Design and Construction of a Sensitive Capasitive Sensor System (RESEARCH NOTE)

Design and construction of a capacitor sensor with a precise readout system are described in this paper. A variable air-gap capacitive transducer was constructed and for signalmeasurement a successful attempt was made to exploit the capacitance changes in terms of the phase angle of the detected signal. The reported setup makes it possible to detect capacitance changes of the order of 2.5 fF in a 4.5 pF range and the readout circuit can detect a capacitance change of about 22 aF (5 in 106). There is a linear relation between the air-gap distance and the output of the sensor over an experimented gap range of 0.5-4.5 mm. It shows a nonlinearity deviation factor of 1% from the expected theoretical line. The dynamic range of the system is variable from 5 μm to 2.5mm and it mainly depends on the initial air-gap distance. Adding a stray capacitance of 180 pF to the system only causes a 0.1% error in the sensor output. In comparison with the other sensor systems the one reported in this paper is much more sensitive, reliable, and stray-immune to electrical interferences.