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Wireless interrogation of SAW strain sensors for automotive applications using TMS320C28xx DSP

V. Kalinin, J. Beckley.

TI Developer Conference, Feb. 28 - Mar. 2, 2006, Dallas, TX, USA. (invited)


Strain sensors based on surface acoustic wave (SAW) devices have been developed over the past 30 years. Their use has been limited due to strong competition from conventional strain gauges and capacitive sensors. However chances for a high volume application of SAW sensors in a number of fields have dramatically improved for two main reasons: Firstly, SAW devices are very well suited for wireless interrogation since they work in the UHF range and can be easily turned into passive (batteryless) transponders by connecting them to a small antenna. Secondly, huge progress in microelectronics, in particular RF ASIC and DSP design, now allows for a cost-effective solution to a rather complex wireless interrogation. At the same time, a great demand has emerged within the automotive industry for wireless torque sensors, for electrical power assisted steering (EPAS) and driveline applications, as well as wireless sensors for tire pressure and temperature monitoring (TPMS). This presentation gives an overview of such systems developed at Transense Technologies with particular emphasis on the implementation of signal processing in the TI DSP TMS320C28xx.

First of all, advantages and disadvantages of various types of SAW sensing elements are overviewed and a design of torque and TPMS sensors based on SAW resonators is presented. After that, interrogation methods and a block diagram of the wireless interrogator are discussed. Factors affecting resolution and accuracy of the system are analysed.

Following general principles of the system operation, an overview of the DSP functions in the interrogator unit is given and a mathematical algorithm for SAW resonant frequency calculation explained. A description of the top-level structure of the software is given and timing and memory requirements are discussed for torque and TPMS applications. Finally, some experimental data demonstrating the system performance is presented and conclusions are drawn.