Patent Publication Number: US-10759282-B2

Title: Vehicle turning control device

Description:
CROSS REFERENCE TO THE RELATED APPLICATION 
     This application is a continuation application, under 35 U.S.C. § 111(a), of international application No. PCT/JP2017/000111, filed Jan. 5, 2017, which claims Convention priority to Japanese patent application No. 2016-002583, filed Jan. 8, 2016, the entire disclosure of which is herein incorporated by reference as a part of this application. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a vehicle turning control device that improves vehicle turning performance and stabilizes the vehicle attitude at a place having a low road surface frictional coefficient, or the like. 
     Description of Related Art 
     Conventionally, yaw angle model following control for improving turning performance using a braking/driving torque difference between right and left wheels, and yaw moment control such as skid prevention control device (Electronic Stability Control (ESC)) for stabilizing the vehicle attitude, have been proposed. 
     For example, a control device is proposed which improves turning performance by yaw moment control using feedforward control and feedback control in combination and which switches from the yaw moment control to vehicle behavior stabilization control as the turning degree approaches the limit (Patent Document 1). 
     RELATED DOCUMENT 
     Patent Document 
     [Patent Document 1] JP Laid-open Patent Publication No. 2015-120415 
     SUMMARY OF THE INVENTION 
     In the yaw moment control of Patent Document 1, tire grip is not taken into consideration, and therefore the following problem arises. For example, if yaw moment control for improving turning performance is performed when the road surface frictional coefficient of the travelling space is low, tire grip exceeds the limit and the vehicle becomes unstable. 
     If the turning degree approaches the limit and the control is switched to vehicle behavior stabilization control from the yaw moment control, when the yaw moment control for improving turning performance is switched to the vehicle behavior stabilization control, the vehicle turning characteristic with respect to steering varies between before and after the switching. Therefore, uncomfortable feeling might be given to a driver. 
     An object of the present invention is to provide a vehicle turning control device that can stabilize a vehicle by performing yaw moment control considering the tire grip limit and gives no uncomfortable feeling to the driver even if the control is switched from the yaw moment control to control for stabilizing the vehicle attitude. 
     In the following description, the reference characters in the embodiments are used for convenience sake in order to facilitate the understanding. 
     A vehicle turning control device according to one aspect of the present invention is a vehicle turning control device for controlling a turning characteristic of a vehicle  1 , the vehicle including braking/driving sources  4  for independently control braking/driving torques (braking torques and/or driving torques) applied to a plurality of wheels  2 , a vehicle speed detection device  18  for detecting a vehicle speed, and a steering angle detection device  19  for detecting a steering angle, the vehicle turning control device including: a target yaw rate calculation module  25  configured to calculate a target yaw rate for the vehicle based on a yaw response characteristic, with respect to the vehicle speed detected by the vehicle speed detection device  18  and the steering angle detected by the steering angle detection device  19 ; a yaw moment calculation module  27  configured to calculate the braking/driving torques developed at the plurality of wheels  2  based on the target yaw rate calculated by the target yaw rate calculation module  25 , so as to calculate a yaw moment predicted to act on the vehicle  1 ; a yaw rate deviation-and/or-road surface friction calculation module  40  including one or both of a yaw rate deviation calculation module  29  configured to calculate a yaw rate deviation, from a difference between the target yaw rate and an actual yaw rate detected by a yaw rate detection device  20  provided to the vehicle  1 , and a road surface frictional coefficient calculation module  24  configured to estimate a road surface frictional coefficient; and a yaw response characteristic changing module (control gain calculation module  26 ) configured to cause the yaw response characteristic used in the target yaw rate calculation module  25  so as to approach a reference yaw response characteristic from a predetermined yaw response characteristic different from the reference yaw response characteristic, as the yaw rate deviation calculated by the yaw rate deviation calculation module  29  increases or as the road surface frictional coefficient estimated by the road surface frictional coefficient calculation module  24  decreases. 
     The “turning characteristic” indicates not only a characteristic of ease in turning of the vehicle but also stability of the vehicle attitude during turning of the vehicle. 
     The “reference yaw response characteristic” refers to the yaw response characteristic inherent in the vehicle, i.e., a yaw response characteristic based on performance and structure of the vehicle, which is independent from the external environment. 
     The “predetermined yaw response characteristic” is determined by experiment and/or simulation. 
     In this configuration, the target yaw rate calculation module  25  calculates the target yaw rate for the vehicle, based on the yaw response characteristic, with respect to the vehicle speed and the steering angle. The yaw moment calculation module  27  calculates braking/driving torques developed at the plurality of wheels  2  based on the target yaw rate, so as to calculate a yaw moment acting on the vehicle  1 . As the calculated yaw rate deviation increases or as the estimated road surface frictional coefficient decreases, the yaw response characteristic changing module  26  causes the yaw response characteristic to approach the reference yaw response characteristic from the predetermined yaw response characteristic different from the reference yaw response characteristic. In this way, by changing the yaw response characteristic in accordance with the road surface frictional coefficient or the yaw rate deviation, it is possible to suppress change in the attitude of the vehicle  1  due to the yaw moment control. In addition, even if the control is switched from the yaw moment control to the control for stabilizing the attitude of the vehicle  1 , it is possible to prevent uncomfortable feeling from being given to the driver. 
     The yaw response characteristic changing module (control gain calculation module  26 ) may determine a control gain for the yaw response characteristic used in the target yaw rate calculation module  25 , in accordance with one or both of the yaw rate deviation and the road surface frictional coefficient. 
     The yaw response characteristic changing module (control gain calculation module  26 ) may adjust the yaw response characteristic used in the target yaw rate calculation module  25  to the reference yaw response characteristic, when the road surface frictional coefficient is smaller than a road surface frictional coefficient threshold value or when the yaw rate deviation is greater than a yaw rate deviation threshold value. The threshold values are determined by experiment and/or simulation. 
     In this configuration, change in the attitude of the vehicle due to yaw moment control is limited to almost zero, when the road surface frictional coefficient is smaller than the road surface frictional coefficient threshold value and/or when the yaw rate deviation is greater than the yaw rate deviation threshold value. Thus, the attitude of the vehicle  1  can be prevented from becoming unstable. 
     The yaw response characteristic changing module (control gain calculation module  26 ) may be configured such that, when the yaw response characteristic used in the target yaw rate calculation module  25  is changed from the reference yaw response characteristic to the predetermined yaw response characteristic, an amount of change in the yaw response characteristic per time is smaller than when the yaw response characteristic is changed from the predetermined yaw response characteristic to the reference yaw response characteristic. 
     In this case, when the yaw response characteristic is changed from the predetermined yaw response characteristic to the reference yaw response characteristic, the attitude of the vehicle  1  can be stabilized immediately, and when the yaw response characteristic is changed from the reference yaw response characteristic to the predetermined yaw response characteristic, uncomfortable feeling can be prevented from being given to the driver. 
     The yaw response characteristic used in the target yaw rate calculation module  25  may be represented by a calculation expression of a second-order lag system of the target yaw rate relative to an actual steering angle, and the yaw response characteristic may be changed by changing a gain of a natural frequency in a yaw direction and/or a gain of an attenuation coefficient in the yaw direction included in the calculation expression. In this way, by representing the yaw response characteristic used in the target yaw rate calculation module  25  by a general calculation expression of a second-order lag system, the calculation expression can be simplified. In addition, the natural frequency and the attenuation coefficient in the yaw direction can be adjusted individually. 
     The road surface frictional coefficient calculation module  24  may estimate the road surface frictional coefficient from a magnitude of an actual lateral acceleration detected by a lateral acceleration detection device  21  provided to the vehicle  1  when a deviation between a target lateral acceleration and the actual lateral acceleration is greater than a predetermined value. The predetermined value is determined by experiment and/or simulation. In this configuration, the road surface frictional coefficient can be estimated using an existing lateral acceleration detection device  21 , and therefore the control device is obtained at low cost. 
     The vehicle turning control device may further include a vehicle attitude stabilization control device  15  configured to perform control for stabilizing an attitude of the vehicle  1 . The vehicle attitude stabilization control device  15  may use the target yaw rate calculated by the target yaw rate calculation module  25 , for the control for stabilizing the attitude of the vehicle  1 . In this case, even if the control is switched from the yaw moment control to the control for stabilizing the attitude of the vehicle  1 , the same target yaw rate as that used in the yaw moment control is used and thus the turning characteristic of the vehicle  1  with respect to steering stays unchanged. Therefore, no uncomfortable feeling is given to the driver. 
     The braking/driving sources  4  may be electric motors  4  configured to drive the respective wheels  2 . In this case, it is possible to control braking/driving torques and generate a yaw moment, by only the electric motors  4 . 
     Any combination of at least two constructions, disclosed in the appended claims and/or the specification and/or the accompanying drawings should be construed as included within the scope of the present invention. In particular, any combination of two or more of the appended claims should be equally construed as included within the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In any event, the present invention will become more clearly understood from the following description of preferred embodiments thereof, when taken in conjunction with the accompanying drawings. However, the embodiments and the drawings are given only for the purpose of illustration and explanation, and are not to be taken as limiting the scope of the present invention in any way whatsoever, which scope is to be determined by the appended claims. In the accompanying drawings, like reference numerals are used to denote like parts throughout the several views, and: 
         FIG. 1  is a system configuration diagram showing the conceptual configuration of a vehicle turning control device according to the first embodiment of the present invention; 
         FIG. 2  is a sectional view showing an example of an in-wheel motor driving device for a vehicle shown in  FIG. 1 ; 
         FIG. 3  is a block diagram showing a specific example of a part of the turning control device shown in  FIG. 1 ; 
         FIG. 4  shows the relationship between a lateral acceleration deviation and a road surface frictional coefficient in the turning control device shown in  FIG. 1 ; 
         FIG. 5  shows the relationship between the control gain and each of a yaw rate deviation and the road surface frictional coefficient in the turning control device shown in  FIG. 1 ; 
         FIG. 6  shows an example of transition of the control gain in the turning control device shown in  FIG. 1 ; 
         FIG. 7  shows an example in which a yaw moment is generated in such a direction as to improve turning performance, when the vehicle shown in  FIG. 1  turns left; 
         FIG. 8  shows three states of the attitude of the vehicle shown in  FIG. 1 ; 
         FIG. 9  is a block diagram showing a specific example of a part of a vehicle turning control device according to the second embodiment of the present invention; 
         FIG. 10  is a block diagram showing a specific example of a part of a vehicle turning control device according to the third embodiment of the present invention; 
         FIG. 11A  is a schematic diagram showing an example of a way of driving a four-wheel drive vehicle using an internal combustion engine as a driving source in the case where a yaw moment is generated in such a direction as to improve turning performance when the vehicle turns left; 
         FIG. 11B  is a schematic diagram showing an example of a way of driving a front-wheel drive vehicle using an internal combustion engine as a driving source in the case where a yaw moment is generated in such a direction as to improve turning performance when the vehicle turns left; and 
         FIG. 11C  is a schematic diagram showing an example of a way of driving a rear-wheel drive vehicle using an internal combustion engine as a driving source in the case where a yaw moment is generated in such a direction as to improve turning performance when the vehicle turns left. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A vehicle turning control device according to the first embodiment of the present invention will be described with reference to  FIGS. 1 to 8 . As shown in  FIG. 1 , an example in which a vehicle  1  provided with the turning control device is a vehicle of four-wheel independent driven type having in-wheel motor driving devices  5  for all four wheels, will be described. In this vehicle  1 , wheels  2 ,  2  that are right and left rear wheels and wheels  2 ,  2  that are right and left front wheels are independently driven by electric motors  4  as braking/driving sources in the respective in-wheel motor driving devices  5 . 
     As shown in  FIG. 2 , each in-wheel motor driving device  5  has the electric motor  4 , a speed reducer  6 , and a wheel bearing  7 . The speed reducer  6  and the wheel bearing  7  are partially or entirely provided inside the wheel  2 . Rotation of the electric motor  4  is transmitted to the wheel  2  via the speed reducer  6  and the wheel bearing  7 . A brake rotor  8   a  composing a frictional brake device  8  is fixed to a flange portion of a hub ring  7   a  of the wheel bearing  7 . The brake rotor  8   a  rotates integrally with the wheel  2 . The electric motor  4  is, for example, an interior permanent magnet synchronous motor, having permanent magnets embedded in a core portion of a rotor  4   a . The electric motor  4  is a motor in which a radial gap is provided between a stator  4   b , fixed to a housing  4   c , and the rotor  4   a , attached to a rotary output shaft  9 . 
     In  FIG. 1 , a control system will be described. The turning control device of the vehicle  1  includes an ECU (Electric Control Unit)  10 , a plurality of (in this example, four) inverter devices  11 , provided for the respective electric motors  4 , and sensors  12 . The ECU  10  includes a main ECU section  13  having a general function, a yaw moment control device  14 , a vehicle attitude stabilization control device  15 , and an inverter torque command device  16 . The ECU  10  is configured from a computer such as a microcomputer, a program executed by the computer, various electronic circuit, and the like. The ECU  10  and the inverter devices  11  are connected to each other via an in-vehicle communication network such as control area network (CAN). 
     The main ECU section  13  has, as a basic configuration thereof, a function for performing integrated control and cooperative control for the entire vehicle, and a braking/driving command generation function. The braking/driving command generation function is a function of generating torque command values to be distributed to the respective electric motors  4 , from an acceleration command value which is a command value of operation amount detected by an acceleration pedal sensor  17  provided to an acceleration pedal (not shown). When the driver operates the acceleration pedal to command for driving, the acceleration command value is supplied from the acceleration pedal sensor  17  to the main ECU section  13  in accordance with the operation amount of the acceleration pedal. 
     An acceleration torque command value from the main ECU section  13  is sent to each inverter device  11  via the yaw moment control device  14  and the like. Each inverter device  11  is configured so as to convert DC power of a battery (not shown) to AC power for driving the electric motor  4 , and has a control unit (not shown) for controlling the output thereof, thereby controlling the corresponding electric motor  4  in accordance with the torque command value distributed for each wheel  2 . Each inverter device  11  includes a power circuit unit (not shown) such as a bridge circuit of switching elements for performing conversion to AC power, and a control unit (not shown) for controlling the power circuit unit. 
     The sensors  12  includes the acceleration pedal sensor  17 , a vehicle speed sensor (vehicle speed detection device)  18  for detecting the vehicle speed, a steering angle sensor (steering angle detection device)  19  for detecting the steering angle, a yaw rate sensor (yaw rate detection device)  20  for detecting the yaw rate, and a lateral acceleration sensor (lateral acceleration detection device)  21  for detecting the lateral acceleration. The steering angle sensor  19  is, for example, a sensor for detecting the steering angle of a steering wheel or the like (not shown). The steering angle from the steering angle sensor  19 , the vehicle speed from the vehicle speed sensor  18 , the actual lateral acceleration from the lateral acceleration sensor  21 , and the actual yaw rate from the yaw rate sensor  20  are supplied to the main ECU section  13 . These values are supplied from the main ECU section  13  to the yaw moment control device  14  and the vehicle attitude stabilization control device  15 . 
     As shown in  FIG. 3 , the yaw moment control device  14  includes a target lateral acceleration calculation module  22 , a lateral acceleration deviation calculation module  23 , a road surface frictional coefficient calculation module  24 , a target yaw rate calculation module  25 , a control gain calculation module (yaw response characteristic changing module)  26 , a yaw moment calculation module  27 , and a braking/driving torque calculation module  28 . The road surface frictional coefficient calculation module  24  composes a yaw rate deviation-and/or-road surface friction calculation module  40 , together with a yaw rate deviation calculation module  29  of the vehicle attitude stabilization control device  15  described later. 
     The vehicle speed, the steering angle, the actual lateral acceleration, and the acceleration torque command value from the acceleration pedal sensor  17 , are supplied from the main ECU section  13  to the yaw moment control device  14 . 
     The target lateral acceleration calculation module  22  calculates a target lateral acceleration from the vehicle speed, the steering angle, and vehicle parameters such as vehicle mass and wheelbase. The lateral acceleration deviation calculation module  23  calculates a lateral acceleration deviation, from a difference between the target lateral acceleration calculated by the target lateral acceleration calculation module  22  and the actual lateral acceleration supplied from the main ECU section  13 . The road surface frictional coefficient calculation module  24  calculates a road surface frictional coefficient in accordance with the following expressions (1), (2).  FIG. 4  shows the relationship between the lateral acceleration deviation and the road surface frictional coefficient in the turning control device. 
     As shown in  FIG. 3  and  FIG. 4 , if the lateral acceleration deviation obtained by the lateral acceleration deviation calculation module  23  is equal to or smaller than a certain lateral acceleration deviation threshold value Gy c , the road surface frictional coefficient calculation module  24  sets a road surface frictional coefficient μ est  to 1, and if the lateral acceleration deviation is greater than the lateral acceleration deviation threshold value Gy c , the road surface frictional coefficient calculation module  24  calculates the road surface frictional coefficient μ est  from an actual lateral acceleration Gy act . Here, the target lateral acceleration is defined as Gy ref , the actual lateral acceleration (G) is defined as Gy act , the road surface frictional coefficient is defined as μ est , the actual lateral acceleration (m/s 2 ) is defined as G y , a front-rear force is defined as Fx, a lateral force is defined as Fy, an up-down force is defined as Fz, the vehicle weight is defined as m, and the gravitational acceleration is defined as g. 
     In an equilibrium expression
 
(μ est   Fz ) 2   =Fx   2   +Fy   2   (1)′,
 
     if Fx is equal to 0, the following expression is obtained:
 
μ est   |Fz|=|Fy|   (2)′.
 
     From an expression
 
| Fy|=m|Gy|   (3)′, and
 
     expression (2)′, 
     the following expression is obtained:
 
μ est   |Fz|=m|Gy|   (4)′.
 
     Thus, μ est  is represented as follows:
 
μ est   =|Gy   act   |=|Gy|/g   (5)′.
 
     From the above, the road surface frictional coefficient is estimated as follows:
 
If | Gy   ref   |−|Gy   act   |≤|Gy   c |, μ est =1  (1)
 
If | Gy   ref   |−|Gy   act   |&gt;|Gy   c |, μ est   ≥|Gy   act |  (2).
 
     As shown in  FIG. 3 , the target yaw rate calculation module  25  calculates a yaw rate predicted to occur in the vehicle, from a predetermined yaw response characteristic, with respect to at least the vehicle speed and the steering angle. Specifically, the target yaw rate calculation module  25  performs calculation using a transfer function of a second-order lag system or a time lag system of second order of a target yaw rate r(s) relative to an actual steering angle δ(s), shown in expression (3), for example. 
     
       
         
           
             
               
                 
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     In expression (3), Gδ r (0) is a yaw angular velocity gain constant (yaw rate gain constant), ω n  is a natural frequency in the yaw direction, ζ is an attenuation coefficient in the yaw direction, Tr is a yaw angular velocity time constant (yaw rate time constant), s is the Laplace operator, α is a control gain of the natural frequency ω n , and λ is a control gain of the attenuation coefficient ζ. The yaw angular velocity gain constant Gδ r (0), the natural frequency ω n  in the yaw direction, the attenuation coefficient ζ in the yaw direction, and the yaw angular velocity time constant Tr are calculated from the vehicle speed and vehicle parameters such as vehicle mass and wheelbase. 
     When the control gain α of the natural frequency ω n  or the control gain λ of the attenuation coefficient ζ is set at greater than 1, rise time of the target yaw rate becomes shorter (i.e., quick responsiveness for the target yaw rate is enhanced), and when the control gains α and λ are set at 1, the yaw response characteristic becomes that the vehicle has, that is, the yaw response characteristic inherent in the vehicle (a reference yaw response characteristic). 
     In this example, the control gain calculation module  26  calculates the control gains α, λ in accordance with one or both of the road surface frictional coefficient calculated by the road surface frictional coefficient calculation module  24  and a yaw rate deviation calculated by the yaw rate deviation calculation module  29  described later. As described below, with reference to  FIG. 5  and  FIG. 6 , the case of the control gain α of the natural frequency ω n  will be described as an example, but the control gain λ, of the attenuation coefficient ζ also has substantially the same characteristic as the control gain α, and therefore the description of the control gain λ is omitted. 
     As shown in  FIG. 5 , two threshold values may be provided for the road surface frictional coefficient and the yaw rate deviation. For example, if the road surface frictional coefficient becomes smaller than the first road surface frictional coefficient threshold value μ a  or the yaw rate deviation becomes greater than the first yaw rate deviation threshold value r a , the control gain calculation module  26  causes the control gain α to approach 1 from the initial value (predetermined yaw response characteristic) α 0 . That is, the control gain calculation module  26  sets a value calculated by α=−((α 0 −1)/(r b −r a ))(r−r a )+α 0  or α=−((α 0 −1)/(μ b −μ a )(μ−μ a )+α 0 , as the control gain α. On the other hand, if the road surface frictional coefficient becomes smaller than the second road surface frictional coefficient threshold value μ b  or the yaw rate deviation becomes greater than the second yaw rate deviation threshold value r b , the control gain calculation module  26  causes the control gain α to approach the initial value α 0  from 1. 
     Here, as described later, the amount of change in the control gain α per time when the control gain α is returned from 1 to the initial value α 0  is small, and therefore the control gain calculation module  26  sets a value calculated by α=−((α 0 −1)/(r b −r c ))(r−r c )+α 0  or a value calculated by α=−((α 0 −1)/(μ b −α c ))(μ−μ c )+α 0 , as the control gain α, wherein the yaw rate deviation value r c  satisfies r c &gt;r a , and the road surface frictional coefficient value μ c  satisfies μ c &lt;μ a . If the road surface frictional coefficient is smaller than the second road surface frictional coefficient threshold value μ b  or the yaw rate deviation is greater than the second yaw rate deviation threshold value r b , the control gain calculation module  26  sets 1 as the control gain α. If the road surface frictional coefficient is equal to or greater than the first road surface frictional coefficient threshold value μ a  or the yaw rate deviation is equal to or smaller than the first yaw rate deviation threshold value r a , the control gain calculation module  26  sets the initial value α 0  as the control gain α. 
     The control gain calculation module  26  in  FIG. 3  is configured such that, when changing the yaw response characteristic from the yaw response characteristic inherent in the vehicle to the predetermined yaw response characteristic, the amount of change in the yaw response characteristic per time is smaller than when changing the yaw response characteristic from the predetermined yaw response characteristic to the yaw response characteristic inherent in the vehicle. Specifically, as shown in  FIG. 6 , as compared to the case where the control gain α decreases from the initial value α 0 , which forms the predetermined yaw response characteristic, to 1, which forms the yaw response characteristic inherent in the vehicle, the amount of change per time is smaller when the control gain α is returned from 1 to the initial value α 0 . That is, the control gain α is changed more gradually. 
     At a place with a low road surface frictional coefficient, the tire grip force is likely to be greatly reduced, and therefore the control gain calculation module  26  ( FIG. 3 ) immediately decreases the control gain α, to decrease the braking/driving torque in the yaw moment control. As shown in  FIG. 6 , at a place with a high road surface frictional coefficient, the tire grip force is restored, and therefore the control gain α is gradually returned to the initial value α 0 , thereby increasing the braking/driving torque in the yaw moment control. By the gradual return, no uncomfortable feeling is given to the driver. 
     As shown in  FIG. 3 , the yaw moment calculation module  27  calculates a braking/driving torque which is a braking torque or a driving torque for each wheel  2  on the basis of the target yaw rate calculated by the target yaw rate calculation module  25 , thereby calculating a yaw moment predicted to act on the vehicle. 
     The yaw moment calculation module  27  performs calculation using a transfer function of a third-order lag system or a time lag system of third order of a yaw moment M z (s) relative to the actual steering angle δ(s), shown in expression (4), for example. 
     
       
         
           
             
               
                 
                   
                       
                   
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                           + 
                           
                             2 
                             ⁢ 
                             ζ 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               ω 
                               n 
                             
                             ⁢ 
                             s 
                           
                           + 
                           
                             s 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     In expression (4), Gδ r (0) is a yaw angular velocity gain constant (yaw rate gain constant), ω n  is a natural frequency in the yaw direction, ζ is an attenuation coefficient in the yaw direction, Tr is a yaw angular velocity time constant (yaw rate time constant), GM r (0) is a yaw moment gain constant, T M  is a yaw moment time constant, s is the Laplace operator, α is a control gain of the natural frequency ω n , and λ is a control gain of the attenuation coefficient ζ. The yaw angular velocity gain constant Gδ r (0), the natural frequency ω n  in the yaw direction, the attenuation coefficient ζ in the yaw direction, the yaw angular velocity time constant Tr, the yaw moment gain constant GM r (0), and the yaw moment time constant T M  are calculated from the vehicle speed and vehicle parameters such as vehicle mass and wheelbase. 
     With reference to expression (4), it is found that the yaw moment M z (s) relative to the actual steering angle δ(s) is zero if the control gains α and λ are 1. The braking/driving torque calculation module  28  determines braking/driving torques for the four wheels on the basis of the acceleration torque command value supplied from the main ECU section  13  and the yaw moment calculated by expression (4), and issues a torque command value Y to the inverter torque command device  16 . If no vehicle attitude stabilization control is performed, the torque command value Y is used as a last command torque. 
       FIG. 7  shows an example in which a yaw moment is generated in such a direction as to improve turning performance when the vehicle  1  is turned left. In  FIG. 7 , a solid-line thin arrow indicates a driving torque by the driving source. A broken-line thin arrow indicates a braking torque by the driving source (the same applies to  FIGS. 11A to 11C ). A solid-line thick arrow indicates a total value of driving torques and a broken-line thick arrow indicates a total value of braking torques (the same applies to  FIGS. 11A to 11C ). 
     In the vehicle shown in  FIG. 7 , a yaw moment is generated by developing a driving torque at the turning outer wheels and developing a braking torque at the turning inner wheels. If the driver performs an accelerating operation or a braking operation during turning of the vehicle  1 , a driving torque or a braking torque is applied and therefore the vehicle  1  is accelerated or decelerated. 
     As shown in  FIG. 3 , the vehicle attitude stabilization control device  15  performs control for stabilizing the attitude of the vehicle. The actual yaw rate is supplied from the main ECU section  13  to the vehicle attitude stabilization control device  15 . The vehicle attitude stabilization control device  15  includes a yaw rate deviation calculation module  29 , a vehicle attitude state calculation module  30 , and a braking/driving torque calculation module  31 . The yaw rate deviation calculation module  29  calculates the yaw rate deviation, from a difference between the actual yaw rate and the target yaw rate calculated by the target yaw rate calculation module  25 . In other words, the yaw rate deviation calculation module  29  uses the target yaw rate calculated by the target yaw rate calculation module  25 , as a target yaw rate in the vehicle attitude stabilization control. 
     The vehicle attitude state calculation module  30  calculates the attitude state of the vehicle from the magnitude of the yaw rate deviation calculated by the yaw rate deviation calculation module  29 . 
       FIG. 8  shows three states of the attitude of the vehicle  1 . In the case where the target yaw rate and the actual yaw rate are substantially equal to each other, the above yaw moment control device  14  ( FIG. 3 ) performs the yaw moment control to command braking torques or driving torques in the same direction for the front and rear wheels on one side as shown in  FIG. 8 , thereby generating a yaw moment. 
     On the other hand, at a place with a low road surface frictional coefficient, or the like, it is likely that the vehicle fails to take a curve, or spins. Where the target yaw rate is r ref , the actual yaw rate is r act , and the yaw rate threshold value is r b , the vehicle attitude state calculation module  30  ( FIG. 3 ) determines that the vehicle is in an understeer state if expression (5) is satisfied. The vehicle attitude state calculation module  30  ( FIG. 3 ) determines that the vehicle is in an oversteer state if expression (6) is satisfied. That is, the determination is performed as follows.
 
Understeer state, if  r   ref   &gt;r   act  and | r   ref   −r   act   |&gt;r   b   (5)
 
Oversteer state, if  r   ref   &lt;r   act  and | r   ref   −r   act   |&gt;rb   (6)
 
     A yaw moment is generated using the rear wheels as control wheels in the case of understeer state or using the front wheels as control wheels in the case of oversteer state, whereby the attitude of the vehicle  1  is stabilized. 
     As shown in  FIG. 3 , the braking/driving torque calculation module  31  of the vehicle attitude stabilization control device  15  calculates a braking torque and a driving torque on the basis of the road surface frictional coefficient calculated by the road surface frictional coefficient calculation module  24 , the vehicle attitude state calculated by the vehicle attitude state calculation module  30 , and the target yaw rate calculated by the target yaw rate calculation module  25 , so that the calculated braking torque and driving torque are issued as a torque command value E. 
     The torque command value Y and the torque command value E are supplied to the inverter torque command device  16  which calculates a last torque command value. The inverter torque command device  16  issues the last torque commands value calculated from the torque command value Y and the torque command value E, to the respective inverter devices  11 . Each of the inverter devices  11  controls current so as to achieve the last torque command value, thereby driving the in-wheel motor driving device  5 . 
     In the vehicle turning control device described above, the target yaw rate calculation module  25  calculates a target yaw rate for the vehicle  1 , from the yaw response characteristic with respect to the vehicle speed and the steering angle. The yaw moment calculation module  27  calculates braking/driving torques for the plurality of wheels  2  on the basis of the target yaw rate, thereby calculating a yaw moment for the vehicle  1 . As the estimated road surface frictional coefficient decreases or as the calculated yaw rate deviation increases, the control gain calculation module  26  causes the yaw response characteristic to approach the reference yaw response characteristic from the predetermined yaw response characteristic, which is different from the reference yaw response characteristic. In this way, by changing the yaw response characteristic in accordance with the road surface frictional coefficient or the yaw rate deviation, it is possible to suppress change in the attitude of the vehicle  1  due to the yaw moment control. In addition, even if the control is switched from the yaw moment control to the control for stabilizing the attitude of the vehicle  1 , it is possible to prevent uncomfortable feeling from being given to the driver. 
     When the road surface frictional coefficient is smaller than the road surface frictional coefficient threshold value and/or when the yaw rate deviation is greater than the yaw rate deviation threshold value, the control gain calculation module  26  uses the vehicle reference yaw response characteristic as the yaw response characteristic used in the target yaw rate calculation module  25 . Thus, change in the attitude of the vehicle  1  due to yaw moment control is limited to almost zero, when the road surface frictional coefficient is smaller than the road surface frictional coefficient threshold value and/or when the yaw rate deviation is greater than the yaw rate deviation threshold value. Thus, the attitude of the vehicle  1  can be prevented from becoming unstable. 
     The control gain calculation module  26  is configured such that, when changing the yaw response characteristic used in the target yaw rate calculation module  25  from the reference yaw response characteristic to the predetermined yaw response characteristic, the amount of change in the yaw response characteristic per time is smaller than when changing the yaw response characteristic from the predetermined yaw response characteristic to the reference yaw response characteristic. Therefore, when the yaw response characteristic is changed from the predetermined yaw response characteristic to the reference yaw response characteristic, the attitude of the vehicle  1  can be stabilized immediately, and when the yaw response characteristic is changed from the reference yaw response characteristic to the predetermined yaw response characteristic, uncomfortable feeling can be prevented from being given to the driver. 
     The yaw response characteristic used in the target yaw rate calculation module  25  is represented by a calculation expression of a second-order lag system of the target yaw rate relative to the actual steering angle, and the yaw response characteristic is changed by changing the gain of the natural frequency in the yaw direction and/or the gain of the attenuation coefficient in the yaw direction included in the calculation expression. In this way, by representing the yaw response characteristic used in the target yaw rate calculation module  25  by a general calculation expression of a second-order lag system, the calculation expression can be simplified. In the calculation expression, the natural frequency and the attenuation coefficient in the yaw direction can be adjusted individually. 
     The vehicle attitude stabilization control device  15  uses the target yaw rate calculated by the target yaw rate calculation module  25 , for the control for stabilizing the attitude of the vehicle  1 . Therefore, even if the control is switched from the yaw moment control to the control for stabilizing the attitude of the vehicle  1 , the same target yaw rate as that used in the yaw moment control is used and thus the turning characteristic of the vehicle  1  with respect to steering stays unchanged. Therefore, no uncomfortable feeling is given to the driver. 
     The second embodiment will be described. As shown in  FIG. 9 , the control gain calculation module  26  may calculate the control gain in accordance with only the road surface frictional coefficient calculated by the road surface frictional coefficient calculation module  24 , without using the yaw rate deviation. That is, the yaw rate deviation-and/or-road surface friction calculation module  40  includes only the road surface frictional coefficient calculation module  24 . 
     As in the third embodiment shown in  FIG. 10 , the control gain calculation module  26  may calculate the control gain in accordance with only the yaw rate deviation calculated by the yaw rate deviation calculation module  29 , without using the road surface frictional coefficient. That is, the yaw rate deviation-and/or-road surface friction calculation module  40  includes only the yaw rate deviation calculation module  29 . 
     The configuration shown in  FIG. 9  or  FIG. 10  can reduce the calculation processing load of the turning control device as compared to the configuration shown in  FIG. 3 . 
     As the vehicle, other than the vehicle including the in-wheel motor driving devices for all four wheels as described above, a vehicle using a friction brake for a braking torque may be applied. 
       FIG. 11A  shows a four-wheel drive vehicle using an internal combustion engine as a driving source,  FIG. 11B  shows a front-wheel drive vehicle having the same driving source, and  FIG. 11C  shows a rear-wheel drive vehicle having the same driving source, and in each vehicle, the yaw moment control can be performed using a friction brake. In each vehicle  1  in  FIGS. 11A to 11C , driving torques by the engine are developed at for the turning outer wheels, and greater braking torques by the friction brakes than the driving torques are developed at the turning inner wheels, whereby a yaw moment is generated. 
     In the in-wheel motor driving device, a cycloidal speed reducer, a planetary reducer, a speed reducer with two parallel shafts, or other speed reducer may be provided. Alternatively, in-wheel motor driving device may be a so-called direct motor type in which no speed reducer is used. 
     As the vehicle, the following vehicle may be used: outputs of two motors provided in the vehicle body respectively for the right and left wheels are transmitted to the respective wheels via a drive shaft and the like and braking/driving torques for the wheels are controlled independently. 
     Although the modes for carrying out the present invention have been described on the basis of the embodiments, the embodiments disclosed herein are illustrative in all aspects and not restrictive. The scope of the present invention is indicated by the claims, rather than by the above description, and is intended to include any modifications within the scope and meaning equivalent to the claims. 
     REFERENCE NUMERALS 
     
         
         
           
               1  . . . vehicle 
               2  . . . wheel 
               4  . . . electric motor (braking/driving source) 
               18  . . . vehicle speed sensor (vehicle speed detection device) 
               19  . . . steering angle sensor (steering angle detection device) 
               20  . . . yaw rate sensor (yaw rate detection device) 
               24  . . . road surface frictional coefficient calculation module 
               25  . . . target yaw rate calculation module 
               26  . . . yaw response characteristic changing module (control gain calculation module) 
               27  . . . yaw moment calculation module 
               29  . . . yaw rate deviation calculation module 
               40  . . . yaw rate deviation-and/or-road surface friction calculation module