Abstract:
A driving force transmission apparatus includes: a driving force transmission shaft receiving driving force of a driving source from a rotating member and transmitting the driving force from a main driving wheel side to an auxiliary driving wheel side; a first driving force interruption unit connects/disconnects the driving force transmission shaft to/from the rotating member; a second driving force interruption unit connects/disconnects the driving force transmission shaft to/from at least one of two auxiliary driving wheels so that transmitted torque is variable; and a control unit controls connection and disconnection of the first and second driving force interruption units. The control unit causes the second driving force interruption unit to connect/disconnect the driving force transmission shaft to/from the at least one of the auxiliary driving wheels before causing the first driving force interruption unit to connect/disconnect the driving force transmission shaft to/from the rotating member.

Description:
INCORPORATION BY REFERENCE 
       [0001]    The disclosure of Japanese Patent Application No. 2010-005029 filed on Jan. 13, 2010 and Japanese Patent Application No. 2010-133874 filed on Jun. 11, 2010 respectively including the specifications, drawings and abstracts is incorporated herein by reference in its entirety. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a driving force transmission apparatus equipped for a four-wheel drive vehicle and a control method for the driving force transmission apparatus. 
         [0004]    2. Description of the Related Art 
         [0005]    There is known a driving force transmission apparatus equipped for a four-wheel drive vehicle that is able to shift from four-wheel drive to two-wheel drive or from two-wheel drive to four-wheel drive. The driving force transmission apparatus of this type is able to transmit or interrupt the diving force of a driving source to auxiliary driving wheels during two-wheel drive (for example, see Japanese Patent Application Publication No. 7-215081 (JP-A-7-215081)). 
         [0006]    The driving force transmission apparatus described in JP-A-7-215081 includes a drive member connected to a differential case so as to be able to transmit driving force, a first friction clutch that is frictionally engaged when the drive member is driven, a second friction clutch that is arranged between a driven member connected to axle shafts of auxiliary driving wheels so at to be able to transmit driving force and a side gear shaft of a differential, and cam means that actuates the first friction clutch to press the second friction clutch. The driving force transmission apparatus automatically connects the side gear shaft to the axles during four-wheel drive, and automatically interrupts the side gear shaft from the axles during two-wheel drive. 
         [0007]    However, the driving force transmission apparatus described in JP-A-7-215081 is just driven by the driving force of a propeller shaft and is not able to control the timing at which the driving force is transmitted or interrupted or the amount of transmitted driving force. Thus, depending on a configuration that a driving force interruption unit is arranged at an upstream side of a driving force transmission path with respect to the propeller shaft, shock or vibration may occur at the time of shifting between a four-wheel drive state and a two-wheel drive state. 
         [0008]    In addition, there is a four-wheel drive vehicle that is configured to interrupt an input side of a propeller shaft from an output side thereof so as not to rotate the propeller shaft during running in a two-wheel drive state in order to reduce a power loss due to rotation of the propeller shaft in the two-wheel drive state (for example, see Japanese Patent Application Publication No. 2003-220847 (JP-A-2003-220847)). 
         [0009]    The four-wheel drive vehicle described in JP-A-2003-220847 includes a transfer clutch located on a torque transmission upstream side of a front propeller shaft and an ADD mechanism (interruption mechanism) located on a torque transmission downstream side of the front propeller shaft. When shifting from a two-wheel drive state where both the transfer clutch and the ADD mechanism are released into a four-wheel drive state, the transfer clutch is connected by torque used to rotate the propeller shaft, and, after that, synchronization of the ADD mechanism is checked and then the ADD mechanism is changed from a disconnected state to a locked state. 
         [0010]    However, in the four-wheel drive vehicle described in JP-A-2003-220847, because synchronization of the ADD mechanism is checked and then the ADD mechanism is changed to a locked state, it may take time for shifting into a four-wheel drive state. In addition, if the ADD mechanism is changed to a locked state in a state where the ADD mechanism is not completely synchronized, shock or vibration may occur. 
       SUMMARY OF INVENTION 
       [0011]    The invention provides a driving force transmission apparatus that is able to suppress shock or vibration at the time of shifting between a four-wheel drive state and a two-wheel drive state, and a control method for the driving force transmission apparatus. In addition, the invention further provides a driving force transmission apparatus that is able to suppress shock or vibration at the time of shifting from a two-wheel drive state of a four-wheel drive vehicle to a four-wheel drive state while reducing a time required to shift into the four-wheel drive state, and a control method for the driving force transmission apparatus. 
         [0012]    A first aspect of the invention relates to a driving force transmission apparatus. The driving force transmission apparatus includes: a driving force transmission shaft that receives driving force of a driving source from a rotating member and that transmits the driving force from a side of main driving wheels to a side of auxiliary driving wheels; a first driving force interruption unit that connects or disconnects the driving force transmission shaft to or from the rotating member and that is arranged on the main driving wheels side of the driving force transmission shaft; a second driving force interruption unit that connects or disconnects the driving force transmission shaft to or from at least one of the pair of auxiliary driving wheels so as to variably transmit torque between the driving force transmission shaft and the at least one of the pair of auxiliary driving wheels and that is arranged on the auxiliary driving wheels side of the driving force transmission shaft; and a control unit that controls connection and disconnection of the first driving force interruption unit and second driving force interruption unit. The control unit causes the second driving force interruption unit to connect the driving force transmission shaft to the at least one of the auxiliary driving wheels before causing the first driving force interruption unit to connect the driving force transmission shaft to the rotating member, and causes the second driving force interruption unit to disconnect the driving force transmission shaft from the at least one of the auxiliary driving wheels before causing the first driving force interruption unit to disconnect the driving force transmission shaft from the rotating member. 
         [0013]    According to the above aspect, when the first driving force interruption unit and the second driving force interruption unit that is able to vary transmitted torque are controlled to shift between four-wheel drive and two-wheel drive, connection or disconnection of the second driving force interruption unit is carried out before connection or disconnection of the first driving force interruption unit. 
         [0014]    A second aspect of the invention relates to a control method for controlling a driving force transmission apparatus that includes a driving force transmission shaft that receives driving force of a driving source from a rotating member and that transmits the driving force from a side of main driving wheels to a side of auxiliary driving wheels, a first driving force interruption unit that connects or disconnects the driving force transmission shaft to or from the rotating member and that is arranged on the main driving wheels side of the driving force transmission shaft, and a second driving force interruption unit that connects or disconnects the driving force transmission shaft to or from at least one of the pair of auxiliary driving wheels so as to variably transmit torque between the driving force transmission shaft and the at least one of the pair of auxiliary driving wheels and that is arranged on the auxiliary driving wheels side of the driving force transmission shaft. The control method includes causing the second driving force interruption unit to connect the driving force transmission shaft to the at least one of the auxiliary driving wheels before causing the first driving force interruption unit to connect the driving force transmission shaft to the rotating member, and causing the second driving force interruption unit to disconnect the driving force transmission shaft from the at least one of the auxiliary driving wheels before causing the first driving force interruption unit to disconnect the driving force transmission shaft from the rotating member. 
         [0015]    According to the above aspect, when the first driving force interruption unit and the second driving force interruption unit that is able to vary transmitted torque are controlled to shift between four-wheel drive and two-wheel drive, connection or disconnection of the second driving force interruption unit is carried out before connection or disconnection of the first driving force interruption unit. 
         [0016]    According to the above aspects, it is possible to suppress shock or vibration at the time of shifting between a four-wheel drive state and a two-wheel drive state. In addition, it is possible to suppress shock or vibration at the time of shifting from a two-wheel drive state of a four-wheel drive vehicle into a four-wheel drive state while reducing a time required to shift into the four-wheel drive state. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]    The features, advantages, and technical and industrial significance of this invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
           [0018]      FIG. 1  is a plan view for illustrating the outline of a vehicle equipped with a driving force transmission apparatus according to a first embodiment of the invention; 
           [0019]      FIG. 2  is a sectional view for illustrating a relevant portion of the driving force transmission apparatus according to the first embodiment of the invention; 
           [0020]      FIG. 3  is a plan view for illustrating the outline of a vehicle equipped with a driving force transmission apparatus according to a second embodiment of the invention; 
           [0021]      FIG. 4  is a sectional view for illustrating a relevant portion of the driving force transmission apparatus according to the second embodiment of the invention; 
           [0022]      FIG. 5A  and  FIG. 5B  are sectional views for illustrating a relevant portion of a dog clutch according to a third embodiment of the invention; 
           [0023]      FIG. 6  is a flowchart for illustrating control procedure according to the third embodiment of the invention; and 
           [0024]      FIG. 7A  to  FIG. 7D  are graphs for illustrating an operation example according to the third embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0025]      FIG. 1  shows the outline of a four-wheel drive vehicle  101  according to a first embodiment. As shown in  FIG. 1 , the four-wheel drive vehicle  101  includes a driving force transmission apparatus  1 , an engine  102 , a transmission  103 , a pair of front wheels  104  that serve as main driving wheels and a pair of rear wheels  105   a  and  105   b  that serve as auxiliary driving wheels. 
         [0026]    The driving force transmission apparatus  1  is arranged together with a front differential  106  and a rear differential  107  in a driving force transmission path from the transmission  103  to the rear wheels in the four-wheel drive vehicle  101 , and is mounted on a vehicle body (not shown) of the four-wheel drive vehicle  101 . 
         [0027]    Then, the driving force transmission apparatus  1  includes a propeller shaft (driving force transmission shaft)  2 , a first driving force interruption unit  3  and a second driving force interruption unit  4 , and shifts the four-wheel drive vehicle  101  from four-wheel drive to two-wheel drive or from two-wheel drive to four-wheel drive. 
         [0028]    The front differential  106  includes a pair of side gears  109  connected to front wheel axle shafts  108 , a pair of pinion gears  110  that are in mesh with the pair of side gears  109  so that the gear axes are perpendicular to the gear axes of the side gears  109 , and a front differential case  111  that accommodates the pair of side gears  109 . The front differential  106  is arranged between the transmission  103  and the first driving force interruption unit  3 . 
         [0029]    The rear differential  107  includes a pair of side gears  113  connected to rear wheel axle shafts  112 , a pair of pinion gears  114  that are in mesh with the pair of side gears  113  so that the gears axes are perpendicular to the gear axes of the side gears  113 , a pinion gear support member  115  that supports the pair of pinion gears  114 , and a rear differential case  116  that accommodates the pinion gear support member  115 , the pair of pinion gears  114  and the pair of side gears  113 . The rear differential  107  is arranged between the propeller shaft  2  and the second driving force interruption unit  4 . A side gear shaft  14  is connected to the left side gear  113  of the pair of side gears  113  so that the side gear shaft  14  is relatively non-rotatable. 
         [0030]    The engine  102  outputs driving force to the pair of front wheel axle shafts  108  via the transmission  103  and the front differential  106  to thereby drive the pair of front wheels  104 . 
         [0031]    The engine  102  outputs driving force to the left rear wheel axle shaft  112   a  via the transmission  103 , the first driving force interruption unit  3 , the propeller shaft  2 , the rear differential  107 , the side gear shaft  14  and the second driving force interruption unit  4  to thereby drive the left rear wheel  105   a . In addition, the engine  102  outputs driving force to the right rear wheel axle shaft  112   b  via the transmission  103 , the first driving force interruption unit  3 , the propeller shaft  2  and the rear differential  107  to thereby drive the right rear wheel  105   b.    
         [0032]    As shown in  FIG. 1 , the driving force transmission apparatus  1  roughly includes the propeller shaft  2 , the first driving force interruption unit  3 , the second driving force interruption unit  4  and a vehicle electronic control unit (ECU)  5  that serves as a control unit. 
         [0033]    The propeller shaft  2  is arranged between the first driving force interruption unit  3  and the second driving force interruption unit  4 . Then, the propeller shaft  2  receives the driving force of the engine  102  from the front differential case  111  and then transmits the driving force from the side of the front wheels  104  to the side of the rear wheels  105   a  and  105   b . A front wheel side gear mechanism  6  is arranged at a front wheel side end of the propeller shaft  2 . The front wheel side gear mechanism  6  is formed of a drive pinion  6   a  and a ring gear  6   b  that are in mesh with each other. A gear mechanism  7  is arranged at a rear wheel side end of the propeller shaft  2 . The gear mechanism  7  is formed of a drive pinion  7   a  and a ring gear  7   b  that are in mesh with each other. 
         [0034]    The first driving force interruption unit  3  is, for example, formed of a dog clutch. The first driving force interruption unit  3  is arranged at the side of the front wheels  104  in the four-wheel drive vehicle  101 , and is connected to the ECU  5  via an actuator (not shown). Then, the first driving force interruption unit  3  connects or disconnects the propeller shaft  2  to or from the front differential case  111 . 
         [0035]      FIG. 2  shows the second driving force interruption unit  4 . As shown in  FIG. 2 , the second driving force interruption unit  4  is, for example, formed of a combined clutch that includes a multiple disk clutch  8 , an electromagnetic clutch  9  and a cam mechanism  10 . The second driving force interruption unit  4  is arranged at the side of the rear wheel  105   a  in the four-wheel drive vehicle  101 , and is accommodated in a differential carrier  11 . 
         [0036]    Then, the second driving force interruption unit  4  connects or disconnects the side gear shaft  14  to or from the left rear wheel axle shaft  112   a.    
         [0037]    That is, when the second driving force interruption unit  4  is connected, torque is transmitted from the propeller shaft  2  to the left rear wheel axle shaft  112   a  via the gear mechanism  7 , the rear differential  107  and the side gear shaft  14 . In addition, torque is transmitted from the propeller shaft  2  to the right rear wheel axle shaft  112   b  via the gear mechanism  7  and the rear differential  107 . 
         [0038]    On the other hand, when the second driving force interruption unit  4  is disconnected, the left rear wheel axle shaft  112   a  is disconnected from the propeller shaft  2 , and, accordingly, torque is not transmitted from the propeller shaft  2  to the right rear wheel axle shaft  112   b  as well. Note that the reason why torque is not transmitted to the right rear wheel axle shaft  112   b  as well is due to the characteristic of a general differential device, that is, when one of the side gears rotates at an idle, torque is not transmitted to the other one of the side gears as well. 
         [0039]    The multiple disk clutch  8  is formed of a friction main clutch that includes a plurality of inner clutch plates  8   a  and a plurality of outer clutch plates  8   b , and is arranged between a housing  12  that serves as a first interruption element and an inner shaft  13  that serves as a second interruption element. Then, the multiple disk clutch  8  frictionally engages the adjacent inner and outer clutch plates among the inner clutch plates  8   a  and the outer clutch plates  8   b  or releases the frictional engagement to thereby connect or disconnect the housing  12  to or from the inner shaft  13 . 
         [0040]    The housing  12  is connected to the side gear shaft  14  by, for example, spline fitting so as to be relatively non-rotatable, and is supported inside the differential carrier  11  so as to be rotatable about an axis of the left rear wheel axle shaft  112   a . The inner shaft  13  is arranged on a radially inner side of the housing  12 , and is connected to the rear wheel axle shaft  112  by, for example, spline fitting so as to be relatively non-rotatable. 
         [0041]    The electromagnetic clutch  9  has a coil  9   a  and an armature cam  9   b , and is arranged along the rotation axis of the housing  12 . Then, the electromagnetic clutch  9  moves the armature cam  9   b  toward the coil  9   a  by electromagnetic force generated by the coil  9   a  to thereby connect the armature cam  9   b  to the housing  12 . 
         [0042]    The cam mechanism  10  includes the armature cam  9   b  that serves as a cam member. The cam mechanism  10  has a main cam  10   a  and a cam follower  10   b , and is accommodated inside the housing  12 . The main cam  10   a  is arranged next to the armature cam  9   b  along the rotation axis of the housing  12 . The cam follower  10   b  is interposed between the main cam  10   a  and the armature cam  9   b . Then, in the cam mechanism  10 , the armature cam  9   b  receives rotational force from the housing  12  as the coil  9   a  is supplied with current and then converts the rotational force to pressing force that becomes clutch force of the multiple disk clutch  8 . As the amount of current supplied to the coil  9   a  increases, friction force between the armature cam  9   b  and the housing  12  increases, and the main cam  10   a  further strongly presses the multiple disk clutch  8 . That is, force pressing the multiple disk clutch  8  is controllable in accordance with the amount of current supplied to the coil  9   a , and torque transmitted to the second driving force interruption unit  4  is variable in accordance with the amount of current supplied to the coil  9   a.    
         [0043]    As shown in  FIG. 1 , the ECU  5  is mounted on the vehicle body of the four-wheel drive vehicle  101 , and is connected to the actuator of the first driving force interruption unit  3  and the electromagnetic clutch  9  of the second driving force interruption unit  4 . A rotation sensor  15  and a rotation sensor  16  are connected to the ECU  5 . The rotation sensor  15  detects the rotational speed of the front differential case  111 . The rotation sensor  16  detects the rotational speed of the propeller shaft  2 . 
         [0044]    Then, the ECU  5  inputs an output signal S 3  from the rotation sensor  15  and an output signal S 4  from the rotation sensor  16  at the time of shifting from two-wheel drive to four-wheel drive during running of the four-wheel drive vehicle  101 . In addition, the ECU  5  outputs control signals S 1  and S 2  respectively to the actuator of the first driving force interruption unit  3  and the electromagnetic clutch  9  of the second driving force interruption unit  4 . The control signals S 1  and S 2  are used to connect the second driving force interruption unit  4  before connection of the first driving force interruption unit  3 . 
         [0045]    During two-wheel drive running, the first driving force interruption unit  3  and the second driving force interruption unit  4  are disconnected, so the propeller shaft  2  does not rotate. However, in this case, as the four-wheel drive vehicle  101  runs, the rear wheels  105   a  and  105   b  rotate as driven wheels. Here, when the amount of current supplied to the coil  9   a  is gradually increased to connect the second driving force interruption unit  4 , the rotational torque of the rear wheels  105   a  and  105   b  is transmitted to the propeller shaft  2 , so the propeller shaft  2  rotates. Subsequently, the rotational speed of the front differential case  111  is detected by the rotation sensor  15 , and the rotational speed of the propeller shaft  2  is detected by the rotation sensor  16 . When the ECU  5  determines that the difference between the rotational speed of the front differential case  111  and the rotational speed of the propeller shaft  2  is smaller than or equal to a predetermined threshold, the first driving force interruption unit  3  is connected. Note that the percentage of an increase in the amount of current supplied to the coil  9   a  is appropriately set to an extent such that an occupant does not experience shock or vibration. 
         [0046]    By so doing, the propeller shaft  2  may be connected to the front differential case  111  in such a manner that the propeller shaft  2  is rotated to reduce the difference in rotational speed between the propeller shaft  2  and the front differential case  111 . Thus, the four-wheel drive vehicle  101  smoothly shifts from two-wheel drive to four-wheel drive during running. 
         [0047]    In addition, the ECU  5  outputs control signals S 5  and S 6  respectively to the actuator of the first driving force interruption unit  3  and the electromagnetic clutch  9  of the second driving force interruption unit  4  so as to disconnect the second driving force interruption unit  4  before disconnection of the first driving force interruption unit  3  when the four-wheel drive vehicle  101  shifts from four-wheel drive to two-wheel drive during running. 
         [0048]    At the time of shifting from four-wheel drive to two-wheel drive, when the amount of current supplied to the coil  9   a  is gradually reduced to disconnect the second driving force interruption unit  4 , torque transmitted from the propeller shaft  2  to the rear wheel axle shafts  105   a  and  105   b  is interrupted. By so doing, torsion of the propeller shaft  2 , which has occurred during transmission of torque, is released. After that, the first driving force interruption unit  3  is disconnected. Note that the percentage of a reduction in the amount of current supplied to the coil  9   a  is appropriately set to an extent such that an occupant does not experience shock or vibration. 
         [0049]    By so doing, when the four-wheel drive vehicle  101  shifts from four-wheel drive to two-wheel drive, the propeller shaft  2  may be disconnected from the front differential case  111  so that a load resulting from the torsion of the propeller shaft  2  does not act on the front differential case  111 . Thus, the four-wheel drive vehicle  101  smoothly shifts from four-wheel drive to two-wheel drive during running. 
         [0050]    Next, the operation of the driving force transmission apparatus according to the first embodiment will be described with reference to  FIG. 1  and  FIG. 2 . 
         [0051]    For example, in the case of steady running in which the four-wheel drive vehicle  101  travels straight ahead at a constant speed, the ECU  5  disconnects the first driving force interruption unit  3  and the second driving force interruption unit  4  in order to improve fuel economy by reducing running resistance. In this case, the propeller shaft  2  is not rotated, the driving force of the engine  102  is transmitted to the front differential  106  via the transmission  103 , and further transmitted from the front differential  106  to the pair of front wheels  104  via the pair of front wheel axle shafts  108 . 
         [0052]    In this case, because the coil  9   a  of the electromagnetic clutch  9  in the second driving force interruption unit  4  is not supplied with current, no magnetic circuit is formed in the coil  9   a , and the armature cam  9   b  does not move toward the coil  9   a  to be connected to the housing  12 . Thus, because no pressing force that becomes clutch force of the multiple disk clutch  8  is generated in the cam mechanism  10 , the inner clutch plates  8   a  and outer clutch plates  8   b  of the multiple disk clutch  8  do not frictionally engage each other. 
         [0053]    When the four-wheel drive vehicle  101  is shifted from two-wheel drive to four-wheel drive, the second driving force interruption unit  4  is initially used to connect the propeller shaft  2  to the pair of rear wheel axle shafts  112   a  and  112   b  and then the first driving force interruption unit  3  is used to connect the front differential case  111  to the propeller shaft  2 . At this time, the ECU  5  inputs the output signal S 3  from the rotation sensor  15  and the output signal S 4  from the rotation sensor  16 , and, after the difference in rotational speed, indicated by both output signals, is smaller than or equal to a predetermined threshold, the ECU  5  outputs the control signal S 1  to the actuator of the first driving force interruption unit  3 . 
         [0054]    The driving force of the engine  102  is transmitted to the propeller shaft  2  via the transmission  103 , the front differential case  111 , the first driving force interruption unit  3  and the gear mechanism  6 , and further transmitted from the propeller shaft  2  to the rear wheels  105   a  and  105   b  via the gear mechanism  7 , the rear differential  107  and the rear wheel axle shafts  112   a  and  112   b.    
         [0055]    When the ECU  5  outputs the control signal S 2  to supply current to the coil  9   a , a magnetic circuit is formed in the coil  9   a , and the armature cam  9   b  moves in a direction to be connected to the housing  12 . Therefore, the armature cam  9   b  frictionally slides on the housing  12 , and the rotational force of the housing  12  is transmitted to the armature cam  9   b.    
         [0056]    The rotational force of the armature cam  9   b  is converted to pressing force that becomes the clutch force of the multiple disk clutch  8  by cam action of the cam mechanism  10 , and the main cam  10   a  moves by the pressing force in a direction to frictionally engage the inner and outer clutch plates of the multiple disk clutch  8  with each other. 
         [0057]    Then, the inner and outer clutch plates of the multiple disk clutch  8  are frictionally engaged with each other, so the housing  12  is connected to the inner shaft  13  so that torque is transmittable. 
         [0058]    Note that, in four-wheel drive, the pressing force of the multiple disk clutch  8  may be adjusted by increasing or reducing the amount of current supplied to the coil  9   a , so the amount of torque transmitted to the rear wheels  105   a  and  105   b  may be controlled. That is, the ECU  5  increases or reduces the amount of current supplied to the coil  9   a  on the basis of a vehicle running state, such as a wheel speed of each wheel and a steering angle, to thereby make it possible to control the distribution of driving force between the front wheels  104  and  104  and the rear wheels  105   a  and  105   b.    
         [0059]    On the other hand, when the four-wheel drive vehicle  101  shifts from four-wheel drive to two-wheel drive, the second driving force interruption unit  4  is initially used to disconnect one of the rear wheel axle shafts  112  from the propeller shaft  2 , and then the first driving force interruption unit  3  is used to disconnect the propeller shaft  2  from the front differential case  111 . At this time, the ECU  5  outputs the control signal S 6  to the electromagnetic clutch  9  of the second driving force interruption unit  22 , and outputs the control signal S 5  to the actuator of the first driving force interruption unit  3 . 
         [0060]    No rotation driving force of the engine  102  is transmitted to the propeller shaft  2  via the transmission  103 , the front differential case  111 , the first driving force interruption unit  3  and the gear mechanism  6 . 
         [0061]    According to the above described first embodiment, the following advantageous effects may be obtained. 
         [0062]    The four-wheel drive vehicle  101  is able to smoothly shift from four-wheel drive to two-wheel drive and shift from two-wheel drive to four-wheel drive. 
         [0063]    The second driving force interruption unit  4  is able to adjust the pressing force of the multiple disk clutch  8  by the amount of current supplied to the coil  9   a , so the amount of supplied current is gradually increased or reduced at the time of shifting between four-wheel drive and two-wheel drive to thereby make it possible to further suppress shock or vibration at the time of shifting. 
         [0064]    Next, a driving force transmission apparatus  21  according to a second embodiment of the invention will be described with reference to  FIG. 3  and  FIG. 4 .  FIG. 3  shows the outline of the four-wheel drive vehicle  101 .  FIG. 4  shows a second driving force interruption unit. In  FIG. 3  and  FIG. 4 , like reference numerals denote the same or equivalent components to those in  FIG. 1  and  FIG. 2 , and the detailed description is omitted. 
         [0065]    As shown in  FIG. 3  and  FIG. 4 , the driving force transmission apparatus  21  according to the second embodiment of the invention includes a second driving force interruption unit  22  at the side of rear wheels. The second driving force interruption unit  22  includes a housing  12  that serves as a first interruption element connected to a propeller shaft  2  via a gear mechanism  7  and an inner shaft  13  that serves as a second interruption element connected to the pair of rear wheels  105  via a pair of rear wheel axle shafts  112  and a rear differential  107 . 
         [0066]    The housing  12  is formed of a first housing element  12 A and a second housing element  12 B, and is fixed to a ring gear  7   b  of the gear mechanism  7 . The first housing element  12 A accommodates a multiple disk clutch  8 , an electromagnetic clutch  9  and a cam mechanism  10 . The second housing element  12 B accommodates the rear differential  107 . The inner shaft  13  is connected to a rear differential case  116  by spline fitting so as to be relatively non-rotatable. In addition, in the rear differential  107 , a pair of side gears  113  are respectively connected to the rear wheel axle shafts  112  by spline fitting. 
         [0067]    Next, the operation of the driving force transmission apparatus according to the second embodiment will be described with reference to  FIG. 3  and  FIG. 4 . 
         [0068]    For example, in the case of steady running in which the four-wheel drive vehicle  101  travels straight ahead at a constant speed, the ECU  5  disconnects the first driving force interruption unit  3  and the second driving force interruption unit  22  in order to improve fuel economy by reducing running resistance. In this case, the propeller shaft  2  is not rotated, the driving force of the engine  102  is transmitted to the front differential  106  via the transmission  103 , and further transmitted from the front differential  106  to the pair of front wheels  104  via the pair of front wheel axle shafts  108 . 
         [0069]    In this case, because the coil  9   a  of the electromagnetic clutch  9  in the second driving force interruption unit  22  is not supplied with current, no magnetic circuit is formed in the coil  9   a , and the armature cam  9   b  does not move toward the coil  9   a  to be connected to the housing  12 . Thus, because no pressing force that becomes clutch force of the multiple disk clutch  8  is generated in the cam mechanism  10 , the inner clutch plates  8   a  and outer clutch plates  8   b  of the multiple disk clutch  8  do not frictionally engage each other. 
         [0070]    When the four-wheel drive vehicle  101  shifts from two-wheel drive to four-wheel drive, the second driving force interruption unit  22  is initially used to connect the propeller shaft  2  to the rear wheel axle shafts  112 , and then the first driving force interruption unit  3  is used to connect the front differential case  111  to the propeller shaft  2 . At this time, the ECU  5  inputs the output signal S 3  from the rotation sensor  15  and the output signal S 4  from the rotation sensor  16 , and, after the difference in rotational speed, indicated by both output signals, is smaller than or equal to a predetermined threshold, the ECU  5  outputs the control signal S 1  to the actuator of the first driving force interruption unit  3 . 
         [0071]    The rotation driving force of the engine  102  is transmitted to the propeller shaft  2  via the transmission  103 , the front differential case  111 , the first driving force interruption unit  3  and the gear mechanism  6 , and further transmitted from the propeller shaft  2  to the rear wheels  105  via the gear mechanism  7 , the rear differential  107  and the rear wheel axle shafts  112 . 
         [0072]    When the ECU  5  outputs the control signal S 2  to supply current to the coil  9   a , a magnetic circuit is formed in the coil  9   a , and the armature cam  9   b  moves in a direction to be connected to the housing  12 . Therefore, the armature cam  9   b  frictionally slides on the housing  12 , and the rotational force of the housing  12  is transmitted to the armature cam  9   b.    
         [0073]    The rotational force of the armature cam  9   b  is converted to pressing force that becomes the clutch force of the multiple disk clutch  8  by cam action of the cam mechanism  10 , and the main cam  10   a  moves by the pressing force in a direction to frictionally engage the inner and outer clutch plates of the multiple disk clutch  8  with each other. 
         [0074]    Then, the inner and outer clutch plates of the multiple disk clutch  8  are frictionally engaged with each other, so the housing  12  is connected to the inner shaft  13  so that torque is transmittable. By so doing, the propeller shaft  2  is connected to the rear differential case  116  of the rear differential  107  so that torque is transmittable. 
         [0075]    Note that, in four-wheel drive, the pressing force of the multiple disk clutch  8  may be adjusted by increasing or reducing the amount of current supplied to the coil  9   a , and the amount of torque transmitted to the rear wheels  105  may be controlled. That is, the ECU  5  increases or reduces the amount of current supplied to the coil  9   a  on the basis of a vehicle running state, such as a wheel speed of each wheel and a steering angle, to thereby make it possible to control the distribution of driving force between the front wheels  104  and the rear wheels  105 . 
         [0076]    On the other hand, when the four-wheel drive vehicle  101  shifts from four-wheel drive to two-wheel drive, the second driving force interruption unit  22  is initially used to disconnect the rear wheel axle shafts  112  from the propeller shaft  2 , and then the first driving force interruption unit  3  is used to disconnect the propeller shaft  2  from the front differential case  111 . At this time, the ECU  5  outputs the control signal S 6  to the electromagnetic clutch  9  of the second driving force interruption unit  22 , and outputs the control signal S 5  to the actuator of the first driving force interruption unit  3 . 
         [0077]    The rotation driving force of the engine  102  is not transmitted to the propeller shaft  2  via the transmission  103 , the front differential case  111 , the first driving force interruption unit  3  and the gear mechanism  6 . 
         [0078]    According to the above described second embodiment, the same advantageous effects as those of the first embodiment may be obtained. 
         [0079]    Next, the operation of a driving force transmission apparatus according to a third embodiment will be described with reference to  FIG. 5A  to  FIG. 7 . 
         [0080]      FIG. 5A  and  FIG. 5B  are sectional views that show an example of the schematic configuration of a first driving force interruption unit  3 . The first driving force interruption unit  3  includes a first rotating member  31 , a second rotating member  32  and a sleeve  33 . The first rotating member  31  is fixed to an end of the front differential case  111 . The second rotating member  32  is relatively rotatable coaxially with the first rotating member  31 . The sleeve  33  is movable in the axial direction around the first rotating member  31  and the second rotating member  32 . 
         [0081]    The first rotating member  31  has an annular shape such that the front wheel axle shaft  108  is inserted. The first rotating member  31  is, for example, fixed to an end of the front differential case  111  by bolt fastening, and integrally rotates with the front differential case  111 . A plurality of mesh teeth  31   a  are formed on the outer periphery of the first rotating member  31 . 
         [0082]    The second rotating member  32  has a cylindrical shape such that one axial end  321  adjacent to a side facing the first rotating member  31  is radially enlarged, and the front wheel axle shaft  108  extends through the center of the second rotating member  32 . A plurality of mesh teeth  32   a  are formed on the outer periphery of the end  321  of the second rotating member  32 . A ring gear  6   b  is fixed to the outer periphery of the other axial end  322  of the second rotating member  32  by, for example, bolt fastening so as to be relatively non-rotatable. 
         [0083]    The second rotating member  32  and the front differential case  111  are supported by a vehicle body via bearings (not shown) independently of each other so as to be rotatable and axially immovable. 
         [0084]    The sleeve  33  has an annular shape. A plurality of mesh teeth  33   a  are formed on the inner peripheral surface of the sleeve  33 . The mesh teeth  33   a  are constantly in mesh with the plurality of mesh teeth  32   a  of the second rotating member  32 , and are also able to be in mesh with the plurality of mesh teeth  31   a  of the first rotating member  31  as the sleeve  33  axially moves along a rotation axis O of the front wheel axle shaft  108 . In addition, an annular groove  33   b  is formed on the outer peripheral side of the sleeve  33 , and a fork  34  is slidably fitted in the groove  33   b . The fork  34  is moved forward or backward in an arrow A direction by an actuator (not shown) together with the sleeve  33 . 
         [0085]    A proximity sensor  35  is arranged at a location facing the first rotating member  31 . The proximity sensor  35  is fitted to the vehicle body. The proximity sensor  35  is, for example, of a high-frequency oscillation type. As the sleeve  33  moves toward the first rotating member  31  (arrow A direction) and then the mesh teeth  31   a  are in mesh with the mesh teeth  33   a , the proximity sensor  35  outputs an on signal. The output signal from the proximity sensor  35  is transferred to the ECU  5  through wiring (not shown). 
         [0086]      FIG. 5B  is a schematic view that shows an example of a mesh state among the plurality of mesh teeth  31   a  of the first rotating member  31 , the plurality of mesh teeth  32   a  of the second rotating member  32  and the plurality of mesh teeth  33   a  of the sleeve  33 . In the state shown in the drawing, the plurality of mesh teeth  32   a  of the second rotating member  32  are in mesh with the plurality of mesh teeth  33   a  of the sleeve  33 ; however, the plurality of mesh teeth  31   a  of the first rotating member  31  are not in mesh with the plurality of mesh teeth  33   a  of the sleeve  33 . Thus, the first driving force interruption unit  3  is in a released state such that the first rotating member  31  is allowed to rotate relative to the second rotating member  32 , and the front differential case  111  is disconnected from the propeller shaft  2 . 
         [0087]    In addition, as the sleeve  33  moves in the arrow A direction from this state, the mesh teeth  33   a  of the sleeve  33  enter between the adjacent mesh teeth  31   a  of the first rotating member  31 , so the mesh teeth  31   a  are in mesh with the mesh teeth  33   a  to enter an engaged state. In this engaged state, the plurality of mesh teeth  33   a  of the sleeve  33  are in mesh with the plurality of mesh teeth  31   a  of the first rotating member  31  and the plurality of mesh teeth  32   a  of the second rotating member  32 , so relative rotation between the first rotating member  31  and the second rotating member  32  is disabled. Thus, the front differential case  111  is connected to the propeller shaft  2  so that torque is transmittable. 
         [0088]    In the third embodiment, when the vehicle shifts from a two-wheel drive state where the propeller shaft  2  is not rotated to a four-wheel drive state where the rear wheels  105   a  and  105   b  are also driven, the ECU  5  increases torque transmitted by the second driving force interruption unit  4  to increase the rotational speed of the propeller shaft  2  and then reduces torque transmitted by the second driving force interruption unit  4 , and controls the first driving force interruption unit  3  in a state where the transmitted torque is reduced to thereby bring the first driving force interruption unit  3  into an engaged state. Then, after the ECU  5  determines that the first driving force interruption unit  3  is in the engaged state, the ECU  5  causes the second driving force interruption unit  4  to generate transmitted torque appropriate for the running state. 
         [0089]      FIG. 6  is a flowchart that shows an example of procedure of the ECU  5  at the time of shifting from a two-wheel drive steady running state to a four-wheel drive state. Note that, in the initial state of process shown in the flowchart, it is assumed that various portions of a driving force transmission system over the second rotating member  32  to the rear differential case  116  do not rotate and a command transmitted torque for the second driving force interruption unit  4  is zero. 
         [0090]    At the time of shifting from a two-wheel drive state to a four-wheel drive state, the ECU  5  initially starts supply of coil current to the coil  9   a  of the second driving force interruption unit  4  (S 01 ). The coil current has a necessary magnitude such that part of torque of the left rear wheel  105   a  that rotates as the four-wheel drive vehicle  101  runs is transmitted to the propeller shaft  2  via the rear differential  107  and the gear mechanism  7  and then the propeller shaft  2  starts rotating. 
         [0091]    Subsequently, the ECU  5  calculates the rotational speed of the front differential case  111  on the basis of a value detected by the rotation sensor  15  (S 02 ), and then calculates the rotational speed of the second rotating member  32  on the basis of a value detected by the rotation sensor  16  (S 03 ). 
         [0092]    After that, the ECU  5  calculates the ratio of the rotational speed of the second rotating member  32 , calculated in step S 03 , to the rotational speed of the front differential case  111 , calculated in step S 02  (S 04 ), and then determines whether the ratio is higher than or equal to a predetermined threshold (S 05 ). The threshold may be, for example, 0.9. In this case, when the second rotating member  32  is rotating at the rotational speed that is 90% or above the rotational speed of the front differential case  111 , the result of determination in step S 05  is affirmative. Note that the threshold may be set to 0.95. 
         [0093]    When the result of determination in step S 05  is negative, the ECU  5  repeats the process in step S 02  and the following processes. 
         [0094]    On the other hand, when the result of determination in step S 05  is affirmative, the ECU  5  reduces the coil current of the second driving force interruption unit  4  (S 06 ). More specifically, the ECU  5  reduces the coil current of the second driving force interruption unit  4  to a value that is smaller than or equal to half the coil current at the time when the result of determination in step S 05  is affirmative. 
         [0095]    Subsequently, the ECU  5  controls the actuator of the first driving force interruption unit  3  to activate the first driving force interruption unit  3  (S 07 ). 
         [0096]    After that, the ECU  5  determines whether the engagement operation of the first driving force interruption unit  3  is complete on the basis of the output signal from the proximity sensor  35  (S 08 ). When the result of determination is negative, the determination is repeatedly made; whereas, when the result of determination is affirmative, the process shown in the flowchart ends. 
         [0097]      FIG. 7A  to  FIG. 7D  are graphs that show temporal changes of various types of signals, or the like, when the ECU  5  executes the process shown in the flowchart of  FIG. 6 .  FIG. 7A  shows the rotational speed of the second rotating member  32 .  FIG. 7B  shows the coil current of the second driving force interruption unit  4 .  FIG. 7C  shows the driving current of the actuator of the first driving force interruption unit  3 .  FIG. 7D  shows an engagement completion flag that indicates that the first driving force interruption unit  3  is in an engaged state. 
         [0098]    As shown in  FIG. 7A  and  FIG. 7B , when supply of coil current I 2  to the second driving force interruption unit  4  is started at time t 1 , the rotational speed of the second rotating member  32  gradually increases from zero. Then, when the rotational speed of the second rotating member  32  becomes higher than or equal to a threshold V S  (V S  is, for example, 90% of V 1 ) lower than the rotational speed V 1  of the front differential case  111  at time t 2 , as shown in  FIG. 7B  and  FIG. 7C , the ECU  5  reduces the coil current supplied to the second driving force interruption unit  4  from I 2  to I 1  (I 1  is, for example, 30% of I 2 ), and supplies driving current to the actuator of the first driving force interruption unit  3 . 
         [0099]    Then, as shown in  FIG. 7D , when the engagement completion flag, which switches between on and off signal statuses on the basis of the output signal from the proximity sensor  35 , turns on at time t 3 , the ECU  5  stops supply of driving current to the actuator of the first driving force interruption unit  3 . Note that it is assumed that the first driving force interruption unit  3  maintains an engaged state even when supply of driving current to the actuator is stopped and enters a released state when inverse driving current is supplied from the ECU  5  to the actuator of the first driving force interruption unit  3 . 
         [0100]    The ECU  5  supplies coil current I 3  to the second driving force interruption unit  4  in order to distribute driving force, corresponding to the running state of the four-wheel drive vehicle  101 , to the rear wheels  105   a  and  105   b  after time t 3 . I 3  is, for example, a current of a magnitude such that driving force may be equally distributed between the front wheels  104  and  104  and the rear wheels  105   a  and  105   b.    
         [0101]    According to the above described third embodiment, the following advantageous effects may be obtained. 
         [0102]    When the first driving force interruption unit  3  is in an engaged state, the coil current of the second driving force interruption unit  4  is reduced, so, in comparison with the case where coil current is not reduced, even when the rotational speed of the propeller shaft  2  steeply varies because of engagement of the first driving force interruption unit  3 , the shock or vibration is hard to be transmitted to the rear wheel axle shafts  112   a  and  112   b  or the rear wheels  105   a  and  105   b . Thus, it is possible to suppress shock or vibration that occurs in the vehicle body of the four-wheel drive vehicle  101 . 
         [0103]    In addition, when the first driving force interruption unit  3  is in an engaged state, the coil current of the second driving force interruption unit  4  is reduced, so, in comparison with the case where coil current is not reduced, moment of inertia of a driving force transmission member reduces in a torque transmission downstream side with respect to the second rotating member  32  of the first driving force interruption unit  3 . Therefore, even when there is a difference in rotation between the first rotating member  31  and the second rotating member  32 , the first driving force interruption unit  3  is easily engaged, and it is possible to further quickly shift from a released state to an engaged state. Thus, for example, when a slip occurs in one of the right and left front wheels  104  and  104  or both in a two-wheel drive state, driving force is promptly transmitted to the rear wheels  105   a  and  105   b  to enter a four-wheel drive state to thereby make it possible to stabilize running. 
         [0104]    In the first and second embodiments, the second driving force interruption unit  4  or  22  is formed of a combined clutch that includes the multiple disk clutch  8 , the electromagnetic clutch  9  and the cam mechanism  10 ; however, the aspect of the invention is not limited to this configuration. For example, the second driving force interruption unit  4  or  22  may be formed of a combined clutch that further includes a pilot clutch in addition to the multiple disk clutch  8 , the electromagnetic clutch  9  and the cam mechanism  10 . 
         [0105]    In the above first and second embodiments, the first driving force interruption unit  3  is formed of a dog clutch; however, the aspect of the invention is not limited to this configuration. The first driving force interruption unit  3  may be formed of a multiple disk clutch. 
         [0106]    In the first embodiment, the second driving force interruption unit  4  is arranged only between the left rear wheel axle shaft  112   a  and the rear differential  107 ; however, the second driving force interruption unit  4  may be additionally arranged between the right rear wheel axle shaft  112   b  and the rear differential  107 . 
         [0107]    In the third embodiment, the coil current I 1  supplied to the second driving force interruption unit  4  between time t 2  and time t 3  shown in  FIG. 7A  to  FIG. 7D  is 30% of the coil current I 2  supplied to the second driving force interruption unit  4  between time t 1  and time t 2 ; however, the configuration is not limited to this. As long as the coil current I 1  is smaller than the coil current I 2 , an advantageous effect corresponding to that condition may be obtained. In addition, the coil current I 1  may be zero. 
         [0108]    In the third embodiment, the coil current supplied to the second driving force interruption unit  4  between time t 1  and time t 2  is constant; however, the configuration is not limited to this. For example, the coil current may be gradually increased from time t 1  to time t 2 . In this case, it is applicable as long as the coil current supplied to the second driving force interruption unit  4  between time t 2  and time t 3  is smaller than a value when the rotational speed of the second rotating member  32  exceeds the threshold V s . 
         [0109]    In the third embodiment, the rotational speed of the front differential case  111  (first rotating member  31 ) is detected by the rotation sensor  15  and the rotational speed of the second rotating member  32  is detected by the rotation sensor  16 ; however, the configuration is not limited to this. For example, it is applicable that the rotational speed of the propeller shaft  2  is detected and then the detected value is multiplied by the gear ratio of the gear mechanism  6  to compute the rotational speed of the second rotating member  32 . In addition, the rotational speed of the first rotating member  31  may be computed by multiplying the rotational speed of the engine  102  by the gear ratio, or the like, of the transmission  103  or may be an average of the front wheel speeds. 
         [0110]    In the third embodiment, determination in step S 05  shown in  FIG. 6  is made on the basis of whether the ratio of the rotational speed of the second rotating member  32  to the rotational speed of the front differential case  111  is higher than or equal to a predefined threshold; however, the determination may be made on the basis of whether the difference in rotational speed between the front differential case  111  and the second rotating member  32  is lower than or equal to a threshold. That is, it is determined whether the difference in rotation between the front differential case  111  and the second rotating member  32  is smaller than or equal to a threshold on the basis of the ratio or difference between the respective rotational speeds, and, when the difference in rotation is smaller than or equal to the threshold, the coil current of the second driving force interruption unit  4  may be reduced and then the first driving force interruption unit  3  may be activated. 
         [0111]    While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the example embodiments are shown in various combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the scope of the invention.