Functional Passive Sensor System Using Reflective SAW Delay Line
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Functional Passive Sensor System Using Reflective SAW Delay Line T. Nomura, T. Kosaka and A. Saitoh Faculty of Engineering, Shibaura Institute of Technology 3-9-14 Shibaura, Minato-ku, Tokyo 108-8548, Japan Abstract A new RFID tag system has been designed by utilizing a reflective surface acoustic wave (SAW) delay line. A shear horizontal mode SAW (SH-SAW) has the unique characteristic of complete reflection at an edge normal to the direction of propagation. The characteristic makes it possible to construct a reflective delay line. In this paper, a reflective SAW delay line is utilized to fabricate a passive sensor and radio frequency identification (RFID) tags. Experimental results showed that the reflective delay lines are very suitable in making SAW RFID tags. Moreover, some applications of the SH-SAW RFID tags are demonstrated for measurement of the acoustic properties of liquids and strain of solid. 1. Introduction Surface acoustic wave (SAW) devices, known as useful electronic devices, are used widely in many fields. Filters used in television sets and cellular phones are just one example of SAW devices. Generally, SAW devices are compact and solid, and show a low aging rate. It is known that SAW electronic devices such as filters are little affected by external influences such as humidity, stress, or electrical and other environmental factors. On the other hand, another application of SAW devices that is rapidly gaining popularity today is sensors [1-4]. Sensor technology is fast growing and SAW devices are finding their way into more and more applications. Recently, many of them, including devices that monitor the environment, perform process control at chemical reaction plants, and conduct medical analysis and bio-sensing. SAW devices have also been applied in a radio frequency identification (RFID) system and in remote sensing [5, 6]. RFID tag technology utilizing SAW devices is a new field of applications for SAW sensors, and has the potential to lead to new sensing systems with desirable features, such as sensor networks. Generally, RFID is a non-contact automatic recognition technology that employs electromagnetic waves. The ID uses microwaves to transmit (read and write) data within a non-contact semiconductor memory as an IC chip, that also utilizes radio waves, etc. Thus, the RFID functions like a barcode [7]. Because it has no need for a power supply or wire, a passive sensor incorporating an SAW device can be attached to a moving or rotating body and can operate under extremely harsh conditions. In this paper, an ID tag is introduced that employs an edge reflection of SH-SAWs. The structure and characteristics of the sensor, as well as its application to passive sensors, is reported.
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2. Surface acoustic wave RFID tag An ID tag, also known as an IC tag or an RFID tag, is an IC chip containing information necessary to recognize an individual and an article as well as the details of what is recognized. Namely, it is a “tag” (like a baggage tag) for recognition. Recognit
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