Load, Pressure, and Torque Measurements: How-To Guide

Publish Date: May 08, 2014 | 17 Ratings | 3.65 out of 5 |  PDF

Overview

Learn how to measure load, pressure, and torque using a Wheatstone bridge as the guiding principle.

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

Table of Contents

  1. Load Cells, Pressure, and Torque Transducers—Overview of Operating Principles

1. Load Cells, Pressure, and Torque 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.

Torque sensors are composed of strain gages that are affixed to a torsion bar. As the bar turns, the gages respond to the bars sheer stress, which is proportional to the torque.

 

 

The following sections describe the principle of operation of bridge-based load cells and how to make a measurement from them, although the same applies for bridge-based pressure and bridge-based torque sensors.

 

How to Make a Bridge Measurement

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

Consider an example, shown below in Figure 1,  of an NI CompactDAQ system with an NI 9237 four-channel simultaneous bridge module.


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 Bridge Measurement

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 bridge 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.

Figure 4. Selecting Channel Connected to Sensor

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 5. 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.

Wiring a Sensor to Your Device

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

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

Figure 6. 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.

Testing the Signal

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 7. 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 a Bridge Measurement

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.

Requirements

Figure 8. NI Example Finder for Measuring Bridge-Based Sensors

Figure 9. LabVIEW Example for Measuring Bridge-Based Sensors

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