Load Cell Measurement
Basic overview of load cells and strain gages and selecting the correct data aquisition hardware.
To fully understand the construction of strain gage load cells, we will need to take a detour and discuss strain gages and strain measurement first.
Strain is the result of a force applied to a solid object. More specifically, strain is defined as the fractional change in length of an object, as depicted in the figure below.
There are two types of strain that can be applied to an object 1.) tensile or compressive and 2.) uniaxial force. For example, assume you have a steel bar cantilevered to the edge of your desk. When you push down on part of the bar hanging off the edge of the desk, you are applying a tensile force to the top of the bar. A compressive force would result when you pull the bar upwards toward you. Both of these strains cause a fractional change in length on the steel bar. On the other hand, a uniaxial force would occur if you grabbed the steel bar with both of your hands and pulled it axially (in the direction parallel to the bar). This would cause the girth (diameter) of the bar to shrink, and the length of the bar to increase (both being very small displacements and require some big muscles). The magnitude of these displacements is material dependent and indicated by the material’s Poisson Ratio (for example, the Poisson’s Ratio for steel ranges from 0.25 to 0.30).
A strain gage is a sensor where the electrical resistance varies in proportion to the amount of strain produced in a device. The picture below is an example of the most commonly used gage, called the bonded metallic strain gage.
All strain gages have a specified gage factor (GF), and this is a fundamental parameter of the gage’s
sensitivity to strain. From a qualitative perspective, the gage factor is defined as the ratio of the
fractional change in electrical resistance to the fractional change in length. The gage factor for a
bonded metallic strain gage (as shown above) is typically around 2.
Strain measurements are very small, so it is extremely important that the gage is capable of measuring very small changes in resistance. A Wheatstone bridge is normally used in order to measure such small variations in resistance. The picture below is an example Wheatstone bridge:
This circuit is ideal at measuring any changes in the resistors. Based on the type of strain measurement, one, two or all four resistors can be replaced with a strain gage, commonly referred to as quarter, half and full bridge, respectively. Remember, a strain gage measures resistivity change as strain is applied to the device.
Load cell (and strain gage) measurements involve sensing small changes in resistance. To properly acquire data from these sensors, the signal must be “conditioned” first. Ideally, the data acquisition hardware should provide the signal conditioning to simplify sensor connectivity. There would be no need to connect separate signal conditioning hardware in addition to data acquisition hardware.
It is important to consider the following when using load cells and / or strain gage sensors:
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