Design and function of a load cell
Load cells are used to measure weight. They are an integral part of our daily life. “In your car or at the cheese counter in the supermarket – we encounter load cells everywhere,” says HBM Product Manager Stefan Schmidt. Of course they are usually not immediately recognizable, because they are hidden in the inner workings of instruments.
Load cells generally consist of a spring element on which strain gauges have been placed. The spring element is usually made of steel or aluminum. That means it is very sturdy, but also minimally elastic. As the name “spring element” suggests, the steel is slightly deformed under load, but then returns to its starting position, responding elastically to every load. These extremely small changes can be acquired with strain gauges. Then finally the deformation of the strain gauge is interpreted by analysis electronics to determine the weight.
To understand this last point, let us consider strain gauges in more detail: They are electrical conductors firmly attached to a film in a meandering pattern. When this film is pulled, it – and the conductors – get longer. When it is contracted, it gets shorter. This causes the resistance in the electrical conductors to change. The strain can be determined on this basis, as resistance increases with strain and diminishes with contraction.
The strain gauges are firmly attached to the spring element, and therefore undergo the same movements it does. These strain gauges are arranged in what is called a bridge circuit, or more precisely a Wheatstone bridge circuit (see diagram). This means that four SGs are connected “in a ring” and the measuring grid of the force being measured is aligned accordingly.
If an object is placed on the load cell or suspended from it, the object’s weight can be determined. The intended load for a load cell is always aligned in the direction of the center of the earth, in other words in the direction of gravity. Only that force component of the load should be acquired. That is not the case for force sensors, which are similar in design, and are also frequently specified as “load cells”: They are usually designed to acquire loads that occur in all directions. The direction of the earth’s gravitational force is not relevant to how they are installed.
In this article, Stefan Schmidt explains how a load cell works. What he connects with load cells:
“Always new and interesting assignments! It never becomes boring.”
There are different types of load cells for different applications. Commonly used ones include:
- Single point load cells: a load cell is located under a platform that is loaded with a weight from above
- Bending beam load cells: several load cells are positioned under a steel structure and are loaded with a weight from above
- Compressive force load cells: several high-capacity load cells are positioned under a steel structure that is loaded with a weight from above
- Tensile load cells: a weight is suspended from one or more load cells
Many load cells also feature special properties such as a special design or material properties. That may be important depending on the application, for example if systems need to be thoroughly cleaned every day. Some load cells can withstand this type of stress without difficulty, others cannot.
Load cells can also be categorized based on the type of signal transmission: Digital load cells have built-in electronics used to process the measurement results and present them in prepared format. For analog load cells, an additional device – a measuring amplifier – is required.
Four strain gauges are positioned on the load cells below at the point where the greatest deformation occurs when force is applied. The arrow is pointing the direction of force application.