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Connect Strain Gages to a Data Acquisition (DAQ)Device

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Before You Begin Back to top

Locating Your DAQ Device Pinout Back to top

Configuring a Strain Measurement Back to top

Included in the Section

Locating Your DAQ Device Pinout

Configuring a Strain Measurement

Wiring a Strain Gage to Your Device

Strain Gage Calibration

Testing the Signal

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

View in-depth description

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 16. Device Terminals Help

You can use NI Measurement & Automation Explorer (MAX) to quickly verify the accuracy of yourmeasurement system setup. Using an NI-DAQmx Global Virtual Channel you can configure a strainmeasurement without any programming. A virtual channel is a concept of the NI-DAQmx driver

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measurement without any programming. A virtual channel is a concept of the NI-DAQmx driverarchitecture used to represent a collection of device property settings that can include a name, aphysical channel, input terminal connections, the type of measurement or generation, and scalinginformation.

Follow these steps to begin:

3. With NI Measurement & Automation Explorer open, select Data Neighborhood and click

Create New.

4. Select NI-DAQmx Global Virtual Channel and click Next.

5. Select Acquire Signals » Analog Input » Strain

Figure 17. Creating an NI-DAQmx Virtual Channel

6. Select ai0 or whichever physical channel you intend to connect 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.

Figure 18. Device Physical Channels

7. Click Next and enter a name for the global virtual channel or leave the default.

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Wiring a Strain Gage to Your Device Back to top

7. Click Next and enter a name for the global virtual channel or leave the default.

8. Click Finish and you should see the following screen in MAX:

Figure 19. Setting up a Strain Channel in MAX

9. On the Settings tab, type in the minimum and maximum strain values you expect to read from

your strain gage (-1m to 1m by default).

10. 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.

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

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

Figure 20. Strain Connection Diagram

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Strain Gage Calibration Back to top

The connection diagram above indicates which pins on your DAQ device should be wired accordingto 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.

When you configure a strain measurement task, you can use the Strain Gage Calibration Wizard tocalibrate your strain gage. Complete the following steps to calibrate the strain gage:

Figure 21. Strain Gage Calibration Wizard

12. Select the Device tab and click Calibrate to launch the Strain Gage Calibration Wizard.

13. Follow the steps on the Setup Hardware window. You can configure the following settings:

i. Enable Offset Nulling Select Enable Offset Nulling to perform an offset nulling calibration

procedure.

ii. Enable Shunt Calibration Select Enable Shunt Calibration to perform a shunt calibration

procedure. If you select this option, configure the following settings:

Shunt Resistor Value—Specify the exact resistor value of the shunt resistor. The default

value is the resistance of the shunt resistor that is the factory shipping default for your

hardware. For the greatest accuracy, use a 6-1/2 digit DMM to measure the res istance

of your entire shunt calibration circuit with the shunt switch engaged and leads

connected. This will increase the accuracy of your measurements by compensating for

the shunt enable switch resistance and any lead resistance caused by long lead wires.

However, since the shunt resistor value is very large, these do not greatly impact the

measurement accuracy.

Shunt Resistor Location—Specify the location on the Wheatstone bridge to which the

shunt resistor is connected. Refer to the figure on the Setup Hardware window for the

relative positions of R1, R2, R3 and R4.

14. Click Next.

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Testing the Signal Back to top

Figure 22. Measure and Calibration Window

15. Use the Measure and Calibrate window to measure and calibrate your strain gage. The

software automatically takes the first measurement, and the table displays the results. The

table shows the following information:

i. Channel Information

Channel Name—The name of the virtual channel on which strain calibration is

performed.

Physical Chan—The physical channel to which the strain gage is connected.

ii. Offset Adjustment—This section shows information about the offset error to help you

determine if you should perform an offset null calibration. Perform an offset null calibration

if you perform a shunt calibration.

Meas Val (strain)—The measured offset value with units of strain. Normally, this value

should be close to zero.

Err (%)—Percentage offset error, determined by the equation: Err (%) = [(offset value *

100) / (Max Range Limit – Min Range Limit)] where Max Range Limit and Min Range

Limit are the maximum and minimum strain range values specified for the given virtual

channel.

Gain Adjustment—This section shows values related to gain error and the shunt

calibration procedure:

Sim Val (strain)—The simulated strain value based on the hardware setup and the

shunt resistor value.

Meas Val (strain)—The measured strain value. Normally, this value should be close

to the simulated strain value.

Gain Adj Val—The gain adjust value. This value is the end result of the shunt

calibration and is used to scale strain measurements on this channel. Normally, this

value is close to 1, which implies that the measured value and simulated value are

equal. Your strain channel stores the gain adjust value as the

AI.Bridge.ShuntCal.GainAdjust attribute.

Err (%)—Percentage gain error, determined by the equation: Err (%) = (measured

value – simulated value) * 100 / (Max – Min) where Max and Min are the maximum

and minimum strain values specified for the given virtual channel.

16. Perform your measurement and calibration with the following buttons:

i. Click Measure to make a preliminary measurement using the offset and shunt calibration

data from a previously run strain gage procedure.

ii. Click Reset Data to reset the offset and shunt calibration data and make a measurement

using their default values.

iii. Click Calibrate to perform the offset and/or shunt calibration. The table shows the resulting

measurements.

17. Click Finish when you are done. The calibration data is saved as part of your virtual channel.

On successful completion, the software automatically uses the calibration data when scaling

voltage to strains on this virtual channel.

NI-DAQmx global virtual channels allow you to preview your measurements.

18. With MAX still open, click back on the NI-DAQmx Global Channel tab and click on the Run

button. You will see the strain value of your measurement displayed at the top of the screen.

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Previous Step Next Step

Connect and Setup Hardware Introduction to LabVIEW

Figure 23. 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 DisplayType dropdown. You also have the option of saving your NI-DAQmx Global Virtual Channel shouldyou wish to refer to this configuration screen again in the future.

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