Low-frequency RFID signal and power transfer circuitry for capacitive and resistive mixed sensor array

This paper presents a contactless measurement system for a mixed array of resistive and capacitive sensors exploiting a low-frequency radio-frequency identification (RFID)-based approach. The system is composed of a reader unit which provides power to and exchanges measurement data with a battery-less sensor unit. The sensor unit is based on a transponder operating at 134.2 kHz and a microcontroller. The microcontroller sequentially measures the elements of the sensor array composed of n capacitive and m resistive sensors which share a common terminal. The adopted technique measures the charging time of a resistor-capacitor (RC) circuit, where the resistor or the capacitor can be either the sensing element or a reference component. With the proposed approach, the measured values of the resistive or capacitive elements of the sensor array are first-order independent from the supply voltage level. A prototype has been developed and experimentally tested with resistive elements in the range 400 kΩ-1.2 MΩ and capacitive elements in the range 200 pF-1.2 nF showing measurement resolution values of 1 kΩ and 5 pF, respectively. Operative distances up to 3 cm have been achieved, with readings taken faster than one element of the array per second.