Wireless Readout of Resistive Sensors

In this paper we present a method for wireless readout of resistive sensors. Our approach is based on the topology where resistive sensor and reference resistor form a voltage divider network, with reference resistor connected in parallel with a varactor diode. Variation of the sensor resistance causes voltage changes over the varactor, and consequently capacitance of the varactor. Varactor is placed in the LC circuit, thus a change of resonant frequency occurs due to the change of the sensor resistance. Resonant frequency can be detected with external readout inductor connected to the inductance measurement device. The proposed method is theoretically analyzed and experimentally verified with discrete resistors and resistive force sensor. Main advantages of our approach are: (1) it keeps simplicity of the voltage divider network interface with resistive sensors, (2) energy autonomy of sensor tag is extended as energy needed for readout is provided by an external reader, (3) proposed method can be easily adopted for various dynamic range of resistive sensors, and (4) two methods for power supply (battery and energy harvesting with inductive coupling) were analyzed.