ecas trailer

7/25/2019 ECAS Trailer

1/20

ElectronicallyControlled AirSuspension (ECAS) forTrailers

Function and Fitting Instructions

3rd Edition

This publication is not subject to any update service.New versions are available in INFORM atwww.wabco-auto.com

815 010 025 38150100253

Copyright WABCO 2005

" !

Vehicle Control SystemsAn American Standard Company

The right of amendment is reservedVersion 003/10.04en

815 010 025 3


2/20

2

Table of contents

1. Important Remarks and Comments 31.1 Advises for Security and Risks 3

1.2 Range of Application 31.3 Explanation of Symbols 3

2. Introduction 42.1 System Benefits 5

3. System Functions 63.1 Reference Level Control 63.1.1 Driving Level I 73.1.2 Driving Levels II and III 73.1.3 Unloading Level 83.1.4 Memory Level 83.1.5 Automatic Lowering 8

3.2 Height Limitation 93.3 Lateral Stabilisation 93.4 Lifting Axle Control 93.5 Offset Point Adjustment

(Lifting Axle Offset) 93.6 Traction Help 93.7 Overload Protection 93.8 Tyre Deflection Compensation 103.9 Stand-By Function 103.10 Further ECAS-Functions on Bus and

Motor Vehicle 103.10.1 Control of Load Sensing Valve 10

3.10.2 Kneeling 103.10.3 Maximum in Traction Control at all time 10

4. Basic Operation 114.1 Mode of Function of the ECAS Basis

System 11

5. Control Algorithm 125.1 Control Algorithm for Levelling Control 125.2 Control Algorithm for Lifting Axle Control 14

(general)5.2.1 Control Algorithm for Lifting Axle Control 15

(one Lifting Axle)

5.2.2 Control Traction Help 175.2.2.1 Trailing Axle Control (Manoeuver Aid) 185.2.3 Control Algorithm for Lifting Axle Control 18

(two separate Lifting Axles)

6. Tyre Deflection Compensation 21

7. System Configuration 227.1 Control in the Trailer 227.1.1 1-Point Control 227.1.2 2-Point Control 227.1.3 3-Point Control 22

8. Components 24

8.1 Sensors 248.1.1 Height Sensor 441 050 011 0 248.1.2 Pressure Sensor 26

8.2 Electronic Control Unit (ECU)446 055 ... 0 27

8.2.1 Installation 288.2.2 Pin Assignment 30

(ECU variant 446 055 065 0)8.2.3 Power Supply and Diagnostic

Assignment 318.2.4 Battery Mode 338.3 ECAS Solenoid Valve 348.3.1 Spring-Returned Valve 348.3.2 Pulse-Controlled Sliding Valves 358.3.3 Difference between 3/2-, 2/2- and 36

3/3-Way Valve8.3.4 Differentiating of the ECAS Solenoid 37

Valves depending on their location8.4 Remote Control Unit 446 056 116/117 0 398.4.1 Remote Control Unit Function

446 056 117 0 39

8.5 Remote Control Box 428.6 Battery Box 428.7 Pneumatic Components and 42

Installation Instruction

9. Commissioning and Diagnostics 469.1 PC Diagnostics 469.1.1 PIN 479.1.2 Initializing the ECU 47

9.2 Setting Parameters 489.2.1 Option Parameters 489.2.2 Value Parameters 489.2.3 Counts 489.2.4 Explanation of Parameters 499.3 Calibration Process 659.3.1 Height Sensor Calibration 669.3.1.1 Height Sensor Calibration with the PC 679.3.2 Pressure Sensor Calibration 68

10. Troubleshooting 6910.1 Safety Concept 6910.2 Troubleshooting 71

11. Replacement older Components 7311.1 Fitting a new ECU 7311.2 Replacement of the Power Supply

Module 7511.3 Component Replacement 75

12. Annex 79Trailer ECAS Parameter Setting 81Explanation of theExample Parameter Records 83Overview Circuit Diagrams 84Parameters Records + Circuit Diagrams 86

Cable Overview 186Certificate of German TechnicalInspection Agency 188

Table of contents


3/20

3

1. Important Remarks and Comments

1.1 Advises for Security and RisksECAS is a vehicle security system. Changes on thesetting of the system are allowed only to persons withnecessary technical knowledge.

When the ignition is switched on or while diagnosticsstarts unexpected movements of the vehicle or suddenraise/lower of the lifting axle can occur.

If you work on the air suspension system, advise otherpersons by affixing an information sign on the steeringwheel of the vehicle.

It is not allowed to combine ECAS with other airsuspension control systems, since the possibility ofdangerous interactions cannot be excluded.

Following points have to be observed when welding atthe trailer :

The electronics have to disconnect from power supply(break off pin 31, 15 and 30). At least, the supply linebetween the towing vehicle and trailer is to be takenoff.

System components (ECU, sensors, actuators, linesetc.) are not being contacted with welding electrodes.

Never drive with the superstructure lowered onto thebuffer, because vehicle and loading can be badlydamaged.

1.2 Range of Applicat ion

ECAS was designed only for suspension control inair sprung vehicles.It is not permitted to place more than one ECASsystem to a trailer.

To avoid dangerous interaction, combination withother air suspension control systems are notadmissible.

Important Basic Requirements for Operation withECAS:

Compressed air supply must be realized sufficiently.

Power supply has to be ensured.

ABS connection or EBS connection must be plug in.

To work on the ECAS system, take onlyinformation shown on circuit diagrams identifiedwith a ten-digit WABCO number.Circuit diagrams without a WABCO number maybe incorrect. They are considered to be diagramsfor which no release from WABCO exists.WABCO does not assume any warranty forsystems which are designed in another way thandescribed here.

You need the approval of WABCO for the:

Usage of components other than those shown in thecircuit diagrams (cables, valves, sensors, remote

control units), Inclusion of any appliances by other manufacturers in

the system or

Implementation of other functions than thosedescribed above.

The structure of the ECAS system is shown by manycircuit diagrams in the annex.

1.3 Explanat ion of Symbols

Possible dangers,Personal injury or material loss

Additional hints, info, tips

C WABCO experimental value(s),recommendation

enumeration

action step

refer to (prior paragraph, prior chapter, prior illustration/table)

refer to (prior paragraph, prior chapter, prior illustration/table)

!

!

!

Important Remarks and Comments ECAS 1.


4/20

4

Introduction2. ECAS

2. Introduction

Air suspension systems have been used on motorvehicles since the 50s - especially on buses. They clearlyimprove ride comfort.

On lorries and trailers, air suspension systems are beingincreasingly used, particularly on vehicles designed tocarry heavy loads. The main reasons for this are specialdesign criteria for the chassis. As static loads can varysignificantly on the motor vehicle's real axle, and on allaxles of a trailer from the unladen vehicle to the fullyladen vehicle. Steel suspension can cause problemswhen the vehicle is unladen or partially laden. Thesuspension behaviour deteriorates. In addition, ridecomfort plays an important role, not only on buses.

The Benefits of Air Suspension Systems over LeafSpring Suspension Systems

The whole of spring travel is available for balancingdynamic axle load cycles. Static axle load cycles arecompensated by means of pressure changes. This inturn achieving additional height for the design of thesuperstructure.

The best possible suspension regardless of the roadcondition and the load carried improves ride comfortand protects the load. No rolling noise of the vehicle

is transmitted. The wheels run evenly on the road surfaces; this

improves braking performance and steerability andconsiderably extends the life of the vehicle's tyres.

Accurate load-dependent control of the compressed-air braking system by using the bellows pressure asthe actuating pressure for the load sensing valve.

Constant vehicle height is irrespective of the staticload.

Controlled raising and lowering processes for loadingramp and container operation.

Control of lifting axles. Individual control of the bellows pressure to

compensate lateral forces (e.g. when negotiatingbends).

Protecting the road surface.

Disadvantages of Conventional Air SuspensionSystems compared to Leaf Spring SuspensionSystems

More expensive system,

More complicated axle systems due to the use of axle

steering and axle stabilisers, A large number of parts due to numerous air

components,

Significant wear of control valves due to constantincrease or decrease in pressure; shorter life due tocontinuously alternating stress

control of cornering angles.

Designing the control system initially with puremechanically operating air suspension valves, soonafterwards electromechanic control systems weredeveloped. This served to enhance operating comfortand to facilitate raising/lowering processes.

ECAS is the most advanced development along theselines. The use of an ECU has achieved major

improvements over the conventional system.

ECAS - Electronically Cont rolled Air Suspension

(Electronically controlled air suspension system)

ECAS is an electronically controlled air suspensionsystem for vehicles with a large number of functions. Ithas been used on motor vehicle since about the early80s.

In the mechanically controlled air suspension systemsthe device which measures the height also controls the

air spring. While ECAS achieves control by means of anECU. It actuates the air spring via solenoid valves, usinginformation received from sensors.

In addition to controlling the vehicle's level, the ECU,together with the remote control unit, also controlsfunctions which, if implemented with conventional airsuspension systems, require a large number ofcomponents.

With ECAS, additional functions can be implementedwhich cannot be achieved by conventional means.

ECAS essentially operates only when the ignition is on.However, is a storage battery is used, stand-by operationcan be activated.

For trailers, power is supplied from the ABS or the EBSsystem. In addition, ABS provides the so-called C3signal, i.e. information on the vehicle's current speed. Ontrailers where ECAS works with EBS, the ECAS systemreceives information about the vehicle's speed and axleload from the EBS-ECU via a data circuit which is alsoknown as the K-line.

A storage battery is provided for additional power supplyin the trailer. Due to this the trailer separated from thetowning vehicle can be adjusted in the height.


5/20

5

Introduction 2.ECAS

2.1 Sys tem Benefits

ECAS offers the following benefits:

Advantages for the Trailer Manufac turer Easy installation due to pre-wired components

Reduced installation time due to a smaller number ofcomponents, screw-in units, less piping

Quicker system start-up via PC end-of-line.

Benefits to the Carrier

Highly service-friendly,

Low error liability

Short laidup times,

Various safety functions for the vehicle (e.g.compensating for tyre deflection, time delay for theunloading level, overload protection),

Automatic return to driving level,

Highly variable operation due to different drivinglevels,

Very rapid raising and lowering times,

High level of comfort and safety due to traction helpfor semi-trailer trains and automatic lifting axlefunction,

Can easily be retrofitted,

Low air consumption due to rugged control concept,

Reduction of the amount of manoeuvring and loadingtimes and thus shorter dispatching times throughstorage of loading levels which are easy to adress viathe remote control unit plus the option of raising orlowering only one side of the superstructure.

Advantages for the Owner o f t he Cargo

Protection of the load,

Adaption of the vehicle to the carrying task due to theuniversal nature of the system,

Reduction of the amount of manoeuvring and loadingtimes and thus shorter dispatching times throughstorage of loading levels which are easy to call up viathe remote control unit.


6/20

6

3. System Functions

Before the system functions for achieving the abovesystem advantage are explained, please note thefollowing basic definitions to facilitate comprehension.

Trailers can have different Types of Ax les

The Main Axle (also known as the Leading Axle)The axle is always on the the ground and cannot besteered. Every trailer has a leading axle. On drawbartrailers the rear axle is the leading axle.

Steering AxleThe steering axle is an axle on a trailer which issteerable. On drawbar trailers, the front axle is the

steering axle. Semi-trailers can have a the trailing axlesas a steering axle.

Lifting Axle

The lifting axle is usually combined with the leading axleto form an axle assembly. When the vehicle exceeds adefined axle load on its leading axle, the lifting axle islowered and can be raised again when the axle load fallsbelow a defined level.

Air Suspension bellows in Air Suspension Sys tems

Supporting BellowsSupporting bellows are the commonly known airsuspension bellows on the axles. They provide thevehicle's suspension. The supporting bellows on theaxles which are in contact with the ground are alwaysfilled with a bellows pressure which is proportional to theapplicable wheel load while the vehicle is in operation.The supporting bellows of raised axles aredepressurised. Supporting bellows are found on all thetypes of axles described above.

Lifting Bellows

Lifting bellows are firmly connected to a lever system ofthe lifting axle. They raise or lower the lifting axle whenthe pressure exceeds or falls below a defined limitpressure in the supporting bellows of the axle assembly'sdriven axle.

ECAS is a control system consisting of at least onecontrol loop. A reference value is specified in a controlloop. A sensor is adapted to the system due to acalibration process when the system is taken intooperation. This sensor measures the actual value of thesystem and sends it to an electronic control unit (ECU).

The ECU compares the actual value to the referencevalue. During this process it is possible to determinecontrol deviations .

That means, that the actual value lies outside a definedreference range.

In the event of a deviation the ECU initiates thecorrection of the reference value via an actuator.

Reference Values are:

Certain distances (levels) of the vehicle'ssuperstructure above the vehicles's axle,

Axle load vehicle conditions (e.g. traction help, limitpressure for lifting axle control).

Two Ways to Submiss ion the Reference Value to the

ECU: Fixing preference values by the vehicle manufacturer

during the systems startup by setting parameters andby calibration.

The user setting the values via the remote controlunit.

Please note that not all the functions described in thisdocument have to be available, depending on the type ofsystem realized. The type of system (amount of liftingaxles, with/without front axle air suspension) determineswhether or not the functions can be implemented.

ECAS can be fitted to every vehicle type withoutproblems. Due to the modular construction varioussystem usages according to customer requirements arepossible.

3.1 Controlling the Reference Level

The reference level is the value for the distance betweenthe vehicle's superstructure and the axle. It is defined bycalibration, by setting parameters or by definding a value

using a remote control unit. Correcting a reference levelis the basic functions of ECAS.

The actuator is a solenoid valve by increasing anddecreasing of the pressure in the supporting bellowsbringing the actual level into line with the reference level.This occurs if there are:

Deviations over a certain tolerance range,

Alteration of the reference value.

Unlike conventional air suspension systems, ECAScontrols not only the driving level but also any other

predefined level. Therefore a level which is set forloading or unloading processes is assumed to be thereference level.

System FunctionsECAS3.


7/20

7

Differentiation of Static/Dynamic Changes in theWheel Load

By using the speed signal ECAS differentiates betweenstatic and dynamic changes in the wheel load, unlikeconventional air suspension systems. Due to thisdifferentiation the system can react optimally to changesin the wheel load.

Static Wheel Load ChangesThe static wheel load changes occur if the vehicle'sloading state changes when it is stationary or movingslowly. This requires the reference value in thecorresponding air suspension bellows to be checked atshort intervals and adjusted if necessary by increasing orreducing the air pressure. ECAS does this performingchecks every second. This inspection cycle can be set in

the parameters.

Dynamic Wheel Load ChangesThe dynamic wheel load changes are mainly caused byuneven road surfaces and is more likely to occur by highspeed. Dynamic wheel load changes are usuallybalanced by the compliance behaviour of the supportingbellows. In this case, bellow pressurizing or ventingwould not be desirable because only the shut-off bellowhas an almost constant compliance character. For thisreason, the regulation is checked in larger intervals whenthe vehicle is moving at higher speed - usually every 60seconds. The comparison of actual value to the

reference value is made continuously.

It is possible to avoid unwanted corrections of dynamicwheel load changes during braking, so no air is pumpedinto or vented from the bellows, when the ECU receivesthe brake light signal.

Driving Level

The driving level (also known as normal level) adjustsitself when the vehicle moves on a higher speed. Amaximum of 3 driving levels can be set for ECAS.

3.1.1. Dr iv ing Level I

Driving level I referes to the reference level defined bythe vehicle manufacturer for driving under optimalconditions. This driving level is the basis for thedimensions of the vehicle's overall height and thevehicle's theoretical centre of gravity. It has a specialmeaning compared to other driving levels.

Driving level I is described as the basic design parameterfor the vehicle.

Please note to take the legally permitted maximuminto account considered to the total height.

The vehicle's theoretical centre of gravity is a referencevalue for calculation the vehicle's braking.

Inform the system over the driving level I only bycalibration process.

Control the driving level I over the vehicle's speedand/or over the remote control unit during the journey.

Set speed value for the point of actuation forcontrolling the parameters.

3.1.2 Dr iv ing Levels II and III

Both driving levels differ to driving level I. This may be

necessary:

For semi-trailers with varying truck heights to keep thetrailer in horizontal level while driving,

For trailers to lowering the superstructure in order toreduce fuel consumption,

For improving lateral stability at higher speed.

By lowering the superstructure depending on thevehicle's speed, is based on the assumption that higherspeeds are achieved on sound road surfaces which donot require the whole of the bellows' spring stroke to beutilised.

Store the values of driving level I and II in the systemby setting the parameters and as defined variancefrom driving level I.

This driving level can optionally be set by:

Switch,

Remote control unit,

Driving speed (only driving level II).

The chosen driving level remains until another drivinglevel is selected.

To start up with the actual driving level touch shortlythe button driving level.

Set the values for the type and the switching points foractuation during the parameterization,

Define driving level III as the highest driving level.!

System Functions 3.ECAS


8/20

8

3.1.3 Unloading Lev el

The unloading level is used for loading or unloading avehicle, while it is stationary or moving at a slow speed.It permits the vehicle's superstructure to be brought to alevel suitable for the loading or unloading process. Theunloading level can be stipulated separately for adrawbar trailer's front and rear axles. This can be useful,for instance, for completely emptying of a tank trailer.

The values of these unloading level are defined as thevariance from driving level I and store in the systemas part of the procedure for setting the parameters.

If an unloading level function is desired, no drivinglevel III can be set.

The unloading level is addressed via an on the ECU

connected switching contact, the so-called unloadinglevel switch.

This switching contact can either be actuated manuallyor via an automatic connection to the unloading fixture(e.g. a tank valve) to readdress the current driving level.

If the unloading level is active, the vehicle will return to itsdriving level as soon as a limit speed is exceeded. Whenthe speed drops below that speed, the unloading level isset once again.

The unloading level can be activated from any preset

level.

3.1.4 Memory Level

Two different memory levels can be used per system.The memory level applies to the whole of thevehicle. The use of the memory function requires aremote control unit.

The Memory Level can be adressed:

For loading or unloading a vehicle while it is stationaryor

moving at a slow speed.

This level is used in order to set a level for thesuperstructure which is useful for loading or unloadingthe vehicle.Unlike the unloading level, which is firmly stored in theECU, the memory level can be defined and changed anytime. Once defined, the system will store any memorylevel until it is changed by the user, i.e. even when theignition has been switched off.

3.1.5 Automati c Loweri ng

If required, raised lifting axles can be lowered

automatically by the driver.

This may be necessary:

when the vehicle is been serviced,

for improving stability on poor road surfaces.

If the vehicle has two separate controlled lifting axles,anoption parameter can be set to determine whether:

Both lifting axles is to be lowered.

Only the second lifting axles is to be lowered.

Options for Lowering Raised Lifting Axles:

1. Lowering Command via Remote Control Unit

Push the Lower button on the remote control unit ofthe preselected lifting axle.

The lifting axles lowers and will stay down until thesystem is reset by switching the ignition off and then on

again.

Push the Raise button on the remote control unit onthe preselected lifting axle.

The lifting axles will raise again.

2. Lowering Command via Traction Help Key

Hold button Traction Help down for longer then 5seconds.

The lifting axles will be lowered and stay down until reset.The automatic lowering function remains activated, until

the ignition is turned off or the button is pushed againlonger then 5 seconds.

All lifting axles are always lowered independent ofparameter 4 Bit 4.

3. By using a Separate Switch and a Line to theSwitching Input Port for Automatic Lowering onthe ECU.

Make sure the ground is connected to the contact.

The lifting axles are lowered. Automatic lowering remainsactive even after the ignition is switched off or on. Thefunction remains active as long as the switch is active.

Open the contact.

The lifting axles will raise again.

These lowering processes are all permissible up to alimiting speed defined by parameter. If fully automaticlifting axle operating is in place, raising will, depending onthe parameter setting, occur while the vehicle isstationary or when it exceeds a defined speed.

A vehicle with two separate lifting axles whoseparameters have been set for lowering both axles and for

raising them only when the vehicle begins to move, hasits lifting axles raised as follows:

!

System Functions3. ECAS


9/20

9

Deactivate automatic lowering switch.

Axle 1 will be raised immediately.

Axle 2 will be raised once the vehicle has been stopped

and its speed subsequently rises above that required forraising the lifting axles.

3.2 Height Limitat ion

The ECU automatically aborts any change in the level ifthe defined value for the upper and lower height limitshave been reached. These values can be freely selected.This is in order to prevent excessive strain on the rubberbuffers and height limit stops (e.g. bellows, limitingcables).

3.3 Lateral Stabil ization

Vehicles expected to carry loads which are unevenlydistributed across its axles (e.g. loading on one side ofthe vehicle only), can have variable spring rates providedon the supporting bellows of one axle by separating theactuation for the individual bellows.

These vehicles should be fitted with 2-point control (7. system configuration).

This is not required for vehicles carrying evenlydistributed loads (e.g. road tanks).

3.4 L ift ing Axle Contro l

When the vehicle is stationary, its lifting axle willautomatically be lowered or weight shifted to the trailingaxle, when the permissible axle load of the leading axleis exceeded. The corresponding signal reaches the ECUfrom the pressure sensor ( 8.1.2 Pressure sensor) orthe pressure switch at the suspension bellows of theleading axle. The lifting axle cannot automatically belowered when the vehicle is in motion, even whenpressure spikes occur.

The speed up to which the lifting axle can be lowered canbe set by parameters.

For safety reasons, the lifting axle is lowered when thevehicle is parked and the ignition is switched off.Optionally, the lifting axle may stay up.

Pressure Sensor SystemIn systems with pressure sensors, the lifting axle can belowered and also raised automatically after the vehiclehas been unloaded. The term used for this is 'fully

automatic lifting axle'.

Pressure Switches/-Buttons systemThe lifting axle is lowered automatically. The raising isachieved manually by means of the ECAS remote controlunit or a separate button/switch.

A raised lifting axle can be lowered by way of anautomatic lowering function.

3.5 Offset Point Adjustment (Lifting AxleOffset)

When the lifting axle is raised, the driving level can beincreased automatically. Due to this a better clearenceof the lifting axle's wheels is achieved. This applies to thewhole of the vehicle.

3.6 Traction Help

On semi-trailers, traction help can be implemented if thefully automatic lifting axle facility has been selected andthe load is heavy enough. By releasing the pressure onthe lifting axle's supporting bellows or raising the liftingaxle, the load on the towing vehicle's semi-trailercoupling and on the leading axle in increased. Thismeans that the load on the driving axle of the towingvehicle is increased, the purpose being to improve itstraction force.

Activate the traction help function by using the remotecontrol unit or by a rocking Switch.

At present, the applicable national legal provisions metby setting the corresponding parameters accordingly(with/without time speed, load limits, with/without forcedinterval). Upon EC Guideline 97/27/EC comes cominginto force, this results in changes which have been takenin account when setting the parameters.

3.7 Overload Protection

By stipulating a maximum permissible pressure for thesupporting ballows, overload protection can be activated.

This protection leads to a lowering of the vehicle'ssuperstructure down to the rubber buffers, if thesupporting bellow pressure was exceeded byoverloading.

You need to unload the vehicle until the vehicle'ssuperstructure can be lifted by the remote control unit.

Do not drive with the lowered superstructure,

vehicle and loading can be badly damaged.

System Functions 3.ECAS


10/20

10

3.8 Tyre Deflection Compensating

For vehicles with a particularly great overall height, notonly small wheels are used to maintain the vehicle

height, but a very short ride clearance is selected for theempty vehicle.

However, as the vehicle is being loaded, the requiredride clearance increases. It is possible to add the tyredeflection caused by increasing the load to the possibleride clearance, at the same time keeping the overallheight of the vehicle constant.

The vehicle height required by law has to bemaintained.

3.9 Stand-By FunctionIf a separate trailer battery and sufficient air supply areavailable, stand-by operation can be achieved for amaximum of 63.5 hours. For this purpose, the trailer doesnot have to be attached to any towing vehicle, and it willat that time address the last level before the ignition wasswitched off as the referance level. To reduce the controlcycles to a minimum it is possible to increase thetolerance range.

3.10 Further ECAS Functions on Bus andMotor Vehicle

Additional functions, although not of importance fortrailers, can be implemented with the assistance ofECAS on buses and motor vehicles.

3.10.1 Control of Load Sensing Valve

Towing vehicles and trailers with air suspension systemsand a conventional braking system have a load sensingvalve fitted which is controlled by the bellows pressure.

In the event of a rapid loss in pressure in the bellows (e.g.bellows leaking badly or severely damaged), the loadsensing valve would assume an empty vehicle in spite ofit being fully loaded. As a consequence, underbraking

would ensue, and with it excessive stopping distances.ECAS contains a facility for recognizing this so that whenit occurs, the supply pressure of the air suspensionsystem would be passed to load sensing control port 41/42. This results in a pass through of the load sensingvalve. ECAS also provides this facilities for trailers.Therefore a 3/2 solenoid valve has to be installed incontrol line between air suspension bellows and the loadsensing control port 41/42.

This option is mainly being used in towing vehicles.

3.10.2 Kneel ingKneeling is a special function for buses. With the busstationary at the bus stop, the whole of its nearside islowered facilitate getting on and off the bus. It "kneels" infront of the passenger, so to speak. A "kerb sensor" i.e.contact strip below the entrance, prevents it fromtouching down on any obstacle.

3.10.3 Maximum in Traction Control at al l time

ECAS offers the following possibilities on motor vehicles

with lifting axle:

The lifting axle is lowered in load conditions. Thedistribution of the axle load across the rear axleassembly can be controlled in such a way that the drivingaxle (within the scope of legal provisions and thestipulatons by the axle manufacturers) carries thegreatest possible load, with the lifting axle supporting theresidual load. Thus the driving forces which can betransferred to the driven axle are always at theirmaximum, thereby permitting good traction.

!

System Functions3. ECAS


11/20

11

4. Basic OperationIn this chapter the operation of ECAS will now be lookedat in a little more detail.

The basic purpose of ECAS is to balance any controldeviations. Control deviations are caused either bydisturbances (such as a change in the load) or bychanges in the reference values (e.g. by way of theremote control unit). These control devariation cause thedistance between the vehicle's axle and itssuperstructure to change. ECAS balances these controldeviations by means of levelling control.

4.1 Functioning of the ECAS Basis

System( Fig. 1)

1. A height sensor (1) which is mounted on the vehicle'ssuperstructure and connected to its axle via a leversystem. The height sensor picks up the distancebetween the axle and the superstructure, or

bodywork. The intervals depend on the vehicles typeof operating (driving or loading operation).

2. This measured value is used as the actual value in thecontrol loop and is sent to the electronic control unit(2).

3. The ECU compares this actual value to the referencevalue stored.

4. In the event of a control deviation the ECAS solenoidvalve (3) receives an actuating signal.

5. Depending on the type of actuating signal received,the ECAS solenoid valve now increases or decreasesthe air pressure in the supporting bellows (4). Thechange in pressure in the supporting bellows changesthe distance between the axle and the superstructure.

6. The new distance is again detected by the heightsensor, and the cycle begins again.

The remote control unit (5) is no belonger part of theECAS basic system. It is still mentioned because itallows the user actively to change the reference level.

Basic Operation 4.ECAS

Fig. 1 Basic Operation of the ECAS System

1

23

4

5

Basic System:

1 Height Sensor

2 Electronic (ECU)

3 ECAS Solenoid Valve

4 Supporting Bellows

5 Remote Control Unit (optional)

DistanceBody/Axle


12/20

12

Control Algorithm5. ECAS

5. Control Algorithm

5.1 Control Algorithm for LevellingControl

In levelling control, the distance between a vehicle'ssuperstructure and axle is controlled. The levellingcontrol is the basic function of ECAS.It may be necessary to adjust that distance due to theinfluence disturbances, or because the reference valuehas to be changed.

In order effectively to describe how ECAS controls thelevelling process, the basic physics of the air suspensionsystem are described below.

General Comments on the Physics of ECAS

The basic problem in any control system when a controldeviation occurs is to determine the best possibleresponse time. This is the time from when the referencevalue starts to change up until the time when the actualvalue is no longer outside the tolerance range for thereference value ( Fig. 2). Until this is achieved, thecontrol process continues and thus consumes air.

Long control times are the result of slow controladjustments of the actual value to the new referencevalue. Although this does achieve an excellent control

performance, it also takes some time.

If control times are shorter, the time until the newreference value is achieved is reduced although thismeans that the system's tendency to oscillate increases.

The large nominal width of the ECAS solenoid valves,which is beneficial for adjusting small differences inreference values, is detrimental if the differences inreference values are great. In the latter results in thetendency in excessive oscillation increases.

Oscillation Damping and Damping Force

During the control process, the role of the oscillationdamper has to be taken into account. Conventionaloscillation dampers can be designed for one operatingpoint only. The damping force for the vehicle is designedfor the upper loading range. This means that for vehicleswhich are only partially laden or empty the part of thedamping force which has to be overcome together with achange in the reference value is comparatively high. Thecould be compensated by variable damping control. Thisis available from WABCO under the name of ESAC.However, ESAC will not be taken into account in thisdocument.

Fig. 2 Example for a Control Process for a Change in the Reference Value

Control Behaviour with Non-Pulsed ECASSolenoid Valve (Overshoot)

Reference Value Tolerance Range

Reference Level B

Reference level A

Control Behaviour with Pulsed

Time [s]

Length of the Pulse Period

ON

OFF

ECASSolenoidValve

D

istance

VehicleBody-VehicleAxle

1.ControlDeviation

2.ControlDeviation

3.ControlDeviation

4.

Control

5

.Control

Reference Value Leap

....... Pulse Period

Deviation

Deviation

ECAS Solenoid Valve


13/20

13

Control Algorithm 5.ECAS

The further the load is removed from the damperoperating point, the greater the effect of the excessdamping force will be.

This issue becomes clear when looking at the way the

oscillation damper works. Inside the damper, oil needsto pass from one chamber via a small throttling port intoanother chamber. The resistance this causes is knownas the damping force. A rapid change in the distancebetween the vehicle's superstructure and the axle alsocauses this damping force to rise rapidly.

Thus it is mainly the change in that distance which isresponsible for building up the damping force.

As the distance between the superstructure and theaxle changes, the damping force is simultaneouslyreduced by the damper oil flowing through the throttle.

The time for this reduction is defined by the design ofthe damper (e.g. throttle diameter, viscosity of oil).

Now the damping force is a force counteracting themotion of the superstructure, and it prevents oscillationof the superstructure and the wheel losing contact withthe road. However, it thus also counteracts the changein the level.

This damping force which varies over time represents aproblem for the control process.

Control Proc ess for Changes in the Reference

ValueWhen the forces of ECAS are balanced, the wheel loadacts on the supporting bellows of the axle. Any axlesteering transmission must be taken into account.

The pressure in the supporting bellows multiplied bythe effective cross-sectional area of the bellows (Itcannot be computed directly from the diameter of thesupporting bellows!) counteracts the wheel load. Thepressure in the supporting bellows depends only on thewheel load, not the level.

As the level is changed as a result of the change in thereference value (e.g. by using the remote control unit),the pressure in the bellows is increased or decreaseduntil the actual value for the distance between thesuperstructure and the axle corresponds to the newreference value. This is a dynamic process. Thegreater the desired change in the reference value, thegreater the acceleration which can be achieved by thecontrol process. The system shows a tendency tooscillate. Overshooting may occur.

This tendency to overshoot particularly occurs onempty vehicles. On the one hand the steep static

pressure gradient between the supply and bellowspressure sides within the ECAS solenoid valve causeshigh flow rates as the bellows are being filled.

On the other hand the damping force to be overcome isgreatest. Thus the hazard of the control loop oscillatingis great. The result is an unnecessarily large number ofcontrol cycles within the ECAS solenoid valve which in

turn reduces its useful life.

If the tolerance range for the reference value is definedwidely enough, undesired oscillations can beprevented. However, this has a negative impact on therepetitive accuracy of the control process at similarreference values.

If, however, a specific dimension should be adhered to,the control process must be changed in such a way thatthe influx of air is reduced even before the referencelevel is reached. This would reduce the speed at whichthe superstructure is raised, and the excessive

tendency to oscillate is prevented.

As the solenoid valve can only switch the air stream offor on but not reduce it, the solenoid current of the ECASsolenoid valve is pulsed. This pulsing action brieflyinterrupts the air stream. Thereby achieving a throttlingeffect which prevents excessive oscillation, i.e.overshooting.

Length of Pulsing Period and Pulse Length

For the pulsing process of the curve, the terms of

'length of the pulsing period' and 'pulse length' need tobe defined:

Length of the pulse periodThe length of the pulsing period is a fixed value whichis stored in the ECU as part of the procedure for settingthe parameters. The beginning of the pulsing period isassumed to be the actuating pulse for the valvesolenoid. The length of the pulsing period itself is thenthe period of time before the valve solenoid receivesthe next actuating pulse. ( Fig. 2).

Pulse Length

The pulse length describes the length of time for whichthe valve solenoid receives the actuating pulse. Thisvalue is variable and is recomputed for each pulsingperiod. The computation of the pulse length by the ECUis done as a ratio of the existing control deviation, i.e.the difference between the reference level and theactual level.This type of control is called 'proportional differentialcontrol' (or 'PD control' for short). The control processis achieved as a ratio of the control deviation and thespeed at which the control deviation changes.Major control deviations cause great pulse lengths. Ifthe computed pulse length is greater than the stored

length of the pulsing period, the valve solenoid isenergized continuously. Thus the change in the controldeviation is at its greatest.


14/20

14

Control AlgorithmECAS5.

As the large flow diameter causes the subsequentcontrol process to be rapid as the vehicle'ssuperstructure is being raised, it is slowed down justbefore the new reference value is reached; for this

purpose, the speed at which the control deviation ischanging is analysed and included in the controlprocess. Control deviations changing at high speedscause the pulse length to be reduced.

Equation f or Calculating the Pulse Length by"Raising the Superstructure while Stationary"

Pulse length = (| control deviation x KP | - | speed atwhich control deviation is changing x KD |) x programcyclus time ( 25 ms)

KP (Proportional coefficient) and K D (Differential

coefficient) are important for describing the controlcycle and are stored in the ECU as part of theprocedure for setting the parameters.

In case of "lowering of the superstructure atstandstill" and always by "regelation when driving"set the speed at which control deviation is changingto 0.

The equation shows:

For KPgreat control deviations or high values resultin prolonged pulse lengths at equal controldeviations.

Whilst prolonged times for changes in the controldeviation or high values at KD at equal controldeviations shorten the pulse length.

The pulse length is recomputed for each length of thepulsing period. A pulse length which exceeds the lengthof the pulsing period causes the solenoid to beenergized continuously ('continuous pulse'). Theshortest pulse length provided is 75 milliseconds (0.075seconds).

Shorter pulse times would jeopardize theswitching process of the solenoid valve attemperatures below - 40C.

Determini ng the Parameters for the Proport ionaland Differential Coefficients

These factors have to be determined by way of trials onthe vehicle. Like the other parameters, they lie withinthe scope of responsibility of the vehicle manufacturer.After the KP-values have been finally determined forfront and rear axle and stored in the ECU, the vehicle istaken to a level which is below the tolerance band forreference value.

If the driving level is reached without any excessiveoscillation and without repeated pulsing of the solenoidvalve, the setting for the reference level tolerance and

the proportional coefficient are acceptable.

The control process is now checked against a majorchange in the reference value. Now the differential

coefficients KDcan be set. The object of establishing K Dis to suppress any tendency to oscillate excessively.Pulsing of the ECAS solenoid valve begins sooner asthe KDvalue increases. Thus the raising process of thesuperstructure is slowed down.

If it turns out that the KP- and KD values establishedaccording to these recommendations do not provideany satisfactory results for the control process. Theflow diameter in the pneumatic portion may be reduced.Usually it is sufficient to fit a throttle in the air linebetween the solenoid valve and the supporting bellowsunit of the axle concerned.

( 9.4.1 Explanation of Parameters)

Summary

Thus it is concluded that the user can adjust the controlprocess for the distance between the vehicle'ssuperstructure and the axle by way of the followingsettings:

Length of pulse period T,

Reference value tolerance Ds,

Proportional coefficient KP,

Differential coefficient KD, .

To see which from values are to recommend andhow they are to determine is descibed in paragraph9.4 "Setting Parameters".

5.2 Control Algorithm for Lift ing AxleControl (General)

Vehicles with a lifting axle can be equipped with a liftingaxle control facility. This is an optional extra and is notnecessarily included in every system.

Lifting axle control is used to control the position of thelifting axle(s). For this purpose, ECAS decides whetherthe lifting axle(s) has (have) to be down or raised. It isdisturbance factors - usually changes in the load -which make it necessary to control the lifting axle(s).

With ECAS, a maximum of two lifting axles can beseparately controlled. Experience has shown thatvehicles with one lifting axle are more common. For thisreason, the control of one lifting axle will be explainedin order to illustrate the basic principle. Two separatelyactuated lifting axles are treated as one axle for thispurpose. Having to separately control two lifting axle isthe exception rather than the rule. This control principleis based on one lifting axle and is explained below.

=


15/20

15

Control Algorithm ECAS 5.

General Comments on Lifting Axle Control

The subject of "lifting axle control" also includes subjectslike "traction help" and "overload protection". These willalso be covered in this context.

Controlling the position of the lifting axle is based on thepressure in the supporting bellows on the leading axle,this is picked up by one pressure sensor on thesupporting bellows. The pressure reading is comparedby the ECU to different reference values. Thesereference values can be defined as part of thecommissioning process for the ECU. These referencevalues determine the following limits:

Lowering or raising pressure for the lifting axle,

Maximum permissible traction help pressure,

Maximum permissible load pressure.Thus every pressure value has a certain condition of theaxle assembly allocated to it.

5.2.1 Control Algori thm for Lif t ing Axle Control(one Lifting Axle)

The thick line in the diagram (Fig. 3) shows the curve forthe supporting bellows pressure on the trailer's leadingaxle as a ratio of the trailer's load. As the trailer is beingloaded or unloaded, this line passes through differentimportant points. Some of the pressures in the

supporting bellows on the leading axle have to be storedin the ECAS system as part of the procedure for settingthe parameters. Other pressures are the result ofreactions of the lifting axle and can thus not be influenced

- such pressures have been marked with an asterisk (*).

Proper lifting axle control requires a sufficient supplyof compressed air and power.

For the purposes of the comments below, pleaseimagine a tanker semi-trailer or trailer would becontinuously filled with, or drained of, a liquid ( Fig. 3).

1. The filling process begins in Point . The trailer has aunladen weight mEmpty. This unladen weight is thecombination of: Mass of the trailer's superstructure and

the lifting axle mass proportion mLA.The corresponding supporting bellows pressurepEmpty is shown on the reference plate for the loadsensing valve.

2. The filling process which now begins increases theweight on the trailer until Point is reached. At thispoint, the lifting axle is lowered. Let us call thecorresponding supporting bellows pressure pLA Lowerthis being the lowering pressure for the first liftingaxle. The supporting bellows pressure has to bestored in the ECU when the parameters are set. Thestandard value for this pressure is the permissiblenominal value p100%of the pressure in the supportingbellows when the vehicle is fully laden. This value isalso shown on the reference plate for the load sensing

valve. The lowering pressure for the lifting axle canalso be selected at a point which is lower than thepermissible nominal pressure if the customerconsiders this necessary.

!

Fig. 3 Lifting Axle Function for Trailers with one Lifting Axle

upporng

eows

ressure

pOverload

p130%

pLA LOWER= p100%

p*LA UP

p*LA RAISEp LA DOWN

pEmpty

pB2 = p130%

p A2< p 130%

mLA

mLA

Mass Proportion of the Lifting Axle mLA

Axle Load of

m

Empty

mLADown

m*LAU

P

m*LARAISE

mLA1LO

WER

mMax.

mov

erload

Leading axle

Traction Help Limit Pressure

Limit of UnladenWeight BellowsPressure

permissible


16/20

16

Control Algorithm5. ECAS

3. When the lifting axle has been lowered, the loadvalues change. The load is reduced by the lifting axlemass mLA. The pressure in the leading axle'ssupporting bellows also falls as the axle load is

distributed across the supporting bellows of the mainand lifting axles. The bellow pressure curve runs fromPoint to Point . The resulting pressure in thesupporting bellows, p*LA DOWN, thus cannot beinfluenced by the user.

4. As the tanker is filled further, the pressure in theleading axle's supporting bellows again rises to itsmaximum permissible value, Point .

5. It goes through the maximum permissible pressurefor the bellows on the supporting axle when traction

help is activaded, p130% Point .

6. Finally it reaches a pressure pOverload at whichoverload protection commences.

7. Overload protection means that when this pressurepOverload is reached, the pressure in the supportingbellows of all axles which are in contact with theground is reduced and the vehicle's superstructure islowered until it sits on the buffer, Point . This is toprevent the vehicle being driven as long there is anexcessive load on the superstructure. Any furtherincrease in the load takes place while the supportingbellows are evacuated. The ECU must be informed ofthe pressure value for pOverload. For this purpose, theaxle manufacturer's data and any legal provisions onthe vehicle's weight must be taken into account.

8. The supporting bellows will not be filled again until theaxle load falls below the pressure pOverloadonce more,e.g. as a result of unloading the vehicle or of reducingthe pressure. That means, when the pressure in thesupporting bellows is reduced when locked at Point . Switching the ignition OFF and ON again issufficient to fill the supporting bellows.

9. Continuing with the above example, the pressure inthe supporting bellows falls below when moreliquid is drained from the vehicle's superstructure .At this point the pressure in the supporting bellows onthe leading axle is so low that it makes sense to raisethe lifting axle. Let us call this pressure in thesupporting bellows on the leadind axle pLA Raise. It hasto be stored in the ECU as part of the procedure forsetting the parameters.

For proper Functioning of this Feature, the followingRules must be observed:

pEmpty< p LA Raise < p LA Lower < p 130%< p Overload(control condition)

pLA Lower p 100% pLA Raise= 0,9 * p LA Lower * (number of nonraised axles/

total numer of axles) at 2 lifting axles working inparallel 0,8 *...

If these requirements are not taken into account,malfunctions of the lifting axle (e.g. continuousraising and lowering) may occur.

10. When the raising pressure has been reached, thelifting axle is raised and the supporting bellows on theleading axle alone support the axle load. The liftingaxle mass proportion mLAnow again forms part of the

load. The pressure curve for the supporting bellowsruns from to where the resulting pressure in thesupporting bellows p*LA UPcannot be influenced.

11. Now the unloading process has been fullycompleted, the pressure curve for the supportingbellows is once again at point .

Summary

The user can set up the control of a lifting axle, includingoverload protection, by setting the following:

Lowering pressure of the lifting axle pLA DOWN Overload protection pressure pOverload Raising pressure of the lifting axle pLA UP

These setting pressures are pointed out in fig. 3.

Fig. 4 and 5 show an unloading and loading process fora semi-trailer with one lifting axle and one with two liftingaxles working parallel.

Fig. 4 Control of one Lifting Axle

3a

3a 2a

unladen

laden

Parameter 29Lowering the

e.g.16.2 t

e.g.18 t

Parameter 28

e.g.

27 t

e.g.

27 t

RollingAxleLoad

pBellows Load x 0.9 xNumber of Non-Raised Axles /Total Number of Axle x (16)

pBellows Load x (16)Lifting Axle

Raising the Lifting Axle


17/20

17

Control Algorithm 5.ECAS

Fig. 5 Control of two Lifting Axles operating in parallel

5.2.2 Traction Help

Generally the traction help facility can be used only if thepressure in the supporting bellows as shown in the fig. 3between and can be associated. That means thatthe lifting axle must be down. The description follows therequirements of EC Guideline 97/27/EC (EC 97/27).

Fig. 3 uses two examples, starting at Points and ,to show how the traction help works for the different loadconditions.

Example 1In point the pressure in the supporting bellows of the

lifting axle is evacuated completely. The lifting axle israised in the process.

One supporting bellows pressure creates itself onto theleading axle lies on the elongation of the line between and and is described with . After deactivation of thetraction help, the pressure in the supporting bellows willonce again be as shown in .

The weight resting on the leading axle as a result oftraction help being activated is limited by EC 97/27. Theaxle load must not exceed the lower of that laid down bythe axle manufacturer or applicable maximum axle load

as defined by law (in Germany, this is defined in section34 of the Motor Vehicle Construction and UseRegulations (StVZO)) by more than 30 %. In addition, itmust be ensured that after activating traction help, theresidual axle weight on the front axle of the motor vehicleis more than 0.

The maximum permissible pressure in the supportingbellows on the leading axle when traction help isactivated, i.e. p130%, which meets these requirementsmust be stored in the ECAS-ECU when the parametersare set. To allow the system to actuate in condition, acontrol range Dp

130%

must be stored in the ECU withinwhich limiting pressure for the traction help, p130% iscontrolled.

If traction help and overload protection are to be providedfor an ECAS system, the following rule applies:

1. pLA Lower< p 130%< p Overload(control condition)

2. p130% p 100%* 1.33. p130% = p LA Lower* 1.3

The control range Dp130% is only effective below p 130%.

According to applicable law, traction help may onlybe effective at speeds below 30 km/h.

Example 2The second example shows the behaviour when thetraction help limiting pressure p130% is reached. Startingat the process of evacuating the supporting belows ofthe lifting axles begins again. When the traction help

limiting pressure is reached, evacuation of the liftingbellows stops and the pressure in the supporting bellowsof the leading axle does not increase any further . Inthis case, the lifting axle stays down. The excess load istaken up by the supporting bellows of the lifting axle.After deactivation of the traction help, the pressure in thesupporting bellows will once again be as shown in .

To summarize, the following values have to be stored inthe ECU for the traction help facility:

max. permissible pressure in the leading axle'ssupporting bellows when traction help is activatedp130% (traction help limiting pressure),

Control range Dp130% ("pressure hysteresis"),

Limiting speed for traction help.

In addition, parameters can be set for the timesapplicable to the length of actuation and the intervalsbetween actuations. However, these parameters are of asubordinate nature for traction help control as such.

Example calculation

This example shows lifting axle control being set for onelifting axle on a 3-axle semi-trailer with ABS-VCS/ECAS.

The reference plate for the load sensing valve shows thatthe bellows pressure pEmpty for the unladen vehicle is 0.7bar and p100% is 4.7 bar for the laden vehicle. Thelowering pressure for the lifting axle pLA Lower is to beequal p100%. If we follow the rules for the traction help andthe overload pressure, we arrive at the followingpressures:

pEmpty= 0.7 bar

pLA Lower= p 100%= 4.7 bar

p130% = 4.7 bar x 1.3 = 6.11 bar

pLA Raise= 0.9 x 4.7 bar x 2/3 = 2.82 bar

The control condition is being met, because 0.7 bar

ecas trailer

Documents