1. Load Cells and Pressure Transducers—Overview of Operating Principles
A load cell is a transducer that converts mechanical force into electrical signals. There are many different types of load cells that operate in different ways, but the most commonly used load cell today is the strain gage (or strain gauge) load cell. As their name implies, strain gage load cells use an array of strain gages to measure the deformation of a structural member and convert it into an electrical signal.
Pressure transducers operate under the same principle. Strain gages, mounted on a diaphragm where the pressure is applied, measure the deformation of the diaphragm that is proportional to the pressure.
The following sections describe the principle of operation of strain gage load cells and how to make a measurement from them, although the same applies for strain gage pressure transducers.
How to Make a Force Measurement
Most strain gage measuring solutions provide an option to measure quarter-, half-, and full-bridge configurations.
Consider an 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
- Locating Your DAQ Device Pinout
- Configuring a Measurement
- Wiring a Sensor to Your Device
- Testing the Signal
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.
Before connecting any signals, locate your device pinout.
- Open NI Measurement & Automation Explorer (MAX) and expand Devices and Interfaces.
- Right-click on your device name, and select “Device Pinouts.”
Figure 2. Device Terminals Help
You can use MAX to quickly verify the accuracy of your measurement system setup. Using 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»Force»Force (Bridge).
Figure 3. Creating an NI-DAQmx Virtual Channel
4. Select ai0 or 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.
5. Click Next and enter a name for the global virtual channel or leave the default.
6. Click Finish and you should see the following screen in MAX:
Figure 4. Setting Up a Force Channel in MAX
7. On the Settings tab, type in the minimum and maximum values you expect to read from your sensor.
8. Configure the Bridge Type and configure a scale to map physical values to electrical values. There are options for 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.
The next step is to physically connect the sensor to your DAQ device.
9. Click the Connection Diagram tab in MAX to continue.
Figure 5. Sensor 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.
With NI-DAQmx global virtual channels, you can preview your measurements.
10. With MAX still open, click back on the NI-DAQmx Global Channel tab and click on the Run button. You see the value of your measurement displayed at the top of the screen.
Figure 6. Previewing a Force 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 Load and Force
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 bridge-based sensors continuously. You can 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.
- Bridge - Continuous Input for LabVIEW Version 10
- LabVIEW Version 10 or Later and NI-DAQmx
- Bridge - Continuous Input for LabVIEW Version 12
- LabVIEW Version 12 or Later and NI-DAQmx
Figure 7. LabVIEW Example for Measuring Bridge-Based Sensors
Figure 8. NI Example Finder for Measuring Bridge-Based Sensors