ZF Active Steering in the Steering Columnfor Mid-Size and Luxury Cars

2accomplished by a complete basissteering system, which has theadvantage that it can be matedwith either an electric or conven-tional hydraulic system. An elec-tro-mechanical steering angleactuator is integrated into thesystem between the steeringwheel and the pinion of the steer-ing gear. Through a gearset, thisactuator can, if required, add orsubtract an additional steeringangle in addition to the angleimparted by the driver, or it cangenerate a steering angle that isindependent of the driver. If theparticular driving condition doesnot require an additional angle,the electric motor of the steeringactuator remains still. In thatinstance, there is a direct mechan-ical connection from the steer-ing wheel to the road wheels, as

Steering actively

Installation schematic of a ZF ActiveSteering in the steering column,with ECU, electric power steeringand steering intermediate shaft

ZF Active Steering for increaseddriving comfort and active safety.

Modern car manufacturing withits ongoing development makesvery high demands on both theoverall concept and on systemsand single components in thevehicle. ZF Lenksysteme GmbH, ajoint venture of Robert BoschGmbH and ZF FriedrichshafenAG, equips renowned vehiclemanufacturers with both high-

quality single components andcomplete steering systems. TheZF Active Steering (AFS = ActiveFront Steering), which was firstintroduced in the market in 2003,highlights these features as welland gives an example of modernand innovative developments inautomotive technology.

The ZF Active Steering is a steeringconcept by which the steeringangle input of the driver can beincreased or decreased. This is

Steering actively

3is the case on conventional steer-ing gears.

The additional degree of free-dom permits continuous and situ-ation-dependent variation of thesteering ratio. While convention-al steering systems are, over thewhole speed range, always de-signed with a constant steeringratio between the steering wheeland the road wheels, the ratiocan be changed actively anddynamically with the aid of theZF Active Steering. This possibilityenables new driver assistanceand vehicle dynamic functionswith the aim of increasing drivercomfort and safety.

Steeringwheel

Motor

Pinion

= 1

Steering actively

Pinion Steeringwheel

iD+ 1

iM Motor

Principle of angle superposition onthe Active Steering

iD = ratio between steering wheel and pinion

iM = ratio between motor shaft and pinion

41

2

6

3

4

5

System Configuration

Steering wheel angle

Motor angle(superpositionangle)

Wheel angle

Wheel angle

Principle diagram of a ZF ActiveSteering in the steering column withbasic steering ZF Servolectric ofparaxial design

1 ZF Servolectric2 Steering column3 Actuator4 Steering intermediate shaft5 Steering pinion6 Electronic control unit

Servolectric is a registered trademark ofZF Lenksysteme GmbH

System Configuration

5Compact, advantageous package.

At the heart of the ZF ActiveSteering in the steering column(2) lies the actuator (3), formedby a superposition gear system(wave gear) and an electricmotor. The Active Steering unittransmits both the steering angleinput by the driver via the steer-ing wheel and the additionalmotor angle (superpositionangle), which is generated by theelectric motor, via the steeringintermediate shaft (4) to thesteering pinion (5) of the powersteering gear (1). This meansthat, via the superposition gearsystem, a front wheel steeringangle can be generated both bythe driver and the electric motor,independently of one another.

The electric control unit (6) eval-uates the instantaneous drivingcondition with the aid of sensorinformation and, depending onthe result controls the superposi-tion angle.

The actuator with superpositiongear system can be integratedinto the steering column or alter-natively into the steering gear.

Integration of the Active Steer-ing actuator into the steeringgear is advantageous, as there isno influence on the frictiondown to the steering valve andthe acoustic emission of noiseinto the engine compartment isless noticeable. Mechanical trans-mission takes place by a com-bined worm and planetary geartrain.

The steering column solutionoffers considerable advantagesregarding package design. Itscompact dimensions permit theuse of the Active Steering evenon vehicles where there is notsufficient installation space forthe steering gear variant. Thedynamic requirements and theefficiency of each of the two vari-ants are comparable.

Figure at top:Actuator variant in the steeringcolumn

Figure at bottom:Actuator variant in the steering gear

System Configuration

63 9 2 4

10

5

6

8 1

7

X

X

superposition gear system, enclos-ed in a lightweight metal hous-ing, the Active Steering unit isable to generate, within splitseconds, the additional steeringangle requested by the vehicleelectronics as required by the cur-rent situation. The conventionalsteering feel is not adverselyaffected by this.

The superposition gear systemdesign is based on a wave gearprinciple. It is basically composedof three concentrically arrangedparts: the elliptical wave genera-tor (2), the flex spline (a flexiblecylinder) with external teeth (3),and a circular spline (a solid steelring) with internal teeth (4).

The hollow shaft (2), which iscentrically seated in the electricmotor forms, together with thewave generator, a single unit. Aflexible ball bearing (5) causesthe area of the teeth of the flexspline to be deformed in linewith the elliptical external con-tour of the wave generator. Thiscauses the teeth in the oppositeareas of the major axis of theellipse (11) to be always safely inmesh with the solid circular

Actuator

Superior interaction of mechani-cal components, electrics andelectronics.

The actuator is the defining com-ponent of the ZF Active Steering

in the steering column. Troughan additional motor angle, theactuator produces a kinematicsuperposition to the steeringwheel angle. With its main subas-semblies electric motor (1) and

Section view of an actuator for theActive Steering in the steeringcolumn

1 Electric motor2 Wave generator/hollow shaft3 Flex spline

4 Circular spline/connection for steering intermediate shaft

5 Flexible ball bearing6 Universal shaft7 Electromechanical locking unit8 Motor angle sensor9 Connection of sensor cable

10 Connection of power supply

Figure on page 7:Partial section of sectional plane X-X

11 Major axis of the ellipse12 Minor axis of the ellipse

Actuator

712

4

5

2

113

6

spline with internal teeth. In thearea of the minor axis of theellipse (12), however, there is nocontact among the two toothsystems. The flex spline can bedriven by both the circular splineand the universal shaft (6) that ispositively connected with it or bythe wave generator.

When the wave generator rota-tes, the major axis of the ellipseshifts and with it, the toothengagement area. Because thecircular spline has two teethmore (number of teeth = 102)than the flex spline (number ofteeth = 100), there results, afterhalf a rotation of the wave gene-rator, a relative movement ofone tooth between the partsand, after a full rotation, a rela-tive movement of two teeth. Forinstance, 51 revolutions of theelectric motor are necessary,assuming the steering wheel isstatic, to move the circular spline

and thus the steering pinion byone rotation (ratio i = 1:51).

If the driving situation does notrequire an additional angle, theinterconnected wave genera-tor/electric motor unit does notmove. In that position, for onerotation of the steering pinion1.02 steering wheel turns arerequired (ratio i = 1:1.02). In prac-tice, this minimal ratio differenceprovides the direct mechanicalconnection when the motor doesnot provide a motion.

The steering wheel angle inputby the driver and the additionalangle from the motor are com-bined at the circular spline andtransmitted to the rack via thesteering intermediate shaft andthe steering pinion. The displace-ment of the rack results in aneffective steering angle at theroad wheel that may be larger orsmaller than determined by the

base mechanical ratio, depend-ing on the direction of rotationof the motor.

A brushless, electrically commu-tated direct-current motor (BLDCmotor) is used. For the control ofthe motor angle, a motor anglesensor (8) is used to determinethe instantaneous motor posi-tion. The performance of thecontrol system is very high: it isboth very precise and providesthe ability to generate a definedmotor angle within split seconds.

Partial section X-X

Steering system (speed)

without ActiveSteering

(whole speed range)

with Active Steering(at 20 km/h)

with Active Steering(at 150 km/h)

DriverSteering wheel

angle

100

ActuatorMotor angle

not available

+ 25

- 25

Steering gearPinion angle

100

125

75

VehicleWheel angle

6

7,5

4,5

Actuator

EffectAdvantages

Same relationship between steering wheel angleand wheel angle over the whole speed range

Larger wheel angle at the same steering wheel angleIncreased driving comfort

Smaller wheel angle at the same steering wheel angleIncreased driving safety

Practical example: Driver turns steering wheel through 100

8System Control

Intelligent and highly dynamic.

The electronic control unit de-veloped for the ZF Active Steer-ing in the steering column estab-lishes the connection betweenthe electrical system of the vehicle,the vehicle sensors, as well as thesteering sensors and the actuator.All the components required forthe control of the ZF ActiveSteering are installed on a multi-layer board. The control unit isdesigned for operation atambient temperatures of -40 Cto +70 C.

The core component of the elec-tronic control unit is a high-capac-ity microprocessor with the per-tinent smart watchdog for moni-toring. This processor evaluatesthe signals from the vehicle andsteering system sensors and cal-culates the control outputs forthe actuator. Via the power out-put stages, which are also inte-grated into the ECU, the electricmotor is then driven, and theactuator is controlled such thatthe desired position is reached. Ifa hydraulic power steering isinstalled, a controllable ECOvalve at the steering pump can

System Control

Electronic control unit, Type 7831

9additionally be actuated (ECO =Electronically Controlled Orifice).

To meet the high safety require-ments for the steering system,the electronic control unit moni-tors all connected componentsfor correct function. Additionally,all functions which have aninfluence on the motor angle arecalculated redundantly on themicroprocessor. If an error isdetected, countermeasures are

initiated immediately. The func-tional safety is achieved by apply-ing ISO 26262.

The program memory of the ECUuses flash technology. This allowsthe programming of the wholesoftware by means of a centralplug in the vehicle. The high flex-ibility ensured by this makes itpossible to incorporate updatedor extended software levelsduring service visits, therefore

ensuring state-of-the-art technol-ogy for the functions of theActive Steering.

Modular AFS system conceptlinked with a vehicle dynamics controller

Vehicle dynamics controller (ECU, OEM)

Requested by the driver (e.g. steering wheel angle)

signal processing

signal processing

steering assistance functions

stabilizing functions

monitoring andsafety functions /

error strategy

monitoring andsafety functions /

error strategy

System Control

Vehicle signals

Controlled system

AFSsteering system vehicle

AFS ECU

actuator control

With ZF Active Steering, ZFLenksysteme makes an ActiveSteering system which is capable

of implementing both steeringassistance functions and driv-

ing stabilizing functions. The various steering assis-tance functions can indi-vidually be parameter-ized and adapted to thevehicle by the vehiclemanufacturers. Definingthe functions and re-sponsibility for thesecan either lie with ZFLenksysteme or withthe vehicle manufactur-er concerned, depend-ing on their design.

Beyond being a classic steer-ing aid, the ZF Active

Steering can provide a varia-ble steering ratio, which

depends on, for example, vehiclespeed. Depending on the drivingsituation, the effective steeringangle at the road wheels be-comes larger or smaller than theangle defined by the driver at thesteering wheel. When drivingslowly in road traffic, a directsteering ratio facilitates parkingand negotiating sharp turns. The

10

Steering Assistance andStabilizing Functions

c.g.

Steering Assistance and Stabilizing Functions

Basic diagram illustrating the generation of a yaw torque and its compensation

Braking forces Lateral forces

c.g. = Vehicle center of gravity AFS intervention

11

150

300

Larger steering wheel motionwithout Active Steering

150

0

85

170

Smaller steering wheel motionwith Active Steering

85

0

Obstacle in view

Obstacle in view

car reacts more directly and com-fortably and is easier to handle.At high speeds, the electricmotor intervenes in a directionopposite to the movement of thesteering wheel, thus slightlyreducing the wheel turningmotion. The steering systembecomes more indirect, and thecar is more stabile and safer. Thedriver needs no longer fear that

he will lose control of the vehicleas a result of an inadvertent sharpsteering motion. The ECU decidesif, and by what amount, the steer-ing angle has to be altered.

Another comfort feature is thesteering lead. Depending on therate at which the driver turns thesteering wheel, an extra angle oflead is generated by the Active

Steering Assistance and Stabilizing Functions

Basic diagram of different vehiclehandling in an avoidance maneuver

Steering. The faster the driverturns the steering wheel, the larger that angle becomes. Thiscan significantly reduce the existing vehicle response time asa result of a steering motion.

Also, steering interventions toimprove vehicle stability are possi-ble. In addition to the stabilizingbraking interventions of theElectronic Stability Program (ESP),steering interventions by theActive Steering can be carried outfor stabilization in safety-criticaldriving situations. Thus, vehicledynamic systems will be able toinfluence the vehicle no longeronly by braking interventions, butalso by the steering system.Advantage: the intervention inthe steering gear is very fast andhardly noticeable to the driver.This linking of Active Steering andESP demonstrably contributes toincreased driving safety.

The combined use of ActiveSteering with an electric steeringgear offers potential for futuresteering functions. Their network-ing makes it possible to freelycreate both the effective steeringangle and the steering torque of

12

500

0

100 150

a steering motion. These function-al degrees of freedom allow theachievement of steer-by-wirefunctions without having toseparate the mechanical connec-tion between the steering wheeland the steered wheels. Due tothese possibilities and the greatappeal for the final customer, ZFActive Steering will gain wide-spread acceptance.

Like any product developed by ZFLenksysteme, Active Steering

too, must be in agreement withvehicle manufacturers goals andsupport. At this early stage, thereis a vision of a completely inte-grated chassis management with-in the automotive industry. Anoverriding electronic control unitwill identify critical driving condi-tions and activate the correspon-ding subsystems such as ESP, ZFActive Steering, Rear Axle Steer-ing or Active Roll Control to makethe best contribution to safevehicle handling. These net-

working possibilities show thepotential for manufacturer-spe-cific combinations of suspensioncontrol systems.

Steering ratio as a function of vehicle speedi_V

i_V max

i_V passive

i_V min

Comfort

Speed (km/h)

Steering Assistance and Stabilizing Functions

Basic diagram of the speed-sensitiveratio of the ZF Active Steering

Active Steering is active(variable steering ratio)

Stability

Agility Active Steering is passive(purely mechanical steering)

13

Our Company is headquarteredin Schwbisch Gmnd, Germany.This is also where developmentwork on the ZF Active Steering iscarried out. By concentrating ourexpertise in mechanical systems,software, electrics, electronics andelectric motors in one location,we are able to respond to ourcustomers needs more 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 companys performance,

while ground-breaking technolo-gical solutions form the basis forthe market success of its prod-ucts. The successful ActiveSteering, which is available formany vehicle types, is an impres-sive example of this.

Comprehensive Expertise forActive Steering Development

Steering system (electric/hydraulic)

Steering column

AFS Development

Actuator

Sensor

Gear system

Motor

Locking unit

Mechanical interfaces

Acoustics

Electronic controlunit (ECU)

Software

Functions

Safety

Electrical interfaces

Vehicle test

Electronics

Comprehensive Expertise

All AFS development areas are inone location

[Steering the right way]A Joint Venture of

Robert Bosch GmbH and

ZF Friedrichshafen AG

Tech

nic

al m

od

ific

atio

ns

rese

rved

.

ZF Lenksysteme GmbH:the systems partner

ZF Lenksysteme GmbH is one ofthe largest independent manu-facturers of power steeringsystems for passenger cars andcommercial vehicles. Renownedautomotive manufacturers fromall over the world value us as acreative and efficient systemspartner for the development ofnew and 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 or entire 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 Schwbisch GmndGermanyPhone: +49 (0) 71 71) 31 - 0Telefax: +49 (0) 71 71) 31 - 32 22www.zf-lenksysteme.com

ZFLS

703

0P-M

BA

09/

11e

Pri

nte

d o

n p

aper

ble

ach

ed w

ith

ou

t u

se o

f ch

lori

ne