Abstract:
In a rear wheel steering control system for a vehicle, a rear wheel steering control unit ( 53 ) forces the toe angle of each rear wheel ( 3   rl,    3   rr ) to a substantially neutral position or a slightly toe-in position when a road condition estimating unit ( 63 ) has detected a rough road surface. Thereby, when the vehicle is traveling over a rough road surface, the actuator is forced to the neutral position, and the rear wheels are brought to a neutral position so that the changes in the wheel geometry (tread and/or alignment) of the rear wheels at the time of a bump or a rebound can be avoided. Therefore, the ride quality of a vehicle equipped with the rear wheel steering control system is favorably maintained even when the vehicle is traveling over a road surface, and the rear wheels undergo large vertical displacements.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a rear wheel steering control system that can change the toe angles of rear wheels of a vehicle. 
       BACKGROUND OF THE INVENTION 
       [0002]    It has been proposed to fit a four-wheeled vehicle with a rear wheel steering control system in addition to a more conventional front wheel steering system for the purpose of improving the driving stability of the vehicle. Typically, in association with each rear wheel is provided an electric linear actuator having an output rod that can be selectively extended and retracted so that the two rear wheels may be steered individually. See Japanese patent laid open publication No. 9-030438, for instance. 
         [0003]    The linear actuator may be advantageously incorporated into one of the lateral arms that form a part of the wheel suspension system of the corresponding rear wheel. In such a case, changing the toe angle of the rear wheel is effected by changing the length of such a lateral arm. Therefore, as the toe angle of the rear wheel is changed, the geometry of the wheel, in particular the tread of the rear wheels, changes at the times of bump and rebound, and this may adversely affect the ride quality of the vehicle. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    In view of such problems of the prior art, a primary object of the present invention is to provide a rear wheel steering control system that can improve the ride quality of a vehicle fitted with a rear wheel steering control system. 
         [0005]    A second object of the present invention is to provide a rear wheel steering control system which is highly simple and compact in structure. 
         [0006]    According to the present invention, such objects can be at least partly accomplished by providing a rear wheel steering control system for a vehicle, comprising; an actuator for changing a toe angle of each rear wheel; a rear wheel steering control unit for activating the actuator according to a prescribed plan; and a road condition estimating unit for estimating a state of a road surface over which the vehicle is traveling; wherein the rear wheel steering control unit forces the toe angle of each rear wheel to a substantially neutral position or a slightly toe-in position when the road condition estimating unit has detected a rough road surface. 
         [0007]    Thereby, when the vehicle is traveling over a rough road surface, the actuator is forced to the neutral position, and the rear wheels are brought to a neutral position so that the changes in the wheel geometry (tread and/or alignment) of the rear wheels at the time of a bump or a rebound can be avoided. Therefore, the ride quality of a vehicle equipped with the rear wheel steering control system is favorably maintained even when the vehicle is traveling over a road surface, and the rear wheels undergo large vertical displacements. 
         [0008]    According to a preferred embodiment of the present invention, the vehicle is additionally fitted with an unsprung mass control unit, and the road condition estimating unit is enabled to determine the state of road condition from a manipulated variable of the unsprung mass control unit. Thereby, the state of the road surface can be accurately determined in a simple and inexpensive manner. 
         [0009]    According to a particularly preferred embodiment of the present invention, the unsprung mass control system forms a part of a damper control system including a variable damping force damper, and is configured to supply a control current corresponding to a product of a stroke and a stroke speed of the damper to the variable damping force damper. The variable damping force damper may consist of a telescopic damper using MRF (magneto-rheological fluid) for the working fluid thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Now the present invention is described in the following with reference to the appended drawings, in which: 
           [0011]      FIG. 1  is a diagram of a four-wheeled vehicle incorporated with a rear wheel steering control system embodying the present invention; 
           [0012]      FIG. 2  is a fragmentary perspective view of a left rear wheel suspension system; 
           [0013]      FIG. 3  is a vertical sectional view of a linear electric actuator of the rear wheel steering control system; 
           [0014]      FIG. 4  is a block diagram of an essential part of an ECU used in the rear wheel steering control system; and 
           [0015]      FIG. 5  is a flowchart showing the rear wheel steering control process of the illustrated embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    A preferred embodiment of the rear wheel steering control system of the present invention is described in the following with reference to  FIG. 1 . In  FIG. 1 , some of the components thereof are associated with each wheel, and such component parts are denoted with suffices fl, fr, rl and rr to indicate with which wheel the particular component is associated. For instance, a front left wheel is denoted with  3   fl , a front right wheel with  3   fr , a rear left wheel with  3   rl  and rear right wheel with  3   rr . When a particular component is collectively referred to, it may be denoted with the corresponding numeral without the suffix. For instance, the wheels of the vehicle may be referred to simply as  3  depending on the situation. 
         [0017]    Referring to  FIG. 1 , the illustrated vehicle V has a vehicle body  1  which has four wheels  3  each fitted with a pneumatic tire  2 . Each wheel  3  is rotatably supported by a knuckle  7  which is in turn supported by wheel suspension system  5  including suspension arms, a spring and a variable damping force damper  4 . The variable damper  4  essentially consists of a telescopic piston and cylinder, and uses MRF (magneto-rheological fluid) for the working fluid thereof. By controlling a magnetic fluid valve incorporated in a piston of the damper  4 , the damping force for the given stroke speed thereof can be changed both quickly and continuously. 
         [0018]    The vehicle V is fitted with a front wheel steering system  9  that allows the right and left front wheels  3   fl  and  3   fr  to be steered by turning a steering wheel  19  with the aid of a rack and pinion gear mechanism, and a rear wheel steering control system  6  that allows each rear wheel  3   rl ,  3   rr  to be steered individually by a corresponding electric actuator  8   l ,  8   r  provided in association with the corresponding rear wheel. Each electric actuator  8  consists of a linear actuator that has a housing attached to a part of the vehicle body  1  and an output rod connected to the knuckle  7  and configured to extend and retract according to an electric current supplied thereto. 
         [0019]    The rear wheel steering control system  6  allows the toe-in and toe-out of the rear wheels  3  to be changes by steering the rear wheels in a mutually symmetric relationship in a corresponding direction, and the rear wheels to be steered by a same steering angle by extending the output rod of one of the actuators and retracting the output rod of the other actuator in an opposite direction by a same stroke. 
         [0020]    The various onboard control systems including the dampers  4  rear wheel steering control system  6  are centrally controlled by an onboard ECU (electronic control unit)  20  which essentially consists of a microcomputer incorporated with ROM, RAM, interface circuits, input and output interfaces and drivers, and is connected to various sensors (which will be described hereinafter), the dampers  4  and the electric actuators  8  via a communication line such as CAN (controlled area network). 
         [0021]    The vehicle V is provided with a steering angle sensor  10  for detecting a steering angle of the steering wheel  19 , a vehicle speed sensor  11  for detecting a traveling speed of the vehicle, a lateral G sensor  12  for detecting a lateral acceleration of the vehicle, a fore-and-aft acceleration sensor  13  for detecting a fore-and-aft acceleration of the vehicle and a yaw rate sensor  14  for detecting a yaw rate of the vehicle which are arranged in appropriate parts of the vehicle V. The vehicle V is additionally provided with a vertical G sensor  15  attached to a part of each wheel house for detecting a vertical acceleration of the corresponding part of the vehicle and a stroke sensor  16  for detecting a vertical stroke of each wheel. Each electric actuator  8  is provided with a position sensor (linear encoder)  17  for detecting the output stroke of the actuator, and the knuckle  7  of each rear wheel  3   rl ,  3   rr  carries an unsprung mass G sensor  18  for detecting the vertical acceleration of the knuckle  7  (unsprung mass acceleration). 
         [0022]      FIG. 2  is a perspective view of a left rear wheel suspension system  5   rl . The right rear wheel suspension system  5   rr  can be given as a mirror image of the left rear wheel suspension system  5   rl.    
         [0023]    As shown in  FIG. 2 , this rear suspension system  5   rl  is of a double wishbone type, and comprises a knuckle  7  rotatably supporting the rear wheel  3   rl , an upper and lower arms  21  and  22  joining the knuckle  7  to the vehicle body in a vertically moveable manner, an electric actuator  8   l  joining the knuckle  7  to the vehicle body so as to allow the toe angle of the rear wheel  3   rl  to be varied, a suspension spring  4   a  resiliently supporting the rear wheel to the vehicle body and a damper  5  to apply a damping force to the vertical movement of the knuckle  7 . 
         [0024]    The upper arm  21  is attached to a part of the vehicle body  1  via a rubber bush joint  23  at the base end thereof and to an upper part of the knuckle  7  via a ball joint  25 , and the lower arm  22  is attached to a part of the vehicle body  1  via a rubber bush joint  24  at the base end thereof and to a lower part of the knuckle  7  via a ball joint  26 . The housing of the electric actuator  8   l  is attached to the vehicle body  1  via a rubber bush joint  27 , and the output rod of the electric actuator  8   l  is connected to a rear part of the knuckle  7  via a rubber bush joint  28 . The damper  4  is connected to the vehicle body  1  via a rubber bush not shown in the drawings at the upper end thereof, and to an upper part of the knuckle  7  via a rubber bush joint  29  at the lower end thereof. 
         [0025]    Thus, when the output rod of the electric actuator  8   l  is extended, the rear part of the knuckle  7  moves laterally outward so that a toe-in movement of the rear wheel  3   rl  is effected. Conversely, when the output rod of the electric actuator  8   l  is retracted, the rear part of the knuckle  7  moves laterally inward so that a toe-out movement of the rear wheel  3   rl  is effected. 
         [0026]      FIG. 3  is a vertical sectional view of the electric actuator  8   l  of the illustrated embodiment. As shown in  FIG. 3 , the electric actuator  8   l  comprises a first housing  30   a  integrally formed with the rubber bush joint (vehicle body)  27 , a second housing  30   b  connected to the first housing  27  by a plurality of threaded bolts  31  and forming a whole housing  30  jointly with the first housing  20   a  and an output rod  32  extending out of the second housing  30   b  and having the rubber bush joint (knuckle)  28  formed at the free end thereof. The first housing  30   a  receives therein a brushless DC motor  34  serving as a power source and fixedly attached to the first housing  30   a  by using threaded bolts  35 . The second housing  30   b  receives therein a planetary gear type reduction gear unit  36 , an elastic coupling  37  and a feed screw mechanism  38  using a trapezoidal thread. 
         [0027]    When the DC motor  34  is actuated, the rotation of the output shaft  34   a  thereof is reduced in speed by the reduction gear unit  36 , and is then converted into a linear motion of the output rod  32  by the feed screw mechanism  38 . 
         [0028]    The position sensor  17  provided on the outer periphery of the second housing  30   b  essentially consists of a magnet piece  41  fixedly attached to the output rod  32  by a threaded bolt  39  and a differential transformer  43  received in a sensor housing  42  which is in turn attached to the second housing  30   b  so as to oppose the magnet piece  41 . The differential transformer  43  includes a primary winding and a pair of secondary windings, and a differential voltage produced between the secondary windings provides a measure of a linear displacement of the output rod  32 . 
         [0029]      FIG. 4  is a block diagram of an essential part of the ECU  20  used in the illustrated embodiment. The ECU  20  includes a damping force control unit  52  for controlling the damping action of the dampers  4 , a rear wheel steering control unit  53  for controlling the steering action of the electric actuators  8 , an input interface  51  interfacing the various sensors  10 - 18  with the damping force control unit  52  and rear wheel steering control unit  53 , and an output interface  54  interfacing the damping force control unit  52  and rear wheel steering control unit  53  with the respective actuators. 
         [0030]    The damping force control unit  52  comprises an attitude control unit  55 , a first control current setting unit  56 , an unsprung mass control unit  57 , a second control current setting unit  58  and a target current selecting unit  59 . The attitude control unit  55  comprises a skyhook control unit  60 , a roll control unit  61  and a pitch control unit  62  which produce a skyhook control target value Dsh, a roll control target value Dr and a pitch control target value Dp, respectively, according to the detection signals of the various sensors  10 - 16 . The first control current setting unit  56  selects one of the three control target values Dsh, Dr and Dp which is the same in sign as the stroke speed of the damper  4  and largest in absolute value as the first target damping force Dtgt1, and looks up a first control current Itb1 from a prescribed first control current map for the given first target damping force Dtgt1 and stroke speed obtained from the stroke sensor  16 . 
         [0031]    The unsprung mass control unit  57  computes an unsprung mass control target value Dw for each damper  4  according to the vehicle speed obtained from the vehicle speed sensor  11  and stroke position obtained from the stroke sensor  16 . The second control current setting unit  58  sets the unsprung mass control target value Dw as the second target damping force Dtgt2, and looks up a second control current Itb2 from a prescribed second control current map for the given second target damping force Dtgt2 and stroke speed obtained from the stroke sensor  16 . 
         [0032]    The skyhook control unit  60  is configured to control the oscillation of the sprung mass, and is effective in suppressing the resonant oscillation of the sprung mass which is about 1 Hz, but is relatively ineffective in suppressing the resonant oscillation of the unsprung mass which is about 10 Hz. In the unsprung mass control, when the damper stroke and stroke speed are high, an unsprung mass control current computed by multiplying a prescribed constant, the damper stroke speed and damper stroke is used instead of the skyhook control current (or added to the target current value required by the skyhook control) to provide the final target current for the damper. As a result, independently of the skyhook control, the resonant oscillation of the unsprung mass in a frequency range around 10 Hz can be effectively suppressed. For more details of the unsprung mass control, reference may be made to Japanese patent laid open publication No. 2006-321259, and U.S. Pat. No. 7,406,371. 
         [0033]    The target current selecting unit  59  compares the obtained first control current Itb1 and second control current Itb2 with each other, and sets one of them having a greater absolute value as the target current Itgt, and supplies a drive current corresponding to the target current Itgt to the magnetic fluid valve of each damper  4  so that a desired damping control may be accomplished. 
         [0034]    The rear wheel steering control unit  53  comprises a road condition estimating unit  63 , a target steering angle setting unit  64 , a target displacement setting unit  65  and a drive current setting unit  66 . The road condition estimating unit  63  estimates the state of the road surface on which the vehicle is traveling according to the unsprung mass control target value Dw computed by the unsprung mass control unit  57 , and forwards the estimated state of the road surface to the target steering angle setting unit  64 . The target steering angle setting unit  64  then determines a rear wheel target steering angle according to the detection signals of the steering angle sensor  10  and yaw rate sensor  14  and the estimated state of the road surface. The target displacement setting unit  65  determines a target displacement of the electric actuator  8  according to the difference between the target rear wheel steering angle and actual rear wheel steering angle obtained from the output of the position sensor  17 . The drive current setting unit  66  supplies a drive current for the electric actuator  8  according to the target displacement. 
         [0035]    The mode of operation of the illustrated embodiment is described in the following. When the operation of the vehicle V has started, the ECU  20  executes the damping force control and a rear wheel steering control at a prescribed control interval (2 ms, for instance). 
         [0036]    The damping control is executed by the damping force control unit  52 . Upon determining the operating condition of the vehicle according to the detection signals of the various sensors  10 - 16 , the damping force control unit  52  computes a skyhook control target value Dsh, a roll control target value Dr and a pitch control target value Dp for each wheel according to the determined operating condition of the vehicle V. The first control current setting unit  56  selects one of these target values which has the same sign as the stroke speed of the damper and the largest absolute value as a first target damping force Dtgt1, and looks up a first target current map to determine a first control current Itb1 according to the first target damping force Dtgt1 and the stroke speed of the damper  4 . The unsprung mass control unit  57  computes an unsprung mass control target value Dw according to the vehicle speed and stroke position of the damper  4 . The second control current setting unit  58  then sets the unsprung mass control target value Dw as a second target damping force Dtgt2, and looks up a second target current map to determine a second control current Itb2 according to the second target damping force Dtgt2 and the stroke speed of the damper  4 . The target current selecting unit  59  selects one of the first target damping force Dtgt1 and second target damping force Dtgt2 which is greater in absolute value as a target current Itgt which is supplied to the damper  4  for controlling the damping force thereof. 
         [0037]    The rear wheel steering control of the illustrated embodiment is described in the following with reference to the flowchart of  FIG. 5 . The rear wheel steering control is performed on each of the rear wheels  3   rr  and  3   rl  in a similar manner, and the following description is limited to that for the left rear wheel  3   rl  for the convenience of description. 
         [0038]    The ECU  20  executes the rear wheel steering control illustrated in the flowchart of  FIG. 5  concurrently with the damping control described above. The road condition estimating unit  63  determines if the absolute value of the unsprung mass control target value Dw forwarded from the unsprung mass control unit  57  at a regular control interval is greater than a prescribed threshold value S 1  in step ST 1 . If the determination result is Yes, a current cumulative value I n  is computed by adding “1” to the previous cumulative value I n−1  in step ST 2 . If the determination result is No, the current cumulative value I n  is computed by subtracting “1” from the previous cumulative value I n−1  in step ST 3 . In the latter case, it is determined if the current cumulative value I n  is smaller than zero in step ST 4 . If it is the case, the current cumulative value I n  is set to zero in step ST 5 . Thus, the minimum value of the current cumulative value I n  is zero owing to the process executed in steps ST 4  and ST 5 . 
         [0039]    Following steps ST 2 , ST 4  or ST 5 , the road condition estimating unit  63  determines if the current cumulative value I n  is greater than a second threshold value S 2  in step ST 6 . If the determination result of this step is Yes or if the vehicle V is traveling over a rough road surface, the absolute value of unsprung mass control target value |Dw|increases owing to the need to control the vibration of the unsprung mass. Therefore, the state of the road surface can be evaluated by determining if the current cumulative value I n  has exceeded the second threshold value S 2 . The determination result of the road condition estimating unit  63  is forwarded to the estimated road surface signal to the target steering angle setting unit  64 . 
         [0040]    When the estimated road surface signal indicates a rough road surface, the target steering angle setting unit  64  set the rear wheel target steering angle to zero in step ST 7 . Each actuator  8  is configured such that the rear wheel steering angle is zero when the actuator is in a neutral state without being extended or retracted. The target displacement setting unit  65  and a drive current setting unit  66  control each electric actuator  8  so that the rear steering angle is maintained at zero. As a result, when the vehicle is traveling over a rough a rough road surface, each electric actuator  8  is maintained in a neutral position so that the impairment of the ride quality which may be otherwise caused by the changes in the tread of the vehicle at the time bump and rebound conditions owing to the extension or retraction of the electric actuator  8 . 
         [0041]    When the estimated road surface signal does not indicate a rough road surface, the rear wheel target steering angle is set in a normal way according to the detection signals of the steering angle sensor  10  and yaw rate sensor  14  in step ST 8 . The target displacement setting unit  65  and a drive current setting unit  66  control each electric actuator  8  so that the actual rear wheel steering angle agrees with the target rear wheel steering angle. When the vehicle is not traveling over a rough road surface, extending or retracting the electric actuator  8  does not cause changes in the tread at the time of bump or rebound, and the actuation of the electric actuator  8  does not impair the ride quality. 
         [0042]    As a slightly modified embodiment of the present invention, the rear wheel target steering angle may be set to a slightly toe-in angle by extending the electric actuator  8 . By forcing the rear wheel target steering angle to a slightly toe-in angle from a normally controlled value, the changes in the tread at the time of the bump and rebound can be controlled, and the ride quality of the vehicle is prevented from being impaired. 
         [0043]    Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims. For instance, the state of the road surface was estimated from the unsprung mass target value Dw in the foregoing embodiment, but may also be estimated from other data such as the detection signal of the vertical G sensor  15  and image information obtained by a camera. 
         [0044]    The contents of the original Japanese patent application on which the Paris Convention priority claim is made for the present application, as well as the contents of any publications mentioned in this disclosure, are incorporated in this application by reference.