The tire pressure monitoring system (TPMS) is a relatively old application of pressure
sensors. Initially, it was used primarily in high-end and high-performance vehicles
because of its high cost. Recent U.S. regulations have boosted this application
by recognizing that underinflated tires can cause accidents, and the National Highway
Traffic Safety Administration has mandated that auto manufacturers install a TPMS
in all cars and light trucks.
Two TPMS types are available. The first uses indirect measurement, which relies
on slight changes in wheel diameter in response to pressure loss. The system detects
and computes the changes based on wheel speed measurements provided by the antilock
braking system. No pressure sensors are required. However, this technology cannot
detect simultaneous pressure loss in all four tires and, therefore, is insufficient
to meet the requirements of the U.S. mandate. Consequently, the second type of system,
based on direct measurement, must be used. In this instance, an electronic device
containing a pressure sensor, motion switch, and wireless transmitter is installed
in the tire's valve stem or wheel to make the measurement. The sensor sends information
on inflation pressure levels by radio signals to a receiving unit inside the vehicle.
Direct vs. Indirect TPMS Approach
Both TPMS types have a dashboard indicator to alert drivers that the pressure level
in one or more tires has dropped below 25% of the recommended pressure. The direct
method is considered to be more accurate than the indirect approach, but one industry
expert notes that the advantage of greater accuracy is offset by a cost of $65 to
$80 per vehicle. A market-friendly price for a system consisting of a pressure sensor,
electronics, and a wireless transmitter is $12–$15 per wheel. High-volume manufacturing
should reduce the price to below $10.
Because a typical tire pressure sensor system is battery powered, it operates at
voltages from below 2.5 V to 3.6 V, with a low-voltage alert if the voltage drops
below 2.5 V. The sensor's range is 4.5 to 8 bar absolute pressure, with a temperature
range of –40°C to 125°C.
The SM5108 is a extremely small (0.65 mm x 0.65 mm) silicon micromachined piezoresistive
pressure sensing chip that has been optimized to provide the highest possible accuracy
for a die of this size. This makes it ideal for high-volume applications such as
TPMS and consumer tire pressure gauges, where cost is a critical factor The small
die results in a significant cost saving when compared to larger sensor die. Over
24,000 die fit on a 150 mm wafer. The SM5108 is available as an absolute pressure
sensor in a full-scale range of 0-60 PSI.
Similarly, SMI’s SM5420 pressure sensor is the packaged version of the die and can
be directly mounted on a printed circuit board. It is available in pressure ranges
of 15 PSI, 30 PSI and 100 PSI.
For more information contact SMI sales at sales@si-micro.com.