Trailer curvature control with adaptive trailer length estimation

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.

Claims

1. An apparatus for determining jackknife conditions of a vehicle comprising: a hitch sensor configured to identify a hitch angle; a processor operable to: set an operating range of the hitch angle to a minimum range; monitor the hitch angle while the vehicle is operated in a forward direction; increase the operating range to an increased range according to the hitch angle detected during the monitoring; set a maximum hitch angle to the increased range; and output a control signal indicating a jackknife condition in response to the hitch angle exceeding the maximum hitch angle during a guided reverse operation of the vehicle.

2. The apparatus according to claim 1, wherein the operating range corresponds to the hitch angle of a trailer relative to the vehicle such that the hitch angle does not reach the jackknife condition.

3. The apparatus according to claim 1, wherein the operating range is increased to a maximum hitch angle observed during the monitoring.

4. The apparatus according to claim 1, wherein the vehicle comprises a trailer backup assist apparatus operable to maneuver the vehicle and the trailer in reverse and control the hitch angle.

5. The apparatus according to claim 1, wherein the minimum range corresponds to a minimum expected trailer length.

6. The apparatus according to claim 1, wherein a trailer length is calculated based on a maximum hitch angle observed and a maximum steering angle for the vehicle.

7. The apparatus according to claim 6, wherein the maximum hitch angle is utilized to calculate the trailer length.

8. The apparatus according to claim 1, wherein the processor is in communication with a steering angle detection apparatus operable to measure a steering angle of the vehicle.

9. The apparatus according to claim 1, wherein the processor is in communication with a hitch angle detection apparatus operable to measure the hitch angle of the trailer relative to the vehicle.

10. The apparatus according to claim 1, wherein the minimum range corresponds to minimum input trailer length for determining the jackknife condition.

11. A method for controlling an operating range of a hitch angle comprising: receiving an input identifying a selected trailer length; comparing the selected trailer length to a minimum trailer length, wherein the minimum trailer length corresponds to a predetermined value; setting the operating range to a minimum range for a trailer in response to the selected trailer length being less than the minimum trailer length; detecting the hitch angle with a hitch sensor; and limiting the hitch angle to the minimum range thereby controlling a reverse operation of the vehicle.

12. The method according to claim 11, further comprising monitoring the hitch angle while the vehicle is operated in a forward direction.

13. The method according to claim 12, further comprising increasing the operating range based on the hitch angle observed during the monitoring.

14. The method according to claim 13, wherein increasing the operating range comprises identifying a maximum hitch angle achieved while the vehicle is operated in the forward direction.

15. The method according to claim 14, wherein the operating range of the hitch angle is set to the maximum hitch angle achieved in the forward direction.

16. The method according to claim 15, further comprising utilizing the operating range of the hitch angle to prevent a jackknife condition during the guided reverse operation of the vehicle.

17. An apparatus for determining jackknife conditions of a vehicle and trailer comprising: a processor in communication with: a steering angle detection apparatus operable to measure a steering angle of the vehicle; and a hitch angle detection apparatus operable to measure a hitch angle of the trailer relative to the vehicle, wherein, the processor is operable to: set an operating range of the hitch angle to a minimum range for a trailer; monitor the hitch angle; increase the operating range to an increased range based on the hitch angle observed during the monitoring; set a maximum hitch angle to the increased range, and output a control signal indicating the hitch angle exceeding the increased range to prevent a jackknife condition during a guided reverse operation of the vehicle.

18. The apparatus according to claim 17, wherein the operating range of the hitch angle is set to the maximum hitch angle achieved in a forward direction.

19. The apparatus according to claim 18, wherein the processor is further operable to: calculate a trailer length based on the maximum hitch angle and a maximum steering angle for the vehicle.

20. The apparatus according to claim 17, wherein the processor is in communication with a trailer backup assist apparatus operable to maneuver the vehicle and the trailer in reverse and control the hitch angle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings:

(2) FIG. 1 is a perspective view of a vehicle and a trailer comprising a trailer backup assist system;

(3) FIG. 2 is a top detail view of a vehicle and a trailer demonstrating a kinematic model of a vehicle and a trailer;

(4) FIG. 3 is a diagram demonstrating a radius of curvature of a vehicle;

(5) FIG. 4 is a block diagram of a trailer backup assist system;

(6) FIG. 5 is a diagram of a center console of a vehicle comprising a steering input apparatus; and

(7) 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

(8) 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.

(9) 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.

(10) 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.

(11) 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.

(12) 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 . The hitch angle 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 , it may be challenging for the operator of the vehicle to determine if the hitch angle is approaching a jackknife condition and a corresponding maximum hitch angle .sub.max.

(13) 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 .sub.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.

(14) 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 . 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.

(15) 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 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.

(16) 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 .sub.max relative to the vehicle 2. The kinematic model 30 parameters include:

(17) : steering angle at front wheels 34 of the vehicle 2;

(18) : hitch angle between the vehicle 2 and the trailer 4;

(19) : maximum hitch angle of a particular vehicle 2 and trailer 4;

(20) : remaining hitch angle;

(21) W: wheel base of the vehicle 2;

(22) L: length between a hitch point 6 and a rear axle center-line 36 of the vehicle 2;

(23) 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

(24) R: radius of curvature of the vehicle 2.

(25) The kinematic model 30 of FIG. 2 relates the dimensions of the vehicle 2 and the trailer 4 to the steering angle and the hitch angle . The steering angle and the hitch angle 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 .sub.max and a trailer length D may be determined for a particular vehicle 2 and trailer 4 combination. The maximum hitch angle .sub.max and trailer length D may be determined based on a relationship of the steering angle and the hitch angle in relation to the radius of curvature R of the vehicle 2. A simplified diagram 40 demonstrating the relationship of the steering angle and the hitch angle in relation to the radius of curvature R of the vehicle 2 is shown in FIG. 3.

(26) Based on the relationships shown in FIG. 3, the minimum radius of curvature R.sub.min for the vehicle 2 is dependent on a maximum steering angle .sub.max and the wheel base W of the vehicle 2. The maximum hitch angle .sub.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.sub.min. As such, the trailer length D and the .sub.max may be determined based on the trigonometric relationship shown demonstrated in Eq. 1.

(27) $\begin{array}{cc}\tan {}_{\max }=\frac{W\sin {}_{\max }}{D+L\cos {}_{\max }}& \left(\mathrm{Eq}.1\right)\end{array}$

(28) The wheel base W, the maximum steering angle .sub.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 .sub.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.

(29) $\begin{array}{cc}\cos {}_{\max }=\frac{-b\sqrt{b^2-4\mathrm{ac}}}{2a},\mathrm{wherein}\text{:}a=L^2{\tan }^2{}_{\max }+W^2;b=2\mathrm{LD}{\tan }^2{}_{\max };\mathrm{and}c=D^2{\tan }^2{}_{\max }-W^2.& \left(\mathrm{Eq}.2\right)\end{array}$

(30) Based on Eq. 2, it is shown that the maximum hitch angle .sub.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 .sub.max.

(31) 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 .sub.max is underestimated for the vehicle 2 and trailer 4. As such, the maximum hitch angle .sub.max calculated based on the minimum trailer length ensures that the controller of the trailer backup assist system 12 will control the hitch angle 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.

(32) While underestimating the trailer length D and the corresponding maximum hitch angle .sub.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 .sub.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 of the trailer 4.

(33) $\begin{array}{cc}{D}_{\mathrm{calc}}=\left(\frac{W\sin {}_{\max }}{\tan {}_{\max }}\right)-L\cos {}_{\max }& \left(\mathrm{Eq}.3\right)\end{array}$

(34) The estimated trailer length as D.sub.calc is calculated by measuring and updating the maximum hitch angle .sub.max of the trailer 4 during forward motion of the vehicle 2. By monitoring and updating the maximum hitch angle .sub.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 .sub.max. By updating and calculating the max hitch angle .sub.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 .sub.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 .sub.max by operating the vehicle 2 in the forward direction through a range of steering angles and corresponding hitch angles.

(35) 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.

(36) 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 . 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.

(37) 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 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 of the trailer 4 relative to the vehicle in response to a hitch angle 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.

(38) 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 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 . 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 during a reverse operation.

(39) 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.

(40) 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.

(41) 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 .sub.max that may be achieved to maneuver the trailer 4.

(42) 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.

(43) 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.

(44) 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.

(45) 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 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 .sub.max with the trailer heading 10 (e.g. a jack knife condition). As the hitch angle decreases, the audible tone may change from continuous or high frequency tones to less frequent tones until the hitch angle is approximately zero and the tone stops.

(46) In some implementations, a steering warning may be displayed on the screen 18 alerting the operator of the vehicle 2 that the hitch angle is approaching the maximum hitch angle .sub.max. Additionally, a steering error may be displayed on the screen 18 alerting the operator that the hitch angle has exceeded the maximum hitch angle .sub.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 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.

(47) 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.sub.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.

(48) In decision step 100, the received trailer length D may be compared to an error threshold or the minimum trailer length D.sub.min. If the received trailer length D is less than the minimum trailer length D.sub.min, the method 90 may set the trailer length D to the minimum trailer length D.sub.min by proceeding to step 98. If the received trailer length D is not less than the minimum trailer length D.sub.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 .sub.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.

(49) The minimum trailer length D.sub.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.sub.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.sub.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.sub.min may be approximately 1 m. Accordingly, the system is configured to underestimate the maximum hitch angle .sub.max to ensure safe operation.

(50) 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 when the vehicle 2 is traveling in the forward direction. The maximum observed value of the hitch angle 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 .sub.max. The maximum hitch angle .sub.max may be changed in response to identifying an increased range or increased maximum hitch angle .sub.max. Based on the updated maximum hitch angle .sub.max from step 104, the system may further determine a calculated trailer length D.sub.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 .sub.max may be improved and increased in response to observed hitch angles identified while the vehicle 2 is operating in the forward direction.

(51) As an additional safety precaution, the system 12 may continue to decision step 108 to determine if the calculated trailer length D.sub.calc is less than the error threshold or the minimum trailer length D.sub.min. If the calculated trailer length D.sub.calc is not less than the minimum trailer length D.sub.min, the method 90 may continue to step 110 to set the trailer length D to the calculated trailer length D.sub.calc. If the calculated trailer length D.sub.calc is less than the minimum trailer length D.sub.min, the method 90 may continue to decision step 112 to determine if the value of D.sub.calc converges toward a value less than the minimum trailer length D.sub.min. If the calculated trailer length D.sub.calc converges toward a value less than the minimum trailer length D.sub.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.sub.min in step 114.

(52) If in decision step 112, the control module 54 does not identify that the calculated trailer length D.sub.calc is converging toward a value less than the minimum trailer length D.sub.min, the control module 54 may continue to step 104 to update and observe hitch angle 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 .sub.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.

(53) 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.