Stacked coupled-coil approach for multi-parameter passive wireless sensing

This paper presents a stacked multiple coil design for simultaneous multi-parameter narrow-band sensing with RLC-type near-field coupled resonators. The resonant frequency and Q-factor of each RLC sensor is dependent on change of stored energy and loss in response to the parameter of interest. One challenge with these types of sensors is mutual coupling between coils and thus interdependence of parameters. In order to null mutual coupling between coils and reduce sensing area the coils are stacked in a designed overlapping configuration. The configuration enables the response of each sensor and thus sensed parameters to be independent. The swept frequency impedance response of an interrogator coil, inductively coupled to the sensors, remotely tracks the parameters of each sensor. EM simulation and experiments with a two-parameter (relative humidity and temperature) sensor shows the stacked overlapping structure accomplishes narrow-band multi-parameter sensing without substantial increase in size. The approach has application in flexible thin-film substrate printed sensors.