Load Cells

Anyone interested in working in industrial electronics will need a knowledge base about load cells. In general, load cells are electronic devices, often called transducers, that are used to convert a force into an electrical signal. This conversion is indirect and happens in two stages.

The first stage is through a mechanical arrangement. The force to be sensed is deformed by a strain gauge. The strain gauge converts this deformation, which is also known as the strain, to electrical signals.

In most cases, load cells consist of four strain gauges in what is known as a Wheatstone bridge configuration. It is possible, however, to have them one or two strain gauges. The electrical signal output (the second stage) is generally on the order of a few milli-volts and requires amplification by an instrumentation amplifier before it can be used.

The output of the transducer (or the load cell) is then hooked up to an algorithm to calculate the force applied to the transducer.

Although strain gauge load cells are the most common types and are used in many applications, there are other types of load cells as well.

In industrial applications, hydraulic (or hydrostatic) is probably the second most common types of load cells. These are often used to eliminate some of the problems with strain gauge load cell devices. An example of this might be the use of a hydraulic load cell in outdoor environments. Because it is immune to lightning it will be more effective and safer to use.

Other types of load cells include piezo-electric load cells and vibrating wire load cells. These are useful in geo-mechanical applications because of their low amounts of drift.

It should be noted that every type of load cell is subject to "ringing" when pushed with abrupt load changes. This is caused by the spring-like behavior of load cells. In order for load cells to measure the loads, they have to deform. Load cells have a finite stiffness to them and therefore have a spring-like behavior when pressured. This results in vibrations which cause a frequency signal.

This can be seen and measured with instruments. An oscillating data pattern can be the result of ringing. It should also be noted that ringing can be suppressed somewhat by passive means. The better method, however, is to use a control system with an actuator to actively damp out the ringing of the load cells. This method offers better performance but it is more complex to create and use.