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A second generation in-car tire pressure monitoring system based on wireless passive SAW sensors

B. Dixon, V. Kalinin, J. Beckley, R. Lohr.

Proc. of 2006 IEEE Int. Freq. Control Symposium, 5-7 Jun. 2006, Miami, FL, USA, pp. 374 - 380.


There has been increased interest in tire pressure monitoring systems (TPMS) for the automotive industry since the adoption of the TREAD Act by the US government 1, 1st November 2000. The first generation of TPMS has been based on MEMS pressure sensors and battery-powered active transmitters positioned in every wheel. Concerns about obtaining 10-year life and safe disposal of batteries have led to a requirement for batteryless TPMS. The use of passive (back scattering) resonant SAW sensors in 2nd generation TPMS offers one of the most promising solutions.

In a number of previous publications, both SAW reflective delay lines2 and SAW resonators3 have been used to measure pressure and temperature of the air inside car tires. Although these reports demonstrated the feasibility of SAW based TPMS, they were far from being fully-fledged installations. The aim of this paper is to present a pre-production prototype of the full in-car TPMS employing 433 MHz SAW resonators as sensing elements.

First, the SAW sensing element incorporating three one-port resonators fabricated on a single substrate and connected to an antenna is described. Calibration models for the sensor are discussed. An issue of sensor antenna tuning and sensor matching is investigated experimentally.

After that, a method of SAW sensor wireless interrogation is presented and the electronic interrogation unit is described. Experimental data characterizing accuracy of the remote resonant frequency measurement is presented. It is shown that the standard deviation of the measured difference frequency in realistic TPMS conditions does not exceed 550 Hz while it is ensured that the system complies with FCC and European regulations.

The paper also presents an architecture of the in-car TPMS and top-level algorithm for the resonant frequency measurement and calculation of pressure and temperature. Experimental data show that the typical signal coverage versus wheel angle is from 90 to 250 depending on the vehicle. However the algorithm used for data collection ensures 100% success rate for a moving vehicle or after one rotation of each wheel. The system provides a typical pressure resolution of 0.025 bar (10 bar sensor) and a temperature resolution of 0.4C.


1R. Lohr, "Wireless sensors", in "McGraw-Hill Yearbook of Science and Technology", McGraw-Hill, 2005, pp. 389-391.
2A. Pohl, G. Ostermayer, L. Reindl, F. Seifert, "Monitoring of the tire pressure at cars using passive SAW sensors", 1997 IEEE Ultrasonics Symposium, pp. 471-474.
3W. Buff, M. Rusko, M. Goroll, J. Ehrenpfordt, T. Vandahl, "Universal pressure and temperature SAW sensor for wireless applications", 1997 IEEE Ultrasonics Symposium, pp. 359-362.