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. The target of a TPMS is avoiding traffic accidents, poor fuel economy, and increased tire wear due to under-inflated tires through early recognition of a hazardous state of the tires.

History

Initial adoption

Due to the influence tire pressure has on vehicle safety and efficiency, tire-pressure monitoring (TPM) was first adopted by the European market as an optional feature for luxury passenger vehicles in the 1980s. Some claim the efficiency gains are negligible. The first passenger vehicle to adopt 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, TPM was introduced by General Motors for the 1991 model year for the Corvette in conjunction with Goodyear run-flat tires. The system uses sensors in the wheels and a driver display which can show tire pressure at any wheel, plus warnings for both high and low pressure. It has been standard on Corvettes ever since.

Firestone recall and legal mandates

The Firestone recall in the late 1990s (which was linked to more than 100 deaths from rollovers following tire tread-separation), pushed the United States Congress 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 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 a TPMS. For N1 vehicles, TPMS are not mandatory, but if a TPMS is fitted, it must comply with the regulation.

On July 13, 2010, the South Korean Ministry of Land, Transport and Maritime Affairs announced a pending partial-revision to the Korea Motor Vehicle Safety Standards (KMVSS), specifying that "TPMS shall be installed to passenger vehicles and vehicles of GVW 3.5 tons or less, ... [effective] on January 1, 2013 for new models and on June 30, 2014 for existing models".^[2] Japan is expected to adopt European Union legislation approximately one year after European Union implementation. Further countries to make TPMS mandatory include Russia, Indonesia, the Philippines, Israel, Malaysia and Turkey.^[3]

After the TREAD Act was passed, many companies responded to the market opportunity by releasing TPMS products using battery-powered radio transmitter wheel modules.

Run-flat tires

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 (49.7 mph) and 80 km (49.7 miles) 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 are based on the principle that under-inflated tires have a slightly smaller diameter (and hence higher angular 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 eigenforms 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 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.^[4]

Various iTPMS have since the mandatory fitment of TPMS in the EU in November 2014 been type-approved according to R64. Examples for this are most of the VW group models, but also numerous Volvo, Opel, Ford, Mazda, PSA, FIAT and Renault models. iTPMS are quickly gaining market shares in the EU and are expected to become the dominating TPMS technology in the near future.

iTPMS are regarded as inaccurate by some stances due to their nature, but given that simple ambient temperature variations can lead to pressure variations of the same magnitude as the legal detection thresholds, many vehicle manufacturers and customers value the ease of use and tire/wheel change higher than the theoretical accuracy of dTPMS.

Direct TPMS

Direct TPMS employ pressure sensors on each wheel, 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) 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 exposure to hostile environments. The majority are powered by batteries which limit their useful life. Some sensors utilise a wireless power system similar to that used in RFID tag reading which solves the problem of limited battery life by electromagnetic induction. This also increases the frequency of data transmission up to 40 Hz and reduces the sensor weight which can be important in motorsport applications. If the sensors are mounted on the outside of the wheel, as are some aftermarket systems, they are subject to mechanical damage, aggressive fluids, as well as theft. When mounted on the inside of the rim, they are no longer easily accessible for battery change and the RF link must overcome the attenuating effects of the tire which increases the energy need.

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 do not transmit information when not rotating (which eliminates spare tire monitoring) or apply a complex, expensive two-way communication which enables wake-up of the sensor. For OEM auto dTPMS units to work properly, they need to recognize the sensor positions and must ignore the signals from other vehicles. There are numerous tools and procedures to make the dTPMS "learn" or "re-learn" this information, some driver implemented, others done by workshops. The cost and variety of spare parts, procedures and tools has led to trouble and confusion 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 commercial vehicles. Many aftermarket dTPMS units do not require specialized tools to program or reset, making them much simpler to use.

First-generation dTPMS sensors that are integral with the valve stem have suffered from corrosion.^[5][6] Metallic valve caps can become seized to the valve stem because of galvanic corrosion of dissimilar metals, and efforts to remove it can break the stem, destroying the sensor. A similar fate may befall an after-market brass valve core inside the stem that may have been installed by unwary technician, replacing the original specialized nickel-coated cores. (They can be distinguished by the yellowish colour of the brass.) Seizure of the valve can complicate repair of a tire leak, possibly requiring replacement of the entire sensor.

There is controversy regarding the compatibility of after-market tire sealants with dTPMS that employ sensors mounted inside the tire. Some manufacturers of sealants assert that their products are indeed compatible,^[7] but others warned that the "sealant may come in contact with the sensor in a way that renders the sensor TEMPORARILY inoperable until it is properly cleaned, inspected and re-installed by a tyre care professional".^[8] Such doubts are also reported by others.^[9][10] Use of such sealants may void the TPMS sensor warranty.^[7]

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 m³) 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 pounds per square inch (69 kPa; 0.69 bar) in pressure on a set of duals literally drags the lower pressured tire 2.5 metres per kilometre (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 26 billion kilograms (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.

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 CO₂ into the atmosphere, and for 200 million tires being prematurely wasted worldwide.

Privacy concerns regarding TPMS

Because each tire transmits a unique identifier, vehicles may be easily tracked using existing sensors along the roadway.^[11] This concern could be addressed by encrypting the radio communications from the sensors but such privacy provisions were not stipulated by the NHTSA.

Heavy-duty vehicles

U.S. National Highway Traffic Safety Administration regulations ^[12] only apply to vehicles under 10,000 pounds. For heavy duty vehicles (Classes 7 and 8, gross vehicle weight [GVW] greater than 26,000 pounds), most of the above-mentioned systems don't work well, requiring the development of other systems.

The US Department of Transportation has commissioned several studies to find systems that work on the heavy duty market specifying some goals that were needed in this market.^[13][14]

The SAE has tried to disseminate best practices, since legal regulations for heavy vehicles has been lagging.^[15]

One problem is lack of standardization. Tires are often purchased in bulk and moved between tractors over time, so a given TPMS system can only work with compatible sensors in the tires, creating logistic problems. RF systems for these units must also work over much longer ranges, which may force repeater systems to be installed on the tractor or trailer. It is expected that battery lives on these systems should be in the five- to seven-year range, since the cost of breaking down a tire can be so much more expensive. The U.S. Department of Transportation's maximum-loading requirements force trailer manufacturers to spread loads over multiple axles, giving rise to trailers with typically 8 to 12 tires, but as many as 96 tires on specialty haulers.

Tire casings can have typical lifetimes of ten or more years, through multiple retreading processes. This has given rise to a specialized industry that focuses solely on issues found in the trucking industry.

Central inflation systems originally claimed to eliminate the need for pressure-monitoring systems. Some major inflation systems are Meritor PSI, Hendrickson International, Stemco AERIS and Vagia (used mostly in South America). They have not yielded a complete solution, since they do not solve all the issues (i.e., no support for the steerable axle), and they bring new issues with maintenance of the rotary couplings in the hub caps. Inflation systems can sometimes shorten the life of tires by concealing slow leaks caused by embedded objects, which drivers would otherwise remove after inspecting the problem tire.

For a tire-pressure sensor to be completely effective, it must have several capabilities to allow for the various maintenance personnel groups to use them.

First, each driver is required to do a pre-trip inspection, so it is beneficial if the tire-pressure monitor has an indicator that can be read without tools.

Second, it usually should have the ability to cover dual sets of tires in some fashion. It is also beneficial if the fill points can be centralized so that inflation can be accomplished easily without reaching through the small hand holes in the rims.

Third, it needs to have a wireless communication system that has an appropriate range and battery life. It is important that sensors regularly communicate an "I'm alive" condition, since having a dead sensor can be worse than having no sensor at all.

Fourth, these systems should have the capability to adapt to the changing of tires and trailers with minimal operator intervention. It is important to use a system having a longer range, since a repeater increases cost.

These requirements can be met by systems with external pressure sensors that connect to the valve stem on each tire. When tires are replaced, the sensor is moved to the new tire.

Although these systems can alert a driver to a hazardous blowout condition, they may not help fleets deal with slow-leaking tires, unless the driver reports them to fleet-maintenance personnel before it is too late. This has given rise in recent years to monitoring solutions that track the tire condition and send alerts to fleet-maintenance personnel. This allows them to schedule maintenance on a slow-leaking tire on an exception basis, instead of having to check each tire manually. Many fleets today admit that tire-pressure checking is a major problem in enforcement. Most have policies in place requiring the regular check of every tire, however, the practice is not terribly effective because of the sheer scope of the issue, and the fact that it is hard to get a complete record of all tire checking.

Today, the best systems employ automated data collection. Some of these use gate readers that automate the collection of tire data to a database, or to a web portal, that allows maintenance operators to see data for the entire fleet at a glance. For long-haul fleets that may not see their vehicles for long periods of time, a centralized reading system may not work, but there are emerging systems that aggregate the tire-pressure-sensor data back to the asset-tracking system so that alerts can be sent back to the main office when an issue arises. For small fleets, handheld devices exist that allow a person checking tires to simply walk around vehicles and collect data for downloading to a central database, allowing enforcement and trending to be done without errors.

Some automotive manufacturers have attempted to broaden their scope into the heavy-duty markets, a few manufacturers have focused solely on this market.

Icons

TPMS system dashboard icons
TPMS low pressure warning icon  TPMS system failure icon

See also

• Central Tire Inflation System
• Cold inflation pressure
• NIRA Dynamics AB
• Tire-pressure gauge
• Automobile safety

References