Patent Publication Number: US-2010121546-A1

Title: Drive force control apparatus for vehicle

Description:
TECHNICAL HELD 
     The present invention relates to a drive force control apparatus for a vehicle configured to control transmitting of a drive force generated by a drive power source, to right and left drive wheels, more particularly, to improvement in responsibility. 
     BACKGROUND ART 
     There is known a drive force control apparatus for a vehicle, having at least one engaging element and configured to implement drive force control to control transmitting of the drive force generated by the drive power source, to the left and right drive wheels by controlling the engaged state of the at least one engaging element in accordance with a demand for control. For instance, it corresponds to the drive force distributing apparatus disclosed in the below Patent Literature 1. This technique permits it to distribute the drive force generated by the drive power source to the left and right drive wheels by the differential device constituted of at least one planetary gear set, and it includes a transmission mechanism in which a pair of planetary gear sets cooperates, coaxially disposed to the adjacent differential device, and a pair of clutches to selectively transmit the output from the transmission mechanism to the carrier and sun gear of the aforementioned differential device, and the drive force transmitted through the transmission mechanism to the sun gear or carrier to be distributed, by slippably engaging any of the pair of clutches if required. Thus, control of distribution of the torque transmitted to the left and right drive wheels can be preferably implemented upon such as turning of the vehicle. 
     Patent Literature 1: JP 11-105573 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the aforementioned conventional technique results in occurrence of a response delay when control of engaging elements is initiated after a real demand for control. Then, technique to improve the responsibility was desired in the drive force control apparatus for the vehicle configured to control transmitting of the drive force generated by the drive power source to the left and right drive wheels. 
     It is therefore an object of the present invention to provide an improvement of the drive force control apparatus for the vehicle in responsibility of control of transmitting the drive force to the left and right drive wheels. 
     Solution to Problem 
     The object indicated above may be achieved according to the present invention, which provides a drive force control apparatus for a vehicle, having at least one engaging element and configured to implement drive force control to control transmitting of a drive force generated by a drive power source, to left and right drive wheels by controlling an engaged state of the at least one engaging element in accordance with a demand for control of the drive force, is characterized in that possibility that the demand for control of the drive force is made is determined on the basis of a predetermined standard, and if it is determined that it is possible that the demand for control would be made, preliminary control to control the engaged state of the at least one engaging element is implemented to a stage prior to implementing of the drive force control. 
     ADVANTAGEOUS EFFECTS OF INVENTION 
     The aforementioned invention provides the drive force control apparatus for a vehicle, wherein the possibility that the demand for the drive force control is made is determined on the basis of the predetermined standard, and if it is determined that it is possible that the demand for control would be made, the preliminary control to control the engaged states of the engaging elements is implemented to the stage prior to implementing of the drive force control. Consequently, the drive force control is implemented with superior responsibility upon the real demand for control. 
     Preferably, the aforementioned preliminary control controls the engaged states of the engaging elements such that the torque transmitting capacity is lower than that in the drive force control. Then the drive force control is implemented with superior responsibility upon the real demand for control by the preliminary control of the practical embodiment. 
     Preferably, the aforementioned preliminary control controls the engaged states of fewer engaging elements than engaging elements in the drive force control. Then the drive force control is implemented with superior responsibility upon the real demand for control by the preliminary control of the practical embodiment. 
     Preferably, the aforementioned drive force control is the differential limitation control that limits the differential control of the left and right drive wheels, and the predetermined standard is whether the vehicle speed is equal to or lower than the predetermined one. Then the responsibility can be improved in the practical embodiment, with respect to the differential limitation control of the left and right drive wheels. 
     Preferably, the aforementioned drive force control is the drive force distributing control that distributes the drive force to the left and right drive wheels in a predetermined ratio, and the predetermined standard is whether the vehicle is turning. Then the responsibility can be improved in the practical embodiment, with respect to the drive force distributing control to the left and right drive wheels. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is the schematic view for explaining a system of a front- and rear-wheel drive vehicle which is typical front-engine and front-wheel-drive, and is provided with a drive force transmitting apparatus to which the present invention is applicable. 
         FIG. 2  is the schematic view for explaining a system of the drive force distributing device provided in the drive force transmitting apparatus of  FIG. 1 . 
         FIG. 3  is a collinear chart indicating rotating speeds of the rotating elements in the drive force distributing device of  FIG. 2  upon noncontrolling. 
         FIG. 4  is a collinear chart indicating rotating speeds of the rotating elements in the drive force distributing device of  FIG. 2  upon left and right wheel torque difference control. 
         FIG. 5  is a collinear chart indicating rotating speeds of the rotating elements in the drive force distributing device of  FIG. 2  upon left and right wheel torque difference control. 
         FIG. 6  is a collinear chart indicating rotating speeds of the rotating elements in the drive force distributing device of  FIG. 2  upon differential limitation control. 
         FIG. 7  is a table indicating engaged states of the engaging elements in the preliminary control of the drive force control by the drive force distributing controller provided in the vehicle in  FIG. 1 . 
         FIG. 8  is a flowchart explaining a major part of the drive force distributing preliminary control by the aforementioned drive force distributing controller provided in the vehicle in  FIG. 1 . 
     
    
    
     REFERENCE SIGNS LIST 
     
         
         
           
               12 : Engine (Drive power source) 
               30 : Rear wheels (Drive wheels) 
             B: Brake (Engaging element) 
             C 1 : First clutch (Engaging element) 
             C 2 : Second clutch (Engaging element) 
           
         
       
    
     DESCRIPTION OF EMBODIMENTS 
     Referring to the drawings, there will be described in detail a preferred embodiment of the present invention. 
     Examples 
     Referring to the schematic view of  FIG. 1  for explaining a system of a front- and rear-wheel drive vehicle which is a typical front-engine and front-wheel-drive (FF) one, and is provided with a drive force transmitting apparatus  10  to which the present invention is applicable. As shown in  FIG. 1 , a drive force (torque) generated by a drive power source in the form of ad engine  12  is transmitted to a pair of front wheels  20   l ,  20   r  (hereinafter, referred to “front wheels  20 ” where it is not specified) at left and right sides, respectively, through an automatic transmission  14 , a differential gear set  16  for front wheels and a pair of front wheel axles  18   l ,  18   r  (hereinafter, referred to “front wheel axles  18 ” where it is not specified) at left and right sides, respectively. And the drive force is also transmitted to a pair of rear wheels  30   l ,  30   r  (hereinafter, referred to “rear wheels  30 ” where it is not specified) at left and right sides, respectively, through a central differential gear set (center diff.)  22 , a drive force transmitting shaft in the form of a propeller shaft  24 , a drive force distributing device  26  and a pair of rear wheel axles  28   l ,  28   r  (hereinafter, referred to “rear wheel axles  18 ” where it is not specified) at left and right sides, respectively. 
       FIG. 1  illustrates the drive force transmitting apparatus  10  in which rotating axes of the rear wheels  30 , that is, drive wheels to which the drive force is distributed by the drive force distributing device  26 , and of the propeller shaft  24  are perpendicularly disposed to each other. Furthermore, the drive force transmitting apparatus  10  is provided with a hydraulic circuit  34  for controlling such as a hydraulic pressure to control an engaged state of an engaging element provided in the drive force distributing device  26 , and a drive force distributing controller  36  for controlling such as a hydraulic pressure supplied from the hydraulic circuit  34  into the drive force distributing device  26 , through such as an electromagnetic control valve (not shown) provided in the hydraulic circuit  34 . In  FIG. 1  a path of a hydraulic pressure output from the hydraulic circuit  34  is represented by a thin broken line with an arrow, and each path of a control signal (control command) output from the controller  36  and each input signal from various sensors is represented by a thin solid line with an arrow. The engine  12  is, for instance, an internal combustion engine such as a gasoline engine or diesel engine which generates a drive force by combustion of a fuel injected into a cylinder. The aforementioned automatic transmission  14  is, for instance, a step-variable automatic transmission which converts a torque input from the engine  12  into a converted torque by a predetermined speed ratio γ in a shifting-down or shifting-up action and outputs the converted torque. The automatic transmission  14  selectively establishes one of forward, rearward or neutral gear positions, and changes the vehicle speed by conversion in accordance with the respective speed ratio γ. An input shaft of the automatic transmission  14  is connected to an output shaft of the engine  12  through such as a torque converter (not shown). 
     The aforementioned drive force distributing controller  36  includes such as a CPU, ROM, RAM and input/output interface, and is a microcomputer which implements signal processing in accordance with programs stored in the ROM utilizing the temporary storage function of the RAM. For instance, the controller  36  implements such as differential limitation control or yaw control as described below by controlling a hydraulic pressure supplied to the engaging element provided in the drive force distributing device  26 , by controlling a commanded value of a current supplied to the electromagnetic control valve provided in the hydraulic circuit  34 . In order to implement these controls, the controller  36  is functionally provided with a vehicle speed determining means  38 , a turning determining means  40  and an engagement control means  42 . These control functions will be described below by reference to such as the flowchart in  FIG. 8 . The drive force transmitting apparatus  10  is provided with various sensors such as a wheel rotating speed sensor  44  for detecting a real rotating speed of the rear wheels  30  corresponding to the vehicle speed, a steering angle sensor  46  for detecting a steering angle of a steering wheel (not shown), and an accelerator operation amount sensor  48  for detecting an operation amount of an accelerator pedal (not shown) corresponding to a depressed amount. The drive force distributing controller  36  receives such as a signal indicative of the vehicle speed, a signal indicative of the steering angle of the steering wheel and a signal indicative of the operation amount of the accelerator pedal from these sensors. 
     Referring to the schematic view of  FIG. 2  for illustrating an example of the aforementioned drive force distributing device  26 , the device  26  is, for instance, provided with a bevel gear  50  connected to an end portion of the propeller shaft  24  that is rotatably driven by the engine  12  through the central differential gear set  22 , and a bevel gear  52  that is meshed with the bevel gear  50 , and the device  26  is constituted such that the drive force is input from the propeller shaft  24  through a set of the bevel gears  50 ,  52 . Further, the drive force distributing device  26  is provided with a differential device  54  for distributing the transmitted drive force from the propeller shaft  24  through the bevel gears  50 ,  52 , to the left and right rear wheels  30   l ,  30   r , a transmission device  56  disposed adjacent to the differential device  54  and such that its axis is coaxial with that of the rear wheel axles  28   l ,  28   r , and engaging elements in the forms of a first and second clutches C 1 , C 2  for selectively transmitting an output from the transmission device  56  to the differential device  54 . 
     The aforementioned differential device  54  is a planetary gear set of a double pinion type including a first rotating element RE 1  in the form of a ring gear R 1 , a plurality of pairs of pinions P 1  meshing with each other, a second rotating element RE 2  in the form of a carrier CA 1  supporting the pinions P 1  such that each pinion is rotatable about its axis and about the axis of the carrier CA 1 , and a third rotating element RE 3  in the form of a sun gear S 1  meshing with the ring gear R 1  through the aforementioned plurality of pairs of pinions P 1 . The gear ratio ρ (=Number of teeth of sun gear S 1 /Number of teeth of ring gear R 1 ) is determined, for instance, such that it corresponds to 0.5. The ring gear R 1  is integrally fixed to a casing  58  of the differential device  54  within the casing  58 , and a torque of the propeller shaft  24  is transmitted to the ring gear R 1  such that the rotating speed of the propeller shaft  24  is reduced by the bevel gears  50 ,  52 . The carrier CA 1  is connected to the left rear wheel  30   l  through the left rear wheel axle  28   l , and the sun gear S 1  is connected to the right rear wheel  30   r  through the right rear wheel axle  28   r . The second rotating element RE 2  and the third rotating element are exchangeable in both of this and the aftermentioned examples. 
     The aforementioned transmission device  56  is provided with a planetary gear set of a single pinion type including a fifth rotating element RE 5  in the form of a sun gear S 2 , a pinion P 2 , a sixth rotating element RE 6  in the form of a carrier CA 2  supporting the pinion P 2  such that each pinion is rotatable about its axis and about the axis of the carrier CA 2 , and a fourth rotating element RE 4  in the form of a ring gear R 2  meshing with the sun gear S 2  through the aforementioned pinion P 2 . The fifth rotating element RE 5  is connected to the first rotating element RE 1  and functions as an input member of the transmission device  56 . The sixth rotating element RE 6  is connected to an engaging element for switching a torque transmitting path in the form of a brake B, and is selectively connected to an unrotatable member  60  through the brake B. The fourth rotating element RE 4  functions as an output member of the transmission device  56 . The fourth rotating element RE 4  is selectively slippably engaged with the second rotating element RE 2  in the form of the carrier CA 1  in the differential device  54  and the left rear wheel axle  28   l , through the first clutch C 1 , and, concurrently, selectively slippably engaged with the third rotating element RE 3  in the form of the sun gear S 1  in the differential device  54  and the right rear wheel axle  28   r , through the second clutch C 2 . Preferably, the brake B, first clutch C 1  and second clutch C 2  are multi-plated frictional engaging devices that are configured to slippably engage. They are engaged or released by a hydraulic pressure output from the hydraulic circuit  34  that is controlled in accordance with a control command output from the drive force distributing controller  36 , and, concurrently, a transmitted torque upon a slipping engagement is controlled by implementing hydraulic control if required. 
     Referring back to  FIG. 1 , the engagement control means  42  provided in the drive force distributing controller  36  implements drive force control to control transmitting the drive force generated by the engine  12  to the left and right rear wheels  30   l ,  30   r , by controlling respective engaged states of the engaging elements in the forms of the first clutch C 1 , second clutch C 2  and brake B, through the hydraulic circuit  34 . The drive force generated by the engine  12  is transmitted to the differential device  54  to rotatably drive the casing  58 , through such as the automatic transmission  14 , the central differential gear set  22  and the propeller shaft  24 . The ring gear R 1  of the differential device  54  is provided integral to the casing  58 , and, accordingly, the drive force from the propeller shaft  24  is input to the differential device  54  through the ring gear R 1  and, sequentially, an input member in the form of the casing  58 . The engagement control means  42  controls the engaging elements in the form of the first clutch C 1 , second clutch C 2  and brake B, to place each of them in an engaged, slippably engaged or released state, by controlling hydraulic pressures supplied from the hydraulic circuit  34  to the respective engaging elements, through such as an electromagnetic control valve (not shown) provided in the hydraulic circuit  34 . Thus, the control of the respective engaged/released states of the first clutch C 1 , second clutch C 2  and brake B permits the control of distribution of the drive force input to the differential device  54  to the left and right rear wheels  30   l ,  30   r . Hereinafter, there will be detailed the distribution of the drive force to the left and right rear wheels  30   l ,  30   r  by the drive force distributing device  26 . 
     Reference is now made to the collinear charts of  FIGS. 3 to 6  indicating rotating speeds of a plurality of the rotating elements in the differential device  42  in the drive force distributing device  26 . In these collinear charts, the rotating speed Nl of the second rotating element RE 2  in the form of the carrier CA 1  connected to the left rear wheel  30   l  is taken along the left vertical axis, the rotating speed Nr of the third rotating element RE 3  connected to the right rear wheel  30   r  is taken along the right vertical axis, and the rotating speed Nc of the fourth rotating element RE 4  and the rotating speed Ni of the first rotating element RE 1  in the form of the ring gear R 1  integrally rotatable to the casing  58  is taken along the central vertical axis. Tables at the right of the collinear charts show the engaged/released states of the brake B, first clutch C 1  and second clutch C 2 , where shown are “O” indicative of the engaged state and “X” indicative of the released state. The connecting line between the points of the rotating speeds Nl and Nc indicates the state of the first clutch C 1 , where shown are a solid line indicative of the slippably engaged state and a broken line indicative of the released state. As well, the connecting line between the points of the rotating speeds Nc and Nr indicates the state of the second clutch C 2 , where shown are a solid line indicative of the slippably engaged state and a broken line indicative of the released state. 
       FIG. 3  illustrates the collinear chart of the drive force distributing device  26  upon noncontrolling. The drive force distributing device  26  upon noncontrolling places the engaging elements in the forms of the brake B, first clutch C 1  and second clutch C 2  all in the released states. In this state, only the differential device  54  functions and the transmission device  56  is placed in an drag state, and accordingly, the drive force is equally distributed to the left and right rear wheels  30   l ,  30   r . Consequently, no change of the torque transmitting path and no differential limitation are implemented in the drive force distributing device  26 , and the device  26  functions as a conventional open differential one. Upon forwarding straight, as shown in  FIG. 3 , the differential device  54  is integrally rotated and, accordingly, the numbers Nl and Nr of rotation of the left and right rear wheels  30   l  and  30   r , respectively, are substantially equal to each other. 
       FIG. 4  illustrates an example of the collinear chart upon yaw-controlling, that is, upon left and right wheel torque difference control. It is a collinear chart in the state that the drive force of the right rear wheel  30   r  is increased, for instance, in turning to the left, to restrain understeering. In this example in  FIG. 4 , the brake B is engaged, the first clutch C 1  is slippably engaged and the second clutch C 2  is released concurrently. The engagement of the brake B permits fixation of the carrier CA 2  in the transmission device  56 , the number Nc of rotation of the fourth rotating element RE 4  is reduced in the reverse direction and output. The slippable engagement of the first clutch C 1  permits transmitting of an output of the fourth rotating element RE 4  to the second rotating element RE 2 . Since the number Nc of rotation of the fourth rotating element RE 4  is reduced in the reverse direction, the drive force of the left rear wheel  30   l  is reduced by the slippable engagement of the first clutch C 1 , while the drive force of the right rear wheel  30   r  is relatively increased. Since the number Nl of rotation of the left rear wheel  30   l  is reduced by the slippable engagement, the number of rotation of the right rear wheel  30   r  is increased by the differential device  54 . 
     As shown in  FIG. 5 , increasing of the drive force of the left rear wheel  30   l , for instance, in turning to the right, permits to restrain understeering. In the example in  FIG. 5 , the brake B is engaged, the second clutch C 2  is slippably engaged and the first clutch C 1  is released concurrently. As well as the example in  FIG. 4 , the engagement of the brake B permits fixation of the carrier CA 2  in the transmission device  56 , the number Nc of rotation of the fourth rotating element RE 4  is reduced in the reverse direction and output. The slippable engagement of the second clutch C 2  permits transmitting of an output of the fourth rotating element RE 4  to the third rotating element RE 3 . Since the number of rotation of the fourth rotating element RE 4  is reduced in the reverse direction, the drive force of the right rear wheel  30   r  is reduced by the slippable engagement of the second clutch C 2 , while the drive force of the left rear wheel  30   l  is relatively increased. Since the number Nr of rotation of the right rear wheel  30   r  is reduced by the slippable engagement, the number of rotation of the left rear wheel  30   l  is increased by the differential device  54 . 
       FIG. 6  illustrates a collinear chart upon differential limitation control, where the brake B is released and concurrently the first and second clutches C 1 , C 2  are engaged. Thus, the concurrent engagement of both the first and second clutches C 1 , C 2  causes limitation of the relative rotation of the left and right rear wheel axles  28   l ,  28   r , and, it causes the differential limitation of the left and right rear wheels  30   l ,  30   r . When the first and second clutches C 1 , C 2  are fully engaged, the drive force distributing device  56  functions as a non-slipping differential one, and, accordingly, the left and right rear wheels  30   l ,  30   r  rotate in the same rotating speed. The differential limitation force can be determined at discretion, being proportional to the clutch control torque. 
     Referring back to  FIG. 1 , the aforementioned engagement control means  42  determines the possibility that a demand for the drive force control, that is, the transmitting control of the drive force to the left and right rear wheels  30   l ,  30   r  would be made on the basis of a predetermined standard. If the means  42  determines that it is possible that the demand for control would be made, the means  42  implements preliminary control to control the engaged states of the engaging elements, that is, the brake B, first clutch C 1  and second clutch C 2 , to a stage prior to implementing of the drive force control. The vehicle speed determining means  38  and turning determining means  40  function as a control demand determining means to determine whether it is possible that the demand for control due to the preliminary control would be made. The vehicle speed determining means  38  determines whether a value of the vehicle speed is lower than a predetermined one, on the basis of a signal representing the vehicle speed supplied from the wheel rotating speed sensor  44 . The turning determining means  40  determines whether the vehicle is turning, on the basis of a signal representing the steering angle supplied from the steering angle sensor  46 . 
     The engagement control means  42  implements preliminary control to control the engaged state of the engaging elements provided in the drive force distributing device  26 , to a stage prior to implementing of the differential limitation control to limit the differential control of the left and right rear wheels  30   l ,  30   r  if it is determined that it is possible that the demand for the drive force control, corresponding to the differential limitation control would be made, that is, if the determination by the vehicle speed determining means  38  is affirmative. Described in detail, the respective engaged states of the engaging elements, that is, the first and second clutches C 1 , C 2  affecting the differential limitation control, is controlled such that a torque transmitting capacity is lower than that in the differential limitation control. This torque transmitting capacity upon the preliminary control, preferably, corresponds to a compressed force to advance the piston for engaging the engaging elements to a position that the piston compresses the frictional plate, that is, a compressed force to achieve the movement to eliminate backlashes, and hydraulic pressures supplied to hydraulic actuators of the first and second clutches C 1 , C 2  are controlled such that possibly higher torque transmitting capacity is achieved within the limitation that the effects on the differential states of the left and right rear wheels  30   l ,  30   r  are disregardable. In the preliminary control of the differential limitation control, the brake B is not controlled and remains released. 
     The engagement control means  42  implements preliminary control to control the engaged state of the engaging elements provided in the drive force distributing device  26 , to a stage prior to implementing of the left-and-right-wheels torque difference control (yaw control) of the left and right rear wheels  30   l ,  30   r  if it is determined that it is possible that the demand for the drive force control corresponding to the torque difference control (yaw control) of the left and right rear wheels  30   l ,  30   r  would be made, that is, if the determination by the turning determining means  40  is affirmative. Described in detail, fewer engaging elements that are selected from the engaging elements, that is, the brake B, first clutch C 1  and second clutch C 2  affecting the left-and-right-wheels torque difference control, than engaging elements upon the left-and-right-wheels torque difference control, for instance, only the brake B is fully engaged. In this preliminary control of the left-and-right-wheels torque difference control, the engaged states of the first and second clutches C 1 , C 2  are not necessary to control, however, as described above, engagement control to achieve the movement to eliminate backlashes of the first and second clutches C 1 , C 2  may be implemented. 
     The vehicle speed determining means  38 , preferably, determines whether the vehicle speed is higher than a predetermined one (different from a determination standard in the preliminary control of the differential limitation control), and the engagement control means  42  may implement the preliminary control of the left-and-right-wheels torque difference control as described above if the determination by the vehicle speed determining means  38  is affirmative. 
       FIG. 7  is a table indicating engaged states of the engaging elements in the preliminary control by the engagement control means  42 . As shown in  FIG. 7 , in the preliminary control of the present embodiment, the first and second clutches C 1 , C 2  are placed in a relatively weakly engaged state, that is, the engaged state in which the movement to eliminate backlashes is achieved, upon starting to low-speed running of the vehicle. Upon running straight such as when starting to low-speed running, a traction performance is often required and the demand for the differential limitation control is often output, then, the achievement of the movement to eliminate backlashes in advance permits to reduce the duration from the output of the real demand for control to the starting of the substantive engagement control of the engaging elements, and, accordingly, permits to improve responsibility of the differential limitation control. As shown in  FIG. 7 , in the preliminary control of the present embodiment, the brake B is fully engaged upon the mid-speed to high-speed running of the vehicle. Upon the mid-speed to high-speed running of the vehicle, steering follow-up characteristic is often required and the demand for the left-and-right-wheels torque difference control (yaw control) is often output, then, the full engagement of the brake B permits to reduce the duration from the output of the real demand for control to the completion of the substantive engagement of the engaging elements, and, accordingly, permits to improve responsibility of the left-and-right-wheels torque difference control. 
     The flowchart of  FIG. 8  illustrates a major part of the drive force distributing preliminary control by the aforementioned drive force distributing controller  36 , and this control routine is repeatedly executed in a predetermined cycle. 
     The control routine is initiated with step S 1  (hereinafter, “step” being omitted) corresponding to the action of the vehicle speed determining means  38 , to determine whether the vehicle speed is equal to or lower than a predetermined one, on the basis of the signal indicative of the vehicle speed supplied from the wheel rotating speed sensor  44 . If a negative determination is obtained in S 1 , the control flow goes to S 4 , otherwise, it goes to S 2  in which the settings for controlling the engaged states of the engaging elements in the drive force distributing device  26  are determined such that traction weighs, and it goes to S 3  corresponding to the action of the engagement control means  42  in which the preliminary control of the engaged states of the engaging elements in the drive force distributing device  26  is implemented. Concretely, the engagement control to achieve the movement to eliminate backlashes of the first and second clutches C 1 , C 2  is implemented, and, then, this routine is terminated. S 4  corresponding to the action of the turning determining means  40 , to determine whether the vehicle is turning, is executed on the basis of the signal indicative of the steering angle supplied from the steering angle sensor  46 . If a negative determination is obtained in S 4 , this routine is terminated, otherwise, the control flow goes to S 5  in which the settings for controlling the engaged states of the engaging elements in the drive force distributing device  26  are determined such that steering follow-up characteristic weighs, and it goes to S 3  in which the preliminary control of the engaged states of the engaging elements in the drive force distributing device  26  is implemented. Concretely, the brake B is fully engaged, and, then, this routine is terminated. 
     Thus, according to the present embodiment, the possibility that the demand for the drive force control is made is determined on the basis of the predetermined standard, and if it is determined that it is possible that the demand for control would be made, the preliminary control to control the engaged states of the engaging elements, that is, the brake B, first clutch C 1  and second clutch C 2  is implemented to the stage prior to implementing of the drive force control. Consequently, the drive force control is implemented with superior responsibility upon the real demand for control. Accordingly, the drive force control apparatus for a vehicle configured to improve responsibility in control of transmitting the drive forces to the left and right drive wheels can be provided. 
     Since the aforementioned preliminary control controls the engaged states of the brake B, first clutch C 1  and second clutch C 2  such that the torque transmitting capacity is lower than that in the drive force control, the drive force control is implemented with superior responsibility upon the real demand for control by the preliminary control of the practical embodiment. 
     Since the aforementioned preliminary control controls the engaged states of fewer engaging elements that are selected from the brake B, first clutch C 1  and second clutch C 2  than engaging elements in the drive force control, the drive force control is implemented with superior responsibility upon the real demand for control by the preliminary control of the practical embodiment. 
     Since the aforementioned drive force control is the differential limitation control that limits the differential control of the left and right rear wheels  30   l ,  30   r  and the predetermined standard is whether the vehicle speed is equal to or lower than the predetermined one, the responsibility can be improved in the practical embodiment, with respect to the differential limitation control of the left and right rear wheels  30   l ,  30   r.    
     Since the aforementioned drive force control is the drive force distributing control that distributes the drive force to the left and right rear wheels  30   l ,  30   r  in a predetermined ratio and the predetermined standard is whether the vehicle is turning, the responsibility can be improved in the practical embodiment, with respect to the drive force distributing control to the left and right rear wheels  30   l ,  30   r.    
     While the preferred embodiment of this invention has been described above in detail by reference to the drawings, it is to be understood that the invention may be otherwise embodied. 
     For instance, in the aforementioned embodiment, the engagement control means  42  implements the preliminary control of the differential limitation control when the determination by the vehicle speed determining means  38  is affirmative, that is, when it is determined that the vehicle speed is equal to or lower than the predetermined one. And the present invention may be applied to other embodiments, and, for example, after it is determined whether it is starting of the vehicle on the basis of the signal indicative of the accelerator operation amount supplied from the accelerator operation amount sensor  48 , if the determination is affirmative, the preliminary control of the differential limitation control may be implemented. Not only the example of the standard indicated in the aforementioned embodiment but various standards determined on the basis of the signals supplied from the various sensors may be available for a determination standard for implementing the preliminary control of the drive force control. 
     In the aforementioned embodiment, the example is detailed in which such as the drive force distributing device  26  is applied to the front- and rear-wheels drive vehicle that is a fundamentally front-engine front-wheel-drive vehicle. The drive force control device is properly applicable to various vehicles such as a front-engine front-wheel-drive (FF) vehicle, a front-engine rear-wheel-drive (FR) vehicle or a front- and rear-wheel-drive vehicle that is a fundamentally front-engine rear-wheel-drive vehicle. 
     In the aforementioned embodiment, the example is detailed in which the present invention is applied to the vehicle that is provided with an internal combustion engine such as a gasoline or diesel engine for the drive power source, and this invention may be properly applied to a vehicle provided with another drive power source such as an electric motor. 
     In the aforementioned embodiment, the transmission device  56  is constituted of one planetary gear set or two planetary gear sets, and, furthermore, various transmission devices such as a structure including more than two planetary gear sets or a structure including a double-pinion type and a single-pinion type planetary gear sets may be applied. A planetary gear set having connection relationship among the sun gear, carrier and ring gear that is optionally changed without inconsistency may be applied, and one having the position of the brake B changed without inconsistency may be applied. 
     In the aforementioned embodiment, the drive force distributing controller  36  selectively implements the left-and-right-wheel torque difference control or differential limitation control in accordance with the signals supplied from the various sensors, that is, the running state of the vehicle, and, furthermore, a structure in which any of those controls is selected by an operator with using such as a switch may be applied. 
     In the aforementioned embodiment, the differential device  54  includes the double-pinion type planetary gear set, and a differential device including a single-pinion type planetary gear set may be applied. 
     In the left-and-right-wheel torque difference control of the aforementioned embodiment, the brake B is fully engaged, and a structure may be applied in which the engagement control means  42  controls the engaged state of the brake B such that the brake B is placed in a semi-engaged state in the left-and-right-wheel torque difference control and its preliminary control because the semi-engaged brake B permits the left-and-right-wheel torque difference control. 
     In the aforementioned embodiment, the engaged elements, that is, the first clutch C 1 , second clutch C 2  and brake B provided with the drive force distributing device  26  are all hydraulic frictional engaging devices, and other types of clutches and brakes such as a magnetic particle type clutch or an electromagnetic clutch are available. 
     It is to be understood that the present invention may be embodied with other changes, improvements, and modifications that may occur to a person skilled in the art without departing from the scope and spirit of the invention defined in the appended claims.