Patent Publication Number: US-8532876-B2

Title: Vehicle steering control apparatus

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to vehicle steering control technique (apparatus and/or method) for varying a steering gear ratio of a steering wheel angle to a wheel steer angle of steerable wheel of a vehicle. 
     A published Japanese patent application, Publication No. JP2005-170129A shows a vehicle steering control system arranged to increase a servo gain of an angle inputting actuator in order to improve a steering stiffness in the vicinity of a steering neutral position where a deviation of an actual rotation angle from a target rotation angle of the actuator becomes smaller. 
     SUMMARY OF THE INVENTION 
     However, in an operation to set the steering gear ratio to a slow side by rotating the actuator (or a rotating element of the actuator) in the opposite direction opposite to the steering direction of the steering wheel, the actuator tends to produce an actuator torque in the opposite direction opposite to the steering direction at the start of a steering operation, and thereby provide the driver an unnatural feeling of decrease or letdown of the steering reaction. 
     Therefore, it is an object of the present invention to provide apparatus and/or method to prevent undesired decrease or letdown of the steering reaction at the start of a steering operation. 
     According to one aspect of the present invention, a vehicle steering control apparatus comprises: an actuator to vary a steering gear ratio of a vehicle wheel steer angle of a steerable wheel to a steering wheel angle; and a controller to control the actuator to produce an actuator torque in a steering direction at the start of a driver&#39;s steering operation. 
     According to another aspect of the invention, a vehicle steering control method comprises: a first method element of producing a control signal to control an actuator to vary a steering gear ratio of a vehicle wheel steer angle of a steerable wheel to a steering wheel angle; and modifying the control signal so as to prevent a torque from being produced by the actuator in an opposite direction opposite to a steering direction at the start of a driver&#39;s steering operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing a vehicle steering control system according to a first embodiment of the present invention. 
         FIG. 2  is a block diagram showing a gear ratio controller  10  shown in  FIG. 1 . 
         FIG. 3  is a flowchart showing a steering gear ratio control process performed by the controller according to the first embodiment in the case of an angle decreasing or subtracting (or returning) operation of a steering gear ratio varying actuator. 
         FIGS. 4 and 5  are schematic views for illustrating a basic concept of the present invention.  FIG. 4  shows a state in which an actuator torque (motor torque) is produced in the same direction as a steering direction.  FIG. 5  shows a state in which the actuator torque (motor torque) is produced in the direction opposite to the steering direction. 
         FIG. 6  is a sectional view showing the gear ratio varying actuator  6  employed in the preceding embodiments. 
         FIG. 7  ( 7 A,  7 B,  7 C and  7 D) is a time chart showing operating conditions in an angle increasing (adding) operation of the actuator. 
         FIG. 8  ( 8 A,  8 B,  8 C and  8 D) is a time chart showing the operating conditions in the angle decreasing (subtracting) operation of the actuator. 
         FIG. 9  is a graph showing a steering gear ratio variation in the gear ratio control according to the first embodiment. 
         FIG. 10  ( 10 A,  10 B,  10 C and  10 D) is a time chart showing the operating conditions in the gear ratio control according to the first embodiment (and a second embodiment). 
         FIG. 11  is a flowchart showing a steering gear ratio control process performed by the controller according to a third embodiment of the present invention. 
         FIG. 12  ( 12 A,  12 B,  12 C and  12 D) is a time chart showing the operating conditions in the gear ratio control according to the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically shows a vehicle steering control apparatus or system according to a first embodiment of the present invention. A vehicle shown in  FIG. 1  includes a steering wheel  1 , front wheels (steerable wheels)  2  and a steering mechanism  3  which, in this example, includes a rack and pinion type steering gear for steering the front wheels  2 . Steering wheel  1  is connected with steering mechanism  3  by a column shaft  4  and a pinion shaft  5 . Column shaft  4  and pinion shaft  5  are connected through a gear ratio varying actuator or variable gear ratio actuator (angle inputting actuator)  6 . In column shaft  4 , there is provided a steering angle sensor  7  for sensing a rotation angle of column shaft  4  (steering wheel angle). Steering angle sensor  7  of this example uses a pulse encoder for sensing the steering angle. Gear ratio varying actuator  6  of this example includes a motor  20  (shown in  FIG. 6 ). A sensor  8  is a motor rotation angle sensor for sensing a motor rotation angle of motor (or angle inputting motor)  20 . 
     Gear ratio varying actuator  6  is an actuator to vary a steering gear ratio (also referred to as a gear ratio) which is a ratio of a rotation angle of pinion shaft  5  to a rotation angle of column shaft  4 , or a ratio of the front wheel steer angle to the steering wheel angle. In this example, gear ratio varying actuator  6  is an actuator of rotation angle adding/subtracting type for producing an output rotation by adding or subtracting a rotation of motor  20  (or an angular input produced by the actuator), to or from a rotation angle of column shaft  4  (or a driver&#39;s angular input or steering input), and deliver the output rotation to pinion shaft  5  toward front wheels  2 . A gear ratio controller  10  (which can serve as gear ratio controlling means) controls motor  20  by sending a command current to motor  20 . 
       FIG. 6  shows, in section, gear ratio varying actuator  6  employed in this example. An input shaft  22  is fixed with an upper shaft  21  which is connected with column shaft  4 . Input shaft  22  is a hollow shaft connected with upper shaft  21  end to end by forcibly fitting over upper shaft  21 . Guide grooves or slots  22   a  are formed entire in the inside circumference of a forward portion of input shaft  22 . An output shaft  25  is connected with pinion shaft  5 . Output shaft  25  is received in input shaft  22  and arranged to rotate relative to input shaft  22 . Output shaft  25  includes a ball screw groove  30  formed in the outside circumferential surface. 
     A slider  23  is disposed radially between input shaft  22  and output shaft  25  so that there is formed a small clearance between an inside circumferential surface of slider  23  and the outside circumferential surface of output shaft  25 , and slider  23  is arranged to be movable axially. Slider  23  has a forward portion which is inserted in steering column  33  and which includes an outside circumferential surface formed with axially extending guide projections engaging with guide grooves  22   a  of input shaft  22  and thereby allowing axial movement of slider  23  relative to input shaft  22 . 
     Slider  23  includes a plurality of ball retaining holes  29  for retaining a plurality of balls  24  so that each ball can roll. Each ball  24  is disposed between the hole  29  of slider  23  and ball screw groove  30  of output shaft  25  so that each ball  24  can roll therebetween. 
     Slider  23  is rotatably supported by a slider arm  28 . Slider arm  28  is screwed over a lead screw shaft  27  so that slider arm  28  is movable axially. A helical gear  32  is fixed to one end of lead screw shaft  28 . This helical gear  32  is engaged with a pinion  31  connected with the rotation shaft of motor  20 . 
     The thus-constructed gear ratio varying actuator  6  is operated as follows: When motor  20  is not operated, an operation force inputted to steering wheel  1  is transmitted from upper shaft  21 , through input shaft  22 , slider  23  and balls  24 , to output shaft  25 . Therefore, in this state, the gear ratio varying actuator  6  transmits motion through pinion shaft  5  to front wheels  2  without producing a rotation angle difference between the input and output. 
     When motor  20  is operated, the lead screw shaft  27  is rotated in one direction, and therefore the slider arm  28  moves axially, and pushes balls  24  in one direction with the edges of the retaining holes  29  of slider  23 . Therefore, balls  24  roll, move helically along the ball screw groove  30 , and thereby impart rotational torque of a predetermined speed to output shaft  25 . Output shaft  25 , therefore, rotates in one direction through an angular displacement greater than the angular displacement of steering wheel  1 , and steers the front wheels  2  through pinion shaft  5  through a greater steer angle. Gear ratio controller  10  can controls motor  20  by supplying a command current, and thereby achieve a desired gear ratio. 
     Gear ratio varying actuator  6  is not limited to the mechanism shown in  FIG. 6 . It is optional to employ various other actuators having different constructions, such as actuators of rotation angle adding and subtracting type, to receive an input rotation from a steering wheel and to deliver an output rotation toward a steerable wheel so that an output rotation angle is made greater or smaller than an input rotation angle. 
     Gear ratio controller  10  receives the steering wheel angle sensed by steering angle sensor  7 , a motor rotation angle (actual rotation angle) sensed by motor rotation angle sensor  8 , and a vehicle speed or a vehicle body speed sensed by a vehicle speed sensor  11 . 
     In accordance with the signals supplied from the sensors, the gear ratio controller  10  calculates a value of the command current to achieve a desired gear ratio, and controls the gear ratio varying actuator  6 . In this example, the desired gear ratio is made greater in a low vehicle speed region and made smaller in a high vehicle speed region to achieve both the turning performance in the low vehicle speed region and the vehicle stability in the high vehicle speed region. 
       FIG. 2  shows the gear ratio controller  10  of this example in the form of control block diagram. Gear ratio controller  10  includes a desired target rotation angle calculating section  10   a  and a steer angle servo control section  10   b.    
     Desired target rotation angle calculating section  10   a  determines a desired target vehicle yaw rate in accordance with the steering wheel angle supplied from steering angle sensor  7  and the vehicle speed supplied from vehicle speed sensor  11 , sets a desired target gear ratio to achieve the target yaw rate, and calculates a desired target rotation angle for gear ratio varying actuator  6 , to achieve the target gear ratio. 
     Steer angle servo control section  10   b  controls the command current to gear ratio varying actuator  6  so as to reduce a deviation of the actual rotation angle of actuator  6  sensed by motor rotation angle sensor  8 , from the target rotation angle determined by desired rotation angle calculating section  10   a.    
     In this embodiment, the desired target gear ratio is set smaller than a mechanical gear ratio (on a slow side) in the high vehicle speed region, and, in the actuator angle decreasing operation or returning operation to drive the actuator  6  in a counter rotational direction opposite to the steering direction, the control system of this embodiment performs a gear ratio controlling process to set the gear ratio to a quick side once at a start of a steering operation from a position near the neutral steering position, and then to vary the gear ratio gradually to a value smaller than the mechanical gear ratio (on the slow side). The slow side is a side on which the gear ratio is smaller than the mechanical gear ratio, and the quick side is a side on which the gear ratio is greater than the mechanical gear ratio. The mechanical gear ratio is a gear ratio in an inoperative state in which gear ratio varying actuator  6  is inoperative and the column shaft  4  and pinion shaft  5  are rotated at a speed ratio of 1:1 (as a unit). The mechanical gear ratio is determined by specification data of steering mechanism  3 . 
     The following is a process of calculating the command current employed in this example. Target rotation angle calculating section  10   a  first calculates vehicle parameters by using the following vehicle model. 
     In general, assuming a two-wheel vehicle model, it is possible to express the vehicle yaw rate and lateral acceleration by the following mathematical expression (1). 
                       [           φ   ″                 V   ′     ⁢   y           ]     =         [           a   11           a   12               a   21           a   22           ]     ⁡     [           φ   ′             Vy         ]       +       [           b     f   ⁢           ⁢   1                 b     f   ⁢           ⁢   2             ]     ⁢   θ     +       [           b     r   ⁢           ⁢   1                 b     r   ⁢           ⁢   2             ]     ⁢   δ         ⁢     
     ⁢       a   11     =         -   2     ⁢     (         K   f     ⁢     L   f   2       +       K   r     ⁢     L   r   2         )           I   z     ⁢     V   x           ⁢     
     ⁢       a   12     =         -   2     ⁢     (         K   f     ⁢     L   f       -       K   r     ⁢     L   r         )           I   z     ⁢     V   x           ⁢     
     ⁢       a   21     =           -   2     ⁢     (         K   f     ⁢     L   f       -       K   r     ⁢     L   r         )         MV   x       -     V   x         ⁢     
     ⁢       a   22     =         -   2     ⁢     (       K   f     +     K   r       )         MV   x         ⁢     
     ⁢         b     f   ⁢           ⁢   1       =       2   ⁢           ⁢     K   f     ⁢     L   f           I   z     ⁢   N         ,       b     f   ⁢           ⁢   2       =       2   ⁢           ⁢     K   f       MN         ⁢     
     ⁢         b     r   ⁢           ⁢   1       =         -   2     ⁢           ⁢     K   r     ⁢     L   r         I   z         ,       b     r   ⁢           ⁢   2       =       2   ⁢           ⁢     K   r       M                 (   1   )               
In these equations, φ′=yaw rate (φ is yaw angle, and the superscript sign ′ indicates differentiation), Vy=lateral speed, θ=front wheel steer angle, δ=rear wheel steer angle, Iz=vehicle&#39;s inertia moment, M=vehicle weight, Lf=distance between the front axle and the center of gravity, Lr=distance between the center of gravity and the rear axle, N=gear ratio, Vx=longitudinal speed, Kf=front wheel cornering power, Kr=rear wheel cornering power, Cf=front wheel cornering force, Cr=rear wheel cornering force, and s=differential operator.
 
     To obtain a transfer function of the yaw rate to the front wheel steer angle from the state equation, the following mathematical expression (2) is obtained. 
                         φ   ′     ⁡     (   s   )         θ   ⁡     (   s   )         =           H   f     ⁡     (   s   )         G   ⁡     (   s   )         =           b     f   ⁢           ⁢   1       ⁢   s     +     (         a   12     ⁢     b     f   ⁢           ⁢   2         -       a   22     ⁢     b     f   ⁢           ⁢   1           )         G   ⁡     (   s   )                   (   2   )               
In this equation, G(s)=s 2 −(a 11 +a 22 )s+(a 11 a 22 −a 12 a 21 ).
 
     From expression (2), the yaw rate transfer function is expressed by: 
                       φ   ′     ⁡     (   s   )       =               ω     φ   ′       ⁡     (   V   )         φ   ′     2     ⁢     (         T   ⁡     (   V   )       ⁢   s     +       g     φ   ′       ⁡     (   V   )         )           s   2     +     2   ⁢           ⁢       ζ     φ   ′       ⁡     (   V   )       ⁢       ω     φ   ′       ⁡     (   V   )       ⁢   s     +         ω     φ   ′       ⁡     (   V   )       2         ⁢     θ   ⁡     (   s   )                 (   3   )               
In this equation,
 
                 g     φ   ′       ⁡     (   V   )       =           a   12     ⁢     b     f   ⁢           ⁢   2         -       a   22     ⁢     b     f   ⁢           ⁢   1                 a   11     ⁢     a   22       -       a   12     ⁢     a   21                         2   ⁢           ⁢       ζ     φ   ′       ⁡     (   V   )       ⁢       ω     φ   ′       ⁡     (   V   )         =       -     a   11       -     a   22                         ω     φ   ′       ⁡     (   V   )       2     =         a   11     ⁢     a   22       -       a   12     ⁢     a   21                         T     φ   ′       ⁡     (   V   )       =       b     f   ⁢           ⁢   1             a   11     ⁢     a   22       -       a   12     ⁢     a   21                 
Thus, it is possible to determine the vehicle parameters g φ′ (V), ζ φ′ (V), ω φ′ (V) and T φ′ (V).
 
     Then, target rotation angle calculating section  10   a  determines a desired target yaw rate φ′* from the vehicle speed V, the vehicle parameters, and target parameters (mentioned later). 
     From mathematical expression (3), a desired target yaw acceleration φ″* (asterisk indicates a target value) is obtained as follows: 
     
       
         
           
             
               
                 
                   
                     
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     The (target) parameters of target yaw rate φ′* are given by the following equations.
 
 g φ′*( V )= g φ″( V )× y rate_gain_map
 
ωφ′*( V )=ωφ′( V )× y rate_omega_map
 
ζφ′*( V )=ζφ′( V )× y rate_zeta_map
 
 T φ′*( V )= T φ′( V )× y rate_zero_map
 
In these equations, yrate_gain_map, yrate_omega_map, yrate_zeta_map, yrate_zero_map are tuning parameters.
 
     Therefore, the target yaw rate is given by:
 
φ′*( s )=φ″*( s )/ s   (5)
 
The target front wheel steer angleθ* is given by:
 
φ″*= a   11 φ′*+ a   12   Vy+b   f1 θ*  (6)
 
θ*=(φ″*− a   11 φ′*− a   12   Vy )/ b   f1   (7)
 
Target rotation angle calculating section  10   a  calculates the target rotation angle θ h  of gear ratio varying actuator  6 , required to achieve the target front wheel steer angle θ*.
 
     Steer angle servo control section  10   b  calculates a deviation e of actual rotation angle θ a  of gear ratio varying actuator  6  from target rotation angle θ h  by using the following equation (8), and then calculates a current command I θ  from the deviation e, a proportional gain P, a derivative gain D and an integral gain I by using the following equation (9).
 
 e=θ   h −θ a   (8)
 
 I   θ   =P×e+P×de/dt+I×∫edt   (9)
 
The proportional gain P, derivative gain D and integral gain I are tuning constants.
 
       FIG. 3  shows a gear ratio control process according to the first embodiment in the form of a flowchart. Gear ratio controller  10  performs this control process in the case of the returning operation (or angle decreasing or subtracting operation) of gear ratio varying actuator  6 . 
     At a step S 1 , controller  10  reads the steering wheel angle θw sensed by steering angle sensor  7  and the vehicle speed (vehicle body speed) V sensed by vehicle speed sensor  11 , for the target rotation angle calculation section  10   a . After S 1 , controller  10  proceeds to a step S 2 . 
     At step S 2 , controller  10  examines whether the steering wheel  1  is in a neutral zone (at or near the neutral position) or not, by examining whether steering wheel angle θw obtained at S 1  is greater than a predetermined value −θ 1  and at the same time the steering wheel angle θw is smaller than a predetermined value θ 1 (−θ 1 &lt;θ&lt;θ 1 ?), as a function of target rotation angle calculating section  10   a . From S 2 , controller  10  proceeds to a step S 7  when the answer of S 2  is affirmative, and to a step S 3  when the sensed steering wheel angle is not in the neutral zone (between −θ 1  and θ 1 ). In this example, step S 2  examines whether the absolute value of steering wheel angle θw is smaller than a (positive) predetermined first angle (θ 1 ). 
     At step S 3 , controller  10  examines, as a function of target rotation angle calculating section  10   a , whether the steering wheel is operated beyond an adjacent zone or not. The adjacent zone is a zone to vary the gear ratio gradually. In this example, controller  10  examines whether the sensed steering wheel angle θw is smaller than a predetermined value −θ 2  or the steering wheel angle θw is greater than a predetermined value θ 2 . From S 3 , controller  10  proceeds to a step S 8  when the answer of S 3  is affirmative, and to a step S 4  when the steering wheel angle is still within the adjacent zone (between −θ 2  and θ 2 , outside the neutral zone). In this example, step S 3  examines whether the absolute value of steering wheel angle θw is greater than a (positive) predetermined second angle (θ 2 ) greater than the predetermined first angle (θ 1 ). 
     At step S 4 , controller  10  sets the target gear ratio G to a ratio value G 3  which is intermediate between ratio values G 1  and G 2 , as a function of target rotation angle calculating section  10   a , and then proceeds to a step S 5 . In this case, controller  10  determines the gear ratio so as prevent a variation of the gear ratio with respect to the steering wheel angle from causing unnatural feeling to the driver, by using, for example, technique of liner interpolation between G 1  and G 2  and θ 1  and θ 2 . Thereafter, the gear ratio G is set finally to the ratio value G 3  (G=G 3 ). The first ratio value G 1  is greater than the mechanical gear ratio whereas the second ratio value G 2  is smaller than the mechanical gear ratio (as shown in  FIG. 9 ). Gear ratio value G 3  is a value derived from G 1  and G 2 . 
     At step S 5 , controller  10  calculates the target rotation angle θh from the target gear ratio G determined at S 4 , S 7  or S 8 , as a function of target rotation angle calculating section  10   a , and then proceeds to a step S 6 . Thus, at S 5 , controller  10  determines the command θh to achieve target gear ratio G. 
     At step S 6 , controller  10  performs a servo control operation to reduce the deviation between the actual rotation angle θa sensed by rotation angle sensor  8  and the target rotation angle θh determined at S 5 , as a function of servo control section  10   b , and then proceeds to an end (RETURN) of the process of  FIG. 3 . 
     At step S 7 , controller  10  sets target gear ratio G to the first ratio value G 1  on the quick side of the mechanical gear ratio, and then proceeds to step S 5 . At step S 8 , controller  10  sets target gear ratio G to the second ratio value G 2  on the slow side of the mechanical gear ratio, and then proceeds to step S 5 . 
     The thus-constructed steering control system is operated as follows: In the steering control system of the before-mentioned Japanese patent document JP2005-170129, there is provided, in a linkage between a steering wheel and front wheels, a gear ratio varying or variable gear ratio actuator having a coaxial motor for varying the steering gear ratio by adding or subtracting a rotation angle to or from the steering wheel angle inputted through the steering wheel. This control system increases a steering stiffness or rigidity by increasing a servo gain in the vicinity of the steering neutral position. 
     This gear ratio varying actuator outputs a torque in accordance with a product obtained by multiplying the deviation between the target rotation angle and actual rotation angle, by the servo gain. However, in a small steering wheel angle zone at or near the steering neutral position, the target rotation angle is small, and the deviation of the actual rotation angle from the target rotation angle tends to be so small as to make it difficult to produce an actuator torque. Accordingly, the actuator torque tends to yield to a steering reaction force which is a load applied from the road surface, and the gear ratio varying actuator tends to be somewhat poor in following up performance or response. 
     Therefore, the control system of the above-mentioned Japanese patent document is arranged to increase the servo gain in the case of a steering operation within the vicinity of the neutral position, and thereby produces a greater steering torque even if the deviation is very small, thereby to cause the actual rotation angle to faithfully follow the target rotation angle. By increasing the actuator torque with a greater servo gain, the system can improve the response of the gear ratio varying actuator and improve a feeling of steering stiffness near the steering neutral position. 
     In this system, however, the gear ratio varying actuator is arranged to produce an actuator torque in a direction to reduce the deviation between the actual rotation angle and the target rotation angle. Therefore, in the case of an actuator&#39;s returning operation of decreasing the gear ratio below the mechanical gear ratio, the actuator torque acts in the direction opposite to the steering direction at the beginning of a steering operation from the neutral position, and tends to cause undesired letdown or decrease of the steering reaction. 
     The steering reaction is let down or decreased in the following manner. When, as shown in  FIG. 4 , the direction of the motor torque (actuator torque) is the same as the direction of the steering force (the steering direction), the motor reaction (actuator reaction) acts in the direction opposite to the steering force, and thereby provides a feeling of sufficient response or resistance to the driver. When, on the other hand, as shown in  FIG. 5 , the motor torque and the steering force act in the opposite directions, the motor reaction acts in the same direction as the steering force, and thereby decreases the response or resistance to the driver. When the motor torque is produced in the opposite direction to the steering direction at the start of a steering operation from the steering neutral position, the gear ratio varying actuator may provide the driver a feeling of letdown or loss of the response or the steering reaction. 
     In the variable gear ratio control, in general, the gear ratio is set to the quick side (an angle increase or addition by the actuator) by addition of the actuator rotation angle to the steering angle in a low and medium vehicle speed region. In a high vehicle speed region, the gear ratio is set to the slow side (an angle decrease or subtraction by the actuator) by subtraction of the actuator rotation angle from the steering wheel angle. 
     In the situation of the actuator&#39;s angle increase operation to set the gear ratio to the quick side, the control system determines, by calculation, the target rotation angle to drive the gear ratio varying actuator in the same direction as the steering direction. Therefore, the actuator torque is produced in the same direction as the steering direction, as shown in  FIGS. 7A˜7D . From the start of a steering operation, the actuator torque rises in the steering direction. 
     In the situation of the actuator&#39;s angle decrease or subtracting operation to set the gear ratio to the slow side, however, the actuator torque behaves in a different manner as shown in  FIGS. 8A˜8D . Because of an internal gear friction inside the gear ratio varying actuator, the gear ratio varying actuator is rotated in the steering direction together by the gear friction at the start of the steering operation. 
     Thereafter, the actuator torque of the gear ratio varying actuator starts following the target rotation angle with a delay. In this case, the direction of the actuator torque is dependent on the direction of the deviation of the actual rotation angle from the target rotation angle, and the torque is produced in the direction to reduce the deviation. As a result, the actuator torque is produced initially in the opposite direction to the steering direction. Thus, the rotation of the actuator is delayed, as shown in  FIG. 8B , because a force is transmitted by the gear friction within the actuator at the initial stage of the steering operation. The torque is generated in the direction opposite to the direction of the steering force (steering direction) due to the delay in the rotation, as shown in  FIG. 8C . Consequently, the steering force could be decreased temporarily, as a letdown in force, as shown in  FIG. 8D . 
     Because the direction of the actuator torque is determined by the direction of the deviation, the system merely arranged to increase the servo gain near the steering neutral position as disclosed in the before-mentioned Japanese patent document JP2005-170129 is unable to avoid the opposition between the direction of the actuator torque and the steering direction, and accordingly unable to prevent undesired initial letdown of the steering reaction. 
     By contrast, the vehicle steering control system according to the first embodiment prevents the letdown of the steering reaction at the initial stage of a steering operation in the following manner. As shown in  FIG. 9 , for improvement against the initial letdown of the steering reaction at the initial stage of the steering operation, the gear ratio G is set to the quick side (G 1 ) only in the neutral zone (−θ 1 &lt;θ&lt;θ 1 )(S 1 →S 2 →S 7 →S 5 →S 6 ). 
     When steering wheel  1  is turned from a position near the neutral position, therefore, the control system can increase the actuator torque in the steering direction from the start of the steering operation, and thereby prevent undesired letdown of the steering reaction, as shown in  FIGS. 10A˜10D . 
     Then, as the steering wheel angle (the absolute value of the steering wheel angle) increases beyond the neutral zone (−θ 1 &lt;θ&lt;θ 1 ), the control system varies the gear ratio G to the slow side in such a gradual manner as to prevent an unnatural feeling to the driver (S 1 →S 2 →S 3 →S 4 →S 5 →S 6 ). 
     When the steering wheel angle (the absolute value of the steering wheel angle) is further increased (−θ 2 &gt;θ or θ 2 &lt;θ), the control system sets the gear ratio G to the slow side (G 2 ) (S 1 →S 2 →S 3 →S 8 →S 5 →S 6 ). If the gear ratio is held on the quick side even in a larger steering wheel angle region, there arises a possibility that the yawing response is held high, the vehicle yaw rate is produced beyond the target yaw rate, and the vehicle stability is degraded. Therefore, when the steering wheel angle increases to some degree, the steering control system of this embodiment decreases the yawing response, and improve the stability by varying the steering gear ratio G to the slow side corresponding to the target yaw rate. 
     The steering control system according to this embodiment performs the operation to produce the actuator torque in the steering direction only in the case of the angle decreasing or returning actuator operation of gear ratio varying actuator  6 . If gear ratio G is adjusted to the quick side in the angle increasing actuator operation, there is a possibility that the yawing response becomes too high, and the vehicle behavior become unstable. Therefore, the steering control system of this embodiment improves the stability of the yawing response by holding the gear ratio G unchanged at the time of the angle increasing actuator operation. 
     The first embodiment can provide the following effects at least. 
     (1) First, gear ratio controller  10  produces an actuator torque of gear ratio varying actuator  6  in the direction identical to the steering direction at an initial stage of a driver&#39;s steering operation. Therefore, the control system according to the first embodiment can prevent underside letdown of the steering reaction by matching the direction of the actuator torque to the steering direction in response to the driver&#39;s steering operation. 
     (2) Second, gear ratio controller  10  produces an actuator torque in the same direction as the steering direction at the start of the steering operation in the neutral zone or small steering angle zone where the absolute value of the steering wheel angle θw is small (smaller than predetermined value θ 1 ). Therefore, the control system of the first embodiment can prevent undesired letdown of the steering reaction, without exerting undesired influence on the vehicle behavior or target vehicle behavior (such as yaw rate or target yaw rate), at the initial stage of the steering operation. 
     (3) Gear ratio controller  10  produces an actuator torque in the same direction as the steering direction only when the actuator varies the gear ratio by subtracting a rotation angle from the steering input. Therefore, the control system of the first embodiment can prevent undesired letdown of the steering reaction without exerting undesired influence on the vehicle behavior or target vehicle behavior (such as yaw rate or target yaw rate) at the initial stage of the steering operation. 
     (4) Gear ratio controller  10  produces an actuator torque in the same direction as the steering direction by adding an actuator&#39;s rotation angle or an actuator&#39;s angular input, to the driver&#39;s steering angular input. Therefore, the control system can hold the deviation e of the actual rotation angle θ a  of the actuator from the target rotation angle θ h  always positive, and thereby matches the direction of the actuator torque to the steering direction. 
     In a vehicle steering control system according to a second embodiment of the present invention, the control system directs the actuator torque in the same direction as the steering direction, by an offsetting operation to add a modification angle corresponding to the gear ratio, to the target rotation angle θ h . The steering control system of the second embodiment is substantially identical in construction to the control system shown in  FIGS. 1 and 2 , so that repetitive explanation is omitted. 
     While the gear ratio G is varied in accordance with the steering wheel angle θ w  in the first embodiment, the target rotation angle θ h  is increased or offset by addition of the modification angle corresponding to a variation of the gear ratio. With this offsetting operation, the control system performs the steer angle increasing actuator operation as in the first embodiment at the initial stage of the steering operation from the neutral position. 
     Therefore, in the control system of the second embodiment, the target rotation angle θ h  rises in the steering direction as shown in  FIG. 10B  when the steering wheel is turned from the neutral position, and the deviation between the actual rotation angle θ a  and target rotation angle θ h  is held positive invariably. Thus, the control system of the second embodiment can match the direction of the actuator torque to the steering direction. 
     The second embodiment can provide the following effect in addition to the first, second and third effects (1)˜(3) of the first embodiment. 
     Gear ratio controller  10  according to the second embodiment modifies the target rotation angle θ h  of gear ratio varying actuator  6  in the vicinity of the neutral position (or in the neutral zone of −θ 1 &lt;θ&lt;θ 1 ) so that target rotation angle θ h  rises initially in the same direction as the steering direction. Therefore, the control system of the second embodiment can hold the deviation e of the actual rotation angle θ a  of the actuator from the target rotation angle θ h  always positive, and thereby match the direction of the actuator torque to the steering direction. 
       FIG. 11  is a view for illustrating a steering control system according to a third embodiment. In the third embodiment, the control system is arranged to match the direction of the actuator torque to the steering direction by modifying the deviation of actual rotation angle θ a  from target rotation angle θ h . The steering control system of the third embodiment is substantially identical in construction to the control system shown in  FIGS. 1 and 2 ; so that repetitive explanation is omitted. The gear ratio controller  10  according to the third embodiment performs a gear ratio control process shown in  FIG. 11   
     At a step S 11 , controller  10  ascertains the target rotation angle θ h  and actual rotation angle θ a  as a function of steer angle servo control section  10   b , and then proceeds to a next step S 12 . At step S 12 , controller  10  calculates the deviation e from target rotation angle θ h  and actual rotation angle θa according to the equation (8)(e=θ h −θ a ), and then proceeds to a step S 13 . 
     At step S 13 , controller  10  detects the steering direction, and then proceeds to a step S 14 . It is possible to detect the steering direction by checking the direction of a produced steering torque, or the direction of a steering angular speed, for example. 
     At step S 14 , controller  10  examines whether the direction of the deviation e is in agreement with the steering direction determined at S 13 , or not. From S 14 , controller  10  proceeds to a step S 16  when the direction of the deviation e is identical to the steering direction, and proceeds to a step S 15  when the direction of the deviation e is opposite to the steering direction. 
     At step S 15 , controller  10  modifies the deviation e toward a region in which the direction of the deviation is the same as the steering direction. Then, controller  10  proceeds from S 15  to step S 16 . 
     At step S 16 , controller  10  calculates the current command I θ  according to the equation (9) from the deviation e determined at S 12  or modified at S 15 , and then terminates the process of  FIG. 11 . 
     The thus-constructed steering control system of the third embodiment modifies the value of deviation e of actual rotation angle (θ a ) from target rotation angle (θ h ), so as to change the direction of the deviation e toward the steering direction, if the direction of the deviation is opposite to the steering direction, to prevent an actuator torque from being produced in the opposite direction to the steering direction at the start of a steering operation from the steering neutral position, by taking the course of S 11 →S 2 →S 13 →S 14 →S 15 →S 16 . By so doing, the control system can prevent the actuator torque from being produced in the opposite direction to the steering direction, and thereby prevent undesired decrease or letdown of the steering reaction at the initial stage of a steering operation, as shown in  FIGS. 12A ,  12 B,  12 C and  12 D. 
     The third embodiment can provide the following effect in addition to the first, second and third effects (1)˜(3) of the first embodiment. 1 
     Gear ratio controller  10  modifies the control deviation e of gear ratio varying actuator  6  in response to a driver&#39;s steering operation from the neutral position, so as to produce an actuator torque in the same direction as the steering direction. Therefore, the control system of the third embodiment can restrain the direction of the actuator torque from becoming opposite to the steering direction, and thereby prevent undesired decrease or letdown of the steering reaction. 
     The present invention is not limited to the first, second and third embodiments. Various variations and modifications are possible within the purview of the present invention. For example, instead of sensing the actual rotation angle of gear ratio varying actuator  6  with the motor rotation sensor  8 , it is possible to employ an arrangement to sense an actual front wheel steer angle, for example, from the rotation angle of the pinion shaft, and to calculate the actual rotation angle of the gear ratio varying actuator from the actual front wheel steer angle. 
     According to the first, second and third embodiments, a vehicle steering control apparatus comprises: an actuator to vary a steering gear ratio of a wheel steer angle of a vehicle to a steering wheel angle; and a controller to control the actuator so as to reduce a deviation of an actual rotation angle of the actuator from a target rotation angle to achieve a desired target steering gear ratio, and to modify the deviation so as to prevent a torque from being produced by the actuator in an opposite direction opposite to a steering direction at the start of a driver&#39;s steering operation. The controller may be configured to modify the deviation so as to prevent a torque from being produced by the actuator in the opposite direction opposite to the steering direction only when (the absolute value of) the steering wheel angle is smaller than a predetermined angle (such as θ 2  or θ 1 ). The steering control apparatus may further comprise a vehicle condition sensor (such as sensor  7  and/or sensor  11 ) to sense a vehicle operating condition; and the controller may be configured to determine the target rotation angle in accordance with the vehicle operating condition. 
     According to the first, second and third embodiments, a vehicle steering control method comprises: producing a control signal to control an actuator to vary a steering gear ratio of a vehicle wheel steer angle to a steering wheel angle; and modifying the control signal so as to prevent a torque from being produced by the actuator in an opposite direction opposite to a steering direction at the start of a driver&#39;s steering operation. The method element of producing the control signal to control the actuator for varying the steering gear ratio may comprise (i) determining a target rotation angle of the actuator in accordance with a vehicle operating condition, (ii) determining a deviation of an actual rotation angle of the actuator from the target rotation angle, and (iii) producing the control signal in a feedback control mode to reduce the deviation; and the method element of modifying the control signal may comprise modifying the deviation so as to prevent a torque from being produced by the actuator in the opposite direction opposite to the steering direction at the start of a driver&#39;s steering operation. 
     According to one aspect of the present invention, a vehicle steering control apparatus comprises: means for controlling an actuator to vary a steering gear ratio of a vehicle wheel steer angle to a steering wheel angle by producing a control signal; and means for modifying the control signal so as to prevent a torque from being produced by the actuator in an opposite direction opposite to a steering direction at the start of a driver&#39;s steering operation. The controlling means may comprise means for determining a target rotation angle of the actuator in accordance with a vehicle operating condition, means for determining a deviation of an actual rotation angle of the actuator from the target rotation angle, and means for producing the control signal in a feedback control mode to reduce the deviation; and the modifying means may comprise means for modifying the deviation so as to prevent a torque from being produced by the actuator in the opposite direction opposite to the steering direction at the start of a driver&#39;s steering operation. 
     This application is based on a prior Japanese Patent Application No. 2007-201478 filed on Aug. 2, 2007. The entire contents of this Japanese Patent Application No. 2007-201478 are hereby incorporated by reference. 
     Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art in light of the above teachings. The scope of the invention is defined with reference to the following claims.