INCORPORATION BY REFERENCE  
         [0001]    The disclosure of Japanese Patent Application No. 2001-046376 filed on Feb. 22, 2001, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates to an accessory drive apparatus, and more particularly to a driving mechanism for an accessory system for driving a power source, such as a fuel cell system.  
           [0004]    2. Description of Related Art  
           [0005]    Japanese Laid-open Patent Publication No. 6-229447 discloses a transmission device for transmitting rotary power of an engine to an engine accessory while changing the speed of rotation, so as to drive the engine accessory. The transmission device disclosed in this publication includes a hollow rotating member, a first gear, a second gear, and a clutch. The hollow rotating member, which is rotatably supported on a drive shaft of the accessory, has an input engaging portion formed at an outer circumferential surface thereof, and a planetary gear rotatably attached to an inner circumferential surface thereof. In operation, the rotary power is transmitted from the engine to the input engaging portion of the hollow rotating member. The first gear is fixedly attached to the drive shaft of the accessory, and meshes with the planetary gear in the hollow rotating member. The second gear is freely rotatably attached to the drive shaft of the accessory, and meshes with the planetary gear in the hollow rotating member. The clutch engages/disengages a stationary or fixed portion of the accessory with/from the second gear, so as to apply braking force to the second gear. When the clutch is in the released or unengaged state, rotation of the second gear is not restricted, and only the revolution of the planetary gear around the drive shaft contributes to rotation of the hollow rotating member, whereby the accessory rotates at the same speed in the same direction as the hollow rotating member. When the clutch is in the engaged state, on the other hand, rotation of the second gear is restricted, and the first gear is rotated at an increased rotational speed achieved by adding the rotation of the planetary gear on its own axis to the revolution thereof around the drive shaft of the accessory. As a result, the accessory rotates at an increased speed as compared with the speed of rotation of the hollow rotating member. By suitably controlling the engaging state of the clutch, therefore, the rotational speed of the accessory can be controlled as desired.  
           [0006]    In the above-described arrangement in which the driving force applied to the accessory is controlled or adjusted by means of the clutch, loss of power is caused by so-called clutch slipping when the clutch is in an intermediate slipping state between a fully engaged position and a fully released position, resulting in a reduced efficiency in driving the accessory.  
           [0007]    Another driving source may be installed for driving an accessory or accessories. In this case, however, the driving source must be large-sized so as to provide sufficiently large power or driving force, resulting in an increased cost. In particular, while fuel-cell vehicles have been increasingly developed in recent years, a system for driving an accessory or accessories for a fuel cell system, which requires a large number of motors and inverters, is undesirably large-sized and complicated in construction. Thus, it has been considered important to simplify the system for driving the accessory.  
         SUMMARY OF THE INVENTION  
         [0008]    It is one object of the invention to provide an accessory drive apparatus with which an accessory driving system can be simplified, and the cost of the system can be reduced.  
           [0009]    To accomplish the above and/or other objects, there is provided an accessory drive apparatus according to the invention, which includes a power source, an accessory system connected to provide an input to the power source, an accessory driving source, and a power combination/distribution mechanism that is connected to the power source and the accessory system, wherein the power combination/distribution mechanism is adapted to transmit power to the accessory system from both the power source and the accessory driving source, so that the accessory system is driven by a combination of power received from the accessory driving source and the power source.  
           [0010]    In one preferred embodiment of the invention, a planetary gear unit including a sun gear, a carrier and a ring gear may be used as the power combination/distribution mechanism. In this case, the accessory driving source or the accessory system is connected to the sun gear, and a part of the power from the power source is transmitted to the carrier, while the accessory system or the accessory driving source is connected to the ring gear.  
           [0011]    In another preferred embodiment of the invention, the power source includes a fuel cell system and a drive motor coupled to the fuel cell system, and the accessory system includes a pump that supplies a fuel gas to the fuel cell system, while the accessory driving source includes a motor that drives the pump. With this arrangement, a part of an output of the drive motor is transmitted to the power combination/distribution mechanism.  
           [0012]    In a further embodiment of the invention, a differential gear unit or assembly may be used as the power combination/distribution mechanism.  
           [0013]    With the accessory drive apparatus constructed as described above, a part of the power received from the power source is used for driving the accessory system, so that less power must be generated by the accessory driving source for driving the accessory system. For example, a planetary gear unit serving as the power combination/distribution mechanism is favorably used for transmitting a part of the power from the power source to the accessory system, whereby the accessory system can be highly efficiently driven with almost no transmission loss. Furthermore, required power to be generated by the accessory driving source is reduced, so that the accessory driving source can be made compact or simplified. While any device may be used as the accessory driving source, a typical example thereof is a motor.  
           [0014]    When the planetary gear unit is in a steady-state operation, a relationship that the torque of the carrier is greater than that of the ring gear and the torque of the ring gear is greater than that of the sun gear is generally established. By connecting the auxiliary drive source to the sun gear and transmitting power from the power source to the carrier, therefore, the torque of the auxiliary driving source can be reduced. For example, the power of the power source may used for running a motor vehicle, as well as driving the auxiliary system. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The foregoing and/or further objects, features and advantages of the invention will become more apparent from the following description of a preferred embodiment with reference to the accompanying drawings, in which like numerals are used to represent like elements and wherein:  
         [0016]    [0016]FIG. 1 is a view showing the construction of a principal part of a fuel-cell vehicle equipped with an accessory drive apparatus according to one preferred embodiment of the invention;  
         [0017]    [0017]FIG. 2 is a view showing the construction of a planetary gear unit of the accessory drive apparatus of FIG. 1;  
         [0018]    [0018]FIG. 3 is a view for explaining an operating region of the planetary gear unit;  
         [0019]    [0019]FIG. 4 is a graph showing a relationship between an angular velocity of a drive motor and power of a pump motor; and  
         [0020]    [0020]FIG. 5 is a graph showing a relationship between the angular velocity of the drive motor and the power of the pump motor.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]    An exemplary preferred embodiment of the invention, when it is employed in a fuel-cell vehicle, will be hereinafter described with reference to the drawings.  
         [0022]    [0022]FIG. 1 illustrates a principal part of a fuel-cell vehicle equipped with an accessory drive apparatus according to the preferred embodiment of the invention. A fuel cell system (FC)  10  installed on the vehicle is operable to supply electric power required for running the vehicle to a drive motor  12 . An output shaft of the drive motor  12  is connected to vehicle wheels  14 , and is also connected to a carrier  16  of a planetary gear unit. With this arrangement, driving force or power generated by the drive motor  12  is transmitted to the wheels  14 , and is also transmitted to the planetary gear unit.  
         [0023]    The planetary gear unit includes the carrier  16 , a pinion gear  18  provided on the carrier  16 , a sun gear  20 , and a ring gear  24 . The sun gear  20  and the pinion gear  18 , and the ring gear  24  and the pinion gear  18 , respectively engage or mesh with each other. The sun gear  20  is supported to be freely rotatable about a center axis of the gear unit, and the pinion gear  18  is supported such that it rotates on its axis while revolving around the sun gear  20 , while the ring gear  24  is supported to be freely rotatable about the same center axis of the gear unit.  
         [0024]    As described above, a part of the driving force or power generated by the drive motor  12  is transmitted to the pinion gear  18  of the planetary gear unit. On the other hand, a pump motor  22  serving as an accessory driving source is connected to the sun gear  20  of the planetary gear unit, as shown in detail in FIG. 2. In addition, an accessory pump  26  for supplying a fuel gas to the fuel cell system  10  is connected to the ring gear  24  of the planetary gear unit. The pump motor  22  is driven with electric power supplied from an accessory battery  28 .  
         [0025]    In the accessory drive apparatus as described above, the pump motor  22  alone may be driven to rotate the sun gear  20 , thereby to rotate the pinion gear  18  and the ring gear  24 , so that the accessory pump  26  can operate to supply fuel gas to the fuel cell system  10 . In this case, however, the size of the pump motor  22  serving as a single driving source needs to be large enough to provide the accessory pump  26  with required power.  
         [0026]    In the present embodiment, therefore, a part of the driving force of the drive motor  12  is supplied to the carrier  16  of the planetary gear unit, so as to be added to torque generated by the pump motor  22 . With this arrangement, a load of the pump motor  22  can be advantageously reduced.  
         [0027]    An operation of the embodiment will be hereinafter described in detail.  
         [0028]    The accessory pump  26  operates substantially in accordance with the vehicle speed, and can be thus linked to the output shaft of the drive motor  12  that controls the vehicle speed. This is a precondition for control as described below. Various symbols used in the following description are defined as follows:  
                                                       S:   sun gear           C:   carrier           R:   ring gear           M t :   load motor (i.e., drive motor)           M p :   drive motor for air pump (i.e., accessory pump)           P:   air pump (i.e., accessory pump)           W:   output shaft of load motor (i.e., drive motor)           ω S :   angular velocity of sun gear           ω C :   angular velocity of carrier           ω R :   angular velocity of ring gear           τ S :   torque applied to sun gear           τ C :   torque applied to carrier           τ R :   torque applied to ring gear           P P1 :   power required for driving air pump (or accessory pump)           ω P1 :   angular velocity of pump at P P1             τ P1 :   load torque applied to pump at P P1             α:   proportional coefficient of τ P1  with respect to ω P1             ω C1     angular velocity of load motor (or drive motor)                      
 
         [0029]    When the planetary gear unit is in a steady-state operation, the following relationships (1), (2) are established. 
         τ C &gt;τ R &gt;τ S   (1) 
         ω R &gt;ω C &gt;ω S   (2) 
         [0030]    [0030]FIG. 3 is a chart showing limits of the rotational speeds of respective components of the planetary gear unit as shown in FIG. 2. In FIG. 3, the horizontal axis P represents the speed of the accessory pump (air pump) 26, and the vertical axis M t  represents the speed of the drive motor (load motor)  12 . A hatched area in FIG. 3 represents an operating region of the planetary gear unit, which region is defined by operating ranges of the pump motor (air pump)  22  that drives the sun gear  20 , the drive motor (load motor)  12  that drives the pinion gear  18 , and the accessory pump  26  connected to the ring gear  24 .  
         [0031]    If the gear ratio ρ is defined as the ratio of the number of gear teeth of the sun gear to the number of gear teeth of the ring gear, the torque relationship among the gears and carrier in a steady-state operation of the planetary gear unit is represented by:  
               τ   R     =       -     1     1   +   ρ              τ   C               (   3   )                 τ   S     =       -     ρ     1   +   ρ              τ   C               (   4   )                               
 
         [0032]    In the meantime, the relationship of the rotational speeds of the sun gear, the ring gear and the carrier is represented by: 
         ω R =(1+ρ)ω C −ρω S   (5) 
         [0033]    The driving torque of the accessory pump  26  is supposed to be proportional to the rotational speed of the pump  26  as indicated in the following expression (6): 
         τ P =αω P   (6) 
         [0034]    where τ P  represents load torque of the accessory pump  26  (&lt;0), and ω P  represents rotational speed of the accessory pump  26 , and α represents proportional coefficient (&lt;0).  
         [0035]    Assuming that the pump motor (i.e., drive motor for the air pump)  22  is operated to drive the accessory pump  26  as desired, and a vehicle speed and a required pump power (which can be calculated by a control unit not illustrated) are given, the following expressions (7) and (8) are derived from the above expression (6), where ω P1  represents the angular velocity of the drive motor  12 , and Pp 1  (&lt;0) represents the required pump power: 
         P P1 =τ P1 ω P1 =1/ατ P1   2   (7) 
         P P1 =αω P1   2   (8) 
         [0036]    Accordingly, the following relationships (9), (10) are obtained:  
               τ   P1     =       -       α                 P                 p1                         (       τ   P1     &lt;   0     )               (   9   )                 ω   P1     =         P                 p1     α               (   10   )                               
 
         [0037]    According to the above-indicated expressions (3) and (4), the torque of the drive motor  12  and the torque of the pump motor  22  are expressed as follows: 
         τ R =−1/1+ρτ C τ P1 =−ρ/1+ρτ C τ C =−(1+ρ)τ P1   (11) 
         [0038]    Using the above expressions, the following relationship (12) is obtained. 
         τ S =−ρ/1+ρτ C =−ρ/1+ρ(−(1+ρ)τ P1 )=ρτ P1   (12) 
         [0039]    Thus, the torque applied to each of the sun gear, carrier and the ring gear is determined.  
         [0040]    In the meantime, the following relationships regarding the rotational speeds of the accessory pump  26  and the sun gear  20  are derived from the above-indicated expressions (5) and (10):  
                 ω   P1     =         (     1   +   ρ     )          ω   C1       -     p                   ω   s                
                  ω   s     =           1   +   ρ     ρ          ω   C1       -       1   ρ          ω   P1                     =           1   +   ρ     ρ          ω   C1       -       1   ρ              P                 p1     α                           (   13   )                               
 
         [0041]    Using the above expressions (12) and (13), the power of the pump motor  22  is calculated as follows:  
                       τ   s          ω   s       =       ρτ   P1          (           1   +   ρ     ρ          ω   C1       -       1   ρ              P                 p1     α           )                   =       -   ρ            α                 P                 p1                       (           1   +   ρ     ρ          ω   C1       -       1   ρ              P                 p1     α           )                   =       -     P   P1       -       (     1   +   ρ     )          ω   C1            α                 P                 p1                         (   14   )                               
 
         [0042]    Thus, the following expression (15) is obtained, where Ps represents the power of the pump motor 22: 
               P   S     =         -     (     1   +   ρ     )              α                 P                 p1            ω   C1       -     P   P1               (   15   )                               
 
         [0043]    [0043]FIG. 4 shows a relationship (as indicated in the above expression (15)) between the angular velocity of the drive motor  12  and the power of the pump motor  22 , using the power of the accessory pump  26  as a parameter. In FIG. 4, the horizontal axis represents the angular velocity of the drive motor  12 , and the vertical axis represents the power of the pump motor  22 . Here, the power of the pump motor  22  is represented by a straight line having an intercept of −P P1  and a slope of −( 1 +ρ) (αP P   1 ) 0.5 , with the intercept and slope varying with P P1 . The power of the pump motor  22  required for providing the same power of the accessory pump  26  is different between the case where the angular velocity of the drive motor  12  is equal to zero (i.e., where torque assist is not performed by the drive motor  12 ), and the case where the angular velocity of the drive motor  12  is not equal to zero (but, for example, is equal to 400 rad/s). It will be understood from FIG. 4 that the required power of the pump motor  22  decreases as the angular velocity of the drive motor  12  increases.  
         [0044]    It is possible to provide characteristics as shown in FIG. 5, by adjusting ρ (gear ratio) and α (proportional coefficient) in FIG. 4. The slope of each straight line in FIG. 5 is inclined more sharply than that of a corresponding line in FIG. 4, and a difference in the power of the pump motor  22  between the case where the angular velocity of the drive motor  12  is equal to zero and the case where it is not equal to zero becomes remarkable. It follows that, owing to the torque assist of the drive motor  12 , the required power of the pump motor  22  decreases. By plotting the required power for driving the accessory pump  26  in relation to the angular velocity of the drive motor  12  in FIG. 5, a rated power of the pump motor  22  can be uniquely determined.  
         [0045]    In operation, a target torque of the drive motor  12  is determined by adding torque obtained according to the above-indicated expression (11), to required torque to be transmitted to the output shaft connected to the wheels (i.e., torque for running the vehicle), and torque control is performed by a control unit (not shown), based on the target torque thus determined. On the other hand, torque control for the pump motor  22  is performed so that the rotational speed of the pump motor  22  (i.e., the sun gear) becomes equal to a target rotational speed that is determined according to the above-indicated expression (13).  
         [0046]    In the present embodiment, therefore, the power to be generated by the pump motor  22  can be reduced with respect to the required power of the accessory pump  26  for driving the fuel cell system  10 . Accordingly, the size of the pump motor  22  and that of an inverter or inverters can be advantageously reduced.  
         [0047]    Further, the planetary gear unit, rather than a clutch, is used for transmitting a part of the power or driving force of the drive motor  12  to the accessory system, and therefore the accessory pump  26  can be efficiently driven with almost no loss of power.  
         [0048]    Since the drive motor  12  used in the present embodiment is capable of generating sufficiently large toque, the size of the drive motor  12  or the capacity of the inverter(s) need not be increased even if extra torque is required for driving accessories.  
         [0049]    While the accessory drive apparatus is installed on the fuel-cell vehicle in the illustrated embodiment, it is to be understood that the invention is not limited to this application, but may also be applied to any system equipped with an accessory system for driving a power source.
An accessory drive apparatus includes a power source, an accessory system that provides an input to the power source, an accessory driving source that drives the accessory system, and a power combination/distribution mechanism that is connected to the power source and the accessory system to transmit power from both the power source and the accessory driving source to the accessory system. With this arrangement, the accessory system is driven by a combination of power received from the accessory driving source and the power source.