Novel telemetric technique for passive resistive sensors based on impedance phase angle measurement at constant frequency

摘要

Telemetric systems are a valid alternative to measurement devices exploiting batteries or cabled connections, which may be not suitable in some cases, such as in harsh or hermetic measurement environments. They permit either to supply a passive sensor or to carry out monitoring tasks without contact. In this paper, we propose a novel measurement technique for telemetric systems working with a resistive sensing element. Such technique is based on a reading of system impedance phase angle always at sensing inductor's resonant frequency, while relative distance between system inductors is kept fixed. Analytical expressions have been derived to calculate sensor resistive output from phase angle value. They have been obtained from a proper circuit model of the telemetric device. The proposed method has been applied to the wireless measurement of temperature inside an oven. Experimental tests have been carried out, by putting a resistive sensor inside the oven and increasing its internal temperature from about 28 °C to about 134 °C. Preliminary results show that calculated temperature values present an average deviation from reference equal to 1.7 °C. Furthermore, uncertainty related to such values is 1.2 °C. Therefore, they prove the feasibility of implementing the proposed technique in all the applications that require the use of telemetric devices with passive resistive sensors.