GB2574394A

GB2574394A - An apparatus and a method for controlling steering

Info

Publication number: GB2574394A
Application number: GB1809000.1A
Authority: GB
Inventors: Solomon Nick; Ward William; Armstrong David
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2018-06-01
Filing date: 2018-06-01
Publication date: 2019-12-11
Anticipated expiration: 2038-06-01
Also published as: GB201809000D0; GB2574394B

Abstract

A method 1900 for controlling steering of rear wheels of a vehicle includes receiving a first signal relating to front steering angle 1901 and determining whether the vehicle has low traction 1902. A proposed rear wheel steering angle is determined depending on the signal and whether the vehicle has low traction 1903. An output signal is provided to control rear wheel steering in dependence on the proposed rear wheel steering angle 1904. The rear wheels may be steered in or out of phase with the front wheels dependent on the traction of the vehicle or a selected mode, such as normal, grass, gravel, snow or sand. The rear steering angle may also be dependent on the product of the front steering angle and whether the traction of the vehicle is low or the yaw rate of the vehicle. The modes may be selected by a user or implemented automatically by a terrain sensing means and may control accelerator and transmission maps or stability control settings.

Description

AN APPARATUS AND A METHOD FOR CONTROLLING STEERING

TECHNICAL FIELD

The present disclosure relates to an apparatus and a method for controlling steering. In particular, but not exclusively it relates to an apparatus and a method for controlling steering in a road vehicle, such as a car.

Aspects of the invention relate to an apparatus, a system, a vehicle, a method, a computer program and a non-transitory computer-readable storage medium having instructions stored therein.

BACKGROUND

A problem with road vehicles, including four wheel drive vehicles, is that they can occasionally get stuck when driving off-road on deformable surfaces, such as sand or mud. It is an aim of the present invention to address this problem.

SUMMARY OF THE INVENTION

Aspects and embodiments of the invention provide an apparatus, a system, a vehicle, a method, a computer program and a non-transitory computer-readable storage medium as claimed in the appended claims.

According to an aspect of the invention there is provided an apparatus for controlling steering of rear wheels of a vehicle, the apparatus comprising a control means configured to: determine from received signals whether the vehicle is in a low traction condition; receive a first signal indicative of a front wheel steering angle; determine a proposed rear wheel steering angle in dependence on the first signal and whether the vehicle is determined to be in a low traction condition; and provide an output signal configured to cause steering of the rear wheels at the proposed rear wheel steering angle. This provides the advantage that, if the vehicle is stuck or only making slow progress due to poor traction provided by the terrain on which the vehicle is driving, cyclical steering of the front wheels that is designed to create additional grip may be emulated by the steering of the rear wheels. For example, a driver may oscillate the steering wheel left and right while requesting torque from the powertrain to scrub away a loose surface layer under the front wheels to gain better traction on a harder surface below. This action may be emulated by the rear wheels to additionally provide better traction between the rear wheels and the ground.

In some embodiments the proposed rear wheel steering angle is the product of the front wheel steering angle and a gain value; the control means is configured to select a first gain value in dependence on the vehicle being in a low traction condition and to select a second gain value in dependence on the vehicle not being in a low traction condition; and the first gain value is greater than the second gain value. This provides the advantage that the rear wheels may be made to oscillate at angles that are more like those of the front wheels when attempting to increase traction.

In some embodiments the control means is configured to determine that the vehicle is in a low traction condition in dependence on a set of criteria being met. This provides the advantage that the vehicle is able to automatically determine a proposed rear wheel steering angle appropriate for a low traction condition. For example, it does not require a user to provide an input at a user input device in order for the rear wheels to be steered with a larger second gain value that is suitable for increasing traction.

In some embodiments one of the criteria comprises a determination that the front wheel steering angle is oscillating with a frequency above a threshold frequency.

In some embodiments one of the criteria comprises an indication of torque being requested from a powertrain.

In some embodiments the control means is configured to receive a signal indicative of a measured yaw rate of the vehicle, and to calculate a target yaw rate in dependence on the front wheel steering angle, the rear wheel steering angle and an indication of speed of the wheels; and one of the criteria comprises a difference between the measured yaw of the vehicle and the target yaw being greater than a threshold value. This provides the advantage that the control means is able to accurately determine whether the vehicle is in a low traction condition.

In some embodiments the control means is configured to receive an indication of a selected mode, and, when the vehicle is determined to be in a low traction condition, the control means is configured to determine the proposed steering angles for the rear wheels in dependence on the selected mode. This provides the advantage that, for example, rear wheel steering that would be helpful in freeing a vehicle when stuck in sand or mud, does not have to be used when it is not appropriate in a currently selected mode, such as a mode used on rough rocky terrain.

In some embodiments the selected mode is selected in response to a user input or in response to terrain sensor signals.

In some embodiments, when the vehicle is determined to be in a low traction condition, the control means is arranged to provide an output signal that is configured to cause steering of the rear wheels out of phase with steering of the front wheels. This provides the advantage that when the traction of the vehicle is improved, additional steering in the rear wheels that is used to obtain increased traction, only causes the vehicle to turn more rapidly, whereas if the rear wheels were steered in phase with the front wheels, when the traction of the vehicle improves, the vehicle might suddenly move to one side in the direction of steer.

In some embodiments the control means is configured to determine a current state of the vehicle as one of a plurality of predefined states and determine a proposed rear wheel steering angle that depends on the current state; and the predefined states comprise at least one of: oriented with a roll angle above a threshold roll angle; moving backwards down an incline with a pitch angle above a threshold pitch angle. This provides the advantage that the steering of the rear wheels may be optimized for the current condition of the vehicle.

In some embodiments the control means comprises an electronic memory device and having instructions stored therein; and an electronic processor electrically coupled to the electronic memory device and configured to access the electronic memory device and execute the instructions

According to another aspect of the invention there is provided a system for controlling steering of rear wheels of a vehicle, the system comprising the apparatus of any one of the previous paragraphs and at least one actuator for controlling a steering angle of the rear wheels of the vehicle in response to the output signal.

In some embodiments the system comprises sensing means configured to sense yaw rate of the vehicle.

In some embodiments the sensing means comprises a gyroscope.

According to a further aspect of the invention there is provided a vehicle comprising the apparatus of any one of the previous paragraphs or the system of any one of the previous paragraphs.

In some embodiments the vehicle comprises a steering mechanism for controlling the steering angle of the front wheels of the vehicle comprising a manually operable device configured to enable adjustment of the steering mechanism.

According to another aspect of the invention there is provided a method of controlling steering of rear wheels of a vehicle with front wheel steer and rear wheel steer, the method comprising: determining from received signals whether the vehicle is in a low traction condition; receiving a first signal indicative of a front wheel steering angle; determining a proposed rear wheel steering angle in dependence on the first signal and whether the vehicle is determined to be in a low traction condition; and causing steering of the rear wheels at the proposed rear wheel steering angle. This provides the advantage that, if the vehicle is stuck or only making slow progress due to poor traction provided by the terrain on which the vehicle is driving, cyclical steering of the front wheels that is designed to create additional grip may be emulated by the steering of the rear wheels.

In some embodiments the proposed rear wheel steering angle is the product of the front wheel steering angle and a gain value; a first gain value is selected in dependence on a determination of the vehicle being in a low traction condition; a second gain value is selected in dependence on a determination of the vehicle not being in a low traction condition; and the first gain value is greater than the second gain value. This provides the advantage that the rear wheels are made to oscillate at angles that are more like those of the front wheels when attempting to increase traction.

In some embodiments the vehicle is determined to be in a low traction condition in dependence on a set of criteria being met. This provides the advantage that a proposed rear wheel steering angle appropriate for a low traction condition is automatically determined.

In some embodiments one of the criteria comprises an indication of torque being requested.

In some embodiments the method comprises receiving a signal indicative of a measured yaw rate of the vehicle, and calculating a target yaw rate in dependence on the front wheel steering angle, the rear wheel steering angle and a received indication of a speed of the wheels; and one of the criteria comprises a difference between the measured yaw of the vehicle and the target yaw being greater than a threshold value. This provides the advantage that the control means is able to accurately determine whether the vehicle is in a low traction condition.

In some embodiments the method comprises receiving an indication of a selected mode, and, determining the proposed steering angles for the rear wheels in dependence on the selected mode. This provides the advantage that, for example, rear wheel steering that is helpful in freeing a vehicle when stuck in sand or mud, may not be used when it is not appropriate in a currently selected mode, such as a mode used on rough rocky terrain.

In some embodiments, when the vehicle is determined to be in a low traction condition, the method comprises causing steering of the rear wheels out of phase with steering of the front wheels. This provides the advantage that when the traction of the vehicle is improved, additional steering in the rear wheels that is used to obtain increased traction, only causes the vehicle to turn more rapidly, whereas if the rear wheels were steered in phase with the front wheels, when the traction of the vehicle improves, the vehicle might suddenly move to one side in the direction of steer.

According to yet another aspect of the invention there is provided a computer program which when executed by a processor causes the processor to perform the method according to any one of the previous paragraphs.

According to yet a further aspect of the invention there is provided a non-transitory computerreadable storage medium having instructions stored therein which when executed on a processor cause the processor to perform the method according to any one of the previous paragraphs.

Within the scope of this application it is expressly intended 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. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

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 block diagram illustrating the functions performed by the control means illustrated in Fig. 3;

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

Fig. 6 shows a plan view of a vehicle travelling at a relatively low speed;

Fig. 7 shows a plan view of the vehicle moving forwards at a relatively low speed;

Figs. 8 and 9 show plan views of the vehicle during a procedure to free the vehicle when it has become stuck or is making only very slow progress due to low traction;

Fig. 10 shows a diagram illustrating operation of the steering angle determination means and the state detection means when it determines that the vehicle is in a LOW TRACTION condition;

Fig. 11 shows a flowchart illustrating a method embodying the present invention and performable by the control means to control steering of rear wheels of the vehicle when it is in a LOW TRACTION condition;

Fig. 12 shows a flowchart illustrating a method that provides an example of the method illustrated by Fig. 11; and

Fig. 13 shows a flowchart illustrating an example of the process at block 2002 of determining whether the vehicle 100 is in a LOW TRACTION condition.

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 shows, somewhat schematically, a 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 an apparatus 104 comprising a control means 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 sensor 119 is configured to sense the orientation of the steering wheel 108 and provide signals to the control means 105 indicative of the orientation of the steering wheel 108 and therefore also indicative of the orientation of the front road wheels 106. The control means 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 control means 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 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 control means 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 system 101 of Fig. 1, and that of Fig. 2, is illustrated by the block diagram shown in Fig. 3. The control means 105 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 processor and memory device are illustrated in Fig. 3, it will be understood that the control means 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 means 105 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 control means 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 control means 105.

The control means 105 also receives signals from an inertial measurement unit (IMU) 305, which in the present embodiment comprises a six degrees of freedom IMU. The IMU 305 comprises accelerometers configured to measure longitudinal acceleration (a_x), lateral acceleration (a_y) and vertical acceleration (a_z) of the vehicle 100, and gyroscopes configured to measure a rate of roll (ω_χ), a rate of pitch (ω_γ) and a rate of yaw (ω_ζ) of the vehicle 100. The IMU 305 is configured to provide indications of the measured accelerations (a_x, a_y, a_z) and angular velocities (ω_χ, ω_γ, ω_ζ) to the control means 105.

In the present embodiment, the vehicle 100 comprises several electronic control units for controlling subsystems of the vehicle 100. For example, the vehicle 100 comprises: an engine control unit (ECU) 307 for controlling operation of an engine (not shown) of the vehicle 100; a transmission control unit (TCU) 308 for controlling gear selection; and a suspension control unit (SCU) 309 for controlling properties of a suspension subsystem (not shown). Each of the subsystems is capable of working in several different modes, and the vehicle 100 comprises a vehicle control system 310 configured to control the mode in which the subsystems operate.

For example, the engine control unit 307 may be controlled by the vehicle control system 310 to operate using an accelerator pedal map selected from several different maps; the transmission control unit 308 may be controlled to operate using a transmission map selected from several different maps; and the suspension control unit 309 may be controlled to operate using a set of stability control settings selected from several different sets.

Depending upon a user’s style of driving or a type of terrain on which the vehicle 100 is travelling, 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 driving 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 control means 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 driving 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 driving 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 control means 105, and may be used to determine signals provided to the first steering actuator 102 and/or the second steering actuator 202.

A block diagram illustrating the functions performed by the control means 105 is shown in Fig. 4. The control means 105 may comprise a vehicle state estimation means 401 which receives signals from the IMU 305 comprising measurements of at least the longitudinal acceleration (a_x), the lateral acceleration (a_y), the rate of roll (ω_χ), the rate of pitch (ω_γ) and the rate of yaw (ω_ζ) of the vehicle 100. The vehicle state estimation means 401 also receives an indication of the currently selected gear, for example from the TCU 308 via a CAN (Controller Area

Network) bus. The vehicle state estimation means 401 also receives signals from the steering input sensor 119 indicative of a requested steering angle and the wheel speed sensing means 304 comprising measurements of the angular velocity of the wheels 103, 106.

The vehicle state estimation means 401 processes the received data (i.e. the selected gear, the requested steering angle and measurements from the IMU 305 and wheel speed sensing means 304) to determine and repeatedly update a plurality of state values that provide an estimate of a current state of the vehicle 100. In the present embodiment, the vehicle state estimation means 401 comprises a Kalman filter into which the received data is input and which generates at least some of the state values. The state values comprise estimates of the roll angle (θ_χ), the pitch angle (0_y), the longitudinal velocity (V_x), longitudinal acceleration (a_x) and centripetal acceleration of the vehicle over the ground, as well as a yaw rate target, a yaw rate measurement, a steering angle and a vehicle direction indication, which indicates if a reverse gear is currently selected.

The yaw rate target is an estimate of the current rate of yaw of the vehicle 100 and it is calculated from the steering angle and the estimate of the longitudinal velocity (V_x) of the vehicle 100 over the ground using a simple mathematical model commonly referred to as a bicycle model. The yaw rate measurement is the rate of yaw measured by the IMU 305.

The control means 105 comprises a state detection means 402 which receives the state values provided by the vehicle state estimation means 401, as well as an indication of a currently selected driving mode and an indication of a powertrain torque request, such as from a throttle position sensor. The state detection means 402 is configured to analyse the state values, selected driving mode and powertrain torque request to determine whether or not the vehicle 100 is currently in a predefined special condition or alternatively in a standard condition. In the present embodiment, the vehicle state estimation means 401 is configured to determine whether the vehicle 100 is in any one of three special conditions, labelled REVERSE DOWN, LOW TRACTION and BANK in Fig. 4, or in its STANDARD condition.

An indication of whether the vehicle 100 is determined to be in one of the predefined special conditions or in the STANDARD condition is provided to a steering angle determination means 403. One or more of the state values, such as longitudinal velocity (V_x) or roll angle (θ_χ), is also received by the steering angle determination means 403 along with the requested steering angle received from the steering input sensor 119. The steering angle determination means 403 is configured to determine a proposed rear wheel steering angle in dependence on at least the requested steering angle received from the steering input sensor 119, the state of the vehicle 100 determined by the state detection means 402 and received state values. The control means 105 is configured to provide an output signal to the first steering actuator 102 to control rear wheel steering in dependence on the proposed rear wheel steering angle.

In an embodiment, such as that of Fig. 2, in which the vehicle 100 is steer-by-wire, the steering angle determination means 403 may be configured to additionally determine a proposed front wheel steering angle in dependence on at least the requested steering angle, the state of the vehicle 100 determined by the state detection means 402 and received state values. The control means 105 is then configured to provide an output signal to the second steering actuator 202 to control front wheel steering in dependence on the proposed front wheel steering angle.

Further details of how the LOW TRACTION condition is detected and how the proposed steering angle is determined will be described below. However, the STANDARD condition, which is established when none of the defined special conditions are detected, will firstly be described with reference to Figs. 5 and 6.

Figs. 5 and 6 show plan views of the vehicle 100 travelling at a relatively high speed and a relatively low speed respectively. In both Figs. 5 and 6 the front wheels 106 are turned approximately 15 degrees relative to the longitudinal axis 501 of the vehicle 100 to cause the vehicle 100 to turn leftwards. In Fig. 5, 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. 5, 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. 6 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.

A special condition of the vehicle 100, labelled LOW TRACTION in Fig. 4 will now be described with reference to Figs. 7 to 13. Fig. 7 shows a plan view of the vehicle 100 moving forwards at a relatively low speed. The vehicle 100 is in its STANDARD condition and its front wheels 106 are turned to the left to enable the vehicle to be steered leftwards. The rear wheels 103 are being steered out of phase with the front wheels 106 to enhance the manoeuvrability of the vehicle 100. As described above, the angle through which the rear wheels 103 are turned is the product of the angle through which the front wheels are turned and a gain value.

Figs. 8 and 9 show plan views of the vehicle 100 during a procedure to free the vehicle when it is unable to make forward progress due to one or more of the road wheels 103, 106 being located in a depression in the ground and the ground providing very low friction and/or being deformable. For example, the ground surface may be formed of mud, snow or sand. A similar procedure may be performed when the vehicle 100 is making very slow progress due to only very low traction being obtainable by the tyres of the vehicle 100. In order to free the vehicle from the location in which it is stuck and/or enable the vehicle to make progress, the driver of the vehicle 100 requests torque from the powertrain of the vehicle 102 by actuation of its accelerator pedal 1601 and oscillates its steering wheel 108 to cause the front wheels 106 to oscillate between a leftwards orientation shown in Fig. 8 and a rightwards orientation shown in Fig. 9. The oscillation of the front wheels assists the front wheels 106 to obtain additional grip and/or reduce the depression in which the wheels 106 are located. For example, on muddy ground additional grip may be obtained by the action of the side walls of the tyres of the wheels 106 against ruts in which the tyres are located, or on loose sand, a depression in which the wheels 106 reside may be reduced in depth by the action of the tyres pulling additional sand into the depression.

In Figs. 8 and 9, the control means 105 has identified that the vehicle 100 is in a LOW TRACTION condition and consequently the angle through which the rear wheels 103 are steered is determined using a relatively large gain value compared to the gain value used in the STANDARD condition illustrated in Fig. 7. In the present embodiment, a gain value of -1 is used to determine the proposed steering angle for the rear wheels 103 so that the rear wheels 103 are steered in a similar way to the front wheels 106 but out of phase with the front wheels 106. Consequently, the oscillation of the rear wheels 103 assists in the process of enabling the vehicle 100 to make forward progress in a similar way to the oscillation of the front wheels 106.

Operation of the steering angle determination means 403 and the state detection means 402 when it determines that the vehicle 100 is in a LOW TRACTION condition is illustrated in Fig. 10. The state detection means 402 is configured to receive indications of the longitudinal velocity (V_x) of the vehicle 100, the requested steering angle, a throttle position (or powertrain torque request), a yaw rate target and a measured yaw rate of the vehicle 100. From these indications, the state detection means 402 determines whether the vehicle 100 is in a LOW TRACTION condition in dependence on a set of criteria being met.

A first one of the criteria is a determination that the front wheel steering angle is oscillating with a frequency above a threshold frequency, i.e. the front wheels are being steered repeatedly leftwards and then rightwards at a frequency that is greater than a threshold frequency. In an embodiment, the threshold frequency is 0.5Hz.

A second criterion comprises that the requested torque from the powertrain is above a threshold torque value, or that the accelerator pedal is being actuated by the driver of the vehicle.

As mentioned above, a yaw rate target is determined by the vehicle state estimation means 401 from the steering angle and the estimate of the longitudinal velocity (V_x) of the vehicle over the ground using a mathematical model. The state detection means 402 is configured to calculate a yaw rate error by determining the difference between the measured yaw rate and the target yaw rate. If the vehicle 100 is travelling over the ground at a speed that is approximately equal to that measured by the wheel speed sensing means 304, the yaw rate target should be approximately equal to the measured yaw rate, i.e. the yaw rate error should be very small. However, if the vehicle 100 is stuck, or is making only very slow progress, while to the road wheels 103, 106 are spinning at a relatively high rate, the measured yaw rate becomes much lower than the yaw rate target, i.e. the yaw rate error is large. A third criterion to determine that the vehicle is stuck, or is not making good progress due to low grip, is that the yaw rate error is above a threshold error value. In an embodiment, the threshold error value is 8 degrees per second, but in other embodiments the threshold error values are between 5 and 10 degrees per second.

An additional criterion may comprise a determination that the pitch angle of the vehicle 100 is not above a threshold pitch angle, because then the lack of forward movement of the vehicle 100 may be caused by a failed climb. In which case, the REVERSE DOWN condition may be determined in preference to the LOW TRACTION condition, so that the steering of the rear wheels 103 is configured to safely enable the vehicle 100 to be steered backwards down the slope.

If the criteria are met, the state detection means 402 determines a LOW TRACTION condition and the steering angle determination means 403 determines a proposed rear wheel steering angle in dependence on the requested steering angle and a relatively high negative gain value. In an embodiment, whenever a LOW TRACTION condition is determined, the steering angle determination means 403 calculates the proposed rear wheel steering angle by multiplying the requested steering angle by a high negative gain value, such as -1. However, in the present embodiment, the gain value that is used also depends on the currently selected driving mode. For a driving mode useable for driving on rough high friction surfaces, such as rocky ground, the magnitude of the gain value may be chosen to be relatively low, e.g. less than 0.2, and possibly the gain value may be kept at the standard gain value used for the vehicle in its STANDARD condition. For all other driving modes a relatively high gain value, such as -1, may be used.

The actuator 102 for steering the rear wheels 103 may not enable the rear wheels 103 to be steered at such large angles as those enabled by the front wheel steering mechanism. However, having determined the proposed rear wheel steering angle, the steering angle determination means 403 provides an output to the actuator 102 to cause steering at the proposed rear wheel steering angle, as far as possible.

A flowchart illustrating a method 1900 embodying the present invention and performable by the control means 105 to control steering of rear wheels 103 of the vehicle 100 is shown in Fig. 11. At block 1901 of the method 1900 a first signal is received that is indicative of a requested steering angle, and at block 1902 it is determined from received signals whether the vehicle 100 is in a LOW TRACTION condition. At block 1903 a proposed rear wheel steering angle is determined in dependence on the first signal and whether the vehicle 100 is determined to be in a LOW TRACTION condition, and at block 1904 an output signal is provided, for example to the actuator 102, to cause steering of the rear wheels at the proposed rear wheel steering angle.

A method 2000 providing an example of the method 1900 is illustrated by the flowchart shown in Fig. 12. At block 2001 a first signal is received that is indicative of a requested steering angle. At block 2002 it is determined whether the vehicle 100 is in a LOW TRACTION condition and, if it is, a first gain value is selected at block 2003. Alternatively if it is determined that the vehicle 100 is not in a LOW TRACTION condition, a second gain value that is smaller than the first gain value is selected at block 2004. As mentioned above, the first gain value may be -1 or close to -1 (e.g. at least -0.5) while the second gain value is comparatively small being the gain value that is selected when the vehicle 100 is determined to be in another state, such as its STANDARD condition.

Whichever gain value is selected, a proposed rear wheel steering angle is determined at block 2005 by multiplying the front wheel steering angle by the selected gain value. An output signal is then provided at block 2006 to cause steering of the rear wheels 103 at the proposed rear wheel steering angle.

An example of the process at block 2002 of determining whether the vehicle 100 is in a LOW TRACTION condition is illustrated in the flowchart of Fig. 13. At block 2101 of the process, it is determined whether torque from the powertrain of the vehicle 100 is being requested. If torque is being requested, it is determined at block 2102 whether a difference between a measured yaw rate of the vehicle 100 and a target yaw rate is greater than a threshold error value. If the difference is greater than the threshold error value then it is determined at block 2103 whether the front wheel steering angle is oscillating with a frequency that is greater than a threshold frequency. If the oscillation has a frequency that is greater than the threshold frequency then the vehicle 100 is determined to be in a LOW TRACTION condition and the first gain value is selected at block 2003.

If any of the determinations at blocks 2101 to 2103 provide a negative result the vehicle 100 is determined not to be in a LOW TRACTION condition and the second gain value is selected at block 2004.

For purposes of this disclosure, it is to be understood that the control means/controller(s) described herein can each comprise a control unit or computational device having one or more electronic processors. A vehicle and/or a system thereof may comprise a single control unit or electronic controller or alternatively different functions of the controller(s) may be embodied in, or hosted in, different control units or controllers. A set of instructions could be provided which, when executed, cause said controller(s) or control unit(s) to implement the control techniques described herein (including the described method(s)). The set of instructions may be embedded in one or more electronic processors, or alternatively, the set of instructions could be provided as software to be executed by one or more electronic processor(s). For example, a first controller may be implemented in software run on one or more electronic processors, and one or more other controllers may also be implemented in software run on or more electronic processors, optionally the same one or more processors as the first controller. It will be appreciated, however, that other arrangements are also useful, and therefore, the present disclosure is not intended to be limited to any particular arrangement. In any event, the set of instructions described above may be embedded in a computer-readable storage medium (e.g., a non-transitory computer-readable storage medium) that may comprise any mechanism for storing information in a form readable by a machine or electronic processors/computational device, including, without limitation: a magnetic storage medium (e.g., floppy diskette); optical storage medium (e.g., CD-ROM); magneto optical storage medium; read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g., EPROM or EEPROM); flash memory; or electrical or other types of medium for storing such information/instructions.

The blocks illustrated in the Figs. 11 to 13 may represent steps in a method and/or sections of code in the computer program 303. The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied. Furthermore, it may be possible for some steps to be omitted.

Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. For example, in alternative embodiments, the control means 105 may be configured to control steering of rear wheels 103 of the vehicle 100 in the LOW TRACTION condition, as described with reference to Figs. 7 to 13, but not in any, or only in selected ones, of the other special conditions. It will also be understood that the control means 105 may be configured to detect other special conditions, in addition to those described, and to control rear wheel steering in a manner that is customized for those other special conditions.

Features described in the preceding description may be used in combinations other than the combinations explicitly described.

Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not.

Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not.

Whilst endeavoring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

1. An apparatus for controlling steering of rear wheels of a vehicle, the apparatus comprising a control means configured to:

determine from received signals whether the vehicle is in a low traction condition; receive a first signal indicative of a front wheel steering angle;

determine a proposed rear wheel steering angle in dependence on the first signal and whether the vehicle is determined to be in a low traction condition; and provide an output signal configured to cause steering of the rear wheels at the proposed rear wheel steering angle.

2. An apparatus according to claim 1, wherein: the proposed rear wheel steering angle is the product of the front wheel steering angle and a gain value; the control means is configured to select a first gain value in dependence on the vehicle being in a low traction condition and to select a second gain value in dependence on the vehicle not being in a low traction condition; and the first gain value is greater than the second gain value.

3. An apparatus according to claim 1 or claim 2, wherein the control means is configured to determine that the vehicle is in a low traction condition in dependence on a set of criteria being met.

4. An apparatus according to claim 3, wherein one of the criteria comprises a determination that the front wheel steering angle is oscillating with a frequency above a threshold frequency.

5. An apparatus according to claim 3 or claim 4, wherein one of the criteria comprises an indication of torque being requested from a powertrain.

6. An apparatus according to any one of claims 3 to 5, wherein: the control means is configured to receive a signal indicative of a measured yaw rate of the vehicle, and to calculate a target yaw rate in dependence on the front wheel steering angle, the rear wheel steering angle and an indication of speed of the wheels; and one of the criteria comprises a difference between the measured yaw of the vehicle and the target yaw being greater than a threshold value.

7. An apparatus according to any one of claims 1 to 6, wherein the control means is configured to receive an indication of a selected mode, and, when the vehicle is determined to be in a low traction condition, the control means is configured to determine the proposed steering angles for the rear wheels in dependence on the selected mode.

8. An apparatus according to claim 7, wherein the selected mode is selected in response to a user input or in response to terrain sensor signals.

9. An apparatus according to any one of claims 1 to 8, wherein, when the vehicle is determined to be in a low traction condition, the control means is arranged to provide an output signal that is configured to cause steering of the rear wheels out of phase with steering of the front wheels.

10. An apparatus according to any one of claims 1 to 9, wherein the control means comprises an electronic memory device and having instructions stored therein; and an electronic processor electrically coupled to the electronic memory device and configured to access the electronic memory device and execute the instructions.

11. A system for controlling steering of rear wheels of a vehicle, the system comprising the apparatus of any one of claims 1 to 11 and at least one actuator for controlling a steering angle of the rear wheels of the vehicle in response to the output signal.

12. A system according to claim 11, wherein the system comprises sensing means configured to sense yaw rate of the vehicle.

13. A system according to claim 12, wherein the sensing means comprises a gyroscope.

14. A vehicle comprising the apparatus of any one of claims 1 to 10 or the system of any one of claims 11 to 13.

15. A vehicle according to claim 14 comprising a steering mechanism for controlling the steering angle of the front wheels of the vehicle comprising a manually operable device configured to enable adjustment of the steering mechanism.

16. A method of controlling steering of rear wheels of a vehicle with front wheel steer and rear wheel steer, the method comprising:

determining from received signals whether the vehicle is in a low traction condition; receiving a first signal indicative of a front wheel steering angle;

determining a proposed rear wheel steering angle in dependence on the first signal and whether the vehicle is determined to be in a low traction condition; and causing steering of the rear wheels at the proposed rear wheel steering angle.

17. A method according to claim 16, wherein: the proposed rear wheel steering angle is the product of the front wheel steering angle and a gain value; a first gain value is selected in dependence on a determination of the vehicle being in a low traction condition; a second gain value is selected in dependence on a determination of the vehicle not being in a low traction condition; and the first gain value is greater than the second gain value.

18. A method according to claim 16 or claim 17, wherein the vehicle is determined to be in a low traction condition in dependence on a set of criteria being met.

19. A method according to claim 18, wherein one of the criteria comprises a determination that the front wheel steering angle is oscillating with a frequency above a threshold frequency.

20. A method according to claim 18 or claim 19, wherein one of the criteria comprises an indication of torque being requested.

21. A method according to any one of claims 18 to 20, wherein: the method comprises receiving a signal indicative of a measured yaw rate of the vehicle, and calculating a target yaw rate in dependence on the front wheel steering angle, the rear wheel steering angle and a received indication of a speed of the wheels; and one of the criteria comprises a difference between the measured yaw of the vehicle and the target yaw being greater than a threshold value.

22. A method according to any one of claims 18 to 21, wherein the method comprises receiving an indication of a selected mode, and, determining the proposed steering angles for the rear wheels in dependence on the selected mode.

23. A method according to claim 22, wherein the selected mode is selected in response to a user input or in response to terrain sensor signals.

24. A method according to any one of claims 18 to 23, wherein, when the vehicle is 10 determined to be in a low traction condition, the method comprises causing steering of the rear wheels out of phase with steering of the front wheels.

25. A computer program which when executed by a processor causes the processor to perform the method according to any one of claims 16 to 24.