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Wireless SAW strain sensors for automotive applications

V. Kalinin.

345 Wilhelm and Else Heraeus Seminar: Acoustic wave based sensors: fundamentals, concepts, new applications. 11-13 April, 2005, Bad Honnef, Germany. (invited)


SAW devices as mechanical strain sensors were known from early days of SAW technology development in the 60s of the last century. However SAW strain sensors have not found any high volume application yet mostly because it is difficult for them to compete with simple and cheap traditional strain gauges. SAW devices do have one unquestionable advantage - they can be easily converted into passive wirelesses sensors working in the UHF range that do not require any battery. The work on wireless SAW sensors has been going on for more than 15 years and a lot of interesting systems have been proposed. Few of them found relatively small-scale industrial application and only recently increased demand in wireless and contactless mechanical sensors in automotive industry as well as a progress in highly integrated and inexpensive RF ASICs has improved chances for wireless SAW mechanical sensors to go into high volume production.

Despite the fact that general principles of wireless SAW sensors are well known one has to solve a number of theoretical and practical problems during an industrialised prototype development. The paper discusses some of these problems in relation with a tire pressure and temperature measurement system (TPMS) and torque sensors for electrical power assisted steering (EPAS) and driveline applications.

The first question to answer is what type of the SAW devices - reflective delay line or one-port resonator, should be selected as a sensing and/or RF energy storage element. The choice is to be done in conjunction with a selection of a wireless interrogation technique. Theoretical analysis shows that a very good potential resolution and a high dynamic range can be achieved by using a CWFM interrogation of the reflective delay line in the frequency domain and phase delay measurements. Even better potential resolution is achievable in the case of the resonator interrogated in the time domain by RF pulses. Practical resolution is also limited by a synthesiser phase noise and some spurious signals in the receiver. Pulsed interrogation of the resonator requires less time then the CWFM interrogation of the delay line which is very important for automotive applications. For this reason SAW resonators are selected as basic elements for the automotive wireless sensors.

The second question is which substrate material, cut and SAW propagation direction should be selected for the SAW device in order to achieve good strain sensitivity and acceptable temperature stability of both the strain sensitivity and the resonant frequency. Limited theoretical and experimental studies that have been performed so far helped to find an acceptable configuration of the torque sensor on quartz substrate but further optimisation would be desirable, especially for new SAW substrate materials.

There are also a number of important practical issues that need to be addressed during the prototype development. The central one is sensor repeatability, reproducibility and accuracy. It is closely related to the sensor package design and methods of attachment of the sensor to vehicle parts. Another issue is complexity of calibration procedure and storing calibration data. The third one is antenna design for the TPMS sensor and RF rotary coupler design for the torque sensor. Finally a drastic reduction of the interrogator's cost is crucially important for successful industrialisation of wireless SAW sensors. It is achieved by development of the RF ASIC that includes all active and most passive components of the transceiver.