Elimination of shunting conductance effects in a low-cost capacitive-sensor interface

A low-cost and reliable interface for capacitive sensors, based on the use of a relaxation oscillator and a microcontroller, is presented. The novel interface system is designed to measure small capacitances in a reliable and accurate way even when the sensor capacitors are shunted by parasitic conductances. The problem of shunting conductances is solved by performing a series of eight measurements and using a new auto-calibration technique. Moreover, all multiplicative and additive errors of the interface are also eliminated by using this new auto-calibration technique. In the microcontroller, the final result is calculated based on the new measurement algorithm, A prototype has been built and tested. Experimental results show that the interface is able to measure a capacitance of 0-1.2 pF with a shunting conductance of up to 0.42 CLS, with a resolution and a relative accuracy of 0.03% and +/-0.15%, respectively. The measurement time is about 400 ms.