Modelling of a wireless SAW system for multiple parameter measurement

V. Kalinin.

Proc. of 2001 IEEE International Ultrasonics Symposium, October 8-10, Atlanta, USA, 2001, pp. 1790-1793.

Abstract

Wirelessly interrogated passive SAW sensor systems have important advantages over their active electronic counterparts. A number of them have been recently proposed for measurement of temperature, pressure, torque and other physical quantities. Most of the sensors are based on SAW reflective delay lines requiring the bandwidth of about 10-20 MHz. Unfortunately, European ISM band at 433 MHz is considerably narrower and is more suited for the use of SAW resonators.

Various systems based on the SAW resonators were also proposed and analysed, although two rather critical aspects of them were not fully investigated. The first one is related to the influence of the sensor antenna impedance and matching conditions on the performance of the system and the second one is the accuracy of measurement that is achievable using various measurement algorithms. The present paper is focused on these two aspects in application to a tire pressure and temperature monitoring system based on three one-port SAW resonators.

It is shown both analytically and by means of computer simulations that the maximum level of the SAW pulse response received by the interrogation unit does not correspond to a complex conjugate match of the antenna to the SAW device. Moreover, it is undesirable to achieve the maximum received signal since the response length is reduced in this case by a factor of 10-20 and the frequency of natural oscillation is very sensitive to a small variation of the antenna impedance. The results obtained allow choosing the antenna impedance for the SAW resonators connected both in series and in parallel on the basis of the trade-off between the signal level and the response length. Computer simulations also show that the accuracy of measurement of the frequency of natural oscillations taking into account both the mutual influence of the three SAW resonators, the practical value of the Q-factor and level of noise in the system can be around 2 kHz.