Coupled resonant coil sensors with increased interrogation distance

Wireless sensors are being explored for many structural health monitoring applications. Passive wireless sensors have the advantage of not requiring a power source on the sensor, as the energy is provided by the interrogation system. One type of passive wireless sensor, the inductively coupled coil sensor, works by using a resonant coil coupled to a measurand that shifts the sensor coil's resonant frequency. This paper discusses a new interrogation method that improves the accuracy and interrogation range of coupled resonant coil sensors. The method uses time-domain gating to produce measurements that are dominated by the response of the sensor coil and are immune to background interference. Using this method a test sensor coil's resonant peak position was determined within 30 parts per million at a distance of 20 cm. This is much greater than that achievable with the impedance measurement method. With a relative humidity sensor coil, the method was able to detect changes in relative humidity less than 2%.