A Capacitive Sensing System for Non-Contact: Detection of Ice

Abstract

Abstract This paper presents a capacitive sensing system for non-contact detection of ice from a distance of 8 to 10 cm and distinguishes it from water. It is well known that the Relative Permittivity (RP) of ice and water are almost equal at low frequencies, in the range of a few kHz. At a high frequency of excitation , i. e., about 100 kHz and above, the RP of water remains same, but that of ice substantially reduces. A capacitive sensor, wherein, the ice or water layer forms part of the sensing volume can be used to detect presence of ice. It can be distinguished from water by measuring and then comparing the capacitance values at low as well as high frequencies. Such a sensor for road ice detection has been reported, but it requires the sensing electrodes to be in close contact with the ice layer. Recently, a capacitive non-contact ice detector has been reported. The sensing range of this system was limited to 3 cm, even after employing active shielding to reduce the effect of offset capacitance. As the distance between ice layer and sensor increases, the ratio of change in sensor capacitance to offset capacitance becomes very small. This paper reports application of a capacitance measurement circuit that can provide an output as a function of presence of ice. The proposed scheme does not require active shield even when the sensor has large offset capacitance. A prototype of the new scheme has been developed and tested in the laboratory. It detected presence of ice layer of thickness 2 mm from a distance of 10 cm and distinguished it from water.