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Load cells, force sensors for weighing

They have spring bodies, which are pieces of metal that deform slightly under the influence of weight, with strain gauges. This deformation is absorbed by the strain gauge and transformed into an electrical signal. Strain gauges are extremely accurate, durable and offer a good bandwidth for fast measurements. In load cells, the intended load is always directed towards the centre of the earth, i.e. in the direction of gravity. They are the best option for most large measurement tasks in industry. But also for high precision and as special individual solutions this method is still the best choice. Traceability of mechanical quantities to national or international level with extreme precision makes this method so valuable. Of course, simplicity and low costs also play a decisive role. Load cells are therefore often used from the shop counter to the truck scale. The measuring range extends from a few 100 g up to several 1000 tons. The spring bodies include double bending beam, shear beam, compression beam, ring torsion spring body, multi bending beam spring body and diaphragm spring body.

What are the characteristics of load cells?

Depending on which spring body is used, the load cells will also have corresponding characteristics. High efficiency is essential for filling and sorting systems because there is not much time for weighing. Older load cells work with oil fillings as damping. Newer systems use a combination of mechanical load cells and electronically aligned filters in the measuring instrument, which is connected downstream. Metrologically, a distinction is made between the nominal load and the limit load. The first is the load limit up to which the load cell is to be used. The latter is the load under whose load the transducer is permanently destroyed. The characteristic value is the output signal at the nominal load and is usually 2 mV/V. The indication of a measurement error is always given when the load cell is installed in a balance and indicates the number of parts that a balance can achieve with this load cell, e.g. 1000, 3000, 4000, 6000 or 10000 parts according to OIML. For platform scales, the corner load dependency is important. It depends on the size of the surface mounted on the load cell, and there are no unacceptable deviations when a weight is placed in one of the corners. Load creep means that the output signal changes during half an hour and nominal load. The ratio of output resistance to electrical characteristic value is decisive.

What types of load cells are available?

• Platform load cells: Load cell under a platform; load is applied by weight from above
• Bending beam load cells: several load cells placed under a steel frame; weight loaded from above
• Compression load cells: several high capacity load cells placed under a steel frame; weight loaded from above
• Tension load cells: one or more load cells are loaded with a weight

Some load cells have an unusual design or material properties depending on the location and requirements. Depending on signal transmission, a distinction is made between digital and analogue load cells. Digital ones have built-in electronics. Analogue ones need an additional amplifier for processing the measurement results.

What role does the environment play in the measurement?

Load cells and their strain gauges expand when heated and contract when cold. This causes a change in the electrical resistance of the conductor, which is counterproductive for measurements. Nevertheless, load cells should give the correct measurement result. Therefore each load cell has a temperature compensation built in. Load cells must withstand all weather conditions, such as rain, ice, snow, heat, etc. In addition, other loads act on the load cells, such as so-called "parasitic forces". These are forces that load from the side, from below or in another way. For this purpose, there are special built-in parts and weighing modules that minimize these parasitic forces and still provide precise measurement results.

Load cells and their applications

A basic distinction is made between legal-for-trade and non-calibratable ranges. All scales in the trade, but especially in the food trade, are subject to verification. Load cells in this range must exhibit specified characteristics with regard to measurement errors and repeatability. In the food sector it is essential that the load cells are washable. In the chemical industry they must be explosion-proof.

In the non-custody transfer area, load cells are used in hopper scales or conveyor belt scales.
Load cells are also used for the industrial filling of bottles, bags or cans. Of course this also applies to the filling of bags with sweets or chips. All this works with precise weight measurement. However, load cells are also increasingly used in high-performance sports. Triathlon and swimwear are developed using platform load cells. These measure the water resistance of swimsuits for top athletes.

Accuracy classes

Depending on the product, weight is more or less important. For bulk materials such as sand or gravel, accuracy class D is sufficient. Pharmaceuticals, however, must be weighed with the much higher classes A or B. Most other consumer goods such as meat or fruit are weighed with accuracy class C. In mechanical engineering and building material scales for additives, class C is also used. The more accurate the weighing technology, the greater the productivity of the filling processes. The robust, precise and hygienic load cell is the central component of the weighing and packaging processes.

Conclusion

Load cells that operate with electromagnetic force compensation (EMF) score high in precision compared to those based on strain gauges. However, these are expensive, prone to failure and complicated to commission. For this reason load cells using strain gauges are under continuous development. In this way a high natural resonant frequency could be achieved, which leads to a faster settling time. The so-called running-in behaviour therefore no longer needs to be considered. Low-noise and high-resolution measured values are the satisfactory result. Depending on the design and nominal weight, even 100 to 400 measurements per minute are possible. Connectors guarantee a simple and quick exchange. The output is increased with high precision, reduces overfilling in areas for which EMF load cells are too expensive. The stable design ensures fewer failures. The active temperature compensation is responsible for bridging temperature fluctuations in the environment. Temperature fluctuations influence measuring processes to a high degree. Therefore, temperature compensation is extremely important. Digital load cells enable the use of gravimetric instead of volumetric filling of containers, which in turn leads to more accurate filling results.

The gravimetric principle is particularly impressive for microbiologically sensitive goods. The hygienic conditions are much better, because the sensor does not come into direct contact with the filling product. Of course, the use of the method depends on the factors already mentioned, but nevertheless there is no way around the use of load cells. Is the technology of load cells based on strain gauges not already exhausted? No, not by a long shot! The technology continues to enjoy great popularity because there is simply nothing better on the market at the moment.