Direct TPMS, or direct tire pressure monitoring systems (direct sensor TPMS) refers to the use of a pressure sensor directly mounted on the wheels or tires of a vehicle.[1] 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.[2]
direct TPM sensor fitted in valve system, manufacturer 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)[1] pressure sensor and then transmits this and other information to one or more vehicle receivers.[1] Other information can include a serial number, temperature, acceleration, direction of rotation and the status of the complete tire pressure monitoring system.[3] 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,[4]BMW[5] or Toyota[6]) comprises the following components on a vehicle:
A direct TPM sensor fitted to the back of the valve stem on each wheel
Most direct TPMS systems use ultra high frequency (UHF) radio in one of the 'unlicensed' ISM bands (industrial, scientific and medical) for transmitting the data,[7] often around 434 MHz in Europe and 315MHz 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 transmissions.
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[8] or attached using adhesive or to a band which is then securely wrapped around the rim inside the tire, usually in the drop zone.[9]
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.[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.[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.[4]
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.[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.[10]
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 125kHz 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 rotated to even up tread wear.
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.[11]
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 (10−6) 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 signals 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.[12]
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'.[13]
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.[14]
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.[14]
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 systems
Aftermarket valvecap-based dTPMS systems, which require a smartphone and an app or portable display unit, are also available for bicycles,[15] automobiles, and trailers.[16]
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%.[17] 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.[18]
EU and Far East
European Union (EU) and Far East legislators looked at TPMS as a way of reducing CO2 emissions.[19] From November 2014 (ECE-R 64 EU Directive), all new passenger cars models sold in the EU must be equipped with a TPMS.[20]
A communications system or communication system is a collection of individual telecommunications networks, transmission systems, relay stations, tributary stations, and terminal equipment usually capable of interconnection and interoperation to form an integrated whole. The components of a communications system serve a common purpose, are technically compatible, use common procedures, respond to controls, and operate in union.
A transmission medium is a system or substance that can mediate the propagation of signals for the purposes of telecommunication. Signals are typically imposed on a wave of some kind suitable for the chosen medium. For example, data can modulate sound, and a transmission medium for sounds may be air, but solids and liquids may also act as the transmission medium. Vacuum or air constitutes a good transmission medium for electromagnetic waves such as light and radio waves. While material substance is not required for electromagnetic waves to propagate, such waves are usually affected by the transmission media they pass through, for instance, by absorption or reflection or refraction at the interfaces between media. Technical devices can therefore be employed to transmit or guide waves. Thus, an optical fiber or a copper cable is used as transmission media.
Telemetry is the in situ collection of measurements or other data at remote points and their automatic transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Greek roots tele, "remote", and metron, "measure". Systems that need external instructions and data to operate require the counterpart of telemetry, telecommand.
Line-of-sight propagation is a characteristic of electromagnetic radiation or acoustic wave propagation which means waves travel in a direct path from the source to the receiver. Electromagnetic transmission includes light emissions traveling in a straight line. The rays or waves may be diffracted, refracted, reflected, or absorbed by the atmosphere and obstructions with material and generally cannot travel over the horizon or behind obstacles.
In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves.
Very low frequency or VLF is the ITU designation for radio frequencies (RF) in the range of 3–30 kHz, corresponding to wavelengths from 100 to 10 km, respectively. The band is also known as the myriameter band or myriameter wave as the wavelengths range from one to ten myriameters. Due to its limited bandwidth, audio (voice) transmission is highly impractical in this band, and therefore only low data rate coded signals are used. The VLF band is used for a few radio navigation services, government time radio stations and for secure military communication. Since VLF waves can penetrate at least 40 meters (131 ft) into saltwater, they are used for military communication with submarines.
Automatic vehicle location is a means for automatically determining and transmitting the geographic location of a vehicle. This vehicle location data, from one or more vehicles, may then be collected by a vehicle tracking system to manage an overview of vehicle travel. As of 2017, GPS technology has reached the point of having the transmitting device be smaller than the size of a human thumb, able to run 6 months or more between battery charges, easy to communicate with smartphones — all for less than $20 USD.
In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. The antenna intercepts radio waves and converts them to tiny alternating currents which are applied to the receiver, and the receiver extracts the desired information. The receiver uses electronic filters to separate the desired radio frequency signal from all the other signals picked up by the antenna, an electronic amplifier to increase the power of the signal for further processing, and finally recovers the desired information through demodulation.
A two-way radio is a radio that can both transmit and receive radio waves, unlike a broadcast receiver which only receives content. It is an audio (sound) transceiver, a transmitter and receiver in one unit, used for bidirectional person-to-person voice communication with other users with similar radios. Two-way radios are available in stationary, mobile, and hand-held portable models. Hand-held two-way radios are often called walkie-talkies, handie-talkies or hand-helds. Two-way radios are used by groups of geographically separated people who need to keep in continuous voice communication, such as aircraft pilots and air traffic controllers, ship captains and harbormasters, emergency services personnel like firefighters, police officers, and ambulance paramedics, taxi and delivery services, soldiers and military units, fast food and warehouse employees, and radio amateurs.
A security alarm is a system designed to detect intrusion, such as unauthorized entry, into a building or other areas such as a home or school. Security alarms used in residential, commercial, industrial, and military properties protect against burglary (theft) or property damage, as well as personal protection against intruders. Security alerts in neighborhoods show a connection with diminished robbery. Car alarms likewise help protect vehicles and their contents. Prisons also use security systems for the control of inmates.
In data communications, flow control is the process of managing the rate of data transmission between two nodes to prevent a fast sender from overwhelming a slow receiver. Flow control should be distinguished from congestion control, which is used for controlling the flow of data when congestion has actually occurred. Flow control mechanisms can be classified by whether or not the receiving node sends feedback to the sending node.
A tire-pressure gauge, or tyre-pressure gauge, is a pressure gauge used to measure the pressure of tires on a vehicle.
A broadcast transmitter is an electronic device which radiates radio waves modulated with information content intended to be received by the general public. Examples are a radio broadcasting transmitter which transmits audio (sound) to broadcast radio receivers (radios) owned by the public, or a television transmitter, which transmits moving images (video) to television receivers (televisions). The term often includes the antenna which radiates the radio waves, and the building and facilities associated with the transmitter. A broadcasting station consists of a broadcast transmitter along with the production studio which originates the broadcasts. Broadcast transmitters must be licensed by governments, and are restricted to specific frequencies and power levels. Each transmitter is assigned a unique identifier consisting of a string of letters and numbers called a callsign, which must be used in all broadcasts.
A tire-pressure monitoring system (TPMS) monitors the air pressure inside the pneumatic tires on vehicles. A TPMS reports real-time tire-pressure information to the driver, using either 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).
A radio repeater is a combination of a radio receiver and a radio transmitter that receives a signal and retransmits it, so that two-way radio signals can cover longer distances. A repeater sited at a high elevation can allow two mobile stations, otherwise out of line-of-sight propagation range of each other, to communicate. Repeaters are found in professional, commercial, and government mobile radio systems and also in amateur radio.
Tubeless tires are pneumatic tires that do not require a separate inner tube.
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Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connected to an antenna which radiates the waves, and received by another antenna connected to a radio receiver. Radio is very widely used in modern technology, in radio communication, radar, radio navigation, remote control, remote sensing, and other applications.
NIRA Dynamics AB is a Swedish company focusing on research and development of signal processing and control systems for the automotive industry. It supplies automotive original equipment manufacturers (OEMs) and suppliers in Europe, North America, Latin America, and Asia with its products and systems. In 2021, the number of vehicles equipped with NIRA's main product, TPI, exceeded the benchmark of 75,000,000.
An RF module is a (usually) small electronic device used to transmit and/or receive radio signals between two devices. In an embedded system it is often desirable to communicate with another device wirelessly. This wireless communication may be accomplished through optical communication or through radio-frequency (RF) communication. For many applications, the medium of choice is RF since it does not require line of sight. RF communications incorporate a transmitter and a receiver. They are of various types and ranges. Some can transmit up to 500 feet. RF modules are typically fabricated using RF CMOS technology.
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