Fast Fourier transform admittance analysis method applied to thickness-shear-mode acoustic wave sensors

A fast Fourier transform admittance analysis (FFT-AA) method, applied to thickness-shear-mode (TSM) acoustic wave sensors, is first proposed and the corresponding theory is presented. Based on the FFT-AA, an oscillatory waveform with the feature of a damped free oscillation is obtained experimentally. It can illustrate dynamically and directly the vibratory characteristics of TSM sensors, which can be considered as an important advantage over an admittance spectrum obtained by admittance analysis. Compared with the motional resistance in equivalent circuit, the decay time, obtained by Levenberg-Marquardt algorithm through the fit of the oscillatory equation to oscillatory waveform, is more suitable for characterization of the electrical energy dissipated in the quartz crystal. The oscillatory features of TSM sensor are investigated by FFT-AA in different medium. The decay times are 0.1807, 0.0761 and 0.2300 μs with the media being air, water and dry coating film respectively. Finally, the proposed method has been successfully applied to monitor a curing process of coating polymer. It is therefore shown to be a useful technique to study oscillatory behavior of TSM acoustic wave sensors.