GB2465777A

GB2465777A - Drive force distribution control

Info

Publication number: GB2465777A
Application number: GB0821751A
Authority: GB
Inventors: Matthijs Klomp
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2008-11-28
Filing date: 2008-11-28
Publication date: 2010-06-02
Anticipated expiration: 2028-11-28
Also published as: GB0821751D0; GB2465777B

Abstract

Controlling drive force distribution to the wheels of a four-wheel vehicle using a drive force distribution between first and second sides of the vehicle, decided via determination of a grip force margin for each side indicative of the amount by which horizontal forces may be increased before they exceed a skidding threshold S3, the drive force distribution q being changed S4 in a direction in which a fictitious moment, which would be caused by the grip force margins acting on the sides of the vehicle in a same transversal direction, is minimized.

Description

Drive Force Distribution controlling I4ethod DesCr1PtOfl The present invention relates to a method of controlling the distribUtion of a drive force to the wheels of a four-wheel driven vehicle.

When a motor vehicle is cornering, and/or when it is cceleratiflg or eceieratiflg, the fraction of the vehicle weight carried by a given wheel varies, and, accordingly, the friction force between the wheel and ground varies. This friction force defines a threshold which the norm of drive forces and centrifugal forces acting at a given wheel must not exceed. If they do, the wheel will slide. In that case, the remaining wheels have to compensate the drive and centrifugal forces, and if they fail to do so, control over the vehicle may be lost.

It is importafltr therefore, to distribute (accelerating or decelerating) drive forces among the wheels of a vehicle so that a safety margin between the horizontal forces acting at each wheel and the friction threshold of the wheel is as large as possible.

uS 5 742 917 discloses a method for controlling drive force distributiOfl to the wheels of a four-Wheel vehicle in which in a first step a drive force or moment distribution between front and rear axles of the vehicle is decided, and in a second step, a distribution between left and right wheels is decided. The calculations Ofl which these decisions are based are complicated and time-consuming, and it is not apparent which parameter is optimized by the described distribution method.

The object of the present invention is to provide a method for controlling drive force distribution which is effective to prevent individual wheels from skidding involving simple and fast calculations.

The object is achieved by a method in which in a first step of deciding a drive force distribution between first and second sides or pairs of wheels, of the vehicle, a grip force margin indicative of the amount by whIch horizontal forces may be increased before they exceed the above-mentioned friction threshold is determined for each side, and a drive force distribution between said first and second sides is changed in a direction in which a fictitious moment which would be caused by the said grip force margins acting on said first and second sides of the vehicle in a same transversal direction is minimized.

According to a first embodiment, the first step may be divided into sub-steps of calculating a drive force distribution which yields a minimum of said fictitious yaw moment about the vertical axis of the vehicle, and setting drive forces at said first and second sides according to the calculated distribution. In this way, in a single iteration of the method an optimization of the drive force distribution is achieved.

In an alternative embodiment, the first step is divided into the sub-steps of estimating the fictitious yaw moment is based on a current drive force distribution, estimating said fictitious moment based on a modified drive force distribution, and setting the drive forces at said first and second sides according to the modified distribution if said fictitious moment is less with the modified distribution than with the current distribution. According to this method, the modified distribution may not yet be optimal, but it is easily and quickly found by simple trial-and error calculations, which can be repeated at a high rate, which is important for realization in a practical application.

io The distribution can thus be adapted very quickly to changing conditions of motion of the vehicle.

Since the power train of a vehicle usually has at least one drive shaft driving both wheels of a same axle, it is preferred that the first and second sides should correspond to front and rear axles of the vehicle.

In a second step a drive force distribution between left and right wheels may be determined.

In that case, a grip force margin indicative of the amount by which horizontal forces may be increased before they exceed a slipping threshold!s determined for each wheel, and a drive force imbalance between left and right wheels is changed in a direction which a fictitious moment which would be caused by the wheel-related grip force margins acting on the wheels of the vehicle in the same transversal directions and by said imbalance is minimized.

To this effect, in a first embodiment the second step may comprise the sub-steps of calculatIng a drive force imbalance which yields a minimum of said fictitious moment, and of setting drive forces at said left and right wheels according to the calculated imbalance. In this way, the left-/right drive force distribution is optimized in a single calculation.

Alternatively, the second step may comprise the sub-steps of estimating said fictitious yaw moment based on a current drive force imbalance, estimating said fictitious yaw moment based on a modified drive force imbalance, and setting drive forces at said left and right wheels according to the modified imbalance if said fictitious yaw moment is less with the modified imbalance than with the current imbalance.

Preferably, said drive force imbalance should be applied only to the rear wheels of the vehicle, since any torque which would be caused by a drive force imbalance in the front wheels would be felt by a driver as an increase or decrease of counter-torque at the steering wheel.

In case of a wire-steered vehicle in which there is no torque feedback from the front wheels to the steering wheel, one might consider applying a drive force imbalance also to the front wheels.

A four-wheel vehicle according to the invention has a power train controller for controlling the distribution of drive forces to the wheels according to the above-described method.

The four-wheel vehicle may have an engine which is coupled to at least two of said wheels by at least one planet drive. More specifically, there may be a single engine or motor, the drive force of which is distributed between all wheels, preferably by multiple planet drives, or there may be one engine for driving a first wheel pair and a second engine or motor for driving the other.

Said planet drive can have two outputs bridged by at least one reduction gear for modifying the distribution of drive forces between the two outputs.

Alternatively, the vehicle may comprise an auxiliary motor connected to two shafts for applying a forward drive force to one of said shafts and a rearward drive force to the other.

Further features and advantages of the invention will become apparent from the subsequent description of embodiments thereof referring to the appended drawings.

Fig. 1 is a block diagram of a motor vehicle embodying the present invention; Fig. 2 is a diagram of a rear axle differential drive of the vehicle of Fig. 1; Fig. 3 is a diagram of a central differential drive of the vehicle of Fig. 1; Fig. 4 is a diagram of a second embodiment of a rear axle differential drive of the vehicle of Fig. 1; Fig. 5 is a diagram of a third embodiment of a rear axle differential drive; Fig. 6 is a flow chart of a first embodiment of a control method carried out by the power train controller of Fig. 1; and Fig. 7 is a flow chart of a second embodiment of the control method.

Fig. 1 is a schematic diagram of a motor vehicle having a four-wheel drive power train. In a conventional manner, a combustion engine 1 drives a gearbox 2 via a friction clutch 3 which may be operated by a pedal, not shown. An output shaft 4 of gearbox 2 drives a planet drive 5 which distributes the drive force of the engine 1 between a front wheel drive shaft 7 and a rear wheel drive shaft 9.

Front wheel drive shaft 7 drives front wheels 10 by means of a conventional differential drive 11. A second differential drive 12 for ransmittiflg drive force from rear wheel drive shaft 9 to rear wheels 13 is shown in detail in Fig. 2. it comprises meshing conical piniofls 14 on drive shaft 9, and 15 for driving in rotation a planet carrier 16 of a planet drive 8. E'lanet carrier 16 carrieS a plurality of conical planet gearwheels 17, each of which engages two mutually opposing sun gearwheels 18, 19. Each sun gearwheel 18, 19 is mounted on a shaft 20, 21, respectivelY, which drives one of the rear wheels 13 and carries two more gearwheels 22, 23 and 24, 25, respectiVelY. A secondary shaft extending in parallel tO shafts 20, 21 comprises a solid shaft 27 and a hollow shaft 31 extending coaxially around solid shaft 27. Solid shaft 27 carries a gearwheel 28 engaging gearwheel 22 of shaft 20, a gearwheel 29 engaging gearwheel 25 of shaft 21, and a friction clutch 30 for selectively transmitting torque between gearwheelS 28 and 29. Since gearwheel 25 is smaller than gearwheel 22, shafts 20, 21 are coupled in rotation when clutch 30 is closed, with the rotation speed of shaft 20 being less than that of shaft 21.

GearwheelS 22, 28, 29, 25 and clutch 30 thus form a first reduction gear between the output shafts 20, 21 of planet drive 8.

The hollow shaft 31 carries gearwheelS 32, meshing with gearwheel 23 and 33, meshing with gearwheel 24, and a friction clutch 34 for selectively coupling the gearwheels 32, 33. Gearwheels 23, 32, 33, 24 and clutch 34 thus form a second reduction gear. By closing clutch 34, shafts 20, 21 can be coupled in rotation, with the lo rotation speed of shaft 21 being less than that of shaft 20.

As long as both clutches 30, 34 are open, drive force from drive shaft 9 is distributed in equal proportions to the two rear wheels 13. By gradually closing one of the clutches to a greater or lesser extent, one wheel 13 can be provided more drive force than the other, i.e. there is a controlled drive force imbalance between right and left wheel 13.

The differential drive 5 between gearbox output shaft 4 and drive shafts 7, 9 is similar in design to differential drive 12 of Fig. 2, except for the fact that the planet drive need not be symmetric with respect to the two drive shafts 7, 9 but will in most cases be asymmetric so as to provide a larger proportion of the drive force to the front wheel drive shaft 7 than to rear wheel drive shaft 9. The asymmetrY may be created by using a conventional planet drive with one sun gearwheel 18, a hollow ring gearwheel 36 and planet gearwheelS 17 in between, as shown in Fig. 3.

Like the differential drive 12 of Fig. 2, the differential drive 5 of Fig. 3 is shown with two reduction gears comprising gearwheelS 22, 28, 29, 25 and clutch 30, and gearwheelS 23, 32, 33, 24 and clutch 34, respectivelY. Since in differential drive 5 there is no need for the drive force to be distributed equally between output shafts 7 and 9, it is possible to simplify differential drive 5 by cancelling one of the two reduction gears.

Fig. 4 illustrates an alternative embodiment of the rear differential drive 12. Conical pinionS 14, 15 are similar to those of Fig. 2, but the planet drive 8 having io the two facing sun gearwheels 18, 19 is replaced by a mirror-symmetric arrangement of two planet drives BL, BR, the planet carriers 16 of which are connected to pinion 15.

Ring gear 36 of planet drive 8L is coupled directly to a gearwheel 28 of a secondary shaft 27, whereas ring gear 36 of planet drive BR is coupled indirectly, via a reversing shaft 41, to gearwheel 29 of secondary shaft 27. A rotor of an auxiliary electric motor 42 is fixed to secondary shaft 27.

While no net torque is transmitted along secondary shaft 27, the two ring gearwheelS 14 are at rest, and torque from drive shaft 9 is distributed in equal proportions to each vehicle wheel 13. By operating the auxiliary electric motor 42, power train controller 35 can create an arbitrary drive force imbalance between left and right wheels 13.

Again, a setup similar to that of Fig. 4, comprising auxiliary electric motor 42, might also be used in the differential drive 5 in order to control the distribution of drive force between front and rear axles.

In a further alternative embodiment the combustion engine is replaced by two electric motors, one for 9-.

driving the front wheels 10 and the other for the rear wheels 13, which are controlled independentlY from one another by the power train controller 35. In this way, arbitrarY distributions of drive force between front and rear wheels can be achieved.

Fig. 5 schematicallY illustrates a rear axle ccordiflg to this embodiment. As in the embodiment of Fig. 4, there are two planet drives 8L, 8R on the rear axle, which have their sun gearwheels ie connected to the vehicle wheels 13. Planet carriers 16 of the two planet drives 8L, 8R are connected to a rotating shaft of electric motor 43, which is placed between the two planet drives. As in the embodiment of Fig. 4, ring gearwheelS iS 36 are coupled by secondary shaft 27, and an imbalance between the two wheels 13 is controlled by auxiliary electric motor 42.

A setup similar to that of Fig. 5 can also be used at the front axle of the vehicle, the only difference being in that the front axle has no auxiliary electric motor 42 assigned to it. Since the front wheels 10 are steered, a drive force imbalance applied to the front wheels 10 would be felt as a counter torque at the steering wheel, which might be jrritating for the driver.

In a power steering system one might contemPlate otrolliflg a steering force in correlation with an imbalance created by an auxiliary electric motor of the front axle, so that the torque variation caused by the imbalance is not felt by the driver. Moreover, in a steered by wire system in which there is no mechanical coupling between actors ontrolliflg the road angle of the front wheels 10 and the steering wheel, an electric motor 42 can be associated to the front axle, since its effects 33 are not felt at the steering wheel.

-10 -Fig. 6 illustrates a first embodiment of a control method carried out by the power train controller 35. In a first step Sl, the friction coefficient jt of the ground on which the vehicle is moving is estimated by any known method. In the simplest case, jt may be assumed to be constant. Preferably, it is calculated from measured centrifugal forces acting on the vehicle and expected centrifugal forces calculated based on vehicle speed and road angle.

In a second step S2, power train controller 35 determines a desired total drive force The total drive force may be positive or negative, so the power train controller 35 takes account of the positions of accelerator and brake pedals for calculating FXIN.

Step S3 determines the proportions of the total desired drive force generated at the front and rear axles, respectively. In other words, if the total drive force FXIN is the sum of the front axle drive force FXF and rear drive force FXR, the object of step S3 is to find the ideal distribution factor q _ji 4XF X, which will yield the best safety margin against skidding.

The maximum horizontal force which may be applied to the front wheels before these wheels skid is where (2) m being the vehicle mass, 1 the distance between front and rear axles, g the gravity acceleration, lR the distance between the centre of mass of the vehicle and its rear axle, and h the height above ground of the centre of mass.

-11 -Part of this maximum horizontal force is "used up" by the front drive force so that the front axle has a force safety margin of \I(PFFZFY-FXF If this force safety margin was applied as a centrifugal force, perpendicular to the direction of motion, to the front wheels of the vehicle, it would generate a moment about the vertical axis of the vehicle of MF=lFJ(uFFzj2FF (3) In order to maximize the skidding safety, yaw moments acting at front and rear axles of the vehicle should compensate each other, i.e. is MF-MR=lF[(aFFzFY--FF lR$jCuRFzj2xR (4) should be as close to zero as possible. In order to simplify the calculations, step S3 determines the value of q which yields a minimum of r, r -M lF [(IIFFZF)2 -F, ] -1R [(PRFZR)2 -F] * (5) This can be done numerically or by solving analytically the expression dM-M/dq-_O (6) for q and incorporating the solution as program code in the operating program of controller 35.

Step S3 directly yields the ideal value of q, and accordingly controller 35 sets the clutches of differential drive 5 (or the power ratio of front and rear electric motors 43) according to the determined value of q in step S4.

The drive forces FXF and FXR for the front and rear axles having thus been found, it is still desirable to 12 -find an optimum distribution of these drive forces between left and right wheels, since if the vehicle is going through a curve, identical drive forces at left and right wheels might favour over-or under steer. Based on a similar reasoning as in step S3, a margin force is calculated for each wheel. Assuming that the front axle drive force FXF is divided equally between left and right front wheels 10, this margin force is wherein i=1 corresponds to left front wheel 10, i2 corresponds to the right front wheel 10, and mlR( ,2h) mh F2, =-g+(-l) a2}-a (8) U) is the weight carried by each front wheel i=l, 2. Herein, bE is the track width of the front axle and ax, ay are longitudinal and transversal accelerations.

For the rear wheels, the margin force is (9) wherein LFXR denotes the difference between the drive forceS Fx� of the left and right rear wheels 13 caused by the clutches of differential drive 12 or by the auxiliary electric motor 42, j=3 corresponds to the left rear wheel 13, j=4 corresponds to the right rear wheel 13, and the weight carried by the rear wheels is given by mIF( ,2h \ mh F2 g+(-l) -a I+ax (10) Un I wherein i is the distance between the front axle and the centre of mass and b is the track width of the rear axle.

Again, the total yaw moment which would be caused by the force margins is minimized, i.e. step S5 -13 -determines the value of the drive force imbalance LIFXR of the rear wheels which fulfils lF(piFz)2 -F /4_lRI(pj)2 [FXR+ (_1)x] (11) this value usually being found by numerical approximation.

In step S6, controller 35 selects one of clutches 30, 34 according to the sign of LFXR and its degree of closure according to the amount thereof, or it sets rotation direction and power of the auxiliary electric motor 42 according to the value of �XFXR.

Again, the drive force balance is directly set to its ideal level in step S6, so that the method need only be repeated when the conditions of motion of the vehicle have changed.

A different approach is used in the method illustrated referring to the flow chart of Fig. 7. In steps Si, S2, the friction coefficient and the total drive force are determined in the same way as in the method of Fig. 6. In step S3', the controller retrieves from memory a value of the distribution ratio q which had been set in a previous iteration and modifies it by a predetermined amount E in step S4' . Step S5' calculates front and rear drive forces FXF, FXR resulting from the distribution fact q' the corresponding margin forces, the momenta which would result from the margin forces being applied at front and rear axles and calculates for these a quantity r' representative of a squared total torque according to r'= lF2[(upFz)2 -Fl 1R[(/JRFZ)2 -FxR1 (12) -14 -In step S6 t' is compared to a corresponding value t obtained in a previous iteration of the method. If t' is smaller than t, q' is closer to optimum than q, the previouSlY stored value q is overwritten by q' in step S7', it=I t-r' is stored, r is overwritten by r', and the new value of q is set in step S8'.

On the other hand, if t' is larger than t, the modification ifl step S4' apparently went into the wrong direCtion, and the sign of is inverted in step S9' In optional step sio', the amount of c may be updated according to the value of t, i.e. s may be reduced by a predetermined factor if it is below a predetermined threshold, or it may be increased if the difference is above a threshold. In a further optional step 511', is again compared to a threshold, and if it is above, steps Si to slO' are repeated, i.e. a second group of method steps beginning with step s12' will only be carried out if a satisfying value of q has been found and set.

In step S12', controller 35 retrieves a previouslY stored value AFXR of the drive force imbalance between left and right rear wheels 13. This imbalance is modified by e in step 513' . The same expression as in step S5 is evaluated once in step S14', and the value i' obtained therein, (12) is compared to the corresponding value obtained in a previous iteration in step S15' . Similarly to steps S6' to S9' described above, the previoUSlY stored value of is overwritten in step s16' if a' is smaller than a, -15 -and is set in differential drive 12 in step S17', whereas otherwise the sign of F is reversed in step S18'.

In analogy to step SlO', the amount of EF may be adjusted to the discrepancy 1c7=Icr-cr'I between current and previous values a, a' in optional step S19'.

If Sli' is carried out, the method may return from step S19' to S12' for a limited number of times. In this way, a good optimization of the drive force imbalance LFXR can be reached before the method again proceeds to update the distribution factor q.

It will be apparent to the skilled person that the general concepts of the drive force distribution control methods described referring to Figs. 6 and 7 are not limited to the mechanical structures shown in Fig. 1 to 5. In particular, it is conceivable to control the drive force distribution as described above in a vehicle in which each wheel has one, preferably electric, motor associated to it, or in a vehicle in which a main motor, which may be electric or combustion-driven, and which provides the major portion of the drive force, is connected to the front wheels, whereas each rear wheel has an electric motor associated to it for applying mainly or only the drive force imbalance tFxR. Further, in a mainly front-wheel driven vehicle the rear axle might be similar to the setup of Fig. 5, except for the fact that the main electric motor 43 is not provided. In that case, the rear wheels 13 would not contribute to a total forward drive force, but would apply the drive force imbalance FxR created by auxiliary motor 42.

List of reference signs 1. cornbustJ-0fl engine 2. gearboX 3. clutch 4. output shaft 5. differential drive 6.

7. front wheel drive shaft 8. planet drive 9. rear wheel drive shaft io. front wheel 11. differential drive 12. differential drive 13. rear wheels 14. pinion 15. pinion 16. planet carrier 17. planet gearwheels 18. SUfl gearwheel 19. sun gearwheel 20. shaft 21. shaft 22. gearwheel 23. gearwheel 24. gearwheel 25. gearwheel 26.

27. secondary shaft 28. gearwheel 29. gearwheel 30. friction clutch 31. hollOw shaft 32. gearwheel 33. gearwheel 34. friction clutch 35. power train controller 36. ring gearwheel 37. planet drive 38. sun gearwheel 39. planet gearwheel 40. ring gearwheel 41. reversing shaft 42. electric motor 43. electric motor

Claims

cia imS 1. A method of ntrolliflg drive force distributiofl to the wheels of a four-Wheel vehicle, in which in a first step (Sl-S3 sl-S6fl a drive force distribution between first and second sides of the vehicle IS decided, characterized in that in the first step, a grip force margin indicative of the amount by which horizontal forces may be increased before they exceed a skidding threshold is determined for each side (S3; S5'), and the drive force distribution (q) is changed (S4; SB') Ifl a direction in which a fictitiOUS moment which would be caused by the said grip force margins acting on said first and second sides of the vehicle in a same transversal direction is minimized.
2. The method of claim i, wherein said first step comprises the sub-steps of 1culatiflg a drive force distribution which yields a minimum of said fictitiOUS moment (S3), and setting drive forces at said first and second sides (S4) according to the calculated distribution.
3. The method of claim 1, wherein said first step comprises the sub-steps of estimating said fictitiOUS moment based Ofl a current drive force distribution (S5'), stjmatiflg said fictitiOuS moment based on a modified drive force distribution (S5'), and setting the drive forces at said first and second sides according to the modified distribution (SB') if said fictitioUS moment is less with the modified distribution than with the current distribution.
4. The method of any of the preceding claims, wherein said first and second sides correspond to front and rear axles of the vehicle.
5. The method of claim 4, wherein, in a second step (S5; S14'-S15'), a drive force distribution between left and right wheels is determined.
6. The method of claim 5, wherein in said second step a grip force margin indicative of the amount by which horizontal forces may be increased before they exceed a slipping threshold is determined for each wheel, and a drive force imbalance (/FxR) between left and right wheels (13) is changed (S6; S17') in a direction in which a fictitious moment () which would be caused by wheel-related grip force margins acting on the wheels of the vehicle in a same transversal direction and by said imbalance (AFXR) is minimized.
7. The method of claim 6, wherein said second step comprises the sub-steps of calculating a drive force imbalance which yields a minimum of said fictitious moment (S5), and of setting drive forces at said left and right wheels (S6) according to the calculated imbalance.
8. The method of claim 6, wherein said second step comprises the sub-steps of estimating said fictitious moment (a) based on a current drive force imbalance (FxR) (S14'), estimating said fictitious moment (a') based on a modified drive force imbalance (AFxR) (S14'), and setting drive forces at said left and right wheels (S17') according to the modified imbalance if said fictitious moment (a, a')is less with the modified imbalance than with the current imbalance.
9. The method of any of claims 6 to 8, wherein said drive force imbalance (FxR) is applied only to the rear wheels of the vehicle.
10. A data processor program product comprising program code means for enabling a data processor, when carried out on it, to execute the method of one of claims 1 to 9.
11. A data carrier having recorded on it, in computer executable form, the computer program product of claim 10.
12. A four-wheel vehicle comprising a power train controller (35) for controlling the distribution of drive forces to the wheels (10, 13) according to the method of one of claims 1 to 9.
13. The four-wheel vehicle of claim 12, wherein an engine (1; 43) is coupled to at least two of said wheels (10, 13) by at least one planet drive (8; 8L, 8R).
14. The four-wheel vehicle of claim 12 or 13, comprising a first engine for driving front wheels (10) and a second engine (43) for driving rear wheels (13).
15. The four-wheel vehicle of any of claims 12 to 14, further comprising an auxiliary engine (42) connected to two shafts (20, 21; 7, 9) for applying a forward drive force to one of said shafts and a rearward drive force to the other.