Passenger safety is a major element of every automotive design. This focus is supported by ongoing efforts to enhance safety-related features, either as a competitive advantage in the marketplace or in response to government requirements. One such mandated feature is the tire-pressure monitoring system (TPMS), which alerts the driver by providing either real-time pressure readings or under-pressure warnings through readouts on the instrument panel.
Studies have shown that vehicles commonly have under-inflated tires. This issue has unwanted side effects such as increased stress on the steering system, accelerated tire wear and decreased fuel economy. Unfortunately, it also has some very sobering consequences. Statistics show that more than 400 fatal and up to 10,000 non-fatal accidents per year are caused by flat tires or blowouts. Twenty percent of flat tires and blowouts are the result of under-inflation.
For these reasons and more, the United States, the European Union, South Korea, and other nations have, in recent years, enacted legislation requiring that all new passenger cars be equipped with a TPMS. The main purpose is to ensure better handling and greater safety by giving drivers real-time warnings about lost tire pressure.
Comparing two approaches to monitoring
The owner’s manual of any recently produced vehicle includes the recommended tire pressure for the factory-installed tires. As an example, the US legislation sets a threshold of no less than 25 percent deflation from the recommended pressure. Any reading lower than the threshold triggers the TPMS, which will then activate a readout that warns the driver.
Two types of monitoring systems are currently in use: indirect and direct. Indirect systems utilize signals measured by typical antilock braking systems (ABS) that use wheel-speed sensors to regulate ABS operations. Data from those same sensors can be used to compare the rotational speeds of the tires: an under-inflated tire has a smaller circumference and therefore a faster rotational speed than the others. Unfortunately, this method requires that a car be in motion for a bit of distance before a problem becomes apparent. Also, it might go undetected in the rare event that all four tires have deflated by the same amount and are rotating at the same speed.
In comparison, direct monitoring has proven to be a more accurate and reliable way to measure tire pressure. This method uses one base receiver unit that monitors four transmitting pressure sensors, one mounted inside each tire. The receiver unit is commonly placed inside the vehicle and drives an indicator on either the dashboard or a separate display. Each sensor measures pressure and transmits its reading over a radio frequency (RF) link to the receiver. The transmitters may be programmed with a unique code or serial number, allowing the base receiver to identify each tire separately.
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