Abstract:
A drive-force distribution controller for a four-wheel-drive vehicle having a torque distribution unit configured to distribute an output torque transmitted from a prime motor to a first set of wheels to a second set of wheels. The drive-force distribution controller includes a first judging device for judging which of the first set is an inner wheel with respect to a turning of the vehicle, a second judging device for judging whether the turning of the vehicle is a tight turn, a third judging device for judging whether the inner wheel is slipping, a fourth judging device for judging whether an outer wheel of the first set of wheels is slipping, and a controlling device for controlling the torque distribution unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a drive-force distribution controller and a control method for a four-wheel-drive vehicle, and more particularly to a drive-force distribution controller and a control method for a four-wheel-drive vehicle in order to avoid a tight-corner braking phenomenon and a slip state when the vehicle turns. 
     2. Description of the Related Art 
     In a four-wheel-drive vehicle, it is known that a braking application acts on front and rear wheels due to a difference of a traveling distance therebetween when the vehicle travels on a corner whose radius is small in a case that a differential between front and rear wheel is restricted, i.e., a tight-corner braking phenomenon happens, whereby it cause to deteriorate a steering feeling in general. 
     To avoid this tight-corner braking phenomenon, that described in Japanese Patent Publication (Kokai) No. 8-2278 has been proposed, for example. In this related art, a differential speed between an outer front wheel and an inner rear wheel is calculated when the vehicle travels on the corner. In a case that the calculated differential speed is larger than a preset differential speed in a turning radius where the tight-corner braking phenomenon happens, it is judged that the tight-corner braking phenomenon happens, the differential between the front and rear wheel can be accepted by decreasing an engagement force in a differential restricting clutch of a torque distribution unit. 
     However, in the aforementioned art, when any one of the driving wheels travels on a low μ road, e.g., when any one of the driving wheel works in a rut in which water ponds, that wheel may slip. In such a situation, the tight-corner braking phenomenon is detected accidentally, so that the differential between the front and rear wheels may be accepted momentarily. 
     Further, in a case that the vehicle launches from the low μ road, e.g., a deep-snow road or a dirt road, it is generated frequently that any one of the driving wheels slips. In the front-wheel drive base four-wheel drive vehicle, one of the front wheels facilitates to slip to give a large drive force to the front wheels, for example. In this case, the tight-corner braking phenomenon is detected accidentally in the aforementioned art, so that the differential between the front and rear wheels may be accepted momentarily. Thus, in a case the differential between the front and rear wheel is accepted momentarily, a required drive force is not obtained substantially. As a result it may be generated that the vehicle gets free from the rut when traveling on the low μ road, or that the vehicle threatens to launch smoothly when launching on the low μ road. 
     Furthermore, the controller described in Japanese Patent Publication (Kokai) No. 11-1129 is proposed to prevent the tight-corner braking phenomenon when turning at an extremely low speed and to slip. In that controller, a wheel speed difference between the front and rear wheel is calculated based upon a wheel speed of each of the four wheels, an operated steering angle and yaw rate, and the engagement force in the differential restricting clutch of the torque distribution unit is based upon that wheel speed difference. Consequently, the tight-corner braking phenomenon is prevented when the vehicle turns at the extremely low speed, and the slipping is also prevented. In the aforementioned controller, a yaw rate sensor is however required, so that the control operation becomes complex. 
     Further, the aforementioned controller is of the type for controlling the wheel speed difference between the front and rear wheels, in a case that one of the driving wheels slips, the slipping may not be detected. In such a situation, the required drive force is not obtained substantially, so that it may be generated that the vehicle gets free from the rut when traveling on the low μ road, or that the vehicle threatens to launch smoothly when launching on the low μ road. 
     To solve the aforementioned problem, an object of the present invention is to provides a torque distribution controller of a four-wheel drive vehicle which is capable of preventing a generation of a tight-corner braking phenomenon and preventing slipping in one of the driving wheels infallibly. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a drive-force distribution controller for a four-wheel-drive vehicle having a torque distribution unit configured to distribute an output torque transmitted from a prime motor to a first set of wheels to a second set of wheels includes a first judging device for judging which of the first set of wheels is an inner wheel with respect to a turning of the four-wheel-drive vehicle, a second judging device for judging whether the turning of the four-wheel-drive vehicle is a tight turn, a third judging device for judging whether the inner wheel is slipping, a fourth judging device for judging whether an outer wheel of the first set of wheels is slipping, and a controller for controlling the torque distribution unit. The second judging device judges that the four-wheel-drive vehicle is engaged in a tight turn if the turning has a turning radius smaller than a predetermined value. The controller decreases the output torque to the second set of wheels if the second judging device judges that the four-wheel-drive vehicle is engaged in a tight turn and if the third and fourth judging devices judge that the first set of wheels are not slipping. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various other objects, features and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description of the preferred embodiments when considered in connection with the accompanying drawings, in which: 
     FIG. 1 is a diagram schematically showing the structure of a four-wheel-drive vehicle which is provided with a drive-force distribution controller according to first and second embodiments of the present invention; 
     FIG. 2 shows a block diagram of an electrical control circuit according to the present invention; 
     FIG. 3 is an explanatory view a calculation of a turning radius when the vehicle turns left, according to the present invention; 
     FIG. 4 is a flowchart showing a control operation according to the first embodiment of the present invention; 
     FIG. 5 is a flowchart showing a control operation according to the second embodiment of the present invention; 
     FIG. 6 is a diagram schematically showing the structure of a four-wheel-drive vehicle which is provided with a drive-force distribution controller according to third and fourth embodiments of the present invention; 
     FIG. 7 is a flowchart showing a control operation according to the third embodiment of the present invention; and 
     FIG. 8 is a flowchart showing a control operation according to the fourth embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described with reference to the drawings. 
     FIG. 1 is a diagram schematically showing the structure of a four-wheel-drive vehicle which is provided with a torque distribution unit according to first and second embodiments of the present invention. In the first and second embodiments, a description will be given while a four-wheel-drive vehicle which is designed on the basis of front-wheel-drive. 
     [First Embodiment] 
     A trans axle  14  is of unitedly providing a transmission, a transfer and a front differential  15 , in which a drive force (drive power) generated by an engine  12  is transmitted through the front differential  15  of the trans axle  14  to axle shafts  16  to drive left and right front wheels RT 1  and RT 2 , and further transmitted to a first propeller shaft  18 . The first propeller shaft  18  is connected with a second propeller shaft  22  through a torque distribution unit  20  (a coupling). When the first propeller shaft  18  is torque-transmittably connected with the second propeller shaft  22  by the torque distribution unit  20 , the drive force can be transmitted through a rear differential  25  to axle shafts  26  to drive left and right rear wheels RT 3  and RT 4 . 
     The torque distribution unit  20  is of being disposed between the first and second propeller shafts  18  and  22 , and is composed of clutch plates  19  for transmitting a torque from the first propeller shaft  18  to the second propeller shaft  22 , and a pressing mechanism (not illustrated) for engageably pressing the clutch plates  19 . When a command signal from an electrical control circuit  50  is given to the pressing mechanism, a pressing force corresponding to the command signal acts on the clutch plates  19 . In a case that it is so controlled to increase a torque distribution from the first propeller shaft  18  to the second propeller shaft  22  according to the command signal, the clutch plates  19  are directly connected to each other thereby, so that the torque of the first propeller shaft  18  can be directly transmitted to the second propeller shaft  22 , for example. In contrast, when it is so controlled to restrict the torque distribution from the first propeller shaft  18  to the second propeller shaft  22 , the command signal to the pressing mechanism is also restricted, so that the pressing force by the pressing mechanism does not act on the clutch plates  19 . Therefore, the clutch plates  19  are separated from each other, so that the torque from the first propeller shaft  18  is not transmitted to the second propeller shaft  22 . Thus, it is so constructed that the transmissible torque supplied from the first propeller shaft  18  to the second propeller shaft  22  can be adjusted by changing an engagement force of the clutch plates  19  in correspondence with a magnitude of the command signal. 
     On the driving (front) wheels RT 1  and RT 2  (hereinafter, the “driving wheels” mean primary recipients of the drive force from the engine  12 ) and the driven (rear) wheels RT 3  and RT 4  (hereinafter, the “driven wheels” mean secondary recipients of the drive force from the engine  12  via the torque distribution unit  20 ), there are respectively provided brakes B 1 , B 2 , B 3  and B 4 , and wheel speed sensors S 1 , S 2 , S 3  and S 4  for detecting a wheel speed (angular velocity). Besides, each of the speed sensors S 1 , S 2 , S 3  and S 4  is also used as a wheel speed sensor for an anti-lock brake system (ABS) which independently controls each of the brakes B 1 , B 2 , B 3  and B 4 . Further, in a steering column connected to a steering wheel  59  for operating the vehicle, there is provided a steering angular sensor S 5  for detecting an operated steering angle θ of the steering wheel  59 . 
     Next, a the electrical control circuit  50  for controlling the torque distribution unit  20  will be described with reference to FIG.  2 . To electrical control circuit  50 , there is inputted each of signals from the speed sensors S 1 , S 2 , S 3  and S 4 , and from the steering angular sensor S 5 . The electrical control circuit  50  is composed of an input/output circuit  58 , a CPU  52  for executing a variety of calculation and control, a ROM  54  in which control programs are memorized, and a RAM  56  used as work areas of the CPU  52 . In such a construction, a result of calculation in the CPU  52 , as an output signal, is transferred to the torque distribution unit  20  through the input/output circuit  58 . 
     Next, detecting operations of a tight-corner braking phenomenon and slip state in the electrical control circuit  50 , and control operations of the torque distribution unit  20  will be described with reference to FIG.  4 . In first step  510 , into the electrical control unit  50 , there is inputted a signal from each of the, sensors, i.e., wheel speed (angular velocity) ω 1 , ω 2 , ω 3  and ω 4  of the front wheels RT 1  and RT 2  and the rear wheels RT 3  and RT 4  from the wheel speed sensors S 1 , S 2 , S 3  and S 4 , and the operated steering angle θ from the steering angular sensor S 5 . 
     In subsequent step S 20 , it is judged which of the driving wheels (RT 1 , RT 2 ) is an inner wheel. This judgment (so-called “inner-wheel judgment” hereinafter) will be explained with reference to FIG.  3 . In a case that the detected wheel speed ω 3  of the left driven wheel RT 3  is smaller than the detected wheel speed ω 4  of the right driven wheel RT 4 , it is judged that the vehicle turns left. Consequently, the inner wheel is judged to be the left driven wheel RT 3  in which the wheel speed ω 3  is detected. Reversely, in a case that the detected wheel speed ω 4  of the right driven wheel RT 4  is smaller than the detected wheel speed ω 3  of the left driven wheel RT 3 , it is judged that the vehicle turns right. As a result, the inner wheel is judged to be the right driven wheel RT 4  in which the wheel speed ω 4  is detected. 
     Next step S 30  and subsequent steps thereto will be explained in a case that the vehicle is judged to be left-turning in step S 20 . 
     In step S 30 , it is judged whether the tight-corner braking phenomenon happens or not. Namely, it is judged that the tight-corner braking phenomenon does not happen when the operated (detected) steering angle θ is smaller than a preset steering angle θ 1  where the tight-corner braking phenomenon happens, and then the control operation moves to step S 70 . In step S 70 , there is executed a normal control operation for giving a large drive force to the driven (rear) wheels by increasing an engagement force in the clutch plates  19  of the torque distribution unit  20 . 
     Concerning the formal control operation, it is so controlled to adjust the engagement force in the clutch plates  19  of the torque distribution unit  20  in correspondence with the detected wheel speed. When the detected wheel speed is small, the wheels is facilitated to slip, so that it is so controlled to give large drive force to the driven wheels by increasing the engagement force in the clutch plates  19  of the torque distribution unit  20 . As a result, the slip state can be prevented. On the other hand, the engagement force in the clutch plates  19  of the torque distribution unit  20  is decreased as the detected wheel speed becomes large. Thus control operation is referred to as the normal control operation in step S 70 . Subsequently, when the operated (detected) steering angle θ is equal to or larger than the preset steering angle θ 1 , it is judged in step S 30  that the tight-corner braking phenomenon happens, the control operation advances to next step S 40 . 
     In step S 40 , it is judged whether an inner slip state happens or not. Namely, it is judged whether the detected wheel speed ω 1  of the inner driving wheel RT 1  is or not larger than the detected wheel speed ω 2  of the outer driving wheel RT 1 . When it is judged that the inner slip state happens (“NO” in step S 40 ), the control operation is skipped to step S 70  to execute the normal control operation while, when it is judged that the detected wheel speed ω 1  of the inner driving wheel is equal to or smaller than the detected wheel speed ω 2  of the outer driving wheel (“YES” in step S 40 ), the control operation is advanced to subsequent step S 50 . 
     In step S 50 , it is judged whether an outer slip state happens or not. This judgment will be explained with reference to FIG.  3 . In the figure, a radius of the each wheel is presented as “r”, and turning radii of the wheels RT 1 , RT 2 , RT 3  and RT 4  are respectively represented as “R 1 ”, “R 2 ”, “R 3 ” and “R 4 .” Further, a turning angular speed (velocity) of the vehicle is represented as “ω”, a front tread between the inner front wheel RT 1  and outer front wheel RT 2  and a rear tread between the inner rear wheel RT 3  and outer rear wheel RT 4  are respectively represented as “L f ” and “L r .” Moreover, a wheelbase between the driving and driven wheels is represented as “L.” Herein, in a condition that each of the wheels RT 1 , RT 2 , RT 3  and RT 4  do not slip, the turning speed relationship is described with following expressions (1)-(4): 
     
       
           R   1   ·ω=r·ω   1   (1) 
       
     
     
       
           R   2   ·ω=r·ω   2   (2) 
       
     
     
       
           R   3   ·ω=r·ω   3   (3) 
       
     
     
       
           R   4   ·ω=r·ω   4   (4) 
       
     
     Expression (5) is established based upon these expressions (1)-(4). 
     
       
           R   1 /ω 1   =R   2 /ω 2   =R   3 /ω 3   =R   4 /ω 4   (5) 
       
     
     In this situation, the difference between the front tread L f  and the rear tread L r  is extremely small, so that expressions (6) and (7) are derived in which L f ≈L r  is premised. 
     
       
           R   1   2   =R   3   2   +L   2   (6) 
       
     
     
       
           R   2   2   =R   4   2   +L   2   (7) 
       
     
     Herein, expression (8) is derived by subtracting expression (7) from expression (6). 
     
       
           R   1   2   −R   2   2   =R   3   2   −R   4   2   (8) 
       
     
     Where the wheel speed (angular velocity) of the outer driving wheel calculated by expressions (5) and (8) is represented as “ω 2     1     2 .” 
     
       
         ω 2     1     2 =ω 4   2 +ω 1   2 −ω 3   2   (9) 
       
     
     From thus-derived relationship, when the detected wheel speed ω 2   2  of the outer driving wheel is larger than the calculated wheel speed ω 2     1     2  thereof (“No” is judged in step S 50 ), it is judged that the outer wheel slip state happens, and then the control operation advances to step S 70 . In step S 70 , the normal control operation is executed, so that the slip state can be prevented. Further, when the detected wheel speed ω 2   2  of the outer driving wheel is equal to or smaller than the calculated wheel speed ω 2     1     2  thereof (“YES” is judged in step S 50 ), it is judged that the tight-corner braking phenomenon happens, the control operation advances to step S 60 . In step S 60 , the tight-corner braking phenomenon can be prevented by decreasing the engagement force in the clutch plates  19  of the torque distribution unit  20 . 
     Besides, in the judgment whether the tight-corner braking phenomenon happens or not in the aforementioned step S 30  of FIG. 4, it is judged based upon the detected steering angle θ from the steering angular sensor S 5 . However, it is also judged by using the turning radius calculated from the wheel speed, as will be explained hereinafter 
     The turning radius R 4  of the outer rear wheel RT 4  is derived by expression (10) with addition of the rear tread L r  to the turning radius R 3  of the inner rear wheel RT 3 . 
     
       
           R   4   =R   3   +L   r   (10) 
       
     
     Herein, the turning radius R 3  of the inner rear wheel RT 3  can be calculated by following expression (11) with expressions (5) and (10). 
     
       
           R   3   =L   r /((ω 4 /ω 3 )−1)  (11) 
       
     
     Based upon this relationship, it is judged that the tight-corner braking phenomenon does not happen when the tuning radius R 3  of the inner rear wheel RT 3  exceeds a preset turning radius R 31  in which the tight-corner braking phenomenon happens. Thereafter, the control operation advances to step S 70  and then, the normal control operation is executed. On the other hand, when the turning radius R 3  of the inner rear wheel RT 3  is equal to or smaller than the preset turning radius R 31 , it is judged that the tight-corner braking phenomenon happens, so that the control operation moves to step S 40 , 
     Thus, if the generation of the tight-corner braking phenomenon is judged based upon the turning radius R 3  of the inner rear wheel RT 3 , the steering angular sensor S 5  can be eliminated. 
     The above description is of that it is judged that the vehicle turns left in step S 20 . In a case that it is judged that the vehicle turns right in step S 20 , the control operation following step S 30  is similarly explained (flowchart itself is omitted in FIG. 4) by substituting as that the wheel speeds of the inner driving wheel is ω 1 , the wheel speed of the outer driving wheel is ω 2 , the wheel speed of the inner driven wheel is ω 3 , and the wheel speed of the outer driven wheel is ω 4 . 
     According to the aforementioned first embodiment of the present invention, in a front wheel drive base four-wheel drive vehicle, the generation of the tight-corner braking phenomenon is judged in step S 30  of FIG.  4 . In such a situation, the wheel speed of the driven wheel that does not facilitate to slip relative to the driving wheel is used for judgment, so that the generation of the tight-corner braking phenomenon can be precisely judged. 
     [Second Embodiment] 
     Next, the second embodiment according to the present invention will be described hereinafter with reference to FIG. 5 In this second embodiment, steps S 20 , S 30  and S 50  of FIG. 4 are substituted to steps S 120 , S 130  and S 150  of FIG. 5, in which the difference relative to the first embodiment is of using a detected steering angle for control operation. Therefore, the control operations of steps S 120 , S 130  and S 150  different from the first embodiment will be mainly described in the second embodiment. 
     In step S 120 , it is judged which of the driving wheels is an inner wheel. Namely, this inner-wheel judgment is carried out based upon a detected steering angle θ from the steering angle sensor. This judgment may be executed based upon the detected wheel speed of the driven wheel, as described in the first embodiment shown in step S 20  of FIG. 4 
     Next, the control operation of step S 130  and subsequent steps thereto will be described hereinafter in a case that it is judged that the vehicle turns left in step S 120 . 
     In step S 130 , it is judged whether the tight-corner braking phenomenon happens or not, i.e., it is judged that the tight-corner braking phenomenon does not happen when the steering operated angle θ is smaller than a preset angle θ 1  where the tight-corner braking phenomenon happens, and then the control operation moves to step S 70  to execute the normal control operation. In contrast, when the detected steering angle θ is equal to or larger than the preset angle θ 1 , it is judged that the tight-corner braking phenomenon happens. Subsequently, the control operation advances to step S 140  and however, the control operation of step S 140  is similar to that of step S 40  of the first embodiment, so that its explanation is omitted. Next, the control operation of step S 150  will be described hereinafter. 
     In step S 150 , it is judged whether an outer-wheel slip state happens or not. This judgment will be explained with reference to FIG.  3 . In a case that a wheel speed of the outer driving wheel RT 2  is presented as “ω 2   2 ”, an average wheel speed ω 5  between the wheel speeds ω 3  and ω 4  of the inner and outer driven wheels RT 3  and RT 4  is presented by expression (12). 
     
       
         ω 5 =(ω 3 +ω 4 )/2  (12) 
       
     
     Where a steering gear ratio is presented as “N,” and where an actual steered angle is presented as “δ,” the relationship between the steering gear ratio N, steering operated angle θ and actual steered angle δ is presented by “δ=θ/N.” The wheel speed ω 2     2    of the driving wheel calculated due to this relationship is derived by expression (13). 
     
       
         ω 2     2   =ω 5 /cos δ=(ω 3 +ω 4 )/(2·cos δ)  (13) 
       
     
     In the aforementioned expression (13), it is not taken account into such that an influence of a term corresponding to the front tread L f /2 is small and however, more precise judgment can be established by taking account into a calculation in the influence of a term corresponding to the front tread L f /2. 
     Due to these relationships, when the detected wheel speed ω 2  of the outer driving wheel is larger than the calculated wheel speed ω 2     2    thereof (“NO” is judged in step S 150 ), it is judged that the outer-wheel slip state happens, and then the control operation moves to step S 70 . The formal control operation is executed in step S 70 , so that the outer-wheel slip state can be prevented. Reversely, when the detected wheel speed ω 2  of the outer driving wheel is equal to or smaller than the calculated wheel speed ω 2  thereof (“YES” is judged in step S 150 ), it is judged that the tight-corner braking phenomenon happens and then, the control operation moves to step S 60 . In step S 60 , the generation of the tight-corner braking phenomenon can be prevented by decreasing the engagement force in the clutch plates  19  of the torque distribution unit  20 . 
     The aforementioned control operation is explained about a case that it is judged that the vehicle turns left in step S 120 . In a case that it is judged that the vehicle turns right in step S 120 , the control operation is omitted in FIG.  5  and however, that of step S 130  and subsequent steps thereto is similarly explained by respectively substituting the wheel speeds of the inner driving wheel, outer driving wheel, inner driven wheel and outer driven wheel to ω 1 , ω 2 , ω 3  and ω 4 . 
     According to the aforementioned second embodiment, in a front-wheel-drive base four-wheel-drive vehicle, the judgment operation for which is an inner/outer driving wheel shown in step S 120  of FIG. 5 is executed by the detected steering angle, so that the influence of the wheel slip state can be eliminated compared with a case that it is judged by the wheel speed. Therefore, the judgment of the inner/outer driving state can precisely carried out in no connection with the slip state of the wheel. Further, in the outer wheel slip judgment shown in step S 150  of FIG. 5, it is judged due to the wheel speed of the driven wheel that does not facilitate to slip relative to the driving wheel and therefore, the outer wheel slip judgment can be executed precisely. 
     [Third Embodiment] 
     Next, a torque distribution unit of a four-wheel-drive vehicle according to the third embodiment of the present invention will be will be described hereinafter with reference to the drawings. In the third embodiment, FIG. 1 in the first and second embodiments is substituted to FIG. 6, and steps S 20 -S 50  therein are substituted to steps S 220 -S 250  of FIG.  7 . The difference relative to the fist embodiment is to change the vehicle drive type from a front-wheel-drive base to a rear-wheel-drive base. Therefore, the construction and control operation for the third embodiment will be mainly described with reference to FIG.  6  and steps S 220 -S 250  of FIG.  7 . 
     FIG. 6 is a diagram schematically showing the structure of a four-wheel-drive vehicle which is provided with a torque distribution unit according to the third and fourth embodiments of the present invention. A trans axle  14  is of unitedly providing a transmission and a transfer, in which a drive force (drive power) generated by an engine  12  is transmitted from the trans axle  14  to axle shafts  26  via a first propeller shaft  22  and a rear differential  25  to drive left and right rear wheels RT 3  and RT 4 . With this transmission, the drive force is transmitted to a torque distribution unit  20  to which a second propeller shaft  61 . When the trans axle  14  is torque-transmissibly connected to the second propeller shaft  61  by the torque distribution unit  20 , the drive force is transmitted from a front differential  15  to axle shafts  16  to drive left and right front wheels RT 1  and RT 2 . 
     The torque distribution unit  20  is of being disposed between the trans axle  14  and the second propeller shaft  61 , and is composed of clutch plates  19  for transmitting a torque (drive force) from the trans axle  14  to the second propeller shaft  61 , and of a pressing mechanism (not illustrated) for pressing the clutch plates  19 . When a command signal from an electrical control circuit  50  is given to the pressing mechanism, a pressing force corresponding to the command signal acts on the clutch plates  19 . In a case that it is so controlled to increase a torque distribution from the trans axle  14  to the second propeller shaft  61  due to the command signal, the clutch plates  19  are directly connected thereby, so that the torque from the trans axle  14  can be directly transmitted to the second propeller shaft  51 , for example. In contrast, when it is so controlled to restrict the torque distribution from the trans axle  14  to the second propeller shaft  61 , the command signal is also restricted to the pressing mechanism, so that the pressing force by the pressing mechanism does not act on the clutch plates  19 . Therefore, the clutch plates  19  are separated from each other, so that the torque from the trans axle  14  is not transmitted to the second propeller shaft  61 . Thus, it is so constructed that the transmissible torque supplied from the trans axle  14  to the second propeller shaft  61  can be adjusted by changing an engagement force of the clutch plates  19  in correspondence with a magnitude of the command signal. 
     On the driven wheels RT 1  and RT 2  and the driving wheels RT 3  and RT 4 , there are respectively provided brakes B 1 , B 2 , B 3  and B 4 , and wheel speed sensors S 1 , S 2 , S 3  and S 4  for detecting each of wheel speeds (velocity). Besides, each of the speed sensors S 1 , S 2 , S 3  and S 4  also is used for a wheel speed sensor for an anti-lock brake system (ABS) which independently controls each of the brakes B 1 , B 2 , B 3  and B 4 . Further, in a steering column connected to a steering wheel  59  for operating the vehicle, there is provided a steering angular sensor S 5  for detecting an operated steering angle θ of the steering wheel  59 . 
     Herein, a control circuit used in this embodiment is same as that in the first and second embodiments (shown in FIG.  2 ). Further, a control operation in step S 10  of FIG. 7 is also same as that in the first and second embodiments (shown in FIGS.  4  and  5 ). 
     Next, the control operations in subsequent steps S 220 -S 250  in FIG. 7 will be described hereinafter for explanation. 
     In step S 220 , it is judged which of the driving wheels is an inner wheel or an outer wheel. This inner/outer-wheel judgment will be explained with reference to FIG.  3 . In a case that a detected wheel speed ω 1  of the left driven wheel RT 1  is smaller than the detected wheel speed ω 2  of the right driven wheel RT 2 , it is judged that the vehicle turns left. Consequently, the inner wheel is judged to be the left driven wheel RT 1  in which the wheel speed ω 1  is detected. Reversely, in a case that the detected wheel speed ω 2  of the right driven wheel RT 2  is smaller than the detected wheel speed ω 1  of the left driven wheel RT 1 , it is judged that the vehicle turns right. As a result, the inner wheel is judged to be the right driven wheel RT 2  in which the wheel speed ω 2  is detected. 
     Next, the control operation of step, S 230  and subsequent steps thereto will be explained, in which it is judged in step S 220  that the vehicle turns left. 
     In step S 230 , it is judged whether a tight-corner braking phenomenon happens or not. In this judgment, it is judged that the tight-corner braking phenomenon does not happen when the operated steering angle θ is smaller than a predetermined angle θ 1  where the tight-corner braking phenomenon happens, and then the control operation moves to step S 270  in which the formal control is executed. In contrast when the operated steering angle θ is equal to or larger than the predetermined angle θ 1 , it is judged that the tight-corner braking phenomenon happens, and then the control operation moves to next step S 20 . 
     In step S 240 , the inner wheel judgment is executed, i.e., it is judged whether the detected wheel speed ω 3  of the inner driving wheel is larger than the detected wheel speed ω 4  of the driving outer wheel. When “NO” is judged in step S 240 , it judged to be the inner-wheel slip state, the control operation is skipped to step S 70 . In step S 70 , the normal control operation is executed, so that the slip state of the inner wheel can be prevented. Reversely, when it is judged that the detected wheel speed ω 3  of the inner driving wheel is equal to or smaller than the detected wheel speed ω 4  of the outer driving wheel (“YES” in step S 240 ), the control operation is advanced to next step S 250 . 
     In step S 250 , it is judged whether the outer-wheel slip state happens or not. This judgment will be explained with reference to FIG.  3 . However, the relationship between the turning radii (R 1 , R 2 , R 3 , R 4 ), wheel speeds (ω 1 , ω 2 , ω 3 , ω 4 ) and wheelbase (L) is same as that in the first embodiment, the explanation is omitted in calculation of expression (1)-(8). 
     Herein, it is assumed that the wheel speed (angular velocity) of the outer driving wheel calculated by expressions (5) and (8) is represented as “ω 4     1     2 ”, it is derived by expression (14). 
     
       
         ω 4     1     2 =ω 2   2 +ω 3   2 −ω 1   2   (14) 
       
     
     From thus-derived relationship, when the detected wheel (angular) speed ω 4   2  of the outer driving wheel is larger than the calculated wheel (angular) speed ω 4     1     2  thereof (“NO” is judged in step S 250 ), it is judged that the outer-wheel slip state happens, and then the control operation advances to step S 70 . In step S 70 , the normal control operation is executed, so that the slip state can be prevented. Further, when the detected wheel (angular) speed ω 4   2  of the outer driving wheel is equal to or smaller and the calculated wheel (angular) speed ω 4   2  thereof (“YES” is judged in step S 250 ), it is judged that the tight-corner braking phenomenon happens, the control operation advances to step S 260 . In step S 60 , the tight-corner braking phenomenon can be prevented by decreasing the engagement force in the clutch plates  19  of the torque distribution unit  20 . 
     Besides, in the judgment whether the tight-corner braking phenomenon happens or not in the aforementioned step S 230  of FIG. 7, it is judged based upon the detected steering operated angle θ from the steering angular sensor S 5 . However, it is also judged by using the turning radius calculated from the wheel speed, as will be explained hereinafter. 
     As shown in FIG. 3, the front tread between the inner and outer front wheels RT 1  and RT 2 , and the rear tread between the inner and outer wheels RT 3  and RT 4  are respectively represented as “L f ” and “L r .” Further, in a case that an equivalent front tread between the inner and outer front wheels RT 1  and RT 2 , is presented as “L p ”, expression (15) is derived based upon the turning radius of the front wheel and wheel speeds. 
     
       
           R   1   :r·ω   1   =R   1   +L   p   :r·ω   2   (15) 
       
     
     Herein, “L p ≈L f ” is established so that expression (16) is derived from expression (15). 
     
       
           R   1   =L   f   ·r·ω   1 /( r·ω   2   −r·ω   1 )  (16) 
       
     
     Based upon thus-derived relationship, when the calculated turning radius R 1  of the inner front wheel RT 1  exceeds a predetermined turning radius R 1     1    where the tight-corner braking phenomenon happens, it is judged in step S 270  that the tight-corner braking phenomenon does not happen, and then the control operation moves to step S 70  to execute the normal control operation. Reversely, when the calculated turning radius R 1  of the inner front wheel RT 1  exceeds a predetermined turning radius R 1     1   , it is judged in the step that the tight-corner braking phenomenon happens, and then the control operation moves to step S 240 . 
     Thus, the generation of the tight-corner braking phenomenon is judged by the calculated turning radius R 1  of the inner front wheel RT 1 , so that it is possible to eliminate the steering angle sensor S 5 . 
     The above description is of that it is judged that the vehicle turns left in step S 220 . In a case that it is judged that the vehicle turns right in step S 220 , the control operation following step S 230  is similarly explained (flowchart itself is omitted in FIG. 7) by substituting as that the wheel speeds of the inner driven wheel is ω 1 , the wheel speed of the outer driven wheel is ω 2 , the wheel speed of the inner driving wheel is ω 3 , and the wheel speed of the outer driving wheel is ω 4 . 
     According to the aforementioned third embodiment, in a rear-wheel-drive base four-wheel-drive vehicle, the generation of the tight-corner braking phenomenon is judged in step S 230  of FIG.  7 . In such a situation, the wheel speed of the driven wheel that does not facilitate to slip relative to the driving wheel is used for judgment, so that the generation of the tight-corner braking phenomenon can be precisely judged. 
     [Fourth Embodiment] 
     Next, a fourth embodiment according to the present invention will be described hereinafter with reference to FIG.  8 . In this fourth embodiment, steps S 220 , S 230  and S 250  shown in FIG. 7 are substituted to steps S 320 , S 330  and S 350  shown in FIG. 8, in which the difference relative to the embodiment is of using a detected steering operated angle for control operation. Therefore, the control operations of steps S 320 , S 330  and S 350  in FIG. 8 different from the third embodiment will be mainly described in the second embodiment. 
     In step S 320 , it is judged which of the driving wheels is an inner or outer wheel. Namely, the judgment of the inner/outer wheel is carried out based upon a detected steering operated angle θ from the steering angle sensor. This judgment, as described in the third embodiment shown in step S 220  of FIG. 7, may be executed based upon the detected wheel speed of the driven wheel. 
     Next, the control operation of step S 130  and subsequent steps thereto will be described hereinafter in a case that it is judged that the vehicle turns left in step S 320 . 
     In step S 330 , it is judged whether the tight-corner braking phenomenon happens or not. In this judgment, it is judged that the tight-corner braking phenomenon does not happen when the steering operated angle θ is smaller than a predetermined angle θ 1  where the tight-corner braking phenomenon happens, and then the control operation moves to step S 70  in which the normal control operation is executed. In contrast, when the steering operated angle θ is equal to or larger than the predetermined angle θ 1 , it is judged that the tight-corner braking phenomenon happens. Subsequently, the control operation advances to step S 340  and however, the control operation of step S 340  is similar to that of step S 240  of the third embodiment, so that its explanation is omitted. Next, the control operation of step S 350  will be described hereinafter. 
     In step S 350 , it is judged whether an outer-wheel slip state happens or not. In a case that a wheel speed of the outer driving wheel RT 4  is presented as “ω 4     2   ”, an average wheel speed ω 6  between the wheel speeds ω 1  and ω 2  of the inner and outer driven wheels RT 1  and RT 2  is presented by expression (17). 
     
       
         ω 6 =(ω 1 +ω 2 )/2  (17) 
       
     
     Where a steering gear ratio, is presented as “N,” and where an actual steered angle is presented as “δ”, the relationship between the steering gear ratio N, steering operated angle θ and actual steered angle δ is presented by “δ=θN.” The wheel speed ω 4     2    of the driving wheel calculated due to this relationship is presented by expression (18). 
     
       
         ω 4     2   =ω 6 ·cos δ=(ω 1 +ω 2 ) cos δ/2  (18) 
       
     
     Due to these relationships, when the detected wheel speed ω 4  of the outer driving wheel is larger than the calculated wheel speed ω 4     2    thereof (“NO” is judged in step S 350 ), it is judged to be the outer wheel slip state, and then the control operation moves to step S 70 . The formal control operation is executed in step S 70 , so that the slip state can be prevented. Reversely, when the detected wheel speed ω 4  of the outer driving wheel is equal to or smaller than the calculated wheel speed ω 4     2    thereof (“YES” is judged in step S 350 ), it is judged that the tight-corner braking phenomenon happens and then, the control operation moves to step S 60 . In step S 60 , the generation of the tight-corner braking phenomenon can be prevented by decreasing the engagement force in the clutch plates  19  of the torque distribution unit  20 . 
     The aforementioned control operation is explained about a case that it is judged that the vehicle turns left in step S 320 . In a case that it is judged that the vehicle turns right in step S 320 , the control operation is omitted in FIG.  5  and however, that of step S 330  and the following step thereto is similarly explained by respectively substituting the wheel speeds of the inner driven wheel, outer driven wheel, inner driving wheel and outer driving wheel to ω 1 , ω 2 , ω 3  and ω 4 . 
     According to the aforementioned fourth embodiment, in a rear-wheel-drive base four-wheel-drive vehicle, the judgment operation for which is an inner/outer driving wheel shown in step S 120  of FIG. 5 is executed by the steering operated angle, the influence of the wheel slip state can be eliminated compared with a case that it is judged by the wheel speed. Therefore, the judgment of the inner/outer driving state can precisely carried out in no connection with the slip state of the wheel. Further, in the outer wheel slip judgment shown in step S 350  of FIG. 8, it is judged due to the wheel speed of the driven wheel that does not facilitate to slip relative to the driving wheel and therefore, the outer-wheel slip judgment can be executed precisely. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.