EBSElectronically controlledBrake systemin the city bus CITARO / CITO

System and functional description

1. Edition

© Copyright WABCO 2003

Vehicle Control SystemsAn American Standard Company

8150004073

The right of amendment is reservedVersion 001/03.00

2 �����

EBS

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Advantages of EBS. . . . . . . . . . . . . . . . . . . . . . . . . 3

System structure. . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Description of componentsBrake signal transmitter . . . . . . . . . . . . . . . . . . . 5Central module . . . . . . . . . . . . . . . . . . . . . . . . . . 6Proportional relay valve. . . . . . . . . . . . . . . . . . . . 73/2 relay valve . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Axle modulator . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Description of a 4S/4M- and 6S/6M- systemFunction of the electro-pneumatic unit . . . . . . . 10Function of pneumatic redundancy . . . . . . . . . . 10Additional redundancy on the front axle . . . . . . 10Rear axle redundancy . . . . . . . . . . . . . . . . . . . . 11Control functions . . . . . . . . . . . . . . . . . . . . . . . . 11Backup functions. . . . . . . . . . . . . . . . . . . . . . . . 12Halt brake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Error recognition.................................................... 13

EBS ”emergency modes” ..................................... 14

EBS test types........................................................ 14

Table of Contents

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EBS

Introduction

The demands made on braking systems are increasingsteadily. Therefore, the development and introduction ofan electronic braking system (EBS) is a logical step.

EBS increases traffic safety through reduced stoppingdistance and improved brake stability. The full diagnosisand surveillance functions as well as the display of brakelining wear offer an effective fleet logistics.

Advantages of EBS

EBS reduces service costs considerably.

� The electronic braking system has a lot offunctions. The aim is to maximise braking safetyat reduced costs, for instance by optimising wheelbrake lining wear.

� Setting pressure, according to wear criteria, to thefront and rear axle results in uniform lining wear.Overall wear is minimised by making the load onall wheel brakes uniform. Moreover, servicing andlining replacement are done at the same time. Thisreduces down-time costs.

� Depending on the vehicle utilisation profile andother factors, this also means considerablesavings for the vehicle user. In terms of wheelbrake service costs alone, a firsthand owner willsave more money with an electronically brakedbus than with a vehicle with a conventional brakingsystem.

stopping distance

spee

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conventionalbraking system

Introduction / Advantages of EBS

2

4 �����

EBS

WABCO EBS construction kit

The design and structure of WABCO EBS allow highflexibility for vehicle manufacturers during systemconstruction. In terms of range

� subsystem or full system� addition and cut-off redundancy � electrical interfaces

the most complex demands can, therefore, be met. Tomeet the main needs of the vehicle owner, WABCOrecommends an EBS with an individual pressure controlunit on the front and rear axle, and which provides forpneumatic redundancies in all brake circuits.

System structure in the CITARO

The EBS described here consists of a dual-circuit, purelypneumatic unit and a superimposed single-circuit,electro-pneumatic unit. This configuration is described as2P/1E system.The single-circuit, electro-pneumatic unit comprises acentral electronic control device (central module), theaxle modulator with integrated electronic unit for the rearaxle, and, if necessary, the axle modulator for the thirdaxle, a brake signal transmitter with two integrated

desired value sensors and brake switches, as well as aproportional relay valve and two ABS valves for the frontaxle. In terms of structure, the dual-circuit pneumatic unitbasically corresponds to that of a conventional brakingsystem. This unit serves as redundancy and onlybecomes active in case of electro-pneumatic circuitfailure.

2P/1E-EBS

AME = Axle modulatorBWG = Brake signal transmitterECAS = Electronically Air Suspension SystemEMR = Electronically engine controlPRV = Proportional relay valveREDV = 3/2 relay valve (redundancy valve)RET = RetarderRV = Relay valveS = Speed sensorV = Wear indicator

Battery

halt

brak

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supply

third axle

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EBS

Brake signal transmitter480 001/ 002 ... 0

The brake signal transmitter is used to produce electricaland pneumatic signals, and to increase and decrease theair pressure of the electronically controlled braking sys-tem. The device has a dual-circuit pneumatic and a dual-circuit electrical structure. Actuation start is recordedelectronically by a double switch. The operating tappet’sroute is controlled and transmitted as pulse-width modu-lated electrical signal. Further pneumatic redundancypressure is delivered in circuits 1 and 2. The pressure inthe second circuit is retained slightly in the process. Incase of (electrical or pneumatic) failure of a circuit, theother circuits remain functional.Depending on bus type, the brake signal transmitter isactuated via a running plate (480 002 ... 0, e.g. CITO) orvia a hanging pedal and tappet (480 001 ...0 in theCITARO).

Description of components

2

3/2 relay valve (redundancy valve) EBS central module

How it works:

6 �����

EBS Description of components

EBS central module 446 130 ... 0

The central module is used to control and monitor theelectronically controlled braking system. It determinesthe vehicle’s nominal delay from the signals received bythe brake signal transmitter. The nominal delay and thewheel velocity measured by the speed sensors are inputsignal for the electro-pneumatic control unit, which usesit to calculate nominal pressure values for the front axleand rear axle. The front axle’s nominal pressure value isthen compared with the measured actual value, and anyexisting deviations corrected with the help of the propor-tional relay valve. Moreover, the wheel velocity is evalu-ated so that in case of locking, an ABS control can becarried out by modulating the braking pressure in thebrake cylinders. The central module exchanges EBSsystem bus related data with the axle modulators. Thecentral module communicates with other systems (en-gine control unit, retarder, etc.) via a vehicle data bus.

4- and 6-channel

Brake SignalTransmitter

Central module

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Wea

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ABS

Valv

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ProportionalRelay valve

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Wea

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ABS

Valv

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Red

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valv

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eed

sens

or

Spee

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Spee

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Spee

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Wea

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icat

or

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r ind

icat

or

Wea

r ind

icat

or

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Axle Modulator Axle Modulator

third axle *

* optional

IES-

CAN

halt

brak

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ABS

rock

ing

switc

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ASR

rock

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pin

15pi

n 30

api

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pin

31

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EBS

2

Description of components

Proportional relay valve 480 202 ... 0

The proportional relay valve is used in the electronicallycontrolled braking system to modulate the brakingpressure on the front axle.

It comprises the proportional solenoid valve, relay valveand pressure sensor. Electrical drive and monitoringtakes place via the central module of the hybrid system(electro-pneumatically / pneumatically).

The control current impressed by the electronic unit istransformed via the proportional solenoid valve into acontrol pressure for the relay valve. The proportionalrelay valve’s output pressure is proportional to thispressure. The pneumatic drive on the relay valve takesplace via the brake signal transmitter’s redundant (back-up) pressure.

EBS central module

Air Reservoir

Air Reservoir

Rear axle

Brake ActuatorFront axle

Brake signal transmitter

How it works:

8 �����

EBS Description of components

3/2 relay valve 480 205 ... 0

The 3/2 relay valve is used to supply air to and removeair quickly from the brake cylinder on the rear axle in caseof redundancy, and comprises several valve units whichmust fulfil the following functions, among others:

� 3/2 solenoid valve function for restrainingredundancy in intact electro-pneumatic brakingcircuit

� Relay valve function, to improve the timebehaviour of redundancy,

� Pressure retention, to synchronise the beginningof pressure level control on the front and rear axle,in case of redundancy

� In case of redundancy, rear axle is controlled. 1:1

The 2/2 relay valve also has a 3/2 directional controlvalve to which current is supplied in case of ABS and isthus meant to prevent an involuntary drive through of therear axle redundancy pressure during ABS control. Port 43 must be plugged.

EBS central module

Air Reservoir

Axle modulator

How it works:

Brake signal transmitter

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EBSDescription of components

2

Axle modulator 480 103 ... 0

The axle modulator controls the brake cylinder pressureon the two sides of a single or dual axle.

It has two pneumatically independent pressure controlchannels with an air admission and bleeder valve each,individual pressure sensors and a joint electronic controlunit. Desired pressure definition and externalsurveillance take place via the central module.

Moreover, wheel rotation speed is measured andevaluated via two speed sensors. In case of wheel-lockor wheel-spin, the set nominal value is modified.

Provision has been made for the connection of twosensors to detect brake lining wear.

The axle modulator has an additional connection for aredundant pneumatic braking circuit. A two-way checkvalve on each side drives the higher pressure (electro-pneumatic or redundant) through to the brake cylinder.

EBS central module

Air Reservoir

3/2 relay valve (redundancy valve)

Tristop® Cylinder

Tristop® Cylinder

How it works:

10 �����

EBS Description of components

Function of the electro-pneumatic unit

The electro-pneumatic unit of the bus and its signal pathwork via

� Brake signal transmittertwo distance sensors determine the nominal valuewhich is transmitted as pulse-width modulatedsignal; two integrated switches are used fornominal value confirmation, among others

� Central modulefor determining the desired pressure for each axle,and system control

� Proportional relay valvefor pressure control on the front axle

� ABS solenoid valvesfor the quick ABS pressure control cycles on theleft and right wheel brakes of the front axle

� 3/2 relay valve (redundancy valve)for restraining the rear axle redundancy pressure

� Axle modulatorswith integrated control unit for regulating brakepressures on each side of the rear axle(s).

EBS can be activated electrically via the driving switch(pin 15) or by actuating the brake signal transmitter viathe integrated brake switches.

The measured brake pedal distance is interpreted as thedesired delay and converted by the central module intodesired pressure standards for the rear and front axle,using various criteria.

The nominal value standard for the axle modulators istransmitted by the central module via a central module.The axle modulators regulate and record the brakingpressures of the rear axles’ left and right wheel brake.The braking pressure of the front axle is regulated by thecentral module via the proportional relay valve withintegrated pressure sensor.

The wheel rotation speeds are recorded via the sensorsknown to the ABS system and serve, among others, asinput quality for the pressure control algorithms, for theABS function and for the ASR function.

Before carrying out a wear control operation, the brakelining wear sensors analyse the brake lining wear on

each wheel brake. The sensor signals from the front axleare recorded by the central module whereas those fromthe rear axle are recorded by the axle modulators.

Signals are processed and errors monitored for the rearaxles in the axle modulators, so that the sensor valuescan be subsequently transmitted to the central modulevia the data bus.

Function of pneumatic redundancy

Front and rear axle circuits work with differentredundancy methods. The front axle circuit worksaccording to the additional redundancy principle, the rearaxle circuit is equipped with a redundancy unit which canbe activated with a valve.

Additional redundancy on the front axle

The front axle circuit which functions pneumatically andserves as redundancy works via

� Brake signal transmitterwith 2 pneumatic circuits (front and rear axle)

� Proportional relay valverelay valve with combined pre-control viapneumatic front axle circuit and the proportionalsolenoid valve

on the front axle brake cylinder.

Electro-pneumatic pressure is delivered via theproportional valve when the brake signal transmitter isactivated. Depending on the control force, pressure issupplied to the proportional valve by the brake signaltransmitter in a delayed, pneumatically redundantmanner.

This is added to the pressure delivered already electro-pneumatically. The pressure delivered by the proportion-al valve is adjusted to the set desired pressure by varyingthe electro-pneumatic pressure.

In case of electro-pneumatic unit failure, the pneumaticpressure part alone affects the proportional valve whichcan be raised to pmax by actuating the brake pedalfurther.

Due to the need to restrain the front axle redundant brakepressure vis-à-vis the electro-pneumatic pressure output(for instance, measures to optimise wear, or integrationof endurance brake), the ”electrical” nominal value

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EBS

predominates the pneumatic front axle pressure outputon the brake signal transmitter (2nd pneumatic circuit ofthe brake signal transmitter).

Rear axle redundancy

The pneumatic redundancy of the rear axle works via

� Brake signal transmitterwith 2 pneumatic circuits (front and rear axle)

� 3/2 relay valve (redundancy valve)with a 2/2 solenoid valve, a 3/2 way valve and arelay valve

� Relay valvefor the additional axle

� Shuttle valvesintegrated in the rear axle modulator

on the brake cylinder of the rear axles.

During hitch-free EBS operation, i.e. an electronicpressure control is possible on the rear axle, the 3/2solenoid valve in the 3/2 relay valve is set to ”switch offredundancy”, due to the electronically controlledpressure at the left rear wheel.

The brake signal transmitter is separated electrically andhas a dual circuit. The actuation process is recognisedvia two switches. The switches have the followingfunctions:

� Sensing the beginning of the braking process

� activating the EBS (if the driving switch is in the”off” position)

� the offset values of the nominal value sensors arecalibrated and monitored without being activated

The inactivated distance sensors transmit the electricalbrake nominal value as pulse-width modulated signals tothe central module. Both signals of the redundantelectrical transmitter are evaluated equally.

The braking pressure on the front axle is regulated withelectrically controlled proportional valves. The actual-pressure sensors are integrated into the valvesubassemblies. The actual values are transmitted asanalogue signals.

Axle load sensing is not required. The braking pressureon each axle is determined by a special braking forcedistribution function. The valves are actuated by thecentral module.

The EBS system status, for instance available errors, istransmitted by the EBS to a display unit via the vehiclebus (data line).

Potentiometers must be provided (or possibly alternativelimit switches for drum brakes) for sensing brake liningwear, and which are read in for the front axle by thecentral module. The activities of each rear axle wearsensor is recorded by the axle modulator; the results aretransmitted via the system bus brake to the centralmodule. The sensors are supplied individually and peraxle with short circuit-proof 5V.

Control functions

Endurance brake integrationThe braking system has an integrated brakemanagement function which always regulates theendurance brake when the brake pedal is activatedbased on an optimum delay of the vehicle. Optimumservice brake wear is attained through the distribution ofendurance and service brake. This function is animportant part of delay control. The integration ofendurance brake can be deactivated via the switch.

Delay controlDelay control is used to adapt the braking pressure levelto the driver’s desired braking rate (def. As z in %).

When the same amount of pressure is applied on thepedal, the vehicle is often braked in the same manner,irrespective of the amount of load it is carrying.

If the coefficient of friction on a wheel brake changes (forinstance when the vehicle is moving downhill), the delaycontrol unit ends every adaptation process when apredefined, fixed maximum is attained, to enable thedriver to also to feel the deterioration.

Adapting to the braking system hysteresis is also part ofdelay control. Each time the brake is released, thereleasing process is chosen in such a way that animmediate braking force modification is adjusted. Thisfunction results in quick release of the brakes, i.e. carfeeling.

Braking force distributionThe distribution of braking forces on the front and rearaxles depends, among others, on the comparison madein the program range ”Delay control” between the actualand nominal value of vehicle delay. The pressure on thefront and rear axles is set in such a way that there isoptimum braking pressure output on these axles.

Description of a 4S/4M- and 6S/6M- system

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EBS

Brake lining wear control

When the brake is applied partially, the braking pressuredistribution is adjusted, notwithstanding the availablewear signals, i.e. the detected wear difference. Thepressure on the wheel brakes with more lining wear isreduced slightly, whereas the pressure on the wheelbrakes with less lining wear is increased adequately, sothat there is no change in the overall braking raterequired by the driver.

The EBS contains the following renowned functions:

Anti-lock braking system (ABS)

The control logic determines from the wheel rotationspeed whether one or more wheels can be blocked anddecides whether to decrease, maintain, or increase thebraking pressure on it. The rear axle wheels arecontrolled in their optimum area in a similar manner(individual control �IR).

On roads with extremely different adhesion levelsbetween the right and left sides, vehicles areuncontrollable or difficult to control using the differentbraking forces in ABS (yawning moment development).

As a result, the braking pressure on the front axle brakesis not adjusted independent of each other, so that thedriver can have a steering reaction (modified individualcontrol � MIR).

If the driving wheels are locked when the endurancebrake is applied on low adhesion levels, possiblyresulting in vehicle instability, the ABS endurance brakeis deactivated via the vehicle’s CAN bus to maintainvehicle stability.

Traction control system (TCS)

Just like in the ABS function, while the vehicle is inmotion, the electronic control system determineswhether the wheels are in the stable area of the µ slipcurve.

In case of wheel-spin, the engine output and/or drivingaxle wheel braking is adjusted by the axle modulator viathe CAN bus and engine control system. An activatedtraction control system is displayed on the functionsdisplay.

Drag torque controlDrag torque occurs in the drive line due to actuation orchange of gas. The resultant braking torque can lead todriving wheel locking and thus to vehicle instability. Dragtorque control prevents this situation. When a slip state isexceeded, the engine torque increases and the braketorque is reduced, irrespective of the driving wheelvelocity. The function of the traction control system endswhen the driving wheel values become stable again.

Backup functions

Generating brake nominal valueThe brake pedal distance measured by the sensors inthe brake signal transmitter is transmitted to the centralmodule where it is ”processed”. The distance isconverted into a nominal delay, based on thecharacteristic curve described in the chart.

Determining brake nominal value

Pressure control on the front and rear axlesThe nominal pressure derived from the brake nominalvalue using the higher control algorithms is controlled inthe front and rear axle’s pressure control circuits. In orderto improve the pressure control properties, the solenoidcurrent in the solenoid valves are controlled.

Wheel speed sensing and wheel adjustment Wheel speed sensing corresponds to the sensingfunction known to ABS. Automatic wheel adjustmentmakes up for the nominal wheel sizes and thus the rollingcircumferences between the axles. If unacceptablewheel combinations are used, this is recognised as anerror.

If the wheel sizes change, the system requires a changeof parameters.

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znominal

Description of a 4S/4M- and 6S/6M- system

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EBS

Halt brakeWhen the halt brake switch in the dashboard is activated,the command ”Activate halt brake” is transmitted to thecentral module via the FPS and the emergency switch”release halt brake”. This latter sends the command tothe proportional relay valve and axle modulator(s), sothat 2 bar braking pressure is exerted on the brakecylinder of the front and rear axle.Either the halt brake is released or actuation is given viadoor electronic unit and afterwards with the brake signaltransmitter, the requirement ”halt brake” via FPS,emergency switch and central module is cancelled.

Failure diagnostics

Error recognition functions are used to avoid the effectsof system failures and/or to inform the driver aboutfunctional problems. The error recognition principlescorrespond partly to those of a conventional ABS device(monitoring the ABS valves, the wheel speed sensors,the computer hardware).

On the other hand, a big part of the surveillance functionsconcerns EBS-specific functions (EBS-specific sensoranalysis, EBS-specific solenoid control, braking pressurecontrol, data transmission via CAN bus).

In addition to the wheel speed sensor signals, the EBSevaluates many other sensor signals and checks thatthese signals are error-free.

Nominal value sensing (sensors and switches)The brake signal transmitter provides two sensor and twoswitch signals. The (pulse-width modulated) sensorsignals are checked to see whether they conform withthe authorised values, and for mutual deviations. Thecorrectness of the (digital) switch signals are then tested.

Braking pressure sensing (on the front and rearaxles)The pressure sensor’s (analogue) signals in the pressurecontrol circuits are checked to see whether theycorrespond to the authorised values.

Note: The cabling for the two rear axle sensors is notaccessible from outside, since it is an internal axlemodulator cabling.

Wear sensing (on the front and rear axles)The (analogue) signals from the wear sensors arechecked to see whether they correspond to theadmissible values.

The EBS monitors EBS-specific solenoid valve control.

Front axle proportional relay valveThe frequent solenoid (pressure proportional to thesolenoid current) of the front axle proportional relay valveis checked to see whether control is carried out correctly.

Rear axle redundancy valveThe rear axle redundancy valve’s solenoid switch ismonitored to see that control takes place correctly.

Rear axle modulator’s inlet and discharge valve The rear axle’s inlet and discharge valves are locatedinside the axle modulator. The solenoid cables are notaccessible from outside.

The EBS monitors braking pressure control. The electrically controlled braking pressure and the pneumatically redundant pressure are also monitored.

Too low front axle braking pressure The availability of minimum braking pressure (on thefront axle at a certain solenoid current supply level) ischecked.

Too high rear axle pressure deviation (from left toright) In normal braking processes (neither ABS nor tractioncontrol system – TCS -controls) the measured brakingpressure on the left and right sides of the rear axle mustalmost be equal. If the braking pressure deviationexceeds the admissible value, an error is reported.

Rear axle redundancy failsIn certain situations (vehicle stand still, parking brake instop position), an electonically control of brake pressureon front- and rear brake is prevented. If the driver will nowactuate the pedal, brakes on front- and rear brakes arecontrolled via pneumatical redundancy. If the front axlebraking pressure exceeds a certain value, the rear axlemust have a specified minimum pressure. A faiIure isrecognized if this not applies.

Rear axle redundancy cannot be switched offNormally, pneumatic redundancy pressure control in therear axle is prevented by the redundancy valve. If anerror makes this deactivation impossible, the rear axlebraking pressure can no longer be reduced during ABScontrol, etc. (because there is inlet of non-ABS-compatible rear axle redundancy pressure into the rearaxle brake cylinder). Error recognition takes place in thiscase.

Failure diagnostics

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EBS

EBS monitors data transmission� between the EBS central module and the axle

modulator (System bus)

� between the EBS and other system controldevices (vehicle bus)

If communication is impossible or is suddenly cut, anerror is reported.

EBS ”emergency modes”

As a rule, certain EBS functions are deactivated when anerror is detected. Functions not impaired by the failureare maintained. For the EBS-drive with limited functions,the term ”emergency mode” is used.

The following functions can be deactivated if an error occurs:

Operation without ABS functionThe ABS function can be deactivated on a wheel, anaxle, or the entire vehicle. Possible causes: faulty speedsensor signal, ABS valve error, etc.)

Operation without ASR functionThe traction control system can be switched offcompletely or partially. Complete deactivation meansthat both the braking system and the engine control unitare deactivated. Partial deactivation means that only thebraking system is deactivated. Possible causes: (faultyspeed sensor signal, etc.)

Pressure control / auxiliary pressure controlNormally, braking pressure control requires brakingpressure sensor signals. When these signals are nolonger available, electrical braking pressure can beproduced. In this case, we talk in terms of pressurecontrol operation or auxiliary pressure control. However,the accuracy of this pressure production is limited,compared to hitch-free pressure control. Possiblecauses: Pressure sensor signal failure, etc.)

Redundancy operationIf electrical pressure control becomes impossible, thecorresponding axle is braked with the help of thepneumatic redundancy pressure. Possible causes:damaged solenoid, or faulty solenoid cabling, etc.)

EBS test types

The following peculiarities must be observed while testing the electronically controlled braking system:

� Test of the 3/2 relay valve (redundancy valve) A vehicle stand still, with parking brake in stopposition and ignition on together with an actuationat the brake pedal will switch off the axlemodulator. Now, the function of the redundancyvalve can be tested with a pressure gauge,connected to the rear axle's brake cylinder. Thefailure of the electronical circuit is simulated. Themeasured pressure must correspond to the supplypressure.

� Maximum pressure level control: When thebrake is applied > 80% of the pedal stroke andignition is off, the full pressure must work on thefront and rear axles.

� Roller dynanometer test: (Roller dynanometer function)A roller dynanometer function was integrated intothe EBS electronic unit so that an electronicallybraked bus can be tested on a roller dynanometer.This function is used to test the braking pressureat full load (permitted total weight).The roller dynanometer function is activated if theEBS is not switched on via the ignition (pin 15)when the vehicle is braked again, but ratherthrough activation of the brake signal transmittervia the integrated brake switches. The front axleand/or rear axle speed must be < 3 km/h.Special EBS controls such as endurance brakeintegration, delay control and brake lining wearcontrol are not active when the roller dynanometerfunction is active. The maximum braking pressurecan now be measured. The EBS is workingcorrectly if the measured braking pressurecorresponds to the basic loaded vehicle design.

For WABCO Diagnosis tools please refer to brochure820 001 029 3

Emergency modes/control possibilities