Modeling and Testing of Fringe-Field Capacitive Moisture Sensor Under Certain Electrode Area

Soil moisture is closely related to the occurrence frequency and the time of landslides disasters, precisely in situ measurement for soil moisture becomes quite critical. The electrodes of conventional dielectric sensor for soil moisture are fragile, large, and unsuitable for integration. Interdigital (IDT) fringing-field capacitive sensors based on printed circuit board (PCB) technology can be used for noncontact measurement and small-scale packaging. In this article, to optimize the sensitivity and penetration depth of the IDT capacitive sensor with a fixed electrode contour area, the influences of IDT fingers shape and size on the sensitivity and penetration depth are discussed in a wide range by using equivalent capacitance theory. Then, the special working cases are intuitively and accurately analyzed by using the finite element analysis (FEA) method. The results show that the sensor with more electrode fingers and smaller finger gap has high sensitivity, yet has a thinner penetration depth, and it requires a tradeoff between sensitivity and penetration depth in practical applications. In addition, the performance of regular and circular IDT sensors with the same specifications is basically identical, and the circular IDT sensor has a slightly better accuracy. Finally, the proposed IDT sensors with different electrode specifications as well as the associated signal conditioning circuit are fabricated and calibrated to verify the theoretical and simulation results. The performance of the designed sensor was compared with other commercial sensors based on the thermogravimetric method. The experimental results meet the expected requirements well, with a repeatability correlation coefficient of
$R^{2} \,\,=0.992$
and a mean square error of 0.01089.