Abstract:
A vehicle steering system coupled to a steerable wheel, comprising a steering rack linked to said steerable wheel, an electric motor having an output shaft engaged with the steering rack for controlling the steerable wheel. The system also includes a position sensor configured to measure an actual steerable wheel angle of the steering rack and thereby the steerable wheel, a return spring, where the return spring is biased so as to provide a returning force to the steering rack. Finally, the system includes a controller that provides a command to the electric motor resulting in a torque, the command is responsive to the actual steerable wheel angle, a desired steerable wheel angle, and a command direction.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. provisional application No. 60/309,375 filed Aug. 01, 2001 the contents of which are incorporated by reference herein in their entirety. 
     
    
     
       BACKGROUND  
         [0002]    A four wheel steering system steers the front and rear wheels of an automobile. More specifically, an electrically driven, rack and pinion rear wheel steer system produces a desired rear wheel steering angle to improve directional stability at high speeds and steering stability at low speeds. Four wheel steering methods include a “in phase” method and a “reverse phase” steering method. The in phase steering method reduces vehicle yaw (an angular speed about a vertical line passing through a center of gravity of a vehicle) by steering the front and rear wheels in the same direction, and improves vehicle safety due to the reduced yawing. The reverse phase steering method achieves better maneuverability at low speeds by steering the front and rear wheels in differential directions, thereby reducing the turning radius of the vehicle.  
           [0003]    In a four wheel or rear wheel steering system a mechanism (such as a spring) may be employed to assist in ensuring that the rear wheels return to a neutral position under inoperative conditions. However, if the rear wheels overshoot an accurate position due to the force of the return spring, undesirable vehicle characteristics and conditions may result.  
         BRIEF SUMMARY  
         [0004]    A vehicle steering system coupled to a steerable wheel, comprising a steering rack linked to said steerable wheel, an electric motor having an output shaft engaged with the steering rack for controlling the steerable wheel. The system also includes a position sensor configured to measure an actual steerable wheel angle of the steering rack and thereby the steerable wheel, a return spring, where the return spring biased so as to provide a returning force to the steering rack. Finally, the system includes a controller that provides a command to the electric motor resulting in a torque, the command is responsive to the actual steerable wheel angle, a desired steerable wheel angle, and a command direction.  
           [0005]    A method for controlling an angle of a steerable wheel in a vehicle with electric power steering with an electric motor and a return spring, comprising determining a desired steerable wheel angle, obtaining an actual steerable wheel angle of the vehicle, determining a direction indicative of a command direction for the electric motor, generating the command, responsive to the desired steerable wheel angle, the actual steerable wheel angle, and the command direction. Where the command results in a torque generated by the electric motor operably connected to the steerable wheel for controlling the actual steerable wheel angle. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    The present invention will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which:  
         [0007]    [0007]FIG. 1 is a schematic diagram of a four wheel, electric power steering system suitable for implementation with an exemplary embodiment;  
         [0008]    [0008]FIG. 2 is a cutaway view of a rear steering mechanism;  
         [0009]    [0009]FIG. 3 depicts return spring forces as a function of rear wheel rack displacement;  
         [0010]    [0010]FIG. 4 is block diagram of an existing control algorithm for four wheel steering;  
         [0011]    [0011]FIG. 5 is a block diagram illustrating a control algorithm for four wheel steering of an exemplary embodiment; and  
         [0012]    [0012]FIG. 6 depicts an exemplary percentage duty cycle as a function of the servo error command. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]    An exemplary embodiment is described herein by way of illustration as may be applied to a vehicle and more specifically a vehicle steering system. While a preferred embodiment is shown and described, it will be appreciated by those skilled in the art that the invention is not limited to the embodiment and application described herein, but also to any vehicle with steerable wheels and where rear wheel steering is employed. Moreover, while an exemplary embodiment is disclosed and illustrated with reference to a particular implementation, it will be appreciated that such illustration should not be construed as limiting. Those skilled in the art will appreciate that a variety of potential implementations and configurations are possible.  
         [0014]    Referring to FIGS. 1 and 2, a diagram partially depicting a vehicle steering control system  10  is depicted, hereinafter referred to as system  10 . The system  10  includes, but is not limited to, a controller  18  coupled to various sensors and interfaces for performing a variety of processes prescribed by the desired controlling functions. FIG. 1 depicts a vehicle employing rear wheel steering and the apparatus for executing the prescribed steering functions.  
         [0015]    System  10  is incorporable into a motor vehicle  12  to provide directional control to motor vehicle  12  and is configurable to enhance steering of the motor vehicle  12 . System  10  comprises a front steering mechanism, shown generally at  14 , a rear steering mechanism, shown generally at  16 , and a controller  18  in communication with front steering mechanism  14  and rear steering mechanism  16 . Although system  10  is described as being applicable to a rack and pinion steering arrangement, system  10  can be incorporable into other steering arrangements including, but not being limited to, integral gear steering systems.  
         [0016]    Front steering mechanism  14  comprises a rack shaft  20 , a front steering rack  22  disposed intermediate opposing ends of rack shaft  20 , a tie rod  24  disposed on each opposing end of rack shaft  20 , a knuckle arm  26  connected to each tie rod  24 , and a front steerable wheel  28  rotatably disposed on each knuckle arm  26 . Rack shaft  20 , tie rods  24 , and knuckle arms  26  are configured such that front steerable wheels  28  can pivot in unison relative to a body of motor vehicle  12  to steer or to effect a change in the direction of travel of motor vehicle  12  while motor vehicle  12  is moving.  
         [0017]    Front steering mechanism  14  further comprises a mechanism through which an operator can effectuate a desired change in the direction of travel of motor vehicle  12 . Such a mechanism comprises a steering column  30  disposed in operable communication at one end thereof with front steering rack  22  through a pinion  32  and at an opposing end thereof with a steering device  34 . Steering device  34  may be a hand steering wheel. Manipulation of steering device  34 , e.g., rotation of the hand steering wheel, causes the axial rotation of steering column  30 , which in turn causes the rotation of pinion  32 . Rotation of pinion  32 , through the engagement of front steering rack  22  and pinion  32 , effectuates the lateral translation of front steering rack  22  relative to the body of motor vehicle  12 . The lateral translation of front steering rack  22  causes front steerable wheels  28  to angle relative to the body of motor vehicle  12 , thereby altering the direction of travel of motor vehicle  12  while motor vehicle  12  is moving. A power steering assist may also be provided to the front wheels  28  with an electric or hydraulic actuator (not shown). Referring to FIG. 2, a partial cutaway view of a rear steering mechanism is depicted. Rear steering mechanism  16  comprises a rear rack shaft  36 , a rear steering rack  38  disposed intermediate opposing ends of rack shaft  36 , a tie rod  40  disposed on each opposing end of rack shaft  36 , a knuckle arm  42  connected to each tie rod  40 , and a rear steerable wheel  44  rotatably disposed on each knuckle arm  42 . Rear rack shaft  36 , tie rods  40 , and knuckle arms  42  are configured such that rear steerable wheels  44 , like front steerable wheels  28 , can be pivoted in unison relative to the body of motor vehicle  12  to steer motor vehicle  12  upon lateral translation of rear steering rack  38 .  
         [0018]    Rear steering mechanism  16  further comprises a mechanism through which rear steerable wheels  44  can similarly be pivoted. Such a mechanism comprises a motor  46  operably connected to rear steering rack  38  through a drive mechanism  48 . Drive mechanism  48 , through a pinion  49 , transfers the rotational motion of a rotor shaft  47  of motor  46  to linear motion of rear steering rack  38 , which effectuates the lateral motion of rack shaft  36  and, ultimately, the pivoting of rear steerable wheels  44 . In the embodiment depicted in FIG. 1, the controller  18  may also be used to provide a torque command to electric motor  46 . Accordingly, a displacement sensor  58  is used to detect the displacement of rear rack shaft  36 , and hence determine the rear steering angle for feedback to the controller  18 .  
         [0019]    Motor vehicle  12  is further provided with a steering angle sensor  56 , and transmits to controller  18  a handwheel angle signal  66  indicative of a front wheel angle. A rear rack shaft displacement sensor  58  detects the displacement of its corresponding rear rack shaft  36  from a reference position, which is the position in which each rear steerable wheel  44  is aligned and rotatable and transmits a rear wheel angle signal  62  to controller  18 . The front wheel angle signal and rear wheel angle signal  62  may be derived from a variety of sources including, but not be limited to, various encoders, other position sensors such as potentiometers, synchros, resolvers, magnetic and optical transducers, and the like, including combinations of the foregoing  
         [0020]    Controller  18  is disposed in communication with the various systems of motor vehicle  12 . Controller  18  receives informational signals from each of the systems, quantifies the received information, and provides an output command signal in response thereto, in this instance, for example, to the rear steering mechanism  16  through motor  46 . Communications, informational signals and commands as such may be employed by controller  18  to facilitate execution of control algorithms including, but not limited to, four wheel steering control, and/or the dual gain algorithm  100  disclosed herein.  
         [0021]    In order to perform the prescribed functions and desired processing, as well as the computations therefore (e.g., the execution of the dual gain algorithm(s)  100 , and the like), controller  18  may include, but not be limited to, a processor(s), computer(s), memory, storage, register(s), timing, interrupt(s), communication interfaces, and input/output signal interfaces, and the like, as well as combinations comprising at least one of the foregoing. For example, controller  18  may include input signal filtering to enable accurate sampling and conversion or acquisitions of such signals from communications interfaces. Additional features of controller  18  and certain processes therein are thoroughly discussed at a later point herein.  
         [0022]    Continuing with FIGS. 1 and 2, a return spring  70  provides a force that returns the rear steering rack  38  to a center, or neutral position under certain conditions in the rear steering system. The return spring  70 , in an exemplary embodiment also is disposed in the rear steering rack  38  with a bi-directional preload. The preload is configured to force the steering rack  38  to the center position when the motor  46  is not maintaining a particular position, and thereby the rear steering rack  38  in a particular position. FIG. 3 is a graph depicting the spring force as a function of rack displacements typical of an existing system. It will be evident that once the rear steering rack  38  is displaced from the center position, the force required thereafter to displace the rear steering rack  38  is not directionally symmetrical. That is, displacements in the direction of further compression of return spring (e.g., against the return spring  70 ) will require additional force, as the displacement is opposed by the spring force, while displacements in the direction of extension (less compression) of the return spring e.g. with the return spring  70 ) will require less force, as the displacement is aided by the spring force. A system and methodology for including a return spring in a steering system is described in Jung et al. U.S. Pat. No. 5,810,108, dated Sep. 22, 1998.  
         [0023]    [0023]FIG. 4 depicts an existing control block diagram for four wheel steering. A system and methodology for rear wheel steering control was described in commonly assigned U.S. Pat. No. 4,828,061 dated May 09, 1989 by Kimbrough, et al., which is incorporated by reference herein in its entirety. In the existing architecture and in the exemplary embodiment disclosed herein, a hand wheel angle is measured by steering angle sensor  56  and in proportion thereto, a front wheel angle may be determined. Based upon vehicle speed and front wheel angle a desired rear wheel angle  202  is computed. The desired rear wheel angle  202  is compared to the measured, actual rear wheel angle  204  to formulate a servo error  206 . The existing control system while well adapted for its requirements may exhibit the abovementioned overshoot under certain operating conditions. To address this concern, an exemplary embodiment of a dual gain algorithm is disclosed, which incorporates determination of the translational direction of the return spring  70  to facilitate damping and enhance control. Referring now to FIG. 5 an exemplary embodiment of the dual gain algorithm implementation is depicted. It will be evident that the existing control architecture of FIG. 4 has been modified to include the dual gain capability of the disclosed embodiment. It is noteworthy to recognize the control diagram of FIG. 5 is similar to that of FIG. 4 with the inclusion of the spring direction process  110  and a modification of the PWM look up table  80  (FIG. 4).  
         [0024]    Referring now the to spring direction process  110  wherein the translation direction of the return spring  70  and ultimately the rear wheels is determined. A direction signal  208  indicative of the command direction to the motor  46  and therefore the direction of the return spring is generated and transmitted to the motor pulse width modulation (PWM) look up process  120 . In addition the servo error  206  is also transmitted to the pulse width modulation (PWM) look up process  120 . Depending upon the direction and servo error  206  a particular set of look up data is utilized in the PWM look up process  120 . The values of the look up table(s) may then be tuned to eliminate undesirable response characteristics of the motor  46  and system  10 .  
         [0025]    Turning now to a detail view of the direction process  110 , the direction may be ascertained from desired/calculated rear wheel angle and the rear wheel angle signal  62 . If the rear wheel angle signal  62  indicates that the actual rear wheel angle is greater than zero degrees and if the desired rear wheel angle is greater than the actual rear wheel angle, then the direction is against the return spring  70 . However, if the actual rear wheel angle is greater than zero degrees and if the desired rear wheel angle is less than the actual rear wheel angle, then the command direction is with the return spring  70 . Conversely, if the rear wheel angle signal  62  indicates that the actual rear wheel angle is less than zero degrees and if the desired rear wheel angle is less than the actual rear wheel angle, then the command direction is against the return spring  70 . Likewise, if the actual rear wheel angle is less than zero degrees and if the desired rear wheel angle is greater than the actual rear wheel angle, then the direction is with the return spring  70 . A direction signal  208  indicating the direction of travel of the return spring  70  as determined above is transmitted to the motor pulse width modulation (PWM) look up process  120 .  
         [0026]    Continuing now with FIG. 5 a detailed disclosure of the PWM look up process  120  is considered. It will be evident that this process is similar to the PWM look up table  80  depicted in FIG. 4. In an exemplary embodiment, the PWM look up process  120  employs one or more selectable look up tables to facilitate generation of a voltage command  210  for the motor  46  (FIG. 1). A look up table is employed to facilitate generating the desired scheduling and amplification of the servo error  206  as a function of the return spring direction  208 . It will be appreciated that while an exemplary embodiment has been described and illustrated with reference to look up tables, other methodologies for implementing the disclosed functions are possible and may include, but not be limited to multiplication, offsets, integration, scheduling, tables, and the like, including combinations of the foregoing.  
         [0027]    In an exemplary embodiment, the PWM look up process  120  applies variable percentage duty cycle commands to the motor  46  as a function of the direction of travel of the return spring  70 , and the servo error command. In an exemplary embodiment two ratios are applied in a look up table for PWM duty cycles, one for commands with the direction of the return spring  70 , another for commands against the direction of the return spring  70 . FIG. 6 depicts an exemplary percentage duty cycle as a function of the servo error command for commands with and against the return spring  70 . It is noteworthy to appreciate that the direction of motion is accounted for in the two curves depicted. In particular, a shallower gain or curve exhibiting a gain reduced from a nominal for commands with the return spring  70 , and a larger, more aggressive gain exhibiting an increase from a nominal gain for commands against the return spring  70 . The variation in duty cycle facilitates tuning the values of the look up table(s) to eliminate undesirable response characteristics of the motor  46  and system  10 .  
         [0028]    The system and methodology described hereinbefore provides a robust design and method to stabilize the rear wheel steering angle, thereby improving high-speed directional stability and low speed turning of a vehicle. In addition, the disclosed invention may be embodied in the form of computer-implemented processes and apparatuses for practicing those processes. The present invention can also be embodied in the form of computer program code containing instructions embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or any other computer-readable storage medium, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. The present invention can also be embodied in the form of computer program code, for example, whether stored in a storage medium, loaded into and/or executed by a computer, or as data signal transmitted whether a modulated carrier wave or not, over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. When implemented on a general-purpose microprocessor, the computer program code segments configure the microprocessor to create specific logic circuits.  
         [0029]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.