Several mechanical input sensors, as well as some sensors which measure humidity or temperature, operate by the transduced quantity causing a change in capacitance, which in turn, is converted to an analog output voltage by an ac bridge circuit or other electronic system. The capacitive sensors have been designed to measure force by means of displacement of one or two capacitive electrodes.
Capacitive sensors can also be employed in the measurement of acceleration, thickness, depth of a dielectric liquid and pressure.
Principle of Operation of Capacitive Sensors
In its simplest form, the mechanical input quantity causes a change in the separation between two or more capacitor plates.
The capacitance between two parallel, conducting plates, each of area A, separated by a dielectric of thickness d is given by:
Where k is the dielectric constant of the material separating the plates and ε0 is the permittivity of free space (8.85 x 10-12 F/m). Small variations in C, ΔC, which may be measured by several means, may occur if the plate separation is caused by change by some physical quantity or if the dielectric constant, k, changes due to pressure, temperature, mechanical strain, humidity, etc. The overall dielectric constant may also be modulated by sliding a substance with a dielectric constant greater than one between the capacitor plates so that some fraction, α, of A has a dielectric constant of k = k1 and the remaining area between the plates (1 – α)A, has a dielectric constant k = 1. The area of the capacitor plates may be effectively changed by sliding one plate over the other at constant d. A typical example of this means of influencing a ΔC is found in parallel plate, rotary turning capacitors employed in radios.
The means of measurement of capacitance changes include various capacitance bridges or the use of the capacitor in a tuned L-C circuit determining the frequency of an oscillator – the oscillator’s frequency is measured to obtain a signal proportional to:
Pressure measurements can also be made using a capacitor with fixed geometry but with a dielectric constant which changes with pressure. Certain nonpolar liquids, gases and piezoelectric materials can be used for this purpose. Dielectric constants also change with temperature, so a null transducer can be used in a bridge circuit to obtain temperature compensation.
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