The most widely used non-resistive force transducer is the capacitive type which proeuces a change in capacitance proportional to the pressure exerted on the plates. The principle of operation is similar to that of a compression capacitor used to provide small values of variable capacitance in electrical circuits. This principle is illustrated in Figure 1.
The variation in capacitance ΔC depends on the change in distance, d between the capacitor plates and therefore on the compression characteristics of the dielectric material which, in turn, depends on Young’s modulus E, as shown by:
But this relationship is linear for only small strain values. The capacitance variation ΔC can be measured using a non-resistive Wheatstone bridge of a kind similar to those used in most laboratories to measure inductances, as shown in Figure 2 (note the alternating current supply), thus allowing a measurement of the mechanical stress, as required.
Other kinds of non-resistive force transducer use fibre-optic cables which detect the presence of the external physical disturbance by a change in the light transmission characteristics. These and other special-purpose force transducers encountered in industry suffer from limited sensitivity when compared with strain gauges and, at present, are seldom used in robotic applications.