Abstract
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.
Highlights
The presence of sensors in everyday life and devices is becoming ubiquitous, among other reasons due to the emerging IoT scenario, where objects can become nodes of an intercommunicating network [1]
The topology of the mixed sensor array is preconfigured in the reader unit (RU) which manages both the
The humidity sensor inside the box detects an increase of relative humidity (RH) up to about 80% RH, while the response of the external sensor remains almost constant, as it will be throughout all the duration of the test
Summary
The presence of sensors in everyday life and devices is becoming ubiquitous, among other reasons due to the emerging IoT (internet of things) scenario, where objects can become nodes of an intercommunicating network [1]. The measurement principle can exploit both frequency-domain and time-domain techniques [21,22] This approach relies on completely passive sensors; i.e., they do not require any active electronic circuits on board to operate. Ultra-high frequency (UHF) ranges [23] In these cases, the reader energizes the sensor unit with an on-board transponder and they exchange data, exploiting the wireless link. Different microcontroller-based configurations have been proposed to interface resistive or capacitive sensors with low-power approaches [29,30,31,32,33,34,35]. By exploiting an RF link, the interrogation unit transfers energy and exchanges data with the sensor unit composed of a RFID transponder, a microcontroller and the sensor array. The prototyped system has been characterized and successfully applied to the measurement of both humidity and temperature of the air inside a package under different operating conditions
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