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TORQUE VECTORING

Wheals, Jon – Ricardo UK. Leamington SpaBarnbrook, Richard – Ricardo UK. Leamington Spa

Parkinson, Rob – Ricardo UK. Leamington SpaDean, Matt - Ricardo UK. Leamington Spa

Dr. Rolland Donin - Ricardo Deutschland GmbH, Schwaebisch Gmuend

Key words: Torque Vectoring, axle, differential, yaw rate, understeering

Torque VectoringTM denotes the possibility, by using a driveline element, to split the torque not onlybetween the axles of a vehicle but also between the wheels of the same axle. The concept is of interestespecially for AWD manufacturers and mainly for SUVs.Based on an Audi A6 Quattro, Ricardo built a demonstration vehicle with a unique compact TorqueVectoring™ rear axle and the development was targeting both safety and driver appeal. The papergives an overall view of the activities carried out on first demonstration vehicles and analysis in moredetail possible solutions for the future.

Introduction and Ricardo’s Qualification

Ricardo have a history of association with new driveline technologies, from the world’s firstproduction 4WD vehicle (Jensen Interceptor FF, 1966, Figure 1) to the invention of thewidely adopted Viscous Coupling (1971 by FFD, a Ricardo company)[1,2] Figure 2.

Fig. 1 Jensen FF, V8 engine, 300 bhp 4WD,mechanical ABS - really quite advanced in itsday

Fig. 2 Patent scheme of viscous coupling– many millions manufactured by GKNaccording this concept

Core to all Ricardo concepts is the use of a low torque actuator amplified by ingeniousgearing to influence drive torque and thereby exploit the well-known friction ellipse of thetyres either across an axle or between axles in a 4WD configuration. But to distinguish theconcept from mere coupling devices solely aimed at improving traction, a name was needed –one that was descriptive – and Torque Vectoring™ has been used ever since.

Inter-Ing 2007„INTERDISCIPLINARITY IN ENGINEERING”SCIENTIFIC INTERNATIONAL CONFERENCE, TG. MUREŞ – ROMÂNIA, 15 -16 November 2007.

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Design

The first two applications in a car of the device were both for sportscars for which installationcompatibility with transaxles dictated a “cartridge” construction with a radial dimension suchthat it fitted inside the existing crownwheel yet did not significantly extend the width.Avoiding the requirement to reduce half-shaft length to accommodate a wider unit wasimportant to avoid compromising existing preferred suspension geometry.

The fundamental topology of the gear arrangement allows the control elements, twomultiplate wet clutches grounded to act as 200Nm brakes, to be located concentrically – afeature that saved some 29mm in addition to the inherently short geared element. A furtheradvantage of the arrangement arose from the possibility of locating the hydraulic elements(ports and clutches) on one side only - much simplifying the hydraulic circuits and casing shutlines.

Figure 3 shows the main elements as arranged in an early design and Figure 4 shows thetorque flow for the three modes of operation.

Fig. 3: Early section drawing of compact Torque Vectoring axle

The benefit of the intrinsic load sharing between all planets within the Torque Vectoring™Module became apparent both from the viewpoint of torque capacity and fatigue allowing acartridge dimension of 149mm diameter by 189 mm length to be achieved.

The same arrangement of gears (joined suns gears) would be applied for all applications, butthe actuation element would be specific to the class of vehicle. For reasons of compactness,mass, thermal duty cycle and control, conventional hydraulics were applied; but as is

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discussed later, electrical actuation has also been investigated and developed for lower costvariants.

As explained, the mechanical gain between the actuator torque and the torque bias generatedbetween the axle outputs is dependent upon the selection of tooth numbers within the TorqueVectoring™ Module. Conservative vehicle simulation suggested that a value of +/-15% speedchange of either output relative to the crownwheel would provide full influence on allanticipated surfaces – and this was applied to initial designs.

Fig. 4 Illustration of torque flow in axle duringthree modes of operation

Proprietary design tools, includingSabre™, were used to ensure the client-directed duty cycle of 10,000 kmNordschleife and 7,000 full torquelaunches would be met with regard togear, shaft bearing life. Furtherdurability targets were applied topotential torque reversals duringtraction control on mixed surfaces.

Rig Testing & Device Control

The sportscar variant was installed in atransaxle and tested on a 3-machinedynamometer (1 drive, 2 absorbers).The pre-production hardware is shownin Figure 5.After basic lubrication and thermalevaluation, the first dynamicmeasurements of torque bias wererecorded, a sample of which are shownin Figure 6. The plot shows the left andright shaft torques for 10% increases inbias demand followed by a series ofstep-wise bias reversals. Although notcalibrated, the control algorithm is seento achieve good response and accuracy.

Demonstration Vehicles

To prove the concepts during developments leading to the final axle concept, two vehicleshave been built which will now be described.

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A BMW X5 was rebuilt with a Torque Vectoring™ centre differential using a 20 Nmbrushless e-machine as the actuation torque source in conjunction with the amplificationgearing, and was demonstrated in Arjeplog (Sweden) in February 2004 [3], [4], [5], [6].

Fig. 5 Prototype Torque Vectoring“cartridge” assembled into transaxle fortesting and control development

Fig. 6 Recorded data showing step increasesin bias followed by a sequence of stepreversals

Figure 7 shows the end of a stable power slide. This arrangement, which also has significanthybrid potential [7], showed the effectiveness of balancing the front and rear lateral tyreforces under limit handling conditions. Whilst the system had the capability to vary the torquedistribution between 100% FWD and 100% RWD, it was found that the extremes of operationwere not required if the software provided early corrective action. Further work may includeassessment of control software to limit vehicle slip angle to prevent roll-over on high frictionsurfaces.

Fig. 7 BMW X5 demonstration car in a stable powerslide

An Audi Quattro was rebuilt with aTorque Vectoring™ rear differentialusing two small hydraulic clutchesas the actuation torque sources. Thiswas demonstrated with the standardTorsen centre differential, but withESP effectively deactivated bysynthetic CAN signals. The vehiclewas demonstrated publicly in Berlinat the CTI conference and then inAvdsjar(Sweden) during February2007 and showed the dramaticinfluence of lateral torquedistribution.

Control algorithms have now been calibrated for the extremes of ice and tarmac surfaces andit has been found that the value of 15% overspeed relative to the axle speed is excessive forall on-road applications. This permitted the second generation of devices to be designed witha lower mechanical gain which had the benefit of reducing the slip speed in the clutches andhence reduced parasitic energy loss. Named Drivewise, this vehicle is the start of a series of

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demonstrators that will include fault-tolerant integration of networked chassis and telematicsystems including e-steer.

Fig. 8: Low cost actuation concept forreplacing concentric hydraulic pistons

An important question in the development ofinnovative applications in the automotiveindustry is the one of reducing cost. In thiscase the focus is upon the actuation system –since by adopting alternative technologies, thegreatest potential savings may be made.The use of a single, bi-directional electricmotor acting upon two concentric tubes hasbeen developed to replace the concentrichydraulic pistons currently used. Clockwiserotation of the motor causes one tube to moveaxially, leaving the other stationary, and anti-clockwise rotation of the motor causes theother tube to engage with the other clutchpack.

A further significant advantage of the system is that it is impossible to engage both clutchessimultaneously, Figure 12 [8].

Future Applications Beyond Automobiles

Fig. 9: 6x6 concept with pivot-turn capability

It was found during testing of the BMWX5 with the e-machine actuator in theTorque Vectoring™ transfer case, that ifthe e-machine was driven when thegasoline engine was static, then theoutputs were driven by the e-machine indifferent directions. If the same principlewas applied across the axles of a vehicleand a direct drive was applied to thecarrier of the Torque Vectoring™ Module,then the vehicle could be rotatedaccording to the direction of the drive tothe carrier providing a low cost pivot turnif required, or a degree of turn assistance.

Fig. 9 shows an early variant with a second, low torque, propeller shaft. The 7:1 gaingenerated by the gearing module, in conjunction with a direct drive, rather than a smallactuator, enable the vehicle to overcome larger lateral resistance than some electric wheel-motor alternatives, and at lower cost for specialist off-highway vehicles [9].Other significant off-highway investigations include the use of one or more TorqueVectoring™ devices to limit the yaw rate of unstable vehicles to reduce the probability of roll-over following extreme steer inputs by the driver.On-highway, an undriven version of the core differential has been investigated fortrucks/trailer combination to provide trailer stabilisation – with promising initial results toprevent jack-knifing.

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Outlook

The invention, development and application of the Torque Vectoring™ concept have beendescribed. So what are the next steps and evolution of the Torque Vectoring™ concept?• Application of a unique direct acting linear motor previously devised for shift rail

actuation.• The Drivewise Audi A6 will be revised from standard 4WD Quattro with a Torque

Vectoring™ rear axle to Torque Vectoring™ RWD only to assess the capability of thedevice to damp and accelerate yaw.

• The Drivewise vehicle will have a steer-by-wire system fitted with a truly integratedvehicle dynamics controller with the facility of demonstrating fault compensation within anetwork of vehicle control devices.

• Other applications may even include marine drives with twin propellers instead of wheels,and perhaps twin-track snow mobiles.

Acknowledgements

The authors also wish to recognise the significant contribution from the following: MarkSlater, Jim Hey, John Stangle, Seb Drury, Nick Jackson, Andy Turner, Keith Ramsay, WillTurner, John Lipman.

References

[1] Pat. Turner History of Ferguson Formula Developments (FFD). 1910 – 1987[2] Patent GB1252753, Viscous coupling, Fergusson Formula Developments, 1971[3] SAE 2004-01-0863 Torque Vectoring AWD Driveline: Design, Simulation, Capabilitiesand Control, Ricardo plc[4] SAE 2005-01-0553 Torque Vectoring Driveline: SUV-Based Demonstrator and PracticalActuation Technologies, Ricardo plc[5] SAE 2006-01-0818 Design and Simulation of a Torque Vectoring Rear Axle, Ricardo plc[6] R. Donin, JC Wheals et al: Torque Vectoring System von Ricardo, VDI-Berichte 1943,2006[7] J Lipman, JC Wheals, Ricardo plc Patent WO 03/066363, Priority 8-2-2002, VehicleTransmission System (Hybrid Torque Vectoring™), Priority 26-Apr-2002,[8] Ricardo plc, Patent Application , Priority 26-Sep-2005, An Electro Mechanical Actuator[9] Ricardo plc, Patent Application , Priority 28-Sep-2006, A method of improvingmanoeuvrability in 3 axle vehicles specifically military type vehicles.