With automotive tire pressure monitoring system (TPMS) mandates in effect and electronic power steering becoming standard, Transense Technologies plc is using Texas Instruments’ TMS320F28x digital signal controllers as a key component in the automotive industry’s first targeted piezoelectric surface acoustic wave (SAW) sensor-based systems. Operating in environmentally harsh or remote automotive and industrial applications, the Transense sensor units operate wirelessly, require no power source, and are typically 11 x 3 mm and less than 2 grams in weight. Use of the F28x-enabled SAW TPMS reduces problems with braking distances and the risk of accidents due to tire under-inflation or failure. Fuel efficiency is also enhanced by up to 10% with proper tire inflation and engine drag reduction through the elimination of the hydraulic pump in electronic power steering systems.
Using Acoustic Wave for Measurement
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SAW sensors use a radio frequency electric field to generate an acoustic wave, which spreads over the piezoelectric substrate surface, transforming back to an electric field and retransmitting for measurement. Thirty-two-bit digital signal processing performance and high integration of the F28x digital signal controllers 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 handles additional tasks such as system communication via the on-chip CAN BUS, for instance. A radio frequency application-specific IC dual channel controls RF transmission and reception.
SAW Sensors Answer U.S. Mandated Requirements
According to the U.S. Dept of Transportation, 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 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 tire under-inflation.
Most existing TPMS are direct active systems using a silicon micro-electromechanical system-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 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 Transense SAW sensor incorporates a 3-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 RF signal -- no battery is required -- first exciting, then transmitting the three resonant SAW frequencies from which independent pressure and temperature are subsequently determined.
EPS Becoming Standard
Projections are that by 2010 half of all the cars sold in Europe will be equipped with electronic power steering systems that reduce both installation and production time for manufacturers and save fuel and maintenance costs for consumers. A vital part of the EPS control system is a torque sensor that measures the driver steering input. Existing EPS systems typically employ potentiometers or optical transducers mounted on a length of steering shaft with reduced section to increase local twist and hence, measurement sensitivity. This approach tends to reduce driver feel and increases the sensor production cost.
Transense SAW-based sensors, positioned at ±45 deg to the shaft axis, provide direct measurement of torque rather than position, without the need to make expensive modifications to the steering column. When torque is applied to the shaft, one of the SAW resonators compresses while the other extends, leading to a combined frequency shift proportional to the applied torque. The RF signal transmits between rotating and stationary parts of the assembly via a noncontact-coupled transmission line. With the exception of the SAW sensor, no electronic components are mounted on the shaft, maintaining driver feel and keeping costs low.
Switching from a hydraulic steering system to an electromechanical model eliminates the constant drag on the engine while the reduced weight contributes to overall fuel economy; estimates indicate that EPS leads to a fuel economy improvement of approximately 3-4%. TI’s F28x controllers offer high-performance and real-time processing required to manage safety-critical automotive motor control applications such as EPS.
DSP Performance and TI Support Are Key
“We needed a device with the high performance, integration, and price points that could only be found in the F28x controllers,” stated Dr Ray Lohr, Transense Technologies’ Technical Director “The robust, easy-to-use development tools and the hands-on support that we got from TI were also crucial.”
TI’s F28x controllers offer 32-bit DSP performance combined with the peripheral integration and ease-of-use of a microcontroller. All F28x-based devices feature a 32-bit wide data path for superior performance and mixed 16/32-bit instruction set for improved code density. These controllers offer exceptional system integration by providing complete control system capabilities from signal input through the on-chip 12-bit analog to digital converter, quadrature encoder pulse interfaces, and timer captures and compares through signal output with up to 10 independent pulse width modulation channels with 150 picosecond resolution. Depending on the device, communication interfaces include CAN, I2C, UART, and SPI ports. The device can be interfaced to one of TI’s automotive-qualified SN65HVD1040-Q1 or SN65HVD1050-Q1 CAN transceivers, which offer industry-leading electrostatic discharge protection of up to ±8 kV (human body model).