Non-resistive Transducers
Non-resistive Transducers
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.
Figure 1 Simplified diagram of a compression-type variable capacitor
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:
Differentiating with respect to d and taking small increments we can re-write eqn (1) as:
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.
Figure 2 Simplified circuit diagram of a capacitor Wheatstone bridge
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.
