National Instruments SCXI-1520 User Manual

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SCXI

SCXI-1520 User Manual

May 2009 372583E-01

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Support

Worldwide Technical Support and Product Information

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Important Information

Warranty

The SCXI-1520 is warranted against defects in materials and workmanship for a period of one year from the date of shipment, as evidenced by receipts or other documentation. National Instruments will, at its option, repair or replace equipment that proves to be defective during the warranty period. This warranty includes parts and labor.

The media on which you receive National Instruments software are warranted not to fail to execute programming instructions, due to defects in materials and workmanship, for a period of 90 days from date of shipment, as evidenced by receipts or other documentation. National Instruments will, at its option, repair or replace software media that do not execute programming instruc tions if National Instruments receives notice of such defects during the warranty period. National Instruments does not warrant that the operation of the software shall be uninterrupted or error free.

A Return Material Authorization (RMA) number must be obtained from the factory and clearly marked on the outside of the package before any equipment will be accepted for warranty work. National Instruments will pay the shipping costs of returning to the owner parts which are covered by warranty.

National Instruments believes that the information in this document is accurate. The document has been carefully reviewed for technical accuracy. In the event that technical or typographical errors exist, National Instruments reserves the right to make changes to subsequent editions of this document without prior notice to holders of this edition. The reader should consult National Instruments if errors are suspected. In no event shall National Instruments be liable for any damages arising out of or related to this document or the information contained in it.

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(2) IN ANY APPLICATION, INCLUDING THE ABOVE, RELIABILITY OF OPERATION OF THE SOFTWARE PRODUCTS CAN BE IMPAIRED BY ADVERSE FACTORS, INCLUDING BUT NOT LIMITED TO FLUCTUATIONS IN ELECTRICAL POWER SUPPLY, COMPUTER HARDWARE MALFUNCTIONS, COMPUTER OPERATING SYSTEM SOFTWARE FITNESS, FITNESS OF COMPILERS AND DEVELOPMENT SOFTWARE USED TO DEVELOP AN APPLICATION, INSTALLATION ERRORS, SOFTWARE AND HARDWARE COMPATIBILITY PROBLEMS, MALFUNCTIONS OR FAILURES OF ELECTRONIC MONITORING OR CONTROL DEVICES, TRANSIENT FAILURES OF ELECTRONIC SYSTEMS (HARDWARE AND/OR SOFTWARE), UNANTICIPATED USES OR MISUSES, OR ERRORS ON THE PART OF THE USER OR APPLICATIONS DESIGNER (ADVERSE FACTORS SUCH AS THESE ARE HEREAFTER COLLECTIVELY TERMED “SYSTEM FAILURES”). ANY APPLICATION WHERE A SYSTEM FAILURE WOULD CREATE A RISK OF HARM TO PROPERTY OR PERSONS (INCLUDING THE RISK OF BODILY INJURY AND DEATH) SHOULD NOT BE RELIANT SOLELY UPON ONE FORM OF ELECTRONIC SYSTEM DUE TO THE RISK OF SYSTEM FAILURE. TO AVOID DAMAGE, INJURY, OR DEATH, THE USER OR APPLICATION DESIGNER MUST TAKE REASONABLY PRUDENT STEPS TO PROTECT AGAINST SYSTEM FAILURES, INCLUDING BUT NOT LIMITED TO BACK-UP OR SHUT DOWN MECHANISMS. BECAUSE EACH END-USER SYSTEM IS CUSTOMIZED AND DIFFERS FROM NATIONAL INSTRUMENTS' TESTING PLATFORMS AND BECAUSE A USER OR APPLICATION DESIGNER MAY USE NATIONAL INSTRUMENTS PRODUCTS IN COMBINATION WITH OTHER PRODUCTS IN A MANNER NOT EVALUATED OR CONTEMPLATED BY NATIONAL INSTRUMENTS, THE USER OR APPLICATION DESIGNER IS ULTIMATELY RESPONSIBLE FOR VERIFYING AND VALIDATING THE SUITABILITY OF NATIONAL INSTRUMENTS PRODUCTS WHENEVER NATIONAL INSTRUMENTS PRODUCTS ARE INCORPORATED IN A SYSTEM OR APPLICATION, INCLUDING, WITHOUT LIMITATION, THE APPROPRIATE DESIGN, PROCESS AND SAFETY LEVEL OF SUCH SYSTEM OR APPLICATION.

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Conventions

The following conventions are used in this manual:

<> Angle brackets that contain numbers separated by an ellipsis represent a

range of values associated with a bit or signal name—for example, AO <3. .0>.

» The » symbol leads you through nested menu items and dialog box options

to a final action. The sequence File»Page Setup»Options directs you to pull down the File menu, select the Page Setup item, and select Options from the last dialog box.

This icon denotes a note, which alerts you to important information.

This icon denotes a caution, which advises you of precautions to take to avoid injury, data loss, or a system crash. When this symbol is marked on a product, refer to the Read Me First: Safety and Radio-Frequency

Interference for information about 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 concept. Italic text also denotes text that is a placeholder for a word or value that you must supply.

monospace Text in this font denotes text or characters that you should enter from the

keyboard, sections of code, programming examples, and syntax examples. This font is also used for the proper names of disk drives, paths, directories, programs, subprograms, subroutines, device names, functions, operations, variables, filenames, and extensions.

monospace bold Bold text in this font denotes the messages and responses that the computer

automatically prints to the screen. This font also emphasizes lines of code that are different from the other examples.

monospace italic

Italic text in this font denotes text that is a placeholder for a word or value that you must supply.

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Chapter 1 About the SCXI-1520

What You Need to Get Started ......................................................................................1-1

National Instruments Documentation ............................................................................1-3

Installing Application Software, NI-DAQ, and the E/M Series DAQ Device .............. 1-5

Installing the SCXI-1520 Module into the SCXI Chassis...............................1-5

Connecting the SCXI-1520 in an SCXI Chassis to an E/M Series

DAQ Device for Multiplexed Scanning .......................................................1-5

Connecting the SCXI-1520 in a PXI/SCXI Combination Chassis to an

E/M Series DAQ Device for Multiplexed Scanning ....................................1-5

Verifying the SCXI-1520 Installation in Software ........................................................1-6

Installing SCXI Using NI-DAQmx in Software .............................................1-6

Manually Adding Modules in NI-DAQmx .....................................................1-6

Installing SCXI Using Traditional NI-DAQ (Legacy) in Software ................ 1-6

Manually Adding Modules in Traditional NI-DAQ (Legacy) ........................1-6

Verifying and Self-Testing the Installation .....................................................1-6

Troubleshooting the Self-Test Verification ...................................................................1-7

Troubleshooting in NI-DAQmx ......................................................................1-7

Troubleshooting in Traditional NI-DAQ (Legacy) ......................................... 1-8

Chapter 2 Connecting Signals

Connecting Bridge Sensor Signals ................................................................................ 2-1

Quarter-Bridge Type I .....................................................................................2-1

Quarter-Bridge Type II....................................................................................2-2

Half-Bridge Type I ..........................................................................................2-4

Half-Bridge Type II.........................................................................................2-5

Full-Bridge Type I...........................................................................................2-6

Full-Bridge Type II.......................................................................................... 2-7

Full-Bridge Type III ........................................................................................2-8

Remote Sense ..................................................................................................2-9

Pin Assignments ............................................................................................................2-10

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Contents

Chapter 3 Configuring and Testing

SCXI-1520 Software-Configurable Settings .................................................................3-1

Common Software-Configurable Settings ...................................................... 3-1

Bridge Configuration........................................................................ 3-1

Excitation Level................................................................................ 3-2

Filter Bandwidth ............................................................................... 3-2

Gain/Input Range.............................................................................. 3-3

Null Potentiometers .......................................................................... 3-4

Shunt Calibration Switches .............................................................. 3-4

Modes of Operation .......................................................................... 3-5

Simultaneous Sample and Hold........................................................ 3-5

Configurable Settings in MAX...................................................................................... 3-5

NI-DAQmx ..................................................................................................... 3-6

Creating a Strain Global Channel or Task........................................ 3-7

Creating a Custom Voltage with Excitation Global Channel

or Task ........................................................................................... 3-8

Traditional NI-DAQ (Legacy) ........................................................................ 3-9

Configuring Module Property Pages in Traditional NI-DAQ

(Legacy)......................................................................................... 3-10

Creating a Strain Virtual Channel .................................................... 3-11

Calibrating a Strain Virtual Channel ................................................ 3-11

Verifying the Signal ...................................................................................................... 3-12

Verifying the Signal in NI-DAQmx Using a Task or Global Channel........... 3-12

Verifying the Signal in Traditional NI-DAQ (Legacy) .................................. 3-13

Verifying the Signal Using Channel Strings .................................... 3-13

Verifying the Signal Using Strain Virtual Channel .......................... 3-14

Using the Strain Calibration Wizard in NI-DAQmx..................................................... 3-14

Chapter 4 Theory of Operation

Strain-Gauge Theory ..................................................................................................... 4-1

Wheatstone Bridges ........................................................................................ 4-1

Strain Gauges .................................................................................................. 4-2

Acronyms, Formulas, and Variable Definitions ............................................. 4-3

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

SCXI-1520 User Manual vi ni.com

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Full-Bridge Type II.......................................................................................... 4-14

Full-Bridge Type III ........................................................................................4-16

SCXI-1520 Theory of Operation ...................................................................................4-18

Bridge Configuration and Completion ............................................................4-21

Excitation.........................................................................................................4-22

Remote Sense....................................................................................4-23

Gain ................................................................................................................. 4-24

Filter Bandwidth and Cutoff Frequency..........................................................4-25

Offset Null Compensation...............................................................................4-25

Shunt Calibration.............................................................................................4-27

Simultaneous Sample and Hold.......................................................................4-28

Maximum Simultaneous Sample and Hold Sample Rate Using

Maximum Simultaneous Sample and Hold Using Traditional

Modes of Operation......................................................................................... 4-34

Theory of Multiplexed Mode Operation ...........................................4-34

Theory of Parallel Mode Operation ..................................................4-35

Chapter 5 Using the SCXI-1520

Developing Your Application in NI-DAQmx ...............................................................5-1

Typical Program Flowchart.............................................................................5-2

General Discussion of Typical Flowchart .......................................................5-4

Creating a Task Using DAQ Assistant or Programmatically ...........5-4

Adjusting Timing and Triggering ..................................................... 5-4

Configuring Channel Properties........................................................5-5

Performing Offset Null Compensation .............................................5-7

Performing Shunt Calibration ...........................................................5-8

Acquiring, Analyzing, and Presenting ..............................................5-8

Completing the Application .............................................................. 5-9

Developing an Application Using LabVIEW..................................................5-9

Using a DAQmx Channel Property Node in LabVIEW ...................5-11

Specifying Channel Strings in NI-DAQmx.....................................................5-12

Text Based ADEs..............................................................................5-14

Measurement Studio (Visual Basic, .NET, and C#)........................................5-14

Programmable NI-DAQmx Properties..............................................5-16

Developing Your Application in Traditional NI-DAQ (Legacy) ..................................5-18

Traditional NI-DAQ (Legacy) in LabVIEW................................................... 5-19

Typical Program Flow ......................................................................5-20

Configuring the SCXI-1520 Settings Using Traditional NI-DAQ

(Legacy) in LabVIEW ..................................................................................5-22

Contents

NI-DAQmx ....................................................................................4-30

NI-DAQ (Legacy) ..........................................................................4-32

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Contents

Performing Offset Null Compensation Using Traditional NI-DAQ

(Legacy) in LabVIEW.................................................................................. 5-24

Performing Shunt Calibration Using Traditional NI-DAQ (Legacy)

in LabVIEW ................................................................................................. 5-26

Configure, Start Acquisition, and Take Readings Using Traditional

NI-DAQ (Legacy) in LabVIEW .................................................................. 5-27

Converting Scaling Using Traditional NI-DAQ (Legacy) in LabVIEW ........ 5-27

Analyze and Display Using Traditional NI-DAQ (Legacy) in LabVIEW ..... 5-28

Traditional NI-DAQ (Legacy) in Text-Based ADEs ...................................... 5-28

Low-Level DAQ Functions ............................................................................ 5-29

Configuring System Settings Using Traditional NI-DAQ (Legacy) C API ... 5-31 Configuring Module Settings Using Traditional NI-DAQ (Legacy) C API... 5-32 Performing Offset Null Compensation Using Traditional NI-DAQ

(Legacy) C API ............................................................................................ 5-34

Performing Shunt Calibration Using Traditional NI-DAQ

(Legacy) C API ............................................................................................ 5-34

Performing Acquisition Using Traditional NI-DAQ (Legacy) C API............ 5-35

Performing Scaling, Analysis, and Display .................................................... 5-35

Other Application Documentation and Material ........................................................... 5-36

Traditional NI-DAQ (Legacy) CVI Examples................................................ 5-36

Traditional NI-DAQ (Legacy) Measurement Studio Examples ..................... 5-36

Calibrating the Strain System........................................................................................ 5-37

Calibrating the SCXI-1520 ............................................................................. 5-37

Internal Calibration Procedure.......................................................... 5-37

Internal Calibration Using LabVIEW............................................... 5-37

Internal Calibration Using a C-Based ADE ..................................... 5-38

External Calibration.......................................................................... 5-38

Calibrating the System .................................................................................... 5-39

Offset Null Compensation ................................................................ 5-39

Shunt Calibration.............................................................................. 5-39

Appendix A Specifications

Appendix B Using SCXI Channel Strings with Traditional NI-DAQ (Legacy) 7.0 or Later

Appendix C Removing the SCXI-1520

SCXI-1520 User Manual viii ni.com

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Appendix D Common Questions

Glossary

Index

Figures

Figure 2-1. Quarter-Bridge I Circuit Diagram .........................................................2-2

Figure 2-2. Quarter-Bridge II Circuit Diagram........................................................2-3

Figure 2-3. Half-Bridge Type I Circuit Diagram .....................................................2-4

Figure 2-4. Half-Bridge Type II Circuit Diagram....................................................2-5

Figure 2-5. Full-Bridge Type I Circuit Diagram......................................................2-6

Figure 2-6. Full-Bridge Type II Circuit Diagram ....................................................2-7

Figure 2-7. Full-Bridge Type III Circuit Diagram ...................................................2-8

Figure 2-8. Remote-Sense Circuit Diagram.............................................................2-9

Figure 4-1. Basic Wheatstone Bridge Circuit Diagram ...........................................4-1

Figure 4-2. Quarter-Bridge Type I Measuring Axial and Bending Strain ...............4-4

Figure 4-3. Quarter-Bridge I Circuit Diagram .........................................................4-5

Figure 4-4. Quarter-Bridge Type II Measuring Axial and Bending Strain..............4-7

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 Bending Strain..... 4-13

Figure 4-11. Full-Bridge Type I Circuit Diagram......................................................4-13

Figure 4-12. Full-Bridge Type II Rejecting Axial and Measuring Bending Strain ...4-15

Figure 4-13. Full-Bridge Type II Circuit Diagram ....................................................4-15

Figure 4-14. Full-Bridge Type III Measuring Axial and

Rejecting Bending Strain.......................................................................4-17

Figure 4-15. Full-Bridge Type III Circuit Diagram ...................................................4-17

Figure 4-16. Block Diagram of SCXI-1314/SCXI-1520 Combination .....................4-19

Figure 4-17. Signal During Simultaneous Sample-and-Hold Sampling ....................4-29

Contents

Figure 5-1. Typical Program Flowchart...................................................................5-3

Figure 5-2. LabVIEW Channel Property Node with Filtering Enabled

at 10 kHz and SS/H Disabled................................................................5-12

Figure 5-3. Typical SCXI-1520 Program Flow with

Traditional NI-DAQ (Legacy)...............................................................5-21

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Contents

Tables

Figure 5-4. Using the AI Parameter VI to Set Up the SCXI-1520 .......................... 5-24

Figure 5-5. Offset Null and Shunt Calibration Flowchart ....................................... 5-30

Figure C-1. Removing the SCXI-1520..................................................................... C-2

Table 1-1. Accessories Available for the SCXI-1520............................................ 1-2

Table 2-1. Front Signal Pin Assignments ..............................................................2-11

Table 2-2. Rear Signal Pin Assignments................................................................ 2-13

Table 2-3. SCXI-1520 Communication Signals..................................................... 2-14

Table 3-1. Excitation Voltage for Configuration and Gauge Resistances ............. 3-2

Table 4-1. Strain-Gauge Configurations ................................................................4-2

Table 4-2. Control Codes for Coarse and Fine Offset Null Potentiometers........... 4-26

Table 4-3. NI-DAQmx Values Used to Determine Maximum Sample Rate

in Multiplexed Mode............................................................................. 4-31

Table 4-4. NI-DAQmx Values Used to Determine Maximum Sample Rate

in Parallel Mode.................................................................................... 4-32

Table 4-5. Traditional NI-DAQ (Legacy) Values Used to Determine

Maximum Sample Rate in Multiplexed Mode...................................... 4-33

Table 5-1. NI-DAQmx Properties .......................................................................... 5-5

Table 5-2. Programming a Task in LabVIEW ....................................................... 5-9

Table 5-3. NI-DAQmx Properties .......................................................................... 5-16

Table 5-4. Settings for Configuring the SCXI-1520 Through the AI Parameter ...5-22

Table 5-5. Configuration Functions ....................................................................... 5-31

Table 5-6. NI-DAQ Functions Used to Configure SCXI-1520.............................. 5-33

Table D-1. Digital Signals on the SCXI-1520 ....................................................... D-2

SCXI-1520 User Manual x ni.com

Page 11

About the SCXI-1520

The SCXI-1520 module is an eight-channel module for interfacing to strain-gauge bridges and other Wheatstone-bridge based sensors.

Note Descriptions in this chapter explicitly refer to the first channel (channel 0); however,

the same descriptions are applicable to channels <1..7>.

You can configure all settings on a per channel basis in software. The SCXI-1520 is configured using Measurement & Automation Explorer (MAX) or through NI-DAQmx property nodes.

With the SCXI-1520 and the accessory SCXI-1314 terminal block, you can do the following:

• Connect sensors of all bridge configurations, including quarter-, half-, and full-bridge

• Set the DC voltage excitation between 0 and 10 V (increments dependent upon the driver software)

• Programmatically offset null bridge circuits connected to the SCXI-1520

• Set the analog input lowpass filter cut-off frequency to 10 Hz, 100 Hz, 1 kHz, 10 kHz, or bypass

• Set the analog input gain between 1 and 1000 at any one of 49 settings

• Implement shunt calibration using two independent circuits

• Connect the bridge for remote-sense voltage excitation

What You Need to Get Started

To set up and use the SCXI-1520, you need the following:

❑ Hardware

– SCXI-1520 module

– One of the following terminal blocks:

• SCXI-1314 terminal block

• SCXI-1314T terminal block

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Chapter 1 About the SCXI-1520

– SCXI or PXI/SCXI combo chassis

– E/M Series DAQ device

– Computer, if using an SCXI chassis

– Cabling, cable adapter, and sensors as required for your

application

❑ Software

– NI-DAQ 7.0 or later

– Application software, such as LabVIEW, LabWindows

Measurement Studio, or other programming environments

❑ Documentation

– Read Me First: Safety and Radio-Frequency Interference

– DAQ Getting Started Guide

– SCXI Quick Start Guide

– SCXI-1520 User Manual

– Documentation for your hardware

– Documentation for your software

™

/CVI™,

The optional accessories listed in Table 1-1 are available for the SCXI-1520.

Table 1-1. Accessories Available for the SCXI-1520

Accessory Description

SCXI-1314

†

Screw terminal block—Mounts on the front of the SCXI-1520 module. It includes connections and sockets for two shunt calibration resistors and a quarter-bridge completion resistor per channel.

SCXI-1314T

††

RJ-50 terminal block—Mounts on the front of the SCXI-1520 module. It features eight RJ-50 10-position/10-conductor (10p10c) modular plugs for connection to hardware TEDS smart sensors.

SCXI-1520 User Manual 1-2 ni.com

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Chapter 1 About the SCXI-1520

Table 1-1. Accessories Available for the SCXI-1520 (Continued)

Accessory Description

SCXI-1310

‡

Connector and shell assembly—The SCXI-1310 provides 96 eyelet-type terminals for easy hook-and-solder signal connection and custom mass termination connectivity.

TBX-96

‡

DIN-rail mounted terminal block with 96 generic screw terminals.

One of the following cables is required to connect the TBX-96 to an SCXI module:

• SH96-96 shielded cable, 1 m

• R96-96 unshielded ribbon cable, 1 m

†

You must wire a shunt resistor between the pins that correspond to the SCA on the SCXI-1314 and the appropriate legs of

the bridge sensor. Refer to Chapter 4, Theory of Operation, for more information.

††

The SCXI-1314T only supports SCA. It does not support SCB. There are no quarter-bridge completion resistors in the SCXI-1314T. If you are connecting a quarter-bridge sensor to an SCXI-1520 using an SCXI-1314T, you must place an external resistor between the pins that correspond to PX– and SX+ on the terminals. Refer to Chapter 2, Connecting Signals, for more information.

‡

There are no quarter-bridge completion resistors in the SCXI-1310 or TBX-96. If you are connecting a quarter-bridge sensor to an SCXI-1520 using an SCXI-1310 or TBX-96, you must place an external resistor between the pins that correspond to PX– and SX+ on the terminals. Refer to Chapter 2, Connecting Signals, for more information.

National Instruments Documentation

The SCXI-1520 User Manual is one piece of the documentation set for data acquisition (DAQ) systems. You could have any of several types of manuals depending on the hardware and software in the system. Use the manuals you have as follows:

• Getting Started with SCXI—This is the first manual you should read. It gives an overview of the SCXI system and contains the most commonly needed information for the modules, chassis, and software.

• SCXI or PXI/SCXI chassis manual—Read this manual for maintenance information on the chassis and for installation instructions.

• The DAQ Getting Started Guide—This document has information on installing NI-DAQ and the E/M Series DAQ device. Install these before you install the SCXI module.

• The SCXI Quick Start Guide—This document contains a quick overview for setting up an SCXI chassis, installing SCXI modules and terminal blocks, and attaching sensors. It also describes setting up the SCXI system in MAX.

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Chapter 1 About the SCXI-1520

• The SCXI hardware user manuals—Read these manuals for detailed information about signal connections and module configuration. They also explain, in greater detail, how the module works and contain application hints.

• Accessory installation guides or manuals—Read the terminal block and cable assembly installation guides. They explain how to physically connect the relevant pieces of the system. Consult these guides when you are making the connections.

• The E/M Series DAQ device documentation—This documentation has detailed information about the DAQ device that plugs into or is connected to the computer. Use this documentation for hardware installation and configuration instructions, specification information about the DAQ device, and application hints.

• Software documentation—You may have both application software and NI-DAQ software documentation. National Instruments (NI) application software includes LabVIEW, LabWindows/CVI, and Measurement Studio. After you set up the hardware system, use either your application software documentation or the NI-DAQ documentation to help you write your application. If you have a large, complex system, it is worthwhile to look through the software documentation before you configure the hardware.

• One or more of the following help files for software information:

– Start»Programs»National Instruments»NI-DAQ»

NI-DAQmx Help

– Start»Programs»National Instruments»NI-DAQ»

Traditional NI-DAQ User Manual

– Start»Programs»National Instruments»NI-DAQ»

Traditional NI-DAQ Function Reference Help

• NI strain-gauge application notes or tutorials—NI has additional material about strain gauges and strain measurements available at

ni.com/support.

You can download NI documents from the latest version of NI-DAQ, click Download Software at

SCXI-1520 User Manual 1-4 ni.com

ni.com/manuals. To download

ni.com.

Page 15

Chapter 1 About the SCXI-1520

Installing Application Software, NI-DAQ, and the E/M Series DAQ Device

Refer to the DAQ Getting Started Guide packaged with the NI-DAQ software to install your application software, NI-DAQ driver software, and the DAQ device to which you will connect the SCXI-1520. NI-DAQ 7.0 or later is required to configure and program the SCXI-1520 module. If you do not have NI-DAQ 7.0 or later, you can either contact a NI sales representative to request it on a CD or download the latest NI-DAQ version

ni.com.

from

Note Refer to the Read Me First: Radio-Frequency Interference document before

removing equipment covers or connecting or disconnecting any signal wires.

Installing the SCXI-1520 Module into the SCXI Chassis

Refer to the SCXI Quick Start Guide to install your SCXI-1520 module.

Connecting the SCXI-1520 in an SCXI Chassis to an E/M Series DAQ Device for Multiplexed Scanning

Refer to the SCXI Quick Start Guide to install the cable adapter and connect the SCXI modules to the DAQ device.

If you have already installed the appropriate software, refer to Chapter 3,

Configuring and Testing, to configure the SCXI-1520 module(s).

Connecting the SCXI-1520 in a PXI/SCXI Combination Chassis to an E/M Series DAQ Device for Multiplexed Scanning

Refer to the SCXI Quick Start Guide to connect the SCXI modules to the DAQ device.

If you have already installed the appropriate software, refer to Chapter 3,

Configuring and Testing, to configure the SCXI-1520 module(s).

Page 16

Chapter 1 About the SCXI-1520

Verifying the SCXI-1520 Installation in Software

Refer to the SCXI Quick Start Guide for information on verifying the SCXI installation.

Installing SCXI Using NI-DAQmx in Software

Refer to the SCXI Quick Start Guide for information on installing modules using NI-DAQmx in software.

Manually Adding Modules in NI-DAQmx

If you did not auto-detect the SCXI modules, you must manually add each of the modules. Refer to the SCXI Quick Start Guide to manually add modules.

Note NI recommends auto-detecting modules for the first time configuration of the

chassis.

Installing SCXI Using Traditional NI-DAQ (Legacy) in Software

Refer to the SCXI Quick Start Guide for information on installing modules using Traditional NI-DAQ (Legacy) in software.

Manually Adding Modules in Traditional NI-DAQ (Legacy)

If you did not auto-detect the SCXI modules, you must manually add each of the modules. Refer to the SCXI Quick Start Guide to manually add modules.

Note NI recommends auto-detecting modules for the first time configuration of the

chassis.

Verifying and Self-Testing the Installation

The verification procedure for the SCXI chassis is the same for both NI-DAQmx and Traditional NI-DAQ (Legacy). To test the successful installation for the SCXI chassis, refer to the SCXI Quick Start Guide. Verify that the chassis is powered on and correctly connected to an E/M Series DAQ device.

SCXI-1520 User Manual 1-6 ni.com

Page 17

Chapter 1 About the SCXI-1520

After verifying and self-testing the installation, the SCXI system should operate properly with your ADE software. If the test did not complete successfully, refer to Chapter 3, Configuring and Testing, for troubleshooting steps.

Troubleshooting the Self-Test Verification

If the Self-Test Verification did not verify the chassis configuration, complete the steps in this section to troubleshoot the SCXI configuration.

Troubleshooting in NI-DAQmx

• If you get a Verify SCXI Chassis message box showing the SCXI

chassis model number, Chassis ID: x, and one or more messages stating Slot Number: x Configuration has module: SCXI-XXXX or 1520, hardware in chassis is: Empty, take the following troubleshooting actions:

– Make sure the SCXI chassis is powered on.

– Make sure all SCXI modules are properly installed in the chassis.

Refer to the SCXI Quick Start Guide for proper installation instructions.

– Make sure the cable between the SCXI chassis and E/M Series

DAQ device is properly connected.

– Inspect the cable connectors for bent pins.

– Make sure you are using the correct NI cable assembly.

– Test the DAQ device to verify it is working properly. Refer to the

DAQ device help file for more information.

• If you get a Verify SCXI Chassis message box showing the SCXI

chassis model number,

Number:

hardware in chassis is: SCXI-

complete the following troubleshooting steps to correct the error.

1. Expand the list of NI-DAQmx devices by clicking the + next to

2. Right-click the SCXI chassis and click Properties to load the

3. Under the Modules tab, ensure that the cabled module is listed in

Configuration has module: SCXI-

NI-DAQmx Devices.

chassis configurator.

the correct slot.

Chassis ID:

, and the message Slot

YYYY

XXXX

or 1520,

, 1520, or Empty,

Page 18

Chapter 1 About the SCXI-1520

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 is empty, click the drop-down listbox next to the correct slot and select the cabled module. Configure the cabled module following the steps listed in the SCXI Quick Start Guide. Click OK.

b. If another module appears where the cabled module should

be, click the drop-down listbox next to the correct slot and select the cabled module. A message box appears asking you to confirm the module replacement. Click OK. Configure the cabled module following the steps listed in the SCXI Quick Start Guide. Click OK.

• Ensure that you have the highest priority SCXI module cabled to the E/M Series DAQ device. Refer to the SCXI Quick Start Guide to find out which SCXI module in the chassis should be cabled to the DAQ device.

• After checking the preceding items, return to the Troubleshooting the

Self-Test Verification section and retest the SCXI chassis.

If these measures do not successfully configure the SCXI system, contact NI. Refer to the Technical Support Information document for contact information.

Troubleshooting in Traditional NI-DAQ (Legacy)

• If you get the message Unable to test chassis at this time, you have not designated at least one module as connected to a E Series DAQ device. Refer to the Traditional NI-DAQ (Legacy) section of Chapter 3, Configuring and Testing, the cabled module in the system from Connected to: None to Connected to: Device x.

• If you get the message codes and the message

Failed to find followed by the module Unable to communicate with chassis,

take the following troubleshooting actions:

– Make sure the SCXI chassis is powered on.

– Make sure the cable between the SCXI chassis and E Series DAQ

device is properly connected.

– Inspect the cable connectors for bent pins.

– Make sure you are using the correct NI cable assembly.

– Test the DAQ device to verify it is working properly. Refer to the

DAQ device help file for more information.

SCXI-1520 User Manual 1-8 ni.com

and change the configuration of

Page 19

Chapter 1 About the SCXI-1520

• If you get the message Failed to find, followed by module codes and the message

Instead found: module with ID 0Xxx, refer

to the Traditional NI-DAQ (Legacy) section of Chapter 3, Configuring

and Testing, and make sure the correct module is in the specified slot.

Delete the incorrect module as described in Appendix C, Removing the

SCXI-1520, and add the correct module as described in the Traditional NI-DAQ (Legacy) section of Chapter 3, Configuring and Testing.

• If you get the message and the message

Failed to find, followed by a module code

Slot x is empty, make sure the configured module

is installed in the specified slot. If not, install the module by following the instructions in the Installing the SCXI-1520 Module into the SCXI

Chassis section. If the module is installed in the correct slot, power off

the chassis, remove the module as specified in Appendix C, Removing

the SCXI-1520, and verify that no connector pins are bent on the rear

signal connector. Reinstall the module as described in the Installing the

SCXI-1520 Module into the SCXI Chassis section, ensuring

the module is fully inserted and properly aligned in the slot.

• After checking the preceding items, return to the Troubleshooting the

Self-Test Verification section and retest the SCXI chassis.

If these measures do not successfully configure the SCXI system, contact NI. Refer to the Technical Support Information document for contact information.

Page 20

Connecting Signals

This chapter describes how to connect Wheatstone bridge sensors to the SCXI-1520 in quarter-, half-, and full-bridge configurations and for remote sensing. It also provides the front and rear signal pin assignments of the module.

Connecting Bridge Sensor Signals

This section discusses how to connect the signals 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 and shunt calibration. Refer to Chapter 4, Theory of Operation, for a discussion of strain-gauge concepts. Refer to the SCXI-1314 Installation Guide for more signal connection information.

Notes The circuits in this section illustrate circuits using the SCXI-1314 terminal block.

If you are using the SCXI-1314T terminal block, refer to the SCXI-1314T TEDS Bridge Sensor Terminal Block Installation Guide for the permitted circuit configuration diagrams.

Refer to Figure 2-5 and use its wiring diagram for full-bridge sensors such as load, force, torque, and pressure sensors.

Quarter-Bridge Type I

This section provides information for connecting the quarter-bridge strain-gauge configuration type I. Figure 2-1 shows the quarter-bridge type I circuit wiring diagram.

Note S– is left unwired.

Page 21

Chapter 2 Connecting Signals

SCXI-1520 Set Bridge

Configuration to

Quarter Bridge

–

+ –

S+

P+

QTR

SCA

Shunt Cal A

SCXI-1314

(gauge)

You must connect

the shunt calibration

wires externally using

the terminal block

screw connections.

Transducer

Figure 2-1. Quarter-Bridge I Circuit Diagram

Quarter-Bridge Type II

SCXI-1520 User Manual 2-2 ni.com

The following symbols apply to the circuit diagram and equations:

•R

and R2 are half-bridge completion resistors.

•R

is the quarter-bridge completion resistor.

•R

is the active element measuring tensile strain (+ε).

• V

• R

• V

Note The value of the quarter-bridge completion resistor, R

resistance of the strain gauge. NI recommends using a 0.1% precision resistor.

This section provides information for connecting the quarter-bridge strain-gauge configuration type II. Figure 2-2 shows the quarter-bridge type II circuit wiring diagram.

is the excitation voltage.

is the lead resistance.

is the measured voltage.

, must equal the nominal

Page 22

Chapter 2 Connecting Signals

–

S+

P+

SCA

Shunt Cal A

SCXI-1314Transducer

(gauge)

(dummy)

SCXI-1520 Set Bridge

Configuration to Quarter Bridge

Note The quarter-bridge type II configuration is often confused with the more commonly

used half-bridge type I configuration. In the half-bridge type I configuration, the R

element is active and bonded to the strain specimen to measure Poisson's ratio, while in the quarter-bridge type II configuration, the R

element does not actively measure strain, but

is in close thermal contact with the strain specimen. In quarter-bridge type II configuration,

element is not bonded to the specimen. Typically it is either physically close to the

the R

specimen or mounted on the same type material at the same temperature, but is not under strain.

Figure 2-2. Quarter-Bridge II Circuit Diagram

The following symbols apply to the circuit diagram and equations:

Note S– and QTR are left unwired.

•R

and R2 are half-bridge completion resistors.

•R

is the quarter-bridge temperature-sensing element (dummy gauge).

•R

is the active element measuring tensile strain (+ε).

• V

• R

• V

is the excitation voltage.

is the lead resistance.

is the measured voltage.

Page 23

Chapter 2 Connecting Signals

–

S+

P+

SCA

Shunt Cal A

SCXI-1520 Set Bridge

Configuration to Half Bridge

SCXI-1314

Transducer

(gauge)

Half-Bridge Type I

Note S– is left unwired.

This section provides information for connecting the half-bridge strain-gauge configuration type I. Figure 2-3 shows the half-bridge type I circuit wiring diagram.

Figure 2-3. Half-Bridge Type I Circuit Diagram

The following symbols apply to the circuit diagram and equations:

•R

and R2 are half-bridge completion resistors.

•R

is the active element measuring compression from Poisson

effect (–νε).

Note As shown in Figure 2-4, for greatest calibration accuracy, use separate wires

•R

• V

• R

• V

between the bridge and the SCA terminals. Do not directly connect S+ or P– to the

SCXI-1520 User Manual 2-4 ni.com

SCA terminals inside the SCXI-1314 terminal block unless the strain-gauge cable length is very short.

is the active element measuring tensile strain (+ε).

is the excitation voltage.

is the lead resistance.

is the measured voltage.

Page 24

Half-Bridge Type II

–

S+

P+

SCA

Shunt Cal A

–

SCXI-1314Transducer

(gauge)

out

SCXI-1520 Set Bridge

Configuration to Half Bridge

Note S– is left unwired.

Chapter 2 Connecting Signals

This section provides information for connecting the half-bridge strain-gauge configuration type II. Figure 2-4 shows the half-bridge type II circuit wiring diagram.

Figure 2-4. Half-Bridge Type II Circuit Diagram

The following symbols apply to the circuit diagram and equations:

•R

and R2 are half-bridge completion resistors.

•R

is the active element measuring compressive strain (–ε).

•R

Note As shown in Figure 2-3, for greatest calibration accuracy, use separate wires

between the bridge and the SCA terminals. Do not directly connect S+ or P– to the SCA terminals inside the SCXI-1314 terminal block unless the strain-gauge cable length is very short.

is the active element measuring tensile strain (+ε).

• V

• R

• V

is the excitation voltage.

is the lead resistance.

is the measured voltage.

Page 25

Chapter 2 Connecting Signals

–

S+

P+

SCA

Shunt Cal A

–

SCXI-1520 Set Bridge

Configuration to Full BridgeSCXI-1314

Transducer

CH+

Full-Bridge Type I

This section provides information for connecting the full-bridge strain-gauge configuration type I. Figure 2-5 shows the full-bridge type I circuit wiring diagram.

Figure 2-5. Full-Bridge Type I Circuit Diagram

The following symbols apply to the circuit diagram and equations:

•R

is an active element measuring compressive strain (–ε).

•R

is an active element measuring tensile strain (+ε).

•R

is an active element measuring compressive strain (–ε).

•R

is an active element measuring tensile strain (+ε).

SCXI-1520 User Manual 2-6 ni.com

Note As shown in Figure 2-5, for greatest calibration accuracy, use separate wires

between the bridge and the SCA terminals. Do not directly connect S+ or P– to the SCA terminals inside the SCXI-1314 terminal block unless the strain-gauge cable length is very short.

• V

• R

• V

is the excitation voltage.

is the lead resistance.

is the measured voltage.

Page 26

Full-Bridge Type II

–

S+

P+

SCA

Shunt Cal A

–

–v

SCXI-1520 Set Bridge

Configuration to Full BridgeSCXI-1314

Transducer

EX–

Chapter 2 Connecting Signals

This section provides information for connecting the full-bridge strain-gauge configuration type II. Figure 2-6 shows the full-bridge type II circuit wiring diagram.

Figure 2-6. Full-Bridge Type II Circuit Diagram

The following symbols apply to the circuit diagram and equations:

•R

is an active element measuring compressive Poisson effect (–νε).

•R

is an active element measuring tensile Poisson effect (+νε).

•R

is an active element measuring compressive strain (–ε).

Note As shown in Figure 2-6, for greatest calibration accuracy, use separate wires

between the bridge and the SCA terminals. Do not directly connect S+ or P– to the SCA terminals inside the SCXI-1314 terminal block unless the strain-gauge cable length is very short.

• V

• R

• V

•R

is an active element measuring tensile strain (+ε).

is the excitation voltage.

is the lead resistance.

is the measured voltage.

Page 27

Chapter 2 Connecting Signals

–

S+

P+

SCA

Shunt Cal A

–v

SCXI-1520 Set Bridge

Configuration to Full BridgeSCXI-1314

Transducer

EX–

Full-Bridge Type III

This section provides information for connecting the full-bridge strain-gauge configuration type I. The full-bridge type III only measures axial strain. Figure 2-7 shows the full-bridge type III circuit wiring diagram.

Figure 2-7. Full-Bridge Type III Circuit Diagram

The following symbols apply to the circuit diagram and equations:

•R

is an active element measuring compressive Poisson effect (–νε).

•R

is an active element measuring tensile strain (+ε).

•R

is an active element measuring compressive Poisson effect (–νε).

•R

is an active element measuring the tensile strain (+ε).

is the excitation voltage.

is the lead resistance.

is the measured voltage.

Note As shown in Figure 2-7, for greatest calibration accuracy, use separate wires

between the bridge and the SCA terminals. Do not directly connect S+ or P– to the SCA terminals inside the SCXI-1314 terminal block unless the strain-gauge cable

• V

• R

• V

length is very short.

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Page 28

Remote Sense

Note NI recommends using remote sense if your application requires the improved

accuracy. Refer to Chapter 4, Theory of Operation, for more information about using remote sense.

Run Separate Wires Between Remote-Sense Terminals and Bridge

Chapter 2 Connecting Signals

Wire the SCXI-1520 for remote sense as shown in Figure 2-8.

SCXI-1520SCXI-1314Transducer

RS+

P+

–

Feedback

EX–

Figure 2-8. Remote-Sense Circuit Diagram

Note

If you use remote sense, set RL to zero in the equations for measured strain (ε).

Page 29

Chapter 2 Connecting Signals

Pin Assignments

The pin assignments for the SCXI-1520 front signal connector are shown in Table 2-1. The front signal connector is a special 96-pin DIN C male connector through which you make all signal connections. The terminal assignments are as follows:

•SX+ and SX– are for analog input

•RSX+ and RSX– are for remote sense

•PX+ and PX– are for excitation output

•SCAX are for shunt calibration circuit A

•SCBX are for shunt calibration circuit B

where X is the channel number.

The negative terminals are listed in Column B and the positive terminals are listed in Column C. The pins labeled RSVD are reserved. Do not make any connections to the RSVD pins.

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Page 30

Column

A B C

Chapter 2 Connecting Signals

Table 2-1. Front Signal Pin Assignments

Front Connector Diagram Pin Number Column A Column B Column C

32 SCB0 S0– S0+

31 SCB0 RS0– RS0+

30 SCB1 P0– P0+

29 SCB1 SCA0 SCA0

28 RSVD S1– S1+

27 RSVD RS1– RS1+

26 RSVD P1– P1+

25 RSVD SCA1 SCA1

24 SCB2 S2– S2+

23 SCB2 RS2– RS2+

22 SCB3 P2– P2+

21 SCB3 SCA2 SCA2

20 RSVD S3– S3+

19 RSVD RS3– RS3+

18 RSVD P3– P3+

17 RSVD SCA3 SCA3

16 SCB4 S4– S4+

15 SCB4 RS4– RS4+

14 SCB5 P4– P4+

13 SCB5 SCA4 SCA4

12 RSVD S5– S5+

11 RSVD RS5– RS5+

10 RSVD P5– P5+

9 RSVD SCA5 SCA5

8 SCB6 S6– S6+

7 SCB6 RS6– RS6+

6 SCB7 P6– P6+

5 SCB7 SCA6 SCA6

4 RSVD S7– S7+

3 RSVD RS7– RS7+

2 RSVD P7– P7+

1 RSVD SCA7 SCA7

Page 31

Chapter 2 Connecting Signals

The rear signal connector is a 50-pin male cable connector used for analog signal connectivity and communication between the SCXI-1520 and the connected E/M Series DAQ device. The rear signal connector is shown in Table 2-2. The rear signal connector allows the DAQ device to access all eight differential analog output signals from the SCXI-1520. The positive terminal of each analog output is named CHX+ and the negative terminal CHX–.

SCXI-1520 User Manual 2-12 ni.com

Page 32

Rear Connector

Diagram

3 4

7 8

910

11 12

13 14

15 16

17 18

19 20

21 22

23 24

25 26

27 28

29 30

31 32

33 34

35 36

37 38

3940

41 42

43 44

45 46

47 48

49 50

Chapter 2 Connecting Signals

Table 2-2. Rear Signal Pin Assignments

Signal Name Pin Number Pin Number Signal Name

— 1 2 —

CH 0 + 3 4 CH 0 –

CH 1 + 5 6 CH 1 –

CH 2 + 7 8 CH 2 –

CH 3 + 9 10 CH 3 –

CH 4 + 11 12 CH 4 –

CH 5 + 13 14 CH 5 –

CH 6 + 15 16 CH 6 –

CH 7 + 17 18 CH 7 –

— 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 37 38 —

— 39 40 —

— 41 42 —

— 43 44 —

— 45 46 SYNC

— 47 48 —

— 49 50 —

In parallel output mode, channel 0 is selected at the output multiplexer and is connected to CH 0. The seven other channels are directly connected to CH 1 through CH 7, respectively, on the rear connector.

Page 33

Chapter 2 Connecting Signals

In multiplexed mode, the CH 0 signal pair is used for sending all eight channels of the SCXI-1520, and analog signals from other modules, to the connected E/M Series DAQ device. If the module is cabled directly to the DAQ device, the other analog channels of the DAQ device are unavailable for general-purpose analog input because they are connected to the SCXI-1520 amplifier outputs. This means that connecting an SCXI-1180 module to the 50-pin breakout connector of the SCXI-1349, or other cable adapter assembly, may cause interference and incorrect measurements when the DAQ device is cabled to the SCXI-1520.

The communication signals between the DAQ device and the SCXI system are listed in Table 2-3. If the DAQ device is connected to the SCXI-1520, these digital lines are unavailable for general-purpose digital I/O.

Table 2-3. SCXI-1520 Communication Signals

NI-DAQmx

SCXI

24, 33 DIG GND D GND DGND — Digital ground—these

25 SER DAT IN P0.0 DIO0 Input Serial data in—this

26 SER DAT OUT P0.4 DIO4 Output Serial data out—this

27 DAQ D*/A P0.1 DIO1 Input Board data/address

Signal Name

Device Signal

Name

Traditional NI-DAQ

(Legacy) Device

Signal Name

Direction Description

pins supply the reference for E/M Series DAQ device digital signals and are connected to the module digital ground.

signal taps into the SCXIbus MOSI line to send serial input data to a module or Slot 0.

signal taps into the SCXIbus MISO line to accept serial output data from a module.

line—this signal taps into the SCXIbus D*/A line to indicate to the module whether the incoming serial stream is data or address information.

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Page 34

Chapter 2 Connecting Signals

Table 2-3. SCXI-1520 Communication Signals (Continued)

NI-DAQmx

SCXI

29 SLOT0SEL* P0.2 DIO2 Input Slot 0 select—this

36 SCANCLK AI HOLD COMP,

37 SER CLK EXTSTROBE* EXTSTROBE* Input Serial clock—this

46 HOLD TRIG PFI 7/

Signal Name

Device Signal

Name

AI HOLD

AI SAMP CLK,

AI SAMP

Traditional NI-DAQ

(Legacy) Device

Signal Name

SCANCLK Input Scan clock—a rising

PFI7/

START SCAN

Direction Description

signal taps into the SCXIbus INTR* line to indicate whether the information on MOSI is being sent to a module or Slot 0.

edge indicates to the scanned SCXI module that the E/M Series DAQ device has taken a sample and causes the module to advance channels.

signal taps into the SCXIbus SPICLK line to clock the data on the MOSI and MISO lines.

Input Hold trigger—this

signal is used by the MIO to set the track-and-hold state of the module.

Page 35

Configuring and Testing

This chapter discusses configuring the SCXI-1520 in MAX for use with either NI-DAQmx or Traditional NI-DAQ (Legacy), creating and testing a virtual channel, global channel, and/or task.

SCXI-1520 Software-Configurable Settings

This section describes how to set the bridge configuration, voltage excitation level, filter bandwidth, and gain/input signal range, as well as how to use configuration utilities in MAX to programmatically perform offset null compensation and shunt calibration. It also describes how to perform configuration of these settings for the SCXI-1520 in NI-DAQmx and Traditional NI-DAQ (Legacy). For more information on the relationship between the settings 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 software-configurable settings for the SCXI-1520. Refer to Chapter 4, Theory of Operation, for a complete list of software-configurable settings.

Bridge Configuration

Bridge configuration is a software-configurable setting that allows you to connect quarter-, half-, or full-bridge configuration Wheatstone bridge sensors easily. When quarter- or half-bridge configuration is selected, Terminal SX– (where X is a particular channel) is disconnected from the front signal connector and internally connected to a half-bridge completion network. Implementing quarter-bridge completion also involves making field wiring connections to the quarter-bridge completion resistor (QTR) in the terminal block.

Page 36

Chapter 3 Configuring and Testing

Excitation Level

Excitation level is a software-configurable setting that allows you to set the voltage excitation level available on PX+ and PX– (where X is a particular channel). You can choose voltage excitation settings between 0 and 10 V. To prevent the module from overheating, do not set the excitation voltage greater than

Note You need not include the loading effect of the internal half-bridge completion

resistors in the above calculation. When using internal quarter-bridge completion you must include the nominal gauge resistance as well as the quarter-bridge completion resistance

+ R4).

Table 3-1 shows the maximum allowable excitation voltages for standard bridge configurations and resistances.

Table 3-1. Excitation Voltage for Configuration and Gauge Resistances

Configuration/

Sensor

Resistance

(resistance connected between the excitation terminals) × (29.0 mA)

Traditional NI-DAQ

NI-DAQmx Excitation

Vol ta g e R an g e

(Legacy) Excitation

Voltage Range

Quarter- or Half-Bridge

120 Ω ≤6.96 V 0 to 6.875 V

350 Ω 0 to 10 V 0 to 10 V

1000 Ω 0 to 10 V 0 to 10 V

Full-Bridge or Full-Bridge Sensor

120 Ω ≤3.48 V 0 to 3.125 V

350 Ω 0 to 10 V 0 to 10 V

1000 Ω 0 to 10 V 0 to 10 V

Filter Bandwidth

Filter bandwidth is a software-configurable setting that allows you to select a lowpass filter cutoff frequency. You can choose 10 Hz, 100 Hz, 1 kHz, 10 kHz, or filter-bypass mode. If your application requires other cutoff frequencies, refer to Chapter 4, Theory of Operation.

SCXI-1520 User Manual 3-2 ni.com

Page 37

Chapter 3 Configuring and Testing

quarter bridge (max strain) (excitation voltage)× (0.5 ×μV/V/με)=

half bridge (max strain) (excitation voltage)× (1.0 ×μV/V/με)=

full bridge (max strain) (excitation voltage)× (2.0 ×μV/V/με)=

gain

SCXI-1520 output voltage range()10 V()×

input signal voltage()

----------------------------------------------------------------------------------------------------------

≤

(max input signal voltage)

(sensor sensitivity) (excitation voltage)× (maximum input)×

(sensor full-scale input)

-------------------------------------------------------------------------------------------------------------------------------------------------=

12 mV

3.0 mV/V()10 V()200 psi()×× 500 psi()

--------------------------------------------------------------------------------=

Gain/Input Range

Gain/input range is a software-configurable setting that allows you to choose the appropriate amplification to fully utilize the range of the E/M Series DAQ device. In most applications NI-DAQ chooses and sets the gain for you determined by the input range. This feature is described in Chapter 4, Theory of Operation. Otherwise, you should determine the appropriate gain using the input signal voltage range and the full-scale limits of the SCXI-1520 output signal. For common strain-gauge configurations where the Gauge Factor is 2.0, the maximum input signal (in microvolts) is:

When you have determined the input signal voltage you can use the following equation to determine the appropriate gain:

If you are using a bridge-based sensor, use the manufacturer-specified sensitivity (usually expressed in the units of millivolts per volt) to determine the maximum input signal. The maximum input signal is:

For example, if you have a 0 to 500 psi pressure sensor with 3.0 mV/V sensitivity, an excitation voltage of 10 V, and a maximum pressure of 200 psi, the maximum signal is:

Page 38

Chapter 3 Configuring and Testing

833

10 V

12 mV

----------------=

If you are using a DAQ device that has a maximum analog input range of ±10 V and you have a maximum input to the SCXI-1520 of +12 mV, set the gain to the setting closest to

but less than 833. A larger gain setting saturates the DAQ device input for a 12 mV signal. In this example, the closest lesser gain setting for the SCXI-1520 is 750.

Null Potentiometers

Coarse and fine null potentiometers are software-configurable settings that allow you to remove unwanted offset voltage. In most cases, you do not explicitly set the null potentiometers, but instead allow driver software to automatically adjust them for you. However, if you want to explicitly set the null potentiometers, you can write an application program that adjusts the null potentiometers settings. Refer to Chapter 4, Theory of Operation, for more information.

Shunt Calibration Switches

Shunt calibration switches A and B are software control settings that allow you to engage or disengage the shunt calibration resistors in order to perform gain calibration. In most cases, you do not explicitly control the shunt calibration switches, but instead allow driver software to automatically adjust them for you during the automated shunt calibration procedure. However, if you want to explicitly control the calibration switches, you can write an application program that controls the shunt calibration switches. Refer to Chapter 4, Theory of Operation, for more information.

Note The gain adjustment is done for you automatically if you have performed shunt

calibration using the NI-DAQ driver. Refer to the Traditional NI-DAQ (Legacy) section and the NI-DAQmx section for more information about how to perform shunt calibration using the driver.

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Page 39

Chapter 3 Configuring and Testing

Modes of Operation

The SCXI-1520 can operate in multiplexed mode or parallel mode. Using NI-DAQmx, you can operate the SCXI-1520 in either multiplexed or parallel mode. In Traditional NI-DAQ (Legacy), only multiplexed mode is supported. Refer to the Strain-Gauge Theory section of Chapter 4, Theory

of Operation, for more information on multiplexed and parallel mode

operation.

Simultaneous Sample and Hold

When it is critical to measure two or more signals at the same instant in time, simultaneous sample and hold (SS/H) is required. Typical applications that might require SS/H include vibration measurements and phase difference measurements. In NI-DAQmx, 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.

Note You cannot change the simultaneous sampling mode in MAX. You must use an ADE

such as LabVIEW or LabWindows/CVI to configure the setting using NI-DAQmx Channel Property Node. Refer to your ADE help file for more information.

Configurable Settings in MAX

Note If you are not using an NI ADE, using an NI ADE prior to version 7.0, or are using

an unlicensed copy of an NI ADE, additional dialog boxes from the NI License Manager appear allowing you to create 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 where you can access each software-configurable setting for modification in MAX. The location of the settings varies depending on the version of NI-DAQ you use. Refer to either the

NI-DAQmx section or the Traditional NI-DAQ (Legacy) section. You also

can refer to the DAQ Getting Started Guide and the SCXI Quick Start Guide for more information on installing and configuring the hardware. You also can use the DAQ Assistant to graphically configure common measurement tasks, channels, or scales.

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Chapter 3 Configuring and Testing

NI-DAQmx

In NI-DAQmx, you can configure software settings such as bridge configuration, voltage excitation level, filter bandwidth, gain/input signal range, and calibration settings in the following two ways:

• Task or global channel in MAX

• Functions in your application

Note All software-configurable settings are not configurable both ways. This section only

discusses settings in MAX. Refer to Chapter 4, Theory of Operation, for information on using functions in your application.

These sections describe settings that you can change in MAX and where they are located. Strain and custom voltage with excitation are the most commonly used NI-DAQmx Task or NI-DAQmx Global Channel types with the SCXI-1520. Use the Custom Voltage with Excitation NI-DAQmx task or global channel when measuring load, force, torque, pressure, or other bridge-based sensors:

• Bridge configuration—configure using the settings tab of NI-DAQmx

• Voltage excitation—configure using either NI-DAQmx Task or

• Filter bandwidth—configure the Device tab using either NI-DAQmx

• Input signal range—configure the input signal range using either

• Calibration settings—null potentiometer settings and control shunt

Task or NI-DAQmx Global Channel and functions in your application. Channel properties override module properties. The default bridge configuration for NI-DAQmx is full bridge.

NI-DAQmx Global Channel. You also can set the voltage excitation level through your application. In NI-DAQmx, you can choose from contiguous voltages between 0 and 10 V. The default voltage excitation in NI-DAQmx is 0 V.

Task or NI-DAQmx Global Channel. You also can set the value through your application. The default filter cut-off frequency in NI-DAQmx is 10 kHz.

NI-DAQmx Task or NI-DAQmx Global Channel. When you set the minimum and maximum range of NI-DAQmx Task or NI-DAQmx Global Channel, the driver selects the best gain for the measurement. You also can set it through your application. The default gain setting in NI-DAQmx is 1.0. This setting corresponds to an input range of ±10 V.

calibration switches only using Strain NI-DAQmx Task, Strain

NI-DAQmx Global Channel, or your application. The Custom Voltage with Excitation NI-DAQmx Task or NI-DAQmx Global Channel cannot adjust calibration settings in MAX. Adjust calibration

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Chapter 3 Configuring and Testing

settings in your application. The default configuration settings set the null potentiometers at their midpoint, 62 for the coarse potentiometer and 2,047 for the fine potentiometer. The default state of the shunt calibration switches is open. This state leaves the shunt calibration resistor disconnected from the circuit.

• Modes of operation—configure only using chassis installation in software. Refer to Chapter 1, About the SCXI-1520, for more information on chassis installation. The default setting in NI-DAQmx is multiplexed.

• Simultaneous sample-and-hold settings—configure only using your application. The default setting in NI-DAQmx engages SS/H.

Note Refer to Chapter 4, Theory of Operation, for information on configuring the settings

for your application using Traditional NI-DAQ (Legacy).

Creating a Strain Global Channel or Task

To create a new NI-DAQmx strain global task or channel, complete the following steps:

1. Double-click Measurement & Automation on 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.

5. Select Strain.

6. If you are creating a task, you can select a range of channels by holding down the <Shift> key while selecting the channels. You can select multiple individual channels by holding down the <Ctrl> key while selecting channels. If you are creating a channel, you can only select one channel. Click Next.

7. Name the task or channel and click Finish.

8. In the Channel List box, select the channel(s) you want to configure. You can select a range of channels by holding down the <Shift> key while selecting the channels. You can select multiple individual channels by holding down the <Ctrl> key while selecting channels.

9. Enter the specific values for your application in the Settings tab. Context help information for each setting is provided on the right side of the screen.

10. Click the Device tab and select the autozero mode and lowpass filter cutoff frequency.

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Chapter 3 Configuring and Testing

11. Ensure that you have selected the strain channel(s) you wish to

12. On the screen that appears choose to enable offset nulling and/or

13. Measure and Calibrate displays information specific to the strain

14. If you are creating a task and want to set timing or triggering controls,

Creating a Custom Voltage with Excitation Global Channel or Task

Use the Custom Voltage with Excitation NI-DAQmx Task or Global Channel when measuring load, force, torque, pressure, or other bridge-based sensors. To create an NI-DAQmx Custom Voltage with Excitation Task or NI-DAQmx Global Channel, complete the

following steps:

1. Double-click Measurement & Automation on the desktop.

2. Right-click Data Neighborhood and select Create New.

3. Select NI-DAQmx Global Channel or NI-DAQmx Task, and

4. Select Analog Input then select Custom Voltage with Excitation.

5. If you are creating a channel, you can select only one channel. If you

6. Select the name of the task or channel, and click Finish.

7. In the Channel List box, select the channel(s) you want to configure.

8. Enter the specific values for your application in the Settings tab.

calibrate in the Channel List box. Click Calibration to calibrate the strain channel(s).

shunt calibration. Enter the shunt calibration resistor information. Click Next.

channel(s). Click Measure to acquire a signal from the strain channel(s) and Reset Data to reset the values to default. Click Calibrate to calibrate the strain channel(s). When you have completed calibrating the strain channel(s), click Finish.

enter the values in the Task Timing and Task Triggering tabs.

click Next.

are creating a task, select the channels to add to the task. You can select a range of channels by holding down the <Shift> key while selecting the channels. You can select multiple individual channels by holding down the <Ctrl> key while selecting channels. Click Next.

You can select a range of channels by holding down the <Shift> key while selecting the channels. You can select multiple individual channels by holding down the <Ctrl> key while selecting channels.

Context help information for each setting is provided on the right side of the screen.

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9. Click the Device tab and select the autozero mode and lowpass filter cutoff frequency.

10. If you are applying custom scaling, select the Create New drop-down and follow the onscreen 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.

Traditional NI-DAQ (Legacy)

In Traditional NI-DAQ (Legacy), you can configure software settings, such as bridge configuration, voltage excitation level, filter bandwidth, gain/input signal range, and calibration settings, in the following three ways:

• module property pages in MAX

• virtual channels properties in MAX

• functions in your ADE

Note All software-configurable settings are not configurable in all three ways. This

section only discusses settings in MAX. Refer to Chapter 4, Theory of Operation, for information on using functions in your application.

Chapter 3 Configuring and Testing

Most of these settings are available in module properties and/or using virtual channels:

• Bridge configuration—configure using module properties, virtual channels properties, and functions in your application. Virtual channel properties override module properties. The default bridge configuration for Traditional NI-DAQ (Legacy) is quarter bridge.

• Voltage excitation—configure using module properties or virtual channel properties. Virtual channel properties override module properties settings. You also can set excitation through your application. You can choose one of 17 voltage settings between 0 and 10 V. The default voltage excitation level for Traditional NI-DAQ (Legacy) is 2.5 V.

• Filter bandwidth—configure only using module properties. You also can set bandwidth through your application. The default filter bandwidth level for Traditional NI-DAQ (Legacy) is 10 Hz.

• Gain/input signal range—configure gain using module properties. When you set the minimum and maximum range of the virtual channel, the driver selects the best gain. The default gain setting for Traditional NI-DAQ (Legacy) is 100.

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Chapter 3 Configuring and Testing

• Calibration settings—configure null potentiometer settings and

• Modes of operation—configure only using module properties. The

• Simultaneous sample-and-hold settings—can modify this setting only

Note Refer to Chapter 4, Theory of Operation, for information on configuring the settings

for your application using Traditional NI-DAQ (Legacy).

Configuring Module Property Pages in Traditional NI-DAQ (Legacy)

1. Right-click the SCXI-1520 module you want to configure and select

2. If the module you are configuring is connected to an E Series DAQ

3. Click the Channel tab. Select the appropriate gain, filter, voltage

4. Click Accessory. Select the accessory you connected to the module.

control shunt calibration switches only using strain virtual channel or using your application. The default configuration settings set the null potentiometers at their midpoint, 62 for the coarse potentiometer and 2,047 for the fine potentiometer. The default state of the shunt calibration switches is open. This state leaves the shunt calibration resistor disconnected from the circuit.

default setting in Traditional NI-DAQ (Legacy) is multiplexed mode. Parallel mode is not supported in Traditional NI-DAQ (Legacy).

using NI-DAQmx.

Properties. Click General.

device, select that device by using Connected to. If you want this DAQ device to control the chassis, confirm there is a check in the This device will control the chassis checkbox. If the module you are configuring is not connected to an DAQ device, select None.

excitation, and bridge configuration setting for each channel. If you want to configure all the channels at the same time, select the Channel drop-down menu, scroll to the bottom, and select All Channels. Refer to the SCXI-1520 Software-Configurable Settings section for a detailed description of each setting. Click Apply.

When configuration is complete, click OK.

The Traditional NI-DAQ (Legacy) chassis and SCXI-1520 should now be configured properly. If you need to change the module configuration, right-click the module and repeat steps 1 through 4. Test the system following the steps in the Troubleshooting the Self-Test Verification section of Chapter 1, About the SCXI-1520.

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Chapter 3 Configuring and Testing

Creating a Strain Virtual Channel

To create a strain virtual channel, complete the following steps:

1. Right-click Data Neighborhood and select Create New.

2. Select Traditional NI-DAQ Virtual Channel and click Finish.

3. Click Add Channel.

4. Select Analog Input from the drop-down menu and click Next.

5. Enter the Channel Name and Channel Description, and click Next.

6. Select Strain Gauge from the drop-down menu and click Next.

7. Enter the following information:

a. Bridge type from the drop-down menu

b. V init in volts

c. Lead Resistance in Ω

d. Nominal Gauge in Ω

e. Gauge Factor

f. Range min in με

g. Range max in με

8. Click Next.

9. Enter the following information:

a. What DAQ hardware will be used? from the drop-down menu

b. What channel on your DAQ hardware? from the drop-down

c. Which analog input mode will be used? from the drop-down

d. What is the Excitation Voltage’s source and value? from the

drop-down menu

e. Voltage in volts

10. Click Finish.

Calibrating a Strain Virtual Channel

To calibrate a strain virtual channel, complete the following steps:

1. Right-click the virtual channel you want to calibrate and select Properties.

2. Click Calibration.

3. Select the Engineering Units from the drop-down menu.

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Chapter 3 Configuring and Testing

4. Enter the Calibration Data and click Update.

5. Make the selections in Shunt Cal Circuit A and Operation, and click Start.

6. After the calibration is completed, click Exit.

Verifying the Signal

This section describes how to take measurements using test panels in order to verify signal, and configuring and installing a system in NI-DAQmx and Traditional NI-DAQ (Legacy).

Verifying the Signal in NI-DAQmx Using a Task or Global Channel

You can verify the signals on the SCXI-1520 using NI-DAQmx by completing the following steps:

1. Expand the list of tasks and virtual channels by clicking the + next to Data Neighborhood.

2. Click the + next to NI-DAQmx Tasks to expand the list of tasks.

3. Click the task.

4. Add or remove channels, if applicable, in the Channel List. Click the Add Channels button, shown at left, and select the type of channel you want to add.

a. In the window that appears, expand the list of channels by clicking

the + next to the module of interest.

b. Select the channel(s) you want to verify. You can select a block of

channels by holding down the <Shift> key or multiple channels by holding down the <Ctrl> key. Click OK.

5. Enter the appropriate information on the Settings tab.

6. Click the Device tab and enter the appropriate information on the Device tab.

7. Click the Test button to open the test panel.

8. Click the Start button, if necessary.

9. After you have completed verifying the channels, close the test panel window.

You have now verified the SCXI-1520 configuration and signal connection.

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Note For more information on how to further configure the SCXI-1520, or how to use

LabVIEW to configure the module and take measurements, refer to Chapter 4, Theory of

Operation.

Verifying the Signal in Traditional NI-DAQ (Legacy)

This section discusses how to verify the signal in Traditional NI-DAQ (Legacy) using channel strings and virtual channels.

Verifying the Signal Using Channel Strings

The format of the channel string is as follows:

obx

! scy ! mdz ! channel

where

obx is the onboard E Series DAQ device channel, with x representing a particular channel where the multiplexed channels are sent. This value is 0 for DAQ device channel 0 in a single-chassis system. In a multichassis or remote chassis system, the DAQ device channel x corresponds to chassis number n – 1, where DAQ device channel x is used for scanning the nth chassis in the system.

scy is the SCXI chassis ID, where y is the number you chose when configuring the chassis.

mdz is the slot position where the module is located, with z being the particular slot number. The slots in a chassis are numbered from left to right, starting with 1.

channel is the channel that is sampled from module z.

Chapter 3 Configuring and Testing

Use the format single input channel.

Complete the following steps to use channel strings in verifying the signal:

1. Expand the list of tasks and virtual channels by clicking the + next to Devices and Interfaces.

2. Click the + next to Traditional NI-DAQ Devices to expand the device list.

3. Right-click the appropriate E Series DAQ device.

4. Click Test Panels.

5. Enter the channel string.

6. Enter the input limits.

obx ! scy ! mdz ! n to verify the signal, where n is a

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Chapter 3 Configuring and Testing

7. Select the Data Mode.

8. Select the Y Scale Mode.

Refer to the LabVIEW Measurements Manual for more information and for proper formatting of channel strings for different uses.

Verifying the Signal Using Strain Virtual Channel

If you have already created a virtual channel, complete the following steps to verify the signal:

1. Right-click the strain virtual channel you want to verify and

select Test.

2. In Channel Names, select the channel you want to verify.

3. When you have completed verifying the channel, click Close.

Using the Strain Calibration Wizard in NI-DAQmx

When using NI-DAQmx, you can perform an automated strain calibration on one or more channels in your task using the Strain Calibration Wizard in MAX.

Note All channels must be strain channels to use the Strain Calibration Wizard.

Complete the following steps to perform calibration using the Calibration Wizard:

1. Expand the list of devices by clicking the + next to Data Neighborhood.

2. Click the + next to NI-DAQmx Tasks to expand the list of tasks.

3. Select the strain task you previously created.

4. Click the Device tab.

5. Select the Auto Zero Mode and Lowpass Filter Cutoff Frequency from the dropdown lists.

6. Click the Calibration button and follow the onscreen instructions.

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Theory of Operation

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

Strain-Gauge Theory

This section discusses 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 active sensing elements. 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

compose the second voltage divider circuit. The output of a Wheatstone

bridge is measured between the middle nodes of the two voltage dividers.

and R2 compose one voltage divider circuit, and R4 and

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Chapter 4 Theory of Operation

Strain Gauges

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. The Wheatstone bridge configuration is used to help measure the small variations in resistance that the sensing elements produce corresponding to a physical changes in the specimen.

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. Table 4-1 shows the number of active elements in each configuration.

Table 4-1. Strain-Gauge Configurations

Configuration Number of Active Elements

Quarter-bridge 1

Half-bridge 2

Full-bridge 4

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.

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Page 51

Acronyms, Formulas, and Variable Definitions

–

2Rg+

------------------------=

VCHstrained()VCH(unstrained)–

-------------------------------------------------------------------------------





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

ε is the measured strain (+ε is tensile strain and –ε is compressive strain).

is the simulated strain.

GF is the Gauge Factor, which should be specified by the gauge manufacturer.

is the nominal gauge resistance, which should be specified by the gauge

manufacturer.

is the lead resistance. If lead lengths are long, RL can significantly

impact measurement accuracy.

is the shunt calibration resistor value.

U is the ratio of 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:

Chapter 4 Theory of Operation

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.

is the measured voltage of the signal.

is the excitation voltage.

is the voltage ratio that is used in the voltage to strain conversion

equations and is defined by the following equation:

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Chapter 4 Theory of Operation

(+ )

Axial Bending

(+ )

Software Scaling and Equations

After you have acquired the voltage signal VCH, you can scale this voltage to the appropriate strain units in software. This is done automatically for you in NI-DAQmx using a strain task or strain channel and in Traditional NI-DAQ (Legacy) using the strain virtual channel. You also can scale the voltages manually in your application using the voltage-to-strain conversion equations provided in this document for each configuration type.

Finally, there are voltage-to-strain conversion functions included in LabVIEW, NI-DAQmx, and Traditional NI-DAQ (Legacy). In LabVIEW, the conversion function, Convert Strain Gauge Reading VI, is in the Data Acquisition»Signal Conditioning subpalette. The prototypes for the NI-DAQ functions, the header file Refer to the Traditional NI-DAQ User Manual and the LabVIEW Measurements Manual for more information.

The names given the strain-gauge types in these sections directly correspond to bridge selections in MAX and the LabVIEW Convert Strain Gauge Reading VI.

convert.h for C/C++, and convert.bas for Visual Basic.

Strain_Convert and Strain_Buf_Convert, are in

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 configurations. Figure 4-3 shows the quarter-bridge type I circuit wiring diagram.

Figure 4-2. Quarter-Bridge Type I Measuring Axial and Bending Strain

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Page 53

A quarter-bridge type I has the following characteristics:

strain ε()

4– V

GF 12Vr+()

-------------------------------

-------





×=

• A single active strain-gauge element is mounted in the principle direction of axial or bending strain.

• A passive quarter-bridge completion resistor (dummy resistor) is required in addition to half-bridge completion.

• Temperature variation in specimen decreases the accuracy of the measurements.

• Sensitivity at 1000 με is ∼ 0.5 mV

Note S– is left unwired.

Chapter 4 Theory of Operation

/VEX input.

out

–

– +

(+ )

Figure 4-3. Quarter-Bridge I Circuit Diagram

The following symbols apply to the circuit diagram and equations:

•R

and R2 are half-bridge completion resistors.

•R

is the quarter-bridge completion resistor (dummy resistor).

•R

is the active strain-gauge element measuring tensile strain (+ε).

• V

• R

• V

is the excitation voltage.

is the lead resistance.

is the measured voltage.

To convert voltage readings to strain units use the following equation:

where

is the nominal gauge resistance of the sensor.

is the lead resistance.

GF is the Gauge Factor.

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Chapter 4 Theory of Operation

4U–

GF 14U+()

------------------------------=

Notes 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.

To minimize temperature drift errors, the strain gauge must have a self-temperature-compensation (STC) number that corresponds to the thermal expansion coefficient of the material under test. STC gauges have a temperature sensitivity that counteracts the thermal expansion coefficient of the test specimen. The STC number approximately 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 gauge with an STC number of 13.0. If the test specimen is steel, use a gauge with an STC number of 6.0.

To minimize temperature drift errors in the wiring, use the three-wire connection shown in Figure 4-3. The wires connected to terminals S+ and QTR carry the same current and are on opposite sides of the same element of the bridge. Therefore, any temperature-related changes in voltage drop across R leaving V

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.

To simulate the effect on strain of applying a shunt resistor across R3, use the following equation:

caused by temperature variation of the leads cancel out,

Quarter-Bridge Type II

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You can neglect lead resistance (R

) of the wiring if shunt calibration is performed or if

lead length is very short (∼ <10 ft), depending on the wire gauge. For example 10 ft of 24-AWG copper wire has a lead resistance of 0.25 Ω.

This section provides information for the quarter-bridge strain-gauge configuration type II. The quarter-bridge type II measures either axial or bending strain. Figure 4-4 shows how to position a strain-gauge resistor in an axial and bending configurations. Figure 4-5 shows the quarter-bridge type II circuit wiring diagram.

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Chapter 4 Theory of Operation

(+ )

Axial

(+ )

Bending

Figure 4-4. Quarter-Bridge Type II Measuring Axial and Bending Strain

A quarter-bridge type II has the following characteristics:

• One active strain-gauge element and one passive, temperature-sensing quarter-bridge element (dummy gauge). The active element is mounted in the direction of axial or bending strain. The dummy gauge is mounted in close thermal contact with the strain specimen but not bonded to the specimen, and is usually mounted transverse (perpendicular) to the principle axis of strain.

• This configuration is often confused with the half-bridge type I configuration, with the difference being that in the half-bridge type I configuration the R specimen to measure the effect of Poisson’s ratio.

• Completion resistors provide half-bridge completion.

• Compensates for temperature.

• Sensitivity at 1000 με is ∼ 0.5 mV

Note S– and QTR are left unwired.

element is active and bonded to the strain

out/VEX

input.

–

(+ )

Figure 4-5. Quarter-Bridge II Circuit Diagram

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Chapter 4 Theory of Operation

strain ε()

4– V

GF 12Vr+()

-------------------------------

-------+





×=

4U–

GF 14U+()

------------------------------=

The following symbols apply to the circuit diagram and equations:

•R

and R2 are half-bridge completion resistors.

•R

is the quarter-bridge temperature-sensing element (dummy gauge).

•R

is the active strain-gauge element measuring tensile strain (+ε).

• V

• R

• V

is the excitation voltage.

is the lead resistance.

is the measured voltage.

To convert voltage readings to strain units use the following equation:

where

is the nominal gauge resistance.

is the lead resistance.

GF is the Gauge Factor.

To simulate the effect on strain of applying a shunt resistor across R

use the following equation:

Notes The dummy gauge element must always be unstrained and mounted to the same

type of material as the active gauge, but not strained. The dummy gauge temperature must closely track the temperature of the active gauge.

Gauges need not have a STC number corresponding to the material type of the test specimen.

As shown in Figure 4-5, for greatest calibration accuracy, use separate wires between the bridge and the SCA terminals. Do not directly connect S+ or P– to the SCA terminals inside the SCXI-1314 terminal block unless the strain-gauge cable length is very short.

You can neglect lead resistance (R

) of the wiring if shunt calibration is performed or if

lead length is very short (∼ <10 ft), depending on the wire gauge. For example, 10 ft of 24-AWG copper wire has a lead resistance of 0.25 Ω.

The nominal value of R

is equal to Rg.

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Page 57

Half-Bridge Type I

–

(– )

(+ )

Chapter 4 Theory of Operation

This section provides information for the half-bridge strain-gauge configuration type I. The half-bridge type I measures either axial or bending strain. Figure 4-6 shows how to position strain-gauge resistors in an axial and bending configurations. Figure 4-7 shows the half-bridge type I circuit wiring diagram.

(+ )

(– )

Axial

(– )

(+ )

Bending

Figure 4-6. Half-Bridge Type I Measuring Axial and Bending Strain

A half-bridge type I has the following characteristics:

• Two active strain-gauge elements. 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 temperature.

• Compensates for the aggregate effect on the principle strain measurement due to the Poisson’s ratio of the specimen material.

• Sensitivity at 1000 με is ∼ 0.65 mV

out/VEX

input.

Note S– is left unwired.

Figure 4-7. Half-Bridge Type I Circuit Diagram

Page 58

Chapter 4 Theory of Operation

strain ε()

–

GF 1 v+()2V

v 1–()–[]

--------------------------------------------------------------

------+





×=

4U–

GF 1 v+()2Uv 1–()–[]

-------------------------------------------------------------=

The following symbols apply to the circuit diagram and equations:

•R

and R2 are half-bridge completion resistors.

•R

is the active strain-gauge element measuring compression from

Poisson effect (–νε).

•R

is the active strain-gauge element measuring tensile strain (+ε).

• V

• R

• V

is the excitation voltage.

is the lead resistance.

is the measured voltage.

To convert voltage readings to strain units use the following equation:

where

is the nominal gauge resistance.

is the lead resistance.

ν is the Poisson’s ratio.

GF is the Gauge Factor.

Notes In half-bridge type I , R

is mounted transverse to the axis of the stress field. Use this configuration in

and R

applications where no stress exists along the axis of the transverse strain gauge.

Strain gauges need not have a particular STC number.

As shown in Figure 4-9, for greatest calibration accuracy, use separate wires between the bridge and the SCA terminals. Do not directly connect S+ or P– to the SCA terminals inside the SCXI-1314 terminal block unless the strain-gauge cable length is very short.

You can neglect lead resistance (R lead length is very short (∼ <10 ft), depending on the wire gauge. For example 10 ft of 24-AWG copper wire has a lead resistance of 0.25 Ω.

The nominal values of R

SCXI-1520 User Manual 4-10 ni.com

To simulate the effect on strain of applying a shunt resistor across R use the following equation:

is mounted along the principal axis of the stress field

) of the wiring if shunt calibration is performed or if

and R4 equal Rg.

Page 59

Half-Bridge Type II

–

(+ )

(– )

Chapter 4 Theory of Operation

This section provides information for the half-bridge strain-gauge configuration type II. The half-bridge type II only measures bending strain. Figure 4-8 shows how to position strain-gauge resistors in an axial configuration. Figure 4-9 shows the half-bridge type II circuit wiring diagram.

Axial

(+ )

Bending

Figure 4-8. Half-Bridge 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 bending strain.

• Rejects axial strain.

• Compensates for temperature.

• Sensitivity at 1000 με is ∼ 1 mV

Note S– is left unwired.

(– )

out/VEX

input.

Figure 4-9. Half-Bridge Type II Circuit Diagram

Page 60

Chapter 4 Theory of Operation

strain ε()

2– V

------------

------+





×=

2U–

----------=

The following symbols apply to the circuit diagram and equations:

•R

and R2 are half-bridge completion resistors.

•R

is the active strain-gauge element measuring compressive

strain (–ε).

•R

is the active strain-gauge element measuring tensile strain (+ε).

• V

• R

• V

is the excitation voltage.

is the lead resistance.

is the measured voltage.

To convert voltage readings to strain units use the following equation:

where

is the nominal gauge resistance.

is the lead resistance.

GF is the Gauge Factor.

To simulate the effect on strain of applying a shunt resistor across R

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 strain gauges need not have a particular STC number.

As shown in Figure 4-7, for greatest calibration accuracy, use separate wires between the bridge and the SCA terminals. Do not directly connect S+ or P– to the SCA terminals inside the SCXI-1314 terminal block unless the strain-gauge cable length is very short.

You can neglect lead resistance (R

) of the wiring if shunt calibration is performed or if

lead length is very short (∼ <10 ft), depending on the wire gauge. For example 10 ft of 24-AWG copper wire has a lead resistance of 0.25 Ω.

The nominal values of R

SCXI-1520 User Manual 4-12 ni.com

and R4 equal Rg.

Page 61

Full-Bridge Type I

Axial

Bending

(+ )

(– )

(+ )

(– )

–

+–

(+ )

(– )

(+ )

Chapter 4 Theory of Operation

This section provides information for the full-bridge strain-gauge configuration type I. The full-bridge type I only measures bending strain. Figure 4-10 shows how to position strain-gauge resistors in 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 of bending strain on the opposite side (bottom).

• Highly sensitive to bending strain.

• Rejects axial strain.

• Compensates for temperature.

• Compensates for lead resistance.

• Sensitivity at 1000 με is ∼ 2.0 mV

/VEX input.

out

Figure 4-11. Full-Bridge Type I Circuit Diagram

Page 62

Chapter 4 Theory of Operation

strain ε()

V–

--------=

U–

--------=

The following symbols apply to the circuit diagram and equations:

•R

is an active strain-gauge element measuring compressive

strain (–ε).

•R

is an active strain-gauge element measuring tensile strain (+ε).

•R

is an active strain-gauge element measuring compressive

strain (–ε).

•R

is an active strain-gauge element measuring tensile strain (+ε).

• V

• R

• V

is the excitation voltage.

is the lead resistance.

is the measured voltage.

To convert voltage readings to strain units use the following equation:

where

GF is the Gauge Factor.

Notes Strain gauges need not have a particular STC number.

As shown in Figure 4-11, for greatest calibration accuracy, use separate wires between the bridge and the SCA terminals. Do not directly connect S+ or P– to the SCA terminals inside the SCXI-1314 terminal block unless the strain-gauge cable length is very short.

The nominal values of R

Full-Bridge Type II

To simulate the effect on strain of applying a shunt resistor across R

use the following equation:

, R2, R3, and R4 equal Rg.

This section provides information for the full-bridge type II strain-gauge configuration. The full-bridge type II only measures bending strain. Figure 4-12 shows how to position strain-gauge resistors in a bending configuration. Figure 4-13 shows the full-bridge type II circuit wiring diagram.

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Page 63

Chapter 4 Theory of Operation

(– )

Axial

(+ )

(– )

Bending

Figure 4-12. Full-Bridge Type II Rejecting Axial and Measuring Bending Strain

A full-bridge type II configuration has the following characteristics:

• Four active strain-gauge 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 gauge and are mounted transverse (perpendicular) to the principal axis of strain with one on one side of the strain specimen (top) and the other on the opposite side (bottom).

• Rejects axial strain.

• Compensates for temperature.

• Compensates for the aggregate effect on the principle strain measurement due to the Poisson’s ratio of the specimen material.

• Compensates for lead resistance.

• Sensitivity at 1000 με is ∼ 1.3 mV

/VEX input.

out

(– )

–

(+ )

+–

(– )

Figure 4-13. Full-Bridge Type II Circuit Diagram

The following symbols apply to the circuit diagram and equations:

•R

is an active strain-gauge element measuring compressive Poisson

effect (–νε).

•R

is an active strain-gauge element measuring tensile Poisson

effect (+νε).

Page 64

Chapter 4 Theory of Operation

strain ε()

2– V

GF 1 v+()

-------------------------=

2U–

GF 1 v+()

-------------------------=

•R3 is an active strain-gauge element measuring compressive strain (–ε).

•R

is an active strain-gauge element measuring tensile strain (+ε).

• V

• R

• V

is the excitation voltage.

is the lead resistance.

is the measured voltage.

To convert voltage readings to strain units use the following equation:

where

GF is the Gauge Factor.

ν is the Poisson’s ratio.

Notes Full-bridge type II is sometimes used for strain measurement of bending beams.

and R4 are positioned along the beam axis and on opposite sides of the beam, and

and R2 are positioned transverse to the beam axis and on opposite sides of the beam.

Strain gauges need not have a particular STC number.

As shown in Figure 4-13, for greatest calibration accuracy, use separate wires between the bridge and the SCA terminals. Do not directly connect S+ or P– to the SCA terminals inside the SCXI-1314 terminal block unless the strain-gauge cable length is very short.

The nominal values of R

Full-Bridge Type III

To simulate the effect on strain of applying a shunt resistor across R

use the following equation:

, R2, R3, and R4 equal Rg.

This section provides information for the full-bridge strain-gauge configuration type III. The full-bridge type III only measures axial strain. Figure 4-14 shows how to position strain-gauge resistors in an axial configuration. Figure 4-15 shows the full-bridge type III circuit wiring diagram.

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Page 65

Chapter 4 Theory of Operation

(+ )

(– )

Axial

Bending

Figure 4-14. Full-Bridge Type III Measuring Axial and Rejecting Bending Strain

A full-bridge type III configuration has the following characteristics:

• 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) and the other on the opposite side (bottom).

• Compensates for temperature.

• Rejects bending strain.

• Compensates for the aggregate effect on the principle strain measurement due to the Poisson’s ratio of the specimen material.

• Compensates for lead resistance.

• Sensitivity at 1000 με is ∼ 1.3 mV

/VEX input.

out

(– )

–

(+ )

+–

(– )

Figure 4-15. Full-Bridge Type III Circuit Diagram

The following symbols apply to the circuit diagram and equations:

•R

is an active strain-gauge element measuring compressive Poisson

effect (–νε).

•R

is an active strain-gauge element measuring tensile strain (+ε).

Page 66

Chapter 4 Theory of Operation

strain ε()

2– V

GF v 1+()Vrv 1–()–[]

----------------------------------------------------------=

4U–

GF v 1+()Uv 1–()–[]

---------------------------------------------------------=

•R3 is an active strain-gauge element measuring compressive Poisson effect (–νε).

•R

is an active strain-gauge element measuring the tensile strain (+ε).

• V

• R

• V

is the excitation voltage.

is the lead resistance.

is the measured voltage.

To convert voltage readings to strain units use the following equation:

where

GF is the Gauge Factor.

ν is the Poisson’s ratio.

To simulate the effect on strain of applying a shunt resistor across R use the following equation:

Notes Full-bridge type III is sometimes used for axial strain measurement. R

are positioned along the beam axis and on opposite sides of the beam, and R positioned transverse to the beam axis and on opposite sides of the beam.

Strain gauges need not have a particular STC number.

As shown in Figure 4-15, for greatest calibration accuracy, use separate wires between the bridge and the SCA terminals. Do not directly connect S+ or P– to the SCA terminals inside the SCXI-1314 terminal block unless the strain-gauge cable length is very short.

The nominal values of R

, R2, R3, and R4 equal Rg.

SCXI-1520 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 while reading this section.

and R4

and R3 are

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Page 67

Chapter 4 Theory of Operation

Rear Signal Connector

CH0–

CH0+

CH1+

Buffer

Analog

Multiplexer

S0+

S1+

RS0–

RS0+

Source

Track and

Calibration

SCXI-1520 Module

Signal Conditioning

Lowpass

Variable Gain

Lowpass

Inst. Amp.

CH7+

Hold, X1

Filter

X1 to X50

Filter

X1 or X20

CH7–

Scan Clock

S7+

Bus

Analog

SCXIbus Connector

Bus

Switch

Analog

Control

Multiplexer

Setting

Voltage

Excitation

Null

Coarse

Compensation

and Control

Digital Interface

Fine

Compensation

–

Amp

Output

RS+

Half-

Bridge

Completion

–X1

Output

Amp

RS–

–

Signal Conditioning (see above)

Shunt Calibration Switches

Block

Terminal

SCXI-1314

S0+

S0–

RS0+

P0+

Screw

to Field

Terminals

Quarter-Bridge

Wiring

Completion

QTR

P0–

RS0–

Shunt Cal A

SCA0

Shunt Cal B

SCB0

S1+

S7+

Figure 4-16. Block Diagram of SCXI-1314/SCXI-1520 Combination

Page 68

Chapter 4 Theory of Operation

The analog input signal from the strain gauge or bridge sensor connects to SX+ and SX– of the terminal block. The signal passes through the terminal block to an electronic four-position switch in the module. Figure 4-16 shows the switching stage that controls the input of to the signal conditioning circuitry and the E/M Series DAQ device.

In the switching stage there are four positions. In the first position where SX+ and SX– inputs connect directly to an instrumentation amplifier. This position is for full-bridge connections or general-purpose analog input. The second position connects the negative input to the internal voltage divider between the excitation terminals. The internal voltage divider functions as bridge completion for quarter- and half-bridge configurations. The third switch position connects the amplifier input to a calibration voltage source inside the module for gain calibration, and the fourth position grounds both inputs for offset calibration.

The instrumentation amplifier stage presents a very high input impedance to external signals and passes only the differential signal. The offset null compensation circuitry adjusts the signal voltage by a specified offset after an offset null calibration is performed.

The signal from the instrumentation amplifier stage passes through a lowpass filter stage, a variable gain stage, another lowpass filter stage, and finally a simultaneous sample-and-hold stage before reaching the output multiplexer for multiplex mode operation. The signals S1± through S7± are also directly routed to the rear signal connector for parallel mode operation.

You can set the cutoff frequencies of the lowpass filter stage to one of five settings.You also can bypass the filters for a maximum bandwidth of 20 kHz.

The variable gain stage allows you to set the gain at many discrete settings between 1 and 50. These settings, along with the 1 or 20 setting of the instrumentation amplifier, permit the SCXI-1520 to have 49 gain settings between 1 and 1000.

The simultaneous sample-and-hold stage uses track-and-hold circuitry to perform simultaneous sampling for all channels. When enabled, track-and-hold circuitry holds the signal at the beginning of a scan, and reverts back to track mode at the end of a scan.

The signal then passes from the simultaneous sample-and-hold stage to the multiplexer stage. The modes of operation are determined by the multiplexer stage. If configured for multiplexed mode operation, the multiplexer selects the conditioned analog signal from one of the

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Page 69

Chapter 4 Theory of Operation

eight channels for routing. The signal is routed to the E/M Series DAQ device channel 0 if the SCXI-1520 is the cabled module and/or to the SCXI backplane through the SCXIbus connector. If configured for parallel mode operation, the signals are routed through the rear signal connector to the digitizing DAQ device.

The multiplexer stage is controlled by the digital interface and control circuitry. The digital interface and control circuitry stores the scan list, controls the multiplexer, and allows flexible scanning (random scanning).

Two other circuitry stages that are not directly in the signal path are excitation circuitry and shunt calibration switches. The excitation stage is stable output with a controlled feedback loop called remote sense. The remote sense signal is connected to the analog multiplexer. You can scan remote sense when configured in multiplexed mode operation.

The shunt calibration switches are controlled by the digital interface and control circuity. You must connect the SCX terminals to the bridge for shunt calibration to function correctly. When the switch is closed, a socketed shunt calibration resistor in the SCXI-1314 connects across a leg of a Wheatstone bridge.

For more detailed information about the operation of any of these circuitry stages, refer to the Bridge Configuration and Completion section, the

Excitation section, the Gain section, the Filter Bandwidth and Cutoff Frequency section, the Offset Null Compensation section, the Shunt Calibration section, the Simultaneous Sample and Hold section, and the Modes of Operation section.

Bridge Configuration and Completion

You can configure the SCXI-1520 for use with Wheatstone bridge sensors that require bridge completion. Bridge completion is necessary for quarteror half-bridge sensors. You can set the SCXI-1520 for quarter-, half-, or full-bridge configuration to match the configuration completion requirements of each sensor. When quarter- or half-bridge configuration is selected, Terminal SX– (where X is a particular channel) is disconnected from the front signal connector and internally connected to a half-bridge completion network. When quarter-bridge configuration is selected, a socketed quarter-bridge completion resistor in the terminal block is internally connected between PX– and the QTR terminal. You then field wire the quarter-bridge sensor between PX+ and QTR. Make sure that the value of the precision quarter-bridge completion resistor matches the nominal gauge resistance of the quarter-bridge sensor. The quarter-bridge completion resistor is socketed for easy replacement.

Page 70

Chapter 4 Theory of Operation

Note When using the SCXI-1520 and SCXI-1314 configured for quarter-bridge

completion, do not wire the sensor or any signal to SX–.

Excitation

Refer to the Configurable Settings in MAX section of Chapter 3,

Configuring and Testing, for more information about programmatically

setting bridge completion in MAX. For more information about programmatically setting bridge completion in NI-DAQmx and Traditional NI-DAQ (Legacy), refer to the Developing Your Application in NI-DAQmx section or the Developing Your Application in Traditional NI-DAQ

(Legacy) section, respectively, of Chapter 5, Using the SCXI-1520.

The SCXI-1520 provides DC excitation voltage for a Wheatstone bridge sensor. For half- and full-bridge applications, the excitation voltage is available at terminal block connections PX+ and PX–. For quarter-bridge applications, PX– is not used; instead, wire to terminals PX+ and QTRX. Terminal QTRX internally connects to PX–.

Excitation voltage originates from two output buffers dedicated to each channel. Since each channel is controlled independently, a short circuit across the excitation terminals of one channel has no effect on the excitation of another channel. The output buffers have negative feedback connections at the terminal block remote-sense terminals, RSX+ and RSX–. You can run separate wires from the bridge to these terminals so that the amplifiers obtain feedback directly from the bridge, thereby forcing bridge voltage to equal the desired setting.

Excitation voltage originates from two excitation output buffers per channel. One output buffer generates the positive excitation voltage, and the other output buffer generates the negative excitation voltage. Since each channel is controlled independently, a short circuit across the excitation terminals of one channel has no effect on the excitation of another channel.

PX+ is always positive with respect to ground, and PX– is always negative with respect to ground. The inverting amplifier –X1 forces the voltage at PX– to equal the negative of the voltage at PX+. For example, if you set the module output for +5 V, PX+ is at +2.5 V with respect to ground, and PX– is at –2.5 V with respect to ground. The excitation setting originates from an internal digital-to-analog converter (DAC). You can set the excitation voltage in a near-continuous range using NI-DAQmx and between 0 V and 10 V in 0.625 V increments using Traditional NI-DAQ (Legacy). You can power a 350 Ω full-bridge at 10 V without exceeding the maximum power rating of the excitation source. The excitation outputs are protected with

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Page 71

Chapter 4 Theory of Operation

surge suppressors that prevent either excitation output terminal from exceeding 6 V with respect to chassis ground.

Note Chassis ground is at the same potential as earth ground when the chassis is plugged

into a standard 3-prong AC outlet. If PX– 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 information about programmatically

setting the excitation level in MAX. For more information about programmatically setting the excitation level in NI-DAQmx and Traditional NI-DAQ (Legacy), refer to the Developing Your Application in NI-DAQmx section or the Developing Your Application in Traditional NI-DAQ

(Legacy) section, respectively, of Chapter 5, Using the SCXI-1520.

Remote Sense

The excitation output buffers have negative feedback connections at the terminal block remote-sense terminals, RSX+ and RSX–. You can run separate wires from the bridge to these terminals so that the amplifiers obtain feedback directly from the bridge, forcing bridge voltage to exactly equal the desired setting. This removes unwanted DC offset in the signal and compensates for changes in lead resistance caused by temperature variation in the lead wires.

The SCXI-1520 excitation output circuits set the output voltage by monitoring the remote-sense terminals. Therefore, the SCXI-1520 corrects for a voltage (I

× R) drop in the excitation leads between the module and

the bridge, even if lead resistance changes with temperature.

You can scan the remote sense terminals. The output multiplexer has input connections to the RSX+ and RSX– terminals. You can scan these terminals for monitoring and scaling purposes, even if the remote-sense terminals are not connected. Refer to Chapter 5, Using the SCXI-1520, and Appendix B,

Using SCXI Channel Strings with Traditional NI-DAQ (Legacy) 7.0 or Later, for more information.

Wire the SCXI-1520 for remote sense as shown in Figure 2-8,

Remote-Sense Circuit Diagram. There are no configuration settings you

need to change in the software.

Note If you use remote sense, set R

to zero in the MAX configuration of the channel and

in your application equations for measured strain (ε).

Page 72

Chapter 4 Theory of Operation

Gain Adjusting Factor 1

---------+=

meas

Gain Adjusting Factor×

Note NI recommends that you connect remote sense wires to the sensor directly for

optimal performance.

If you leave the remote-sense terminal unconnected, internal 1 kΩ resistors provide feedback to the buffers from the PX+ and PX– terminals. Therefore, you need not install a jumper wire between RSX+ and PX+, or RSX– and PX–. However, NI recommends performing a shunt calibration to compensate for the voltage drop across lead resistance.

If you are not connecting remote sense and not performing shunt calibration, you must scale the measurements to compensate for the excitation voltage drop across the lead resistance. You should use the following gain adjustment factor:

This gain adjust factor is used in your application to compensate for the voltage drop across the leads as follows:

Gain

SCXI-1520 User Manual 4-24 ni.com

The SCXI-1520 has multiple gain stages to provide optimal overall signal gains appropriate for fully utilizing the range of the digitizing E/M Series DAQ device. The first gain stage (the instrumentation amplifier stage) provides gains of either 1 or 20. The second gain stage provides many discrete settings between 1 and 50. Together these two gain stages combine for 49 overall gain settings with overall gains between 1 and 1000.

For overall 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 20, the first stage is set to 1 and the appropriate second stage gain is applied.

In normal operation of the SCXI-1520, you need not set the gain since NI-DAQ sets the gain based on the range of your virtual channel, task, or global channel, or the input limits set in LabVIEW.

In NI-DAQmx the default setting is 1.0 and in Traditional NI-DAQ (Legacy) the default gain setting is 100.00.

Page 73

Refer to the Configurable Settings in MAX section of Chapter 3,

Configuring and Testing, for more information about programmatically

setting gain using range settings in MAX. For more information about programmatically setting gain using range settings in NI-DAQmx and Traditional NI-DAQ (Legacy), refer to the Developing Your Application in

NI-DAQmx section or the Developing Your Application in Traditional NI-DAQ (Legacy) section, respectively, of Chapter 5, Using the SCXI-1520.

Filter Bandwidth and Cutoff Frequency

The SCXI-1520 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 mode. For additional flexibility in cutoff frequency settings and for greater suppression, NI recommends combining the hardware filtering provided by the SCXI-1520 with digital filtering. NI recommends using the Advanced Analysis functions of LabVIEW, LabWindows/CVI, or Measurement Studio. By combining hardware anti-aliasing with digital filtering, you can choose any cutoff frequency.

The Advanced Analysis functions are only available in LabVIEW Full or Professional Development Systems, and LabWindows/CVI Base or Full Development Systems.

Chapter 4 Theory of Operation

Refer to the Configurable Settings in MAX section of Chapter 3,

Configuring and Testing, for more information about programmatically

setting the cutoff frequency of the filter in MAX. For more information about programmatically setting the cutoff frequency of the filter in NI-DAQmx and Traditional NI-DAQ (Legacy), refer to the Developing

Your Application in NI-DAQmx section or the Developing Your Application in Traditional NI-DAQ (Legacy) section, respectively, of

Chapter 5, Using the SCXI-1520.

Offset Null Compensation

The SCXI-1520 provides offset null compensation to adjust signal voltages to proper levels when the strain gauge or bridge sensor is at rest (unstrained). For most sensors offset null compensation is used to remove an initial voltage offset from the Wheatstone bridge. Many strain-gauge signal conditioning devices use a manually adjusted multi-turn screw potentiometer for offset null compensation. In the SCXI-1520, offset null compensation is performed electronically using software-controlled electronic potentiometers.

Page 74

Chapter 4 Theory of Operation

Table 4-2. Control Codes for Coarse and Fine Offset Null Potentiometers

Offset Null

Potentiometer

(Integer Values)

Two offset null potentiometers are used, one for coarse adjustments and the other for fine adjustments. The sum of the two potentiometer signals is added to the analog input path to adjust the signal voltage to remove the offset, which nulls the strain-gauge channel. The voltage input to the potentiometers is a voltage proportional to the 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 offset null potentiometers changes by the same amount and the offset null is maintained.

The offset null potentiometers are controlled digitally using control codes. The control codes of the offset null potentiometers are set in software using integer values. The coarse potentiometer ranges from 0 to 127 and the fine potentiometer from 0 to 4095. The span of correction (the voltage nulling range) for each potentiometer depends on the channel gain setting. Table 4-2 summarizes the nulling range and scale of the control codes.

Approximate

Range

Mid-Scale

Module

Channel Gain

Settings

Correction

Span at Analog

Input

Coarse 0 to 127 62 ≥20 VEX/10

<20 2 × V

Fine 0 to 4095 2047 ≥ 20 VEX/364

<20 VEX/18

In most cases, you do not explicitly set the offset null potentiometers, but instead allow the NI-DAQ driver software to automatically adjust them for you. You can do this either through MAX or in your application.

Refer to the Configurable Settings in MAX section of Chapter 3,

Configuring and Testing, for more information about programmatically

performing offset null compensation in MAX. For more information about programmatically performing offset null compensation in NI-DAQmx and Traditional NI-DAQ (Legacy), refer to the Developing Your Application in

NI-DAQmx section or the Developing Your Application in Traditional NI-DAQ (Legacy) section, respectively, of Chapter 5, Using the SCXI-1520.

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Page 75

Shunt Calibration

gain adjust factor

simulated signal level

measured signal level with shunt resistor engaged

------------------------------------------------------------------------------------------------------------------------=

Chapter 4 Theory of Operation

Shunt calibration is a process used to obtain a gain adjust factor, which is used to correct for system gain error and discrepancies between nominal Gauge Factor and actual Gauge Factor of the strain gauge.

The gain adjust factor is derived using theoretical (simulated) signal levels that should result from engaging a shunt resistor across one leg 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 each future measurement to obtain highly accurate measurements that are adjusted for any gain errors or any 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 or strain virtual channels using Traditional NI-DAQ (Legacy) and tasks. For more information about programmatically performing shunt calibration switches in NI-DAQmx and Traditional NI-DAQ (Legacy), refer to the Developing

Your Application in NI-DAQmx section or the Developing Your Application in Traditional NI-DAQ (Legacy) section, respectively,

of Chapter 5, Using the SCXI-1520.

The SCXI-1520 has two independent shunt calibration circuits available for each channel at terminal sets SCAX and SCBX on the terminal block. Each shunt calibration circuit consists of a resistor in series with a switch. The SCXI-1520 shunt calibration switch is a long-life solid-state switch. The electronic switch is galvanically isolated from ground; therefore, you can connect the switch across any external bridge element.

Page 76

Chapter 4 Theory of Operation

Notes You can control an individual shunt calibration switch or combination of multiple

shunt calibration switches using NI-DAQ software. The shunt calibration resistors in series with each switch are housed in the SCXI-1314 terminal block. The shunt calibration resistors are socketed for easy replacement. The resistors are RN-55 style (standard 1/4 W) precision resistors. For resistor replacement instructions, refer to the SCXI-1314 Universal Strain Terminal Block Installation Guide.

Perform an offset null compensation just before you perform a shunt calibration. Performing a shunt calibration before an offset null compensation causes improper gain adjustment because the offset signal voltage is compensated multiple times.

Simultaneous Sample and Hold

Simultaneous sample and hold (SS/H) signal conditioning allows multiplexing MIO DAQ devices to return synchronized samples of all channels with negligible skew time between channels. SS/H signal conditioning is performed on the SCXI-1520 with track-and-hold (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 device. All channels with T/H circuitry hold their signal at the same time, even if they are on different SCXI modules. The DAQ device then digitizes the signal of each channel, giving you simultaneous sampling between channels since no time elapsed between the holding of each signal. All signals are then released and the T/H circuitry output returns to tracking the input signal. For accurate measurements, you can use the SS/H equations to calculate the maximum sample rate when scanning SCXI systems with at least one SS/H module in the scan list. Figure 4-17 shows and example of a signal during a SS/H sampling.

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Page 77

Hold Line

Convert

12 3

CH 0

CH 1

CH 2

1 HoldTime 2 Max (Min Settle Time of MIO, Min Settle Time of SCXI) 3 TrackTime

Volts

Time

Chapter 4 Theory of Operation

Figure 4-17. Signal During Simultaneous Sample-and-Hold Sampling

It is possible to enable and disable SS/H programmatically in NI-DAQmx, although NI recommends that you leave SS/H enabled for most applications. Disabling SS/H for one module disables this feature for all modules in all chassis that are controlled by the same DAQ device. You should only disable SS/H if your application does not require simultaneous sampling and requires higher acquisition rates than are possible with SS/H enabled. Refer to the Developing Your Application in NI-DAQmx section of Chapter 5, Using the SCXI-1520, for more information about

Page 78

Chapter 4 Theory of Operation

(HT ( NoC 1)–+ [Maximum of either (MST

MIO

) or ( MST

SCXI

)] TT)+×

--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------=

programmatically enabling and disabling SS/H using NI-DAQmx in your application.

Maximum Simultaneous Sample and Hold Sample Rate Using NI-DAQmx

When using NI-DAQmx you can use the SCXI-1520 in multiplexed or parallel mode.

Multiplexed Mode

Use the following formula to calculate the maximum SS/H sample rate in multiplexed mode:

where

SR is sample rate—frequency of acquisition of all channels

HT is hold time—the time between holding all the SS/H channels and

the first A/D conversion

NoC is number of channels—the total number of channels being sampled in the scan list (SS/H or not)

MST

rate of the MIO (also minimum interchannel delay)

MST

multiplex rate of SCXI (1 ÷ 333 k = 3 μs for 12-bit MIO, 1 ÷ 1000 k = 10 μs for 16-bit MIO)

TT is track time—the minimum time between the last AD conversion of the current scan and engaging the hold signal of SS/H channels of next scan

is minimum settle time of MIO—inverse of maximum sample

MIO

is minimum settle time of SCXI—inverse of maximum

SCXI

SCXI-1520 User Manual 4-30 ni.com

Table 4-3 shows some example values used to determine the SR using the general equation.

Page 79

Chapter 4 Theory of Operation

(HT ( NoC 1)–+(MST

MIO

) TT)+×

----------------------------------------------------------------------------------------=

Table 4-3. NI-DAQmx Values Used to Determine Maximum Sample Rate in Multiplexed Mode

DAQmx SR

Required for

Maximum

Accuracy

(Hz)‡

MIO

Device

Accuracy

(Bits)

Device

Max

Sample

Rate (S/s)

(μs)

NoC

MST

(μs)

MIO

MST

SCXI

(μs)†

Default

NI-DAQmx SR

(Multiplexed)

(Hz)

NI 6070E 12 1250000 3 8 0.8 3 7 32258 32258

32 0.8 3 7 9709 9709

NI 6023E 12 200000 3 8 5 3 7 22222 22222

32 5 3 7 6061 6061

NI 6221 (68-Pin)

16 250000 3 8 4 3 7 26316 5181

32 4 3 7 7463 1486

NI 6289 18 625000 3 8 1.6 3 7 32258 5181

32 1.6 3 7 9709 1486

†

These are the default values used by DAQmx, but these might not be the desired values for your application.

‡

For DAQ boards with a 16 bit or higher ADC, 20 μs was used for MST

Note: To use a MST

value that is different than the 3 μs default, you must explicitly set the AI Convert Clock Rate in

SCXI

and 50 μs for TT.

SCXI

DAQmx. Refer to your ADE help file for details on setting the AI Convert Clock Rate property.

Maximum SS/H Sample Rates in Parallel Mode

Use the following formula to calculate the maximum SS/H sample rate in parallel mode:

where

SR is sample rate—frequency of acquisition of all channels

HT is hold time—the time between holding all the SS/H channels and

the first A/D conversion

NoC is number of channels—the total number of channels being sampled in the scan list (SS/H or not)

MST

rate of the MIO (also minimum interchannel delay)

TT is track time—the minimum time between the last AD conversion of the current scan and engaging the hold signal of SS/H channels of next scan

is minimum settle time of MIO—inverse of maximum sample

MIO

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Chapter 4 Theory of Operation

Table 4-4 shows some example values used to determine the SR using the general equation.

Table 4-4. NI-DAQmx Values Used to Determine Maximum Sample Rate in Parallel Mode

DAQmx SR

Device

MIO

Device

NI 6070E 12 1250000 3 8 0.8 7 64103 64103

NI 6023E 12 200000 3 8 5 7 22222 22222

NI 6221 (68-Pin)

NI 6254 16 1000000 3 8 1 7 58824 16667

NI 6289 18 625000 3 8 1.6 7 47170 15576

†

These are the default values used by DAQmx, but these might not be the desired values for your application.

‡

For DAQ boards with a 16 bit or higher ADC, 20 μs was used for MST

Accuracy

(Bits)

16 250000 3 8 4 7 26316 12346

Max

Sample

Rate (S/s)

(μs)

NoC

3 16 1 7 40000 14706

3 16 1.6 7 29412 12987

MST

(μs)

MIO

(μs)

and 50 μs for TT.

SCXI

†

Default

NI-DAQmx

SR (Parallel)

(Hz)

Required for

Maximum

Accuracy

(Hz)

‡

Maximum Simultaneous Sample and Hold Using Traditional NI-DAQ (Legacy)

When using Traditional NI-DAQ (Legacy), you can only use the SCXI-1520 in multiplexed mode. To use parallel mode, you must use NI-DAQmx.

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Chapter 4 Theory of Operation

NoC (Maximum of either MST

MIO

MST

SCXI

+ or HT ) TT+×

---------------------------------------------------------------------------------------------------------------------------------------------------------------=

Multiplexed Mode

Use the following formula to calculate the maximum SS/H sample rate in multiplexed mode:

where

SR is sample rate—frequency of acquisition of all channels

HT is hold time—the time between holding all the SS/H channels and

the first A/D conversion

NoC is number of channels—the total number of channels being sampled in the scan list (SS/H or not)

MST

rate of the MIO (also minimum interchannel delay)

MST

multiplex rate of SCXI (1 ÷ 333 k = 3 μs for 12-bit MIO, 1 ÷ 1000 k = 10 μs for 16-bit MIO)

TT is track time—the minimum time between the last AD conversion of the current scan and engaging the hold signal of SS/H channels of next scan

is minimum settle time of MIO—inverse of maximum sample

MIO

is minimum settle time of SCXI—inverse of maximum

SCXI

Table 4-5. Traditional NI-DAQ (Legacy) Values Used to Determine Maximum Sample Rate in Multiplexed Mode

MIO

Device

NI 6070E 12 1250000 3 8 1 3 7 30961

NI 6070E 12 1250000 3 32 1 3 7 9243

NI 6023E 12 200000 3 8 5 3 7 18640

NI 6023E 12 200000 3 32 5 3 7 5166

NI 6052E 16 333333 3 8 3 10 7 11047

NI 6052E 16 333333 3 32 3 10 7 2932

NI 6032E 16 100000 3 8 10 10 7 8324

NI 6032E 16 100000 3 32 10 10 7 2176

Device

Accuracy

(Bits)

Table 4-5 shows some example values used to determine the SR using the general equation.

Device Maximum

Sample Rate (S/s)

(µs)

NoC

MST

(µs)

MIO

MST

SCXI

(µs)

Maximum SR

Traditional NI-DAQ

(Legacy) Multiplexed

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Chapter 4 Theory of Operation

Modes of Operation

The SCXI-1520 provides two modes of operation for passing the conditioned signals to the digitizing DAQ device—multiplexed mode and parallel mode.

Theory of Multiplexed Mode Operation

In multiplexed mode, all input channels of an SCXI module are multiplexed into a single analog input channel of the DAQ device. Multiplexed mode operation is ideal for high channel count systems. Multiplexed mode is typically used for performing scanning operations with the SCXI-1520. The power of SCXI multiplexed mode scanning is its ability to route many input channels to a single channel of the DAQ device.

The multiplexing operation of the analog input signals is performed entirely by multiplexers in the SCXI modules, not inside the DAQ device or SCXI chassis. In multiplexed mode, the SCXI-1520 scanned channels are kept by the NI-DAQ driver in a scan list. Immediately prior to a multiplexed scanning operation, the SCXI chassis is programmed with a module scan list that controls which module sends its output to the SCXIbus during a scan through the cabled SCXI module.

The list can contain channels in any physical order and the multiplexer can sequence the channel selection from the scan list in any order. The ordering of scanned channels need not be sequential. 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 device. This is referred to as flexible scanning (random scanning). Not all SCXI modules provide flexible scanning.

The module includes first-in first-out (FIFO) memory for storing the channel scan list defined in your application code. NI-DAQ drivers load the FIFO based on the channel assignments you make in your application. You need not explicitly program the module FIFO as this is done automatically for you by the NI-DAQ driver.

When you configure a module for multiplexed mode operation, the routing of multiplexed signals to the DAQ device depends on which module in the SCXI system is cabled to the DAQ device. There are several possible scenarios for routing signals from the multiplexed modules to the DAQ device.

If the scanned SCXI-1520 module is not directly cabled to the DAQ device, the module sends its signals through the SCXIbus to the cabled module. The cabled module, whose routing is controlled by the SCXI chassis, routes

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Chapter 4 Theory of Operation

the SCXIbus signals to the DAQ device through the CH 0 pin on its rear signal connector.

If the DAQ device scans the cabled module, the module routes its input signals through the CH 0 pin on its rear signal connector to the DAQ device CH 0.

Multiplexed mode scanning acquisition rates have limitations that are determined based on the hardware in the system, the mode of operation, and SS/H. Refer to the Simultaneous Sample and Hold section for equations that can help you determine maximum acquisition rates with SS/H enabled. If SS/H is disabled and the system is configured for multiplexed mode operation, the maximum sampling rate is determined by the slower of the maximum sample rate of the DAQ device and the maximum multiplexing rate of SCXI. The maximum multiplexing rate of SCXI is 333 kHz. If the DAQ device can sample more quickly than 333 kHz, then the maximum multiplexing rate of SCXI is the limiting factor. If the DAQ device can sample at 333 kS/s, then the DAQ device’s sample rate is the limiting factor on the maximum acquisition rate of the system in multiplexed mode operation.

Theory of Parallel Mode Operation

Parallel mode is ideal for high speed acquisitions. In parallel mode, the eight conditioned analog output signals at the rear signal connector of the SCXI-1520, shown in Figure 4-16, are connected directly to the eight analog input channels on the DAQ device. When the SCXI-1520 operates in parallel mode, the DAQ device performs multiplexed scans of the SCXI-1520 parallel outputs. The SCXI-1520 module does not multiplex the channels.

Traditional NI-DAQ (Legacy) driver software can only control the SCXI-1520 module in multiplexed mode. NI-DAQmx can operate the SCXI-1520 in both multiplexed and parallel mode.

In parallel mode, SCXI-1520 channels 0 through 7 conditioned outputs are passed directly to DAQ device channels 0 through 7. The DAQ device channels should be configured for differential input mode.

Scanning remote-sense channels is not possible when operating the SCXI-1520 in parallel mode operation. This is because RSX+ and RSX– terminals are not connected to the 50-pin rear signal connector of the SCXI-1520 as shown in Figure 4-16, but instead are only connected internally to the analog multiplexer. The NI-DAQmx driver can still scan the remote sense channel to utilize for strain task and channel calibration

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Chapter 4 Theory of Operation

Note The remote-sense hardware control loop still functions regardless of the mode of

operation correcting the excitation voltage level at all times, even though you cannot always scan the remote-sense channels.

and scaling in MAX. However, during operation in your application you cannot monitor the remote-sense channels if the SCXI-1520 is operating in parallel mode.

When SS/H is enabled, the parallel output signals are held while the channels are scanned by the DAQ device. The T/H circuit of each channel is in hold mode during this time. This appears as glitches on the parallel outputs as the SCXI-1520 is sampled by the digitizing DAQ device. Due to these signal events, NI recommends that you not scan the SCXI-1520 channels with a DAQ device while simultaneously measuring their parallel outputs with any other analog input device, such as an oscilloscope, unless you disable SS\H.

It is possible to enable and disable SS/H programmatically in NI-DAQmx, although NI recommends that you leave SS/H enabled for most applications. Disabling SS/H for one module disables this feature for all modules in all chassis that are controlled by the same DAQ device. SS/H should only be disabled if your application does not require simultaneous sampling and requires higher acquisition rates than are possible with SS/H enabled. Refer to the Developing Your Application in NI-DAQmx section of Chapter 5, Using the SCXI-1520, for more information about programmatically enabling and disabling SS/H using NI-DAQmx in your application.

For more information about programmatically performing offset null compensation in NI-DAQmx and Traditional NI-DAQ (Legacy), refer to the Developing Your Application in NI-DAQmx section or the Developing

Your Application in Traditional NI-DAQ (Legacy) section, respectively, of

Chapter 5, Using the SCXI-1520.

Parallel mode operation acquisition rates have limitations that are determined based on the DAQ device you are using and SS/H. Refer to the

Simultaneous Sample and Hold section for equations that can help you

determine maximum acquisition rates with SS/H enabled. If SS/H is disabled and the system is configured for parallel mode operation, the maximum sampling rate is determined by the maximum sample rate of the DAQ device. The 333 kHz maximum SCXI multiplexing rate is not a limitation in parallel mode operation. Therefore, if the DAQ device can sample more quickly than 333 kHz, the SCXI-1520 configured for parallel mode operation is not the limiting factor.

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Using the SCXI-1520

This chapter makes suggestions for developing your application and provides basic information regarding calibration.

Caution Electrostatic discharges (ESD) of 4 kV or greater may cause the SCXI-1520 to

stop responding, resulting in a lost or erroneous reading. As a result, a software reset may be required to enhance immunity by restoring functionality in such a condition. A software reset can be accomplished by calling the DAQmx Reset Device function. The DAQmx Reset VI is located in the Measurement I/O»NI-DAQmx»Dev Config palette on the LabVIEW Block Diagram. The Reset Device function is usually used at the beginning or end of a program. If the Reset Device is used in the middle of a program it will immediately abort all active tasks associated with a device, disconnect any routes, and return the device to an initialized state. Aborting a task immediately terminates the currently active operation, such as a read or a write. Aborting a task puts the task into an unstable but recoverable state. To recover the task in DAQmx, use DAQmx Start to restart the task or use DAQmx Stop to reset the task without starting it. The task cannot be recovered in Traditional DAQ so the task will need to be completely re-setup. For more information on these functions please see the DAQmx/Traditional DAQ help files.

Developing Your Application in NI-DAQmx

Note If you are not using an NI ADE, using an NI ADE prior to version 7.0, or are using

This section describes how to configure and use NI-DAQmx to control the SCXI-1520 in LabVIEW, LabWindows/CVI, and Measurement Studio. These ADEs provide greater flexibility and access to more settings than MAX, but you can use ADEs in conjunction with MAX to quickly create a customized application.

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Chapter 5 Using the SCXI-1520

Typical Program Flowchart

Figure 5-1 shows a typical program flowchart for creating a task to configure channels, take a measurement, analyze the data, present the data, stop the measurement, and clear the task.

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Chapter 5 Using the SCXI-1520

Ye s

Create Strain or Custom

Voltage with Excitation Channel

Adjust Timing Settings

Create a Task

Programmatically

Create Task in

DAQ Assistant

or MAX

Hardware

Timing/Triggering?

Create Task Using

DAQ Assistant?

NoYes

Ye s

Configure Channels

Further Configure

Channels?

Start Measurement

Read Measurement

Clear Task

Continue Sampling?

Stop Measurement

Ye s

Perform

Offset

Null?

Perform

Shunt

Calibration?

Bridge Null

Operation

Ye s

Shunt Calibration

Operation

Analyze Data?

Display Data?

Process

Data

Graphical

Display Tools

Ye s

Create Another

Channel?

Ye s

Figure 5-1. Typical Program Flowchart

Page 88

Chapter 5 Using the SCXI-1520

General Discussion of Typical Flowchart

The following sections briefly discuss some considerations for a few of the steps in Figure 5-1. These sections are meant to give an overview of some of the options and features available when programming with NI-DAQmx.

Creating a Task Using DAQ Assistant or Programmatically

When creating an application, you must first decide whether to create the appropriate task using the DAQ Assistant or programmatically in the ADE.

Developing your application using DAQ Assistant gives you the ability to configure most settings such as measurement type, selection of channels, bridge configuration, excitation voltage, signal input limits, task timing, and task triggering. You can access the DAQ Assistant through MAX or your NI ADE. Choosing to use the DAQ Assistant can simplify the development of your application. NI recommends creating tasks using the 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 functions or VIs. If you create a task using the DAQ Assistant, you can still further configure the individual properties of the task programmatically with functions or property nodes in your ADE. NI recommends creating a task programmatically if you need explicit control of programmatically adjustable properties of the DAQ system.

Programmatically adjusting properties for a task created in the DAQ Assistant overrides the original, or default, settings only for that session. The changes are not automatically saved to the task configuration. The next time you load the task, the task uses the settings originally configured in the DAQ Assistant. Refer to the NI-DAQmx Help for information on programmatically saving tasks.

Adjusting Timing and Triggering

There are several timing properties that you can configure through the DAQ Assistant or programmatically using function calls or property nodes. If you create a task in the DAQ Assistant, you can still modify the timing properties of the task programmatically in your application.

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Chapter 5 Using the SCXI-1520

When programmatically adjusting timing settings, you can set the task to acquire continuously, acquire a buffer of samples, or acquire one point at a time. For continuous acquisition, you must use a While Loop around the acquisition components even if you configured the task for continuous acquisition using MAX or the DAQ Assistant. For continuous and buffered acquisitions, you can set the acquisition rate and the number of samples to read in the DAQ Assistant or programmatically in your application. By default, the clock settings are automatically 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 ADEs used to configure the SCXI-1520 access an underlying set of NI-DAQmx properties. Table 5-1 shows some of these properties. You can use Table 5-1 to determine what kind of properties you 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 properties, you

must stop the task, make the adjustment, and re-start the application. Figure 5-1 assumes all properties are configured before the task is started.

Table 5-1. NI-DAQmx Properties

Property Short Name Description

Analog Input»General Properties» Advanced»Range»High

AI.Rng.High Specifies the upper

limit of the input range.

Analog Input»General Properties» Advanced»Range»Low

AI.Rng.Low Specifies the lower

limit of the input range.

Analog Input»General Properties» Filter»Analog Lowpass»Enable

AI.Lowpass.Enable Enables the lowpass

filter of the channel.

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Chapter 5 Using the SCXI-1520

Property Short Name Description

Table 5-1. NI-DAQmx Properties (Continued)

Analog Input»General Properties» Filter»Analog Lowpass»Cutoff Frequency

Analog Input»General Properties» Signal Conditioning»Bridge» Configuration

Analog Input»General Properties» Signal Conditioning»Bridge» Shunt Cal»Shunt Cal Enable

Analog Input»General Properties» Signal Conditioning»Bridge» Shunt Cal»Shunt Cal Select

Analog Input»General Properties» Signal Conditioning»Excitation» Value

Analog Input»Strain»Strain Gage» Configuration

AI.Lowpass.CutoffFreq Specifies in hertz

the frequency corresponding to the –3 dB cutoff of the filter. You can specify 10, 100, 1000, or

10000.

AI.Bridge.Cfg Specifies the sensor

Wheatstone bridge type.

AI.Bridge.ShuntCal.Enable Specifies whether

to place the shunt calibration resistor across one arm of the bridge.

AI.Bridge.ShuntCal.Select Specifies which

calibration switch(es) to enable.

AI.Excit.Val Specifies the amount

of excitation in volts.

AI.StrainGage.Cfg Specifies the

strain-gauge configuration type.

Analog Input»General Properties» Signal Conditioning»Bridge» Nominal Resistance

AI.Bridge.NomResistance Specifies in ohms the

resistance of the bridge in an unloaded condition.

Analog Input»Strain»Strain Gage» Gage Factor

AI.StrainGage.GageFactor Specifies the

sensitivity of the strain gauge.

Analog Input»Strain»Strain Gage» Poisson Ratio

AI.StrainGage.PoissonRatio Specifies the ratio of

lateral strain to axial strain in the specimen material.

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Chapter 5 Using the SCXI-1520

Table 5-1. NI-DAQmx Properties (Continued)

Property Short Name Description

Analog Input»General Properties» Signal Conditioning»Bridge» Initial Bridge Voltage

Analog Input»General Properties» Signal Conditioning»Bridge» Balance»Coarse Potentiometer Property

Analog Input»General Properties» Signal Conditioning»Bridge» Balance»Fine Potentiometer Property

Analog Input»General Properties» Signal Conditioning»Excitation» Source

Analog Input»General Properties» Advanced»Sample and Hold Enable

AI.Bridge.InitialVoltage Specifies in volts the

output voltage of the bridge in the unloaded condition.

AI.Bridge.Balance.CoarsePot Specifies by how much

to compensate for offset in the signal. This value can be between 0 and 127.

AI.Bridge.Balance.FinePot Specifies by how much

to compensate for offset in the signal. This value can be between 0 and 4095.

AI.Excit.Src Specifies the source of

excitation.

AI.SampAndHold.Enable Specifies whether

to enable the sample-and-hold circuitry of the device.

Note This is not a complete list of NI-DAQmx properties and does not include every

property you may need to configure your application. It is a representative sample of important properties to configure for strain and Wheatstone bridge measurements. For a complete list of NI-DAQmx properties and more information about NI-DAQmx properties, refer to your ADE help file.

Performing Offset Null Compensation

The SCXI-1520 provides offset null compensation circuitry to adjust signal voltages to proper levels when the strain gauge or bridge sensor is at rest (unstrained). For most sensors, offset null compensation removes an initial voltage offset from the Wheatstone bridge. If you are measuring strain, you can use a strain task or global channel to perform offset null compensation. The offset null compensation can be performed during the configuration of the global channel(s) or programmatically using the DAQmx Offset Null function (in LabVIEW, use

Daqmx Perform Bridge Offset Nulling

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Chapter 5 Using the SCXI-1520

Calibration.vi; in CVI, use the DAQmxPerformBridgeOffsetNullingCal function). Refer to the

Creating a Strain Global Channel or Task section of Chapter 3, Configuring and Testing, for information about offset null compensation

when in MAX. If you are not measuring strain or would like to adjust the offset to an arbitrary voltage, you can manually adjust the coarse and fine potentiometer settings using properties.

For more information about offset null compensation, refer to the Offset

Null Compensation section of Chapter 4, Theory of Operation.

Performing Shunt Calibration

Shunt calibration is a process used to obtain a gain adjust factor, which corrects for system gain error and discrepancies between the nominal gauge factor and actual gauge factor of the strain gauge. If you are measuring strain, you can use a strain task or global channel to perform shunt calibration. The shunt calibration is performed during the configuration of the global channel(s). Refer to the Creating a Strain Global Channel or

Task section of Chapter 3, Configuring and Testing, for information about

shunt calibration 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 the acquisition, read measurements, analyze the data returned, and display it according to the needs of your application. Typical methods of analysis include digital filtering, averaging data, 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 the nominal excitation voltage output by the SCXI-1520. You can scan the remote sense pins RSX+ and RSX– with the DAQmx physical channels

DevX/_pPos

actual excitation voltage. Take the difference of the two physical channels to determine the actual excitation applied to the bridge and use this value in the scaling equation.

Note If RSX+ and RSX– are not wired to the bridge where PX+ and PX– connect, _ pPos

_pNeg

and

only measure the internal excitation. Measuring this voltage does not correct

for the voltage drop in the excitation leads.

and Dev

/_pNeg

to find the

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Page 93

NI provides powerful analysis toolsets for each NI ADE to help you perform advanced analysis on the data without requiring you to have a programming background. After you acquire the data and perform any required analysis, it is useful to display the data in a graphical form or log it to a file. NI ADEs provide easy-to-use tools for graphical display, such as charts, graphs, slide controls, and gauge indicators. NI ADEs have tools that allow you to easily save the data to files such as spread sheets for easy viewing, ASCII files for universality, or binary files for smaller file sizes.

Completing the Application

After you have completed the measurement, analysis, and presentation of the data, it is important to stop and clear the task. This releases any memory used by the task and frees up the DAQ hardware for use in another task.

Note In LabVIEW, tasks are automatically cleared.

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 SCXI-1520. If you need more information or for further instructions, select Help»VI, Function, & How-To Help from the LabVIEW menu bar.

Chapter 5 Using the SCXI-1520

Note Except where otherwise stated, the VIs in Table 5-2 are located on the Functions»

All Functions»NI Measurements»DAQmx - Data Acquisition subpalette and

accompanying subpalettes in LabVIEW.

Table 5-2. Programming a Task in LabVIEW

Flowchart Step VI or Program Step

Create Task in DAQ Assistant Create a DAQmx Task Name Constant located on the

Controls»All Controls»I/O»DAQmx Name Controls

New Task (DAQ

Create a Task Programmatically (optional)

subpalette, right-click it, and select

Assistant)

DAQmx Create Task.vi—This VI is optional if you created

and configured your task using the DAQ Assistant. However, if you use it in LabVIEW, any changes you make to the task will not be saved to a task in MAX.

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Chapter 5 Using the SCXI-1520

Table 5-2. Programming a Task in LabVIEW (Continued)

Flowchart Step VI or Program Step

Create AI Strain Channel (optional)

Create AI Custom Voltage with Excitation Channel (optional)

Adjust Timing Settings (optional)

Configure Channels (optional)

Perform Offset Null Compensation

DAQmx Create Virtual Channel.vi (AI Voltage by default,

to change to a strain gauge channel, click AI Voltage and select Analog Input»Strain»Strain Gage.)—This VI is optional if

you created and configured your task and channels using the DAQ Assistant. Any channels created with this VI are not saved in the DAQ Assistant. They are only available for the present session of the task in LabVIEW.

DAQmx Timing.vi (Sample Clock by default)—This VI is

optional if you created and configured your task using the DAQ Assistant. Any timing settings modified with this VI are not saved in the DAQ Assistant. They are only available for the present session.

DAQmx Channel Property Node, refer to the Using a DAQmx

Channel Property Node in LabVIEW section for more

information. This step is optional if you created and fully configured the channels 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.

Daqmx Perform Bridge Offset Nulling

Calibration.vi

or you can perform offset null compensation when you create and configure your channels using the DAQ Assistant. Refer to the Creating a Strain Global Channel or Task section of Chapter 3, Configuring and Testing, for information about offset null compensation in MAX.

Perform Shunt Calibration You can perform shunt calibration when you create and configure

your channels using the DAQ Assistant. Refer to the Creating a

Strain Global Channel or Task section 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 include filtering, scaling,

harmonic analysis, or level checking. Some data analysis tools are located on the Functions»Signal Analysis subpalette and on the Functions»All Functions»Analyze subpalette.

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Chapter 5 Using the SCXI-1520

Table 5-2. Programming a Task in LabVIEW (Continued)

Flowchart Step VI or Program Step

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 and Controls» All Controls»Graph subpalette.

Continue Sampling For continuous sampling, use a While Loop. If you are using

hardware timing, you also need to set the

DAQmx Timing.vi

sample mode to Continuous Samples. To do this, right-click the terminal of the

DAQmx Timing.vi labeled sample mode and

click Create»Constant. Click the box that appears and select Continuous Samples.

Stop Measurement DAQmx Stop Task.vi (This VI is optional, clearing the task

automatically stops the task.)

Clear Task DAQmx Clear Task.vi

Using a DAQmx Channel Property Node in LabVIEW

Note With the SCXI-1520, you must use property nodes to disable SS/H.

You can use property nodes in LabVIEW to manually configure the channels. To create a LabVIEW property node, complete the following steps:

1. Launch LabVIEW.

2. Create the property node in a new VI or in an existing VI.

3. Open the block diagram view.

4. From the Functions toolbox, select All Functions» NI Measurements»DAQmx - Data Acquisition, and select

Channel Property Node

DAQmx

5. Configure your channels. The initial ActiveChan item allows you to specify exactly what channel(s) you want to configure. If you want to configure several channels with different properties, 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.

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6. Right-click ActiveChan, select Add Element, and left-click in the new ActiveChan box. Navigate through the menus, and select the property you wish to define.

7. Change the property to read or write to either get the property or write a new value. Right-click the property, go to Change To, and select Write, Read, or Default Value.

8. After you have added the property to the property node, right-click the terminal to change the attributes of the property, add a control, constant, or indicator.

Figure 5-2. LabVIEW Channel Property Node with Filtering Enabled

at 10 kHz and SS/H Disabled

9. To add 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.

Specifying Channel Strings in NI-DAQmx

Use the channel input of DAQmx Create Channel to specify the SCXI-1520 channels. The input control/constant has a pull-down menu showing all available external channels. You can right-click the physical channel input, select I/O Name Filtering, and check Internal Channels. This allows you to select the SCXI-1520 excitation channels. The strings take one of the following forms:

• single device identifier/channel number—for example

• multiple, noncontinuous channels—for example SC1Mod1/ch0,

SC1Mod1/ch4

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SC1Mod1/ch0

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• multiple continuous channels—for example SC1Mod1/ch0:4 (channels 0 through 4)

• positive or negative excitation on a particular SCXI-1520 channel—for example

SC1Mod1/_pPos0, SC1Mod1/_pNeg0

When you have a task containing SCXI-1520 channels, you can set the properties of the channels programmatically using the DAQmx Channel Property Node.

Follow the general programming flowchart or open an example to build a basic strain virtual channel. You can use property nodes in LabVIEW to control, configure, and customize the NI-DAQmx task and SCXI-1520. To create a LabVIEW property node, complete the following steps:

1. Launch LabVIEW.

2. Create the property node in a new Virtual Instrument (VI) or in an existing VI.

3. Open the block diagram view.

4. From the Functions tool bar, select NI Measurements, DAQmx - Data Acquisition, and select the type of property node you wish to configure.

5. The ActiveChan item allows you to specify what channel(s) you want to configure. If you want to configure several channels with different properties, separate the lists of properties with another Active Channels box, and assign the appropriate channel to each list of properties.

6. Right-click ActiveChan and select Add Element. Left-click the new ActiveChan box. Navigate through the menus and select the property you wish to define.

7. You must change the property to read or write to either get the property or write a new value. Right-click the property, go to Change To, and select Write, Read, or Default Value.

8. After you have added the property to the property node, right-click the terminal to change the attributes of the property, add a control, constant, or indicator.

9. To add 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.

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Text Based ADEs

You can use text based ADEs such as LabWindows/CVI, Measurement Studio, Visual Basic, .NET, and C# to create code for using the SCXI-1520.

LabWindows/CVI

LabWindows/CVI works with the DAQ Assistant in MAX to generate code for a strain task. You can then use the appropriate function call to modify the task. To create a configurable channel or task in LabWindows/CVI, complete the following steps:

1. Launch LabWindows/CVI.

2. Open a new or existing project.

3. From the menu bar, select Tools»Create/Edit DAQmx Tasks.

4. Choose Create New Task In MAX or Create New Task In Project

to load the DAQ Assistant.

5. Configure the NI-DAQmx strain task following the instructions in the Creating a Strain Global Channel or Task section of Chapter 3,

Configuring and Testing.

6. The DAQ Assistant creates the code for the task based on the parameters you define in MAX and the device defaults. To change a property of the channel programmatically, use the

DAQmxSetChanAttribute function.

Note Refer to the NI LabWindows/CVI Help for more information on creating NI-DAQmx

tasks in LabWindows/CVI and NI-DAQmx property information.

Measurement Studio (Visual Basic, .NET, and C#)

When creating a strain task in Visual Basic .NET and C#, follow the general programming flow in Figure 5-1. You can then use the appropriate function calls to modify the task. This example creates a new task and configures an NI-DAQmx strain channel on the SCXI-1520. You can use the same functions for Visual Basic .NET and C#.

In this example, an analog input channel object 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.CreateStrainGageChannel(

System.String physicalChannelName,

System.String nameToAssignChannel,

System.Double minVal,

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System.Double maxVal,

AIStrainGageConfiguration strainGageConfiguration,

AIExcitationSource excitationSource,

System.Double excitationValue,

System.Double gageFactor,

System.Double initialBridgeVoltage,

System.Double normalGageResist,

System.Double poissonRatio,

System.Double leadWireResist,

AIStrainUnits units);

To actually create and configure the channel, you would enter something resembling the following example code:

Task myTask = new

NationalInstruments.DAQmx.Task(“myTaskName”);

MyTask.CreateStrainGageChannel (

“SC1Mod1/ai0”,

“strain0”,

-0.001,

0.001,

AIStrainGageConfiguration.FullBridgeIII,

// System.Double minVal

// System.Double maxVal

AIStrainGageConfiguration strainGageConfiguration

AIExcitationSource.Internal,

excitationSource

3.3,

// System.Double excitationValue

2.0,

// System.Double gageFactor

0.0,

// System.Double initialBridgeVoltage

120.0,

0.3,

0.0,

AIStrainUnits.Strain);

// System.Double normalGageResist // System.Double poissonRatio // System.Double leadWireResist

// setting attributes after the channel is created

AIChannel myChannel = myTask.AIChannels[“strain0”];

myChannel.LowpassCutoffFrequency = 10.0;

myChannel.LowpassEnable = true;

myChannel.AutoZeroMode = AIAutoZeroMode.Once;

// System.String physicalChannelName

// System.String nameToAssignChannel

// AIExcitationSource

// AIStrainUnits units

You can change any of the properties at a later time. For example, to change the filter settings of

AIChannel myChannel = myTask.AIChannels[“strain 0”];

myChannel.LowpassCutoffFrequency = 10.0;

myChannel.LowpassEnable = true;

myChannel, enter the following lines:

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Chapter 5 Using the SCXI-1520

Notes You can create and configure the strain task in MAX and load it into your application

with the function call

Refer to the NI Measurement Studio Help for more information on creating NI-DAQmx tasks in LabWindows/CVI and NI-DAQmx property information.

Property Short Name Description

Modify the example code above or the code from one of the shipping examples as needed to suit your application. Refer to the Other Application

Documentation and Material section for the location of program examples.

NationalInstruments.DAQmx.DaqSystem.Local.LoadTask

Programmable NI-DAQmx Properties

All of the different ADEs that configure the SCXI-1520 access an underlying set of NI-DAQmx properties. Table 5-3 provides a list of some of the properties that configure the SCXI-1520. You can use this list to determine what kind of properties you need to set to configure the device for your application. For a complete list of NI-DAQmx properties, refer to your ADE help file.

Table 5-3. NI-DAQmx Properties

Analog Input»General Properties» Advanced»Range»High

AI.Rng.High Specifies the upper limit

of the input range on the digitizer device.

Analog Input»General Properties» Advanced»Range»Low

AI.Rng.Low Specifies the lower limit

of the input range on the digitizer device.

Analog Input»General Properties» Advanced»Sample and Hold Enable

AI.SampAndHold.Enable Specifies whether

to enable the sample-and-hold circuitry of the device.

Analog Input»General Properties» Signal Conditioning»Bridge» Configuration

Analog Input»General Properties» Signal Conditioning»Bridge» Shunt Cal»Shunt Cal Enable

AI.Bridge.Cfg Specifies whether the

sensor is a type of Wheatstone bridge.

AI.Bridge.ShuntCal.Enable Specifies whether

to place the shunt calibration resistor across one arm of the bridge.

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National Instruments SCXI-1520 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

SCXI-1520 User Manual

Support

Worldwide Technical Support and Product Information

National Instruments Corporate Headquarters

Worldwide Offices

Important Information

Warranty

Copyright

Trademarks

Patents

WARNING REGARDING USE OF NATIONAL INSTRUMENTS PRODUCTS

Conventions

Contents

What You Need to Get Started

Table 1-1. Accessories Available for the SCXI-1520

National Instruments Documentation

Installing Application Software, NI-DAQ, and the E/M Series DAQ Device

Installing the SCXI-1520 Module into the SCXI Chassis

Connecting the SCXI-1520 in an SCXI Chassis to an E/M Series DAQ Device for Multiplexed Scanning

Connecting the SCXI-1520 in a PXI/SCXI Combination Chassis to an E/M Series DAQ Device for Multiplexed Scanning

Verifying the SCXI-1520 Installation in Software

Installing SCXI Using NI-DAQmx in Software

Manually Adding Modules in NI-DAQmx

Installing SCXI Using Traditional NI-DAQ (Legacy) in Software

Manually Adding Modules in Traditional NI-DAQ (Legacy)

Verifying and Self-Testing the Installation

Troubleshooting the Self-Test Verification

Troubleshooting in NI-DAQmx

Troubleshooting in Traditional NI-DAQ (Legacy)

Connecting Bridge Sensor Signals

Quarter-Bridge Type I

Figure 2-1. Quarter-Bridge I Circuit Diagram

Quarter-Bridge Type II

Figure 2-2. Quarter-Bridge II Circuit Diagram

Half-Bridge Type I

Figure 2-3. Half-Bridge Type I Circuit Diagram

Half-Bridge Type II

Figure 2-4. Half-Bridge Type II Circuit Diagram

Full-Bridge Type I

Figure 2-5. Full-Bridge Type I Circuit Diagram

Full-Bridge Type II

Figure 2-6. Full-Bridge Type II Circuit Diagram

Full-Bridge Type III

Figure 2-7. Full-Bridge Type III Circuit Diagram

Remote Sense

Figure 2-8. Remote-Sense Circuit Diagram

Pin Assignments

Table 2-1. Front Signal Pin Assignments

Table 2-2. Rear Signal Pin Assignments

Table 2-3. SCXI-1520 Communication Signals

SCXI-1520 Software-Configurable Settings

Common Software-Configurable Settings

Bridge Configuration

Excitation Level

Table 3-1. Excitation Voltage for Configuration and Gauge Resistances

Filter Bandwidth

Gain/Input Range

Null Potentiometers

Shunt Calibration Switches

Modes of Operation

Simultaneous Sample and Hold

Configurable Settings in MAX

NI-DAQmx

Creating a Strain Global Channel or Task

Creating a Custom Voltage with Excitation Global Channel or Task

Traditional NI-DAQ (Legacy)

Configuring Module Property Pages in Traditional NI-DAQ (Legacy)

Creating a Strain Virtual Channel

Calibrating a Strain Virtual Channel

Verifying the Signal

Verifying the Signal in NI-DAQmx Using a Task or Global Channel

Verifying the Signal in Traditional NI-DAQ (Legacy)

Verifying the Signal Using Channel Strings

Verifying the Signal Using Strain Virtual Channel

Using the Strain Calibration Wizard in NI-DAQmx

Strain-Gauge Theory