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. 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 pictogram display, or a simple low-pressure warning light. TPMS systems can be divided into two different types; "direct" and "indirect". Direct systems can be further classified into battery powered and battery-less systems. TPMS are provided both at an OEM (factory) level as well as aftermarket solution.
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Due to the significant influence tire pressure has on vehicle safety and efficiency, TPMS was first adopted widely 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 to 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 U.S., as of 2008 and the EU, as of 2012, all new models of passenger cars must be equipped with a TPMS.
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. Today, a plethora of different systems, each with product capabilities and options, are sold in transportation markets worldwide, providing tire pressure monitoring for all vehicle types, sizes and configurations.
TPMS support for OEM (factory) systems has resulted in a confusing patchwork of proprietary implementations in the automotive industry sector which is vast and includes vehicle manufacturers and assembly plants, OEM dealers, tire fitters, auto electricians and the vehicle owners and drivers. Information and training in diagnosing OEM TPMS faults, correctly repairing/replacing the multitude of different TPMS components and in the different TPMS “Learn tools” and activation and reset procedures has become complex.
Regardless of U.S. and EU legislation, the introduction by several tire manufacturers of run flat tires and space saver tires has required TPMS to be mandatory. When under-inflated or "flat" the run-flats are designed to be used at no more than 80 km/h (50 mph) for no more than a distance of 80 km. The requirement is similar for space saver tires. Furthermore, transportation communities are realizing that visual inspections and “tire kicking” are not adequate means of testing, resulting in commercial vehicles utilizing TPMS, although not required by law.
In recent years, several advancements have been made in the TPMS market, especially in the commercial vehicle and aftermarket segments. New developments include battery-less systems, such as those supplied by VisiTyre TPMS,[2] which use electromagnetic coupling technology to power sensors.
STE Engineering has introduced an energy efficient wireless sensor device based on a technology called SPX (Short Pulse Technology) which integrates a hybrid ceramic circuit inside the body of a standard tire stem.
Direct TPMSs employ pressure sensors, either internal or external, on each tire which physically measure the tire pressure in each tire and report that information to the vehicle's instrument cluster or a corresponding monitor. These systems can identify under-inflation situations, simultaneous or singly, in any combination. Though systems vary in reporting options, many TPMS products (especially aftermarket solutions) can display real time tire pressures at each location monitored whether the vehicle is moving or parked.
Direct-sensor TPMS are specifically designed to cope with ambient and road-to-tire friction-based temperature changes, both of which heat up the tire, and increase its pressure. The alarm-activation threshold pressure for OEM systems is usually set according to the manufacturer's recommended "cold placard inflation pressures". Aftermarket system thresholds, which vary per system, can either be set at the factory or can be set by the customer during installation.
Direct sensors require power and may be either battery-powered or battery-less. In order to transfer data to the monitor or display, most direct systems utilize sensors having battery powered radio-frequency (RF) communication to transmit pressure readings and other data collected from the sensors. VisiTyre TPMS is a battery-less technology that employs maintenance free electromagnetic couplings to overcome the tire/chassis rotational boundary and eliminate "learn tool" reset and calibration procedures.
Generally, the pressure sensors used in direct-sensor TPMS can be installed inside the tire (internal sensors) or mounted on the valve stem (external sensor). Internal sensors have to be fitted to the wheel rim or to the in-tire section of the valve stem when the tire is fitted, requiring tires to be dismantled when installing or maintaining sensors. Additional interference from the metal of a tire typically requires that TPMS systems utilizing internal sensors use additional antennae to get signals to the cab on larger vehicle configurations. In comparison, external sensors are installed by being screwed onto the tire’s valve stem, replacing the dust cap, but are vulnerable to theft and physical damage.
Indirect TPMS do not use physical pressure sensors but "infer" air pressures by monitoring individual wheel rotational speeds and other signals available outside of the tire itself. Most indirect TPMS systems operate under the theory of using an under-inflated tire’s slightly smaller diameter (and hence higher angular velocity) to determine inflation. Later 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. The vibration analysis technique may require the use of additional suspension sensors, which result in increased complexity and cost of the overall system, as long as vertical chassis movements are concerned. That is why most current advanced indirect systems use the spectral content of the wheel speed sensor signals, so no additional sensors are needed. The computations can also be carried out by existing computing power, for example in usual ABS or ESC control units.
Indirect TPMS are realized in software algorithms in combination with wheel-speed sensors for anti-lock braking systems, and electronic stability control systems. An advantage of the indirect TPMS is that it needs no sensors, thus decreasing weight and cost as well as increasing customer satisfaction, because sensor-related problems are eliminated. A disadvantage of indirect TPMS is that the driver must calibrate the system by pushing a reset button on the dashboard via an on-board computer, and if this is performed when any tire is in an under inflated condition, then the system will report erroneously. Audi was the first car maker to attempt to comply with the U.S. TPMS legislation (Tread Act), using an indirect system, with the launch of the Audi TT model year 2006. The system - called Tire Pressure Indicator TPI - purports to meet the American FMVSS 138 and the European ECE R-64 safety regulations on tire pressure monitoring systems. Unfortunately, it does not yet comply with the US DOT NHTSA regulations.
Presently indirect TPMS systems do not meet the requirements of the US government NHTSA DOT as a viable solution for TPMS Tread Act requirements under FMVSS 138.
TPMS systems are designed to provide drivers with the tire pressure information and alerts needed to add safety and savings to travel through increased fuel efficiency, extended tire life, decreased downtime and maintenance, improved stability and handling, and decreased emissions. These are significant advantages and 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 USA 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 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.
Decreased Downtime & Maintenance: Under-inflated tires lead to costly hours of downtime and maintenance.
Added 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.
Drive Green: 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 US 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.
On the maintenance side, it is important to realize that fuel efficiency and tire wear are severely affected by under-inflation. In the U.S., 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 EU 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.
In the U.S., the U.S. Department Of Transportation (NHTSA) released the FMVSS No. 138, which requires 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, as of 2007. In the EU, starting in 2012, all new models of passenger cars must be equipped with a TPMS, with even tighter specifications that will be defined by the UNECE Vehicle Regulations (Regulation No. 64). 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 1 January 2013 for new models and on 30 June 2014 for existing models".[3] Japan is expected to adopt EU legislation approximately one year after EU rollout.
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For heavy-duty vehicles (class 7-8, 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 main issues on a large vehicle are:
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 5-7 year range, since the cost of breaking down a tire can be so much more expensive. The Department of Transportation's maximum-loading requirements force trailer manufacturers to spread loads over multiple axles, giving rise to trailers with typically 8-12 tires, but high as 96 tires on specialty haulers.
Tire carcasses can have lifetimes of up to 8 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 and Vagia (used mostly in South America). However, they have not yielded a complete solution, since they do not solve all the issues (i.e., no support for the steer axle), and they bring new issues with maintenance of the rotary couplings in the hub caps. Furthermore, inflation systems can sometimes shorten the life of tires by covering slow leaks caused by embedded objects, which drivers would otherwise remove after inspecting the problem tire.
In order 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 nice if the tire-pressure monitor has some indicator that can be read without tools.
Second, it usually should have the capability to cover dual sets of tires in some fashion. It is also nice if the fill points can be centralized so that airing can be done easily without reaching through the small hand holes in the rims.
Third, it needs to have some 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, if everyone thinks the monitoring is being taken care of automatically.
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 adds to the 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 simply attached 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 back 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.