GB2388348A

GB2388348A - Vehicle differential control

Info

Publication number: GB2388348A
Application number: GB0308514A
Authority: GB
Inventors: David Andrew Clare; Keith Gary Reginald Parsons
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2002-05-02
Filing date: 2003-04-14
Publication date: 2003-11-12
Anticipated expiration: 2023-04-14
Also published as: GB0308514D0; GB2388348B; GB0210084D0

Abstract

A control system for a vehicle controls the distribution of drive torque to its road wheels. The system increases a degree of locking of a differential in response to a detected increase in road roughness. Thus a difference in drive torque supplied to different wheels is reduced. The system is typically applied to a four wheel drive vehicle comprising centre and rear differentials. A controller 28 receives signals from sensors, such as ride height sensors 42,44,46,48 and wheel speed sensors 34,35,38,40. The controller 28 determines from the ride height signals and wheel speed signals the degree of roughness of the surface over which the vehicle is travelling. The degree of locking is increased with increased road roughness to avoid loss of traction.

Description

I VEHICLE DIFFERENTIAL CONTROL

The present invention relates to the control of differentials in vehicle drive trains, and in particular to the control of the degree of locking of such 5 differentials.

It has long been known to provide lockable differentials which can be locked or unlocked so as essentially to prevent or allow different rates of rotation of the wheels of a vehicle. It is also known to provide limited slip 10 differentials (LSDs) in which the two sides of the differential output, which are drivingly connected to different wheels on the vehicle, are connected together, for example by means of some form of clutch device, so that the two sides of the differential can move relative to each other, but only when the torque transmitted between the two sides by the differential exceeds a 15 variable limit set by the clutch.

US 5,471,390 and US 5,685,386 disclose control systems for LSDs which vary the locking torque of the differential in response to changes in various parameters of the vehicle's condition or operation.

The present invention provides a control system for controlling a drive torque distribution means arranged to distribute drive torque between a plurality of wheels of a vehicle, wherein the drive torque distribution means is arranged to provide a variable degree of locking between said 25 wheels, the system comprising measuring means for measuring the degree of roughness of a surface over which the vehicle is travelling and control means arranged to increase said degree of locking in response to an increase in said degree of roughness.

30 The measuring means may be arranged to measure vertical movement of at least one wheel of the vehicle and to determine said roughness at least partly from said vertical movement.

( Alternatively, or in addition, the measuring means may be arranged to measure the speed of rotation of at least one wheel of the vehicle, and to determine said roughness at least partly from said speed of rotation Alternatively, or in addition, the measuring means may be arranged to measure acceleration of at least one point on the vehicle and to determine said roughness at least partly from said acceleration. The acceleration can be lateral acceleration, or a rotational acceleration, for example about a 10 longitudinal axis of the vehicle.

Preferably the degree of locking is variable between a maximum value, at which the drive torque distribution means may be substantially fully locked and a minimum value, at which the drive torque distribution means may be 15 substantially fully unlocked. However, the degree of locking may be variable in one or more discrete steps.

The drive torque distribution means may be a front or rear differential arranged to distribute drive torque between wheels on opposite sides of the 20 vehicle. Alternatively the drive torque distribution means may be centre differential arranged to distribute drive torque between the front wheels and the rear wheels of the vehicle.

Alternatively the drive torque distribution means may be arranged to direct 25 drive torque primarily to a first group of said wheels, but to provide a redistribution of the drive torque to a second group of said wheels under predetermined conditions, for example if the first group of wheels suffers a reduction in traction.

30 In this case the control means is preferably arranged to vary said degree of locking by varying the control of said redistribution.

( Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: 5 Figure 1 is a diagrammatic representation of a vehicle including a differential control system according to a first embodiment of the invention; Figures 2 to 4 are graphs showing operation of the system of Figure 1; and Figure 5 is a diagrammatic representation of a vehicle including a drive torque distribution system according to a second embodiment of the invention. Referring to Figure 1, a four wheel drive vehicle 10 has four wheels 11, 12, 15 13, 14 and a power train 16 for providing driving torque to the wheels. The power train 16 comprises an engine 18, an automatic transmission 20 which transmits drive torque, at any of a number of transmission ratios, to the input side of a centre differential 22. Front and rear differentials 24, 26 receive torque from the centre differential 22 and transmit it to the front 20 wheels 11, 12 and rear wheels 13, 14 respectively. The centre and rear differentials 22, 26 are limited slip differentials in which the degree of locking can be controlled by controlling a clutch pack acting between the two output sides of the differential. This controls the torque difference between the two outputs at which the clutch, and hence the two sides of the 2 output, will start to slip. For torque differences below the locking torque slip will not occur and the differential will behave as if fully locked. If the torque difference exceeds the locking torque the clutch will slip. This torque difference is therefore also the maximum torque that can be transmitted from one output side to the other through the differential. A 30 differential controller 28 controls the locking torque of each of the centre and rear differentials 22, 26.

( The vehicle further comprises a steering wheel 30 for steering the front wheels 11, 12. A steering angle sensor 32 provides a steering angle signal which varies with the steering input from the driver. Four wheel speed 5 sensors 34, 36, 38, 40 each provide a wheel speed signal which varies with the wheel speed of a respective one of the vehicle's wheels 11, 12,13, 14.

Four ride height sensors 42, 44, 46, 48 each provide a ride height signal which varies with the ride height of a respective one of the vehicle's wheels 1 1, 12, 13, 14. These ride height sensors therefore provide a 10 measure of vertical movement of the wheels 11, 12, 13, 14 on the vehicle's suspension 50 relative to the vehicle's body 52.

A yaw sensor 51 provides a yaw signal indicative of the instantaneous yaw rate of the vehicle lO, and a pair of lateral accelerometers 53, 54 provide IS lateral acceleration signals indicative of the instantaneous lateral acceleration at two points on the vehicle which are vertically spaced from each other.

It will be appreciated that, of the vehicle is travelling over rough terrain 20 such that the wheels are moving vertically at frequencies of the order of wheel hop frequency, which can be of the order of 2 to iOHz, then this causes wheel hop and the contact force between the wheels and the ground is reduced significantly for short periods. As this occurs independently for each of the wheels, large differences in traction occur between the wheels 25 for short periods of time, which can result in wheel spin and a reduction in driving torque. Wheel movements at other frequencies resulting from surface roughness can also reduce traction. The controller 28 is therefore arranged to detect the degree of roughness of the surface over which the vehicle is travelling, and to increase the differential locking torque on 30 rough surface as is described in more detail below.

( Referring to Figure 2, a first method of measuring road roughness comprises monitoring the ride height signals from the ride height sensors 42, 44, 46, 48. In Figure 2 variations in the ride height signal H from one of the ride height sensors 42 as the vehicle travels over a surface of varying 5 roughness are shown. The raw ride height signal is analysed by the controller 28 to determine the amplitude of oscillations within the frequency range of interest, in this case 2 to 10 Hz. The amplitude of oscillations in that frequency range is used as a measure of road roughness, and the rate of change of the amplitude of the oscillations is also used as a 10 measure of road roughness.

Referring to Figure 3, a second method of measuring road roughness comprises monitoring the signals from the wheel speed sensors 34, 36, 38, 40. On a smooth surface, if the vehicle is travelling at a constant speed in a 15 straight line, all of the wheels speeds will be equal and constant. However, if the vehicle is travelling over a rough surface each wheel will move vertically, as well as rotating, to accommodate the unevenness of the surface. This results in the wheels travelling different distances and rotating at different speeds, the speeds varying at the frequency of the 20 vertical movements. Figure 3 shows the variation of the wheel speed signal from one of the wheel speed sensors 34 over time as the vehicle travels over surface of varying roughness. As with the ride height signals, the wheel speed signals are analysed to determine the level of road roughness, either by analysing variations in wheel speeds within the frequency range 2 25 to 10 Hz, or by analysing the absolute levels of acceleration and deceleration of the wheels In this embodiment both of the methods described above are used to measure road roughness and the results combined to provide a more 30 accurate measurement. Further methods of measuring the road roughness could also be included, for example using the lateral accelerometers 52, 54, as described in WO 00/ 18595, or measuring roll movements or roll

( accelerations of the vehicle about its longitudinal axis using a rotary accelerometer. Referring to Figure 4, the controller 28 monitors the instantaneous 5 measured road roughness and controls the locking torque of the rear differential 26 in response. As shown in Figure 4, the locking torque T is continuously variable between a minimum level Tmjn where the differential is almost completely unlocked and the rear wheels 13, 14 can rotate at different speeds substantially independent of each other and a maximum 10 level Tmax where the differential is effectively locked and the rear wheels 13,14 can only rotate at substantially equal speeds. Assuming that all other inputs to the controller 28 are constant, as the measure R of road roughness increases from zero, the locking torque increases in a smooth curve from Tmjn to TmaX. Also over that range, the rate at which T increases with road 15 roughness increases as road roughness increases. The result of this is that, on relatively smooth surfaces, the differential 26 can remain unlocked, assuming that it is not being locked for some other reason, but as road roughness increases and the likelihood of loss of traction due to wheel hop increases, the degree of locking of the differential 26 is increased, 20 gradually for low levels of roughness, but more rapidly as roughness Increases. In this embodiment the centre differential is controlled in the same way as the rear differential 26. However, it will be appreciated that the two LSDs 25 might be controlled in different ways under at least some circumstances.

Also, if the front differential 24 were a controllable LSD, it could also be controlled in the same way as the other two. Furthermore, in a two wheel drive vehicle with only one differential, that differential could be controlled as described above.

Referring to Figure 5, in a second embodiment of the invention, the centre differential 22 of the first embodiment is replaced by a drive torque

distribution unit 122 which distributes drive torque between the front and rear differentials 124, 126. The drive torque distribution unit 122 is arranged to direct all drive torque to the front wheels 1 11, 1 12 during normal driving when they have sufficient traction and do not slip. However 5 if the front wheels 1 1 1, 1 12 suffer a partial or complete loss of traction this is detected by the unit 122 by measuring the speeds of its outputs 122a, 122b to the front and rear wheels. A difference in the speeds of the outputs, in particular if the output to the front wheels has a higher speed than that to the rear wheels, this indicates reduction in traction to the front wheels 10 which is affecting the transmission of driving torque to the wheels. The unit 122 therefore starts to transmit a proportion of the driving torque to the rear wheels sufficient to reduce the difference in speeds of the outputs to a predetermined low level. The control unit 128 controls the unit 122 so as to vary the allowable difference in speeds between the front and rear wheels, 15 and hence the degree to which the unit 122 locks the front and rear wheels, and in particular their rotational speeds, together.

The redistribution of drive torque to the rear wheels may also take place under other conditions, for example if the vehicle is driving away with a 20 high engine speed.

By controlling the degree of locking of the front and rear wheels in this manner, the system of Figure operates in the same manner as that of Figure 1.

The invention is also applicable to systems in which the degree of locking of the differential is not continuously variable, but is variable in one or more discrete steps.

Claims

( CLAIMS

1. A control system for controlling a drive torque distribution means arranged to distribute drive torque between a plurality of wheels of a vehicle, wherein the drive torque distribution means is arranged to provide 5 a variable degree of locking between said wheels, the system comprising measuring means for measuring the degree of roughness of a surface over which the vehicle is travelling and control means arranged to increase said degree of locking in response to an increase in said degree of roughness.

10

2. A system according to claim 1 wherein the measuring means is arranged to monitor movement of a part of the vehicle and to determine said road roughness at least partly from said movement.

3 A system according to claim 2 wherein the measuring means 15 includes filtering means arranged to filter out movements or changes in movement of said part which are within a predetermined range of frequencies, and to measure the roughness by measuring said movements or changes in movement.

20

4. A system according to claim 3 wherein said range of frequencies includes frequencies within the range of from 2 to 10 Hz.

5. A system according to claim 4 wherein said range of frequencies is of the order of 2 to 10 Hz.

6. A system according to any foregoing claim wherein the measuring means is arranged to measure vertical movement of at least one wheel of the vehicle and to determine said roughness at least partly from said vertical movement.

7. A system according to any foregoing claim wherein the measuring means is arranged to measure the speed of rotation of at least one wheel of

( the vehicle, and to determine said roughness at least partly from changes in said speed of rotation.

8. A system according to any foregoing claim wherein the measuring means is arranged to measure acceleration of at least one point on the vehicle and to determine said roughness at least partly from said acceleration.

9. A system according to claim 8 wherein said acceleration is a lateral I O acceleration.

10. A system according to claim 8 wherein said acceleration is a rotational acceleration.

15

11. A system according to claim 10 wherein said rotational acceleration is about a longitudinal axis of the vehicle.

12. A system according to any foregoing claim wherein the degree of locking is variable between a maximum value and a minimum value.

13. A system according to claim 12 wherein at the maximum value the drive torque distribution means is substantially fully locked.

14. A system according to claim 12 or claim 13 wherein at the minimum 25 value the drive torque distribution means is substantially fully unlocked.

15. A system according to any of claims 12 to 14 wherein the degree of locking is substantially continuously variable between the maximum value and the minimum value.

16. A system according to any of claims 12 to IS wherein the control means is arranged to control the degree of locking between the minimum value and the maximum value in response to changes in said roughness.

5

17. A system according to any foregoing claim wherein the drive torque distribution means is arranged to distribute drive torque between wheels on opposite sides of the vehicle.

18. A system according to any of claims I to 16 wherein the drive torque 10 distribution mean is arranged to distribute drive torque between the front wheels and the rear wheels of the vehicle.

19. A system according to any foregoing claim wherein the drive torque distribution means is a lockable differential.

20. A system according to any foregoing claim wherein the drive torque distribution means is a limited slip differential and the control means is arranged to vary said degree of locking by varying a locking torque of the differential.

21. A system according to any of claims 1 to 19 wherein the drive torque distribution means is arranged to direct drive torque primarily to a first group of said wheels, but to provide a redistribution of the drive torque to a second group of said wheels under predetermined conditions.

22. A system according to claim 21 wherein the drive torque distribution means is arranged to redistribute the drive torque to the second group of wheels if the first group of wheels suffers a reduction in traction.

30

23. A system according to claim 21 or claim 22 wherein the control means is arranged to vary said degree of locking by varying the control of said redistribution.

(

24. A system according to any foregoing claim wherein the degree of locking comprises a degree to which the rotational speeds of the wheel are locked together.

25. A control system substantially as hereinbefore described with reference to Figures 1, 2 and 4, or Figures l, 3 and 4, or Figures 2, 4 and 5 or Figures 3, 4 and 5 of the accompanying drawings.

lo

26. A vehicle comprising wheels, a drive train arranged to providedrive to the wheels and including drive torque distribution means, and a control system according to any foregoing claim arranged to control the drive torque distribution means.