GB2574453A

GB2574453A - Control system for a steering system of a vehicle

Info

Publication number: GB2574453A
Application number: GB1809351.8A
Authority: GB
Inventors: Solomon Nick; Ward William
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2018-06-07
Filing date: 2018-06-07
Publication date: 2019-12-11
Anticipated expiration: 2038-06-07
Also published as: GB201809351D0; GB2574453B

Abstract

A control system for a vehicle steering system is provided, with controllers configured to compare a deceleration time value, namely a predicted duration until the vehicle speed reaches zero, to a target time value (typically the time taken to steer the wheels to a neutral position) and to output a signal to control a steering angle of a steered wheel 103, 106 based on the comparison so that the steered wheel is in a straight ahead position (for example within 3.5 degrees of being parallel with longitudinal axis 1001) by the time the vehicle speed is zero. The system may avoid the need for dry steering, and is particularly useful for use with vehicles with rear wheel steering.

Description

CONTROL SYSTEM FOR A STEERING SYSTEM OF A VEHICLE

TECHNICAL FIELD

The present invention relates to a control system for a steering system of a vehicle; a steering system; a vehicle; and a method of controlling a steering system of a vehicle.

BACKGROUND OF THE INVENTION

Some vehicle steering systems are electronically controlled, such that the mechanical link between a steering request and a steering output it not present. Such systems are known as ‘steer-by-wire’ systems. An exemplary form of steer-by-wire is in rear wheel steer vehicles. Such steering systems are known to provide varying benefits to a vehicle at differing vehicle speeds. At higher vehicle speeds the rear wheels can be steered in phase with the front wheels, promoting vehicle stability. At higher vehicle speeds the rear wheels can be steered out of phase with the front wheels, providing improved manoeuvrability

Turning the vehicle wheels when stationary (known as dry steering) can produce unwanted results, for example excessive tyre wear and excessive loading in the steering actuator. Further it may not be possible to dry steer if the friction between the wheel and the surface it is on is too high and/or the vehicle mass is large.

One approach is to prevent the wheels from turning below a predetermined threshold speed. The disadvantage of this approach is that the vehicle then loses the manoeuvrability advantages provided by RWS systems at low speed.

It is an aim of the present invention to address the aforementioned problems.

SUMMARY OF THE INVENTION

Aspects of the present invention relate to a control system for a steering system of a vehicle; a steering system; a vehicle; and a method of controlling a steering system of a vehicle.

According to an aspect of the present invention there is provided a control system for a steering system of a vehicle, the control system comprising one or more controllers, the control system configured to compare a deceleration time value to a target time value, wherein the deceleration time value is indicative of a predicted duration until the vehicle’s speed reaches zero, and output a control signal to control a steering angle of a steered wheel in dependence on the comparison, such that the steered wheel is controlled to turn towards a straight ahead condition by the time the vehicle speed reaches zero.

This provides the benefit that the wheels are not subjected to ‘dry steering’ and the further benefit that the steering angle is not unduly limited such that manoeuvrability is improved. This is particularly advantageous in control systems in which the steering system is a rear wheel steering system, as the components and actuators may be of smaller mechanical and electrical capacities than in a front wheel steering system.

In some embodiments the control system is configured to output the control signal in dependence on the deceleration time value being equal to or less than the target time value, or in other words, in dependence on the target time value being equal to or greater than the deceleration time value

In an embodiment of the invention the target time value may be a predetermined time value stored within a memory of the one or more controllers.

In an alternative embodiment to the preceding embodiment the target time value comprises an actuator time value, the actuator time value being indicative of a duration for a steered wheel of the vehicle to return to the straight ahead condition. Preferably the actuator time value is determined in dependence on a current actuator displacement and an actuator rate.

The actuator rate may be a predetermined value stored in a memory of the one or more controllers, alternatively the actuator rate may be determined in dependence on at least one of a surface friction value and a vehicle mass value.

The target time value may further comprise a tuneable time value, preferably determined at least in part in dependence on an estimated latency within the control system.

The tuneable time value may be further determined dynamically at least in part in dependence on at least one of: a current vehicle speed, a current vehicle acceleration, surface friction value and a vehicle mass value.

The tuneable time value may be a predetermined time value stored within a memory of the one or more controllers.

In some embodiments of the invention the straight ahead condition may be within the range of -3.5 to 3.5 degrees from parallel to a longitudinal axis of the vehicle.

In an embodiment of the invention the control system may be configured to receive a speed signal indicative of a current vehicle speed, an acceleration signal of a current vehicle acceleration, and determine the deceleration time value in dependence on the received speed signal and acceleration signal.

Alternatively the control system may be configured to receive a plurality of speed signals indicative of vehicle speeds over a time period, and determine the deceleration time value in dependence on the received plurality of speed signals.

In some embodiments the control signal overrides a normal use control signal.

In some embodiments the control system is configured to receive a drive mode signal indicative of a drive mode of a vehicle, not output the control signal in dependence on the drive mode signal. The driving mode may be a rock crawl mode.

According to an aspect of the present invention there is provided a steering system comprising the control system of the preceding aspect

In some embodiments the steering system comprises a steering actuator.

According to an aspect of the present invention there is provided a vehicle comprising the control system or the steering system of the preceding aspects. Preferably the vehicle is a rear wheel steer or all wheel steer vehicle.

According to an aspect of the present invention there is provided a method of controlling a steering system of a vehicle, the method comprising: comparing a deceleration time value to a target time value, wherein the deceleration time value is indicative of a predicted duration until the vehicle’s speed reaches zero, and controlling a steering angle of the steered wheel in dependence on the comparison, such that the steered wheel is returned to the straight ahead condition by the time the vehicle speed reaches zero.

Any controller or controllers described herein may suitably comprise a control unit or computational device having one or more electronic processors. Thus the system may comprise a single control unit or electronic controller or alternatively different functions of the controller may be embodied in, or hosted in, different control units or controllers. As used herein the term “controller” or “control unit” will be understood to include both a single control unit or controller and a plurality of control units or controllers collectively operating to provide any stated control functionality. To configure a controller, a suitable set of instructions may be provided which, when executed, cause said control unit or computational device to implement the control techniques specified herein. The set of instructions may suitably be embedded in said one or more electronic processors. Alternatively, the set of instructions may be provided as software saved on one or more memory associated with said controller to be executed on said computational device. A first controller may be implemented in software run on one or more processors. One or more other controllers may be implemented in software run on one or more processors, optionally the same one or more processors as the first controller. Other suitable arrangements may also be used.

Within the scope of this application it is expressly envisaged that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 shows a top view of a vehicle embodying the present invention;

Fig. 2 shows a top view of another vehicle embodying the present invention;

Fig. 3 shows a block diagram illustrating a system enabling steering of the vehicles of Figs. 1 and 2;

Fig. 4 shows a plan view of a vehicle travelling at a relatively high speed;

Fig. 5 shows a plan view of a vehicle travelling at a relatively low speed;

Fig. 6a shows a block diagram illustrating a control system enabling steering of the vehicles of Figs. 1 and 2;

Fig. 6b shows a block diagram illustrating a control system enabling steering of the vehicles of Figs. 1 and 2;

Fig. 6c shows a block diagram illustrating a control system enabling steering of the vehicles of Figs. 1 and 2;

Fig. 7a shows a block diagram illustrating a control system enabling steering of the vehicles of Figs. 1 and 2;

Fig. 7b shows a block diagram illustrating a control system enabling steering of the vehicles of Figs. 1 and 2;

Fig. 7c shows a block diagram illustrating a control system enabling steering of the vehicles of Figs. 1 and 2;

Fig. 7d shows a block diagram illustrating a control system enabling steering of the vehicles of Figs. 1 and 2;

DETAILED DESCRIPTION

A vehicle 100 embodying the present invention is shown in a top view in Fig. 1. The vehicle 100 is a car that is configured for use both on roads and off-road on various types of terrain. In the present embodiment, the vehicle 100 is a four wheel drive vehicle, but it will be appreciated that many of the features of the vehicle 100 described below are also applicable to rear wheeled drive vehicles.

Fig. 1 also schematically shows a steering system 101 configured to enable steering of the vehicle 100. The system 101 comprises an actuator 102 configured to cause steering of rear road wheels 103 of the vehicle 100, and also includes a control system 104 comprising a control means in the form of a controller 105 for controlling the operation of the actuator 102.

In the present embodiment, front road wheels 106 of the vehicle 100 are steered by means of a mechanism 107 comprising a steering wheel 108, which is connected to a pinion 109 via a steering column 110. The pinion 109 engages a rack 111 which is connected to steering knuckles 112 by tie rods 113.

The rear wheels 103 are steerable by a mechanism 114 which is operated by the actuator 102. In the present embodiment the actuator 102 is configured to drive a second pinion 115 associated with a second rack 116 which provides forces to steering knuckles 117 of the rear wheels 103 via tie rods 118.

A steering input or position sensor 119 is configured to sense the orientation of the steering wheel 108 and provide signals to the controller 105 indicative of the orientation of the steering wheel 108 and therefore also indicative of the orientation of the front road wheels 106. The controller 105 is configured to provide output signals to the actuator 102 to cause steering of the rear wheels 103 in dependence of the signals received from the steering input sensor 119. However, the output signals provided to the actuator 102 are also dependent on other signals received by the controller 105, as will be described in detail below.

An alternative vehicle 100 embodying the present invention is shown in Fig. 2, in which a system 101 enables “steer-by-wire” of all wheels 103,106 of the vehicle 100. The vehicle 100 of Fig. 2 has many features in common with that of Fig. 1, which have been provided with the same reference signs. Thus, like the vehicle 100 of Fig. 1, the vehicle 100 of Fig. 2 comprises a steering system 101 comprising pinion 109 and a rack 112 configured to operate steering knuckles 112 via tie rods 113, in order to steer the front wheels 106. A first actuator 102 is configured to drive a second pinion 115 associated with a second rack 116 which provides forces to steering knuckles 117 of the rear wheels 103 via tie rods 118.

However, in the embodiment of Fig. 2, the pinion 109 for driving the front wheels 106 is driven by a second actuator 202. The steering wheel 108 is mounted on a rotatable shaft 201 but it is not mechanically connected to the pinion 109. Instead, as well as providing signals to the actuator 102 for causing steering of the rear wheels 103, the controller 105 is also configured to provide signals to the second actuator 202 to cause steering of the front wheels 106 in dependence on signals it receives from the steering input sensor 119 located on the shaft 201 of the steering wheel 108.

In an alternative embodiment, the vehicle 100 has front wheels that are steer-by-wire, like those of Fig. 2, but the rear wheels 103 are not steerable.

The steering system 101 of Fig. 1, and that of Fig. 2, is illustrated by the block diagram shown in Fig. 3. Fig. 3 also illustrates some exemplary vehicle systems that may be in communication with the steering system 101. The control system 104 comprises a controller 105 which itself comprises an electronic processor 301 and an electronic memory device 302 which stores instructions 303 performable by the processor 301 to cause the processor 301 to perform the method described below and output signals to the first steering actuator 102 to cause steering of the rear wheels 103. In the case of the vehicle 100 of Fig. 2, the processor 301 also provides signals to the second steering actuator 202 for steering the front wheels 106. Although only one controller, processor and memory device are illustrated in Fig. 3, it will be understood that the control system 104 may comprise several controllers 105 and each controller 105 may comprise several processors 301 and/or several electronic memory devices 302, so that the processing as described below may be distributed over several processors.

As well as receiving signals from the steering input sensor 119, the control system 104 receives signals from wheel speed sensing means 304 indicative of a speed of rotation of each road wheel 103, 106. The wheel speed sensing means 304 may comprise wheel speed sensors, each of which is arranged to measure a speed of rotation of a respective one of the wheels 103, 106 and to provide a value for the speed of rotation directly to the controller 105. Alternatively, the wheel speed sensors may form a part of another system such as an antilock braking system (not shown) comprising a control unit configured to receive the signals from the wheel speed sensors and provide wheel speed values to the controller 105.

Figs. 4 and 5 show plan views of the vehicle 100 travelling at a relatively high speed and a relatively low speed respectively. In both Figs. 4 and 5 the front wheels 106 are turned approximately 15 degrees relative to the longitudinal axis 1001 of the vehicle 100 to cause the vehicle 100 to turn leftwards. In Fig. 4, the current speed of the vehicle 100, as determined from the wheel speed sensing means 304, is above a threshold speed and consequently the rear wheels 103 have been steered in phase with the front wheels 106. That is, because the front wheels 106 have been turned to the left, the rear wheels 103 are also turned to the left. As is known, steering the rear wheels 103 in phase with the front wheels 106 provides the vehicle 100 with increased stability, which is advantageous at high speeds.

In Fig. 4, the rear wheels 103 have only been steered leftwards by about 1.5 degrees, i.e. a tenth of the angle turned by the front wheels 106. The proportion of the front wheel steering angle by which the rear wheels 103 have been steered is referred to herein as the gain value. Thus, in this example the rear wheel steering has a gain value of +0.1 (= 1.5/15).

In Fig. 5 the current speed of the vehicle 100 is below the threshold speed and consequently the rear wheels 103 have been steered out of phase with the front wheels 106. That is, because the front wheels 106 have been turned to the left, the rear wheels 103 have been turned to the right. Stability of the vehicle 100 is not an issue at low speeds and, as is known, steering the rear wheels 103 out of phase with the front wheels 106 provides the vehicle 100 with increased agility.

The rear wheels 103 have been steered rightwards by about 3 degrees, i.e. a fifth of the angle turned by the front wheels 106. Thus, in this example the rear wheel steering has a gain value of -0.2 (= -3 /15). I.e. the absolute value (0.2) of the gain value is higher than the gain value for speeds above the threshold speed, but the gain value is negative due to the rear wheels 103 being turned out of phase with the front wheels 106.

Depending upon a user’s style of driving or a type of terrain on which the vehicle 100 is travelling, a particular set of vehicle characteristics may be most appropriate, for example one particular accelerator pedal map may be more appropriate than others, and similarly one particular transmission map and one particular set of stability control settings may be most appropriate. To enable a user to select the most appropriate settings for a chosen style of driving or a particular terrain, the vehicle 100 also comprises a user input device (UID) 311 configured to enable a user to indicate to the vehicle control system 310 a selected drive mode. For example, the user may select a standard mode (or normal mode) when driving on tarmac roads and the vehicle control system 310 controls the ECU 307, the TCU 308 and the SCU 309 to operate in a mode suitable for the tarmac road surface. Alternatively the user may select another mode, such as a grass, gravel and snow mode for driving over a terrain that provides a low coefficient of friction, or a sand mode for driving on a deformable surface such as sand, which provides a very low coefficient of friction, or a rock crawl mode for driving on rough surfaces with high friction. In response to such a user indication, the vehicle control system 310 controls the ECU 307, the TCU 308 and the SCU 309 to operate in a mode suitable for the indicated type of terrain. The mode selected by the use of the user input device 311 is also provided to the controller 105, and may be used to determine signals provided to the first steering actuator 102 and/or the second steering actuator 202.

The user input device 311 may comprise a switch or switches, a touch screen device, or other electrical or electronic device suitable for enabling a user to provide an indication of a mode they wish to select.

The vehicle control system 310 may comprise a terrain estimation system (TES) 306. Such a system is known and described in the applicant’s UK patent GB2492655B and US patent application published as US2014350789A1. The terrain estimation system 310 is configured to select a drive mode that is the most appropriate mode for the subsystems 307, 308, 309 based on measurements indicative of the terrain on which the vehicle 100 is travelling, to enable the vehicle control system 310 to automatically control the subsystems 307, 308, 309 to operate in the selected mode.

The TES 306 receives signals from terrain sensing means 312 comprising various different sensors and devices for providing information indicating the type of terrain on which the vehicle 100 is travelling. The terrain sensing means 312 may include the aforementioned IMU 305, wheel speed sensing means 304, steering input sensor 119, as well as other sensors (not shown), such as an ambient temperature sensor, an atmospheric pressure sensor, an engine torque sensor, a brake pedal position sensor, an acceleration pedal position sensor, ride height sensors, etc. Various outputs from the terrain sensing means 312 are used by the terrain estimation system 310 to derive a number of terrain indicators. For example, a vehicle speed is derived from the wheel speed sensors, wheel acceleration is derived from the wheel speed sensors, the longitudinal force on the wheels is derived from the IMU 305, and the torque at which wheel slip occurs (if wheel slip occurs) is derived from the motion sensors of the IMU 305 to detect yaw, pitch and roll. The terrain indicators are then processed to determine a probability that each of the different drive modes is appropriate, and thereby determine which of the modes is most appropriate for the operation of the subsystems. In its automatic mode, the terrain estimation system 310 continually determines for each mode the probability that it is appropriate and in dependence on another mode having a consistently higher probability than the currently selected control mode, the vehicle control system 310 commands the subsystems to operate in accordance with that other mode.

The mode determined automatically by the terrain estimation system 306, or selected by the use of the user input device 311, is also provided to the controller 105, and may be used to determine signals provided to the first steering actuator 102 and/or the second steering actuator 202.

The first steering actuator 102 is operable to provide a torque sufficient to turn the wheels 103 of the vehicle 100 at the lower and higher speeds as described above in relation to Fig. 3 and Fig. 4. When stationary this torque may not be sufficient to overcome the friction between the wheels 103 and a driven surface however.

An alternate solution presented by the present invention is to return the wheels 103 to a straight ahead condition in dependence on a determined time value at which the vehicle speed will reach zero.

In such scenarios the control system 104 may be configured to implement a ‘return to zero’ function. This function comprises two main steps as shown in Fig. 6a. The first step is performed at determination block 501, in which the control system determines a requirement for the steered wheel ‘return to zero’, and comprises comparing an estimated time until the vehicle speed will reach 0, i.e. a deceleration time value T_d, to a target time value T_T. The deceleration time value T_d is indicative of a predicted duration until the vehicle’s speed reaches zero, i.e. a measure of time from a current point in time to a point in time at which the vehicle is predicted to have zero speed. The second step is performed at control block 502, in which if it is determined at determination block 501 that there is a ‘return to zero’ requirement then the control system outputs a control signal to control the steered wheels 103 to turn towards the straight ahead condition. In some example these blocks are part of a single controller 105 of the control system 104, in other examples they are part of separates controllers 105 within the control system 104.

The comparison of the deceleration time value T_d to a target time value T_T in order to determine whether the wheels should be controlled to a straight ahead condition comprises monitoring for the condition at which the deceleration time value T_d is less than or equal to the target time value Tt:

T_d<T_T

Monitoring for the condition at which the target time value T_T is greater than or equal to the deceleration time value T_d would also be appropriate.

T_T>T_d

Both of the time values T_d, T_T may be provided as part of signals received at an electrical input (not shown) of the controller 105. These signals may be provided to the control system 104 by another vehicle system as per Fig. 6b or determined by another controller 105 of the control system 104 as per Fig. 6c.

For example the deceleration time value T_d may be received by the control system 104 as part of a signal from another controller or system of the vehicle. A vehicle having an autonomy level of 2 or higher (as defined by SAE) may have a vehicle level controller and/or trajectory planner operable to determine and/or plan a point in time at which the vehicle will be stationary and this, or a derivative of it, could be provided as a value for the deceleration time value T_d.

One or both of the time values T_d, Tt may be determined within the control system 104.

Fig. 7b provides an example of the control system 104 in which the deceleration time value T_dis determined in dependence on a current vehicle speed vand a current vehicle acceleration a at determination block 503.

A positive velocity coupled with zero or positive acceleration will be indicative that the vehicle is travelling forwards and not decelerating and thus there will be no ‘return to zero’ requirement. Likewise a negative velocity coupled with zero or negative acceleration will be indicative that the vehicle is travelling in reverse and not decelerating. In both of these cases the standard control as described in relation to figures 4 and 5 will be maintained.

If the velocity is positive and the acceleration is negative, or the velocity is negative and the acceleration is positive, then it is indicative that the vehicle is decelerating and a deceleration time value T_d may be determined:

The deceleration time value T_d may instead be determined at 503 by receiving a plurality of vehicle velocity values v_b v₂... v_n at corresponding time intervals as per Fig. 7b. These values can then be extrapolated such that an estimated time at which the vehicle velocity is equal to 0 can be provided. The deceleration time value T_d can therefore be calculated as the difference between the current time and the estimated time at which the vehicle velocity is equal to 0

The target time value T_t may be a predetermined value stored within a memory 302 of the at least one controller 105.

Alternatively the target time value T_T may comprise an actuator time value T_a such that in some examples:

T_T = T_a

In some examples the actuator time value T_a may be dynamically determined, as shown at determination block 505 in Fig. 7c. This determination may be done through comparing a current actuator displacement 0_a (proportional to a current steered angle) to an actuator rate m_a, i.e. the speed at which the actuator can rotate the wheels in radians per second or degrees per second. The equation for said determination is:

For example if the current angular displacement of the wheels is 5° and the rate of actuator rate is 10 degrees per second then the actuator time value T_a will be half a second.

The actuator rate m_a may be a predetermined value stored within a memory 302 of the at least one controller 105 or it may be dynamically determined. Either determination may be done in dependence on various vehicle and environmental variables. These variables may include a vehicle mass value, the tyre characteristics of the vehicle, a surface friction value, and a drive mode of the vehicle. The vehicle mass value may be a pre-determined value stored within the memory device 102, alternatively it may be received from a vehicle mass estimator within the control system 104 or another system of the vehicle 100. The surface friction value is indicative of a coefficient of friction between the tyre and the driven surface, it may be determined by and received from the TES 306, or received directly from the terrain sensing means 312 and determined within the control system.

In some examples the control system 104 is configured to retrieve an actuator rate m_a from a look-up table stored within the memory device 302 in dependence on signals received indicative of the various vehicle and environmental variables.

The actuator rates ω₃ may fall within the range of 0 rad/s to 0.5 rad/s. Lower values would be in situations with very high vehicle loading a high surface friction such that the actuator may not be able to provide enough torque to turn the wheels 103. Higher values would be in situations in which there is little to no friction or vehicle loading. Maximum rates would be available with the vehicle wheels out of contact with the surface. Average angular rates ω₃may fall within the range of 0.15 rad/s to 0.25 rad/s with 0.2 rad/s being the rate expected under normal vehicle usage.

It will be appreciated that these angular displacements and rates 0_a, ω₃ could be replaced by linear values, for example in relation to the position of a vehicle steering rack, without departing from the scope of the invention.

The target time value T_T may comprise further time values as per Fig. 7d, in which the target time value Tt is determined at determination block 504 in dependence on the further time values. For example the target time value T_T may comprise a latency or tuneable time value T_L, such that:

T_T = T_a + T_L

The tuneable time value T_L may be a predetermined value stored within a memory 302 of the at least one controller 105. The value may fall within the range of 0 to 2 seconds with a preferred value of 1 second. The tuneable time value allows for mitigations against possible, estimated latencies in the vehicle systems, as well as providing a larger window within which the actuator 102 can return the wheels 103 to the straight ahead condition.

The tuneable time value T_L may be modified or determined in dependence on various factors. These factors may include the current vehicle speed, the current vehicle acceleration, the vehicle mass value, the surface friction value, and the drive mode of the vehicle.

Upon the determination that ‘return to zero’ is required the control system then provides control of the wheels 103 to return to a straight ahead condition. The straight ahead condition may be defined as the wheels 103 being in parallel with a longitudinal axis 1001 of the vehicle 100 +/a predefined tolerance.

The tolerance may be defined in dependence on the characteristics of components of the vehicle associated with the steering and suspension systems. The straight ahead condition may be defined as 0°+/-3.5°.

The controller 105 provides a control signal to the steering actuator 102. This control signal may override the control that is provided in normal driving, i.e. by a normal use control signal. Alternatively it may be output in its place is determined within the same controller 105.

Once the vehicle 100 is determined to be in motion again the control system 104 may resume its normal function as defined above in reference to figures 4 and 5. The determination that the vehicle is in motion may comprise receiving an indication that the vehicle speed is greater than 0.2 m/s.

The 'return to zero’ function may be modified or disabled in dependence on the currently selected or determined drive mode. For example when driving in a rock crawl scenario, and therefore when most likely operating in the rock crawl mode, the movement of the vehicle is characterised as being at very low speed. The terrain is also likely to have a very high level of surface friction and the wheels 103 may have varying loads. These conditions are most likely to exceed the capabilities of the actuator 102 and the variations at low speed may cause the wheels 103 to dither as the vehicle is continuously determined to be decelerating to 0 and then accelerating again.

The 'return to zero’ function may also be disabled in dependence on other determined driving scenarios. For example in high speed track driving vehicle decelerations may be very high, and thus the deceleration time value T_d may be found to be less than the target time value T_T, but the vehicle will not actually be braking to stationary. In order to prevent activation of the ‘return to zero’ function vehicle speed and acceleration thresholds may be put in place, such that the function does not activate at high vehicle speeds or high decelerations.

It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

CLAIMS:

1. A control system for a steering system of a vehicle, the control system comprising one or more controllers, the control system configured to:

compare a deceleration time value to a target time value, wherein the deceleration time value is indicative of a predicted duration until the vehicle’s speed reaches zero; and output a control signal to control a steering angle of a steered wheel in dependence on the comparison, such that the steered wheel is controlled to turn towards a straight ahead condition by the time the vehicle speed reaches zero.

2. The control system of claim 1 wherein the one or more controllers collectively comprise:

at least one electronic processor configured to access at least one electronic memory device and execute the instructions thereon so as to compare the deceleration time value to the target time value; and an electrical output configured to output the control signal to a steering actuator of the steering system.

3. A control system according to claim 1 or claim 2 wherein the steering system is a rear wheel steering system.

4. A control system according to any preceding claim wherein the control system is configured to output the control signal in dependence on the deceleration time value being equal to or less than the target time value.

5. A control system according to any preceding claim wherein the target time value is a predetermined time value stored within a memory of the one or more controllers.

6. A control system according to any one of claims 1 to 4 wherein the target time value comprises an actuator time value, the actuator time value being indicative of a duration for a steered wheel of the vehicle to return to the straight ahead condition.

7. A control system according to claim 6 configured to determine the actuator time value in dependence on a current actuator displacement and an actuator rate.

8. A control system according to claim 7 wherein the actuator rate is a predetermined value stored in a memory of the one or more controllers.

9. A control system according to claim 7 wherein the actuator rate is determined in dependence on at least one of a surface friction value and a vehicle mass value.

10. A control system according to any one of claims 6 to 9 wherein the target time value further comprises a tuneable time value.

11. A control system according to claim 10 wherein the tuneable time value is determined at least in part in dependence on an estimated latency within the control system.

12. A control system according to claim 10 or claim 11 wherein the tuneable time value is determined dynamically at least in part in dependence on at least one of: a current vehicle speed, a current vehicle acceleration, a surface friction value and a vehicle mass value.

13. A control system according to claim 8 or claim 9 wherein the tuneable time value is a predetermined time value stored within a memory of the one or more controllers.

14. A control system according to any preceding claim wherein the straight ahead condition is within the range of -3.5 to 3.5 degrees from parallel to a longitudinal axis of the vehicle.

15. A control system according to any preceding claim configured to:

receive a speed signal indicative of a current vehicle speed; receive an acceleration signal of a current vehicle acceleration;

determine the deceleration time value in dependence on the received speed signal and acceleration signal.

16. A control system according to any one of claims 1 to 10 configured to:

receive a plurality of speed signals indicative of vehicle speeds over a time period; and determine the deceleration time value in dependence on the received plurality of speed signals.

17. A control system according to any preceding claim wherein the control signal overrides a normal use control signal.

18. A control system according to any preceding claim wherein the control system is configured to:

receive a drive mode signal indicative of a drive mode of the vehicle; and not output the control signal in dependence on the drive mode signal.

19. A control system according to claim 17 wherein the drive mode is a rock crawl mode.

20. A steering system comprising the control system of any preceding claim.

21. A steering system according to claim 19 further comprising a steering actuator.

22. A vehicle comprising the control system of any one of claims 1 to 19 or the steering system of claim 20 or claim 21.

23. A vehicle according to claim 22 wherein the vehicle is a rear wheel steer or all wheel steer vehicle.

24. A method of controlling a steering system of a vehicle, the method comprising:

comparing a deceleration time value to a target time value, wherein the deceleration time value is indicative of a predicted duration until the vehicle’s speed reaches zero; and controlling a steering angle of a steered wheel in dependence on the comparison, such that the steered wheel is returned to the straight ahead condition by the time the vehicle speed reaches zero.

25. A non-transitory computer readable medium comprising computer readable instructions that, when executed by a processor, cause performance of the method of claim