AU2022263482A1

AU2022263482A1 - Improved Controlled Differential

Info

Publication number: AU2022263482A1
Application number: AU2022263482A
Authority: AU
Inventors: Ian Spark
Original assignee: Spark Ian James Dr
Current assignee: Spark Ian James Dr
Priority date: 2021-11-02
Filing date: 2022-11-01
Publication date: 2023-05-18

Abstract

: Improved controlled Differential Vehicles that maximise traction and efficiency by employing the principle of cooperative redundant multiple steering systems (CRMSS) can best be designed with independent drives to the driven wheels. An on-board computer deduces the correct wheel speeds and wheel angles and control systems implement these speeds and angles. Vehicles with independent drives to all wheels are therefore best designed from scratch. Up to now it has been difficult to retro-fit CRMSS to existing vehicles such as traditional 4WDs. However, one way to retro-fit the CRMSS to existing 4WDs is to replace the traditional live drive axles with new drive axles that contain controlled differentials, where an on-board computer deduces the correct speed difference between the left-hand and right-hand drive shafts and a control system enforces this difference. Ideally a third controlled differential should be used to enforce the correct speed difference between the forward and rearward drive shafts. This means that there will now be six wheel-speed steering effects and sixwheel-angle steering effects acting on the vehicle, where these 12 steering effects have a single theoretical instant centre. This means that when one steering effect starts to fail (as conditions become more difficult), it is backed up (or reinforced) by the other steering effects. As there is no conflict between the steering effects, ground damage, tyre wear and fuel consumption will be minimised. The maximum torque difference between a loaded wheel and an unloaded wheel can be increased by positively rotating the planetary gears in the controlled differential with high displacement hydraulic motors, which are driven by a computer controlled reversible hydraulic pump. In a variant of the invention cooperative redundancy of multiple steering systems is achieved by replacing the computer-controlled hydraulic motors located within the body of the controlled differential with a computer-controlled brake alsolocated within the body of the controlled differential. VFL FR VFR t(a) RFL IC E G - Vo-- -- --d COV VRL7 RRL V RR S tR CD Figure 1. (a) Hybrid drive sytem for four wheel drive/four wheel steering (4WD/4W) vehicle; (b) alternative driver interfaces.

Description

VFL FR VFR RFL IC E G

COV VRL7 t(a) RRL

V RR S

tR CD

- Vo-- d -- --

Figure 1. (a) Hybrid drive sytem for four wheel drive/four wheel steering (4WD/4W) vehicle; (b) alternative driver interfaces.

IMPROVED CONTROLLED DIFFERENTIAL

Technical Field

The invention relates to a computer-controlled differential where the steering effect of positively controllingthe difference in the angularvelocity of all the driven wheels is made identical to the steering effect of all the wheel angles.

The are basically two ways of steering a wheeled vehicle. One way is to turn one or more steerable wheels about a substantially vertical axis. The otherway is to drive one or more left-hand wheels independently of one or more right-hand wheels. If both steering systems are enabled, they will generally try to impose different steering effects on the vehicle. This conflict causes ground damage, tyre wear and increased fuel consumption -since the energy required to inflict ground damage and tyre wear comes from burning more fuel. In traditional road cars and tractors, this conflict is avoided by disablingthe drive wheel speed steering effect. This is achieved by installing one or more open differentials in the drive train to the driven wheels.

The disadvantage of the open differential is that if one drive wheel has reduced grip on the ground, the force that the otherwheels can exert on the ground is also reduced. Attempts to increase the net force acting on the vehicle will be unsuccessful, with spinning of the wheel with the least grip on the ground. This problem can be overcome to some extent by replacingthe open differential (or differentials) with limited slip differentials. In alimited slip differential, frictional resistance in the gear train of the differential is increased so that force exerted on the vehicle by the non -spinning drive wheels can now significantly increase relative to the force that can be exerted by the spinning wheel. Ideally this force (ortorque) difference should increase with the rate of slip of the limited slip differential. This significantly improves the traction of the vehicle on steep and slippery slopes.

A bettersolution to the above traction problem is to make the steering effect of the difference in angularvelocities of the driven wheels identical to the steering effect of all the wheel angles. In this case we have cooperative redundancy between the drive -wheel speed steering effect and the wheel angle steering effect. Now the two steering effects reinforce each other. If one steering system starts to fail, it is reinforced by the othersystem. This wil significantly increase the operational envelope of the vehicle in difficult conditions.

Definitions

Differentials in general:

Differentials are angularvelocity averaging machines. The angularvelocity of the body of the differential (orinput) is generallythe average of the angularvelocities of the left and right drive shafts (or outputs). This averaging effect will be referred to asthe differential action. The specific angularvelocities of the output shafts cannot be determined from the angularvelocity of the input shaft.

DifferentialsalsoobeyNewtonssecondlaw.Powerin=powerout+frictionallosses.Therefore;

(Input angularvelocity)* (Inputtorque) = (LH ang vel)*(LH outputtorque) + (RH ang vel)*(RH output torque) +frictional losses.

Open Differential:

The ideal open differential is alsoatorque equalization machine. If there are nofrictional losses within the differential, the LH and RH outputtorques must be equal. This creates a problem if the reaction torque that can be applied to one drive wheel is reduced. Then the reaction torque that can be applied to the otherdrive wheel is also reduced to the same value. If the combined torques are insufficientto move the vehicle, the wheel with the reduced grip on the ground will spin and the vehicle is bogged. An extreme case is where one wheel loses contact with the ground. In this case the raised wheel willspin and almost zero force will be exerted on the vehicle by the non -spinning wheel.

Limited Slip Differential:

In a limited slip differential, a difference is created between the torque that can be exerted on the vehicle by the non-spinningand spinningwheels. This torque difference can be created by increasing the frictional resistance in the geartrain that enables the differential action. Ideallythis torque difference should increase rapidlywith the rate of differential slip. The inventorhas previously proposed a limited slip differential where the torque difference is proportional tothe square of the difference in angularvelocities.

Lockable Differential:

In a lockable differential it is possibleto make the differential bodyand the twooutputshafts rotate in unison. In short it is a wheel speed equalization machine. Itis usuallyachieved by lockingone outputshaftto the differential body. The differential action ensuresthatthe other output shaft must rotate at the same speed. The steering effect of a locked differential can only be identical to the wheel angle steering effect when the vehicle is proceedingin a straight line. Forall otherpaths there will be conflict between the two steering systems. This conflict increases asthe desired path curvature increases. This makes manoeuvringof avehicle with alocked differential difficult.

Controlled Differential:

In a controlled differential the angularvelocity difference between the twooutputshafts is positively controlled. This control usually ensures that the steering effect of the drive wheel speeds is identical tothe steering effect of all the wheel angles.This ensuresthatthe two steering systems alwayswork in unison, regardless of the curvature of the path selected bythe driver. This max imises the stability, traction and stabilityof the vehicle while mininising ground damage, tyre wearand fuel consumption. Italso rendersthe vehicle kinematically determinate since it now has onlyone theoretical instantcentre - sothiswould be expected to be the actual instantcentre. On the other hand, if the vehicle has multiple theoretical instantcentres, dynamicanalysis (orexperiment) must be used to determine the actual instantcentre, and therebythe actual path of the vehicle. Note that a rigid body can only have one actual instant centre.

Prior Art

The primary function of a differential is to enable the left-hand and right-hand drive wheels to rotate at different speeds when the vehicle is turningleft or right. The sharperthe turn, the gre aterwill be the speed difference between the wheels. The differential achieves this objective by being a torque equalization machine, where the same torque must be delivered to the left-hand and right-hand drive wheels (if we neglect frictional losses). N ote that the average drive wheel speed is unchanged by the differential.

However, this torque equalization effect causes a problem if one wheel starts to spin. In this case the traction force exerted on the vehicle bythe non -spinning wheel will be the same as the traction force exerted on the vehicle bythe spinningwheel. If the sum of these forces is insufficientto move the vehicle, the vehicle will be bogged. An extreme case is where one drive wheel is not in contact with the ground. In this case the traction force exerted on the vehicle will be close to zero.

Many attempts have been made to overcome this problem by the production oflimited slip differentials (LSDs). These LSDs produce atorque difference between the torque deliveredtothe spinning wheel and the non-spinning wheel. Generally, this torque increment increases with the speed difference between the spinningandthe non -spinning wheel.

This raises two questions.

1. What is the ideal relationship between the torque difference and the speed difference? 2. How can this ideal performance characteristic be achieved?

The answerto the first question is that the steering effect of the difference in the speeds of the drive wheels (as characterized by a theoretical instant centre (TICs)) is identical to the wheel angle steering effect (as characterized by a theoretical instant centre (TICa)).

If the single theoretical instant centre selected by the driverwith his/hersteeringwheel has the coordinates Rx and Ry relative to the centre of the vehicle, then the correctwheel speeds and wheel angles will be given by equations given below.

The answerto the second question is that the ideal performance characteristics can best be achieved by drivingthe left and right drive wheels independently at the correct speeds. This system is referred to as a Cooperative Redundant Multiple Steering System (CRMSS). This system is described and claimed in the following patents:

Spark I, Improved off road vehicle. US patent 7,191,865

Spark 1. Improved off road vehicle. US patent 7,646,785

Spark I, Improved off road vehicle. US patent 7,857,085

Spark I, Improved off road vehicle. Australian patent 2001293507

Spark, I Improved off road vehicle. Australian patent 2003201206

Spark I, Off road vehicle. Australian patent 2005235431

In the CRMSS, the differential is generally discarded in favorof independent drives to the drive wheels.

Although the CRMSS is superiorto an LSD system, it is difficult to retrofit to an existing vehicle. CRMSS vehicles are best designed from scratch.

In principle it would be possibleto achieve perfect speed control of the drive wheels (i.e. CRMSS) by replacing a standard drive axle containing a standard differential with a drive axle containing a controlled differential. In a controlled differential the speed difference between the left hand and right-hand wheels is indirectly controlled by the driverwith his/her steering wheel.

Controlled differentials are used on military tanks and otherskid steervehicles. However, the se are generally too bulky and heavy to be able to directly replace a traditional differential. They are generally an assembly of two orthree differentials and a steering motor (as well as the main motor and gearbox). The steering motor is generally a high torque/low speed hydraulic motorwhich is indirectly controlled by the driver. When the steering motoris stationary, the speed of the left -hand and right-hand tracks or wheels will be the same, so the vehicle will proceed in a straightline.

Although a controlled differential limits slip, itshould not be regarded as an LSD. A controlled differential positively controls the speed difference between the left hand and right -hand wheels. An LSD cannot control this speed difference.

The Problem to be solved:

To produce a drive axle containing a controlled differential that can directly replace a drive axle containing a conventional (or normal) differential.

The Solution Proposed:

The invention is a drive axle containing a controlled differential that can directly replace a drive axle containingaconventionaldifferential.

The essential feature of the controlled differential is that the bevel gears usually found in a conventional differential are replaced with straight cut spurgears with parallel rotational axes. These spurgears are keyed to parallel shafts that rotate within the body of the controlled differential. The body of the controlled differential is such that there is minimum space that is not occupied by the spur gears and theirshafts. This enables the spurgears to be transformed into a multitude of gear motors with inlet and outlet ports built into the body of the controlled differential. All the inlet ports are connected in parallel. All the outlet ports are also connected in parallel. Workingfluid is transferred from a stationary inlet spigot passing through the differential housingto a stationary ringwhich rotates relative to the rotating body of the controlled differential. Similarly, the workingfluid is transferred to a stationary outlet s pigot passing through the differential housing to a stationary ringwhich rotates relative to the rotating body of the controlled differential. The inside of both rings are grooved to allow workingfluid to flow from the inlet spigotto the inlet ports of the gear motors, and to the outlet spigot from the outlet ports of the gearmotors. Note that the inlet and output spigots and ports are interchangeable depending on whetherwe require the left hand drive shaft to rotate fasterthan the left-hand drive shaft orvice versa.

The two spigots are connected toa reversible hydraulicfixed displacement pump located within the body of the vehicle by means of hydrauliclines and hoses. This means we can positively control the speed difference between the two drive wheels by controllingthe speed and direction of rotation of the hydraulic steering pump. Alternatively, we could use a reversible variable displacement pump drivenata speed proportionalto the speed of the crown wheel. In this case the speed difference generated by the controlled differential will be varied by varying the displacement of the variable displacement pump. The latterconfiguration has the advantage that the pump speed is automatically compensated forchanges in vehicle speed.

The driver determines the path of the vehicle with his/her steering wheel. The angle of the steering wheel ideally determines the curvature of the path of a nominated location on the vehicle. On 2WD/4WD vehicles the midpoint of the non-steered axle is nominated as the nominated reference point in this invention. The path of the nominated point on the vehicle determines the desired instant centre (IC) of the vehicle. Note that each two wheels of the vehicle produce a wheel angle steering effect characterized by a theoretical instant centre (TICa). If the vehicle has N wheels it will be subjected to N(N-1)/2wheel-angle steering effects (and TICas). If the vehicle has N' positively driven wheels it will be subjected to N'(N'-1)/2wheel-speed steering effects (and TICs)

In the ideal vehicle all the steering effects (and TICs) will be identical. This means that all the steering effects areall trying to impose the same path on the vehicle. Traction is maximized because when one steering effect starts to fail, it is backed up by the other steering effects. Furthermore, elimination of conflict minimizes scuffingand skidding. This minimizesground damage, tyre wear and fuel consumption. Note that the energy required to inflictground damage and tyre wear comes from burning more fuel. This ideal vehicle is said to have cooperative redundant multiple steering effects (CRMSE)(or systems (CRMSS)).

In an ideal hydrostatic circuit the speed difference between the left hand and right hand drive wheels could be directly controlled bythe speed of the hydraulic pump. Howeverreal hydrostatic circuits are subjectto internal and external leakage of the hydraulicfluid. In this case speed sensors could be installed to measure the speeds of the left hand and right-hand wheel speeds. If the desired wheel speed difference is notattained the speed of the hydraulic pump can be adjusted accordingly.

The invention may be better understood by referencetothe following figures.

Figure 1shows the ideal relationship between speed of the drive wheels and the angle of all of the wheels, forthe general case where a single instant centre can be located anywhere outside the vehicle.

Figure 2 shows the ideal relationship between speed of the drive wheels and the angle of all of the wheels, forthe special case where a single instant centre can be located anywhere on the axis of the rear un-steered wheels (except inside the vehicle).

Figure 3 is a schematic drawing of the moving parts of the controlled differential. The length of the spur gears has been shorted and the length of the associated shafts has been increased in the interests of clarity.

Figure 4 is an assembly drawingof the proposed controlled differential.

Figure 5 shows sections through the seven layers that comprise the body of the controlled differential.

Figure 6 shows sections through the six gear motors and depicts the flow of the workingfluid through these gear motors.

Figure 7 shows the flow of fluid through the six gear motors via the stationary rings when the body of the controlled differential is rotating.

Figure 8 shows means of compensating for leakage in the hydrostatic circuit.

Figure9showsa meansof preventing wind-up in a 4WD/2WS vehicle with front and rearcontrolled differentials.

Figure 10 shows the moving parts of a controlled differential which includes two extra high displacement gerotor hydraulic motors and two extra high displacement lobe hydraulic motors.

Figure 11 shows sections perpendicularto the longitudinal axis of the controlled differential shown in Figure 10 which pass through eachof the eleven layers that makeup the controlled differential.

Figure 12 shows a front wheel steering4WD with a single theoretical instantcentre. Equationsfor the required wheel angles and wheel speedsaregiven.

Figure 13 shows an open differential with spurgears.

Figure 14 shows a means of multiplyingthe difference between the angularvelocity of the left -hand drive shaft and the body of the differential with one or more step-up epicyclicgeartrains.

Figure 15 shows means of increasing the frictional resistance to the operation of the epicyclic gear train with a multi-discwet brake.

Figure 16 shows means of connecting the left-hand output shaft of the differential to the epicyclic gear train.

Figure 17 shows means of varyingthe clampingforce acting on the wet multi-disc brake.

Figure 18 shows a schematic assembly drawing of the proposed computer-controlled differential.

Detailed Description

The wheel anglesrequired to produce the desired instant centre (R,,Ry) are given below: (see Fig1). Subscripts FR, FL, RR, RL refertothe front right, front left, rearright and rear leftwheels respectively. Subscript 0 refers to a notional wheellocated atthe centre of the vehicle (COV).

tan OFR = (b/2 - Ry)/(R.- t/2) (1)

tanFL= (b/2 - Ry)/( R.+ tF/2) (2)

tanORR=(b/2+Ry)/(R.- tF/2) (3)

tan ORL= (b/2 + Ry)/(R.,+ tF/2) (4)

Andtano =(Ry/Ry)= tano, therefore 00=i (5,6)

where R,=R sini and Ry=Rsin1P; (7,8)

where |RI is the radius of curvature (ROC) of the path of the centre of the vehicle (COV), b is the wheelbaseof the vehicle, 0 is the angle between the direction of displacementof the three-axis joystick and the longitudinal axis of the vehicle, where an anticlockwise angle is deemed positive.

The relationship between the clockwise rotation of the rotatable joystick 0 and the ROCR is given by: R0 -= cot[9 0°( )] = cot[90° 0'] (9) t Omax

Where t is the average track of the vehicle, Omax is the maximum rotation of the joystick and 0' is the relative rotation of the joystick. Note that R will be negative when 0 and0' are negative. Alternative functions could be used, as long as they produce +/-infinitywhen 0'equals Oand 0 when 0' is+/-1.0.

Figure 1(b) shows three possible driver interfaces. The first is a three- axis joystick. The second is a steering wheel plus a two-axis joystick. The third is twojoysticks.

Note that the above equations will only yield angles between -90° and +900 with 180 steps. To obtain a continuous range of wheel angles of 180°, 180° needsto be subtracted oradded tothe raw angle if it is outside the continuous angle range that passes through the origin. The wheel speeds required to produce the desired instant centre (R, Ry) aregivenbelow:

VFR/RMSWS=IRFR I/RMSR where RR= - Ry)2+ (R)- 2 (10, 11)

VFL/RMSWS= IRFLI/RMSR where RFL= (- Ry )2+(Rx + )2 (12, 13)

VRR/RMSWS= IRRRI/RMSR where RRR =(2+ Ry )2+(Rx - 2 (14,15)

VRL/RMSWS=IRRL|VRMSR where RL (+ Ry)2+(Rx+ )2 and (16,17)

VO/RMSWS=IRI/RMSR whereR2 = (Ry)2+(R.)2 (18,19)

RMSR2 = R2+ Ry +t2 /8+ t2/8 +b 2/4 where RMSR is the root mean square of the distances of each kingpin from the instant centre selected by the driver (20)

Where RMSWS = Kd, where d is selected by the driver by displacing the rotatable joystick or equivalent. If p is greaterthan 90degrees and less than 270degrees, d is negative so all wheel speeds are multiplied by -1. Note that the centre of the vehicle now moves in the direction of displacement of the rotatable joystick or equivalent. Also note that although values R and R, can have negative values, the modulus of R and R, must be used to calculate the required wheel speeds. This happens automatically if Pythagoras' equation is used to determine Ror R,. Note IRI = (R2).

In this paperthe rollingspeed of the wheel across the ground V, is used. This makes iteasierto handle wheels with different rolling radii. Ambiguities arise if the driver selects an instant centre inside the rectangle defined by the track and wheelbase of the vehicle. In this paper track is defined as the distance betweenthe centres of thecontact patches of the LH and RHtyres. If the frontand reartracks are differentwe will have a trapezium.

Note that the above equations for the modulus of wheel velocities are correct regardless of the quadrant selected by the driver for the instant centre. However, Pythagoras' equation only yields positive radii, and therefore positive wheel velocities. These will be correct so long as the instant centre selected by the driver is outside the rectangle (ortrapezium) defined by the kingpins of the wheels. However negative velocities are sometimes required if the instant centre selected is inside the vehicle. This problem is overcome by "if, then, else" statements.

In Figure 1 the rear standard differential has been replaced with two reverse differentials close coupled to the drive wheels. The outputsof the left and right reverse differentials are connected to the leftand rightwheel hubs respectively.One inputtothe reverse differentials is provided bythe left and rightdrive shaftswhich are drivenatthe root mean square wheel speed(RMSWS) selected bythe driver. The otherinputto the reverse differentials is provided byspeed correcting hydraulic motors. If the left hand and right-hand speed correcting motors are stationary, the speed of the left hand and right rear wheels will be the same. However, we can correct the speeds of the left and right rear wheels bydrivingthe speed correctingmotors atspeedswhich produce the desiredtheoretical instant centre (TIC).

The easiestway of controllingthe speed of the speed correcting hydraulic motors is to connectthem in series with variable displacement pumps (VDPs) driven at a speed proportional to the RMSWS selected bythe driver. The speed of the speed correcting hydraulic motors can now be controlled by varyingthe displacement of the VDPs. Bearing in mind that negative strokes will reversethe direction of flow of the workingfluid.

The arrangement described above has the same effect as a single controlled differential. However, it would be difficult to replace a standard (free) differential with such an arrangement. A similar arrangement can be used to positively control the speed of the frontwheels.

Let us now consider the special case of a 4WD vehicle with unsteered rear wheels, where the front and rear tracks are the same. That is tf = tr = t. In this case Ry = -b/2. See Figure 2.

The above equations now become:

tan ©FR = b/(Rx - ) (21) t tan FL = b/(Rx +-1) (22) 2 0 tan (DRR~ tanRL (23)

RFR 2 b 2 + (Rx - t/2)2 So VFR = (RMSWS/RMSR)[Rx +b 2 + t 2/4 - Rxt]°-5 2 (24, 25)

RFL 2 =b 2+(Rx+t/2) 2 So VFL=(RMSWS/RMSR)[Rx 2 +b 2 +t 2/4+Rxt]1 0 5 (26, 27)

RRR2 = (Rx - t/2)2 So VRR =(RMSWS/RMSR) [Rx - t/2] (28, 29)

RRL2 = (Rx + t/2)2 So VRL =(RMSWS/RMSR)[Rx +t/2] where RMSR= [RX 2 +t 2 /2+ b 2/4 05 . (30,31)

Therefore VFL- VFL = (RMSWS/RMSR)([RX2 + b2 +t 2/4+ Rx t]-05- [RX2 + b2 + t 2 /4 - Rx t] 0 -5) (32)

And average (VFL + VFJ/2= (RMSWS/2*RMSR)([Rx2 + b 2 +t 2/4+ Rx t] 05 + [RX2 + b 2 + t 2/4 - Rx t] 0 -5) (33)

Also VRL- VRL = (RMSWS/RMSR) [t] (34)

And average (VRL + VRL)/2= (RMSWS/RMSR) [Rx] (35)

Note that the average front wheel speed will be greaterthan the average rearwheel speed.

The difference between the average speed of the front axle and the average speed of the rearaxle is; 05 (VFL + VFL/2- (VFL + VFJ/2 = (RMSWS/2*RMSR)([RX2 + b2 + t 2/4 + Rx t] 05 + [RX2 + b2 + t 2/4 - Rx t] - - 2R) (36)

Note that the front wheel angles OFLand OFR are approximated by a four-bar linkage mehanically connectedtothe steeringwheel.

Figure3 showsthegears and shaftsof a schematic spur gear differential. The length ofthegearshas been reduced andthe length of the shafts increased inthe interestsof clarity.All shaftsare rotatably supported bythe bodyof the differential which is not shown.A crown wheel is usually bolted to the body of the differential. This crown wheel is rotated by a pinion. Onlythe pitch cylindersof the spur gears are shown. In Figure 3 only the pitch cylinders of the gears are shown. The gearteeth are not shown in the interestof clarity.

The operation of the differential is best explained if the body of the differential is stationary. In this case, rotation of the left-hand drive gear will cause the right-hand drive gear to rotate an equal amountin the opposite direction. Note thatthe average rotation of the drivegearswill be zero, thus matchingthe rotation of the body ofthe differential. Ifwe rotatethe left-hand drive gear1clockwise (lookingfrom the left) this will cause an engaged planetary gear 2 to rotate counterclockwise. The planetarygear2 is keyedtothe shaft3, as isan identical gear4, which engageswith athird planetary gear 5 which is caused to rotate clockwise. The third planetary gear 5 is keyed to its shaft 6, as is a fourth planetary gear 7. The fourth planetary gear 7 engages the right-hand drive gear 8 which is caused to rotate counter clockwise. A second set of four planetary gears are located diametrically opposite the first set. This second gear train will share the torque that is transmitted from the left hand drive gear to the right-hand drive gear. Itwill also balance the first set of gears and shafts.

If the body of the differential is such that all the space not occupied bythe shafts and rotatinggears is occupied by solid material, the engaginggear pairs are transformed into six gearmotors.

Figure 4 shows how such an assembly of gear motors could be assembled. The assembly consists of seven layers. There are threegearmotor bodies,each containingtwogearmotors. Thereare two end plates and two separating plates. The diameterof the cylindrical bores in the gear motor bodies will be slightly greater than the addendum of the gears. The crown wheel (not shown) is bolted to a flange on the left-hand gear motor body. Only the pitch cylinders of the gears are shown.

Figure 4 shows two sections through the axis of the controlleddifferential.Section HH to the left is a vertical section. Section JJ is rotated 60degreescounterclockwisefromthevertical.

Each gear motor will require inlet and outlet portslocatedintheend plates or separating plates. These are notshown.

All gears are keyed to theirshafts. Although some gears could be integral with theirshafts, others must be keyed to theirshafts to allow assembly of the controlled differential. The end plate bores contain an internal groove 10to accommodate O-rings (notshown).The ends of the end plates are recessed lto accommodateoil seals(notshown)

Thesecond and third cylindrical gear motor bodies are encompassed bycylindrical rings 12,13 which can rotate relativetothe bodies.The insideofthese ringsis grooved to allowthe workingfluid to flow around the grooves. Each ring has a spigot 14 which passes through the housing (not shown) of the controlled differential. These rings 12 and 13 allowthe workingfluid to flow between a reversible hydraulic pump 35located inthe bodyof the vehicle and the gearmotors in the controlled differential.

Figure5showsthe layered structure of the controlled differential that enables it to be assembled one layerat a time. All sectionsare perpendicularto the axis of the controlled differential. Onlypitchcirdes of the gears are shown. See also Fig4.

Figure5(a) isa section AA through the left-handend plate 18. It shows the cylindrical borewhich acts as a bearingforthe shaft 22 of the left-hand drive gearland the diametrically opposite bores 23, 24 which act as bearings forthe shafts 3 and 3' keyedtothe firstpairof planetarygears 2and 2'.

Figure 5(b) is a section BB through the first gear motor body 15. It shows the left-hand drivegear 1 which meshes with two diametrically opposite planetary spur gears 2and2'.There isasmall clearance betweenthe addenda of the spur gears and the intersecting cylindrical bores. This body has a flange 9 towhichthe crownwheel 25will be bolted.

Figure 5© is a section CCthrough the first separating plate 20. It shows the bores 26, 27, 28 and 29 which act as right-hand bearings forthe shafts 3 and 3' keyed to the first set of planetarygears 2 and 2' and the left-hand drive gear1. The bores that act as the left-hand bearingsforthe shafts 6and 6' that are keyedtothethird pairof planetarygears 5and 5' arealso shown.

Figure 5(d) is a section DD through the second gear motor body 16. It shows intersecting bores which rotatably accommodate planetary gears 4 and 4'. It also shows the two intersecting bores which rotatably accommodate planetary gears 5 and 5'.

Figure 5 (e) is a section EE through the second separating plate 21. It shows the bores which act as bearings forthe right-hand end of the shafts 3and 3'that are keyed to planetary gears 4and 4'. It also shows the bores that act as bearings forthe left-hand end of shafts 6 and 6' keyed to planetary gears and 5'.

Figure 5(f) is a section FF through the third gear motorbody 17. It shows the right-hand drive gear 8 which meshes with diametrically opposite planetary spur gears 7 and 7'. There is a small clearance between the addenda of the three gears 8, 7 and 7' and the three intersecting bores.

Figure 5(g) is a section GG through the right-hand end plate 19. It shows the three cylindrical bores that act as bearings for the right-hand ends of the three shafts 6, 6' and 32 that are keyed to right hand drive gear8 and the diametrically opposite planetary gears 7 and 7'.

Figure 6 shows three schematic sections through the bodies of the three gear motors 15, 16 and 17 and a hydraulic pump 35, which show the flowof the workingfluid through thegear motors and pump 35. The six gearmotors are driven by the flow of fluid caused by the hydraulic pump 35located in the body of the vehicle and connected to the controlled differential by hydraulic lines and hoses. In this respect it resemblesthe wheel braking system. With regard to the hydraulic circuit, high pressure lines are denoted by solid (red) lines, return lines by dashed (blue) lines and ducts within the body of the controlled differential as dotted lines. In Figure 6 the pitch circle, addendum and dedendum cirdes are shown. Note thatthe pitch circles of meshinggears should touch.

Note thatthe controlled differentialcan be initiallytested with the differential stationary. In this case the hydraulicpump 35would cause the leftand right drive shafts land 32to rotate at the same speed but in opposite directions. Thisgreatly simplifiesthe hydraulic circuit. The three gear motor bodies 15, 16 and 17 make up six gear motors, each having a inlet port and an outlet port. These ports will be located in the end plates and the two separation plates. All inlet ports will be connected in parallel by ducts in the end plates and separation plates. All outlet ports will also be connected in parallel. Note thatthe inlet and outlet ports will be reversed if the hydraulic pump isdriven in the opposite direction.

An on-board computerwill calculate the correct speed difference forthe left and right drive shafts 1 and 32 from the driver inputs. The computer will then calculate the correct speed for the reversible hydraulic pump 35. This speed will be implemented by a suitable control system.

Figure 7 shows how the working fluid can be transferred to and from the rotating controlled differential. In normal operation the body of the controlled differential flange 9will be rotated bythe attached crown wheel 25. In this case cylindrical ringsl2and 13 are rotatably located on thecylindrical bodies of the second and third gear motor bodies 16 and 17. The inside of these rigs is grooved to allowthe workingfluid to flow around the stationary rings and to and from the hydraulic pump 35 via the hydraulic hoses and lines and spigots 14 that pass through the differential housing. The inlet and outlet ports of the gear motors will be connected to holes in the gear motor bodies that are covered by the cylindrical rings 12 and13.

A possible hydraulic circuit is shown in Figure7. Hydraulic pump 35pumps the workingfluidfrom right to left. The pressurised fluid flows via tubes and hoses tothe inletcollection ring 13 which delivers it to the rotating controlled differential. Internal ducts in the body of the controlled differential deliver the fluid to the inlet ports of the six gear motors. It then does work on the spur gears until it reaches the outlet ports. It then flows via internal ducts in the rotating controlled differential to the outlet collection ring12. The fluid is then returned tothe inlet of the hydraulic pump 35via tubes and hoses. Both the inletand outlet lines are connected to a reservoir 36 via separate non-return valves 37 and 38 which prevent pressurised fluid flowing intothe reservoir36. However, if a vacuum is generated in the inlet line fluid will be sucked intothe circuitfrom the reservoir 36.

Both the inlet and outlet lines are connected to the reservoir via pressure relief valves 39 and 40. These relief valves will prevent dangerous pressures building up in the hydraulic circuit. If a pressure relief valve is open, the controlled differential is converted to alimited slip differential (LSD).

A small bleed valve 60 may connect the inlet and outlet lines. This will enable a small flow of fluid to lubricate the controlled differential or the hydraulic pump when either the planetary gears or the hydraulic pump 35 is stationary.

A by-passvalve4lis located on a linewhich connects the inlet and outletlines. If this by-pass valve is open the controlled differential is converted to a normal differential.

Note that, if the direction of rotation of the hydraulicpump is reversed, the inlet and outletlines will be reversed and the speed difference between the left and right drive shafts will also be reversed.

In the interest of clarity drain lines have not been shown.

Figure 8 shows a method of compensating for internal leakagewithin and external leakagefrom the hydrauliccircuit. The above Figures assume that the volumetricefficiency of the hydrauliccircuit is 100%. However real hydraulic circuits are subject to both internal (by-pass) leakage and external leakage to a sump. We can calculate (in real time) the flow rate required to produce the required speed difference between the left-hand and right-hand drive shafts. However, if there is significant leakage the speed difference produced bythe controlled differential will be lessthan thatrequiredto produce cooperative redundancy (CRMSS). Asthe volumetric efficiency is dependenton the pressure within the hydraulic circuit, afeedback loop is required.

Figure 9 showstwo methodsof preventing wind-up if controlled differentials are installed in both the frontand back axles of a 4WD vehicle with unsteered rearwheels.

Figure 9(a) shows a4WD frontwheel steered vehicle with controlled differentials installedin the front and rear axles. Here engine 43 drives gearbox 44 then transfer case 45. A normal differential 46 is mounted to the transfer case 45 to enable the forward drive shaft 47 to rotate faster than the rear drive 48. Note that the average speed of the forward and rear drive shafts 47 and 48 remains unchanged. The forward drive shaft 47 provides the input to the controlled differential 49 mounted in the front axle. The rear drive shaft 48 provides the input to the controlled differential 50 mounted in the rear axle. Speed sensors 51record the speeds of the left and right front driveshafts 52 and 53 and the left and right rear drive shafts 54 and 55. An on-board computer (not shown) calculates the speed difference that is required to achieve the required front drive wheel speed steering effect. This is achieved by driving hydraulic pump 56at the correct speed. The on-board computeralso calculates the speed difference that is required to achieve the required rearwheel drive steering effect. This is achieved by driving hydraulic pump 57 at the correct speed. In this case the third normal differential 46 will allow the speedsof the front and reardrive shafts to adjust to eliminatewindup. Although this configuration will eliminate wind up, it sacrifices fourof the six drive wheel speed steering effects. As a consequence, traction and manoeuvrability in difficult conditions will be reduced.

A better solution is shown in Figure 9(b). This issimilarto the configuration shown in Figure 9(a) except that the third normal differential 46 is replaced with a controlled differential 59 which positively enforces the correct speed difference between the forward and rear drive shafts 47 and 48. The advantage of this configuration isthatall six drive -wheel speed steering effects are nowactingon the vehicle. This will maximise traction and manoeuvrabilityin difficult conditions.

We have already shown that the magnitude of the required average speed of the front wheels will generally be greater than the magnitude of the required average speed of the rear wheels. This front/rear speed difference will be zero wheel the vehicle is proceeding in a straightline and increase as the curvature of the path required by the driver increases. If the speeds of the front and rearcrown wheels are correct when the vehicle is proceeding in a straight line, as the curvature of the chosen path increases, the speed of the frontwheels will be lessthan that required to produce cooperative redundancy of all four drive wheels. This will cause "windup" in the powertrain to the fourwheels. Windup is accommodated by elastic twisting of the shafts in the powertrain. The windupwill i ncrease with the distance travelled along a curved path. Note that the effect ofleft-hand and right-hand turns do not cancel out.

One method of eliminatingwind up is to drive the vehicle around a curved path in reverse.

Another method is to stop the vehicle and jack one wheel off the ground.

Another method is to drive the vehicle on a gravel road. In this case windup is eliminated by micro skiddingand micro spinningof the wheels.

A better method of eliminating windup is to instal a normal differential 46 in the drive train to the front and rear wheels. This will effectively reduce the number of drive wheel speed steering effects acting on the vehicle from six to two. However, the two remaining steering effects are the most powerful. Thissystem will onlyfail when both thefrontwheels orboth back wheelslose their traction. In this rare event lockingthe differential would restore traction tothe non -spinning wheels.

The best method of eliminatingwindup is to replace the normal differential 46with a third controlled differential 59. The correct speed difference between the frontand rearcrown wheels isgiven bythe equation;

(VFL+ VFL/2- (VFL+ VFL)/2 = (RMSWS/2RMSR)([Rx 2 +b 2+ t /4+ 2 Rx t]5 + [Rx 2 + b 2 + t2/4 - Rx t] 0 5 - 2Rx)

Note that micro-windup could occur over long distances on straight or curved roads if the rolling diameterof the tyres is different from the ideal rolling diameters. Itwill beworstif thefront wheels are biggerand the rear wheels smaller, or vice versa. Such micro-windup could be eliminated by opening a bypass valve 42 between the inlet and outlet hydraulic lines of the third controlled differential 59for say one second every kilometre travelled. Also note that leakage in the hydraulic circuitwould alsotend to eliminate micro-windup.

Let usconsiderthe special case of a 4WD vehiclewherethe desired path isa straightline. For cooperative redundancy between the wheel angle steeringsystem andthe wheelspeed steering system, all the drivewheels mustbe positively driven atthe same speed.The control system would read the zerofrontwheel angle and deduce thatthe drive wheels should be driven atthe same speed.Thiswould requirethatthe speed ofthe hydraulicmotors be zero. If the reactiontorque actingon both drivewheels is similar, then then zerospeed of the hydraulic motors could be achieved bydrivingthe reversible hydraulic pump at zero speed. However, if one drivewheel isoff thegroundthe reactivetorque actingon itmust be zero. In a normal differential, thetorque that could be exerted onthe otherwheel would also be zero(if the effectof internal friction is neglected). Inthe controlled differential, rotating the wheel in contact with the ground requiresthat a torque difference is developed betweenthe non-loaded wheel andthe loaded wheel. This torque is provided bythe hydraulic motors. In an ideal hydraulic circuit, where I eakageiszero, zero difference inwheel speedswould require zero movement of the hydraulic motors. However, in a real hydraulic circuit, fluid would tend to leak from high pressure regions to low pressure regions. As thetorque required torotatethe loadedwheel atthe same speed asthe unloadedwheel increases the pressure difference, and thereforethe hydraulic leakage will increase. This would allowthe loadedwheel toslowdown andthe unloaded wheel to speed up.This leakagewould allowthe hydraulic motors to rotate backwards.This problem can be overcomewith a secondary control system. Herethe difference (orerror) inwheelspeed isdetected, andthe hydraulicpump issped up in order to reduce the errorto zero. The pump effectively compensates forthe leakage in the hydraulic motors. If the pressure in the high-pressure region reaches an unsafe level, one of the pressure reliefvalves willopen and the controlled differential will default to a LSD.

The main disadvantage of gear motors is their ow displacement for a given overall size. Thislimits the torque that can be delivered to their output shaft, for a given maximum pressure. One way of increasing theirdisplacement is to increase theiraxiallength. However, such lengthening islimited if the controlled differential is to fit into the axle housing of a normal open differential.

One way of increasing the displacement of the hydraulic motors is to interpose hydraulic motors of greaterdisplacementintothedifferential gear train. Figure 10showshow two gerotor hydraulic motors 60 and 61 can belocated in the controlleddifferential where theirexternal gears (or stars) 62 and 63 are coaxial with the left-handpair of planetary gears 2 and 2'. Note that in Figure 10 only the pitch cylinders of the spur gears 2 and 2' are shown. The internal gears of the gerotor hydraulic motors 64 and 65 are free to rotate in cylindrical bores in the body of the differential 66. These internal gears64 and 65 float between the differential body 66 and the external gears (or stars) 62 and 63. In Figure 10 the external gears 62 and 63 and internal gears 64 and 65 have six and seven teeth or lobes respectively. This means that one rotation of the stars 62 and 63 is accompanied by a 6/7 rotation of the internal gears 64 and 65. The six pockets located between the meshing external and internal gears vary theirvolumes from almost zero to a maximum (at 180 degree rotation) and back to almost zero for each rotation of the external gears (orstars) 62 and 63. A disc valve (not shown) enables hydraulicfluid to enterand exit these pockets. The pressure difference between the expanding and shrinking pockets applies a torque to the external gears (orstars) 62 and 63.

Anotherway of increasingthe displacement of the hydraulic motors is to interpose lobe hydraulic motors with higher displacement between the second set of planetary gears and the third set of planetary gears. Lobe motors are effectively gear motors with few teeth (or lobes). As these do not function well as gearteeth, parallel geartrains are used to synchronise the rotation of the lobes. In principle, the lobes could be rotated without actually touching each other. Furthermore, as both lobes are connected to axial shafts, in principle they could be rotated without touchingthe cylindrical bores in the body of the controlled differential 68.

The operation of the controlled differential will now be described with reference to Figures 10and 11. The body of the differential is rotated by a crown wheel 67connected to the layerof the controlled differential 66containing the two bores that rotatably accommodate the two gerotor hydraulic motors. See 11(b). Two parallel geartrains enable the left-hand drive gear land the right hand drive gear 8 to rotate at different speeds. However, the average speed of the drive gears must be the speed of the crown wheel and thereby the speed of the body of the differential. Let us assume that the body of the differential rotates clockwise lookingfrom left to right. If the left-hand drive gear 1 rotates fasterthan the body of the differential 66 and 68, the left-hand planetary gears 2 and 2' will rotate counterclockwise relative to the body of the differential 66. Central planetary gears 4 and 4' are fixed to the same shafts 3 and 3' respectively. Gears 4and 4' mesh with planetary gears 5 and 5' which rotate clockwise. Gears Sand 5' are fixed to the same shafts 6 and 6' as gears 7 and 7' respectively which must also rotate clockwise. The right-hand pairof planetary gears 7 and 7' mesh with the right-hand drive gear8 which now must rotate slowerthan the speed of the body of the differential 66.

Note that if the external gear(orstar) is also connected to shaft 3, then the speed of shaft 3 will be proportional tothe flow of fluid through the gerotor hydraulic motor. This fluid is supplied by a reversible hydraulic pump 35 (not shown in Figures 10 and 11 but shown in Figures 6 and 7).

Note that shaft 6 can also be driven a bi-wing rotor68 of a hydrauliclobe motor. The meshing bi wing rotor 69 also drives central planetary gear8' and right-hand planetarygear7. Bi-wing rotors 68 and 69 are kept in phase (i.e. synchronised) bythe meshingof gears 5 and 6. Thus the speed of shafts 6 and 6' can be positively controlled by controlling the speed of the bi-wing rotor hydraulic motor 70. This in turn can be controlled by controlling the speed of a reversible hydraulic pump 35. Note that no torque needs to be transferred between themeshingbi-wing rotors.

Alternatively, shafts 2' and 6' could be positively driven by a pair of three lobe rotors 71 and 72. These two three lobe rotors could formpart of a hydraulic motor 73 which could be positively driven in either direction by a reversable hydraulic pump 35 shown in Figures 6 and 7. In this case the lobes are synchronised by meshing gears 4' and 5'which are fixed to shafts 3' and 6' respectively. Note that no torque needs to be transferred between the rotors 71and 72. In principle a small clearance could be maintained between the two meshing rotors, and between the rotors and the cylindrical bores in the body of the controlled differential 68.

Figures 11(a) to 11(e) show sections through the layers of the controlled differential that enclose rotating components otherthan shafts.

Figures 11(f) and 11(k) show sections through the left-hand and right-hand end plates respectively.

Figures 11(g) to 11(j) show sections through the plates that separate the coaxial layers of the controlled differential that contain the rotating components. These separating plates prevent axial leakage of fluid between the layers of the controlled differential. They also provide bearings forthe shafts that support the rotating components of the differential.

The end plate shown in Figure 11(f) supports the left-hand drive shaft land the shafts keyed to the left-hand planetary gears 2and 2'. The end plate shown in Figure 11(k) supports the right-hand drive shaft and the shafts keyed to the right-hand planetary gears 7 and 7'.

The separation plates shown in Figures 11(g) to11(j) and the end plates shown in Figures 11(f) and 11(K) are shown as single plates in the interests of clarity. However, each plate could be made up of a sandwich of two or more parallel plates. This would facilitate the provision of blind bores in the plates, which would prevent axial leakage offluid through the clearance gap between the bearings and the shafts they support. This avoids the need to machine blind bores into one -piece plates.

Note that inlet and outlet ducts must be incorporated into the separating plates. The provision of these ducts could be simplified if the separation plates were a sandwich of three or more parallel plates.

Although three types oflarger displacement hydraulic motors are shown in Figures 10 and 11, only one type of larger displacement hydraulic motorwould be used in practice. This would be the most cost effective. The controlled differential could be simplified if the left, centre and right sets of planetary gears were not converted into hydraulicgear motors. This would reduce the total displacement of all the hydraulic motors slightly, and therebythe size of the torque difference between the left hand and right-hand drive shafts.

Figure 12 shows a 4WD vehicle with steerable front wheels. The track t is the distance between the centres of the contact patches of the left and right hand tyres. In the vehicle shown, the front and rear tracks are the same. The wheelbase b is the distance between the centres of the contact patches of the front and rear tyres. The left and right wheel bases are the same. The instant centre IC of the vehicle is assumed tolie on the axis of the un -steered rearwheels. Rx is the distance of the IC to the right of the longitudinal axis of the vehicle. If Rx is positive the vehiclewill execute a right hand turn.

Ry is the distance of the IC ahead of the centre of the vehicle COV-which is deemed to be the origin of the X and Y coordinates. For the vehicle shown Ry=-b/2.

Wheel angles are measuredclockwisefromthe straight-ahead position. If all wheels are to have the single instantcentre shown, they are given by the following equations;

Tan IFR= b/(Rx - t/2)

Tan FL = b/(Rx+ t/2)

And Tan FL = Tan OFR=0

The distance of each wheel from the IC is given by the Pythagoras equations; + RFR2 2 (Rx - t/2) 2

+ RFL2 2 (Rx + t/2) 2

RRR2 = (Rx - t/2) 2

RRL2 = (Rx+ t/2) 2

Note that although Rx can be negative R, must generally be positive. Pythagoras' equation only gives positive values.

If the speed of each driven wheel is to produce the same IC as the wheel angles, then the velocity of each wheel relative to the ground must satisfy the following equations;

I VI= RMSWS*IR lI/RMSR WhereI V IandI R, I are the velocity and distance of the nth wheel from the single theoretical IC;

Where the root mean square radius RMSR = (RX 2 +b 2 /2+ t 2 /4)o

Therefore, the correct speed difference between the velocities of the front wheels to a chieve cooperative redundancy is given by the equation;

VFL - VFR = [RMSWS/RMSR]*[ (RX2 + b 2 /2 + t 2 /4 + Rxt)0 -5- (RX2 + b 2/2 + t 2/4 - Rxt) 0 -5] And the average velocity of the front wheels is;

05 (VFL+ VFR)/2= [RMSWS/2*RMSR]*[ (RX 2 +b 2/2 + t 2 /4+Rxt) + (RX2 +b 2 /2 + t 2/4 -Rxt) 0 -5]

Similarly, the correct speed difference between the velocity of the rearwheels to achieve cooperative redundancy is;

VRL - VRR = [RMSWS/RMSR]*[t] And the average velocity of the rearwheels is;

(VRL + VRR)/2= [RMSWS/2*RMSR]*[ 2Rx] Therefore, the correct velocity difference between the average speeds of the front and rear wheels is;

(VFL + VFR)/2- (VRL+ VRR)/2 = [RMSWS/2*RMSR]*[ (RX2 +b /2 + t /4 + Rxt) 2 2 05 + (RX2 +b 2/2 + t 2/4 - Rxt) 0 5 2Rx]

The driver determines the speed and path of the vehicle with the accelerator and steering wheel respectively. The driver effectively determinesthe location of the single IC. Ingeneral R will be a function of the angle of the steering wheel I clockwise from the straight -ahead position. A suitable function is Rx/t= cot [(/max)*7t/2] =cot[(')*7t/2]

Where emax is the maximum angle of the steering wheel from the straight-aheadposition.

The above equations can handle ICs outside the vehicle. These require front wheel angles in the -7/2 to +7u/2 range. As the correct angle of the inner front wheel can be more than three times the correct angle of the outer front wheel, each front wheel can best be turned with a dedicated actuator. However, if the front wheel angles are limited to+/ -7t/4radians, the correct wheel angles are approximately proportional to the steering wheel angle. In this case the front wheels can be turnedbya single four-barlinkage which can approximate pure Ackermann steering.

Initially the controlled differential operates as an open differential. Sensors are required to monitor the velocities of the fourwheels. If the velocity difference between the two wheels linked by the differentialis greaterthan the correct velocity difference (as given by the equations above) a dog clutch is engaged which converts the open differential to alimited slip differential. If the speed difference still exceeds the correct speed difference a wet multi-disc brake is engaged to reduce the speed difference to the correct value.

Figure 13 shows the moving parts of an open differential comprising two trains of spurgears. Only the pitch cylinders of the spurgears are shown. A left-hand sun gear 74 engages a planetary gear 75 the length of which is overtwice the length of the sun gear 74. The long planetary gear 75 engages a short planetary gear 76 which is coaxial with an identical planetary gear 77. This identical gear 77 engages with a right-hand sun gear 78. The shafts of all gears are rotatably supported by bearings in the body of the open differential -which is not shown. This arrangement ensures that if the left hand sun gear 74 rotates clockwise relative to the body of the differential, the right-hand sun gear 78 must rotate at the same speed in the opposite direction. This differential action ensures that the angularvelocity of the differential body is the average of the angularvelocities of the left and right sungears74and78 respectively -and thereby the left and right wheels.

Onlytwo parallel geartrains are shown in Figure 13 in the interests of clarity. However, three, four, five, six oreight parallel trains could be employed to share the torque transmitted through the differential.

The advantage of the arrangement shown in Figure 13 is that means of controlling the differential actioncan be located between the left and right sun gears 74 and 78.

Figure14showsa meansof magnifyingthe angularvelocity difference between the left-hand sun gear 74 and the body of the differential. The shaft 79 of the left-hand sun gear74 contains a coaxial external spline 80which can be ejected from the right-hand end of this shaft 79 by air or oil pressure (see Figure 16). The ejected spline 80 engages the cage 81 of an epicyclicgeartrain. The right-hand endofthiscage81 has two stub axles 82 which rotatably support two planetary gears 83. These planetary gears 83 engage an annulargear 84 which is fixed to the body of the differential. They also engage a sun gear 85 which isfixed tothe cage 86 of a second epicyclicgeartrain. If the planetary andsungears83and85 respectively have the same number of teeth, then the output sun gear 85 will rotate fourtimes as fast as the input cage 81. Asplitbush87 is used to rotatably support the first and second sun gears 85 and 88.

Figure 15 shows means of controllingthe magnifiedspeeddifference between the left-hand sun gear 74 and the body of the differential. The sun gear 88 of the second epicyclictrain is coaxially connected to a spline 89which rotates 16 times faster than the left-hand sun gear 74. This spline is coaxially locatedwithin a brake body that is fixed to the body of the differential. This brake body 90 contains an internal spline 91.Stationary brake discs 92 are free to slide axially within the brake body 90, but not rotate. Interposed between the stationary brake disks 92are disks 93 which are splined to the rotating shaft 89. Pressure can be applied to the right-most stationary disc by means of an annular piston 94. The speed difference between the stationary and rotating discs 92 and 93 respectively can be reduced by increasing the pressure applied to the annular piston 94.

Figure 16 shows across section of left-hand sun gear 74 and its integral left and right hand shafts 79 that passes through the longitudinal axis of the cylindrical shafts 79. The left-hand shaft 79 contains an internal spline 95 which would engage the external spline of the left-hand drive shaft (not shown). The right-hand shaft 79 contains an internal spline 96and a cylindrical bore 97 that extends towards the left-hand spline 95. A piston 98 is slidably located within the cylindrical bore 97. This piston 98 is integral with an external spline 98'that is slidably located within internal spline 96. The top part of the section shows the piston/spline 98/98' pushed to the left by a coil spring 100. Inthis condition the external spline 97is enclosed by the internal spline 96. The bottom part of the section shows the piston/spine 98/98' pushed to the right by oil pressure orair pressure. In this condition the external spline 97engages an internal spline 101located in the left-hand shaft of a rotatable cage 81 with two diametrically opposite stub axles 82 projecting the right. These stub axles 82 rotatably support planetary spurgears 83 which engage with an annulargear 84 which is stationary with respect to the body of the differential. The planetary gears 83 engage an axial sun gear 85 which is integral with the cage 86 of a second epicyclicgeartrain.

Figure 17 shows a section through the axis of a computer-controlled multi-discwet brake which is used to control the angularvelocity of the left-hand sun gear74 (and drive shaft) relative to the body of the differential. The axial shaft of this disc brake comprises four parts. The sun gear 102 of the preceding epicyclicgeartrain is located at left-hand end of the shaft 103. To the right of this is a cylindrical bearing 103'which is rotatably located within a split bearing bush 87. This bush 87 is split to enable it to be assembled on the shaft 103 and then located in the stationary separation plate 104 by insertion from the right. The next section of the shaft 89 is an external spline 89'which slidably engages the rotating discs 93 of the wet brake. The stationary discs 92 of the wet disc brake are interposed with the rotating discs 93. These stationary discs 92 slidably engage with an internal spline 91in the body of the wet brake 90. The right-hand end of the shaft 89 is a cylindrical bearing 105 which is rotatably supported in the stationary end plate 106 of the wet brake. This end plate 106 contains an annularcylinder 107 in which an annular piston 94 is slidably located.

If air or oil is injected into the space between the end plate and the piston, the latterwill be pushed to the left which will cause the stationary discs to clamp the rotating discs. The angularvelocity of the rotating discs can be controlled by computer-control of the oil pressure orair pressure.

Figure 18 shows an assembly drawing of the complete computer-controlled differential. It is a section through the rotational axis of the differential. Cylindrical elements that are coaxial are not sectioned. To simplify the drawing only the pitch cylinders of the gears are shown. Clamping bolts are not shown. Norare rotary seals that are used to transferair or oil to ducts in the body of the differential.

The body of the differential is a sandwich of eleven layers 108to 118. The second layerfrom the left 109 includes a flange 119 to which a crown wheel gear(not shown) can be bolted. The left-hand sun gear 74 engages a long planetary gear 75 which is mounted on a short shaft 120. Long planetary gear engages short planetary gear 76 which is mounted tolong shaft 121. Planetary gear 77 is also mounted tolong shaft 121 by means of a key and keyway. This will facilitate assembly of the differential. Planetary gear 77 engages right hand sun gear 78. This epicyclicgeartrain achieves the differential action where the average angularvelocity of the left and right sun gears 74 and 78 respectively is equal to the angularvelocity of the body of the differential.

The left-hand sun gear 74 can be connected to the cage 81of the first step-up epicyclicgeartrain by means of a spline 99that can be ejected from the right end of the left sun gear 74. Planetarygears 83 rotatably mounted on the stub axles 84 of the cage engage a stationary annulargear84. Planetary gears 83 also engage with sun gear 85 which is attached to the cage of the second step -up epicyclicyeartrain. Once again planetary gears 83 rotatably mounted on the stub axles 82 of the cage 86 engage a stationary annulargear84. Planetary gears 83 also engage with sun gear 88 which is attached to the spline 89of a multi-discwet brake. The wet brake comprises alternating stationary discs 92 and rotating discs 93 which can slide on internal and external splines 91and 89' respectively. A clamping force is provided by applying oil or air pressure to an annular piston 94.

To summarise, if the external spline 99engages the cage 81 of the first epicyclicgeartrain, and the clamping force applied to the multi-disc brake is controlled by the computerto ensure that the angularvelocity difference between the body of the differential and the left-hand drive shaft is correct, then we will have cooperative redundancy between the wheel speed steering effect and the wheel angle steeringeffect.

If the external spline 99does notengage the cage 81 of the first epicyclicgeartrain, the controlled differential will operate as an open differential.

If the external spline 99does engage the cage 81 of the first epicyclicgeartrain and the multi-disc brake is notactivated, then the controlled differential will operate as alimited slip differential. Slip could be further restricted byapplyinga constant clampingforce to the multi-disc brake.

If the external spline 99does engage the cage 81 of the first epicyclicgeartrain and the multi-disc brake is locked, then the controlled differential will operate as alocked differential.

Claims

The claims defining the invention are:

1. A four wheel drive, fourwheel steering vehicle where power is transmitted from the output of an engine-driven gearbox to a first differential, the two outputs of which constitute the input of a front differential and the input of a rear differential respectively, where the outputs of the front differential drive the left and right frontwheels and the outputs of the rear differential drive the left and right rearwheels, where the speed difference of the two outputs of the three differentials is positively controlled so that the wheel speed steering effect of each combination of two wheels is identical, and the angles of all wheels are also controlled so that the wheel angle steering effects of each combination of two wheels are also identical, wherethe singlewheel speed steering effect is also identical to the single wheel angle steering effect

2. A fourwheel drive, front wheel steeringvehicle where poweris transmitted from the output of an engine-driven gearbox to a first differential, the two outputs of which constitute the input of a front differential and the input of a rear differential respectively, where the outputs of the front differential drive the left and right frontwheels and the outputs of the rear differential drive the left and right rearwheels, wherethe speed difference of the two outputs of the three differentials is positively controlled so that the wheel speed steering effect of each combination of two wheels is identical, and the angles of the front wheels are also controlled so that the wheel angle steering effects of each combination of two wheels are also identical, where the single wheel speed steering effect is also identical to the single wheel angle steering effect

3. A vehicle according to claim 1where the speed difference between the front leftwheeland the front right wheel is given by; 2 VFL-VFR=(RMSWS/RMSR)[((b/2-Ry) +(Rx+t/2) 2) 0. 5-((b/2-Ry) 2 +(Rx-t/2) 2 ) 0 .5 ]andthespeed difference between the rearleft wheeland the rear right wheel is given by;

VRL-VRR=(RMSWS/RMSR)[((b/2+ (Rx+t/2) 2) 0.5- ((b/2 + Ry)2 + (Rx - t/2) 2 ) 0.5 ] and the speed y)2+

difference between the average frontwheel speed and the average rearwheel speed is given by; 205 (VFL + VFR)/2- (VRL -VRR)/2 = (RMSWS/2RMSR) [((b/2 - Ry) 2 + (Rx + t/2) 2 ) 0. 5+ ((b/2- Ry) 2 + (R. - t/2) ) . - ((b/2+ Ry) 2 + (Rx+ t/2) 2 ) 0. 5- ((b/2+ Ry) 2 + (Rx - t/2)2 )0.5 ] where;

RMSR = (R 2 + Ry2 +b 2/4+ t 2/4) 0 5.

b and t are the wheelbaseand track respectively

Rx is the distance of the instantcentre of the vehicle tothe rightof the centre of the vehicle

Ry isthe distance of the instantcentre of the vehicle forward of the centre of the vehicle and

RMSWS is the root mean square speed of the fourwheels -which is selected bythe driver.

4. A vehicle accordingto claim 2 where Ry = -b/2 so that the rear wheels are un-steered, and the speed difference between the front left wheel and the front right wheel is given by;

VFL-VFR=(RMSWS/RMSR)[(b 2 +(Rx+t/2) 2)o.s- (b2 +(Rx-t/2) 2 )0 5.

and the speed difference between the rearleft wheel and the rear right wheel is given by;

VRL -VRR = (RMSWS/RMSR)[t] and the speed difference between the average front wheel speed and the average rear wheel speed is given by;

(VFL+VFR)/2- (VRL+VRR/2= (RMSWS/2RMSR)[((b 2+(R,+t/2) 2) 0.5+((b 2 +(R.- t/2)2 )0.5 -2Rj]where;

RMSR = (R) 2 + b 2/2 + t2 /4) 0 5.

b and t are the wheelbase and track respectively

R, is the distance of the instant centre to the right of the centre of the vehicle and

RMSWS is the root mean square speed of the fourwheels.

5. A vehicle according to any one of claims 1to 4 where the differentials are transformed into hydraulic gear motors by eliminating any volume in the differe ntial housing that is no t occupied by the rotating spur gears, where these gear motors are caused to rotate by a flow of oil supplied by a hydraulic pump located in the body of the vehicle, where the discharge rate of each hydraulic pump is controlled d by an on-board computer so as to positively drive each differential so as to satisfy the above equations.

6. A controlled differential according to claims 1to 5 where the working fluid is transferred to or from the rotating controlled differential by means of a pair of stationary rotary seals.

7. A controlled differential according to claims 1to 6 where the two stationary rotary seals comprise circular rings located on the cylindrical housing of the rotating controlled differential, where the inner cylindrical surface of the stationary rings is coaxially grooved, so that the workingfluid can flow to or from holes in the cylindrical body to orfrom the hydraulic gear motors and to or from the reversible hydraulic pumplocated in the body of the vehicle by means of two hydrauliclines and two hydraulic hoses.

8. A controlled differential according to claims 1to 7 where pressure relief valves are provided to limit the hydraulic pressure generated, to a safe level.

9. A controlled differential according to claims 1to 8 where means are provided of replenishing any working fluid that leaks from the hydraulic system.

10. A controlled differential according to claims 1to 9 where means of cooling the working fluid is provided when required.

11. A controlled differential according to claims 1to 10 where a feedback loop in the speed control system of the reversible hydraulic pump is used to compensate for leakage in the hydraulic circuit of the controlled differential.

12. A ve hicle whe re controlle ddifferentials according to claims 1 to 11 are fitted to both the front and rear axles, where windup is prevented by installation of a third (normal) differential between the forward and rearward drive shafts to the front and rear axles.

13. A vehicle where controlled differentials according to claims 1to 11 are fitted to both the front and rear axles, where windup is prevented by installation of a third controlled differential between the forward and rearward drive shafts to the front and rear axles, where the speed of a third hydraulic pump is used to enforce the correct speed difference between the average wheel speed of the front axle and the average speed of the rear wheels, so that the six wheel -speed steering effects of all fourwheels have a single theoretical instant centre, which is identical to the theoretical instant centre of the four wheel angles.

14. A vehicle according to claims 1to 11 where each controlled differential can be converted to a normal differential byopeningavalve in a hydraulicline thatconnectsthe outletand inlet lines of the dedicated hydraulicpump.

15. A vehicle accordingto claims 1 to 5 where the shafts keyed tothe planetarygears of each controlled differential are alsodriven by large displacement hydraulicmotors also keyed to said shafts where said motors are driven by a dedicated computer controlled reversible hydraulic pump.

16. A vehicle according to claim 15 where the large displacement hydraulic motors are gerotor motors keyed to the shafts connecting the left-hand set of planetary gears to the central set of planetary gears, or keyed to the shafts connecting the central setof planetary gears to the right-hand set of planetarygears. Of each of the controlled differentials.

17. A vehicle according to claim 15 where thelarge displacement hydraulic motors are hydraulic motors with two bi-wing rotors keyed to the shafts connectingthe left-hand set of planetary gears to the central set of planetarygears, or keyed to the shafts connectingthecentral set of planetarygears to the right-hand set of planetarygears of each controlleddifferential.

18. A vehicle according to claim 15 where thelarge displacement hydraulic motors are hydraulic motors with a pair of three lobe rotors keyed to the shafts connectingthe left-hand set of planetarygears to the central set of planetarygears, or keyed to the shafts connectingthe central setof planetary gears to the right-handsetof planetarygears of each controlled differential.

19. A vehicle according to claims 6 to 14, where the shafts connecting the coaxial planetary gears are alsodriven by largedisplacementhydraulicmotors are interposed between the left-hand set of planetary gears and the central set of planetary gears, or between the centralsetof planetary gears and the right-hand set of planetary gears of each controlled differential, where these hydraulic motors are driven by computer controlled reversible hydraulic pumps.

20. A vehicle according to claim 19 where the large displacement hydraulic motors are gerotor hydraulicmotors, or motors with a pair of bi-wing rotors, or motors with a pair of threelobe rotors.

21. Avehicle according to anyone of claims 1 to 4 where a traditional spur gear differential is transformed into a computer-controlleddifferential by positively controlling the angular velocitydifference between the body of the differentialand one of the two outputshafts by first magnifyingthis velocity difference byrmeansof one ormore step -up epicyclic gear trains and then controllingthe magnified velocitydifferencewith a computer controlled internal brake so that the steering effect of the peripheral velocities of all the positively driven wheels are identical, and the steering effect of all the wheel angles are (as far as possible) also identical, and where the single driven-wheel-speed steering effect is the same as the single wheel-angle steering effect.

22. A vehicle according to anyone of claims 1to 4 where a traditional spurgear differential is transformed into a computer-controlled differential by positively controlling the angular velocity difference between the body of the differentialand one of the two output shafts by first magnifying this velocity difference by means of one or more step -up epicyclic gear trains and then controlling the magnified velocity difference with a computer controlled internal brake so that the theoretical instant centres of the peripheral velocities of all the positively driven wheels are identical, and the theoretical instant centres of all the wheel angles are (as far as possible) also identical, and where the single driven -wheel-speed instant centre is the same as the single wheel-angle instant centre.

23. A vehicle accordingto claims 21or 22 where one controlled differential output shaft is connected to the velocity difference magnifying step-up epicyclicgeartrains by means of a slidable external spline.

24. A vehicle according to anyone of claims 21 to 23 where the slidable spline is integral with an axial piston, where the piston and spline can be moved axially to connect the differential output shaft to the cage of the first epicyclicgeartrain by the application of air pressure or oil pressure to the face of the piston, where a compression spring retracts the spline in the absence of air or oil pressure.

25. Avehicle according to anyone of claims 21 to 24, where the sun gear of the laststep-up epicyclic gear train is connected to the external spline of amulti-disc brake, where the rotating even discs are slidably connected to the external spline and the stationary odd discs are slidably connected to an internal spline that is connected to the body of the controlled differential.

26. Avehicle according to anyone of claims 21 to 25 where the angularvelocitycdifference between the body of each controlled differential and the rotating external spline of the last epicyclic gearbox is controlled by controlling the clamping force applied to the sandwich of rotating and stationary discs by a piston which can be moved axially by the application of oil pressure orair pressure, where this pressure iscontrolled byanon-board computer.

27. Avehicle according to anyone of claims 21 to 26 where rotary seals are located on the body of each controlled differential toenable pressurised air or oil to be transferred from the stationaryhousingwhich encloses the rotating differential body and ducts within the rotating differential body that lead to the axial pistons.

28. A controlled differential according to anyone of claims 21 to 27 that is suitable for the front axle of a four-wheel drive where the difference in angular velocity between the left and right drive shafts VFL- VFR isgiven be the equation;

VFL- VFR = [RMSWS/RMSR]*[(RX2 +b 2/2 + t 2/4 +Rxt) 0 -5 - (RX2 +b 2/2 + t 2/4 -Rxt) 0-5] where; .0 5 RMSR = (RX 2 +b 2 /2+ t 2 /4) and the average angularvelocity of the front axles is given by;

29. A controlled differential according to anyone of claims 21 to 27 that is suitable forthe rear axle of a four-wheel drive where the difference in angularvelocity between the left and right drive shafts VRL- VRR isgiven be the equation;

VRL-VFR = [RMSWS/RMSR]*[t] where; RMSR = (RX 2 + b 2/2+ t2 /4) 0 5 . and the average angularvelocity of the rearaxles is given by;

(VRL+ VRR)/2= [RMSWS/2RMSR]*[2Rx)= RMSWS/RMSR]*[Rx)

30. A controlled differential according to anyone of claims 21 to 27 that is suitable to replace an open differential whose input shaft is connected to the output shaft of the gearbox and whose two output shafts are connected to the input shafts to the front and rear differentials, where the difference in angularvelocity between the front and rear drive shafts (VFL+ VFR)/2- (VRL+VRR)/2 is given be the equation; 05 (VFL+VFR)/2- (VRL+VRR)/2= [RMSWS/2RMSR]*[ (RX2 +b 2 /2 + t 2/4 +Rxt) + (RX 2 +b2/2 + t 2/4 -Rxt) 0 -5 -2Rx]

31. A controlled differential accordingto any one of claims 21 to 30 which can be converted to a locked differential by supplying sufficient clampingforce to the multi-disc brake to lock this brake.

32. A controlled differential accordingto any one of claims 21 to 30 which can be converted to a limited slip differential by applying a constant clampingforce to the multi-disc brake.

33. A controlled differential accordingto any one of claims 21 to 30 which can be converted to an open differential by disengagingthe external spline from the cage of the first epicyclic step-up geartrain.