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Tire pressure monitoring system

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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.

1 Types
- 1.1 Direct
- 1.2 Indirect
2 History
3 Technology
4 Setting up the system
5 Performance
6 See also
7 External links

Types

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.

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 by pushing a "Calibration Button" when the tires are changed or re-inflated—forgetting to perform this initialization leads to potentially dangerous false or missing alerts. Another disadvantage of indirect TPMS is that if the Calibration Reset Button is pressed when one or more tires are under-inflated then the system accepts this under-inflation as normal and the driver will be unaware of potentially dangerous tire pressures.

History

The first passenger vehicle to adopt tire pressure monitoring (TPM) was the Porsche 959 in 1986, using a system developed by PSK. Major TPMS manufacturers are SmarTire Systems, Shanghai Baolong Industries, Wi-Gauge wireless TPMS, Siemens VDO, Beru AG, TRW Inc, ETV Corporation PL, Pacific Industries, Schrader Electronics, EnTire Solutions LLC, NIRA Dynamics AB, Transense Technologies plc, and IQ-mobil GmbH (RDS®), APRI s.r.o. 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 the 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 1990s that was attributed to more than 100 deaths from roll-overs following a tire tread-separation, 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 a severe under-inflation condition of their tires. This act affects all light motor vehicles (<10,000 lb) sold after September 1, 2007. Phase-in started in October 2005 at 20% and reaches 100% for models produced after September 2007. Whilst in the US TPMS legislation grew from safety-related motives, European Union (EU) and Far East legislators are looking at TPMS as a way of reducing CO₂ emissions and are presently considering mandatory tire pressure monitoring from this environmental stance. Regardless of US and EU 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 are designed to be used at no more than 80 km/h (50 mph) for no more than a distance of 80 km and this is why it is mandatory that runflats are monitored by TPMS. They have received a mixed reception from the public due to their impact on comfort with a harder ride. Lastly, the most recent advance with TPMS technology is the introduction of Battery-less Direct sensor sytems which require zero maintenance and are very reliable. VisiTyre is the first of this new class of battery-less TPMS which allows pressure on demand readings immediately from ignition and unlike radio frequency TPMS systems is also transparent to all tire construction types.

Technology

Early TPM systems were implemented using radio frequency (RF) 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 functionality to interpret pressure data coming from battery powered sensor transmitters within tire cavities. The system delivers alerts and warnings to the driver. Companies like Schrader Electronics designed first generation TPM systems using battery powered radio transmitters with sensors mounted on a standard tire valve and a chassis mounted radio frequency receiver, whose functions can also be integrated in other radio-frequency units mounted on the vehicle, such as Remote Keyless Entry receivers and Body Control Units. Typical RF TPM systems employ four or five battery powered 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. Each tire pressure sensor can periodically trigger a transmission of pressure status, or be polled continuously on demand. The most technologically challenging part of the system is the conservation of battery power used by the RF transmitter-sensor. Most RF based TPM sensors on the market today use a battery, a silicon-based pressure sensor and an RF oscillator either SAW- or PLL–based. Automakers require a battery lifespan of between seven and ten years, so TPMS system designers use power saving techniques to extend the battery life. The heart of the sensor is a silicon application-specific integrated circuit (ASIC) chip which can manage critical power saving algorithms and other functions of the sensor. However there remains the fundamental problem that all batteries eventually become exhausted and this represents safety and replacement cost issues for the consumer. In the US there are approximately 16 million new passenger vehicles manufactured annually which must ultimately comply with the legislative requirements of the TREAD Act and be fitted with TPMS. If each vehicle has four or five wheels fitted with battery-powered RF TPMS wheel modules there could be more than 65 million batteries introduced annually into the environment. Consumer abuse and disposal phases of the batteries' life cycles in such a widespread consumer applications is of minor environmental concern when juxtaposed to the proliferation of general application batteries such as AA size. A new generation of battery-less TPMS is being developed by two companies using quite different technologies. Transense uses a SAW-based technology and VisiTyre use an electromagnetic close coupling technology to effectively eliminate the battery. VisiTyre battery-less TPMS is in pre-production preparation for supply and integration into model year 2009 vehicle platforms. Transense has licenced its technology to several automotive companies but it is not yet commercially available for OE passenger vehicles. Pirelli announced in Spring 2007 they had developed a new energy harvesting device which they say will allow them to produce a battery-less TPMS tire. Recently STE Engineering, a company based in the North of Italy, introduced a new class OEM oriented TPMS tire stem whose concept is basically the integration of a simple hybrid ceramic circuit inside the body of a standard tire stem, as opposed to traditional TPMS applications implementing electronic PCB mechanically residing in a dedicated plastic box, just beneath the tyre stem itself. Advantages of this solution are obviously connected to the use of an ISO-TS qualified tire stem, as normally used in the automotive market, allowing huge cost savings and enabling a real standard in matter of remote direct TPMS. Due to extremely reduced power consumption, measured to be about three orders of magnitude low power less than standard technologies, this new application allows use of reduced size battery cells: infact now a 12.5mm diameter standard cell replace the common used 20mm normally used; STE says that, being able to "survive" feeded by the very small energy harvesters devices normally generate, this new technology supports "Battery-less" operating applications and, at the same time, introducing a new methodology which sees "in-tyre" electronics implementation rather than "attached-to-the-stem" technology. Other benefits are: reduced overall weight, mechanical robustness, minimization of electronic and consequent reduction of costs, extended temperature range accordingly to automotive requirements (-40° +125°C).

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. In the case of battery powered RF systems, 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. Similarly the car dealer needs to have portable tools available to extract the wheel sensor ID and in the case of battery powered TPM systems, reprogram the car's ECU as would be required for wheel sensor module replacement when a battery fails. Service to the vehicle tires may also result in required replacement of a TPM due to valve core corrosion, broken band or other issue. When a TPM is replaced it is important to understand your vehicle. Every manufacturer has a different method with which to reprogram the vehicle. Some vehicles simply reprogram themselves while you drive. Others require a method with which to perform this service, such as turning the key and stomping a peddle or using the key-fob to trigger a re-learn mode. Not all vehicles may be placed into a re-learn mode, some require an extra interface to the vehicles OBDII/CAN-BUS to speak with the vehicle ECU, BCM or other device. Vehicles requiring this interface force you to return to your dealership for a simple tire rotation.

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% of 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 realize 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 20 and 60 kilopascals. 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 40 kPa 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^[vague] of CO₂ in the atmosphere and 200 million tires prematurely wasted in the world. For these safety and environmental reasons the US Federal government has ruled to mandate the use of a TPM system and other countries should follow closely. The TPMS mandated by the US law must warn the driver when a tire is under-inflated by as much as 25%. However, since the recommended tire pressures for most vehicles are more than 160 kPa (23 psi), a deflation of 40 kPa would be within the 25% allowance and would not trigger TPMS warning mandated by the U.S. law at all. In other words, the mandated TPMS is mainly designed for safety and is unlikely to deliver the above benefits. The drivers are still advised to manually check their tire pressure often to maintain optimal performance. In the case of battery powered TPMS at some point in time, and within the vehicle's lifetime, every battery will ultimately become exhausted and there will be an ‘unsafe’ window where the system is unavailable. Battery lifetime is adversely affected by sub zero temperature extremes which occur in Europe and North America. Hence, vehicle manufacturers are showing great interest in the next generation of battery-less TPM systems being developed by Transense and VisiTyre. Generally speaking direct tire pressure monitoring systems offer the following features:

Measure (and may 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 (optional).
Locate Tire involved in pressure defect (optional).
React to fast and slow leaks (< 5 s) 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.