Abstract:
An apparatus for determining a jackknife condition of a vehicle and trailer is disclosed. The apparatus comprises a processor operable to set an operating range of a hitch angle to a minimum range. The processor monitors the hitch angle while the vehicle is operated in a forward direction and increases the operating range based on the hitch angle observed during the monitoring. The processor is further operable to utilize the increased operating range to prevent a jackknife condition during a guided reverse operation of the vehicle.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is a continuation-in-part of U.S. Pat. No. 9,335,162, which was filed on Jun. 3, 2014, entitled “TRAILER LENGTH ESTIMATION IN HITCH ANGLE APPLICATIONS.” The aforementioned related application is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The disclosure relates generally to a method for modeling the operation of a vehicle, and more particularly, to a method for estimating a dimension of a trailer to control the operation of the vehicle. 
     BACKGROUND OF THE INVENTION 
     Reversing a vehicle while towing a trailer is very challenging for many drivers. This is particularly true for drivers that are unskilled at backing vehicles with attached trailers, which may include those that drive with a trailer on an infrequent basis (e.g., have rented a trailer, use a personal trailer on an infrequent basis, etc.). One reason for such difficulty is that backing a vehicle with an attached trailer requires steering inputs that are opposite to normal steering when backing the vehicle without a trailer attached. Another reason for such difficulty is that small errors in steering while backing a vehicle with an attached trailer are amplified thereby causing the trailer to depart from a desired path. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, an apparatus for determining a jackknife condition of a vehicle and trailer is disclosed. The apparatus comprises a processor operable to set an operating range of a hitch angle to a minimum range. The processor monitors the hitch angle while the vehicle is operated in a forward direction and increases the operating range based on the hitch angle observed during the monitoring. The processor is further operable to utilize the increased operating range to prevent a jackknife condition during a guided reverse operation of the vehicle. 
     According to another aspect of the present invention, a method for controlling an operating range of a hitch angle of a vehicle and trailer is disclosed. The method comprises receiving an input to set a trailer length and comparing the input to a minimum trailer length. The method continues by setting the operating range to a minimum range for a trailer in response to the input being less than the minimum trailer length. The hitch angle of the vehicle relative to the trailer is then limited to a minimum range during a guided reverse operation of the vehicle. 
     According to a further aspect of the present invention, an apparatus for determining jackknife conditions of a vehicle and trailer is disclosed. The apparatus comprises a steering angle detection apparatus operable to measure a steering angle of the vehicle and a hitch angle detection apparatus operable to measure the hitch angle of the trailer relative to the vehicle. Each of the steering angle detection apparatus and the hitch angle detection apparatus are in communication with a processor. The processor is operable to set an operating range of the hitch angle to a minimum range for a trailer and monitor the hitch angle. Based on the hitch angle observed during the monitoring, the operating range of the hitch angle is increased. The processor is further operable to utilize the increased operating range as a maximum hitch angle to prevent a jackknife condition during a guided reverse operation of the vehicle. 
     These and other aspects, objects, and features of the present invention will be understood and appreciated by those skilled in the art upon studying the following specification, claims, and appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a perspective view of a vehicle and a trailer comprising a trailer backup assist system; 
         FIG. 2  is a top detail view of a vehicle and a trailer demonstrating a kinematic model of a vehicle and a trailer; 
         FIG. 3  is a diagram demonstrating a radius of curvature of a vehicle; 
         FIG. 4  is a block diagram of a trailer backup assist system; 
         FIG. 5  is a diagram of a center console of a vehicle comprising a steering input apparatus; and 
         FIG. 6  is a flow chart of a method for estimating a trailer length based on a hitch angle of a trailer relative to a vehicle in accordance with the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While various aspects of the inventive subject matter are described with reference to a particular illustrative embodiment, the inventive subject matter is not limited to such embodiments, and additional modifications, applications, and embodiments may be implemented without departing from the inventive subject matter. In the figures, like reference numbers will be used to illustrate the same components. Those skilled in the art will recognize that the various components set forth herein may be altered without varying from the scope of the inventive subject matter. 
     The disclosed subject matter is directed to providing trailer backup assist functionality in a manner that is relatively low cost and that offers an intuitive user interface. In particular, such trailer backup assist functionality provides for controlling curvature of a path of travel of a trailer attached to a vehicle (i.e., trailer path curvature control by allowing a driver of the vehicle to specify a desired path of the trailer by inputting a desired trailer path curvature as the backup maneuver of the vehicle and trailer progresses). The various systems and methods disclosed herein may provide audible and/or visual information to the operator of a trailer backup assist system. 
     Particularly, the methods described herein are directed to a method of utilizing a trailer backup assist system or various systems that may be operable to measure a hitch angle of a trailer relative to a vehicle to determine a maximum controllable hitch angle. The maximum controllable hitch angle may correspond to a maximum angle of a trailer relative to a vehicle undertaking a reverse or backup maneuver based on various dimensional and functional characteristics of the vehicle and the trailer. The maximum controllable hitch angle may be determined by the method while the vehicle and the trailer are operating in a forward direction by monitoring the hitch angle. As such, under steady state conditions, the measurement of the hitch angle of the trailer relative to the vehicle may be utilized to determine the maximum controllable hitch angle. The measurement of the hitch angle of the trailer relative to the vehicle may also be utilized to estimate a length of a trailer. 
     In various embodiments, the method may provide for a trailer backup assist system to learn or correct a trailer length input by an operator of a vehicle or stored in a memory of a trailer backup assist system. In this way, the systems and methods disclosed provide for a method of setup for a trailer backup assist system that is operable to both learn a trailer length of a trailer utilized by the system, but also is operable to correct a trailer length inputted or stored in a trailer backup assist system. As such, the disclosure provides for improved safety and accuracy in setting up and operating a trailer backup assist system by safely and accurately determining a trailer length and a corresponding maximum hitch angle of a trailer relative to a vehicle. 
     Referring to  FIG. 1 , a schematic diagram illustrating a vehicle  2  coupled to a trailer  4  is shown in accordance with the disclosure. The vehicle  2  and the trailer  4  are coupled about a hitch point  6  and are shown in a turning configuration angled at a hitch angle γ. The hitch angle γ is defined by the difference between a vehicle heading  8  and a trailer heading  10  about the hitch point  6 . When the trailer  4  is angled relative to the vehicle  2  at the hitch angle γ, it may be challenging for the operator of the vehicle to determine if the hitch angle γ is approaching a jackknife condition and a corresponding maximum hitch angle γ max . 
     The vehicle  2  may be equipped with a trailer backup assist system  12  configured to control the vehicle  2  during a reversing or backup operation of the trailer  4 . Based on the particular dimensional and functional characteristics of each combination of vehicle and trailer, the trailer backup assist system  12  is operable to maneuver the trailer according to specific dimensional limitations, such as the maximum hitch angle γ max . As such, for the trailer backup assist system  12  to account for the specific dimensional and functional characteristics of the vehicle and the trailer, certain dimensions must be input and/or identified by alternative measure techniques. The disclosure provides for various methods and techniques that may be utilized to safely determine such dimensions and ensure efficient and safe operation of the trailer backup assist system  12 . 
     The backup assist system  12  is controlled by the operator of the vehicle  2  via an interface configured to receive a directional input, for example a steering input apparatus  14  disposed in a passenger compartment  16  of the vehicle  2 . The steering input apparatus  14  may be configured to control a reversing operation of the vehicle  2  and the trailer  4  by receiving a rotational input corresponding to the hitch angle γ. As referred to herein, the trailer heading  10  may refer to a trailer heading that will result from a vehicle operator maintaining a current control input into the steering input apparatus  14 . The trailer heading  10 , the vehicle heading  8 , and additional heading information discussed herein may be updated by the trailer backup assist system  12  in response to a detected change in the steering input apparatus  14 . 
     The vehicle  2  is further equipped with a display or screen  18  disposed in the passenger compartment  16 . The screen  18  is operably coupled to a display controller  20 . In response to the trailer hitch angle γ and other kinematic properties of the vehicle  2  and the trailer  4 , the display controller  20  may be operable to generate and display a graphical representation of the vehicle heading  8 , the trailer heading  10 , and in some implementations, may be operable to display a predicted heading on the screen  18 . The graphical representation provides a reference for the vehicle operator to utilize to ensure safe operation of the steering input apparatus to maneuver the vehicle  2  and the trailer  4 . 
     Referring to  FIGS. 2 and 3 , a kinematic model  30  of the vehicle  2  coupled to the trailer  4  is shown. The kinematic model  30  is based on various parameters associated with the vehicle  2  and the trailer  4 . From the kinematic model  30 , a maximum trailer heading  32  is shown at a maximum hitch angle γ max  relative to the vehicle  2 . The kinematic model  30  parameters include: 
     δ: steering angle at front wheels  34  of the vehicle  2 ; 
     γ: hitch angle between the vehicle  2  and the trailer  4 ; 
     γ: maximum hitch angle of a particular vehicle  2  and trailer  4 ; 
     β: remaining hitch angle; 
     W: wheel base of the vehicle  2 ; 
     L: length between a hitch point  6  and a rear axle center-line  36  of the vehicle  2 ; 
     D: length between hitch point  6  and a trailer axle center-line  38 , wherein the position of the rear axle center-line  36  may be an effective, or equivalent, axle length for a trailer having a multiple axle configuration; and 
     R: radius of curvature of the vehicle  2 . 
     The kinematic model  30  of  FIG. 2  relates the dimensions of the vehicle  2  and the trailer  4  to the steering angle δ and the hitch angle γ. The steering angle δ and the hitch angle γ may be measured by a plurality of sensors of the trailer backup assist system  12  as discussed further in reference to  FIG. 4 . From the kinematic model  30 , a maximum hitch angle γ max  and a trailer length D may be determined for a particular vehicle  2  and trailer  4  combination. The maximum hitch angle γ max  and trailer length D may be determined based on a relationship of the steering angle δ and the hitch angle γ in relation to the radius of curvature R of the vehicle  2 . A simplified diagram  40  demonstrating the relationship of the steering angle δ and the hitch angle γ in relation to the radius of curvature R of the vehicle  2  is shown in  FIG. 3 . 
     Based on the relationships shown in  FIG. 3 , the minimum radius of curvature R min  for the vehicle  2  is dependent on a maximum steering angle δ max  and the wheel base W of the vehicle  2 . The maximum hitch angle γ max  for the vehicle  2  and the trailer  4  corresponds to the vehicle  2  and the trailer  4  turning at the minimum radius of curvature R min . As such, the trailer length D and the δ max  may be determined based on the trigonometric relationship shown demonstrated in Eq. 1. 
     
       
         
           
             
               
                 
                   
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     The wheel base W, the maximum steering angle γ max , and length L correspond to static dimensions that may not change when changing from a first trailer to a different, second trailer. The static dimensions of the vehicle  2  may correspond to dimensions that are not generally subject to change based on many common hitching configurations. As such, a control module of the trailer backup assist system  12  may be configured to calculate the maximum hitch angle γ max  using Eq. 2. It is noted that the methods and equations discussed may be utilized similarly for other common hitching configurations, such as fifth wheel hitching configurations. 
     
       
         
           
             
               
                 
                   
                     
                       
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     Based on Eq. 2, it is shown that the maximum hitch angle γ max  may be determined based on the trailer length D and the static dimensions of the vehicle  2 . In this way, the trailer length D may be input by an operator of the trailer backup assist system  12  in order to calculate the maximum hitch angle γ max . 
     In operation, the trailer backup assist system  12  may be configured to underestimate the trailer length in order to ensure that safe operation of a trailer backup assist function may be accomplished even if the trailer length is unknown. For example, if the trailer length is unknown, the system  12  may be configured to assign a minimum trailer length as the trailer length D. By utilizing the minimum trailer length as the trailer length in Eq. 2, the maximum hitch angle γ max  is underestimated for the vehicle  2  and trailer  4 . As such, the maximum hitch angle γ max  calculated based on the minimum trailer length ensures that the controller of the trailer backup assist system  12  will control the hitch angle γ within an underestimated range. Underestimating the safe operating range of the hitch angle  8  may ensure that the trailer  4  is not accidentally placed in a jackknife condition during a reversing operation. 
     While underestimating the trailer length D and the corresponding maximum hitch angle γ max  may ensure safe operation of the trailer backup assist system  12 , it may also limit the utility of the system  12  by limiting the maximum hitch angle γ max . To ensure that safe operation and maximum performance are achieved, the system  12  provides for improving the trailer length D programmed into the system  12  by estimating the trailer length by utilizing Eq. 3. Eq. 3 may be used to update and improve the trailer length D during forward operation of the vehicle  2  while monitoring the hitch angle γ of the trailer  4 . 
     
       
         
           
             
               
                 
                   
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     The estimated trailer length as D calc  is calculated by measuring and updating the maximum hitch angle γ max  of the trailer  4  during forward motion of the vehicle  2 . By monitoring and updating the maximum hitch angle γ max  the trailer length D corresponding to the actual dimensions of the trailer may be improved. According to Eq. 2, an increase in the trailer length D results in an increase in the maximum hitch angle γ max . By updating and calculating the max hitch angle γ max  and the trailer length D, the system  12  is operable to improve the performance of a reverse or backup operation of the vehicle  2  and the trailer  4 . The performance is improved by accurately estimating the trailer length D and consequently increasing the maximum hitch angle γ max . In operation, this means that system  12  is operable to automatically configure the kinematic model  30  including the trailer length D and the maximum hitch angle γ max  by operating the vehicle  2  in the forward direction through a range of steering angles and corresponding hitch angles. 
     Referring to  FIG. 4 , a block diagram of the trailer backup assist system  12  of the vehicle  2  is shown. The trailer backup assist system  12  is operable to control the curvature of path of the trailer  4  by adjusting the vehicle  2  in response to the steering input apparatus  14 . The backup assist system  12  operates by controlling the steering of the vehicle  2  via a power steering assist system  52 . The steering input apparatus  14  may comprise a touchscreen, knob or other various forms of input devices, and in some implementations may be in communication with a human machine interface (HMI) coupled to the screen  18 . 
     The trailer backup assist system  12  includes a trailer backup assist control module  54 , the trailer backup steering input apparatus  14 , and a hitch angle detection apparatus  58  operable to monitor the hitch angle γ. The trailer backup assist control module  54  is in communication with the trailer backup steering input apparatus  14  and the hitch angle detection apparatus  58 . The control module  54  of the trailer backup assist system  12  is further in communication with a power steering assist control module  60  and may be indirectly in communication with a steering angle detection apparatus  62  of the power steering assist system  52 . The trailer backup assist system  12  may also in communication with a brake system control module  64  and a powertrain control module  66  for controlling motion of the vehicle  2  and the trailer  4 . 
     The trailer backup assist control module  54  (e.g., a trailer curvature algorithm thereof) is operable to generate vehicle steering information as a function of information received from the trailer backup steering input apparatus  14 , the hitch angle detection apparatus  58 , the power steering assist control module  60 , the brake system control module  64 , and the powertrain control module  66 . In operation, the trailer backup assist control module  54  is operable to maneuver the vehicle  2  to achieve a commanded curvature of a path for the trailer  4 . The path of travel and the hitch angle γ are adjusted in response to an operator input into the steering input apparatus  14 . The control module is further operable to adjust the hitch angle γ of the trailer  4  relative to the vehicle in response to a hitch angle γ received from the hitch angle detection apparatus  58 . Further detailed implementations of a trailer backup assist module are described in further detail in U.S. patent application Ser. No. 14/294,489, which is incorporated herein by reference in its entirety. 
     The hitch angle detection apparatus  58  may operate in conjunction with a hitch angle detection component  68  which may be coupled to the vehicle  2  or the trailer  4 . The hitch angle detection apparatus  58  may be utilized in combination with the hitch angle detection component  68  to communicate information relating to the hitch angle γ to the trailer backup assist control module  54 . The hitch angle detection apparatus  58  may be implemented by proximity or distance sensors (e.g. an ultrasonic sensor), a camera-based sensor configured to visually monitor a target, or any angular measurement device. The hitch angle detection apparatus  58  may also be implemented as a device mounted proximate the hitch point  6  to measure the hitch angle γ. The trailer backup assist system  12  as discussed herein provides an intuitive system for maneuvering the vehicle  2  and the trailer  4  by monitoring and controlling the hitch angle γ during a reverse operation. 
     Referring now to  FIG. 5 , the steering input apparatus  14  is shown as a component of an interface  74  configured to receive a directional input to control the trailer backup assist system  12 . The steering input apparatus  14  may be disposed in a center console portion  76  of the passenger compartment  16  of the vehicle  2  as an input device in communication with an HMI  78 . The HMI  78  may further be in communication with the display controller  20  and the screen  18  to provide the operator of the vehicle  2  with reference information generated by the display controller  20 . The reference information may include a graphical representation  80  of the vehicle  2  and the trailer  4  including the maximum trailer heading  32  to assist the operator of the vehicle in utilizing the steering input apparatus  14 . 
     In some implementations, the steering input apparatus  14  may comprise a rotatable control element in the form of a knob  82 . The knob  82  is further coupled to a movement sensing device  84 . The knob  82  may be biased (e.g., by a spring return) to an at-rest position P(AR) between opposing rotational ranges of motion R(R), R(L). A force that biases the knob  82  toward the at-rest position P(AR) can increase (e.g., non-linearly) as a function of the amount of rotation of the knob  82  with respect to the at-rest position P(AR). Even in a spring biased configuration, an operator may have difficulty determining a relative position of the knob  82  and a corresponding trailer heading  10  in response to an input. The graphical representation  80  provides visual feedback to the operator to improve the intuitive nature of the steering input apparatus  14 . 
     For example, as shown in  FIG. 5 , the knob  82  is rotated in the direction of the right rotational range R(R). In response to the rotation detected by the sensing device  84  of the steering input apparatus  14 , the trailer backup assist control module  54  has positioned the vehicle such that the trailer  4  is angled toward a passenger side of the vehicle  2  as shown in the graphical representation  80 . To assist the driver in operation of the vehicle  2 , the display controller  20  includes the vehicle heading  8 , the trailer heading  10 , and the maximum trailer heading  32 , as calculated from Eq. 2. The maximum trailer heading  32  may notify the operator of the vehicle  2  of a maximum hitch angle γ max  that may be achieved to maneuver the trailer  4 . 
     Though the steering input apparatus  14  is discussed in detail in reference to the knob  82  and a corresponding rotating configuration, the steering input apparatus  14  may be implemented by any form of user input configured to direct the vehicle  2  and the trailer  4 . For example, in some implementations, the screen  18  may be configured as a touchscreen. The touchscreen may be of any type suited to a particular application and may be resistive, capacitive, surface acoustic wave, infrared, or optical. The touchscreen may utilize a plurality of soft keys in communication with the display controller  20  and the trailer backup assist system  12  to select a location or path for the vehicle  2  and the trailer  4 . The touchscreen may further provide options for the operator to select the vehicle  2  or the trailer  4  and control a direction of each via a plurality of directional inputs  86 . 
     In some implementations, the HMI  78  may provide feedback to an operator of the vehicle  2  while the operator is waiting for the vehicle  2  to complete a command received by the trailer backup assist control module  54 . For example, the HMI  78  may provide feedback to the operator during control tasks and maneuvers of the vehicle  2  and the trailer  4  that may require an extended period to execute. In this way, the HMI  78  may provide a reassurance to the driver that the trailer backup assist control module  54  is functioning properly. The feedback may also serve to limit an operator from prematurely adjusting an input to the steering input apparatus  14  prior to the completion of a control task. 
     The HMI  78  and the knob  82  may be configured to provide feedback to the operator of the vehicle  2  in a variety of ways. For example, a notification may be displayed on the screen  18  showing a remaining change in the trailer heading  10  based on an input received by the steering input apparatus. In some implementations, the remaining change in the trailer heading  10  may be displayed numerically on the screen  18  as an angle. The remaining change may also be displayed by updating the graphical representation  80  and/or the direction of the arrows denoting the trailer heading  10 . The graphical representation  80  may further be configured to flash on and off during the completion of a control task. One or more icons or symbols may also be overlaid on the screen notifying the operator that the trailer backup assist system  12  is active. 
     The operator of the vehicle  2  may further be provided feedback for a turning operation of the trailer backup assist system  12  by audible or tactile feedback that may be provided by the HMI  78  and/or additional systems in the vehicle  2 . In some implementations, a steering wheel of the vehicle may vibrate or oscillate in response to conditions requiring that the steering angle δ be maintained at a maximum steering angle to complete a steering maneuver. Also, periodic audible tones may be provided through one or more speakers in the vehicle  2 . The audible tones may increase in frequency as the vehicle heading  8  approaches the maximum hitch angle γ max  with the trailer heading  10  (e.g. a jack knife condition). As the hitch angle γ decreases, the audible tone may change from continuous or high frequency tones to less frequent tones until the hitch angle γ is approximately zero and the tone stops. 
     In some implementations, a steering warning may be displayed on the screen  18  alerting the operator of the vehicle  2  that the hitch angle γ is approaching the maximum hitch angle γ max . Additionally, a steering error may be displayed on the screen  18  alerting the operator that the hitch angle γ has exceeded the maximum hitch angle γ max . The steering error displayed on the screen  18  may inform the operator that the vehicle  2  must be pulled forward to avoid a jackknife condition. In this way, the system  12  may alert the operator of the vehicle  2  that the steering angle γ as calculated by the method disclosed herein may be exceeded such that the operator may correct a current direction of the trailer  4  to avoid an error condition. 
     Referring now to  FIG. 6 , a method  90  for operating the trailer backup assist system  12  is shown. The method may begin by initializing the trailer backup assist system  12  ( 92 ). The trailer backup assist system  12  may be initialized in response to the connection of a trailer  4  to the hitch of the vehicle  2 . In response to the initialization of the trailer backup assist system  12 , the control module  54  may cause the display controller  20  to display a prompt on the screen  18  requesting that the operator input a trailer length D ( 94 ). In decision block  96 , if the trailer length D is not received, the trailer length D may be set to a minimum trailer length D min  by proceeding to step  98 . In decision block  96 , if the trailer length D is received, the method  90  may complete an additional decision step  100 . 
     In decision step  100 , the received trailer length D may be compared to an error threshold or the minimum trailer length D min . If the received trailer length D is less than the minimum trailer length D min , the method  90  may set the trailer length D to the minimum trailer length D min  by proceeding to step  98 . If the received trailer length D is not less than the minimum trailer length D min , the method  90  may set the trailer length D to the received trailer length by proceeding to step  102 . Steps  92  to  102  may serve as initialization or initial setup steps for the trailer length D. Based on these steps it may be noted that the trailer length may initially set to a low estimate or minimum trailer length to ensure that the maximum hitch angle γ max  is underestimated. In this configuration, the trailer backup assist system  12  can avoid approaching a jackknife condition even if the trailer length D is unknown. 
     The minimum trailer length D min  may correspond to a variety of lengths that may correspond to a particular style and/or type of vehicle  2  utilizing the trailer backup assist system  12 . In some embodiments, a minimum trailer length D min  may correspond to a minimum length of trailer that is supported for backup assistance by the trailer backup assist system  12 . The minimum trailer length D min  may also correspond to an average minimum trailer length based on customer surveys for a particular make and model of the vehicle  2 . In an exemplary embodiment, the minimum trailer length D min  may be approximately 1 m. Accordingly, the system is configured to underestimate the maximum hitch angle γ max  to ensure safe operation. 
     Following steps  98  or  102 , the method  90  may continue to step  104 . In step  104 , the control module  54  may receive updated hitch angle data from the hitch angle detection apparatus  58  identifying an operating range of the hitch angle γ when the vehicle  2  is traveling in the forward direction. The maximum observed value of the hitch angle γ of the trailer  4  identified when the vehicle  2  is traveling in the forward direction may be set by the control module to update the maximum hitch angle γ max . The maximum hitch angle γ max  may be changed in response to identifying an increased range or increased maximum hitch angle γ max . Based on the updated maximum hitch angle γ max  from step  104 , the system may further determine a calculated trailer length D calc  by utilizing Eq. 3 ( 106 ). In this way, the system is operable to improve an input or calculated trailer length D such that the operating range corresponding to the maximum hitch angle γ max  may be improved and increased in response to observed hitch angles γ identified while the vehicle  2  is operating in the forward direction. 
     As an additional safety precaution, the system  12  may continue to decision step  108  to determine if the calculated trailer length D calc  is less than the error threshold or the minimum trailer length D min . If the calculated trailer length D calc  is not less than the minimum trailer length D min , the method  90  may continue to step  110  to set the trailer length D to the calculated trailer length D calc . If the calculated trailer length D calc  is less than the minimum trailer length D min , the method  90  may continue to decision step  112  to determine if the value of D calc  converges toward a value less than the minimum trailer length D min . If the calculated trailer length D calc  converges toward a value less than the minimum trailer length D min , for a plurality of cycles or calculations over time, the control module  54  may set the trailer length D to a value less than the minimum trailer length D min  in step  114 . 
     If in decision step  112 , the control module  54  does not identify that the calculated trailer length D calc  is converging toward a value less than the minimum trailer length D min , the control module  54  may continue to step  104  to update and observe hitch angle γ while the vehicle is operating in the forward direction. Over time the trailer length may converge toward an increased trailer length. The increased trailer length will allow the trailer backup assist system  12  to increase an operating range for maneuvering by estimating the maximum hitch angle γ max  as discussed herein. In this way the system  12  may provide for an accurate estimation of a trailer length and improve a maneuvering range while avoid jackknife conditions. 
     It is to be understood that variations and modifications can be made on the aforementioned structure without departing from the concepts of the present invention, and further it is to be understood that such concepts are intended to be covered by the following claims unless these claims by their language expressly state otherwise.