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

Wireless operation of LC resonant sensors 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 and detect the sensor's response. Wireless LC resonant sensors are used in many applications where cable connections for powering the sensor and acquiring its response are inconvenient. This review focuses on the fundamental operating principles of wireless LC resonant sensors, their measurement techniques and challenges, as well as potential solutions. The main challenge in wireless measurement of the sensors is to accurately measure the resonance 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 the wireless resonant sensor, 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 measurement of the sensors in frequency domain phase dip technique. 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 the frequency and time domains, resulting in an inaccurate measurement of the sensors' resonance 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 sensors.