GB2414525A

GB2414525A - Vehicle differential control

Info

Publication number: GB2414525A
Application number: GB0412008A
Authority: GB
Inventors: Jan Pieter Prins
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2004-05-28
Filing date: 2004-05-28
Publication date: 2005-11-30
Anticipated expiration: 2024-05-28
Also published as: GB0412008D0; GB2414525B

Abstract

A vehicle comprises a differential controller 28 arranged to control the degree of locking of its centre 22, front 24 and rear 26 differentials, and a brake control system 60. If the brake control system 60 reduces braking to a slipping wheel and the wheel continues to slip, it sends a signal to the differential controller 28 which locks the differential to increase the speed of rotation of the slipping wheel.

Description

24 1 4525

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 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 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 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.

Anti-lock braking systems are also well known in which the speed of rotation of the wheels is monitored during braking and, if one of the wheels starts to slip, i.e. to rotate at a speed lower than it should be for the speed of the vehicle, the brake of the slipping wheels is released. Normally this allows the speed of the slipping wheel to increase which helps to maintain lateral grip and therefore stability. However, if a vehicle is travelling over a very uneven or low friction surface, the speed of the slipping wheel does not always increase sufficiently quickly when its brake is released, and therefore it may remain in slip. This can reduce the lateral grip of the wheel and therefore the driver's control over the vehicle.

Accordingly the present invention provides a brake control system for controlling the braking of a plurality of wheels of a vehicle, the system being arranged to apply a braking torque to the wheels, but to produce a reduction in S the braking torque to any one of the wheels that is detected to be slipping and, to send signals to a differential controller arranged to control a differential that transmits drive torque to each of said wheels, wherein the brake control system is further arranged to send a signal to the differential controller to increase the degree of locking of the differential thereby to increase the speed of rotation of the slipping wheel if, despite said reduction, the slipping wheel continues to slip.

Increasing the degree of locking of the slipping wheel effectively links it, at least partially, to the other wheel or wheels which are still rotating.

Preferably the system is arranged to determine the speed of rotation of another wheel, to which the slipping wheel can be connected through the differential, and to control the degree of locking in dependence on the speed of the slipping wheel and the speed of the other wheel. For example, the system may be arranged to increase the degree of locking only if the speed of the other wheel is greater than the speed of the slipping wheel.

Preferably the system is arranged to monitor the speeds of the slipping wheel and the other wheel to detect whether increasing the degree of locking increases slipping of the other wheel.

If increasing the locking of the differential has the effect of increasing the slipping of other wheels, rather than decreasing the slipping of the initially slipping wheel, then it is undesirable. However, if all of the vehicle's wheels are slipping, this cannot be detected by wheel speed sensors.

Therefore the system preferably further comprises vehicle speed sensing means arranged to determine the speed of the vehicle independently of the speed of rotation of the wheels such that the degree of slipping of each of the wheels can be determined independently of the speed of the other S wheels.

The differential may be a front or rear differential arranged to distribute torque between wheels on opposite sides of the vehicle. In this case a slipping wheel on one side of the vehicle can be partially locked to a non slipping wheel on the other side of the vehicle. Alternatively the differential may be a centre differential arranged to distribute torque between the front wheels of the vehicle and the rear wheels of the vehicle.

[n this case if both the front wheels are slipping they can be partially locked to the rear wheels, or vice versa.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure I is a diagrammatic representation of a vehicle including a control system according to an embodiment of the invention.

Referring to Figure 1, a four wheel drive vehicle 10 has four wheels 11, 12, 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 via front and rear prop shafts 25, 27 from the centre differential 22 and transmit it to the front wheels 11, 12 and rear wheels 13, 14 respectively via drive shafts 29. 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 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 differential controller 28 controls the locking torque of each of the centre and rear differentials 22, 26. It will be appreciated that the front differential 24 could also be a lo limited slip differential if required.

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 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 measure the ride height of a respective one of the vehicle's wheels 11, 12, 13, 14. These ride height sensors therefore provide a measure of vertical movement of the wheels 11, 12, 13, 14 relative to the vehicle's body 52 which is allowed by compression and expansion of the air suspension cylinders 50.

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

The vehicle also comprises a brake control system 60 which performs a dynamic stability control (DSC) function and an anti-lock brake (ABS) function. The brake control system 60 receives signals from the wheel speed sensors 34, 36, 38, 40, and the steering angle sensor 32, as well as the yaw sensor 51 and the lateral accelerometers 53, 54.

In performing the DSC function, the brake control system 60 monitors the yaw rate of the vehicle as measured by the yaw sensor 51 and compares it with an expected yaw rate derived from the wheel speeds as measured by the wheel speed sensors 34, 36, 38, 40 and the steering angle as measured by the steering angle sensor. If the measured yaw rate is different from the expected yaw rate, the DSC function operates to correct the yaw rate towards the expected yaw rate. This is achieved by controlling the braking torques at the respective wheels so that they are not all equal, which induces a yaw moment to correct the vehicle yaw. While the DSC control is occurring, the drive torque produced by the engine 18 is reduced so that it is not working against the braking produced by the DSC system.

In performing the ABS function the brake control system 60 monitors the speed of each of the wheels 11, 12, 13, 14 using the wheel speed sensors 34, 36, 38, 40 to detect when one of them is in slip, i.e. when it is rotating at a speed which is slower than expected for the vehicle speed. When slip of a wheel is detected, the brake of that wheel is partially or fully released, to a sufficient extent to stop the wheel from slipping and allow it to rotate at a speed commensurate with the speed of the vehicle.

In order for the brake control system 60 to perform the ABS and DSC functions most effectively it is desirable to reduce the locking torque in the centre and rear differentials 22, 26 as this allows the system to control the braking torques to each wheel of the vehicle independently. 'I'herefore when the brake control system 60 needs to control the braking torque to the wheels it needs to communicate with the differential controller 28 to request that one or both of the differentials be opened.

The differential controller 28 and the brake control system 60 are each connected to a CAN bus 64 which allows them to communicate with each other so that the differential controller 28 can receive signals from the brake control system 60 requesting opening of the differentials. The differential controller 28 and brake control system 60 can also communicate with other systems on the vehicle such as the engine management system. In particular each system makes available on the CAN bus data from the sensors associated with it, data relating to its own operation, and data relating to the vehicle's operation which it has derived from the information available to it. Therefore the differential controller receives over the CAN bus the signals from the wheel speed sensors 34, 36, 38, 40, the accelerometers 53, 54 and the yaw sensor 51.

The brake control system 60 also receives signals over the CAN bus 64 from a brake pedal sensor 70 which senses movement of the brake pedal 72 of the vehicle. A vehicle speed sensor, in the form of a longitudinal accelerometer 66 is also provided which provides to the brake control system 60 a longitudinal acceleration signal. This is integrated by the brake control system 60 to produce a vehicle speed signal which is also transmitted onto the CAN bus 64. The differential controller 28 receives signals from the ride height sensors 42, 44, 46, 48.

Under normal operation the differential controller 28 controls the degree of locking of the centre and rear differentials 22, 26 in response to various functions of the operation of the vehicle. For example, the differentials are arranged to be more locked when the vehicle is travelling at low speeds on very rough surfaces, as detected by the wheel speed sensors 34, 36, 38, 40 and the ride height sensors 42, 44, 46, 48, and to be less locked when the vehicle is travelling at higher speeds on smooth surfaces. The differential controller 28 also responds to signals from the brake control system 60 to reduce or increase the locking of one or both of the centre and rear differentials 22, 26 when this is required to control the vehicle's stability.

When the brake control system 60 detects that one of the wheels is slipping and that it needs to provide the ABS function it communicates this to the differential controller 28, over the CAN bus 64, which responds by decreasing the degree of locking of the differentials 22, 26 provided this is acceptable from a safety and vehicle stability point of view. When the brake control system 60 actively reduces the braking torque to one of the wheels 11, 12, 13, 14 in order to reduce slip of that wheel, it monitors the speed of the slipping wheel and also the speed of the other wheel on the to same axle as the slipping wheel, i.e. the other front wheel 11, 12 if the slipping wheel is a front wheel 11, 12 and the other rear wheel 13, 14 if the slipping wheel is a rear wheel 13, 14. If that other wheel is not slipping, and the slipping wheel does not increase in speed to the speed of the other wheel within a predetermined time when its brake is released, then the brake control system 60 sends a signal to the differential controller 28 causing it to start to increase the degree of locking of the differential between the slipping wheel and the other wheel. For example if the left rear wheel 13 is slipping and does not come back up to vehicle speed when its brake is released, then, provided the right rear wheel 14 is not slipping, the locking of the rear differential 26 is increased. Provided the right rear wheel 14 has sufficient traction, this will bring the left rear wheel 13 back up to a speed commensurate with the vehicle speed.

When the locking of the differential 26 has been increased in this way, the brake control system 60 continues to monitor the speeds of the two wheels which have been partially locked together by the differential 26, i.e. the two rear wheels 13, 14, to check that the non-slipping wheel, i. e. the right rear wheel, is not caused to slip. In order to do this, as the speeds of the two rear wheels 13, 14 approach each other, the brake control system 60 compares them to a reference speed, which is a speed of rotation related to the vehicle speed as determined from the longitudinal accelerometer 66.

Provided the speeds of the two wheels 13, 14 converge towards the reference speed, then the differential locking is kept at an increased level until the wheel speeds are equal to each other. It then reduces the differential locking again to its previous level.

If, when the differential locking is increased the speeds of the two wheels 13, 14 converge but towards a speed lower than the reference speed, this suggests that the slipping wheel is in fact reducing the speed of the other wheel. In this case the speeds of the remaining wheels of the vehicle, in this case the front wheels 11, 12 are also checked. Provided they are rotating at a speed closer to the reference speed than the two rear wheels, they can be used to speed up both the rear wheels 13, 14. Therefore the brake control system 60 sends a signal to the differential controller 28 whereupon locking of the centre differential 22 is increased and the speeds of all of the wheels monitored further. Provided the speeds of all the wheels 11, 12, 13, 14 converge towards the reference speed, the locking of the centre differential 22 is maintained until the wheel speeds have all substantially reached the reference speed. It is then unlocked again to its previous degree of locking in response to a further signal from the brake control system 60 to the differential controller 28. If the speeds of all of the wheels start to converge towards a speed substantially lower than the reference speed, then this indicates that all of the wheels are starting to slip. In this case the degree of locking of both the centre and rear differentials 22, 26 is decreased again by the differential controller 28 in response to a further signal from the brake control system 60, so that if any one of the wheels can start to increase in speed again it is free to do so.

If, when one Off the wheels 13 is first detected to be slipping, the other wheel on the same axle is also found to be slipping to the same or a greater degree, then increasing the degree of locking of the differential 22 between them clearly will not help to bring either of them up to vehicle speed. The brake control system 60 therefore sends a signal to the differential controller 28 whereupon the locking of the centre differential 22 is increased immediately, provided the other two wheels 11, 12 on the other axle are not also in slip, so that both of the slipping wheels 13, 14 can be brought out of slip.

It will be appreciated that the system described above can be used in conjunction with a traction control function that increases the torque provided by the engine 18 if all of the wheels are in slip in order to accelerate them back up to vehicle speed. Since slip usually occurs when the brakes are being applied, and therefore when it is not desired to accelerate the vehicle, the use of the differentials as far as possible to bring the wheels out of slip can delay, or avoid altogether, an increase in driving torque until it is becomes essential. Also use of the differentials to increase the speed of a slipping wheel according to the invention allows the drive torque to each of the wheels to be controlled more accurately than systems which simply increase the drive torque in response to a slipping wheel.

Claims

1. A brake control system for controlling the braking of a plurality of wheels of a vehicle, the system being arranged to apply a braking torque to the wheels, but to produce a reduction in the braking torque to any one of the wheels that is detected to be slipping and, to send signals to a differential controller arranged to control a differential that transmits drive torque to each of said wheels, wherein the brake control system is further arranged to send a signal to the differential controller to increase the degree of locking of the differential thereby to increase the speed of rotation of the slipping wheel if, despite said reduction, the slipping wheel continues to slip.

2. A system according to claim I arranged to determine the speed of rotation of another wheel, to which the slipping wheel can be connected through the differential, and to control the degree of locking in dependence on the speed of the slipping wheel and the speed of the other wheel.

3. A system according to claim 2 arranged to increase the degree of locking only if the speed of the other wheel is greater than the speed of the slipping wheel.

4. A system according to claim 2 or claim 3 arranged to monitor the speeds of the slipping wheel and the other wheel to detect whether increasing the degree of locking increases slipping of the other wheel.

5. A system according to claim 1 or claim 2 further comprising vehicle speed sensing means arranged to determine the speed of the vehicle independently of the speed of rotation of the wheels such that the degree of slipping of each of the wheels can be determined independently of the speed of the other wheels.

6. A system according to any foregoing claim wherein the differential is a front or rear differential arranged to distribute torque between wheels on opposite sides of the vehicle.

7. A system according to any of claims I to 5 wherein the differential is a centre differential arranged to distribute torque between the front wheels of the vehicle and the rear wheels of the vehicle.

8. A vehicle control system comprising a brake control system according to any foregoing claim, and a differential controller.

9. A vehicle comprising a control system according to claim 8, and a differential.

10. A drive system substantially as hereinbefore described with reference to the accompanying drawing.