RFID powered system for contactless measurement of a resistive sensor array

This paper presents a contactless measurement system for an array of resistive sensors that exploits a low-frequency (LF) RFID-based approach. The system is composed of an interrogation unit (reader) which powers the sensor unit and exchanges measurement data with it. The sensor unit exploits a transponder operating at 134.2 kHz and a microcontroller. The transponder receives power from the reader and transfers data through the RF-link. The microcontroller sequentially measures the elements of a sensor array composed of six resistors with one common terminal. The adopted technique relies on the evaluation of the charging time of a capacitor through the measured resistor. By exploiting a known reference resistor this technique is theoretically independent from the values of the capacitor and of the supply voltage. A prototype has been developed showing operative distances of up to few centimeters and reads faster than one element of the array per second.