Strain Measurements With Strain Gages: How-To Guide

Publish Date: Aug 09, 2013 | 31 Ratings | 3.71 out of 5 |  PDF

Overview

This document is part of the How-To Guide for Most Common Measurements centralized resource portal.

Table of Contents

How to Make a Strain Gage Measurement

Most strain gage measuring solutions provide an option to measure quarter-, half-, and full-bridge configurations.

Consider the following example of an NI CompactDAQ system with an NI 9237 four-channel simultaneous bridge module (Figure 1).


Figure 1. NI CompactDAQ and NI 9237 Bridge Module

Included in the  Section

Before You Begin
This document provides step-by-step instructions for wiring and configuring your NI data acquisition device for strain gage measurements. Before you begin using your NI data acquisition hardware, you must install your application development environment (ADE) and NI-DAQmx driver software. Refer to the Installing NI LabVIEW and NI-DAQmx document for more information.

Locating Your DAQ Device Pinout

Before connecting any signals, locate your device pinout.

  1. Open NI Measurement & Automation Explorer (MAX) and expand Devices and Interfaces.
  2. Right-click on your device name, and select “Device Pinouts.”

Figure 2. Device Terminals Help

Configuring a Strain Measurement

You can use MAX to quickly verify the accuracy of your measurement system setup. With an NI-DAQmx global virtual  channel, you can configure a strain measurement without any programming. A virtual channel is a concept of the NI-DAQmx driver architecture used to represent a collection of device property settings that can include a name, a physical channel, input terminal connections, the type of measurement or generation, and scaling information.

Follow these steps to begin:

  1. With MAX open, select Data Neighborhood and click Create New.
  2. Select NI-DAQmx Global Virtual Channel and click Next.
  3. Select Acquire Signals»Analog Input»Strain.

Figure 3. Creating an NI-DAQmx Virtual Channel

  1. Select ai0 or whichever physical channel you intend to connect to your strain gage. A physical channel is a terminal or pin at which you can measure or generate an analog or digital signal. A single physical channel can include more than one terminal or pin, as in the case of a differential input channel.
  2. Click Next and enter a name for the global virtual channel or leave the default.
  3. Click Finish and you should see the following screen in MAX:

Figure 4. Setting Up a Strain Channel in MAX

  1. On the Settings tab, type in the minimum and maximum strain values you expect to read from your strain gage (-1 m to 1 m by default).
  2. Configure the Gage Factor, Gage Resistance, Initial Voltage, VEX Source, VEX Value (V), Lead Resistance, and Strain Configuration based on your sensor specifications and the information discussed above in the Strain Measurement Fundamentals section.

Wiring a Strain Gage to Your Device

The next step is to physically connect the strain gage to your DAQ device.

  1. Click the Connection Diagram tab in MAX to continue.

Figure 5. Strain Connection Diagram

The connection diagram in Figure 5 indicates which pins on your DAQ device should be wired according to the physical channel you selected. In this example, a full-bridge type I configuration uses pins 2, 3, 6, and 7, corresponding to AI+, AI-, EX+, and EX- on an NI 9237 C Series module.

Testing the Signal

With NI-DAQmx global virtual channels, you can preview your measurements.

  1. With MAX still open, click back on the NI-DAQmx Global Channel tab and click on the Run button. You see the strain value of your measurement displayed at the top of the screen.

Figure 6. Previewing a Strain Measurement in MAX

You can choose to view the signal in tabular form or as a graph by selecting Graph from the Display Type pull-down menu. You also have the option of saving your NI-DAQmx global virtual channel should you wish to refer to this configuration screen again in the future.

LabVIEW Examples for Measuring Strain

Now that you have configured your measurement device, connected the sensor, and tested the signal, you are ready to make a measurement using LabVIEW. Included in the attachments section are two VIs for measuring strain continuously. Find these in LabVIEW by going to Help»Find Examples... to open the NI Example Finder. The settings on the front panel are configured in the same way that you set up the global virtual channel above.

Requirements

 

Figure 7. LabVIEW Example for Measuring Strain

Figure 8. NI Example Finder for Measuring Strain


Other Strain Measurement Resources

  1. Understanding Strain and Choosing Strain Gages
  2. Choosing a Strain Gage Measurement System
  3. Making a Strain Measurement With LabVIEW
  4. Getting to a Better Strain Measurement

 

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