Mutual inductance suppressed stacked inductors for passive wireless multi-parameter sensors

In an LC-type passive wireless sensor system for multi-parameter monitoring, it is desirable for the inductors to be coaxially aligned stacked in order to save area. However, the transmitting signals affect each other due to mutual coupling between the stacked inductors. This paper presents a novel structure of inductor which is in a specific winding to achieve the minimized mutual inductance. Simulation results of three stacked inductors each connected with a variable capacitor indicate that the mutual inductance between the inductors can actually be greatly suppressed. This phenomenon has also been verified through multi-layer PCB inductors. Three capacitive sensors for temperature, pressure and relative humidity were linked to the three stacked inductors to implement the simultaneous measurements of humidity, temperature and pressure. The measurement results indicate that the sensitivity of the humidity sensor is about 53.33 kHz/%RH between 15%RH to 90%RH, the sensitivity of the temperature sensor is about 41.67 kHz/°C between -20°C to 100°C while the sensitivity of the pressure sensor is about -133.33 kHz/kPa between 50kPa to 110 kPa.