Abstract
Soil moisture sensing plays an important role in agriculture, especially in greenhouses or vertical farms. However, existing soil moisture sensing systems are either expensive and require batteries or suffer from low accuracy, preventing their real-world applications. This paper introduces SoilTAG, a battery-free, chipless tag-based high accuracy soil moisture sensing system. The key insight is that the tag's resonator can convert changes in soil moisture levels into changes in the tag's frequency response. However, two challenges need to be addressed before applying the system to the real world. First, how to design a resonator whose frequency response is sensitive to even small moisture changes, which is the basis for high-precision moisture sensing. To solve the challenge, we design a special structure (i.e., a defected ground structure) as the tag's resonator and optimize its key parameters to increase the frequency response sensitivity for different soil moisture levels. Second, how to design a robust soil moisture feature that is independent of the tag's location changes, since the frequency response varies by both the tag location and soil moisture. To deal with this challenge, we introduce a relative frequency response feature whose amplitude ratio is only related to soil moisture levels and independent of the tag location changes. Extensive experiments show that SoilTAG achieves <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$90th$</tex-math></inline-formula> percentile moisture sensing error of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$2\%$</tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$3.64\%$</tex-math></inline-formula> , and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$8\%$</tex-math></inline-formula> when the distance between transmitter and tag is 6 m, 10 m, and 13.9 m. Compared to current commodity sensors, SoilTAG saves the cost per sensor by more than 70%.
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