The electronic tire pressure monitoringsystem (TPMS) detects pressure drops in car tires while driving. Corresponding systems are based on pressure monitoring at the valve or on measuring the tire circumference.
In the case of direct pressure monitoring at the tire valves, these are equipped with tire pressure sensors and radio transmitters, which then transmit the tire pressure to the on- board computer in real time via radio. To ensure that the pressure sensors do not require their own power supply, there is a system that operates in the ISM band, in which the pressure sensor obtains the necessary energy for measurement and transmission from the carrier frequency transmitted by a transponder fitted in the car. These systems are relatively precise and will be mandatory in the U.S. for trucks weighing 4.5 t or more starting in 2008.
With indirect tire pressure monitoring, the tire circumference and the travel distance resulting from the wheel speed are calculated and constantlychecked. If the pressure drops, the tire circumference decreases and the wheel speed increases for a certain distance. This method has the advantage that it does not require additional tire pressure sensors or other electronic components and the calculation can be based on the existing values. However, only sharp drops in pressure are registered.