Strain gauges for data acquisition

- an overview or tutorial about strain gauges used to measure the strain of elements in a system, and often found in data acquisition systems.

Strain gauges are used in a variety of applications, including data acquisition, for measuring the strain or deformation of a mechanical element. Essentially a strain gauge is a resistive transducer that has an electrical output that is proportional to the amount it has been deformed under strain. If the strain gauge is attached to a mechanical element, then it will be deformed along with it and provide an output according to the strain that it undergoes.

Strain gauges are normally fairly simple in their concept. They may simply comprise of a thin resistive element, often foil, that may be mounted using an adhesive to the element under test. As it deforms, so does the strain gauge whose resistance changes, even if by a small amount, so that the change can be measured and converted to give an indication of the strain.

The basic strain gauge load cell can easily be used in such a way that it can be used to give an indication of pressure, force or tension in a network. As a result strain gauges are widely used in many areas of industry and the data may be captured using a data acquisition system.

What is strain?

In order to look at strain gauges in more detail, it is necessary to first take a look at what exactly strain is.

There are many definitions of strain that can be seen in literature of all sorts. In scientific and engineering applications strain is defined as the deformation caused by the action of stress (i.e. force per unit area on a given plane) on a body. Strain is therefore manifested by a change in shape and or size. Thus stress can be defined as the amount of deformation a material experiences per unit of original length in response to stress.

Practical implementation of strain gauges

It is found that the resistance changes on a strain gauge element are exceedingly small. As a result it is necessary to take account of this in the circuitry used to ensure that the greatest levels of accuracy can be obtained.

Although it is possible to detect the resistance changes on a strain gauge relatively easily with modern equipment, it is necessary to ensure that the measurements that are taken are as accurate as possible. It is also necessary to ensure that changes resulting from other environmental conditions such as temperature are compensated for. To achieve all of this, a technique that has been in use for many years is employed and a Wheatstone bridge circuit is used for each load cell.

The Wheatstone bridge circuit itself compromises four resistors. Other resistors are used to calibration resistors and temperature dependent elements are used to compensate and calibrate the bridge output signal. Thus each strain gauge cell has four wires: two for the input power, and two for the output signal.

By Ian Poole

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