Multi-Frame Fringing Field Capacitive Soil Moisture Sensor With Enhanced Sensitivity and Penetration Depth

Recently, 2-frame interdigital fringe-field (ID-FF)-based capacitive sensors have been widely employed for soil moisture measurement. The present challenge with this design is to achieve both penetration depth and sensitivity simultaneously. In this work, keeping the electrode contour area fixed, multi-frame ID-FF capacitive soil moisture sensors are proposed and designed to enhance sensitivity and penetration depth, simultaneously. A theoretical model for capacitance is extended to a multi-frame configuration. The estimated capacitance from this extended theoretical model is compared against simulation results and found to be well-correlated. Multi-frame ID-FF configurations with 2, 3, 4, 6, 8, and 9 frames are used to develop single-sided (s-s) and dual-sided (d-s) ID-FF capacitive soil moisture sensor models. Simulation results show an increase in the sensitivity and penetration depth with reference to the generic 2-frame ID-FF capacitive sensor configuration. The models with distinct multi-frame configurations and an associated signal-conditioning circuit are also fabricated. Experiments for sensitivity indicate an average error of less than 6% with reference to simulation, which is lower than that reported in the recent literature. One of the fabricated sensors is calibrated using the thermogravimetric method. The performance of the fabricated sensors is then compared with a standard commercial sensor with respect to the thermogravimetric method and is found to meet the expected requirements with a correlation coefficient of 0.997.