Abstract:
The objective of the present invention is to provide a control device for maintaining vehicle behavior stability by suppressing the transfer of excess driving force to the rear wheels even when the traction control is not effective. According to the present invention, there is provided a control device for a four-wheel drive vehicle, including: a yaw moment control unit for selecting a wheel based on a driving state and generating yaw moment on the vehicle by applying braking force to the selected wheel; a driving force control unit for identifying excess driving force based on the driving state and reducing the identified excess driving force; a limited slip differential control unit for limiting a differential between a front right wheel and a front left wheel; a front and rear driving force distribution control unit for controlling engagement torque of a clutch unit which varies torque distribution between front and rear axles via a center differential; and a switch unit for selecting an operation state or a non-operation state of the driving force control unit, wherein the front and rear driving force distribution control unit sets the engagement torque of the clutch unit at a value when the yaw moment control unit operates while the driving force control unit is in the non-operation state, the value being greater than that of the engagement torque of the clutch unit when the yaw moment control operates while the driving force control unit is in the operation state.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority under 35 U.S.C. 119 based upon Japanese Patent Application Serial No. 2005-018804, filed on Jan. 26, 2005. The entire disclosure of the aforesaid application is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a control device for a four-wheel drive vehicle, comprising yaw moment control for generating yaw moment on the vehicle, driving force control for suppressing excess driving force, and limited slip differential control for limiting differential motion between the right and left wheels.  
         [0004]     2. Description of the Related Art  
         [0005]     Four-wheel drive vehicles manufactured in recent years are equipped with a number of vehicle behavior control devices such as yaw moment control for generating yaw moment on the vehicle, driving force control (traction control) for suppressing excess driving force, limited slip differential control (LSD control) for limiting differential motion between the right and left wheels, and the like. Since these controls often interfere with each other, a wide variety of techniques have been developed and utilized in order for these controls to effectively work together to their full potential.  
         [0006]     For example, Japanese Laid-open Publication 2004-106649 discloses a technique wherein transfer clutch torque is adjusted to be small enough to avoid the interference of the behavior controls such as the yaw moment control, the traction control, or the ABS (Anti-lock Brake System) operation in a four-wheel drive vehicle where the driving force is distributed between front and rear axles by, instead of a center differential, a transfer clutch that can vary the transfer capacity. The entire disclosure of the aforesaid publication is incorporated herein by reference.  
         [0007]     For example, when the yaw moment control operates to prevent an over-steering tendency, the braking force is applied to the front wheel located at the outer side of turning. In this case, the braking force due to the limited slip differential control is applied to the front wheel located at the inner side of the turning, because a difference in wheel velocity has been generated between the right and left front wheels. While the cornering force generated by the front tires is maintained owing to the braking forces applied to the two front wheels, the driving force is transferred to the two rear wheels. If the traction control via a switch means is effective, the engine control prevents the transfer of excess driving force to the two rear wheels. However, if the traction control is not effective, the driving force is excessively transferred to the two rear wheels so that the cornering force generated by the rear tires tends to decrease. As a result, the yaw moment along the same direction as the vehicle&#39;s yaw moment is generated because of the reduction in the cornering force at the two rear wheels while the cornering force at the two front wheels is being maintained. Thus, the over-steering tendency cannot be prevented after all. In this case, if a transfer clutch that can vary the transfer capacity is used for the driving force distribution instead of a center differential, as disclosed in the aforesaid Japanese Laid-open publication 2004-106649, it is possible to reduce the torque transfer from the front wheels to the rear wheels by reducing the transfer clutch torque, thereby preventing the interference of the yaw moment control with the rear wheels. However, in the center differential mechanism wherein the torque distribution between the front and rear axles is controlled by means of a clutch that can vary the transfer capacity, reducing the engagement torque of the clutch may permit the transfer of excess driving force to the rear wheels, likely to cause unstable vehicle behaviors.  
       SUMMARY OF THE INVENION  
       [0008]     In view of the above circumstances, the object of the present invention is to provide a control device for a four-wheel drive vehicle for suppressing the transfer of excess driving force to the rear wheels so as to maintain the vehicle behavior stability even when the traction control is not effective.  
         [0009]     According to the present invention, there is provided a control device for a four-wheel drive vehicle, comprising: a yaw moment control unit for selecting a wheel based on a driving state and generating yaw moment on the vehicle by applying braking force to the selected wheel; a driving force control unit for identifying excess driving force based on the driving state and reducing the identified excess driving force; a limited slip differential control unit for limiting a differential between a front right wheel and a front left wheel; a front and rear driving force distribution control unit for controlling engagement torque of a clutch unit which varies torque distribution between front and rear axles via a center differential; and a switch unit for selecting an operation state or a non-operation state of the driving force control unit, wherein the front and rear driving force distribution control unit sets the engagement torque of the clutch unit at a value when the yaw moment control unit operates while the driving force control unit is in the non-operation state, the value being greater than that of the engagement torque of the clutch unit when the yaw moment control operates while the driving force control unit is in the operation state.  
         [0010]     By use of the control device for a four-wheel drive vehicle according the present invention, it is possible to maintain vehicle behavior stability since the transfer of excess driving force to the rear wheels is suppressed even when the traction control is not effective.  
         [0011]     Those skilled in the art will appreciate these and other advantages and benefits of various embodiments of the invention upon reading the following detailed description of the preferred embodiments with reference to the below-listed drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is an explanatory diagram showing the schematic structure of the vehicle provided with a plurality of behavior control units.  
         [0013]      FIG. 2  is a functional block diagram of the center differential limited slip differential control unit.  
         [0014]      FIG. 3  is an explanatory diagram showing the target front and rear differential rotation frequency and the front and rear differential rotation frequency for starting the control vs. the vehicle velocity.  
         [0015]      FIG. 4  is an explanatory diagram showing the front and rear differential rotation frequency for starting the control vs. the steering angle.  
         [0016]      FIG. 5  is a flowchart showing the center differential limited slip differential control program.  
         [0017]      FIG. 6  is a flowchart showing the computation routine for the front and rear limited slip differential torque Tlsdctr. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     Embodiments of the present invention are described below with reference to the accompanying drawings to facilitate understanding of the present invention.  
         [0019]     In  FIG. 1 , the reference numeral  1  refers to an engine mounted on the front of a vehicle. The driving force generated by the engine  1  is transmitted through an automatic transmission system  2  (shown including a torque converter and other parts) in the rear of the engine and through a transmission output axle  2   a  to a center differential system  3 .  
         [0020]     The driving force transmitted to the center differential system  3  is inputted to a rear wheel final reduction device  7  through a rear drive axle  4 , a propeller shaft  5 , and a drive pinion axle, as well as to a front wheel final reduction device  11  through a transfer drive gear  8 , a transfer driven gear  9 , and a front drive axle  10  which serves as a drive pinion axle. The automatic transmission system  2 , the center differential system  3 , and the front wheel final reduction device  11  are housed in a case  12 .  
         [0021]     The driving force transmitted to the rear wheel final reduction device  7  is further transmitted to a left rear wheel  14   rl  through a left rear wheel drive axle  13   rl  as well as to a right rear wheel  14   rr  through a right rear wheel drive axle  13   rr . Similarly, the driving force transmitted to the front wheel final reduction device  11  is further transmitted to a left front wheel  14   fl  through a left front wheel drive axle  13   fl  as well as to a right front wheel  14   fr  through a right front wheel drive axle  13   fr.    
         [0022]     The center differential system  3  includes a first sun gear  15  with a large diameter at the transmission output axle  2   a , the first sun gear  15  engaged with a first pinion  16  with a small diameter to form a first gear array.  
         [0023]     At the rear drive axle  4 , a second sun gear  17  with a small diameter is provided, the second sun gear  17  engaged with a second pinion  18  with a large diameter to form a second gear array.  
         [0024]     First and second pinions  16  and  18  are integrally formed on a pinion section  19 , a plurality of which (three, for example) are rotatably supported at a fixed shaft provided for a carrier  20 . At the front end of the carrier  20 , the transfer drive gear  8  is attached for outputting the driving force to the front wheels.  
         [0025]     The transmission output axle  2   a  is rotatably inserted into the carrier  20  from the front, and the rear drive axle  4  is rotatably inserted into the carrier  20  from the rear. In the space between the two, the first sun gear  15  and the second sun gear  17  are housed. The first and second pinions  16  and  18  at each pinion section  19  are engaged with the first and second sun gears  15  and  17 , respectively.  
         [0026]     As seen above, compound planetary gears without ring gears are provided with respect to the first sun gear  15  at the input side with one output side through the first and second pinions  16  and  18  and the second sun gear  17 , and the other output side through the carrier  20  having the first and second pinions  16  and  18 .  
         [0027]     The center differential system  3  with such compound planetary gears is made to exhibit a differential function by properly setting the number of teeth of the first and second sun gears  15  and  17 , as well as of the plurality of the first and second pinions  16  and  18  surrounding the sun gears  15  and  17 .  
         [0028]     Furthermore, a standard torque distribution (for example, an unequal torque distribution with more torque on the rear wheels) can be predetermined by properly setting the pitch radii of the engagement between the first and second pinions  16  and  18  and the first and second sun gears  15  and  17 , respectively.  
         [0029]     In the center differential system  3 , helix gears, for example, may be used for the first and second pinions  16  and  18  and the first and second sun gears  15  and  17 , and the torsion angle may be made different between the first and second gear arrays so as not to cancel the thrust load of each other. Furthermore, the force generated by combining the separation load and the tangential load, acting on the engagement point on the surface of the fixed shaft provided for the carrier  20  and the first and second pinions  16  and  18 , is made to act on both ends of the pinion section  19  so as to generate friction torque thereon. The center differential system  3  is made to exhibit a limited slip differential function by obtaining the limited slip differential torque to be proportional to the input torque.  
         [0030]     A center differential clutch  21  is provided between the two output sections of the center differential system  3 , i.e. the carrier  20  and the rear drive axle  4 . A hydraulic multi-board clutch is employed for the center differential clutch  21 , which variably controls the driving force distribution between the front and rear axles. By controlling the engagement torque of the center differential clutch  21 , it is possible to vary the torque distribution ratio between the front and rear wheels from 50:50 representing a four-wheel drive with directly connected front and rear wheels to the value (35:65, for example) set by the center differential system  3 .  
         [0031]     The center differential clutch  21  is connected to a center differential clutch driving unit  41 , which comprises a hydraulic circuit with a plurality of solenoid valves. Engagement and release of the clutch are controlled by the hydraulic pressure generated at the center differential clutch driving unit  41 . The control signal for driving the center differential clutch driving unit  41  (a signal to each solenoid valve) is outputted from a center differential limited slip differential control unit  40 , which is described later.  
         [0032]     The reference numeral  51  refers to a brake driving unit, which is connected to a master cylinder (not shown in the figure) connected to a brake pedal operated by the driver. When the driver operates the brake pedal, the master cylinder applies brake pressure via the brake drive unit  51  to wheel cylinders  31   fl ,  31   fr ,  31   rl , and  31   rr  of wheels  14   fl ,  14   fr ,  14   rl , and  14   rr , respectively, for braking the four wheels. The brake driving unit  51  is a hydraulic unit equipped with a source for applying pressure, a valve for reducing pressure, a valve for increasing pressure, and the like. Brake pressure can be individually applied to each of the wheel cylinders  31   fl ,  31   fr ,  31   rl , and  31   rr  according to the input signals from a yaw moment control unit  50  and a limited slip differential control unit  80 , which are described later, in addition to the driver&#39;s brake operation as above.  
         [0033]     The yaw moment control unit  50  is provided as a yaw moment control means, which performs automatic brake control as below based on, for example, the wheel velocity obtained by each of wheel velocity sensors  32   fl ,  32   fr ,  32   rl , and  32   rr , the steering angle obtained by a steering angle sensor  33 , the yaw rates obtained by a yaw rate sensor  34 , and so on.  
         [0034]     The differential value of a target yaw rate, the differential value of an expected yaw rate during running on a low-μ road, and the difference between the two differential values as well as the difference between the actual and target yaw rates are computed. Based on these values, target braking force for correcting an under-steering tendency or an over-steering tendency is computed. The yaw moment control unit  50  outputs a control signal to the brake drive unit  51  to apply the target braking force to a selected wheel for the yaw moment control. The selected wheel is a rear wheel located at the inner side of turning if the under-steering tendency needs to be corrected, and it is a front wheel located at the outer side of turning if the over-steering tendency needs to be corrected. The signal indicating operation or non-operation of the yaw moment control unit  50  is also outputted to the center differential limited slip differential control unit  40 .  
         [0035]     The reference numeral  60  refers to a traction control unit as a driving force control means, which detects a slip rate of each wheel based on signals from an accelerator open degree sensor  36  and the wheel velocity sensors  32   fl ,  32   fr ,  32   rl , and  32   rr . When the slip rate becomes greater than or equal to a predetermined judgment value while the acceleration open degree becomes greater than or equal to 0, the traction control  60  outputs a control signal to an engine control unit  70  to perform various controls such as fuel emission control so as to decrease the torque of the engine  1 . The signals indicating operation or non-operation of the traction control unit  50  are also outputted to the center differential limited slip differential control unit  40 .  
         [0036]     A traction control OFF switch  35  is provided as a switch means, and is connected to the traction control unit  60 , enabling the driver to select between operation and non-operation of the traction control. The signals from the traction control OFF switch  35  are also outputted to the center differential limited slip differential control unit  40 .  
         [0037]     The reference numeral  80  refers to the limited slip differential (LSD) control unit as a limited slip differential means, which computes the difference in velocity between the right and left wheels at each of the front and rear based on the signals from the wheel velocity sensors  32   fl ,  32   fr ,  32   rl , and  32   rr , determines the target braking force according to the absolute value of the difference, and outputs a signal to the brake drive unit  51  to apply the target braking force to the wheel of a greater wheel velocity.  
         [0038]     Inputs to the center differential limited slip differential control unit  40  are: the wheel velocities ωfl, ωfr, ωrl, and ωrr from the wheel velocity sensors  32   fl ,  32   fr ,  32   rl , and  32   rr , respectively, the steering angle from the steering angle sensor  33 , the selected state of the traction control from the traction control OFF switch  35 , the operation or non-operation state of the yaw moment control from the yaw moment control unit  50 , and the operation or non-operation state of the traction control from the traction control unite  60 .  
         [0039]     Based on these inputs, when the yaw moment control or the traction control is executed while the operation state of the traction control is selected by the traction control OFF switch  35 , a predetermined constant TCL is set as a front and rear limited slip differential torque Tlsdctr. On the other hand, when the yaw moment control is executed while the non-operation state of the traction control is selected by the traction control OFF switch  35 , a predetermined constant TCH which is larger than the above TCL is set as the front and rear limited slip differential torque Tlsdctr. In the cases other than the above, the front and rear limited slip differential torque Tlsdctr, which was computed based on the actual differential rotation frequency between the front and rear axles and the target differential rotation frequency, is set as the front and rear limited slip differential torque Tlsdctr. The limited slip differential force corresponding to the front and rear limited slip differential torque Tlsdctr determined as above is outputted to the center differential clutch driving unit  41  to control the engagement torque of the center differential clutch  21 .  
         [0040]     The center differential limited slip differential control unit  40  is provided as a front and rear driving force distribution control means, comprising, as shown in  FIG. 2 , a microcomputer and its peripherals, a vehicle velocity computing unit  40   a , an actual front and rear differential rotation frequency computing unit  40   b , a target front and rear differential rotation frequency setting unit  40   c , a front and rear differential rotation frequency difference computing unit  40   d , a start-control front and rear differential rotation frequency setting unit  40   e , a front and rear start-control conditions judging unit  40   f , a first front and rear limited slip differential torque computing unit  40   g , a second front and rear limited slip differential torque computing unit  40   h , a front and rear limited slip differential torque computing unit  40   i , and a front and rear limited slip differential torque setting unit  40   j.    
         [0041]     The wheel velocity sensors  32   fl ,  32   fr ,  32   rl , and  32   rr  input the wheel velocities ωfl, ωfr, ωrl, and ωrr of the wheels  14   fl ,  14   fr ,  14   rl , and  14   rr , respectively, into the vehicle velocity computing unit  40   a  The vehicle velocity V is obtained by taking the average of these four values, for example, and is outputted to the target front and rear differential rotation frequency setting unit  40   c  and the start-control front and rear differential rotation frequency setting unit  40   e.    
         [0042]     The wheel velocity sensors  32   fl ,  32   fr ,  32   rl , and  32   rr  input the wheel velocities ωfl, ωfr, ωrl, and ωrr of the wheels  14   fl ,  14   fr ,  14   rl , and  14   rr , respectively, into the actual front and rear differential rotation frequency computing unit  40   b , which computes the actual front and rear differential rotation frequency Δωctr as in Eq. (1):  
               Δ   ⁢           ⁢   ω   ⁢           ⁢   ctr     =              (       ω   ⁢           ⁢   fl     +     ω   ⁢           ⁢   fr       )     2     -       (       ω   ⁢           ⁢   rl     +     ω   ⁢           ⁢   rr       )     2                    (   1   )             
 
 which is then outputted to the front and rear differential rotation frequency difference computing unit  40   d  and the front and rear start-control conditions judging unit  40   f.  
 
         [0043]     The vehicle velocity computing unit  40   a  inputs the velocity V into the target front and rear differential rotation frequency setting unit  40   c , which determines the target front and rear differential rotation frequency Δωctrt corresponding to the vehicle velocity V by referencing a map showing Δωctrt vs. V obtained in advance through experiments and/or simulations.  
         [0044]     An example of the map is shown in  FIG. 3 . The target front and rear differential rotation frequency Δωctrt decreases as the vehicle velocity V increases, and is determined by taking into account various errors during actual running based on a number of factors.  
         [0045]     The target front and rear differential rotation frequency setting unit  40   c  inputs the target front and rear differential rotation frequency Δωctrt into the front and rear differential rotation frequency difference computing unit  40   d , and the actual front and rear differential rotation frequency computing unit  40   b  inputs the actual front and rear differential rotation frequency Δωctr into the front and rear differential rotation frequency difference computing unit  40   d . Then, the difference between the two (a front and rear differential rotation frequency difference) εctr is computed as in the following Eq. (2): 
 
ε ctr=Δωctr−≢ωctrt   (2) 
 
 which is then outputted to the first and second front and rear limited slip differential torque computing units  40   g  and  40   h.  
 
         [0046]     The vehicle velocity computing unit  40   a  inputs the velocity V into the start-control front and rear differential rotation frequency setting unit  40   e , which determines a start-control front and rear differential rotation frequency Δωctrs corresponding to the vehicle velocity V by referencing a map showing Δωctrs vs. V obtained in advance through experiments and/or simulations. An example of the map is shown in  FIG. 3 . The target front and rear differential rotation frequency Δωctrt decreases as the vehicle velocity V increases, and is determined by taking into account various errors during actual running based on a number of factors.  
         [0047]     The start-control front and rear differential rotation frequency Δωctrs is smaller than the target front and rear differential rotation frequency Δωctrt, and serves as the lower limit of the actual front and rear differential rotation frequency Δωctr.  
         [0048]     In the present embodiment, the steering angle sensor inputs the steering angle into the start-control front and rear differential rotation frequency setting unit  40   e , which corrects the start-control front and rear differential rotation frequency Δωctrs, which has been determined based on the velocity V, further based on the steering angle for better accuracy. The correction through the steering angle may be carried out by using a map shown in  FIG. 4 , for example. The larger the steering angle is, the larger the correction of the start-control front and rear differential rotation frequency Δωctrs is.  
         [0049]     The actual front and rear differential rotation frequency computing unit  40   b  inputs the actual front and rear differential rotation frequency Δωctr into the front and rear start-control conditions judging unit  40   f , and the start-control front and rear differential rotation frequency setting unit  40   e  inputs the start-control front and rear differential rotation frequency Δωctrs into the front and rear start-control conditions judging unit  40   f . Then, the front and rear start-control conditions judging unit  40   f  determines if the front and rear conditions for starting the control have been met by comparing Δωctr to Δωctrs.  
         [0050]     If Δωctr is greater than Δωctrs, the front and rear start-control conditions judging unit  40   f  determines that the conditions for starting the control are met, and outputs the judgment result to the first and second front and rear limited slip differential torque computing units  40   g  and  40   h.    
         [0051]     Into the first front and rear limited slip differential torque computing unit  40   g , the front and rear differential rotation frequency difference εctr is inputted from the front and rear differential rotation frequency difference computing unit  40   d , and the judgment result is inputted from the front and rear start-control conditions judging unit  40   f  as to whether the conditions for starting the control are met. Then, the first front and rear limited slip differential torque computing unit  40   g  computes a first front and rear limited slip differential torque Tsmcctr as shown in Eqs. (3) and (4):  
             sctr   =       ɛ   ⁢           ⁢   ctr     +     k   ⁢           ⁢   i   ⁢           ⁢     ctr   ·     ∫       (     ɛ   ⁢           ⁢   ctr     )     ⁢     ⅆ   t                       (   3   )               x   =       k   ⁢           ⁢   ω   ⁢           ⁢     ctr   ·   j     ⁢           ⁢   ω   ⁢           ⁢     ctr   ·     (       ⅆ     (     ɛ   ⁢           ⁢   ctr     )         ⅆ   t       )         +     Tsgctr   ·     (     sctr     (          sctr        +     δ   ⁢           ⁢   ctr       )       )                 (   4   )             
 
 where the integration range is from 0 to t, kωtr is a gain for the integral term, kωctr is a gain for the differential term, jωctr is a moment of inertia, Tsgctr is a switching gain, δctr is a constant for making the limited slip differential force continuous so as to prevent chattering. 
 
         [0052]     If x&gt;0, Tsmcctr=x; and if x≦0, Tsmcctr=0. Based on the judgment result from the front and rear start-control conditions judging unit  40   f , if the conditions are not met with Δωctr≦Δωctrs for starting the control, resetting is made so as to have Tsmccrt=0 and ∫ (εctr) dt=0 even if Tsmcctr=x in the above.  
         [0053]     That is, if the actual front and rear differential rotation frequency Δωctr is smaller than or equal to the start-control front and rear differential rotation frequency Δωctrs, which is the lower limit of Δωctr, the first front and rear limited slip differential torque Tsmcctr is set to be 0 so as to avoid the friction-induced locking state of the center differential clutch  21 . Furthermore, resetting the integral ∫ (εctr) dt to 0 prevents an abnormally low integral value, which could greatly delay the control when the center differential clutch  21  resumes slipping, resulting in a prolonged stick slip of the clutch. Thereafter, Tsmcctr so obtained is outputted to the front and rear limited slip differential torque computing unit  40   i.    
         [0054]     The front and rear differential rotation frequency difference computing unit  40   d  inputs εctr into the second front and rear limited slip differential torque computing unit  40   h , and the front and rear start-control conditions judging unit  40   f  inputs the judgment result as to whether the conditions for starting the control are met into the second front and rear limited slip differential torque computing unit  40   h . Then, the second front and rear limited slip differential torque computing unit  40   h  computes a second front and rear limited slip differential torque Tpcctr as follows.  
         [0055]     If εctr&gt;0, Tpcctr=kpctr·εctr; and if εctr≦0, Tpcctr=0. Here, kpctr is a gain for the proportional term. Based on the judgment result from the front and rear start-control conditions judging unit  40   f , if the conditions are not met with Δωctr≦Δωctrs for starting the control, resetting is made so as to have Tpccrt=0 even if Tpcctr=kpctr·εctr in the above in order to avoid a friction-induced locking state of the center differential clutch  21 .  
         [0056]     The first front and rear limited slip differential torque computing unit  40   g  inputs Tsmcctr into the front and rear limited slip differential torque computing unit  40   i , and the second front and rear limited slip differential torque computing unit  40   h  inputs Tpcctr into the front and rear limited slip differential torque computing unit  40   i . The front and rear limited slip differential torque Tlsdctr is then obtained as in Eq. (5): 
 
 Tlsdctr=Tsmcctr+Tpcctr   (5) 
 
 which is then outputted to the front and rear limited slip differential torque setting unit  40   j.  
 
         [0057]     The front and rear limited slip differential torque Tlsdctr may be computed by the vehicle velocity computing unit  40   a —the front and rear limited slip differential torque computing unit  40   i  as needed by the front and rear limited slip differential torque setting unit  40   j , or may be computed in advance and read in as needed by it.  
         [0058]     Into the front and rear limited slip differential torque setting unit  40   j , the selected state of the traction control is inputted from the traction control OFF switch  35 , the operation or non-operation state of the yaw moment control is inputted from the yaw moment control unit  50 , and the operation or non-operation state of the traction control is inputted from the traction control unite  60 , and Tlsdctr is inputted from the front and rear limited slip differential torque computing unit  40   i  as needed.  
         [0059]     When the yaw moment control or the traction control is executed while the traction control is in the operation state, the predetermined constant TCL (between 0.5-1.0 Nm, for example) is assigned to Tlsdctr.  
         [0060]     When the yaw moment control is executed while the traction control is in the non-operation state, the predetermined constant TCH (between 90-110 Nm, for example), which is larger than the above TCL, is assigned to Tlsdctr.  
         [0061]     In the other cases than the above two, Tlsdctr from the front and rear limited slip differential torque computing unit  40   i  is directly assigned to Tlsdctr.  
         [0062]     The limited slip differential value corresponding to the front and rear limited slip differential torque Tlsdctr determined as above is then outputted to the center differential clutch driving unit  41 .  
         [0063]     The processing flow at the center differential limited slip differential control unit  40  is explained below with reference to the flowchart shown in  FIG. 5 .  
         [0064]     In the step S 101 , the wheel velocities ωfl, ωfr, ωrl, and ωrr from the wheel velocity sensors  32   fl ,  32   fr ,  32   rl , and  32   rr , respectively, the steering angle from the steering angle sensor  33 , the selected state of the traction control from the traction control OFF switch  35 , the operation or non-operation state of the yaw moment control from the yaw moment control unit  50 , the operation or non-operation state of the traction control from the traction control unite  60 , and other necessary parameters are read in.  
         [0065]     In the step S 102 , the selected state of the traction control by the traction control OFF switch  35  is judged. If the traction control OFF switch is not operating, that is the operable state of the traction control is selected, the process flow goes to the step S 103  where it is determined if the yaw moment control is operating.  
         [0066]     If the yaw moment control is not operating, the process flow goes to the step S 1104  where it is determined if the traction control is operating. If not, the process flow goes to the step S 105 .  
         [0067]     In the step S 105 , Tlsdctr is computed via a front and rear limited slip differential torque computation routine, the flowchart of which is shown in  FIG. 6 . In the step S 106 , the limited slip differential value corresponding to Tlsdctr obtained at the step S 105  is outputted to the center differential clutch driving unit  41 , and then the process flow exits the program.  
         [0068]     If it is judged that the yaw moment control is operating in the step S 103 , or the traction control is operating in the step S 104 , the process flow goes to the step S 107  where the predetermined constant TCL is assigned to Tlsdctr (Tlsdctr=TCL). Then, in the step S 106 , the limited slip differential value corresponding to Tlsdctr set in the step S 107  is outputted to the center differential clutch driving unit  41 , and then the process flow exits the program.  
         [0069]     If it is judged that the traction control OFF switch  35  is operating, that is, the non-operable state of the traction control is selected, the process flow goes to the step S 108  where it is determined if the yaw moment control is operating.  
         [0070]     If the yaw moment control is not operating, the process flow goes to the step S 105  where Tlsdctr is computed via the front and rear limited slip differential torque computation routine, the flowchart of which is shown in  FIG. 6 . In the step S 106 , the limited slip differential value corresponding to Tlsdctr obtained at the step S 105  is outputted to the center differential clutch driving unit  41 , and then the process flow exits the program.  
         [0071]     On the other hand, if the yaw moment is operating, the process flow goes to the step S 109  where the predetermined constant TCH, which is larger than the above TCL, is assigned to Tlsdctr (Tlsdctr=TCH). Then, in the step S 1106 , the limited slip differential value corresponding to Tlsdctr obtained at the step S 109  is outputted to the center differential clutch driving unit  41 , and then the process flow exits the program.  
         [0072]      FIG. 6  shows the flowchart of the front and rear limited slip differential torque computation routine, which is executed in the above step S 105 . Specifically, this figure explains how Tlsdctr is computed by the vehicle velocity computing unit  40   a —the front and rear limited slip differential torque computing unit  40   i , which are shown in the block diagram in  FIG. 2 . Incidentally, the process shown in this flowchart may be initiated upon receiving a trigger signal from the front and rear limited slip differential torque setting unit  40   j , or may be routinely executed.  
         [0073]     In the step S 201 , the vehicle velocity V is computed by the vehicle velocity computing unit  40   a . In the step S 202 , the target front and rear differential rotation frequency Δωctrt corresponding to V is determined by the target front and rear differential rotation frequency setting unit  40   c  based on the map showing Δωctrt vs. V.  
         [0074]     In the step S 203 , the start-control front and rear differential rotation frequency Δωctrs is determined by the start-control front and rear differential rotation frequency setting unit  40   e  based on the map showing Δωctrs vs. V with the steering angle correction.  
         [0075]     In the step S 204 , the actual front and rear differential rotation frequency Δωctr is computed by the actual front and rear differential rotation frequency computing unit  40   b  based on Eq. (1).  
         [0076]     In the step S 205 , Δωctr and Δωctrs are compared to each other by the front and rear start-control conditions judging unit  40   f . If Δωctr is greater than Δωctrs, it is judged that the front and rear conditions for starting the control are met, and then the process flow goes to the step S 206 .  
         [0077]     In the step S 206 , the front and rear differential rotation frequency difference εctr is computed by the front and rear differential rotation frequency difference computing unit  40   d  based on Eq. (2), and then the process flow goes to the step S 207 .  
         [0078]     In the step S 207 , the integration of εctr, i.e. ∫ (εctr) dt, from t=0 to t=t is carried out by the first front and rear limited slip differential torque computing unit  40   g . In the step S 208 , the first front and rear limited slip differential torque Tsmcctr is computed by the first front and rear limited slip differential torque computing unit  40   g . The result depends on x in Eq. (4): if x&gt;0, Tsmcctr=x; and if x≦0, Tsmcctr=0.  
         [0079]     In the step S 209 , the second front and rear limited slip differential torque Tpcctr is computed by the second front and rear limited slip differential torque computing unit  40   h . Specifically, If εctr&gt;0, Tpcctr=kpctr·εctr; and if εctr≦0, Tpcctr=0.  
         [0080]     In the step S 210 , the summation of Tsmcctr and Tpcctr is carried out by the front and rear limited slip differential torque computing unit  40   i  to give Tlsdctr as in Eq. (5), and then the process flow exits the routine.  
         [0081]     In the above step S 205 , if Δωctr is less than or equal to Δωctrs, it is judged that the front and rear conditions for starting the control are not met, and the process flow goes to the step S 211 .  
         [0082]     In the step S 211 , Tsmcctr, Tpcctr, and Tlsdctr are all set to 0.  
         [0083]     Thereafter, in the step S 212 , the integral of εctr is reset to 0 by the first front and rear limited slip differential torque computing unit  40   g , and the process flow exits the routine.  
         [0084]     As seen above, according to the present embodiment, when the yaw moment control is operating while the non-operation state of the traction control is selected via the traction control OFF switch  35 , the front and rear limited slip differential torque Tlsdctr is assigned with the constant TCH. When either the yaw moment control or the traction control is operating while the operation state of the traction control is selected, Tlsdctr is assigned with the constant TCL, which is smaller than TCH. Therefore, if over steering occurs while the non-operation state of the traction control is selected by the traction control OFF switch  35 , the braking force is applied to the front wheel at the outer side of turning by the yaw moment control, the other braking force is applied to the front wheel at the inner side of the turning by the limited slip differential control, and the driving force is applied to the two rear wheels. However, since the front and rear limited slip differential torque of a clutch means, which varies the torque distribution between the front and rear axles via the center differential, is set at a large value, transfer of excess driving force via the center differential from the front wheels to the rear wheels or vice versa will be suppressed. Thus, unstable vehicle behaviors are definitely prevented.  
         [0085]     Although, in the present embodiment, the center differential system  3  comprises the compound planetary gears without ring gears, compound planetary gears with ring gears or bevel gears may be utilized.  
         [0086]     Furthermore, although, in the present invention, the brake driving unit  51  as the limited slip differential control means operates to apply the target braking force to the wheel with a greater wheel velocity, a limited slip differential system such as a clutch incorporated in the front wheel final reduction device  11  may be utilized.  
         [0087]     It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can be readily devised by those skilled in the art without departing from the spirit and scope of the invention.