Robust Analog Multi-Sensory Array System for Lossy Capacitive Sensors Over Long Distances

A robust capacitive multi-sensory readout system is presented in this paper. The system consists of a few thousand lossy capacitive-based sensors which can be modeled as a parallel connection of sensor capacitor and undesirable leakage resistance. The real system in our application will consist of a high number of capacitive sensors used for the monitoring of relative humidity in a High Energy detector at CERN. However, our findings apply to any set of capacitive devices aimed to any other sensing application. Furthermore, the proposed system is intended for harsh environments (e.g. with high radiation levels and magnetic field presence) where the readout system is separated from the sensor and placed far away, connecting to them via cables. In such cases, the presence of cables brings another issue to the readout system – parasitic capacitance. Using long cables, we introduce high values of parasitic capacitance that make the sensor capacitance practically invisible to the readout unit. This paper presents an effective, robust, analog method, based on a quadrature-phase shifter and an auto-balancing bridge, used to overcome the sensor leakage resistance and parasitic capacitance caused by the cables. The experiments for various values of capacitance and leakage resistance at different frequencies and the influence of different cable lengths and types are analyzed and discussed.