here are two kinds of TPMS employed in motor vehicles, indirect and direct. Indirect TPMS uses the speed sensors in the ABS (anti-lock braking system) to compare wheel rotation speeds to determine under-inflated conditions. An under-inflated tire has a slightly smaller diameter than correctly inflated tires, thus it rolls at a different rate from properly inflated tires when the vehicle is in motion. The system alerts the driver when it detects an under-inflation condition. However, it cannot generally detect tire deflation until the tire is at least 25 percent under-inflated.
Indirect TPMS is inexpensive and easy to install, but it is not as accurate as direct TPMS. What's more, the user has to reset the system every time the tires are changed, rotated, or re-inflated.
Direct TPMS employs pressure sensors installed on the wheel rims to provide independent, real-time air pressure measurements for each tire that can be transmitted to the vehicle instrument cluster to instantly inform the driver. Direct TPMS can also employ additional components, such as MCUs and RF devices to expand the system's capabilities. The optimal TPMS solution includes:
Sensors for measuring pressure and temperature inside the tire
Controller with time base for periodic measurements
Means to identify which tire is providing the data
Data output to the vehicle chassis
Command input for diagnostics and wake up
The Freescale direct tire pressure monitoring system
Freescale's MPXY8300 TPMS (see Figure 1) is the first of its kind to integrate a pressure sensor, an 8-bit MCU, an RF transmitter, and a two-axis (XZ) accelerometer all in one package.
Precise pressure measurements: The low-power, surface micromachined, capacitive pressure sensor's p-cell (pressure cell) measures pressure range from 100 to 800 kilopascale (kPA). 101.3 kPa is equal to one atmosphere pressure. The TPMS also offers high-pressure range p-cells for truck tires (100 to 1,400 kPa) and optional reduced accuracy calibration for lower cost applications. Freescale's capacitive surface Microelectromechanical Systems (MEMS) pressure sensing technology also offers significant power usage advantages over piezoresistive bulk MEMS. These include 0.14 µA supply current (3 V, 30 KHz) versus 600 to 10,000 µA and 0.9 nano-amp-second (nAs) minimum charge per reading versus 60 to 1,000 nAs for piezoresistive bulk MEMS.