High Precision Wireless Measurement of Temperature by Using Surface Acoustic Waves Sensors

摘要

Surface acoustic wave devices can be used as wireless sensor elements (SAW transponders) for measuring physical quantities such as temperature, that do not need any power supply and may be accessed wirelessly. The complete wireless sensor system consists of such a SAW transponder and a local radar transceiver. An RF burst in the VHF/UHF region transmitted by the radar transceiver is received by the antenna of the 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. The amplitude and phase of this RF response signal carry information on the positioning Surface acoustic wave (SAW) devices can be used as sensor elements that do not need any power supply and may be read out wirelessly. The reader unit performs a radar measurement of the impulse response of the SAW transponder via a high-frequency electromagnetic radio link. A temperature variation changes the SAW velocity and thereby the response pattern of the SAW device. By analyzing the time delay between backscattered pulses with different time delays we get a coarse estimation of the temperature of the SAW transponder. By using this information the ambiguity of +- 2π in the phase differences between the pulses can be eliminated, which enables to an overall temperature resolution of +- 0.2°C.