National Physical Laboratory

What is the general principle of a force transducer? (FAQ - Force)

In short ...

A transducer is a device that provides an output quantity having a determined relationship to the parameter being measured - force in this case.

If you apply a force to a block of metal (ie 'squeeze' it) it will change its shape, albeit not very much, and if you are able to measure the shape-change (deformation) - perhaps with a micrometer - you will have made a mechanical force transducer. Provided that you calibrate the block - that is measure how much it deforms when subjected to a sequence of known forces - you can subsequently use it to measure forces by measuring its deformation.

Alternatively, some materials change their electrical resistance when mechanically deformed and can thus be used as force transducers provided there is a means available to measure the resistance change. Unfortunately they can only withstand very small forces before breaking but gluing such a component to the side of the metal block in the above example effectively scales-down the 'squeezing' force applied to it - allowing it to survive much higher forces. So, providing you have the means to measure the changes in resistance as the combination is subjected to various forces (and again it has been calibrated), you will have converted your mechanical force tranducer into an electrical force tranducer by joining two very different force transducers together.

There are many other methods available and they are described under types of force transducer and there is also a page summarising their operating characteristics.

More technically ...

Many force transducers employ an elastic load-bearing element or combination of elements. Application of force to the elastic element causes it to deflect and this deflection is then sensed by a secondary transducer which converts it into a measurable output. The output may be in the form of electrical signal as in strain gauge and linear variable differential transducer (LVDT) type load cells or mechanical indications as in proving rings and spring balances. Such transducers are known generically as elastic devices, and form the bulk of all commonly used force transducers. There are a number of different elastic transducer elements but generally they consist of circular rings, cylinders or beams.

Transducer Element

The fundamental nature of elastic devices is illustrated opposite where, in this case, the elastic element is a circular cylinder made of a metal such as steel. The force is applied to the end of the cylinder and the deformation is measured as the difference between the uncompressed and compressed length. The cylinder also expands outwards as it is compressed, the ratio of outward expansion to longitudinal compression being determined by the nature of the material (through Poisson's ratio). The force may also be applied such that the device is in tension, rather than compression, and in that case the distortion is simply reversed.

All elastic devices share this common basis, but the method of measuring the distortion of the elastic element varies considerably. The most frequent method is to make measurements of the longitudinal and lateral strain, and when this is undertaken by electrical resistance strain gauges such a transducer is known as a strain gauge load cell. These are the most common commercially available type of force transducer and are described, with many others, under types of force transducer with a separate page summarising their operating characteristics.

Last Updated: 25 Mar 2010
Created: 8 Oct 2007

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