Multi-Coil-Based Inductive Power Transfer System For Insitu Soil Sensing Application

A 3-coil-based inductive power transfer system was designed and demonstrated for energizing a 20 mW underground battery-free soil moisture sensor with an improved efficiency and voltage gain compared to a 2-coil-based implementation. The 3-coil-based design employs an intermediate tuned coil, which is critically coupled to the last-stage coil interfacing the sensor, thus effectively mitigating the loading effect from the sensor to ensure an enhanced performance provided certain design conditions are met. The prototype system achieves an inductive power transfer efficiency and a voltage gain of 4.2% and 4.6, respectively, over a 30 cm distance in air. The system further demonstrates an efficiency of 3.2% and a voltage gain of 4 across a 30 cm depth of soil. The prototype system is also capable of wirelessly transferring 40 mW of power over the same distance with an efficiency of 4.8%, limited by the input stage power driving capability. The multi-coil-based system design approach is desirable for enabling advanced, wireless, and battery-free underground soil monitoring systems, which may demand high power dissipation.