Fringing Field Capacitive Sensor for Measuring Soil Water Content: Design, Manufacture, and Testing

Capacitors are used as sensors in several applications. Typically, the quantity to be measured changes either the geometric properties of the structure used as a capacitive sensor or the physical properties of the dielectric between the electrodes of that structure. On the other hand, if the sensor geometry maximizes the capacitance due to fringing, then the possibilities are totally different. In this case, the electric field can be projected onto the test sample. This paper covers the usage of fringing field sensors to measure the water present in the soil. We examine how the geometry of the PCB-made interdigited electrode structure influences the characteristics of the sensor. However, fringing field sensors are difficult to be analytically modeled due to its intrinsic nonlinear characteristic. Therefore, our conclusions were mainly based on results of simulations, but we also tried to reproduce the results of the simulations experimentally. Also, we discuss a circuit that converts the changes in the sensor´s capacitance into variations in the width of a pulse signal. Finally, the proposed solution has been experimentally compared with the standard gravimetric method to quantify the water content in the soil. The results were encouraging since the proposed sensor´s measurements fit to the experimental data with a coefficient of determination R²= 0.94.