Tire Pressure Monitoring System
From Wikipedia, the free encyclopedia
A tire pressure monitoring system (TPMS) is an electronic system to monitor the air pressure inside a pneumatic tire. They are sometimes referred to as remote tire pressure monitoring systems (RTPMS) or simply as Run-flat indicators.
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[edit] Types
[edit] Direct
Direct TPMS delivers real time tire pressure information to the driver of the vehicle - either via a gauge or a simple low pressure warning light. These systems employ physical pressure sensors inside each tire and a means of sending that information from inside the tire to the vehicle instrument cluster.
[edit] Indirect
Indirect TPMS measures the air pressure indirectly by monitoring individual wheel speeds and other signals available in the vehicle. Most indirect TPMS uses the fact that an under-inflated tire has a slightly smaller diameter than a correctly inflated tire and therefore has to rotate more times to cover a specific distance to detect under-inflation. Such TPMS can detect under-inflation in up to three tires simultaneously but not in all four since the operating principle of these systems is to compare the different wheel speeds and if all four tires lose the same amount of air the relative change will be zero. Newer developments of indirect TPMS can also detect simultaneous under-inflation in all four tires thanks to vibration analysis of individual wheels or analysis of load shift effects during acceleration and/or cornering. Indirect TPMS is cheap and easy to implement since most modern vehicles already have wheel speed sensors for anti-lock braking systems and electronic stability control systems. The disadvantage is that they rely on the user resetting the system when the tires are changed or re-inflated - forgetting to perform this initialization leads to potentially dangerous false or missing alerts.
[edit] History
The first passenger vehicle to adopt TPMS was the Porsche 959 in 1986, using a system developed by PSK. Major TPMS manufacturers are SmarTire Systems, Wi-Gauge by Banyan International, Siemens VDO, Beru AG, TRW Inc, ETV Corporation PL, Pacific Industries, and Schrader Electronics. Due to vehicle safety and maintenance economy, TPMS appeared more widely in Europe as an optional feature for top range passenger vehicles like the Mercedes S-Class and the BMW 7-series. In 1999 that PSA (Peugeot and Citroën Group) decided to adopt TPM as a standard feature on the Peugeot 607. The following year Renault launched the Laguna II, the first high volume mid-size passenger vehicle in the world to be equipped with TPM as a standard feature. The Firestone recall in the United States in the late 90s that was attributed to more than 100 deaths from roll-overs following tire blow out, pushed the Clinton administration to publish the “TREAD ACT”. This act mandates the use of a suitable TPM technology in order to alert drivers of severe under inflation condition of their tires. This act affects all light motor vehicles (<10,000 lbs) sold after September 1, 2007. Phase-in starts in October 2005 at 20% and ramps up to 70% after September 2006. European and Far East legislators are also expected to rule on the subject even if no signs of legislative activity are forecasted in a near future.
Regardless of U.S. legislation the introduction by several tire manufacturers of run-flat tires makes it mandatory for car manufacturers to fit a system where the drivers are made aware the run-flat has been damaged. The run-flats have received a mixed reception from the public as, along with the harder ride, the TPM systems have not been very accurate.
[edit] Technology
TPM systems are often implemented using Radio Frequency technology to avoid expensive and rather complicated rotating contact wiring, together with an electronic control unit fitted inside the vehicle which provides the necessary processing power to interpret pressure data coming from the sensors and delivers alerts and warnings to the driver. Companies like Schrader have designed a system made of Radio Transmitters sensors mounted on a standard tire valve and a dedicated Radio frequency receiver, whose functions can also be integrated in other Radio Frequency units mounted on the vehicle, such as Remote Keyless Entry receivers, Body Control Units or else.
A typical TPM system employs four or five transmitter-sensors, one RF receiver either stand-alone or integrated in other vehicle electronics, and some other satellite hardware which can absolve to the function of identifying the tire position involved in the inflation anomaly. This satellite hardware can be either active, like a device which can trigger a transmission, or passive, like a simple short range RF listening device. The most technologically challenging part of the system is the TPM transmitter-sensor, or simply TPM sensor. Most TPM sensors on the market today use a lithium battery, a silicon based pressure sensor and an RF oscillator either SAW or PLL based. The heart of the sensor is a silicon ASIC (Application Specific Circuit) chip which can manage all different components of the sensor.
A new generation of battery-less TPMS is being developed. Using SAW technology patented by Transense Technology Limited, a small UK based company, this is now coming to market. Michelin announced in November 2006 that it is using Transense technology in its EtireII commercial tires, and Lear has announced that it will also be using Transense SAW sensors. Other licensees of Transense include Honeywell and Melexis. Texas Instruments has worked closely with Transense to enable its DSP's to intepret information provided by both the TPMS and other SAW based systems.
Respected commentators consider that Transense SAW based technology will soon become the industry standard
[edit] Setting up the system
The TPM system has to be installed and tested in the plant environment. The process is generally as follows. The TPM sensors are attached to the wheel during the Wheel and Tire assembly process. The wheels are then attached to the vehicle. This is the first point at which the TPMS can be clearly associated with the vehicle. It is on this final car assembly line that RF antennas are used to extract the unique IDs of the TPMS. These IDs and their associated wheel position on the car are downloaded to the vehicle Engine Management Unit. This enables warnings to be associated with wheel position in the event of under or over pressure alarms. Similarly the car dealer needs to have portable tools available to extract the TPM ID and reprogram the car's ECU in the event of TPM replacement.
[edit] Performance
A TPM system improves vehicle safety and aids drivers in maintaining their vehicle tires. Properly maintained tires ensure vehicle safety, performance and economy. In the US the National Highway Traffic Safety Administration (NHTSA) has estimated that every year 533 fatalities are caused by tire defects in road accidents. Adding TPM to all vehicles could avoid 120 of the 533 yearly victims and spare as many as 8,400 injuries every year. The French institution called Sécurité Routière (meaning: Road Safety) estimates that 9% off all road accidents involving fatalities are attributable to tire under-inflation and the German DEKRA estimated that 41% of accidents with physical injuries are linked to tire problems. On the maintenance side it is important to remind that fuel efficiency and tire wear are severely affected by under-inflation. Tires leak air naturally and over a year a typical new tire can lose between 200 to 600 milli-bars. If we also consider that over 40% of vehicle owners in Europe and North America check their tires less than once a year it is conceivable to think that 40% or more of currently used vehicles in those areas are running with underinflated tires. If we consider that an average under-inflation of 400 milli-bars produces an increase of fuel consumption of 2% and a decrease of tire life of 25% we can conclude that tire under-inflation today is responsible for over 20 million liters of unnecessary burned fuel dumping over 2 million tons of CO2 in the atmosphere and 200 million tires wasted in the world. For these reasons the US Federal government has ruled to mandate the use of a TPM system and other countries should follow closely.
Generally speaking direct tire pressure monitoring systems offer the following features:
- Measure and display Tire air Pressure with an accuracy able to detect under-inflation conditions of less than 25% of the recommended cold inflation pressure
- Measure and display Tire air Temperature.
- Locate Tire involved in pressure defect (optional).
- React to Fast and Slow leaks (<5secs) for early warning.
- Warn for Punctures.
- Alert for proper tire Maintenance (Air transpiration).
- Do not require initialization or zero button, i.e., self learning. (optional)
- Can monitor spare tire pressure.
- Can monitor tire pressure when stationary and deliver key-on information to the driver.
