Abstract:
A control system for estimating the tongue length of a trailer being towed by a vehicle in connection with a front wheel steering with or without coordinated rear wheel steering associated with the vehicle. The control system employs an algorithm that calculates an estimated trailer yaw rate based on a corrected tongue length, a front wheel steering angle, a rear wheel steering angle, vehicle speed and a vehicle yaw rate. The estimated trailer yaw rate is compared to a measured trailer yaw rate to generate a yaw rate error that is converted to a tongue length error. The tongue length error is compared to the estimated tongue length to become a corrected estimated tongue length for a next computation period. After a few seconds of processing, the corrected estimated tongue length will be the actual tongue length of the trailer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a control system for estimating the tongue length of a trailer being towed by a vehicle and, more particularly, to a control system for estimating the tongue length of a trailer being towed by a vehicle where the trailer includes a yaw rate sensor, and where the vehicle includes driver operated front-wheel steering with or without computer controlled rear-wheel steering. 
     2. Discussion of the Related Art 
     Automotive vehicles that employ coordinated front-wheel steering and rear-wheel steering systems are known in the art. Typically in such coordinated vehicle wheel steering systems, the driver controls the steering of the vehicles front wheels and a computer-based on-board steering controller controls the steering of the vehicles rear wheels in response thereto. In one example, the computer controlled rear-wheel steering system employs an electric motor-driven rack and pinion rear-wheel steering actuator. 
     Backing up a vehicle-trailer is typically a complex task, and requires a significant level of skill. It is heretofore been known in the art to employ a coordinated front and rear-wheel steering system to assist a driver operating a vehicle pulling a trailer. Particularly, U.S. Pat. No. 6,292,094, issued Sep. 18, 2001 to Deng et al., assigned to the assignee of this application and herein incorporated by reference, discloses a vehicle/trailer backing up control system in connection with a computer controlled rear-wheel steering system. The &#39;094 patent employs an algorithm that uses front-wheel angle, vehicle speed, vehicle yaw rate and hitch angle to control the rear-wheel steering angle to assist the operator in backing up the vehicle-trailer. 
     The vehicle-trailer back-up control system disclosed in the &#39;094 patent has been shown to be effective in assisting the vehicle operator when backing up a trailer. However, because trailers come in a variety of different lengths, the performance of the back-up control system can be improved by providing an input to the system that gives an estimation of the tongue length of the trailer. The algorithm in the &#39;094 patent uses the same average tongue length for trailers of all lengths. 
     U.S. patent application Ser. No. 10/336,120, filed Jan. 3, 2003, titled “Trailer Tongue Length Estimation Using a Hitch Angle Sensor,” assigned to the assignee of this application, and herein incorporated by reference, discloses a system that estimates the tongue length of a trailer being towed by using a hitch angle sensor that provides a measurement of the hitch angle between the vehicle and the trailer to determine the tongue length. That system has also been shown to be effective in estimating the tongue length of the trailer to improve the ability of the control system to assist the driver in backing-up the trailer. However, further improvements can be made to make the system more practical and more cost effective. For example, because trailer hitches come in a variety of styles, sizes, etc., providing a hitch angle sensor that accurately measures the hitch angle for all of the various types of hitches may be impractical and costly. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a control system is disclosed for estimating the tongue length of a trailer being towed by a vehicle in connection with a coordinated front and rear-wheel steering system. The control system employs an algorithm that calculates an estimate of the yaw rate of the trailer based on a corrected trailer tongue length, a steering wheel angle, a rear-wheel angle, vehicle speed and vehicle yaw rate applied to a vehicle-trailer kinematics model. The estimated trailer yaw rate is compared to the actual trailer yaw rate measured by a trailer yaw rate sensor to generate a yaw rate error signal. The yaw rate error signal is converted to a tongue length error signal by a PID controller. The tongue length error signal is subtracted from an estimated tongue length to give the corrected trailer tongue length for the next computation period. After a few seconds of processing, the yaw rate error signal will be nearly zero and the tongue length error signal will be nearly zero, and thus, the corrected tongue length will be the actual tongue length of the trailer. 
     Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a vehicle towing a trailer, where the vehicle includes a coordinated front and rear-wheel steering system that provides an estimation of the tongue length of the trailer, according to an embodiment of the present invention; 
         FIG. 2  is a kinematics model of a vehicle-trailer system for the algorithm of the invention; and 
         FIG. 3  is a block diagram of a control system employing an algorithm for estimating the tongue length of the trailer shown in  FIG. 1  by the kinematics model shown in  FIG. 2 , according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following discussion of the embodiments of the invention directed to a control system for estimating the tongue length of a trailer being towed by a vehicle employing a coordinated front and rear-wheel steering system is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
       FIG. 1  is a plan view of a vehicle-trailer system  10  that estimates a tongue length (TL) of a trailer  12  being towed by a vehicle  14 . The system  10  is a variation of the backing up control system disclosed in the &#39;094 patent that uses the coordinated front and rear wheel steering system to provide an intelligent vehicle-trailer backing-up system. The system  10  includes a similar controller  18  as the control system in the &#39;094 patent. Further, the system  10  includes various other vehicle sensors used in the control system disclosed in the &#39;094 patent, as will be apparent from the discussion below. The various vehicle sensors discussed below can be any sensor suitable for the purposes discussed herein, and need not be specifically limited to those types of sensors disclosed in the &#39;094 patent. In other embodiments, the tongue length estimation process of the invention can be used in vehicles that do not have assisted rear-wheel steering. 
     The trailer  12  includes a trailer hitch post  20 , a trailer bed  22  and trailer wheels  30  rotatably mounted to a trailer axle  32 . In other embodiments, the trailer  12  may include more than one axle each including trailer wheels. The center of the turning radius for those trailers may be between the axles. The vehicle  14  includes a vehicle hitch post  24  having a hitch  26  that couples the hitch post  24  to the hitch post  20  in any known manner that allows the trailer  12  to be towed by the vehicle  14 . As defined herein, the tongue length of the trailer  12  is the distance from the pivot location at the hitch  26  to the rotating center or turning radius of the trailer  12 . When the vehicle  14  and the trailer  12  are backing up at a low speed and there is no side slip at the wheels  30 , the tongue length of the trailer  12  is the distance from the hitch  26  to the center of the trailer axle  32 . 
     The vehicle  14  includes a steering wheel  40  mounted to a steering column  42  that allows a vehicle operator to steer front wheels  44  of the vehicle  14  through a steering linkage and a front wheel axle  46 . A steering wheel angle sensor  48  is mounted to the steering column  42  to provide a front wheel angle signal δ f (t) indicative of the steering direction of the wheels  44 . The maximum angular movement for a particular vehicle&#39;s front wheels is generally fixed, and may be, for example, about +33° to the left or −33° to the right. The signal from the steering wheel angle sensor  48  is provided to the controller  18 . 
     The vehicle  14  also includes rear wheels  52  mounted to a rear wheel axle  54 . The rear wheels  52  are turned by an electric motor  56  in connection with a rack and pinion steering mechanism  58  mounted to the axle  54 . A rear wheel angle sensor  62  is mounted in combination with the rack and pinion steering mechanism  58 , and provides a rear wheel angle signal δ r (t) to the controller  18  indicative of the angle of the rear wheels  52 . 
     The vehicle  14  also includes a vehicle speed sensor  64  that measures the speed of the vehicle  14  and provides a vehicle speed signal V x (t) to the controller  18 . The vehicle  14  further includes a vehicle yaw rate sensor  66  that measures the yaw rate of the vehicle  14  and provides a vehicle yaw rate signal r v (t) to the controller  18 . The speed sensor  64  and the yaw rate sensor  66  can be any sensor suitable for the purposes described herein. Further, the trailer  12  includes a trailer yaw rate sensor  28  that measure the yaw rate of the trailer  12  and provides a trailer yaw rate signal r t (t) to the controller  18 . 
     The controller  18  provides driver signals and commands to a driver advisor  68 , including a suitable display, indicative of the operation of the system  10 . For example, the vehicle  14  needs to be turning for some period of time to provide the necessary signals to calculate the estimated tongue length of the trailer  12 . The driver advisor  68  can be used to instruct the driver to make the necessary turns when the trailer  12  is first connected to the vehicle  14 , and tell the driver that the estimated tongue length of the trailer  12  has been calculated thereafter. In one embodiment, the driver advisor  68  is part of an ultrasound rear parking aid (URPA) alarm system. 
     According to the invention, the vehicle speed signal V x (t), the front wheel angle signal δ f (t), the rear wheel angle signal δ r (t), the trailer yaw rate signal r t (t) and the vehicle yaw rate signal r v (t) are used to calculate an estimated trailer tongue length. The process of determining the tongue length is discussed below with reference to a kinematics model of a vehicle-trailer system  72 , shown in  FIG. 2 , where reference number  74  represents the vehicle  14  and reference number  76  represents the trailer  12 . 
       FIG. 3  is a block diagram of a trailer tongue length estimation system  80  to be used in connection with the system  10 . The tongue length estimation system  80  would be included in the controller  18 . An initial or previous tongue length estimation signal {circumflex over (T)}L(t−Δt) is applied to a comparator, such as a summer  82 . The initial tongue length estimation signal TL (t−Δt) can be based on an average trailer tongue length, for example, 10–12 feet. A tongue length error signal ΔTL(t), described below, is subtracted from the tongue length estimation signal {circumflex over (T)}L(t−Δt) in the summer  82  to provide a corrected tongue length estimation signal {circumflex over (T)}L(t). 
     When the tongue length estimation process is first initiated, the tongue length error signal ΔTL(t) is zero, and thus the initial tongue length estimation signal {circumflex over (T)}L(t−Δt) outputted from the summer  82  is the corrected tongue length estimation signal {circumflex over (T)}L(t). For subsequent calculation periods, the corrected tongue length estimation signal {circumflex over (T)}L(t) will be closer to the actual trailer tongue length than the initial tongue length estimation signal. 
     The corrected tongue length estimation signal {circumflex over (T)}L(t) is applied to a vehicle-trailer kinematics model controller  84  that calculates variables for determining an estimated trailer yaw rate based on the kinematics model shown in  FIG. 2 . At time t, the input signals of the vehicle speed along the x axis V x (t), the front wheel angle δ f (t), the rear wheel angle δ r (t), the vehicle yaw rate r v (t), and the trailer yaw rate r t (t) are provided to the system  80 . The controller  84  receives the steering wheel angle signal δ f (t) from the sensor  48 , the rear-wheel angle signal δ r (t) from the sensor  62 , the vehicle speed signal V x (t) from the sensor  64  and the vehicle yaw rate signal r v (t) from the sensor  66 . 
     The controller  84  calculates the lateral velocity component at the hitch  26  for the vehicle side as: 
                 V   yh     ⁡     (   t   )       =       -     (     H   +   B   +         A   ⁢           ⁢     tan   ⁡     (       δ   r     ⁡     (   t   )       )         -     B   ⁢           ⁢     tan   ⁡     (       δ   f     ⁡     (   t   )       )               -     tan   ⁡     (       δ   r     ⁡     (   t   )       )         +     tan   ⁡     (       δ   f     ⁡     (   t   )       )             )       *       r   v     ⁡     (   t   )                 (   1   )             
 
A is the distance from the center of the front axle  46  to the center of the gravity point of the vehicle  14 , B is the distance from the center of gravity point to the center of the rear axle  54 , and H is the distance from the center of the rear axle  54  to the hitch  26 .
 
     The longitudinal velocity of the trailer  12  at the hitch  26  is:
 
 V   xh ( t )= V   x ( t )  (2)
 
     The magnitude of the hitch velocity is:
 
 V   h ( t )=√{square root over ( V   xh   2 ( t )+ V   yh   2 ( t ))}{square root over ( V   xh   2 ( t )+ V   yh   2 ( t ))}  (3)
 
     Using the corrected tongue length signal, the lateral hitch velocity for the trailer  12  can be written as:
 
 U   yh ( t )= {circumflex over (T)}L ( t )* r   t ( t )  (4)
 
     From the relationship between the vehicle side hitch velocities and the trailer side hitch velocities:
 
 U   yh ( t )=− V   xh ( t )*sin(θ est ( t ))+ V   yh ( t )*cos(θ est ( t ))  (5)
 
     A hitch angle estimation θ est (t) can then be calculated as:
 
θ est ( t )=−sin −1 ( U   yh ( t )/ V   h ( t ))+sin( V   yh ( t )/ V   h ( t ))  (6)
 
     The hitch angle rate can be estimated by differentiating the hitch angle estimation θ est (t) from equation (6) by: 
                   θ   .     est     ⁡     (   t   )       =           θ   est     ⁡     (   t   )       -       θ   est     ⁡     (     t   -     Δ   ⁢           ⁢   t       )           Δ   ⁢           ⁢   t               (   7   )             
 
     The estimated hitch angle rate {dot over (θ)} est (t) is then applied to a controller  86  that estimates the trailer yaw rate. The estimated trailer yaw rate r t     est    is calculated in the controller  86  as:
 
 r   t     est   ( t )= r   v ( t )+{dot over (θ)} est ( t )  (8)
 
     The estimated trailer yaw rate r t     est    is then applied to a subtractor  88  to compare the estimated trailer yaw rate r t     est    to the measured trailer yaw rate r t (t) from the sensor  28 . This difference is a yaw rate error signal Δr t (t) as determined by:
 
Δ r   t ( t )= r   t ( t )− r   t     est   ( t )= r   t ( t )−( r   v ( t )+θ est ( t ))=( r   t ( t )− r   v ( t ))+θ est ( t )  (9)
 
     The yaw rate error signal Δr t (t) is applied to a PID controller  90  to generate a tongue length estimation value as: 
               Δ   ⁢           ⁢     TL   ⁡     (   t   )         =         K   p     *   Δ   ⁢           ⁢       r   t     ⁡     (   t   )         +       K   i     ⁢     ∫     Δ   ⁢           ⁢       r   t     ⁡     (   t   )       ⁢     ⅆ   t           +       K   d     ⁢       ⅆ     (     Δ   ⁢           ⁢       r   t     ⁡     (   t   )         )         ⅆ   t                   (   10   )             
 TL ( t )= TL ( t−Δt )+Δ TL ( t )(11) 
     K p  is a proportional gain constant, K i  is an integral gain constant and K d  is a derivative gain constant. The PID control gains (K p , K i , K d ) are assigned using the following PID gain assignment rule. If |Δr t (t)|&gt;0.1 degree/sec, then the PID gains are:
 
 K   p =0.1 , K   i =0.03 , K   d =0.00005  (12)
 
otherwise,
 
 K   p =0.1 , K   i =0.2 , K   d =0.000001  (13)
 
     The PID controller  90  provides the tongue length error signal ΔTL(t) in this embodiment. However, other controllers may output a different type of signal that needs to be modified to get the tongue length error signal ΔTL(t). In those embodiments, a tongue length modification system  92  can be employed to convert the output of the PID controller  90  to the tongue length error signal ΔTL(t). 
     In order to start the estimation process, some of the input parameters need to be greater than a certain value because the present invention may not work if the vehicle-trailer is moving in a straight path. In other words, the vehicle-trailer needs to move in a circular path to produce a vehicle-trailer yaw rate. For example, the conditions to start the estimation process can be:
 
| V   x ( t )|&gt;0.05 (kph),
 
 |r   v ( t )|&gt;0.005 (deg/sec), and  (14)
 
| r   t ( t )|&gt;0.005 (deg/sec).
 
     Although it has been observed that the best maneuver to produce an accurate tongue length estimation is a step steer forward or backward, other turning or sinusoidal steering maneuvers can be used for the estimation process. There is also a stop condition for the estimation process. When the trailer yaw rate estimation error is within a reasonable range for a certain period, the estimated tongue length is accepted as a true value. The stop conditions are dependent upon the noise characteristics of the input data as:
 
|Δ r   t ( t )|&lt;0.005 (deg/sec) for  t   duration &gt;100 Δt   (15)
 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.