GB2414217A

GB2414217A - Vehicle differential control

Info

Publication number: GB2414217A
Application number: GB0411345A
Authority: GB
Inventors: John Lawrence Beaty; Jan Pieter Prins
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2004-05-21
Filing date: 2004-05-21
Publication date: 2005-11-23
Anticipated expiration: 2024-05-21
Also published as: GB0411345D0; GB2414217B

Abstract

A vehicle comprises a differential controller 28 arranged to control the degree of locking of its centre differential 22. The controller 28 anticipates wheel slip caused by braking, on the basis of the level of braking and of a parameter indicative of the amount of weight transfer between the front and rear of the vehicle. An increased degree of locking may reduce the likelihood of wheel slip. The measure of the level of braking may be provided directly by a brake control system 60, by using a sensor 70 associated with the brake pedal 72 or by measuring deceleration. Ride height sensors 42-48 and wheel speed sensors 34-40 may be provided.

Description

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.

The present invention provides a drive system for driving the front and rear wheels of a vehicle, the system comprising a differential arranged to transmit drive torque to said front and rear wheels, and control means arranged to control the degree of locking of the differential, wherein the control means is arranged to detect the application of a brake of the vehicle, to monitor at least one parameter indicative of the degree of weight transfer between the front and rear wheels of the vehicle darling braking, and to respond by increasing the locking of the differential by an amount which is dependent on the at least one parameter. 30,.

The application of the brake may be detected directly, for example Dam a brake control system, or indirectly, for example by measuring the amount of slip produced at one or more of the wheels by the braking, or by measuring the level of braking demanded by the driver, such as by measuring the position of the brake pedal.

The at least one parameter may include the degree of braking, which can be determined, for example, by measuring the deceleration rate of the vehicle.

Alternatively, or in addition, said at least one parameter may include the load on at least one wheel of the vehicle, a load change on at least one wheel or the loads on at least two wheels.

The control means may be arranged to monitor a difference between the load on the first wheel or wheels and the load on the second wheel or wheels and to increase the locking by an amount which is dependent upon the difference between the loads. Where there are two first wheels and two second wheels, the control means may be arranged to monitor a difference between the average load on the first wheels and the average load on the second wheels and to increase the locking by an amount which is dependent upon the difference between the average loads.

The control means may be arranged to estimate a level of braking at which slip of at least one of the wheels is likely to occur, and to increase the degree of locking when that level of braking is reached. This estimate may be made on the basis of the load on the wheel, and any other available information such as the level of friction of the surface over which the vehicle is travelling.

The control means may be arranged to estimate, for the level of braking being provided, a degree of locking of the differential which is likely to prevent said at least one of the wheels from slipping, and to increase the locking to that degree. This can ensure that the level of locking provided is approximately the minimum sufficient to avoid locking of any of the wheels.

The control means may be arranged to detect a fault in the operation of a brake torque distribution system arranged to control the distribution of brake torque between the wheels of the vehicle, and to increase the locking of the differential in response to the application of a brake of the vehicle only when such a fault is detected. This can allow the control of the differentials to take over, to some degree, the stability control functions of the brake system when the brake system is not functioning correctly.

]O Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure l is a diagrammatic representation of a vehicle including a ] 5 differential control system according to a first embodiment of the invention.

Referring to Figure], a four wheel drive vehicle 10 has four wheels 11, ] 2, 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 1], ] 2 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.

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 suspension sensors 42, 44, 46, 48 each measure the load on a respective one of the vehicle's wheels 11, 12, 13, 14. The suspension sensors 42, 44, 46, 48 in this case comprise air pressure sensors arranged to measure the air pressure in air suspension cylinders 50. However, they may comprise ride height sensors arranged to provide a measure of vertical movement of the wheels 11, 12, 13, 14 relative to the vehicle's body. These height sensors will only provide an accurate load measurement function in some cases, for example where the vehicle does not include an automatic levelling system. However they will always be able to give an indication of drive under braking which will give some information about weight distribution. The suspension sensors may even comprise a combination of air pressure sensors and ride height sensors, but in any event they need to be arranged to produce signals indicative of the load on the respective wheels 11, 12, 13, 14.

A yaw sensor 51 provides a yaw signal indicative of the instantaneous yaw rate and yaw acceleration of the vehicle 10, 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, an anti-lock brake (ABS) function, and an electronic brake distribution (EBD) 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 performing the EBD function, the brake control system 60 is arranged to control the distribution of braking torque between the front wheels 11, 12, 13, 14 on the basis of the difference between the average speed of the front wheels and the average speed of the rear wheels. This is to ensure that lightly loaded wheels are braked less than heavily loaded wheels thereby allowing the maximum braking to be applied to each wheel without causing it to slip. The main function of the EBD function is to bias the braking towards the front wheels 11, 12 during heavy braking when the weight of the vehicle is transferred towards the front wheels l l, 12. However, it can also bias the braking towards the rear wheels if the front wheels start to slip more than the rear wheels. This can happen, for example, if the vehicle is reversing or in extreme load cases such as when the vehicle is towing a trailer.

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. Therefore 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 controller 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 differential controller 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, and from the ride height sensors 42, 44, 46, 48. It is also provided with direct connections to a driver display 68 and a separate vehicle speed sensor 66 which provides an indication of the vehicle's speed from the speed of rotation of the input to the centre differential 22.

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 monitors the level of slip as it occurs, using signals from the wheel speed sensors 34, 36, 38, 40, and when slip increases above an acceptable level, responds by increasing the degree of locking of the appropriate differential 22, 26 to reduce it to an acceptable level or eliminate it altogether. In particular the onset of slip of the rear wheels during braking can be detected by comparing the average slip of the front wheels with the average slip of the rear wheels to determine the difference between these averages. When that difference reaches a predetermined level, the centre differential 22 can start to be locked. For the rear differential 26, the levels of slip of the two rear wheels are compared and, when the difference between them reaches a predetermined level, the locking of the rear differential 26 is increased. The level of braking is still monitored and the differentials only used to control the wheel slip when the braking is above a predetermined level.

The differential controller 28 also determines in a pre-emptive manner when the rear (or front) wheels are likely to slip due to weight transfer between the front and rear of the vehicle caused by braking. When the driver brakes the vehicle, the differential controller 28 detects this via the CAN bus 64 and is arranged to increase the locking of the centre differential 22, provided various criteria are met which indicate that to do so will not interfere with the stability of the vehicle. These criteria will be described below. Assuming that it is safe to increase the locking of the centre differential, the differential controller 28 estimates a level of braking at which the rear wheels will start to slip. When the braking applied to the wheels approaches this critical level, the differential starts to increase the locking of the centre differential 22. This helps to prevent differences in speed between the front and rear wheels and therefore reduces the likelihood of the rear wheels slipping. The level of braking is preferably defined in terms of, and measured by means of, the deceleration rate of the vehicle. This is because the. deceleration rate gives the best indication of the longitudinal forces between the ground and the wheels. Other measures of braking can be used, such as the pressure of the brake fluid. However this does not always give a precise measurement of the forces between the wheels and the ground or road surface, for example because the brakes can be acting against the driving torque of the engine, resulting in little or no resultant force on the wheels.

In determining the critical level of braking the differential controller 28 needs to take account of the loading on the wheels 11, 12, 13, 14.

Specifically as it is the wheels with the lowest loading that are likely to slip first, the differential controller 28 needs to identify the wheel or wheels with the lowest loading on them, estimate what that loading is, and control the differentials accordingly. In this embodiment, wheel loads are measured directly. In order to do this, the differential controller monitors the signals from the suspension sensors 42, 44, 46, 48 and determines from them the average loading on the front wheels 11, 12 and the average loading on the rear wheels 13, 14. From these it can determine whether the front or rear wheels are the least loaded, and also the absolute loading on the least loaded wheels. It can therefore determine the critical level of braking at which those wheels are likely to slip. Typically if the vehicle is moving forwards and braking, the rear wheels l 3, l 4 will be the least loaded wheels. However, if the vehicle is reversing, in particular down a steep slope and with a heavy load towards its rear end, then the front wheels may be the least loaded. When the critical level of braking has been reached, if the level of braking increases above the critical level, then the degree of locking of the centre differential 26 which is required to prevent slipping of the rear wheels will increase accordingly. Therefore the differential controller 28 is arranged to continually monitor the level of braking and the to wheel loads and determine from them when the onset of wheel slip is likely. This may be simply by detecting when a predetermined level of braking is exceeded. It then determines the degree of locking of the centre differential 26 required to prevent the rear wheels from slipping, and sets the degree of locking to that level.

In an alternative arrangement, the differential controller does not monitor wheel loads, but controls the differential locking to avoid wheel locking under braking purely on the basis of the level of braking. When a predetermined level of braking is exceeded, the locking of the centre differential 26 starts to be increased, and the degree of locking is increased further as the level of braking increases further. The appropriate degree of locking can be determined assuming some selected condition of the vehicle, such as carrying an average load, and calculating the degree of weight transfer that will occur between the front and rear axles for any given level of braking deceleration.

As well as pre-emptive operation in support of the EBD function, a further function of the differential controller 28 is to provide, to a certain degree, the EBD function if the brake controller 60 is unable to provide it due to a fault. As described above, during normal operation, the degree of locking of the differentials 22, 26 is reduced if the DSC or ABS functions are in operation. However, if those functions cannot be provided, then the need to unlock the differentials is not present. Furthermore, the instability which these functions normally control is partly due to differences in traction between the wheels which results in them rotating at different speeds from each other. These wheel speed differences can be avoided, to a certain extent, by increasing the locking of the differentials 22, 26 when they are likely to occur.

A further possibility is for the differential controller 28 only to respond to braking when a fault with the normal EBD function of the brake control system 60 is detected.

When a fault, either within the brake control system or elsewhere in the vehicle, occurs that prevents normal operation of the brake control system, this is detected, for example by the brake controller 60 itself, and indicated to the differential controller 28 over the CAN bus 64. The differential controller 28 then monitors the level of braking of the vehicle and, as the level of braking increases, increases the degree of locking of the centre and rear differentials 22, 26. Nominally the degree of locking can be increased progressively from zero to a maximum level as the level of braking increases from zero. However, the system is also arranged to take into account the load on each of the wheels ll, 12, 13, 14 as will now be descri bed.

While the fault is present, the differential is arranged to monitor the load on each of the wheels I l, 12, 13, 14. For each wheel, the likelihood of slip occurring is determined from the load on the wheel and the level of braking of the wheel. If the chances of slip increase for one of the wheels to a level that is higher than one or more of the other wheels, then the locking of the differential or differentials between the wheel that is likely to slip and the other wheel or wheels is increased. This helps to keep the wheels rotating at the same speed as each other, and hence reduces the chances of slip ll In a further modification the slip prediction method and slip measurement method are both used to control the differential locking.

The method of measuring the level of braking can be direct, with the brake control system 60 providing over the CAN bus to the differential controller 28 an indication of the total level of braking, or even the level of braking of each of the wheels individually. Alternatively the level of braking can be measured indirectly, either by measuring the deceleration of the vehicle, for example using a longitudinal accelerometer, or by measuring the amount by which the brake pedal 72 is depressed, as indicated by the brake pedal sensor 70. As mentioned above, deceleration based braking measurement is preferred, as it is the deceleration that actually determines the amount of weight transfer which will occur, and also the deceleration that gives the best indication of the longitudinal forces between the wheels and the ground or surface over which the vehicle is travelling.

Claims

1. A drive system for driving the front and rear wheels of a vehicle, the system comprising a differential arranged to transmit drive torque to said the front and rear wheels, and control means arranged to control the degree of locking of the differential, wherein the control means is arranged to detect the application of a brake of the vehicle, to monitor at least one parameter indicative of the degree of weight transfer between the front and rear wheels of the vehicle during braking, and to respond by increasing the locking of the differential by an amount which is dependent on the at least one parameter.

2. A system according to claim l wherein the at least one parameter includes the level of braking of the vehicle.

3. A system according to claim 2 wherein the control means is arranged to measure the level of braking by measuring the deceleration of the vehicle.

4. A drive system according to any foregoing claim wherein the at least one parameter includes the load on at least one of the wheels, and the control means is arranged to measure the load on said at least one of the wheels.

5. A drive system according to claim 4 wherein the at least one parameter includes a difference between the load on a first wheel or wheels connected to one side of the differential and the load on a second wheel or wheels connected to the other side of the differential.

6. A drive system according to claim 5 wherein there are two first wheels and two second wheels, and the at least one parameter includes a difference between the average load on the first wheels and the average load on the second wheels.

7. A system according to any of claims 5 to 7 further comprising load measuring means arranged to measure the load on said wheel or wheels.

8. A drive system according to any foregoing claim wherein the control means is arranged to estimate a level of braking at which slip of at least one of the wheels is likely to occur, and to increase the degree of locking when that level of braking is reached.

9. A drive system according to claim 8 wherein the control means is arranged to estimate, for the level of braking being provided, a degree of locking of the differential which is likely to prevent said at least one of the wheels from slipping, and to increase the locking to that degree.

10. A drive system according to any foregoing claim wherein the control means is arranged to detect a fault in the operation of a brake torque distribution system arranged to control the distribution of brake torque between the wheels of the vehicle, and to increase the locking of the differential in response to the application of a brake of the vehicle only when such a fault is detected.

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