Wikipedia threw up this:
A Tire Pressure Monitoring System (TPMS) is generally an electronic system designed to monitor the air pressure inside all the pneumatic tires on automobiles, aeroplane undercarriages, straddle lift carriers, forklifts and other vehicles. The system is also sometimes referred to as a Tire Pressure Indication System (TPIS). These systems report real time tire pressure information to the driver of the vehicle - either via a gauge, a pictogramme display, or a simple low pressure warning light.
Types
Direct
Direct sensor TPMS: These systems employ physical pressure sensors inside each tire, and a means of processing and sending that information from inside the tire to the vehicle's instrument cluster.
Direct Sensor TPMS can identify simultaneous underinflation in all four tires in any combination.
Direct sensor TPMS are specifically designed to cope with the effects of changes in tire pressure due to ambient temperature changes and road to tire friction based temperature changes. Friction between the tire and road surface heats up the tire and increases the pressure in the tire. The alarm activation threshold pressures are usually set according to the manufacturers recommended "cold placard inflation pressures".
In order to transfer data from a rotating wheel, Direct Sensor TPMS may use a Radio Frequency (RF) communication channel or an electromagnetic coupling means to overcome the tire/chassis rotational boundary.
The pressure sensor devices of Direct sensor TPMS may be either battery powered or batteryless: Battery powered Radio Frequncy based TPMS have several disadvantages. Batteries eventually become exhausted and represent a maintenance cost to the consumer. Batteries are chemical systems with lifetimes that unfortunately perform very poorly in extreme temperature environments typical of North America and European climates and in aerospace applications. In order to conserve battery life and to conform to various countries communication authority standards for short range radio communications, the power levels of battery powered TPMS are kept very low . As a consequence of these very low power levels, the construction of certain steel belted radial replacement tires and vehicle metallic geometries can block the low power signal transmission paths.
Other disavantage of battery powered direct sensor TPMS are that their physical sensors are quite large and are either mounted on the end of valve stems or by a steel band around a rim's dropwell center. In both cases, these sensors affect a wheel's balance are subject to damage during tire removal and fitting procedures. Banded sensors may also damage the tire bead's air seal.
On the other hand Batteryless Direct Sensor TPMS overcome these limitations and have the advantage of being maintenance free.
Indirect
Indirect TPMS do not use physical pressure sensors. Indirect TPMS measure the "apparent" air pressure, by monitoring individual wheel rotational speeds, and other signals available outside the tire itself. Most indirect TPMS use the fact that an under-inflated tire has a slightly smaller diameter than a correctly inflated tire and therefore has to rotate at a higher angular velocity to cover the same distance as a correctly inflated tire. Newer developments of indirect TPMS can also detect simultaneous under-inflation in up to all four tires using vibration analysis of individual wheels or analysis of load shift effects during acceleration and/or cornering, which can be realized in software using advanced signal processing techniques. However the vibration analysis technique requires the use of additional suspension sensors which result in increased complexity and cost of the overall system.
Indirect TPMS are realized in software in combination with wheel speed sensors for anti-lock braking systems, and electronic stability control systems. A disadvantage of indirect TPMS is that the driver must calibrate the system by pushing a reset button on the dashboard or through the on-board computer and if this is performed when any tire is in an underinflated condition then the system will not report correctly.
Direct TPMS Technology
In the early days of development TPMSs were 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 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 TPMSs 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,[6] one RF receiver (either stand-alone or integrated in other vehicle electronics), and some other satellite hardware which can perform 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 with the result that the consumer is faced with flat battery problems as well as flat tire problem. The battery represents safety and replacement cost issues for the consumer. Vehicle manufacturers are also concerned about costly warranty claims and litigation that may result if injury and loss of property occur as a consequence of RF based TPMS battery failures.[6]
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. Disposal of the batteries in such a widespread consumer application is a minor environmental concern compared to the other wastes from automobile operation.
To overcome the battery issues a new generation of batteryless TPMS is being developed by two companies using quite different technologies. Transense is promoting a SAW-based technology. VisiTyre is using an electromagnetic close-coupling technology to effectively eliminate the battery. VisiTyre batteryless 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 OEM passenger vehicles.
Other developments with TPMS include research into the use of energy harvesting devices which may lead to future types of batteryless TPMS.
Recently STE Engineering, a company based in the North of Italy, introduced a new class of 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 which have an electronic PCB located in a dedicated plastic box, just beneath the tire 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 standardization 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 in fact, now a 12.5 mm diameter standard cell can replace the 20 mm cell normally used. STE says that, being able to "survive" fed by the very small energy harvesters devices are able to generate, this new technology approaches the highly desirable "Battery-less" operating condition Bridgestone, Eoplex and, at the same time, introduces a new methodology which sees in-stem electronics rather than "attached-to-the-stem" technology. Other benefits are: reduced overall weight, mechanical robustness, cost reduction, and extended temperature range (-40° +125°C). Because the limited consumption this technology enable "battery-less" "in-tyre" units.
Setting up the system
The TPMS has to be installed and tested in the vehicle manufacturing environment. The process is generally as follows. The TPMS 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[citation needed]. 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[citation needed]. This enables warnings to be associated with wheel position.
Similarly, the franchised car dealer workshop needs to have portable tools available to extract the wheel sensor ID, and in the case of battery powered TPMS, 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 require replacement of a TPMS sensor due to valve core corrosion, a broken band, or other issues.
When a TPMS sensor is replaced, it is important to understand your vehicle. Every manufacturer has a different method to reprogram the vehicle. Some vehicles simply reprogram themselves while you drive. Others require the user to perform an action, such as turning the key and pressing a pedal, 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[citation needed]. Vehicles with this interface require the user to return to the dealership for a tire rotation[citation needed].
Direct sensor batteryless TPMSs, such as the VisiTyre system, are zero-maintenance direct sensor TPMS that do not require any recalibration after tire replacement or tire rotation.
Performance
A TPMS helps to improve vehicle safety, and aids drivers in maintaining their vehicle tire pressures. Properly maintained tires help with 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[citation needed]. Adding TPMS to all vehicles could avoid 120 of the 533 yearly victims and spare as many as 8,400 injuries every year[citation needed].
The French Sécurité Routière (Road Safety organization) estimates that 9% of all road accidents involving fatalities are attributable to tire under-inflation[citation needed], and the German DEKRA estimated that 41% of accidents with physical injuries are linked to tire problems[citation needed].
On the maintenance side, it is important to realize that fuel efficiency, and tire wear are severely affected by under-inflation. In the US NHTSA data relates that 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[citation needed], it is conceivable that 40% or more of vehicles currently in use in those areas are running with underinflated tires.
The European Union reports that an average under-inflation of 40 kPa produces an increase of fuel consumption of 2% and a decrease of tire life of 25%. The EU concludes conclude that tire under-inflation today is responsible for over 20 million liters of unnecessary burned fuel, dumping over 2 million tonnes of COâ‚‚in the atmosphere, and 200 million tires prematurely wasted in the world.
For these safety and environmental reasons, the US Federal government has mandated the use of TPMS, 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% {Ref: US DOT NHTSA Docket No 2005-20586}. 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 the TPMS warning mandated by the U.S. law. Therefore, the mandated TPMS is mainly designed for safety and is unlikely to deliver the above benefits. 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 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 certain areas of Europe and North America. Hence, vehicle manufacturers are showing a great interest in the next generation of battery-less TPM systems being developed by Transense and VisiTyre.
Generally speaking, direct tire pressure monitoring systems may 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
Do not require initialization or zero button, i.e., self learning (optional)
Can monitor spare tire pressure{Ref: VisiTyre Batteryless TPMS}
Can monitor tire pressure when stationary (Direct TPMS only).
