Tyre pressure monitoring systems have been widely deployed to address safety and environmental issues on passenger cars. There have been many studies which have led to these systems becoming mandatory on vehicles.
- An under inflated tyre gives rise to higher fuel cost, (up to 4% savings for properly inflated tyres) and reduces the lifetime of the tyre, (25- 30% reduction in tyre lifetime).
- A recent NHTSA report found that driving on tyre under inflated by more than 25% are three times more likely to be involved in a crash related to tyre problems.
This paper looks at the history of TPMS, the two competing systems, (direct and indirect), a comparison and use case implications for each. Factory fit and aftermarket requirements, auto- locate methods, energy harvesting challenges and future features and markets for TPMS.
History and Legal Requirements
Tyre pressure monitoring systems, (TPMS), were first deployed in Europe on luxury cars. After a spate of accidents due to tyre failure in the US, legislation was introduced, which made the systems mandatory on certain classes of motor vehicles, (the Transportation Recall Enhanced Accountability and Documentation – TREAD act from 2007). This led to the growth in their deployment. The legislation mandates a minimal system to report tyre deflation to the driver under a certain set of conditions. This involves a warning indication being lit and maintained (even after the engine has been switched off), for a certain percentage deflation, against a recommended inflation pressure within a set time limit and speed, being maintained until the deflation is corrected. The amounts of deflation, speed and time limits vary dependent on the legislative body which set them.
Europe has legislation being introduced for new cars in 2014, with China and Russia expected to follow. This has led to a significant market for these devices, and a complex set of use cases being developed as there is a wide range of passenger cars available.
Scenarios
When considering the different systems it is worth looking at the scenarios that they should attempt to cover.
- A tyre will always leak air, as it diffuses through the rubber so there will be a natural deflation of all four tyres.
- A slow deflation due to a puncture
- Rapid deflation, (a blowout is a significant event, which will be noticed quickly by the driver)
There are other factors which will influence the tyre pressure, which should not cause a false trigger, such as temperature, (a car parked could have some tyres being warmed by the sun, whilst others are in the shade, leading to a differential in pressures), loading of the car and road surface. Tyre pressure information should be available at start of engine, but one could argue that a driver would notice a flat before commencing a journey, (it is worth noting hear that the time to report tyre pressure is critical especially in the performance car market, as these can get to high speeds quickly). If a tyre’s pressure falls below a pre-determined threshold, (for the US, 25% below the manufacturer’s recommended Cold Inflation Pressure, CIP, 20% threshold for the EU but with a 5% tolerance for type approval tests), a yellow warning indicator light will alight on the dashboard to alert the driver.
Some standard icons are shown below:-
Indirect System
There are two methods of monitoring the tyre pressure, direct and indirect. The indirect system relies on the fact that an underinflated tyre will rotate faster than a properly inflated tyre to cover the same distance as its diameter is smaller. Rotational speed sensors are located at each wheel and comparing the speed of each wheel to the average speed of all the wheels it is possible to determine if one is rotating significantly faster than the others. Initially the detection was done by employing diagonal, two tyre or combination algorithms to ascertain the under inflated tyre or tyres.
However there are certain circumstances where these algorithms would not detect the under inflation, e.g. all four tyres under inflated. In more recent times more detailed signal processing has been applied to these systems to make them more robust, however they still rely on proper inflation at calibration time.
The indirect system has a price advantage if you consider that it will operate on existing features of the vehicle, ( i.e. it is a software implementation onto an existing ECU and possible uses the infotainment console for calibration and reset), and if tyres are changed there has to be a reset mechanism, (e.g. if a different tyre is fitted), and also if the system has been triggered. Also original equipment specific tyres are required to be fitted when replacements are needed as the tyres characteristics are programmed into the systems software. The cost of these should be included in any comparisons but is often omitted.
Typically, routine maintenance of the indirect TPMS is not necessary. However, as with any system, introduction of new components such as different sized wheels and/or tyres may cause undesired results. The threshold for a warning to be displayed may be increased or decreased, causing the system to become either hyper-sensitive or not sensitive enough when it comes to alerting the driver to a potential inflation problem. If different sized wheels and/or tyres are installed on the vehicle, it may be necessary to recalibrate the system.
Direct System – The Swindon Silicon Systems approach
The direct system has sensors within the tyre. These can be mounted either on the rim (strapped to the rim) or utilising the valve. The sensor will take a direct pressure reading and then there is a mechanism, (RF link), for reporting the information back to the central ECU. These are definitive readings, but there will be a cost associated with each sensor and RF ECU.
The sensors have to be powered and will have a battery associated with them. It is desirable to have a long battery life for these sensors (10 years – which is the recommended fitted lifetime of a tyre, even if it was never driven). To achieve this, it is necessary to design the system with ultra-low power usage. This is done by using a dedicated ASIC, (Application Specific Integrated Circuit), which has been carefully designed to provide optimum performance for minimum price.
The design will go into sleep mode, consuming only nano amps whilst in this mode, and will wake up when required, to quickly transmit the tyre pressure, whilst compensating for temperature changes so the sensor provides accurate information. This system can obviously transmit tyre pressure at rest.
The direct system does require unique ID’s for each sensor and these are factory programmed. If a sensor is replaced, a tool to read the existing sensor information and program the new sensor is required. When changing a tyre, the valve can be serviced.
ASIC Functional Requirements
The TPMS device is required to transmit pressure information back to the driver. The ideal scenario for the most accurate information would be continuous transmission. However this is not conducive to a long battery life so other methods have to be used. Any vehicle will have periods of rest along with periods of movement. In heavy traffic, it is possible to have short rest periods which have to be accounted for and not mistaken for parked mode. This means the device has to have a motion sensor (roll sensor) as well as the pressure sensor.
These devices are also usually sealed units including the battery. This has several implications. There is a need to indicate when the battery is low. The battery voltage is also affected by temperature, so a temperature sensor has to be included, which can then be used in the low voltage algorithm. The device also has to have a very low quiescent current when in sleep mode to maximise the battery life, and the device has to enter this when the vehicle comes to a stop, but not for traffic. The device therefore has to recognise the difference between these situations.
When the unit is manufactured, the part could be on the shelf for some time before being deployed, so there has to be a mode of operation where it is inert until programmed.
Once deployed the device has to transmit the information in a timely manner and in an optimum way. This involves reading the temperature and pressure, applying any filtering and performing any corrections before transmitting the information in a digital format. Given that the once the pressure has been established, it is differences in pressure that should be logged, it is possible to limit the transmit data packets and so save power by setting thresholds. However, the system should also respond quickly to any rapid pressure changes, so automatically up its transmission rates when required. Each unit will transmit its information at required intervals to the central ECU.
There is a potential for packet clashes, so a mechanism for avoiding these has to be included, so even if two sensors decided to read the data at the same time, the transmit time would be different. Also each sensor has a unique ID associated with it and this will be transmitted to assure that the ECU does not pick up erroneous date from other vehicles. The packet lengths are kept to a minimum. Packet information is proprietary, so different sensor manufactures devices will not normally interoperate correctly. A checksum is incorporated to validate the data.
The preamble allows the receiver to synchronise ready for the data payload.
The device operates in an adverse environment and has to account for variations in manufactured parts, so each one has to be calibrated and programmed before it is fitted. It then is required to maintain its accuracy over its lifetime, whilst coping with the typical automotive requirements and conditions. There may also be a need to force the device into various behaviours during its manufacture, shipping and deployment, so a mechanism for this has to be provided, but not accessible by mistake or malicious act. After market tools are required to be provided that can accomplish this.
The size of the device is also critical as any weight and size is more susceptible to G-Force and cost. Initial designs were based on state machines, but as more functionality has been included a proprietary micro with limited instruction set that can be used in the pressure calculations and testing is desirable. Obviously any device which has various states, analogue functionality and RF will require an oscillator and frequency locking mechanism. Physically the TPMS device needs to securely mounted, (as discussed previously either rim or valve mounted).
In order to develop such a device, many key skills are required, besides the analogue, RF and digital design skills, testing and system knowledge are also required. There also has to be quality metrics in place to meet the strict automotive standards and respond to any critical failures.
Typically field failure rates of one part per million have to be maintained and evidence provided. As more functionality is added to the device, (for developing use cases), there may be a need to add further signal processing. This is always a balance, as the low power requirements of the device has to be maintained.
Other Features
Positional Information methods
In early systems, there is no indication of which tyre has a problem, so the driver will have to check all the tyres. A desirable feature would be to report the position of the problem tyre. The first impact this has on the system is the icon has to have the positional information so it becomes more extensive, usually a car icon with the problem tyre indicated in some manner, for example:-
This ICON could also have the actual pressure displayed next to each tyre. Ideally the positioning should be automatic, so the system can cope with aftermarket tyre changes, (e.g. winter and summer tyres, tyre rotation, new tyres). Several techniques can be used for this:-
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[acc_item title=”ABS” style=”background:#c0c0c0;padding-top:50px;”]If you can get wheel rotation information from two independent sources it is possible to correlate this information and so ascertain the wheel position provided one of the sources has positional information. This relies on the fact that the wheels on a vehicle will normally rotate at slightly different speeds due to vehicle loading, road condition, turning corners, tyre wear etc.
A first source that may be used to gather information regarding the rotational period of each wheel is the ABS sensors which are mounted to the vehicle. This system consists of a toothed ring and an associated sensor, which is hard-wired to each wheel location. As this is a hard-wired system, information is available to a vehicle’s Body Control Module (BCM) on a per wheel basis. ABS rotation information is typically updated in the order of tens of milliseconds. The TPM sensor can also provide rotational information for each wheel but it does not know which wheel it is. So comparing the rotational information for the ABS system to the TPMS information it is possible to deduce the wheel position for the TPMS.
This system is self-learning so even if there has been a change of tyre, it will be able to correlate and correct after a few turns of the wheel.[/acc_item]
[acc_item title=”USING RECEIVED SIGNAL STRENGTH INDICATION (RSSI)”]Each transmitter will have a received signal strength associated with it. The difficultly comes in that these are in a constant state of flux due to environmental conditions, (as the wheel rotates the body work and other metal parts will cause changes in the RF field pattern), so can have a wide range of values, even to a point where the closest transmitter does not have the highest signal strength. To overcome this, a local antenna can be placed near to each tyre and an average maintained. If left and right positional information is available this can be done with just two antennae, however if this is not available the minimum is three. This has the disadvantage of additional costs.[/acc_item]
[acc_item title=”FACTORY SETTING AND TOOL FOR ASSIGNMENT”]Tyre position can obviously be assigned during factory fit. This can be maintained if an after-market tool is available for programming the TPMS device and an infrastructure provided. This will have to be controlled by authorised technicians, as there would be implications of letting the general public interfere with a safety system on a vehicle.[/acc_item]
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Future Developments
There are two major developments coming…
Energy harvesting replacing the battery
As there is a constant movement and rotational force, it is natural to consider the possibility of energy harvesting to power the TPMS device. However, even with piezoelectric type devices, the G-Force which tyre can undergo is significant and causes considerable mechanical stress on the structures leading to failures. So finding a reliable method for energy harvesting is difficult. As technology advances these devices may get mounted into the rubber of the tyre and so this may become more prevalent, but at the moment efforts are focused on reducing the power requirements for the sensor system, which will of course be of benefit to energy harvesting systems.
Correct pressure under load
The loading of a car changes with different numbers of passengers and luggage. It is important to maintain the correct pressure under load and with the direct TPMS system this information can be available and warnings issued to the driver.
After Market
Tyres are constantly being changed on vehicles to avoid wear and also for different driving conditions. Any system has to be able to cope with this, or else the TPMS benefits are lost. The TPMS system is also a safety system on the vehicle and as such should not be able to be disabled or interfered with. This means that there has to be a significant investment in training and support of aftermarket tyre shops, so that wheels can be changed easily.
There also has to be a programme of driver education about TPMS benefits and any pitfalls of home servicing.
Conclusions
The TPMS market is a multi-million pound market which will significantly grow as more regions introduce legislation to adopt the system, current estimates are $3billion by 2017. The preferred choice is the direct TPMS system, which allows additional use cases to be deployed and gives accurate pressure information, whilst the costs are comparable to the other systems if you include the restrictions in tyre choice.
The systems do require a change in behaviour from the drivers, but the benefits that can be achieved should easily make the case for these systems. There will be a constant drive to include more and more functionality in these devices whilst capping the power budget. This requires specialist knowledge to achieve and the leading solutions are ASIC based.
Any ASIC provider not only has to be capable of designing to the strict automotive standards but also has to be able to support the devices over their lifetime. The tyres on vehicles are not a fit and forget type of part and are crucial to the safety of road users and as such the TPMS system has a lot of use cases to cope with. Removal of the battery is an ultimate goal, (for weight, cost and environmental disposal reasons), and this will also keep the low power goal as a critical priority
References
- Evaluation of indirect tyre pressure monitoring systems using data from NCSA’S tyre pressure special study
Kristin K. Thiriez National Highway Traffic Safety Administration United States of America Paper Number 259 - System and method for performing auto-location of a tyre pressure monitoring sensor arranged with a vehicle wheel Patent number: 8332104 Type: Grant Filed: August 31, 2011 Issued: December 11, 2012
Assignee: Schrader Electronics Ltd. Inventors: John Greer, Samuel Strahan - Vehicle wheel auto-location using wheel phase angle information
Patent number: 8332103 Type: Grant Filed: September 22, 2010 Issued: December 11, 2012
Assignee: Schrader Electronics Ltd. Inventors: John Greer, Paul McGrotty, Samuel Strahan
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The major decisions in ASIC development
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The chart below details the approach form our initial contact through to the delivered product.
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About Swindon Silicon Systems
Swindon, the leader in high quality, high performance mixed signal ASICs (Application Specific Integrated Circuits), is part of Schrader International, the global automotive and industrial system component supplier. The UK based Swindon design team has developed ultra-low power, high speed ASICs for a wide range of applications from consumer electronics to defence and medicine, and the company’s state of the art wafer probe and ATE test facilities ensure an unrivalled ASIC solution offering. Amongst many high quality innovative and cost effective developments for the automotive industry, the Swindon Tyre Pressure Monitoring System ASICs meet over 50% of the global demand. Swindon is TS 19449 Accredited.