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Car tyres monitor their own pressure

July 01, 2003

Whether inserted in the cover of a book or moulded in a car key, transponders prefer to lead an inconspicuous life. It's only when the retail store theft alarm screams or when an automobile anti-theft system prevents a car from being stolen that their active presence is noticed.

But strictly speaking, the tiny electronic circuits are always passive, even when transmitting their short bursts of identification data over a wireless link. Powering transponders with long-life batteries though, especially when transmitting over short distances, is neither cost-effective nor essential. To power and activate passive transponders, an external electromagnetic field is perfectly sufficient. In the case of the sensors that provide tire pressure and temperature readings via wireless transmission in modern top-of-the-range cars, there are other reasons to take into consideration.

"The batteries used in conventional active systems make them more maintenance-intensive," explains Dr. Gerd vom Bögel, a transponder specialist at the Fraunhofer Institute for Microelectronic Circuits and Systems IMS in Duisburg. "Energy storage devices are also temperature-sensitive. In warm weather and under fast driving conditions, tyres can heat up to more than 100 degrees centigrade."

Meanwhile, such chips are being mass-produced using low-cost CMOS processes. In the passive variety developed at the IMS, all of the circuit components are integrated on a single silicon chip: energy extractor, sensor elements, A/D converter, microprocessor, memory for calibration data and identification number, and finally the transmitter itself, all on an area measuring just 3.2 x 4.7 square millimetres.

There are many good reasons for incorporating such "secret detectives" in tyres. Linked to the onboard computer system, they can constantly monitor the condition of the tyres. In the event of a sudden loss of tyre pressure accompanied by a rise in temperature, the driver receives a timely warning enabling him to stop the car before the tyre explodes. Tyre failures and higher gas consumption can often be traced back to a slow and gradual decrease in tyre pressure. For drivers who out of sheer absentmindedness or laziness neglect to regularly check the tyre pressure, low-cost systems with only one passive transponder in each tyre are sufficient. It's conceivable that one day the tyre pressure values could be read and displayed directly on the petrol pump during the next fill-up. One way or the other, dirty fingers through the time-consuming, conventional method of checking tyre pressure could soon be a thing of the past.

Fraunhofer-Gesellschaft