Influence of the electrodes configuration on a differential capacitive rain sensor performances

The dimensioning and experimental tests of a differential capacitive rain sensor without sensitive layer are presented. The detection principle relies on the change in electric permittivity induced by water on the active area of the sensor. This sensor is made of three protected coplanar electrodes defining two capacitors: a sensitive capacitor which capacitance (C1) varies with water, and an insensitive-reference-capacitor (C2). In dry condition, both capacitances have the same value. Imbalance between C1 and C2 reveals the presence of water on the sensor. Two types of electrodes are studied: rectangular and interdigital. The sensors geometrical configurations are first refined by numerical simulation using a finite element method (FEM). The sensors behavior in different rain conditions is then simulated using the same FEM software. Finally, sensors prototypes are manufactured and tested. The rectangular electrodes sensor exhibits a significant sensitivity to water due to both a maximum variation of the sensitive capacitance and a negligible variation of the reference capacitance, while the interdigital electrodes sensor appears to be less sensitive.