Introduction
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The NHTSA, which is responsible for establishing and enforcing vehicle
safety standards, published a mandate requiring that the TPM system
must be capable of monitoring all four tires and alerting the driver if a tire
is under inflated by 25% or more of the OEM’s recommended tire
pressure. Testing showed that proper tire inflation significantly improves
vehicle handling safety, decreases tread wear, improves fuel economy,
and decreases stopping distances. Underinflated tires are dangerous
because they build up internal heat that can damage the tire structure,
potentially resulting in fatal blowouts.
Direct Versus Indirect Systems
The NHTSA’s mandate on TPM systems allows two system architectures.
The first, called the “direct” system, uses a direct RF link from each wheel
to a central receiving control unit, and the second, called the "indirect"
system, works in conjunction with the vehicle's antilock-braking (ABS)
system.
Direct Systems. Wireless sensor in or on each wheel that
measures internal tire air pressure and temperature separately
and communicates the readings by RF signals to a
receiver/controller control unit at regular intervals. The sensors
have an integral low frequency transmitter and are typically built
into or mounted on the valve stem. The end of the valve stem
serves as the sensor’s antenna. Most direct systems share the
receiver control unit with the vehicle’s keyless entry system.
Indirect Systems. Monitors rotation of tires via the vehicle’s
antilock braking system (ABS) to calculate whether they are
properly inflated. Does not measure tire pressure or
temperature. The system measures the wheel speed of all four
tires and calculates a “rolling circumference” for each one. In
the event of a tire pressure loss, the rolling circumference of
that tire is reduced, producing a change in the tire’s rotational
speed.
The NHTSA found that direct TPM systems are preferred
because they are better able to detect under- or over-inflation
and are quicker to provide alerts. Virtually all vehicle
manufacturers today have moved to direct systems.
