Patent Description:
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. Cruise control speed (or cruise set-speed) is settable by the vehicle driver, typically by pressing a button when the vehicle is at the desired speed. Plus and minus buttons provide for incremental speed variation whilst the cruise control is set.

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

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

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

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

Some vehicles are adapted for off-highway use, and it would be desirable to provide low-speed cruise control for such vehicles so as to permit progress to be maintained over rough terrain. In off-highway conditions cruise control may permit a driver, particularly a novice driver, to concentrate upon activities such as steering.

It is against this background that the present invention has been conceived. Embodiments of the invention may provide an apparatus, a method or a vehicle which addresses the above problems. Other aims and advantages of the invention will become apparent from the following description, claims and drawings.

It is also known to provide a control system for a motor vehicle for controlling one or more vehicle subsystems. <CIT> discloses a vehicle control system comprising a plurality of subsystem controllers including an engine management system, a transmission controller, a steering controller, a brakes controller and a suspension controller. The subsystem controllers are each operable in a plurality of subsystem function or configuration modes. The subsystem controllers are connected to a vehicle mode controller which controls the subsystem controllers to assume a required function mode so as to provide a number of driving modes for the vehicle. Each of the driving modes corresponds to a particular driving condition or set of driving conditions, and in each mode each of the sub-systems is set to the function mode most appropriate to those conditions. Such conditions are linked to types of terrain over which the vehicle may be driven such as grass/gravel/snow, mud and ruts, rock crawl, sand and a highway mode known as 'special programs off' (SPO). The vehicle mode controller may be referred to as a Terrain Response (TR) (RTM) System or controller. The driving modes may also be referred to as terrain modes, terrain response modes, or control modes.

For further information related to vehicle speed control systems, the reader is directed to International patent publication number <CIT>, which relates to a method for operating a speed control system comprising detecting an external force acting on the vehicle. <CIT> describes detecting an external force on a vehicle by detecting deviation of a longitudinal acceleration of the vehicle from an expected acceleration profile. The rate of change of a component of the net torque on the wheels is modulated to compensate for the external force.

Furthermore, International patent publication number <CIT> relates to a system for controlling the speed of a vehicle where, in response to a change in the user set-speed, the vehicle accelerates from the current set-speed to the changed set-speed in accordance with an acceleration profile which depends on the selected terrain mode.

In one aspect of the invention there is provided a speed control system for a vehicle according to claim <NUM>.

This has the advantage that the responsiveness of a vehicle to deviations of the actual speed of the vehicle from the target speed value may be adjusted in dependence on the driving surface gradient in order to enhance vehicle composure and driver enjoyment of the vehicle. It is to be understood that, if driving uphill over relatively slippery terrain, an increased responsiveness to deviations of vehicle speed below the target speed reduces the risk that the vehicle fails to make adequate progress over the terrain, particularly if deviations below the target speed (known as 'undershoot') occur.

The torque control means is configured to attempt to cause the vehicle to travel at the target speed, the torque control means being configured to control the rate of change of the amount of torque applied to the one or more wheels, in order to attempt to maintain the vehicle traveling at the target speed value, in dependence at least in part on the gradient of the driving surface.

Optionally, the torque control means is configured wherein when actual vehicle speed is less than the target speed value and the information indicative of surface gradient indicates the vehicle is traveling uphill, the torque control means attempts to cause the vehicle to accelerate towards the target speed value at a rate that is higher than when driving on a horizontal surface.

In other words, when the vehicle is travelling uphill, i.e. on a driving surface having a positive gradient relative to a horizontal driving surface, the torque control means may be configured to attempt to cause the vehicle to accelerate towards the target speed value when speed is below the target speed value at a rate that is higher than when driving on a horizontal surface. That is, the torque control means may be configured to attempt to cause the vehicle to accelerate towards the target speed value when speed is below the target speed value at a more aggressive rate than when driving on a horizontal surface.

Optionally, the torque control means is configured wherein when actual vehicle speed is less than the target speed value, the torque control means attempts to cause the vehicle to accelerate towards the target speed value at a rate that is progressively higher for progressively higher values of uphill driving surface gradient.

Thus, the torque control means may attempt to increase the amount of torque at a rate that increases with increasing positive (uphill) driving surface gradient. That is, the torque control means may attempt to increase the amount of torque at a rate that is more aggressive with increasing gradient steepness.

Alternatively, the torque control means may be configured wherein when actual vehicle speed is less than the target speed value and the information indicative of surface gradient indicates the vehicle is traveling uphill, the torque control means attempts to cause the vehicle to accelerate towards the target speed value at a rate that is lower than when driving on a horizontal surface.

In other words, when the vehicle is travelling uphill, i.e. on a driving surface having a positive gradient relative to a substantially horizontal driving surface, the torque control means may be configured to attempt to cause the vehicle to accelerate towards the target speed value when speed is below the target speed value at a rate that is less than the corresponding rate when driving on a substantially horizontal surface. That is, the torque control means may be configured to attempt to cause the vehicle to accelerate towards the target speed value when speed is below the target speed value at a less aggressive rate than when driving on a substantially horizontal surface, for given values of actual vehicle speed and target speed.

Optionally, the torque control means is configured wherein when actual vehicle speed is less than the target speed value, the torque control means attempts to cause the vehicle to accelerate towards the target speed value at a rate that is increasingly lower for increasingly higher values of uphill driving surface gradient.

Thus, the torque control means may attempt to increase the amount of torque at a rate that decreases with increasing positive (uphill) driving surface gradient. That is, the torque control means may attempt to increase the amount of torque at a rate that is progressively less aggressive with progressively increasing gradient steepness.

Optionally, the torque control means is configured wherein when actual vehicle speed is greater than the target speed value, the torque control means causes a reduction in torque applied to the one or more wheels in order to attempt to cause the vehicle to travel at the target speed at a rate that is lower for a given deviation in speed above the target speed than in the case of a corresponding deviation below the target speed value.

Thus, the control system may respond to deviations above the target speed (referred to as 'overshoot') less aggressively than deviations below the target speed.

The torque control means may be configured wherein when actual vehicle speed is greater than the target speed value and the vehicle is traveling uphill, the torque control means causes a reduction in torque applied to the one or more wheels in order to attempt to cause the vehicle to travel at the target speed at a rate that is equal to or lower than in the case of a corresponding deviation in vehicle speed above the target speed whilst traveling over a horizontal driving surface.

Optionally, the torque control means is configured wherein when actual vehicle speed is greater than the target speed value and the vehicle is travelling downhill, the torque control means causes a reduction in torque applied to the one or more wheels in order to attempt to cause the vehicle to travel at the target speed at a rate that is substantially equal to or lower than in the case of a corresponding deviation in vehicle speed above the target speed whilst traveling over a horizontal driving surface.

Thus, the control system may respond to vehicle speed deviations above the target speed (referred to as 'overshoot') less aggressively when going uphill and/or downhill relative to driving over substantially horizontal surfaces. Alternatively the control system may respond more aggressively. Other arrangements may be useful.

Optionally, the torque control means is configured wherein when actual vehicle speed is greater than the target speed value and the vehicle is travelling downhill, the torque control means causes a reduction in torque applied to the one or more wheels, in order to attempt to cause the vehicle to travel at the target speed, at a rate that is greater than in the case of a corresponding deviation in vehicle speed above the target speed whilst traveling over a horizontal driving surface.

Thus, the control system may attempt to correct vehicle speed more aggressively in the event of overshoot when travelling downhill due to the effect of gravity in tending to promote overshoot. Accordingly, the amount of any further increase in vehicle speed may be reduced due to the relatively aggressive response.

According to the invention, the control system is configured to cause the vehicle to accelerate from a first speed to the target speed value, where the first speed is less than the target speed value, at least in part according to stored data in respect of target rate of acceleration as a function of speed, wherein the value of target rate of acceleration according to which the vehicle is caused to accelerate is determined in further dependence at least in part on the driving surface gradient.

The control system may be configured to cause a vehicle to decelerate from a second speed to the target speed value, where the second speed is greater than the target speed value, according to stored data in respect of target rate of deceleration as a function of speed, wherein the target rate of deceleration according to which the vehicle is caused to decelerate is determined in further dependence at least in part on the driving surface gradient.

The control system may be configured to control a rate of change of vehicle speed towards the target speed iteratively by causing the vehicle to attempt to achieve an intermediate instant target speed, the value of intermediate instant target speed and therefore vehicle speed being caused to change in an iterative manner towards the target speed value at a required rate.

The control system may be operable to control a rate of change of vehicle speed so as not to exceed a prescribed jerk value.

Optionally, the prescribed jerk value is set in dependence on the gradient of the driving surface.

Optionally, the terrain mode is one of a plurality of driving modes in which each one of a plurality of vehicle subsystems is caused to operate in one of a plurality of configuration modes of that subsystem, the subsystem configuration mode being determined in dependence on the selected driving mode.

The control system may be configured wherein the torque control means is operable to control the rate of change of the amount of torque applied to the one or more wheels, in order to attempt to maintain the vehicle traveling in accordance with the target speed value, in dependence at least in part on the information indicative of driving surface gradient only if the vehicle is operating in a driving mode that is a member of a predetermined group of one or more of the plurality of driving modes.

The control system may be configured wherein when actual vehicle speed is less than the target speed value and the information indicative of surface gradient indicates the vehicle is traveling uphill, the torque control means attempts to cause the vehicle to accelerate towards the target speed value at a rate that is lower than when driving on a horizontal surface if the vehicle is operating in a driving mode that is a member of a first group of one or more of the driving modes and is not a driving mode that is not a member of the first group.

Optionally, the first group of driving modes comprises at least one driving mode adapted for driving on a driving surface of relatively low surface coefficient of friction.

Optionally, the first group of driving modes comprises at least one driving mode adapted for driving on a driving surface of relatively low surface coefficient of friction excluding a mode adapted for driving on sand.

It is to be understood that sand represents a surface that is characterised by relatively high drag and relatively highly deformability as well as relatively low surface coefficient of friction. Accordingly, when driving uphill over sand and undershoot occurs, it is advisable to respond with a relatively rapid, i.e. aggressive, increasing in torque applied to the one or more wheels, in order to reduce the risk of the vehicle becoming bogged down or otherwise immobilised. Slip of wheels when travelling over sand can be effective in enhancing traction by enabling the wheels to 'excavate' sand, compounding the material to improve tractive force.

In contrast, when driving over a surface of relatively low surface coefficient other than sand, such as surfaces that are of relatively low drag, a less aggressive increase in torque may be advisable when undershoot occurs in order to reduce the risk of excessive wheel spin and potential resultant loss of traction. Furthermore, undesirable modification of the driving surface can occur as a consequence of excessive wheel slip. In the case of a grassy surface, being typically of relatively low surface coefficient of friction, modification of the surface can result in the exposure of a surface of even lower surface coefficient of friction such as mud, making progress over the surface even more difficult.

Optionally, the first group of driving modes comprises at least one driving mode adapted for driving on at least one of a snowy surface, an icy surface, grass, gravel, snow and mud.

The first group of driving modes may include a snow and ice mode, a grass/gravel/snow mode and/or a mud/ruts mode. Other modes may be useful in addition or instead.

In one aspect of the invention there is provided a method according to claim <NUM>.

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> shows a vehicle <NUM> according to an embodiment of the present invention. The vehicle <NUM> has a powertrain <NUM> that includes an engine <NUM> that is connected to a driveline <NUM> having an automatic transmission <NUM>. 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> the transmission <NUM> may be set to one of a plurality of transmission operating modes, being a park mode, a reverse mode, a neutral mode, a drive mode or a sport mode, by means of a transmission mode selector dial <NUM>. The selector dial <NUM> provides an output signal to a powertrain controller <NUM> in response to which the powertrain controller <NUM> causes the transmission <NUM> to operate in accordance with the selected transmission mode.

The driveline <NUM> is arranged to drive a pair of front vehicle wheels <NUM>,<NUM> by means of a front differential <NUM> and a pair of front drive shafts <NUM>. The driveline <NUM> also comprises an auxiliary driveline portion <NUM> arranged to drive a pair of rear wheels <NUM>, <NUM> by means of an auxiliary driveshaft or prop-shaft <NUM>, a rear differential <NUM> and a pair of rear driveshafts <NUM>.

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> the transmission <NUM> is releasably connectable to the auxiliary driveline portion <NUM> by means of a power transfer unit (PTU) 131P, allowing operation in a two wheel drive mode or a four wheel drive mode. It is to be understood that embodiments of the invention may be suitable for vehicles having more than four wheels or where only two wheels are driven, for example two wheels of a three wheeled vehicle or four wheeled vehicle or a vehicle with more than four wheels.

A control system for the vehicle engine <NUM> includes a central controller <NUM>, referred to as a vehicle control unit (VCU) <NUM>, the powertrain controller <NUM>, a brake controller <NUM> (an anti-lock braking system (ABS) controller) and a steering controller 170C. The ABS controller <NUM> forms part of a braking system <NUM> (<FIG>). The VCU <NUM> receives and outputs a plurality of signals to and from various sensors and subsystems (not shown) provided on the vehicle. The VCU <NUM> includes a low-speed progress (LSP) control system <NUM> shown in <FIG>, a stability control system (SCS) <NUM>, a cruise control system <NUM> and a hill descent control (HDC) system 12HD. The SCS <NUM> improves the safety of the vehicle <NUM> by detecting and managing loss of traction. When a reduction in traction or steering control is detected, the SCS <NUM> is operable automatically to command the ABS controller <NUM> to apply one or more brakes of the vehicle to help to steer the vehicle <NUM> in the direction the user wishes to travel. In the embodiment shown the SCS <NUM> is implemented by the VCU <NUM>. In some alternative embodiments the SCS <NUM> may be implemented by the ABS controller <NUM>.

Although not shown in detail in <FIG>, the VCU <NUM> further includes a Traction Control (TC) function block. The TC function block is implemented in software code run by a computing device of the VCU <NUM>. The ABS controller <NUM> and TC function block provide outputs indicative of, for example, TC activity, ABS activity, brake interventions on individual wheels and engine torque requests from the VCU <NUM> to the engine <NUM> in the event a wheel slip event occurs. Each of the aforementioned events indicate that a wheel slip event has occurred. In some embodiments the ABS controller <NUM> implements the TC function block. Other vehicle sub-systems such as a roll stability control system or the like may also be included.

As noted above the vehicle <NUM> also includes a cruise control system <NUM> which is operable to automatically maintain vehicle speed at a selected speed when the vehicle is travelling at speeds in excess of <NUM> kph. The cruise control system <NUM> is provided with a cruise control HMI (human machine interface) <NUM> by which means the user can input a target vehicle speed to the cruise control system <NUM> in a known manner. In one embodiment of the invention, cruise control system input controls are mounted to a steering wheel <NUM> (<FIG>). The cruise control system <NUM> may be switched on by pressing a cruise control system selector button <NUM>. When the cruise control system <NUM> is switched on, depression of a 'set-speed' control <NUM> sets the current value of a cruise control set-speed parameter, cruise_set-speed to the current vehicle speed. Depression of a '+' button <NUM> allows the value of cruise_set-speed to be increased whilst depression of a '-' button <NUM> allows the value of cruise_set-speed to be decreased. A resume button 173R is provided that is operable to control the cruise control system <NUM> 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 <NUM> 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, control of vehicle speed by the cruise control system <NUM> is cancelled and the vehicle <NUM> reverts to a manual mode of operation which requires accelerator or brake 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 control of vehicle speed by the cruise control system <NUM>. Speed control by the system <NUM> is resumed if the driver subsequently depresses the resume button 173R.

The cruise control system <NUM> 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 <NUM> kph. In other words, the cruise control system is ineffective at speeds lower than <NUM> kph. The cruise control HMI <NUM> may also be configured to provide an alert to the user about the status of the cruise control system <NUM> via a visual display of the HMI <NUM>. In the present embodiment the cruise control system <NUM> is configured to allow the value of cruise_set-speed to be set to any value in the range <NUM>-150kph.

The LSP control system <NUM> 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 to maintain vehicle speed. Low-speed speed control (or progress control) functionality is not provided by the on-highway cruise control system <NUM> which operates only at speeds above <NUM> kph.

In the present embodiment, the LSP control system <NUM> is activated by pressing a HDC system selector button <NUM> mounted on steering wheel <NUM> for less than a prescribed time period (in the present embodiment the prescribed time period is <NUM> although other values are also useful), and subsequently pressing the 'set +' button <NUM>. In some embodiments a dedicated LSP control system selector button is mounted on the steering wheel <NUM>, by means of which the LSP control system <NUM> is activated. The system <NUM> is operable to apply selective powertrain, traction control and braking actions to one or more wheels of the vehicle <NUM>, collectively or individually.

The LSP control system <NUM> is configured to allow a user to input a desired value of set-speed parameter, user_set-speed to the LSP control system <NUM> via a low-speed progress control HMI (LSP HMI) <NUM> (<FIG>, <FIG>) which shares certain input buttons <NUM>-<NUM> with the cruise control system <NUM> and HDC control system 12HD. Provided the vehicle speed is within the allowable range of operation of the LSP control system <NUM> (which is the range from <NUM> to 30kph in the present embodiment although other ranges are also useful) and no other constraint on vehicle speed exists whilst under the control of the LSP control system <NUM>, the LSP control system <NUM> controls vehicle speed in accordance with a LSP control system set-speed value LSP_set-speed which is set substantially equal to user_set-speed. Unlike the cruise control system <NUM>, the LSP control system <NUM> is configured to operate independently of the occurrence of a traction event. That is, the LSP control system <NUM> does not cancel speed control upon detection of wheel slip. Rather, the LSP control system <NUM> actively manages vehicle behaviour when slip is detected.

The LSP control HMI <NUM> is provided in the vehicle cabin so as to be readily accessible to the user. The user of the vehicle <NUM> is able to input to the LSP control system <NUM>, via the LSP HMI <NUM>, 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 <NUM> and the '+'/ '-' buttons <NUM>, <NUM> in a similar manner to the cruise control system <NUM>. The LSP HMI <NUM> also includes a visual display by means of which information and guidance can be provided to the user about the status of the LSP control system <NUM>.

The LSP control system <NUM> receives an input from the ABS controller <NUM> of the braking system <NUM> of the vehicle indicative of the extent to which the user has applied braking by means of the brake pedal <NUM>. The LSP control system <NUM> also receives an input from an accelerator pedal <NUM> indicative of the extent to which the user has depressed the accelerator pedal <NUM>, and an input from the transmission or gearbox <NUM>. This latter input may include signals representative of, for example, the speed of an output shaft of the gearbox <NUM>, an amount of torque converter slip and a gear ratio request. Other inputs to the LSP control system <NUM> include an input from the cruise control HMI <NUM> which is representative of the status (ON/OFF) of the cruise control system <NUM>, an input from the LSP control HMI <NUM>, and an input from a gradient sensor <NUM> indicative of the gradient of the driving surface over which the vehicle <NUM> is driving. In the present embodiment the gradient sensor is a gyroscopic sensor. In some alternative embodiments the LSP control system <NUM> receives a signal indicative of driving surface gradient from another controller such as the ABS controller <NUM>. The ABS controller <NUM> may determine gradient based on a plurality of inputs, optionally based at least in part on signals indicative of vehicle longitudinal and lateral acceleration and a signal indicative of vehicle reference speed (v_actual) being a signal indicative of actual vehicle speed over ground. Methods for the calculation of vehicle reference speed based for example on vehicle wheel speeds are well known. For example in some known vehicles the vehicle reference speed may be determined to be the speed of the second slowest turning wheel, or the average speed of all the wheels. Other ways of calculating vehicle reference speed may be useful in some embodiments, incuding by means of a camera device or radar sensor.

When the HDC system 12HD is active, the system 12HD controls the braking system <NUM> 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 parameter may also be referred to as an HDC target speed. Provided the user does not override the HDC system 12HD by depressing the accelerator pedal <NUM> when the HDC system 12HD is active, the HDC system 12HD controls the braking system <NUM> (<FIG>) to prevent vehicle speed from exceeding HDC_set-speed. In the present embodiment the HDC system 12HD is not operable to apply positive drive torque. Rather, the HDC system 12HD is only operable to cause negative brake torque to be applied, via the braking system <NUM>.

A HDC system HMI 20HD is provided by means of which a user may control the HDC system 12HD, including setting the value of HDC_set-speed. The HDC system is activated by depressing the HDC selector button <NUM> for more than the prescribed period (<NUM> in the present embodiment as noted above).

As noted above, the HDC system 12HD 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 <NUM> and LSP control system <NUM>. Thus, in the present embodiment, when the HDC system 12HD 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 <NUM> and LSP control system, using the same control buttons <NUM>, 173R, <NUM>, <NUM>. The HDC system 12HD is operable to allow the value of HDC_set-speed to be set to any value in the range from <NUM>-30kph.

If the HDC system 12HD is selected when the vehicle <NUM> is travelling at a speed of 50kph or less and no other speed control system is in operation, the HDC system 12HD sets the value of HDC_set-speed to a value selected from a look-up table. The value output by the look-up table is determined in dependence on the identity of the currently selected transmission gear, the currently selected PTU gear ratio (Hi/LO) and the currently selected driving mode. The HDC system 12HD then causes the powertrain <NUM> and/or braking system <NUM> (via signal <NUM>, <FIG>) to slow the vehicle <NUM> to the HDC system set-speed provided the driver does not override the HDC system 12HD by depressing the accelerator pedal <NUM>. It is to be understood that the HDC system 12HD may cause the powertrain <NUM> to apply negative torque to one or more wheels, for example by engine over-run braking, but cannot cause the powertrain <NUM> to apply a positive torque to a wheel.

If actual vehicle speed v_actual exceeds the set-speed value HDC_set-speed, the HDC system 12HD is configured to slow the vehicle <NUM> to the set-speed value at a deceleration rate not exceeding a maximum allowable rate. The rate is set as <NUM>-<NUM> in the present embodiment, however other values are also useful. If the user subsequently presses the 'set-speed' button <NUM> the HDC system 12HD sets the value of HDC_set-speed to the instant vehicle speed provided the instant speed is 30kph or less.

If the HDC system 12HD is selected (by depressing the HDC selector button <NUM> for more than the prescribed period when the HDC system 12HD and LSP control system <NUM> are switched off) and the vehicle <NUM> is travelling at a speed exceeding 50kph, the HDC system 12HD ignores the request and provides an indication to the user that the request has been ignored.

It is to be understood that the VCU <NUM> is configured to implement a known Terrain Response (TR) (RTM) System of the kind described above in which the VCU <NUM> controls settings of one or more vehicle systems or sub-systems such as the powertrain controller <NUM> in dependence on a selected driving mode. The driving mode may be selected by a user by means of a driving mode selector <NUM> (<FIG>). The driving modes may also be referred to as terrain modes, terrain response (TR) modes, or control modes.

In the embodiment of <FIG> 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, being terrain characterised at least in part by relatively high drag, relatively high deformability or compliance and relatively low surface coefficient of friction; a 'grass, gravel or snow' (GGS) driving mode suitable for driving over grass, gravel or snow, being relatively slippery surfaces (i.e. having a relatively low coefficient of friction between surface and wheel and, typically, lower drag than sand); a 'rock crawl' (RC) driving mode suitable for driving slowly over a rocky surface; and a 'mud and ruts' (MR) driving mode suitable for driving in muddy, rutted terrain. Other driving modes may be provided in addition or instead. In the present embodiment the selector <NUM> also allows a user to select an 'automatic driving mode selection condition' in which the VCU <NUM> selects automatically the most appropriate driving mode as described in more detail below. The on-highway driving mode may be referred to as a 'special programs off' (SPO) mode in some embodiments since it corresponds to a standard or default driving mode, and is not required to take account of special factors such as relatively low surface coefficient of friction, or surfaces of high roughness.

In some embodiments, including the present embodiment, the LSP control system <NUM> may be in either one of an active condition, a standby condition and an 'off' condition at a given moment in time. In the active condition, the LSP control system <NUM> actively manages vehicle speed by controlling powertrain torque and braking system torque. In the standby condition, the LSP control system <NUM> does not control vehicle speed until a user presses the resume button 173R or the 'set speed' button <NUM>. In the off condition the LSP control system <NUM> is not responsive to input controls.

In the present embodiment the LSP control system <NUM> is also operable to assume an intermediate mode or condition similar to that of the active mode but in which the LSP control system <NUM> is prevented from commanding the application of positive drive torque to one or more wheels of the vehicle <NUM> by the powertrain <NUM>. Thus, only braking torque may be applied, by means of the braking system <NUM> and/or powertrain <NUM>. In the present embodiment, the intermediate mode is implemented by causing the HDC control system 12HD to control vehicle speed, with the value HDC_set-speed set substantially equal to LSP_set-speed. Other arrangements are also useful.

With the LSP control system <NUM> in the active condition, the user may increase or decrease the vehicle set-speed by means of the '+' and '-' buttons <NUM>, <NUM>. In addition, the user may also increase or decrease the vehicle set-speed by lightly pressing the accelerator or brake pedals <NUM>, <NUM> respectively. In some embodiments, with the LSP control system <NUM> in the active condition the '+' and '-' buttons <NUM>, <NUM> are disabled such that adjustment of the value of LSP_set-speed can only be made by means of the accelerator and brake pedals <NUM>, <NUM>. This latter feature may prevent unintentional changes in set-speed from occurring, for example due to accidental pressing of one of the '+' or '-' buttons <NUM>, <NUM>. 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 <NUM> is operable to cause the vehicle to travel in accordance with a value of set-speed in the range from <NUM>-30kph whilst the cruise control system <NUM> is operable to cause the vehicle to travel in accordance with a value of set-speed in the range from <NUM>-150kph although other values are also useful. If the LSP control system <NUM> is selected when the vehicle speed is above 30kph but less than or substantially equal to 50kph, the LSP control system <NUM> assumes the intermediate mode. In the intermediate mode, if the driver releases the accelerator pedal <NUM> whilst travelling above 30kph the LSP control system <NUM> deploys the braking system <NUM> to gently slow the vehicle <NUM> to a value of set-speed corresponding to the value of parameter LSP_set-speed. Once the vehicle speed falls to 30kph or below, the LSP control system <NUM> assumes the active condition in which it is operable to apply positive drive torque via the powertrain <NUM>, as well as brake torque via the powertrain <NUM> (via engine braking) and the braking system <NUM> in order to control the vehicle in accordance with the LSP_set-speed value. If the LSP control system <NUM> is selected and no LSP set-speed value has been set, the LSP control system <NUM> assumes the standby mode, the system <NUM> becoming active once the 'set +' button <NUM> is depressed. In some embodiments, if the LSP control system <NUM> is selected when the vehicle speed is above 30kph but less than or substantially equal to 50kph, the system <NUM> deploys the braking system <NUM> to slow the vehicle <NUM> to 30kph and prevents vehicle speed from exceeding 30kph unless the driver over-rides the system <NUM> by depressing the accelerator pedal <NUM> or switching off the system <NUM>.

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

In the present embodiment, as noted above the cruise control HMI <NUM> and the LSP control HMI <NUM> are configured within the same hardware so that the speed selection is input via the same hardware.

<FIG> illustrates the means by which vehicle speed is controlled when the LSP control system <NUM> is in the active mode. When in the active mode the LSP control system determines the amount of positive drive torque to be applied by the powertrain <NUM>, LSP_PT_TQ, and causes the powertrain <NUM> to deliver this amount of torque by communicating the value of LSP_PT_TQ to the powertrain controller <NUM>. The value of LSP_PT_TQ may be communicated to the powertrain controller <NUM> via the TC function block, which may arbitrate the value of LSP_PT_TQ in dependence on the amount of slip experienced by a driving wheel. Thus, the TC function block may reduce the value of LSP_PT_TQ output to the powertrain controller <NUM> when excessive slip is experienced.

When the LSP control system <NUM> is active, the amount of brake torque to be applied by the braking system <NUM>, LSP_BRK_TQ, is determined by the HDC control system 12HD, which is effectively 'slaved' to the LSP control system <NUM> when the LSP control system <NUM> is active. The HDC system 12HD causes the braking system <NUM> to deliver this amount of brake torque by communicating the value of LSP_BRK_TQ to the ABS controller <NUM>. It is to be understood that the LSP control system <NUM> may cause the HDC control system 12HD to command a non-zero value of LSP_BRK_TQ whilst the LSP control system <NUM> is commanding application of positive (or negative) powertrain torque, LSP_PT_TQ, in an automated implementation of 'two pedalling' where both brake and accelerator pedals are depressed by a driver to reduce wheel slip.

As shown in <FIG>, the LSP control system <NUM> has an input function block 12a that receives the following signals: a signal HDC_button indicating whether HDC system selector button <NUM> is currently pressed; a signal set_plus indicating whether the 'set +' button <NUM> is currently pressed; and a signal Resume_button indicating whether the resume button 173R is currently pressed.

In the embodiment of <FIG>, the LSP control system <NUM> is configured to become active and command application of positive powertrain torque as required if the HDC selector button <NUM> is pressed for less than three seconds whilst the LSP control system is not active and the 'set +' button is subsequently pressed within <NUM> seconds of release of the HDC selector button <NUM>. Other time periods are also useful.

The LSP control system input function block 12a is arranged to communicate with a corresponding input function block 12HDa of the HDC control system 12HD. If the LSP control system assumes the active mode, the LSP control system input function block 12a provides a signal LSP_active to the HDC system 12HD signalling that the LSP control system <NUM> is in the active state. With the LSP control system <NUM> in the active state, the HDC system 12HD is configured to set the value of HDC_set-speed to the value of LSP_set-speed and to operate in a slave mode to the LSP control system <NUM>. That is, the HDC control system 12HD is operable to command application of brake torque by the ABS controller <NUM> when commanded to do so by the LSP control system <NUM>.

If neither the LSP control system <NUM> nor the HDC system 12HD are active and the HDC selector button <NUM> is pressed for <NUM> or longer, the HDC system 12HD becomes active. Under such circumstances the HDC system 12HD is not slaved to the LSP control system <NUM> and the LSP control system <NUM> remains inactive.

If either the LSP control system <NUM> or the HDC system 12HD is active and the HDC selector button is pressed for less than <NUM>, the active system <NUM>, 12HD is deactivated.

As noted above, the HDC system 12HD is operable to apply brake torque to prevent vehicle speed exceeding HDC_set-speed (which is set equal to LSP_set-speed when the LSP control system is active), but not to apply positive powertrain torque.

The HDC control system input function block 12HDa is configured to output a value of LSP_set-speed to a target speed trajectory profile function block 12b of the LSP control system <NUM> as well as to a target speed trajectory profile function block 12HDb of the HDC control system 12HD. If the LSP control system <NUM> is activated with the vehicle substantially stationary, the value of LSP_set-speed is set to the minimum value at which the LSP control system <NUM> may cause a vehicle <NUM> to operate. In the present embodiment this speed is substantially 2kph. Other speeds may be set instead of 2kph.

If the LSP control system <NUM> is activated whilst the vehicle <NUM> is moving, the value of LSP_set-speed may be set to the instant vehicle speed, v_actual as determined by the VCU <NUM>.

Function block 12b also receives as an input a signal TR_mode indicative of the driving mode (or 'TR mode') in which the vehicle <NUM> is currently operating, and signal v_actual, indicating the speed of the vehicle <NUM> over ground as determined by the VCU <NUM>.

The function block 12b is configured to determine a target instant speed value LSP_V_T and a target acceleration value LSP_A_T being, respectively, an instant speed at which the vehicle <NUM> is required to travel and an instant rate at which the vehicle is required to accelerate to the instant speed, respectively. The function block 12b receives as inputs the values of LSP_set-speed, TR_mode and v_actual. The value of each of these parameters is input to a look-up table which generates the values of LSP_V_T and LSP_A_T. The values of the parameters LSP_V_T and LSP_A_T are input to a PI (proportional-integral) control module 12c to generate a value of LSP_PT_TQ that is output to the powertrain controller <NUM>. Function block 12b controls the value of LSP_V_T and the value of LSP_A_T such that the target speed gradually becomes equal to LSP_set-speed according to target speed trajectory profiles stored in a memory thereof.

The PI control module 12c also receives as an input a value corresponding to the instant value of torque, PT_trq, being generated by the powertrain <NUM>, a value of a parameter A_actual corresponding to the actual instant rate of acceleration of the vehicle <NUM>, the signal TR_mode and a value of a parameter 'slope' corresponding to a steepness of a slope on which the vehicle <NUM> is driving. It is to be understood that A_actual may be positive or negative depending on whether the vehicle <NUM> is accelerating or decelerating. The value of 'slope' may be positive or negative depending on whether the vehicle <NUM> is ascending or descending a slope.

It is to be understood that in the present embodiment the values of proportional feedback gain and integral feedback gain are adjusted in dependence on the TR mode in which the vehicle <NUM> is operating, as determined by reference to parameter TR_mode, and the driving surface gradient, as determined by reference to parameter slope.

It is to be understood therefore, that the data stored in the look-up table associated with function block 12b is able to take account of differences in the optimum rates of acceleration and deceleration for the different TR modes. Function block 12c, in turn, adjusts these rates in dependence on the gradient of the driving surface according to stored data. The manner in which the rates are adjusted in dependence on slope is further dependent on the TR mode, and therefore the function block 12c receives the signals indicative of TR mode and driving surface gradient.

In the present embodiment the values are adjusted such that when the vehicle is in the 'Sand' TR mode and ascending a slope, the rate at which the value of LSP_PT_TQ increases when an increase in powertrain torque is required, due to target speed undershoot, is greater (i.e. more aggressive) than that when the vehicle is traversing level ground. When the vehicle is in the 'Sand' TR mode and ascending a slope and a decrease in powertrain torque is required, due to target speed overshoot, the rate at which LSP_PT_TQ decreases is lower (i.e. less aggressive) than in the case where the vehicle is traversing level ground. This is because gravity is acting in favour of reducing vehicle speed even in the absence of brake torque from the braking system <NUM>, such that vehicle speed will reduce at a greater rate than if the vehicle <NUM> were travelling over level ground.

In contrast, if the vehicle <NUM> is in the GGS or the MR TR mode and ascending a slope (i.e. over a driving surface with a positive gradient), the values of proportional feedback gain and integral feedback gain are adjusted such that the rate at which the amount of commanded powertrain torque increases, when an increase in powertrain torque is required due to target speed undershoot, is lower (i.e. less aggressive) than that when the vehicle is traversing level ground. This reduces the risk of excessive wheel slip, and therefore loss of traction.

When the vehicle <NUM> is in the GGS or the MR TR mode and ascending a slope and a decrease in powertrain torque is required, due to target speed overshoot, the rate at which LSP_PT_TQ decreases is lower (i.e. less aggressive) than in the case where the vehicle is traversing level ground. This is at least in part because gravity is acting in favour of reducing vehicle speed even in the absence of brake torque from the braking system <NUM>, such that vehicle speed will reduce at a greater rate than if the vehicle <NUM> were travelling over level ground.

<FIG> illustrates schematically the general form of three gain profiles A, B, C corresponding to data stored by function block 12b for use when operating in the sand TR mode when actual vehicle speed falls below the target speed, i.e. target speed undershoot occurs. The gain profiles are employed by the function block 12c to control the rate of increase of the amount of tractive torque that the powertrain controller <NUM> is requested to deliver.

Profile B is a baseline profile and is used when travelling over a substantially horizontal surface and target speed undershoot occurs. Profile A is used when travelling uphill and target speed undershoot occurs; it can be seen that the value of gain employed as a function of speed can be seen to be higher when travelling uphill compared to when travelling over a substantially horizontal surface. That is, the function block 12c is caused to command a more aggressive increase in powertrain torque when undershoot occurs when travelling uphill, compared with travel over a substantially horizontal surface.

Profile C is used when travelling downhill and target speed undershoot occurs. It can be seen that the value of gain employed as a function of speed is lower when travelling downhill compared to when travelling over a substantially horizontal surface. That is, the function block 12c is caused to command a less aggressive increase in powertrain torque when undershoot occurs when travelling downhill, compared with travel over a substantially horizontal surface. This is due at least in part to the fact that gravity will tend to assist in the correction undershoot in the case of downhill driving whilst gravity will tend to oppose correction of undershoot when travelling uphill.

<FIG> illustrates schematically the general form of three gain profiles A, B, C corresponding to data stored by function block 12b for use when operating in the GGS mode when actual vehicle speed falls below the target speed, i.e. target speed undershoot occurs. The gain profiles are employed by the function block 12c to increase the amount of tractive torque that the powertrain controller <NUM> is requested to deliver.

Profile B is a baseline profile and is used when travelling over a substantially horizontal surface and target speed undershoot occurs. Profile A is used when travelling uphill and target speed undershoot occurs; it can be seen that the value of gain employed as a function of speed can be seen to be lower when travelling uphill compared to when travelling over a substantially horizontal surface. That is, the function block 12c is caused to command a less aggressive increase in powertrain torque when undershoot occurs when travelling uphill, compared with travel over a substantially horizontal surface. This has the advantage that the wheels are less likely to experience excessive slip, increasing the risk that the vehicle fails to make adequate progress over terrain, and/or the risk that undesirable modification of the driving surface occurs due to wheel slip. Surface modification is typically less problematic in the case of driving over sand.

Profile C is used when travelling downhill and target speed undershoot occurs. It can be seen that the value of gain employed as a function of speed is lower when travelling downhill compared to when travelling over a substantially horizontal surface, in a similar manner to the case when travelling uphill in the GGS mode. That is, the function block 12c is caused to command a less aggressive increase in powertrain torque when undershoot occurs when travelling downhill, compared with travel over a substantially horizontal surface.

In the embodiment illustrated in <FIG> the value of gain is lower, for travel downhill over a driving surface having a given value of slope, compared with the gain value for travel uphill over a driving surface having the same value of slope as shown by the fact that profile C is below profile A in the figure. However, the value of gain for travel downhill may be higher than, or substantially the same as, the value for travel uphill, in some alternative embodiments.

If the vehicle is operating in the MR mode, the general relative form of the gain profiles for speed undershoot when travelling uphill, over substantially horizontal ground, or downhill, have a similar relationship to that illustrated in <FIG>, i.e. the gain values decrease when travelling uphill or downhill compared to travel over substantially level ground.

In order to prevent or at least reduce passenger discomfort due to rapid changes in acceleration rate (jerk), the LSP control system <NUM> limits the rate of change of acceleration of the vehicle <NUM>, LSP_A_T, such that it does not exceed a prescribed maximum value. The value of LSP_A_T is set in dependence on TR mode, the value for TR_mode=sand being higher than the value for TR_mode=SPO, GGS or MR due to the higher drag imposed on a vehicle <NUM> traversing sand compared with a vehicle traversing a dry asphalt highway surface, a grass, gravel or snow surface, or a muddy or rutted surface.

Furthermore, the value of LSP_A_T is controlled such that a steady state rate of acceleration is established the value of which is determined according to the value of TR_mode. The steady state rate of acceleration is higher for high-drag surfaces such as sand compared with lower drag surfaces in order to reduce a risk that a vehicle becomes stuck, i.e. unable to make adequate progress across terrain.

Turning to the HDC control system 12HD, the system 12HD has a function block 12HDb similar to the function block 12b of the LSP control system <NUM> that also receives signals TR_mode, v_actual and A_actual. Function block 12HDb is configured to determine, by reference to a look-up table, an instant value of a parameter HDC_V_T and parameter HDC_A_T based on the signals TR_mode, v_actual and A_actual, and to output the value of parameters HDC_V_T and HDC_A_T to a PI control module 12HDc. The value of parameter HDC_V_T corresponds to a required target instant speed of the vehicle <NUM> and the value of parameter HDC_A_T corresponds to a target instant rate of deceleration of the vehicle <NUM>. Function block 12HDb controls the value of HDC_V_T and the value of HDC_A_T such that the target speed gradually becomes equal to HDC_set-speed according to trajectory profiles stored in a memory thereof.

The value of HDC_A_T is controlled such that a maximum allowable rate of change of acceleration of the vehicle (referred to as a maximum jerk value) is not exceeded, the maximum allowed value of HDC_A_T when TR_mode=sand being lower than that when TR_mode=SPO, GGS or MR due to the more rapid deceleration of the vehicle when travelling over high drag terrain such as sand compared with lower drag terrain, when the amount of drive torque to a wheel is reduced, due to the increased drag. Furthermore, the value of HDC_A_T is controlled such that a steady state rate of deceleration is established the value of which is determined according to the value of TR_mode. The steady state rate of deceleration is arranged to be lower for high-drag surfaces such as sand compared with low-drag asphalt surfaces in order to reduce a risk that sand displaced by a wheel builds up in front of a wheel and causes abrupt deceleration. Abrupt deceleration typically compromises vehicle composure and is therefore typically undesirable.

The values of HDC_A_T and HDC_V_T are input to a PI (proportional-integral) control module 12HDc which generates a value of HDC_BRK_TQ that is output to the ABS controller <NUM>.

The PI control module 12HDc also receives as an input a value corresponding to the instant value of brake torque, BRK_trq, being generated by the braking system <NUM>, along with values of A_actual, 'slope' and TR_mode. It is to be understood that the value of A_actual may be positive or negative depending on whether the vehicle <NUM> is accelerating or decelerating. The value of 'slope' is used to adjust a value of proportional feedback gain and integral feedback gain of the PI control module 12HDc according to the slope of the driving surface and the TR_mode in which the vehicle <NUM> is driving. Thus, the function block 12HDc adjusts the values of proportional and integral feedback gain constants employed by PI control module 12c in dependence on the gradient of the driving surface and TR mode.

As noted above, when the LSP control system <NUM> is active, the HDC control system 12HD is slaved to the LSP control system <NUM> and is configured to apply brake torque to the wheels as required. The LSP control system <NUM> is configured to command less aggressive application of brake torque by the HDC control system 12HD when the vehicle is operating in the sand mode compared to the SPO mode in order to reduce the risk that one or more wheels sink into the relatively compliant surface and cause a relatively abrupt and substantial increase in resistance to vehicle movement. In the case of operation in the GGS or MR TR modes,.

When the vehicle <NUM> is travelling uphill in one of the GGS or MR TR modes and target speed overshoot occurs, the LSP control system <NUM> is configured to cause the PI control module 12HDc of the HDC control system 12HD to operate with reduced gain values in order to reduce vehicle speed (in the event braking is required) due to the effect of gravity in assisting deceleration of the vehicle <NUM>.

In contrast, when the vehicle is travelling downhill in one of the GGS or MR TR modes and target speed overshoot occurs, the LSP control system <NUM> is configured to cause the PI control module 12HDc of the HDC control system 12HD to operate with increased gain values relative to those employed when travelling over substantially horizontal terrain. This results in more aggressive braking when travelling downhill, and assists in preventing excessive target overshoot due to the effect of gravity in resisting deceleration of the vehicle <NUM>. However, the gains are not set to excessively high values, in order to reduce the risk of excessive wheel slip due to the relatively low surface coefficient of friction of the surfaces for which the GGS and MR TR modes are optimised, relative to dry tarmac surfaces for which the SPO mode is optimised.

It is to be understood that in some embodiments in which a powertrain <NUM> has one or more electric machines operable as a generator, negative torque may be applied by the powertrain <NUM> to one or more wheels by the one or more electric machines. 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 <NUM> 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 when positive drive torque is commanded by the driver or LSP control system <NUM>.

In order to cause application of the necessary positive or negative torque to the wheels, the VCU <NUM> may command that positive or negative torque is applied to the vehicle wheels by the powertrain <NUM> and/or that a braking force is applied to the vehicle wheels by the braking system <NUM>, 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 some embodiments torque is applied to the vehicle wheels individually, for example by powertrain torque vectoring, so as to maintain the vehicle at the required speed. Alternatively, in some embodiments torque may be applied to the wheels collectively to maintain the required speed, for example in vehicles having drivelines where torque vectoring is not possible. In some embodiments, the powertrain controller <NUM> may be operable to implement torque vectoring to control an amount of torque applied to one or more wheels by controlling a driveline component such as a rear drive unit, front drive unit, differential or any other suitable component. For example, one or more components of the driveline <NUM> may include one or more clutches operable to allow an amount of torque applied to one or more wheels to be varied. Other arrangements may also be useful.

Where a powertrain <NUM> includes one or more electric machines, for example one or more propulsion motors and/or generators, the powertrain controller <NUM> may be operable to modulate torque applied to one or more wheels in order to implement torque vectoring by means of one or more electric machines.

In some embodiments the LSP control system <NUM> may receive a signal wheel_slip (also labelled <NUM> in <FIG> and <FIG>) indicative of a wheel slip event having occurred. This signal <NUM> is also supplied to the on-highway cruise control system <NUM> of the vehicle, and which in the case of the latter triggers an override or inhibit mode of operation in the on-highway cruise control system <NUM> so that automatic control of vehicle speed by the on-highway cruise control system <NUM> is suspended or cancelled. However, the LSP control system <NUM> is not arranged to cancel or suspend operation on receipt of wheel_slip signal <NUM>. Rather, the system <NUM> is arranged to monitor and subsequently manage wheel slip so as to reduce driver workload. During a slip event, the LSP control system <NUM> 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 (by the powertrain <NUM> and braking system <NUM>) so as to maintain vehicle speed at the selected value. It is to be understood therefore that the LSP control system <NUM> is configured differently to the cruise control system <NUM>, for which a wheel slip event has the effect of overriding the cruise control function so that manual operation of the vehicle must be resumed, or speed control by the cruise control system <NUM> resumed by pressing the resume button 173R or set-speed button <NUM>.

In a further embodiment of the present invention (not shown) a wheel slip signal <NUM> is derived not just from a comparison of wheel speeds, but further refined using sensor data indicative of the vehicle's speed over ground. Such a speed over ground determination may be made via global positioning (GPS) data, or via a vehicle mounted radar or laser based system arranged to determine the relative movement of the vehicle <NUM> 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 function by depressing the accelerator pedal <NUM> and/or brake pedal <NUM> to adjust the vehicle speed in a positive or negative sense. However, in the event that a wheel slip event is detected via signal <NUM>, the LSP control system <NUM> remains active and control of vehicle speed by the LSP control system <NUM> is not suspended. As shown in <FIG>, this may be implemented by providing a wheel slip event signal <NUM> to the LSP control system <NUM>, wheel slip then being managed by the LSP control system <NUM>. In the present embodiment the SCS <NUM> generates the wheel slip event signal <NUM> and supplies it to the LSP control system <NUM> and cruise control system <NUM>. In some embodiments the ABS controller <NUM> generates the wheel slip event signal <NUM>. Other arrangements may be useful.

A wheel slip event is triggered when a loss of traction occurs at any one of the vehicle wheels. Wheels and tyres may be more prone to losing traction when travelling for example on snow, ice, mud or sand and/or on steep gradients or cross-slopes. A vehicle <NUM> may also be more prone to losing traction in other 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 <NUM> 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 <NUM> is also provided with additional sensors (not shown) which are representative of a variety of different parameters associated with vehicle motion and status. These may be inertial systems unique to the LSP or HDC control systems <NUM>, 12HD or part of an occupant restraint system or any other sub-system which may provide data from sensors such as gyros and/or accelerometers that may be indicative of vehicle body movement and may provide a useful input to the LSP and/or HDC control systems <NUM>, 12HD. The signals from the sensors provide, or are used to calculate, a plurality of driving condition indicators (also referred to as terrain indicators) which are indicative of the nature of the terrain conditions over which the vehicle <NUM> is travelling.

The sensors (not shown) on the vehicle <NUM> include, but are not limited to, sensors which provide continuous sensor outputs to the VCU <NUM>, including wheel speed sensors, as mentioned previously, an ambient temperature sensor, an atmospheric pressure sensor, tyre pressure sensors, wheel articulation sensors, gyroscopic sensors to detect vehicular yaw, roll and pitch angle and rate, a vehicle speed sensor, a longitudinal acceleration sensor, an engine torque sensor (or engine torque estimator), a steering angle sensor, a steering wheel speed sensor, a gradient sensor (or gradient estimator), a lateral acceleration sensor which may be part of the SCS <NUM>, a brake pedal position sensor, a brake pressure sensor, an accelerator pedal position sensor, longitudinal, lateral and vertical motion sensors, and water detection sensors forming part of a vehicle wading assistance system (not shown). In other embodiments, only a selection of the aforementioned sensors may be used.

The VCU <NUM> also receives a signal from the steering controller 170C. The steering controller 170C is in the form of an electronic power assisted steering unit (ePAS unit) 170C. The steering controller 170C provides a signal to the VCU <NUM> indicative of the steering force being applied to steerable road wheels <NUM>, <NUM> of the vehicle <NUM>. This force corresponds to that applied by a user to the steering wheel <NUM> in combination with steering force generated by the ePAS unit 170C.

The VCU <NUM> 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 the automatic driving mode selection condition, the VCU <NUM> 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. <CIT>, <CIT> and <CIT>, the contents of each of which is incorporated herein by reference.

As indicated above, the nature of the terrain over which the vehicle is travelling (as determined by reference to the selected control mode) may also be utilised in the LSP control system <NUM> to determine an appropriate increase or decrease in vehicle speed. For example, if the user selects a value of user_set-speed that is not suitable for the nature of the terrain over which the vehicle is travelling, the system <NUM> 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 <NUM> selects a set-speed (a value of LSP_set-speed) that differs from the user-selected set-speed user_set-speed, a visual indication of the speed constraint is provided to the user via the LSP HMI <NUM> to indicate that an alternative speed has been adopted.

It is to be understood that, when driving downhill on sand, it may be desirable not to apply negative torque to wheels of the vehicle <NUM>. As described above, this is because the wheels will have a tendency to dig into the sand, the effect being enhanced by the nose-down, weight forward condition during vehicle descent. This may be achieved by relaxing the rate at which negative torque is applied by a braking system <NUM>, in the present embodiment by reducing the proportional and integral feedback gain values of the PI control module 12HDc.

In some embodiments, the VCU <NUM> may be configured such that the LSP control system <NUM> tends to allow the value of v_actual to increase to become substantially equal to LSP_set-speed by coasting rather than by applying positive powertrain torque. in order to achieve this, in the present embodiment the proportional and integral feedback gain values of the PI control module 12c are set to relatively low values when the value of 'slope' indicates a downhill slope. The actual proportional and integral feedback gain values may in some embodiments be arranged to become progressively higher as the value of 'slope' indicates an increasingly steep downhill slope. In some embodiments the actual proportional and integral feedback gain values are set to sufficiently low values that they substantially prevent application of positive torque as the vehicle accelerates downhill towards LSP_set-speed.

It is to be understood that, in some embodiments, in addition to providing a signal TR_mode to the function blocks 12b, 12HDb, a parameter indicative of an actual amount of drag on a vehicle due to external forces may be provided. The function blocks 12b, 12HDb may be arranged to determine, respectively, the values of LSP_V_T, LSP_A_T and HDC_V_T, HDC_A_T in dependence on the amount of drag as well as or instead of the selected TR mode. It is to be understood that travel over sand corresponds to travel over terrain for which the amount of external drag is relatively high. Means for measuring external drag forces on a vehicle are well known.

In some situations, a vehicle <NUM> may descend an incline at a speed below LSP_set-speed and the LSP control system <NUM> may be required to cause application of positive powertrain drive torque to accelerate the vehicle <NUM> to LSP_set-speed. In such circumstances, in some embodiments function blocks 12b, 12c may be configured to set the value of LSP_PT_TQ to a value corresponding to substantially no positive powertrain drive torque prior to v_actual attaining LSP_set-speed. This is so as to prevent excessive overshoot of LSP_set-speed by v_actual, and be performed in dependence on the value of 'slope' and TR mode. This procedure may enable the vehicle <NUM> to descend the slope without a requirement to apply brake torque to one or more wheels. Application of brake torque may give rise to sudden, undesirably high deceleration and degrade vehicle composure. It is to be understood that the LSP control system <NUM> may take advantage of a drag force on the vehicle <NUM> due to the high drag terrain to mitigate excessive over-speed as the vehicle descends the slope. Should excessive overshoot occur, the HDC control system 12HD may be arranged to cause application of brake torque in a more gentle manner (by appropriate control of the values of HDC_V_T and HDC_A_T).

Some embodiments of the present invention enable vehicle operation with enhanced composure on driving surfaces of different gradients.

In addition, some embodiments of the present invention have the advantage that sudden over-braking on high drag terrain such as sand may be prevented. Some embodiments of the present invention give rise to greatly enhanced vehicle composure when driving across varied terrain, especially over high drag, deformable surfaces such as sand.

It will be understood that the embodiments described above are given by way of example only and are not intended to limit the invention, the scope of which is defined in the appended claims.

Claim 1:
A speed control system for a vehicle, comprising:
torque control means (<NUM>) for automatically causing application of positive and negative torque to one or more wheels (<NUM>, <NUM>, <NUM>, <NUM>) of a vehicle (<NUM>) to cause a vehicle to travel at a speed substantially equal to the target speed value; and
means for receiving information indicative of a gradient of a driving surface over which the vehicle is driving,
the torque control means (<NUM>) being configured to control the rate of change of the amount of torque applied to the one or more wheels (<NUM>, <NUM>, <NUM>, <NUM>), in order to attempt to maintain the vehicle traveling substantially at the target speed value, in dependence at least in part on the gradient of the driving surface,
characterized in that the torque control means is further configured to cause the vehicle to accelerate from a first speed to the target speed value, where the first speed is less than the target speed value, at least in part according to stored data in respect of target rate of acceleration as a function of speed, wherein the value of target rate of acceleration according to which the vehicle is caused to accelerate is determined in further dependence at least in part on the driving surface gradient.