In order to meet upcoming US 2004 legislation and a wider perceived global market, Netherlands contractor Royal Philips Electronics has launched a chip-based Tire Pressure Monitoring System (TPMS) that provides the automobile driver with measurements of individual tire pressures and notification of pressure/inflation problems as they arise.
TPMS applications have major safety and cost implications in terms of tire-related accidents, levels of tire wear and fuel consumption values. Goodyear calculates that at any given time, one in five vehicular tires is under-inflated by as much as 40 percent. Such irregularities lead to significant alterations in road-holding and braking characteristics, a significant shortening of tire life and an increase in fuel consumption of roughly one percent for every three pounds/in2 of under-inflation.
The Philips TPMS uses a direct monitoring approach and takes the form of a Tire Module (TM) that broadcasts RF data to a central receiver. Each TM typically comprises an analogue piezo-resistive pressure sensor, a pressure sensor signal conditioning chip and an RF transmitter unit. As applied to the tire, the TM has to withstand a temperature range of from as low as -40° to as high as +150°C and acceleration rates of up to 2000 G. Functionally, the signal from the pressure sensor is amplified and digitised and the full device calibrated and initialised. A P2SC signal processing chip picks up the signal from a sensor bridge, digitises it, measures chip temperature and performs all the required calibration and initialisation. The P2SC chip includes a field proven Reduced Instruction Set Computer (RISC) microcontroller core, minimum power consumption and a wheel identification feature to solve issues of auto-rotation.
After calibration and initialisation, each tire sends its pressure data to the automobile's dashboard and a controller recognises which tire the signal is coming from. A low frequency wake-up approach has been used for tire location and is described as providing immediate and reliable identification. Small, 125 kHz wheel arch antennas are used to send the wake-up signal to the specific TM, which responds via the RF link. Each tire is woken up each time the ignition is switched on. Thereafter, tire pressure is checked at regular intervals and in the event of a sudden pressure drop, a warning is relayed to the driver without the need to wake the system up. Looking to the future, Philips notes that Bluetooth™ technology could have a TPMS application, as could inductive coupling on passive GHz technologies to overcome the need for internal batteries.