Pressure and Load Measurements: How-To Guide

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

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.

View a 60-second video on how to take a Load Cell/Pressure Measurement

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.

To understand how a load cell works, you need to first understand the basic theory behind the operating principles. As stated before, strain gages measure deformation, or strain, to determine the force (load) applied. Strain is defined as the fractional change in length. More specifically, strain is the change in length, dL, divided by the original length, L, and it varies directly proportional with the applied load. Figure 1 illustrates this concept. By sensing the strain and knowing the physical characteristics of the structural member to which the load is applied, you can accurately calculate the force.

 

Figure 1. Strain

While there are several methods of measuring strain, the most common is with a strain gage, a device whose electrical resistance varies in proportion to the amount of strain in the device. The most widely used gage is the bonded metallic strain gage as shown in Figure 2.

 

Figure 2. Bonded Metallic Strain Gage

Because the changes in strain, and therefore resistance, are extremely small, you have to use additional circuitry to amplify the changes in resistance. The most common circuit configuration in a load cell is called a Wheatstone bridge. The general Wheatstone bridge, illustrated in Figure 3, consists of four resistive arms with an excitation voltage, V_EX, that is applied across the bridge.

 

 Figure 3. Wheatstone Bridge

The output voltage of the bridge, V_O, is equal to:

 

Load cells typically use four strain gages in a Wheatstone bridge configuration, meaning that each resistive leg of the circuit is active. This configuration is called full-bridge. Using a full-bridge configuration greatly increases the sensitivity of the circuit to changes in strain, providing more accurate measurements. Although there is more in-depth theory about Wheatstone bridges, you do not need to know it because load cells are usually a “black box” with two wires for excitation (0 V and V_ex) and two wires for the output signal (AI+ and AI-). Load cell manufacturers provide a calibration curve for every load cell, which correlates the output voltage to a specific amount of force.

 

How to Make a Load Cell/ Pressure Measurement

The following section describes the necessary data acquisition and signal conditioning equipment to make an effective load cell/pressure transducer measurement. The basic requirements to make a load cell/pressure transducer measurement are excitation, signal amplification, and bridge balancing.

Bridge Excitation
Load cell signal conditioners typically provide a constant voltage source to power the bridge. While there is no standard voltage level that is recognized industry wide, excitation voltage levels around 3 to 10 V are common. While a higher excitation voltage generates a proportionately higher output voltage, the higher voltage can also cause larger errors due to self-heating. It is very important that the excitation voltage be very accurate and stable.

Signal Amplification
The output of load cells and bridges is relatively small. In practice, most load cells and load-based transducers output less than 10 mV/V (10 mV of output per volt of excitation voltage). With a 10 V excitation voltage, the output signal is 100 mV. Therefore, load cell signal conditioners usually include amplifiers to boost the signal level to increase measurement resolution and improve signal-to-noise ratios.

Bridge Balancing, Offset Nulling

When a bridge is installed, it is very unlikely that the bridge outputs exactly 0 V when no strain is applied. Rather, slight variations in resistance among the bridge arms and lead resistance generate some nonzero initial offset voltage. There are a few different ways that a system can handle this initial offset voltage.

1. Software compensation – The first method compensates for the initial voltage in software. With this method, you take an initial measurement before you apply the strain input. This is also referred to as auto-zero. This method is simple, fast, and requires no manual adjustments. The disadvantage of the software compensation method is that the offset of the bridge is not removed. If the offset is large enough, it limits the amplifier gain you can apply to the output voltage, therefore limiting the dynamic range of the measurement.
2. Offset-nulling circuit – The second balancing method uses an adjustable resistance, or potentiometer, to physically adjust the output of the bridge to 0 V. By varying the position of the potentiometer, you can control the level of the bridge output – set the output to 0 V initially.
3. Buffered offset nulling – The third method, like the software method, does not affect the bridge directly. With buffered nulling, a nulling circuit adds an adjustable DC voltage to the output of the instrumentation amplifier.

Connecting an Load Cell or Pressure Transducer to an Instrument

For this section, consider an example using the NI cDAQ-9172 chassis and the NI 9237 C Series strain gage module (see Figure 4). Similar procedures apply when using a different measurement instrument.

Figure 4. NI CompactDAQ System

 

Required equipment includes the following:

-          cDAQ-9172 eight-slot Hi-Speed USB chassis for NI CompactDAQ

-          NI 9237 four-channel, ±25 mV/V, 24-bit simultaneous bridge module

-          Full-bridge load cell

 

The NI 9237 has four RJ-50 receptacles that provide connections for four half or full bridges. Figure 5 lists the signal names of the terminals for each connector, and shows the correlation between the pin numbers of the RJ-50 10-position/10-conductor (10p10c) modular plug and the NI 9237 receptacle. The NI 9237 also has a four-position connector you can use to connect an external excitation voltage source to the module. Figure 6 shows where that connector is located, on the bottom of the NI 9237 module. It also displays the necessary connections for a full-bridge configuration.

Figure 5. NI 9237 Terminal Names 

 

Figure 6. Wiring in Full-Bridge configuration

Getting to See Your Measurement: NI LabVIEW

Now that you have connected your load cell to the measurement device, you can use LabVIEW graphical programming software to transfer the data into the computer for visualization and analysis.

Figure 7 shows an example of displaying measured strain data on a chart indicator inside the LabVIEW programming environment. 

[+] Enlarge Image
 Figure 7. LabVIEW Front Panel Showing Load Data

 

Recommended Hardware and Software

Example Load/Pressure Measurement System

NI CompactDAQ: 3-minute “out of the box” video

Take a virtual tour of NI CompactDAQ measurement hardware

Learn about and test-drive LabVIEW software for free

 

Load-Cell/Pressure Webcasts, Tutorials, and Other How-To

Load, Force, and Torque Measurements

Measuring Pressure with Pressure Sensors

 

Reader Comments | Submit a comment »

example code/application needed
Please include example code to download that included shunt and null calibration.
- Apr 23, 2008