Contactless Readout of Passive LC Sensors with Compensation Circuit for Distance-Independent Measurements

Marco Baù,Marco Demori,Vittorio Ferrari,Marco Ferrari

doi:10.3390/proceedings2130842

Abstract

Contactless readout of passive LC sensors composed of a capacitance sensor connected to a coil can be performed through an electromagnetically coupled readout coil set at distance d. Resonant frequency fs and Q-factor QS of the LC sensor can be extracted from the measurement of the impedance at the readout coil by using a technique theoretically independent of d. This work investigates the effects on the measurement accuracy due to the unavoidable parasitic capacitance CP in parallel to the readout coil, which makes the measured values of fs and QS dependent on d. Numerical analysis and experimental tests confirm such dependence. To overcome this limitation, a novel electronic circuit topology for the compensation of CP is proposed. The experimental results on assembled prototypes show that for a LC sensor with fs ≈ 5.48 MHz a variation of less than 200 ppm across an interrogation distance between 2 and 18 mm is achieved with the proposed compensation circuit.

Highlights

Passive LC sensors can be obtained by connecting a capacitive sensor to a coil, forming a resonant LC circuit where the measurement information is related to the resonant frequency and the quality factor of the LC circuit
It is composed of the interrogation unit made up by an impedance analyzer and the readout coil represented by inductance L1 and series resistance R1
To avoid the dependence of fmP on CP and on the distance, the proposed compensation circuit of Figure 1b is connected in parallel to the readout coil

Summary

Introduction

Passive LC sensors can be obtained by connecting a capacitive sensor to a coil, forming a resonant LC circuit where the measurement information is related to the resonant frequency and the quality factor of the LC circuit. The lack of wired connection to the sensor is attractive in applications where cabled solutions are difficult or unpractical, such as measurement in sealed environments or inside animal/human bodies [1,2]. These applications can take advantage on the absence of on-board power supply, allowing in principle for unattended unlimited operation. Approaches theoretically independent of the distance have been proposed both in the time-domain [3,4] and in the frequency-domain [2] In the latter case, measuring the real part of the impedance at the readout coil allows to obtain the resonant frequency and the quality factor of the LC sensor [5,6]. A circuit is proposed to compensate for this unwanted effect

Operating Principle

Circuit Prototype and Experimental Results

Conclusions