Direct TPMS

'''Direct TPMS''', or direct [[Tire-pressure monitoring system|tire pressure monitoring systems]] (direct sensor TPMS) refers to the use of a [[pressure sensor]] directly mounted on the wheels or [[tire]]s of a vehicle.<ref name="Hawes, p. 1">Hawes, p. 1.</ref> The pressure inside the tire is measured using a [[pressure transducer]] with the pressure information being subsequently sent to the vehicle to warn the driver of under or over inflation of a tire. The pressure information is commonly transmitted to the vehicle using [[radio frequency]] (RF) technology, though systems using mechanical, [[electrical]] or [[magnetic]] methods have been used over recent years.<ref>Hawes, p. 2.</ref>

==Typical system== [[File:TPMS front side.jpg|thumb|''direct TPM'' sensor fitted in valve system, manufacturer [[VDO (company)|VDO]]]]

In most current designs of direct TPMS, a small electronic assembly which is rugged enough to be mounted inside a tire, measures the [[pressure]] using a [[microelectromechanical system]] (MEMS)<ref name="Hawes, p. 1"/> pressure sensor and then transmits this and other information to one or more vehicle [[Receiver (information theory)|receivers]].<ref name="Hawes, p. 1"/> Other information can include a serial number, [[temperature]], [[acceleration]], direction of rotation and the status of the complete tire pressure monitoring system.<ref>Hawes, p. 123.</ref> The purpose of the serial number is to allow the vehicle to ignore transmissions from other vehicles and operate with a unique data field. A typical direct TPMS (e.g. [[Ford Motor Company|Ford]],<ref name="Hawes, p. 185">Hawes, p. 185.</ref> [[BMW]]<ref>Hawes, p. 51.</ref> or [[Toyota]]<ref>Hawes, p. 690.</ref>) comprises the following components on a vehicle:

* A direct TPM sensor fitted to the back of the [[valve stem]] on each wheel * A TPM Warning Light * [[Unique identifier]] (IDs) for which tire is providing the data including [[speed]] and the direction of [[rotation]] * A tire pressure monitor [[electronic control unit]] (ECU) * Antenna(s) * Controller for periodic measurements * Source of power * Diagnostics and wake up system

Most direct TPMS systems use [[ultra high frequency]] (UHF) radio in one of the 'unlicensed' [[ISM band]]s (industrial, scientific and medical) for transmitting the data,<ref>{{cite web|url=http://www.itu.int/ITU-R/terrestrial/faq/index.html |title=Frequently asked questions |publisher=Itu.int |date=2001-06-01 |accessdate=2021-12-03}}</ref> often around 433 M[[Hertz|Hz]] in Europe and 315 MHz in much of the rest of the world. On some systems there is a separate receiver or antenna near each wheel whilst more commonly there is a single receiver which receives data from all of the wheels on the vehicle. Commonly this receiver is also used for [[remote keyless entry system]] (RKE) as this also usually uses UHF [[radio transmission]]s.

TPM sensors can be fitted to the wheels in a number of ways. They can be mounted on the back of the tire's [[valve stem]]<ref>Hawes, p. 3.</ref> or attached using adhesive or to a band which is then securely wrapped around the rim inside the tire, usually in the drop zone.<ref>Hawes, p. 459.</ref>

===Direct tire pressure monitor system warning light=== When the direct TPMS warning light comes on, either one of the tires is under-inflated, severely over-inflated, or there is a fault with the system.<ref name="Hawes, p. 1"/> If the light is constant then inflating to the correct [[placard]] pressure should turn it off. If this is not the case then this indicates a puncture. If the light is intermittent or if it stays on after correct inflation or replacement of a punctured tire then this indicates a fault with the direct TPMS system.<ref name="Hawes, p. 1"/>

===Registration of direct TPMS IDs=== When the direct TPMS system is fitted at the factory the unique ID numbers of the TPM (tire pressure monitoring) sensors have to be registered along with their position on the vehicle with the tire pressure monitor ECU. This is also the case if any of the system components are subsequently changed e.g. in the event of rotating the tires, changing sensors, replacing the ECU etc.<ref name="Hawes, p. 185"/>

This process requires the activation of the direct TPMS sensor using [[low frequency]] (LF) radio and the capture of the UHF data transmitted. This data includes the direct TPMS ID, the pressure and the temperature. In automotive [[manufacturing plants]], the activation is carried out using large antenna systems whilst in the dealerships and tire shops, hand tools are used. These tools can also be used to check the direct TPMS for faults prior to disassembly. If a TPM sensor or its position on the car are changed without re-registering the IDs, then the TPMS warning light will turn on and stay on until the IDs are re-registered.<ref name="Hawes, p. 1"/>

==Localization== If there are multiple antennas or receivers, this permits localization of the TPM such that the vehicle can tell from which wheel the pressure data has come. As an alternative to this method, the vehicle can be programmed at the time of manufacture with the position of the tire together with its TPM serial number. This allows the vehicle to display which tire has low pressure.<ref>{{cite web|url=https://patentscope.wipo.int/search/en/detail.jsf?docId=WO2006104484 |title=WO2006104484 AUTO-LOCATION FOR TIRE PRESSURE MONITORING SYSTEMS |publisher=Wipo.int |date=2005-03-28 |accessdate=2025-06-03}}</ref>

Also, some vehicles have low frequency radio transmitters mounted near to each wheel which can be used to force the individual TPMs to transmit at will. These typically use similar technology to 125 kHz [[RFID]] tags where the transmitted field is predominantly magnetic and can be easily detected by a small LF antenna located in the TPM. This method of localization is often referred to as a high line system.

The LF antenna is also often used by the TPM for configuration and to force transmission so that localization can be re-learned by the vehicle if a sensor is changed or the wheels are rotated.

A third method uses the UHF signal strength which is proportional to the distance of the TPM from the receiver. If the receiver is located towards the front of the vehicle, the signal from the front wheel TPM's will be stronger than that from the wheels at the rear.

In those systems, the TPM's have a method of detecting the rotational direction of the wheel, which forms part of the message transmitted to the receiver. This information allows the receiver to identify which side of the vehicle the TPMS transmits from. Combined with the signal strength, the correct wheel can be identified.

==TPM sensor features== The TPM sensors currently fitted to high volume production cars worldwide are battery-powered, self-contained units which periodically measure tire pressure, and often temperature and acceleration. The sensor is equipped with an RF transmitter circuit which is used to broadcast the measured pressure etc. within the tire.<ref>{{cite web|url=http://www.freescale.com/webapp/sps/site/overview.jsp?nodeId=02Wcbf123F2D5D |title=NXP® Semiconductors Official Site | Home |publisher=Freescale.com |date=2021-11-23 |accessdate=2021-12-03}}</ref>

The TPM is designed to use as little power as possible to give maximum battery life. This is done by using very low power circuitry and transmitting the data as infrequently as possible and with as low power as possible. The UHF transmitter in a TPM typically transmits around 250μW (1μW is equal to one millionth (10<sup>−6</sup>) of a [[watt]]).

TPMs do not have UHF receivers built in due to the relatively high power requirements of this technology. This means that they can't tell that they are transmitting at the same time as another TPM. Most TPMs do have LF receivers as this uses little or no power.

The pressure, temperature and [[acceleration]] sensors generate [[analog signal]]s which are converted to their digital equivalents using analog to digital converters. The acceleration sensor measures the centrifugal force generated when the wheel rotates. This force is proportional to the rotational speed. The [[acceleration]] sensor may be a simple switch rather than an analog [[transducer]] (accelerometer). This is usually referred to as a roll [[switch]]. The acceleration sensor allows the TPM to be placed in a [[low-power communication device]] mode, when the vehicle is stationary which can extend the battery life. The advantage of a roll switch over an [[accelerometer]] is that the switch is purely mechanical and doesn't use any power to take a measurement.<ref>{{cite web |url=http://www.yole.fr/pagesAn/products/Report_sample/TPMS_tire_pressure_system_market_report.pdf |title=Archived copy |access-date=2009-01-15 |archive-url=https://web.archive.org/web/20090205123036/http://yole.fr/pagesAn/products/Report_sample/TPMS_tire_pressure_system_market_report.pdf |archive-date=2009-02-05 |url-status=dead }}</ref>

When the vehicle is stationary, the TPM may periodically transmit to the vehicle. This allows (as long as the vehicle receiver is always on) the driver or vehicle operator to be warned of low pressure as soon as the Ignition system is switched on rather than having to wait until the vehicle is moving.

All TPM units on a vehicle operate on the same RF channel frequency and each message includes pressure data, temperature data, a unique ID code, operating state data, status information and check digits. The [[check digit]] is either a [[checksum]] or a [[cyclic redundancy check]] (CRC).

The TPM does not usually have information about the tire's correct pressure as this would be very difficult and possibly dangerous to support. However it may have an [[algorithm]] contained within it which detects both slow and rapid changes in pressure. This condition may be transmitted as part of the TPM's status. It may also cause the TPM to transmit more frequently.

==Direct tire pressure monitoring systems== There are two main types of direct tire pressure monitoring system currently in use. These are known as 'high line' and 'low line'.<ref>Hawes, p. 361.</ref>

===High line system=== If the vehicle is fitted with low frequency (LF) transmitters near each wheel, the vehicle may use these to force the sensors to transmit. In this case, the TPM may not transmit on its own, but the vehicle will periodically command the sensors to send their information.<ref name="Hawes, p. 363">Hawes, p. 363.</ref>

In addition, the TPMs will be forced to transmit when the ignition is switched on. This will give an early indication of low pressure without having to have the vehicle's receiver switched on when the vehicle is not in use. The transmitters are usually activated one at a time in sequence so that the vehicle can inform the driver of the location of the wheel with low pressure. This information can then be used later for localisation by matching the TPM's unique ID with its position discovered by this sequential activation. This method is used on some high line systems where the TPM also transmits periodically.

On some vehicles only three LF transmitters are used in order to save money. The vehicle assumes that transmissions from a nearby TPM which has not been woken up by the LF belong to the TPM located where there is no LF transmitter.

High line systems are inherently more expensive than low line systems but they have the advantage of the vehicle knowing the pressure when started without draining the vehicle's main battery and providing localisation. These systems tend to be used on higher end models.

===Low line system=== In this system, the TPM units transmit on their own at fixed or random intervals. As the individual TPMs on the vehicle do not know if another TPM is transmitting at the same time, it is possible to have collisions between messages transmitted. Measures have to be taken to ensure that the message is received by the vehicle. On some systems the message is re-transmitted multiple times to reduce the effect of [[interference (communication)]]. The transmission pattern can be [[random]] or [[pseudo random]] to reduce the chance of collisions between transmissions from the sensors on the vehicle.<ref name="Hawes, p. 363"/>

Another method of attempting to avoid collisions is simply to transmit more frequently such as once per minute. In addition, if the TPM detects a rapid change in pressure or too high a temperature, it will start to transmit more frequently so that the vehicle has more chance of receiving the information. The low line system is used on the majority of vehicles due to its lower cost.

===Aftermarket TPMS=== Aftermarket TPMS systems are designed to be retrofitted to vehicles that were not originally equipped with the technology. These systems are available for a wide range of vehicles, from bicycles and trailers to heavy-duty trucks.<ref>{{Cite book |last=Dissanayake |first=Don |url=https://books.google.com/books?id=NuqODwAAQBAJ&pg=PA344 |title=Acoustic Waves |date=2010-09-28 |publisher=BoD – Books on Demand |isbn=978-953-307-111-4 |pages=344 |language=en}}</ref>

In commercial fleets, aftermarket TPMS can be integrated into a [[fleet telematics system]] and [[fleet digitalization]]. As part of a [[vehicle tracking system]], the sensors on each tire wirelessly transmit real-time pressure and temperature data to a central [[telematic control unit]] or [[GPS tracking unit]] in the vehicle.<ref>{{cite journal |url=https://www.researchgate.net/publication/330645322_An_Innovative_Decision_Rule_Approach_to_Tyre_Pressure_Monitoring |title=An Innovative Decision Rule Approach to Tyre Pressure Monitoring |author1=A. Gentile |author2=S. L. R. C. Pinto |date=January 2019 |journal=SAE Technical Paper |doi=10.4271/2019-01-0731}}</ref> This [[telematics|telematics data]] can be combined with [[GPS tracking unit|GPS data]] for [[automatic vehicle location]] and [[track and trace]] operations. The data is relayed to a [[fleet management software|fleet management platform]], allowing [[fleet management|fleet managers]] to remotely monitor tire health as a form of [[asset tracking]]. This monitoring helps prevent blowouts and can improve fuel economy by ensuring correct tire inflation; this is often paired with data from a [[fuel level sensor]] to help monitor overall fuel consumption.<ref>{{cite web |url=https://wialon.com/en/case-studies/tire-pressure-and-temperature-control |title=Tire pressure and temperature control on semi-trailers: a Wialon-based solution |website=Wialon |access-date=September 30, 2025}}</ref> Some systems integrate TPMS data with [[video telematics]] from a [[dashcam]] as part of a [[driver scoring]] model.

==Legal status==

===United States=== The use of TPMS in vehicles in the USA was mandated in 2008 by the [[Transportation Recall Enhancement, Accountability and Documentation Act]] (or TREAD). This states that the driver of a vehicle must be warned of under inflation by 25%.<ref>{{cite web |url=http://www.nhtsa.dot.gov/staticfiles/DOT/NHTSA/Rulemaking/Rules/Associated%20Files/TPMS-2005-FMVSS-No138.pdf |title=Archived copy |access-date=2008-11-06 |archive-url=https://web.archive.org/web/20080921185722/http://www.nhtsa.dot.gov/staticfiles/DOT/NHTSA/Rulemaking/Rules/Associated%20Files/TPMS-2005-FMVSS-No138.pdf |archive-date=2008-09-21 |url-status=dead }}</ref> The warning is displayed to the driver in line with 49 CFR Part 571 [[Federal Motor Vehicle Safety Standards]] (FMVSS). The minimum requirement for the warning is in the form of a simple lit symbol on the dashboard, and this is often augmented by the display of a graphic of the car showing the wheel position to which the warning light corresponds. All new model year 2008 light vehicles shall have a TPMS (direct or indirect) fitted which is capable of detecting when one or more of the vehicle's tires, up to all four tires, is 25% or more below the manufacturer's recommended inflation pressure (placard pressure) or a minimum activation pressure specified in the standard, whichever is higher.<ref>{{Cite web |url=http://www.nhtsa.dot.gov/cars/rules/rulings/tirepresfinal/index.html |title=Tire Pressure Monitoring Final Rule |access-date=2009-01-15 |archive-date=2009-02-09 |archive-url=https://web.archive.org/web/20090209215948/http://www.nhtsa.dot.gov/cars/rules/rulings/TirePresFinal/Index.html |url-status=dead }}</ref>

===EU and Far East=== European Union (EU) and Far East legislators looked at TPMS as a way of reducing [[Greenhouse gas emissions|carbon dioxide emissions]].<ref>{{Cite web |url=http://www.iea.org/Textbase/work/2005/EnerEffTyre/stock.pdf |title=Archived copy |access-date=2009-01-15 |archive-date=2009-01-17 |archive-url=https://web.archive.org/web/20090117163723/http://www.iea.org/Textbase/work/2005/EnerEffTyre/stock.pdf |url-status=dead }}</ref> From November 2014 (ECE-R 64 EU Directive), all new passenger cars models sold in the EU must be equipped with a TPMS.<ref>[https://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2010:310:0018:0040:EN:PDF Regulation No 64 of the Economic Commission for Europe of the United Nations]</ref>

==See also== *[[Central tire inflation system]] *[[Cold inflation pressure]] *[[Run-flat tire]] *[[Tire-pressure gauge]] *[[Car safety]] {{Fleet digitalization|state=expanded}} ==Notes== {{reflist|33em}}

==Bibliography== *{{Citation | last1 = Hawes | first1 = James | last2 = Fisher | first2 = John | last3 = Mercer | first3 = Todd | title = Tire Pressure Monitoring Systems Guide | publisher = Mitchel1 | year = 2008 | isbn = 978-1-58718-177-1}}.

==External links== *[https://web.archive.org/web/20120616210316/http://ftp.cse.sc.edu/reports/drafts/2010-002-tpms.pdf Security and Privacy Vulnerabilities of In-Car Wireless Networks: A Tire Pressure Monitoring System Case Study] *[https://web.archive.org/web/20081120010841/http://www.bartecusa.com/what_is_tpms.htm Media file presenting information on direct tpms, tpm sensors and tpms safety tool] *[http://www.nhtsa.gov/DOT/NHTSA/Rulemaking/Rules/Associated%20Files/TPMS-2005-FMVSS-No138.pdf NHTSA impact analysis report for tpms] {{Webarchive|url=https://web.archive.org/web/20140506184414/http://www.nhtsa.gov/DOT/NHTSA/Rulemaking/Rules/Associated%20Files/TPMS-2005-FMVSS-No138.pdf |date=2014-05-06 }}

{{DEFAULTSORT:Direct Tpms}} [[Category:Automotive technologies]] [[Category:Tires]] [[Category:Automatic identification and data capture]] [[Category:Fleet management]]