SAW-Based Wireless Measurements of the Fast-Varying Deformations in Rotating Vibrating Objects

Measurement of vibration characteristics with SAW-sensors is attractive in applications for power generation, transport and aviation. The possibility of wireless data acquisition is especially useful for interrogation of sensors fixed on rotating vibrating objects, such as turbine blades for example. This paper reports on details of experimental wireless acquisition of vibration related information from rotating tuning forks which simulate the behavior of the vibrating objects at room temperature. SAW-resonators are used as sensors for fast-varying deformations measurements. Tuning forks with known frequencies of mechanical resonances equal to 512 Hz were fixed on a rotating shaft of an electrical motor. Commercial 224 MHz quartz SAW resonators were glue-bonded to tuning forks shoulders in the area of maximal strain. The resonators were wire-bonded to intermediate PC-boards. They could be connected either as feedback circuits to a miniature oscillator circuit with the output connected to rotating electromagnetic couplers (near-field antennas), or directly to the couplers for interrogation by external signals without the battery and the oscillator electronics. The oscillator had a miniature battery fixed on the same shaft and its output signal was frequency modulated by the tuning fork vibration, inducing fast varying deformations in the SAW resonator. The 512 Hz vibration of rotating at 900- 2700 rpm tuning forks was excited by compressed air flow or by hammer shocks. After demodulation of the oscillator signal about 15 s long exponentially decaying bursts were digitized by a computer sound card and recorded. This approach to data acquisition, although applicable only to low temperature experiments, has shown a better frequency resolution (0.005% in initial experiments) than the interrogation with external signals, and a much larger working distance: above 10 m with vibratortype antennas and oscillator power of only 10 μW. The spectrum of the output signal contained the rotation related contribution.