Abstract:
A vehicle steering control device capable of executing automatic steering and manual steering includes: an automatic control amount-setting device that sets a target automatic control amount based on information about vehicle behavior and ambient environment; a steering control amount-setting device that sets a target steering amount based on a manual steering amount input by a driver and the set target automatic control amount; and a steering device that steers based on the set target steering amount. When the target automatic control amount contributes more in the setting of the target steering amount than the manual steering amount does, the steering control amount-setting device sets the target steering amount so as to better the responsiveness in the lateral shift of the vehicle relatively to the responsiveness in the turning of the vehicle, in comparison with when the target automatic amount does not contribute more than the manual steering amount.

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2006-280330 filed on Oct. 13, 2006 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a vehicle steering control device and a vehicle steering control method and, more particularly, to a vehicle steering control device and vehicle steering control method that change control rules at the time of automatic steering. 
     2. Description of the Related Art 
     As a related-art vehicle steering control device, Japanese Patent Application Publication No. 2005-324744 (JP-A-2005-324744), for example, discloses a vehicular automatic steering device capable of selecting and disengaging an automatic steering control. In this disclosed technology, when the automatic steering control is selected, a front wheel steering control device starts the automatic steering control after a rear wheel steering control has stopped at a neutral position. When the selection of the automatic steering control is cancelled, the rear wheel steering control device starts the rear wheel steering control after the automatic steering control has stopped. Thus, this technology substantially avoids the discomfort caused to the driver and the like and prevents the wobbling of the vehicle at the time of the automatic steering control. 
     However, even with the technology in which the start timing of the automatic steering control is prescribed as described above, it sometimes happens that at the time of the automatic steering, the vehicle is not stable and the driver feels discomfort, in comparison with the time of the manual steering performed by the driver. 
     SUMMARY OF THE INVENTION 
     The invention provides a vehicle steering control device that can further stabilize the vehicle and make the driver feel secured and comfortable at the time of automatic steering. 
     A first aspect of the invention provides a vehicle steering control device capable of executing automatic steering and manual steering characterized by including: an automatic control amount-setting device that sets a target automatic control amount based on information about behavior of a vehicle and about ambient environment; a steering control amount-setting device that sets a target steering amount based on a manual steering amount input by a driver and the target automatic control amount set by the automatic steering amount-setting device; and a steering device that steers based on the target steering amount set by the steering control amount-setting device, wherein when a contribution of the target automatic control amount is larger in setting of the target steering amount than the contribution of the manual steering amount, the steering control amount-setting device sets the target steering amount so as to better a responsiveness in a lateral shift of the vehicle relatively to the responsiveness in turning of the vehicle, in comparison with when the contribution of the target automatic control amount is equal to or smaller in the setting of the target steering amount than the contribution of the manual steering amount. 
     According to this construction, when the contribution of the target automatic control amount is larger in the setting of the target steering amount than the contribution of the manual steering amount, the steering control amount-setting device sets the target steering amount so as to better the responsiveness in the lateral shift of the vehicle relatively to the responsiveness in the turning of the vehicle, in comparison with when the contribution of the target automatic control amount is equal to or smaller in the setting of the target steering amount than the contribution of the manual steering amount. When the construction of the target automatic control amount is larger in the setting of the target steering amount than the contribution of the manual steering amount, that is, when the vehicle is running under the automatic control, it is generally often the case that the responsiveness in the lateral shift is required more than the responsiveness in the turning at the time of the execution of a constant-speed run control or the like. Therefore, since during the automatic control, the steering control amount-setting device sets the target steering amount so as to better the responsiveness in the lateral shift relatively to the responsiveness of the turning, the stabilization of the vehicle can be further improved and the driver can be caused to feel secured and comfortable during the automatic steering. 
     The automatic control amount-setting device may set the target automatic control amount so that a predetermined vehicle speed is maintained, and therefore can be applied to the time of the CC control (cruise control). 
     Furthermore, the automatic control amount-setting device may set the target automatic control amount so that an inter-vehicle distance to a preceding vehicle is kept at a predetermined value, and therefore can be applied to the time of the ACC control (adaptive cruise control). 
     Furthermore, the automatic control amount-setting device may set the target automatic control amount so that the vehicle stays and runs in a predetermined lane, and therefore can be applied to the time of the LKA (lane keeping assist) control. 
     Furthermore, when the contribution of the target automatic control amount is larger in the setting of the target steering amount than the contribution of the manual steering amount, the steering control amount-setting device may set the target steering amount so that a phase delay of a lateral acceleration of the vehicle relative to the steering amount becomes small, in comparison with when the contribution of the target automatic control amount is equal to or smaller in the setting of the target steering amount than the contribution of the manual steering amount. 
     According to this construction, since during the automatic control, the steering control amount-setting device sets the target steering amount so that the phase delay of the lateral acceleration relative to the steering amount becomes small, it is possible to improve the responsiveness in the lateral shift during the automatic control. 
     Furthermore, the steering device may steer a front wheel and a rear wheel individually based on the target steering amount, and when the contribution of the target automatic control amount is larger in the setting of the target steering amount than the contribution of the manual steering amount, the steering control amount-setting device may set the target steering amount so that the steering amount of the rear wheel becomes large, in comparison with when the contribution of the target automatic control amount is equal to or smaller in the setting of the target steering amount than the contribution of the manual steering amount. 
     According to this construction, the steering device steers the front wheel and the rear wheel individually based on the target steering amount, and the steering control amount-setting device sets the target steering amount so that the steering amount of the rear wheel becomes large at the time of the automatic control. Therefore, the amount of the turning is restrained and, on the other hand, the responsiveness in the lateral shift further improves, so that the stabilization of the vehicle can be further improved and the driver can be caused to feel secured and comfortable. 
     A second aspect of the invention provides a vehicle steering control method capable of executing automatic steering and manual steering characterized by including: setting a target automatic control amount based on information about behavior of a vehicle and about ambient environment; setting a target steering amount based on a manual steering amount input by a driver and the set target automatic control amount; and steering based on the set target steering amount, wherein when a contribution of the target automatic control amount is larger in setting of the target steering amount than the contribution of the manual steering amount, the target steering amount is set so as to better a responsiveness in a lateral shift of the vehicle relatively to the responsiveness in turning of the vehicle, in comparison with when the contribution of the target automatic control amount is equal to or smaller in the setting of the target steering amount than the contribution of the manual steering amount. 
     According to the vehicle steering control device and method of the aspects of the invention, it is possible to further improve the stability of the vehicle at the time of the automatic steering and cause the driver to feel secured and comfortable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and further objects, features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: 
         FIG. 1  is block diagram showing a construction of a vehicle steering control device in accordance with a first embodiment of the invention; 
         FIG. 2  is a block diagram showing a construction of an A4WS-ECU in accordance with the first embodiment; 
         FIG. 3  is a flowchart showing an operation of the vehicle steering control device in accordance with the first embodiment; 
         FIG. 4  is a block diagram showing a control rule of a computation portion at the time of the manual steering in accordance with the first embodiment; 
         FIG. 5  is a block diagram showing a control rule of the computation portion at the time of the automatic steering in accordance with the first embodiment; 
         FIG. 6  is a Bode diagram showing a frequency response of the lateral acceleration of the control rule in accordance with the first embodiment; 
         FIG. 7  is a diagram showing a control map of the yaw rate gain in accordance with a second embodiment; 
         FIG. 8  is a diagram showing a control map of the slip angle gain in accordance with the second embodiment; 
         FIG. 9  is a diagram showing a control map of the slip angle advancement time in accordance with the second embodiment; 
         FIG. 10  is a block diagram showing a control rule of a computation portion at the time of the automatic steering in accordance with the second embodiment; 
         FIG. 11  is a Bode diagram showing a frequency response of the lateral acceleration of the control rule in accordance with the second embodiment; 
         FIG. 12  is a block diagram showing a construction of an A4WS-ECU in accordance with a third embodiment; and 
         FIG. 13  is a flowchart showing an operation of the vehicle steering control device in accordance with the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, vehicle steering control devices in accordance with embodiments of the invention will be described with reference to the accompanying drawings. 
       FIG. 1  is a block diagram showing a construction of a vehicle steering control device in accordance with a first embodiment of the invention. A vehicle steering control device of this embodiment is constructed so that either one of the manual steering and the automatic steering can be executed during the four-wheel steering, and steering control can be performed so as not to cause uncomfortable feeling to a driver at the time of the automatic steering. 
     As shown in  FIG. 1 , a vehicle steering control device  10  of this embodiment includes a steering handle  12  that a driver operates at the time of the manual steering, a VGRS (variable gear ratio steering) actuator  22  that changes the gear ratio of the front wheel steering, and a front steer actuator  24  that performs the steering of the front wheels. The vehicle steering control device  10  also includes a steering angle sensor  14  that detects the steering angle, and a vehicle speed sensor  16  that detects the vehicle speed. The information detected by these sensors is output to an A4WS (active four-wheel steering)-ECU  18 . 
     The vehicle steering control device  10  includes a CC-ECU  30  that performs the CC control (cruise control), an ACC-ECU  32  that performs the ACC control (adaptive cruise control), and an LKA-ECU  34  that performs the LKA control (lane keeping assist control). These ECUs set predetermined target automatic control amounts on the basis of the vehicle behaviors and ambient environments detected by sensors and the like, such as the steering angle sensor  14 , the vehicle speed sensor  16 , etc. The CC-ECU  30 , the ACC-ECU  32  and the LKA control LKA-ECU  34  function as an automatic control amount-setting device. The target automatic control amounts set by the CC-ECU  30 , the ACC-ECU  32  and the LKA control LKA-ECU  34  are output to the A4WS-ECU  18 . 
     The A4WS-ECU  18  is provided for setting target steering amounts on the basis of the manual steering amount input by the driver via the steering handle  12  as well as the target automatic control amounts set by the CC-ECU  30 , the ACC-ECU  32  and the LKA-ECU  34 . The A4WS-ECU  18  functions as a steering control amount-setting device. The A4WS-ECU  18  outputs the target steering amount set in this manner to a VGRS-ECU  20 . The VGRS-ECU  20  outputs a drive signal to the VGRS actuator  22  so as to drive the front steer actuator  24 . In this manner, the VGRS-ECU  20  performs the automatic steering of the front wheels. Besides, the A4WS-ECU  18  outputs the target steering amount set in the foregoing manner to a rear steer ECU  26 . The rear steer ECU  26  performs the automatic steering of the rear wheels by outputting a drive signal to a rear steer actuator  28  to drive the rear steer actuator  28 . Therefore, the front steer actuator  24  and the rear steer actuator  28  function as a steering device. 
     Next, operations of the vehicle steering control device of the embodiment will be described. Firstly, a steering control in the two-wheel steering will be described. The control rule in the two-wheel steering is expressed by the following expressions (1) to (3). 
     
       
         
           
             
               
                 
                   EXPRESSION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       γ 
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                     
                       δ 
                       ⁡ 
                       
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                         s 
                         ) 
                       
                     
                   
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                         ω 
                         n 
                         2 
                       
                       
                         
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                           2 
                         
                         + 
                         
                           2 
                           · 
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                           · 
                           
                             ω 
                             n 
                           
                           · 
                           s 
                         
                         + 
                         
                           ω 
                           n 
                           2 
                         
                       
                     
                     · 
                     
                       G 
                       γ 
                     
                     · 
                     
                       ( 
                       
                         1 
                         + 
                         
                           
                             T 
                             γ 
                           
                           · 
                           s 
                         
                       
                       ) 
                     
                     · 
                     
                       
                         
                           δ 
                           f 
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                       
                         δ 
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       β 
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                     
                       δ 
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                   
                   = 
                   
                     
                       
                         ω 
                         n 
                         2 
                       
                       
                         
                           s 
                           2 
                         
                         + 
                         
                           2 
                           · 
                           ζ 
                           · 
                           
                             ω 
                             n 
                           
                           · 
                           s 
                         
                         + 
                         
                           ω 
                           n 
                           2 
                         
                       
                     
                     · 
                     
                       G 
                       β 
                     
                     · 
                     
                       ( 
                       
                         1 
                         + 
                         
                           
                             T 
                             β 
                           
                           · 
                           s 
                         
                       
                       ) 
                     
                     · 
                     
                       
                         
                           δ 
                           f 
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                       
                         δ 
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       L 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         A 
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                     
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                       ⁡ 
                       
                         ( 
                         s 
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                         ω 
                         n 
                         2 
                       
                       
                         
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                           2 
                         
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                           · 
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                           · 
                           
                             ω 
                             n 
                           
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                         + 
                         
                           ω 
                           n 
                           2 
                         
                       
                     
                     · 
                     
                       G 
                       LA 
                     
                     · 
                     
                       ( 
                       
                         
                           
                             
                               
                                 
                                   T 
                                   
                                     LA 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                   
                                 
                                 · 
                                 
                                   s 
                                   2 
                                 
                               
                               + 
                             
                           
                         
                         
                           
                             
                               
                                 
                                   T 
                                   
                                     LA 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     1 
                                   
                                 
                                 · 
                                 s 
                               
                               + 
                               1 
                             
                           
                         
                       
                       ) 
                     
                     · 
                     
                       
                         
                           δ 
                           f 
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                       
                         δ 
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     In the foregoing expression, γ is the yaw rate; β is the slip angle; LA is the lateral acceleration; ζ is the damping coefficient determined from the vehicle specifications and the vehicle speed; ω n  is the eigenfrequency determined from the vehicle specifications and the vehicle speed; G γ  is the yaw rate gain determined from the vehicle specifications and the vehicle speed; G β  is the slip angle gain determined from the vehicle specifications and the vehicle speed; G LA  is the lateral acceleration gain determined from the vehicle specifications and the vehicle speed; T γ  is the yaw rate advancement time determined from the vehicle specifications and the vehicle speed; T β  is the slip angle advancement time determined from the vehicle specifications and the vehicle speed; T LA1  is the lateral acceleration advancement time 1 determined from the vehicle specifications and the vehicle speed; T LA2  is the lateral acceleration advancement time 2 determined from the vehicle specifications; δ is the steering handle angle; and δ f  is the front wheel steering angle. 
     The value δ/δ f  is the steering gear ratio. For example, δ/δ f =18.1. In the case of two-wheel steering, when the steering handle is turned or operated, the front wheel steering angle is determined, so that the yaw rate, the slip angle and the lateral acceleration are determined. 
       FIG. 2  is a block diagram showing a construction of the A4WS-ECU in accordance with the first embodiment. In the case of four-wheel steering as in this embodiment, if the vehicle speed V, the steering handle angle δ, a target yaw rate γ* and a target slip angle β* are given to a computation portion  36  of the A4WS-ECU  18 , the front wheel steering angle δ f  and the rear wheel steering angle δ r  are determined. 
       FIG. 3  is a flowchart showing an operation of the vehicle steering control device in accordance with the first embodiment. This control operation is repeatedly executed at predetermined timing during a period from the turning on of the electric power source of the vehicle until the turning off thereof. As shown in  FIG. 3 , in the case where the vehicle steering control device  10  of the embodiment is performing an arbitrary four-wheel steering control (S 11 ), the target yaw rate γ* and the target slip angle β* become as shown in the expressions (4) and (5) below. 
     
       
         
           
             
               
                 
                   EXPRESSION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         γ 
                         * 
                       
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                     
                       δ 
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                   
                   = 
                   
                     
                       G 
                       γ 
                       * 
                     
                     · 
                     
                       1 
                       
                         1 
                         + 
                         
                           T 
                           · 
                           s 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         β 
                         * 
                       
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                     
                       δ 
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                   
                   = 
                   
                     
                       G 
                       β 
                       * 
                     
                     · 
                     
                       1 
                       
                         1 
                         + 
                         
                           T 
                           · 
                           s 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     At this time, the control rule of the target lateral acceleration LA* becomes as shown in the expression (6) below. As shown in  FIG. 4 , the computation portion  36  of the A4WS-ECU  18  calculates a front wheel steering angle δ* f  and a rear wheel steering angle δ* r  corresponding to the target lateral acceleration LA*. 
     
       
         
           
             
               
                 
                   EXPRESSION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         
                           
                             L 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 A 
                                 * 
                               
                               ⁡ 
                               
                                 ( 
                                 s 
                                 ) 
                               
                             
                           
                           
                             δ 
                             ⁡ 
                             
                               ( 
                               s 
                               ) 
                             
                           
                         
                         = 
                         
                           V 
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   
                                     γ 
                                     * 
                                   
                                   ⁡ 
                                   
                                     ( 
                                     s 
                                     ) 
                                   
                                 
                                 
                                   δ 
                                   ⁡ 
                                   
                                     ( 
                                     s 
                                     ) 
                                   
                                 
                               
                               + 
                               
                                 s 
                                 · 
                                 
                                   
                                     
                                       β 
                                       * 
                                     
                                     ⁡ 
                                     
                                       ( 
                                       s 
                                       ) 
                                     
                                   
                                   
                                     δ 
                                     ⁡ 
                                     
                                       ( 
                                       s 
                                       ) 
                                     
                                   
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           V 
                           · 
                           
                             
                               
                                 G 
                                 γ 
                                 * 
                               
                               + 
                               
                                 
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                                   β 
                                   * 
                                 
                                 · 
                                 s 
                               
                             
                             
                               1 
                               + 
                               
                                 T 
                                 · 
                                 s 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     Incidentally, in the expression (6), the relationship of the following expression (7) holds.
 
EXPRESSION 4
 
 G*   62    ≠T·G*   γ   (7)
 
     In the vehicle steering control device  10  of the embodiment, when the contribution of the target automatic control amount is less than or equal to the manual steering amount in the setting of the target steering amount, that is, when the manual control is equivalent or more dominant in the setting of the target steering amount than the automatic control, in other words, when the steering is based on the manual control, target values are prepared as in the foregoing expressions (4) to (6). During this state, the lateral shift and the turning of the vehicle become substantially equal in responsiveness.  FIG. 6  is a Bode diagram showing a frequency response of the lateral acceleration of the control rule in accordance with the first embodiment. As shown in  FIG. 6 , regarding a curve A of the frequency response of the lateral acceleration at the time of the manual steering, the higher the frequency, the lower the gain becomes, and the greater the phase delay becomes. In  FIG. 6 , a curve  2 WS of the lateral acceleration of the two-wheel steering is also shown for reference. 
     As shown in  FIG. 3 , when any of the CC control, the ACC control and the LKA control is on (YES in S 12 ), the vehicle steering control device changes the steering control rule to another so as to restrain the responsiveness in the turning of the vehicle and raise the responsiveness in the lateral shift of the vehicle (S 13 ). Or, the vehicle steering control device may change steering control rules so as to restrain the responsiveness in the turning and raise the responsiveness in the lateral shift when the vehicle speed becomes equal to or higher than 80 km/h. 
     In that case, the relationship of the following expression (8) is assumed to hold, and the expression (9) is obtained so that the responsiveness in the lateral shift becomes maximum. Then, the control rule of the target lateral acceleration LA** becomes as shown in the expression (10), so that as shown in  FIG. 5 , the computation portion  36  of the A4WS-ECU  18  calculates a front wheel steering angle δ** f  and a rear wheel steering angle δ** r  corresponding to the target lateral acceleration LA**. 
     
       
         
           
             
               
                 
                   EXPRESSION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   5 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     G 
                     β 
                     * 
                   
                   ≠ 
                   
                     T 
                     · 
                     
                       G 
                       γ 
                       * 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
             
               
                 
                   EXPRESSION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   6 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         β 
                         ** 
                       
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                     
                       δ 
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                   
                   = 
                   
                     T 
                     · 
                     
                       G 
                       β 
                       * 
                     
                     · 
                     
                       1 
                       
                         1 
                         + 
                         
                           T 
                           · 
                           s 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
             
               
                 
                   EXPRESSION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   7 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         
                           
                             
                               LA 
                               ** 
                             
                             ⁡ 
                             
                               ( 
                               s 
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                             δ 
                             ⁡ 
                             
                               ( 
                               s 
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                         = 
                         
                           V 
                           · 
                           
                             ( 
                             
                               
                                 
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                                   γ 
                                   * 
                                 
                                 + 
                                 
                                   T 
                                   · 
                                   
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                                     γ 
                                     * 
                                   
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                                   · 
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                         = 
                         
                           V 
                           · 
                           
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                             γ 
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                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     In this case, the phase delay of the lateral acceleration relative to the steering handle angle disappears, and the frequency response of the lateral acceleration becomes as shown by a curve B in  FIG. 6 . 
     According to this embodiment, during the execution of the CC control, the ACC control or the LKA control, during which the contribution of the target automatic control amount is greater in the setting of the target steering amount than the contribution of the manual steering amount, that is, when the automatic control is more dominant in the setting of the target steering amount than the manual control, the A4WS-ECU  18  sets the target steering amount so as to better the responsiveness in the lateral shift relatively to the responsiveness in the turning, in comparison with when the contribution of the target automatic control amount is less than or equal to the manual steering amount in the setting of the target steering amount. When the contribution of the target automatic control amount is larger in the setting of the target steering amount than that of the manual steering amount, that is, when the vehicle is running under an automatic control, such as the CC control, the ACC control, the LKA control, etc., it is generally often the case that the responsiveness in the lateral shift is required more than the responsiveness in the turning during the execution of a constant-speed run control or the like. Therefore, since during the automatic control, the A4WS-ECU  18  sets the target steering amount so as to better the responsiveness in the lateral shift relatively to the responsiveness in the turning, the stabilization of the vehicle can be further improved and the driver can be caused to feel secured and comfortable during the automatic steering. 
     Particularly, in this embodiment, since during the automatic control, the A4WS-ECU  18  sets the target steering amount so that the phase delay of the lateral acceleration relative to the steering amount becomes small, it is possible to improve the responsiveness in the lateral shift during the automatic control. 
     A second embodiment of the invention will be described below. This embodiment is different from the first embodiment in that the responsiveness in the lateral shift is improved by changing control maps at the time of the automatic steering. 
     In this embodiment, as shown in  FIG. 3 , in the case where the vehicle steering control device  10  of this embodiment is performing an arbitrary four-wheel steering control (S 11 ), if the target yaw rate δ* and the target slip angle β* are prepared as in the following expressions (11) and (12), the control rule of the target lateral acceleration LA* becomes as shown in the following expression (13). 
     
       
         
           
             
               
                 
                   EXPRESSION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   8 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         γ 
                         * 
                       
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                     
                       δ 
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                   
                   = 
                   
                     
                       
                         ω 
                         n 
                         
                           * 
                           2 
                         
                       
                       
                         
                           s 
                           2 
                         
                         + 
                         
                           2 
                           · 
                           
                             ζ 
                             * 
                           
                           · 
                           
                             ω 
                             n 
                             * 
                           
                           · 
                           s 
                         
                         + 
                         
                           ω 
                           n 
                           
                             * 
                             2 
                           
                         
                       
                     
                     · 
                     
                       ( 
                       
                         1 
                         + 
                         
                           
                             T 
                             γ 
                             * 
                           
                           · 
                           s 
                         
                       
                       ) 
                     
                     · 
                     
                       G 
                       γ 
                       * 
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         β 
                         * 
                       
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                     
                       δ 
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                   
                   = 
                   
                     
                       
                         ω 
                         n 
                         
                           * 
                           2 
                         
                       
                       
                         
                           s 
                           2 
                         
                         + 
                         
                           2 
                           · 
                           
                             ζ 
                             * 
                           
                           · 
                           
                             ω 
                             n 
                             * 
                           
                           · 
                           s 
                         
                         + 
                         
                           ω 
                           n 
                           
                             * 
                             2 
                           
                         
                       
                     
                     · 
                     
                       ( 
                       
                         1 
                         + 
                         
                           
                             T 
                             β 
                             * 
                           
                           · 
                           s 
                         
                       
                       ) 
                     
                     · 
                     
                       G 
                       β 
                       * 
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
             
               
                 
                   EXPRESSION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   9 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       L 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           A 
                           * 
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                     
                       δ 
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                   
                   = 
                   
                     V 
                     · 
                     
                       ( 
                       
                         
                           
                             
                               γ 
                               * 
                             
                             ⁡ 
                             
                               ( 
                               s 
                               ) 
                             
                           
                           
                             δ 
                             ⁡ 
                             
                               ( 
                               s 
                               ) 
                             
                           
                         
                         + 
                         
                           s 
                           · 
                           
                             
                               
                                 β 
                                 * 
                               
                               ⁡ 
                               
                                 ( 
                                 s 
                                 ) 
                               
                             
                             
                               δ 
                               ⁡ 
                               
                                 ( 
                                 s 
                                 ) 
                               
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     During this state, the lateral shift and the turning become substantially equal in responsiveness. As shown in  FIG. 11 , regarding a curve C of the frequency response of the lateral acceleration at the time of the manual steering, the higher the frequency, the lower the gain becomes, and the greater the phase delay becomes. 
     Furthermore, as shown in  FIG. 3 , if any of the CC control, the ACC control and the LKA control is on (YES in S 12 ), the control map is changed to another so as to restrain the responsiveness in the turning and raise the responsiveness in the lateral shift (S 13 ). 
       FIG. 7  is a diagram showing a control map of the yaw rate gain in accordance with the second embodiment, and  FIG. 8  is a diagram showing a control map of the slip angle gain, and  FIG. 9  is a diagram showing a control map of the slip angle advancement time. As shown in  FIG. 7 , when any of the CC control, the ACC control and the LKA control is performed, the yaw rate gain is decreased to restrain the responsiveness in the turning. On the other hand, as shown in  FIGS. 8 and 9 , the slip angle gain and the slip angle advancement time are increased to raise the responsiveness in the lateral shift when any of the CC control, the ACC control and the LKA control is performed. 
     The target yaw rate γ** and the target slip angle β** in the case where the control map has been changed become as shown in the following expressions (14) and (15). 
     
       
         
           
             
               
                 
                   EXPRESSION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   10 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       
                         γ 
                         ** 
                       
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                     
                       δ 
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                   
                   = 
                   
                     
                       
                         ω 
                         n 
                         
                           * 
                           2 
                         
                       
                       
                         
                           s 
                           2 
                         
                         + 
                         
                           2 
                           · 
                           
                             ζ 
                             * 
                           
                           · 
                           
                             ω 
                             n 
                             * 
                           
                           · 
                           s 
                         
                         + 
                         
                           ω 
                           n 
                           
                             * 
                             2 
                           
                         
                       
                     
                     · 
                     
                       ( 
                       
                         1 
                         + 
                         
                           
                             T 
                             γ 
                             * 
                           
                           · 
                           s 
                         
                       
                       ) 
                     
                     · 
                     
                       G 
                       γ 
                       ** 
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         β 
                         ** 
                       
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                     
                       δ 
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                   
                   = 
                   
                     
                       
                         ω 
                         n 
                         
                           * 
                           2 
                         
                       
                       
                         
                           s 
                           2 
                         
                         + 
                         
                           2 
                           · 
                           
                             ζ 
                             * 
                           
                           · 
                           
                             ω 
                             n 
                             * 
                           
                           · 
                           s 
                         
                         + 
                         
                           ω 
                           n 
                           
                             * 
                             2 
                           
                         
                       
                     
                     · 
                     
                       ( 
                       
                         1 
                         + 
                         
                           
                             T 
                             β 
                             * 
                           
                           · 
                           s 
                         
                       
                       ) 
                     
                     · 
                     
                       G 
                       β 
                       ** 
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     At this time, the control rule of the target lateral acceleration LA** becomes as shown in the following expression (16). As shown in  FIG. 10 , the computation portion  36  of the A4WS-ECU  18  calculates a front wheel steering angle δ** f  and a rear wheel steering angle δ** r  corresponding to the target lateral acceleration LA**. 
     
       
         
           
             
               
                 
                   EXPRESSION 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   11 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     
                       L 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           A 
                           ** 
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                     
                     
                       δ 
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                   
                   = 
                   
                     V 
                     · 
                     
                       ( 
                       
                         
                           
                             
                               γ 
                               ** 
                             
                             ⁡ 
                             
                               ( 
                               s 
                               ) 
                             
                           
                           
                             δ 
                             ⁡ 
                             
                               ( 
                               s 
                               ) 
                             
                           
                         
                         + 
                         
                           s 
                           · 
                           
                             
                               
                                 β 
                                 ** 
                               
                               ⁡ 
                               
                                 ( 
                                 s 
                                 ) 
                               
                             
                             
                               δ 
                               ⁡ 
                               
                                 ( 
                                 s 
                                 ) 
                               
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
     In this case, the phase delay of the lateral acceleration relative to the steering handle angle becomes less, and the frequency response of the lateral acceleration becomes as shown by a curve D in  FIG. 11 . 
     According to this embodiment, during the execution of the CC control, the ACC control or the LKA control, during which the contribution of the target automatic control amount is larger in the setting of the target steering amount than the contribution of the manual steering amount, the A4WS-ECU  18  changes control maps so as to better the responsiveness in the lateral shift relatively to the responsiveness in the turning, in comparison with during the manual steering. Therefore, since during the automatic control, the A4WS-ECU  18  sets the target steering amount so as to better the responsiveness in the lateral shift relatively to the responsiveness in the turning, the stabilization of the vehicle can be further improved and the driver can be caused to feel secured and comfortable during the automatic steering. 
     A third embodiment of the invention will be described. This embodiment is different from the above-described first embodiment in that during the automatic steering, the responsiveness in the turning is restrained and the responsiveness in the lateral shift is raised by performing such a correction that the rear wheel steer angle becomes large on the basis of the LKA-ESP torque. 
       FIG. 12  is a block diagram showing a construction of an A4WS-ECU in accordance with the third embodiment. As shown in  FIG. 12 , the A4WS-ECU  18  of this embodiment includes not only the computation portion  36  but also a correction coefficient-calculating portion  38  that calculates a rear wheel steer angle correction coefficient Z from the LKA-ESP torque output by the LKA-ECU  34 , and a final rear wheel steer angle-calculating portion  40  that calculates a final rear wheel steer angle DR from the rear wheel steering angle δ r  calculated by the computation portion  36 . The final rear wheel steer angle DR calculated by the final rear wheel steer angle-calculating portion  40  is output to the rear steer ECU  26 , so that the rear steer actuator  28  is accordingly driven. 
       FIG. 13  is a flowchart showing an operation of the vehicle steering control device in accordance with the third embodiment. As shown in  FIG. 13 , if the LKA control becomes on (S 22 ) from a state where the vehicle steering control device is performing an arbitrary four-wheel steering control (S 21 ), the correction coefficient-calculating portion  38  of the A4WS-ECU  18  calculates the rear wheel steer angle correction coefficient Z through a function of the rear wheel steer angle correction coefficient Z that uses the LKA-ESP torque as a parameter (S 23 ). 
     The final rear wheel steer angle-calculating portion  40  of the A4WS-ECU  18  calculates a final rear wheel steer angle DR using a correction expression DR=δr•(1+Z) from the rear wheel steering angle δ r  calculated by the computation portion  36 , and the rear wheel steer angle correction coefficient Z calculated by the correction coefficient-calculating portion  38  (S 24 ). 
     The rear steer ECU  26 , after receiving the final rear wheel steer angle DR from the final rear wheel steer angle-calculating portion  40 , performs such a steering control as to change the rear wheel steer angle amount to an increased amount and thus restrain the responsiveness in the turning and raise the responsiveness in the lateral shift (S 25 ). 
     According to the embodiment, during the automatic control, the A4WS-ECU  18  sets the target steering amount so that the steering amount of the rear wheels becomes relatively large. Therefore, the amount of the turning is restrained and, on the other hand, the responsiveness in the lateral shift is further improved. Hence, the stabilization of the vehicle can be further improved, and the driver can be caused to feel secured and comfortable. 
     While embodiments of the invention have been described above, the invention is not limited to the foregoing embodiments, but may also be modified in various manners.