Patent Publication Number: US-9902425-B2

Title: System for guiding trailer along target route during reversing maneuver

Description:
RELATED APPLICATIONS 
     This application is a 35 U.S.C. § 371 national stage application of PCT Application No. PCT/EP2014/064270, filed on Jul. 3, 2014, which claims priority from Great Britain Patent Application No. 1312040.7 filed on Jul. 4, 2013, the contents of which are incorporated herein by reference in their entireties. The above-referenced PCT International Application was published in the English language as International Publication No. WO 2015/001066 A9 on Jan. 8, 2015. 
     TECHNICAL FIELD 
     The present disclosure relates to a vehicle control system; a vehicle; and a method of controlling a vehicle. 
     BACKGROUND OF THE INVENTION 
     It is known from GB 2447672 to provide a computing unit to calculate a hitch angle between a trailer and a vehicle. The computing unit uses image data from three video cameras, fitted respectively to the rear of the vehicle and one on each door mirror, to track the position of a trailer-mounted marker to determine the position of the trailer relative to the vehicle. The computing unit can also predict a trailer target route and this is output to the driver on a display as guide lines overlaid onto a camera view. 
     At least in certain embodiments, the present invention relates to an improved control system. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention provide a system, a vehicle and a method as claimed in the appended claims. 
     According to a further aspect of the present invention there is provided a vehicle control system for controlling a vehicle to facilitate reversing a trailer coupled to the vehicle, the vehicle control system comprising a processor configured to:
         receive a target trailer destination;   determine a current trailer position ; and   model a target route from the current trailer position to the target trailer destination;   wherein the processor is configured to output a vehicle control signal for controlling the vehicle to guide the trailer along the target route. The target trailer destination may be spatially displaced from the current trailer position and the controller may be operative to guide the trailer from its current position to the target destination. The target route may be modelled to enable the trailer to be reversed from the current trailer position to the target trailer destination. Rather than directly control the direction of travel of the vehicle or the trailer, the user may specify the target destination for the trailer and the processor may be configured to generate a target route. In particular, the processor may be configured to model the target route for the trailer to follow as it travels from its current position to the specified target destination.       

     The vehicle control signal can define the required steering angle inputs for the vehicle to guide the trailer along the target route. These could be output, for example to an audio or visual system, to prompt the user to make appropriate changes to the steering angle. Alternatively, the vehicle control signal can be output to steering control means for controlling the steering angle. The steering control means can, for example, comprise electronic power assisted steering (EPAS). 
     The current trailer position can define a current hitch angle (between the vehicle and the trailer) which determines an actual travel direction of the trailer. The current trailer position can be determined based on one or more sensor signals. The current trailer position can be measured by one or more sensors, such as a rotary sensor, provided at the coupling between the vehicle and the trailer for measuring a hitch angle of the trailer. The current trailer position could be determined by measuring the orientation of the trailer in relation to the vehicle, for example employing one or more ultrasonic sensors or one or more image sensors (for example disposed in a camera). The image data generated by said one or more image sensors could be analyzed to track the trailer, for example by identifying a feature of the trailer and/or a target provided on the trailer. 
     The target trailer destination can comprise position (i.e. spatial) data; and/or orientation (i.e. angular) data. The processor can determine the spatial and/or angular relationship between the current trailer position and the target trailer destination. 
     The target route can comprise one or more curves; and/or one or more linear sections. The target route can comprise only one reversing movement; or can comprise a sequence of forward and reversing movements. The user could specify whether a single reversing movement or multiple reversing movements are performed. The processor could be configured to output braking signals and/or acceleration signals for controlling the braking and acceleration respectively of the vehicle. Alternatively, the driver of the vehicle can control one or both of vehicle acceleration and braking. The processor can be configured to output driver prompts, for example to perform one or more of the following functions: accelerate, decelerate, change gear (such as select Drive or Reverse), engage parking brake and so on. The processor could be configured to inhibit the maximum speed of the vehicle, for example to 10 km/h. 
     The vehicle control signal can map a center point of the trailer to the target route. The vehicle control signal can be generated to maintain the center point of the trailer substantially coincident with the target route. 
     The target route can define a target trailer travel direction at each point along its length. The processor can be configured to control the vehicle to match the actual trailer travel direction to the target trailer travel direction as it travels along the target route. The vehicle control signal can control the steering angle of the vehicle to maintain the actual trailer travel direction at least substantially coincident with the target trailer travel direction. The actual trailer travel direction can thereby match the target trailer travel direction. 
     The processor can be configured to identify permissible target trailer destinations. The permissible target trailer destinations could be based on the identification of one or more obstructions. The processor can be configured to identify said permissible target trailer destinations based on one or more of the following: a maximum permissible hitch angle; the detected space available for the trailer and/or the vehicle; a detected terrain parameter (such as an incline or gradient). The processor can be configured to inhibit selection of an impermissible target trailer destination. 
     The processor can be configured to receive image data from one or more video cameras disposed on the vehicle and/or the trailer. The processor can comprise an image processing module for identifying obstruction(s) in video image data obtained from one or more optical cameras. The image processing module can be configured to identify one or more obstructions proximal to the trailer and/or the vehicle. Alternatively, or in addition, ultrasonic and/or laser sensor systems could be provided for detecting said obstruction(s). The processor could be configured dynamically to modify the target route if an obstruction is identified. 
     The processor could be configured to determine if there is sufficient space available to manoeuvre the trailer to the target trailer destination. Based on predefined trailer parameters (such as the trailer length, width and axle location), the processor can calculate a route corridor required to reverse the trailer. The imaging system can then identify obstacles proximal to the vehicle and/or the trailer and determine if there is sufficient space available to reverse the trailer. The processor can be configured to modify the target route of the trailer to remain within the calculated route corridor. 
     The processor can be configured to output the target trailer destination and/or the target route to a display. The target trailer destination and/or the target route could, for example, be overlaid onto a video image from one or more of said video camera(s). 
     The target trailer destination can be specified by a user. A human machine interface can be provided for specifying the target trailer destination. The human machine interface can, for example, comprise a display device, such as an LCD screen. An input means can be provided for the human machine interface. The input means can, for example, comprise one or more of the following: a touchscreen, a capacitive sensor, a joystick, a rotary wheel, a trackball, voice recognition, gesture recognition, and eye tracking. A graphical image can be displayed on the display device to represent the target trailer destination. The position and/or the orientation of the graphical image can be controlled using the input means. The position and/or the orientation of the graphical image can be converted to data to identify said target trailer destination and output to the processor. 
     A scaling function can be applied to the graphical image as it is moved around the screen. The scaling function has particular application when the graphical image is displayed in a perspective view, for example as an overlay onto a video image from a camera disposed on the vehicle or the trailer. 
     The graphical image can represent a footprint of the trailer, for example representing an outline of the trailer on the ground. The graphical image could, for example, comprise a rectangle. Alternatively, the graphical image could provide a 3-dimensional representation of the trailer, for example a vector image or a rendering of the trailer. The graphical image can provide a scale representation of the trailer. 
     The vehicle control signal can comprise a steering control signal for controlling a steering angle of the vehicle. 
     According to a still further aspect of the present invention there is provided a vehicle control system for controlling a vehicle to facilitate reversing a trailer coupled to the vehicle, the vehicle control system comprising a processor configured to:
         generate a target route from the current trailer position to the target trailer destination;   wherein the processor is configured to generate the target route by modifying a pre-defined path based on a determined angular offset and/or positional offset between the current trailer position and the target trailer destination.       

     The pre-defined path can comprise first and second curves. The first and second curves can be arranged in an S-shape. The processor can be configured to increase or decrease an included angle of one or both of said first and second curves in dependence on said angular offset. The processor can be configured to define a minimum radius of curvature in the target route based on a jack-knife angle of the trailer. 
     According to a further aspect of the present invention there is provided a vehicle comprising a vehicle control system of the type described herein. 
     According to a still further aspect of the present invention there is provided a method of controlling a vehicle to reverse a trailer to a specified target trailer destination, the method comprising:
         receiving an input to define a target trailer destination;   determining a current trailer position;   modelling a target route from the current trailer position to the target trailer destination; and   outputting a vehicle control signal for controlling the vehicle to guide the trailer along the target route.       

     According to a yet further aspect of the present invention there is provided a method of generating a target route for reversing a trailer from a current position to a target destination; the method comprising:
         modifying a pre-defined path based on a determined angular offset and/or positional offset between the current position and the target destination to generate the target route. The pre-defined path can be stored in memory. The pre-defined path can, for example, comprise first and second curves, for example arranged in an S-shape. The method can comprise applying a minimum radius of curvature to the target route, the minimum radius of curvature being based on the jack-knife angle of the trailer.       

     The methods described herein can each be computer-implemented, for example on a computational apparatus comprising one or more microprocessors. According to a yet further aspect of the present invention there is provided a computer program product comprising a computer readable storage medium including computer readable program code, where the computer readable program code when executed on a computer causes the computer to perform the method(s) described herein. 
     The term processor used herein is to be understood as covering both single processors and multiple processors. 
     Within the scope of this application it is expressly envisaged that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying figures, in which: 
         FIG. 1  shows a plan view of a trailer coupled to a vehicle incorporating a vehicle control system in accordance with an embodiment of the present invention; 
         FIG. 2  shows a schematic representation of the vehicle and the control system in accordance with an embodiment of the present invention; 
         FIG. 3  shows a plan view of the vehicle and trailer with the target trailer destination and the target route indicated; 
         FIG. 4  shows a plan view of the vehicle and trailer shown in  FIG. 3  with the path to be followed by the wheels of the trailer indicated; 
         FIG. 5  illustrates the routes traced by the rotational centers of the vehicle and the trailer and the trailer hitch; 
         FIGS. 6A-6E  illustrate the generation of the target route in different scenarios; 
         FIGS. 7A and 7B  illustrate the control strategy for guiding the trailer along the target route; 
         FIG. 8  illustrates the vehicle and trailer dimensions and associated nomenclature; and 
         FIG. 9  shows a display image from the rear camera on the trailer with a target trailer destination indicated. 
     
    
    
     DETAILED DESCRIPTION OF AN EMBODIMENT 
     A vehicle control system  1  for assisting with the reversing of a vehicle  3  and a trailer  5  in accordance with an aspect of the present invention will now be described with reference to  FIGS. 1 to 9 . 
     The vehicle control system is operable to facilitate reversing of the trailer  5  when it is coupled to the vehicle  3  (the combination of the vehicle  3  and the trailer  5  can be referred to as ‘a rig’). More specifically, the vehicle control system  1  is configured to control the steering of the vehicle  3  such that the trailer  5  is reversed to a target trailer destination D TAR  specified by the user. In the present embodiment, the operation of the throttle and brakes of the vehicle  3  are controlled by the user, but these control functions could be automated. 
     A plan view of the vehicle  3  and the trailer  5  is shown in  FIG. 1 . The vehicle  3  has two front wheels W 1 , W 2  and two rear wheels W 3 ,  4 . As illustrated in  FIG. 2 , the front wheels W 1 , W 2  of the vehicle  3  are steerable in conventional manner to define a steering angle θ. The trailer  5  has two wheels TW 1 , TW 2  which are not steerable. 
     As shown in  FIGS. 1 and 2 , the vehicle  3  is provided with a vehicle imaging system comprising a centrally mounted rear vehicle camera  9 ; and left and right side vehicle cameras  10 ,  11  mounted in respective wing mirrors  13 ,  15 . The vehicle cameras  9 ,  10 ,  11  are optical cameras arranged to face to the rear of the vehicle  3  and their fields of view V 1 , V 2 , V 3  are illustrated by dashed triangles. The vehicle  3  optionally also comprises an obstruction detection system, for example side radar systems  7 A,  7 B for detecting the presence of an obstruction in the rear three quarters position of the vehicle  3 . 
     A tow hitch  17  is mounted to the vehicle  3  for coupling to a trailer coupling  19  mounted to the trailer  5 . The tow hitch  17  is an upwardly projecting tow ball in the present embodiment. The trailer coupling  19  is mounted to a hitch frame  21  disposed at the front of the trailer  5 . In the present embodiment, the hitch frame  21  is an A-frame having a front apex  23  to which the trailer coupling  19  is mounted. A target  25  is mounted to a front face  27  of the trailer  5  to enable the position of the trailer  5  relative to the vehicle  3  to be determined, as described herein. The target  25  is a visible image comprising three circles arranged in a triangular formation. It will be appreciated that the present invention can be implemented with other targets  25 , for example comprising different symbols/images or non-visible targets. 
     The trailer  5  is provided with a trailer imaging system comprising a centrally mounted rear trailer camera  29 ; and left and right side trailer cameras  30 ,  31  mounted on respective sides of the trailer  5 . The trailer cameras  29 ,  30 ,  31  are optical cameras arranged to provide video images of the area around the trailer  5  which may otherwise be obscured from view. The fields of view TV 1 , TV 2 , TV 3  of the trailer cameras  29 ,  30 ,  31  are illustrated by dashed triangles in  FIGS. 3 and 4 . The trailer  5  optionally also comprises an obstruction detection system, for example parking sensors  32 A,  32 B, for detecting the presence of obstructions at the rear of the trailer  5 . The image data from the trailer cameras  29 ,  30 ,  31  and/or obstruction data from the obstruction detection system is transmitted to the vehicle  3  over either a wired connection or a wireless communication channel. 
     The vehicle control system  1  comprises an electronic control unit (ECU) having an electronic processor  33 , as shown schematically in  FIG. 2 . The processor  33  comprises image processing means in the form of an image processing module  33 A for analysing the image data. The vehicle cameras  9 ,  10 ,  11  and the trailer cameras  29 ,  30 ,  31  each output image data to the image processing module  33 A for analysis. In use, the image processing module  33 A analyses the image data from the vehicle cameras  9 ,  10 ,  11  to identify the target  25  to determine the current position of the trailer  5  in relation to the vehicle  3 . In particular, the image processing module determines the current orientation of the trailer  5  relative to the vehicle  3 . Moreover, the image processing module  33 A analyses the image data from the trailer cameras  29 ,  30 ,  31  to identify obstructions proximal to the trailer  5 . The image processing module  33 A can optionally analyse the image data from the vehicle cameras  9 ,  10 ,  11  to identify obstructions proximal to the vehicle  3  and trailer  5 . 
     The image processing module  33 A is configured to multiplex the images from the trailer cameras  29 ,  30 ,  31  to generate a composite image  35  showing a panoramic view to the rear of the trailer  5 , as shown in  FIG. 9 . The resulting composite image  35  can be output to a display screen  37  to be viewed by the user substantially in real-time. The display screen  37  can be provided in an instrument cluster or in an infotainment console. The image data from the vehicle cameras  9 ,  10 ,  11  and the trailer cameras  29 ,  30 ,  31  could be combined to provide a plan view of the vehicle  3  and the trailer  5  (as if viewed from a position above the vehicle  3  and the trailer  5 ). 
     The processor  33  further comprises vehicle/trailer guidance means in the form of a guidance module  33 B. The guidance module  33 B is provided to assist with guiding the trailer  5  when the vehicle  3  and the trailer  5  are being reversed together. In particular, the guidance module  33 B is configured to output a control signal for controlling the steering angle θ of the front wheels W 1 , W 2  of the vehicle  3  to guide the trailer  5  along a target route R. The target route R is generated by the guidance module  33 B to guide the trailer  5  from its current position to a target destination D TAR  specified by the user. The target route R is illustrated by a dashed line in  FIG. 3  and the path to be followed by the wheels W 1 , W 2  of the trailer  3  are represented by arcuate lines in  FIG. 4 . 
     The vehicle control system  1  comprises a human machine interface (HMI) module  39 . The HMI module  39  is coupled to the display screen  37  and is configured to superimpose a graphical image  41 , representing the target destination D TAR  of the trailer  5 , onto the composite image  35 . In the present embodiment the graphical image  41  comprises a rectangle  43  which represents the footprint of the trailer  5 . The graphical image  41  is sized to provide a scale representation of the trailer  5  within the composite image  35 . To provide an accurate representation of the trailer  5 , the trailer dimensions can be specified by the user and stored in a data file accessible to the HMI module  39 . In the present embodiment a portion of the graphical image  41  is semi-transparent to reduce obfuscation of objects in the composite image  35 . It will be appreciated that the present invention is not limited to the display of a rectangle  43  and other graphical images  39  can be used to represent the trailer  5 . 
     The HMI module  39  is coupled to input means  45  in the form of a touch screen and a rotary dial. The input means  45  can be operated by the user to position the graphical image  41  within the composite image  35 . The user can thereby position the graphical image  41  to specify the target destination D TAR  for the trailer  5 , typically corresponding to a location where the trailer  5  is to be parked. The input means  45  can optionally also enable the user to define the orientation of the trailer  5  in relation to the vehicle  3 , for example by adjusting the angular orientation of the rectangle  43 . 
     The HMI module  39  performs a scaling operation on the graphical image  41  to adjust its size based on its position within the composite image  35 , for example to increase/decrease its size in dependence on its virtual position in relation to the camera generating the video image. The graphical image  41  can optionally also be modified to alter its perspective, for example to simulate foreshortening, as it is moved within the composite image  35 . The graphical image  41  can thereby provide an accurate representation of the size of the trailer  5  within the composite image  35 . The processor  33  could be configured to identify areas or regions which are unsuitable for positioning the trailer  5 . The HMI module  39  can be configured to represent any such areas as an overlay on the composite image  35 . Alternatively, or in addition, the HMI module  39  can be configured to prevent the graphical image  41  being moved to locations identified by the processor  33  as being unsuitable for positioning the trailer  5 . For example, the processor  33  could determine that a region displayed in the composite image  35  is unsuitable due to the detection of one or more obstructions by the image processing module  33 A. Alternatively, or in addition, the processor  33  could assess one or more terrain parameters, such as incline or gradient, to identify unsuitable regions for the trailer  5 . The terrain parameters could be assessed from the image data derived from the vehicle cameras  9 ,  10 ,  11  and/or the trailer cameras  29 ,  30 ,  31 . Alternatively, or in addition, one or more of the following types of sensor could be employed to monitor the terrain: ultrasonic sensors/cameras, laser scanners and radar. The resulting terrain data could be used to restrict selection of the target destination D TAR ; and/or control vehicle operating parameters, for example to engage four wheel drive or to select a low transfer range; and/or request user confirmation of the selected target destination D TAR . 
     The guidance module  33 B is configured to generate the target route R to guide the trailer  5  from its current location to the target destination D TAR  specified by the user. The target route R defines a target trailer travel direction T TAR  for guiding the trailer  5  from its current position to the target destination D TAR . The target route R can comprise rectilinear and/or curved sections. The target route R is arranged coincident with a midpoint of the trailer  5  in its current position. The current position of the trailer  5  is monitored compared to the originally calculated target route R. Small deviations are managed within the vehicle control system  1 . Larger deviations can trigger a recalculation of the target route R. If the target destination D TAR  becomes unachievable from the current position, the user is alerted and a corrective manoeuvre is suggested (for example travel forward a short distance). 
     The user positions the graphical image  41  within the composite image  35  to define the target destination D TAR  of the trailer  5 . The guidance module  33 B determines the target route R between the current trailer position and the specified target destination D TAR . As illustrated in  FIG. 5 , the vehicle steering angle θ is controlled such that a center of rotation  49  of the trailer  5  defines a first path  51  which at least substantially matches the target route R. The trailer hitch  19  defines a second path  53  as the trailer  5  follows the target route R; and a center of rotation  55  of the vehicle  3  defines a third path  57  as the trailer  5  follows the target route R. In an alternate embodiment, the guidance module  33 B could be configured to generate the second or third paths  53 ,  55  to guide the trailer  5  to the target destination D TAR  and to control the vehicle steering angle θ to follow said second or third paths  53 ,  55 . The target route R is a straight line if the target destination D TAR  is in line with the trailer  5 . Otherwise, the target route R comprises a curve based on the geometry of the vehicle and trailer and location of the target destination. 
     With reference to  FIGS. 6A-6E , the guidance module  33 B implements a geometric algorithm to generate the target route R. The guidance module  33 B utilises the angular offset between the current trailer longitudinal axis X 2  and the target trailer longitudinal axis X 2   TAR ; and the lateral offset between the current trailer position and the target destination D TAR  (measured perpendicular to the trailer longitudinal axis X 2 . As shown in  FIG. 6A , the algorithm defines a default S-shaped path  55  (shown in phantom in  FIGS. 6B-6E ) comprising first and second curves A, B. The first curve A defines a first included angle of +45°; and the second curve B defines a second included angle of −45°. If required, a scaling function (along an X axis and/or a Y axis) is applied to the S-shaped path  55  to link the actual longitudinal axis X 2  of the trailer  5  to the target longitudinal axis X 2   TAR  of the trailer  5  when it is in the target destination D TAR . The guidance module  33 B can optionally be configured to define a minimum radius of curvature for the first and second curves A, B to ensure that the hitch angle φ does not equal or exceed the jack-knife angle. 
     The S-shaped path  55  is positioned to link the actual longitudinal axis X 2  of the trailer  5  to the target longitudinal axis X 2   TAR . A scaling function and/or a rotation function can be applied to the S-shaped path  55  depending on the position and/or orientation of the target destination D TAR  in relation to the current trailer position. The S-shaped path  55  is then modified by adjusting the included angle of either the first curve A or the second curve B in dependence on the angular offset between the actual longitudinal axis X 2  and the target longitudinal axis X 2   TAR . 
     In the present embodiment, the guidance module  33 B identifies which of the first and second curves A, B opens onto the side of the S-shaped path  55  to which the target longitudinal axis X 2   TAR  is oriented. The guidance module  33 B then modifies the included angle of the identified curve A, B corresponding to the angular offset between the actual longitudinal axis X 2  and the target longitudinal axis X 2   TAR . This technique will now be described with reference to  FIGS. 6B-6E . 
     In the scenario illustrated in  FIG. 6B , the target longitudinal axis X 2   TAR  is offset by −45° and is directed to the left side of the S-shaped path  55 . Accordingly, the included angle of the first curve A (which opens to the left side of the S-shaped path  55 ) is reduced by 45°. As the pre-defined included angle is +45°, the first curve A is effectively removed from the S-shaped path  55  leaving only the second curve B. The resulting modified curve is defined as the target route R. 
     In the scenario illustrated in  FIG. 6C , the target longitudinal axis X 2   TAR  is offset by −90° and is directed to the left side of the S-shaped path  55 . Accordingly, the included angle of the first curve A is reduced by 90° to define an included angle of −45°, thereby reversing the curvature direction of the first curve A. The resulting modified curve is defined as the target route R. 
     In the scenario illustrated in  FIG. 6D , the target longitudinal axis X 2   TAR  is offset by +45° and is directed to the right side of the S-shaped path  55 . Accordingly, the included angle of the second curve B (which opens to the right side of the S-shaped path  55 ) is increased by 45°. As the pre-defined included angle is −45°, the second curve B is effectively removed from the S-shaped path  55  leaving only the first curve A. The resulting modified curve is defined as the target route R. 
     In the scenario illustrated in  FIG. 6E , the target longitudinal axis X 2   TAR  is offset by +90° and is directed to the right side of the S-shaped path  55 . Accordingly, the included angle of the second curve B is increased. However, to compensate for the lateral offset of the trailer  5 , the second curve B is increased by 45° to 0°, thereby removing the second curve B. The first curve A is also increased by 45° to 90° (again to compensate for the lateral offset of the target destination D TAR ). The resulting modified curve is defined as the target route R. 
     In a second process step, the vehicle control system  1  attempts to identify the presence of any obstructions (denoted generally by the reference numeral  46  in  FIGS. 7A and 7B ). Typical obstructions include kerbs, walls, vehicles, etc. The vehicle control system  1  can optionally also determine terrain parameters, for example to detect a metalled surface indicating a road; and/or terrain features, for example gradient changes. As described herein, the obstructions  46  can be identified by the image processing module  33 A and/or other detection systems. The guidance module  33 B modifies the target route R generated in the first process step. By way of example, the guidance module  33 B will give a higher priority to a route which guides the trailer  5  to the target destination D TAR  whilst remaining on a road than a route which provides the shortest achievable path. 
     With reference to  FIGS. 7A and 7B , in order to guide the trailer  5  the guidance module  33 B calculates the required vehicle steering angle θ to achieve the appropriate trailer angle δ at intervals x, 2x, 3x . . . nx along the route R, where x equals half the combined length of the vehicle  3  and the trailer  5 . The interval distance x is configurable and can be reduced, for example to ¼ the combined length of the vehicle  3  and the trailer  5  to provide improved accuracy, albeit at the expense of increased processing load on the processor  33  The trailer angle δ is the angle between an expected angular position of the trailer  5  at a particular interval and the position of the trailer  5  before the manoeuvre starts. Once the guidance module  33 B has calculated the required trailer angle δ at the calculating points (defined by the n intervals along the target route R), the trailer angles δ n , δ n+1  at adjacent calculation points are compared to determine the difference between the trailer hitch angles φ n , φ n+1  at adjacent interval points. 
     If the hitch angle φ can be achieved between the two calculation points, the guidance module  33 B stores the values in memory and, during the manoeuvre, accesses the stored values after each interval to set the next interval value as the desired hitch angle φ. If the guidance module  33 B detects a large deviation between the present hitch angle φ and the expected hitch angle φ it restarts the calculation to avoid failure. 
     If the hitch angle φ between the two calculation points cannot be achieved (for example based on the maximum steering angle θ of the vehicle and the trailer geometry), the guidance module  33 B compares the required hitch angle φ n , φ n+1  between first and second intervals x to establish if this hitch angle φ can be achieved while considering surrounding conditions. If the comparison of the hitch angle φ n , φ n+1  between the first and second intervals x is possible, the guidance module  33 B stores the values in memory and, during the manoeuvre, accesses the stored values after each interval to set the next interval value as the desired hitch angle φ. If the guidance module  33 B fails to achieve the required hitch angle φ for four (4) consecutive interval points (i.e. x to 4x), an alternative target route R is calculated while considering space requirement and the surrounding obstructions  46 . If the guidance module  33 B is unable to establish a viable alternative target route R, the guidance module  33 B calculates an alternate position between failure points identified by means of driving forward in the available space to complete the manoeuvre. During the route calculation the guidance module  33 B determines whether the hitch angle φ change between two intervals can be achieved. If the hitch angle φ cannot be achieved then the calculation needs to check if the hitch angle φ change calculated in alternative routes in two previous and two subsequent intervals can be achieved. If this fails then the calculation starts again or requests an initial drive forwards to reduce the rate of change of hitch angle. A forward shuffle could also be requested mid-manoeuvre. 
     The angular change Ω in the trailer angle δ at intervals along the target route R is illustrated in  FIG. 7B . The angular change Ω is illustrated with reference to a first time t and a second time t+1, but the calculation of the required angle change is based on distance rather than time. 
     As described herein, the steering angle θ of the vehicle  3  is controlled to maintain the actual travel direction T ACT  of the trailer  5  substantially coincident with the target trailer travel direction T TAR  during the reversing manoeuvre along said target route R. The guidance module  33 B can control the steering angle θ of the front wheels W 1 , W 2  to guide the trailer  5  along the target travel direction T TAR . 
     The control algorithm for generating a control signal to guide the vehicle  3  along the target route R will now be described with reference to  FIG. 9 . The vehicle  3  has a first longitudinal axis X 1  and the trailer  5  has a second longitudinal axis X 2 . The angular offset between the first and second longitudinal axes X 1 , X 2  is referred to as the hitch angle φ. During reversing, the trailer  5  travels in a direction T ACT  corresponding to the hitch angle φ (unless the hitch angle φ exceeds a jack-knife angle for the trailer  5 , as described herein). 
     The image processing module  33 A calculates the hitch angle φ with reference to the target  25  and outputs a hitch angle signal to the guidance module  33 B. When reversing, the guidance module  33 B calculates the required steering angle θ based on the following equation:
 
θ t+1 =θ t +min(max( k (φ req −φ cur ),−α),α)
     Where: θ t+1  and θ t  are the steering angles of the vehicle  3  at frame t+1 and t (auto steering command from the algorithm and current steering from the CAN respectively);
       φ req  and φ cur  are the requested and current hitch angles;   α is the maximum steering offset value; and   k is a constant multiplier.   
       

     The value of the gain k can be calculated based on the relationship between θ and φ, as shown in  FIG. 8 . When the trailer hitch length L plus the tow bar offset of the vehicle h is equal to the vehicle wheelbase d, then the relationship between θ and φ is one (1) for small angles and so the gain k can be set to a value of one (1). The gain k can therefore be calculated based on the following equation: 
     
       
         
           
             k 
             = 
             
               
                 L 
                 + 
                 h 
               
               d 
             
           
         
       
         
         Where: L is the hitch length of the trailer  5 ;
       h is the tow bar offset of the vehicle  3 ;   d is the wheelbase of the vehicle  3 ;   
     
       
    
     The gain k thereby compensates for the fact that longer trailers take longer to reach the requested hitch angle φ. 
     The guidance module  33 B is configured to calculate a maximum permissible hitch angle φ MAX . If the hitch angle φ exceeds the maximum permissible hitch angle φ MAX  (which in the present embodiment is the jack-knife angle of the trailer  5 ), it is no longer possible to reduce the hitch angle φ by continuing to reverse (i.e. the trailer  5  has jack-knifed). If the detected hitch angle φ is equal to or exceeds the calculated jack-knife angle, the vehicle control system  1  will advise the user to drive forward until it determines that the trailer angle δ at the next interval is achievable while considering space and obstructions. With reference to  FIG. 8 , the guidance module  33 B calculates the maximum permissible hitch angle φ MAX  by applying the following set of equations: 
     
       
         
           
             R 
             = 
             
               d 
               
                 tan 
                 ⁡ 
                 
                   ( 
                   θ 
                   ) 
                 
               
             
           
         
       
       
         
           
             ϕ 
             = 
             
               
                 
                   cos 
                   
                     - 
                     1 
                   
                 
                 ⁡ 
                 
                   ( 
                   
                     
                       
                         - 
                         Lh 
                       
                       + 
                       
                         R 
                         ⁢ 
                         
                           
                             
                               R 
                               2 
                             
                             + 
                             
                               h 
                               2 
                             
                             - 
                             
                               L 
                               2 
                             
                           
                         
                       
                     
                     
                       
                         R 
                         2 
                       
                       + 
                       
                         h 
                         2 
                       
                     
                   
                   ) 
                 
               
               ⁢ 
               
                   
               
               ⁢ 
               for 
               ⁢ 
               
                   
               
               ⁢ 
               positive 
               ⁢ 
               
                   
               
               ⁢ 
               θ 
             
           
         
       
       
         
           
             ϕ 
             = 
             
               
                 - 
                 
                   
                     cos 
                     
                       - 
                       1 
                     
                   
                   ⁡ 
                   
                     ( 
                     
                       
                         
                           - 
                           Lh 
                         
                         - 
                         
                           R 
                           ⁢ 
                           
                             
                               
                                 R 
                                 2 
                               
                               + 
                               
                                 h 
                                 2 
                               
                               - 
                               
                                 L 
                                 2 
                               
                             
                           
                         
                       
                       
                         
                           R 
                           2 
                         
                         + 
                         
                           h 
                           2 
                         
                       
                     
                     ) 
                   
                 
               
               ⁢ 
               
                   
               
               ⁢ 
               for 
               ⁢ 
               
                   
               
               ⁢ 
               negative 
               ⁢ 
               
                   
               
               ⁢ 
               θ 
             
           
         
       
         
         Where: R is the turning radius;
       θ is the steering angle of the vehicle  3 ;   d is the wheelbase of the vehicle  3 ;   h is the tow bar offset of the vehicle  3 ;   L is the hitch length of the trailer  5 ;   
     
       
    
     The wheelbase d, the tow bar offset h and the maximum steering angle θ MAX  of the vehicle  3  are defined for the vehicle  3 . The hitch length L of the trailer  5  is entered during trailer setup by the user (or can be determined during a calibration exercise). The guidance module  33 B outputs a maximum hitch angle signal to indicate the maximum permissible hitch angle φ MAX  for the current steering angle θ. The guidance module  33 B is configured to inhibit the trailer travel direction T ACT  to values which are less than the maximum permissible hitch angle φ MAX . A minimum radius of curvature of the target route R can be defined to ensure that the trailer travel direction T ACT  is less than or equal to the maximum permissible hitch angle φ MAX . 
     The guidance module  33 B calculates the initial required steering angle θ to achieve the desired hitch angle φ, then iteratively calculates the required steering angle θ and hitch angle φ. In use, the guidance module  33 B is configured to output a steering control signal operative to control the steering angle θ of the front wheels W 1 , W 2  to adjust the hitch angle φ, thereby modifying the actual trailer travel direction T ACT . Specifically, the guidance module  33 B adjusts the steering angle θ (which can be positive or negative in relation to a reference point) to increase or decrease the hitch angle φ. The actual trailer travel direction T ACT  can thereby be matched to the target trailer travel direction T TAR  required to follow the target route R. The hitch angle signal is updated at least substantially in real time and the guidance module  33 B makes corresponding real time adjustments to the steering control signal. The guidance module  33 B controls the hitch angle φ so that the difference between the actual trailer travel direction T ACT  and the target trailer travel direction T TAR  is at least substantially zero. A tolerance of +/−0.5° between the actual trailer travel direction T ACT  and the target trailer travel direction T TAR  provides a good result. 
     The steering angle control signal is output to an electronic power assisted steering (EPAS) module  45  to control the angle of a steering wheel  47  to provide a required steering angle θ for the front wheels W 1 , W 2 . The EPAS module  45  automatically adjusts the angular orientation of the steering wheel  47  to provide the appropriate steering angle θ at the front wheels W 1 , W 2  to control the vehicle  3  to match the actual trailer travel direction T ACT  with the target trailer travel direction T TAR . 
     In use, the user operates the HMI module  39  to position the graphical image  41  at the target destination D TAR  for the trailer  5 , as illustrated in  FIG. 9 . Specifically, the user operates the input means  45  to guide the graphical image  41  to the desired location on the video image  35 . The input means  45  can be operated to adjust the position and/or orientation of the target destination D TAR  for the trailer  5 . The user then confirms the target destination D TAR  for the trailer  5 . 
     The image processing module  33 A analyses the image data from the vehicle cameras  9 ,  10 ,  11  to identify the target  25  on the trailer  5  and to determine the current position of the trailer  5  in relation to the vehicle  3 . The guidance module  33 B then models a target route R for the trailer  5  from its current position to the target trailer destination D TAR . A vehicle control signal is output by the guidance module  33 B to the EPAS module  45  to adjust the steering angle θ to control the vehicle  3  to match the actual travel direction T ACT  of the trailer  5  with the target travel direction T TAR . The HMI module  39  can optionally output instructions to prompt the user to select the appropriate drive gear and to operate the vehicle brakes and throttle. The EPAS module  45 , in dependence on the vehicle control signal output from the guidance module  33 B, then steers the vehicle  3  to guide the trailer  5  along the target route R. A notification can be output by the HMI module  39  to notify the user when the trailer  5  is in the target destination D TAR . The vehicle control system  1  can thereby facilitate reversing the trailer  5  to the target destination D TAR . 
     The present invention is applicable to a range of different types of trailer  5 . For example, the trailer  5  can be a caravan, a goods trailer, a flatbed trailer, a van trailer, a livestock trailer and so on. Similarly, the present invention is applicable to a range of different types of vehicles  3 . For example, the vehicle  3  can be a motor vehicle, such as an off-road or sports utility vehicle; or a towing engine or tractor for a semi-trailer truck. 
     The control techniques described herein are a development of those described in the Applicant&#39;s earlier UK patent applications GB1307524.7 (filed on 26 Apr. 2013) and GB1307525.4 (filed on 26 Apr. 2013), the contents of which are incorporated herein in their entirety by reference. Furthermore, the contents of the Applicant&#39;s related application entitled “VEHICLE CONTROL SYSTEM” filed on the same day as the present application at the UK Intellectual Property Office are hereby incorporated in their entirety into the present application by reference. 
     It will be appreciated that various changes and modifications can be made to the vehicle control system  1  described herein without departing from the present invention. Rather than specify the target trailer destination D TAR  by positioning the graphical image  41  on a video image, the graphical image  41  could be positioned on a static image such as a grid or mesh representing a predefined region around the vehicle  3 . 
     Furthermore, although the vehicle control system  1  has been described with reference to a set of vehicle cameras  9 ,  10 ,  11  and a set of trailer cameras  29 ,  30 ,  31 , it will be appreciated that a single camera could be provided on the vehicle  3  and/or the trailer  5 . In a further alternative, the trailer cameras  29 ,  30 ,  31  could be omitted. A set of parking sensors provided on the trailer  5  could, for example, provide notification to the driver of any obstructions. The hitch angle φ could be measured directly, for example by a rotary encoder. 
     In a further alternative, a video image could be transmitted to the vehicle  3  from a remote camera (for example provided in a cellular telephone) over a wireless network such as Bluetooth®.