Analysis and Validation of Contactless Time-Gated Interrogation Technique for Quartz Resonator Sensors

Abstract

A technique for contactless electromagnetic interrogation of AT-cut quartz piezoelectric resonator sensors is proposed based on a primary coil electromagnetically air-coupled to a secondary coil connected to the electrodes of the resonator. The interrogation technique periodically switches between interleaved excitation and detection phases. During the excitation phase, the resonator is set into vibration by a driving voltage applied to the primary coil, whereas in the detection phase, the excitation signal is turned off and the transient decaying response of the resonator is sensed without contact by measuring the voltage induced back across the primary coil. This approach ensures that the readout frequency of the sensor signal is to a first order approximation independent of the interrogation distance between the primary and secondary coils. A detailed theoretical analysis of the interrogation principle based on a lumped-element equivalent circuit is presented. The analysis has been experimentally validated on a 4.432 MHz AT-cut quartz crystal resonator, demonstrating the accurate readout of the series resonant frequency and quality factor over an interrogation distance of up to 2 cm. As an application, the technique has been applied to the measurement of liquid microdroplets deposited on a 4.8 MHz AT-cut quartz crystal. More generally, the proposed technique can be exploited for the measurement of any physical or chemical quantities affecting the resonant response of quartz resonator sensors.

Interrogation Techniques and Interface Circuits for Coil-Coupled Passive Sensors

“…This approach is robust because it is unaffected by the disturbances, such as noise and electromagnetic interferences, which typically affect the signal amplitude. Specifically, two kinds of sensors are investigated, as introduced in Section 2 , namely, capacitive sensors, which form a resonant LC circuit with the secondary coil, and piezoelectric resonators, such as Quartz Crystal Resonators (QCRs) [ 21 ] or ceramic Resonant Piezo Layers (RPLs) [ 22 ].…”

“…Two readout techniques, that are virtually independent of the coupling, are presented and discussed in detail in Section 3 . In particular, a frequency-domain technique based on impedance measurements [ 20 ] and a time-domain technique called time-gated technique [ 21 ] are discussed. Both techniques suffer from significant accuracy degradation, due to the unavoidable parasitic capacitance in parallel to the readout coil that introduces a dependence of the readings on the interrogation distance.…”

Contactless Interrogation System for Capacitive Sensors with Time-Gated Technique

“…The exponential decay time τ is related to the quality factor by Q = πfRτ; while Q is determined mainly by the parasitic series resistance of L2 [5,6]. The additional term in Equation (2) is a voltage pulse which accounts for the initial current in L1.…”

“…Passive wireless humidity sensors [1] and non-contact passive electromagnetic interfaces especially dedicated to capacitive sensors [2][3][4] have been demonstrated, but these adopted measurement techniques generally suffer from a dependency on the interrogation distance, which makes them difficult to apply in real operating conditions. Differently, this paper presents a contactless interrogation system for capacitive sensors that exploits a specific time-gated technique firstly developed for piezoelectric resonant sensors [5,6]. As a fundamental advantage compared to techniques based on reflected impedance, this novel technique ensures that the readout frequency is to first order independent of the interrogation distance between the two coils.…”

Low-Frequency RFID Signal and Power Transfer Circuitry for Capacitive and Resistive Mixed Sensor Array

“…Similarly, contactless interrogation can be achieved through an electromagnetic link between two coupled coils [17,18]. In [19][20][21][22], a primary coil is connected to the interrogation circuit, while a secondary coil is connected to a resonant sensor; i.e., a mechanical resonator such as a quartz crystal microbalance (QCM), a micro electro-mechanical system (MEMS) or an electrical resonant sensor such as an LC-tank circuit. The measurement principle can exploit both frequency-domain and time-domain techniques [21,22].…”