As automotive tire pressure monitoring system (TPMS) mandates come into effect and torque sensing applications like electronic power steering (EPS) become standard on even low- to mid-range cars, Texas Instruments Incorporated (TI) has announced that Transense Technologies plc is using their TMS320F28x digital signal controllers as a key component in the automotive industry's first targeted piezo-electric 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 mm by 3 mm and less than 2 grams in weight. By using the F28x-enabled SAW TPMS, braking distances and the risk of accidents due to tire under inflation or failure are reduced. Fuel efficiency is also enhanced by up to ten percent through properly inflated tires and engine drag reduction through the elimination of the hydraulic pump in EPS systems.
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. 32-bit DSP 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 can handle additional tasks such as system communication via the on-chip CAN BUS for instance. A radio frequency (RF) application specific IC (ASIC) dual channel controls RF transmission and reception.
According to the US Department of Transportation (DOT), up to 27 percent of passenger cars and 33 percent of light trucks operate with under inflated tires, resulting in an estimated 23,000 crashes and 535 fatalities each year.
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 Transense 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 RF signal - no battery is required - first exciting, then transmitting the three resonant SAW frequencies from which independent pressure and temperature are subsequently determined.
By 2010 half of all the cars sold in Europe are projected to be equipped with electronic power steering (EPS) 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 degrees 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 non-contact 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 three to four percent. TI's F28x controllers offer high performance and real time processing required to manage safety critical automotive motor control applications such as EPS.
Texas Instruments
