Abstract:
An apparatus for controlling a drive force produced between a road surface and each of two drive wheels of a motor vehicle having a differential disposed between the drive wheels and a drive power source and connecting the drive wheels in a differential manner, and two brakes for braking the respective drive wheels, independently of each other, the apparatus including two wheel speed sensors for detecting the speeds of the respective drive wheels, and a brake control device responsive to the outputs of the sensors, for activating one of the two brakes which corresponds to one of the drive wheels which has a smaller critical value of the drive force with respect to the road surface, to thereby apply a braking torque to the one drive wheel for increasing an apparent value of the drive force of that one drive wheel, in order to increase the actual value of the drive force of the other drive wheel which has a larger critical value of the drive force.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to an apparatus for controlling an actual drive force to be applied to drive wheels of a motor vehicle, and more particularly to techniques for applying a braking torque to at least one drive wheel to control the actual drive forces of the drive wheels. 
     2. Discussion of the Related Art 
     Generally, such a wheel drive force control apparatus is provided for a motor vehicle which includes (a) a drive power source such as an engine and/or an electric motor, (b) at least one pair of drive wheels driven by the drive power source, (c) a differential gear unit disposed between the drive power source and each pair of drive wheels, for connecting the right and left drive wheels of the pair so as to permit the drive wheels to be rotated at different speeds and to equally distribute the driving torque of the drive power source to the drive wheels, so that a drive force is produced between each drive wheel and a road surface, and (d) a pair of brakes for braking the respective drive wheels, independently of each other. 
     Since the driving torque values distributed to the two drive wheels of each pair from the drive power source through the differential gear unit are always equal to each other, the sum of the actual drive forces of the two drive wheels is two times the actual drive force of the wheel whose critical drive force with respect to the road surface is smaller than that of the other drive wheel. The critical drive force is the maximum drive force that can be produced between the drive wheel and the road surface without slipping of the drive wheel on the road surface, while the differential action of the drive wheels by the differential gear unit is not limited. The critical drive force of each drive wheel depends upon a friction coefficient μ of the road surface and a vertical load acting on the drive wheel. The critical drive force of one of the two drive wheels of the pair may become smaller than that of the other drive wheel, due to a lower value of the friction coefficient μ of the road surface area on which the above-indicated one drive wheel is running, and/or a smaller value of the vertical load acting on the above-indicated one drive wheel, than the values of the other drive wheel. In this case, the above-indicated one drive wheel slips on the road surface, and the actual drive force of that drive wheel decreases, so that the driving torque distributed to the other drive wheel by the differential gear unit is reduced. As a result, the actual drive force of the above-indicated other drive wheel decreases, whereby the sum of the actual drive forces of the two drive wheels decreases, making it difficult to achieve smooth starting or acceleration of the vehicle. Namely, in the presence of a difference between the critical drive forces of the two drive wheels, the driving torque to be distributed by the differential gear unit to the drive wheel whose critical drive force is larger cannot be made large enough to assure the actual drive force of that drive wheel that is sufficiently close to its critical drive force, even while the critical drive force of that drive wheel is considerably larger than the actual drive force. 
     In the light of the above drawback where there exists a difference between the critical drive forces of the right and left drive wheels, the differential gear unit is conventionally provided with an exclusively designed limited slip differential (LSD) mechanism which is adapted to limit the differential action of the differential gear unit, for increasing the actual drive of the drive wheel whose critical drive force is larger, to a value close to the critical value. 
     However, the provision of such a LSD mechanism in the differential gear unit for each pair of drive wheels inevitably results in a considerable increase in the cost of manufacture and the weight of the differential gear unit. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an apparatus for controlling actual drive forces to be applied to drive wheels of a motor vehicle, so as to maximize the actual drive forces, by utilizing a brake provided for each drive wheel and without using an exclusively designed limited slip differential mechanism. 
     The above object may be achieved according to a first aspect of the present invention, which provides an apparatus for controlling an actual value of a drive force produced between a road surface and each of a pair of drive wheels of a motor vehicle including (a) a drive power source for driving the pair of drive wheels, (b) a differential disposed between the drive power source and the pair of drive wheels and connecting the pair of drive wheels so as to permit a difference between rotating speeds of the drive wheels and to evenly distribute a driving torque of the drive power source to the pair of drive wheels, for thereby producing the drive force between the road surface and each drive wheel, and (c) a pair of brakes for braking the pair of drive wheels, respectively, independently of each other, the apparatus being characterized by comprising: a pair of wheel speed sensors for detecting the rotating speeds of the pair of drive wheels; and a brake control device responsive to output signals of the pair of wheel speed sensors, for activating one of the pair of brakes which corresponds to one of the drive wheels which has a smaller critical value of the drive force with respect to the road surface than the other of the drive wheels, to thereby apply a braking torque to the above-indicated one drive wheel for increasing an apparent value of the drive force of that one drive wheel, in order to increase the actual value of the other drive wheel which has a larger critical value of the drive force. 
     The critical drive force of one of the two drive wheels may become smaller than that of the other drive wheels, due to a lower value of the friction coefficient μ of the road surface area on which the above-indicated one drive wheel is running, and/or a smaller value of the vertical load acting on the above-indicated one drive wheel, than the values of the other drive wheel. The vertical load may be reduced upon vertical movement or jumping of the drive wheel away from the road surface. If the brake for the above-indicated one drive wheel having the smaller critical drive force value is activated in the above condition, a braking torque is applied to that one drive wheel in the same direction as the actual drive torque based on an actual friction force between the road surface and the drive wheel in question. Namely, the braking torque and the actual drive torque are simultaneously applied to that drive wheel in the same direction. As a result, the apparent value of the drive force of the drive wheel having the smaller critical drive force, as seen from the other drive wheel having the larger critical drive force, is increased, and the slipping of the drive wheel having the smaller drive force is restricted, whereby the speed difference between the two drive wheels is reduced. Accordingly, the drive torque to be applied to the drive wheel having the larger critical drive force is increased by an amount corresponding to the braking torque applied to the drive wheel having the smaller critical drive force, so that the actual drive force of the drive wheel having the larger critical drive force is increased toward the critical value. 
     The wheel drive force control apparatus according to the first aspect of this invention was developed based on the above finding. In the present apparatus, the brake control device is adapted to activate the appropriate brake for braking the drive wheel having the smaller critical drive force value, when the vehicle is running with the two drive wheels having the different critical drive force values with respect to the road surface. The application of a braking torque to the drive wheel having the smaller critical drive force results in an increase in the apparent value of the drive force of that drive wheel, thereby increasing the actual value of the drive force of the other drive wheel having the larger critical drive force value. Therefore, the brake control device provides substantially the same effect as provided where the apparatus is equipped with a limited slip differential mechanism. 
     Thus, the present wheel drive force control apparatus is capable of maximizing the actual drive force of each drive wheel, without using an exclusively designed limited slip differential mechanism. 
     The present first aspect of this invention is applied to a front-drive motor vehicle, a rear-drive motor vehicle, or a four-wheel-drive motor vehicle which will be described. The four-wheel-drive motor vehicle may be a full-time or a part-time four-wheel drive type. 
     Generally, the four-wheel-drive motor vehicle indicated above includes (a) drive power source for driving the four drive wheels, (b) a front differential connecting the front right and left drive wheels so as to permit these drive wheels to rotate at different speeds, (c) a rear differential connecting the rear right and left drive wheels so as to permit these drive wheels to rotate at different speeds, (d) a center differential connecting input shafts of the front and rear differentials so as to permit these input shafts to rotate at different speeds, and (e) four brakes for braking the respective four drive wheels, independently of each other. The center differential is adapted to distribute a driving torque of the drive power source to the input shafts of the front and rear differentials, and the front differential is adapted to evenly distribute a torque received by the input shaft thereof to the front right and left drive wheels, for thereby producing the drive force between the road surface and each of the front right and left wheels, while the rear differential is adapted to evenly distribute a torque received by the input shaft thereof to the rear right and left drive wheels, for thereby producing the drive force between the road surface and each of the rear right and left drive wheels. 
     In the four-wheel-drive motor vehicle, slipping of one of the four drive wheels will result in a decrease in the actual drive forces of all of the four drive wheels. To improve the starting and driving performance of the four-wheel-drive motor vehicle on a road surface having a relatively low friction coefficient μ or on a bad road surface, a differential lock mechanism is conventionally provided for at least one of the three differentials indicated above. Where the differential lock mechanism is provided for the center differential, the slipping of one of the four drive wheels will not cause a decrease in the actual drive forces of all of the four drive wheels. If, however, one of the front right and left drive wheels and one of the rear right and left wheels suffer from slipping, the drive forces of all of the four drive wheels decrease. If the differential lock mechanisms are provided for the center differential and one of the front and rear differentials, the actual drive forces of all of the four drive wheels will not decrease even if one of the front wheels and one of the rear wheels suffer from slipping. However, slipping of the three wheels of the four wheels will cause a decrease of the actual drive forces of all of the four wheels. If the differential lock mechanisms are provided for all of the center, front and rear differentials, the slipping of the three wheels will not cause a decrease in the actual drive forces of all of the four wheels. 
     Thus, the conventional four-wheel-drive motor vehicle has a limited slip differential mechanism in the form of a differential lock mechanism for at least one of the center, front and rear differentials, in order to improve the starting and driving performance of the vehicle on the low-friction-coefficient or bad road surface. The provision of the limited slip differential mechanism inevitably results in considerable increase in the cost of manufacture and the weight of the vehicle. 
     It is therefore another object of the present invention to provide an apparatus for controlling actual drive forces to be applied to drive wheels of a four-wheel-drive motor vehicle, so as to maximize the actual drive forces, by utilizing a brake provided for each drive wheel and without using an exclusively designed limited slip differential mechanism. 
     The above object may be achieved according to a second aspect of this invention, which provides an apparatus for controlling an actual value of a drive force produced between a road surface and each of four drive wheels consisting of a front right drive wheel, a front left drive wheel, a rear right drive wheel and a rear left drive wheel of a four-wheel drive motor vehicle including (a) a drive power source for driving the four drive wheels, (b) a front differential connecting the front right and left drive wheels so as to permit a difference between rotating speeds of the front right and left drive wheels, (c) a rear differential connecting the rear right and left drive wheels so as to a difference between rotating speeds of the rear right and left drive wheels, (d) a center differential connecting input shafts of the front and rear differentials so as to permit a difference between rotating speeds of the input shafts, and (e) four brakes for braking the four drive wheels, respectively, independently of each other, the center differential distributing a driving torque of the drive power source to the input shafts of the front and rear differentials, the front differential evenly distributing a torque received by the input shaft thereof to the front right and left drive wheels, for thereby producing the drive force between the road surface and each of the front right and left drive wheels, the rear differential evenly distributing a torque received by the input shaft thereof to the rear right and left drive wheels, for thereby producing the drive force between the road surface and each of the rear right and left drive wheels, the apparatus being characterized by comprising: four wheel speed sensors for detecting the rotating speeds of the four drive wheels, respectively; and a brake control device responsive to output signals of the four wheel speed sensors, and operable when the four drive wheels have a first and a second drive wheel having different critical values of the drive force with respect to the road surface, the critical value of the drive force of the first drive wheel being smaller than that of the second drive wheel, the brake control device activating one of the four brakes which corresponds to the first drive wheel, to thereby apply a braking torque to the first drive wheel for increasing an apparent value of the drive force of the first drive wheel, in order to increase the actual value of the second drive wheel. 
     The wheel drive force control apparatus constructed according to the second aspect of the invention described above is also capable of maximizing the actual drive force of each of the four drive wheels, without having to use a limited slip differential mechanism, for the same reasons as described above with respect to the first aspect of the invention. 
     In the present apparatus, the center differential may be arranged to either evenly or unevenly distribute the input drive torque to the front and rear differentials. Where the center differential is arranged for uneven distribution of the input drive torque to the front and rear differentials, the distribution ratio may be either fixed or variable. 
     According to a first preferred form of the apparatus of the second aspect of this invention, the brake control device includes control means operable when the four drive wheels includes a lowest-speed drive wheel whose rotating speed is the lowest of all of the four drive wheels. The control means is adapted to control three brakes of the four brakes which three brakes correspond to respective three drive wheels that are the four drive wheels except the lowest-speed drive wheel. The control means controls the three brakes, by using the lowest rotating speed of the lowest-speed drive wheel as a reference wheel speed, so as to substantially zero a difference of the rotating speed of each of the three drive wheels from the reference wheel speed. 
     In the above first preferred form of the apparatus, the lowest rotating speed of the lowest-speed drive wheel is used as the reference wheel speed for controlling the brakes for the other three drive wheels, such that the rotating speeds of these three drive wheels are controlled to approach the reference wheel speed. Accordingly, the difference of the rotating speeds of the front right and left wheels connected to each other by the front differential is reduced toward zero, and the difference of the rotating speeds of the rear right and left wheels connected to each other by the rear differential is also reduced toward zero. Further, the difference of the rotating speeds of the output shafts of the center differential (input shafts of the front and rear differentials) is also reduced toward zero. Thus, the present form of the apparatus provides substantially the same effect as provided by the limited slip differential mechanisms for all of the center, front and rear differentials. 
     Accordingly, the wheel drive force control apparatus according to the first preferred form of the second aspect of this invention assures improved starting and driving performance of the four-wheel-drive motor vehicle. 
     The wheel drive force control apparatus according to the first preferred form, which is arranged to improve the starting and driving performance of the motor vehicle as described above, does not permit the four drive wheels to rotate at different speeds, and therefore tends to suffer from a so-called “tight corner braking phenomenon” during turning of the vehicle. Further, this apparatus does not permit the vehicle operator to recognize a slipping or spinning action of a given drive wheel or wheels which may occur due to jumping thereof away from the road surface (e.g., bumpy road surface). In other words, the apparatus does not permit the vehicle operator to obtain sufficient information on the road surface condition. 
     It is therefore an optional object of the invention to permit some degree of difference of the rotating speeds of the four drive wheels, for thereby preventing the tight corner braking phenomenon and enabling the vehicle operator to obtain sufficient information on the road surface condition. 
     The above optional object may be achieved according to a second preferred form of the second aspect of this invention, wherein the brake control device includes control means for calculating a total speed control error for each of the front right and left drive wheels, and a total speed control error for each of the rear right and left drive wheels. The total speed control error for each front drive wheel consists of a first error component which is a difference obtained by subtracting a half of an average speed of the rear right and left drive wheels from a half of an average speed of the front right and left drive wheels, and a second error component which is a difference obtained by subtracting the average speed of the front right and left drive wheels from the speed of each front drive wheel. The total speed control error for each rear drive wheel consists of a first error component which is a difference obtained by subtracting a half of an average speed of the front right and left drive wheels from a half of an average speed of the rear right and left drive wheels, and a second error component which is a difference obtained by subtracting the average speed of the rear right and left drive wheels from the speed of each rear drive wheel. The control means is adapted to control each of the four brakes which corresponds to each of the four drive wheels whose total speed control error is a positive value, such that the positive total speed control error of each drive wheel is substantially zeroed. 
     In the wheel drive force control apparatus according to the second preferred form of the invention described above, the total speed control error for each of the front right and left drive wheels is obtained as the sum of the first error component and the second error component, as the control amounts suitable for controlling the front wheel brakes, so as to zero the speed difference of the center differential and the speed difference of the front differential. The first error component is obtained by subtracting a half of the average speed of the rear right and left wheels from a half of the average speed of the front right and left drive wheels, while the second error component is obtained by subtracting the average speed of the front drive wheels from the speed of each front drive wheel. On the other hand, the total speed control error for each of the rear right and left drive wheels is obtained as the sum of the first error component and the second error components, as the control amounts suitable for controlling the rear wheel brakes, so as to zero the speed difference of the center differential and the speed difference of the rear differential. The first error component for the rear drive wheels is obtained by subtracting a half of the average speed of the front drive wheels from a half of the average speed of the rear drive wheels, while the second error component for the rear drive wheels is obtained by subtracting the average speed of the rear drive wheels from the speed of each rear drive wheel. However, the rotating speeds of the drive wheels can be controlled by the brakes only where the total speed control error is a positive value. That is, the brakes can be used only when the speeds of the drive wheels are lowered, and cannot be used to raise the speeds of the drive wheels. Thus, the present form of the invention permits some degrees of difference between the speeds of the front and rear wheels, difference between the speeds of the front right and left wheels, and difference between the speeds of the rear right and left wheels. In other words, the four drive wheels are permitted to some extent to rotate at speeds different from each other. 
     Therefore, the apparatus according to the second preferred form of the invention described above prevents the tight corner braking phenomenon and enables the vehicle operator to obtain sufficient information on the road surface condition. 
     While the apparatus according to the first preferred form of the invention provides substantially the same effect as provided when the limited slip differential mechanisms are provided for the center, front and rear differentials, as described above, the apparatus according to the second preferred form provides substantially the same effect as provided when the limited slip differential mechanism is provided for only the center differential, but this effect is smaller than that provided when the limited slip differential mechanisms are provided for all of the three differentials. 
     According to a third preferred form of the second aspect of this invention, the brake control device includes control means for controlling two brakes of the four brakes which correspond to one of a first pair of wheel consisting of the front right and left wheels and a second pair of wheels consisting of the rear right and left drive wheels. The above-indicated one of the first and second pairs has a higher average speed than the other of the first and second pairs. In this case, the control means controls the two brakes, such that the average speed of the one of the first and second pairs is reduced toward the average speed of the other of the first and second pairs. 
     The wheel drive force control apparatus according to the third preferred form of the invention described above is adapted to control the brakes, by taking into account the speed difference between the front and rear drive wheels, namely, the speed difference of the center differential, but not taking into account the speed difference between the right and left drive wheels. 
     This third preferred form of the invention provides substantially the same effect as provided when the limited slip differential mechanism is provided for only the center differential. 
     According to a fourth preferred form of the second aspect of the invention, the brake control device includes at least two control means selected from the control means according to the first preferred form, the control means according to the second preferred form and the control means according to the third preferred form. In this case, the brake control device further includes selectively enabling means for selectively enabling one of the at least two control means to operate. 
     The wheeld drive force control apparatus according to the fourth preferred form described above provides an increased degree of freedom in controlling the brakes so as to control the drive forces of the drive wheels, than the apparatus according to the first, second or third preferred form, which includes only one of the three control means described above. 
     The selectively enabling means may include determining means for determining whether the motor vehicle is placed in a condition in which starting of the motor vehicle is difficult, and selecting means for selecting the control means according to the first preferred form if the determining means determines that the motor vehicle is placed in the above-indicated condition, and selecting one of the control means according to the second and third preferred forms if the determining means determines that the motor vehicle is not placed in the above-indicated condition. In this case, the selectively enabling means may include means for determining whether the rotating speed of any one of the four drive wheels is substantially zero, and determining means for determining whether an average rotating speed of the front right and left drive wheels is substantially equal to an average rotating speed of the rear right and left drive wheels. The determining means determines that the motor vehicle is placed in the above-indicated condition if the rotating speed of any one of the four drive wheels is substantially zero and if the average rotating speeds of the front right and left drive wheels and the rear right and left drive wheels are substantially equal to each other. 
     Alternatively, the selectively enabling means may include determining means for determining whether any one of at least one operator-controlled member is placed in a position for selecting the control means according to the first preferred form, and means for enabling the control means according to the first preferred form if the determining means determines that any one of the above-indicated at least one operator-controlled member is placed in said position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and optional objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments or modes of the invention, when considered in connection with the accompanying drawings in which: 
     FIG. 1 is a plan view schematically showing a drive system of a four-wheel-drive motor vehicle equipped with a wheel drive force control apparatus constructed according to one embodiment of this invention; 
     FIG. 2 is a view illustrating a braking system in the four-wheel drive motor vehicle of FIG. 1; 
     FIG. 3 is a block diagram showing an electrical arrangement of the wheel drive force control apparatus; 
     FIG. 4 is a flow chart illustrating a wheel drive force control routine executed according to a program stored in a read-only memory of the wheel drive force control apparatus; 
     FIG. 5 is a view for explaining the wheel drive force control according to the routine of FIG. 4 in one example of a running condition of the motor vehicle; 
     FIG. 6 is another view for explaining the wheel drive force control routine of FIG. 4; 
     FIG. 7 is a further view for explaining the wheel drive force control routine; 
     FIG. 8 is a still further view for explaining the wheel drive force control routine; 
     FIG. 9 is a yet further view for explaining the wheel drive force control routine; 
     FIG. 10 is another view for explaining the wheel drive force control routine; 
     FIGS. 11 and 12 are further views for explaining the wheel drive force control routine in another example of a running condition of the motor vehicle. 
     FIG. 13 is a flow chart illustrating a wheel drive force control routine executed according to a program stored in the read-only memory of a wheel drive force control apparatus constructed according to a second embodiment of the present invention ; 
     FIG. 14 is a view for explaining the relationship between the embodiments of FIGS. 1 and 13; 
     FIG. 15 is a view explaining the relationship between the embodiments of FIGS. 1 and 13; 
     FIG. 16 is a flow chart illustrating a wheel drive force control routine executed according to a program stored in the read-only memory of a wheel drive force control apparatus constructed according to a third embodiment of this invention; 
     FIG. 17 is a flow chart illustrating a second speed control mode selecting routine executed in the third embodiment, to select the speed control mode used in the routine of FIG. 16; 
     FIG. 18 is a block diagram illustrating an electrical arrangement of a wheel drive force control apparatus constructed according to a fourth embodiment of this invention; and 
     FIG. 19 is a flow chart illustrating a wheel drive force control routine executed according to a program stored in the read-only memory of the apparatus of the fourth embodiment of FIG.  18 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIG. 1, there is schematically shown a drive system of a four-wheel-drive motor vehicle equipped with a wheel drive force control apparatus constructed according to a first embodiment of this invention. The motor vehicle has a pair of front wheels, namely, right and left front wheels  10  which are connected to each other by a front differential gear unit  12  (hereinafter referred to as “front differential  12 ”), and a pair of rear wheels, namely, right and left rear wheels  14  which are connected to each other by a rear differential gear unit  16  (hereinafter referred to as rear differential  16 ”). These front and rear differentials  12 ,  16  are connected to each other by a center differential gear unit  18  (hereinafter referred to as “center differential  18 ”). The center differential  18  has a front output shaft  20  and a rear output shaft  22 , which serve as an input shaft of the front differential  12 , and an input shaft of the rear differential  16 , respectively. 
     The center differential  18  has an input shaft  23  which receives a torque from a drive power source in the form of an engine (E/G)  24  through a transmission (T/M)  26  and a transfer  28  in the order of description. The torque received by the input shaft  23  is equally distributed by the center differential  18  to the front and rear differentials  12 ,  16 . The transfer  28  has a HIGH-GEAR TRANSFER state and a LOW-GEAR TRANSFER state in which the torque of the engine  24  is transmitted to the center differential  18 , with a, high-gear ratio and a low-gear ratio. The transfer  28  is selectively placed in the HIGH-GEAR or LOW-GEAR TRANSFER state, by an operation of an operator-controlled transfer switching member  30 . 
     Each wheel  10 ,  14  of the motor vehicle is provided with a hydraulically operated disc brake  40 , which includes a disc rotor  42  rotating with the wheel  10 ,  14 , and a hydraulic actuator  44  which includes a pair of friction pads and a wheel brake cylinder  56  hydraulically operated to force the friction pads onto the opposite surfaces of the disc rotor  42 , to thereby brake the wheel  10 ,  14 . 
     That is, the motor vehicle has a braking system as shown in FIG. 2, which includes the four brakes  40  for the respective four drive wheels  10 ,  14 . The braking system further includes an operator-controlled brake operating member in the form of a brake pedal  50 , a master cylinder  54  operatively connected to the brake pedal  50  through a brake booster  52 . The master cylinder  54  is of a tandem type having two mutually independent pressurizing chambers disposed in series. Hydraulic pressure is mechanically generated in the two pressurizing chambers of the master cylinder  54  such that the generated hydraulic pressure changes with an operating-force acting on the brake pedal  50 . One of the two pressurizing chambers of the master cylinder  54  is connected to the brake cylinders  56  of the brakes  40  for the front drive wheels  10  through a fluid passage  58 , while the other pressurizing chamber is connected to the brake cylinders  56  of the brakes  40  for the rear drive wheels  14  through another fluid passage  58 . A hydraulic pressure control device  60  is connected to the fluid passages  58 , for controlling the hydraulic pressures to be applied to the individual brakes  40 . 
     The hydraulic pressure control device  60  has a plurality of operating states including a pressure-increase state for fluid communication between the master cylinder  54  and the wheel brake cylinder  56 , and a pressure-decrease state for fluid communication between the wheel brake cylinder  56  and a reservoir (not shown). In the present embodiment, the hydraulic pressure control device  60  has three operating states consisting of the pressure-increase and pressure-decrease states indicated above, and a pressure-hold state in which the wheel brake cylinder  56  is disconnected from both the master cylinder  54  and the reservoir. These three operating states are selectively established by solenoid-operated valves when the hydraulic pressure control device  60  is placed in a drive force control mode. Normally, the control device  60  is placed in the pressure-increase mode in which the hydraulic braking pressure applied to each wheel brake cylinder  56  is controlled according to the operating force acting on the brake pedal  50 . 
     The hydraulic pressure control device  60  may use a three-position valve having the pressure-increase, pressure-decrease and pressure-hold states, for the brake cylinder  56  for each of the four drive wheels  10 ,  14 . Alternatively, the control device  60  may use a two-position valve having the pressure-increase and pressure-hold states, and another two-position valve having the pressure-decrease and pressure-hold states, for each drive wheel  10 ,  14 . The control device  60  may be adapted such that the brake cylinder  56  for each drive wheel  10 ,  14  is connected to a pressure control chamber which is partially defined by a control piston that is fluid-tightly and slidably received in a bore formed in a housing. In this case, the control piston is driven by a suitable actuator adapted to convert an electric energy into a mechanical energy. The actuator may be an electric motor or a piezoelectric element, for example. The hydraulic pressure in each wheel brake cylinder  56  can be controlled by positioning the control piston. 
     Referring next to the block diagram of FIG. 3, there is illustrated an electrical arrangement of the present wheel drive force control apparatus, which has a controller  70  principally constituted by a computer  78  which incorporates a central processing unit (CPU)  72 , a read-only memory (ROM)  74  and a random-access memory (RAM)  76 . To the input of the controller  70 , there are connected four wheel speed sensors  80 , an accelerator sensor  82  and a brake pedal sensor  84 . The wheel speed sensors  80  are adapted to detect rotating speeds of the respective drive wheels  10 ,  14 . The accelerator sensor  82  is adapted to detect an operation of an accelerator pedal or other vehicle accelerating member by the operator of the vehicle. The brake pedal sensor  84  is adapted to detect an operation of the brake pedal  50  by the vehicle operator. To the output of the controller  70 , there is connected the above-indicated hydraulic pressure control device  60 . The ROM  74  stores various control programs, and the CPU  72  operates according to these control programs while utilizing a temporary data storage function of the RAM  76 , to electrically control the hydraulic pressure control device  60  for thereby controlling the drive forces of the drive wheels  10 ,  14 . 
     The ROM  74  of the controller  70  stores a control program for executing a wheel drive force control routine as illustrated in the flow chart of FIG.  4 . To begin with, the concept underlying this wheel drive force control routine will be explained. 
     The wheel drive force control routine of FIG. 4 is formulated to minimize a difference between the speeds of the front and rear output shafts  20 ,  22  of the center differential  18 , a difference between the speeds of the output shafts of the front differential  12  (a difference between the speeds of the front right and left wheels  10 ), and a difference between the speeds of the output shafts of the rear differential  16  (a difference between the speeds of the rear right and left wheels  14 ). 
     The wheel drive force control routine of FIG. 4 will be explained with respect to a specific example of running condition of the motor vehicle as indicated in FIG.  5 . In this example, the rotating speeds of the four drive wheels  10 ,  14  are different from each other. More specifically, the rotating speeds of the front left and right wheels  10  are indicated as “4” and “3”, respectively, while the rotating speeds of the rear left and right wheels  14  are indicated as “1” and “2”, respectively. Accordingly, the speed of the front output shaft  20  of the center differential  18  is “3.5”, while the speed of the rear output shaft  22  of the center differential  18  is “1.5”, so that the speed difference of the center differential  18  is equal to “2”. The speed difference of the front differential  12  is “1”, and that of the rear differential  16  is also equal to “1”. 
     The speed difference of the center differential  18  and the speed difference of the front differential  12  can be reduced by suitably controlling the brakes  40  for the front drive wheels  10 , more specifically, by suitably controlling the hydraulic pressures in the brake cylinders  56  for the front drive wheels  10 , so as to change (lower) the rotating speeds of these wheels  10 . The amounts of change of the speeds of the wheels  10  by operation of the brakes  40  will be referred to as “speed control amounts” of the wheels  10 . The term “speed control amounts” will be also used for the rear drive wheels  14 . 
     To eliminate the speed difference “2” of the center differential  18 , the speeds of the front right and left wheels  10  must be lowered by a total amount of “2”. Since the front differential  12  is adapted to evenly distribute the input torque from the center differential  18  to the front right and left wheels  10 , the speed of each front wheel  10  must be lowered by “1”. That is, the speed control amount for each front drive wheel  10  is “1”, as indicated in FIG.  6 . Where the speed control amount is a positive value, the speed is lowered. Where the speed control amount is a negative value, the speed is raised. In this respect, it is noted that the speed can be lowered by brake application to the wheel, but cannot be raised by the brake application. 
     To eliminate the speed difference “1” of the front differential  12 , the speed control amount for the front left wheel  10  must be “+0.5” while the speed control amount for the front right wheel  10  must be “−0.5”, as indicated in FIG.  7 . In other words, the speed of the front left wheel  10  must be lowered by “0.5” while the speed of the front right wheel  10  must be raised by “0.5”. 
     Accordingly, the overall speed control amount for the front left wheel  10  is “1.5” while the overall speed control amount for the front right wheel  10  is “0.5”, as indicated in FIG.  8 . 
     Where the speed difference “2” of the center differential  18  and the speed difference “1” of the rear differential  16  are eliminated by controlling the speeds of the rear left and right wheels  14 , the overall speed control amount for the rear left wheel  14  must be “−1.5”=−1−0.5, while the overall speed control amount for the rear right wheel  14  must be “−0.5”=−1+0.5, as indicated in FIGS. 9 and 10. However, the speeds of the wheels  10 ,  14  cannot be raised by controlling the brakes  40 , as indicated above. Therefore, the final speed control amounts for the rear drive wheels  14  are both zeroed, as indicated in FIG.  11 . 
     In the light of the above, the present embodiment is adapted to obtain a total speed control error for each of the drive wheels  10 ,  14 . The total speed control error is a sum of a first error component and a second error component. The first error component for each front drive wheel  10  is a difference obtained by subtracting a half of the average speed of the two rear wheels  14  from a half of the average speed of the two front wheels  10 . The second error component for each front drive wheel  10  is a difference obtained by subtracting the average speed V FM  of the two front drive wheels from the speed of the front drive wheel  10  in question. The first error component for each rear drive wheel  14  is a difference obtained by subtracting a half of the average speed of the two front wheels  10  from a half of the average speed of the two rear wheels  14 , while the second error component for each rear drive wheel  14  is a difference obtained by subtracting the average speed V RM  of the two rear drive wheel  14  from the speed of the rear drive wheel  14  in question. The brake  40  for each wheel  10 ,  40  whose total speed error is a positive value is controlled so that the total speed error is substantially zeroed. 
     The specific example of FIGS. 5-11 provides substantially the same result as provided where the limited slip differential mechanism is provided for each of the center and front differentials  18 ,  12 . In an example of FIG. 12, only the speed difference of the center differential  18  is zeroed, while the speed differences of the front and rear differentials  12 ,  16  are not zeroed. In this example of FIG. 12, the present embodiment provides substantially the same result as provided where the limited slip differential mechanism is provided for only the center differential  18 . In other words, the effect provided by the wheel drive force control device adapted to control the brakes  40  for controlling the wheel drive forces according to the present embodiment is at least the same as the effect provided where the limited slip differential mechanism is provided for only the center differential  18 , but is smaller than the effect provided where the limited slip differential mechanism is provided for each of the three differentials  12 ,  16 ,  18 . 
     Referring back to the flow chart of FIG. 4, the wheel drive force control routine is executed at a predetermined cycle time while the operation of the vehicle accelerating member is detected by the accelerator sensor  82  but the operation of the brake pedal  50  is not detected by the brake pedal sensor  84 , after the ignition switch of the vehicle is turned ON. The routine is initiated with step S 1  to determine whether the control of the wheel drive force is permitted by the vehicle operator. If a negative decision (NO) is obtained in step S 1 , one cycle of execution of the routine is terminated. If the control of the wheel drive force is permitted by the operator, that is, if an affirmative decision (YES) is obtained in step S 1 , the control flow goes to step S 2  to read in the rotating speeds V** (V FR , V FL , V RR , V RL ) detected by the wheel speed sensors  80 , and calculate the speed control amounts for the individual drive wheels  10 ,  14 , that is, ΔVFR for the front right wheel  10 , ΔVFL for the front left wheel  10 , ΔVRR for the rear right wheel  14 , and ΔVRL of the rear left wheel  14 , as explained above, according to the following equations: 
     
       
         ΔV FR [(V FR +V FL )−(V RR +V RL )]/4+(V FR −V FM ) 
       
     
     
       
         ΔV FL [(V FR +V FL )−(V RR +V RL )]/4+(V FL −V FM ) 
       
     
     
       
         ΔV RR [(V RR +V RL )−(V FR +V FL )]/4+(V RR −V RM ) 
       
     
     
       
         ΔV RL [(V RR +V RL )−(V RR +V FL )]/4+(V RL −V RM ) 
       
     
     In the above equations, the term [(V FR +V FL )−(V RR +V RL )]/4 is the first error component for each front drive wheel  10 , and the term [(V RR +V RL )−(V FR +V FL )]/4 is the first error component for each rear drive wheel  14 . Further, the terms (V FR −V FM ) and (V FL −V FM ) are the second error components for the front right and left drive wheels  10 , respectively, while the terms (V RR −V RM ) and (V RL −V RM ) are the second error components for the rear right and left drive wheels  14 , respectively. 
     If the speed control amount of a given drive wheel  10 ,  14  calculated according to the appropriate equation is a positive value, this value is determined as the final speed control amount. If the calculated speed control amount is a negative value, it means that the speed of the drive wheel in question must be raised, and therefore the final speed control amount is determined to be zero. 
     Then, the control flow goes to step S 3  in which the braking pressure to be applied to the brake cylinder  56  of the brake  44  of each drive wheel  10 ,  14  is determined on the basis of the final speed control amount determined for that drive wheel, and the hydraulic pressure control device  60  is controlled so that the determined braking pressure is applied to the brake cylinder  56 . Thus, one cycle of execution of the wheel drive force control routine of FIG. 4 is terminated. 
     It will be understood from the foregoing description of the present embodiment that the hydraulic pressure control device  60  and a portion of the controller  70  assigned to execute the wheel drive force control routine of FIG. 4 cooperate to provide a brake control device including control means for controlling the brakes  40  so as to substantially zero the total speed control error for each wheel  10 ,  14 . 
     Referring to FIG. 13, there will be described a wheel drive force control apparatus constructed according to a second embodiment of the present invention. While the first embodiment is adapted to control the rotating speeds of the drive wheels  10 ,  14 , so as to minimize the speed differences of all of the three differentials  12 ,  16 ,  18 , the present second embodiment is adapted to control the rotating speeds of the drive wheels  10 ,  14  so as to minimize the speed difference of only the center differential  18 , according to a wheel drive force control routine illustrated in the flow chart of FIG.  13 . In this case, the speed control amounts for the drive wheels  10 ,  14  are calculated according to the following equations, in step S 52  implemented where the affirmative decision (YES) is obtained in step S 51  which is identical with step S 1  of FIG.  4 . 
     
       
         ΔV FR =[(V FR +V FL )−(V RR +V RL )]/4 
       
     
     
       
         ΔV FL =[(V FR +V FL )−(V RR +V RL )]/4 
       
     
     
       
         ΔV RR =[(V RR +V RL )−(V FR +V FL )]/4 
       
     
     
       
         ΔV RL =[(V RR +V RL )−(V RR +V FL )]/4 
       
     
     Step S 52  is followed by step S 53  identical with step S 3  of FIG.  4 . 
     As in the first embodiment, the calculated speed control amount which is positive is determined as the final speed control amount, but the final speed control amount is zeroed if the calculated speed control amount is a negative value. 
     It will be understood that the hydraulic pressure control device  60  and a portion of the controller  70  assigned to implement the wheel drive force control routine of FIG. 13 cooperate to provide the brake control device including the total control means for controlling the brakes  40  so as to substantially zero the speed control error for each wheel  10 ,  14 . 
     The present second embodiment is adapted to minimize the speed difference of only the center differential  18  while the speed differences of the front and rear differentials  12 ,  16  are not taken into account for controlling the drive forces of the drive wheels  10 ,  14 . This embodiment is capable of suitably dealing with the running condition of the motor vehicle in which the front right and left wheels  10  connected to each other by the front differential  12  have almost the same degrees of slipping tendency, as indicated in FIG.  14 . However, the present embodiment is not capable of suitably dealing with the running condition of the motor vehicle in which the front left drive wheel  10  has a larger degree of slipping tendency than the front right drive wheel  10 , as indicated in FIG.  15 . In this case, the drive torque to be applied to the front right drive wheel  10  having a smaller degree of slipping tendency is undesirably reduced. In this respect, it is noted that the first embodiment assures a sufficient drive torque of the front right drive wheel  10  even in the case of FIG. 15, since the speeds of the wheels  10 ,  14  are controlled so as to minimize the speed differences of the front and rear differentials  12 ,  16  as well as the speed difference of the center differential  18 . 
     A third embodiment of this invention will be described by reference to FIGS. 16 and 17. The same reference numerals as used in the first embodiment will be used in the third embodiment, to identify the same elements, which will be not be described redundantly. 
     In the first embodiment adapted to control the brakes  40  so as to substantially zero the speed difference between the center differential  18 , the speed control amounts for the drive wheels  10 ,  14  may be extremely small, making it difficult to smoothly start the motor vehicle, in a running condition of the vehicle in which the slipping state of the front wheels  10  is substantially the same as that of the rear wheels  14 . 
     In the light of the above running condition, the present third embodiment is adapted to control the speeds of the drive wheels  10 ,  14  in a suitable one of two speed control modes, namely, a first speed control mode according to a first rule used in the first embodiment to calculate the speed control amounts for the drive wheels  10 ,  14 , and a second speed control mode according to a second rule in which the speed control amount for each drive wheel  10 ,  14  is calculated as a speed control error obtained by subtracting a minimum value V MIN  of the speeds of the four drive wheels  10 ,  14 , from the speed of each drive wheel  10 ,  14 . The minimum-value V MIN  is the lowest one of the speeds of the four wheels  10 ,  14 , and is used as a reference wheel speed for controlling the brakes  40 . That is, the speed control amount ΔV** for each wheel  10 ,  14  is calculated according to the following equation: 
     
       
         ΔV**=V**−V MIN   
       
     
     The selection of one of the first and second speed control modes (first and second rules) for each drive wheel  10 ,  14  is effected depending upon whether the motor vehicle is in a running condition in which smooth starting of the vehicle is difficult. 
     The determination as to whether smooth starting of the vehicle is difficult may be effected by determining whether the speed of any one of the four wheels  10 ,  14  is substantially zero, that is, whether any one of the four wheels is not rotating. In the present third embodiment, however, another condition is also checked to determine the difficulty of smooth starting of the motor vehicle. That is, the third embodiment is adapted to determine that smooth starting of the vehicle is difficult if any one of the four wheels is stopped and if the speed difference of the center differential  18  is substantially zero. If these two conditions are simultaneously satisfied, it means that the control amounts for the wheels as calculated according to the first rule tend to be extremely small, making it difficult to smoothly start the vehicle. However, the second speed control mode is not selected immediately after the above two conditions are simultaneously satisfied, but is selected only after a predetermined time T has passed with the two conditions being simultaneously satisfied. This arrangement improves the adequacy of selection of the speed control mode. 
     The determination of the difficulty of smooth starting of the vehicle may be effected by using a third condition that the first speed control mode is currently established. In this case, the three conditions must be satisfied simultaneously for the predetermined time T, to select the second speed control mode. Alternatively, the second condition that the speed difference of the center differential  18  is substantially zero may be replaced by the above condition that the first speed control mode is currently established. 
     Thus, the second speed control mode in which the drive wheel speeds are controlled so as to zero the speed control error ΔV**=V**−VMIN is selected when the predetermined conditions are satisfied for the predetermined time T. 
     The wheel drive force control routine illustrated in the flow chart of FIG. 16 is executed according to a control program stored in the ROM  74  of the controller  70 . This routine includes step S 102  in which an affirmative decision (YES) is obtained if the second speed control mode has been selected according to a second speed control mode selecting routine of FIG.  17 . 
     The wheel drive force control routine of FIG. 16 is initiated with step S 101  to determine whether the control of the wheel drive forces is permitted by the vehicle operator. If the affirmative decision (YES) is obtained in step S 101 , the control flow goes to step S 102  to determine whether the second speed control mode has been established according to the routine of FIG.  17 . If a negative decision (NO) is obtained, that is, it means that the first speed control mode must be used to control the brakes  40 . In this case, the control flow goes to step S 103  to calculate the speed control amounts for the individual drive wheels  10 ,  14  according to the first rule as in step S 2  of FIG. 4 of the first embodiment. 
     If the second speed control mode has been established according to the second speed control mode selecting routine of FIG. 17, that is, if an affirmative decision (YES) is obtained in step S 102 , the control flow goes to step S 105  to calculate the speed control amounts for the drive wheels  10 ,  14  according to the second rule, more specifically, calculate the speed control amount for each wheel by subtracting the reference wheel speed, that is, the lowest one V MIN  of the speeds of the four wheels from the speed of that wheel. 
     Steps S 103  and S 105  are followed by step S 104  identical with step S 3  of FIG. 4, in which the hydraulic pressure control device  60  is controlled to control the brakes  40  for the drive wheels  10 ,  14  according to the calculated control amounts. 
     The second speed control mode selecting routine illustrated in the flow chart of FIG. 17 is initiated with step S 201  to determine whether the speed V** of any drive wheel  10 ,  40  is zero (Okm/h), namely, whether any drive wheel is held stationary. If a negative decision (NO) is obtained in step S 201 , the control flow goes to step S 206  to reset a time counter, which is provided in the computer  78 , to measure a time during which an affirmative decision (YES) is obtained in both steps S 201  and S 202 , as described below. In this case, one cycle of execution of the routine of FIG. 17 is terminated. Under this condition, the speed control amounts for the drive wheels  10 ,  14  are calculated in step S 103  according to the first rule. 
     If the speed of any of the drive wheels  10 ,  14  is substantially zero, the affirmative decision (YES) is obtained in step S 201 , and the control flow goes to step S 202  to determine whether the speed difference of the center differential  18  is substantially zero, that is, whether the sum of the speeds of the front left and right wheels  10  is substantially equal to the sum of the speeds of the rear left and right wheels  14 . If a negative decision (NO) is obtained in step S 202 , the control flow goes to step S 206  to reset the time counter as described above, and one cycle of execution of the routine is terminated. 
     If the speed of any wheel  10 ,  14  is substantially zero and if the sum of the front wheels  10  and the sum of the rear wheels  14  are substantially equal to each other, the affirmative decision (YES) is obtained in step S 202 , and the control flow goes to step S 203  to increment the time counter. Then, the control flow goes to step S 204  to determine whether a content C T  of the time counter is equal to or larger than a predetermined value T, that is, determine whether the affirmative decision (YES) has been obtained in both of the steps S 201  and S 202  for at least the predetermined time T. If a negative decision (NO) is obtained in step S 204 , the control flow goes back to step S 201 . 
     When the content C T  of the time counter has reached the predetermined value T during repeated implementation of steps S 201 -S 204 , an affirmative decision (YES) is obtained in step S 204 , and the control flow goes to step S 205  to select the second speed control mode. Thus, one cycle of execution of the routine of FIG. 17 is terminated. 
     It will be understood from the above description of the third embodiment that the hydraulic pressure control device  60  and a portion of the controller  70  assigned to implement step S 103  cooperate to provide control means for controlling the brakes  40  so as to substantially zero the speed control error for each wheel  10 ,  14 . It will also be understood that the hydraulic pressure control device  60  and a portion of the controller  70  assigned to implement step S 105  cooperate to provide control means for controlling the brakes  40  so as to substantially zero the speed difference of the non-stopped wheels  10 ,  14  from the speed (lowest speed V MIN ) of the stopped wheel  10 ,  14  (reference wheel speed V MIN ). It will also be understood that the hydraulic pressure control device  60  and a portion of the controller  70  assigned to implement steps S 101  and S 103 -S 105  cooperate to provide at least two control means for controlling the brakes  40  in a selected one of a plurality of speed control modes. It will further be understood that the wheel speed sensors  80  and a portion of the controller  70  assigned to implement the second speed control mode selecting routine of FIG. 17 cooperate to provide selectively enabling means for selectively enabling one of the above-indicated at least two control means to operate. 
     Referring next to FIGS. 18 and 19, a fourth embodiment of this invention will be described. The same reference numerals as used in the first embodiment will be used in the fourth embodiment, to identify the same elements, which will not be described redundantly. 
     Reference is made to the block diagram of FIG. 18, there is shown an electrical arrangement of the wheel drive force control apparatus, including the controller  70  which is adapted to receive output signals of a LOW-GEAR TRANSFER sensor  92  and a SPEED CONTROL MODE sensor  94 . The LOW-GEAR TRANSFER sensor  92  is connected to the operator-controlled transfer switching member  30 , to detect that the switching member  30  is placed in its position for placing the transfer  28  in a LOW-GEAR transfer state. The SPEED CONTROL MODE sensor  94  is connected to an operator-controlled SPEED CONTROL MODE selector switch  96  which is provided in the driver&#39;s compartment of the motor vehicle and has a FIRST SPEED CONTROL mode position and a SECOND SPEED CONTROL mode position for selecting the respective first and second speed control modes in which the speed control amounts for the drive wheels  10 ,  14  are calculated according to the respective first and second rules described above. The SPEED CONTROL MODE sensor  94  is adapted to detect that the selector switch  96  is placed in the SECOND SPEED CONTROL MODE position. 
     The ROM  74  of the controller  70  stores a control program for executing a wheel drive force control routine illustrated in the flow chart of FIG.  19 . This routine is initiated with step S 301  to determine whether the control of the wheel drive forces is permitted by the vehicle operator. If the affirmative decision (YES) is obtained in step S 301 , the control flow goes to step S 302  to determine whether the transfer  28  is placed in the LOW-GEAR transfer state. The determination in this step S 302  is effected based on the output signal of the LOW-GEAR TRANSFER sensor  92 . If an affirmative decision (YES) is obtained in step S 302 , the control flow goes to step S 306 . If a negative decision (NO) is obtained in step S 302 , the control flow goes to step S 303  to determine whether the SPEED CONTROL MODE selector switch  96  is placed in the SECOND SPEED CONTROL MODE position. The determination in step S 303  is effected based on the output signal of the SPEED CONTROL MODE sensor  94 . If a negative decision (NO) is obtained in step S 303 , the control flow goes to step S 304  to calculate the speed control amounts for the drive wheels  10 ,  14  according to the first rule described above with respect to step S 2 . If an affirmative decision (YES) is obtained in step S 303 , the control flow goes to step S 306 , as in the case where the affirmative decision is obtained in step S 302 . Step S 306  is provided to calculate the speed control amounts for the drive wheels  10 ,  14  according to the second rule described above with respect to step S 105 . Steps S 304  and S 306  are followed by step S 305  identical with steps S 3 , S 53  and S 104 , wherein the hydraulic pressure control device  60  is controlled according to the calculated speed control amounts, to control the brakes  40 . Thus, one cycle of execution of the routine of FIG. 19 is terminated. 
     It will be understood from the above description of the fourth embodiment that the hydraulic pressure control device  60  and a portion of the controller  70  assigned to implement step S 304  of FIG. 19 cooperate to provide control means for controlling the brakes  40  so as to substantially zero the speed control error for each wheel  10 ,  14 . It will also be understood that the hydraulic pressure control device  60  and a portion of the controller  70  assigned to implement step S 306  of FIG. 19 cooperate to provide control means for controlling the brakes  40  so as to substantially zero the speed difference of the non-stopped wheels  10 ,  14  from the speed (lowest speed or reference speed V MIN ) of the stopped wheel  10 ,  14 . It will also be understood that the hydraulic pressure control device  60  and a portion of the controller  70  assigned to implement steps S 101  and S 304 -S 306  cooperate to provide at least two control means for controlling the brakes  40  in a selected one of a plurality of speed control modes. It will further be understood that the LOW-GEAR TRANSFER sensor  92 , SPEED CONTROL MODE sensor  94  and a portion of the controller  70  assigned to implement steps S 302  and S 303  of FIG. 19 cooperate to provide selectively enabling means for selectively enabling one of the above-indicated at least two control means to operate. 
     While the illustrated embodiments are adapted to control the drive forces of the drive wheels  10 ,  14  by controlling the brakes  40 , the wheel drive forces may be controlled by controlling the engine  24  so as to control the output torque of the engine  24 , as well as the brakes  40 , so as to permit smooth starting of the vehicle. In this case, the control of the engine  24  may be initiated simultaneously with the control of the brakes  40 , or after the initiation of the control of the brakes  40  in the case where the vehicle cannot be smoothly started by controlling only the brakes  40 . 
     While the several presently preferred embodiments of the present invention have been described above in detail by reference to the accompanying drawings, it is to be understood that the present invention may be embodied with various changes, modifications and improvements, which may occur to those skilled in the art, without departing from the spirit and scope of the invention defined in the following claims.