Tire pressure monitoring system (TPMS)

Thursday, 04 January 2007

Tire pressure monitoring systems (TPMS) are commonly used to monitor the internal air pressure in individual pneumatic tires of a vehicle and to provide a warning signal to the driver whenever the internal air pressure in one or more of the vehicle tires is dangerously low or high. Motor vehicles are supported by inflatable tires having a desired inflation pressure. Monitoring and maintaining proper tire air pressure for vehicles has long been an important maintenance item carried out by a vehicle owner. Improper inflation of a tire can lead to poor gas mileage and increased tire wear. The loss of pressure in one or more tires on the vehicle may result in less than optimum driver control. Inadvertent over-inflation of one or more tires may also result in less than optimum vehicle handling. Operating a vehicle with tire pressures outside the recommended inflation range can also reduce tire life due to excessive flexing and heating and resultant fatigue or wear. Low tire pressure causes the tire tread to wear non-uniformly which may also cause premature failure of the tire. Tire costs, particularly for the trucking industry, are the second highest maintenance expense behind fuel. Tire costs even exceed the cost for drivers of the trucks. Loss of tire pressure has been recognized to be a significant factor in road traffic accidents. Most of tire failures are a consequence of a creeping pressure loss. Correctly adjusted tire pressure ensures optimal driving comfort at all times in terms of rolling noise, vertical bumpiness and sensitivity to transverse expansion joints. Maintaining tire inflation within an acceptable range can alleviate these issues. It is known to equip vehicle tires with tire pressure monitors that sense the air pressure in a tire and to transmit that tire pressure information to a vehicle operator via electronic circuits and radio transmitters. The tire pressure monitors are mounted within the tire air valve stem, and they transmit pressure readings at predetermined time intervals using radio frequency signals directly to a centralized tire pressure monitoring receiver.

Tire pressure monitoring systems are becoming widely utilized in modern vehicles. By means of such tire pressure sensor systems, pressure changes are to be passed to a central unit, e.g. the on-board computer of the vehicle, as early as possible in order to thus recognize damage of a tire as early as possible or warn the driver sufficiently early when tire pressure changes are present, because these indicate gas loss or abnormal deformation of the tire. Various types of pressure sensing systems for monitoring the pressure within the tires of an automotive vehicle have been developed. While several tire pressure-monitoring methods have been proposed, two general approaches have been favored. One is indirect and relies on the determination of the rolling radius of each hub and tire assembly (and often linked to the anti-lock brake system (ABS) of the vehicle). The rolling radius method relies on the signals generated by wheel rotation sensors, typically installed as part of an anti-lock braking system. The rolling radius represents the actual radius from the center of the tire to the generally flattened area of the tire in contact with the ground. Additional sensor signals from active dampers (e.g., vertical displacement sensors) may be used in indirect monitoring systems to increase accuracy, and may utilize dynamic frequency evaluation of wheel speed sensors using fast fourier transforms (FFTs). However, this system of tire pressure monitoring (ABS-tire pressure monitoring) does not provide absolute values of pressure, nor does it provide tire temperature information. The second is direct and relies on the wireless transmission of a signal from a transducer module installed in the pressurized cavity of each tire. Direct measurement systems include pressure sensors and temperature sensors mounted in the tire in which the signals are wirelessly transmitted to a control unit. Tire pressure sensors have long been used to sense the pressure of tires to indicate when the tire is below a predetermine tire pressure. These sensors use various means, typically diaphragms screwed into tire value stems and responsive to tire pressure for activating an electrical switch for generating an alarm. Sensors are mounted within each of the tires and sense the pressure within the tire. The direct sensing method requires the installation of a wireless module in contact with the gas within the pressurized cavity of each tire and hub assembly. A matched receiving module is installed on a fixed portion of the vehicle such that the transmitted data may be processed and presented to the vehicle operator as required. The direct system offers accuracy and fast response compared to systems employing an ABS wheel speed sensor to detect tire pressure. The disadvantage of the direct system is that the cost is relative high. The communication link between the sensor and the external receiver may be wireless, with radio frequency signals and/or infrared or optical signals being the most common forms. Although the utility of wireless communication in these direct systems is severely limited due to reliance on battery power, these sensors can physically measure the tire pressure and transmit the tire pressure, temperature, battery level, sensor ID number, and even location information out of the rotating tire by RF signal. Typically, the tire pressure sensors are powered by long-life batteries also disposed within the tires. The battery is permanently attached to the sensor circuit board and installed in the sensor housing to protect it from the relatively harsh environment within the tire.

A typical tire pressure monitor system is a direct measurement system that includes four transmitters correspondingly disposed within the four tires of a vehicle. In each tire, the tire pressure sensed by the tire pressure sensor is transmitted by the transmitter to a receiver located on-board the vehicle. Remote tire pressure sensors have been developed which sense tire pressure and transmit, such as via a radio frequency link, the tire pressure to a vehicle-based controller. The sensor has a transmitter to communicate through a wireless link, typically an RF link, with a control on the vehicle. The tire pressure information delivered to the receiver is subsequently conveyed to a vehicle operator or occupant, typically in the form of a display. The control is able to display any tire that has a low pressure condition to the vehicle operator. The information regarding tire pressure may be used to control vehicle safety-based devices, such as ride and comfort control systems, road-handling systems, braking systems, and the like. It is also known to utilize such sensors in combination with tires that can run without inflation in order to warn the driver that the tire is operating in an emergency mode and should be driven at a limited speed for a limited distance. A typical tire monitoring system has a receiver with one or more antennas for receiving signals from the tags in each of the tires on the vehicle. The tags may be packaged and mounted within the tire cavity or, alternatively mounted to the valve stem in communication with the tire cavity. In order to report tire conditions that are properly identified with each of the tires on the vehicle, the monitoring system receiver must be able to determine from which tire the received RF signals originates. In tire pressure monitoring systems, tire pressure sensors and radio frequency (RF) transmitters are mounted inside each tire, typically adjacent the inflation valve stem. Since each wheel typically has its own respective sensor unit which communicates with a single control module and since nearby vehicles may use the same transmission frequencies and protocols, the sensor transmissions are differentiated by providing a unique ID code within each sensor which is included in the transmissions. The control module must be configured to associate certain ID codes with respective wheel positions on the vehicle. When a particular pressure sensor indicates that an inadequate pressure is being sensed in its respective tire, the control module can alert the driver as to which tire is affected. To recognize the particular tire location (e.g., front left (FL), front right (FR), rear left (RL), rear right (RR)) associated with an RF signal received from a tire transmitter, such tire pressure monitoring systems are programmed in an initialization or sign-up operation. In order to provide a vehicle operator with information specific to each vehicle tire, programming of the tire pressure monitoring system must be undertaken by a technician or vehicle owner so that each RF signal from a tire transmitter will be associated with a particular tire location.

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