Philips Semiconductors has developed a chip for direct measurementof vehicle tire pressure and identification. The signal conditioningchip will boost safety in the car by communicating whether tirepressures are at their correct level and will increase conveniencefor the driver by reducing the need for regular manual checking oftire pressure.
Direct or indirect tire pressure monitoring systems (TPMS) willbecome a standard feature on cars within the next ten years and arerequired by US law from 2004 onwards. Direct measurement provides amuch more accurate indication of tire pressure than is possible usingan indirect measurement system, with the advantage that the signal isalways available even when the vehicle is parked.
The P2SC signal conditioning chip provides the link between thetire module and the driver interface, enabling early warning ofincorrect tire pressure to the driver. The wireless link of the P2SCchip solution uses Philips' Passive Keyless Entry (PKE) RFIDtechnology and is already field proven and qualified for automotiveuse.
Tire pressure monitoring systems were implemented a number ofyears ago but only on high-end luxury vehicles. The currentgeneration of systems is based on a pressure sensor, which includesan ASIC for conditioning the pressure and temperature signal.
The next generation, currently under development, will replace theASIC with a micro-controller, such as the Philips P2SC.
“Tire Pressure Monitoring is set to become the fastest growingautomotive electronics system over the next few years,” according toSimon Schofield, industry analyst with Strategy Analytics' AutomotivePractice. Strategy Analytics expects the worldwide vehicle OEM marketfor direct tire pressure monitoring to have reached 15 millionsystems by 2006, growing to over 22 million systems by the end of thedecade.
The P2SC chip provides low frequency wake-up and high frequencyreturn, enabling the system to 'ask' each tire for its currentpressure and position and relay this information to the driver. Eachtire is 'woken-up' every time the ignition is switched on – to givethe driver status information on the tires before starting todrive.
While driving, an adaptive wake-up pattern ensures regular statusupdates for the driver. In the event of a sudden drop in pressure,the tire will automatically relay this information to the driverwithout first needing to be woken-up. The driver would be made awareof the tire pressure through different systems such as an icondisplay or virtual car on the dashboard.
Overcoming auto-rotation issues, Philips' P2SC uses low-frequencywake-up to automatically identify the correct wheel transmitter, evenafter a tire position has changed during maintenance. This savestime-consuming reprogramming as required for other systems after tirerotations or when new tires are mounted.
The signal conditioning chip is mounted directly onto the tirerim. Small low frequency (125kHz) wheelhouse antennae send a wake-uppattern to the specific tire module, which responds via the RF link.The LF wake-up has to bridge a distance from the wheelhouse antennato the tire module of about 1m, which is already proven to befeasible, using the Philips Semiconductors PKE technology, where asimilar distance has to be bridged to open a car remotely. Athree-dimensional (3D) interface in the Tire Module guarantees anorientation independent sensitivity for incoming wake-up pattern.
Incorrect tire pressure can compromise the stability of a vehicle,its handling and braking, since low tire pressure can result inincreased stopping distances.
One in five tires is under-inflated by up to 40%. This leads to asignificant decrease in the life of the tire and an increase in fuelconsumption by around one per cent for every 3PSI under-inflation,according to recent data from Goodyear.
A direct tire pressure monitoring system is one which alwaysmeasures the pressure directly inside the tire. For this, the tiremodules are located in the tire – usually attached to the inflationvalves – and broadcast their data via RF to a central receiver. ThisRF link uses the same RF principles and frequency range as the RemoteKeyless Entry systems usually already found in a modern vehiclereducing the overall system cost.
The tire module itself consists of three parts:
- A pressure sensor (usually piezo-resistive analog)
- A pressure sensor signal conditioning chip (which can beintegrated in the sensor)
- An RF transmitter unit
The tire modules have to withstand temperatures from -40°C toover 150°C in combination with acceleration ranges of up to2000g. In this harsh environment, special components will be used toinsure a lifetime of up to ten years.
Signals from sensor
The signal from the silicon sensor has to be amplified anddigitized and the full device calibrated and initialized. Philips'sensor signal processing chip – P2SC – picks up the signal from thesensor bridge, changes it to digital, measures the temperaturedirectly on chip and performs all the calibration and initializationneeds. The P2SC includes the STARC based reduced instruction setcomputer (RISC) microcontroller core, which is field proven in RKEapplications.
Since cost and lifetime of the batteries in the tire modules willremain an issue, the automotive electronics industry will continueworking on batteryless solutions, such as using inductive coupling onpassive GHz technologies. Future systems may also make use ofBluetooth, once this technology is established in the vehicle.