A wireless interrogation system exploiting narrowband acoustic resonator for remote physical quantity measurement

Abstract

Monitoring physical quantities using acoustic wave devices can be advantageously achieved using the wave characteristic dependence to various parametric perturbations (temperature, stress, and pressure). Surface acoustic wave (SAW) resonators are particularly well suited to such applications as their resonance frequency is directly influenced by these perturbations, modifying both the phase velocity and resonance conditions. Moreover, the intrinsic radio frequency (rf) nature of these devices makes them ideal for wireless applications, mainly exploiting antennas reciprocity and piezoelectric reversibility. In this paper, we present a wireless SAW sensor interrogation unit operating in the 434 MHz centered ISM band--selected as a tradeoff between antenna dimensions and electromagnetic wave penetration in dielectric media--based on the principles of a frequency sweep network analyzer. We particularly focus on the compliance with the ISM standard which reveals complicated by the need for switching from emission to reception modes similarly to radar operation. In this matter, we propose a fully digital rf synthesis chain to develop various interrogation strategies to overcome the corresponding difficulties and comply with the above-mentioned standard. We finally assess the reader interrogation range, accuracy, and dynamics.

Near-field interrogation of SAW resonators on rotating machinery

“…In this work, frequency domain measurements by searching the maximal insertion were used loss, as described in Friedt et al (2010). Pulses long enough for their spectral width to be narrower than the resonator bandwidth transfer all incoming energy to the resonator at a known accurate emitted pulse frequency.…”

High temperature packaging for surface acoustic wave transducers acting as passive wireless sensors

“…The exponential decay of the power returned by the resonator, assuming a constant radiofrequency link budget (constant distance and constant antenna efficiency), is affected by the resonator quality factor variation Q through loss = 2 ×8.7 · f · Q/Q 2 dB/s or, since the sampling date after switching from emission to reception is 1 s to get rid of clutter, loss = 2 × 8.7 · f · Q/Q 2 dB. The leading coefficient 2 reflects that our reader electronics receiver stage is based on a power detector [21], a quantity related to the returned voltage by a squared function. The signal loss in the resonator acts in a similar way as the radar cross section in the classical radar equation, and the interrogation range is dependent on this loss through a fourth power law [26].…”

“…Consequently, a flexible algorithm has been developed to probe the whole frequency range from 430.5 to 434.5 MHz and to record the returned power. The frequency sweep step is selected to be equal to one third of the width at half height of the resonance, a tradeoff between immunity to measured power noise and validity of a second order polynomial approximation of the resonance shape [21]. For Q = 6000, the width at half height f is (f/Q) = (434 MHz/6000) = 72 kHz, and sweeping the 4 MHz wide band with steps of f/3 requires storing 166 samples to be processed in order to find the two resonance frequencies.…”

High-Overtone Bulk Acoustic Resonator

“…With classic method it is possible to obtain 100Hz of resolution for 434MHz sensors [28]. If electronics is improving, we can achieve 5Hz of resolution for 434MHz sensors [29].…”

“…Compared to normal oscillator use in frequency/time applications, some specific operation regimes must be considered for sensors [28]. Particularly, the resonance frequency is assumed to drift along the measured parameter on a large frequency domain.…”