GB2452277A

GB2452277A - Vehicle steering control system

Info

Publication number: GB2452277A
Application number: GB0716747A
Authority: GB
Inventors: Stuart Perrott; Elliot Duncan Hemes; David Armstrong; Andrew Richard Lowis; David Edward John Pook; Karl Gordon Richards; Andrew Gavin Maskell
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2007-08-30
Filing date: 2007-08-30
Publication date: 2009-03-04
Anticipated expiration: 2027-08-30
Also published as: GB2452277B; GB0716747D0

Abstract

A steering system is able to switch between different power assistance steering maps associated with different driving modes, such as a sport mode or an economy mode. In order to prevent the driver from being unnerved by a sudden shift from one level of assistance to another, the invention provides a gradual change from one steering map to another over a specified time period. Additional safety provisions prevent a change if a dynamic stability control event is operative. A response characteristic of the steering system can change by a constant amount per unit time and can be changed from a first characteristic to a second characteristic over a specified time period. One of the characteristics may be applicable to low speed maneuvering for parking or alternatively for minimum assistance during high speed driving. Each characteristic may define a relationship between steering ratio and a steering wheel angle.

Description

1 2452277 Vehicle Steering Control Systems This invention relates to steering control systems for vehicles.

Vehicle steering systems with variable power assistance are known. See for example SAE Technical Paper 93161.7 published October 1993. Therein is described a system which provides a high level of assistance at low vehicle speeds i.e. for parking manoeuvres, and a decreasing level at higher speeds. Such behaviour can be described by a steering "map" represented by the curve A in Fig. 1 (which is a plot of vehicle speed against level of assistance) Some vehicles currently in production are provided with a driver-operable mode selection switch whereby the driver may change from a "normal" or "economy" mode to a "sport" mode or, as a further example from an "on-road" mode to an "off-road" mode. These different modes typically trigger some change in the vehicle's response characteristics; for example, by altering the shift points of an automatic transmission or the throttle response. It is envisaged that selection of a particular mode can also effect a change from one power assisted steering map to another. For example a "sport" mode may be characterised by a steering map which provides less assistance at most vehicle speeds compared with a "normal mode", in order to give the driver more "feel".

Such a characteristic is represented by curve B in Fig. 1.

As can be seen from Fig. 1, two maps can give a significantly different level of assistance at a particular speed. For example at speed Q, an instantaneous changeover from map A to* map B (i.e. a level of assistance switching from a higher value (c) to a lower value (ci)) would mean a sudden reduction in steering assistance. It would be preferable for the change to be gradual and therefore almost imperceptible to the driver.

Steering systems with variable ratios (dependent on steering wheel angle, for example) are also known. See for example SAE Technical Paper 200-01-0821 published March 2000 and US 7,073,622.

Fig. 2 illustrates two differing steering ratio maps, C and D plotted on a graph of steering wheel angle against steering ratio. Map C is appropriate for an on-road driving mode and has a maximum ratio when the steering wheel is close to the straight ahead position, i.e. steering wheel angle is substantially zero. This gives an initially slow response of the steered wheels to driver input starting from the straight ahead position in order to give the driver better control because the vehicle is usually travelling fast when steering input is small. The ratio becomes linear as the steering wheel angle increases in order to assist low speed parking manoeuvre s.

Curve D is a steering map appropriate for an off-road driving mode and is linear. This allows the steered wheels to react quickly to driver input.

An instantaneous changeover from map D to map C at low steering wheel angle (e.g. from point (e) to point (f)) would result in a significant difference in steering ratio which could be disconcerting to the driver. A gradual change would be preferable.

When a switch between steering maps is made, e.g. following a driving mode change brought about by the driver operating a mode selection switch, an instantaneous change in power assistance level and/or steering ratio can be unnerving to the driver. The present invention aims to mitigate this problem.

In a first aspect, the invention consists of an apparatus for controlling a vehicle steering system wherein the system has a response characteristic which varies in a plurality of selectable ways dependent on a vehicle parameter, and wherein the apparatus comprises a control means for effecting a gradual changeover from a first selected response characteristic to a second selected response characteristic by varying the steering system's response to the vehicle parameter by a constant amount per unit time.

In a second aspect, the invention consists of apparatus for controlling a vehicle steering system wherein the system has a response characteristic which varies in a plurality of selectable ways dependent on a vehicle parameter, and wherein the apparatus comprises a control means for effecting a gradual changeover from a first selected response characteristic to a second selected response characteristic by varying the steering system's response to the vehicle parameter over a specified time period.

In a third aspect, the invention consists of apparatus for controlling a vehicle steering system wherein the system has a response characteristic which varies in a plurality of selectable ways dependent on a vehicle parameter, and wherein the apparatus comprises a control means for effecting a gradual changeover from a first selected response characteristic to a second selected response characteristic during a specified time period by varying the steering system's response to the vehicle parameter at a rate dependent upon a measurable parameter associated with the vehicle which is measured at discrete time intervals during said changeover.

The length of the specified time period may have different values depending on vehicle speed and steering angle.

For example, the specified time period may be longer in instances where vehicle speed is relatively high compared with instances where vehicle speed is relatively low.

The specified time period may be longer in instances where there is little steering wheel angular movement (i.e. steering angle is small, close to a straight ahead position) compared with instances where the angle is large.

In these two instances, there is very little feedback to the driver from the steering system and so a long changeover period is less disconcerting than a short one.

The response characteristic may be a level of steering assistance.

The vehicle parameter may be vehicle speed. The measurable parameter may be a vehicle speed or a value relating to torque applied to the vehicle's steering wheel by the driver. Such a value may be obtained by monitoring the electrical or hydraulic power demanded by a steering pump,

for example.

Alternatively, the response characteristic may be a steering ratio. The vehicle parameter may be steering wheel angle. The measurable parameter may be steering wheel angle.

In one embodiment, the changeover is effected in response to a driving mode request signal from a driver-operated driving mode selection switch. tn another embodiment, a changeover is effected in response to a signal from an on-board sensor, e.g. one which detects a transition between on-road and off-road driving.

Preferably, for safety reasons, the control means inhibits a changeover by ignoring a driving mode request signal if certain vehicle dynamic parameters are detected to be in a pre-determined state. For example, a changeover is inhibited if the vehicle is experiencing a stability control, traction control or anti-lock braking event.

A changeover may also be inhibited if steering wheel angle is proportionately large with respect to vehicle speed. A changeover may also be inhibited if steering wheel angle velocity is proportionately large with respect to vehicle speed.

The state of the vehicle dynamic parameters may be determined by on-board sensors and the control means may be made responsive to the outputs of one or more of these sensors.

As a further safety measure, the control means may be adapted to select the map having least assistance or most linear ratio if a fault condition is detected.

Some embodiments of the invention will now be described, by way of example only, with reference to the drawings of which; Fig. 1 is a graph showing two typical steering maps - (vehicle speed against level of assistance); Fig. 2 is a graph showing two further steering maps -(steer�ng wheel angle against steering ratio); Fig. 3 is a schematic block diagram of steering system control apparatus in accordance with an embodiment of the invention; Fig. 4 is a flow chart showing operation of the embodiment of Fig. 1; Fig. 5 is a graph of steering wheel angle against vehicle speed; and Fig. 6 is a graph of steering wheel angular velocity against vehicle speed.

In Fig. 3 a steering mechanism 1 is connected to a pair of wheels 2, 3 of a vehicle (not shown) and to a steering actuator 4. The steering mechanism 1 is shown in the drawing as a rack and pinion type but other mechanisms can be utilised.

The steering actuator 4 is able to vary the level of steering assistance and in a further embodiment is also able to vary the steering ratio. The mechanisms for achieving these capabilities are well known arid will not be described herein and the illustrated element given the reference numeral 4 in Fig. 3 merely represents the functionality of such steering mechanisms.

Linked to the steering actuator 4 is a steering wheel 5 whose angular position and angular velocity are monitored by a steering wheel sensor 6.

An output of the steering wheel sensor 6 is connected to a controller 7. The controller also receives outputs from a plurality of on-board sensors 8. The on-board sensors 8 provide the controller with the following information; vehicle speed and whether or not dynamic stability control, traction control and/or anti-lock braking are activated at any given instant.

The controller 7 is connected to the steering actuator 4 and also to a switch 9 which is driver-operable. The switch 9 permits the driver to select between an "economy" driving mode and a "sport" mode.

The controller is connected with a store 10 which contains steering maps as illustrated in Fig. 1 viz, vehicle speed versus steering assistance maps for economy or sport mode, and in the further embodiment, steering angle versus steering ratio maps e.g. for an on-road and an off-road mode.

Operation of the embodiment of Fig. 3 will now be described with reference to Figs. 4, 5 and 6.

Initially, the controller 7 controls the actuator 4 in accordance with steering map A accessed from the store 10 i.e. the current driving mode is set to "economy".

At step 11 the driver sets the switch 9 to "sport" and the switch sends a signal to the controller 7 notifying it of this mode change request.

At step 12, the controller 7 determines, from the on-board sensors 8, if the current speed of the vehicle is either so low that maximum possible steering assistance is already being provided, or so high that the minimum possible steering assistance is being provided. If either of these criteria is met, then the controiJer 7 proceeds to step 13 to effect a steering map change in one of three alternative manners to be described herein below. If, on the other hand, the vehicle speed falls between these two limits then the operation proceeds to step 14.

At step 14, the controller 7 determines, from the on-board sensors 8 if a dynamic stability control, anti-lock braking or traction control event are currently active. If any one of these events is active, then no steering map change is effected and the operation reverts to step 12. On the other hand, if none of these vehicle control events is active then the operation proceeds to step 15.

At step 15, the controller 7 interrogates the steering wheel sensor 6 to determine if the current steering wheel angle is within the range specified by the area under the curve of Fg. 5 or if the current steering wheel angular velocity is within the range specified by the area under the curve of Fig. 6.

If both angle and velocity are within their respective specified ranges then the controller proceeds to step 13 for effecting a steering map change in a manner to be described.

However, if either angle or velocity are outside their respective specified ranges then no steering map change is effected and the operation reverts to step 12.

Once step 13 has been reached, the controller has to access steering map B from the store 10 and control the steering actuator 4 in accordance therewith. In accordance with an advantageous feature of the invention, the controller 7 is adapted to effect the changeover from one steering map to another in a gradual fashion so that the driver is not disconcerted by a sudden step change in behaviour of the steering assistance (or ratio) . This gradual changeover can be achieved in one of three ways.

Say, for example that at the point when step 13 has been reached, the vehicle's speed is equal to Q (see Fig. 1) and so the level of assistance has to move from the value at point c to the value at point d.

In a first option, the controller 7 sends a signal to the actuator 4 causing it to reduce the level of steering assistance at a fixed, pre-deterrnined amount per unit time until the value at point d is reached, whereupon the controller 7 controls the actuator 4 in accordance with steering map B. -11 -In a second option, the controller sends a signal to the actuator 4 causing it to reduce the level of steering assistance to value d', linearly over a fixed pre-set time period. The time period may typically be set at 2 minutes if the vehicle speed is high and there is very little or no steering input from the driver. The time period may be set to just a few seconds in cases where vehicle speed is low or large steering manoeuvres are taking place. On expiry of the time period, the controller 7 controls the actuator 4 in accordance with steering map B. In a third option, the controller sends a signal to the actuator 4 causing it to reduce the level of steering assistance in a fixed time period by a variable amount which depends on the value of vehicle speed measured at discrete time intervals during the time period. Say for example that the time period is set at three seconds and the time interval between speed measurements is one second.

As in the previous examples, say that when step 13 has been reached, the vehicle's speed is measured to have the value Q and so the level of steering assistance has to gradually be reduced from a value c to the value d. The controller 7 knows that it has 3 seconds to effect this change and so sends a signal to the actuator to gradually reduce the level at a rate so that a value d will be reached after 3 seconds. However, say that the driver wishes to increase his speed and so after 1 second, when the vehicle speed is measured again, it is now greater than Q which corresponds to a new point g' on curve B. Point g is now a new target level and so the controller 7 has to cause the actuator 4 to adjust its rate of decrease of assistance level so that it can get to point g' in the two seconds remaining.

After 2 seconds have elapsed a further speed measurement is taken and any necessary further adjustments to the rate of change of steering assistance level are made.

After 3 seconds have elapsed and the assistance level has reached the appropriate point on curve B commensurate with the vehicle's speed, the controller continues to control the actuator 4 in accordance with steering map B.

Claims

1. An apparatus for controlling a vehicle steering system wherein the system has a response characteristic which varies in a plurality of selectable ways dependent on a vehicle parameter, and wherein the apparatus comprises a control means for effecting a gradual changeover from a first selected response characteristic to a second selected response characteristic by varying the steering system's response to the vehicle parameter by a constant amount per unit time.

2. An apparatus for controlling a vehicle steering system wherein the system has a response characteristic which varies in a plurality of selectable ways dependent on a vehicle parameter, and wherein the apparatus comprises a control means for effecting a gradual changeover from a first selected response characteristic to a second selected response characteristic by varying the steering system's response to the vehicle parameter over a specified time period.

3. An apparatus for controlling a vehicle steering system wherein the system has a response characteristic which varies in a plurality of selectable ways dependent on a vehicle parameter, and wherein the apparatus comprises a control means for effecting a gradual changeover from a first selected response characteristic to a second selected response characteristic during a specified time period by varying the steering system's response to the vehicle parameter at a rate dependent upon a measurable parameter associated with the vehicle which is measured at discrete time intervals during said changeover.

4. Apparatus as claimed in any preceding claim in which the response characteristic is a level of steering assistance.

5. Apparatus as claimed in any preceding claim in which the vehicle parameter is vehicle speed.

6. Apparatus as claimed in any preceding claim in which the measurable parameter is vehicle speed.

7. Apparatus as claimed in claim 1, claim 2 or claim 3 in which the response characteristic is a steering ratio.

8. Apparatus as claimed in claim 1, 2, 3 or 7 in which the vehicle parameter is a measurement of the angular position of the vehicle's steering wheel.

9. Apparatus as claimed in claims 1, 2, 3, 7 and 8 in.

which the measurable parameter is steering wheel angle.

10. Apparatus as claimed in any preceding claim and further including a driver-operated mode selection switch for generating a driving mode request signal and in which the control means is adapted to effect said gradual changeover in response to said signal.

11. Apparatus as claimed in claim 10 wherein the control means is also responsive to outputs from one or more on-board vehicle sensors for determining one or more vehicle dynamic parameters and adapted to ignore a driving mode request signal if one or more of said vehicle dynamic parameters is detected to be in a pre-determined state.

12. Apparatus for controlling a vehicle steering system substantially as herein before described with reference to the drawings.