Patent Publication Number: US-2010114447-A1

Title: Automobile and control device for automobile

Description:
TECHNICAL FIELD 
     The present invention relates to an automobile and a control device for an automobile, and in particular, relates to an automobile and a control device for an automobile that perform a driving torque control and a braking torque control for suppressing an idling of wheels for the automobile with respect to a traveling road face. 
     BACKGROUND ART 
     When accelerating a vehicle such as an automobile, an outer circumferential velocity of the wheels becomes higher than the car body velocity due to a driving torque applied to the wheels, and a frictional force between the wheels and the road face due to slipping is caused. This frictional force generates a driving force to accelerate the vehicle. Likely, during deceleration a frictional force between the wheels and the road face due to slipping becomes a braking force to decelerate or stop the vehicle. 
     As an index representing a degree of slipping between wheels and a road face, a slip rate SR is proposed as defined by the following equations (1) and (2). Wherein equation (1) is the slip rate SR at the time of driving and equation (2) is the slip rate SR at the time of braking. 
         SR =( VW−VG )/ VW   (1) 
         SR =( VG−VW )/ VG   (2) 
     Wherein, VG is a car body velocity, and VW is an outer circumferential velocity of a wheel. 
     A frictional force due to slipping depends on a friction coefficient μ between a wheel and a road face, and the friction coefficient μ varies in response to a slip rate SR. 
     As shown in  FIG. 11 , in a region where a slip rate SR is low, a friction coefficient μ increases together with an increase of a slip rate SR. And in this region, a frictional force in response to a driving toque or a braking torque can be obtained. 
     However, a friction coefficient μ assumes the maximum value at a certain slip rate SRP (generally recognized as about 0.05˜0.30), and when the slip rate SR increases beyond the value, a friction coefficient μ decreases. Thereby, a frictional force in response to a driving force or a braking force cannot be obtained in this region. 
     For this reason, in order to obtain a frictional force in response to a driving force or a braking force, it is necessary to suppress the slip late SR below the slip rate SRP where the friction coefficient μ maximizes. 
     As a braking force control device for an automobile, an anti lock brake device is proposed in which a wheel velocity at the slip rate where the friction coefficient μ between the wheels and the road face maximizes is computed by presumption as a target wheel velocity, and the braking force is controlled so that the wheel velocity computed by presumption coincides with the target wheel velocity (for example, disclosed in patent document 1). In this control device, a target wheel acceleration velocity is computed by differentiating the target wheel velocity, and through comparison of a wheel acceleration velocity with the target wheel acceleration velocity, a setting of a control mode for braking force is performed. 
     In a driving force control device for a four wheel drive vehicle, generally, a presumed car body velocity is calculated by making use of a car body velocity presuming means, and a slip rate of the drive wheels is controlled. And as a drive force control device of this kind, a drive force control device is proposed in which after comparing a velocity determined by adding a limit acceleration velocity to a previously presumed car body velocity, a velocity determined by subtracting a limit deceleration velocity from the previously presumed car body velocity and the maximum velocity among the four wheel velocities, an intermediate velocity (intermediate value) thereof is determined as the currently presumed car body velocity (for example, disclosed in patent document 2). 
     Patent document 1: JP-B-3304575 
     Patent document 2: JP-A-11-189150 
     SUMMARY OF THE INVENTION 
     Tasks to be Solved by the Invention 
     However, with the braking force control device as disclosed in patent document 1, when a wheel velocity is larger than a target wheel velocity, since a pressure increasing mode is set with no relation to the wheel acceleration velocity and the braking torque is increased regardless to an indication of a tendency where the wheel acceleration velocity decreases and the slip rate increases, it is highly possible that a slip rate exceeds beyond the slip rate where the friction coefficient μ between the wheels and the road face maximizes because of dispersion of the friction coefficients μ of the road faces and a delay from a braking force command to an actual braking force generation. 
     Particularly, when a slip rate exceeds beyond the slip rate where the friction coefficient μ between the wheels and the road face maximizes such as on a compacted snow road and a frozen road, the friction coefficient μ between the wheels and the road face further lowers because of melting of the snow and ice to deteriorate the braking performance. 
     Further, in the driving force control device as disclosed in patent document 2, since the car body velocity is presumed based on the wheel velocity of the four wheels, when all of the slip rates of the four wheels gradually increase, the presumption error of the car body velocity enlarges which makes difficult to properly control the slip rates of the driving wheels. 
     The present invention is carried out in view of the above tasks to be solved, and an object of the present invention is to provide an automobile and a control device for the automobile, which permits to obtain a driving performance and a braking performance close to their limits achievable on the concerned road even such as on a compacted snow road and a frozen road. 
     (Countermeasure for Solving the Tasks) 
     A control device for an automobile according to the present invention for achieving the above object is a control device for an automobile for controlling a driving torque and/or a braking torque for wheels of the automobile which is characterized by controlling the driving torque or the braking torque in such a manner that when a slip rate of the wheels is kept in a range smaller than a value where a friction coefficient between the wheels and the road face maximizes, and when a command for increasing the driving torque or the braking torque is input to the control device, a first control mode for gradually increasing the driving torque or the braking torque and a second control mode for gradually decreasing the driving torque or the braking torque are selectively used in response to a slip condition. 
     Further, a control device for an automobile according to the present invention for achieving the above object is a control device for an automobile for controlling a driving torque and/or a braking torque for wheels of the automobile which is characterized by being provided with a slip judgment means for judging an increase of a slip rate of the wheels, a torque target value calculation means for calculating a first torque target value of increasing the driving torque or the braking torque, when a command for increasing the driving torque or the braking torque is input, a torque limiting means for calculating a second torque target value obtained by subtracting a predetermined value from the first torque target value calculated by the torque target value calculation means, a wheel acceleration velocity information acquiring means for measuring or presuming a wheel acceleration velocity, a car body acceleration velocity information acquiring means for measuring or presuming a car body acceleration velocity and an acceleration velocity feed back control means for calculating a third torque target value for performing a torque control so that the wheel acceleration velocity measured or presumed by the wheel acceleration velocity information acquiring means coincides with a wheel acceleration velocity target value obtained by multiplying a predetermined coefficient by the car body acceleration velocity measured or presumed by the car body acceleration velocity information acquiring means, and is characterized by controlling the driving torque or the braking torque by selecting of any one of the first torque target value, the second torque target value and the third torque target value in response to the judgment result by the slip judgment means and a relationship between magnitudes of the first torque target value and the third torque target value. 
     Further, A control device for an automobile according to the present invention for achieving the above object is a control device for an automobile for controlling a driving torque and/or a braking torque for wheels of the automobile driven or braked by four wheels which is characterized by being provided with a wheel acceleration velocity information acquiring means for measuring or presuming a wheel acceleration velocity, a car body acceleration velocity information acquiring means for measuring or presuming a car body acceleration velocity and an acceleration velocity feed back control means for controlling the driving torque or the braking torque so that the wheel acceleration velocity measured or presumed by the wheel acceleration velocity information acquiring means coincides with a wheel acceleration velocity target value obtained by multiplying a predetermined coefficient by the car body acceleration velocity measured or presumed by the car body acceleration velocity information acquiring means. 
     A control device for an automobile according to the present invention is desirably characterized by setting the coefficient to be multiplied by the car body acceleration velocity in response to at least one of an acceleration pedal opening degree, a brake pedal depressing amount and a steering angle. 
     A control device for an automobile according to the present invention is desirably characterized by being provided with a wheel velocity sensor for measuring a velocity of at least one of the wheels and a wheel torque information acquiring means for measuring or presuming the driving torque or the braking torque of at least one of the wheels and characterized in that the car body acceleration velocity information acquiring means presumes the car body acceleration velocity by making use of the wheel velocity measured by the wheel velocity sensor and the driving torque or the braking torque measured or presumed by the wheel torque information acquiring means. 
     A control device for an automobile according to the present invention is desirably characterized in that the car body acceleration velocity information acquiring means is provided with an effective driving and braking force calculation means for assuming a value obtained by multiplying a preset constant by a differentiation of the wheel velocity as an idling torque and for calculation an effective driving force or an effective braking force transmitted from a road face obtained by dividing a difference between the driving torque or the braking torque and the idling torque with the wheel radius, a pseudo car body velocity calculation means for calculating a pseudo car body velocity based on at least one of wheel velocities, a car body velocity calculation means for calculating a car body velocity by integrating a car body acceleration velocity calculated by a car body acceleration velocity presumption value calculation means, an external force calculation means for calculating an external force in response to a difference between the car body velocity calculated by the car body velocity calculation means and the pseudo car body velocity calculated by the pseudo car body velocity calculation means, and the car body acceleration velocity presumption value calculation means for calculating a value obtained by dividing a sum of the effective driving force or the effective braking force calculated by the effective driving and braking force calculation means and the external force calculated by the external force calculation means with the vehicle weight as the car body acceleration velocity. 
     A control device for an automobile according to the present invention is desirably characterized in that the pseudo car body velocity calculation means calculates the pseudo car body velocity by subjecting the at least one of the wheel velocities measured by the wheel velocity sensor to at least one of a delay processing and a smoothing processing. 
     An automobile according to the present invention is an automobile performing a control with a control device of a driving torque and/or a braking torque for wheels of the automobile which is characterized by controlling the driving torque or the braking torque in such a manner that when a slip rate of the wheels is kept in a range smaller than a value where a frictional coefficient between the wheels and the road face maximizes, and when a command for increasing the driving torque or the braking torque is input to the control device, a first control mode for gradually increasing the driving torque or the braking torque and a second control mode for gradually decreasing the driving torque or the braking torque are selectively used in response to a slip condition. 
     ADVANTAGES OF THE INVENTION 
     According to the present invention, even when a slip rate of the wheels is in a range smaller than a value where the frictional force between the wheels and the road face maximizes, since the control of gradually decreasing the driving torque or the braking torque is performed, the idling of the wheels can be properly suppressed even such as on a compacted snow road and a frozen road. 
     Further, through presuming the car body acceleration velocity from the wheel velocity and the driving torque or the braking torque, setting the wheel acceleration velocity target value based on the presumed car body acceleration velocity and controlling the driving torque or the braking torque so that the actual wheel acceleration velocity coincides with the wheel acceleration velocity target value, the driving performance and the braking performance close to their limits achievable on the concerned road can be obtained while suppressing the idling of the wheels even such as on a compacted snow road and a frozen road. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a constitutional diagram showing an embodiment of an automobile to which a device according to the present invention is applied; 
         FIG. 2  is a system block diagram showing an embodiment of a 4WD controller to which a device according to the present invention is applied; 
         FIG. 3  is a block diagram of a traction control portion in the 4WD controller of the present embodiment; 
         FIG. 4  is a block diagram of a torque control unit in the 4WD controller of the present embodiment; 
         FIG. 5  is a mode transition diagram of a TCS mode judgment unit in the 4WD controller of the present embodiment; 
         FIG. 6  is a block diagram of a car body acceleration velocity observer in the 4WD controller of the present embodiment; 
         FIG. 7  is a graph showing an example of pseudo car body velocity producing methods by a pseudo car body velocity producing unit in the 4WD controller of the present embodiment; 
         FIG. 8  is a block diagram of an acceleration velocity feed back controlling unit in the 4WD controller of the present embodiment; 
         FIG. 9  is a graph showing a control characteristic when a torque control is performed by a conventional control device; 
         FIG. 10  is a graph showing a control characteristic when a torque control is performed by the control device of the present embodiment; and 
         FIG. 11  is a graph showing a general relationship between slip rate and friction coefficient. 
     
    
    
     EXPLANATION OF REFERENCE NUMERALS 
       1  . . . Engine,  2  . . . Transmission,  3  . . . Electric generator,  4   a ,  4   b  . . . Front wheel shaft,  4   c ,  4   d  . . . Rear wheel shaft,  5   a ,  5   b  . . . Front wheel,  5   c ,  5   d  . . . Rear wheel,  6  . . . Electric power line,  7  . . . Electric motor,  8  . . . Rear differential device,  9   a ,  9   b ,  9   c ,  9   d  . . . Wheel velocity sensor,  100  . . . 4WD controller,  101  . . . Motor torque target value calculating portion,  102  . . . Motor torque presumption value calculating portion,  103  . . . Traction control portion,  104  . . . Field current control portion,  110  . . . Rear wheel slip judgment unit,  111  . . . Slip judgment unit by front and rear wheel velocity difference,  112  . . . Slip judgment unit by rear left and right wheel velocity difference,  113  . . . Rear wheel acceleration velocity slip judgment unit,  114  . . . Vehicle velocity slip judgment unit,  120  . . . Torque control unit,  121  . . . TCS mode judgment unit,  122  . . . Car body acceleration velocity observer,  123  . . . Torque limiter,  124  . . . Acceleration velocity feed back control unit,  125  . . . Torque switching unit,  137 ,  138 ,  139  . . . Summing point,  141  . . . Idling torque calculating portion,  142  . . . Wheel shaft driving torque calculating portion,  143  . . . Effective driving force calculating portion,  144  . . . Car body acceleration velocity presumption value calculating portion,  145  . . . Car body velocity presumption value calculating portion,  146  . . . Pseudo car body velocity producing unit,  147  . . . External force calculating portion,  148  . . . Rear wheel acceleration velocity calculating portion,  151  . . . Limiter,  152  . . . Rear wheel acceleration velocity target value calculating portion,  153  . . . Gain setting unit,  154  . . . Limiter,  155 , 156  . . . Summing point,  157  . . . Rear wheel acceleration velocity calculating portion. 
     BEST MODE FOR EMBODYING THE INVENTION 
     At first, an embodiment of an automobile to which a control device of the present invention is applied will be explained with reference to the drawings. 
     As shown in  FIG. 1 , an automobile representing an embodiment of the present invention is a four wheel driven type automobile and includes an engine (internal combustion engine)  1 , a transmission  2 , an electric generator  3  and an electric motor  7 . 
     A motive power generated by the engine  1  is transmitted to the transmission  2  and the electric generator  3 . The motive power transmitted to the transmission  2  is speed changed to those suitable to the traveling condition and is transmitted to left and right front wheel shafts  4   a ,  4   b  to drive left and right front wheels  5   a ,  5   b.    
     A motive power transmitted to the electric generator  3  is converted to an electric power and is fed to the electric motor  7  through an electric power line  6 . The electric motor  7  is a field coil type DC motor and generates a torque by making use of the fed electric power. The torque generated by the electric motor  7  is distributed left and right through a rear differential device  8  and transmitted to rear wheel shafts  4   c ,  4   d  to drive left and right rear wheels  5   c ,  5   d.    
     Wheel velocity sensors  9   a ,  9   b ,  9   c ,  9   d  are attached respectively to the front wheels  5   a ,  5   b  and the rear wheels  5   c ,  5   d  and detect individual wheel velocities of the respective wheels (the front wheels  5   a ,  5   b , the rear wheels  5   c ,  5   d ). 
     The automobile of the present embodiment is provided with a 4WD controller  100 . The 4WD controller  100  is an electronic control type constituted by a microcomputer and controls the electric generator  3  and the electric motor  7 . 
     The 4WD controller  100  is provided with a motor torque target value calculating portion  101 , a motor torque presumption value calculating portion  102 , a traction control portion  103  and a field current control portion  104  as shown in  FIG. 2 . 
     The 4WD controller  100  receives acceleration pedal opening degree signals APO from an acceleration pedal not shown, wheel velocity signals VW from the wheel velocity sensors  9   a ,  9   b ,  9   c ,  9   d  and motor rotation number signals Nm and motor armature current signals Ia from the electric motor  7 . 
     The 4WD controller  100  outputs generator field current control signals Ifg to the electric generator  3  and motor field current control signals Ifm to the electric motor  7 . 
     A torque of the electric motor  7  is generally calculated by a product of a motor torque constant determined by the field of the electric motor  7  and an armature current flowing through the electric motor  7 , and with regard to a field coil type motor, the motor torque constant can be controlled by adjusting the field. 
     Namely, the 4WD controller  100  controls an electric power generation amount of the electric generator  3  with the generator field current control signals Ifg and in addition controls a torque to be output by the electric motor  7  through controlling the field of the electric motor  7  with the motor field current control signals Ifm. 
     The motor torque target value calculating portion  101  determines a motor torque target value MTt in response to a difference between an average value VWF of the left and right front wheel velocities and an average value VWR of the left and right rear wheel velocities. Namely, the larger a value obtained by subtracting the average value VWR of the left and right rear wheel velocities from the average value VWF of the left and right front wheel velocities is, the larger the motor torque target value (first torque target value) MTt becomes. When a command for increasing the driving torque is input, the motor torque target value MTt is to be gradually increased. 
     The motor torque presumption value calculating portion  102  is a wheel torque information acquiring means, calculates a motor torque constant Kt from the motor field current control signals by making use of a map obtained in advance through such as experiments and further calculates a product of the motor torque constant Kt and the motor armature current Ia as a motor torque presumption value Tmpr. 
     The traction control portion  103  calculate a motor torque command value MTtcs that matches the road face condition and the vehicle condition based on the motor torque target value MTt calculated by the motor torque target value calculating portion  101 . The details of the traction control portion  103  will be explained later. 
     The field current control portion  104 , when the motor rotation number Nm is below a first threshold value tNm 1  determined in advance, sets as the motor field current Ifm=cIfm 1 , when the motor rotation number Nm is above a second threshold value tNm 2  determined in advance, sets as the motor field current Ifm=cIfm 2  and when the motor rotation number is between the first threshold value tNm 1  and the second threshold value tNm 2 , successively decreases the motor field current Ifm from cIfm 1  to cIfm 2 . Wherein, tNm 1 &lt;tNm 2 , cIfm 1 &gt;cIfm 2 . 
     The field current control portion  104  calculates a motor torque constant Kt from the motor field current Ifm by making use of a map obtained in advance through such as experiments, further calculates a motor armature current target value Iat obtained by dividing the motor torque command value MTtcs with the motor torque constant Kt and adjusts the generator field current Ifg so that an actual motor armature current Ia coincides with the motor armature current target value Iat. 
       FIG. 3  shows a detailed constitutional structure of the traction control portion  103 . 
     The traction control portion  103  is provided with a rear wheel slip judgment unit  110  and a torque control unit  120 . 
     The rear wheel slip judgment unit  110  calculates a rear wheel slip flag RSLIP from an acceleration pedal opening degree APO, a wheel velocity VW, an observer low gain flag FGLow and a car body velocity presumption value Vpr. As the observer low gain flag FGLow and the car body velocity presumption value Vpr, the values output from the torque control unit  120  and delayed by one sampling period with sample delays DLY 1 . DLY 2  are used. Subsequently, the details of the rear wheel slip judgment unit  110  will be explained. 
     The rear wheel slip judgment unit  110  is constituted from a slip judgment unit  111  by the front and rear wheel velocity difference, a slip judgment unit  112  by the rear left and right wheel velocity difference, a rear wheel acceleration velocity slip judgment unit  113  and a vehicle velocity slip judgment unit  114 . 
     In the slip judgment unit  111  by the front and rear wheel velocity difference, when a value DVW obtained by subtracting from a larger one of the left and right rear wheel velocities a smaller one of the left and right front wheel velocities becomes larger than a threshold value tDVW 1  set in advance, it is judged that the slip rate of the rear wheels increased, and a rear wheel slip flag RSLIP is set. Then, when a value DVW obtained by subtracting from a larger one of the left and right rear wheel velocities a smaller one of the left and right front wheel velocities becomes smaller than a threshold value tDVW 2  set in advance, it is judged that the slip rate of the rear wheels decreased, and the rear wheel slip flag RSLIP is cleared. 
     The slip judgment unit  112  by the rear left and right wheel velocity difference compares rear left and right wheel velocities VWRL, VWRR and when the velocity difference DVWRL of both becomes larger than a threshold value tDVWRL 1  set in advance, judges that the rear wheel slip rate increased and sets the rear wheel slip flag RSLIP. Then, when the velocity difference DVWRL becomes smaller than a threshold value tDVWRL 2  set in advance, the unit  112  judges that the rear wheel slip rate sufficiently decreased and clears the rear wheel slip flag RSLIP. 
     The rear wheel acceleration velocity slip judgment unit  113  calculates a rear wheel acceleration velocity average value GWR after differentiating an average value of the left and right rear wheel velocities VWRL, VWRR, and when the rear wheel acceleration velocity average value GWR becomes larger than a threshold value tGWR 1  set in advance, judges that the rear wheel slip rate increased and sets the rear wheel slip flag RSLIP. The threshold tGWR 1  is what is set in response to the acceleration pedal opening degree APO, therefore, the threshold value tGWR 1  is set in such a manner that when the acceleration pedal opening degree APO is large, the threshold value tGWR 1  becomes large and when the acceleration pedal opening degree APO is small, the threshold value tGWR 1  becomes small. Then, when the rear wheel acceleration velocity average value GWR becomes smaller than a threshold value tGWR 2  set in advance, the unit  113  judges that the rear wheel slip rate sufficiently decreased and clears the rear wheel slip flag RSLIP. 
     The vehicle velocity slip judgment unit  114 , when a ratio of the car body velocity presumption value Vpr and the left and right rear wheel velocities VWRL, VWRR, namely, any one of or both of VWRL/Vpr, VWRR/Vpr becomes larger than a threshold value tVWSR set in advance as well as the observer low gain flag FGLow is set, judges that the rear wheel slip rate increased and sets the rear wheel slip flag RSLIP. Then, when a time tTVWSR set in advance has passed after the rear wheel slip flag RSLIP was set, the rear wheel slip flag RSLIP is cleared. 
     Namely, when at least one of the slip judgment unit  111  by the front and rear wheel velocity difference, the slip judgment unit  112  by the rear left and right wheel velocity difference, the rear wheel acceleration velocity slip judgment unit  113  and the vehicle velocity slip judgment unit  114  judges that the rear wheel slip rate increased, the rear wheel slip judgment unit  110  sets the rear wheel slip flag RSLIP. 
     The torque control unit  120  calculates a motor torque command value MTtcs from an acceleration pedal opening degree APO, a wheel velocity VW, a rear wheel slip flag RSLIP, a motor torque target value MTt and a motor torque presumption value Tmpr as well as calculates an observer low gain flag GFLow and a car body velocity presumption value Vpr. 
       FIG. 4  shows a detailed constitutional structure of the torque control unit  120 . 
     The torque control unit  120  includes a TCS mode judgment unit  121 , a car body acceleration velocity observer  122 , a torque limiter  123 , an acceleration velocity feed back control unit  124  and a torque switching unit  125 . 
     The TCS mode judgment unit  121  determines a TCS mode from a motor torque target value MTt, a rear wheel slip flag RSLIP, a car body acceleration velocity presumption value Gpr and a motor torque command value MTtcs and outputs a torque switching command SWtcs, a torque limiting command TRQCHG and a observer low gain flag FGLow. In the TCS mode judgment unit  121 , as the motor torque command value MTtcs and as the car body acceleration velocity presumption value Gpr, the value output from the torque switching unit  125  and the value output from the car body acceleration velocity observer  122  are respectively used while delaying the same by one sampling period with sample delays DLY 3 , DLY 4 . 
     Details of the TCS mode judgment performed by the TCS mode judgment unit  121  will be explained later. 
     The car body acceleration velocity observer  122  outputs a car body acceleration velocity presumption value Gpr and a car body velocity presumption value Vpr from a rear wheel slip flag RSLIP, an observer low gain flag FGLow output from the TCS mode judgment unit  121 , a wheel velocity VW and a motor torque presumption value Tmpr. The details of the car body acceleration velocity observer  122  will also be explained later. 
     The torque limiter  123  subtracts a fixed value set in advance from the motor torque command value MTtcs or maintains the motor torque command value MTtcs and outputs the same as a motor torque second target value (second torque target value) MTt 2  according to the torque limiting command TRQCHG output from the TCS mode judgment unit  121 . As the motor torque command value MTtcs, a value output from the torque switching unit  125  is used while delaying the same by one sampling period with a sample delay DLY 3 . The motor torque second target value MTt 2  is a value obtained by subtracting a fixed value from the motor torque target value MTt. 
     Namely, the torque limiter  123  calculates a second torque target value obtained by subtracting a predetermined value from the first torque target value calculated by the torque target value calculating means. 
     The acceleration velocity feed back control unit  124  outputs a motor torque third target value MTt 3  from an acceleration pedal opening degree APO, a wheel velocity VW, a car body acceleration velocity presumption value Gpr output from the car body acceleration velocity observer  122  and a motor torque command value MTtcs. In the acceleration velocity feed back control unit  124 , as the motor torque command value MTtcs a value output from the torque switching unit  125  is used while delaying the same by one sampling period with a sample delay DLY 3 . Details of the acceleration velocity feed back control unit  124  will also be explained later. 
     The torque switching unit  125  outputs any one of the motor torque target value MTt, the motor torque second target value MTt 2  and the motor torque third target value MTt 3  as the motor torque command value MTtcs according to the torque switching command SWtcs output from the TCS mode judgment unit  121 . 
     The TCS mode judgment performed by the TCS mode judgment unit  121  will be explained with reference to  FIG. 5 . 
     In this TCS mode judgment, a default mode mDef is at first selected. In the default mode mDef, the torque switching command SWtcs is set so that the motor torque target value MTt assumes the motor torque command value MTtcs, the torque limiting command TRQCHG is set so that the motor torque command value MTtcs is maintained and the observer low gain flag FGLow is cleared. 
     During when the default mode mDef is selected, when the rear wheel slip flag RSLIP is set, a slip mode mSlp is selected. In the slip mode mSlp, the torque switching command SWtcs is set so that the motor torque second target value MTt 2  assumes the motor torque command value MTtcs and the observer low gain flag FGLow is kept cleared. 
     Further, the slip mode mSlp includes a torque down mode mTd and a torque keep mode mTk, and at first the torque down mode mTd is selected and the torque limiting command TRQCHG is set so that a value obtained by subtracting a fixed value set in advance from the motor torque command value MTtcs is output as the second target value MTt 2 . 
     During when the torque down mode mTd is selected, when the motor torque command value MTtcs reduces below a value tMTmin set in advance, the torque keep mode mTk is selected, the torque limiting command TRQCHG is set so that the motor torque command value MTtcs is maintained and the same is output as the second target value MTt 2 . 
     Namely, during when the slip mode mSlp is selected, a control of lowering the motor torque command value MTtcs down to the fixed value tMTmin with a fixed speed is performed. 
     During when the slip mode mSlp is selected, when the rear wheel slip flag RSLIP is cleared, an acceleration velocity feed back control mode mAfb is selected, the torque switching command SWtcs is set so that the motor torque third target value MTt 3  is output as the motor torque command value MTtcs and the torque limiting command TRQCHG is set so that the motor torque command value MTtcs is maintained. 
     The acceleration velocity feed back control mode mAfb includes a low acceleration velocity mode mLa, a delay mode mDy and an observer low gain mode mLg, and at first, the low acceleration velocity mode mLa is selected and the observer low gain flag FGLow is kept cleared. 
     During when the low acceleration velocity mode mLa is selected, when the car body acceleration velocity presumption value Gpr exceeds a threshold value tGpr 1  set in advance, the delay mode mDy is selected and the observer low gain flag FGLow is kept cleared. 
     Under the condition that the delay mode mDy is selected, when the car body acceleration velocity presumption value Gpr lowers below a threshold value tGpr 2  set in advance, the low acceleration velocity mode mLa is selected. 
     Under the condition that the delay mode mDy is selected, when the car body acceleration velocity presumption value Gpr lowers below a threshold value tGpr 2  set in advance as well as when a time Tdly set in advance has passed, the observer low gain mode mLg is selected and the observer low gain flag FGLow is set. 
     During when the acceleration velocity feed back control mode mSlp is selected, when the motor torque command value MTtcs exceeds the motor torque target value MTt, the default mode mDef is selected. Namely, the default mode mDef is selected in response to the magnitude relationship between the motor torque target value MTt and the motor torque third target value MTt 3 . 
     The TCS mode judgment unit  121  selects the respective modes as above and outputs the torque switching command SWtcs, the torque limiting command TRQCHG and the observer low gain flag FGLow in response to the respective modes. 
       FIG. 6  shows details of the car body acceleration velocity observer  122 . 
     The car body acceleration velocity observer  122  representing a car body acceleration velocity information acquiring means differentiates an average value VWR of left and right rear wheel velocities with a rear wheel acceleration velocity calculating portion (differentiator)  148  and calculates a rear wheel acceleration velocity average value GWR. Subsequently, in an idling torque calculating portion  141 , an idling torque Trs is calculated by multiplying the rear wheel acceleration velocity average value GWR by an inertia moment I of such as the rear wheels, the rear wheel shafts  4   c ,  4   d  rotating together with the rear wheels and the rear differential shaft  8  and then dividing the same with the wheel radius r, in a wheel shaft driving torque calculating portion  142 , a driving torque Tr on the wheel shaft is calculated by multiplying the motor torque presumption value Tmpr by a reduction gear ratio RG of the differential shaft  8 , in a summing point, from the driving torque Tr the idling torque Trs is subtracted, and in an effective driving force calculating portion  143 , an effective driving force Fd is calculated by dividing the above sum with the wheel radius r. 
     Herein, the effective driving force Fd implies an effective driving force transferred to the road face, as explained above the effective driving force Fd can be calculated by subtracting a torque consumed only for velocity increase of the wheels from the output torque of the electric motor  7  and then by dividing the same with the wheel radius r. 
     Subsequently, in a summing point  138 , the effective driving force Fd and an external force (force acting in front and back direction of the vehicle other than the effective driving force) Fdis and the sum is divided by the weight m of the vehicle in car body acceleration velocity presumption value calculating portion  144  to obtain a car body acceleration velocity presumption value Gpr. 
     In a car body velocity presumption value calculating portion (integrator)  145 , the car body velocity presumption value Vpr is calculated by integrating the car body acceleration velocity Gpr, the value obtained in a summing point  137  by subtracting the car body velocity presumption value Vpr from the pseudo car body velocity Vps calculated in a pseudo car body velocity producing unit  146  is multiplied by a gain Gdis in the external force calculating portion  147  to adjust the external force Fdis. The pseudo car body velocity producing unit  146  will be explained later. 
     The gain Gdis is switched in response to the TCS modes, and during when the slip mode mSlp and the observer low gain mode mLg are selected, the observer low gain flag FGLow is set and the gain Gdis is reduced in comparison with when other modes are selected. 
     For example, when the observer low gain flag FGLow is cleared, Gdis=1000, and when the observer low gain flag FGLow is set, Gdis=100. By reducing the gain Gdis, the variation of the external force Fdis is reduced, and since the variation of the car body acceleration velocity presumption value Gpr greatly depends on the variation of the effective driving force Fd, an actual car body acceleration velocity follow-up performance when the effective driving force Fd reduces due to slip of the rear wheels can be improved. 
     Herein, the gain Gdis is divided into two steps, however, the gain Gdis can be set in further multiple steps in response to car body velocity by making use of a map, in such instance, it is desirable to set the gain Gdis larger when the car body velocity is large so that the larger the car body velocity is, the larger the variation of the external force becomes. Further, a value obtained by integrating a product of a value obtained by subtracting the car body velocity presumption value Vpr from the pseudo car body velocity Vps and a value of the gain Gdis can be used as the external force Fdis. 
       FIG. 7  shows an example of methods of producing a pseudo car body velocity by the pseudo car body velocity producing unit  146 . The pseudo car body velocity producing unit  146  calculates as the pseudo car body velocity Vps a value obtained by subjecting to a well known low pass filter a smaller wheel velocity among a minimum wheel velocity VWmin representing the smallest wheel velocity among the four wheel velocities and a minimum wheel velocity VWmind obtained after subjecting the minimum wheel velocity VWmin to a delay processing. 
     Further, when the rear wheel slip flag RSLIP is set, since the wheel velocity is hold, the slip rate SR of the front wheels is already increased, a large deviation of the pseudo car body velocity Vps from the actual car body velocity can be prevented even when the slip rate SR of the rear wheels increases. 
     Further, the delay processing is not limited to one time, the wheel velocity can be subjected to the delay processing in a multiple of times to obtain a plurality of wheel velocities, and a value obtained by subjecting the smallest wheel velocity among the above obtained plurality of wheel velocities can be subjected to a smoothing processing through a well known low pass filter to obtain a value to be used as the pseudo car body velocity Vps. 
       FIG. 8  shows details the acceleration velocity feed back control performed in the acceleration velocity feed back controlling unit  124 . 
     The acceleration velocity feed back control  124  limits the car body acceleration velocity presumption value Gpr above a value cGmin set in advance by the limiter  151  and calculates the rear wheel acceleration velocity target value DGWR by multiplying the car body acceleration velocity presumption value Gpr subjected to the limiting processing by a gain k (coefficient). The gain k is calculated according to the following equation (3). 
         k= 1/(1 −DSR )  (3) 
     Wherein, DSR is a target slip rate and can be a constant, however, the target slip rate DSR is set according to an acceleration pedal opening degree APO by making use of a map. Herein, the larger the acceleration pedal opening degree APO is, the larger the target slip rate DSR is set. 
     Subsequently, in a summing point  155 , a difference (deviation) between the rear wheel acceleration velocity target value DGWR and the rear wheel acceleration velocity average value GWR calculated through differential operation by a rear wheel acceleration velocity calculating portion (differentiator)  157  is obtained, the difference is multiplied by a gain G in a gain setting unit  153 , the output therefrom is limited by limiter  154  in a proper range in view of a response characteristic of the electric motor  7  and in a summing point  156  a value obtained by adding the motor torque command value MTtcs is calculated as the motor torque third target value MTt 3 . 
     Namely, in the acceleration velocity feed back control, the target wheel acceleration velocity is calculated based on the car body acceleration velocity so that the variation speed of the slip rate SR assumes zero when the slip rate SR coincides with the target slip rate DSR and the wheel torque is controlled so that the actual wheel acceleration velocity coincides with the target wheel acceleration velocity. 
       FIGS. 9 and 10  respectively show relationships between velocity (rear wheel velocity average value VWR, car body velocity Vr), acceleration velocity (rear wheel acceleration velocity average value GWR, car body acceleration velocity Gr), slip rate (rear wheel slip rate) SR and motor torque command value MTtcs, when the torque is controlled respectively by a conventional control device and the control device of the present embodiment. 
     As shown in  FIG. 9 , with the conventional control device, the rear wheel acceleration velocity average value GWR begins to depart from the car body acceleration velocity Gr at time point t 1 . Although this shows signs of increasing the slip rate SR, since the difference between the rear wheel velocity average value VWR and the car body velocity is small and the slip rate SR does not exceed the slip rate SRP where the friction coefficient maximizes, the motor torque command value MTtcs is not decreased by the conventional control method. 
     Subsequently, at time point t 2 , since the slip rate SR exceeds the slip rate SRP where the friction coefficient maximizes, the motor torque command value MTtcs is decreased. However, because of dispersion of friction coefficient on road faces and a delay from a decrease of the motor torque command value MTtcs to an actual motor torque decrease, since the slip rate is caused to be increased, the snow and ice on the road face melt, which is feared to further reduce the friction coefficient of the road. 
     With the present embodiment, as shown in  FIG. 10 , since the rear wheel acceleration velocity average value GWR increases and exceeds the above explained rear wheel acceleration velocity target value DGWR at time point t 3 , the motor torque command value MTtcs is gradually decreased. 
     Subsequently, in response to the event that the slip rate SR exceeds the slip rate SRP where the friction coefficient maximizes and the car body acceleration velocity Gr begins to decrease at time point t 4 , the motor torque command value MTtcs is further decreased. 
     In the present embodiment, before the slip rate SR exceeds the slip rate SRP where the friction coefficient maximizes, since an increase of the slip rate SR is estimated from the relationship between the rear wheel acceleration velocity average value GWR and the car body acceleration velocity Gr, the motor torque command value MTtcs is decreased in advance, an increase of the slip rate SR can be suppressed. 
     In the present embodiment as has been explained hitherto, the car body acceleration velocity is presumed from a wheel velocity and a torque, and the target wheel acceleration velocity is set based on an acceleration pedal opening degree and a presumed car body acceleration velocity, then a motor torque, namely a rear wheel driving force is controlled so that an actual wheel acceleration velocity coincides with the target wheel acceleration velocity, thereby, even on such as a compacted snow road and frozen road, a driving performance close to a limit achievable on the concerned road can be obtained. 
     In the above embodiment, although an example in which the present invention is applied to a four wheel drive vehicle wherein the front wheels  5   a ,  5   b  are driven by the engine  1  and the rear wheels  5   c ,  5   d  are driven by the electric motor  7  was shown, the present invention is not limited thereto, the present invention can be applied to an automobile wherein the four wheels are driven by an engine as well as to an automobile wherein the four wheels are driven by an electric motor. Further, the present invention is not limited to the driving operation but can be applied to an automobile wherein the braking operation is performed for the four wheels thereof. 
     Further, in the above embodiment, although the target slip rate DSR is set in response to an acceleration pedal opening degree APO, but not being limited to the acceleration pedal opening degree APO, the target slip rate DSR can be set in response to a steering angle. 
     Further, in the above embodiment, although an example was shown in which the present invention is applied only to the operation during driving, however, when controlling a braking torque, the target slip rate DSR can be set in response to a brake pedal depressing amount as well as even when the brake pedal is not depressed, the braking torque can be controlled so that a deceleration corresponding to an engine brake can be obtained. 
     Since the car body acceleration velocity assumes a negative value during braking operation, when calculating the pseudo car body velocity Vps with the pseudo car body velocity producing unit  146 , it is desirable to switch the pseudo car body velocity producing method in such a manner that in place of the minimum wheel velocity Vwmin representing the smallest wheel velocity among the four wheel velocities, the maximum wheel velocity Vwmax representing the largest wheel velocity among the four wheel velocities. Further, with regard to the limiter  151  in the acceleration velocity feed back control unit  124 , it is desirable to switch the limiter  151  so that the car body acceleration velocity presumption value Gpr is limited below the value cGmax set in advance. 
     Further, in the above embodiment, although an example was shown in which the present invention is applied only to the operation during driving of an automobile provided with wheels driven by an electric motor, the present invention can be applied to when a negative driving torque is generated due to a regenerative braking by an electric motor. In such instance like the operation during braking as explained above, since the car body acceleration velocity becomes negative, it is desirable to effect the switching like the operation during braking as explained above.