Tire pressure monitoring system - AbsoluteAstronomy.com

General

A Tire Pressure Monitoring System (TPMS) is a system designed to monitor the air pressure inside the pneumatic tires on various types of vehicles. These systems report the tire pressure information to the driver of the vehicle.

Target

Avoiding traffic accidents due to under-inflated tires by early recognition of the malfunction of tires
Reducing CO2 emission and reducing tire abrasion by an optimal inflation

System solutions

To approach the targets, two different systems are currently available, direct system and indirect system.

Direct systems

Direct TPMS (based on MEMS technology) employ physical pressure sensors inside each tire and a means of processing and sending that information to a central processing unit in the vehicle. These systems can recognize simultaneously under-inflation in all four tires in any combination.
Direct TPMS are specifically designed to cope with temperature changes which influence the tire pressure. The pressure thresholds for the warning activation are usually derived from the manufacturers recommended "cold placard inflation pressures".
In order to transfer the data from a rotating wheel, a direct TPMS use a radiofrequency communication channel.
A direct TPMS sensor consists of following main functions requiring only a few external components (e.g. battery, housing, PCB) to get the sensor module which is mounted to the valve inside the tire:

Pressure sensor
Analog-Digital Converter
Microcontroller
System Controller
Oscillator
Radio Frequency Transmitter
Low Frequency Receiver
Voltage regulator (battery management)

To prevent corrosion due to electrolysis

Electrolysis

In chemistry and manufacturing, electrolysis is a method of using a direct electric current to drive an otherwise non-spontaneous chemical reaction...

and malfunction of the electronics in a direct TPMS and avoid costly repairs, metal valve caps without insulating plastic linings should not be used on direct-TMPS-equipped tire valve stems http://www.31inc.com/catalog.

Indirect systems

Indirect TPMS do not use physical pressure sensors. By comparing individual wheel rotational speeds (provided by the ABS), the indirect TPMS determines a deviation of an under-inflated tire due to its higher rotational speed. Depending on the computational effort and the use of additional signals existing in the vehicle, the reliability and accuracy of an indirect system can be further improved e.g. applying a frequency analysis of the jitter of the ABS signal. There is still considerable debate as to whether the indirect systems will be accurate enough to achieve the objectives of the mandatory regulations.

Advantages and disadvantages

	Direct TPMS	Indirect TPMS
Accuracy	Higher	Lower
Compensation with the in-tire temperature	Yes	No
Availability of pressure value	Yes	No
Additional components	Yes	No
Required interaction with the driver (e.g. after tire type change, inflation change, uneven load distribution)	Low	Higher
Simultaneous recognition of more than one under-inflated tire in any combination	Yes	Yes (with frequency analysis of ABS signal jitter)
Reliable under all driving conditions (sportive, offroad, slippery)	Yes	No
Detection time	Faster	Slower
Localization of the under-inflated tire	Yes (e.g. with combination of the ABS signal)	Yes

Legislation

In the U.S., the U.S. Department Of Transportation (NHTSA) released the FMVSS No. 138, which rules an installation of a Tire Pressure Monitoring System to all new passenger cars, multipurpose passenger vehicles, trucks, and buses that have a gross vehicle weight rating (GVWR) of 4,536 kg (10,000 lbs.) or less, except those vehicles with dual wheels on an axle in year 2007.
In EU, starting 2012, all new models of passenger cars must be equipped with a TPMS with even tighter specification that will be defined by the UNECE Vehicle Regulations (Regulation No. 64).
Korea will also be seen to introduce TPMS, likely in 2013 and Japan will be expected to adapt EU legislation approximately one year after EU rollout.