GB2344326A

GB2344326A - Vehicle steering control

Info

Publication number: GB2344326A
Application number: GB9826587A
Authority: GB
Inventors: Carl Charles Bourne; Martin Ranson; James Marco
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1998-12-04
Filing date: 1998-12-04
Publication date: 2000-06-07
Also published as: GB9826587D0

Abstract

A vehicle has an electric steering motor 34 which, in response to signals from a control unit 22, controls the steering angle of the driven front wheels. In soft terrain the steering angle, and optionally also the driving torque, is oscillated about the steering angle requested by the driver to improve traction.

Description

Vehicle Steering Control The present invention relates to steering control systems for vehicles.

The present invention provides a steering control system for a vehicle having a steerable wheel and drive means for supplying a driving torque to the steerable wheel, the system comprising steering angle request means operable by a driver to request a steering angle of the steerable wheel, and control means for controlling said steering angle wherein the control means is operable to produce an oscillation of the steering angle while the requested steering angle remains constant.

The steering angle control can be provided by controlling the motor of a fully electric power steering system, or by controlling the actuation valve in a hydraulic power steering system independently of the driver's input through the steering shaft.

The present invention further provides a traction control system for a vehicle having a steerable wheel and drive means for supplying a driving torque to the steerable wheel, the system comprising steering angle request means operable by a driver to request a steering angle of the steerable wheel, and control means for controlling said steering angle wherein the control means is operable to produce synchronized oscillation of the steering angle and said driving torque while the requested steering angle remains constant.

The drive torque can be varied by simultaneous oscillations in the torque to all or some of the driven wheels. This can be achieved either by simple control of the traction motor or motors if the vehicle is an electric or hybrid vehicle, or by control of the fuelling of the internal combustion engine in a conventional power train.

If the vehicle is an electric vehicle having independent motors for each wheel, these can be controlled to provide the oscillations in drive torque.

Alternatively the drive torque oscillations can be produced using the centre differential of a four wheel drive vehicle which distributes torque between the front and rear axles. In this case the oscillations in torque provided to the front wheels will be accompanied by oscillations, in anti-phase, in torque provided to the rear wheels. As a further alternative the oscillations in drive torque could be provided by the engine management system of a vehicle driven by a conventional internal combustion engine.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of a vehicle including a steering system according to the invention, Figure 2 is a graph showing variations over time in requested steering angle and actual steering angle in the system of Figure 1 in a first mode of operation, Figure 3 shows variations over time in actual steering angle and drive torque in the system of Figure 1, in a second mode of operation, and Figures 4a, 4b and 4c show variations in time of requested steering angle, actual steering angle and drive torque in the system of Figure 1, in a third mode of operation.

Referring to Figure 1, a vehicle 10 has a steerable front pair of wheels 12a, 12b and a non-steerable rear pair of wheels 14a, 14b. Each of the wheels 12,14 is driven by an independent electric motor 16a, 16b, 16c, 16d.

The power for the motors 16 is derived via a generator 18 from an internal combustion engine 20 and distributed in the desired proportions to the motors 16 by a control unit 22. The polarity of the power supply to each of the motors can be reversed by the control unit 22 so each motor can apply a braking torque as well as a driving torque to its respective wheel 12. The direction and magnitude of the torque supplied to each wheel is therefore controllable independently. A battery 24 is also provided to act as a buffer for the storage of excess energy produced and to smooth over irregularities in the power of the engine 20. An accelerator pedal 26 has a potentiometer 28 connected to it which produces a torque demand signal dependent on the pedal position and inputs it to the control unit 22. A steering angle request sensor 30 senses the rotational position of the steering shaft 32 in the vehicle steering column and provides a steering angle request signal to the control unit 22 indicative of the steering angle requested by the driver. The control unit 22 controls an electric steering motor 34 operating on a steering rack 36 thereby to control the actual steering angle of the front wheels 12a, 12b.

The control unit 22 is further connected to a traction assist switch 40 which can be operated by the driver to request traction assistance, the effect of which will be described below.

During normal operation the control unit monitors the steering angle request signal and controls the steering motor 34 so as to provide the requested steering angle.

If the driver is approaching terrain where he believes that the vehicle may lose traction, for example if he is approaching an uphill gradient on soft ground such as mud or sand, he can activate the traction assistance function of the steering system by operation of the switch 40.

Referring to Figure 2, if the traction assistance function is selected, the control unit 22 monitors the requested steering angle, shown as a broken line, and controls the actual steering angle so as to oscillate about the requested angle as shown by the solid line. The mean steering angle therefore follows the requested steering angle. The effect of this is that, on soft ground where the front wheels 12a, 12b have sunk into the surface, the side walls of the tyres are pressed against material forming the surface, for example sand or mud, thereby increasing the contact force between the tyres and the surface above that provided solely by the weight of the vehicle.

The maximum driving torque which can be provided to the wheels without them losing traction is therefore increased.

Whilst this oscillation of the steering angle can improve traction it will obviously affect the feel of the vehicle to the driver and may cause undesirable damage to the ground, therefore the magnitude and frequency of the oscillations is arranged to increase as the drive torque requested by the driver increases. The oscillations are therefore only significant when they are in fact needed.

The optimum frequency and magnitude of the oscillations will also vary depending on the type of surface the vehicle is travelling on. For example on soft sand, which tends to have a relatively constant consistency, high amplitude oscillations may be necessary to produce any appreciable effect, whereas in heavy mud, which tends to be less consistent, high amplitude oscillations might throw the vehicle body from side to side in an undesirable manner, and produce undesirable steering effects for example if a swing to one side of the front wheels was resisted significantly more than a subsequent swing to the other side. However, systems are known in which the vehicle wheel speeds and their variations in response to changes in driving torque can be used to analyse the type of surface the vehicle is travelling on. The control unit 22 is therefore arranged to analyse the surface from the wheel speed sensors 38a, 38b, 38c, 38d and the driving torques of the motors 16a, 16b, 16c, 16d and to control the frequency and amplitude of the steering angle oscillations accordingly.

With reference to Figure 3, in a modified operating mode, the control unit 22 is arranged to oscillate the driving torque applied to the front wheels at essentially twice the frequency of the steering angle oscillations.

The driving torque is arranged to peak when the steering angle is at either extreme of the range in which it is oscillating, and to be lowest when the steering angle is in the middle of the range, i. e. equal to the requested steering angle. This is because, at least on some surfaces such as heavy mud where the wheel tends to cut a permanent rut, the greatest force on the tyre side walls occurs when the wheels are at their extremes of steering travel.

However, on other surfaces which are more fluid, such as sand, the greatest force on the side walls occurs when the wheel is turning fastest, i. e. when it is substantially at the requested steering angle between its extremes of movement. Therefore in these circumstances the oscillations in drive torque are arranged such that it peaks when the steering angle is in the middle of its range of oscillation and is lowest when it is at the ends of that range. The selection of which is appropriate is made on the basis of the surface analysis described above.

A further function of the system will now be described with reference to Figures 4a, 4b and 4c. This function is used for the specific circumstances when the vehicle wheels have become stuck in a rut which tends to limit the steerability of the vehicle so that it is at least very difficult to steer out of the rut. At time to the vehicle is driving forwards in a straight line but with its wheels stuck in a rut. The driving torque is constant and corresponds to that requested by the driver. In order to get out of the rut, at time ta the driver turns the steering wheel to the left and activates the system. The subsequent constant steering angle request is shown in Figure 4a. The control unit 22 responds by turning the steering angle towards the left and increasing the driving torque. This will steer the vehicle to the left and it will start to climb out of the rut. However, when the steering angle reaches a predetermined angle a without the vehicle climbing right out of the rut, the steering angle starts to return to its original value (straight ahead) and then continues on to the same angle a to the right. While the steering angle is to the right the drive torque is dropped to a level which is below the level requested by the driver. The wheels are then turned back to the left, up to the steering angle a and the drive torque again increased, and then the wheels turned back to the right and the driving torque again decreased.

This synchronized oscillation of steering angle and drive torque, which is independent of the drivers steering angle request and drive torque request which both remain constant, causes the front wheels to'rock'from side to side in the rut and the rocking builds up with each oscillation. Therefore as shown here, on the third'rock'to the left the vehicle climbs out of the rut at time tx. This is detected, for example by an inclinometer or by the change in acceleration of the vehicle in response to the drive torque, and the steering angle is returned to the requested steering angle and the drive torque returned to the requested drive torque.

It will be appreciated that this system enables a driver to get a vehicle out of a rut with minimal damage to the environment, in particular minimum damage to the ground and minimum fuel consumption.

In a modification to this function, the magnitude of the steering angle oscillations is not predetermined, and the control unit 22 increases the steering angle until the steered wheels contact the side of the rut. This can be detected by monitoring the resistance to the increase in steering angle, which can be determined, for example, by monitoring the current supplied to the steering motor 34. The control unit therefore increases the steering angle in one direction, at a suitable predetermined rate, until the resistance to further increase reaches a predetermined level and then reverses, returning the wheels to straight ahead and then increasing the steering angle in the other direction until the predetermined resistance is again reached. The frequency and amplitude of the steering angle oscillations are therefore determined by the speed of operation of the steering motor and the width of the rut, and also by the speed of the vehicle. The oscillations in drive torque can then be controlled so as to be synchronized with those in the steering angle so that the rocking effect described above is produced.

Indeed this method of determining when the change of steering angle should change direction can also be used as part of the traction assistance function described above as a further or alternative control on the amplitude of the steering angle oscillations.

Claims

CLAIMS 1. A steering control system for a vehicle having a steerable wheel and drive means for supplying a driving torque to the steerable wheel, the system comprising steering angle request means operable by a driver to request a steering angle of the steerable wheel, and control means for controlling said steering angle wherein the control means is operable to produce an oscillation of the steering angle while the requested steering angle remains constant.
2. A system according to claim 1 wherein the control means is arranged to produce an oscillation of the steering angle about a means steering angle which varies with the requested steering angle.
3. A system according to claim 1 wherein the control means is arranged to produce an oscillation of the steering angle about the requested steering angle.
4. A system according to any foregoing claim wherein the control means is arranged to produce said oscillation of the steering angle for any requested steering angle within at least a range of requested steering angles.
5. A traction control system for a vehicle having a steerable wheel and drive means for supplying a driving torque to the steerable wheel, the system comprising steering angle request means operable by a driver to request a steering angle of the steerable wheel, and control means for controlling said steering angle wherein the control means is operable to produce synchronized oscillation of the steering angle and said driving torque while the requested steering angle remains constant.
6. A system according to claim 5 wherein the frequency or amplitude of the drive torque oscillations is variable by the control means in response to variations in steering angle request.
7. A system according to claim 5 or claim 6 wherein the frequency or amplitude of the drive torque oscillations is variable by the control means in response to variations in a driving torque requested by a driver.
8. A system according to any one of claims 5 to 7 further comprising surface analysing means for analysing the surface over which the vehicle is travelling, wherein the control means is arranged to vary the frequency or amplitude of the drive torque in response to changes in the nature of surface.
9. A system according to any one of claims 5 to 8 wherein the oscillations in driving torque are arranged to be of twice the frequency of those in the steering angle so that the average steering effect of the driving torque oscillations is substantially zero.
10. A system according to claim 9 wherein the driving torque is arranged to be a maximum at substantially the same time as the steering angle reaches the extremes of the range in which it is oscillating.
11. A system according to claim 9 wherein the driving torque is arranged to be a maximum at substantially the same time as the rate of change of steering angle is a maximum.
12. A system according to any one of claims 5 to 11 wherein the control means can vary the phase relationship between the steering angle oscillations and the drive torque oscillations.
13. A system according to claim 12 further comprising surface analysing means for analysing the surface over which the vehicle is travelling, wherein the control meansis arranged to vary said phase relationship depending on the nature of said surface.
14. A system according to any foregoing claim wherein the frequency or amplitude of the steering angle oscillations is variable by the control means in response to variations in steering angle request.
15. A system according to any foregoing claim wherein the frequency or amplitude of the steering angle oscillations is variable by the control means in response to variations in a driving torque requested by a driver.
16. A system according to any foregoing claim further comprising means for measuring the resistance to changes in the steering angle, wherein the changes of direction producing the steering angle oscillations are each produced in response to the detection of increases in the resistance to changes of the steering angle.
17. A system according to any one of claims 5 to 8 wherein the oscillations in driving torque are arranged so that the driving torque is greater when the steerable wheel is at one end of the range within which it is oscillating than when it is at the other end of said range.
18. A system according to claim 17 wherein the oscillations in driving torque are arranged to be of the same frequency as those in the steering angle.
19. A system according to claim 17 or claim 18 which is arranged to detect when the vehicle leaves a rut in the ground on which it is travelling, wherein the steering angle is arranged to oscillate about a mean steering angle which is different from the requested steering angle until the vehicle is detected as having left the rut.
20. A system according to claim 19 wherein the control unit is arranged, on detection of the vehicle leaving the rut, to re-align the steering angle with the requested steering angle.
21. A steering control system substantially as hereinbefore described with reference to the accompanying drawings.