A load cell is used to determine mass: The weight force exerted by the sample is converted into an electrical signal by the load cell. Load cells are therefore the most important components of an electronic scale. One of the most widely used technologies is based on strain gauge technology: an analog load cell consists of a measuring element (so-called spring body) made of steel or aluminum, on which a strain gauge (Wheatstone bridge) is mounted. Each electronic scale has an integrated load cell that ensures that a weight can be measured.
DMS load cells are the most widely used, but there are other technologies for weight determination in weighing technology such as EMFC, electromagnetic force compensation, in which mass determination takes place completely without friction loss. Generally, load cells are installed in industrial plants, such as filling plants, weighing silos or checking the filling level of tanks.
What types of load cells are there?
There are many different types of load cells, depending on the application they are designed for. Bending beam or shear beam load cells are often used, for example, in platform scales. Compression load cells, on the other hand, are usually installed under a structure (container, silo, etc.) that is loaded with a weight from above and are often designed for higher loads. With tensile load cells, on the other hand, a weight is attached to the load cell. Load cells are the most important component of a scale, be it a platform, floor or bench scale.
How does a load cell work?
Although the material of the spring body is stable, it is also minimally elastic. This means that when a weight is placed on the scale, this force acts on the spring body. The term spring body derives from the fact that the material deforms minimally as a result of this effect, but returns to its original shape - or "springs" as soon as no force is applied to it. This is an elastic deformation of the measuring element. This change is detected by the strain gauges mounted on the load cell. A strain gage, or "DMS" for short, is an electrical conductor. Because it is firmly attached to the spring body, any force acting directly on the measuring strip extends or shortens it. The electrical resistance increases when the measuring strip is stretched, but decreases when it is compressed. This change in resistance changes the output signal of the load cell and thus makes it possible to determine which force is acting on the load cell - in other words, which weight is currently being weighed.
In the next step, the electrical signal is transmitted to weighing electronics or weighing terminals (which could also be evaluation and display devices). The measured weight can then be read on the display of a display device, indicator or weighing controller.
Important features of load cells
In addition to the various types of load cells, these differ in the manufacturing materials used, mostly aluminium and stainless steel. In addition to the material, the environment in which the cell is used must also be taken into account: Especially the ambient temperatures are an important factor: Every material changes due to heat or cold, which also applies to load cells and strain gages. In order for the exact weight to be displayed, the load cell must be able to compensate for external interference.
Accuracy is particularly important when weighing: load cells are classified into different accuracy classes. These are divided into classes A to D, where A is the best possible class. Strain gage load cells are mostly produced in classes C and D. Depending on the area of application, the requirements for the required accuracy vary: For example, the pharmaceutical industry requires different accuracies and process measurement uncertainties than for instance recyling or retail.