DMS (DehnungsMessStreifen – the German for strain gauge) load cells are the most common form of electro-mechanical scales. They use electrical signals to determine mass.
A basic distinction is made between electronic scales which determine mass without mechanical intermediate stages or parameters (electronic force compensation) and electro-mechanical scales which do use mechanical intermediate stages and parameters (e.g. strain gauges).
DMS load cells belong to the group of electro-mechanical scales based on the strain gauge converter principle.
The DMS measuring principle follows the same principle as spring scales. These adjust themselves to a balanced state and show the mass on an appropriate display. The function of the spring is taken by a malleable and flexible body (a special measuring spring), a hollow cylinder, a rod or a ring. The measuring body displays particular spring rigidity. This is designed to be particularly generous (c>5∙104N/mm) in order to minimise the deflection of the weighed material or object.
The weight itself causes the measuring body to deflect and/or its length to change within limits set by Hooke’s Law. Sensors on the measuring body very accurately measure the body’s change in length.
These sensors are strain gauges.
The physical principle is based on the fact that an electric conductor changes its electrical resistance during mechanical deformation.
Put simply, the electric conductor is a metal foil applied to a special carrier material. Meandering or ring-shaped conductor paths are etched into this to produce the maximum line length possible in the smallest area. This results in the greatest possible change in resistance with the smallest change in length. These strain gauges are shaped like a grid iron or a very fine grille for instance.
Using this basic principle, we encounter DMS load cells of different levels of sensitivity, which are ideal for use in areas at risk of explosion with very high temperatures and extreme ambient requirements or for several cells in a parallel circuit.
DMS load cells are very robust. They are easy to construct, hermetically encapsulated and can be operated with intrinsic safety in areas at risk of explosion. Their nominal loads range from around 1 kilogram to several thousand tons.
In industrial scenarios, several DMS load cells are often used together. For example in proven process scales, there are normally three load cells connected in an electric parallel circuit.
Signal processing for simple measurements is analogue. Nowadays further processing is usually digital to ensure maximum accuracy. Also refer to A/D converter.
For more system descriptions, technical properties, specifications and examples of applications, search for load cell.
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