TIRE PRESSURE MONITORING SYSTEM
A
tire pressure monitoring system (
TPMS) is an
electronic system designed to monitor the
air pressure inside the
pneumatic tires on various types of vehicles. TPMS report real-time tire-pressure information to the driver of the
vehicle, either via a gauge, a pictogram display, or a simple low-pressure warning light. TPMS can be divided into two different types —
direct (dTPMS) and indirect (iTPMS). TPMS are provided both at an OEM (factory) level as well as an aftermarket solution.
History
Due to the significant influence tire pressure has on vehicle safety and efficiency, TPMS was first adopted by the European market as an optional feature for
luxury passenger vehicles in the 1980s. The first
passenger vehicle to adopt tire-pressure monitoring (TPM) was the
Porsche 959 in 1986, using a hollow spoke wheel system developed by PSK. In 1996 Renault used the Michelin
PAX system[1] for the Scenic and in 1999 the
PSA Peugeot Citroën decided to adopt TPM as a standard feature on the
Peugeot 607. The following year (2000),
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.
In the United States, the
Firestone recall in the late 1990s (which was linked to more than 100 deaths from
rollovers following tire tread-separation), pushed the
Clinton administration to legislate the
TREAD Act. The Act mandated the use of a suitable TPMS technology in all light motor vehicles (under 10,000 pounds), to help alert drivers of severe under-inflation events. This act affects all light motor vehicles sold after September 1, 2007. Phase-in started in October 2005 at 20%, and reached 100% for models produced after September 2007. In the United States, as of 2008 and the European Union, as of November 1, 2012, all new passenger car models (M1) released must be equipped with a TPMS. From November 1, 2014, all new passenger cars sold in the European Union must be equipped with TPMS. For N1 vehicles, TPMS are not mandatory, but if a TPMS is fitted, it must comply with the regulation.
After the Tread Act was passed, many companies responded to the new market opportunity by releasing TPMS products that use an obvious means of getting tire pressure and temperature data across a vehicle's rotating wheel-chassis boundary — battery-powered radio transmitter wheel modules.
The introduction of
run-flat tires and emergency spare tires by several tire and vehicle manufacturers has motivated to make at least some basic TPMS mandatory when using run flat tires. With run flat tires, the driver will most likely not notice that a tire is running flat, hence the so-called "run flat warning systems" were introduced. These are most often first generation, purely roll-radius based iTPMS, which ensure that run-flat tires are not used beyond their limitations, usually 80 km/h and 80 km driving distance. The iTPMS market has progressed as well. Indirect TPMS are able to detect under-inflation through combined use of roll radius and spectrum analysis and hence four-wheel monitoring has become feasible. With this breakthrough, meeting the legal requirements is possible also with iTPMS.
Direct vs. indirect
Indirect TPMS
Indirect TPMS do not use physical pressure sensors but measure air pressures by monitoring individual wheel rotational speeds and other signals available outside of the tire itself. First generation iTPMS systems utilize the effect that an under-inflated tire has a slightly smaller diameter (and hence lower tangential velocity) than a correctly inflated one. These differences are measurable through the wheel speed sensors of ABS/ESC systems. Second generation iTPMS can also detect simultaneous under-inflation in up to all four tires using spectrum analysis of individual wheels, which can be realized in software using advanced signal processing techniques. The spectrum analysis is based on the principle that certain Eigen forms and frequencies of the tire/wheel assembly are highly sensitive to the inflation pressure. These oscillations can hence be monitored through advanced signal processing of the wheel speed signals. Current
[when?] iTPMS consist of software modules being integrated into the ABS/ESC units.
iTPMS cannot measure or display absolute pressure values, they are relative by nature and have to be reset by the driver once the tires are checked and all pressures adjusted correctly. The reset is normally done either by a physical button or in a menu of the on-board computer. iTPMS are, compared to dTPMS, more sensitive to the influences of different tires and external influences like road surfaces and driving speed or style. The reset procedure, followed by an automatic learning phase of typically 20 to 60 minutes of driving under which the iTPMS learns and stores the reference parameters before it becomes fully active, cancels out many, but not all of these. As iTPMS do not involve any additional hardware, spare parts, electronic or toxic waste as well as service whatsoever (beyond the regular reset), they are regarded as easy to handle and very customer friendly.
[2]
According to
Nira, based on their request to
TÜV SÜD to do a pre-test according to similar requirements of the EU legislation, the iTPMS system passed that pre-test.
[3] However, the full test procedure as required by the EU regulation, completed by the regulatory body assigned to make the homologation, has not yet been done. Manufacturers like Dunlop Tech also claim their products to fulfill the regulations.
[4]
iTPMS are widely regarded as inaccurate due to the nature of which they obtain their pressure readings. As such, most TPMS units now on the market are of the Direct type.
Direct TPMS
Direct TPMS employ pressure sensors on each tire, either internal or external. The sensors physically measure the tire pressure in each tire and report it to the vehicle's instrument cluster or a corresponding monitor. Some units also measure and alert temperatures of the tire as well. These systems can identify under-inflation in any combination, be it one tire or all, simultaneously. Although the systems vary in transmitting options, many TPMS products (both OEM and aftermarket solutions) can display real time tire pressures at each location monitored whether the vehicle is moving or parked. There are many different solutions but all of them have to face the problems of limited battery lifetime and exposure to tough environments. If the sensors are mounted on the outside of the wheel, which is the case for some aftermarket systems, they are in danger of mechanical damage, aggressive fluids and other substances as well as theft. If they are mounted on the inside of the rim, they are no longer easily accessible for service like battery change and additionally, the RF communication has to overcome the damping effects of the tire which additionally increases the need for energy.
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 that is mounted to the valve stem inside the tire:
- pressure sensor;
- analog-digital converter;
- microcontroller;
- system controller;
- oscillator;
- radio frequency transmitter;
- low frequency receiver, and
- voltage regulator (battery management).
Most originally fitted dTPMS have the sensors mounted on the inside of the rims and the batteries are not exchangeable. With a battery change then meaning that the whole sensor will have to be replaced and the exchange being possible only with the tires dismounted, the lifetime of the battery becomes a crucial parameter. To save energy and prolong battery life, many dTPMS sensors hence do not transmit information when not rotating (which also keeps the spare tire from being monitored) or apply a complex and expensive two-way communication which enables an active wake-up of the sensor by the vehicle. For OEM auto dTPMS units to work properly, they need to recognize the sensor positions and have to ignore the signals from other vehicles' sensors. There are hence numerous tools and procedures to make the dTPMS "learn" or "re-learn" this information, some of them can be carried out by the driver, others need to be done by the workshops or even require special electronic tools. The cost and variety of spare parts, procedures and tools has led to much trouble and confusion both for customers and workshops.
Aftermarket dTPMS units not only transmit while vehicles are moving or parked, but also provide users with numerous advanced monitoring options including data logging, remote monitoring options and more. They are available for all types of vehicles, from motorcycles to heavy equipment, and can monitor up to 64 tires at a time, which is important for the commercial vehicle markets. Many aftermarket dTPMS units do not require specialized tools to program or reset, making them much simpler to use.
Benefits of TPMS
The dynamic behavior of a pneumatic tire is closely connected to its inflation pressure. Key factors like braking distance and lateral stability require the inflation pressures to be adjusted and kept as specified by the vehicle manufacturer. Extreme under-inflation can even lead to thermal and mechanical overload caused by overheating and subsequent, sudden destruction of the tire itself. Additionally, fuel efficiency and tire wear are severely affected by under-inflation. Tires do not only leak air if punctured, they also leak air naturally, and over a year, even a typical new, properly mounted tire can lose from 20 to 60
kPa (3 to 9
psi), roughly 10% or even more of its initial pressure.
The significant advantages of TPMS are summarized as follows:
Fuel savings: According to the GITI, for every 10% of under-inflation on each tire on a vehicle, a 1% reduction in fuel economy will occur. In the United States alone, the Department of Transportation estimates that under inflated tires waste 2 billion US gallons (7,600,000 m
3) of fuel each year.
Extended tire life: Under inflated tires are the #1 cause of tire failure and contribute to tire disintegration, heat buildup, ply separation and sidewall/casing breakdowns. Further, a difference of 10 lbs. in pressure on a set of duals literally drags the lower pressured tire 13 feet per mile. Moreover, running a tire even briefly on inadequate pressure breaks down the casing and prevents the ability to retread. It is important to note that not all sudden tire failures are caused by under-inflation. Structural damages caused, for example, by hitting sharp curbs or potholes, can also lead to sudden tire failures, even a certain time after the damaging incident. These cannot be proactively detected by any TPMS.
Decreased downtime and maintenance: Under-inflated tires lead to costly hours of downtime and maintenance.
Improved safety: Under-inflated tires lead to tread separation and tire failure, resulting in 40,000 accidents, 33,000 injuries and over 650 deaths per year. Further, tires properly inflated add greater stability, handling and braking efficiencies and provide greater safety for the driver, the vehicle, the loads and others on the road.
Environmental efficiency: Under-inflated tires, as estimated by the Department of Transportation, release over 57.5 billion pounds of unnecessary carbon-monoxide pollutants into the atmosphere each year in the United States alone.
Further statistics include:
The French
Sécurité Routière, a road safety organization, estimates that 9% of all road accidents involving fatalities are attributable to tire under-inflation, and the German
DEKRA, a product safety organization, 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 United States, NHTSA data relate that tires leak air naturally, and over a year, a typical new tire can lose from 20 to 60
kPa (3 to 9
psi), roughly 10% or more of its initial pressure.
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 European Union concludes that tire under-inflation today is responsible for over 20 million liters of unnecessarily-burned fuel, dumping over 2 million tonnes of CO
2 into the atmosphere, and for 200 million tires being prematurely wasted worldwide.
