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DEVELOPMENT Tires and Wheels Current and Future Developments in Tire Pressure Monitoring Systems By Harald Bochmann, Ralf Kessler and Gunter Schulze Tire pressure monitoring systems offer more safety, greater comfort and increased efficiency. They are also increasingly in demand for high-volume cars. In principle, two different technologies are competing in this field: direct measurement and indirect measurement systems. With its Tire Safety System (TSS), Beru AG is throwing its weight behind direct measurement technology and is now presenting its third generation. 2 ATZ 2/2005 Jahrgang 107
1 Introduction Tire pressure monitoring systems (TPMS) are increasingly being seen as standard features in cars. Complex insurance cases with defect tires lend added impetus to this development. For example, this issue prompted the government in the United States to introduce legislation for all cars and light commercial vehicles to be fitted with these systems in the future. The minimum requirements according to the National Highway Traffic Safety Administration (NHTSA) include the specification that an alarm be sounded if there is a 25 % loss in pressure compared to the target pressure. The reduction in pressure must be identified and displayed within no more than ten minutes [1]. The speed at which the NHTSA aims to introduce these requirements is remarkable: from 1. 9. 2005 onwards, 50 % of all new vehicles are to be fitted with TPMS; this figure will rise to 90 % from 1. 9. 2006 and to 100 % from 1. 9. 2007 onwards. It is not clear whether other countries will also follow, but it is fair to assume that it will be the case given the practical experience with other safety-related products. Both the government and customers alike expect systems of this kind to primarily contribute to vehicle safety. The first indications of pressure loss are indicated in advance and the driver is prompted by an alarm to replenish air in the tire or to stop the vehicle, which in most cases will prevent subsequent damage or accidents. This kind of control facility is indispensable for vehicles with run flat tires, so as to prevent the driver unknowingly proceeding with defective or insufficiently inflated tires. Over and above the safety element, drivers appreciate the benefits of tire pressure monitoring. The pressure display in the vehicle is updated at the press of a button. In view of the fact that tires are designed to last for their service life if they are properly inflated, TPMS users benefit from the increased efficiency in addition to the safety aspect. System costs have plummeted over the last few years. Accordingly, TPMS is rapidly becoming a standard feature even in midsize vehicles. 2 System Variations In principle, there are two standard methods of determining tire pressure: ■ Indirect measurement systems draw on measurement values provided by the ABS and/or dynamic driving systems in order to calculate changes in the tire pressure on the basis of algorithms. ATZ 2/2005 Jahrgang 107 ■ Direct measurement systems measure the actual pressure and temperature values of the air in the tires, which is then sent to a module for evaluation. 2.1 Indirect Measurement Systems Indirect measurement TPMS utilize the ABS speed sensors in the vehicles to assess differential speeds of the wheels. If the relative differential speed of one wheel suddenly exceeds a calculated threshold compared to the other wheels, the system assumes that the air pressure in the wheel in question has fallen. An alarm is then triggered. In this context, the threshold value must be aligned dynamically in order to compensate for the changes in wheel circumference due to tire wear and stress. For example, the stress differs when driving round bends or in a winter environment, in which the wheels turn at different speeds due to the differing friction. In these cases, the warning capacity of the indirect measurement TPMS is significantly restricted. Although this type of operation can be integrated cost-efficiently, such as in a software module within the ESP or ABS control units, its use is restricted to warnings in the event of a sudden drop in pressure in individual tires due to damage. However, the system will not recognize a gradual loss in pressure in all tires – for example due to diffusion – nor deviations of less than 25 % from the target value. Furthermore, it is not possible to measure the absolute pressure. Therefore, the indirect measurement systems currently on offer do not satisfy the NHTSA demands. In all probability, this applies equally to another indirect measurement TPMS version, which is currently under development. It draws on changes in the resonant frequency of the wheel/tire system depending on the tire pressure. In this system, a measurement of the rotary oscillation, which overlies the wheel rotation as interference, is used to calculate the pressure values. However, the computing of the Kalman filter used to identify the parameters is considerable and will make itself noticeable in the hardware costs [2]. Accordingly, the application possibilities for indirect measurement systems are fundamentally limited. 2.2 Direct Measurement Systems Drawing on sensors located on the inside of the wheels, direct measurement systems determine the actual individual wheel values for pressure and temperature. The mea- The Authors Dr. Harald Bochmann, is technical director at Beru Electronics GmbH, Bretten. Ralf Kessler, is head of the product group development of tire pressure monitoring systems at Beru Electronics GmbH, Bretten. Gunter Schulze, he is head of the component development for Tire pressure monitoring systems at Beru Electronics GmbH, Bretten. sured values are sent by remote transmission together with an identification code to a control unit fitted in the vehicle [3]. The pressure sensor elements generally operate on a micro-mechanical basis as part of an integrated circuit. The temperature is measured on the chip using a bandgap reference. Depending on the remote transmission conditions in the country of use, the data transmission takes place via frequency shift keying modulation (FSK) as a burst with 19 kbit/s in each case in the 433 MHz or 315 MHz band. The telegram length is 96 bits, Table 1. Error protection is provided by the cyclic redundancy check algorithm (CRC). Special lithium batteries are used to power the tire electronics. Combined with the energy-optimised sensor electronics, they allow a service life of up to ten years. The required service life of the batteries can be achieved by careful circuit design and a needs-based control of the measurement cycles and transmitter activity. 3
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