Electromagnetic contactless interrogation technique for quartz resonator sensors

A technique for contactless readout of AT-cut quartz resonator sensors is proposed and validated. The technique is based on the separation in time of the excitation and detection phases, exploiting the sensing of the transient response of the resonator. An external primary coil is electromagnetically air-coupled to a secondary coil connected to the electrodes of the resonator. During the excitation phase the fundamental thickness shear mode of the resonator is excited, while in the detection phase the excitation signal is turned off and the transient response of the resonator is contactless sensed by measuring the voltage induced back across the primary coil. A lumped-element equivalent circuit has been studied in order to model the interrogation principle. Experimental results validate the theoretical model and show the successful detection of relative humidity changes by a quartz resonator sensitized with a hygroscopic coating. More generally the proposed technique can be exploited for the measurement of physical or chemical quantities affecting the resonant response of quartz resonator sensors.