Development Of Point-Contact Surface Acoustic Wave Based Sensor System

Abstract

Surface Acoustic Waves (SAW) fall under a special category of elastic waves that need a material medium to propagate. The energy of these waves is confined to a limited depth below the surface over which they propagate, and their amplitudes decay with increasing depth. As a consequence of their being a surface phenomenon, they are easily accessible for transduction. Due to this reason, a lot of research has been carried out in the area, which has resulted in two very popular applications of SAW - SAW devices and in Non-Destructive Testing and Evaluation.

A major restriction of SAW devices is that the SAW need a piezoelectric medium for generation, propagation and reception. This thesis reports the attempt made to overcome this restriction and utilize the SAW on non-piezoelectric substrates for sensing capabilities. The velocity of the SAW is known to be dependent purely on the material properties, specifically the elastic constants and material density. This dependence is the motivation for the sensor system developed in the present work.

Information on the survey of the methods suitable for the generation and reception of SAW on non-piezoelectric substrates has been included in the thesis. This is followed by the theoretical and practical details of the method chosen for the present work - the point source/point receiver method. Advantages of this method include a simple and inexpensive fabrication procedure, easy customizability and the absence of restrictions due to directivity of the SAW generated. The transducers consist of a conically shaped PZT element attached to a backing material. When the piezoelectric material on the transmitter side is electrically excited, they undergo mechanical oscillations. When coupled to the surface of a solid, the oscillations are transferred onto the solid, which then acts as a point source for SAW. At the receiver, placed at a distance from the source on the same side, the received mechanical oscillations are converted into an electrical signal as a consequence of the direct piezoelectric effect. The details of the fabrication and preliminary trials conducted on metallic as well as non-metallic samples are given.

Various applications have been envisaged for this relatively simple sensor system. One of them is in the field of pressure sensing. Experiments have been carried out to employ the acoustoelastic property of a flexible diaphragm made of silicone rubber sheet to measure pressure. The diaphragm, when exposed to a pressure on one side, experiences a varying strain field on the surface. The velocity of SAW generated on the stressed surface varies in accordance with the applied stress, and the consequent strain field generated. To verify the acoustoelastic phenomenon in silicone rubber, SAW velocities have been measured in longitudinal and transverse directions with respect to that of the applied tensile strain. Similar measurements are carried out with a pressure variant inducing the strain. The non-invasive nature of this setup lends it to be used for in situ measurement of pressure.

The second application is in the field of elastography. Traditional methods of diagnosis to detect the presence of sub-epidermal lesions, some tumors of the breast, liver and prostate, intensity of skin irritation etc have been mainly by palpation. The sensor system developed in this work enables to overcome the restrictive usage and occasional failure to detect minute abnormal symptoms. In vitro trials have been conducted on tissue phantoms made out of poly (vinyl alcohol) (PVA-C) samples of varying stiffnesses. The results obtained and a discussion on the same are presented.