ZF Active Steering in the Steering Column for Mid-Size and Luxury Cars
Sep 26, 2015
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
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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
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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
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