GB2574258A

GB2574258A - An apparatus and a method for controlling steering

Info

Publication number: GB2574258A
Application number: GB1809001.9A
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: 2019-12-04
Anticipated expiration: 2038-06-01
Also published as: GB201809001D0; GB2574258B

Abstract

A steering control device 104 for a vehicle 100 includes a control unit 105 receiving a first signal indicating a requested steering angle and a second signal indicating a roll angle. A proposed steering angle for steerable wheels 103, 106 is determined dependent on the first signal and an output provided to cause the steerable wheels to steer. The proposed steering angle may be dependent on the second signal. Typically, the apparatus may steer rear wheels 103 based on the roll angle, requested steering angle and/or a terrain mode; the requested steering angle may typically be a front wheel steering angle.

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

In some situations, it is desirable to drive a wheeled vehicle along a bank at a substantially constant height. For example, when driving on a terrain comprising sand dunes, good progress may be made by driving on a side of a dune. Depending upon the steepness of the bank and the composition of its surface, it may be necessary to steer up the bank to make forward progress along it.

It is known for some road vehicles to have four steerable wheels, which include rear wheels that are steered out of phase with the front wheels at low speeds, in order to make the vehicle more agile. However, such steering of the rear wheels can make it difficult to keep the vehicle on a straight path along the bank.

It is an aim of the present invention to address disadvantages of the prior art.

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 a vehicle, the apparatus comprising a control means configured to: receive a first signal indicative of a requested steering angle; receive a second signal indicative of a roll angle of the vehicle; determine a proposed steering angle for steerable wheels of the vehicle in dependence on the first signal; and provide an output signal configured to cause steering of the steerable wheels at the proposed steering angle; wherein, in dependence on a set of criteria being met, the control means is configured to determine the proposed steering angle in dependence on the second signal. This provides the advantage that it is easier for a driver to maintain a desired path along a bank, for example at a constant height up the bank.

In some embodiments the control means is configured to increase proposed steering angles in dependence on receiving second signals indicative of increasing roll angles. This provides the advantage that as the steepness of the bank varies, the control means is able to automatically compensate for the varying effect of the bank on the vehicle, and so it is easier to maintain a desired path along the bank.

In some embodiments the first signal is received from a steering input sensor configured to receive user requested steering angles.

In some embodiments the apparatus is configured to control steering of rear wheels of the vehicle.

In some embodiments the first signal is indicative of a front wheel steering angle and the proposed steering angle is a proposed rear wheel steering angle.

In some embodiments, in dependence on the criteria being met, the control means is configured to determine a non-zero rear wheel steering angle in dependence on the first signal indicating a front wheel steering angle of zero. This provides the advantage that the driver is able to maintain a straight path along a bank without the steering wheel being turned.

In some embodiments the non-zero rear wheel steering angle is arranged to steer the rear of the vehicle to the right in dependence on the roll angle being positive, corresponding to the left side of the vehicle being raised relative to the right side of the vehicle, and the non-zero rear wheel steering angle is arranged to steer the rear of the vehicle to the left in dependence on the roll angle being negative. This provides the advantage that the rear wheels steer the rear of the vehicle down the bank so that the vehicle is angled slightly up the bank and cause the vehicle to progress on a path along the bank.

In some embodiments, in dependence on the criteria being met, the control means is configured to decrease proposed steering angles for the rear steerable wheels in dependence on receiving first signals indicating increasing front wheel steering angles that are below a threshold angle. This provides the advantage that the rear wheel steering is able to smoothly transition into a standard mode to allow a driver to smoothly alter the course of the vehicle, for example, off the bank.

In some embodiments, in dependence on the criteria being met, the control means is configured to increase proposed steering angles for the rear steerable wheels in dependence on receiving first signals indicating increasing front wheel steering angles that are above the threshold angle. This provides the advantage that the control of the steering transitions to a more agile mode when steering angles are above the threshold angle.

In some embodiments, in dependence on one of the set of criteria not being met, the control means is configured to determine a zero rear wheel steering angle when the first signal indicates a front wheel steering angle of zero. This provides the advantage that unless all of the set of criteria are met, which indicates that the vehicle is traversing a bank, the rear wheel steering is able to operate in a conventional manner.

In some embodiments, in dependence on one of the set of criteria not being met, the control means is configured to increase proposed steering angles for the rear steerable wheels when received first signals indicate increasing front wheel steering angles that are above or below the threshold angle.

In some embodiments the control means is configured to receive a mode signal, and, when the criteria are met, the control means is configured to determine the proposed steering angles for the rear steerable wheels in dependence on the mode signal. This provides the advantage that the rear wheel steering may be controlled in a manner that is expected to most appropriately compensate for the effect of the bank on the vehicle. For example, when the vehicle is driven along a bank and the front wheels are steered at zero degrees, the rear wheels may be steered at a relatively large angle in a mode used for driving on sand and a relatively small angle, or zero degrees, in a mode used for driving on hard ground.

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

In some embodiments the control means is configured to determine a proposed steering angle for the rear steerable wheels that are the product of the front wheel steering angle and a gain value; the gain value depends on whether all of the criteria are met; and the control means is configured to cause a gradual transition between a first gain value and a second gain value in dependence on a change from one criteria not being met to all criteria being met. This provides the advantage that the steering automatically adjusts as the vehicle travels onto a bank in a smooth manner so that the user is easily able to maintain control of the vehicle’s direction.

In some embodiments the set of criteria comprises the second signal being indicative of a roll angle that is larger than a threshold angle.

In some embodiments the set of criteria comprises the roll angle having been continuously above the threshold angle for a defined period of time. This provides the advantage that the steering does rapidly fluctuate if the vehicle travels along a bank at angles that oscillate from above to below the threshold angle.

In some embodiments the control means is configured to: receive a speed signal indicative of current speed of the vehicle; and the set of criteria comprises the speed signal being indicative of a current speed below a threshold speed.

In some embodiments: the control means is configured to determine a current condition of the vehicle as one of a plurality of predefined conditions and determine a proposed rear wheel steering angle that depends on the current condition; and the predefined conditions comprise at least one of: a low traction condition; 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 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 wheels of the vehicle in response to the output signal.

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

According to a further aspect of the invention there is provided a method of controlling steering of a vehicle, the method comprising: determining a proposed steering angle for steerable wheels of the vehicle in dependence a requested steering angle; and providing an output signal configured to cause steering of the steerable wheels at the proposed steering angle; wherein, in dependence on a set of criteria being met, the proposed steering angle is determined in dependence on a roll angle of the vehicle. This provides the advantage that it is easier for a driver to maintain a desired path along a bank, for example at a constant height up the bank.

In some embodiments the method comprises increasing proposed steering angles in dependence on receiving second signals indicative of increasing roll angles. This provides the advantage that as the steepness of the bank varies, the varying effect of the bank on the vehicle is compensated for, and so it is easier to maintain a desired path along the bank.

In some embodiments the output signal is configured to control steering of rear wheels of the vehicle.

In some embodiments, in dependence on the criteria being met, the method comprises determining a non-zero rear wheel steering angle in dependence on the first signal indicating a front wheel steering angle of zero, and decreasing proposed steering angles for the rear steerable wheels in dependence on receiving first signals indicating increasing front wheel steering angles that are below a threshold angle. This provides the advantage that the driver is able to maintain a straight path along a bank without the steering wheel being turned, and to smoothly transition into a standard mode to allow a driver to smoothly alter the course of the vehicle, for example, off the bank.

In some embodiments one of the set of criteria is that the second signal is indicative of a roll angle that is larger than a threshold angle.

In some embodiments one of the set of criteria is that the roll angle has been continuously above the threshold angle for a defined period of time. This provides the advantage that the steering does rapidly fluctuate if the vehicle travels along a bank at angles that oscillate from above to below the threshold angle.

In some embodiments one of the set of criteria is that the current speed is below a threshold speed.

According to a further 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 another 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 travelling in a forward direction in its STANDARD condition; Fig. 8 shows a plan view of the vehicle travelling in a forward direction in its BANK condition; Fig. 9 shows a diagram illustrating the operation of the steering angle determination means and the state detection means when it determines that the vehicle is in its BANK state;

Fig. 10 shows an example of the operation of the steering angle determination means for such a vehicle with steer-by-wire front wheels;

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 in its BANK condition;

Fig. 12 shows a flowchart illustrating processes performed in the method of Fig. 11;

Fig. 13 shows a flowchart illustrating other processes performed in the method of Fig. 11; and Fig. 14 shows a flowchart illustrating operations performed within a process illustrated in Fig. 13.

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 (u>_y) 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. The vehicle state estimation means 401 may be 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 predefined BANK 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 BANK in Fig. 4 will now be described with reference to Figs. 7 to 14. Fig. 7 shows a plan view of the vehicle 100 travelling in a forward direction in its STANDARD condition. The front wheels 106 are oriented with a steering angle of zero degrees and consequently the rear wheels 103 are also oriented with a steering angle of zero degrees.

A plan view of the vehicle 100 is shown in Fig. 8 travelling in a forward direction in its BANK condition. In Fig. 8 the vehicle 100 is travelling along a bank 2301 with the left side of the vehicle 100 higher up the bank 2301 than the right side of the vehicle 100. The arrows 2302 in Fig. 8 indicate steepest directions down the bank 2301, and the vehicle 100 is travelling in a direction that is substantially perpendicular to the arrows 2302 to maintain a constant height up the bank 2301.

When travelling along a bank in this way, there is a tendency for gravity to have an effect on a vehicle, such that the front of a conventional vehicle is caused to slide lower down the bank than the rear of the vehicle and the vehicle is pulled down the bank. To compensate for this effect, an experienced driver may steer slightly up the bank in order to keep the vehicle moving in a desired direction along the bank. However, as shown in Fig. 8, the vehicle 100 provides steering of the rear wheels 103 so that the direction of the vehicle 100 is along the bank 2301 as desired by its driver, without the driver having to provide steering to compensate for the effect of the bank 2301. Specifically the rear wheels 103 are steered (in this example to the right) to direct the rear 2303 of the vehicle 100 lower down the bank 2301 than the front 2304 of the vehicle 100, while the front wheels 106 remain at a zero steering angle. Thus, the vehicle 100 is oriented so that its longitudinal axis is pointing slightly up the slope but, due to the effect of gravity, the vehicle 100 travels along the bank 2301 at a constant height, as desired.

The control means 105 is configured to automatically cause steering of the rear wheels 103 in this manner when it detects that the vehicle 100 is travelling on a bank 2301 that is sloped at an angle that is greater than a threshold angle. The magnitude of the angle of steering of the rear wheels 103, while the front wheels 106 are steered at zero degrees, depends upon how steep the bank 2301 is. It also depends on what driving mode is currently selected. For example, on a bank 2301 formed of sand, e.g. a side of a sand dune, the steering angles of the rear wheels 103 may be arranged to be relatively large to compensate for the relatively large effect that the bank 2301 has on the vehicle 100 when compared to the effect that a solid bank 2301 with a high friction surface would have.

While the vehicle 100 is travelling along a bank 2301, as shown in Fig. 8, if a steering input is received to turn the vehicle 100 either up or down the bank 2301, for example by a driver using the steering wheel 108, the front wheels 106 are steered in accordance with the steering input and the control means 105 reduces the angle of steer of the rear wheels 103 by a proportional amount. Thus, the rear wheel steering angle has a maximum angle when the front wheel steering angle is zero degrees. As the steering input causes the front wheel steering angle to increase further, the angle of steering of the rear wheels 103 is further proportionally reduced, until the rear wheel steering angle is zero when the front wheel steering angle is at a threshold steering angle. The threshold steering angle may be between about 0 and 15 degrees, depending on the bank angle. (It should be noted that this threshold angle is a road wheel angle of the front wheels, and not a steering wheel angle.) As the front wheel steering angle increases above the threshold angle, the rear wheel steering angle may be maintained at zero, until the steering angle of the front wheels 103 reaches a second, higher threshold angle, at which point the control means 105 reverts to the standard condition in which the rear wheel steering angle is proportional to the front wheel steering angle. For example, at relatively low speeds on a sand dune, the rear wheels 103 are steered out of phase with the front wheels 106 to provide the vehicle 100 with enhanced maneuverability, which may enable the driver of the vehicle 100 to steer off the bank 2301.

Operation of the steering angle determination means 403 and the state detection means 402 when it determines that the vehicle 100 is in its BANK state is illustrated in Fig. 9. The state detection means 402 receives an estimated roll angle (θ_χ) of the vehicle 100, from the vehicle state estimation means 401. The estimated roll angle (θ_χ) is calculated by the vehicle state estimation means 401 by sensor fusion of the measurements of rate of roll (ω_χ) and lateral acceleration (a_y) received from the IMU 305. The BANK condition is only determined by the state detection means 402 when the received roll angle (θ_χ) exceeds a threshold roll angle for more than a predefined period of time.

In the present embodiment the state detection means 402 also receives indications of the centripetal acceleration, longitudinal velocity (V_x) of the vehicle 100 and requested steering angle. The centripetal acceleration may be calculated by the vehicle state estimation means 401 from the rate of yaw (ω_ζ) and the longitudinal velocity (V_x), as is known. In the present embodiment, the state detection means 402 only determines that the vehicle 100 is in the BANK condition when the received roll angle (θ_χ) exceeds the threshold roll angle while the centripetal acceleration is below a threshold value for more than the predefined period of time. By requiring the centripetal acceleration to be below a threshold value enables the state detection means 402 to avoid identifying a roll angle caused by centripetal acceleration as a roll angle caused by the vehicle 100 being on a bank.

In the present embodiment, the state detection means 402 also only determines that the vehicle 100 is in the BANK condition when the current longitudinal velocity (or speed) is below a maximum speed threshold and the requested steering angle is smaller than a maximum steering angle.

When the BANK condition is determined by the state detection means 402, the steering angle determination means 403 determines a proposed rear wheel steering angle in dependence on the requested steering angle, the roll angle (θ_χ) and, in the present embodiment, the currently selected driving mode.

To achieve this, the received roll angle (θ_χ) is multiplied by a bank gain value to determine a bank steering angle 2401 at which the rear wheels 103 may be steered when the requested steering angle is zero. In the present embodiment, the bank gain value is selected in dependence on the currently selected driving mode. The bank gain value may be selected in dependence on the friction provided by the surface on which the vehicle 100 is travelling. Typically the selected bank gain value is relatively large when the driving mode is selected for travelling over a terrain formed of deformable material such as sand, and may be less when the terrain comprises a low friction surface such as grass, gravel or snow. For a solid high friction surface, such as tarmac, the bank gain value may be relatively very small. Alternatively, it may be a criterion for determination of the BANK condition by the state detection means 402 that the selected driving mode is not one that is selected for solid, high friction surfaces such as tarmac, i.e. For such a surface the vehicle 100 continues to remain in its STANDARD condition when travelling along banks.

The requested steering angle is multiplied by a correction coefficient to determine a correction angle 2402. The magnitude of the bank steering angle 2401 is then reduced by the correction angle 2402 to determine a corrected bank steering angle 2403. i.e. whether the requested steering angle is up or down the bank, the magnitude of the bank steering angle 2401 is reduced by an amount equal to the correction angle 2402.

When the vehicle 100 has been travelling along a bank for more than a short period of time, for example more than 2 seconds, the rear wheels 103 may be successfully steered at the corrected bank steering angle 2403. However, in order to provide stability to the vehicle 100 and enable the driver to easily keep control as the vehicle 100 drives onto a bank, the steering angle determination means 403 is configured to provide a smooth transition from the steering angle of the rear wheels 103 in the STANDARD condition and the steering angle of the rear wheels 103 in the BANK condition. To achieve this, when the BANK condition is detected, the steering angle determination means 403 continues to calculate a standard rear wheel steering angle 2404 by multiplying the requested steering angle by a standard gain value, i.e. as described above with reference to Figs. 5 and 6. The corrected bank steering angle 2403 and the standard rear wheel steering angle 2404 are then combined by a blend function 2405 to produce the proposed rear wheel steering angle.

The blend function 2405 is configured to produce the proposed rear wheel steering angle by adding a portion of the corrected bank steering angle 2403 to a portion of the standard rear wheel steering angle 2404 during an initial period following an indication that the BANK condition is detected. Over that initial period, the portion of the corrected bank steering angle 2403 is steadily increased with time from zero, while the portion of the standard rear wheel steering angle 2404 is steadily decreased with time down to zero. After the initial period the blend function 2405 determines the proposed rear wheel steering angle to be equal to the corrected bank steering angle 2403.

The steering angle determination means 403 provides output signals to the actuator 102 to cause the actuator 102 to steer the rear wheels 103 at the proposed rear wheel steering angle.

In an embodiment, such as that of Fig. 2, in which the vehicle 100 has steer-by-wire front wheels 106, the steering angle determination means 403 may be configured to automatically control steering of the front wheels 106 in dependence on the roll angle of the vehicle 100, in addition to, or instead of, controlling steering the rear wheels 103. In this embodiment, when the vehicle 100 is driven along a bank at a substantially constant height with one side of the vehicle 100 higher than the other side (i.e. so that the roll angle is not zero), the front wheels 106 are automatically steered to cause the front of the vehicle 100 to be raised higher on the bank than the rear of the vehicle 100, without the driver providing a steering input.

An example of the operation of the steering angle determination means 403 for such a vehicle 100 with steer-by-wire front wheels 106 is shown in Fig. 10. When the vehicle 100 is determined to be in its BANK condition, a bank steering angle 2501 is determined by multiplying the roll angle of the vehicle 100 by a front wheel bank gain value. A fraction of the bank steering angle 2501 is added to the requested steering angle by a blend function 2502 to produce the proposed front wheel steering angle. The fraction of the bank steering angle 2501 that is added by the blend function 2502 is steadily increased from zero to one over a predefined period, of about 1 second, following the time at which the BANK condition is detected. After that predefined period has elapsed, the whole of the bank steering angle is added to the requested steering angle.

It should be understood that the addition takes account of the sign (positive or negative) of the angles, so that a negative bank steering angle when added to a positive requested steering angle results in a proposed front wheel steering angle with a magnitude that is the difference in the magnitudes of the requested steering angle and the bank steering angle.

It will be appreciated that such automated steering of front wheels in response to detection of a BANK condition may also be applied to a vehicle having front wheels that are steer-by-wire and rear wheels that are not steerable.

A flowchart illustrating a method 2600 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 2601 of the method 2600, a first signal indicative of a requested steering angle is received, and at block 2602 a second signal indicative of a roll angle of the vehicle is received. At block 2603 it is determined whether a set of predefine criteria have been met, which indicate that the vehicle 100 is in a BANK condition. If not all of the criteria have been met, then a proposed steering angle for steerable wheels of the vehicle 100 is determined in dependence on the first signal and without regard to the second signal, at block 2605. Alternatively, if all the criteria are found to have been met at block 2603, indicating that the vehicle 100 is in a BANK condition, then a proposed steering angle for steerable wheels of the vehicle 100 is determined in dependence on the first signal (indicative of the requested steering angle) and the second signal (indicative of the roll angle of the vehicle) at block 2604. Following the process at block 2604 or block 2605, an output signal is provided at block 2606 that is configured to cause steering of the steerable wheels at the proposed steering angle. The method 2600 is repeatedly performed in order to control the steerable wheels of the vehicle 100 during travel.

The processes performed at block 2603 of the method 2600 are illustrated in the flowchart of Fig. 12. At block 2701 a first process of block 2603 is performed, in which it is determined whether the second signal is indicative of a roll angle that is larger than a threshold roll angle. If it is, at block 2702 it is determined whether the roll angle has been continuously above the threshold angle for a predefined period of time. If it has, it is determined at block 2703 whether the centripetal force on the vehicle 100 has been continuously less than a threshold value during the predefined period. If it has, it is determined if the speed (or longitudinal velocity) of the vehicle 100 is less than a threshold speed at block 2704. If it is, it is determined at block 2705 whether the requested steering angle is less than a threshold steering angle. If it is, then all criteria have been met at block 2603, i.e. the vehicle 100 is determined to be in the BANK condition, and consequently the process at block 2604 is performed.

If a negative result is determined at any one of blocks 2701 to 2705, indicating that at least one criterion of the set of criteria has not been met, then the process at block 2605 is performed.

The processes performed at block 2605 of the method 2600 are illustrated in the flowchart of Fig. 13, for a vehicle 100 in which the rear wheels 103 are automatically steered in dependence on the roll angle of the vehicle 100. At block 2801 a first process of block 2605 is performed, in which a bank gain value is determined in dependence on the currently selected driving mode of the vehicle 100. At block 2802 a bank steering angle 2401 is determined by multiplying the bank gain value by the roll angle of the vehicle 100. At block 2603 the requested steering angle, which was indicated by the received first signal, is multiplied by a steering correction coefficient to determine a correction angle. At block 2804, the bank steering angle, which was determined at block 2602, is then added to the correction angle, taking account of signs of the angles, to produce a corrected bank steering angle. Finally at block 2805, a proposed rear wheel steering angle is determined at block 2805 in dependence on the corrected bank steering angle.

The processes performed at block 2805 are illustrated in the flowchart shown in Fig. 14. Initially within block 2805, at block 2901, it is determined how much time, t, has elapsed since a BANK condition was first detected i.e. since all the criteria were first met at block 2603. At block 2902 it is determined whether the elapsed time, t, since the BANK condition was first determined is greater than a predefined blend period, te. If it is, then at block 2905 the proposed rear wheel steering angle is determined to be equal to the corrected bank steering angle that was produced at block 2804. Otherwise, at block 2903 a standard rear wheel steering angle is determined by multiplying the requested steering angle by a standard gain value, i.e. the standard rear wheel steering angle is determined in just the same way as the proposed rear wheel steering angle when the vehicle is in its STANDARD condition.

Having determined the standard rear wheel steering angle at block 2903, a proposed rear wheel steering angle is determined by a blend function at block 2904. The proposed rear wheel steering angle is calculated by multiplying the corrected bank steering angle by the fraction (t/te) of the blend period, te, that has elapsed and adding the result to the product produced by multiplying the standard rear wheel steering angle by the fraction ((te - t)/te) of the blend period, te, that remains.

After determining the proposed rear wheel steering angle at either block 2904 or 2905, the process at block 2606 is performed to complete the method 2600.

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 14 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 BANK condition, as described with reference to Figs. 7 to 14, 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 a vehicle, the apparatus comprising a control means configured to:

receive a first signal indicative of a requested steering angle;

receive a second signal indicative of a roll angle of the vehicle;

determine a proposed steering angle for steerable wheels of the vehicle in dependence on the first signal; and provide an output signal configured to cause steering of the steerable wheels at the proposed steering angle;

wherein, in dependence on a set of criteria being met, the control means is configured to determine the proposed steering angle in dependence on the second signal.

2. An apparatus according to claim 1, wherein the control means is configured to increase proposed steering angles in dependence on receiving second signals indicative of increasing roll angles.

3. An apparatus according to claim 1 or claim 2, wherein the first signal is received from a steering input sensor configured to receive user requested steering angles.

4. An apparatus according to any one of claims 1 to 3, wherein the apparatus is configured to control steering of rear wheels of the vehicle.

5. An apparatus according to claim 4, wherein the first signal is indicative of a front wheel steering angle and the proposed steering angle is a proposed rear wheel steering angle.

6. An apparatus according to claim 5, wherein, in dependence on the criteria being met, the control means is configured to determine a non-zero rear wheel steering angle in dependence on the first signal indicating a front wheel steering angle of zero.

7. An apparatus according to claim 6, wherein the non-zero rear wheel steering angle is arranged to steer the rear of the vehicle to the right in dependence on the roll angle being positive, corresponding to the left side of the vehicle being raised relative to the right side of the vehicle, and the non-zero rear wheel steering angle is arranged to steer the rear of the vehicle to the left in dependence on the roll angle being negative.

8. An apparatus according to claim 6 or claim 7, wherein, in dependence on the criteria being met, the control means is configured to decrease proposed steering angles for the rear steerable wheels in dependence on receiving first signals indicating increasing front wheel steering angles that are below a threshold angle.

9. An apparatus according to claim 8, wherein, in dependence on the criteria being met, the control means is configured to increase proposed steering angles for the rear steerable wheels in dependence on receiving first signals indicating increasing front wheel steering angles that are above the threshold angle.

10. An apparatus according to any one of claims 5 to 9, wherein the control means is configured to receive a mode signal generated in response to a user input or generated in response to terrain sensor signals, and, when the criteria are met, the control means is configured to determine the proposed steering angles for the rear steerable wheels in dependence on the mode signal.

11. An apparatus according to any one of claims 1 to 10, wherein the control means is configured to determine a proposed steering angle for the rear steerable wheels that are the product of the front wheel steering angle and a gain value; the gain value depends on whether all of the criteria are met; and the control means is configured to cause a gradual transition between a first gain value and a second gain value in dependence on a change from one criteria not being met to all criteria being met.

12. An apparatus according to any one of claims 1 to 11, wherein the set of criteria comprises the second signal being indicative of a roll angle that is larger than a threshold angle.

13. An apparatus according to any one of claims 1 to 12, wherein the set of criteria comprises the roll angle having been continuously above the threshold angle for a defined period of time.

14. An apparatus according to any one of claims 1 to 13, wherein the control means is configured to: receive a speed signal indicative of current speed of the vehicle; and the set of criteria comprises the speed signal being indicative of a current speed below a threshold speed.

15. An apparatus according to any one of claims 1 to 14, wherein: the control means is configured to determine a current condition of the vehicle as one of a plurality of predefined conditions and determine a proposed rear wheel steering angle that depends on the current condition; and the predefined conditions comprise at least one of: a low traction condition; moving backwards down an incline with a pitch angle above a threshold pitch angle.

16. A system for controlling steering of a vehicle, the system comprising the apparatus of any one of claims 1 to 15 and at least one actuator for controlling a steering angle of wheels of the vehicle in response to the output signal.

17. A vehicle comprising the apparatus of any one of claims 1 to 15 or the system of claim 16.

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

determining a proposed steering angle for steerable wheels of the vehicle in dependence a requested steering angle; and providing an output signal configured to cause steering of the steerable wheels at the proposed steering angle;

wherein, in dependence on a set of criteria being met, the proposed steering angle is determined in dependence on a roll angle of the vehicle.

19. A method according to claim 18, wherein the method comprises increasing proposed steering angles in dependence on receiving second signals indicative of increasing roll angles.

20. A method according to claim 18 or claim 19, wherein the output signal is configured to control steering of rear wheels of the vehicle.

21. A method according to claim 20, wherein the first signal is indicative of a front wheel steering angle and the proposed steering angle is a proposed rear wheel steering angle.

22. A method according to claim 21, wherein, in dependence on the criteria being met, the

5 method comprises determining a non-zero rear wheel steering angle in dependence on the first signal indicating a front wheel steering angle of zero, and decreasing proposed steering angles for the rear steerable wheels in dependence on receiving first signals indicating increasing front wheel steering angles that are below a threshold angle.

10

23. A method according to any one of claims 18 to 22, wherein one of the set of criteria is that the second signal is indicative of a roll angle that is larger than a threshold angle.

24. A method according to claim 23, wherein one of the set of criteria is that the roll angle has been continuously above the threshold angle for a defined period of time.

25. A computer program which when executed by a processor causes the processor to perform the method according to any one of claims 18 to 24.