ZF Servolectric®

Electric Power Steering System for Passenger Carsand Light Commercial Vehicles

2

The vehicle’s ride characteristics areenhanced considerably because theelectrical control perfectly respondsto vehicle dynamics and handling.At the same time, vehicle manufac-turers can increase their productiv-ity thanks to easy system integra-tion, fast installation and optimumadaptation to the vehicle.

Exciting benefits:more useful functions, lesspower consumption

Depending on vehicle type, ZFServolectric consumes over 90 %less power than hydraulic steeringsolutions. This means in particularthat, for the NEDC driving cycle,the fuel consumption is reducedby 0.4 l/100 km, and city drivingup to 0.8 l/100 km fuel savings canbe realized. The reason: ZF Servo-lectric only requires power whenthe vehicle is actually being steered(in contrast to conventional powersteering, permanent oil pressure isnot required). The servo unit aside,there are no major components:steering valve, steering pump, oilreservoir and high-pressure hosesare all dispensed with. This savesweight and facilitates installationin the vehicle.

Electric Power Steering System ZF Servolectric®

Servolectric® is a registered trademark ofZF Lenksysteme GmbH

An electrifying power solution

Top ride comfort and unsurpassedprecision – easy installation andminimum power consumption: ZFLenksysteme GmbH has brought anew quality to steering technolo-gy for passenger cars and lightcommercial vehicles. Its name: ZF Servolectric.

ZF Servolectric is superior to con-ventional hydraulic power steer-ing in many respects. It works byusing an electronically controlledelectric motor which replaces theconventional hydraulic system.Control and steering assistanceare powered by a single, uncom-plicated source: electric current.

Contents Page

ZF Servolectric®/Servo Unit on the Steering Column ....... 4

ZF Servolectric®/Servo Unit on a second Pinion................ 6

ZF Servolectric®/Paraxial Servo Unit................................... 8

The System ........................................................................10

The Power Pack ................................................................12

The Torque Sensor ............................................................14

Unique matching Options ................................................15

Comprehensive Expertise for EPS Development ............16

Production Locations ........................................................17

Worth knowing

3

Safety • Stabilization functions• Lane departure warning• Obstacle avoidance assistance• …

Comfort • Straight line running correction• Parking assistant• Lane keeping assistant• …

Steering • Steering feel• Steering performance• Noise performance• …

Emissions • CO2 saving: 10 g/km*)

20 g/km**)

Consumption • Fuel saving: 0.4 l/100 km*)

0.8 l/100 km**)

ing angle sensor, which canoptionally be integrated, under-lines the high value added by thissteering technology.

Installation variantsfor all classes

With just 3 basic types, ZF Servo-lectric covers all vehicle classes,from subcompact to mid-size andupper-class cars and vans. Theservo unit is mounted on thesteering column (EPSc), on a sec-ond pinion (EPSdp) or in parallelto the steering rack (EPSapa),depending on the installationspace available, the vehiclepower supply and the steeringrack force required.

assistance can be easily adaptedto suit any vehicle type. And theelectronic control unit registers awide range of parameters such asroad speed, steering angle, steer-ing torque or steering speed, forexample. Meanwhile, a program-mable damping characteristicguarantees superb absorption ofroad bumps. Moreover, a steer-

*) New European Driving Cycle (NEDC) with 2-liter spark ignition engine**) only city traffic

Steering speed at standstill100°/s

360°/s

3200

400

600

800

1000

Steering rack force (kN)4 5 6 7 8 9 10 11 12 13 14 15 16

Subcompact cars Mid-size cars Upper classCompact cars

EPSc

EPSapa

Upper mid-size class Luxury classOff-road vehicles

Perf

orm

ance

(W

)

EPSdp

Standard values of steering rack force and mechanical performance for all vehicle classes

Summary of advantages for the customerThe right solution for every vehicle

Worth knowing

Easier installation –no maintenance

Insert, fasten, connect up theelectronics. It really is that easy: ZFServolectric is supplied as a com-plete, fully functional systemmodule. Dispensing with oil andhydraulics makes it absolutelymaintenance free. A diagnosticsystem enables checking of thefunctions. And it can be integrat-ed in electronic stability systems(e.g. ABS, ESP etc.) without anyproblems.

Enhanced steering comfort,extreme precision

ZF Servolectric operates complete-ly independently of the vehicle’sengine. This means the power

4

ZF Servolectric®

Servo Unit on the Steering Column

and its electronic control unit areintegrated in the steering column.

Ideal for vehicle manufacturers:The steering column and servounit are delivered to the produc-tion line ready-to-install. Theyare connected to the mechanicalrack and pinion steering gear viathe intermediate shaft with uni-versal joints. The sensors and tor-sion bar are located next to theworm gear. The torque producedby the electric motor is converted,via a worm gear, into an assis-tance torque and transmitted tothe intermediate shaft (see alsodescription on page 7).

Combined with a mechanical rackand pinion steering gear, this“wheel-to-wheel” steering sys-tem impressively demonstratesthe thorough system competenceof ZF Lenksysteme GmbH.

The result:Drivers have all the benefits of ZFServolectric with an extremelylightweight design that requiresvery little space.

2

7 6 1

3

5

8 4

Configuration of ZF Servolectricwith servo unit on the steeringcolumn (- - - non-visible part orinternal part)

1 Upper steering column2 Steering intermediate shaft3 Serrations for steering wheel4 Torque sensor5 Sensor cable6 Electronic control unit7 Electric motor8 Worm gear

Picture on the right side of page 5:“Wheel-to-wheel” steering systemwith ZF Servolectric and mechanicalrack and pinion steering

Servo Unit on the Steering Column

This is why ZF Lenksysteme GmbHdeveloped a Servolectric variantthat is specially tailored for vehi-cles with lower steering effort.The basic principle: the servo unit

Even subcompact and compact cardrivers have now come to expectsupreme steering comfort – whenmaneuvering through city trafficor when parking, for example.

5

An ideal combination

To achieve appropriate adaptationof the electric power steering sys-tem to the vehicle, modular servounits are available for different per-formance requirements. Anotherway is to separate the electroniccontrol unit from the servo unit andposition it in a different place (seebelow).

The ideal base for these steeringvariants is the mechanical rack andpinion steering from ZF Lenksys-teme. High stiffness, good efficiencyand lightweight design requiring

little space are the features of thiswell-proven component of whichmillions have been made. The ratioof the rack and pinion steering caneither be constant or variable.

5 Yoke6 Bellows7 Tie rod

Configuration of the mechanicalrack and pinion steering (- - - non-visible part or internal part)

1 Housing2 Steering rack3 Input shaft connection 4 Drive pinion

Servo Unit on the Steering Column

7 6 1

4

3

5

2

6

ZF Servolectric®

Servo Unit on a second Pinion

The servo unit on a second pinionis a further superb option, de-signed for mid-size or upper mid-size cars. An option that allowsthe ZF Servolectric to shine and toshow just how versatile it really is.

The installation of the servo uniton the second pinion enables thephysical separation of the sensor and the drive unit.

The fact that the drive pinionratio is independent of the steer-ing ratio creates the opportunityfor a performance-optimized con-figuration. The increase in systemperformance is in the order of 10 -15 %. And superb crash safetyis guaranteed thanks to optimumuse of the available installationspace. By allowing a rotation ofthe servo unit of 360° about theaxes of the rack and the drivepinion, the position of the servounit can be defined to fit the par-ticular application.

Configuration of ZF Servolectricwith servo unit on a secondpinion (- - - non-visible part orinternal part)

1 Housing2 Steering rack3 Input shaft connection4 Torque sensor5 Sensor cable6 Steering pinion7 Yoke

8 Electronic control unit9 Electric motor

10 Worm gear11 Bellows12 Tie rod13 Drive pinion (second pinion)

12 11 10 9

12137 5 3 4

8 6

7

Servo Unit on a second Pinion

7

Excellent power transmission

The function of the sturdy wormgear is to convert the torque pro-vided by the electric motor intoassist torque and to transmit it tothe steering rack. The demandsmade in the process on perform-ance and comfort are very high.To achieve this, it must beensured that the teeth of theworm (5) and helical gear (1)remain in mesh without backlashduring the entire service life ofthe steering gear. For this pur-pose, a specially developedspring damper element (2) isused which provides optimumspring load to the worm in anydriving situation.

To make sure that the worm canmove as necessary, a novel fixedbearing (4) is fitted. Thanks to itsconvex outer race, which isaccommodated in a concave steelring, the ball bearing can absorbhigh axial and radial loads andcan nevertheless move easily.

In unusual driving situations andin case of abuse, the worm gearis subjected to extreme loads. Toprevent damage to and anyblockage of the worm gear, inthose rare cases, an overload

Configuration of the worm gear

1 Helical gear2 Spring damper element3 Overload safety device4 Fixed bearing5 Worm6 Drive pinion7 Housing

safety device (3) is providedbetween the helical gear anddrive pinion (6, universal shaft insteering column version). Thisprovides an defined limit to thetorque that can be transmitted.

Servo Unit on a second Pinion

127 5

4 6 3

8

ZF Servolectric®

Paraxial Servo Unit

ZF Servolectric with paraxial driveis characterized by low systemfriction and high efficiency. Thepossible customer applicationsrange from dynamic sports carsand upper mid-size cars to high-load vehicles such as off-roadvehicles and vans.

Due to the combination of recir-culating ball gear and toothed-belt drive, ZF Servolectric withparaxial drive is ideally suited forthe customer’s differing perform-ance requirements.

The wide range of positioningpossibilities of the servo unitallows optimum use of the instal-lation space on the vehicle. Thus,this steering system helps to meetthe high crash requirements ofthe automotive industry.

As with all products of ZF Lenksys-teme GmbH, the focus for ZFServolectric with paraxial drive ison full compliance with the cus-tomer requirements with respectto function and cost. This goal isachieved by a modular concept,which minimizes the amount ofdevelopment and applicationwork.

Configuration of ZF Servolectric withparaxial servo unit (- - - non-visiblepart or internal part)

1 Housing2 Steering rack3 Input shaft connection4 Torque sensor

5 Sensor cable6 Steering pinion7 Yoke8 Electronic control unit9 Electric motor

10 Servo gear system11 Bellows12 Tie rod

12 11 10 9

1 52 3 4

8 7 6

Paraxial Servo Unit

9

Minimum sliding forces withlow noise performance

To transform the rotationalmovement of the electric motorinto a linear movement of thesteering rack (1), ZF Servolectricwith paraxial drive uses a driveconcept consisting of toothed-belt drive (2, 3, 4) and recirculat-ing ball gear (5, 6, 7). Both drivestages work with a very high effi-ciency.

The recirculating ball gear is asystem in which the ball chain (7)is returned through a channel (6)integrated in the ball recirculat-ing nut (5).

The noise performance of theball thread during operation isgood. The low noise generationof the recirculating ball gearallows rigid connection of thesteering gear to the subframe ofthe vehicle, which gives a verydirect steering feel.

Despite its ability to transmithigh torques safely, the slip-freetoothed belt (2) is characterizedby good noise performance.

Configuration of the servo gearsystem

1 Steering rack2 Toothed belt3 Toothed disc, small4 Toothed disc, big5 Ball recirculation nut6 Ball return channel7 Balls/ball chain

7

1

2

3

5 4

6

Paraxial Servo Unit

10 System

Electric motor

Electronic control unit

Engine speedDiagnosis Ignition

Perfect harmony

Two aspects are decisive in deter-mining the effectiveness of ZFServolectric: on the one hand, naturally, thesuperior quality of the centralcomponents; on the other hand,perfect interaction betweenthese components and a wholehost of external signals. ZF Servo-lectric flexibly responds to anytechnical environment and anydriving situation.

The System

11

Intelligence: the key to easiersteering

An ingenious system provides forremarkably uncomplicated pro-cesses: ZF Servolectric operatesbased on highly developed elec-tronics and thus makes theprocess of power steering fasci-natingly easy – whichever instal-lation variant you choose.

The rack and pinion principle hasproved the ideal steering solu-tion to date, and the ZF Servo-lectric provides a modern inter-pretation of this proven concept.

What happens, step by step:As soon as the driver turns thesteering wheel, sensors registerthe corresponding steering torqueand steering speed with absoluteprecision.

System

The registered data are used tocalculate the required steeringassistance and, based on thecalculated results, to control theservomotor.

In the final step, the motor trans-mits the optimum servo torque –via worm gear or recirculatingball gear – to the steering column(EPSc), second pinion (EPSdp) orsteering rack (EPSapa) in themechanical rack and pinion steer-ing system.

If the vehicle power supply fails,the driver can continue to steerthe vehicle, due to the mechani-cal connection between the steer-ing wheel and the steeredwheels.

Torque sensor

Power supply

Vehicle speed

Vehicle speed, power supply,engine speed, ignition

Electric motor

Gearbox

Steering force

Motor torque

Torque applied by driver

Torque sensor

Torque applied by driver

Mechanicalsteering unit

Electroniccontrol unit

Wheels,vehicle

mechanical flow flow of current

Functional block diagram of ZF Servolectric

12 Power Pack

Configuration of the power pack fora paraxial drive

1 Brushless electric motor2 Electronic control unit 3 Toothed disc, small

Example of a power pack for thedrive of a worm gear

The Power Pack

with respect to noise performanceand torque ripple for the require-ments of an electric power steer-ing system.

On the paraxial variant, themotor torque is transmitted, via atoothed disc (3) on the motorshaft, to the servo gear system(toothed-belt drive and recircu-lating ball gear). On the steeringcolumn variant and the doublepinion variant, the torque istransmitted to the worm gear bymeans of a coupling.

Thanks to their graded perform-ance classes, the power packranges of the modular design canbe adapted, as needed, to a widevariety of customer requirements.

A high-precision converterof electrical energy

The electronically commutatedbrushless electric motor (1) con-verts the current defined by theelectronic control unit (2) into atorque.

The newly developed generationof brushless motors provides theassist calculated by the electroniccontrol unit with high precisionin any driving condition. The latestmaterials enable high efficiencywith extremely small size. Inaddition, the drive was optimized

3 1

2

13

Electronic control unit:the heart of precision

At the heart of ZF Servolectric isthe highly precise electronic con-trol unit: an extremely high-per-formance microhybrid unit thathas a multi-layer ceramic base.

The ECU houses all the compo-nents – 32-bit microcontroller withon-chip flash memory and RAM, aswell as the output stage for themotor – in what is a fully integrat-ed, hermetically sealed unit.

In order to ensure optimumresponse, the electronic controlunit has all the relevant informa-tion permanently “in view”, includ-ing stored vehicle data as well ascurrent operating parameters(such as vehicle speed, enginestatus or steering speed).

And, as one would expect, ZFServolectric can be networked withvehicle management systems viathe CAN bus or FlexRayTM withoutany problems whatsoever: a newdimension in intelligent driving!

Power Pack

The steering signal is taken by thetorque sensor and passed on to theECU.Optimum assistance and motor con-trol are calculated.

Configuration of the electronic con-trol unit

1 Control electronics with heat sinkand intermediate plate

2 Input filter unit

1 2

Torsion bar angle [°]

-10

2.0

1.0

0.0

3.0

4.0

5.0

6.0

-4

-8

-12

0

4

8

12

-8 -6 -4 -2 0 2 4 6 8 10

Sig

nal

2 [

V]

Sig

nal

1 [

V]

Stee

rin

g t

orq

ue

[N

m]

14

then calculated by means of aninverse tangent function.

The high safety demands on elec-tric steering systems requiredetection of all faults occurringon the sensor and creation of asafe condition of the steering sys-tem. The sensor data are trans-mitted to the electronic controlunit via a very rugged digitalinterface.

Optionally, the torque sensor canalso accommodate an index mag-net (7) and an index sensor (8).The index sensor delivers a signalto the ECU for each full steeringwheel turn. In combination withthe data from the rotor positionsensor and the wheel speeds, theelectronic control unit is able tocalculate the steering angle witha resolution < 0.05°.

which is connected to the steer-ing pinion by means of the tor-sion bar (1). The measuring rangecovered by the sensor is between+/- 8 and +/- 10 Nm. If the steeringtorques are higher, a mechanicalangle limiter prevents overload

of the torsion bar. When thedriver applies a torque on

the steering wheel, thetorsion bar is rotated

as is the magnet rel-ative to the sensor.

The sensor con-sists of magneto-resistive elementswhich changetheir resistancewhen the fielddirection changes.In the process,

the voltage fol-lows a sine and

cosine curve whenthe magnet is rotat-

ed. The direct rotationangle of the torsion bar is

The torque sensor measures thetorque applied by the driver atthe steering wheel. Based on this,the control unit calculates thesteering assistance for the motor.

The torque sensor sits on thesteering pinion (5). A pole wheel(9) is fitted on the input shaft (2)

Torque Sensor

The Torque Sensor

Configuration of the torque sensoron the steering pinion (- - - non-visible part or internal part)

1 Torsion bar2 Input shaft3 Sensor module

4 Clock spring5 Steering pinion6 Plug7 Index magnet (optional)8 Index sensor (optional)9 Pole wheel

5 7 8 6

12

94 3

15

The use of ZF Servolectric allowsone to adjust the steering charac-teristics (e.g. steering effort, self-centering and damping) by pro-gramming the control unit, thusadapting them perfectly to dif-ferent vehicle philosophies. Byusing identical hardware compo-nents, this electronic program-ming enables universal adapta-tion of the steering system tovery different types of vehiclesand applications (for examplesedan, SUV, sports coupé or van).The result is a reduction in themultitude of parts and part num-bers.

A sporty steering system is char-acterized by best possible steer-ing precision and handling char-

acteristics. One factor for achiev-ing this character is the ability toreduce the steering assist; whenthe driver must apply more steer-ing effort, road feel and roadfeedback become more direct,which is desired. Conversely,higher steering assistance is pro-grammed when the vehicle isdesigned for more comfortabledriving.

Matching Options

Unique matching Options

Steering torque [Nm]

Matched for sporty driving

Vehicle speedVehicle speed

0 km/h0 km/h

max

max

Matched for comfort

0

1

0

2

3

4

87654321

Ass

ista

nce

to

rqu

e [N

m]

Examples of characteristics of steering system matching on vehicles

16Comprehensive Expertise

Our company is headquartered inSchwäbisch Gmünd, Germany.This is also where all develop-ment work on the Servoelectricsystem is carried out. By concen-trating our expertise in mechani-cal systems, software, electrics,electronics and electric motors inone location, we are able torespond to our customers’ needsmore quickly.

Positioned as the technology andinnovation leader in the steeringmarket, ZF Lenksysteme serves asa reliable and professional part-ner to vehicle manufacturersaround the globe. A rigorousfocus on satisfying the needs ofvehicle manufacturers and endcustomers lays the foundationfor the company’s performance,while ground-breaking techno-

logical solutions form the basisfor the market success of itsproducts. The Servolectric electro-mechanical steering system, ofwhich we have already producedmillions of units, is an impressiveexample.

Comprehensive Expertise for EPS Development

Calculation,simulation Mechanical

design

Development of control units

Sensor system development

Benchmarking

Systemnetworking

Motordevelopment

Softwaredevelopment

Analysis,inspection

NVHtesting

Vehicletesting

Safety

Schwäbisch Gmünd

Comprehensive expertise for EPS development at one location

17

Production Locations

16 locations9 countries4 regions

Production Locations

Locations Hydraulic EPS Pumps Steering columns

steering gears

Cars CVs Cars Cars CVs Cars CVs

Germany • • • • • • •France • • •Great Britain •U.S.A. • • • •Brazil • • • • •China • • •• • • •Malaysia • •*)

Hungary • • •India**) • ••• planned *) only mechanical rack and pinion steering gears **) minority holding

ZF Lenksysteme GmbH:the systems partner

ZF Lenksysteme GmbH is one ofthe largest independent manu-facturers of power steering sys-tems for passenger cars and com-mercial vehicles. Renowned auto-motive manufacturers from allover the world value us as a cre-ative and efficient systems part-ner for the development of newand innovative solutions.

As a joint venture of RobertBosch GmbH and ZF Friedrichs-hafen AG, ZF Lenksysteme GmbHoffers its customers a uniquesource of expertise when itcomes to integrating a widerange of top technologies inmodules, system modules orentire chassis systems.

The benefits for the manufactur-er are clear to see: even shorterdevelopment times and opti-mized production processes –with quality standards which justget better and better.

ZF Lenksysteme GmbHRichard-Bullinger-Straße 77D-73527 Schwäbisch GmündGermanyPhone: +49 (0)71 71) 31 - 0Telefax: +49 (0)71 71) 31 - 32 22www.zf-lenksysteme.com

A Joint Venture of

Robert Bosch GmbH and

ZF Friedrichshafen AG

[Steering the right way]

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