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Chapter 4

Theory of Operation

NI PXI-4220 User Manual

4-2

ni.com

Strain Gauges

Strain-gauge configurations are arranged as Wheatstone bridges. The 
gauge is the collection of all of the active elements of the Wheatstone 
bridge. There are three types of strain-gauge configurations: quarter-, half-, 
and full-bridge. The number of active element legs in the Wheatstone 
bridge determines the kind of bridge configuration. Refer to Table 4-1 
to see how many active elements are in each configuration.

Each of these configurations is subdivided into multiple configuration 
types. The orientation of the active elements and the kind of strain 
measured determines the configuration type. NI supports seven 
configuration types in software. However, with custom software scaling 
you can use all Wheatstone bridge configuration types with any 
NI hardware product that supports the gauge configuration type.

The supported strain gauge configuration types measure axial strain, 
bending strain, or both. While you can use some similar configuration types 
to measure torsional strain, NI software scaling does not support these 
configuration types. It is possible to use NI products to measure torsional 
strain, but to properly scale these configuration types you must create a 
custom scale in MAX or perform scaling in your software application.

This document discusses all of the mechanical, electrical, and scaling 
considerations of each strain-gauge configuration type supported by NI.

Acronyms, Formulas, and Variable Definitions

In the figures and equations in this document, the acronyms, formulas, and 
variables are defined as:

ε

 is the measured strain (+

ε

 is tensile strain and –

ε

 is compressive strain).

ε

S

 is the simulated strain.

Table 4-1.  

Strain-Gauge Configurations

Configuration

Number of Active Elements

Quarter-bridge

1

Half-bridge

2

Full-bridge

4

Содержание NI PXI-4220

Страница 1: ...PXI NI PXI 4220 User Manual NI PXI 4220 User Manual July 2003 Edition Part Number 323530A 01 ...

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Страница 3: ...r maintenance instructions owner s modification of the product owner s abuse misuse or negligent acts and power failure or surges fire flood accident actions of third parties or other events outside reasonable control Copyright Under the copyright laws this publication may not be reproduced or transmitted in any form electronic or mechanical including photocopying recording storing in an informati...

Страница 4: ... marked on the product refer to the Read Me First Safety and Radio Frequency Interference document shipped with the product for precautions to take bold Bold text denotes items that you must select or click in the software such as menu items and dialog box options Bold text also denotes parameter names italic Italic text denotes variables emphasis a cross reference or an introduction to a key conc...

Страница 5: ... Self Test Verification 1 8 Removing the NI PXI 4220 1 8 Removing the PXI 4220 from a PXI Chassis 1 8 Removing the NI PXI 4220 from MAX 1 9 Chapter 2 Connecting Signals Connecting Bridge Sensor Signals to the NI PXI 4220 2 1 Quarter Bridge Type I 2 4 Quarter Bridge Type II 2 5 Half Bridge Type I 2 6 Half Bridge Type II 2 7 Full Bridge Type I 2 8 Full Bridge Type II 2 9 Full Bridge Type III 2 10 Re...

Страница 6: ...s Formulas and Variable Definitions 4 2 Software Scaling and Equations 4 4 Quarter Bridge Type I 4 4 Quarter Bridge Type II 4 6 Half Bridge Type I 4 9 Half Bridge Type II 4 11 Full Bridge Type I 4 13 Full Bridge Type II 4 15 Full Bridge Type III 4 17 NI PXI 4220 Theory of Operation 4 19 Bridge Configuration and Completion 4 22 Excitation 4 22 Remote Sense 4 23 Gain Input Range 4 25 Filter 4 26 Off...

Страница 7: ...g and Presenting 5 7 Completing the Application 5 8 Developing an Application Using LabVIEW 5 8 Using a DAQmx Channel Property Node in LabVIEW 5 10 Synchronization and Triggering 5 11 Synchronizing the NI PXI 4220 5 12 Synchronizing the NI PXI 4220 Using LabVIEW 5 14 Other Application Documentation and Material 5 15 Calibrating the NI PXI 4220 5 16 Calibrating the NI PXI 4220 5 16 Internal Calibra...

Страница 8: ...asuring Axial and Bending Strain 4 6 Figure 4 5 Quarter Bridge II Circuit Diagram 4 7 Figure 4 6 Half Bridge Type I Measuring Axial and Bending Strain 4 9 Figure 4 7 Half Bridge Type I Circuit Diagram 4 9 Figure 4 8 Half Bridge Type II Rejecting Axial and Measuring Bending Strain 4 11 Figure 4 9 Half Bridge Type II Circuit Diagram 4 11 Figure 4 10 Full Bridge Type I Rejecting Axial and Measuring B...

Страница 9: ... 6 Figure B 9 AI CONV CLK Input Signal Timing B 7 Figure B 10 AI CONV CLK Output Signal Timing B 8 Figure B 11 AI SAMPLE CLK TIMEBASE Signal Timing B 9 Figure B 12 AI HOLD COMPLETE Signal Timing B 10 Tables Table 3 1 Excitation Voltage for Configuration and Gauge Resistances 3 3 Table 4 1 Strain Gauge Configurations 4 2 Table 4 2 Control Codes for Coarse and Fine Null Potentiometers 4 27 Table 4 3...

Страница 10: ...that guarantees fast settling times at all gain settings The NI PXI 4220 also uses the NI DAQmx DAQ Assistant specifically the Strain Gage Calibration wizard to easily perform offset null compensation and shunt calibration This chapter provides an introduction to the NI PXI 4220 device its installation and removal About the NI PXI 4220 The NI PXI 4220 is a two channel module for interfacing to Whe...

Страница 11: ...e The NI PXI 4220 is configured using Measurement Automation Explorer MAX or through function calls to NI DAQmx What You Need to Get Started To set up and use the NI PXI 4220 you need the following NI PXI 4220 device NI PXI 4220 User Manual NI DAQ 7 0 or later DAQ Quick Start Guide Read Me First Safety and Radio Frequency Interference PXI chassis and documentation 1 8 in flathead screwdriver Natio...

Страница 12: ...bVIEW features interactive graphics a state of the art interface and a powerful graphical programming language The LabVIEW NI DAQmx Data Acquisition VI Library a series of virtual instruments for using LabVIEW with National Instruments DAQ hardware is included with LabVIEW LabVIEW is also capable of automatic code generation with the DAQ Assistant LabWindows CVI is a complete ANSI C ADE that featu...

Страница 13: ...click Download Software at ni com Installing the Software You can control the NI PXI 4220 in an ADE using NI DAQ 7 0 The supported ADEs include LabVIEW ANSI C and NET environments Complete the following steps to install your NI PXI 4220 software 1 Install your ADE if you have not already done so NI ADEs have release notes containing software installation instructions Note Your ADE version may not ...

Страница 14: ...ce for loose components or any sign of damage Notify NI if the device appears damaged in any way Do not install a damaged device into the PXI chassis Store the NI PXI 4220 device in the antistatic package when not in use Installing the NI PXI 4220 Device into a PXI Chassis Caution Power off the chassis before installing CompactPCI or PXI devices To install a NI PXI 4220 device into a PXI chassis s...

Страница 15: ...ect the card into the chassis 6 Secure the device front panel to the chassis using the front panel mounting screws The NI PXI 4220 is now installed Figure 1 1 Installing the NI PXI 4220 Device 1 PXI Chassis 2 PXI System Controller 3 NI PXI 4220 Module 4 Injector Ejector Handle 5 Front Panel Mounting Screws 6 Guides 7 Power Switch 3 1 2 4 6 PXI 1000B 7 5 ...

Страница 16: ...detection of the NI PXI 4220 complete the following steps Note If you need help during the verification process open the Measurement Automation Help by selecting Help Help Topics 1 Double click the Measurement Automation Explorer icon on the desktop 2 Display the list of devices and interfaces by clicking the next to the Devices and Interfaces icon 3 Display the list of NI DAQmx devices by clickin...

Страница 17: ...into a PXI Chassis section If these measures do not cause the operating system to detect the card or the self test to complete successfully contact NI Refer to the Signal Conditioning Technical Support Information document for contact information Removing the NI PXI 4220 This section provides details for removing an NI PXI 4220 device from MAX and from a PXI chassis Note You must physically remove...

Страница 18: ...Device Removal Removing the NI PXI 4220 from MAX To remove an NI PXI 4220 device from MAX complete the following steps after launching MAX 1 Display the list of installed devices and interfaces by clicking the next to the Devices and Interfaces icon The NI PXI 4220 should have a red circle with a white X inside it next to the device indicating it has been physically removed from the chassis 2 Righ...

Страница 19: ... of supported strain gauge configuration types as well as full bridge sensors such as load force torque and pressure sensors It also discusses connecting leads for remote sensing Refer to Chapter 4 Theory of Operation for a discussion of strain gauge concepts Caution Refer to the Read Me First Safety and Radio Frequency Interference document before removing equipment covers or connecting disconnec...

Страница 20: ...and calibration signals only Voltages greater than 10 V can damage the device Figure 2 2 shows the pin signal assignments for each of the NI PXI 4220 DSUB connectors Refer to Figure 2 2 when constructing DSUB connection leads to ensure the signal wires are routed correctly In the electrical connection diagrams each signal connection node is labelled with a pin number to indicate which pin carries ...

Страница 21: ...signments 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 Signal S Excitation P Remote Sense RS Shunt Cal A SCA Signal S Excitation P Remote Sense RS QTR Shunt Cal B QTR SCB Shunt Cal COM SCCOM Channel 0 Channel 1 Signal S Excitation P Remote Sense RS Shunt Cal A SCA Signal S Excitation P Remote Sense RS QTR Shunt Cal B QTR SCB Shunt Cal COM SCCOM ...

Страница 22: ...uit diagram and equations R1 and R2 are half bridge completion resistors R3 is the quarter bridge completion resistor dummy resistor R4 is the active element measuring tensile strain ε VEX is the excitation voltage RL is the lead resistance VCH is the measured voltage RS is shunt calibration resistor A Note The value of the quarter bridge completion resistor dummy resistor must equal the nominal r...

Страница 23: ... to the circuit diagram and equations R1 and R2 are half bridge completion resistors R3 is the quarter bridge temperature sensing element dummy gauge R4 is the active element measuring tensile strain ε VEX is the excitation voltage RL is the lead resistance VCH is the voltage measured RS is shunt calibration resistor A Note The quarter bridge type II is often confused with the more commonly used h...

Страница 24: ...e information Figure 2 5 Half Bridge Type I Circuit Diagram The following symbols apply to the circuit diagram and equations R1 and R2 are half bridge completion resistors R3 is an active element measuring compression from Poisson effect νε R4 is an active element measuring tensile strain ε VEX is the excitation voltage RL is the lead resistance VCH is the voltage measured RS is shunt calibration ...

Страница 25: ...y of Operation for more information Figure 2 6 Half Bridge Type II Circuit Diagram The following symbols apply to the circuit diagram and equations R1 and R2 are half bridge completion resistors R3 is an active element measuring compressive strain ε R4 is an active element measuring tensile strain ε VEX is the excitation voltage RL is the lead resistance VCH is the voltage measured RS is shunt cal...

Страница 26: ...ull Bridge Type I Circuit Diagram The following symbols apply to the circuit diagram and equations R1 is an active element measuring compressive strain ε R2 is an active element measuring tensile strain ε R3 is an active element measuring compressive strain ε R4 is an active element measuring tensile strain ε VEX is the excitation voltage RL is the lead resistance VCH is the measured voltage RS is...

Страница 27: ...2 8 Full Bridge Type II Circuit Diagram The following symbols apply to the circuit diagram and equations R1 is an active element measuring compressive Poisson effect νε R2 is an active element measuring tensile Poisson effect νε R3 is an active element measuring compressive strain ε R4 is an active element measuring tensile strain ε VEX is the excitation voltage RL is the lead resistance VCH is th...

Страница 28: ...tion Figure 2 9 Full Bridge Type III Circuit Diagram The following symbols apply to the circuit diagram and equations R1 is an active element measuring compressive Poisson effect νε R2 is an active element measuring tensile strain ε R3 is an active element measuring compressive Poisson effect νε R4 is an active element measuring the tensile strain ε VEX is the excitation voltage RL is the lead res...

Страница 29: ...t Diagram The following symbols apply to the circuit diagram and equations R1 R2 R3 and R4 are the transducer bridge elements VCH is the measured voltage VEX is the excitation voltage RL is the lead resistance VRSENSE is the measured bridge excitation Note Remote sense is illustrated using a full bridge circuit diagram for simplicity but in general the VRSENSE terminals should be connected to your...

Страница 30: ...st panels to verify the device is measuring signals properly The test panels allow you to measure the signal connected to the NI PXI 4220 directly as well as configure some of the properties of your measurement To open the NI PXI 4220 device test panels when in MAX complete the following steps 1 Display the list of devices and interfaces by clicking the next to the Devices and Interfaces icon 2 Di...

Страница 31: ...onfigurable settings for the NI PXI 4220 Bridge Configuration and Completion Bridge configuration is a software configurable setting that allows you to connect quarter half or full bridge sensors configurations Refer to Chapter 4 Theory of Operation for more information Excitation Level Excitation level is a software configurable setting that allows you to set the voltage excitation level on Pins ...

Страница 32: ...ration for more information Otherwise you should determine the appropriate gain using the input signal voltage range and the full scale limits of the NI PXI 4220 signal conditioning output signal Offset Null Compensation Potentiometers Coarse and fine offset null compensation potentiometers are software configurable settings that allow you to remove unwanted offset voltage In most cases you do not...

Страница 33: ...s sample and hold SS H is required Typical applications that might require SS H include vibration measurements and phase difference measurements You can disable this setting through your application if you require scan rates beyond the maximum allowable with SS H engaged NI recommends leaving SS H engaged Disabling SS H introduces a small offset voltage You can compensate for this offset by perfor...

Страница 34: ...onfiguration set through the settings tab of your NI DAQmx Task or NI DAQmx Global Channel and functions in your application Virtual channel properties override module properties The default bridge configuration for NI DAQmx is full bridge Voltage Excitation set either through NI DAQmx Task or NI DAQmx Global Channel You also can set the voltage excitation level through your application In NI DAQm...

Страница 35: ...the desktop 2 Right click Data Neighborhood and select Create New 3 Select NI DAQmx Task or NI DAQmx Global Channel and click Next 4 Select Analog Input and then select Strain 5 If you are creating a task keep the Create New Local Channels selected and select the channels to add to the task You can select blocks of channels by pressing the Shift key while making the selections or individual channe...

Страница 36: ... values to default Click Calibrate to calibrate the strain channel s When you have completed calibrating the strain channel s click Finish Note For offset null compensation and shunt calibration of quarter bridge configuration types the value of the quarter bridge completion resistor must equal the nominal resistance of the strain gauge 13 If you are creating a task and want to set timing or trigg...

Страница 37: ...e applying custom scaling select Create New from the Custom Scaling drop down and follow the on screen wizard 11 If you are creating a task and want to set timing or triggering controls enter the values in the Task Timing and Task Triggering tabs Verifying the Signal This section describes how to take measurements using test panels in order to verify signal connection system configuration and devi...

Страница 38: ...t button 11 Click the Start button 12 After you have completed verifying the channels click the Stop button You have now verified the NI PXI 4220 configuration and signal connection Note For more information on how to further configure the NI PXI 4220 or how to use LabVIEW to configure the module and take measurements refer to Chapter 5 Developing Your Application ...

Страница 39: ... 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 R1 and R2 compose one voltage divider circuit and R4 and R3 compose the second voltage divider circuit The output of a Wheatstone bridge is measured between the middle nodes of the two voltage dividers A...

Страница 40: ...pes with any NI hardware product that supports the gauge configuration type The supported strain gauge configuration types measure axial strain bending strain or both While you can use some similar configuration types to measure torsional strain NI software scaling does not support these configuration types It is possible to use NI products to measure torsional strain but to properly scale these c...

Страница 41: ...f expected signal voltage to excitation voltage with the shunt calibration circuit engaged Parameter U appears in the equations for simulated strain and is defined by the following equation ν is the 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 is the measured signa...

Страница 42: ...ypes in the following sections directly correspond to bridge selections in MAX and the LabVIEW Convert Strain Gauge Reading VI Quarter Bridge Type I This section provides information for the quarter bridge strain gauge configuration type I The quarter bridge type I measures either axial or bending strain Figure 4 2 shows how to position a strain gauge resistor in an axial and bending configuration...

Страница 43: ...is the nominal gauge resistance of the sensor RL is the lead resistance GF is the gauge factor To simulate the effect on strain of applying a shunt resistor across R3 use the following equation The value of the quarter bridge completion resistor dummy resistor must equal the nominal resistance of the strain gauge NI recommends using a 0 1 precision resistor If a less precise resistor is used offse...

Страница 44: ...by temperature variation of the leads cancel out leaving VCH unchanged The voltage drop across the lead resistance on a quarter bridge type I configuration is uncompensated in hardware It is important to accurately determine the gauge lead resistance and enter it in MAX or in the application software equation so the software can compensate for the voltage drop You can neglect lead resistance RL of...

Страница 45: ...th the half bridge type I configuration with the difference being that in the half bridge type I configuration the R3 element is active and bonded to the strain specimen to measure the effect of Poisson s ratio Completion resistors provide half bridge completion Compensates for specimen temperature variation Sensitivity at 1000 µε is 0 5 mVout VEX input Figure 4 5 Quarter Bridge II Circuit Diagram...

Страница 46: ...e gauge The nominal value of R3 is equal to Rg Gauges need not have a STC number corresponding to the material type of the test specimen As shown in Figure 2 4 Quarter Bridge II Circuit Diagram for greatest calibration accuracy use separate wires between the bridge and the shunt calibration pins SCA pin 4 and SCCOM pin 5 Do not directly short SCA pin 4 or SCCOM pin 5 inside your connector unless t...

Страница 47: ...ents One is mounted in the direction of axial strain the other acts as a Poisson gauge and is mounted transverse perpendicular to the principal axis of strain Completion resistors provide half bridge completion Sensitive to both axial and bending strain Compensates for specimen temperature variation Compensates for the aggregate effect on the principle strain measurement due to the Poisson s ratio...

Страница 48: ...s of the stress field Use this configuration in applications where no stress exists along the axis of the transverse strain gauge The nominal values of R3 and R4 equal Rg Strain gauges need not have a particular STC number As shown in Figure 2 5 Half Bridge Type I Circuit Diagram for greatest calibration accuracy use separate wires between the bridge and the shunt calibration pins SCA pin 4 and SC...

Страница 49: ...e Type II Rejecting Axial and Measuring Bending Strain A half bridge type II configuration has the following characteristics Two active strain gauge elements One is mounted in the direction of bending strain on one side of the strain specimen top the other is mounted in the direction of bending strain on the opposite side bottom Completion resistors provide half bridge completion Sensitive to bend...

Страница 50: ...ing a shunt resistor across R3 use the following equation Notes Half bridge type II requires one strain gauge to undergo tensile strain while the other strain gauge undergoes compressive strain of the same magnitude This configuration is often used to measure bending strain where the strain gauges are mounted on opposite sides of a beam The nominal values of R3 and R4 equal Rg The strain gauges ne...

Страница 51: ... a bending configuration Figure 4 11 shows the full bridge type I circuit wiring diagram Figure 4 10 Full Bridge Type I Rejecting Axial and Measuring Bending Strain A full bridge type I configuration has the following characteristics Four active strain gauge elements Two are mounted in the direction of bending strain on one side of the strain specimen top the other two are mounted in the direction...

Страница 52: ...red voltage To convert voltage readings to strain units use the following equation where GF is the gauge factor To simulate the effect on strain of applying a shunt resistor across R3 use the following equation Notes The nominal values of R1 R2 R3 and R4 equal Rg Strain gauges need not have a particular STC number As shown in Figure 2 7 Full Bridge Type I Circuit Diagram for greatest calibration a...

Страница 53: ...Four active strain gauge elements Two are mounted in the direction of bending strain with one on one side of the strain specimen top the other on the opposite side bottom The other two act together as a Poisson gauge and are mounted transverse perpendicular to the principal axis of strain with one on one side of the strain specimen top the other on the opposite side bottom Rejects axial strain Com...

Страница 54: ...tio To simulate the effect on strain of applying a shunt resistor across R3 use the following equation Notes Full bridge type II is sometimes used for strain measurement of bending beams R3 and R4 are positioned along the beam axis and on opposite sides of the beam and R1 and R2 are positioned transverse to the beam axis and on opposite sides of the beam The nominal values of R1 R2 R3 and R4 equal...

Страница 55: ... Four active strain gauge elements Two are mounted in the direction of axial strain with one on one side of the strain specimen top the other on the opposite side bottom The other two act together as a Poisson gauge and are mounted transverse perpendicular to the principal axis of strain with one on one side of the strain specimen top the other on the opposite side bottom Compensates for specimen ...

Страница 56: ...tio To simulate the effect on strain of applying a shunt resistor across R3 use the following equation Notes Full bridge type III is sometimes used for axial strain measurement R2 and R4 are positioned along the beam axis and on opposite sides of the beam and R1 and R3 are positioned transverse to the beam axis and on opposite sides of the beam The nominal values of R1 R2 R3 and R4 equal Rg Strain...

Страница 57: ...tion 4 19 NI PXI 4220 User Manual NI PXI 4220 Theory of Operation This section includes a brief overview and a detailed discussion of the circuit features of the module Refer to Figure 4 16 which is a block diagram of the NI PXI 4220 while reading this section ...

Страница 58: ... Bus Inte rface c PCI Bus Interface EEPROM EEPROM ADC FIFO A D Converter AI Control IRQ DMA NI PGA Analog Mode Multiplexer Voltage Reference Calibration DACs Control Address Data PXI Connector RTSI Data Cal MUX Temperature Sensor Signal Conditioning References SMB PFI0 Amplifier Input Cal Multiplexer S 0 S 0 Lowpass Filter Variable Gain Amp Simultaneous Sample Hold DAC DAC SCCOM 0 SCA 0 SCB 0 RS 0...

Страница 59: ... H This allows you to acquire synchronized measurements from both channels You cannot enable or disable SS H on a per channel basis Disabling SS H results in higher maximum sample rates Disabling SS H introduces a small offset voltage You can compensate for this offset by performing offset null calibration The NI PXI 4220 uses a multiplexed architecture that enables the measurement of multiple cha...

Страница 60: ...and PX pin 2 with a third lead connected to QTR SCBX pin 9 as shown in Figure 2 3 Quarter Bridge I Circuit Diagram Make sure that the value of the precision quarter bridge completion resistor exactly matches the nominal gauge resistance of the strain gauge The quarter bridge completion resistor is socketed for easy replacement The quarter bridge completion resistor for AI 0 is located on the NI PX...

Страница 61: ...r excitation output pin from exceeding 6 V with respect to chassis ground Note Chassis ground is at the same potential as earth ground when the PXI chassis is plugged into a standard 3 prong AC outlet If pin PX pin 7 is connected to earth ground the excitation source does not function properly Refer to the Configurable Settings in MAX section of Chapter 3 Configuring and Testing for more informati...

Страница 62: ...oftware Note If you use remote sense set RL to zero in the MAX configuration of the channel and in your application equations for measured strain ε If you leave the remote sense pin unconnected internal 1 kΩ resistors provide feedback to the buffers from pins PX pin 2 and PX pin 7 Therefore you need not install a jumper wire between RSX pin 3 and PX pin 2 or RSX pin 8 and PX pin 7 when you do not ...

Страница 63: ...module gain settings equal to or greater than 20 the gain of the first stage is set to 20 so that the noise and offset drift of later stages is small in comparison to this stage The instrumentation amplifier stage uses operational amplifiers with very low temperature drift and noise characteristics If overall module gain is less than twenty the first stage is set to 1 and the appropriate second st...

Страница 64: ... Application Filter The NI PXI 4220 provides two filtering stages with an overall response of a four pole Butterworth filter You can control the cutoff frequency of the filter through software You can choose 10 Hz 100 Hz 1 kHz 10 kHz or filter bypass to disable the filter For additional flexibility in cutoff frequency settings and for greater upper band suppression in the stop band NI recommends c...

Страница 65: ...he excitation voltage setting Therefore if the excitation voltage changes by a small amount due to changes such as temperature and sensor loading the correction signal produced by the potentiometers changes by the same amount and the offset null compensation is maintained The offset null compensation potentiometers are controlled digitally using control codes The control codes of the potentiometer...

Страница 66: ... adjusted for any gain errors or discrepancies in the nominal gauge factor Refer to the Configurable Settings in MAX section of Chapter 3 Configuring and Testing for more information about performing shunt calibration automatically in global channels using NI DAQmx in MAX For more information about programmatically performing shunt calibration in NI DAQmx refer to the Developing Your Application i...

Страница 67: ...nnels SS H signal conditioning is performed on the NI PXI 4220 with T H circuitry The outputs of the T H amplifiers follow their inputs also called tracking the inputs until they receive a hold signal from the DAQ circuitry Both channels on the NI PXI 4220 hold their signal at the same time Multiple NI PXI 4220 devices can be synchronized allowing you to simultaneously sample channels on multiple ...

Страница 68: ...nds that you leave SS H enabled for most applications By default the NI PXI 4220 has SS H enabled You should only disable SS H if your application does not require simultaneous Hold Line Convert Clock 1 2 3 CH0 CH1 1 HoldTime 2 Max Min Settle Time of MIO Min Settle Time of SCXI 3 TrackTime Signals Held Channels Sampled Signals Released Channels Return to Tracking Voltage output of the T H circuitr...

Страница 69: ...re differential In general a differential measurement system is preferable because it rejects not only common mode voltages and ground loop induced errors but also the noise picked up in the environment to a certain degree Common Mode Rejection Ratio The ability of a measurement device to reject voltages that are common to both input pins is referred to as the common mode rejection ratio CMRR and ...

Страница 70: ...s The DAQ STC makes possible applications such as internal hardware timing or multiple point acquisitions equivalent time sampling and seamless changing of the sampling rate The NI PXI 4220 uses the PXI trigger bus to easily synchronize several measurement functions between multiple NI PXI 4220 devices to a common trigger or timing event The PXI trigger bus is connected through the rear signal con...

Страница 71: ...20 Software can select PFI0 as the external source for a given timing signal Any timing signal can use the PFI0 pin as an input and multiple timing signals can simultaneously use the same PFI This flexible routing scheme reduces the need to change physical connections to the I O connector for different applications The front SMB connector also uses the programmable functionality of PFI0 to route p...

Страница 72: ...other device programmed to receive this timebase signal This clock source whether local or from the PXI trigger bus is used directly by the device as the primary frequency source The default configuration is to use the internal timebase without sharing the clock over the PXI trigger bus timebase signal The NI PXI 4220 can use the PXI_Trig 7 line to synchronize Master Timebase with other devices Fo...

Страница 73: ...his is the source for the analog input digital start trigger the trigger that begins an acquisition This sends out the actual analog input start trigger Input Output Input Output AI REF TRIG Input Output This is the trigger that creates the reference point between the pretrigger samples and the posttrigger samples Input Input Output AI SAMP CLK Input Output This clock controls the time interval be...

Страница 74: ...put Input Output AI PAUSE TRIG Input This signal can pause and resume acquisition Input Input AI SAMPLE CLK TIMEBASE Input This timebase provides the master clock from which the sample clocks are derived Input Input Table 4 4 PXI Trigger Bus Timing Signals Continued Signal Direction Description Availability on PFI0 SMB Availability on PXI Trigger Bus ...

Страница 75: ...ll advantage of the NI DAQ Assistant This section describes how to configure and use NI DAQmx to control the NI PXI 4220 in LabVIEW LabVIEW provides greater flexibility and access to more settings than MAX but you can use LabVIEW in conjunction with MAX to quickly create a customized application Typical Program Flow Figure 5 1 shows a typical program flow chart for creating a task configuring chan...

Страница 76: ...ant 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 Stop Measurement Yes No Perform Offset Null Compensation Perform Shunt Calibration Bridge Null Operation Yes No Yes No Shunt Calibration Operation Analyze Data Display Data No No Process Data Graphical D...

Страница 77: ...DAQ Assistant for ease of use when using a sensor that requires complex scaling or when many properties differ between channels in the same 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 using function or VI calls If you create a task using the DAQ As...

Страница 78: ...ly set by an internal clock based on the requested sample rate You also can select advanced features such as clock settings that specify an external clock source internal routing of the clock source or select the active edge of the clock signal Configuring Channel Properties All of the different ADEs used to configure the NI PXI 4220 access an underlying set of NI DAQmx properties Table 5 1 is a l...

Страница 79: ... AI Bridge ShuntCal Enable Specifies whether to place the shunt calibration resistor across one arm of the bridge Analog Input General Properties Signal Conditioning Excitation Value AI Excit Val Specifies the amount of excitation in volts Analog Input Strain Strain Gage Configuration AI StrainGage Cfg Specifies the strain gauge configuration type Analog Input General Properties Signal Conditionin...

Страница 80: ...nsation is performed during the configuration of the global channel s Refer to the Creating a Strain Analog Input General Properties Signal Conditioning Bridge Initial Bridge Voltage AI Bridge InitialVoltage Specifies in volts the output voltage of the bridge in the unloaded condition Analog Input General Properties Signal Conditioning Bridge Balance Coarse Potentiometer Property AI Bridge Balance...

Страница 81: ...ng and Testing for information about shunt calibration when in MAX To manually perform shunt calibration refer to the Shunt Calibration section of Chapter 4 Theory of Operation Acquiring Analyzing and Presenting After configuring the task and channels you can start your acquisition read measurements analyze the data returned and display it according to the needs of your application Typical methods...

Страница 82: ...ardware for use in another task Developing an Application Using LabVIEW This section describes in more detail the steps shown in the typical program flowchart in Figure 5 1 such as how to create a task in LabVIEW and configure the channels of the NI PXI 4220 If you need more information or for further instructions select Help VI Function How To Help from the LabVIEW menu bar Note Except where othe...

Страница 83: ...nd fully configured the channels in your task using the DAQ Assistant Any channel modifications made with a channel property node are not saved in the task in the DAQ Assistant they are only available for the present session Perform Offset Null Compensation You can perform offset null compensation when you create and configure your channels using the DAQ Assistant Refer to the Creating a Strain Gl...

Страница 84: ... separate the lists of properties with 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 Display Data You can use graphical tools such as charts gauges and graphs to display your data Some display tools are located on the Controls Numeric Indicators subpalette an...

Страница 85: ... another property to the property node right click an existing property and left click Add Element To change the new property left click it and select the property you wish to define Note Refer to the LabVIEW Help for information about property nodes and specific NI DAQmx properties Synchronization and Triggering If you have multiple NI PXI 4220 devices you can synchronize them to acquire samples ...

Страница 86: ...d start trigger each slave NI PXI 4220 still generates a hardware sample clock but it generates the clock using the timebase and start trigger of the master NI PXI 4220 This causes the slave device to acquire samples at the same time as the master The preferred method of synchronization is to use a shared timebase but it is also possible to synchronize multiple NI PXI 4220 devices by sharing the s...

Страница 87: ...hannels Configure Master Timing Create a Slave Task Create Slave Channels Set Slave to Use Timebase from Master Start Slave Measurement s Start Master Measurement Configure Master Channel Continue Sampling Yes No Clear Master Task Clear Slave Task Configure Slave Timing Configure Slave Channel More Slave Tasks Get Master Timebase Source and Rate from Master Task Configure Slave Triggering Yes No ...

Страница 88: ...irtual Channel vi AI Voltage by default To change to a strain gauge channel click AI Voltage and select Analog Input Strain Strain Gage Configure Master Channels Use a DAQmx Channel Property Node Refer to the Using a DAQmx Channel Property Node in LabVIEW section for more information Configure Master Timing DAQmx Timing vi Sample Clock by default Get Master Timebase Source and Rate from Master Tas...

Страница 89: ...Use a DAQmx Timing Property Node to set MasterTimebase Src and MasterTimebase Rate to the values retrieved from the master task in the Get Master Timebase Source and Rate from Master Task step Configure Slave Triggering DAQmx Trigger vi Start Digital Edge Use MasterDevice ai StartTrigger as the source substituting the master device identifier for MasterDevice Start Slave Measurement s DAQmx Start ...

Страница 90: ...uting channel inputs to ground or to an onboard high precision reference voltage NI driver software disconnects the channel inputs from the front signal connector and reconnects the inputs to ground or to an onboard reference when performing an internal two point calibration The driver software determines the correct adjustment factors for offset and gain error and stores them as calibration const...

Страница 91: ...oning Technical Support Information document Calibrating the System You should perform offset null compensation or shunt calibration on transducers in your system to improve accuracy This is considered system or end to end calibration Offset Null Compensation Performing an offset null compensation removes offset in the measurement system Factors such as imperfections in the strain gauge electrical...

Страница 92: ...t Number of input channels 2 Voltage gain settings X1 to X1000 with the following gain settings 1 1 15 1 3 1 5 1 8 2 2 2 2 4 2 7 3 1 3 6 4 2 5 6 6 5 7 5 8 7 10 11 5 13 15 18 20 22 24 27 31 36 42 56 65 75 87 100 115 130 150 180 200 220 240 270 310 360 420 560 650 750 870 1 000 Resolution 16 bits 1 in 65 536 Input coupling DC Bandwidth 20 kHz filter bypassed Table A 1 Maximum Sampling Rates Number o...

Страница 93: ...1000 100 0 0300 25 00 2 4 mV 1 0 V 10 0 1 300 500 50 0 0300 25 00 1 4 mV 500 0 mV 20 0 1 150 250 25 0 0300 5 00 675 µV 180 0 mV 42 0 1 75 100 10 0 0300 5 00 265 µV 100 0 mV 100 0 1 50 50 5 0 0 0300 5 00 155 µV 50 0 mV 200 0 1 50 25 2 5 0 0300 5 00 103 µV 18 0 mV 420 0 1 50 10 1 0 0 0300 5 00 69 µV 10 0 mV 1000 0 1 50 10 1 0 0 0300 5 00 61 µV Accuracies are valid for measurements following an inter...

Страница 94: ...30 mV s typ 100 mV s max Interchannel skew 200 ns typ Intermodule skew 250 ns typ Transfer Characteristics Offset error 150 µV G 20 3 mV G 20 Gain error 0 1 Non linearity Better than 0 02 of FSR DNL 0 5 LSB typ 1 LSB max No missing codes 16 bits guaranteed Calibration Recommended warm up time 15 minutes External calibration interval 1 year Onboard calibration reference Level 5 000 V 1 mV actual va...

Страница 95: ...ain 200 95 dB Dynamic Characteristics Minimum scan interval 0 0015 relative accuracy 16 bit 5 µs Wideband noise gain 200 10 kHz filter 40 µVpp Noise RTI gain 200 0 1 to 10 Hz 1 5 µVpp Spot noise RTI gain 200 1000 Hz 16nV Filter Characteristics Lowpass filter type 4 pole Butterworth 24 dB octave rolloff Lowpass filter settings 10 Hz 100 Hz 1 kHz 10 kHz or bypass Bandwidth filter bypassed 3 db at 20...

Страница 96: ...typ 25 µV C max Offset drift without self calibration at temperature Gain 20 18 µV C typ 23 µV C max Gain 20 350 µV C typ 375 µV C max Null Compensation Range 4 of excitation voltage 20 000 counts of resolution 80 000 microstrain null compensation range 4 microstrain resolution for quarter bridge GF 2 0 Excitation Type Constant voltage Settings 0 0 to 10 0 V in 1024 equally spaced increments Error...

Страница 97: ...s Resistor 100 kΩ resistors socketed on PXI 4220 Switch resistance 32 Ω typical 50 Ω max Switch off leakage 1 nA Switch break down voltage 60 V 1 Half bridge completion is performed inside the module and configured under software control The quarter bridge completion resistor is located near the front of the PXI 4220 and is socketed Resistors shipped with the PXI 4220 are 350 Ω installed and 120 Ω...

Страница 98: ...e Compatibility 5 V TTL Response Rising or falling edge Pulse width 10 ns min Impedance 10 kΩ Coupling DC PXI Trigger Bus Trigger lines 6 Star trigger 1 FIFO buffer size 512 samples Data transfers DMA interrupts programmed I O DMA modes Scatter gather single transfer demand transfer Configuration memory size 512 words Bus interface Master slave Power requirements 1 A at 5 VDC 5 Physical Dimensions...

Страница 99: ...following standards of safety for electrical equipment for measurement control and laboratory use IEC 61010 1 EN 61010 1 UL 3111 1 UL 61010B 1 CAN CSA C22 2 No 1010 1 Note For UL and other safety certifications refer to the product label or to ni com Electromagnetic Compatibility Emissions EN 55011 Class A at 10 m FCC Part 15A above 1 GHz Immunity EN 61326 1997 A2 2001 Table 1 EMC EMI CE C Tick an...

Страница 100: ...ropean Directives as amended for CE marking as follows Low Voltage Directive safety 73 23 EEC Electromagnetic Compatibility Directive EMC 89 336 EEC Note Refer to the Declaration of Conformity DoC for this product for any additional regulatory compliance information To obtain the DoC for this product click Declarations of Conformity Information at ni com hardref nsf ...

Страница 101: ...ces the need to change the physical wiring to the device I O connector for applications requiring alternative wiring As an input each PFI signal can be individually configured for edge or level detection and polarity selection You can use the polarity selection for any timing signal but the edge or level detection depends on the particular timing signal being controlled The detection requirements ...

Страница 102: ... a trigger event is received Figure B 1 shows a typical posttriggered sequence Figure B 1 Typical Posttriggered Sequence Pretriggered DAQ allows you to view data that is acquired before the trigger of interest in addition to data acquired after the trigger Figure B 2 shows a typical pretriggered sequence Figure B 2 Typical Pretriggered Sequence 1 3 0 4 2 AI START TRIG AI SAMP CLK AI CONV CLK Scan ...

Страница 103: ...relationship of AI START TRIG to the sequence As an output AI START TRIG reflects the action that initiates a sequence even if the acquisition is externally triggered by another PFI The output is an active high pulse with a pulse width of 50 to 100 ns This output is set to high impedance at startup Figures B 3 and B 4 show the input and output timing requirements for AI START TRIG Figure B 3 AI ST...

Страница 104: ...AI REF TRIG is recognized After the SC decrements to zero it is loaded with the number of posttrigger scans to acquire while the acquisition continues The device ignores AI REF TRIG if it is asserted prior to the SC decrementing to zero After the selected edge of AI REF TRIG is received the device acquires a fixed number of scans and the acquisition stops In pretriggered mode the device acquires d...

Страница 105: ...can The SI2 counter starts if you select an internally triggered AI CONV CLK As an output AI SAMP CLK reflects the actual start pulse that initiates a scan even if the starts are externally triggered by another PFI PXI_Trig 0 5 or PXI_Star You have two output options The first option is an active high pulse with a pulse width of 50 to 100 ns which indicates the start of the scan The second option ...

Страница 106: ...ted conversions the first AI CONV CLK appears when the onboard SI2 counter reaches zero If you select an external AI CONV CLK the first external pulse after AI SAMP CLK generates a conversion Separate the AI SAMP CLK pulses by at least one scan period Rising Edge Polarity Falling Edge Polarity tw 10 ns minimum tw b Scan in Progress Two Conversions per Scan Start Pulse AI CONV REF AI SAMP CLK toff ...

Страница 107: ...t the AI CONV CLK signal which is also available as an output on PXI_Trig 0 5 Refer to Figures B 1 and B 2 for the relationship of AI CONV CLK to the sequence As an input AI CONV CLK is configured in edge detection mode You can configure the polarity selection for either rising or falling edge The selected edge of AI CONV CLK initiates an A D conversion As an output AI CONV CLK reflects the actual...

Страница 108: ...uence Scans occurring within a sequence can be gated by either the hardware AI PAUSE TRIG signal or the software command register gate AI PAUSE TRIG Signal PFI0 PXI_Trig 0 5 or PXI_Star can externally input the AI PAUSE TRIG signal which is not available as an output on the I O connector AI PAUSE TRIG can mask off scans in a sequence You can configure the pin you select as the source for AI PAUSE ...

Страница 109: ...or low There is no minimum frequency Either the 20 MHz or 100 kHz internal timebase generates AI SAMPLE CLK TIMEBASE unless you select an external source Figure B 11 shows the timing requirements for AI SAMPLE CLK TIMEBASE Figure B 11 AI SAMPLE CLK TIMEBASE Signal Timing AI HOLD COMPLETE Signal AI HOLD COMPLETE is an output only signal that generates a pulse with the leading edge occurring approxi...

Страница 110: ...anual B 10 ni com Note The polarity of AI HOLD COMPLETE is not software selectable when programmed using NI DAQmx It is a positive polarity pulse Figure B 12 AI HOLD COMPLETE Signal Timing tw 400 to 500 ns td 50 to 100 ns AI CONV CLK AI HOLD COMPLETE td tw ...

Страница 111: ...t to the positive or negative voltage rail How do I control the gain excitation voltage filter setting bridge configuration potentiometer settings and shunt calibration switches from LabVIEW The gain of each NI PXI 4220 channel is automatically set based on the channel limits used in setting up the acquisition Although excitation voltage filter setting and bridge configuration are usually set usin...

Страница 112: ...ransducers with an external voltage source what voltage setting should I set on the NI PXI 4220 Even with an external voltage source for the transducers internal excitation voltage is still used by the offset null compensation circuitry inside the NI PXI 4220 Set the internal excitation voltage equal to the external excitation voltage to ensure the accuracy of offset null compensation How do I per...

Страница 113: ...micro 10 6 m milli 10 3 k kilo 103 M mega 106 G giga 109 Symbols percent positive of or plus negative of or minus per degree Ω ohm A A amperes A D analog to digital AC alternating current ADC analog to digital converter an electronic device often an integrated circuit that converts an analog voltage to a digital number ...

Страница 114: ... which a measuring device can respond bipolar a signal range that includes both positive and negative values for example 5 to 5 V breakdown voltage the voltage high enough to cause breakdown of optical isolation semiconductors or dielectric materials See working voltage bus the group of conductors that interconnect individual circuitry in a computer Typically a bus is the expansion vehicle to whic...

Страница 115: ...common mode signal any voltage present at the instrumentation amplifier inputs with respect to amplifier ground counter timer a circuit that counts external pulses or clock pulses timing CTR counter D D A digital to analog DAC digital to analog converter an electronic device often an integrated circuit that converts a digital number into a corresponding analog voltage or current DAQ data acquisiti...

Страница 116: ... from computer memory from to a device or memory on the bus while the processor does something else DMA is the fastest method of transferring data to from computer memory DNL differential nonlinearity a measure in least significant bit of the worst case deviation of code widths from their ideal value of 1 LSB driver software that controls a specific hardware device such as a DAQ device E EEPROM el...

Страница 117: ...ure of deviation of the gain of an amplifier from the ideal gain grounded measurement system See referenced single ended configuration H h hour Hz hertz the number of scans read or updates written per second I I O input output the transfer of data to from a computer system involving communications channels operator interface devices and or DAQ and control interfaces in inches INL integral nonlinea...

Страница 118: ... The interchannel delay is regulated by AI CONV CLK K k kilo the standard metric prefix for 1 000 or 103 used with units of measure such as volts hertz and meters kS 1 000 samples L LabVIEW Laboratory Virtual Instrument Engineering Workbench a program development application based on the programming language G and used commonly for test and measurement purposes LED light emitting diode linearity t...

Страница 119: ... functions and development tools for controlling measurement devices noise an undesirable electrical signal noise comes from external sources such as the AC power line motors generators transformers fluorescent lights soldering irons CRT displays computers electrical storms welders radio transmitters and internal sources such as semiconductors resistors and capacitors Noise corrupts signals you ar...

Страница 120: ...e RTSI bus for PCI DAQ devices R referenced single ended configuration RSE all measurements are made with respect to a common reference measurement system or ground also called a grounded measurement system relative accuracy a measure in LSB of the accuracy of an ADC It includes all nonlinearity and quantization errors It does not include offset and gain errors of the circuitry feeding the ADC res...

Страница 121: ...qual to the number of channels in the input group For example one pulse from the scan clock produces one scan which acquires one new sample from every analog input channel in the group scan clock the clock controlling the time interval between scans scan interval controls how often a scan is initialized The scan interval is regulated by AI SAMP CLK scan rate reciprocal of the scan interval self ca...

Страница 122: ...ar transistors wired in a certain manner V V volts VCC positive supply voltage VDC volts direct current VI virtual instrument 1 a combination of hardware and or software elements typically used with a PC that has the functionality of a classic stand alone instrument 2 a LabVIEW software module VI which consists of a front panel user interface and a block diagram program VIN volts in Vm measured vo...

Страница 123: ...installing modules figure 1 6 configuration settings excitation level 3 2 filter bandwidth 3 3 gain 3 3 4 25 null potentiometers 3 3 conventions used in the manual iv CONVERT signal See also PFI2 CONVERT signal DAQ timing connections B 7 signal routing figure 4 33 D DAQ timing connections AIGATE signal B 8 CONVERT signal B 7 SCANCLK signal B 9 SISOURCE signal B 9 STARTSCAN signal B 5 TRIG1 signal ...

Страница 124: ...ration I 2 8 4 13 full bridge configuration II 2 9 4 15 full bridge configuration III 2 10 4 17 half bridge configuration I 2 6 4 9 half bridge configuration II 2 7 4 11 quarter bridge configuration I 2 8 4 13 quarter bridge configuration II 2 9 4 15 remote sense 2 11 N NI DAQ software C 1 null compensation theory of operation 4 27 null potentiometers coarse and fine control codes table 4 27 confi...

Страница 125: ...CE signal B 9 software NI DAQ version required C 1 software programming choices National Instruments application software 1 3 NI DAQ 1 4 specifications electromagnetic compatibility A 8 regulatory compliance A 9 safety A 8 STARTSCAN signal B 5 See also PFI7 STARTSCAN signal strain gauge connections full bridge configuration I 2 8 4 13 full bridge configuration II 2 9 4 15 full bridge configuration...

Страница 126: ...B 1 programmable function input connections B 1 timing signal routing CONVERT signal routing figure 4 33 device and RTSI clocks 4 34 programmable function inputs 4 33 TRIG1 signal B 3 See also PFI0 TRIG1 signal TRIG2 signal B 4 See also PFI1 TRIG2 signal troubleshooting questions and answers C 1 ...

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