Patent Publication Number: US-10328936-B2

Title: Vehicle speed control system and method for limiting the rate of acceleration when changing from a first target speed to a second one due to a request from the accelerator pedal

Description:
INCORPORATION BY REFERENCE 
     The content of patent applications WO2014/027069, GB2492748, GB2492655 and GB2499252 is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to vehicle speed control systems. In particular but not exclusively the invention relates to monitoring of vehicle speed control systems to ensure correct operation. 
     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, referred to as a set-speed, and the vehicle is maintained at a target speed that is set equal to the set-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 by a sufficient amount 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 (set-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 (TC system or TCS) or stability control system (SCS). Accordingly, they are not well suited to maintaining vehicle progress when driving in off road conditions where such events may be relatively common. 
     It is an aim of embodiments of the present invention to address disadvantages associated with the prior art. 
     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 one aspect of the invention for which protection is sought there is provided a speed control system for a motor vehicle, the speed control system being configured to cause automatic operation of the vehicle in accordance with a first target speed value stored by the system at least in part by controlling automatically an amount of drive torque applied to one or more wheels by a powertrain, the speed control system being configured to receive an instantaneous driver drive demand signal in response to a prevailing driver displacement of an accelerator pedal; and wherein the speed control system is configured to determine a second target speed value greater than the first target speed value in dependence on the instantaneous driver drive demand signal; the system being further configured to cause automatic acceleration of the vehicle from the first target speed value to the second target speed value at a rate not exceeding a maximum acceleration. 
     This feature has the advantage that the speed control system continues to control vehicle speed even when a driver overrides the speed control system by depressing the accelerator pedal, i.e. changes in speed during a driver speed override intervention are controlled by the speed control system. Consequently, the speed control system is able to ensure that the rate of acceleration of the vehicle does not exceed a maximum acceleration value. This enables vehicle composure to be maintained even when the driver overrides the speed control system. 
     In some embodiments, if the instantaneous driver drive demand signal indicates a sufficiently large drive demand, such as a sufficiently large torque demand, the system may suspend limiting the rate of acceleration to the maximum acceleration value and permit the amount of drive demanded from the powertrain to correspond substantially to that instantaneously demanded by the driver drive demand signal. For example, the driver drive demand signal may correspond to a powertrain torque demand, and the driver drive demand signal may be conveyed to a powertrain controller which may in turn cause the powertrain to develop an amount of torque that is substantially equal to that demanded by a driver via the accelerator pedal. 
     Optionally, the system may be configured to cause the vehicle to accelerate from the first target speed value to the second target speed value according to a predetermined speed or acceleration profile. 
     Optionally, the predetermined speed or acceleration profile may be dependent on at least one vehicle parameter. 
     Optionally, the predetermined speed or acceleration profile may be dependent on the identity of a driving mode, selected from a plurality of driving modes, in which the vehicle is operating. 
     This feature has the advantage that the speed or acceleration profile may be selected to be one that is appropriate for the driving mode in which the vehicle is operating. For example, in the case the vehicle is operating in a driving mode appropriate to operation on a driving surface of relatively high surface coefficient of friction, or a driving mode appropriate to operation on sand, the speed or acceleration profile may correspond to a higher rate of acceleration than if the vehicle is operating in a driving mode appropriate to operation on a driving surface of relatively low surface coefficient of friction such as grass, gravel or snow, mud and ruts, or ice. 
     Optionally, the system may be configured to receive a signal indicative of the driving mode in which a user requires the vehicle to operate. 
     The system may be provided in combination with a user-operable driving mode control by means of which the signal indicative of the driving mode in which a user requires the vehicle to operate is generated. 
     The system may be provided in combination with automatic driving mode selection means configured to select automatically a driving mode appropriate to a driving surface over which a vehicle is driving when the vehicle is operated in an automatic driving mode selection mode. 
     The system may be provided in further combination with a user-operable automatic driving mode control input for selecting operation of the vehicle in the automatic driving mode selection mode. 
     Optionally, the system may be configured wherein the driving modes are control modes of at least one subsystem of a vehicle selected from amongst an engine management system, a transmission system, a steering system, a brakes system and a suspension system, the speed control system being provided in combination with a subsystem controller for initiating control of the or each of the vehicle subsystems in the selected one of the plurality of subsystem control modes, each of the subsystem control modes corresponding to one or more different driving surfaces. 
     It is to be understood that subsystem control modes may be provided that correspond to driving style, or any other suitable parameter. Driving style may for example be determined by the control system in dependence on user selection of a required driving style such as a performance oriented driving style, or an economy oriented driving style. 
     Optionally, the system may be configured to determine the maximum rate of acceleration of the vehicle in dependence at least in part on the first and second target speed values, the system being configured to limit the rate of acceleration according to the maximum rate. 
     Optionally, the system may be configured to monitor an amount of slip of one or more wheels of the vehicle whilst causing automatic acceleration of the vehicle from the first target speed value to the second target speed value, the system being configured to: reduce a net torque applied to one or more wheels in dependence on the amount of slip of one or more wheels thereby to reduce slip when slip exceeds a predetermined amount, or control a net torque applied to one or more wheels in dependence on the amount of slip of one or more wheels thereby to prevent slip from exceeding a predetermined amount. 
     The predetermined amount may be dependent on one or more vehicle parameters such as a speed of travel of a vehicle over a driving surface. The speed of travel over a driving surface may be determined by reference to one or more parameters such as a camera device, a radar system, a speed of one or more wheels such as an average speed of one or more wheels or a speed of a wheel that is not driven by a powertrain. Other methods of determining vehicle speed may be useful in addition or instead. 
     Thus it is to be understood that the system may cause the vehicle to accelerate to the second target speed value whilst actively managing the acceleration rate to maintain traction. 
     Optionally, the system may be configured wherein the second target speed value is a target speed value stored by the system. 
     Optionally, the second target speed value may be stored in a memory of the system. 
     The system may be configured to cause a powertrain to commence causing the vehicle to operate in accordance with the second target speed value in dependence on the driver drive demand signal when the driver demand signal corresponds to a greater drive torque than that required by the system to cause the vehicle to operate in accordance with the first target speed value. 
     It is to be understood that in some embodiments the speed control system may be configured to generate a virtual driver drive demand signal and to supply this signal to a powertrain controller, for example an engine controller. The instantaneous driver drive demand signal may correspond to the position of an accelerator pedal that is actuated by a driver. The virtual driver drive demand signal may correspond to a virtual accelerator pedal position. The control system may be configured to compare the instantaneous driver drive demand signal with the virtual driver drive demand signal, and cause the vehicle to operate in accordance with the second target speed value in dependence on whether the instantaneous driver drive demand signal corresponds to a greater amount of torque than the virtual driver drive demand signal. The speed control system may calculate the second target speed value in dependence on the instantaneous driver drive demand signal when the instantaneous driver drive demand signal exceeds the virtual driver drive demand signal. 
     The system may be configured to calculate a rate of acceleration of a vehicle that would be expected according to the instantaneous driver drive demand signal when not operating in a speed control mode, and to calculate the second target speed value in dependence on the expected rate of acceleration. 
     Thus, the system may calculate a value of acceleration of the vehicle in dependence on the instantaneous driver demand signal. The value of acceleration may be that which would be experienced if the speed control system was not causing the vehicle to operate in accordance with a target speed value, for example if the vehicle control system was in the first mode and the instantaneous driver drive demand signal was controlling directly the amount of drive torque applied to one or more wheels by a powertrain without the speed control system acting to control powertrain torque. 
     Optionally, the second target speed value corresponds to an integrated value of the expected rate of acceleration of a vehicle according to the prevailing instantaneous driver drive demand signal. 
     Optionally, the system may be configured to recalculate the second target speed value substantially continuously. 
     Optionally, the system may be configured to recalculate the second target speed value substantially continuously when the instantaneous driver drive demand signal corresponds to an amount of drive torque that is greater than an amount of drive torque required by the system to cause the vehicle to operate in accordance with the first target speed value. 
     Optionally, the system may be configured to cause the second target speed value gradually to become equal to the first target speed value when the driver demand signal corresponds to an amount of drive torque substantially equal to or less than that required by the system to cause the vehicle to operate in accordance with the first target speed value. 
     This feature has the advantage that the system may smoothly accommodate changes in target speed value due to changes in the value of driver drive demand signal when the control system is operating in the second mode. 
     Optionally, the system may be configured to cause the second target speed value gradually to become equal to the first target speed value when the driver demand signal indicates a driver is demanding substantially no drive torque. 
     Optionally, the system may be configured to cause the second target speed value gradually to become equal to the first target speed value by blending the first and second target speed values over time when the driver demand signal corresponds to an amount of drive torque substantially equal to or less than that required by the system to cause the vehicle to operate in accordance with the first target speed value. 
     Optionally, the system may be configured to cause a vehicle to operate in accordance with a target speed value by causing a vehicle to travel at a speed substantially equal to the target speed value. 
     Optionally, the system may be configured to allow a user to input a target speed value, the system being configured to set the first target speed value equal to the target speed value input by the user. 
     Optionally, the system may be configured to set the first target speed value to a value lower than the target speed value input by the user in dependence on one or more parameters. 
     Optionally, the system may be configured to set the first target speed value to a value lower than the target speed value input by the user in dependence on one or more parameters the parameters being selected from amongst one or more parameters indicative of movement of at least a portion of a body of a vehicle, one or more parameters indicative of movement of at least a portion of a body of an occupant, and one or more parameters indicative of the nature of terrain over which a vehicle is driving. 
     The one or more parameters indicative of movement of at least a portion of a body of a vehicle may include lateral, longitudinal and or vertical acceleration. Other parameters may also be useful in addition or instead. The system may be configured to reduce the first target speed value to a value lower than the value input by the user so as to enhance vehicle composure and user enjoyment of a vehicle when travelling over relatively rough terrain. 
     The nature of the terrain over which the vehicle is driving may be determined by reference to a user input indicative of terrain such as a user operable terrain response (TR) mode selector or a signal indicative of a terrain response mode selected automatically by a controller. It is to be understood that vehicles with user selectable TR (RTM) modes are known and more recently vehicle with automatic TR mode selection functionality have become known. 
     Optionally, the speed control system comprises a speed controller configured to receive the driver drive demand signal and cause the vehicle to operate in accordance with the second target speed value greater than the first in dependence at least in part on the driver drive demand signal. 
     Optionally, the first controller is hosted by an anti-lock braking system (ABS) controller. 
     It is to be understood that the controller or controllers described herein may comprise a control unit or computational device having one or more electronic processors. 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 “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 the stated control functionality. A set of instructions could be provided which, when executed, cause said computational device to implement the control techniques described herein. The set of instructions could be embedded in said one or more electronic processors. Alternatively, the set of instructions could be provided as software to be executed on said computational device. The 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 controller. Other arrangements are also useful. 
     In a further aspect of the invention for which protection is sought there is provided a vehicle comprising a body, a plurality of wheels, a powertrain to drive said wheels, a braking system to brake said wheels, and a system according to another aspect. 
     In an 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 first target speed value stored by the system at least in part by controlling automatically an amount of drive torque applied to one or more wheels by a powertrain; 
     receiving an instantaneous driver drive demand signal in response to a prevailing driver displacement of an accelerator pedal; 
     determining a second target speed value greater than the first target speed value in dependence on the instantaneous driver drive demand signal; and 
     causing automatic acceleration of the vehicle from the first target speed value to the second target speed value at a rate not exceeding a maximum acceleration. 
     The method may comprise causing the vehicle to accelerate from the first target speed value to the second target speed value according to a predetermined speed or acceleration profile. 
     The method may comprise determining the speed or acceleration profile in dependence on at least one vehicle parameter. 
     The method may comprise determining the speed or acceleration profile in dependence on the identity of the driving mode, selected from a plurality of driving modes, in which the vehicle is operating. 
     The method may comprise receiving a signal indicative of the driving mode in which a user requires the vehicle to operate. 
     The method may comprise receiving a signal indicative of the driving mode in which a user requires the vehicle to operate by means of a user-operable driving mode control. 
     The method may comprise selecting automatically a driving mode appropriate to a driving surface over which a vehicle is driving when the vehicle is operated in an automatic driving mode selection mode. 
     Optionally the driving modes are control modes of at least one subsystem of a vehicle selected from amongst an engine management system, a transmission system, a steering system, a brakes system and a suspension system, 
     the method comprising causing a subsystem controller to control one or more vehicle subsystems in the selected one of the plurality of subsystem control modes, each of the subsystem control modes corresponding to one or more different driving surfaces. 
     The method may comprise determining the maximum rate of acceleration of the vehicle in dependence at least in part on the first and second target speed values, the method comprising limiting the rate of acceleration according to the maximum rate. 
     The method may comprise monitoring slip of one or more wheels of the vehicle whilst causing automatic acceleration of the vehicle from the first target speed value to the second target speed value, and 
     reducing a net torque applied to one or more wheels in dependence on the amount of slip of one or more wheels thereby to reduce slip when slip exceeds a predetermined amount, or 
     controlling net torque applied to one or more wheels in dependence on the amount of slip of one or more wheels thereby to prevent slip from exceeding a predetermined amount. 
     Optionally, the method may comprise setting the first target speed value to a value input by a user. 
     In one aspect of the invention for which protection is sought there is provided a system comprising a speed control system configured automatically to cause a vehicle to operate in accordance with a first target speed value stored by the system at least in part by controlling an amount of drive torque applied to one or more wheels by a powertrain, the system being configured to receive a driver drive demand signal indicative of an amount of driver demanded drive torque to be applied to one or more wheels, the system being configured to cause the vehicle to operate in accordance with a second target speed value greater than the first in dependence at least in part on the driver drive demand signal. 
     The second target speed value may also be a target speed value stored by the system, for example in a memory of the system. 
     Optionally the system is configured to cause a powertrain to commence causing the vehicle to operate in accordance with the second target speed value in dependence on the driver drive demand signal when the driver demand signal corresponds to a greater drive torque than that required by the system to cause the vehicle to operate in accordance with the first target speed value. 
     Optionally the system is configured to calculate an expected rate of acceleration of a vehicle according to the prevailing driver drive demand and to calculate the second target speed value in dependence on the expected rate of acceleration. 
     Thus, the system calculates a value of acceleration of the vehicle in dependence on the driver demand signal. The value of acceleration may be that which would be experienced if the speed control system were not causing the vehicle to operate in accordance with a target speed value, and the driver drive demand signal was controlling directly the amount of drive torque applied to one or more wheels by a powertrain. 
     Optionally the second target speed value corresponds to an integrated value of the expected rate of acceleration of a vehicle according to the prevailing driver drive demand signal. 
     Optionally the system is configured to recalculate the second target speed value substantially continuously. 
     In a further aspect of the invention for which protection is sought there is provided a method of operating a vehicle implemented by means of a control system comprising: 
     automatically causing a vehicle to operate in accordance with a first target speed value stored by the system at least in part by controlling an amount of drive torque applied to one or more wheels by a powertrain; 
     receiving a driver drive demand signal indicative of an amount of driver demanded drive torque to be applied to one or more wheels; and 
     causing the vehicle to operate in accordance with a second target speed value greater than the first in dependence at least in part on the driver drive demand signal. 
     In one aspect of the invention for which protection is sought there is provided a carrier medium carrying computer readable code for controlling a vehicle to carry out the method of another aspect. 
     In one aspect of the invention for which protection is sought there is provided a computer program product executable on a processor so as to implement the method of another aspect. 
     In one aspect of the invention for which protection is sought there is provided a computer readable medium loaded with the computer program product of another aspect. 
     In one aspect of the invention for which protection is sought there is provided a processor arranged to implement the method of another aspect. 
     In one aspect of the invention for which protection is sought there is provided a motor vehicle control system comprising a speed control system, the system being operable in one of at least a first mode and a second mode, 
     wherein in the first mode the speed control system does not control vehicle speed and an amount of drive torque applied to one or more wheels by a powertrain is controlled at least in part by reference to a driver drive demand signal indicative of an amount of driver demanded drive torque to be applied to one or more wheels, and 
     in the second mode the speed control system is configured automatically to cause a vehicle to operate in accordance with a first target speed value stored by the system at least in part by controlling automatically an amount of drive torque applied to one or more wheels by a powertrain, the speed control system being configured to receive the driver drive demand signal and to cause the vehicle to operate in accordance with a second target speed value greater than the first in dependence at least in part on the driver drive demand signal. 
     The second target speed value may also be a target speed value stored by the system, for example in a memory of the system. 
     The system may be configured when in the second mode to cause a powertrain to commence causing the vehicle to operate in accordance with the second target speed value in dependence on the driver drive demand signal when the driver demand signal corresponds to a greater drive torque than that required by the system to cause the vehicle to operate in accordance with the first target speed value. 
     In an aspect of the invention for which protection is sought there is provided a method of operating a vehicle implemented by means of a control system comprising: 
     in a first mode of operation of the control system, controlling an amount of drive torque applied to one or more wheels by a powertrain at least in part by reference to a driver drive demand signal indicative of an amount of driver demanded drive torque to be applied to one or more wheels; and 
     in a second mode of operation of the control system, automatically causing a vehicle to operate in accordance with a first target speed value stored by the system at least in part by controlling automatically an amount of drive torque applied to one or more wheels by a powertrain, 
     the method comprising, when in the second mode, causing the vehicle to operate in accordance with a second target speed value greater than the first in dependence at least in part on the driver drive demand signal. 
     The method may comprise setting the first target speed value to a value input by a user. 
     In one aspect of the invention for which protection is sought there is provided a system comprising a speed control system configured automatically to cause a vehicle to operate in accordance with a first target speed value stored by the system at least in part by controlling an amount of drive torque applied to one or more wheels by a powertrain, the system being configured to receive a driver drive demand signal indicative of an amount of driver demanded drive torque to be applied to one or more wheels, the system being configured to cause the vehicle to operate in accordance with a second target speed value greater than the first in dependence at least in part on the driver drive demand signal 
     Other arrangements may also be useful. 
     In one aspect of the invention for which protection is sought there is provided a motor vehicle comprising a system according to another aspect. 
     In one aspect of the invention for which protection is sought there is provided a vehicle comprising a chassis, a body attached to said chassis, a plurality of wheels, a powertrain to drive said wheels, a braking system to brake said wheels, and a system according to another aspect. 
     Within the scope of this application it is envisaged that the various aspects, embodiments, examples and alternatives, and in particular the individual features thereof, set out in the preceding paragraphs, in the claims and/or in the following description and drawings, may be taken independently or in any combination. For example features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. 
     For the avoidance of doubt, it is to be understood that features described with respect to one aspect of the invention may be included within any other aspect of the invention, alone or in appropriate combination with one or more other features. 
    
    
     
       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 figures in which: 
         FIG. 1  is a schematic illustration of a vehicle according to an embodiment of the invention in plan view; 
         FIG. 2  shows the vehicle of  FIG. 1  in side view; 
         FIG. 3  is a high level schematic diagram of an embodiment of the vehicle speed control system of the present invention, including a cruise control system and a low-speed progress control system; 
         FIG. 4  is a schematic diagram of further features of the vehicle speed control system in  FIG. 3 ; 
         FIG. 5  illustrates a steering wheel and brake and accelerator pedals of a vehicle according to an embodiment of the present invention; 
         FIG. 6  is a schematic illustration of a known key fob for use with the vehicle of  FIG. 1 ; 
         FIG. 7  is a schematic illustration of a portion of a controller according to an embodiment of the present invention; 
         FIG. 8  is a schematic illustration of a portion of a controller according to an embodiments of the present invention in further detail; and 
         FIG. 9  is a schematic illustration of a portion of a controller according to a further embodiment of the present invention. 
     
    
    
     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. 1  shows a vehicle  100  according to an embodiment of the present invention. The vehicle  100  has a powertrain  129  that includes an engine  121  that is connected to a driveline  130  having an automatic transmission  124 . 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 of  FIG. 1  the transmission  124  may be set to one of a plurality of transmission operating modes, being a park mode P, a reverse mode R, a neutral mode N, a drive mode D or a sport mode S, by means of a transmission mode selector dial  124 S. The selector dial  124 S provides an output signal to a powertrain controller  11  in response to which the powertrain controller  11  causes the transmission  124  to operate in accordance with the selected transmission mode. 
     The driveline  130  is arranged to drive a pair of front vehicle wheels  111 , 112  by means of a front differential  137  and a pair of front drive shafts  118 . The driveline  130  also comprises an auxiliary driveline portion  131  arranged to drive a pair of rear wheels  114 ,  115  by means of an auxiliary driveshaft or prop-shaft  132 , a rear differential  135  and a pair of rear driveshafts  139 . The front wheels  111 ,  112  in combination with the front drive shafts  118  and front differential  137  may be referred to as a front axle  136 F. The rear wheels  114 ,  115  in combination with rear drive shafts  139  and rear differential  135  may be referred to as a rear axle  136 R. 
     The wheels  111 ,  112 ,  114 ,  115  each have a respective brake  111 B,  112 B,  114 B,  115 B. Respective speed sensors  111 S,  112 S,  114 S,  115 S are associated with each wheel  111 ,  112 ,  114 ,  115  of the vehicle  100 . The sensors  111 S,  112 S,  114 S,  115 S are mounted to a chassis  100 C of the vehicle  100  and arranged to measure a speed of the corresponding wheel. 
     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 drive/four wheel drive vehicles. In the embodiment of  FIG. 1  the transmission  124  is releasably connectable to the auxiliary driveline portion  131  by means of a power transfer unit (PTU)  131 P, 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 engine  121  includes a central controller  10 , referred to as a vehicle control unit (VCU) 10, the powertrain controller  11 , a brake controller  13  and a steering controller  170 C. The brake controller  13  is an anti-lock braking system (ABS) controller  13  and forms part of a braking system  22  ( FIG. 3 ). The VCU 10 receives and outputs a plurality of signals to and from various sensors and subsystems (not shown) provided on the vehicle. The VCU 10 includes a low-speed progress (LSP) control system  12  shown in  FIG. 3 , a stability control system (SCS)  14 S, a traction control system (TCS)  14 T, a cruise control system  16  and a Hill Descent Control (HDC) system  12 HD. The SCS  14 S improves stability of the vehicle  100  by detecting and managing loss of traction when cornering. When a reduction in steering control is detected, the SCS  14 S is configured automatically to command a brake controller  13  to apply one or more brakes  111 B,  112 B,  114 B,  115 B of the vehicle  100  to help to steer the vehicle  100  in the direction the user wishes to travel. In some embodiments, in addition or instead the SCS  14 S may reduce powertrain torque to one or more wheels. If excessive wheel spin is detected, the TCS  14 T is configured to reduce wheel spin by application of brake force in combination with a reduction in powertrain drive torque. In some embodiments, in addition or instead the TCS  14 T may cause application of brake force without reducing powertrain drive torque. In the embodiment shown the SCS  14 S and TCS  14 T are implemented by the VCU 10. In some alternative embodiments the SCS  14 S and/or TCS  14 T may be implemented by the brake controller  13 . Further alternatively, the SCS  14 S and/or TCS  14 T may be implemented by separate controllers. 
     Similarly, one or more of the controllers  10 ,  11 ,  13 ,  170 C may be implemented in software run on a respective one or more computing devices such as one or more electronic control units (ECUs). In some embodiments two or more controllers may be implemented in software run on one or more common computing devices. Two or more controllers may be implemented in software in the form of a combined software module, or a plurality of respective modules each implementing only one controller. 
     One or more computing devices may be configured to permit a plurality of software modules to be run on the same computing device without interference between the modules. For example the computing devices may be configured to allow the modules to run such that if execution of software code embodying one module terminates erroneously, or the computing device enters an unintended endless loop in respect of one of the modules, it does not affect execution by one or more computing devices of software code comprised by a software module embodying the second controller. 
     It is to be understood that one or more of the controllers  10 ,  11 ,  13 ,  170 C may be configured to have substantially no single point failure modes, i.e. one or more of the controllers may have dual or multiple redundancy. It is to be understood that robust partitioning technologies are known for enabling redundancy to be introduced, such as technologies enabling isolation of software modules being executed on a common computing device. It is to be understood that the common computing device will typically comprise at least one microprocessor, optionally a plurality of processors, which may operate in parallel with one another. In some embodiments a monitor may be provided, the monitor being optionally implemented in software code and configured to raise an alert in the event a software module is determined to have malfunctioned. 
     The SCS  14 S, TCS  14 T, ABS controller  22 C and HDC system  12 HD provide outputs indicative of, for example, SCS activity, TCS activity and ABS activity including brake interventions on individual wheels and engine torque requests from the VCU 10 to the engine  121 , for example in 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 present. 
     As noted above the vehicle  100  includes a cruise control system  16  which 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 system  16  is 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  in a known manner. In one embodiment of the invention, cruise control system input controls are mounted to a steering wheel  171  ( FIG. 5 ). The cruise control system  16  may be switched on by pressing a cruise control system selector button  176 . When the cruise control system  16  is switched on, depression of a ‘set-speed’ control  173  sets the current value of a cruise control set-speed parameter, cruise_set-speed to the current vehicle speed. Depression of a ‘+’ button  174  allows the value of cruise_set-speed to be increased whilst depression of a ‘−’ button  175  allows the value of cruise_set-speed to be decreased. A resume button  173 R is provided that is operable to control the cruise control system  16  to 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 system  16  are configured so that, in the event that the user depresses the brake or, in the case of vehicles with a manual transmission, a clutch pedal, the cruise control function is cancelled and the vehicle  100  reverts 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 system  16  is resumed if the driver subsequently depresses the resume button  173 R. 
     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. In other words, the cruise control system is ineffective at speeds lower than 25 kph. The cruise control HMI  18  may also 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 . In the present embodiment the cruise control system  16  is configured to allow the value of cruise_set-speed to be set to any value in the range 25-150 kph. 
     The LSP control system  12  also 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 system  16  which operates only at speeds above 25 kph. 
     The LSP control system  12  is activated by means of a LSP control system selector button  172  mounted on the steering wheel  171 . The system  12  is operable to apply selective powertrain, traction control and braking actions to one or more wheels of the vehicle  100 , collectively or individually, to maintain the vehicle  100  at the desired speed. It is to be understood that in some embodiments the LSP control system selector button  172  may be mounted in a location other than on the steering wheel  171 , such as in a dashboard or any other suitable location. 
     The LSP control system  12  is configured to allow a user to input a desired value of set-speed parameter, user_set-speed to the LSP control system  12  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  12 HD. 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 system  12  controls vehicle speed in accordance with the value of user_set-speed by setting a parameter LSP_set-speed equal to the value of user_set-speed. Unlike the cruise control system  16 , the LSP control system  12  is configured to operate independently of the occurrence of a traction event. That is, the LSP control system  12  does not cancel speed control upon detection of wheel slip. Rather, the LSP control system  12  actively manages vehicle behavior when slip is detected. 
     The LSP control HMI  20  is provided in the vehicle cabin so as to be readily accessible to the user. The user of the vehicle  100  is able to input to the LSP control system  12 , via the LSP HMI  20 , 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’ button  173  and the ‘+’/‘−’ buttons  174 ,  175  in a similar manner to the cruise control system  16 . The LSP HMI  20  also includes a visual display upon which information and guidance can be provided to the user about the status of the LSP control system  12 . 
     The LSP control system  12  receives an input from 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  also receives an input from an accelerator pedal  161  indicative of the extent to which the user has depressed the accelerator pedal  161 . An input is also provided to the LSP control system  12  from the transmission or gearbox  124 . This input may include signals representative of, for example, the speed of an output shaft of the gearbox  124 , torque converter slip and a gear ratio request. Other inputs to the LSP control system  12  include an input from the cruise control HMI  18  which is representative of the status (ON/OFF) of the cruise control system  16 , and an input from the LSP control HMI  20 . 
     The HDC system  12 HD is configured to limit vehicle speed when descending a gradient. When the HDC system  12 HD is active, the system  12 HD controls the braking system  22  (via brake controller  13 ) 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  12 HD is active, the HDC system  12 HD controls the braking system  22  to prevent vehicle speed from exceeding the value of HDC_set-speed. In the present embodiment the HDC system  12 HD is not operable to apply positive drive torque. Rather, the HDC system  12 HD is only operable to apply negative brake torque by means of the braking system  22 . 
     A HDC system HMI  20 HD is provided by means of which a user may control the HDC system  12 HD, including setting the value of HDC_set-speed. An HDC system selector button  177  is provided on the steering wheel  171  by means of which a user may activate the HDC system  12 HD to control vehicle speed. 
     As noted above, the HDC system  12 HD is operable to allow a user to set a value of HDC set-speed parameter HDC_set-speed and to adjust the value of HDC_set-speed using the same controls as the cruise control system  16  and LSP control system  12 . Thus, in the present embodiment, when the HDC system  12 HD 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 system  16  and LSP control system  12 , using the same control buttons  173 ,  173 R,  174 ,  175 . The HDC system  12 HD 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 system  12 HD is selected when the vehicle  100  is travelling at a speed of 50 kph or less and no other speed control system is in operation, the HDC system  12 HD 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 as discussed in further detail below. The HDC system  12 HD then applies the powertrain  129  and/or braking system  22  to slow the vehicle  100  to the HDC system set-speed provided the driver does not override the HDC system  12 HD by depressing the accelerator pedal  161 . The HDC system  12 HD is configured to slow the vehicle  100  to the set-speed value at a deceleration rate not exceeding a maximum allowable rate although as noted elsewhere the HDC system  12 HD is not able to cause positive drive torque to be applied by the powertrain  129  in order to reduce a rate of deceleration of the vehicle  100 . The rate is set at 1.25 ms-2 in the present embodiment, however other values are also useful. If the user subsequently presses the ‘set-speed’ button  173  the HDC system  12 HD sets the value of HDC_set-speed to the instant vehicle speed provided the instant speed is 30 kph or less. If the HDC system  12 HD is selected when the vehicle  100  is travelling at a speed exceeding 50 kph, the HDC system  12 HD ignores the request and provides an indication to the user that the request has been ignored. 
     In the present embodiment the vehicle  100  is configured to assume one of a plurality of power modes PM at a given moment in time. In each power mode the vehicle  100  may be operable to allow a predetermined set of one or more operations to be performed. For example, the vehicle  100  may allow a predetermined one or more vehicle subsystems such as an infotainment system, a windscreen demist subsystem and a windscreen wiper control system to be activated only in a respective one or more predetermined power modes. In one or more of the power modes the vehicle  100  may be configured to inhibit one or more operations, such as turning on of the infotainment system. 
     The identity of the power mode in which the vehicle  100  is to operate at a given moment in time is transmitted to each controller  10 ,  11 ,  12 ,  13 ,  14 ,  16 ,  12 HD, of the vehicle  100  by the central controller  10 . The controllers respond by assuming a predetermined state associated with that power mode and that controller. In the present embodiment each controller may assume an ON state in which the controller is configured to execute computer program code associated with that controller, and an OFF state in which supply of power to the controller is terminated. In the present embodiment, the central controller  10  is also operable to assume a quiescent state. The quiescent state is assumed by the central controller  10  when the vehicle is in power mode PM 0  and the controller  10  has confirmed that the other controllers  11 ,  12 ,  13 ,  14 ,  16 ,  12 HD have successfully assumed the OFF state following receipt of the command to assume power mode PM 0 . 
     In the present embodiment the vehicle  100  is provided with a known key fob  190  ( FIG. 6 ) that has a radio frequency identification device (RFID)  190 R embedded therein. The key fob  190  has first and second control buttons  191 ,  192 . The key fob  100  is configured to generate a respective electromagnetic signal in response to depression of the first or second control buttons  191 ,  192 . The central controller  10  detects the electromagnetic signal by means of a receiver module forming part of the controller  10  and triggers locking or unlocking of door locks  182 L of the vehicle  100 . Each door  100 D of the vehicle  100  is provided with a respective door lock  182 L as shown in  FIG. 2 . 
     Pressing of the first control button  191  generates a door unlock signal, which triggers unlocking of the door locks  182 L, whilst pressing of the second control button  192  triggers a door lock signal, which triggers locking of the door locks  182 L. 
     When the controller  10  is in the quiescent state, consumption of power by the central controller  10  is reduced and the controller  10  monitors receipt of a door unlock signal from the key fob  190 . It is to be understood that in some embodiments one or more vehicle controllers may be configured to remain in the ON or quiescent state, to allow one or more essential functions to be performed, when the vehicle is in power mode PM 0 . For example in vehicles fitted with an intruder alarm system an intruder alarm controller may be permitted to remain in the ON or a quiescent state pending detection of an intrusion. Upon detection of an intrusion the intruder alarm controller may cause the central controller  10  to assume the ON state if it is not already in that state. 
     The central controller  10  is also configured to transmit a radio frequency (RF) ‘interrogation’ signal that causes the RFID device  190 R of the key fob  190  to generate an RF ‘acknowledgement’ signal in response to receipt of the interrogation signal. In the present embodiment the RFID device  190 R is a passive device, not requiring battery power in order to generate the acknowledgement signal. The controller  10  is configured to detect the acknowledgment signal transmitted by the RFID device  190 R provided the RFID device  190 R is within range. By the term ‘within range’ is meant that the RFID device  190 R or fob  190  is sufficiently close to the controller  10  to receive the interrogation signal and generate an acknowledgement signal that is detectable by the controller  10 . 
     The vehicle  100  is also provided with a start/stop button  181 . The start/stop button  181  is configured to transmit a signal to the central controller  10  when pressed in order to trigger an engine start operation, provided certain predetermined conditions are met. In response to pressing of the start/stop button  181  the central controller  10  causes the vehicle  100  to be placed in a condition in which if the transmission  124  is subsequently placed in the forward operating mode D or reverse operating mode R, the vehicle  100  may be driven by depressing accelerator pedal  161 . In the present embodiment, the central controller  10  is configured to perform a pre-start verification operation before commanding the powertrain controller  11  to trigger an engine start operation. In performing the pre-start verification operation the controller  10  verifies (a) that the vehicle  100  is in a predetermined power mode as described in more detail below, (b) that the controller  10  is receiving an acknowledgement signal from the key fob  190  in response to transmission of the interrogation signal by the controller  10 , and (c) that the transmission  124  is in either the park P or neutral N modes. Thus, the controller  10  requires that the RFID device  190 R is within range of the controller  10  before permitting an engine start. If any of conditions (a) to (c) are not met the controller causes the vehicle  100  to remain in its current power mode. 
     It is to be understood that the central controller  10  is configured to cause the vehicle  100  to assume a predetermined one of a plurality of power modes in dependence at least in part on actuation of a control button  191 ,  192  of the key fob  190  and actuation of the start/stop button  181 . In some embodiments the vehicle  100  may be configured such that the central controller  10  responds to voice commands from a user in addition to or instead of signals received from the key fob  190 . 
     The various power modes in which the vehicle  100  of the embodiment of  FIG. 1  may be operated will now be described. As noted above, the key fob  190  is operable to cause the door locks  182 L of the vehicle  100  to be locked and unlocked. When the doors  100 D of the vehicle  100  ( FIG. 2 ) are closed and the locks  182 L are in the locked condition, the vehicle  100  assumes power mode PM 0 . 
     If the first button  191  of the key fob  190  is subsequently actuated, the controller  10  causes the door locks  182 L to assume the unlocked condition. Once the door locks  182 L are in the unlocked condition and the controller  10  detects the acknowledgement signal from the key fob  190 , the controller  10  causes the vehicle  100  to assume power mode PM 4 . In power mode PM 4  the controller  10  permits a predetermined number of electrical systems to become active, including an infotainment system. Power mode PM 4  may also be referred to as a convenience mode or accessory mode. If a user subsequently presses the second button  192  of the key fob  190 , the controller  10  causes the vehicle  100  to revert to power mode PM 0 . 
     If, whilst the vehicle is in power mode PM 4  a user presses the starter button  181  and maintains the button  181  in a depressed condition, the controller  10  performs the pre-start verification operation described above. Provided conditions (a) to (c) of the pre-start verification operation are met, the controller  10  places the vehicle  100  in power mode PM 6 . When the vehicle  100  is in power mode PM 6  the powertrain controller  11  is permitted to activate a starter device. In the present embodiment the starter device is a starter motor  121 M. The powertrain controller  11  is then commanded to perform an engine start operation in which the engine  121  is cranked by means of the starter motor  121 M to cause the engine  121  to start. Once the controller  10  determines that the engine  121  is running, the controller  10  places the vehicle  100  in power mode PM 7 . 
     In power mode PM 6  the controller  10  disables certain non-critical electrical systems including the infotainment system. This is at least in part so as to reduce the magnitude of the electrical load on a battery  100 B of the vehicle during cranking in order to permit an increase in the amount of electrical current available for engine starting. Isolation of non-critical electrical systems also reduces a risk of damage to the systems when a relatively large current drain is placed on the battery  100 B by the starter motor  121 M. 
     If whilst the vehicle is in power mode PM 7 , with the engine  121  running, a user again actuates the start/stop button  181 , the controller  10  causes the powertrain controller  11  to switch off the engine  121  and the controller  10  causes the vehicle  100  to transition to power mode PM 4 . A user may then cause the vehicle to assume power mode PM 0  by pressing the first button  191  of the key fob  190  provided each of the doors  100 D is closed. It is to be understood that in some embodiments the user may trigger assumption of power mode PM 0  whilst remaining in the vehicle  100  and locking the doors  181  by means of the key fob  190 . In some embodiments the vehicle  100  may be configured to assume power mode PM 0  regardless of whether the controller is receiving the acknowledgement signal from the key fob  190 . Other arrangements are also useful. 
     It is to be understood that assumption of power mode PM 0  by the vehicle  100  may be referred to as ‘key off’, whilst assumption of power mode PM 4  from power mode PM 0  may be referred to as ‘key on’. A sequence of transitions of the vehicle from power mode PM 0  to PM 4 , and back to power mode PM 0 , optionally including one or more transitions to power mode PM 6  and power mode PM 7  prior to assumption of power mode PM 0 , may be referred to as a ‘key cycle’. Thus a key cycle begins and ends with the vehicle  100  in power mode PM 0 . In some embodiments, assumption of power mode PM 6  or PM 7  from power mode PM 0  may be required in order to complete a key cycle, starting with power mode PM 0 . 
     It is to be understood that the VCU 10 is configured to implement a known Terrain Response (TR) (RTM) System of the kind described above 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 in which the VCU 10 causing the one or more systems or sub-systems to operate is indicated by a parameter driving_mode. The driving mode may be selected by a user by means of a driving mode selector  141 S ( FIG. 1 ). The driving modes may also be referred to as terrain modes, terrain response modes, or control modes. In the embodiment of  FIG. 1  four 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 the present embodiment, at any given moment in time the LSP control system  12  is in one of a plurality of allowable ‘on’ modes (also referred to as conditions or states) selected from amongst an active or full function (FF) mode, a descent control (DC) mode, also referred to as an intermediate mode and a standby mode. The LSP control system may also assume an ‘off’ mode or condition. The active mode, DC mode and standby mode may be considered to be different ‘on’ modes or conditions of the vehicle, i.e. different modes in which the LSP control system is in an ‘on’ mode or condition as opposed to an ‘off’ mode or condition. In the off condition the LSP control system  12  only responds to pressing of the LSP selector button  172 , which causes the LSP control system  12  to assume the on condition and the DC mode. When the LSP control system  12  assumes the on mode from the off mode in response to pressing of the LSP selector button, the value of user_set-speed is set to the instant speed of the vehicle  100  provided it is in the allowable range of speeds for operation of the LSP control system  12 . If the vehicle speed  100  is above this range the value of user_set-speed is set to the highest allowable speed for operation of the LSP control system  12 , i.e. 30 kph. 
     In the active or full function mode, the LSP control system  12  actively manages vehicle speed in accordance with the value of LSP set-speed, LSP_set-speed, by causing the application of positive powertrain drive torque to one or more driving wheels or negative braking system torque to one or more braked wheels. 
     In the DC mode the LSP control system  12  operates in a similar manner to that in which it operates when in the active mode except that the LSP control system  12  is prevented from commanding the application of positive drive torque by means of the powertrain  129 . Rather, only braking torque may be applied, by means of the braking system  22  and/or powertrain  129 . The LSP control system  12  is configured to increase or decrease the amount of brake torque applied to one or more wheels in order to cause the vehicle to maintain the LSP set-speed to the extent possible without application of positive drive torque. It is to be understood that, in the present embodiment, operation of the LSP control system  12  in the DC mode is very similar to operation of the HDC system  12 HD, except that the LSP control system  12  continues to employ the LSP control system  12  set-speed value LSP_set-speed rather than the HDC control system set-speed value HDC_set-speed. 
     In the standby mode, the LSP control system  12  is unable to cause application of positive drive torque or negative brake torque to a wheel. 
     As noted above, in the ‘off’ mode the LSP control system  12  is not responsive to any LSP input controls except the LSP control system selector button  172 . Pressing of the LSP control system selector button  172  when the system  12  is in the off mode causes the system  12  to assume the ‘on’ condition and the DC mode. The value of LSP_set-speed is set to the instant speed of the vehicle  100  provided it is in the allowable range of speeds for operation of the LSP control system  12 . If the vehicle speed  100  is above this range the value of LSP_set-speed is set to the highest allowable speed for operation of the LSP control system  12 , i.e. 30 kph. 
     If whilst in DC mode the ‘set+’ button  174  is pressed, the LSP control system  12  sets the value of user_set-speed to the instant value of vehicle speed according to vehicle speed signal  36  ( FIG. 4 , discussed in more detail below) and assumes the active mode. If the vehicle speed is above 30 kph, being the maximum allowable value of user_set-speed and LSP_set-speed, the LSP control system  12  remains in the DC mode and ignores the request to assume the active mode. A signal may be provided to the driver indicating that the LSP control system  12  cannot be activated due to the vehicle speed exceeding the maximum allowable value of LSP_set-speed. The signal may be provided by means of a text message provided on the LSP control HMI  18 , by means of an indicator lamp, an audible alert or any other suitable means. 
     If the resume button  173 R is depressed whilst in the DC mode, the LSP control system assumes the active mode and causes the vehicle to operate in accordance with the stored value of user_set-speed, i.e. LSP_set-speed is set to the stored value of user_set-speed unless a lower value of LSP_set-speed is required, provided the vehicle speed does not exceed 30 kph. The manner in which lower values of LSP_set-speed may be set is discussed in more detail below with respect to  FIG. 7 . 
     If vehicle speed is above 30 kph but less than or substantially equal to 50 kph when the resume button  173 R is pressed the LSP control system  12  remains in the DC mode until vehicle speed falls below 30 kph. In the DC mode, provided the driver does not depress the accelerator pedal  161  the LSP control system  12  deploys the braking system  22  to slow the vehicle  100  to 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 system  12  assumes the active mode in which it is operable to cause a required amount of positive powertrain drive torque to be applied to one or more wheels via the powertrain  129 , as well as negative torque via the powertrain  129  (via engine braking) and brake torque via the braking system  22  in order to control the vehicle in accordance with the LSP_set-speed value. The LSP control system  12  may generate a virtual accelerator pedal signal in order to cause the powertrain  129  to develop a required amount of powertrain torque in some embodiments. The virtual accelerator pedal signal may correspond to that which would be generated by an accelerator pedal controller in response to depression of the accelerator pedal  161  by an amount corresponding to the amount of powertrain torque required at a given moment in time. The accelerator pedal controller may form part of a powertrain controller  11  although other arrangements are also useful. 
     With the LSP control system  12  in the active mode, the user may increase or decrease the value of user_set-speed by means of the ‘+’ and ‘−’ buttons  174 ,  175 . In addition, the user may optionally also increase or decrease the value of user_set-speed by lightly pressing the accelerator or brake pedals  161 ,  163  respectively. In some embodiments, with the LSP control system  12  in the active mode the ‘+’ and ‘−’ buttons  174 ,  175  may be disabled such that adjustment of the value of user_set-speed can only be made by means of the accelerator and brake pedals  161 ,  163 . This latter feature may prevent unintentional changes in set-speed from occurring, for example due to accidental pressing of one of the ‘+’ or ‘−’ buttons  174 ,  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 system  12  is 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, such as 30-120 kph or any other suitable range of values. 
     It is to be understood that if the LSP control system  12  is in the active mode, operation of the cruise control system  16  is inhibited. The two speed control systems  12 ,  16  therefore operate independently of one another, so that only one can be operable at any one time. 
     In some embodiments, the cruise control HMI  18  and the LSP control HMI  20  may 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 control HMI  20  and the cruise control HMI  18 . 
     When in the active mode, the LSP control system  12  is configured to command application of positive powertrain torque and negative brake torque, as required, by transmitting a request for (positive) drive torque in the form of a powertrain torque signal and/or a request for (negative) brake torque in the form of a brake torque signal to the brake controller  13 . The brake controller  13  arbitrates any demand for positive powertrain torque, determining whether the request for positive powertrain torque is allowable. If a request for positive powertrain torque is allowable the brake controller  13  issues the request to the powertrain controller  11 . In some embodiments, the request for brake torque may correspond to an amount of brake torque (or brake fluid pressure) to be developed by the braking system  22 . In some alternative embodiments the request for brake torque may be for an amount of negative torque to be applied to one or more wheels. The brake controller  13  may in some embodiments determine whether the requested negative torque is to be supplied by means of powertrain braking alone, for example engine overrun braking, by means of powertrain braking and brake torque developed by the braking system  22 , or by means of the braking system  22  alone. In some embodiments the brake controller  13  or LSP control system  12  may be configured to cause a required amount of net negative torque to be applied to one or more wheels by applying negative torque by means of the braking system  22  against positive drive torque generated by the powertrain  129 . Application of positive drive torque generated by means of the powertrain  129  against negative brake torque generated by means of the braking system  22  may be made in order to reduce wheel flare when driving on surfaces of relatively low surface coefficient of friction such as during off-road driving. By wheel flare is meant excessive wheel slip as a result of the application of excess positive net torque to a wheel. 
     In the present embodiment the brake controller  13  also receives from the LSP control system  12  a signal S_mode indicative of the mode in which the LSP control system  12  is operating, i.e. whether the LSP control system  12  is operating in the active mode, DC mode, standby mode or off mode. 
     If the brake controller  13  receives a signal S_mode indicating that the LSP control system  12  is operating in the DC mode, standby mode or off mode, the brake controller  13  sets a powertrain torque request inhibit flag in a memory thereof. The powertrain torque request inhibit flag indicates that positive torque requests to the powertrain controller  11  from the brake controller  13  in response to positive torque requests from the LSP control system  12  are forbidden. Accordingly, if a request for positive powertrain torque is received by the brake controller  13  from the LSP control system  12  whilst the LSP control system  12  is operating in the DC mode, standby mode or off mode, the positive torque request is ignored by the brake controller  13 . 
     In some embodiments, the powertrain controller  11  is also provided with signal S_mode indicating the mode in which the LSP control system  12  is operating. If the LSP control system  12  is operating in a mode other than the active mode, such as the DC mode, standby mode or off mode, positive powertrain torque requests received as a consequence of a command from the LSP control system  12  are ignored by the powertrain controller  11 . 
     In some embodiments, if the powertrain controller  11  receives a request for positive powertrain torque from the brake controller  13  as a consequence of a command from the LSP control system  12  and the request is received more than a predetermined period after the LSP control system  12  has transitioned to a mode other than the active mode, the powertrain controller  11  causes the LSP control system  12  to assume a disabled off mode. In the disabled off mode the LSP control system  12  is effectively locked into the off condition or mode for the remainder of the current key cycle and the LSP control system  12  does not assume the DC mode in response to pressing of the LSP selector button  172 . The predetermined period may be any suitable period such as 50 ms, 100 ms, 500 ms, 1000 ms or any other suitable period. The period may be set to a value such that any delay in receipt of a positive torque request issued by the LSP control system  12  immediately prior to a transition from the active mode to a mode other than the active mode (and in which positive torque requests are not permitted) that is consistent with normal system operation will not trigger a transition to the disabled off mode. However, the powertrain controller  11  is configured such that any request for positive powertrain torque received by the powertrain controller  11  as a consequence of a request issued by the LSP control system  12  after assuming a mode other than the active mode (and in which positive torque requests are not permitted) will trigger a transition to the disabled off mode. 
     It is to be understood that other arrangements may also be useful. For example, in some embodiments, in the disabled off mode the LSP control system  12  may be configured not to respond to the LSP selector button  172  by assuming the DC mode until after the vehicle has transitioned from power mode PM 7  to power mode PM 4 . As described above, a transition from power mode PM 7  to power mode PM 4  may be accomplished by depressing the start/stop button  124 S. When the vehicle  100  is subsequently restarted and assumes power mode PM 7 , the LSP control system  12  may be permitted to assume operation in the active mode as required. 
     It is to be understood that some vehicles may be provided with known automatic engine stop/start functionality. In vehicles with this functionality, the powertrain controller  11  is configured to command stopping and starting of the engine  121  according to a stop/start control methodology when the vehicle  100  is being held stationary by means of brake pedal  163  with the transmission in the drive mode D. The process of automatically commanding stopping and restarting of the engine  121  may be referred to as an automatic stop/start cycle. In vehicles having automatic engine stop/start functionality, the controller  10  may be configured to cause the vehicle  100  to assume a power mode PM 6 A when the engine  121  is stopped during a stop/start cycle. Power mode PM 6 A is similar to power mode  6 , except that disabling of certain vehicle systems such as the infotainment system is not performed when in power mode PM 6 A. In power mode PM 6 A, the powertrain controller  11  is configured to restart the engine  121  upon receipt of a signal indicating a user has released the brake pedal  163 . It is to be understood that in some embodiments, a vehicle  100  may be configured to require an engine restart before the LSP control system  12  may exit the DC fault mode but an engine restart as part of an automatic stop/start cycle may be configured not to qualify as an engine restart permitting the system  12  to exit the DC fault mode. In some embodiments therefore, a transition from power mode PM 7  to power mode PM 6 A and back to power mode PM 7  does not permit the LSP control system  12  to exit the disabled off mode. 
     In some embodiments the LSP control system  12  may be configured such that it can assume one of a number of different further modes such as:
     (i) DC fault mode   (ii) DC fault mode fade-out mode   (iii) DC mode fade-out mode   (iv) Active standby mode   (v) DC standby mode   

     The DC fault mode corresponds to the DC mode except that if the DC fault mode is assumed by the LSP control system  12 , the LSP control system  12  is unable subsequently to assume the active mode for the remainder of the current key cycle. Thus, when the next key-on procedure is performed, following the next key-off procedure, the LSP control system  12  is permitted to assume the active mode when required. The vehicle  100  may be configured wherein the LSP control system  12  may assume the DC fault mode if a fault is detected indicating that the LSP control system  12  should not be permitted to request positive powertrain drive torque but where it is determined that it may be desirable for the benefits of DC mode to be enjoyed. Thus a transition from active mode to DC fault mode may be preferable to a transition to the off mode, particularly when negotiating off road conditions, in the event of a relatively minor fault in respect of the LSP control system  12 . 
     In some embodiments, if a transition to DC fault mode occurs with more than a predetermined frequency, the LSP control system  12  may become latched in the DC fault mode until a reset procedure is performed requiring action other than a key-off and subsequent key-on procedure in order to permit the active mode to be assumed again. In some embodiments, the LSP control system  12  may require a predetermined code to be provided to it. In some embodiments, the LSP control system  12  may be configured to receive the code via a computing device external to the vehicle  100  that temporarily communicates with the LSP control system  12  in order to provide the code. The computing device may be a device maintained by a vehicle servicing organization such as a dealer certified by a manufacturer of the vehicle  100 . The computing device may be in the form of a laptop or other computing device, and be configured to communicate wirelessly with the LSP control system  12  or via a wired connection. 
     The predetermined frequency may be defined in terms of a predetermined number of occurrences in a predetermined number of key cycles, or a predetermined distance driven, or be time based such as a predetermined number of occurrences in a predetermined period in which the vehicle is in power mode  7  (or power mode  6 A in addition to power mode  7 , in the case of a vehicle with stop/start functionality) over one or more key cycles, or a predetermined number of occurrences in a given calendar period, such as a day, a week, a month or any other suitable frequency. 
     The DC fault mode fade-out mode is a mode assumed by the LSP control system  12  when transitioning from the DC fault mode to an off mode such as disabled off, unless an immediate (binary′) transition to an off mode is required in which case the DC fault mode fade-out mode is not assumed. Thus, under certain conditions, rather than abruptly terminate commanding application of brake torque by means of the braking system  22  when ceasing operation in the DC fault mode and transitioning to an off mode such as ‘off’ or ‘disabled off’, the LSP control system  12  gradually fades out the application of any brake torque applied by the braking system  22  as a consequence of being in the DC fault mode, before assuming the off or disabled off mode. This is at least in part so as to allow a driver time to adapt to driving without the system  12  applying brake torque automatically. 
     Similarly, if the LSP control system  12  transitions from the DC mode to a mode in which the LSP control system  12  is unable to command application of brake torque such as the standby mode, off mode or disabled off mode, the LSP control system  12  may assume the DC fade-out mode as an intermediate mode. In the DC fade-out mode, like the DC fault mode fade-out mode, the LSP control system  12  gradually reduces the amount of any brake torque commanded by the LSP control system  12 , before assuming the target mode such as standby mode, off mode or disabled off mode. 
     The active standby mode is a mode assumed by the LSP control system  12  from the active mode if the driver over-rides the LSP control system  12  by depressing the accelerator pedal  161  to increase vehicle speed. If the driver subsequently releases the accelerator pedal with vehicle speed within the allowable range for the LSP control system  12  to operate in the active mode (i.e. a speed in the range 2-30 kph), the LSP control system  12  resumes operation in the active mode. If the speed is above the allowable range when the driver releases the accelerator pedal, the LSP control system  12  remains in the active standby mode until vehicle speed falls to a value within the allowable range. The LSP control system  12  then assumes the active mode. In some alternative embodiments the LSP control system  12  may remain in the active standby mode until vehicle speed falls to the prevailing value of LSP_set-speed, before assuming the active mode. Other arrangements may also be useful. 
     The DC standby mode is a mode assumed by the LSP control system  12  if whilst operating in the DC mode the driver over-rides the LSP control system  12  by depressing the accelerator pedal  161 . If the driver subsequently releases the accelerator pedal, then when vehicle speed is within the allowable range for the LSP control system  12  to operate in the DC mode (i.e. a speed in the range 2-30 kph), the LSP control system  12  resumes operation in the DC mode. Other arrangements are also useful. In some embodiments the LSP control system  12  may be configured to assume DC mode from the DC standby mode and cause application of brake torque to slow the vehicle  100  when a driver releases the accelerator pedal  161  even at speeds above 30 kph. In some embodiments the LSP control system  12  may be configured to cause application of brake torque at speeds of up to 50 kph, 80 kph or any other suitable speed in order to cause vehicle speed to reduce to the LSP target speed LSP_set-speed. The LSP control system  12  may be configured to take into account negative torque applied by a powertrain due for example to engine over-run braking in determining an amount of brake torque required in order to cause a vehicle to slow at a desired rate. The LSP control system  12  may be configured to cause a vehicle to slow at a desired rate according to a predetermined deceleration profile. 
     In some embodiments, if the powertrain controller  11  receives a request for positive powertrain torque from the brake controller  13  as a consequence of a command from the LSP control system  12  and the LSP control system  12  is in the DC mode, the powertrain controller  11  causes the LSP control system  12  to assume the DC fault mode if the positive torque request is received more than a predetermined period after the LSP control system  12  has transitioned to the DC mode. As noted above, in the DC fault mode the LSP control system  12  is permitted to cause application of brake torque by the braking system  22  to control vehicle speed but is prevented from assuming the active or FF mode for the remainder of the current key cycle. In these circumstances, the LSP control system  12  assumes the DC fault mode substantially immediately with no requirement to blend the transition between the DC mode and DC fault mode. 
     As noted above, the predetermined period may be any suitable period such as 50 ms, 100 ms, 500 ms, 1000 ms or any other suitable period. The period may be set to a value such that any inherent system delay in receipt by the powertrain controller  11  of a torque request from the brake controller  13  as a consequence of a request issued by the LSP control system  12  prior to a transition from the active mode to the DC mode will not trigger a transition to the DC fault mode. It is to be understood that by inherent system delay is meant a delay in signal receipt that occurs during normal operation, for example due to a requirement to synchronize timing signals, or to transmit commands from the LSP control system  12  to the powertrain controller  11  at predetermined intervals as part of an inter-controller communications protocol. 
     In some embodiments, if the powertrain controller  11  receives a request for positive powertrain torque from the brake controller  13  as a consequence of a command from the LSP control system  12  and the LSP control system  12  is in the DC fault mode or DC fault mode fade out mode only, the powertrain controller  11  causes the LSP control system  12  to assume the disabled off mode if the positive torque request is received more than a predetermined period after the LSP control system  12  has transitioned to the DC fault mode or DC fault mode fade out mode. In the present embodiment the predetermined period is a period of 500 ms. However the predetermined period may be any suitable period such as 50 ms, 100 ms, 1000 ms or any other suitable period. The LSP control system  12  may be configured substantially abruptly to terminate application of any negative (brake) torque requested by the LSP control system  12  when the transition to the disabled off mode is commanded even if the system  12  is in the processes of fading out any negative brake torque that is being applied as a result of a request issued by the LSP control system  12   
     In some embodiments, in addition or instead, if the powertrain controller  11  receives a request for positive powertrain torque from the brake controller  13  as a consequence of a command from the LSP control system  12  and the signal S_mode indicates that the LSP control system  12  is in the DC mode, DC standby mode, DC mode fade-out mode or active standby mode, the powertrain controller  11  causes the LSP control system  12  to assume the disabled off mode if the positive torque request is received over a sustained period of more than a predetermined period. In the present embodiment the predetermined period is substantially 500 ms. However the predetermined period may be any suitable period such as 100 ms, 1000 ms or any other suitable period. The LSP control system  12  is configured gradually to cause fade-out of any negative (brake) torque being applied as a consequence of a command from the LSP control system  12  when the transition to the disabled off mode is commanded. The fade-out of brake torque may be accomplished by assuming the DC mode fade-out mode or DC fault mode fade-out mode if they have not already been assumed. 
     In some embodiments, the LSP control system  12  is caused to assume the disabled off mode if the powertrain controller  11  receives a request for positive powertrain torque from the brake controller  13  as a consequence of a command from the LSP control system  12  and signal S_mode indicates that the LSP control system  12  is in the DC fault mode or DC fault mode fade-out mode, as well as when the signal indicates the LSP control system  12  is in the DC mode, DC standby mode, DC mode fade-out mode or active standby mode. 
     It is to be understood that in some embodiments, instead of gradually fading out negative brake torque, the LSP control system  12  may be configured to abruptly terminate application of any negative brake torque as a consequence of a command by the LSP control system  12 . Thus, if a request for positive powertrain torque is received over a sustained period of more than the predetermined period when the LSP control system  12  is in the DC mode, DC standby mode, DC fault mode, DC mode fade-out mode, DC fault mode fade-out mode or active standby mode the system may abruptly terminate application of brake torque caused by the LSP control system  12 . It is to be understood that the braking system  12  continues to respond to driver brake commands via the brake pedal  163 . 
     It is to be understood that in the present embodiment if a driver switches off the LSP control system  12  manually, the LSP control system  12  is configured gradually to cause fade-out of any negative (brake) torque being applied as a consequence of a command from the LSP control system  12 . This feature has the advantage that vehicle composure may be enhanced. 
       FIG. 4  illustrates the means by which vehicle speed is controlled in the LSP control system  12 . As described above, a speed selected by a user (set-speed) is input to the LSP control system  12  via the LSP control HMI  20 . A vehicle speed calculator  34  provides a vehicle speed signal  36  indicative of vehicle speed to the LSP control system  12 . The speed calculator  34  determines vehicle speed based on wheel speed signals provided by wheel speed sensors  111 S,  112 S,  114 S,  115 S. The LSP control system  12  includes a comparator  28  which compares the LSP control system set-speed LSP_set-speed  38  (also referred to as a ‘target speed’  38 ) selected by the user with the measured speed  36  and provides an output signal  30  indicative of the comparison. The output signal  30  is provided to an evaluator unit  40  of the VCU 10 which interprets the output signal  30  as either a demand for additional torque to be applied to the vehicle wheels  111 - 115 , or for a reduction in torque applied to the vehicle wheels  111 - 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 the amount of any positive powertrain torque delivered to a wheel, by increasing the amount of any negative powertrain torque delivered to a wheel, for example by reducing an amount of air and/or fuel supplied to an engine  121 , and/or by increasing a braking force on a wheel. It is to be understood that in some embodiments in which a powertrain  129  has one or more electric machines operable as a generator, negative torque may be applied by the powertrain  129  to one or more wheels by means of the electric machine. As noted above 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 vehicle  100  is 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 output  42  from the evaluator unit  40  is provided to the brake controller  13 . The brake controller  13  in turn controls a net torque applied to the vehicle wheels  111 - 115  by commanding application of brake torque via the brakes  111 B,  112 B,  114 B,  115 B and/or positive drive torque by commanding powertrain controller  11  to deliver a required amount of powertrain torque. The net torque may be increased or decreased depending on whether the evaluator unit  40  demands positive or negative torque. In order to cause application of the necessary positive or negative torque to the wheels, the brake controller  13  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. In the illustrated embodiment the torque is applied to the vehicle wheels individually so as to maintain the vehicle  100  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 controller  11  may be operable to control an amount of torque applied to one or more wheels at least in part 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 driveline  130  may include one or more clutches operable to allow an amount of torque applied to wheels of a given axle to be controlled independently of the torque applied to wheels of another axle, and/or the amount of torque applied to one or more individual wheels to be controlled independently of other wheels. Other arrangements are also useful. 
     Where a powertrain  129  includes one or more electric machines, for example one or more propulsion motors and/or generators, the powertrain controller  11  may be operable to modulate or control the amount of torque applied to one or more wheels at least in part by means of the one or more electric machines. 
     The LSP control system  12  also receives a signal  48  indicative of a wheel slip event having occurred. This may be the same signal  48  that is supplied to the on-highway cruise control system  16  of the vehicle, and which in the case of the latter triggers an override or inhibit mode of operation of the on-highway cruise control system  16  so that automatic control of vehicle speed by the on-highway cruise control system  16  is suspended or cancelled. However, the LSP control system  12  is not arranged to cancel or suspend operation in dependence on receipt of a wheel slip signal  48  indicative of wheel slip. Rather, the system  12  is arranged to monitor and subsequently manage wheel slip so as to reduce driver workload. During a slip event, the LSP control system  12  continues 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 system  12  is configured differently to the cruise control system  16 , for which a wheel slip event has the effect of overriding the cruise control function so that manual operation of the vehicle  100  must be resumed, or speed control by the cruise control system  12  resumed by pressing the resume button  173 R or set-speed button  173 . 
     In a further embodiment of the present invention (not shown) a wheel slip signal  48  is derived not just from a comparison of wheel speeds, but further refined using sensor data indicative of the vehicle&#39;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 vehicle  100  and 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 LSP function by depressing the accelerator pedal  161  and/or brake pedal  163  to adjust the vehicle speed in a positive or negative sense. However, absent any override by a user, in the event that a wheel slip event is detected via signal  48 , the LSP control system  12  remains active and control of vehicle speed by the LSP control system  12  is not terminated. As shown in  FIG. 4 , this may be implemented by providing a wheel slip event signal  48  to the LSP control system  12  which is then managed by the LSP control system  12  and/or brake controller  13 . In the embodiment shown in  FIG. 1  the SCS  14 S generates the wheel slip event signal  48  and supplies it to the LSP control system  12  and cruise control system  16 . 
     A wheel slip event is triggered when a loss of traction occurs at any one of the vehicle wheels. Wheels and tires 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 vehicle  100  may 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 vehicle  100  is 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 vehicle  100  is also provided with additional sensors (not shown) which are representative of a variety of different parameters associated with vehicle motion and status. 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. Suitable sensor data may be provided by inertial systems unique to the LSP or HDC control system  12 ,  12 HD or systems that form part of another vehicle sub-system such as 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 systems  12 ,  12 HD. 
     The sensors on the vehicle  100  include sensors which provide continuous sensor outputs to the VCU 10, including wheel speed sensors, as mentioned previously and as shown in  FIG. 1 , and other sensors (not shown) such as an ambient temperature sensor, an atmospheric pressure sensor, tire 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 SCS  14 S, 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. Other sensors may be useful in addition or instead in some embodiments. 
     The VCU 10 also receives a signal from the steering controller  170 C. The steering controller  170 C is in the form of an electronic power assisted steering unit (ePAS unit). The steering controller  170 C provides a signal to the VCU 10 indicative of the steering force being applied to steerable road wheels  111 ,  112  of the vehicle  100 . This force corresponds to that applied by a user to the steering wheel  171  in combination with steering force generated by the ePAS unit  170 C. 
     The VCU 10 evaluates 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 VCU 10 then 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. GB2492748, GB2492655 and GB2499252, 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 utilized in the LSP control system  12  to determine an appropriate increase or decrease in vehicle speed. 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 system  12  is 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 system  12  selects 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 HMI  20  to indicate that an alternative speed has been adopted. 
     A-438 Driver Throttle Override 
     In the present embodiment, the LSP control system  12  is configured to determine the speed at which the vehicle  100  is to be caused to operate at a given moment in time, LSP_set-speed, in dependence on the value of user_set-speed and a number of parameters associated with the vehicle  100 . In particular, the LSP control system  12  is configured to cause the vehicle  100  to operate in accordance with the lowest of six values of vehicle set speed unless the driver overrides control of vehicle speed by depressing the accelerator pedal  161  by a sufficient amount. That is, provided the driver does not override vehicle speed control by means of the accelerator pedal  161 , the LSP control system  12  sets the value of target_speed to the lowest of six values of speed. The six speed values are (a) a value of target speed requested by a user, user_set-speed, described above; (b) a maximum vehicle speed Psng_Excit_v calculated in dependence on the value of an occupant excitation parameter Psng_Excit, the value of Psng_Excit being set in dependence on vehicle pitch acceleration, roll acceleration and heave acceleration; (c) a maximum speed steering_angle_v that is set in dependence on steering angle and vehicle speed; (d) a maximum speed sideslope_v that is set in dependence on a value of surface side slope; (e) a maximum speed grad_v that is set in dependence on surface gradient; and (f) a maximum speed warp_v or warp speed that is set in dependence on vehicle suspension articulation, also referred to as suspension warp. Optionally, the inputs may include a further maximum speed value that is set in dependence on whether the vehicle is wading. In some embodiments this maximum speed value may be set in dependence at least in part on a depth of liquid through which the vehicle is wading. Other parameters are also useful. Other speed values may also be useful. 
     Calculation of the values of the six values of target speed are also discussed in co-pending international patent application WO2014/027069, the content of which is hereby incorporated by reference. 
     The LSP control system  12  is configured to receive inputs corresponding to a number of vehicle parameters. The parameters include: (a) a current vehicle reference value of surface coefficient of friction, being a value calculated based on values of one or more parameters such as an amount of torque applied to a wheel at which excessive wheel slip has been induced; (b) a value of expected surface coefficient of friction corresponding to a currently selected vehicle driving mode, being a prescribed value for each driving mode; (c) a current value of steering angle, corresponding to a steerable road wheel angle or steering wheel position; (d) a current yaw rate of the vehicle (determined by reference to an output of an accelerometer); (e) a current measured value of lateral acceleration (also determined by reference to an output of an accelerometer); (f) a current measured value of surface roughness (determined by reference to suspension articulation); (g) a current location of the vehicle (determined by reference to a global satellite positioning system (GPS) output); and (h) information obtained by means of a camera system. The preceding list is intended to be illustrative of an example only and is not intended to be limiting, other inputs also being useful in addition or instead. 
     The information obtained by means of a camera system may include for example an alert in the event that it is determined that the vehicle  10  may be about to depart from an off-road lane or track. In some embodiments, one or more systems of the vehicle  100 , optionally the LSP control system  12 , may be configured to detect terrain ahead of the vehicle that may affect the value of Psng_Excit. That is, the LSP control system  12  may predict that occupant excitation may be adversely or positively affected by terrain ahead of the vehicle based on analysis of one or more images of terrain in a path of the vehicle  100 . Thus the LSP control system  12  may be configured to change the value of Psng_Excit or otherwise affect vehicle speed in anticipation of a change in the value of Psng_Excit if the vehicle  100  continues at its current rate of progress. This is in contrast to reactive evaluation of terrain by reference to the vehicle parameters discussed above. 
     It is to be understood that in some embodiments a controller or system other than the LSP control system  12  may be configured to determine the value of Psng_Excit. 
     It is to be understood that if when the value of pedal_tq_rq is greater than that of LSP_tq_rq, i.e. when a driver overrides the LSP control system  12  by forcing the powertrain  129  to develop a greater amount of torque, the LSP control system  12  could be caused to assume a standby mode during the period of override, in which the LSP control system  12  is not involved in the determination of the amount of powertrain torque required. However, the present applicant has recognized this may result in a form of response of the powertrain  129  to the increase in torque demand during driver accelerator pedal override that is different from that which would have occurred if a driver had increased the value of user_set-speed in order to cause an increase in vehicle speed by means of the LSP control system  12  rather than by depressing the accelerator pedal  161 . 
     This difference in form of response may cause a reduction in vehicle composure at least in part because in the present embodiment when in the active mode the LSP control system  12  limits the rate of acceleration and deceleration of the vehicle  100 . 
     In order to mitigate this problem the present applicant has conceived a system and method of operation of a vehicle  100  in which the speed control system may remain actively involved in the control of the amount of torque generated by the powertrain  129 , so as to reduce or prevent loss of vehicle composure, during accelerator pedal override. In the present embodiment the LSP control system  12  remains in the active mode during accelerator pedal override and continues to limit the rate of acceleration and deceleration of the vehicle  100 . 
       FIG. 7  is a schematic illustration of the manner in which the LSP control system  12  causes the vehicle  100  to operate in accordance with the value of LSP_set-speed when the LSP control system  12  is in the active mode. 
     Function block  110  is a minimizer function block  110  that receives six signal inputs corresponding to the six values of set-speed described above: user_set-speed, steering-angle_v, sideslope_v, gradient_v, warp_v and psng_excit_v. The minimizer function block  110  outputs a signal LSP_raw_set-speed that is substantially equal to the lower of the six signals input to the block  110 . 
     Function block  120  receives an input signal LSP_tq_rq being a virtual accelerator pedal position signal generated by the LSP control system  12 , a signal pedal_tq_rq indicative of an actual position of the accelerator pedal  161 , and a signal v_instant indicative of an instant speed of the vehicle  100 . In the present embodiment, the signal LSP_tq_req is generated by the LSP control system  12  in order to cause the powertrain  129  to develop an amount of powertrain torque required to cause the vehicle  100  to operate in accordance with the prevailing value of LSP_set-speed. By generating the signal LSP_tq_rq, an interface between the powertrain controller  11  and LSP control system  12  may be simplified, since in some embodiments the powertrain controller  11  need only receive signal LSP_tq_rq from the LSP control system  12  in order to cause the powertrain  129  to develop the required amount of torque. It is to be understood that the powertrain controller  11  may in some embodiments be configured to generate an amount of torque according to the signal LSP_tq_rq only if the LSP control system  12  is in the active mode. 
     The signal v_instant corresponds substantially to vehicle speed signal  36 . 
     Function block  120  is configured to compare the signals LSP_tq_rq and pedal_tq_rq. If the signal LSP_tq_rq is greater than the signal pedal_tq_rq the function block  120  outputs to function block  130  a signal override_set-speed that is set substantially to zero. If the signal pedal_tq_rq is greater than the signal LSP_tq_rq, function block  120  feeds the signal pedal_tq_rq to a sub-block  120   a  of the function block  120 . The LSP control system  12  remains in the active mode even when pedal_tq_rq_ is greater than LSP_tq_rq unless the accelerator pedal  161  is depressed by more than 80% of its maximum range of travel. In this latter case, the LSP control system  12  assumes the standby mode and the signal pedal_tq_rq is fed directly to the powertrain controller  11 . 
       FIG. 8  illustrates four sub-blocks  120   a - d  of the function block  120 . The purpose of sub-blocks  120   a - d  is to generate an ‘effective’ value of set-speed value, herein referred to as an accelerator pedal override set-speed value, override_set-speed, which is then used to cause the LSP control system  12  to cause the vehicle  100  to experience an increase in speed in accordance with driver torque demand as determined by reference to pedal_tq_rq. This is done by setting the parameter LSP_set-speed equal to override_set-speed. 
     Sub-block  120   a  is configured to feed the value of parameter pedal_tq_rq into a look-up table in order to generate a value of an amount of powertrain torque, Tq_PT, corresponding to the value of pedal_tq_rq. The value of PT_tq is output to a function block  120   c.    
     In the present embodiment the value of Tq_PT corresponds substantially to that which would be developed by the powertrain controller  11  if the LSP control system were to assume the standby mode and the signal pedal_tq_req were fed directly to the powertrain controller  11  rather than to the function block  120 . 
     In some embodiments the value of Tq_PT may be different from that which would be developed by the powertrain controller  11  if the signal pedal_tq_req were fed directly to the powertrain controller  11 . Other arrangements may also be useful. 
     Function block  120   b  receives as an input a value of instant vehicle speed, v_instant, corresponding to vehicle speed signal  36 . Based on v_instant, function block  120   b  calculates a retarding torque on the powertrain of the vehicle, Tq_retard, due to aerodynamic drag and optionally one or more other drag torques acting on a vehicle powertrain  129  such as hydraulic and frictional torques associated with the transmission  124  and driveline  131 . The value of Tq_retard is fed as an input to function block  120   c.    
     Function block  120   c  calculates a value of acceleration of the vehicle  100 , a_target, that would be expected to result from the value of Tq_PT in combination with the value of Tq_retard. The value of a_target is fed to an integrator function block  120   d  that calculates an expected instant value of speed of the vehicle based on the value of a_target by integrating the value of a_target over time. The integrator function block  120   d  sets the value of a parameter override_set-speed substantially equal to this expected instant value of speed and outputs this parameter to function block  130  of  FIG. 7 . It is to be understood that, when the value of pedal_tq_rq initially exceeds LSP_tq_rq, during a given period in which pedal_tq_rq exceeds LSP_tq_rq, the value of override_set-speed is set substantially equal to the instant vehicle speed v_instant in the present embodiment. 
     The value of override_set-speed is fed to function block  130  of  FIG. 7 . 
     As noted above function block  130  receives input signals LSP_raw_set-speed and override_set-speed. 
     Block  130  takes the larger of the two values override_set-speed and LSP_raw_set-speed and sets the value of LSP_set-speed to the larger of the two. The value LSP_set-speed is the output from the function block  130  and the LSP control system  12  attempts to cause the vehicle  100  to travel at a speed substantially equal to LSP_set-speed. 
     In attempting to cause the vehicle  100  to travel at the speed LSP_set-speed, the system  12  attempts to ensure that a rate of acceleration and deceleration of the vehicle  100  does not exceed a saturation rate of 1.25 ms −2 . Other values of positive or negative acceleration, i.e. maximum values of positive vehicle and acceleration and deceleration, may also be useful. 
     When the value of pedal_tq_rq falls below LSP_tq_rq for more than a predetermined period of time, the LSP control system  12  causes the value of LSP_set-speed to transition smoothly back to the value of LSP_raw_set-speed. In some embodiments this may be performed by blending the values of override_set-speed and LSP_raw_set-speed. 
       FIG. 9  is a schematic illustration of a portion of an LSP control system according to a further embodiment of the present invention, and corresponds to the portion of the LSP control system  12  of the previous embodiment illustrated in  FIG. 7 . Like features of the portion of  FIG. 9  to those of the portion of  FIG. 7  are shown with like reference numerals incremented by 100. 
     The portion of the control system illustrated in  FIG. 9  differs from that of the embodiment of  FIG. 7  in that minimizer function block  110  of the embodiment of  FIG. 7  is replaced by a function block  210  which receives as an input the value of user_set-speed and outputs to function block  130  a signal LSP_raw_set-speed which is set substantially equal to the value of user_set-speed. It is to be understood that in some embodiments function block  210  may receive as inputs signals corresponding to a plurality of speed values, as in the case of minimizer function block  120  of the embodiment of  FIG. 7 , and output a value of LSP_raw_set-speed which is set substantially equal to the lower of the speeds input thereto. 
     The portion of the control system illustrated in  FIG. 9  also differs from that of the embodiment of  FIG. 7  in that function block  230  additionally receives as an input the signal driving_mode. The function block  230  is configured to select an acceleration profile (which may also be referred to as a speed profile) from a plurality of stored profiles in dependence on the signal driving_mode. The acceleration or speed profiles are stored in a memory of the LSP control system  12  and the profile to be employed at a given moment in time is determined in dependence on driving_mode by reference to a look-up table. In use, the control system  12  causes the vehicle to operate in accordance with the value of user_set-speed unless it is determined that the control system  12  is to cause the vehicle to operate in accordance with the value of override_set-speed because override_set-speed exceeds LSP_raw_set-speed in the manner described above with respect to  FIG. 7  and  FIG. 8 . If the control system  12  is causing the vehicle to operate in accordance with the value of override_set-speed and the value of v_instant is less than the value of override_set-speed, the control system  12  causes the vehicle to accelerate from the instant speed v_instant to the speed override_set-speed at a rate determined according to the acceleration or speed profile that has been selected by the function block  230  in dependence on the parameter driving_mode. 
     In the present embodiment the VCU 10 stores a plurality of acceleration profiles corresponding to positive values of vehicle acceleration. These profiles are employed when v_ref is less than LSP_set-speed and an increase in v_ref is required. In some embodiments, in addition the VCU 10 stores a plurality of acceleration profiles corresponding to negative values of vehicle acceleration, employed when v_ref is greater than LSP_set-speed and a decrease in v_ref is required. 
     A plurality of acceleration profiles in respect of positive vehicle acceleration are stored in the present embodiment because in certain terrain conditions such as when driving over sand, it may be desirable to have relative high acceleration profiles that demand higher rates of acceleration whilst in certain other terrain conditions it may be desirable to have positive acceleration profiles that demand lower rates of acceleration, such as when driving over grass, gravel or snow. 
     Selection from a plurality of positive and negative acceleration profiles is particularly advantageous. For example, when an increase in vehicle speed is required when driving on sandy terrain, it is typically desirable to accelerate at a relatively high rate of acceleration, whilst when a decrease in vehicle speed is required when driving on sandy terrain it is typically desirable to decelerate at a relatively low rate in order to reduce a risk of sink of one or more wheels into the driving surface. Sink of one or more wheels into the driving surface may result from skid associated with excessive braking on deformable surfaces such as sand. In contrast, when operating on surfaces such as grass, relatively low levels of positive and negative acceleration are desirable in order to reduce slip and reduce the extent to which the surface is modified as the vehicle progresses over the surface. 
     It will be appreciated that, alternatively, a single common acceleration profile may be used for positive and negative acceleration. When negative acceleration is required, the VCU 10 may simply reverse the sign of the stored positive acceleration profile in order to determine the rate of negative acceleration (or the speed of the vehicle) at a given moment in time. 
     Some embodiments of the present invention have the advantage that a speed control system such as LSP control system  12  may continue to control vehicle speed even when a driver overrides the speed control system by depressing the accelerator pedal. Consequently, the speed control system is able to ensure that the rate of acceleration of the vehicle does not exceed a maximum acceleration value. This enables vehicle composure to be maintained even when the driver overrides the speed control system. Furthermore, in some embodiments the speed control system may set the maximum acceleration rate, and/or an acceleration profile, according to a vehicle parameter such as a driving mode in which a vehicle is operating, optionally according to the nature of terrain over which the vehicle is operating. Thus, when operating in off-road conditions, the LSP control system  12  may adapt the acceleration profile according to the prevailing terrain even if a driver overrides the speed control system by demanding an amount of torque exceeding that which the speed control system has demanded in order to control vehicle speed according to the prevailing set-speed value. As described above, the prevailing set-speed value may be a user set set-speed value or a lower set-speed value if otherwise determined to be appropriate, for example due to the terrain over which the vehicle is travelling or one or more other factors such as a value or state of one or more vehicle parameters. 
     Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps. 
     Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
     Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.