Frequency evaluation of SAW torque response signal using Hilbert envelope-demodulation

When detecting the strain with passive wireless SAW (Surface Acoustic Wave) resonator sensor, the response signal is of narrow band, high frequency, low SNR and transient attenuation. The response signal is produced only in the case that the interrogation covers the operational frequency band of the SAW resonator. Burst of sinusoidal is used in the experiment to excite the resonator, and analysis of the sensing signal reveals that the response signal is an exponential decay signal of single frequency, changes of strain lead to a shift of the resonance frequency. Torque applied to the shaft can be acquired from changes of the center frequency of the resonator. The frequency resolution of traditional FFT spectrum analysis method is limited by sampling length, which can't meet the accuracy. Parameter estimation methods, such as MLE (Maximum Likelihood Estimate) or LSE (Least Square Estimate) can be applied, but it is time-consuming to satisfy a real-time demand. In this paper, a new method of Hilbert envelope-demodulation is employed to estimate the center frequency, by which response signal can be converted to constant amplitude sinusoid. This can simplify the waveform greatly, and make it easy to determine the center frequency of the sensor. A problem of frequency detuning in envelope-demodulation is solved in this paper. For the demodulated single frequency sine wave, the resonance frequency can be got directly in time domain by finding the extreme of the waveform. The results show that this method can estimate the frequency more accurately and faster.