SEMA eNews Vol. 10, No. 21, May 24, 2007

NO BATTERIES NEEDED FOR NEW TPMS SENSOR

As automotive tire-pressure monitoring system (TPMS) mandates come into effect, Texas Instruments Incorporated and Transense Technologies PLC have developed what they say is the automotive industry’s first targeted piezo-electric surface acoustic wave (SAW) sensor based system for TPMS applications. According to a recent Texas Instruments press release, the sensor units operate wirelessly, require no power source and are typically 11 millimeters x 3 millimeters in size and less than 2 grams in weight.
 
By using the company’s new sensor in tire-pressure monitoring systems, braking distances and the risk of accidents due to tire under inflation or failure are reduced, TI says. The company also says that fuel efficiency is also enhanced by up to 10% through properly inflated tires.

SAW sensors utilize a radio frequency electric field to generate an acoustic wave which spreads over the piezo-electric substrate surface, transforming back to an electric field and re-transmitting for measurement. Texas Instruments says these new sensors utilize “32-bit DSP performance and high integration of the F28x digital signal controllers” in order to perform essential real-time data handling, calculation and reporting functions. The F28x device calculates the spectrum of the SAW impulse response, finds the frequency of natural oscillations of the SAW sensor and can handle additional tasks, such as system communication via the on-chip CAN BUS, for instance. A radio-frequency application-specific IC (ASIC) dual-channel controls radio-frequency transmission and reception.

Most existing TPMS are direct active systems utilizing a silicon micro-electro-mechanical system (MEMS)-based sensor inside each tire powered by a battery. Pressure and temperature information is transmitted by radio from each of the wheels to an electronic control unit (ECU) and displayed as either a number or a warning indicator. Batteries inside tires add weight, have limited life and cannot be replaced. With 1.2 billion tires sold annually, this waste represents an increasing environmental hazard.

The passive SAW sensor incorporates a three element die within a small gastight capsule. Pressure is transmitted via a diaphragm to deform the die and mechanically strain one of the elements, while all three elements see thermal strains. The sensor is interrogated by an radio-frequency signal—no battery is required—first exciting, then transmitting the three resonant SAW frequencies from which independent pressure and temperature are subsequently determined.

According to the U.S. Department of Transportation (DOT), up to 27% of passenger cars and 33% of light trucks operate with under inflated tires, resulting in an estimated 23,000 crashes and 535 fatalities each year. As part of the November-2000-enacted Transportation, Recall Enhancement, Accountability and Documentation (TREAD) Act—which affects all light motor vehicles registered after September 1, 2007—TPMS technology must alert drivers of significant under-inflation of their tires.

Source: Texas Instruments Incorporated. (May 14, 2007). “First Automotive Surface Acoustic Wave (SAW) Sensor Coupled With TI Controller Enhances Fuel Efficiency and Safety.” Texas Instruments Incorporated press release courtesy of PR Newswire.

 

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