A <i>k</i>-Independent Wireless Readout Technique for Resistive Sensors in Series-<i>RLC</i> Configuration: Design, Implementation, and Challenges

This article presents a wireless readout circuit designed to measure the resistance of a resistive sensor connected in a series RLC configuration. In this setup, the sensor is placed in the secondary circuit, where its resistance varies with respect to the measurand. The proposed readout technique involves measuring the reflected impedance from the primary side, and the sensor resistance is calculated using the real and imaginary components of the reflected impedance. The key advantage of this method is that the resistance measurement is independent of the coupling coefficient (k) between the primary and the secondary, over a considerable range of k, except in the cases of very weak coupling (low values of k). The design was validated using analytical equations, simulation studies, and later confirmed through experimental studies on the readout prototype built in the laboratory. Initially, the system was tested using commercially available standard resistors, followed by tests with a thermistor under varying temperature conditions. The results demonstrate excellent performance, with the output showing high linearity and a worst case error of less than 1.4%. The key challenges in implementing the proposed readout circuit, along with the various sources of errors, have been identified, analyzed, and presented in this article.