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.
[] IndirectIndirect 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 and . The disadvantage is that they rely on the user resetting the system by pushing a "Calibration Button" when the tires are changed or re-inflatedforgetting 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.
Technology
Early TPM systems were implemented using (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 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 or based.
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" fed by the very small energy harvesters devices are able to 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).
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 S�curit� Routi�re (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 . 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[] 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:
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Tire Pressure Monitoring System
