SAW Sensors measure tire pressure and temperature.
(Photo: Transense Technologies)
Most existing TPMS are direct active systems using a battery-powered, silicon micro-electromechanical system (MEMS)-based sensor inside each tire. 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 a limited lifespan 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 gas-tight 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, which requires no battery, is interrogated by a radio-frequency (RF) signal that generates an acoustic wave, which spreads over the piezoelectric substrate surface. The wave transforms into an electric field and retransmits pressure and temperature measurements. The sensors can be attached to the back of the tire valve; attached or embedded in the wheel itself; or embedded in the tire.
Directly measuring torque in EPS systems EPS systems, which are projected by 2010 to be standard equipment in more than half the vehicles sold, reduce both installation and production time for manufacturers, as well as saving fuel and maintenance costs for consumers. Combined with high-performance digital controllers, SAW sensors offer the real-time processing required to manage critical automotive safety motor control applications such as EPS. 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.
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. Also, 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 percent to 4 percent.
In addition to TPMS and EPS systems, a number of other automotive applications for the sensors are being explored. For example, Transense is researching the measurement of tire wear and tread adhesion, suspension and braking forces, bearing stresses and bumper proximity sensing. For today's technicians, it is important to understand that not only are vehicles coming that have increased numbers of sensors and telematic functionality, but that the very nature of those sensors is evolving at an accelerating rate.
(Sources: Texas Instruments
Inc., Transense Technologies
PLC)
