The automotive electronics industry has in recent years been able to help make significant improvements in vehicle safety. One application that looks to ramp up on a mass scale is tire pressure monitoring (TPM) systems.
The high incidence of serious and fatal accidents due directly to tire blowouts has created an acute need for a low cost, low power, reliable small form factor method of monitoring vehicle tire pressures. The arrival of such technology that also integrates low-data-rate wireless capability will, over the next few years, see TPMS propagate through OEM vehicle offerings.
Some of the data that underlines the need for TPMS are alarming. For example, statistics from Renault indicate that 6% of all fatal motorway accidents are caused by the sudden failure of under-inflated tires. Also, a survey by Michelin shows that 30% of U.K. drivers rely on garage servicing for their tire pressure checksbut on some of the newest vehicles, service intervals can be as long as 18,000 miles (25,000 km)!
Many new high-end vehicles list tire pressure monitoring as part of standard equipment. In the past this was typically achieved by enhancing the capabilities of the antilock braking system (ABS) using differential (wheel-to-wheel) rotational speed measurement as a criteria to check if one tire was under inflated. The accuracy and response time of this type of system unfortunately was not high enough to satisfy legislation requirements. It was also incapable of providing a reliable warning if all the tires were under inflated as it relied on a differential measurement. On such systems, user intervention was required to reset the system after any tire maintenance period.
The circuit solution shown in below is based on a system that monitors pressure, motion, temperature, and battery voltage at each tire (see above). This information is processed and data transmitted from each wheel to the vehicle's central controller where, if necessary, the driver is warned of an unsafe condition. The information provided to the driver can be as simple as an alarm or as complex as a dashboard digital display of each individual tire's pressure. Some systems may also include the capability of a requested read back or pressure on demand (POD).
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AMI Semiconductor, has developed and manufactured a number of low-data-rate wireless solutions for TPMS that support sub-gigahertz frequency applications. The expected operating frequencies for TPMS will likely continue in the unlicensed frequency bands of 315 MHz for US and 434 MHz for Europe to enable TPMS systems to share the remote keyless entry (RKE) receiver.