Proximity Distance-Independent Contactless Interrogation of a Passive Sensor Unit Based on a Resistive Sensing Element

A contactless interrogation technique for a passive sensor unit (SU) built around a resistive sensing element is presented and validated. The proposed approach exploits the resonant frequency stability and the high quality factor of a quartz crystal resonator (QCR), used as an auxiliary resonant element, to measure a series-connected resistor, which is the target sensing element. The technique is based on the measurement of the quality factor of the passive SU composed of the QCR, the resistive sensing element, and the sensor coil (SC) necessary for the proximity inductive coupling with a contactless interrogation unit. Advantageously, as a key feature and aspect of novelty of the method, independence from the interrogation distance across the working range is achieved. The operating principle is described analytically, demonstrated through circuit simulations, and validated experimentally by means of a tailored experimental setup, with the resistive sensing element ranging from 100 to 200 Ω. The experimental results have shown a linear relationship between the reciprocal of the quality factor and the sensor resistance, in good agreement with theoretical predictions and simulation results. Within the tested range, an equivalent resolution of 9.1 m Ω has been obtained over repeated measurements across an interrogation distance of few centimeters.