Rene Fachberger

Wireless SAW Temperature Sensor System for High-Speed High-Voltage Motors

This paper gives an overview on the system development of a wireless surface acoustic wave (SAW) temperature measurement application in high-speed high-voltage motors. The measurement location of interest is on the rotor in the vicinity of the copper squirrel cage. To fit the sensor in a 4 mm bore, a custom packaging with outer dimensions of 10.0 × 3.1 × 2.0 mm3 has been developed. For the purpose of interrogation a mechanically robust metal slot antenna has been designed to withstand rotation speeds up to 15 000 rpm. As the sensor interacts only during a very short time of a few 100 μs with the reader antenna a fast frequency modulated continuous wave (FMCW) reader was developed. The reader operates in the ISM band of 2.4 GHz and achieves a typical sweep time for data acquisition of 100 μs. The sensor system has been tested both in test bed and real engine conditions and performed stable read outs, although in the real engine signal behavior is disturbed by reflections and possibly electromagnetic field bursts caused by the power converter.

SAW Transponder – RFID for Extreme Conditions

Harsh or hazardous environments, e.g. continuous furnaces, process chambers, rotating or moving objects, require a robust wireless passive transponder technology for sensor and RFID applications. The transponders’ operating temperature often exceeds 200°C in these applications and is way above the thermal limit of CMOS devices. Surface acoustic wave (SAW) devices are excellent candidates for high temperature applications as their operation has been shown at temperatures of 1000°C (Hornsteiner, 1997). With the use of an RF (radio frequency) antenna SAW devices can be interrogated passively and wirelessly. The main advantage of surface acoustic wave sensors is their outstanding thermal stability specially compared to semiconductors. The sensors utilize the piezo-effect that creates socalled surface acoustic waves by means of a transducer structure on the surface of the sensor. Metallization gratings, so called reflectors are used to supply the device with a unique identification code (ID), achieved by pulse position coding (Reindl, 1998). This allows using the device as a high temperature stable RFID transponder (radio frequency identification). Depending on temperature or mechanical strain the surface acoustic waves are also affected. These changes on the surface acoustic waves can be used to implement an additional sensor functionality. In this way pressure sensors in combination with temperature sensing have been demonstrated [Pohl,1997; Kalinin, 2004]. In ideal application fields of SAW based RFID systems environmental conditions like high temperature or high doses of γ-radiation exist. Successful application examples are the automatic identification of pressure sensors, vehicle identification in paint shops and several tagging tasks in the steel industry. Often the temperature information contained in the response signal gives valuable additional process information of the tagged goods. This chapter gives an overview of SAW based RFID transponders made for extreme conditions like temperatures up to 400°C or cryogenic temperatures down to –196°C. Their function principle and system performance is explained and pertinent application examples are given.