-4330/4331 User Manual

2-2

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The Wheatstone bridge is the electrical equivalent of two parallel voltage divider circuits. R

1

and R

2

compose one voltage divider circuit, and R

4

and R

3

compose the second voltage

divider circuit. The output of a Wheatstone bridge is measured between the middle nodes of
the two voltage dividers. A physical phenomena, such as a change in strain or temperature
applied to a specimen, changes the resistance of the sensing elements in the Wheatstone
bridge, resulting in a bridge output voltage that is proportional to the physical phenomena.
The output voltage of the bridge scales with the excitation voltage. However, the ratio of the
bridge output (

V

CH

) and the excitation voltage (

V

EX

) remains fixed over variations in

excitation voltage, and it is this unitless ratio (

V

CH

/

V

EX

) that is of interest. To accurately

measure the ratiometric output of a bridge based sensor both the bridge output voltage (

V

CH

)

and the excitation voltage must be known. Determination of the excitation voltage can be
accomplished by either using an accurate voltage source or by measuring it. The
NI PXIe-4330/4331 uses circuitry that continuously measures the excitation voltage and
applies it as a reference to its analog-to-digital converter (ADC). In this way, variations in the
excitation voltage are compensated for, and the module returns data as a ratio of the bridge
output voltage and the excitation voltage.

Connection Options to Correct for Resistance Errors

The basic Wheatstone bridge in Figure 2-1 shows the excitation voltage impressed directly
across the bridge. However, field wiring used to connect sensors to measurement devices have
a non-zero resistance, and this resistance can create errors in bridge circuit measurements.
The NI PXIe-4330/4331 provides two mechanisms to correct for these errors: remote sensing
and shunt calibration.

Remote Sensing

Remote sensing continuously and automatically corrects for errors in excitation leads, and
generally is most appropriate for half- and full-bridge sensors. Moreover, its use is most
critical in applications that employ long wires and/or small gauge wires, as these have
greater resistance. The resistance in the wires that connect the excitation voltage to the bridge
causes a voltage drop, which is a source of gain error. The NI PXIe-4330/4331 includes
remote sensing to compensate for this gain error. Connect remote sense inputs of the
NI PXIe-4330/4331 to the excitation voltage wires of the sensor as close to the bridge circuit
as possible. Refer to the full-bridge diagram in Figure 2-2 for an illustration of how to connect
remote sense wires.

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Содержание SC Express PXIe-4330

Страница 1: ...NI SC Express NI PXIe 4330 4331 User Manual NI PXIe 4330 4331 User Manual April 2010 373029A 01 ...

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Страница 3: ...rmation retrieval system or translating in whole or in part without the prior written consent of National Instruments Corporation National Instruments respects the intellectual property of others and we ask our users to do the same NI software is protected by copyright and other intellectual property laws Where NI software may be used to reproduce software or other materials belonging to others yo...

Страница 4: ...6 Half Bridge Type I 2 8 Half Bridge Type II 2 10 Full Bridge Type I 2 12 Full Bridge Type II 2 14 Full Bridge Type III 2 16 Force Pressure and Torque Sensor Configurations 2 18 Common Mode Voltage Considerations 2 20 Shielding and Grounding Considerations 2 21 Module Pinout 2 21 I O Connector Signal Description 2 23 NI PXIe 4330 4331 Block Diagram 2 23 Signal Acquisition Considerations 2 25 Softw...

Страница 5: ...r Filters 3 3 Appendix A Offset Nulling Bridge Balancing Appendix B Technical Support and Professional Services Figures Figure 2 1 Basic Wheatstone Bridge Circuit Diagram 2 1 Figure 2 2 Connecting Remote Sense Wires to the NI PXIe 4330 4331 2 3 Figure 2 3 Quarter Bridge Type I Measuring Axial and Bending Strain 2 4 Figure 2 4 Quarter Bridge I Circuit Diagram 2 5 Figure 2 5 Quarter Bridge Type II M...

Страница 6: ...Bridge Type III Circuit Diagram 2 17 Figure 2 17 Force Pressure and Torque Sensor Circuit Diagram 2 18 Figure 2 18 NI PXIe 4330 4331 Signal Conditioning Block Diagram 2 24 Figure 2 19 NI PXIe 4330 4331 Digital Back End Block Diagram 2 25 Figure 2 20 Analog Level Trigger on Rising Slope 2 31 Figure 2 21 Analog Edge Triggering with Hysteresis on Rising Slope 2 31 Figure 2 22 Analog Edge Triggering w...

Страница 7: ...er half and full bridge Setting the DC voltage excitation between 0 625 V and 10 V Selecting 120 Ω 350 Ω and 1 kΩ quarter bridge completion resistors Programmable bridge offset null and shunt calibration Analog and digital filtering to reject out of band signals Remote sense of bridge excitation Installation Refer to the NI SC Express 4330 4331 Installation Guide and Terminal Block Specifications ...

Страница 8: ...er to the NI SC Express 4330 4331 Installation Guide and Terminal Block Specifications for more signal connection information Wheatstone Bridges Many sensors including strain gages load cells pressure sensors and torque sensors are based on the concept of a Wheatstone bridge There are four elements or legs in a Wheatstone bridge In general these elements can be resistive or reactive but in the maj...

Страница 9: ...plies it as a reference to its analog to digital converter ADC In this way variations in the excitation voltage are compensated for and the module returns data as a ratio of the bridge output voltage and the excitation voltage Connection Options to Correct for Resistance Errors The basic Wheatstone bridge in Figure 2 1 shows the excitation voltage impressed directly across the bridge However field...

Страница 10: ... across the excitation lead wire resistance Rlead If you do not use remote sensing of the actual bridge voltage the resulting reduction in gain is given by the following equations for half bridge sensors for full bridge sensors If you connect the remote sense signals directly to the bridge resistors the NI PXIe 4330 4331 senses the actual bridge voltage using high impedance RS leads and eliminates...

Страница 11: ...f strain by changing the resistance of an arm in the bridge by some known amount This is accomplished by shunting or connecting a large resistor of known value across one arm of the bridge creating a known change in the bridge output You can then measure the output of the bridge and compare it to the expected bridge output value You can use the results to correct gain errors in the entire measurem...

Страница 12: ...etion resistor located inside the NI PXIe 4330 4331 R3 must be equal to the nominal resistance of the active gage R4 RS Shunt calibration resistor located inside the NI PXIe 4330 4331 R4 Active element measuring tensile strain ε You provide this element RL Lead resistance The resistance in the EX and QTR SC field wiring should match GF Gage Factor specified by the gage manufacturer VCH Measured vo...

Страница 13: ...ing configurations Figure 2 6 shows the quarter bridge type II circuit wiring diagram Figure 2 5 Quarter Bridge Type II Measuring Axial and Bending Strain A quarter bridge type II has the following characteristics One active strain gage element and one passive quarter bridge element dummy gage used for temperature compensation The active element R4 is mounted in the direction of axial or bending s...

Страница 14: ...stor located inside the NI PXIe 4330 4331 R4 Active element measuring tensile strain ε RL Lead resistance GF Gage Factor specified by the gage manufacturer VCH Measured voltage of the bridge VEX Excitation voltage provided by the NI PXIe 4330 4331 Vr Offset compensated ratiometric bridge output defined by the following equation Note The ratio of the bridge output voltage and the excitation voltage...

Страница 15: ... Figure 2 8 shows the half bridge type I circuit wiring diagram Figure 2 7 Half Bridge Type I Measuring Axial and Bending Strain A half bridge type I has the following characteristics Two active strain gage elements One strain gage element is mounted in the direction of axial strain while the other acts as a Poisson gage and is mounted perpendicular to the principal axis of strain Half bridge comp...

Страница 16: ...ring tensile strain ε RL Lead resistance GF Gage Factor specified by the gage manufacturer ν Poisson s ratio defined as the negative ratio of transverse strain to axial strain longitudinal strain Poisson s ratio is a material property of the specimen you are measuring VCH Measured voltage of the bridge VEX Excitation voltage provided by the NI PXIe 4330 4331 Vr Offset compensated ratiometric bridg...

Страница 17: ...ration Figure 2 10 shows the half bridge type II circuit wiring diagram Figure 2 9 Half Bridge Type II Rejecting Axial and Measuring Bending Strain A half bridge type II configuration has the following characteristics Two active strain gage elements One strain gage element is mounted in the direction of bending strain on one side of the strain specimen top while the other is mounted in the directi...

Страница 18: ...4331 R4 Active element measuring tensile strain ε RL Lead resistance GF Gage Factor specified by the gage manufacturer VCH Measured voltage of the bridge VEX Excitation voltage provided by the NI PXIe 4330 4331 Vr Offset compensated ratiometric bridge output defined by the following equation Note The ratio of the bridge output voltage and the excitation voltage is done internally on the NI PXIe 43...

Страница 19: ...the full bridge type I circuit wiring diagram Figure 2 11 Full Bridge Type I Rejecting Axial and Measuring Bending Strain A full bridge type I configuration has the following characteristics Four active strain gage elements Two strain gage elements are mounted in the direction of bending strain on one side of the strain specimen top while the other two are mounted in the direction of bending strai...

Страница 20: ...nt calibration resistor located inside the NI PXIe 4330 4331 RL Lead resistance GF Gage Factor specified by the gage manufacturer VCH Measured voltage of the bridge VEX Excitation voltage provided by the NI PXIe 4330 4331 Vr Offset compensated ratiometric bridge output defined by the following equation Note The ratio of the bridge output voltage and the excitation voltage is done internally on the...

Страница 21: ...2 13 Full Bridge Type II Rejecting Axial and Measuring Bending Strain A full bridge type II configuration has the following characteristics Four active strain gage elements Two are mounted in the direction of bending strain with one on one side of the strain specimen top and the other on the opposite side bottom The other two act together as a Poisson gage and are mounted transverse perpendicular ...

Страница 22: ...located inside the NI PXIe 4330 4331 RL Lead resistance GF Gage Factor specified by the gage manufacturer ν Poisson s ratio defined as the negative ratio of transverse strain to axial strain longitudinal strain Poisson s ratio is a material property of the specimen you are measuring VCH Measured voltage of the bridge VEX Excitation voltage provided by the NI PXIe 4330 4331 Vr Offset compensated ra...

Страница 23: ...e type III circuit wiring diagram Figure 2 15 Full Bridge Type III Measuring Axial and Rejecting Bending Strain A full bridge type III configuration has the following characteristics Four active strain gage elements Two strain gage elements are mounted in the direction of axial strain with one on one side of the strain specimen top while the other is on the opposite side bottom The other two act t...

Страница 24: ...Factor specified by the gage manufacturer ν Poisson s ratio defined as the negative ratio of transverse strain to axial strain longitudinal strain Poisson s ratio is a material property of the specimen you are measuring Rs Shunt calibration resistor located inside the NI PXIe 4330 4331 VCH Measured voltage of the bridge VEX Excitation voltage provided by the NI PXIe 4330 4331 Vr Offset compensated...

Страница 25: ...ors typically use a full bridge configuration with a 350 Ω nominal bridge resistance but other configurations and nominal bridge resistances can be used Figure 2 17 shows the force pressure torque sensor circuit diagram Figure 2 17 Force Pressure and Torque Sensor Circuit Diagram 1 Sensors that measure force pressure or torque through other means for example piezoelectric force sensors cannot be u...

Страница 26: ... are specified as pairs of corresponding physical and electrical values electrical1 physical1 and electrical2 physical2 These should be based on the calibration certificate of the sensor if one is available otherwise they can be based on the specifications or data sheet of the sensor Any two points can be used assuming that they are far enough apart to accurately determine the slope of the linear ...

Страница 27: ...e Considerations The NI PXIe 4330 4331 supports common mode voltages between 2 V The majority of full bridge sensors are balanced between the upper and lower arms of the bridge resulting in a common mode voltage equal to one half the excitation voltage when connected to a single ended excitation voltage or 0 V when connected to a balanced differential excitation voltage Since the NI PXIe 4330 4331...

Страница 28: ...o reduce the excitation voltage to a level that results in a common mode voltage that is within range To find the maximum excitation voltage that meets this criterion solve the following equation setting Vcm equal to 2 V if r 0 5 or setting Vcm equal to 2 V if r 0 5 Shielding and Grounding Considerations Note For proper electromagnetic compatibility EMC performance use shielded wire and connect th...

Страница 29: ...D EX AI 2 25 QTR SC EX AI 24 QTR SC RS RS 23 AIGND EX AI 3 22 QTR SC EX AI 21 QTR SC RS RS 20 DGND T0 T1 No Channel 19 T3 T0 T1 18 T3 T2 T2 17 DGND T4 T5 16 T7 T4 T5 15 T7 T6 T6 14 AIGND EX AI 4 13 QTR SC EX AI 12 QTR SC RS RS 11 AIGND EX AI 5 10 QTR SC EX AI 9 QTR SC RS RS 8 AIGND EX AI 6 7 QTR SC EX AI 6 QTR SC RS RS 5 AIGND EX AI 7 4 QTR SC EX AI 3 QTR SC RS RS 2 RSVD DGND RSVD No Channel 1 RSV...

Страница 30: ...lected based on the selected excitation voltage level Table 2 2 I O Connector Signal Descriptions Signal Names Direction Description AIGND Analog Input Ground AI 0 7 AI 0 7 Input Analog Input Channels 0 to 7 AI and AI are the positive and negative inputs of the differential analog input QTR SC 0 7 These pins provide the connection point for quarter bridge completion and shunt calibration EX 0 7 EX...

Страница 31: ...nfigure the NI PXIe 4330 4331 to select between internal 120 Ω 350 Ω and 1 kΩ quarter bridge completion resistors In addition you can enable programmable shunt calibration resistors individually or in parallel to provide three different possible shunt calibration resistor values Each channel also provides a transducer electronic data sheet TEDS interface This interface can read the TEDS informatio...

Страница 32: ...n order to acquire data at the specified rate The DDS clock can be generated either using an onboard oscillator or using the 100 MHz backplane clock provided by PXI Express Running the DDS clock from the 100 MHz backplane clock allows for synchronization of multiple NI PXIe 4330 4331 modules In addition to sending and receiving triggers from the PXIe backplane the NI PXIe 4330 4331 can also be con...

Страница 33: ...gnals with frequencies lower than fs 2 This maximum frequency is known as the Nyquist frequency The bandwidth from 0 Hz to the Nyquist frequency is the Nyquist bandwidth ADC The NI PXIe 4330 4331 ADCs use a conversion method known as delta sigma modulation This approach involves oversampling the input signal at a higher rate and then decimating and filtering the resulting data to achieve the desir...

Страница 34: ...resented correctly while others contain aliased artifacts Lowpass filtering to eliminate components above the Nyquist frequency either before or during the digitization process can guarantee that the digitized data set is free of aliased components The NI PXIe 4330 4331 modules employ both digital and analog lowpass filters to achieve this protection The NI PXIe 4330 4331 modules include an oversa...

Страница 35: ...tal filtering performed by the NI PXIe 4330 4331 produces a delay of many samples worth of time between when an event occurs on the input signal going into the NI PXIe 4330 4331 and when the data associated with that event is available at the output of the acquisition and filtering process This delay is called the group delay In order to simplify the process of acquiring data from the NI PXIe 4330...

Страница 36: ...he maximum rate for the module Timing and Triggering This section contains information about timing and triggering theory of operation Sample Clock Timebase The ADCs require an oversample clock to drive the conversion The oversample clock frequency is greater than the sample rate On the NI PXIe 4330 4331 modules the oversample clock is produced from a DDS clock generation circuit The DDS in turn i...

Страница 37: ...trigger samples The analog trigger on the NI PXIe 4330 4331 cannot be used as a start trigger This restriction is a result of the way the module compensates for the filter group delay When using an analog reference trigger the module first waits for the specified number of pre trigger samples to be acquired Once enough pre trigger samples are acquired the reference trigger will occur the next time...

Страница 38: ...dge triggering you add a window above or below the trigger level This triggering mode often is used to reduce false triggering due to noise or jitter in the signal For example if you add a hysteresis of 1 mV V to the example in Figure 2 20 which uses a level of 3 2 mV V the signal must start at or drop below 2 2 mV V to arm the trigger The analog comparison becomes true when the signal rises above...

Страница 39: ...ises above 4 2 mV V as shown in Figure 2 22 Figure 2 22 Analog Edge Triggering with Hysteresis on Falling Slope Window Triggering A window trigger occurs when an analog signal either passes into enters or passes out of leaves a window defined by two levels Specify the levels by setting a value for the top and bottom window boundaries Figure 2 23 demonstrates a trigger that acquires data when the s...

Страница 40: ...minate clock drift between modules in long duration operations You can synchronize the analog input operations on two or more NI PXIe 4330 4331 modules to extend the channel count for your measurements In addition the NI PXIe 4330 4331 can synchronize with certain other DSA modules such as the NI PXIe 449x modules using Reference Clock Synchronization You can also synchronize modules that support ...

Страница 41: ...r slaves at DAQmx Trigger Advanced Synchronization Synchronization Type to Slave and that of the Master to Master Also query DAQmx Timing More Synchronization Pulse Synchronization Time on all modules being synchronized choose the maximum value and set that as the DAQmx Timing More Synchronization Pulse Minimum Delay To Start on all modules 7 Commit all of the sync pulse slave module tasks using t...

Страница 42: ... pop up menu To programmatically configure TEDS call the DAQmx Configure TEDS VI Accessory Auto Detection NI SC Express modules automatically detect compatible accessories or terminal blocks The RSVD pins on the I O connector provide power to the accessories as well as digital communication lines This allows software to detect when accessories are inserted or removed In addition software can autom...

Страница 43: ...dules in a PXI Express measurement or control system The PXI Express backplane is responsible for generating PXIe_SYNC100 independently to each peripheral slot in a PXI Express chassis For more information refer to the PXI Express Specification at www pxisa org PXI_CLK10 PXI_CLK10 is a common low skew 10 MHz reference clock for synchronization of multiple modules in a PXI measurement or control sy...

Страница 44: ...d as a trigger signal for input operations An SC Express module is not a System timing controller An SC Express module can be used in the system timing slot of a PXI system but the system will not be able to use the Star Trigger feature PXIe_DSTAR A C PXI Express devices can provide high quality and high frequency point to point connections between each slot and a system timing slot These connecti...

Страница 45: ... When the filter clock has sampled the signal high on N consecutive edges the low to high transition is propagated to the rest of the circuit The value of N depends on the filter setting Refer to Table 3 2 The filter settling for each input can be configured independently At power on the filters are disabled Enabling filters introduces jitter on the input signal The maximum jitter is one period of...

Страница 46: ...he bridge while not under load and then use this measurement as the initial bridge voltage when scaling readings from the bridge This method is simple fast and requires no manual adjustments The disadvantage of the software compensation method in contrast to hardware compensation is that software compensation does not physically remove the offset of the bridge If the offset is large enough it limi...

Страница 47: ... you may renew to continue your benefits For information about other technical support options in your area visit ni com services or contact your local office at ni com contact Training and Certification Visit ni com training for self paced training eLearning virtual classrooms interactive CDs and Certification program information You also can register for instructor led hands on courses at locati...

Страница 48: ... PXIe 4330 4331 User Manual B 2 ni com this manual You also can visit the Worldwide Offices section of ni com niglobal to access the branch office Web sites which provide up to date contact information support phone numbers email addresses and current events ...

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