A low cost and reliable interface for capacitive sensors

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 a new measurement algorithm. A prototype has been built and tested. Experimental results show that the interface is able to measure a capacitance of 0 to 2 pF with a shunting conductance of up to 0.16 μS, with a resolution and a relative accuracy of 0.03% and ±72%, respectively. The measurement time is about 400 ms