Wireless Remote 2-D Strain Sensor Using SAW Delay Line
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1129-V04-17
Wireless Remote 2-D Strain Sensor Using SAW Delay Line T. Nomura, and A. Saitoh Faculty of Engineering, Shibaura Institute of Technology 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan ABSTRACT Two-dimensional (2-D) strain sensor utilizing surface acoustic wave (SAW) devices is demonstrated. SAW devices offer many attractive features for applications as chemical and physical sensors. In this paper, a novel SAW strain sensor that employs SAW delay lines has been designed. Two crossed delay lines were used to measure the two-dimensional strain. A wireless sensing system is also proposed for effective operation of the strain sensor. INTRODUCTION In many fields, a measurement of strain and its distribution is required. It is the need for health and safety monitoring for the sake of safety and comfort, which requires constant monitoring of strains within complex structures and buildings as well as the strain distributions in mobile objects, typically the wings of an aircraft [1], [5], [8], [9]. The traditional method to measure strain is using a resistance strain gauge, which is mainly structured to measure unidirectional strains. However, as is the case when measuring stress, rosette analysis is required when measuring strains on a plane. Rosette analysis requires multiple elements to be positioned together at a single location and used simultaneously. Consequently there a large number of elements and accordingly a large number of electrical supply lines, making the measurement process very complex. A surface acoustic wave (SAW) can be easily excited on a piezoelectric substrate using an interdigital transducer (IDT). Its propagation characteristics are easily affected by external physical and electrical changes. This is why applications of SAW’s include sensors for measuring physical parameters. Recently, there have been many reports on SAW sensors that take advantage of SAW’s propagation characteristics, which change easily in response to external factors [2], [3], [4], [5],. In this paper, we first clarify the relationship between changes in the propagation path length of a SAW and the phase of the wave. We then discuss a strain sensor that utilizes changes in the velocity of a SAW caused by strain in the SAW device. Although some reports on sensors using SAWs have been published, they mostly discussed one-dimensional strains [6], [7], [9]. We herein propose a two-dimensional (2-D) strain sensor that simultaneously measures strains in two directions using two delay lines that cross each other at right angles. These two delay lines exist on an anisotropic piezoelectric substrate (128° Y-cut, X-propagating LiNbO3). The delay lines are used as a 2-D strain sensor. The 2-D strain sensor is passive sensor that can be attached to a structure and the remotely interrogated though a wireless interfaces. As its working principle, this strain sensor monitors changes in the propagation path length caused by strains applied to the piezoelectric substrate.
MEASUREMENT OF STRAIN USNG SAW Principle of measurement Figure
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