Capacitive Tactile Sensors
Capacitive Tactile Sensors
This is not a popular technology for tactile sensors at present but may be developed more in future in view of its potential for measuring higher pressures. This is afforded by the inherently less malleable and therefore more mechanically robust construction of capacitive traI)sducers compared with resistive ones, but at the expense of a bigger tactel size and therefore lower x-y tactel resolution.
Figure 1. Side view of tactile sensor based on capacitive tactels array
The principle of operation of each tactel is predictably the same as that of a capacitive force transducer, though based on an x-y array of capacitive tactels, as shown in Figure 1 :
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.
Figure 2. 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 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 3 (note the alternating current supply), thus allowing a measurement of the mechanical stress, as required.
Figure 3. Simplified circuit diagram of a capacitor Wheatstone bridge
