Positionally-independent and extended read range resonant sensors applied to deep soil moisture monitoring

Abstract

Here we detail an inductively coupled extender (ICE) and resonant (LC) sensor to monitor soil moisture using a portable reader. Significant advances of this ICE-LC design are extending typical LC sensor read range over a meter and reducing positional alignment sensitivity between reader and sensor. An analytical model validates the working principle and feasibility of the ICE-LC system. Prototypes of the ICE-LC sensor were built and optimized in terms of sensitivity and power transfer (single and four turns for ICE top and bottom coils, respectively). Soil moisture tests validated the ICE-LC improvements on minimized positional alignment sensitivity and extended read range, transducing a decrease in resonant frequency with increasing soil moisture. When calibrating with existing wired approaches, the ICE-LC sensor had a reproducible, linear sensor gain of 4.52%moisture content/MHz with an R2 of 0.745 and RMSE of 2.41%. A smaller, planar form factor of the ICE-LC sensor was also tested and exhibited reduced positional alignment sensitivity between reader and sensor at shorter read ranges. This initial study demonstrates the feasibility of the ICE-LC resonant sensor as a cost-effective method to monitor soil moisture content throughout the growing season at many field locations.