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 os cillatory waveform with the feature of a damped free oscillation is ob tained experimentally. It can illustrate dynamically and directly the vibratory characteristics of TSM sensors, which can be considered as a n important advantage over an admittance spectrum obtained by admittan ce analysis. Compared with the motional resistance in equivalent circu it, the decay time, obtained by Levenberg-Marquardt algorithm through the fit of the oscillatory equation to oscillatory waveform, is more s uitable for characterization of the electrical energy dissipated in th e quartz crystal. The oscillatory features of TSM sensor are investiga ted by FFT-AA in different medium. The decay times are 0.1807, 0.0761 and 0.2300 mu s with the media being air, water and dry coating film r espectively. Finally, the proposed method has been successfully applie d to monitor a curing process of coating polymer. It is therefore show n to be a useful technique to study oscillatory behavior of TSM acoust ic wave sensors. (C) 1997 Elsevier Science B.V.