Principle:
The principle of variable capacitance is used in displacement measuring transducers in various ways. Capacitance is a function of effective area of conductor, the separation between the conductors and the dielectric strength of the material. It is described in the equation below:
d - Distance between the two parallel electrodes. ε - Dielectric constant, permittivity, of the dielectric medium A - Area of the electrode.
Working:
There are three way to change the capacitance, changing in the area, changing the distance between the electrode plates and changing the material between the electrodes thus permittivity changes.
One of the two electrodes is made fixed and the other is made movable for measure displacement. Displacement to be measured is applied to the movable metal plate, as the plate moves the distance between the plates increases and this changes the capacitance measurement. Thus the change in the capacitance will be the function of the displacement of the electrode.
The capacitor plates are formed by two concentric, hollow, metal cylinders. The displacement to be measured is applied to the inner cylinder, which alters the capacitance.
Example: capacitance transducer for Level measurement:
Here both the electrodes are fixed so the distance is constant. One electrode is dipped into the liquid and another one is fixed to the wall of the tank. As the liquid level increases the permittivity changes and thus the capacitance changes.
Advantages:
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It produces an accurate frequency response to both static and dynamic measurements
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Negligible loading effects
Disadvantage:
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Accuracy can be affected by change in temperature
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As the lead is lengthy it can cause errors or distortion in signals