Patent Grant
7154385
1. Field of the Invention
This invention relates generally to a vehicle back-up control system for assisting a vehicle operator in backing up a vehicle-trailer combination and, more particularly, to a vehicle back-up control system that assists a vehicle operator in backing up a vehicle-trailer combination, where the system employs a hitch angle sensor and an active front steering sub-system.
2. Discussion of the Related Art
Backing up a vehicle-trailer combination requires proper actions by steering and/or braking the vehicle to slow down and/or stabilize the vehicle-trailer combination before a jack-knife condition occurs. Particularly, in order to position the trailer toward the target direction, the vehicle operator typically needs to provide counter-steering inputs, which are opposite to normal steering. The jack-knife condition occurs when the vehicle-trailer combination is moving away from its equilibrium position and the system becomes unstable. In other words, the relative angle between the vehicle and the trailer is diverging from the driver's intended target angle, which usually increases when proper steering and/or braking actions are not taken.
U.S. Pat. No. 6,292,094 discloses a system for controlling a backing maneuver of a vehicle-trailer combination, where the vehicle includes operator-actuated front wheel steering and microprocessor-actuated electric motor driven rear-wheel steering that uses the hand-wheel angle for a desired driver command. In this system, the hand-wheel is mechanically coupled to the road wheels through the steering mechanism. When the vehicle operator turns the hand-wheel to provide the command to the controller, the front wheels turn accordingly regardless of the driver's intention. Therefore, the front wheels may turn in-phase with the rear wheels when the driver's intention is to provide counter steering, which would be required to provide out-of-phase steering between the front and rear wheels.
Some state of the art vehicles employ an active front steering (AFS) system, known to those skilled in the art, in combination with a steer-by-wire system where the steering hand-wheel is mechanically decoupled from the vehicle wheels. In other words, the turning of the hand-wheel is electronically detected, where a controller operates the steering gear to turn the front wheels based on the detected signal. The prior art has proposed utilizing a true steer-by-wire mechanism, where the desired front wheel angle is generated based on the driver command provided through the steering hand-wheel. However, this system is limited to a theoretical control feasibility using a steer-by-wire system and instrumented hitch.
Because the known vehicle-trailer back-up control systems only interpret the driver intentions based on the hand-wheel angle, a counter steering command may produce a wrong wheel angle command that is opposite to the driver's intention. The driver should not perform a counter steer if he/she is using this back-up control feature because the counter steer is performed by the AFS. Therefore, perceiving a driver's intention accurately during trailer back-up is important for controlling the backing up motion of a vehicle-trailer combination.
In accordance with the teachings of the present invention, a vehicle-trailer back-up control system is disclosed that employs an active front wheel steer sub-system. The system includes a hand-wheel sensor for measuring the steering hand-wheel angle input from the vehicle operator, and an active front wheel steering actuator for converting the measured hand-wheel angle signal to an applicable steering signal for the vehicle. The system further includes a smart hitch controller that receives a vehicle speed signal and the hand-wheel angle signal, and, using a bicycle model or a kinematics model, calculates a hitch angle command signal. The system also includes a hitch angle sensor that measures the hitch angle between the vehicle and the trailer that is compared to the hitch angle command signal to generate a hitch angle error signal. A PID control unit receives the hitch angle error signal, and generates a corrected road wheel angle signal based on proportional and derivative gains. The corrected road wheel angle signal is applied to a process block for generating a motor angle signal that is applied to the steering actuator to be combined with the steering angle signal to generate a front wheel steering signal during a back-up maneuver.
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.
The following discussion of the embodiments of the invention directed to an AFS based vehicle-trailer back-up control system employing a hitch angle sensor is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.
As will be discussed in detail below, the present invention proposes an AFS based vehicle-trailer back-up control system employing a hitch angle sensor. The vehicle-trailer back-up control system of the invention only requires that the vehicle operator provide the desired command because it interprets and converts the command to the necessary steering control signals, including counter-steering. The steering gear ratio is controlled to give the effective road wheel angle. The system uses the front-wheel angle as a control input and the hand-wheel angle as a command, where the mechanical connection between the hand-wheel and the road wheels is decoupled. Although the steering wheel is still connected mechanically to the road wheels, it behaves as if it is disconnected functionally by a steer-by-wire mechanism. Therefore, the vehicle operator doesn't need to have knowledge or experience for counter steering when backing up a vehicle-trailer.
The trailer 12 includes a trailer hitch post 20, a trailer bed 22 and trailer wheels 30 rotatably mounted to a trailer axle 32. 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. The hitch 26 includes a hitch angle sensor 28 that provides an electrical signal indicative of the angle between the hitch post 24 and the hitch post 20 to the controller 18.
The steering hand-wheel 16 is mounted to a steering column 42 that allows a vehicle operator to steer front wheels 44 of the vehicle 14 through a steering linkage, the steering gear and a front wheel axle 46. A hand-wheel angle sensor 48 is mounted to the steering column 42 and provides a hand-wheel steering angle signal δsw(t) indicative of the operator's intended steering direction of the wheels 44 to the controller 18. The maximum angle movement for a particular vehicle's front wheels is generally fixed, and may be, for example, about +34° to the left and −34° to the right. The vehicle 14 also includes a vehicle speed sensor 40 that measures the speed of the vehicle 14 and provides a vehicle speed signal Vx to the controller 18. Additionally, the system 10 may include an ultrasound rear parking aid (URPA) alarm system in combination with an optional warning device 56.
The system 10 also includes a front wheel angle sensor 34 and an active front steering electric motor 36. As will be discussed in detail below, based on the steering hand-wheel angle signal δsw(t), the controller 18 computes the intended or desired hitch angle for achieving the desired backing curvature of the trailer 12. Based on the desired hitch angle and the actual hitch angle, the controller 18 provides a signal to the electric motor 36 that provides active front wheel steering assist, including counter steering, so that the desired hitch angle substantially matches the actual hitch angle. The vehicle 14 also includes a smart hitch switch 38 that allows the vehicle operator to disengage or engage the active front wheel steering when performing a back-up maneuver.
The desired hitch angle is computed using a kinematics model 50, shown in
The following nomenclature is used in the kinematics model and bicycle model calculations discussed below.
The steering hand-wheel signal measured by the sensor 48 is applied to the actuator 64 since there is a direct linkage and a bicycle model process block 66, or command interpreter, within the controller 62. The bicycle model process block 66 uses the steering hand-wheel angle signal δsw(t) and the vehicle speed signal Vx to generate a desired hitch angle command signal θcm(t) as discussed below. The linear bicycle model can be formulated in a matrix form as:
M{dot over (x)}=A1x+B1U (1)
or {dot over (x)}=Ax+BU (2)
Where x=[v r φ θ]T of are state variables and U=[δsw0]T is a control input.
The system matrix A and the input matrix B are defined as:
A hitch equilibrium hitch angle {overscore (θ)}eq is calculated by solving the algebraic equations (1) and (2) a {dot over (x)}=0. From the linear bicycle model and the equations (1) and (2), the equilibrium point can be obtained as:
{overscore (x)}eq=−A−1BU (6)
{overscore (θ)}eq={overscore (x)}eq[4]=f(A,B,U)=f(Γ,u,δf)={overscore (θ)}cmd (7)
Where Γ represents the dynamic and kinematic parameters of the vehicle-trailer combination, and u is the vehicle traveling speed. The system 10 has only one equilibrium point whether the vehicle-trailer combination moves forward or backward. The hitch equilibrium angle {overscore (θ)}eq is chosen as the desired hitch angle command θcmd(t).
The hitch angle command signal θcmd(t) from the process block 66 is compared to the measured hitch angle θmsrd(t) from the hitch angle sensor 28 in a summer 70 to compute a hitch angle error signal Δθ(t) as:
Δθ(t)=θcmd(t)−θmsrd(t) (8)
In this manner, the hitch angle error signal Δθ(t) is minimized by properly controlling the front wheel steering angle. Particularly, the system 60 attempts to maintain the hitch angle command signal θcmd(t) as close to the measured hitch angle θmsrd(t) as possible.
The hitch angle error signal Δθ(t) is applied to a PID control unit 72 to compute a corrected front road wheel angle δf
The proportional and derivative gains are used to compute the desired front wheel angle. The proportional gain Kp is adjusted based on the vehicle longitudinal speed, so that it is at a maximum when the vehicle speed is close to zero and varies according to the change in the vehicle speed. The derivative gain Kd is a constant.
The corrected front road wheel angle δf
The front road wheel angle δfw(t) is controlled using the electric motor 36 that modifies the hand-wheel angle signal δsw(t) from the sensor 48 according to the gain schedule:
The open-loop AFS control gain Kvr is typically a function of the vehicle speed signal Vx. As a result, the electric motor 36 operates on a variable ratio as a function of the vehicle speed signal Vx as:
A typical Gv curve is designed to have a value lower than the standard gear ratio Go at lower speeds and greater than the standard gear ratio Go at higher speeds.
When the time comes to use the AFS back-up control for trailer backing up, the controller 62 receives the steering hand wheel angle signal δsw(t) to use the electric motor 36 in a different way than it's normally used in the equation (10). When the steering hand-wheel angle signal δsw(t) is received, the AFS control enters a trailer back-up mode to control the motor angle signal δm(t). The driver's desire for using the AFS back-up control can be facilitated by the switch 38 and by activating a switch normally used for other functionalities. For example, the driver intent can be recognized by prescribing a sequence of switch activations, such as for example, putting the transmission in the reverse gear and turning on the switch 38.
When the AFS back-up control is on, equation (12) below should hold between the hand-wheel angle signal δsw(t) and the front road wheel angle signal δfw(t).
Where G0 is a fixed gear ratio. Therefore, the motor angle signal δm(t) provided to the AFS actuator 64 should be:
δm(t)=G0*δf
If the smart hitch switch 38 is on at the decision diamond 90, the control algorithm knows that the vehicle operator just activated it. The controller algorithm then determines whether the vehicle 14 is in the reverse gear at decision diamond 92. If the vehicle 14 is in the reverse gear, then the control algorithm sets the smart hitch flag true at box 94 and computes the hitch angle command signal θcmd in the process block 66 at box 96. The control algorithm then determines the difference between the hitch angle command signal θcmd and the measured hitch angle θcmd in the summation device 70 at box 98. The control algorithm then determines the PID control in the PID process box 72 at box 100, the motor angle signal δm(t) from the process block 74 at box 102 and the steering signal at box 104.
If the switch 38 is not on at the decision diamond 90 or the vehicle 14 is not in the reverse gear at the decision diamond 92, then the control algorithm sets the smart hitch flag false at box 106, and computes the active steering signal with the back-up control off from the equation (9) at box 108. The control algorithm then proceeds to determining the motor angle signal δm(t) at the box 102 skipping the processing for the smart hitch calculations.
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.
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| Number | Date | Country | |
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| 20060103511 A1 | May 2006 | US |
