Development of a Novel Sensor For Soil Moisture's Profile

Abstract

Monitoring of soil’s water content has been an important part of the irrigation system in farming, regional resources development, rainfall detection as well as disaster prevention. There has been studies and developments in this area over the decades. However, most of the works are focused on sensing the soil as a homogeneous medium, which does not accurately reflect the heterogeneous structure of soil. Works on multi-layer soil sensing have been researched and developed. Though, the current design is complex, not easy to produce and assemble. This work presents a novel low-cost and simple soil sensor design to detect the soil’s water content at different layers. The design is capacitive approach. In this method, soil acts as the medium whose dielectric values vary with its water content. These changes result in shifts in resonant frequencies of the sensor’s structure, which can be used to determine the water content of the soil. This sensing approach is simple, inexpensive, easy to set up and portable. For soil profile sensing purposes, the soil depth’s information is encoded in the sensor’s physical structure. Instead of using a rectangular plate, the sensor’s shape is varied exponentially. As the capacitive effect depends on the area of the conductor, this design allows for more changes in soil profile to be captured. A theoretical model for this sensor design was developed with Transmission Line and ABCD Network in MATLAB. A similar model was set up in High-Frequency Simulation Software (HFSS) for comparison. The deign was tested in medium which consists of two different soil profiles. The boundary between the two profiles varied along the length of the sensor. The results in theoretical and simulating model showed corresponding changes in resonant frequencies as the soil boundary changes in three different cases. A lab measurement was arranged to compare the performance of this proposed design (exponential shape) with the standard one (rectangular shape). This setup was also modeled in simulating software for comparison. The results showed that proposed sensor design had higher sensitivity when the boundary layer varied toward end of the sensor. Finally, the effect of the proposed sensor design’s parameter was studied in HFSS and the design for the sensor was finalized to achieve the highest sensitivity.