- Tyre Pressure Inflation System

7/29/2019 - Tyre Pressure Inflation System

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Perfection built in

Tire Pressure Monitoring Systemfor all Vehicle Categories

Special reprint fromAutomobiltechnische Zeitschrift (ATZ)

A T ZR e p r i n t


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ATZ worldwide

1 Technical Status

Current series-production systems performthe following tasks:

continuous tire pressure monitoringboth during the journey and at a stand-still

early warning for the driver of pressureloss, tires running at too low a pressureand tire failure

prevention of errors when setting thenominal tire pressure value and wheninflating tires

automatic identification and location ofthe vehicles wheels

system and component diagnosis invehicle production and in the repairshop.

The Beru tire pressure monitoring system(TPMS) uses battery-powered wheel elec-

tronics which measure the air pressure andtemperature inside the tire at short inter-vals. Together with the individual identifi-cation coding of the wheel electronics andinformation on battery life, these valuesare transmitted as data telegrams by radioto a radio frequency antenna mounted inthe wheel arch. From there they are relayedby cable to the central electronic controlunit, Figure 1.

The control unit evaluates the data tele-grams, identifies the point from which they

were transmitted and decides whether thedriver should be informed. Each tire is mon-itored separately. Its air pressure is convert-

Development Tires

By Norbert NormannTire inflation pressure is an important factor for road safety, ride quali-ty, tire life and the vehicles fuel consumption. Despite the evident im-portance of being able to monitor tire pressures during the journeyand the extensive development work that has been devoted to such asystem, it has only recently proved possible to design and manufactureone that is suitable for series production.The Beru company suppliesthe only tire pressure monitoring system approved by German auto-mobile manufacturers as original equipment which is capable of avoid-ing tire failures and maintaining the vehicles mobility.This series-pro-duction system is currently undergoing further development to reduceits cost and weight without affecting its functional efficiency. It can alsobe adapted to match the vehicles cost level and thus offered for sale inhigh-volume cars.The distinctions in such cases relate only to operat-ing convenience,not to safety.

Tire Pressure Monitoring System

for all Vehicle Categories

2

Dr. rer. nat.NorbertNormann is Electronics De-velopment Manager at Beru

AG with responsibility for the

Tire Pressure Monitoring Sys-tems Product Group.

The author


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3ATZ Automobiltechnische Zeitschrift 102 (2000) 11

ed to the standard pressure by means of atemperature characteristic.

Either the driver or the cars manufacturerinputs the nominal tire pressure value as adesired figure specific to the car. If the dri-ver makes this input, the system checks itfor plausibility to avoid setting-up errors.After the value has been checked, it is usedfor tire pressure monitoring. The systemautomatically identifies the wheels on the

vehicle to which it is fitted, and is thereforenot confused by installing new or differentwheels (for example with winter tires), orby radio signals from the tire pressure mon-itoring system of an adjacent system. It iseven capable of identifying the point on thecar, for instance on the right at the rear,where a new wheel has been fitted. As anoption, the spare wheel can also be incorpo-rated into the pressure monitoring system.

The current series-production system,without spare wheel monitoring, consistsof four aluminium valves, the wheel elec-

tronics, the radio frequency (RF) signal re-ception antennas and specially shielded ca-bles resistant to interference for connectingthe four antennas to the central controlunit. Table 1 summarises the systems func-tions, operation and components.

2 Future Versions of the System

Future versions of the system will take theneeds of specific vehicles into account.They will differ from the current series-pro-

duction system in terms of function (basicor luxury versions) and equipment level(standard or optional).

2.1 Luxury Version(Optional Equipment)

The luxury version (the schematic circuitdiagram is shown in Figure 2) has all thefunctions of the current series-productionsystem but does not need a radio frequencywiring harness and can also call up specificdata telegrams from the wheel electronicsthat would otherwise be transmitted auto-matically. The RF wiring harness is replaced

by a LIN bus that connects the antennas tothe control unit. The antennas are equippedwith an RF receiver and trigger transmit-ters. Monitor of trailer tires is an optionequipped with its own RF receiver, triggertransmitter and bus interface. Table 2shows the features of this system withchanges in relation to the current systemshown in bold type.

2.2 Luxury Version(Standard Equipment)

If tire pressure monitoring is to be stan-dard equipment on the vehicle, the RF re-ceiver for the radio-controlled locking sys-tem control unit already installed on thevehicle is used for radio frequency signal

reception. The TPMS functional softwarealso runs in this control unit, thus avoidingthe hardware costs of a separate TPMS con-trol unit. Trigger transmitters are activatedat the installation points to permit fully-automatic location. Figure 3 is a schematiccircuit diagram of the forthcoming luxurysystem for use as standard equipment onthe vehicle.

DevelopmentTires

Table 1: Current series-production tire pressuremonitoring system

Figure 1: Schematic diagram of current tire pressure monitoring system

Figure 2: Schematic diagram of luxury system for installation as an optional extra


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4 ATZ worldwide

2.3 Basic Version

Figure 3 also shows the block circuit dia-gram for the basic version. This version wasdeveloped to make the system of interest

for high-volume midsize and compact carswith a full equipment specification. Forcost reasons, the basic version has no auto-matic identification of the wheels fittedpositions, so that the trigger transmittersneeded for localisation purposes can beomitted. Whenever a wheel is changed, ad-ditional action on the drivers part is there-fore necessary.

Table 3 summarises the systems features.The successive development stages fromthe current series-production system to theluxury and basic versions described here

can best be seen by comparing Tables 1 to 3.The changes printed in bold type always re-fer to the immediately preceding system.

3 Components

3.1 Wheel Electronics

The further developed wheel electronicsused in all versions of the system have ahigher overall level of integration and re-

quire less battery power. They are accord-ingly smaller, flatter and lighter than thecurrent units. This has advantages in terms

of the force exerted at the wheel rim, thebalancing weights needed and when fit-ting the tires.

The wheel electronics consist primarily of asensor, the signal processing circuit, the RFtransmitter stage, the battery and the hous-ing; Figure 4 is a block circuit diagram of

the wheel electronics and Table 4 sum-marises their features. Only a very smallSMD antenna is needed to pick up the trig-ger signal. The key element in the wheelelectronics is the sensor, which is an intel-ligent type specially developed for this ap-plication.

The SMD housing contains a pressure sen-sor, a temperature sensor and the integrat-ed circuit that undertakes the completemeasured-value signal recording and pro-cessing. The sensor directly activates a dig-ital module with integrated RF transmitter

stage.

The power supply to the sensor and thetransmitter stage comes from an environ-mentally acceptable lithium battery thathas been developed specifically for the highstandards of resistance to temperaturechanges and acceleration forces and thelong life called for in these operating condi-tions.

The construction of the wheel electronicsand the connecting techniques adopted

here are capable of withstanding the ex-tremely severe operating conditions in thetire for a period of 7 years, that is to say

temperatures from minus 40 to +150C oreven 170C for short periods, dynamic peakacceleration values of up to 2,000 g, ex-treme moisture and condensation and alsoother solid or liquid substances in the tiresuch as those used to simplify tire fitting.

The wheel electronics and the tire valve

form a single compact unit that can be in-stalled on almost all the customary wheelrim contours, Figure 5.

Development Tires

Table 3:Tire pressure monitoring system,basic version (changes compared with luxuryversion,Table 2,shown in bold type)

Table 2:Tire pressure monitoring system,luxury version for use as an optional extra(changes compared with current system,Table 1,shown in bold type)

Figure 3: Schematic diagram of the luxury system for series-production and (in blue) the basic version


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3.2 Control Unit

Unlike the current control unit, the one thathas now been developed is capable of call-ing up information specifically from thewheel electronics. This greatly improves

the process of locating individual wheelsand the standard of tire status information.The digital control unit consists of an eval-uating unit with microprocessor controland data bus and diagnostic interfaces, Fig-ure 2. It evaluates the data it has receivedand transmits messages to the driver infor-mation system if necessary.

Control unit functions can be divided intotwo groups: tire pressure monitoring (thesystem section) and the vehicles specificdata bus connection with the associatedcontrol and warning routines and diagnos-

tic facility (the communication section).These two functional areas are obtainedwith the aid of two separate, autonomousprogram modules which make it relativelysimple to adapt the equipment to suit man-ufacturers specific bus protocols.

A separate control unit is only used for theluxury optional-equipment version (see2.1). For the other system versions, namelythe luxury standard-equipment (2.2) andbasic versions (2.3) the control functions areintegrated into the existing radio central

locking control unit.

3.3 Antenna

On the current system, the radio frequencysignals are converted into digital informa-tion in the RF receiver of the central controlunit. In future systems, this processing taskwill be transferred to the antenna, whichwill therefore be equipped with a radio fre-quency receiver, a low-cost micro-con-troller and a digital interface for connectionto a LIN bus, Figure 2. This will avoid theneed for a complex RF wiring harness.

For calling up data telegrams, the antennasare equipped with a simple trigger transmit-ter which, in response to control unit com-mands, causes the wheel electronics totransmit their signal. The trigger function isperformed by a simplified form of transpon-der technique, as already widely used todayfor cableless information transmission. Thisis referred to as a simplified technique be-cause no energy has to be transmitted, thisbeing already available from the batteries inthe wheel electronics. It is even possible to

use trigger transmitters already present onthe vehicle, for example those for a radio-op-erated central locking system. Modules for

the transponder technique are manufac-tured in large numbers and are thereforeavailable at correspondingly low cost.

4 Functions

4.1 Data Recording in the Tire

A tire pressure monitoring system has toperform two main functions: measurement

of the absolute tire pressure value andidentification of pressure loss. For the ab-solute measurements, data can be trans-mitted at relatively infrequent intervals,whereas a loss of pressure, on the otherhand, has to be transmitted immediately.In order to obtain these functions withoutexcessive current consumption, the wheelelectronics themselves decide when a datatelegram should be transmitted. Each

wheel electronic unit normally measurestire pressure and the approximate temper-ature in the tire every three seconds, but ifthe measured values are stable, the infor-mation is transmitted relatively infre-quently (typically every 54 seconds). If,however, the pressure has dropped com-pared with the previous measurement, thewheel electronics send their signals moreoften, the actual interval (down to every 0.8s) depending on the degree of change in the

tire pressure.

Each wheel electronic unit has its individ-ual identification code (ID) which is trans-mitted together with the data. The datatelegram also contains information on theremaining battery life. The measured datafrom the wheel are transmitted by an inte-grated RF transmission stage in the wheelelectronics.

DevelopmentTires

Table 4: Features of thewheel electronics

Figure 4: Schematic diagram of wheel electronics


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4.2 Data Transmission

A unidirectional radio frequency mea-sured-data transmission principle is usedfrom the wheel to the vehicle. The RF sys-tem is designed to ensure reliable transmis-sion of the data telegrams in a wide varietyof conditions, for example statistical recep-tion amplitude fluctuations caused byvarying reception conditions at the anten-na. The transmissions take place in the 433MHz range, the so-called ISM waveband, orfor certain countries in the 315 MHz wave-

band. In the near future, a further transmis-sion zone will become available: the 868MHz waveband. The data transmitted bythe wheel electronics of the luxury versionare processed directly at the receptionpoints by the radio frequency receivers andmicro-controllers in the antennas. Process-ing involves converting the signals into asuitable data bus format and supplyingthem to the control unit by LIN bus (Figure2). On the luxury standard-equipment andbasic versions, RF reception is by means ofthe RF receiver for the radio-operated cen-tral locking RF system, Figure 3.

4.2.1 Calling Up Data

By making provision for data telegrams tobe called up at any time, the wheel elec-tronics transmitting intervals (normally inthe region of one minute) can be consider-ably lengthened. The current series-produc-tion system uses regular one-minute trans-missions as a means of detecting possiblewheel electronics defects after a certain pe-riod, which must of course be long enoughto bridge unavoidable gaps caused by losttelegrams. The time in question is deter-

mined by the vehicle manufacturer and isnormally between eight and ten minutes.The trigger function enables the system to

be tested if telegrams are not received, toensure that the wheel electronics are func-tioning correctly. This is done for exampleby calling for a series of transmission oper-ations. The basic transmission frequencycan thus be reduced, for example to twominutes. This halves energy consumptionwithout endangering the systems safety.The battery capacity rendered unnecessaryin this way is used primarily to reduce theweight and installed bulk of the equipmentin the tire, these being the critical factors.

4.3 Nominal Pressure Input

The actual tire pressure after adjustment iseither initialised by the driver pressing abutton or the system adopts nominal pres-sures memorised in it for the vehicle type towhich it is fitted. The system checks theplausibility of pressure inputs, for examplethe minimum pressure or differences be-tween the front and rear, left and righttires. If the tire has been inflated correctly,its pressure is then monitored; if not, thedriver is requested to correct the pressure.Correct tire pressures are essential for opti-

mum ride comfort in the sense of lowrolling noise, freedom from sharp verticalmovements and sensitivity to transverseroad-surface joints. After the vehicleswheels have been changed (for examplewinter tires fitted), the system is capable ofidentifying the new wheels and their fittedpositions.

4.4 Pressure Loss Warning

In regular vehicle operation, tire pressureschange only gradually. The typical slight

loss of pressure due to air diffusion is reli-ably identified by the system as soon as thecorresponding warning thresholds are ex-

ceeded. The driver is then reminded to cor-rect the tire pressures at the next conve-nient opportunity. Tire failures that resultfrom driving fast with the tires insuffi-ciently inflated, as a result of the excessivecarcass flexing that then takes place, can be

avoided in this way. 85% of all tire failuresare a consequence of this kind of gradualpressure loss.

In the event of tire failure during the jour-ney, the driver is warned immediately. If atotal loss of pressure occurs while the vehi-cle is standing still, the driver is warned assoon as the vehicle is next driven.

4.4.1 Warning Limits

The isochoric relationship between pressureand temperature is used when computingthe warning limits. The same warning in-

tensity can thus be achieved over a broadtemperature range. With the aid of the nom-inal pressure and the temperature when cal-ibrated, the system calculates the limit val-ues valid for the actual tire temperature. Ifthey are exceeded, the control unit transmitssignals to the driver information system ac-cording to the degree of pressure change. Adistinction is made between two types oflow-pressure warning: Reminder (a softwarning) and Failure (a hard warning).

In the case of Reminder warnings, the ve-

hicle remains safe to drive. When the igni-tion is next switched on, or during the jour-ney, the driver is reminded to correct thetire pressure at the next opportunity.

In the case of Failure warnings, the vehi-cle may not be safe to drive. The driver is in-structed to stop immediately and check thecondition of the tire. It is then up to the dri-ver to decide whether the journey can becontinued at slow speed as far as the nextrepair shop, or whether the wheel must bechanged or the tire repaired with sealanton the spot.

4.5 Identification of the VehiclesOwn Wheels and Location ofIndividual Wheels

One of the systems important functions isautomatic identification and location of thecars wheels. Each wheel electronic unittransmits a different and unmistakablecode signal. By statistical evaluation of theinformation it receives, the system identi-fies the wheel electronics installed on the

car (identification of the cars own wheels)and where they are fitted (position identifi-cation).

Development Tires

Figure 5:Wheel electronics with tire valve fitted to wheel rim


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4.5.1 Four-position Identification

On the current system and on the futureluxury optional-extra equipment, wheelposition is identified by statistical evalua-tion of reception signal strengths as re-ceived at the four individual antennas in

the wheel arches. When wheel location hasbeen reliably identified, the positions aresaved to a non-volatile memory.

4.5.2 Four-position Identificationby Interrogation

By interrogating the wheel sensors, a sec-ond method of identifying wheel locationentirely separate from the first can be builtup. In this case, the antenna is not only ableto call up data telegrams but also specificwheel identification codes.

Four-position identification can also take

place by means of the trigger transmittersalone, in which case an RF receiver is not es-sential. In this case, the data are received atthe central control unit, which can also bean existing one, for instance for a radio-op-erated central locking system. Such a sys-tem costs only a fraction of the current one,yet includes all the features called for on aluxury car.

4.5.3 Visual Position Identification

The use of only one antenna, as on the basicversion, reduces costs still further. If the dri-

ver is supplied with a warning, he or shemust then check all the tires. When a wheelis changed, the nominal pressure must bereset, since in this case the system does notidentify wheel changes automatically.

In order to make the system available at apracticable price for high-volume compactcars, simplification can even go a stage fur-ther. Provided that the car has an RF receiv-er, this can act as the reception stage forwheel signals. In this minimum systemspecification, the system hardware consistsonly of the four wheel sensors. The function

software is integrated into an existing con-trol unit with a radio frequency receptionfacility.

5 Monitoring at a Standstill

When the vehicles ignition is switched off,the control unit switches over to an econo-my mode. It is then activated only shortlybefore the regular data telegrams have tobe received from the wheels, and reverts tothe economy mode after receiving them.

This makes it possible to perform two addi-tional functions when the ignition isswitched off:

warning of tire failure before the jour-ney starts: the measured pressure valueis subjected to the regular evaluationprocedure as soon as the ignition isswitched on and the appropriate warn-ing issued if necessary

warning that the tire has been slashed.For this purpose, the tire pressure moni-toring system is coupled to an alarm sys-tem.

Monitoring while the car is at a standstill,which is an important safety feature, needsa considerable proportion of the battery ca-pacity for signal transmission. Interroga-tion as a means of supplying the driver ef-fectively with a status report before thejourney starts enables the transmission in-terval to be greatly extended, for examplefrom the current value of one minute to a

quarter of an hour. This saves a very largeamount of current without any loss of in-formation, and thus enables a smaller bat-tery to be used and the weight of the wheelelectronics reduced.

6 Functional ChecksDuring Production andin the Workshop

In accordance with modern standards ofmanufacturing technology, the systems

functions can be checked during the indi-vidual wheel fitting stages. A succession oftests is provided for this purpose. In the firststage, the wheel electronics are checked af-ter fitting the wheel and tire, and directlyafter inflation. This procedure needs verylittle time, since the rapid rise in air pres-sure causes a large number of datatelegrams to be transmitted. The secondstage is to check that the correct wheelshave been fitted. Since a large number ofdata telegrams can be called up by interro-gation, this check takes only a fraction of asecond, and can be performed directly dur-

ing the fitting operation. This is not possi-ble on the current system because thecheck takes longer than the time providedfor in the assembly-line cycle; the check istherefore postponed until the end of the as-sembly line is reached.

Systematic functional checks are also ofgreat importance in the repair shop. If thecurrent system reports a fault in the RFtransmission path, the workshop normallystarts a replacement process involving thecontrol unit, the antenna and, if the fault

has still not been rectified, the wheel elec-tronics. Sometimes the mechanic discoversan error that can only be regarded as banal

in nature, but which occurs quite often: thewheel in question is not fitted with tiremonitoring electronics at all!

On the new system, the wheel electronicscan be checked separately and from outside

the vehicle, provided that the diagnosticunit has a triggering function. The check isthus relocated to component level, whichavoids the poorly directed and costlyprocess of replacing other assemblies.

7 Use of the System

The Beru tire pressure monitoring system isalready being offered as an optional extraon a series of automobile models: the BMW3, 5 and 7 Series and the new X5; the Daim-lerChrysler S Class, CL Coup and AMG E

Class; the Audi A8 and the new AllroadQuattro. Various manufacturers have plansto fit it to other models.

The versions and equipment levels whichthe new system permits make it suitablefor use in all vehicle categories. Further im-petus will come from the adoption of tireswith a run-flat characteristic, which makethe installation of a tire pressure monitor-ing system effectively mandatory. This isbecause, in the absence of such a system,there is a risk of the run-flat condition,

which is difficult for the driver to identify,being used for a longer period and at high-er speeds than these tires permit.

DevelopmentTires

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