Use NI DAQ Devices for Load, Pressure, and Torque Measurements

The article walks through how to make a measurement from bridge-based load cells, a step-by-step instructions for wiring and configuring your NI data acquisition device for strain gauge measurements. The principle also applies for bridge-based pressure and bridge-based torque sensors. The steps will be using an NI CompactDAQ system with an NI 9237 four-channel simultaneous bridge module and walk through setting up a Global Virtual Bridge Channel in NI Measurement and Automation Explorer (MAX) and testing the signal measurement and then where to find a LabVIEW example to start you application.

This article assumed basic knowledge of load cells, pressure, and torque transducer operating principles. Please review Measuring Load and Torque with Bridge-Based Sensors, to understand fundamentals of load and torque measurements and how different sensor specifications impact load cell or torque sensor performance in your application, and Pressure Measurement Overview, to understand basic pressure concepts and how different pressure sensors work.

1. Confirm that you have a physical DAQ device connected, the compatible N-DAQmx driver installed and you can see the device in NI MAX  or have created simulated NI-DAQmx device available in MAX.
2. Locating Your DAQ Device Pinout
   1. Launch NI Measurement & Automation Explorer (MAX).
   2. On the left pannel expand My System >> Devices and Interfaces.
   3. Right-click on your device name, and select Device Pinouts. This will launch the NI-DAQmx Device Terminals Help window.

3. Once you know the location of the the of the DAQ Device Pinout, you can configure a Bridge Measurement in NI MAX. Under My System, right-click Data Neighborhood and click Create New. This will launch a a Create New.. window
4. Select NI-DAQmx Global Virtual Channel and click Next.
5. Select Acquire Signals » Analog Input » Force » Force (Bridge).

6. Select whichever physical channel you intend to connect to your sensor. 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.

7. Click Next and enter a unique name for the global virtual channel (recommended) or leave the default.
8. Click Finish and you should see the completed Global Virtual Channel screen

8. On the Main (Settings) tab, type in the minimum (Min) and maximum (Max) values you expect to read from your sensor and configure the Bridge Type
9. Click Configure Scale... to configure a scale to map physical values to electrical values. There are 3 options: Two-Point Linear, Table, and Polynomial. The information for these scales can be found in the data sheet for the sensor. The excitation and Bridge Resistance also need to be verified.
10. After finishing the configuration of the Global Virtual Channel, navigate to the Connection Diagram tab in MAX. The connection diagram 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. Confirm the connections are correct.

11. Navigate back to the NI-DAQmx Global Channel tab. You can now preview your measurements buy clicking on the Run button at the top of the page. You see the value of your measurement displayed at the top of the screen. You can choose to view the signal by clicking the Display Type drop-down; the options you have are Tabular, Chart, or Graph.

11. Once you have finish testing, click the Save option at the top of the page to save NI-DAQmx global virtual channel in NI MAX to refer to it in the future.
12. Now that you have configured your measurement device, connected the sensor, and tested the signal, you are ready to make a measurement using LabVIEW. 
    1. Launch LabVIEW
    2. Navigate to Help >> Find Examples... to open the NI Example Finder window
    3. Confirm you are in the Browse tab and Navigate to Hardware Input and Output >> DAQmx >> Analog Input
    4. Double-Click the Bridge related example to open the example. Be sure to Save-As the example and use the newly saved copy to edit.