Capacitance Pickup is a transducer that converts angular displacement into electrical signal. It consists of ganged capacitance and is based on the principle of variation of effective area of conductors, when other parameters such as separation, distance and dielectric strength being kept constant.
The basis of angular displacement measurement with the help of capacitance pickup is frequency modulation system. Two sets of ganged identical condensers form a part of weinbridge oscillator. The frequency of oscillation f = 1/2πRC. If “C” is varied typically from 550pf to 50pf we get a frequency variation in the range of 110.
The following figure represents the block diagram of capacitance pickup:
Procedure for finding characteristics of Capacitance Pickup:-
- Connect the capacitance pickup cable to the input socket of the main unit.
- Keep the input angular displacement to zero position.
- Check the DPM readings for zero indication or obtain the same by adjusting pot P2.
- Now, turn the shaft to fully anti-clockwise position and to obtain the DPM indication of 180° by operating knob P2 if necessary.
- Note down the DPM indication for different input angular displacements of capacitance pickup.
- Draw the graph of output versus input angular displacements. Its expected waveform is as given below:
The main precaution to be taken while conducting this study experiment is not to stretch the wire coming from capacitance sensor.