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.
Highlights
AT-cut quartz crystal resonators (QCRs) are thickness-shear-mode (TSM) acoustic-wave resonators in which a thin quartz disk, obtained from a quartz rod sliced at an angle of 35.25◦ with respect to its optical axis, is sandwiched between two metal electrodes [1]
Quartz crystal microbalances (QCMs) are the QCRs commonly employed as mass sensors in gas phase, in vacuum, and in contact with liquids in many bio-analytic applications [4,5,6,7,8,9]
LmiLm represent the initial conditions of the electric capacitance Cd of the detection circuit input stage, V m0 = qCm−1 − Lm iLm represent the initial conditions of the electric capacitance Cd of the of the primary and secondary coils L1 and L2, of the QCR electric capacitance C0 and of the QCR
Summary
AT-cut quartz crystal resonators (QCRs) are thickness-shear-mode (TSM) acoustic-wave resonators in which a thin quartz disk, obtained from a quartz rod sliced at an angle of 35.25◦ with respect to its optical axis, is sandwiched between two metal electrodes [1]. The contactless electromagnetic principle proposed in this paper advantageously employs QCR crystals with ordinary electrodes It grants a first order independence from the interrogation distance, since the mutual inductance between the coils acts only as a scaling factor on the signal amplitude. To this purpose, dedicated post-readout techniques, based on autocorrelation analysis suitable for the implementation into embedded systems, have been developed for measuring the significant parameters of QCR sensors [27] This contactless interrogation technique can simultaneously measure both the resonant frequency and quality factor (Q = 1/D) of the sensor, making it somewhat related to the QCM-D technique, but with the advantage of contactless operation. F exc approaches r, connects the primary coil to the readout circuit In this condition, the QCR undergoes decaying the effectiveness of the excitation is increased and the amplitude of the detected signal rises.
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