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©

 National Instruments Corporation

4-1

SCXI-1521/B User Manual

4

Theory of Operation

This chapter discusses strain-gauge concepts and the theory of operational 
measurement concepts.

Strain-Gauge Theory

This section describes how to arrange, connect, and scale signals from 
bridge-based sensors—especially strain gauges.

Wheatstone Bridges

All strain-gauge configurations are based on the concept of a Wheatstone 
bridge. A Wheatstone bridge is a network of four resistive legs. One or 
more of these legs can be an active sensing element. Figure 4-1 shows a 
Wheatstone bridge circuit diagram.

Figure 4-1.  

Basic Wheatstone Bridge Circuit Diagram

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 phenomenon, such as a change in strain or temperature applied 
to a specimen, changes the resistance of the sensing elements in the 

+

V

CH

R

1

R

2

R

4

R

3

V

EX 

+

–

Содержание SCXI-1521

Страница 1: ...SCXI TM SCXI 1521 B User Manual SCXI 1521 B User Manual March 2006 373749B 01...

Страница 2: ...3451 3400 Lebanon 961 0 1 33 28 28 Malaysia 1800 887710 Mexico 01 800 010 0793 Netherlands 31 0 348 433 466 New Zealand 0800 553 322 Norway 47 0 66 90 76 60 Poland 48 22 3390150 Portugal 351 210 311...

Страница 3: ...he copyright laws this publication may not be reproduced or transmitted in any form electronic or mechanical including photocopying recording storing in an information retrieval system or translating...

Страница 4: ...bol is marked on a product it denotes a component that may be hot Touching this component may result in bodily injury bold Bold text denotes items that you must select or click in the software such as...

Страница 5: ...Self Test Verification 1 6 Chapter 2 Connecting Signals Pin Assignments 2 1 Chapter 3 Configuring and Testing SCXI 1521 B Software Configurable Settings 3 1 Common Software Configurable Settings 3 1 B...

Страница 6: ...ting Timing and Triggering 5 3 Configuring Channel Properties 5 4 Perform Offset Null Compensation 5 5 Perform Shunt Calibration 5 6 Acquiring Analyzing and Presenting 5 6 Completing the Application 5...

Страница 7: ...317 and SCXI 1521 B Combination 4 6 Figure 5 1 Typical Program Flowchart 5 2 Figure A 1 SCXI 1521 B Dimensions A 4 Figure B 1 Removing the SCXI 1521 B B 2 Tables Table 1 1 Accessories Available for th...

Страница 8: ...ng Measurement Automation Explorer MAX or through NI DAQmx property nodes The SCXI 1521 has 350 completion resistors while the SCXI 1521B has 120 completion resistors The SCXI 1521 B includes the foll...

Страница 9: ...re available for the SCXI 1521 B Table 1 1 Accessories Available for the SCXI 1521 B Accessory Description SCXI 1317 Screw terminal block Mounts on the front of the SCXI 1521 B module and provides eas...

Страница 10: ...module configuration They also explain in greater detail how the module works and contain application hints Accessory installation guides or manuals If you are using accessory products read the termi...

Страница 11: ...NI sales representative to request it on a CD or download the latest NI DAQ version from ni com Installing the SCXI 1521 B Note Refer to the Read Me First Radio Frequency Interference document before...

Страница 12: ...1 Make sure the appropriate software is already installed or install it 2 If the E M Series DAQ device for controlling the SCXI system is not installed in the right most slot of the PXI 1010 combinati...

Страница 13: ...rs you have successfully installed the module The SCXI system should now operate properly with your ADE software If the test did not complete successfully refer to Chapter 3 Configuring and Testing fo...

Страница 14: ...listed in the correct slot 4 If the cabled module is not listed in the correct slot complete the following troubleshooting steps a If the cabled module is not listed in the correct slot and the slot...

Страница 15: ...ock Installation Guide Refer to Chapter 4 Theory of Operation for a discussion of strain gauge concepts Pin Assignments The pin assignments for the SCXI 1521 B front signal connector are shown in Tabl...

Страница 16: ...I 7 22 EX 8 QTR 8 AI 8 21 EX 9 QTR 9 AI 9 20 RSVD QTR 10 AI 10 19 RSVD QTR 11 AI 11 18 RSVD EX 10 RSVD 17 RSVD EX 11 RSVD 16 EX 12 QTR 12 AI 12 15 EX 13 QTR 13 AI 13 14 EX 14 QTR 14 AI 14 13 EX 15 QTR...

Страница 17: ...d for analog signal connectivity and communication between the SCXI 1521 B and the connected E M Series DAQ device Table 2 2 shows the rear signal connector and lists the pin assignments for the SCXI...

Страница 18: ...AI 0 3 4 AI 0 5 6 7 8 9 10 11 12 13 14 15 16 17 18 OUT REF 19 20 21 22 23 24 DIG GND SER DAT IN 25 26 SER DAT OUT DAQ D A 27 28 SLOT 0 SEL 29 30 31 32 DIG GND 33 34 35 36 AI HOLD COMP AI HOLD SER CLK...

Страница 19: ...neral purpose digital I O Table 2 3 SCXI 1521 B Communication Signals Pin SCXI Signal Name NI DAQmx Device Signal Name Direction Description 24 33 DIG GND D GND Digital ground These pins supply the re...

Страница 20: ...I module that the E M Series DAQ device has taken a sample and causes the module to advance channels 37 SER CLK EXTROBE Input Serial clock This signal taps into the SCXIbus SPI CLK line to clock the d...

Страница 21: ...and the measurements and how to configure settings in your application refer to Chapter 4 Theory of Operation Common Software Configurable Settings This section describes the most frequently used soft...

Страница 22: ...alibration Switches The SCXI 1521 B includes a software controllable shunt calibration switch on each channel of the module that is used to engage or disengage a fixed shunt calibration resistor Note...

Страница 23: ...nimum and maximum range of NI DAQmx Task or NI DAQmx Global Channel the driver selects the best gain for the measurement Calibration settings fine null potentiometer settings and control shunt calibra...

Страница 24: ...s of various measurement types to the same task you must complete this task in your application Refer to the Developing Your Application in NI DAQmx section of Chapter 5 Using the SCXI 1521 B for more...

Страница 25: ...asurements using test panels in order to verify signal and configuring and installing a system in NI DAQmx Verifying the Signal in NI DAQmx Using a Task or Global Channel Complete the following steps...

Страница 26: ...ork of four resistive legs One or more of these legs can be an active sensing element Figure 4 1 shows a Wheatstone bridge circuit diagram Figure 4 1 Basic Wheatstone Bridge Circuit Diagram The Wheats...

Страница 27: ...d be specified by the gauge manufacturer RL is the lead resistance If lead lengths are long RL can significantly affect measurement accuracy Rs is the shunt calibration resistor value U is the ratio o...

Страница 28: ...t and Strain_Buf_Convert are in the header file convert h for C C and convert bas for Visual Basic The names of the strain gauge types in these sections directly correspond to bridge selections in MAX...

Страница 29: ...idge completion resistor dummy resistor R4 is the active strain gauge element measuring tensile strain VEX is the excitation voltage RL is the lead resistance VCH is the measured voltage Vr is the vol...

Страница 30: ...pproximately equals the thermally induced change in strain with change in temperature and is expressed in units of microstrain per degree Fahrenheit For example if the test specimen is aluminum use a...

Страница 31: ...SCXI 1521 B Module Analog Multiplexer Scan Clock Rear Signal Connector SCXIbus Connector Buffer Digital Interface and Control Analog Bus Switch Multiplexer Control To Analog Bus Excitation Voltage Ref...

Страница 32: ...lexer stage The modes of operation are determined by the multiplexer stage The signal is routed to the E M Series DAQ device channel 0 if the SCXI 1521 B is the cabled module and or to the SCXI backpl...

Страница 33: ...exists in the strain gauge when it is unstrained to adjust signal voltages to proper levels Hardware offset nulling is performed programmatically using software The fine null potentiometer is used for...

Страница 34: ...of a bridge sensor and the measured signal levels with the shunt resistor actually engaged Use the following formula to calculate the gain adjust factor The gain adjust factor is then multiplied by e...

Страница 35: ...ial Channels can occur multiple times in a single scan list The scan list can contain an arbitrary number of channels for each module entry in the scan list limited to a total of 512 channels per DAQ...

Страница 36: ...te a task or global channel in unlicensed mode These messages continue to appear until you install version 7 0 or later of an NI ADE This section describes how to configure and use NI DAQmx to control...

Страница 37: ...istant or MAX No Hardware Timing Triggering Create Task Using DAQ Assistant No Yes No Yes Configure Channels Further Configure Channels Start Measurement Read Measurement Clear Task Continue Sampling...

Страница 38: ...e task If you are using an ADE other than an NI ADE or if you want to explicitly create and configure a task for a certain type of acquisition you can programmatically create the task from your ADE us...

Страница 39: ...need to set to configure the module for your application For a complete list of NI DAQmx properties refer to your ADE help file Note You cannot adjust some properties while a task is running For these...

Страница 40: ...ring and Testing for information about offset null compensation when in MAX For more information about offset null compensation refer to the Offset Null Compensation section of Chapter 4 Theory of Ope...

Страница 41: ...a performing harmonic analysis applying a custom scale or adjusting measurements mathematically Some custom scaling applications require the actual excitation voltage applied to the bridge instead of...

Страница 42: ...click it and select New Task DAQ Assistant Create a Task Programmatically optional DAQmx Create Task vi This VI is optional if you created and configured your task using the DAQ Assistant However if y...

Страница 43: ...n of Chapter 3 Configuring and Testing for information about shunt calibration in MAX Start Measurement DAQmx Start Task vi Read Measurement DAQmx Read vi Analyze Data Some examples of data analysis i...

Страница 44: ...ith another Active Channels box and assign the appropriate channel to each list of properties Note If you do not use Active Channels the properties are set on all of the channels in the task 6 Right c...

Страница 45: ...a task containing SCXI 1521 B channels you can set the properties of the channels programmatically using the DAQmx Channel Property Node Text Based ADEs You can use text based ADEs such as LabWindows...

Страница 46: ...ect is used since reading the voltage from a Wheatstone bridge configuration is an analog input operation The following text is a function prototype example void AIChannelCollection CreateStrainGageCh...

Страница 47: ...AX and load it into your application with the function call NationalInstruments DAQmx DaqSystem Local LoadTask Refer to NI Measurement Studio Help for more information about creating NI DAQmx tasks in...

Страница 48: ...NI recommends performing an external calibration once a year Calibrating the System You should perform offset null compensation or shunt calibration on transducers in the system to improve accuracy T...

Страница 49: ...ervoltage protection 11 V powered on and off Inputs protected AI 0 24 Transfer Nonlinearity 0 02 max Gain error After calibration1 0 1 reading max Before calibration2 4 0 of reading max Input offset e...

Страница 50: ...ndwidth Filter Lowpass filter type 2 pole Butterworth 12 dB octave rolloff 3 dB bandwidth 10 Hz Stability Recommended warm up time 30 minutes Gain drift 11 ppm C max 4 ppm C typ Offset drift 3 V C max...

Страница 51: ...0025 C 30 V C max Protection Surge arrestors in parallel with excitation terminals shunt to ground short circuit protected Bridge Completion SCXI 1521 350 0 1 2 ppm C max 55 mW at 50 C SCXI 1521 long...

Страница 52: ...l A 4 ni com Power Requirements from SCXI Backplane V 18 5 to 25 VDC 170 mA min V 18 5 to 25 VDC 170 mA min 5 V 4 75 to 5 25 VDC 50 mA min Physical Figure A 1 SCXI 1521 B Dimensions Weight 748 gm 26 4...

Страница 53: ...tegories II III or IV Environmental Operating temperature 0 to 50 C Storage temperature 20 to 70 C Humidity 10 to 90 RH noncondensing Maximum altitude 2 000 m Pollution Degree indoor use only 2 Safety...

Страница 54: ...CE Compliance The SCXI 1521 B meets the essential requirements of applicable European Directives as amended for CE marking as follows Low Voltage Directive safety 73 23 EEC Electromagnetic Compatibil...

Страница 55: ...ion window Click Yes to continue deleting the module or chassis or No to cancel this action Note Deleting the SCXI chassis deletes all modules in the chassis All configuration information for these mo...

Страница 56: ...terclockwise until they are loose but do not completely remove the thumbscrews 5 Remove the SCXI 1521 B by pulling steadily on both thumbscrews until the module slides completely out Figure B 1 Removi...

Страница 57: ...nately there is always the chance that one or more components in the system are not operating correctly that requires you to call or email a technical support representative who can often suggest addi...

Страница 58: ...utputs of SCXI 1521 B channels 1 through 7 Can I configure the SCXI 1521 B for use in parallel mode No The SCXI 1521 B does not support parallel mode Which digital lines are unavailable on the E M Ser...

Страница 59: ...0 1531 or SCXI 1140 module at least one of these modules must be the cabled module A cabled module is the module connected directly to the E M Series DAQ device Can I use the SCXI 1521 B with a versio...

Страница 60: ...ntrol the excitation voltage filter setting potentiometer settings and shunt calibration switches from LabVIEW Refer to Chapter 3 Configuring and Testing for more information What should I take into c...

Страница 61: ...lossary Symbol Prefix Value micro 10 6 m milli 10 3 k kilo 103 M mega 106 G giga 109 Numbers Symbols Percent Positive of or plus Negative of or minus Plus or minus Less than Per Degree Strain s Simula...

Страница 62: ...signal AI HOLD COMP AI HOLD Scan clock signal used to increment to the next channel after each E M Series DAQ device analog to digital conversion amplification A type of signal conditioning that impr...

Страница 63: ...lts from an object being compressed has a negative value current excitation A source that supplies the current needed by a sensor for its proper operation cutoff frequency The frequency that defines t...

Страница 64: ...lified DIN Deutsche Industrie Norme German Industrial Standard DIO Digital input output drivers driver software Software that controls a specific hardware device such as an E M Series DAQ device E EEP...

Страница 65: ...s a measure of strain gauge sensitivity GF See Gauge Factor H half bridge A Wheatstone bridge consisting of two active strain gauges and two passive fixed valued resistors Hz Hertz Cycles per second o...

Страница 66: ...a or computer memory max Maximum maximum working voltage The highest voltage with respect to ground that can be applied to an input terminal without damaging the device Includes both the signal and co...

Страница 67: ...t of measured output voltage to measured input voltage with the ends of the line connecting the extremes of the plot at the full scale limits O offset error The output of a system with a zero volt inp...

Страница 68: ...er bridge configuration the quarter bridge completion resistor must have the same nominal resistance value as the strain gauge R Rg Gauge resistance RL Line resistance RMA Return Material Authorizatio...

Страница 69: ...nnects modules in the same chassis to each other sensor A device that converts a physical phenomenon into an electrical signal SER CLK Serial clock signal used to synchronize digital data transfers ov...

Страница 70: ...t or an ADC when the analog inputs are grounded T tensile strain Strain that results from an object being stretched has a positive value typ Typical U UL Underwriters Laboratory V V Volts VDC Volts di...

Страница 71: ...eatstone bridge A circuit arrangement consisting of four resistive elements in a diamond pattern with excitation voltage applied across two opposing terminals small resistance changes in the elements...

Страница 72: ...ration settings excitation level 3 1 gain 3 2 null potentiometers 3 2 conventions used in the manual iv D DAQ devices cabling restrictions with plug in E Series DAQ devices C 3 connecting to SCXI 1521...

Страница 73: ...assignments 2 1 multiplexed mode operation connecting to SCXI 1521 B for DAQ device in PXI combination 1 4 questions about C 3 N NI DAQ software C 1 C 3 null compensation specifications A 2 theory of...

Страница 74: ...r pin assignments table 2 2 software application development environments supported 1 4 installation 1 4 NI DAQ version required C 1 specifications analog input A 1 bridge completion A 3 dynamic A 2 e...

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