Abstract:
To enhance durability of a gear transmission mechanism by eliminating play in a gear engagement due to the backlash between the gears (gear clearance) of the gear transmission mechanism in a variable pitch mechanism of blades or a nacelle turning mechanism and further to reduce damages to a gear tooth surface caused by slide of the tooth surface or impact thereon due to slight vibration, the gear transmission mechanism is constructed such that a first pinion gear  20   a  and a second pinion gear  20   b  are provided to mesh with a gear wheel  18  respectively and rotated by a first drive motor  32   a  and a second drive motor  32   b  respectively, and a controller  36  controls so as to eliminate play in the engagement of the internal gear and the pinion gear due to backlash between the internal gear  18  and the first pinion gear  20   a , thereby avoiding generation of a slide and impact force on the gear transmission mechanism even when the external force or the wind acts on the blade  16.

Description:
RELATED APPLICATIONS 
     The present application is based on, and claims priority from PCT Application Number PCT/JP2010/059225, filed May 31, 2010, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wind turbine generator comprising a nacelle supported on a tower so as to perform yaw control, or a plurality of blades which are supported on a hub so as to perform pitch control of the blades, with a gear transmission mechanism provided for controlling the pitch of the blade or the yaw of the nacelle, and a method of controlling the wind turbine generator. More specifically, the present invention relates to a wind turbine generator with a gear transmission mechanism comprising pitch wheels provided on a shaft side to be controlled and pinion gears provided so as to mesh with the gears, and the controlling method thereof. 
     2. Description of the Related Art 
     In reference to  FIG. 11 , a schematic structure of the wind turbine generator is explained. In  FIG. 11 , the wind turbine generator  100  comprises a tower  102  installed upright on a foundation B, a nacelle  102  provided on a top end of the tower  102 , a hub  106  fixed to the nacelle  104 , and a plurality of blades  108  fixed to the hub  106 . 
     The nacelle  104  supports the rotor hub  106  and houses a rotation shaft  110  and a generator  112  therein. The rotation shaft  110  transmits the torque from the rotor hub side to the generator side. 
     In this type of wind turbine generator, Patent Documents listed below disclose to perform the pitch control of the blades  108  with respect to the rotor hub  106  or the yaw control of the nacelle  104  with respect to the tower  102  via the gear transmission mechanism. 
     For instance, a variable pitch mechanism disclosed in Patent Document 1 is explained in reference to  FIG. 12 . The variable pitch mechanism  120  comprises an internal gear  122   a  formed on an inner circumference of a blade ring  122  of the blade  108 , and a gear reducer  136  for transmitting the drive power from the motor driving the pinion gear  124 . The pinion gear  124  is rotated so as to change the pitch of the blade  108 . 
     Moreover, Patent Document 2 discloses a power transmission mechanism of a planetary gear type arranged such that a plurality of shafts are provided around a pitch shaft center of the blade at a base of the blade and planetary gears are arranged therein supported rotatably. The planetary gear revolves and rotates so as to transmit the motion of the blade to the rotor and also the blade pitch is controlled by controlling the rotation of the planetary gear by a rotation member of a ring shaped motor.
     [Patent Document 1] JP2003-222070A   [Patent Document 2] US2009/0016885A1   

     SUMMARY OF THE INVENTION 
     When the pitch control is performed with the gear transmission mechanism using one pinion gear as disclosed in Patent Document 1 or using the plurality of pinion gears as disclosed in Patent Document 2, there is backlash (clearance between mating gears) between the gears and thus, even when the pinion gear is stopped and the pitch of the blade is fixed, a slight vibration of the pitch due to the play in the amount of the clearance still takes place by the external force. And this can cause damage to the tooth surfaces of the gears from a slide of the tooth surface or impact thereon and lead to a durability problem. 
     Particularly, the blades of the wind turbine generator is constantly subjected to the external force in turn due to the rotation or the wind force and thus if the slide of the tooth surface or impact thereon due to movement in the amount corresponding to the backlash continues over a long period of time, the tooth surface is prone to the damage. Moreover, in recent years the wind turbine generator are getting bigger in size, by which the blades are more subjected to the external force, the force against each tooth surface inevitably becomes large and the gear transmission mechanism is even more prone to the damage. 
     Moreover, this problem is common not only to the variable pitch mechanism members as described above but also to a nacelle turning mechanism members (yaw control) having similar mechanism. 
     In other words, the yaw control mechanism is structured such that the gear wheel is formed on a bearing arranged on the bottom of the nacelle and the gear wheel engages with the pinion gear so as to perform the yaw control by the rotation of the pinion gear. In most of the wind turbine generator, when the direction of the nacelle (yaw direction) is fixed, a hydraulic brake is arranged besides the gear mechanism so as to stop the vibration in the amount corresponding to the backlash of the gear. However, when a brake for stopping the turning of the nacelle is not put on, the external force of the wind and the dynamic force of the wind turbine generator causes the gear mechanism for the yaw control to slide in the amount corresponding to the backlash and thus the force of the impact is loaded on the tooth surfaces. 
     In view of the above problems, in a wind turbine generator having a gear transmission mechanism as a variable pitch mechanism of blades or as a nacelle turning mechanism, an object of the present invention is to eliminate play in the gear engagement due to the backlash between the gears (gear clearance) by adjusting the engagement of the gears and reduce the damages to a gear tooth surface caused by slide of the tooth surface or impact thereon due to slight vibration, so as to enhance a durability of the gear transmission mechanism. 
     The present invention proposes a gear transmission mechanism of a wind turbine generator used as a variable pitch mechanism or a nacelle turning mechanism, the gear transmission mechanism comprising: a plurality of pinion gears; a plurality of motors which drive the plurality of pinion gears respectively and independently; a gear wheel which meshes with the plurality of pinion gears; and a controller which controls the plurality of pinion gears individually to adjust an engagement of the pinion gear and the gear wheel so as to eliminate play due to backlash between the pinion gear and the gear wheel during operation of the pinion gears or during non-operation of the pinion gears. 
     Further, the present invention also proposes a method of controlling an engagement of gears of a gear transmission mechanism which is used as a variable pitch mechanism or a nacelle turning mechanism of a wind turbine generator and which includes a plurality of pinion gears which are respectively and independently rotated by a plurality of motors and a gear wheel meshing with the plurality of pinion gears, the method comprising the step of: controlling the plurality of pinion gears individually to adjust an engagement of the pinion gear and the gear wheel so as to eliminate play due to backlash between the pinion gear and the gear wheel during operation of the pinion gears or during non-operation of the pinion gears. 
     According to the above gear transmission mechanism and method of controlling the engagement of the gears, to take a measure against the backlash formed in the engagement of the plurality of pinion gears driven respectively and independently by the plurality of motors and the mating gears, the plurality of pinion gears are individually controlled to adjust the engagement of the pinion gear and the gear wheel so as to eliminate play due to backlash between the pinion gear and the gear wheel during operation of the pinion gears or during non-operation of the pinion gears. 
     When performing the pitch control of the blade or the yaw control, play in a small clearance due to backlash between the pinion gear and the gear wheel is eliminated during the performance of the pitch/yaw control, during the transition period of the pinion gears being stopped, and particularly after the stopping the rotation of the pinion gears, so as to suppress the damage to the gear tooth surface caused from a slide of the tooth surface or impact thereon and improve the durability of the gear transmission mechanism. 
     To eliminate the backlash between the plurality of the pinion gears and the gear wheel, the gear transmission mechanism of the wind turbine generator of the first aspect of the present invention may include such mechanism that achieve the control procedure listed below. To eliminate the backlash between the plurality of the pinion gears and the gear wheel, the method of controlling the engagement of the gears of the gear transmission mechanism as the second aspect of the present invention may include such steps that achieve the control procedure as listed below. Particularly, it is important to eliminate the backlash when stopping the gears, and it is possible to perform one step or more than one step simultaneously.
     (1) Controlling the first pinion gear and the second pinion gear such that the first pinion gear rotates at a circumferential speed different from that of the second pinion gear during the operation of the pinion gears or during a transition period when the pinion gears are to be stopped so as to eliminate the backlash.   (2) Controlling the first pinion gear and the second pinion gear such that the first pinion gear stops at a timing different from that of the second pinion gear during the transition period from the operation to the stopping of the pinion gears so as to eliminate the backlash.   (3) Controlling the pinion gears such that the second pinion gear is rotated in a reverse direction after the first pinion gear is stopped so as to eliminate the backlash.   

     As a third aspect of the present invention, the present invention also provides a wind turbine generator comprising the gear transmission mechanism as described above. With the structure, the durability of the gear transmission mechanism for performing the pitch control of the variable pitch control mechanism and the nacelle turning mechanism is improved. 
     Effects of the Invention 
     According to the present invention, play in a small clearance due to backlash between the pinion gear and the gear wheel is eliminated against the backlash formed between the plurality of pinion gears respectively and independently rotated by the plurality of motors, and thus the damage to the gear tooth surface caused from a slide thereof or impact thereon is suppressed, resulting in improving the durability thereof. 
     Specifically, in the case of the pinion gears for the pitch control mechanism, one pinion gear rotates at a circumferential speed different from that of another pinion gear or one pinion gear stops at a timing different from that of another pinion gear during the operation of the blade pitch control, during the transition period when the pinion gear is to be stopped, during the operation of the yaw control of the nacelle, or during the non-operation of the pinion gear. Alternatively, one pinion gear is rotated in a reverse direction after the plurality of the pinion gears is stopped. In this manner, the pinion gears and the gear engage such that there is no play between the gears due to the backlash formed between the pinion gear and the gear wheel, resulting in suppressing the damage to the gear tooth surface caused from a slide of the tooth surface or impact thereon and improving the durability thereof. 
     Moreover, with the wind turbine generator of the present invention, the durability of the gear transmission mechanism for performing the blade pitch control by the variable pitch mechanism and the yaw control by the nacelle turning mechanism is enhanced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  A perspective view showing a partial cross sectional view of a blade when the gear transmission mechanism of the present invention is applied to the members of the pitch control mechanism. 
         FIG. 2  An enlarged sectional view of a part indicated by X in  FIG. 1  with an explanatory diagram of a controller. 
         FIG. 3  (A) is a fragmentary view taken in the direction of Y of  FIG. 2  and (B) is a partial enlarged view of  FIG. 3(A) . 
         FIG. 4  A graphical illustration of an engagement of mating gears in relation to each preferred embodiment of the first device of the present invention. 
         FIG. 5  A flow chart of a first preferred embodiment. 
         FIG. 6  A flow chart of a second preferred embodiment. 
         FIG. 7  A flow chart of a third preferred embodiment. 
         FIG. 8  A flow chart of a fourth preferred embodiment. 
         FIG. 9  A sectional view showing the gear transmission mechanism of the present invention being applied to a nacelle turning mechanism in reference to a fifth preferred embodiment of the present invention. 
         FIG. 10  A fragmentary view taken in the direction of Y of  FIG. 2 , illustrating a sixth embodiment of the present invention. 
         FIG. 11  A side view showing a general structure of a wind turbine generator. 
         FIG. 12  A diagram illustrating a variable pitch mechanism of a conventional wind turbine generator. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will now be described in detail with reference to the accompanying drawings. It is intended, however, that unless particularly specified, dimensions, materials, shape, its relative positions and the like shall be interpreted as illustrative only and not limitative of the scope of the present. 
     First Preferred Embodiment 
     A first embodiment of the present invention is explained in reference to  FIG. 1  to  FIG. 5 . In a wind turbine generator  10  of the present embodiment as shown in  FIG. 1 , an internal gear  18  is formed on an inner circumference of a blade ring  30  of a blade  16  inside a hub  14  arranged at the front of a nacelle  12 , and a first pinion gear  20   a  and a second pinion gear  20   b  are provided which mesh with the internal gear  18  respectively. The first pinion gear  20   a  and the second pinion gear  20   b  have a first pinion shaft  22   a  and a second pinion shaft  22   b  respectively. 
       FIG. 2  illustrates a variable pitch mechanism  41  having a bearing support  26  being fixed to top of the hub  14  by bolts  24 . On an inner circumference side of the bearing support  26 , the blade ring  30  of the blade  16  is rotatably supported via a bearing  28 . On the inner side of the blade ring  30 , the internal gear  18  having a large diameter is formed. 
     In  FIG. 3(A) , on the inner side of the blade ring  30  of the blade  16 , the first pinion gear  20   a  and the second pinion gear  20   b  are arranged symmetrically with respect to a center O of the blade ring  30  and mesh with the internal gear  18 . Herein, a shaft center of the first pinion gear  20   a  is O 1  and that of the second pinion gear  20   b  is O 2 . 
     In this preferred embodiment, the first pinion gear  20   a  and the second pinion gear  20   b  have the same diameter. However, the pinion gears may have different diameter and the first pinion gear  20   a  and the second pinion gear  20   b  may not be arranged symmetrically with respect to the center of the blade ring and three or more pinion gears may be provided. 
     As shown in  FIG. 2 , the hub  14  has an inner portion  14   a  to which a first drive motor  32   a  and a second drive motor  32   b  are fixed. The first pinion gear  20   a  has a first pinion shaft  22   a  which is inserted in one of the holes  34  drilled in the inner portion  14   a  and is connected to the first drive motor  32   a . Further, the second pinion gear  20   b  has a second pinion shaft  22   b  which is inserted in another hole  34  and is connected to the second drive motor  32   a.    
     As already known, the first drive motor  32   a  performs the pitch control of the blade  16 . Depending on the wind direction or strength, the first drive motor  32   a  is operated so as to rotate the blade  16  in the direction to open the blade angle (bigger) or in the direction to close the blade angel (smaller) via the internal gear  18  meshing with the first pinion gear  20   a.    
     A controller  36  controls the operations of the first drive motor  32   a  and the second drive motor  32   b  by its output signals. The controller  36  controls the first drive motor  32   a  and the second drive motor  32   b  respectively and independently. 
     A timing signal is inputted from a timer  38  to the controller  36 . And an encoder  39  detects a rotational displacement per unit time of the first pinion shaft  22   a  and the second pinion shaft  22   b  and inputs a detection signal to the controller  36 . With the structure above, the variable pitch control device  40  for controlling the pitch of the blade  16  is constructed. 
     In the preferred embodiment as shown in  FIG. 3(A) , the first drive motor  32   a  rotates the first pinion gear  20   a  in the direction indicated with an arrow N 1 , and simultaneously the second drive motor  32   b  rotates the second pinion gear  20   b  in the direction N 1 . And the internal gear  18  meshing with the pinion gears rotates in the direction indicated with an arrow N so as to perform the pitch control of the blade  16 . During this process, as shown in  FIG. 3(B) , at a meshing portion where the internal gear  18  meshes with the first pinion gear  20   a , backlash C is formed between a tooth  1 S of the internal gear  18  and a tooth  2 S of the first pinion gear  20   a . In  FIG. 3(B) , the first pinion gear  20   a  has a standard pitch circle  2   q.    
     Next, a control procedure by the controller  36  is explained in reference to  FIG. 5 . The flow chart shows a circumferential speed A of the first pinion gear  20   a  and a circumferential speed B of the second pinion gear  20   b.    
     In  FIG. 5 , first the wind turbine generator  10  is started (step S 10 ) and the controller  36  sends a command to start the variable pitch control (step S 11 ). By this, as shown in  FIG. 3 , the pinion gears  20   a  and  20   b  rotate in the direction N 1  and the internal gear  18  rotates in the direction N. Then, the controller  36  sends a command to stop the variable pitch control (step S 12 ) and the controller  36  sends a command to decrease the circumferential speed B of the second pinion gear  20   b  (step S 13 ). 
     In the step S 13 , the command for reducing the circumferential speed is issued only for the second pinion gear  20   b  so that there is a difference in the circumferential speed between the first pinion gear  20   a  and the second pinion gear  20   b.    
     Next, it is determined whether or not such set amount of time to create the circumferential speed difference between the pinion gears (step S 14 ) has passed. If it is determined that the set amount of time has passed, the brake is put on the first drive motor  32   a  and the second drive motor  32   b  so as to stop the first pinion gear  20   a  and the second pinion gear  20   b  and maintain such a state by keeping the brake on (step S 15 ). 
     By performing the above control, when stopping the variable pitch control, an engagement of the internal gear  18  and the first pinion gear  20   a  and an engagement between the internal gear  18  and the second pinion gear  20   b  can be adjusted so as to eliminate play in the clearance of the backlashes C. As a result, even when the blade is subjected to wind force, there is no major impact on the internal gear  18  or the pinion gears  20   a  and  20   b . Accordingly, there is no damage to the gear transmission mechanism and long durability thereof is maintained. 
     Second Preferred Embodiment 
     Next, a second preferred embodiment is explained in reference to  FIG. 6 . The structure of the device of the present embodiment is the same as that of the first embodiment except for the control by the controller  36 . The operation time of the plurality of the pinion gears is differentiated. The control flow is explained in reference to  FIG. 6 . 
     In  FIG. 6 , the control procedure is the same as that of the first embodiment from the step S 20  to the step S 22 . Subsequently, the controller  36  sets the time T 1  at which the first pinion gear  20   a  stops and the time T 2  at which the second pinion gear  20   b  stops such that the time T 1  is different from the time T 2 , e.g. T 1 &gt;T 2  (step S 23 ), and accordingly controls the circumferential speed of the first drive motor  32   a  and the circumferential speed of the second drive motor  32   b . Further, it is possible to set the time T 1  and the time T 2  in advance before the command for stopping the pitch control is issued. 
     Next, it is determined whether or not the in equation, T 1 &gt;T 2  is satisfied (step S 24 ). If it is not satisfied, the process returns to the step S 24  to allow more time and if it is satisfied, the brake is put on the first drive motor  32   a  and the second drive motor  32   b  so as to stop the first pinion gear  20   a  and the second pinion gear  20   b  and maintain such a state by keeping the brake on (step S 25 ). 
     As described above, after starting the pitch control, the second pinion gear  20   b  is controlled so as to stop before the first pinion gear  20   a  stops. Therefore, as illustrated in  FIG. 4 , the engagement of the internal gear  18  and the first pinion gear  20   a  and the engagement between the internal gear  18  and the second pinion gear  20   b  can be adjusted so as to eliminate play in the clearance of the backlashes C. As a result, even when the blade  16  is subjected to wind force, the impact force against the gear transmission mechanism is can be suppressed. Accordingly, there is no damage to the gear transmission mechanism and long durability thereof is maintained. 
     Third Preferred Embodiment 
     A third preferred embodiment of the present invention is explained in reference to  FIG. 7 . The structure of the device of the present embodiment is the same as that of the first embodiment except for the control by the controller  36 . The control flow is explained in reference to  FIG. 7 . 
     In  FIG. 7 , the control procedure is the same as that of the first embodiment from the step S 30  to the step S 32 . The controller  36  sends a command to stop the variable pitch control (step S 32 ) and accordingly the first pinion gear  20   a  and the second pinion gear  20   b  stop the rotation such that the circumferential speeds thereof become zero (step S 33 ). 
     Next, the second pinion gear  20   b  is rotated in the direction opposite to the first pinion gear  20   a  for a set amount of time (step S 34 ). After the set amount of time has passed, the brake is put on the first drive motor  32   a  and the second drive motor  32   b  so as to stop the first pinion gear  20   a  and the second pinion gear  20   b  and maintain such a state by keeping the brake on (step S 35 ). 
     By performing the above control, as illustrated in  FIG. 4 , after stopping the variable pitch control, the engagement of the internal gear  18  and the first pinion gear  20   a  and the engagement between the internal gear  18  and the second pinion gear  20   b  can be adjusted so as to eliminate play in the clearance of the backlashes C. As a result, even when the blade is subjected to wind force, there is no major impact on the internal gear  18  or the pinion gears  20   a  and  20   b . Accordingly, there is no damage to the gear transmission mechanism and long durability thereof is maintained. 
     Fourth Preferred Embodiment 
     Next, a fourth preferred embodiment of a first device of the present invention is described in reference to  FIG. 8 . In  FIG. 8 , the control procedure is the same as that of the first embodiment from the step S 40  to the step S 42 . Subsequently, the controller  36  sets measurement values L 1  and L 2  measured by the encoder  39  of the circumferential speed A of the first pinion gear  20   a  and the circumferential speed B of the second pinion gear  20   b  respectively (L 1 &gt;L 2  in this example)(step S 43 ). It is also preferable to set the L 1  and L 2  in advance before the command for stopping the pitch control is given. 
     Then, from the start of the pitch control to a set timing Ta, the circumferential speeds of the first pinion gear  20   a  and the second pinion gear  20   b  are set to L 1  based on values from the timer  38 , and from the set timing Ta to a timing Ts, only the circumferential speed B of the second pinion gear  20   b  is set to L 2  (step S 44 ). Subsequently, the brake is put on the first drive motor  32   a  and the second drive motor  32   b  so as to stop the first pinion gear  20   a  and the second pinion gear  20   b  and maintain such a state by keeping the brake on (step S 45 ). The process of controlling the circumferential speed of the pinion gears is illustrated in  FIG. 8  (B). 
     In this manner, the circumferential speed B of the second pinion gear is reduced before the circumferential speed A of the first pinion gear  20   a  so as to eliminate play in the clearance of the backlash C between the internal gear  18  and the first pinion gear  20   a  and between the internal gear  18  and the second pinion gear  20   b  after stopping the pitch control. As a result, even when the blade is subjected to wind force, the impact on gear transmission mechanism can be reduced. Accordingly, there is no damage to the gear transmission mechanism and durability thereof is enhanced. 
     Moreover, in this preferred embodiment, the first pinion gear  20   a  and the second pinion gear  20   b  are stopped at the same time so that the second pinion gear  20   b  is not subjected to the load by continuing rotation of the first pinion gear  20   a . Therefore, there is no extra stress on both of the pinion gears and the first drive motor  32   a  and the second drive motor  32   b.    
     Fifth Preferred Embodiment 
     Now, a fifth preferred embodiment of the present invention is explained in reference to  FIG. 9 . The fifth preferred embodiment is an application to a wind turbine generator  50  having a nacelle turning mechanism for yaw control. 
     A nacelle turning mechanism  52  illustrated in  FIG. 9  comprises a first pinion gear  54   a  being rotated by a first yaw motor  56   a , and an internal gear  58  meshing with the first pinion gear  54   a  in which the first yaw motor  56   a  rotates the first pinion gear  54   a  so as to turn a nacelle  12 . 
     The internal gear  58  is formed on an inner circumference of an inner ring  12   a  of a bearing  12  arranged on the bottom of the nacelle. On the internal gear  58 , the first pinion gear  54   a  and a second pinion gear  54   b  are arranged axisymmetrically. The first pinion gear  54   a  and the second pinion gear  54   b  have the same diameter but may have different diameter. 
     Further, a second yaw motor  56   b  which is controlled independently and in a separate system from the first yaw motor  56   a , is provided and the second pinion gear  54   b  is connected to the second yaw motor  56   b.    
     A controller  60  is electrically connected to each of the first yaw motor  56   a  and the second yaw motor  56   b  so that the first yaw motor  56   a  and the second yaw motor  56   b  are controlled by output signals of the controller  60 . Specifically, the first pinion gear  54   a  and the second pinion gear  54   b  are respectively and independently driven or stopped by the controller  60 . 
     A timing signal is inputted from a timer  64  to the controller  60 . And an encoder  62  detects a rotational displacement per unit time of the first yaw motor  56   a  and the second yaw motor  56   b  and inputs a detection signal to the controller  60 . With the structure above, a nacelle turning control device  53  for controlling the yaw turning of the nacelle  12  is formed. 
     The first pinion gear  54   a , the second pinion gear  54   b , the first yaw motor  56   a , the second yaw motor  56   b  and the internal gear  58  of the present embodiment are equivalent of the first pinion gear  20   a , the second pinion gear  20   b , the first drive motor  32   a , the second drive motor  32   b  and the internal gear  18  of the first to fourth embodiments. 
     Therefore, in the gears of the nacelle turning mechanism, the wind against the blades  16  may causes a major impact due to backlash and thereby damages the gear mechanism. 
     To take a measure against this, the nacelle turning control device  53  of the present embodiment also performs the procedure similar to the variable pitch control device  40  of the first to fourth embodiments. 
     Sixth Preferred Embodiment 
     Now, a sixth preferred embodiment of the present invention is explained in reference to  FIG. 10 .  FIG. 10  corresponds to  FIG. 3(A)  of the first to fourth embodiments. The construction and effects other than shown in  FIG. 10  are the same as the first to fourth embodiments. A variable pitch control mechanism  70  of the present embodiment has a circular drive wheel which is housed in the hub  14  and is integral with the blade  16 . Further, an internal gear  74  is formed on a lower part of the inner circumference of the drive wheel. Furthermore, a first pinion gear  76   a  and a second pinion gear  76   b  are arranged on the internal gear  74  symmetrically with respect to the center O of the bottom of the blade  16 . 
     In the preferred embodiment, the first pinion gear  40   a  has a diameter larger than that of the second pinion gear  40   b . Herein, the internal gear  74  has a rotation number N, the first pinion gear has a rotation number N 1 , and the second pinion gear  76   b  for controlling the backlash has a rotation number N 2 . In the variable pitch control mechanism  70  having the above gear mechanism, the control procedure is similar to the first to fourth embodiments is performed. By this, the impact force on the gear transmission mechanism due to the backlash C is eased. Moreover, the first pinion gear  40   a  has a diameter different from that of the second pinion gear  40   b  so that circumferential speed difference is easily obtained and the process for generating the circumferential speed difference of the second and fourth embodiments is simplified. 
     Further, the second pinion gear  76  has a smaller diameter so that the second pinion gear  76   b  for controlling the backlash is downsized, resulting in reducing the drive torque for driving the second pinion gear  76   b . Therefore, the gear mechanism and the drive mechanism can be made at a lower cost. 
     Furthermore, it is not necessary to arrange the first pinion gear  76   a  and the second pinion gear  76   b  axisymmetrically. 
     INDUSTRIAL APPLICABILITY 
     The present invention relates to a wind turbine generator having a gear transmission mechanism as a variable pitch mechanism of blades or as a yaw control mechanism, and applicable to a wind turbine generator as an engagement of mating gears is controlled so that there is no play due to backlash (gear clearance) between the mating gears, thereby avoiding damages to the gear transmission mechanism and improving durability thereof.