Direct TPMS
Encyclopedia
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. 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.
using a microelectromechanical system (MEMS) pressure sensor and then transmits this and other information to one or more vehicle receivers
. Other information can include a serial number, temperature
, acceleration
and the status of the complete tire pressure monitoring system. 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, BMW
or Toyota) comprises the following components on a vehicle:
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, often around 434 MHz
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 transmissions.
TPM sensors can be fitted to the wheels in a number of ways. They can be mounted on the back of the tire valves stem or attached using adhesive or to a band which is then securely wrapped around the rim inside the tire, usually in the drop zone.
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.
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 ID's, then the TPMS warning light will turn on and stay on until the ID's are re-registered.
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 localisation 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 localisation 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.
The TPM also has a method of detecting the rotational direction of each wheel to identify which side of the vehicle the TPM is located and this information forms part of the message transmitted to the vehicle. This combination allows the correct wheel to be identified.
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
).
TPM's 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 TPM's 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.
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.
In addition, the TPM's 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.
. The transmission pattern can be random or pseudo random to reduce the chance of collisions between transmissions from the sensors on the vehicle.
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.
(or TREAD). This states that the driver of a vehicle must be warned of under inflation by 25%. 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.
Tire
A tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...
pressure monitoring systems (direct sensor TPMS) refers to the use of a pressure sensor
Pressure sensor
A pressure sensor measures pressure, typically of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually acts as a transducer; it generates a signal as a function of the...
directly mounted on the wheels or tires of a vehicle. 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
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) technology, though systems using mechanical, electrical or magnetic methods have been used over recent years.
Typical system
In most current designs of direct TPMS, a small electronic assembly which is rugged enough to be mounted inside a tire, measures the pressurePressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
using a microelectromechanical system (MEMS) pressure sensor and then transmits this and other information to one or more vehicle receivers
Receiver (Information Theory)
The receiver in information theory is the receiving end of a communication channel. It receives decoded messages/information from the sender, who first encoded them. Sometimes the receiver is modeled so as to include the decoder. Real-world receivers like radio receivers or telephones can not be...
. Other information can include a serial number, temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, acceleration
Acceleration
In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...
and the status of the complete tire pressure monitoring system. 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, BMW
BMW
Bayerische Motoren Werke AG is a German automobile, motorcycle and engine manufacturing company founded in 1916. It also owns and produces the Mini marque, and is the parent company of Rolls-Royce Motor Cars. BMW produces motorcycles under BMW Motorrad and Husqvarna brands...
or Toyota) comprises the following components on a vehicle:
- A direct TPM sensor fitted to the back of the valve stemValve stemA valve stem is a self-contained valve which opens to admit gas to a chamber , and is then automatically closed and kept sealed by the pressure in the chamber, or a spring, or both, to prevent the gas from escaping...
on each wheel - A TPM Warning Light
- Unique identifierUnique identifierWith reference to a given set of objects, a unique identifier is any identifier which is guaranteed to be unique among all identifiers used for those objects and for a specific purpose...
(ID's) for which tire is providing the data including speedSpeedIn kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
and the direction of rotationRotationA rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation... - A tire pressure monitor electronic control unitElectronic control unitIn automotive electronics, electronic control unit is a generic term for any embedded system that controls one or more of the electrical systems or subsystems in a motor vehicle....
(ECU) - Antenna(s)
- Controller for periodic measurements
- Source of power
- Diagnostics and wake up system
Most direct TPMS systems use ultra high frequency
Ultra high frequency
Ultra-High Frequency designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz , also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres...
(UHF) radio in one of the 'unlicensed' ISM band
ISM band
The industrial, scientific and medical radio bands are radio bands reserved internationally for the use of radio frequency energy for industrial, scientific and medical purposes other than communications....
s (industrial, scientific and medical) for transmitting the data, often around 434 MHz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
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 transmissions.
TPM sensors can be fitted to the wheels in a number of ways. They can be mounted on the back of the tire valves stem or attached using adhesive or to a band which is then securely wrapped around the rim inside the tire, usually in the drop zone.
Direct tire pressure monitor system warning light
When the direct TPMS warning light comes on, either one of the tires is under-inflated or there is a fault with the system. If the light is constant then inflating to the correct placardPlacard
A placard is a notice installed in a public place, like a small card, sign, or plaque. It can be attached to or hung from a vehicle or building to indicate information about the vehicle operator or contents of a vehicle or building.- Buildings :...
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.
Registration of direct TPMS ID's
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.This process requires the activation of the direct TPMS sensor using low frequency
Low frequency
Low frequency or low freq or LF refers to radio frequencies in the range of 30 kHz–300 kHz. In Europe, and parts of Northern Africa and of Asia, part of the LF spectrum is used for AM broadcasting as the longwave band. In the western hemisphere, its main use is for aircraft beacon,...
(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
Activation
Activation in chemical sciences generally refers to the process whereby something is prepared or excited for a subsequent reaction.- Chemistry :...
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 ID's, then the TPMS warning light will turn on and stay on until the ID's are re-registered.
Localisation
If there are multiple antennas or receivers, this permits localisation 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.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 localisation 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 localisation 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.
The TPM also has a method of detecting the rotational direction of each wheel to identify which side of the vehicle the TPM is located and this information forms part of the message transmitted to the vehicle. This combination allows the correct wheel to 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.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
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
).
TPM's 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 TPM's do have LF receivers as this uses little or no power.
The pressure, temperature and acceleration
Acceleration
In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...
sensors generate Analog signal
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
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
Acceleration
In physics, acceleration is the rate of change of velocity with time. In one dimension, acceleration is the rate at which something speeds up or slows down. However, since velocity is a vector, acceleration describes the rate of change of both the magnitude and the direction of velocity. ...
sensor may be a simple switch rather than an analog transducer
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
(accelerometer). This is usually referred to as a roll switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
. The acceleration sensor allows the TPM to be placed in a low-power communication device
Low-power communication device
In telecommunication, a low-power communication device, also short-range device is a restricted radiation device, exclusive of those employing conducted or guided radio frequency techniques, used for the transmission of signs, signals , writing, images and sounds or intelligence of any nature by...
mode, when the vehicle is stationary which can extend the battery life
Battery life
Battery life can refer to*How long a mobile device can work on a single charge of a rechargeable battery.*Battery lifetime degrading of battery quality over time....
. The advantage of a roll switch over an accelerometer
Accelerometer
An accelerometer is a device that measures proper acceleration, also called the four-acceleration. This is not necessarily the same as the coordinate acceleration , but is rather the type of acceleration associated with the phenomenon of weight experienced by a test mass that resides in the frame...
is that the switch is purely mechanical and doesn't use any power to take a measurement.
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
Check digit
A check digit is a form of redundancy check used for error detection, the decimal equivalent of a binary checksum. It consists of a single digit computed from the other digits in the message....
is either a checksum
Checksum
A checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and...
or a cyclic redundancy check
Cyclic redundancy check
A cyclic redundancy check is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data...
(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
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
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'.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.In addition, the TPM's 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 TPM's 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)Interference (communication)
In communications and electronics, especially in telecommunications, interference is anything which alters, modifies, or disrupts a signal as it travels along a channel between a source and a receiver. The term typically refers to the addition of unwanted signals to a useful signal...
. The transmission pattern can be random or pseudo random to reduce the chance of collisions between transmissions from the sensors on the vehicle.
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.
USA
The use of direct TPM in vehicles in the USA was mandated in 2000 by the Transportation Recall Enhancement, Accountability and Documentation ActTransportation Recall Enhancement, Accountability and Documentation Act
The Transportation Recall Enhancement, Accountability and Documentation Act is a United States federal law enacted in the fall of 2000. This law intends to increase consumer safety through mandates assigned to the National Highway Traffic Safety Administration...
(or TREAD). This states that the driver of a vehicle must be warned of under inflation by 25%. 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.
EU and Far East
European Union (EU) and Far East legislators are looking at TPMS as a way of reducing CO2 emissions, and are presently (as at January 2009) considering compulsory tire pressure monitoring systems from this environmental stance.See also
- Cold inflation pressureCold inflation pressureCold inflation pressure is the inflation pressure of tires before the car is driven and the tires warmed up. Recommended cold inflation pressure is displayed on the owner's manual and on the placard attached to the vehicle door edge, pillar, glovebox door or fuel filler flap...
- Run flat tire
- Tire-pressure gaugeTire-pressure gaugeA tire-pressure gauge is a pressure gauge used to measure the pressure of tires on a vehicle. conditions can introduce a 13 to 15 percent variability in pressure due to temperature , and additional changes can result due to altitude...
- Car safetyCar safetyAutomobile safety is the study and practice of vehicle design, construction, and equipment to minimize the occurrence and consequences of automobile accidents. Automobile safety is the study and practice of vehicle design, construction, and equipment to minimize the occurrence and consequences of...