Strain gage transducers

Extensometers with resistive wires, also known as strain gauges (this expression is not quite correct, but is common), were invented to measure the deformations of structures in order to assess their state of stress. These are small, very thin electrical circuits (figure 1 ) which, when glued to the parts to be studied, undergo deformation, resulting in a variation in their electrical resistance. Electrical measurements can be very precise and sensitive, since deformations of the order of micrometers per meter (µm /m) are easily achieved. These gauges are described in the Extensometry article. .

From the outset, the fidelity of strain gages made it possible to envisage their use in the manufacture of sensors. With the same gauges and the same downstream instrumentation, it is possible to imagine many different types of sensor 1 .

Simple extensometers can be installed even by non-specialists. In fact, it is common practice in workshops and laboratories for users to build their own transducers first, and then have them professionally manufactured once the method has proved satisfactory and can be extended to higher accuracies.

The non-specialist who has built a sensor, simply by buying gauges, adhesives and a few accessories, will be able to define his problem himself, at reduced cost. Then, to move on to definitive use or large quantities, it will of course be in his interest to contact a specialized manufacturer. The choice is a question of relative economy (table 5 ).

The many fixtures cited in the rest of this article are illustrated by examples involving metal screen gauges. Gauges can also be of the silicon semiconductor type. The latter give a higher signal and offer the possibility of a very small footprint (integrated sensors). However, semiconductor gauges have the disadvantage of being sensitive to temperature; this sensitivity can be compensated for by a microprocessor (storage of resistance-temperature curves). The advantage of metal gauges is that, since their characteristics depend on numerous parameters linked to the various types...