Elimination of shunting Conductance Effects in a Low-Cost Capacitive-Sensor

Abstract—A low-cost and reliable interface for capacitive sen-sors, based on the use of a relaxation oscillator and a microcon-troller, is presented. The novel interface system is designed to mea-sure 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 mea-surement algorithm. A prototype has been built and tested. Exper-imental results show that the interface is able to measure a capac-itance of 0–1.2 pF with a shunting conductance of up to 0.42 S, with a resolution and a relative accuracy of 0.03 % and 0.15%, respectively. The measurement time is about 400 ms. Index Terms—Auto-calibration, capacitive sensor, conductance effect, sensor interface. I