Towards a Traffic Measuring System Utilizing Tire Pressure Monitoring Systems (TPMS)

Kurzfassung

The number of wireless devices influencing everyday life communication behaviour increases continuously. This trend encourages traffic engineers to develop systems which utilize the identifier of wireless communication standards to measure traffic. The principle to derive traffic parameters by querying the device’s address via the Bluetooth® interface is well known and frequently tested by the ITS community. Additionally, the DLR develops methods for measuring travel times in a road network on the basis of Wi-Fi. A new field of DLR’s research is to detect and recognize vehicles via wireless networks integrated in modern automobiles. Tire pressure monitoring systems (TPMS) are thought to be promising in-vehicle sensor systems for purposes of traffic. They evaluate inflation pressure of tires and transmit the measured data along with its static identifier to an electronic control unit (ECU) wirelessly. After being mandatory in the U.S., in the European Union all vehicles type approved after November 1st, 2012 have to be equipped with TPMS as well as all vehicles manufactured after November 1st, 2014. Thus, the dissemination level of motor vehicles coming with TPMS will theoretically increase up to 100 percent within the upcoming years. Since it is mandatory for every sold run-flat tire system, already today car dealers sell vehicle equipped with TPMS. For proof of concept the authors selected an aftermarket TPMS consisting of four sensors and display unit. Its sensors were easy to mount by replacing the valve caps with the sensor caps. The sensors proved to be advantageous for our purposes since by removing and reinstating the battery, a measurement and transmission can be triggered. Thus the moment of transmission was known, the raw signal data is easy to capture. Since the protocols do not rely on cryptographic mechanisms, the modulation scheme was determined: the data transmission of the selected TPMS utilizes an amplitude shift keying in the 433 MHz band. Next, we resolved the encoding scheme. The sensors employed Manchester encoding, which is a binary signalling mechanism that combines data and clock into bit-symbols. In order to understand the message’s bitfield, we varied pressure and temperature and observed which bits changed.