Vehicle speed control system and method

Embodiments of the present invention provide a vehicle speed control system operable to cause a vehicle to operate in accordance with a target speed value by controlling an amount of brake torque applied by a braking system and an amount of drive torque applied by a powertrain to one or more wheels of the vehicle, the system being operable to detect a leading wheel step event in which a leading wheel encounters an abrupt increase in surface gradient, the system being operable in dependence on the detection of a leading wheel step event to cause brake torque to be applied against drive torque in anticipation of a trailing wheel step event corresponding to the leading wheel step event, the system being operable to cause the application of brake torque at least until a trailing wheel has travelled a sufficient distance to reach the location at which a leading wheel experienced the leading wheel step event.

FIELD OF THE INVENTION

This disclosure relates to a system for controlling the speed of a vehicle. In particular, but not exclusively, the disclosure relates to a system for controlling the speed of a land-based vehicle which is capable of driving in a variety of different and extreme terrains and conditions. Aspects of the invention relate to a system, to a method and to a vehicle.

The content of co-pending UK patent application no GB81214651.0 and U.S. Pat. No. 7,349,776 are hereby incorporated by reference.

BACKGROUND

In known vehicle speed control systems, typically referred to as cruise control systems, the vehicle speed is maintained on-road once set by the user without further intervention by the user so as to improve the driving experience for the user by reducing workload.

With typical cruise control systems, the user selects a speed at which the vehicle is to be maintained, and the vehicle is maintained at that speed for as long as the user does not apply a brake or, in the case of a vehicle having a manual transmission, depress a clutch pedal. The cruise control system takes its speed signal from a driveshaft speed sensor or wheel speed sensors. When the brake or the clutch is depressed, the cruise control system is disabled so that the user can override the cruise control system to change the vehicle speed without resistance from the system. If the user depresses the accelerator pedal the vehicle speed will increase, but once the user removes his foot from the accelerator pedal the vehicle reverts to the pre-set cruise speed by coasting.

Such systems are usually operable only above a certain speed, typically around 15-20 kph, and are ideal in circumstances in which the vehicle is travelling in steady traffic conditions, and particularly on highways or motorways. In congested traffic conditions, however, where vehicle speed tends to vary widely, cruise control systems are ineffective, and especially where the systems are inoperable because of a minimum speed requirement. A minimum speed requirement is often imposed on cruise control systems so as to reduce the likelihood of low speed collision, for example when parking. Such systems are therefore ineffective in certain driving conditions (e.g. low speed) and are set to be automatically disabled in circumstances in which a user may not consider it to be desirable to do so.

More sophisticated cruise control systems are integrated into the engine management system and may include an adaptive functionality which takes into account the distance to the vehicle in front using a radar-based system. For example, the vehicle may be provided with a forward-looking radar detection system so that the speed and distance of the vehicle in front is detected and a safe following speed and distance is maintained automatically without the need for user input. If the lead vehicle slows down, or another object is detected by the radar detection system, the system sends a signal to the engine or the braking system to slow the vehicle down accordingly, to maintain a safe following distance.

Known cruise control systems also cancel in the event that a wheel slip event is detected requiring intervention by a traction control system (TCS) or stability control system (SCS). Accordingly, they are not well suited to maintaining vehicle progress when driving in oft road conditions where such events may be relatively common.

It is also known to provide a control system for a motor vehicle for controlling one or more vehicle subsystems. U.S. Pat. No. 7,349,776 discloses a vehicle control system comprising a plurality of subsystem controllers including an engine management system, a transmission controller, a steering controller, a brakes controller and a suspension controller. The subsystem controllers are each operable in a plurality of subsystem function modes. The subsystem controllers are connected to a vehicle mode controller which controls the subsystem controllers to assume a required function mode so as to provide a number of driving modes for the vehicle. Each of the driving modes corresponds to a particular driving condition or set of driving conditions, and in each mode each of the sub-systems is set to the function mode most appropriate to those conditions. Such conditions are linked to types of terrain over which the vehicle may be driven such as grass/gravel/snow, mud and ruts, rock crawl, sand and a highway mode known as ‘special programs off’ (SPO). The vehicle mode controller may be referred to as a Terrain Response (TR)® System or controller. The driving modes may also be referred to as terrain modes, terrain response modes, or control modes.

SUMMARY OF THE INVENTION

Embodiments of the invention may be understood with reference to the appended claims.

Aspects of the present invention provide a system, a vehicle and a method.

In an aspect of the invention for which protection is sought there is provided a vehicle speed control system operable to cause a vehicle to operate in accordance with a target speed value by controlling an amount of brake torque applied by a braking system and an amount of drive torque applied by a powertrain to one or more wheels of the vehicle, the system being operable to detect a leading wheel step event in which a leading wheel encounters an obstacle presenting an abrupt increase in surface gradient, the system being operable in dependence on the detection of a leading wheel step event to cause brake torque to be applied against drive torque in anticipation of a corresponding trailing wheel step event, the system being operable to cause the application of brake torque at least until a trailing wheel has travelled a sufficient distance to reach the location at which a leading wheel experienced the leading wheel step event.

It is to be understood that the corresponding trailing wheel step event may include an event in which a trailing wheel encounters the same or a corresponding obstacle to that first encountered by the leading wheel.

The corresponding obstacle may be the same obstacle, especially if the vehicle is travelling in a straight line. The system may be operable to predict when the trailing wheel will encounter the obstacle (which may also be referred to herein as an object) at least in part in dependence on steering angle since a curvature of a path of the vehicle may influence when the vehicle encounters the obstacle.

Embodiments of the present invention have the advantage that a risk of rollback or bounce-back of the vehicle when one or more trailing wheels encounter a step in a driving surface may be reduced.

It is to be understood that if substantially no brake torque is being applied when the leading wheel step event is detected the amount of brake torque may be increased from substantially zero by the control system.

It is to be understood that brake torque requested by the vehicle speed control system may be applied against drive torque to reduce the risk of bounce-back substantially immediately upon detection of the leading wheel step event. Alternatively brake torque may be applied against drive torque to reduce the risk of bounce-back immediately prior to a trailing wheel reaching the location at which the leading wheel experienced the leading wheel step event.

In some embodiments brake torque may be biased selectively to leading wheels of the vehicle in response to detection of a leading wheel step event such that an amount of brake torque applied to the leading wheels is greater than that applied to the trailing wheels. In some embodiments brake torque may be applied substantially only to the leading wheels.

In some alternative embodiments, brake torque is applied selectively to trailing wheels of the vehicle in response to detection of a leading wheel step event such that an amount of brake torque applied to the trailing wheels is greater than that applied to the leading wheels. In some embodiments brake torque may be applied substantially only to the trailing wheels.

The system may be operable to increase the amount of drive torque applied to the one or more wheels to compensate at least in part for the increase in brake torque applied in response to detection of the leading wheel step event.

The system may be operable to detect a trailing wheel slip event.

It is to be understood that a trailing wheel slip event may include an event in which a trailing wheel encounters an abrupt increase in surface gradient.

The system may be operable to decrease the amount of brake torque applied against drive torque once a trailing wheel slip event has been detected.

The system may then continue to operate in accordance with a methodology for negotiating a surface step. However, importantly, the control system anticipates the occurrence of a trailing wheel step event when a leading wheel step event is detected and employs the braking system to act against the powertrain to resist ‘bounce back’ of the vehicle when the surface step is encountered by a trailing wheel.

It is to be understood that in some embodiments the control system may maintain at least some brake torque applied to one or more wheels against drive torque when the trailing wheel slip event is detected so as to reduce lurch of the vehicle when the obstacle triggering the trailing wheel step event has been mounted. It is to be understood that an amount of lurch, if any, of the vehicle as the vehicle crests the obstacle may be more readily managed in the presence of a least some brake torque acting against powertrain torque as the vehicle crests the obstacle. This may be particularly useful in vehicles having motors such as internal combustion engines for which there is a time delay in response of the motor to a change in torque demand due at least in part to inertia of the motor.

The system may be operable to limit a speed of the vehicle to a prescribed surface step speed limit value after a leading wheel surface step is detected and before a trailing wheel reaches the location at which the leading wheel step event was detected, the system being operable to lift the limit on speed when the trailing wheel has passed the location at which the leading wheel step event was detected.

The surface step speed limit value may be determined at least in part in dependence on one or more characteristics of terrain over which the vehicle is travelling.

The one or more characteristics may be determined at least in part by reference to a currently selected driving mode of the vehicle.

The control system may comprise a plurality of subsystem controllers each operable in a plurality of subsystem function modes, the system being operable to cause the vehicle to operate in one a of plurality of driving modes, wherein in each mode each of the sub-systems is set to the function mode most appropriate to those conditions.

The system may be operable to detect the leading wheel step event at least in part in dependence on detection of an abrupt increase in rate of deceleration of the vehicle consistent with a leading wheel step event.

The system may be operable to detect the leading wheel step event at least in part in dependence on detection that a rate of deceleration of the vehicle consistent with a leading wheel step event exceeds a prescribed value over a prescribed period.

It is to be understood that the system may be configured to distinguish between deceleration of the vehicle due to braking, and deceleration due to a surface step event.

Thus in some embodiments if a rate of deceleration of the vehicle exceeds a prescribed value over a prescribed period of time and the rate is not attributable to application of brake torque to slow the vehicle, the system may determine that a leading wheel step event has occurred. The system may in addition or instead monitor suspension articulation and/or vehicle pitch angle or pitch rate in order to verify that an increase in deceleration is consistent with a leading wheel encountering a step and not a trailing wheel encountering a step. Other arrangements are also useful.

One or both of the prescribed value of rate of deceleration and prescribed period may be substantially fixed values regardless of vehicle speed. One or both the prescribed values may be a function of speed or any other suitable parameter. Other arrangements are also useful.

In a further aspect of the invention for which protection is sought there is provided a vehicle comprising a system according to a preceding aspect.

The vehicle may have four wheels, the vehicle being operable in a four wheel drive mode in which each of the four wheels are driven by a powertrain of the vehicle.

The vehicle may be further operable in a two wheel drive mode in which only two wheels of the vehicle are driven by the powertrain.

In one aspect of the invention for which protection is sought there is provided a method of controlling a vehicle comprising:causing the vehicle to operate in accordance with a target speed value by controlling an amount of brake torque applied by a braking system and an amount of drive torque applied by a powertrain to one or more wheels of the vehicle;detecting a leading wheel step event in which a leading wheel encounters an obstacle presenting an abrupt increase in surface gradient; andcausing brake torque to be applied against drive torque in dependence on detection of a leading wheel step event in anticipation of a corresponding trailing wheel step event, the method comprising causing the application of brake torque at least until a trailing wheel has travelled a sufficient distance to reach the location at which a leading wheel experienced the leading wheel step event.

It is to be understood that the corresponding trailing wheel slip event may include an event in which a trailing wheel encounters the same or a corresponding obstacle.

In a further aspect of the invention for which protection is sought there is provided a vehicle speed control system operable to cause a vehicle to operate in accordance with a target speed value by controlling an amount of brake torque applied by a braking system and an amount of drive torque applied by a powertrain to one or more wheels of the vehicle, the system being operable to detect a leading wheel step event in which a leading wheel encounters an obstacle presenting an abrupt increase in surface gradient, the system being operable in dependence on the detection of a leading wheel step event to limit a speed of the vehicle to a prescribed surface step speed limit value after a leading wheel surface step is detected and before a trailing wheel reaches the location at which the leading wheel step event was detected, the system being operable to lift the limit on speed when the trailing wheel has passed the location at which the leading wheel step event was detected.

Optionally, the surface step speed limit value is determined at least in part in dependence on one or more characteristics of terrain over which the vehicle is travelling.

Optionally the one or more characteristics are determined at least in part by reference to a currently selected driving mode of the vehicle.

In one aspect of the invention for which protection is sought there is provided a method of controlling a vehicle comprising:causing the vehicle to operate in accordance with a target speed value by controlling an amount of brake torque applied by a braking system and an amount of drive torque applied by a powertrain to one or more wheels of the vehicle;detecting a leading wheel step event in which a leading wheel encounters an obstacle presenting an abrupt increase in surface gradient; andcausing a speed of the vehicle to be limited to a prescribed surface step speed limit value after a leading wheel surface step is detected and before a trailing wheel reaches the location at which the leading wheel step event was detected, the method comprising lifting the limit on speed when the trailing wheel has passed the location at which the leading wheel step event was detected.

In an aspect of the invention for which protection is sought there is provided a vehicle speed control system operable to cause a vehicle to operate in accordance with a target speed value by controlling an amount of brake torque applied by a braking system and an amount of drive torque applied by a powertrain to one or more wheels of the vehicle, the system being operable to detect a surface step event in which a leading wheel encounters an obstacle presenting an abrupt increase in surface gradient, the system being operable in dependence on the detection of the surface step event to cause brake torque to be applied against drive torque in anticipation of a step event being encountered by a trailing wheel corresponding to the obstacle first encountered by the leading wheel, the system being operable to cause the application of brake torque at least until a trailing wheel has travelled a sufficient distance to reach the location at which the leading wheel experienced the surface step event.

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

DETAILED DESCRIPTION

References herein to a block such as a function block are to be understood to include reference to software code for performing the function or action specified which may be an output that is provided responsive to one or more inputs. The code may be in the form of a software routine or function called by a main computer program, or may be code forming part of a flow of code not being a separate routine or function. Reference to function block is made for ease of explanation of the manner of operation of embodiments of the present invention.

FIG. 1shows a vehicle100according to an embodiment of the present invention. The vehicle100has a powertrain129that includes an engine121that is connected to a driveline130having an automatic transmission124. It is to be understood that embodiments of the present invention are also suitable for use in vehicles with manual transmissions, continuously variable transmissions or any other suitable transmission.

In the embodiment ofFIG. 1the transmission124may be set to one of a plurality of transmission operating modes, being a park mode, a reverse mode, a neutral mode, a drive mode or a sport mode, by means of a transmission mode selector dial124S. The selector dial124S provides an output signal to a powertrain controller11in response to which the powertrain controller11causes the transmission124to operate in accordance with the selected transmission mode.

The driveline130is arranged to drive a pair of front vehicle wheels111,112by means of a front differential137and a pair of front drive shafts118. The driveline130also comprises an auxiliary driveline portion131arranged to drive a pair of rear wheels114,115by means of an auxiliary driveshaft or prop-shaft132, a rear differential135and a pair of rear driveshafts139.

Embodiments of the invention are suitable for use with vehicles in which the transmission is arranged to drive only a pair of front wheels or only a pair of rear wheels (i.e. front wheel drive vehicles or rear wheel drive vehicles) or selectable two wheel drivel/our wheel drive vehicles. In the embodiment ofFIG. 1the transmission124is releasably connectable to the auxiliary driveline portion131by means of a power transfer unit (PTU)131P, allowing operation in a two wheel drive mode or a four wheel drive mode. It is to be understood that embodiments of the invention may be suitable for vehicles having more than four wheels or where only two wheels are driven, for example two wheels of a three wheeled vehicle or four wheeled vehicle or a vehicle with more than four wheels.

A control system for the vehicle engine121includes a central controller10, referred to as a vehicle control unit (VCU)10, the powertrain controller11, a brake controller13and a steering controller170C. The brake controller13forms part of a braking system22(FIG. 3). The VCU10receives and outputs a plurality of signals to and from various sensors and subsystems (not shown) provided on the vehicle. The VCU10includes a low-speed progress (LSP) control system12shown inFIG. 3and a stability control system (SCS)14. The SCS14improves the safety of the vehicle100by detecting and managing loss of traction. When a reduction in traction or steering control is detected, the SCS14is operable automatically to command a brake controller13to apply one or more brakes of the vehicle to help to steer the vehicle100in the direction the user wishes to travel. In the embodiment shown the SCS14is implemented by the VCU10. In some alternative embodiments the SCS14may be implemented by the brake controller13. Further alternatively, the SCS14may be implemented by a separate controller.

Although not shown in detail inFIG. 3, the VCU10further includes a Dynamic Stability Control (DSC) function block, a Traction Control (TC) function block, an Anti-Lock Braking System (ABS) function block and a Hill Descent Control (HDC) function block. These function blocks are implemented in software code run by a computing device of the VCU10and provide outputs indicative of, for example, DSC activity, TC activity, ABS activity, brake interventions on individual wheels and engine torque requests from the VCU10to the engine121in the event a wheel slip event occurs. Each of the aforementioned events indicate that a wheel slip event has occurred. Other vehicle sub-systems such as a roll stability control system or the like may also be useful.

As noted above, the vehicle100also includes a cruise control system16which is 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 system16is provided with a cruise control HMI (human machine interface)18by which means the user can input a target vehicle speed to the cruise control system16in a known manner. In one embodiment of the invention, cruise control system input controls are mounted to a steering wheel171(FIG. 5). The cruise control system16may be switched on by pressing a cruise control system selector button176. When the cruise control system16is switched on, depression of a ‘set-speed’ control173sets the current value of a cruise control set-speed parameter, cruise_set-speed to the current vehicle speed. Depression of a ‘+’ button174allows the value of cruise_set-speed to be increased whilst depression of a ‘−’ button175allows the value of cruise_set-speed to be decreased. A resume button173R is provided that is operable to control the cruise control system16to resume speed control at the instant value of cruise_set-speed following driver over-ride. It is to be understood that known on-highway cruise control systems including the present system16are configured so that, in the event that the user depresses a brake pedal163or, in the case of vehicles with a manual transmission, a clutch pedal (not shown), the cruise control function is cancelled and the vehicle100reverts to a manual mode of operation which requires accelerator pedal input by a user in order to maintain vehicle speed. In addition, detection of a wheel slip event, as may be initiated by a loss of traction, also has the effect of cancelling the cruise control function. Speed control by the system16is resumed if the driver subsequently depresses the resume button173R.

The cruise control system16monitors 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. In other words, the cruise control system is ineffective at speeds lower than 25 kph. The cruise control HMI18may also be configured to provide an alert to the user about the status of the cruise control system16via a visual display of the HMI18. In the present embodiment the cruise control system16is configured to allow the value of cruise_set-speed to be set to any value in the range 25-150 kph.

The LSP control system12also provides 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. Low-speed speed control (or progress control) functionality is not provided by the on-highway cruise control system16which operates only at speeds above 25 kph.

The LSP control system12is activated by means of a LSP control system selector button172mounted on the steering wheel171. The system12is operable to apply selective powertrain, traction control and braking actions to one or more wheels of the vehicle100, collectively or individually, to maintain the vehicle100at the desired speed.

The LSP control system12is configured to allow a user to input a desired value of set-speed parameter, LSP_set-speed to the LSP control system12via a low-speed progress control HMI (LSP HMI)20(FIG. 1,FIG. 3) which shares certain input buttons173-175with the cruise control system16and a hill descent control (HDC) control system12HD.

Provided the vehicle speed is within the allowable range of operation of the LSP control system (which is the range from 2 to 30 kph in the present embodiment although other ranges are also useful) the LSP control system12controls vehicle speed in accordance with the value of LSP_set-speed. Unlike the cruise control system16, the LSP control system12is configured to operate independently of the occurrence of a traction event. That is, the LSP control system12does not cancel speed control upon detection of wheel slip. Rather, the LSP control system12actively manages vehicle behaviour when slip is detected.

The LSP control HMI20is provided in the vehicle cabin so as to be readily accessible to the user. The user of the vehicle100is able to input to the LSP control system12, via the LSP HMI20, an indication of the speed at which the user desires the vehicle to travel (referred to as “the target speed”) by means of the ‘set-speed’ button173and the ‘+’/‘−’ buttons174,175in a similar manner to the cruise control system16. The LSP HMI20also includes a visual display upon which information and guidance can be provided to the user about the status of the LSP control system12.

The LSP control system12receives an input from the braking system22of the vehicle indicative of the extent to which the user has applied braking by means of the brake pedal163. The LSP control system12also receives an input from an accelerator pedal161indicative of the extent to which the user has depressed the accelerator pedal161. An input is also provided to the LSP control system12from the transmission or gearbox124. This input may include signals representative of, for example, the speed of an output shaft of the gearbox124, torque converter slip and a gear ratio request. Other inputs to the LSP control system12include an input from the cruise control HMI18which is representative of the status (ON/OFF) of the cruise control system16, and an input from the LSP control HMI20.

The HDC function block of the VCU10forms part of the HDC system12HD. When the HDC system12HD is active, the system12HD controls the braking system22(of which the ABS function block forms part) in order to limit vehicle speed to a value corresponding to that of a HDC set-speed parameter HDC_set-speed which may be set by a user. The HDC set-speed may also be referred to as an HDC target speed. Provided the user does not override the HDC system by depressing the accelerator pedal when the HDC system is active, the HDC system12HD controls the braking system22(FIG. 3) to prevent vehicle speed from exceeding the HDC_set-speed. In the present embodiment the HDC system12HD is not operable to apply positive drive torque. Rather, the HDC system12HD is only operable to apply negative brake torque.

A HDC system HMI20HD is provided by means of which a user may control the HOC system12HD, including setting the value of HDC_set-speed. An HDC system selector button177is provided on the steering wheel171by means of which a user may activate the HDC system12HD to control vehicle speed.

As noted above, the HDC system12HD is operable to allow a user to set a value of HOC set-speed parameter HDC_set-speed and to adjust the value of HDC_set-speed using the same controls as the cruise control system16and LSP control system12. Thus, in the present embodiment, when the HOC system12HD is controlling vehicle speed, the HDC system set-speed may be increased, decreased or set to an instant speed of the vehicle in a similar manner to the set-speed of the cruise control system16and LSP control system, using the same control buttons173,173R,174,175. The HDC system12HD is operable to allow the value of HDC_set-speed to be set to any value in the range from 2-30 kph.

If the HDC system12HD is selected when the vehicle100is travelling at a speed of 50 kph or less and no other speed control system is in operation, the HDC system12HD sets the value of HDC_set-speed to a value selected from a look-up table. The value output by the look-up table is determined in dependence on the identity of the currently selected transmission gear, the currently selected PTU gear ratio (Hi/LO) and the currently selected driving mode. The HDC system12HD then applies the powertrain129and/or braking system22to slow the vehicle100to the HDC system set-speed provided the driver does not override the HDC system12HD by depressing the accelerator pedal161. The HDC system12HD is configured to slow the vehicle100to the set-speed value at a deceleration rate not exceeding a maximum allowable rate. The rate is set as 1.25 ms-2 in the present embodiment, however other values are also useful. If the user subsequently presses the ‘set-speed’ button173the HDC system12HD sets the value of HDC_set-speed to the instant vehicle speed provided the instant speed is 30 kph or less. If the HDC system12HD is selected when the vehicle100is travelling at a speed exceeding 50 kph, the HDC system12HD ignores the request and provides an indication to the user that the request has been ignored due to the vehicle speed being above the limit for operation of the system HDC12HD.

It is to be understood that the VCU10is configured to implement a known Terrain Response (TR)® System of the kind described above in which the VCU10controls settings of one or more vehicle systems or sub-systems such as the powertrain controller11in dependence on a selected driving mode. The driving mode may be selected by a user by means of a driving mode selector141S (FIG. 1). The driving modes may also be referred to as terrain modes, terrain response modes, or control modes. In the embodiment ofFIG. 1four driving modes are provided: 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; a ‘grass, gravel or snow’ driving mode suitable for driving over grass, gravel or snow, a ‘rock crawl’ driving mode suitable for driving slowly over a rocky surface; and a ‘mud and ruts’ driving mode suitable for driving in muddy, rutted terrain. Other driving modes may be provided in addition or instead.

In some embodiments, the LSP control system12may be in either one of an active condition, a standby condition and an ‘off’ condition. In the active condition, the LSP control system12actively manages vehicle speed by controlling powertrain torque and braking system torque. In the standby condition, the LSP control system12does not control vehicle speed until a user presses the resume button173R or the ‘set speed’ button173. In the off condition the LSP control system12is not responsive to input controls until the LSP control system selector button172is depressed.

In the present embodiment the LSP control system12is also operable to assume an intermediate condition similar to that of the active mode but in which the LSP control system12is prevented from commanding the application of positive drive torque to one or more wheels of the vehicle100by the powertrain129. Thus, only braking torque may be applied, by means of the braking system22and/or powertrain129. Other arrangements are also useful.

With the LSP control system12in the active condition, the user may increase or decrease the vehicle set-speed by means of the ‘+’ and ‘−’ buttons174,175. In addition, the user may also increase or decrease the vehicle set-speed by lightly pressing the accelerator or brake pedals161,163respectively. In some embodiments, with the LSP control system12in the active condition the ‘+’ and ‘−’ buttons174,175are disabled such that adjustment of the value of LSP_set-speed can only be made by means of the accelerator and brake pedals161,163. This latter feature may prevent unintentional changes in set-speed from occurring, for example due to accidental pressing of one of the ‘+’ or ‘−’ buttons174,175. Accidental pressing may occur for example when negotiating difficult terrain where relatively large and frequent changes in steering angle may be required. Other arrangements are also useful.

It is to be understood that in the present embodiment the LSP control system12is operable to cause the vehicle to travel in accordance with a value of set-speed in the range from 2-30 kph whilst the cruise control system is operable to cause the vehicle to travel in accordance with a value of set-speed in the range from 25-150 kph although other values are also useful. If the LSP control system12is selected when the vehicle speed is above 30 kph but less than or substantially equal to 50 kph, the LSP control system12assumes the intermediate mode. In the intermediate mode, if the driver releases the accelerator pedal161whilst travelling above 30 kph the LSP control system12deploys the braking system22to slow the vehicle100to a value of set-speed corresponding to the value of parameter LSP_set-speed. Once the vehicle speed falls to 30 kph or below, the LSP control system12assumes the active condition in which it is operable to apply positive drive torque via the powertrain129, as well as brake torque via the powertrain129(via engine braking) and the braking system22in order to control the vehicle in accordance with the LSP_set-speed value. If no LSP_set-speed value has been set, the LSP control system12assumes the standby mode.

It is to be understood that if the LSP control system12is in the active mode, operation of the cruise control system16is inhibited. The two systems12,16therefore operate independently of one another, so that only one can be operable at any one time, depending on the speed at which the vehicle is travelling.

In some embodiments, the cruise control HMI18and the LSP control HMI20may be configured within the same hardware so that, for example, the speed selection is input via the same hardware, with one or more separate switches being provided to switch between the LSP input and the cruise control input.

FIG. 4illustrates the means by which vehicle speed is controlled in the LSP control system12. As described above, a speed selected by a user (set-speed) is input to the LSP control system12via the LSP control HMI20. A vehicle speed sensor34associated with the powertrain129(shown inFIG. 1) provides a signal36indicative of vehicle speed to the LSP control system12. The LSP control system12includes a comparator28which compares the set-speed38(also referred to as a ‘target speed’38) selected by the user with the measured speed36and provides an output signal30indicative of the comparison. The output signal30is provided to an evaluator unit40of the VCU10which interprets the output signal30as either a demand for additional torque to be applied to the vehicle wheels111-115, or for a reduction in torque applied to the vehicle wheels111-115, depending on whether the vehicle speed needs to be increased or decreased to maintain the speed LSP_set-speed. An increase in torque is generally accomplished by increasing the amount of powertrain torque delivered to a given position of the powertrain, for example an engine output shaft, a wheel or any other suitable location. A decrease in torque at a given wheel to a value that is less positive or more negative may be accomplished by decreasing powertrain torque delivered to a wheel and/or by increasing a braking force on a wheel. It is to be understood that in some embodiments in which a powertrain129has one or more electric machines operable as a generator, negative torque may be applied by the powertrain129to one or more wheels by the electric machine. Negative torque may also be applied by means of engine braking in some circumstances, depending at least in part on the speed at which the vehicle100is moving. If one or more electric machines are provided that are operable as propulsion motors, positive drive torque may be applied by means of the one or more electric machines.

An output42from the evaluator unit40is provided to the powertrain controller11and brake controller13which in turn control a net torque applied to the vehicle wheels111-115. The net torque may be increased or decreased depending on whether the evaluator unit40demands positive or negative torque. In order to cause application of the necessary positive or negative torque to the wheels, the evaluator unit40may command that positive or negative torque is applied to the vehicle wheels by the powertrain129and/or that a braking force is applied to the vehicle wheels by the braking system22, 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. In the illustrated embodiment the torque is applied to the vehicle wheels individually so as to maintain the vehicle at the required speed, but in another embodiment torque may be applied to the wheels collectively to maintain the required speed. In some embodiments, the powertrain controller11may be operable to control an amount of torque applied to one or more wheels by controlling a driveline component such as a rear drive unit, front drive unit, differential or any other suitable component. For example, one or more components of the driveline130may include one or more clutches operable to allow an amount of torque applied to one or more wheels to be varied. Other arrangements are also useful.

Where a powertrain129includes one or more electric machines, for example one or more propulsion motors and/or generators, the powertrain controller11may be operable to modulate torque applied to one or more wheels by means of one or more electric machines. The LSP control system12also receives a signal48indicative of a wheel slip event having occurred. This may be the same signal48that is supplied to the on-highway cruise control system16of the vehicle, and which in the case of the latter triggers an override or inhibit mode of operation in the on-highway cruise control system16so that automatic control of vehicle speed by the on-highway cruise control system16is suspended or cancelled. However, the LSP control system12is not arranged to cancel or suspend operation in dependence on receipt of a wheel slip signal48indicative of wheel slip. Rather, the system12is arranged to monitor and subsequently manage wheel slip so as to reduce driver workload. During a slip event, the LSP control system12continues to compare the measured vehicle speed with the value of LSP_set-speed, and continues to control automatically the torque applied to the vehicle wheels so as to maintain vehicle speed at the selected value. It is to be understood therefore that the LSP control system12is configured differently to the cruise control system16, for which a wheel slip event has the effect of overriding the cruise control function so that manual operation of the vehicle must be resumed, or speed control by the cruise control system12resumed by pressing the resume button173R or set-speed button173.

In a further embodiment of the present invention (not shown) a wheel slip signal48is derived not just from a comparison of wheel speeds, but further refined using sensor data indicative of the vehicle's speed over ground. Such a speed over ground determination may be made via global positioning (GPS) data, or via a vehicle mounted radar or laser based system arranged to determine the relative movement of the vehicle100and the ground over which it is travelling. A camera system may be employed for determining speed over ground in some embodiments.

At any stage of the LSP control process the user can override the function by depressing the accelerator pedal161and/or brake pedal163to adjust the vehicle speed in a positive or negative sense. However, in the event that a wheel slip event is detected via signal48whilst the LSP control system12is active, the LSP control system12remains active and control of vehicle speed by the LSP control system12is not suspended. As shown inFIG. 4, this may be implemented by providing a wheel slip event signal48to the LSP control system12which is then managed by the LSP control system12. In the embodiment shown inFIG. 1the SCS14generates the wheel slip event signal48and supplies it to the LSP control system12and cruise control system16. In some arrangements the SCS14provides the wheel slip event signal48to the LSP control system12or cruise control system16depending on which system is operating at the time. In some arrangements the SCS14broadcasts the signal48on a controller area network (CAN) bus (not shown) with which the LSP control system12and cruise control system16are in communication, whereby the systems12,16may detect the signal48,

A wheel slip event is triggered when a loss of traction occurs at any one of the vehicle wheels. Wheels and tyres may be more prone to losing traction when travelling for example on snow, ice, mud or sand and/or on steep gradients or cross-slopes. A vehicle100may also be more prone to losing traction in environments where the terrain is more uneven or slippery compared with driving on a highway in normal on-road conditions. Embodiments of the present invention therefore find particular benefit when the vehicle100is being driven in an off-road environment, or in conditions in which wheel slip may commonly occur. Manual operation in such conditions can be a difficult and often stressful experience for the driver and may result in an uncomfortable ride.

The vehicle100is also 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 system12,12HD 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 LSP and/or HDC control systems12,12HD. 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 is travelling.

The sensors (not shown) on the vehicle100include, but are not limited to, sensors which provide continuous sensor outputs to the VCU10, including wheel speed sensors, as mentioned previously and as shown inFIG. 5, 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 SCS14, a brake pedal position sensor, a brake pressure sensor, an accelerator pedal position sensor, longitudinal, lateral and vertical motion sensors, and water detection sensors forming part of a vehicle wading assistance system (not shown). In other embodiments, only a selection of the aforementioned sensors may be used.

The VCU10also receives a signal from the steering controller170C. The steering controller170C is in the form of an electronic power assisted steering unit (ePAS unit). The steering controller170C provides a signal to the VCU10indicative of the steering force being applied to steerable road wheels111,112of the vehicle100. This force corresponds to that applied by a user to the steering wheel171in combination with steering force generated by the ePAS unit170C.

The VCU10evaluates the various sensor inputs to determine the probability that each of a plurality of different control modes (driving modes) for the vehicle subsystems is appropriate, with each control mode corresponding to a particular terrain type over which the vehicle is travelling (for example, mud and ruts, sand, grass/gravel/snow).

If the user has selected operation of the vehicle in an automatic driving mode selection condition, the VCU10then selects the most appropriate one of the control modes and is configured automatically to control the subsystems according to the selected mode. This aspect of the invention is described in further detail in our co-pending patent application nos. GB1111288.5, GB1211910.3 and GB1202427.9, the contents of each of which is incorporated herein by reference.

The nature of the terrain over which the vehicle is travelling (as determined by reference to the selected control mode) may also be utilised in the LSP control system12to determine an appropriate increase or decrease in drive torque that is to be applied to the vehicle wheels. For example, if the user selects a value of LSP_set-speed that is not suitable for the nature of the terrain over which the vehicle is travelling, the system12is operable to automatically adjust the vehicle speed downwards by reducing the speed of the vehicle wheels. In some cases, for example, the user selected speed may not be achievable or appropriate over certain terrain types, particularly in the case of uneven or rough surfaces. If the system12selects a set-speed that differs from the user-selected set-speed, a visual indication of the speed constraint is provided to the user via the LSP HMI20to indicate that an alternative speed has been adopted.

When the LSP control system12is controlling vehicle speed, the system12is configured to monitor a rate of acceleration of the vehicle by reference to a signal indicative thereof. The signal may be produced by one or more accelerometers or other suitable sensing means mounted to the vehicle and arranged to measure vehicle acceleration in line with the longitudinal and/or vertical axis of the vehicle. If the rate of longitudinal acceleration indicates that the vehicle100is accelerating or decelerating at a rate consistent with the encountering of an obstacle presenting an abrupt increase in surface gradient such as a step, a pothole, or any other feature, the system12is configured to set a flag indicative of the detection of a leading wheel step event. The control system12then monitors vehicle progress from the location at which the leading wheel step event is detected.

When the vehicle100has travelled a prescribed distance after detection of the leading wheel step event, the system12commands application of a braking system22to oppose vehicle progress by commanding an increase in brake pressure and therefore brake torque of braking system22. Brake pressure is commanded to increase to a prescribed trailing wheel step event amount. Substantially simultaneously, the system12commands an increase in drive torque developed by the powertrain129to compensate for the increase in brake torque. The amount of powertrain torque is increased to compensate for the increase in brake torque such that vehicle speed is substantially unchanged, and any fluctuation or variation of the vehicle speed during the negotiation of the obstacle is controlled. In this way, vehicle ride quality and composure is enhanced when driving off-road, whilst the vehicle speed corresponds to the prevailing target speed value LSP_set-speed, maintained by the system12, reducing driver workload. In the present embodiment the prescribed distance corresponds to substantially half the distance between leading and trailing wheels of the vehicle100, i.e. substantially 50% of the wheelbase length value of the vehicle100. Other distances are useful.

In the present embodiment, the control system12is configured to ensure that the amount of brake torque applied by the braking system22when one or both trailing wheels reach the location at which the leading wheel step event occurred s at least equal to the prescribed trailing wheel step event amount. If the LSP control system12has already commanded application of brake torque by the braking system22for another reason, for example in response to the detection of a wheel slip event, the system12does not command a further increase in brake torque (or brake pressure) or an increase in powertrain torque unless the amount of brake torque (or brake pressure) is less than the prescribed trailing wheel step event amount. If the amount of brake torque (or pressure) is less than the prescribed trailing wheel step event amount, the control system12commands an increase in brake torque or brake pressure, and a corresponding increase in powertrain torque, such that the amount of brake torque or brake pressure is substantially equal to the prescribed trailing wheel step event amount. Thus, the prescribed trailing wheel step event amount may be considered to represent a minimum amount of brake torque or brake pressure for which the trailing wheel is permitted to negotiate the terrain feature. Other arrangements are also useful.

In some alternative embodiments the LSP control system12may be configured to increase the amount of brake torque (or brake pressure) applied by the braking system22in anticipation of a trailing wheel step event regardless of the prevailing amount of brake torque or brake pressure already applied by the braking system22, optionally subject to a prescribed ceiling amount of brake torque or pressure.

The LSP control system12ensures that the value of brake torque or brake pressure is at least equal to the prescribed trailing wheel step event amount and the vehicle is driven by the powertrain129against the brake torque developed by the braking system22. The control system12causes the vehicle100to be driven a sufficient distance to ensure that a trailing wheel of the vehicle100has reached the location at which the leading wheel experienced the leading wheel step event and began to negotiate the obstacle and the attendant increase in gradient. The system12may then reduce the amount of additional brake torque and the amount of additional powertrain torque (if any) that has been applied to resist roll-back. In some embodiments, at least a certain amount of brake pressure may continue to be applied against powertrain drive torque as the obstacle is negotiated in order to prevent excessive acceleration of the vehicle100once the trailing wheel begins to crest the obstacle. By cresting is meant that a pitch angle of the vehicle begins to decrease, i.e. a leading portion of the vehicle begins to pitch in a downward direction, as the gradient of the obstacle begins to decrease as the trailing wheel approaches completion of negotiation of the gradient encountered. Cresting may be detected in some embodiments by monitoring one or more vehicle parameters such as vehicle pitch or pitch rate, vehicle acceleration and/or powertrain torque demand. Cresting may be detected in some embodiments by monitoring powertrain torque demand and detecting an increase in vehicle speed and a corresponding decrease in powertrain torque as the vehicle crests an obstacle. In some embodiments, detection of cresting may be made by means of a combination of powertrain torque demand, vehicle acceleration and changes in pitch angle. Other arrangements are also useful.

In an example, a vehicle is traveling off-road on substantially flat terrain and one of the leading wheels of the vehicle encounters an obstacle, such as an isolated boulder, with an upper surface above the surface of the substantially flat surrounding terrain. The negotiation of the obstacle by the vehicle wheel gives rise to a step-event detectable by the system12. When the leading wheel of the vehicle climbs up the obstacle, the vehicle body will pitch upwards, adopting a ‘nose-up’ attitude, but will reach a maximum pitch angle (nose-up) just before the leading wheel passes over an uppermost portion of the boulder. The vehicle attitude will return towards a being substantially horizontal once the leading wheel clears the boulder. However, if the vehicle proceeds along a path so as to cause a trailing wheel to encounter the same obstacle, the vehicle will pitch downwards adopting a ‘nose-down’ attitude as the trailing wheel climbs the boulder. The vehicle will reach a maximum pitch angle (nose-down) just before the trailing wheel clears the uppermost portion of the boulder. The vehicle attitude will return to being substantially horizontal once the trailing wheel clears the boulder.

Once the leading wheel clears the obstacle, the detected step event is complete, but the system12will maintain brake application to one or more vehicle wheels so as to mitigate the severity of the contact between the trailing wheel and obstacle, which can give rise to an uncomfortable bounce-back or rebound of the wheel (and vehicle) off the obstacle, reducing vehicle momentum and degrading vehicle composure. In a typical off-road vehicle, if the step event occurs whilst ascending a steep slope, the rebound may cause the vehicle to temporarily move backwards down the slope until the driver increases positive torque applied to the wheels, whereas with a vehicle having a system12according to an embodiment of the present invention, the intervention of the system12mitigates this effect and greatly enhances off-road capability by maintaining momentum.

It will be appreciated from the example given above, that as the vehicle wheels encounter an obstacle during a step event, composed progress will be at least temporarily disturbed as the wheel transitions from rolling on a flat surface to climbing the obstacle. This transition results in a variation in vehicle acceleration, in particular parallel to the longitudinal and vertical axes, and variation in vehicle attitude. It is to be understood that the system12monitors and reacts to these variations.

As noted above the terrain presenting the abrupt increase in gradient may be a step, a pothole or other terrain feature.

It is to be understood that the control system12may perform additional control of brake torque applied by the braking system22and powertrain torque applied by the powertrain129in order to reduce rapid increases in acceleration of the vehicle as a vehicle negotiates a step, for example when a leading and/or trailing wheel crests a step or other abrupt terrain feature. Importantly, the system12causes an increase in brake torque prior to a trailing wheel encountering a surface step event corresponding to a surface step that has already been experienced by a leading wheel.

Embodiments of the present invention have the advantage that a control system may reduce a risk of roll-back of a vehicle100when one or both trailing wheels thereof encounter a surface step or other abrupt terrain feature. The LSP control system12employs intelligence gained in respect of terrain encountered by one or both leading wheels of the vehicle100to manage the negotiation of terrain by one or both trailing wheels, reducing or eliminating roll-back of the vehicle100due to trailing wheel step events. Embodiments of the invention enable an increase in vehicle composure to be achieved, resulting in enhancement of occupant enjoyment and a reduction in occupant fatigue.

In some embodiments the control system12may be configured to impose a ceiling on allowable vehicle speed to a prescribed maximum trailing wheel step event speed value when a leading wheel step event is detected. Thus, if the instant vehicle speed exceeds this value the speed is reduced such that the speed is less than or substantially equal to this value. This feature has the advantage of reducing a speed at which the trailing wheel impacts the obstacle, further reducing an amount of any rollback or bounce-back of the vehicle100.

An improvement in vehicle composure may therefore be enjoyed when the one or more trailing wheels experience the trailing wheel step event. It is to be understood that if, in contrast, a user were controlling vehicle speed manually, in the absence of a speed control system12according to an embodiment of the present invention, the vehicle would likely experience severe rollback when the obstacle was encountered. The system12may maintain vehicle speed at or below the prescribed maximum trailing wheel step event speed value at least until one or both trailing wheels have reached the location at which the leading wheel step event was experienced. The system12may then remove the ceiling on allowable vehicle speed imposed following the leading wheel step event, allowing the LSP control system12to determine an appropriate speed for negotiating the obstacle. Other arrangements are also useful. For example, the LSP control system12may maintain the ceiling on allowable vehicle speed until one or both of the trailing wheels have negotiated the obstacle.

It is to be understood that the system12may be operable to detect leading wheel step events in respect of left and right wheels of the vehicle independently of one another. The system12may then coordinate vehicle speed such that respective left and right trailing wheels of the vehicle100encounter the obstacle(s) already encountered by the respective leading wheels at a speed at or below the prescribed maximum trailing wheel step event speed value. Other arrangements are also useful.

FIG. 6illustrates a method of controlling the vehicle100ofFIG. 1according to the present embodiment.

At step S101the vehicle100is travelling over terrain with LSP control system12managing vehicle speed in accordance with the value of LSP_set-speed by controlling the powertrain129and braking system22. That is, the control system12attempts to cause the vehicle to travel at a speed substantially equal to LSP_set-speed subject to any requirement to reduce speed, for example when it is determined that the prevailing value of LSP_set-speed is inappropriate for the prevailing terrain.

At step S103the LSP control system12checks whether a leading wheel step event is detected. The system12does this by monitoring vehicle acceleration; if vehicle acceleration is negative (indicating deceleration of the vehicle100) and the deceleration is not attributable to braking of the vehicle100by the braking system22or powertrain129, the method continues at step S105else the method continues at step S101.

At step S105the LSP control system12starts measuring the distance travelled by the vehicle100since the leading wheel step event was detected.

The LSP control system12also checks the pressure of brake fluid in the braking system22by reference to a brake fluid pressure signal before continuing at step S107.

At step S107the system12determines whether the pressure of brake fluid is less than a prescribed trailing wheel step event amount. If the pressure is less than the prescribed trailing wheel step event amount the method continues at step S109, else the method continues at step S111.

At step S109, when the distance travelled by the vehicle100since the leading wheel step event was detected has reached half the wheelbase value of the vehicle100, the system12commands an increase in brake pressure by the braking system22to a value substantially equal to the prescribed trailing wheel step event amount.

A corresponding increase in powertrain torque is also commanded at this time, in order to cause the vehicle100to continue to operate in accordance with the value of LSP_set-speed. In this way, the system12effectively maintains vehicle composure whilst negotiating step-like obstacles by balancing the positive and negative torque applied to the wheels by the powertrain129and braking system22. This effectively mitigates, at least in part, the effect the obstacle may otherwise have on vehicle progress, composure and occupant comfort.

At step S111the system12checks whether the trailing wheels of the vehicle100have passed the location at which the leading wheel step event was detected. If the trailing wheels have passed this location the method continues at step S113. If the trailing wheels have not passed this location, step S111is repeated. It is to be understood that the system12may be configured to terminate the method if the trailing wheels do not pass the location at which the leading wheel step event was detected within a prescribed period of the detection of the leading wheel step event. The period may be referred to as a ‘time out’ period.

At step S113the LSP control system12commands a reduction in brake pressure of the braking system22to the prevailing value before the leading wheel step event was detected and adjusts powertrain torque accordingly. The method then continues at step S101. In some embodiments the LSP control system12may command a reduction in brake pressure substantially to zero, and a corresponding reduction in powertrain torque before continuing at step S101. It is to be understood, however, that the control system12continues to cause the vehicle100to operate in accordance with the value of LSP_set-speed or a lower limit of vehicle speed if one has been imposed.

Embodiments of the present invention enable a substantial improvement in vehicle composure when negotiating obstacles at least a portion of which presents a relatively abrupt increase in gradient of a driving surface. Embodiments of the invention reduce or eliminate bounce-back or longitudinal rebound of a vehicle when one or more trailing wheels encounter an object previously encountered by a leading wheel. This is accomplished in some embodiments by detecting the presence of an obstacle and applying brake force against driving force ahead of the encounter.

In another embodiment, not shown, the system12is further provided with input means arranged to permit the user to configure the operation of the system12and a memory to store these user defined preferences. In this way, the user may input an indication of an amount of intervention, whether more intervention or less intervention, to suit their driving style. In some embodiments the system12may permit a user to activate or deactivate the system12manually, via the input means. Where the system12is arranged to store user preferences, it may be further configured to store data relating to one or more trailers that the user wishes to tow with the vehicle. By inputting into the system12details specific to the type of trailer, such as drawbar length, number of axles, axle width and total vehicle and trailer unit length, the system12may compensate not only for the trailing wheel of the vehicle but also the wheels of the towed trailer. This approach greatly improves occupant comfort when towing a trailer over rough or varied terrain and reduces wear on both the vehicle and the trailer. The input means may be in the form of one or more physical control dials, switches or knobs, a touchscreen panel, or any other suitable input means.

FIG. 7illustrates a method according to an embodiment of the present invention.

At step S101the vehicle100is travelling over terrain with LSP control system12managing vehicle speed in accordance with the value of LSP_set-speed by controlling the powertrain129and braking system22. That is, the control system12attempts to cause the vehicle to travel at a speed substantially equal to LSP_set-speed subject to any requirement to reduce speed, for example when it is determined that the prevailing value of LSP_set-speed is inappropriate for the prevailing terrain.

At step S103the LSP control system12checks whether a leading wheel step event is detected. The system12does this be monitoring vehicle acceleration; if vehicle acceleration is negative (indicating deceleration of the vehicle100) and the deceleration is not attributable to braking of the vehicle100by the braking system22or powertrain129, the method continues at step S105else the method continues at step S101. As noted above in respect of the method ofFIG. 6, one or more other parameters may be monitored such as pitch rate and suspension articulation in order to distinguish between step events at leading and trailing wheels.

At step S105the LSP control system12determines whether the instant vehicle speed is greater than a prescribed trailing wheel step speed limit value. If the instant speed is greater than this value the method continues at step S107. If the instant speed is not greater than this value the method continues at step S109. The value of trailing wheel step speed limit may be set in dependence on one or more parameters such as terrain type, terrain roughness, and/or a severity of the leading wheel step event. The value of trailing wheel step speed limit may be set so as to achieve a desired balance between a desire for the vehicle to travel at a sufficiently high speed to make satisfactory progress over terrain, and a requirement to maintain vehicle composure when the trailing wheel experiences the trailing wheel step event. The value may be as low as 1 or 2 kph in some embodiments. In some embodiments the value of trailing wheel step speed limit is selected in the range from 3 to 10 kph depending on the severity of the leading wheel step event. Severity of the leading wheel step event may be determined in dependence on the maximum rate of deceleration experienced by the vehicle100when the leading wheel step event is detected.

At step S107the LSP control system12causes the vehicle speed to reduce to a value at or below the trailing wheel step speed limit value. The system12is configured to ensure that the speed of the vehicle100is reduced to such a value before the trailing wheel reaches a location at which the leading wheel step event was detected. The system12then causes the vehicle100to travel at a target speed that is temporarily set substantially equal to the trailing wheel step speed limit value unless the user reduces the target speed below this value, i.e. the user sets the value of LSP_set-speed to a value below the trailing wheel step speed limit value.

As step S109the LSP control system12causes the vehicle100to travel at a speed that does not exceed the trailing wheel step speed limit value. If the value of LSP_set-speed is lower than the trailing wheel step speed limit value the system12causes the vehicle to operate in accordance with a target speed that is equal the value of LSP_set-speed; if the value of LSP_set-speed is subsequently set to a value higher than the trailing wheel step speed limit value before the trailing wheel has reached the location at which the leading wheel step event was detected, the LSP control system12causes a speed of the vehicle100to be limited to the trailing wheel step speed limit value until the trailing wheels have passed the location at which the leading wheel step event was detected.

At step S11the LSP control system12checks whether trailing wheels of the vehicle100have passed the location at which the leading wheel step event was detected. If the vehicle100has passed this location the system12continues at step S113. It the vehicle100has not passed this location the system12continues at step S109.

At step S113the LSP control system12resumes causing the vehicle100to operate in accordance with the value of LSP_set-speed.