Wireless Monitoring of Displacement Using Spiral Resonators

A distance sensor based on a spiral resonator (SR) label printed on a dielectric substrate and remotely interrogated through a small loop antenna acting as reader is presented. The proposed system is working at a single frequency (856.5 MHz) and could be compliant with conventional RFID readers. The measurement of distance is carried out by observing the magnitude variation of the response at resonance, namely the real part of the input impedance of the probing loop. Indeed, the latter response changes as a function of the distance between the tag and the loop due to the variation of mutual coupling between the pair. The proposed sensor is capable of providing estimates of the distance with high resolution within the specified range of operation. The results also show that the sensitivity of the sensor, which is quantified by the change of input impedance over distance, decreases at large displacements, eventually reaching a saturation point which defines the maximum detectable distance. The sensing principle is verified using simulations and measurements. It is also demonstrated that the maximum readable distance is comparable to the size of the SR. Finally, the effect of lateral displacement was investigated experimentally to demonstrate how the sensor performs in conditions where perfect axial alignment cannot be guaranteed. Thanks to its scalability potential, the proposed sensor is suitable for various practical scenarios.