Wireless Passive SAW Identification Marks and Sensors.

Surface acoustic wave (SAW) devices can be turned into identification and sensor elements, so called SAW transponders. Identification elements transmit an individual ID number and sensor elements measure physical quantities such as temperature, pressure, torque, acceleration, humidity, etc. The SAW devices do not require any power supply and may be accessed wirelessly. The complete sensor system consists of such a SAW transponder and a local radar transceiver (reader unit). An RF burst transmitted by the radar transceiver is received by antenna of the passive SAW transponder. The passive transponder responses with an RF signal, like a radar echo, which can be received by the front-end of the local transceiver. Amplitude, frequency, phase and time of arrival of this RF response signal transmit information about the SAW reflection and propagation mechanisms which in many cases can be directly attributed to the sensor effect for a certain measurement value. Due to the high delay time of the SAW transponder in the order of a few microseconds, usually no intersymbol interferences due to environmental reflections occur when the system operates in harsh indoor/outdoor environments. This chapter first surveys about classical wired SAW sensors. Then the functional principles of such wireless SAW transponder systems and the reader systems will be pointed out. Furthermore the design requirements for SAW transponders, such as identification tags and radio sensors, several application examples and their state-of-the-art performances are presented. Keywords: Passive SAW transponder, surface acoustic wave, local radar transceiver, wireless measurement, sensor, identification, temperature, pressure, torque, acceleration, tire-road friction, magnetic field, water content of soil [ABSTRACT FROM AUTHOR]