Abstract
Surface acoustic wave (SAW) resonators can advantageously operate as passive sensors which can be interrogated through a wireless link. Amongst the practical applications of such devices, structural health monitoring through stress measurement and more generally vibration characteristics of mechanical structures benefit from the ability to bury such sensors within the considered structure (wireless and battery-less). However, measurement bandwidth becomes a significant challenge when measuring wideband vibration characteristics of mechanical structures. A fast SAW resonator measurement scheme is demonstrated here. The measurement bandwidth is limited by the physical settling time of the resonator (Q/π periods), requiring only two probe pulses through a monostatic RADAR-like electronic setup to identify the sensor resonance frequency and hence stress on a resonator acting as a strain gauge. A measurement update rate of 4800 Hz using a high quality factor SAW resonator operating in the 434 MHz Industrial, Scientific and Medical band is experimentally demonstrated.
Highlights
Piezoelectric transducers acting as passive sensors probed through a wireless link have demonstrated extended robustness and interrogation range1,2 compared to silicon based radiofrequency identification devices.3 Because such devices act linearly in the conversion of the incoming electromagnetic wave to the acoustic propagating wave, the interrogation range is not limited by the output power reaching a rectifier diode threshold voltage but solely by the receiver noise level and the ability to identify the sensor signal above this noise.Surface acoustic wave (SAW) sensors are designed along two main approaches: 1. Wideband delay lines in which a short pulse launched by an interdigitated transducer (IDT) propagates over the free piezoelectric substrate surface with a velocity dependent on the environmental physical property and a time delay representative of this physical property.2
Narrowband resonators in which an acoustic wave is excited by the above-mentioned IDT placed between two Bragg mirrors, exhibiting a resonant frequency dependent on the physical property under consideration
During an initialization step of the algorithm, the SAW sensor response is probed along a frequency comb with spacing f (Fig. 1) selected so that, under the assumption of the SAW resonator quality factor is known, at least three returned magnitude measurements fit within the bandpass of the resonator
Summary
Piezoelectric transducers acting as passive sensors probed through a wireless link have demonstrated extended robustness and interrogation range compared to silicon based radiofrequency identification devices. Because such devices act linearly in the conversion of the incoming electromagnetic wave to the acoustic propagating wave, the interrogation range is not limited by the output power reaching a rectifier diode threshold voltage but solely by the receiver noise level and the ability to identify the sensor signal above this noise. An acoustic delay line being a wideband device, it should in principle be the class of sensor fastest to probe, with typical time delay in the 5 μs range, yielding 200 kHz measurement rate using high speed analog-to-digital converters and fast storage media. Such hardware is power consuming and hardly compatible with embedded applications.
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