Surface Acoustic Wave Devices for Wireless Strain Measurement

Strain monitoring is a nondestructive inspection method that can reveal the redistribution of internal forces, or the presence of anomalous loadings, in structures. Surface acoustic wave (SAW) devices are small, robust, inexpensive solid-state components in which a wave propagates along the surface of a piezoelectric material, and such devices are used in large numbers commercially as delay devices and as filters. Changes in strain or temperature cause shifts in the acoustic wave speed, by which such SAW devices can also serve as sensors. We present analytical, FEM simulation, and experimental studies on SAW devices fabricated in our laboratory on lithium niobate wafers, with an inter-electrode spacing of 8 micrometers. We discuss the change in wave speed with temperature and with strain, we outline the influence of rotated cuts for the piezoelectric substrate, and we show results of laboratory sensing experiments. Moreover, an electrode on a SAW device can be terminated as an antenna and interrogated with a wireless RF probe to act as a passively-powered device, and we present laboratory results incorporating such wireless performance in our research investigation. We pattern one set of electrodes on the SAW device as a transducer connected to the antenna, and other sets of electrodes on the device acting as reflectors of the surface acoustic wave. At the RF frequencies used for SAW devices, it is realistic to use directional antennas on the probe unit to achieve reasonable stand-off distances.