The modern automobile is witnessing rapid growth in sensor systems spurred by the growing need for higher efficiency and reduced emissions. The progression of sensor applications can be seen from the early days of measuring oil and water pressure/temperature, which were followed by the addition of crank and cam position, air mass flow/temperature/pressure and exhaust gas analysis. On the chassis, anti-lock braking system (ABS) wheel speed sensors compete with direct pressure sensors to determine the status of tire inflation. Accelerometers for airbag deployment have been supplemented by gyro-based inertial module units. The question is: Which sensors will be next to support the advances of vehicle performance and safety?

The drive for reduced exhaust emissions means it is important to control engine performance over the life of the vehicle. Modern engine control is typically based on look-up tables of nominal output torque versus a range of input variables, all of which are derived from dynamometer testing of a few engines. Because engines vary with production tolerances and by wear throughout their service life, torque estimates for control of the engine and automatic transmission gear changes are generally non-optimal.

First-generation tire pressure monitoring systems (TPMS) used ABS wheel sensing (tires with low pressure rotate faster) or battery-powered tire pressure sensors and transmitters. ABS sensors have relatively poor accuracy and the algorithms need minutes to stabilize. Battery-powered sensors provide accuracy but are relatively heavy, have limited service life, and present battery disposal issues. With more than a billion tires purchased annually, discarding batteries is not ecologically sound.

Surface acoustic wave (SAW) technology can be applied as a strain transducer that is lightweight (<2 grams), small, rugged, recyclable and both battery-less and wireless in nature. SAW sensors can readily sense torque and temperature for steering EPS and powertrain applications as well as pressure and temperature for TPMS. SAW is particularly suitable for monitoring rotating components or those difficult or dangerous to access. This article reviews the design and interrogation of SAW sensor systems produced by Honeywell and discusses several key automotive applications.

Figure 1. SAW resonators use small piezoelectric quartz die upon which two or three single-port resonators are fabricated in aluminum using standard photolithographic techniques.

Surface acoustic waves on solids were predicted and characterized in the 19th century but not used in electronic systems until the latter part of the 20th century. A SAW device has the ability to convert an electrical signal into an acoustic signal with the same frequency, but a much smaller wavelength because of a reduction in propagation velocity of about five orders of magnitude; e.g., a 100 MHz electrical signal with a wavelength of about three meters is converted to a wavelength of only 30 microns on the SAW device. This allows manipulation of an RF signal in a very small package. Low propagation velocity enabled the use of SAW devices for time delays and filtering in radar systems and television but their application burgeoned within the mobile phone market. Honeywell SAW sensors use small piezo-electric quartz die upon which two or three single-port resonators, with natural resonant frequencies around 434 MHz, are fabricated in aluminum using standard photo-lithographic techniques (Figure 1). Because of the established high-volume market for SAW filters, no new manufacturing tooling is required to produce SAW resonators, only a new mask, providing benefits of low unit cost and easy second sourcing.

A SAW resonator is excited by a short radio-frequency (RF) burst. The centrally placed interdigital transducer (IDT) converts the electrical input signal to a mechanical wave through the piezo-electric effect. The waves propagate from IDT to reflectors and back until a forced resonance exists as a standing wave. After the transmit signal is switched off, the resonator continues to oscillate but at a frequency modified by any applied mechanical and/or thermal strain. The decaying oscillation is converted back to an electrical signal via the piezo-electric effect and retransmitted to the SAW interrogation board where the frequencies are analyzed and converted to engineering parameters.