Measurement Techniques and Challenges of Wireless LC Resonant Sensors: A Review

Wireless operation of the LC resonant sensor is based on magnetic coupling between two inductive coils, where the inductor of the sensor acts as a secondary coil for the magnetic coupling. An external reader coil is used as a primary coil to interrogate the sensor to detect the sensor’s response. The wireless LC resonant sensor is applied in many applications where the cable connections for powering sensors and acquiring data are inconvenient. This review focuses on the fundamental operating principles of wireless LC resonant sensors, their measurement techniques, the challenges of accurately measuring the LC sensors, and their potential solutions. The main challenge in wireless measurement of the LC resonant sensor is to accurately measure the resonant frequency and the quality factor of the sensor which are solely dependent on the intrinsic parameters of the sensors. For practical wireless applications, it is crucial to interrogate LC resonant sensors regardless of their wireless measurement distances. To interrogate wireless LC resonant sensors, frequency and time domain measurements are commonly used. The coupling coefficient, which is greatly influenced by the geometrical dimensions and alignment of the two inductively coupled coils, has an adverse effect on distance independent readout in phase dip measurement in the frequency domain. Furthermore, the presence of parasitic capacitance that appears in parallel to the readout coil of the sensor has also an adverse effect on distance independent measurement in both frequency and time domain, resulting in an inaccurate measurement of the sensor’s resonant frequency. A parasitic capacitance compensation technique can be employed to reduce or even eliminate the presence of parasitic capacitance in the readout coil, which significantly improves the measurement accuracy of the LC resonant sensor.