GB2584587A

GB2584587A - Control system for a vehicle

Info

Publication number: GB2584587A
Application number: GB1815340.3A
Authority: GB
Inventors: Fairgrieve Andrew; John King Paul
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2018-09-20
Filing date: 2018-09-20
Publication date: 2020-12-16
Anticipated expiration: 2038-09-20
Also published as: GB201815340D0; DE102019212933A1; GB2584587B

Abstract

A control system 10 for a vehicle 100 has one or more controllers and has an autonomous driving mode. The control system is configured to, in the autonomous driving mode, obtain lateral acceleration data indicative of the lateral acceleration of the vehicle. The lateral acceleration data is used to limit a steering angle of one or more wheels 111, 112, 114, and 115 of the vehicle and a longitudinal speed of the vehicle in dependence on the lateral acceleration data. The vehicle lateral acceleration data may be known to the controller either from a lateral acceleration sensor of the vehicle, or by obtaining the product of the longitudinal speed and a yaw rate of the vehicle. The control system may include a low-speed progress (LSP) control system 12, a stability control system (SCS) 14, a cruise control system 16, a hill descent control (HDC) system 12HD, an anti-lock braking system controller (ABS) 13, etc, and the lateral acceleration data may be obtained by sensors linked to at least one of these systems.

Description

CONTROL SYSTEM FOR A VEHICLE

TECHNICAL FIELD

The present disclosure relates to a control system and particularly, but not exclusively, to a control system for a vehicle. Aspects of the invention relate to a control system, a method, a vehicle, a computer program product and a non-transitory computer readable medium.

BACKGROUND

When a vehicle goes into understeer, the grip limit of the tyres has been reached and it is no longer possible to increase lateral acceleration of the vehicle. The vehicle will then follow a path with a radius which is larger than intended. In unstructured road environments (e.g. off road) there is an increased risk of understeer due to reduced traction.

It is an object of embodiments of the invention to at least mitigate one or more of the problems

of the prior art.

SUMMARY OF THE INVENTION

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

According to an aspect of the invention, there is provided a control system for autonomously inhibiting understeer in a vehicle.

According to another aspect of the invention, there is provided a control system for autonomously inhibiting understeer in a vehicle by limiting a steering angle of one or more wheels of the vehicle and limiting the longitudinal speed of the vehicle.

According to another aspect of the invention, there is provided a control system for a vehicle, the control system comprising one or more controllers and having an autonomous driving mode, the control system being configured to, in the autonomous driving mode: obtain lateral acceleration data indicative of a (e.g. current) lateral acceleration of the vehicle; and limit a steering angle of one or more wheels of the vehicle and a longitudinal speed of the vehicle in dependence on the lateral acceleration data.

By limiting a steering angle and a longitudinal speed of the vehicle in dependence on the lateral acceleration data, the control system can reactively inhibit (e.g. reduce) understeer of the vehicle, thereby improving the ride comfort, stability and controllability of the vehicle. This is of particular value when the vehicle is travelling on an unstructured terrain, such as off-road terrain, where the vehicle is more at risk of understeer than for example on a structured highway, for example because the vehicle may be travelling on a surface having a low coefficient of friction.

It may be that the control system is configured to control the longitudinal speed of the vehicle and the steering angle of the vehicle in the autonomous driving mode. It may be that the autonomous driving mode is a driving mode having level 1, 2, 3, 4 or 5 autonomy (e.g. level 2 autonomy). It may be that the control system is configured to limit the steering angle of the one or more wheels and the longitudinal speed of the vehicle in dependence on the lateral acceleration data in an autonomous off-road driving mode, such as an autonomous low-speed cruise control driving mode or in both an autonomous low-speed cruise control driving mode and an off-road driving mode.

It may be that the control system is configured to limit the longitudinal speed of the vehicle and the steering angle of one or more wheels of the vehicle respectively in dependence on the lateral acceleration data to thereby inhibit (e.g. reduce or prevent) understeer of the vehicle.

It may be that the control system is configured to limit the longitudinal speed of the vehicle and the steering angle of one or more wheels of the vehicle respectively in dependence on the lateral acceleration data to thereby limit the lateral acceleration of the vehicle based on a user comfort preference.

It may be that the vehicle comprises one or more lateral acceleration sensors (e.g. accelerometer or gyroscope) configured to measure the lateral acceleration of the vehicle. It may be that the control system is configured to obtain the lateral acceleration data from the one or more lateral acceleration sensors. Additionally or alternatively the vehicle may comprise one or more vehicle speed sensors configured to measure a longitudinal speed of the vehicle and one or more yaw rate sensors configured to measure a yaw rate of the vehicle. In this case, it may be that the control system is configured to obtain longitudinal speed data indicative of a longitudinal speed of the vehicle from one or more longitudinal speed sensors of the vehicle and yaw rate data indicative of a yaw rate of the vehicle from one or more yaw rate sensors of the vehicle, and to determine the lateral acceleration data from the longitudinal speed data and the yaw rate data. For example, it may be that the lateral acceleration data comprises the longitudinal speed data and the yaw rate data or a measure of the lateral acceleration of the vehicle derived therefrom.

It may be that the at least one controller collectively comprises: at least one electronic processor having an input for receiving the lateral acceleration data; and at least one electronic memory device electrically coupled to the at least one electronic processor and having instructions stored therein, wherein the at least one electronic processor is configured to access the at least one memory device and execute the instructions thereon to limit the steering angle and the longitudinal speed of the vehicle in dependence on the lateral acceleration data.

It may be that the at least one controller is configured to limit a steering angle of one or more wheels of the vehicle by way of one or more steering angle command signals. It may be that the at least one controller is configured to limit the steering angle of one or more wheels of the vehicle by sending one or more command signals to thereby limit a rotational range of a steering wheel of the vehicle. Alternatively it may be that the at least one controller is configured to limit the steering angle of one or more wheels of the vehicle by sending one or more command signals to a drive-by-wire controller of the vehicle.

It may be that the control system is configured to limit the steering angle of one or more wheels of the vehicle based on a first steering angle limit, the first steering angle limit being determined in dependence on the lateral acceleration data.

It may be that the first steering angle limit is determined based on a predetermined relationship between the steering angle of one or more wheels of the vehicle and an acceleration of the vehicle, the said acceleration of the vehicle being dependent on the lateral acceleration of the vehicle. Thus, the control system can determine a steering angle limit which is appropriate for the lateral acceleration of the vehicle, for example to inhibit understeer of the vehicle.

It may be that the acceleration of the vehicle is the lateral acceleration of the vehicle.

It may be that the acceleration of the vehicle is a resultant of the lateral acceleration and a longitudinal acceleration of the vehicle. This helps to ensure that the first limit is not unduly low, enabling the vehicle to maximise its available grip. In this case, it may be that the control system is configured to obtain longitudinal acceleration data indicative of a longitudinal acceleration of the vehicle, e.g. from one or more longitudinal acceleration sensors or one or more longitudinal speed sensors of the vehicle. It may be that the control system is configured to determine the said acceleration of the vehicle by determining a resultant of the lateral and longitudinal acceleration of the vehicle based on the longitudinal and lateral acceleration data.

It may be that the first steering angle limit is dependent on the longitudinal speed of the vehicle.

The control system may be configured to obtain longitudinal speed data indicative of a longitudinal speed of the vehicle from one or more longitudinal speed sensors of the vehicle, and to determine the first steering angle limit in dependence on the longitudinal speed data.

It may be that the first steering angle limit is dependent on the yaw rate of the vehicle. The control system may be configured to obtain yaw rate data indicative of a yaw rate of the vehicle from one or more yaw rate sensors of the vehicle and to determine the first steering angle limit in dependence on the yaw rate data.

It may be that the first steering angle limit is dependent on a (e.g. fixed) calibration factor. It may be that the calibration factor is predetermined, for example empirically. This helps to ensure that the first steering angle limit is not applied during normal driving of the vehicle.

It may be that the control system is configured to limit the steering angle of one or more wheels of the vehicle based on a second steering angle limit, the second steering angle limit being based on a predetermined lateral acceleration limit.

By determining a second steering angle limit based on a predetermined lateral acceleration limit, a failsafe steering angle limit can be provided which ensures that extreme steering angles are not implemented by the control system. It may be that the predetermined lateral acceleration limit is based on a user comfort preference. This helps to ensure that not only is the vehicle driven safely, but that it also provides a comfortable ride for passengers.

It may be that the user comfort preference is a predefined user comfort preference associated with a driving mode of the vehicle.

It may be that the predetermined lateral acceleration limit is a fixed predetermined lateral acceleration limit.

It may be that the second steering angle limit is additionally based on a longitudinal speed of the vehicle. This helps to ensure that the second steering angle limit is appropriate for the longitudinal speed of the vehicle.

It may be that the control system is configured to limit the steering angle of one or more wheels of the vehicle to the lower of the first and second steering angle limits. By limiting the steering angle to the lower of the first and second steering angle limits, it can be better ensured that the vehicle can be controlled responsively and that the ride comfort is in accordance with user preferences.

It may be that the control system is configured to limit the longitudinal speed of the vehicle based on a first longitudinal speed limit, the first longitudinal speed limit being dependent on the lateral acceleration data. It may be that the first longitudinal speed limit is dependent on a target steering angle relating to one or more wheels of the vehicle. By basing the first longitudinal speed limit on the lateral acceleration data and on a target steering angle, a longitudinal speed limit appropriate to a path being followed, and the lateral acceleration being experienced, by the vehicle can be determined. This enables the control system to reactively limit the speed of the vehicle to inhibit understeer.

It may be that the control system is configured to determine the first longitudinal speed limit in dependence on an acceleration of the vehicle, the acceleration being dependent on the lateral acceleration of the vehicle. It may be that the acceleration is the lateral acceleration of the vehicle or the resultant of lateral and longitudinal accelerations of the vehicle.

It may be that the target steering angle is a target steering angle required to steer the vehicle onto a target path. It may be that the control system is configured to adjust the target steering angle in accordance with one or more steering angle limits (e.g. the first or second steering angle limit). It may be that the first longitudinal speed limit is determined in dependence on the target steering angle prior to said adjusting. This provides the best estimate of the steering angle required for the said one or more wheels to steer the vehicle onto a target path.

It may be that the control system is configured to limit the longitudinal speed of the vehicle in dependence on the vehicle being in understeer. By limiting the longitudinal speed of the vehicle in dependence on it being in understeer, it can be ensured that the longitudinal speed of the vehicle is not unduly limited.

It may be that the control system is configured to limit the longitudinal speed of the vehicle in dependence on the vehicle being in understeer by limiting the longitudinal speed of the vehicle based on the first longitudinal speed limit. It may be that the first longitudinal speed limit is further dependent on one or more calibration factors.

It may be that the one or more calibration factors comprise a first calibration factor dependent on a difference between the target steering angle of one or more wheels of the vehicle and an actual steering angle applied to the one or more wheels of the vehicle determined in dependence on a steering angle limit (e.g. the first or second steering angle limit, or the lower of the first and second steering angle limits). It may be that as the difference between the demand steering angle and the actual steering angle increases, the significance of the first calibration factor in the determination of the first longitudinal speed limit of the vehicle decreases (and vice versa).

It may be that the one or more calibration factors comprise a second calibration factor dependent on a longitudinal speed of the vehicle. It may be that the second calibration factor accounts for yaw rate sensor noise at the said longitudinal speed of the vehicle, which is typically of greater significance at low longitudinal speeds of the vehicle. Thus, by determining the second calibration factor dependent on a longitudinal speed of the vehicle, it can be ensured that low speed yaw rate sensor measurement noise does not limit the longitudinal speed of the vehicle. It may be that as the longitudinal speed of the vehicle increases, the significance of the second calibration factor in the determination of the first longitudinal speed limit decreases (and vice versa).

It may be that the control system is configured to limit the longitudinal speed of the vehicle in dependence on a second longitudinal speed limit, the second longitudinal speed limit being based on a lateral acceleration limit.

It may be that the lateral acceleration limit is a fixed lateral acceleration limit. It may be that the lateral acceleration limit is based on a user comfort preference. It may be that the user comfort preference is a predefined user comfort preference associated with a driving mode of the vehicle.

It may be that the second longitudinal speed limit is dependent on a (e.g. current) steering angle of one or more wheels of the vehicle. The control system may be configured to obtain steering angle data relating to a (e.g. current) steering angle of one or more wheels of the vehicle, for example from one or more steering angle sensors of the vehicle, and to determine the second longitudinal speed limit in dependence thereon.

It may be that the control system is configured to limit the longitudinal speed of the vehicle to the lower of the first and second longitudinal speed limits. By limiting the longitudinal speed of the vehicle to the lower of the first and second longitudinal speed limits, it can be better ensured that the responsive control of the vehicle is achieved, together with a comfortable ride for passengers.

It may be that the control system is configured to determine a future path for the vehicle. It may be that the control system is configured to control the longitudinal speed of the vehicle in dependence on the future path, for example to pre-emptively inhibit understeer of the vehicle. The control system may be configured to determine a desired longitudinal speed of the vehicle in dependence on the curvature of the said path, and to control the longitudinal speed of the vehicle based on the desired longitudinal speed. By determining a future path for the vehicle and controlling the longitudinal speed of the vehicle in dependence on the future path, the vehicle speed can be controlled pre-emptively, for example prior to reaching a corner or curve in the path. This provides better control of the vehicle, helping the vehicle to follow the future path more closely and with lower lateral acceleration (or a lower resultant of lateral and longitudinal acceleration of the vehicle).

It may be that the control system is configured to: obtain environment data from one or more environment sensors of the vehicle; and determine the future path for the vehicle in dependence on the environment data.

It may be that the control system is configured to send one or more command signals to control torque applied to one or more wheels of the vehicle and/or to send one or more command signals to control braking of one or more wheels of the vehicle to thereby control a longitudinal speed of the vehicle in dependence on the determined vehicle path.

It may be that the control system is configured to: determine a future longitudinal speed for the vehicle in dependence on the said future path (e.g. on the curvature of the future path) and on one or more lateral acceleration limits; and control the longitudinal speed of the vehicle in dependence on the determined future longitudinal speed. This helps to ensure that the longitudinal speed of the vehicle is controlled to a level appropriate to the future path. It may be that the one or more lateral acceleration limits are based on a user comfort preference. It may be that the one or more lateral acceleration limits comprise a fixed lateral acceleration limit, which may be based on a user comfort preference, and a pre-emptive lateral acceleration limit based on a current speed of the vehicle, a desired speed of the vehicle based on the future vehicle path (e.g. on the curvature of the future path) and a desired deceleration distance. It may be that the control system is configured to determine the future longitudinal speed for the vehicle in dependence on the said future path (e.g. on the curvature of the future path) and on the lower of the fixed lateral acceleration limit and the pre-emptive lateral acceleration limit. This helps to ensure that the longitudinal speed of the vehicle is controlled to a level appropriate to provide a comfortable ride in the vehicle along the future path.

It may be that the functionality of the control system is performed by the at least one controller.

It may be that the at least one controller is implemented in hardware, software, firmware or any combination thereof. It may be that the at least one controller comprises one or more electronic processors. It may be that one or more or each of the electronic processors are hardware processors. It may be that the at least one controller comprises or consists of an electronic control unit.

According to another aspect of the invention, there is provided a vehicle comprising a control system as described herein.

According to another aspect of the invention, there is provided a method for operating a vehicle in an autonomous driving mode, the method comprising: obtaining lateral acceleration data indicative of a lateral acceleration of the vehicle; and limiting a steering angle of one or more wheels of the vehicle and a longitudinal speed of the vehicle in dependence on the lateral acceleration data.

It may be that the method comprises limiting the steering angle of one or more wheels of the vehicle based on a first steering angle limit, the first steering angle limit being determined in dependence on the lateral acceleration data.

It may be that the method comprises determining the first steering angle limit based on a predetermined relationship between the steering angle of one or more wheels of the vehicle and an acceleration of the vehicle, the said acceleration of the vehicle being dependent on the lateral acceleration of the vehicle.

It may be that the method comprises limiting the steering angle of one or more wheels of the vehicle based on a second steering angle limit, the second steering angle limit being based on a predetermined lateral acceleration limit.

It may be that the method comprises limiting the steering angle of one or more wheels of the vehicle to the lower of the first and second steering angle limits.

It may be that the method comprises limiting the longitudinal speed of the vehicle based on a first longitudinal speed limit, the first longitudinal speed limit being dependent on the lateral acceleration data and on a target steering angle relating to one or more wheels of the vehicle.

It may be that the method comprises determining the first longitudinal speed limit in dependence on a resultant of the lateral acceleration and a longitudinal acceleration of the vehicle.

It may be that the method comprises limiting the longitudinal speed of the vehicle in dependence on the vehicle being in understeer.

It may be that the method comprises limiting the longitudinal speed of the vehicle in dependence on the vehicle being in understeer by limiting the longitudinal speed of the vehicle based on the first longitudinal speed limit, wherein the first longitudinal speed limit is further dependent on one or more calibration factors.

It may be that the method comprises limiting the longitudinal speed of the vehicle in dependence on a second longitudinal speed limit, the second longitudinal speed limit being based on a lateral acceleration limit. It may be that the lateral acceleration limit is based on a user comfort preference.

It may be that the method comprises limiting the longitudinal speed of the vehicle to the lower of the first and second longitudinal speed limits.

It may be that the method comprises: determining a future path for the vehicle; and controlling the longitudinal speed of the vehicle in dependence on the future path.

It may be that the method comprises: obtaining environment data from one or more environment sensors of the vehicle; and determining the future path for the vehicle in dependence on the environment data.

It may be that the method comprises sending one or more command signals to control torque applied to one or more wheels of the vehicle and/or sending one or more command signals to control braking of one or more wheels of the vehicle to thereby control a longitudinal speed of the vehicle in dependence on the determined vehicle path.

It may be that the method comprises: determining a future longitudinal speed for the vehicle in dependence on the said future path and on one or more lateral acceleration limits; and controlling the longitudinal speed of the vehicle in dependence on the determined future longitudinal speed.

According to another aspect of the invention, there is provided a computer program product comprising executable instructions that, when executed by a computer, cause performance of the method described herein.

According to another aspect of the invention, there is provided a non-transitory computer readable medium comprising computer executable instructions that, when executed by a computer, cause performance of the method as described herein.

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

Within the scope of this application it is expressly 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 or aspect 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 schematic illustration of a vehicle in plan view; Fig. 2 shows the vehicle of Fig. 1 in side view; Fig. 3 is a high level schematic diagram of the vehicle speed control system of the vehicle of Figs 1 and 2, including a cruise control system and a low-speed progress control system; Fig. 4 illustrates a steering wheel of the vehicle of FIGs. 1, 2; Fig. 5 is a flow chart illustrating operation of a control system of the vehicle of FIGs. 1, 2; Fig. 6 illustrates the manner in which a colour and/or texture descriptor pi may be generated; Fig. 7 is a close up schematic view of terrain ahead of the vehicle together with a cost map overlaid thereon; Fig. 8 shows the view and cost map of Fig. 7 with a plurality of candidate trajectories overlaid 25 thereon; Fig. 9 shows the relationship between turning radius, R, wheelbase length, L, and steering angle 6, based on the Ackerman steering model; Fig. 10 shows the resulting force on a tyre of the vehicle of Fig. 1 caused by a combination of lateral and longitudinal accelerations; Fig. 11 shows a plot of steering angle of one or more wheels of the vehicle of Fig. 1 versus resultant acceleration; Fig. 12 is a plot of a first lateral acceleration calibration factor versus the difference between a target and an actual steering angle applied to one or more wheels of the vehicle of Fig 1; Fig. 13 is a plot of a second lateral acceleration calibration factor versus the speed of the vehicle of Fig. 1; and Figs. 14-21 each show various empirically obtained plots relating to the vehicle of Fig. 1 following a sinusoidal vehicle path in an autonomous mode under different control conditions.

DETAILED DESCRIPTION

FIGs. 1 and 2 show a vehicle 100 having wheels 111, 112, 114, 115, each of which is fitted with a respective tyre, and a body 116 carried by the wheels 111, 112, 114, 115. The vehicle 100 has a powertrain 129 that includes an engine 121 that is connected to a driveline 130 having an automatic transmission 124. A control system for the vehicle engine 121 includes a central controller, referred to as a vehicle control unit (VCU) 10, a powertrain controller 11, a brake controller 13 (an anti-lock braking system (ABS) controller) and a steering controller 1700. The ABS controller 13 forms part of a braking system 22 (Fig. 3). Each of the controllers 10, 11, 13, 170 comprises one or more electronic processors and a memory device storing computer program instructions, the one or more electronic processors being configured to access the respective memory device and to execute the computer program instructions stored therein to thereby perform the functionality attributed to that controller 10, 11, 13, 170.

The VCU 10 may receive and output a plurality of signals to and from various sensors and subsystems (not shown) provided on the vehicle. Referring to Fig. 3, the VCU 10 may include a low-speed progress (LSP) control system 12, a stability control system (SOS) 14, a cruise control system 16 and a hill descent control (HDC) system 12HD. The SOS 14 improves the safety of the vehicle 100 by detecting and managing loss of traction or steering control. When a reduction in traction or steering control is detected, the SOS 14 is operable automatically to command the ABS controller 13 to apply one or more brakes of the vehicle to help to steer the vehicle 100 in the direction the user wishes to travel. Although the SOS 14 is implemented by the VCU 10 in this case, the SOS 14 may alternatively be implemented by the ABS controller 13.

The cruise control system 16 may be operable to automatically maintain vehicle speed at a selected speed when the vehicle is travelling at speeds in excess of 25 kph. The cruise control system 16 may be provided with a cruise control HMI (human machine interface) 18 by which means the user can input a target vehicle speed to the cruise control system 16. For example, the cruise control system input controls may be mounted to a steering wheel 171. This is illustrated in Fig. 4. The cruise control system 16 monitors vehicle speed and any deviation from the target vehicle speed is adjusted automatically so that the vehicle speed is maintained at a substantially constant value, typically in excess of 25 kph. The cruise control system 16 is not effective at speeds lower than 25 kph. The cruise control HMI 18 may be configured to provide an alert to the user about the status of the cruise control system 16 via a visual display of the HMI 18.

The LSP control system 12 may also provide a speed-based control system for the user which enables the user to select a very low target speed at which the vehicle can progress without any pedal inputs being required by the user to maintain vehicle speed. It may be that low-speed speed control (or progress control) functionality is not provided by the on-highway cruise control system 16 which may operate only at speeds above 25 kph. The LSP control system 12 may be activated by pressing LSP control system selector button 178 mounted on steering wheel 171. The LSP system 12 is operable to apply selective powertrain, traction control and braking actions to one or more wheels 111, 112, 114, 115 of the vehicle 100, collectively or individually.

The LSP control system 12 is configured to allow a user to input a desired value of vehicle target speed in the form of a set-speed parameter, user set-speed, via a low-speed progress control HMI (LSP HMI) 20 (Fig. 1, Fig. 3) which shares certain input buttons 173-175 with the cruise control system 16 and HDC control system 12HD (Fig. 4). Provided the vehicle speed is within the allowable range of operation of the LSP control system 12 (which may be the range from 2 to 30kph although other ranges may be provided) and no other constraint on vehicle speed exists whilst under the control of the LSP control system 12, the LSP control system 12 controls vehicle speed in accordance with a LSP control system set-speed value LSP set-speed which is set substantially equal to user setspeed. The LSP HMI 20 also includes a visual display by means of which information and guidance can be provided to the user about the status of the LSP control system 12.

The LSP control system 12 may receive an input from the ABS controller 13 of the braking system 22 of the vehicle indicative of the extent to which the user has applied braking by means of the brake pedal 163. The LSP control system 12 may also receive an input from an accelerator pedal 161 indicative of the extent to which the user has depressed the accelerator pedal 161, and an input from the transmission or gearbox 124. Other inputs to the LSP control system 12 may include an input from the cruise control HMI 18 which is representative of the status (ON/OFF) of the cruise control system 16, an input from the LSP control HMI 20, and an input from a gradient sensor 45 indicative of the gradient of the driving surface over which the vehicle 100 is driving. In the present embodiment the gradient sensor 45 is a gyroscopic sensor. Alternatively the [SR control system 12 may receive a signal indicative of driving surface gradient from another controller such as the ABS controller 13. The ABS controller 13 may determine gradient based on a plurality of inputs, optionally based at least in part on signals indicative of vehicle longitudinal and lateral acceleration and a signal indicative of vehicle reference speed (v actual) being a signal indicative of actual vehicle speed over ground. The vehicle reference speed may be determined to be the speed of the second slowest turning wheel, or the average speed of all the wheels. Other ways of calculating vehicle reference speed may be used including by means of a camera device or radar sensor.

The VCU 10 may be configured to implement a Terrain Response (TR) (RTM) System in which the VCU 10 controls settings of one or more vehicle systems or sub-systems such as the powertrain controller 11 in dependence on a selected driving mode. The driving mode may be selected by a user by means of a driving mode selector 141S (Fig. 1) or it may be determined automatically by the VCU 10. The driving modes may also be referred to as terrain modes, terrain response (TR) modes, or control modes. Five driving modes may be provided such as: an 'on-highway' driving mode suitable for driving on a relatively hard, smooth driving surface where a relatively high surface coefficient of friction exists between the driving surface and wheels of the vehicle; a 'sand' driving mode suitable for driving over sandy terrain, being terrain characterised at least in part by relatively high drag, relatively high deformability or compliance and relatively low surface coefficient of friction; a 'grass, gravel or snow' (GGS) driving mode suitable for driving over grass, gravel or snow, being relatively slippery surfaces (i.e. having a relatively low coefficient of friction between surface and wheel and, typically, lower drag than sand); a 'rock crawl' (RC) driving mode suitable for driving slowly over a rocky surface; and a 'mud and ruts' (MR) driving mode suitable for driving in muddy, rutted terrain.

The latter four driving modes may be considered to be off-road driving modes.

In order to cause application of the necessary positive or negative torque to the wheels, the VCU 10 may command that positive or negative torque is applied to the vehicle wheels by the powertrain 129 and/or that a braking force is applied to the vehicle wheels by the braking system 22, either or both of which may be used to implement the change in torque that is necessary to attain and maintain a required vehicle speed.

The vehicle 100 may be provided with additional sensors (not shown) which are representative of a variety of different parameters associated with vehicle motion and status. These may be inertial systems unique to the LSP or HDC control systems 12, 12HD or part of an occupant restraint system or any other sub-system which may provide data from sensors such as gyros and/or accelerometers that may be indicative of vehicle body movement and may provide a useful input to the [SR and/or HDC control systems 12, 12HD. The signals from the sensors provide, or are used to calculate, a plurality of driving condition indicators (also referred to as terrain indicators) which are indicative of the nature of the terrain conditions over which the vehicle 100 is travelling. The sensors (not shown) of the vehicle 100 may include, but are not limited to, sensors which provide continuous sensor outputs to the VCU 10, including any one or more of: wheel speed sensors; an ambient temperature sensor; an atmospheric pressure sensor; tyre pressure sensors; wheel articulation sensors; gyroscopic sensors to detect vehicular yaw, roll and pitch angle and rate; a vehicle speed sensor; a longitudinal acceleration sensor; an engine torque sensor (or engine torque estimator); a steering angle sensor; a steering wheel speed sensor; a gradient sensor (or gradient estimator); a lateral acceleration sensor which may be part of the SOS 14; a brake pedal position sensor; a brake pressure sensor; an accelerator pedal position sensor; longitudinal, lateral and vertical motion sensors; water detection sensors forming part of a vehicle wading assistance system (not shown). The vehicle 100 may further comprise a location sensor, such as a satellite positioning system (e.g. Global Positioning System (GPS), Galileo or GLONASS) receiver configured to receive signals from a plurality of satellites to determine the location of the vehicle.

The vehicle 100 may be provided with a stereoscopic camera system 1850 configured to generate stereo colour image pairs by means of a pair of forward-facing colour video cameras comprised by the system 1850. The system 1850 may further comprise one or more electronic processors and a memory device storing computer program instructions, the one or more electronic processors being configured to access the respective memory device and to execute the computer program instructions stored therein. A stream of dual video image data may be fed from the cameras to the one or more processors of the system 1850 which access and execute instructions stored in the memory of the said system 1850 to process the image data and repeatedly generate a 3D point cloud data set based on the images received. Alternatively the images may be obtained and processed by any processing system of the vehicle 100, such as the VCU 10. Each point in the 3D point cloud data set corresponds to a 3D coordinate of a point on a surface of terrain ahead of the vehicle 100 viewed by each of the forward-facing video cameras of the stereoscopic camera system 1850.

The [SF control system 12 may have an autonomous driving mode in which the VCU 10 controls the steering and speed of the vehicle autonomously. In this case, the [SR HMI 20 may allow the driver to select the autonomous driving mode. The autonomous driving mode may be have a level of automation of level 1 or above by the SAE International standard. It may be that the autonomous mode has level 2 autonomy, that is: the automated system takes full control of the vehicle (accelerating, braking, and steering); the driver must monitor the driving and be prepared to intervene immediately at any time if the automated system fails to respond properly. Thus, the speed of the vehicle 100 set by the user in the LSP mode may be overridden (typically reduced) by the VCU 10, for example, if it is inappropriate for driving conditions (e.g. if there are obstacles in the path, or if the set speed is inappropriate for the curvature of the vehicle path).

It may be that the autonomous LSP driving mode is particularly suitable for controlling the vehicle off-road, where road markings and road signs are either absent or sparsely provided.

Accordingly, as well as operating in the autonomous LSP driving mode, it may be that the vehicle is also operating in one of the off-road TR driving modes. However, the autonomous LSP driving mode may also be suitable for controlling the vehicle on road.

As will be explained below with reference to Fig. 5, when the vehicle is operating in the autonomous LSP driving mode, the stereoscopic camera system 1850 may be configured to use the colour image pairs to determine cost data relating to the terrain and to provide the cost data to the VCU 10. The VCU 10 may then determine a future path of the vehicle 100 in dependence on the cost data and control the vehicle 100 in accordance with the determined path. For example, the steering angle of one or more wheels 111, 112, 114, 115 of the vehicle 100 may be controlled by the VCU 10 outputting a steering angle control signal to the steering controller 1700 dependent on the curvature of the determined path. The VCU 10 may store in an electronic memory thereof a look-up table of predetermined maximum allowable vehicle speeds for different path curvatures, and the VCU 10 may select an appropriate vehicle speed from the look-up table in dependence on the curvature of the determined path, and output this speed to the LSP control system 12 to override the user setspeed.

A method for determining a future path of the vehicle to traverse (e.g. off-road) terrain based on image data from the stereoscopic camera system 1850 will now be explained with reference to Fig. 5. At 500, a stereo colour image pair may be captured by the stereoscopic camera system 1850 and RGB image data from the stereo colour image pair is stored in a memory accessible to the one or more electronic processors of the stereoscopic camera system 1850. At 502a, the stereoscopic camera system 1850 may select the image data of a first image of the image pair (which is a 2D colour image) and at 504a may convert the selected image from the RGB colour space to the LAB colour space (although 504a is not essential, and other colour spaces such as RGB or HSV may alternatively be used). At 502b, the camera system 1850 may compare the image data of the first and second images of the image pair to thereby determine a disparity image.

At 504b, the stereoscopic camera system 1850 may calculate a real-world 3D point cloud based on the disparity image. The 3D point cloud may initially be related to a frame of reference of the camera system 1850, but may then be translated to a frame of reference of the vehicle 100 before being translated to a frame of reference which is fixed with respect to the earth (rather than with respect to the vehicle 100), for example by reference to vehicle orientation information provided by the vehicle's inertial measurement unit (IMU) 23. The 3D point data cloud typically has a high number of points. The number of points of the 3D point data cloud may be reduced by the camera system 1850 determining a 3D grid (by a method such as multi-level surface (MLS)) map from the 3D point data cloud mapped relative to a horizontal ground plane representing the surface of the terrain. The 3D grid map may comprise one or more metrics in respect of each of a plurality of 3D blocks of the 3D point cloud, the metrics typically including information relating to the slope of the terrain and the elevation of the features of the terrain within that block.

At 506, the stereoscopic camera system 1 850 may overlay the LAB (or RGB or HSV) pixels derived from the first image of the stereo image pair onto the 3D grid map.

It may be that the (e.g. off-road) terrain to be traversed by the vehicle comprises a path region (e.g. a paved portion or mud ruts provided through a grass field) and a non-path region (e.g. grass on either side of the ruts, or on either side of the paved portion). At 508a-516a, the one or more processors of the stereoscopic camera system 1850 execute computer program instructions on the image data determined at 506 to determine probabilities that respective portions of the terrain relate to the path region thereof.

The image data may be divided by the stereoscopic camera system 1850 into a plurality of sub-regions. It may be that each of the sub-regions relates to a 25cm x 25cm region of the terrain. Thus, it may be that each of the sub-regions comprise a plurality of pixels of image data. At 508a, an assumption is made that the tyres of the vehicle are located on a path region of the terrain. Because the vehicle 100 is moving, the current location of the vehicle may differ from the location of the vehicle when the image data was captured. Accordingly, the image data may comprise image data corresponding to the current locations of the tyres of the vehicle 100. The camera system 1850 may determine the current location of the vehicle 100 relative to the location of the vehicle 100 when the image data was captured, e.g. by performing visual odometry or inertial odometry on the image data, and/or inertia data from the IMU 23, or using satellite positioning system data (such as Global Positioning System (GPS) data) indicative of the location of the vehicle 100, and to identify one or more sub-regions of the image data corresponding to the locations of the terrain currently contacted by the tyres of the vehicle 100 based on the current location of the vehicle. In the following description it will be assumed that one sub-region of the image data is identified for each tyre, but it will be understood that more than one sub-region may be identified for each tyre (depending on the relative sizes of the portion of the tyre in contact with the terrain and the sub-regions).

At 510a, the stereoscopic camera system 185C may process the sub-regions of the image data corresponding to the locations currently occupied by the tyres of the vehicle 100 to determine tyre region image content data relating to each of those tyre regions. The image content data may comprise colour image content data relating to the colour content of the respective sub-regions. Additionally or alternatively, the image content data may comprise texture data relating to the texture content of the respective sub-regions. Texture is a measure of the local spatial variation in the intensity of the image and is generally measured by subtracting the intensity of a given pixel from the intensity of each of the eight surrounding pixels to provide eight texture descriptors per pixel. It may be that the image content data comprises a colour and texture descriptor, p i, which contains eleven components for each pixel consisting of three L, a, b colour components and eight texture descriptors. An example of how a colour and texture descriptor, p i, may be calculated in shown in Fig. 6, where subject pixel S of intensity Lc is shown surrounded by pixels Si to 58 of respective intensities LI to Lg. Lc, ac and bc are the "LAB" colour components of pixel S. The set of weights Wi, W2 and W3 is used to balance how much to use colour, texture and brightness for image clustering.

By making the assumption that the tyre regions of the image data relate to a path region of the terrain, it is not necessary for the VCU 10 to request user input in order to identify portions of the terrain which relate to path regions. In addition, "path regions" comprising tyre tracks or mud ruts rather than paved paths can automatically be accounted for.

At 512a, the tyre region image content data is merged with a global path model, such as a Gaussian mixture model (GMM), stored in a memory of the VCU 10. In some cases, it may be that more than one path model is provided (e.g. one for colour and one for texture), in which case 512a may be performed for each path model, but it will be assumed in the following description that a single global path GMM is provided. The global path GMM is based on historical image data captured by the stereoscopic camera system 185C relating to historical locations of the terrain of the tyres of the vehicle 100.

When the relevant tyre region image content data is merged with the global path GMM, an updated global path GMM is provided. It will be understood that, the first time the method of Fig. 5 is performed, 512a is omitted. Instead, it may be that the tyre region image content data relating to the locations of each of the tyres are compared to each other and the matching tyre region image content data is used to create a global path GMM.

At 514a, the updated global path GMM may be used to determine probabilities that the respective sub-regions of the image data (not only the tyre regions) relate to the path region of the terrain. The image content of each of the sub-regions of the image data may be compared to the distribution of the updated global path GMM in order to determine a probability that the respective sub-region relates to the path region. Thus, a single path probability value may be determined for each sub-region. The closer the image content to the peak of the distribution of the updated path GMM, the higher the probability that the sub-region relates to a path region of the terrain, and vice versa. It may be that the path probability determined for each sub-region is stored in a memory of the stereoscopic camera system 185C in association with the sub-region of the image data to which it relates.

At 516a, the camera system 1850 may determine a path probability map in dependence on the determined path probabilities. It may be that the path probability map comprises the image data determined at 506 with the path probabilities for each of the sub-regions overlaid thereon.

At 508b-516b, the processor(s) of the stereoscopic camera system 1850 may execute computer program instructions on the image data determined at 506 to determine for each of the sub-regions a non-path probability that the respective sub-region relates to a non-path region of the terrain.

At 508b, two non-path regions laterally offset from the vehicle 100 are selected by the stereoscopic camera system 1850. It may be that the camera system 1850 is configured to identify the non-path regions by determining image content data relating to each of a plurality of sub-regions of image data relating to a first portion of the terrain laterally offset from the vehicle 100 on a first (e.g. left) side of the vehicle 100 and to a second portion of the terrain laterally offset from the vehicle 100 on a second (e.g. right) side of the vehicle opposite the first side. For example, the sub-regions may relate to portions of the terrain between 3m and 8m laterally offset from the centre of the wheelbase line of the vehicle 100 on both sides of the vehicle 100 at the current location thereof (as before the current location of the vehicle may be obtained by visual odometry or inertia odometry or from satellite positioning system data (such as Global Positioning System (GPS) data) indicative of the location of the vehicle).

As before, the image content data may comprise, for example, colour and/or texture data derived from the sub-region of image data It may be that the camera system 185C is configured to compare the image content data relating to each of the selected laterally offset sub-regions to the global path GMM to thereby identify one or more of the sub-regions having an image content which meet one or more dissimilarity criteria with respect to the global path GMM. The camera system 1850 may be configured to determine a lateral offset between the centre of the wheelbase line of the vehicle 100 at its current location and respective portions of the terrain to which the said one or more dissimilar sub-regions relate. For subsequent iterations of the method of Fig. 5 (for at least a limited time), the camera system 1850 may be configured to determine the non-path sub-regions simply by determining sub-regions relating to portions of the terrain laterally offset from the vehicle by the lateral offset. Different lateral offsets may be determined for each side of the vehicle.

It may be that the dissimilarity criteria comprise one or more conditions relating to the image content data and the global path GMM. For example, it may be that the dissimilarity criteria comprise one or more colour and/or texture conditions that a colour and/or texture distribution of the image content data relating to a respective sub-region do not match corresponding distribution of the global path GMM to a given degree.

Thus, sub-regions relating to the non-path region of the terrain may be identified. By identifying the non-path regions with reference to the global path GMM, no user input needs to be requested by the VCU 10 in order to identify the non-path regions.

At 510b, image content data (e.g. the colour and/or texture data) relating to the non-path sub-regions is determined. At 512b, the image content data relating to the non-path sub-regions is merged with a global non-path model, such as a global non-path GMM. In some cases, it may be that more than one non-path model is provided (e.g. one for colour and one for texture), in which case 512b may be performed for each non-path model, but it will be assumed in the following description that a single global non-path GMM is provided. The global non-path GMM is based on historical non-path image content data relating to portions of the terrain laterally offset from the vehicle on either side.

It will be understood that the first time 508b-512b are performed, step 512b may be omitted. In this case, the camera system 1850 may be configured to merge the image content data obtained from the sub-regions laterally offset from the vehicle to form the global non-path GMM.

At 514b, the updated global non-path GMM may be used by the stereoscopic camera system to determine probabilities that the respective sub-regions of the image data (not only the subregions laterally offset from the vehicle) relate to the non-path region of the terrain. In this case, the image content of each of the sub-regions of the image data may be compared to the updated global non-path GMM to determine a probability that the respective sub-region relates to the non-path region of the terrain. The closer the image content data to the peak of the distribution of the updated non-path GMM, the higher the probability that the sub-region relates to the non-path region of the terrain, and vice versa. It may be that the probability is stored in a memory of the VCU 10 in association with the sub-region of the image data to which it relates.

At 516b, a non-path probability map is determined in dependence on the non-path probabilities. It may be that the non-path probability map comprises the image data determined at 506 with the non-path probabilities for each of the sub-regions overlaid thereon. 15 At 518, path and non-path probability data determined during 508a-516a and 508b-516b are combined to provide a final path probability map. The secondary path probability map may be an inverse of the non-path probability map determined by inferring that the sub-regions which have low non-path probabilities have high path probabilities. For example, it may be that the camera system 1850 is configured to infer the secondary path probability, P * secondary_patti, for an ith sub-region from the non-path probability, P * non-path 1, based on: P secondary path i = 1 -Pnonpath _1-Alternatively, the path and non-path probability data may be combined in any other suitable way to determine the final probability map. For example, the probabilities determined at 514a, 514b may be combined, for example by subtracting the non-path probability from the path probability to provide a final path probability for each sub-region. By inferring secondary probability data from the non-path probability data and combining the secondary path probability data with the path probability data, a more confident determination can be made as to whether a portion of the terrain is a path or non-path region of the terrain.

When the final path probability map is determined, it may be merged with a global final path probability map determined from previous frames of image data captured by the stereoscopic camera system 1850.

The stereoscopic camera system 1850 (or any other processing system of the vehicle such as the VCU 10) may determine a cost map relating to the terrain based on the path and non-path probabilities, for example in dependence on the final path probability map. In order to determine the cost map, the stereoscopic camera system 1850 may determine, for each of the sub-regions of the image data, a cost for the vehicle 100 to traverse the respective portion of the terrain to which the sub-region relates in dependence on the path and non-path probabilities determined from 508-518, for example in dependence on the final path probability relating to that sub-region determined at 518. The cost may relate to a penalty or a reward associated with the respective portion of the terrain. An increased cost may relate to an increased penalty or a reduced reward. Similarly a reduced cost may relate to a reduced penalty or an increased reward. However, it will be assumed in the following description that the cost is allocated on a penalty basis.

In an example, for each sub-region, the greater the final path probability, the lower the cost allocated to that sub-region and the lower the final path probability, the greater the cost allocated to that sub-region. It may be that the costs are allocated to sub-regions on a binary basis, for example a low cost for sub-regions having final path probabilities greater than a threshold and a high cost for sub-regions having final path probabilities lower than a threshold.

However, it may be that costs are allocated on a more granular basis. For example, it may be that the cost for the vehicle to traverse a portion of the terrain to which a sub-region relates is determined depending on the final path probability meeting one or more path probability criteria indicating that the sub-region relates to the path region, one or more non-path probability criteria indicating that the sub-region relates to the non-path region or neither the path probability criteria nor the non-path probability criteria.

This is illustrated in Fig. 7 which schematically shows an example terrain ahead of the vehicle 100 with a cost map 800 determined using a three-tiered cost allocation scheme overlaid thereon, the cost map 800 comprising a plurality of cells 802 each of which relates to a sub-region of the image data. The letter L indicates that a relatively low cost has been allocated to the sub-region (which may be a sub-region determined to relate to a path region of the terrain with relatively high confidence), the letter H indicates that a relatively high cost has been allocated to the sub-region (which may be a sub-region determined to relate to a non-path region of the terrain with relatively high confidence), while the letter I indicates that an intermediate cost has been allocated to the sub-region (which may be a sub-region comprising a shadow or puddle (for example) 705 and therefore determined to relate to neither a path region nor a non-path region of the terrain with relatively high confidence). For ease of illustration, the cells 802 of the cost map 800 of Fig. 7 are larger than would normally be used in practice.

The cost map may be transmitted from the stereoscopic camera system 1850 to the VCU 10 which may determine a future path for the vehicle in dependence on the cost map. In order to determine a future path for the vehicle 100, costs for the vehicle to traverse the terrain by each of a plurality of candidate trajectories may be calculated from the cost map 800 (or from a global cost map into which cost map 800 is merged) and a preferred path may be selected from the candidate trajectories in dependence on the calculated costs. This is illustrated in Fig. 8, which shows the same terrain and cost map 800 as Fig. 7 with a plurality of candidate trajectories 810-830 overlaid thereon. If it is assumed that the VCU 10 will want to avoid high cost regions of the terrain, then the VCU 10 may select trajectory 810 as its preferred path as it will have the lowest cost.

It will be understood that, instead of calculating the cost for each of a plurality of candidate trajectories and selecting a preferred path from the candidate trajectories in dependence on the costs, it may be that the future path is determined by analysing the cost map in order to determine the lowest cost route. While this may allow a more optimal route to be determined, it is more processing intensive.

It may be that further cost data, such as obstacle cost data which may for example be derived from the 3D data derived from the stereoscopic image data or from ranging data provided by a ranging system of the vehicle such as a radar-based terrain ranging system, a laser-based terrain ranging (e.g. LIDAR) system or an acoustic ranging system of the vehicle 100, is also taken into account to determine whether the preferred path determined from the path probability data is traversable by the vehicle 100. If not, the next preferred path may be checked and so on until a suitable path is determined.

It will be understood that the future path of the vehicle may be determined in any suitable additional or alternative way. For example, the vehicle path may be determined by (e.g. the camera system 185C or the VCU 10) executing a deep learning algorithm which may involve obtaining one or more images of the terrain ahead of the vehicle (e.g. from one or both image sensors of the stereoscopic camera system), processing the images to determine the edge of the path by edge detection, and estimating the curvature of the path from the determined edges. In another example, the vehicle path may be a determined path provided by a (e.g. a global satellite positioning system (e.g. Global Positioning System, GPS) based) positioning system of the vehicle. For example, it may be that the future path of the vehicle is determined based on the path it is currently on, and it may be that that path is known to the positioning system of the vehicle (e.g. by reference to a mapping database).

The VCU 10 may then control the vehicle 100 in accordance with the determined path. The VCU 10 may determine a required steering angle for one or more wheels of the vehicle 100 in dependence on the curvature of the determined path, and in dependence thereon transmit a steering angle command signal to the steering controller 1700. The steering controller 170C, in turn, may set the angle of the steerable wheels of the vehicle accordingly. The VCU 10 may also determine a recommended speed of the vehicle in dependence on the curvature of the determined path from the relevant look-up table. The VCU 10 may be configured to output the recommended speed to the LSP control system 12 which controls the speed of the vehicle 100 accordingly by changing the user set speed in accordance with the received recommended speed.

In some examples the vehicle 100 is a front wheel drive or four wheel drive vehicle and is therefore at risk of going into understeer, particularly when driving on a surface with a low surface co-efficient of friction. This can cause passengers riding in the vehicle 100 to experience less than optimal comfort levels, and the control of the steering of the wheels of the vehicle by the VCU 10 may be temporarily less responsive. Accordingly, it may be undesirable for the vehicle to be in understeer. In order to ensure that understeering is pre-emptively prevented and/or reactively inhibited, the VCU 10 may be configured to limit the longitudinal speed and/or the steering angle of the vehicle 100. For example, in order to preemptively inhibit understeer, it may be that the VCU 10 is configured to pre-emptively limit the longitudinal speed of the vehicle in dependence on the determined path.

As discussed above, the determined path may be a candidate trajectory selected from a plurality of candidate trajectories in dependence on the costs of the said plurality of candidate trajectories. In this case, each of the candidate trajectories may be a trajectory in accordance with a respective predefined polynomial. Alternatively, a future path determined by a deep learning algorithm or by satellite positioning system with reference to a mapping database (as appropriate) may be expressed as a polynomial. Accordingly, a polynomial defining the determined path may be known to the VCU 10, and the radius of curvature, R, of the determined path can be determined by the VCU 10 using the following equation (in which the x, y plane relates to the ground plane of the terrain, and where y is the polynomial of the determined path as a function of x, y being a lateral distance from the vehicle and x being a longitudinal distance from the vehicle): For example, y(x) may be a third order polynomial: y = ax3 + bx2± cx ±a It will be understood that the radius of curvature of the path may change with distance x and that there may therefore be a plurality of values of R for a given path. The VCU 10 may then determine desired vehicle speeds based upon a target lateral acceleration value and associated path radii by: where: vt is the desired speed; R is the radius of curvature of the determined vehicle path (see above); and ac is the lateral acceleration target.

It may be that the lateral acceleration target, ac, is a fixed value, for example of 1.5, based on a predetermined user comfort preference, which may be stored in a memory in association with the [SR mode and/or the TR mode of the vehicle. Alternatively, the lateral acceleration target may be a pre-emptive deceleration determined based on the radius of curvature of the determined vehicle path. A pre-emptive deceleration may be determined based on a current speed of the vehicle and a desired speed based on the radius of curvature of the determined vehicle path and a desired deceleration distance, which may be obtained based on a user comfort preference (which may be stored in a memory accessible to the processor(s) 19 of the VCU 10, such as in a memory of the VCU 10, typically in association with the LSP mode and/or the TR mode of the vehicle). Thus, a plurality of target speeds, vt, may be determined for the determined path. In some cases, it may be that target lateral acceleration is the lower of a fixed value (e.g. 1.5) and a pre-emptive deceleration determined based on the radius of curvature of the determined vehicle path and a user comfort preference. Put another way, it may be that the target lateral acceleration is the one of the fixed value, based on the predetermined user comfort preference, and the pre-emptive deceleration which requests the vehicle to slow the most.

As discussed above, the VCU 10 may be configured to receive vehicle speed data relating to the vehicle speed from one or more vehicle speed sensors. Additionally or alternatively, the VCU 10 may be configured to receive wheel speed data relating to the wheel speed of one or more wheels of the vehicle. In the latter case, it may be that the VCU 10 is configured to determine the speed of the vehicle based upon the wheel speed data. As mentioned above, the VCU 10 may be further configured to determine a desired acceleration or deceleration distance based on the difference between the current speed of the vehicle and the target speeds vt and based on user comfort information (which may be pre-stored in a memory accessible to the processor(s) 19 of the VCU 10, such as in a memory of the VCU 10, typically in association with the LSP mode and/or the TR mode of the vehicle). Based on the distance and speed information, the VCU 10 may be configured to determine longitudinal acceleration targets of the vehicle 100 using the chain rule: dv dl a dl at where I is the distance along the path and dl/dt is taken as the average vehicle speed between the current velocity of the vehicle and the target velocity L metres ahead, which depends on the deceleration profile.

The VCU 10 may then select the minimum longitudinal acceleration value, a (which may be a maximum deceleration), for the vehicle speeds vt for the determined path (or, as mentioned above, the lower of a fixed value (e.g. 1.5) and the minimum longitudinal acceleration value, a, for the vehicle speeds vt for the determined path) as a longitudinal acceleration target. The VCU 10 may then determine a target (future) longitudinal vehicle speed based on the sum of a previous target longitudinal vehicle speed and the product of the longitudinal acceleration target and the sample rate, the sample rate being the rate at which the vehicle speed is updated. The VCU 10 may then cause application of the necessary positive or negative torque to the wheels by the powertrain 129 and/or a braking force to the vehicle wheels by the braking system 22 in order to adjust the longitudinal vehicle speed in accordance with the target longitudinal speed.

By selecting an appropriate value for ac, it can be ensured that under normal circumstances, the vehicle 100 will not go into understeer and that the ride will be comfortable for passengers.

However, in order to ensure that ride comfort and responsive control of the vehicle 100 is maintained, for example but not exclusively when it is driving on a surface having a low coefficient of friction, the VCU 10 may additionally or alternatively be configured to reactively inhibit understeer. In examples in which the VCU 10 is configured to reactively inhibit understeer without also pre-emptively inhibiting understeer based on a determined future path of the vehicle 100, it may be that the control system does not determine the future path of the vehicle 100.

In order to reactively inhibit understeer On addition to or as an alternative to pre-emptively inhibiting understeer based on a determined future path of the vehicle), the VCU 10 may reactively limit one or both the steering angle of one or more wheels (e.g. the front wheels) of the vehicle 100 and/or the longitudinal speed of the vehicle 100. In order to limit the steering angle of one or more wheels of the vehicle 100, it may be that two steering angle limits are defined: a first steering angle limit which is dependent on the longitudinal speed, yaw rate and lateral acceleration of the vehicle; and a second steering angle limit which is based on a fixed lateral acceleration target. It may be that the first steering angle limit is a friction limit for inhibiting understeer, while the second steering angle limit is determined to prevent extreme steering angles being requested for one or more wheels of the vehicle 100. It may be that the VCU 10 is configured to limit the steering angle of one or more wheels of the vehicle based on the lower of the first and second steering angle limits.

The first steering angle limit, 6, may be determined by: ILO \ atan + k.t.

where L is the vehicle wheel base length (m), v is the current longitudinal speed of the vehicle 100 (m/s), W is the yaw rate (rad/seconds) of the vehicle 100, kug is the understeer gradient and ac is the current lateral acceleration of the vehicle. This takes into account the radius, R, of the turn being driven by the vehicle 100, R being related to the longitudinal speed v and the yaw rate W by:

R

The relationship between R, L and 6, based on the Ackerman steering model, is illustrated in Fig. 9.

The wheel base length may be predefined and known to the VCU 10. The current longitudinal speed of the vehicle may also be known to the VCU 10 (e.g. based on longitudinal speed data from a longitudinal speed sensor of the vehicle, or based on wheel speed data from one or more wheel speed sensors of the vehicle). The yaw rate may also be known to the VCU 10 (based on yaw rate data from a yaw rate sensor of the vehicle). The current lateral acceleration ac of the vehicle 100 may be known to the VCU 10, either from a lateral acceleration sensor of the vehicle or by obtaining the product of the longitudinal speed and yaw rate of the vehicle 100. The understeer gradient [(Lig may be known to the VCU 10 by way of stored calibration data. For example, the understeer gradient, kug, may be predetermined by extracting the gradient of a best fit straight line through a plurality of empirically determined data points of the steering angle of one or more (e.g. front) wheels of the vehicle 100 versus lateral acceleration on a test road surface.

In order to inhibit the first steering angle limit, 6, from coming into effect during normal driving, it may be that an additional calibration factor is added to the value determined above. In one example, a calibration factor of 0.25°to the steering angle of one or more wheels of the vehicle was empirically determined to be effective. This may correspond to a steering wheel rotational angle of 4.25°.

From initial testing, it was found that determining the first steering angle limit in the way described above generally resulted in an overcautious vehicle response which did not maximise the vehicle's available grip. An assumption made in the above equations for the first steering angle limit is that the vehicle 100 is carrying out a constant turning radius at a constant vehicle speed. The inventors have realised, however, that if the vehicle 100 is carrying out a dynamic manoeuvre which contains both lateral and longitudinal acceleration, then the friction limit may be a combination of x and y axis components. Thus, it has been determined that if the resultant acceleration between lateral and longitudinal accelerations is taken into account in the above equations (rather than merely the lateral acceleration), the steering angle can be increased further before the vehicle's friction limit is reached. The resulting force on a tyre of the vehicle 100 caused by a combination of lateral and longitudinal accelerations is illustrated in Fig. 10.

The resultant acceleration of the vehicle 100, a, may be determined as follows: where LP is the yaw rate of the vehicle, a is the longitudinal acceleration of the vehicle and (t) is an estimate of the required vehicle speed, obtained from: naci tko = itan(16D where L is the vehicle wheel base length (m), ac is the current lateral acceleration of the vehicle and 6 is a steering angle target of one or more wheels of the vehicle, which may for example be determined in dependence on the determined path before the steering angle limits are applied thereto.

The longitudinal acceleration of the vehicle 100 may be determined by the VCU 10 from longitudinal acceleration data received from a longitudinal acceleration sensor of the vehicle 100, or from longitudinal speed data received from one or more vehicle speed sensors or derived from wheel speed data received from one or more wheel speed sensors of the vehicle 100.

Using lateral acceleration rather than resultant acceleration, the first steering angle limit, 6, may be determined by: ci = atan Note that the lateral acceleration term, ac, may instead be substituted by resultant acceleration, ar. However, in this case, 1(.9, may be determined by extracting the gradient of a best fit straight line through a plurality of empirically determined data points of the steering angle of one or more wheels (e.g. the front wheels) of the vehicle 100 (or of a vehicle of the same manufacturer and model) versus resultant acceleration on a test road surface. This is illustrated in Fig. 11, the understeer gradient kug being the gradient of the line 899.

The second steering angle limit, 6, may be determined by: (5' atar, where ac is a fixed calibratable lateral acceleration target to ensure that the allowable steering angle is reduced as the speed of the vehicle increases. It may be that the lateral acceleration target is based on a user comfort preference stored in a memory accessible to the processor(s) 19 of the VCU 10 (e.g. a memory of the VCU 10), typically in association with the LSP mode and/or the TR mode of the vehicle. In some examples, it may be that ac is set equal to 0.4*9.81. This limitation is not generally used to inhibit understeer behaviour, but may be included to better ensure that extreme steering angles are not requested.

The VCU 10 may send command signals to the steering controller 170C to reactively limit the steering angle of one or more wheels (e.g. the front wheels) of the vehicle 100 in accordance with the lower of the first and second limits to thereby better ensure that understeer is inhibited and that the vehicle 100 maintains user comfort levels, without unduly limiting the steering angle of the vehicle 100. As explained above, this helps even when the VCU 10 is pre-emptively controlling the vehicle speed in accordance with the determined vehicle path, particularly when the vehicle is travelling on a surface having a relatively low coefficient of friction. It will be understood that the steering controller 170C may limit the steering angle of one or more wheels of the vehicle by limiting a steering angle of the steering wheel of the vehicle 100, or by limiting a steering angle applied by a drive-by-wire steering system of the vehicle 100.

In order to limit the longitudinal speed of the vehicle 100, it may be that two longitudinal speed limits are defined: a first longitudinal speed limit which is dependent on a target steering angle, 6, of one or more wheels of the vehicle and the current lateral acceleration, ac, of the vehicle; and a second longitudinal speed limit which is based on a lateral acceleration target dependent on the current steering angle of one or more wheels of the vehicle 100. It may be that the VCU 10 is configured to limit the longitudinal speed of the vehicle based on the lower of the first and second longitudinal speed limits.

The first longitudinal speed limit may be determined by: I Liaj EKI) itanOSn where, as above, L is the vehicle wheel base length (m), ac is the current lateral acceleration of the vehicle and 6 is a steering angle target of one or more wheels of the vehicle, which may for example be determined in dependence on the determined path before the steering angle limits are applied thereto.

The steering angle target before the steering angle limits are applied thereto may be used in the above equation for the longitudinal speed limit because it may be the best estimate of the steering angle required to steer the vehicle in accordance with the target path.

The second longitudinal speed limit may be determined using the above equation but where ac is a lateral acceleration target chosen based on the current steering angle 6, and L is the vehicle wheel base length (m). The lateral acceleration target may be based on a predetermined user comfort preference for the current steering angle, and which may be stored in a memory accessible to the processor(s) 19 of the VCU 10, such as a memory of the VCU 10, for example in association with the [SR mode and/or the TA mode of the vehicle 100. It may be that the second longitudinal speed limit is not used to inhibit understeer, but rather is a hard speed limit for the vehicle for the current steering angle.

It may be that the VCU 10 is configured to limit the longitudinal speed of the vehicle 100 to the lower of the first and second longitudinal speed limits. It can thus be better ensured that understeer of the vehicle is inhibited and that the vehicle 100 provides a comfortable ride to 35 passengers.

In order to reactively reduce the target speed of the vehicle 100 in dependence on the vehicle 100 going into understeer (rather than significantly before the vehicle 100 goes into understeer), the lateral acceleration variable a, in the above equation for the first longitudinal speed limit may be modified to include one or more calibration factors. For example, the lateral acceleration variable a, may be modified as follows: = ii(Ockti ±f(1) ± 2 where ii(t) is an estimate of the required longitudinal vehicle speed through the corner based on current lateral acceleration and target steering angle (which may be determined using the above-mentioned equation), and f(1) and f(2) are respective first and second calibration factors which artificially increase the lateral acceleration to ensure that a speed reduction is requested in dependence on the vehicle going into understeer.

The first calibration factor, f(1), may be based upon the difference between the target steering angle of one or more wheels of the vehicle prior to the above first and second steering angle limits being applied and the actual steering angle applied to the said one or more wheels of the vehicle after the above first and second steering angle limits have been applied. As shown in Fig. 12, which shows the additional lateral acceleration value provided by the first calibration factor f(1) versus the difference between the target steering angle of one or more wheels of the vehicle prior to the above first and second steering angle limits being applied and the actual steering angle applied to the said one or more wheels of the vehicle after the above first and second steering angle limits have been applied. It can be seen from Fig. 12 that, as the difference between the target and actual steering angles increases, the value of the first calibration factor, and therefore its significance in the determination of the first longitudinal speed limit, decreases. Similarly, as the difference between the target and actual steering angles decreases, the value of the first calibration factor, and therefore its significance in the determination of the first longitudinal speed limit, increases up to a maximum value of 0.5m/s2.

The second calibration factor, f(2), adds additional lateral acceleration when the vehicle speed is low to ensure that low speed yaw rate sensor measurement noise does not inadvertently limit the target vehicle speed. This is illustrated in Fig. 13 which shows the additional lateral acceleration provided by the second calibration factor f(2) versus the vehicle speed v. It can be seen from Fig. 13 that, as the longitudinal vehicle speed increases, the value of the second calibration factor, and therefore its significance in the determination of the first longitudinal speed limit, decreases. Similarly, as the longitudinal speed of the vehicle decreases, the value of the second calibration factor, and therefore its significance in the determination of the first longitudinal speed limit, increases up to a maximum value of 0.5m/s2.

As with the steering angle limits, it has been found in practice that the above equations for the first longitudinal speed limit cause overcautious vehicle behaviour which does not maximise the vehicle's available grip. In order to help maximise the vehicle's available grip, the above modified lateral acceleration term may be modified to become: ) +f(2) where a, is the resultant acceleration defined above with respect to the steering angle limits, and 1(1) and 1(2) are calibration factors similar to those described above with respect to Figs. 14 and 15 but where the values provided by f(1) and f(2) are resultant accelerations rather than lateral accelerations.

The VCU 10 may send command signals indicative of the lower of the first and second longitudinal speed limits to the LSP control system 12 to override the user setspeed to thereby limit the longitudinal speed of the vehicle 100 to inhibit understeer, but without unduly limiting the speed of the vehicle 100. As explained above, this helps even when the VCU 10 is preemptively controlling the vehicle speed in accordance with the determined vehicle path, particularly when the vehicle is travelling on a surface having a relatively low coefficient of friction.

Figs. 14-21 each show various empirically obtained plots relating to a vehicle 100 following a sinusoidal vehicle path in an autonomous LSP mode under different control conditions, with the respective plots in each case being plots (from top to bottom) of: target (dotted line) and actual (solid line) x-axis position of the vehicle 100 versus y-axis position (where the x-axis relates to longitudinal position of the vehicle and the y-axis relates to lateral position of the vehicle); target (dotted line) longitudinal vehicle speeds before any of the first and second longitudinal speed limits have been applied and actual (solid line) longitudinal vehicle speeds after any of the first and second longitudinal speed limits have been applied versus y-axis position; target (dotted line) and actual (solid line) steering angle applied to the front wheels of the vehicle after any of the first and second steering angle limits have been applied; and y-axis position versus x-axis position, with the greyscale shade between black and white of the line relating to the resultant acceleration along the path (the key relating the greyscale shade of the line to a resultant acceleration value being provided on the right hand side of the bottom plot, black being a relatively low resultant acceleration and white being a relatively high resultant acceleration). In the third plot from the top, the dotted line shows the target steering angle for front wheels of the vehicle before any of the first and second steering angle limits have been applied.

In Fig. 14, none of the first and second reactive longitudinal speed or steering angle limits are applied, and no pre-emptive speed control is applied based on path 901. The speed and angle are controlled in accordance with the demanded speed and angle based on the determined vehicle path. Whilst the bottom plot of Fig. 14 appears to show that it is possible to travel at the upper speed limit (30kph in this case) of the LSP system 12 throughout the sinusoidal path 901, from the resultant acceleration values it is apparent that the vehicle 100 is understeering and the overall steering control is becoming more unstable as it travels (from top to bottom on the bottom plot) along the path.

In Fig. 15, the steering angle of the front wheels of the vehicle 100 is controlled in accordance with the reactive second steering angle limit. The first steering angle limit and the first and second longitudinal speed limits are not applied, nor is pre-emptive speed control based on path 901. Fig. 15 shows that, for the road surface used in these tests, the second steering angle limit does not reduce understeer.

In Fig. 16, both the reactive first and second steering angle limits are applied (with resultant rather than lateral acceleration being used to determine the first steering angle limit), but neither of the reactive first and second longitudinal speed limits are applied nor is pre-emptive speed control based on path 901. It can be seen from the dotted line in the plot relating to the steering angle (third from the top) that, before any steering limits are applied, the target steering angle exceeds 20° in some places (which is close to full lock). From the lateral acceleration values, the steering angle limitations appear to have helped to inhibit understeer.

However, the vehicle 100 does not follow the target path 901 particularly accurately.

In Fig. 17, both the first and second steering angle limits and the second longitudinal speed limit are applied (with resultant rather than lateral acceleration being used to determine the first steering angle limit), but the first longitudinal speed limit is not applied nor is pre-emptive speed control based on path 901. The vehicle 100 follows the path 901 more accurately than in Fig. 16 and stops the vehicle 100 understeering when speed and steering are under control, the vehicle 100 being stabilised for the second turn in the path 901.

In Fig. 18, both the first and second steering angle limits and the first and second longitudinal speed limit are applied (with resultant rather than lateral acceleration being used to determine the first steering angle limit and the first longitudinal speed limit), but pre-emptive speed control based on path 901 is not. In this case, further improvement over Fig. 17 is observed in the accuracy to which the vehicle 100 follows the path 901, with understeer and resultant acceleration being reduced similarly to Fig. 17.

In Fig. 19, pre-emptive longitudinal vehicle speed control based on path 901 is applied together with the first and second longitudinal speed and steering angle limits, but with lateral rather than resultant acceleration being used to determine the first steering angle limit and the first longitudinal speed limit. In this case, the vehicle 100 follows the target path 901 accurately and with low resultant acceleration and understeer throughout. However, the desired vehicle speed undershoots the steady-state speed required to stop the vehicle 100 understeering.

That is, the longitudinal speed of the vehicle is reduced overcautiously.

In Figs. 20 and 21, pre-emptive longitudinal vehicle speed control based on path 901 is applied together with the first and second longitudinal speed and steering angle limits, with resultant rather than lateral acceleration being used to determine the first steering angle limit and the first longitudinal speed limit. In each case, the vehicle speed remains higher throughout the path 901 than in Fig. 19, with the vehicle 100 following the path 901 accurately without understeer or uncomfortable resultant accelerations in either case. In Fig. 20 a lower comfort setting (i.e. greater tolerable resultant acceleration, and thus higher second steering and longitudinal speed limits) is applied than in Fig. 21. Accordingly, the resultant acceleration of the vehicle 100 can be seen in Fig. 20 actively trying to find the first steering angle (friction) limit, the resultant acceleration running close to 2.8m/s2 especially at the second turn of the path where the vehicle speed has increased. The resultant accelerations are less in Fig. 21 as a result of the greater comfort setting.

It can therefore be seen that the reactive control of longitudinal speed and steering angle of one or more wheels of the vehicle 100 in accordance with the first and second longitudinal speed and steering angle limits, optionally together with pre-emptive speed control in dependence on the path, provide stable control of the vehicle 100 with comfortable levels of resultant acceleration for passengers of the vehicle. Use of resultant rather than lateral acceleration to determine the first steering angle and longitudinal speed limits also ensures that the vehicle response is not overly cautious.

It will be appreciated that embodiments of the present invention can be realised in the form of hardware, software or a combination of hardware and software. Any such software may be stored in the form of volatile or non-volatile storage such as, for example, a storage device like a ROM, whether erasable or rewritable or not, or in the form of memory such as, for example, RAM, memory chips, device or integrated circuits or on an optically or magnetically readable medium such as, for example, a CD, DVD, magnetic disk or magnetic tape. It will be appreciated that the storage devices and storage media are embodiments of machine-readable storage that are suitable for storing a program or programs that, when executed, implement embodiments of the present invention. Accordingly, embodiments provide a program comprising code for implementing a system or method as claimed in any preceding claim and a machine readable storage storing such a program. Still further, embodiments of the present invention may be conveyed electronically via any medium such as a communication signal carried over a wired or wireless connection and embodiments suitably encompass the same.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.

Claims

CLAIMS1. A control system for a vehicle, the control system comprising one or more controllers and having an autonomous driving mode, the control system being configured to, in the autonomous driving mode: obtain lateral acceleration data indicative of a lateral acceleration of the vehicle; and limit a steering angle of one or more wheels of the vehicle and a longitudinal speed of the vehicle in dependence on the lateral acceleration data.
2. A control system according to claim 1 wherein the at least one controller collectively comprises: at least one electronic processor having an input for receiving the lateral acceleration data; and at least one electronic memory device electrically coupled to the at least one electronic processor and having instructions stored therein, wherein the at least one electronic processor is configured to access the at least one memory device and execute the instructions thereon to limit the steering angle and the longitudinal speed of the vehicle in dependence on the lateral acceleration data.
3. A control system according to any preceding claim wherein the control system is configured to limit the steering angle of one or more wheels of the vehicle based on a first steering angle limit, the first steering angle limit being determined in dependence on the lateral acceleration data.
4. A control system according to claim 3 wherein the first steering angle limit is determined based on a predetermined relationship between the steering angle of one or more wheels of the vehicle and an acceleration of the vehicle, the said acceleration of the vehicle being dependent on the lateral acceleration of the vehicle.
5. A control system according to claim 4 wherein the acceleration is a resultant of the lateral acceleration and a longitudinal acceleration of the vehicle.
6. A control system according to any preceding claim wherein the control system is configured to limit the steering angle of one or more wheels of the vehicle based on a second steering angle limit, the second steering angle limit being based on a predetermined lateral acceleration limit.
7. A control system according to claim 6 wherein the lateral acceleration limit is based on a user comfort preference.
8. A control system according to claim 6 or 7 as dependent on any of claims 3 to 5 wherein the control system is configured to limit the steering angle of one or more wheels of the vehicle to the lower of the first and second steering angle limits.
9. A control system according to any preceding claim wherein the control system is configured to limit the longitudinal speed of the vehicle based on a first longitudinal speed limit, the first longitudinal speed limit being dependent on the lateral acceleration data and on a target steering angle relating to one or more wheels of the vehicle.
10. A control system according to claim 9 wherein the first longitudinal speed limit is dependent on a resultant of the lateral acceleration and a longitudinal acceleration of the vehicle.
11. A control system according to any preceding claim wherein the control system is configured to limit the longitudinal speed of the vehicle in dependence on the vehicle being in 20 understeer.
12. A control system according to claim 11 wherein the control system is configured to limit the longitudinal speed of the vehicle in dependence on the vehicle being in understeer by limiting the longitudinal speed of the vehicle based on a or the first longitudinal speed limit, wherein the first longitudinal speed limit is dependent on the lateral acceleration data, a target steering angle relating to one or more wheels of the vehicle and one or more calibration factors.
13. A control system according to claim 12 wherein the one or more calibration factors comprise a first calibration factor dependent on a difference between the target steering angle of one or more wheels of the vehicle and an actual steering angle applied to the one or more wheels of the vehicle determined in dependence on a steering angle limit.
14. A control system according to claim 12 or claim 13 wherein the one or more calibration factors comprise a second calibration factor dependent on a longitudinal speed of the vehicle.
15. A control system according to any preceding claim wherein the control system is configured to limit the longitudinal speed of the vehicle in dependence on a second longitudinal speed limit, the second longitudinal speed limit being based on a lateral acceleration limit.
16. A control system according to claim 15 wherein the lateral acceleration limit is based on a user comfort preference.
17. A control system according to claim 15 or claim 16 as dependent on claim 9 or claim or on any of claims 12 to 14 wherein the control system is configured to limit the longitudinal speed of the vehicle to the lower of the first and second longitudinal speed limits.
18. A control system according to any preceding claim wherein the control system is configured to: determine a future path for the vehicle; and control the longitudinal speed of the vehicle in dependence on the future path.
19. A control system according to claim 18 wherein the control system is configured to: obtain environment data from one or more environment sensors of the vehicle; and determine the future path for the vehicle in dependence on the environment data.
20. A control system according to claim 18 or claim 19 wherein the control system is configured to send one or more command signals to control torque applied to one or more wheels of the vehicle and/or to send one or more command signals to control braking of one or more wheels of the vehicle to thereby control a longitudinal speed of the vehicle in dependence on the determined vehicle path.
21. A control system according to any of claims 18 to 20 wherein the control system is configured to: determine a future longitudinal speed for the vehicle in dependence on the said future path and on one or more lateral acceleration limits; and control the longitudinal speed of the vehicle in dependence on the determined future longitudinal speed.
22. A control system according to claim 21 wherein the one or more lateral acceleration limits are based on a user comfort preference.
23. A vehicle comprising a control system according to any of claims 1 to 21.
24. A method for operating a vehicle in an autonomous driving mode, the method comprising: obtaining lateral acceleration data indicative of a lateral acceleration of the vehicle; and limiting a steering angle of one or more wheels of the vehicle and a longitudinal speed of the vehicle in dependence on the lateral acceleration data.
25. A non-transitory computer readable medium comprising computer executable instructions that, when executed by a computer, cause performance of the method according to claim 24.