Abstract:
A first member of a robot is fixed to a casing of a speed reducer. A second member is fixed by fitting to a rotating member that rotates relatively to the casing. A motor is mounted on the second member, and an input gear that is connected directly to the shaft of the motor and a spur gear of the speed reducer are made to mesh with each other. A crankshaft that is connected to the spur gear is rotatably mounted on the rotating member through a bearing. As the spur gear and the crankshaft rotate, an external gear rocks eccentrically and rotates for on tooth with respect to an internal gear in the casing. Thereupon, the rotating member rotates relatively to the casing, while the second member rotates relatively to the first member. The speed reducer of the invention, compared with a conventional one, requires no use of a center gear, so that it includes fewer components, and therefore, is lower-priced and more reliable.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a joint structure for transmitting power to a joint portion between movable robot members such as a turning trunk, arms, etc. of a robot.  
           [0003]    2. Description of the Prior Art  
           [0004]    In an industrial robot, speed reducers of various types are used for joint portions between members that move relatively to each other, e.g., between a robot base and a trunk portion turning with respect to the robot base, between the trunk portion and a first arm rocking on the trunk portion, between the first arm and a second arm rocking on the first arm, etc.  
           [0005]    Of these speed reducers, a hollow-type speed reducer is used in order to secure the installation position of a motor for driving each arm or wrist of the robot and a passage for cables and hoses through which energy such as electric power, hydraulic pressure, or pneumatic pressure is supplied to an end effecter that is attached to the distal end of the robot wrist. Since a through hole for the cable or hose passage is provide in the center of rotation of the speed reducer, moreover, the motor is located off the axis of the speed reducer, in general.  
           [0006]    [0006]FIG. 3 is a sectional view of a planetary-gear speed reducer of the eccentric rocking type that is generally used in each joint portion of the robot. FIG. 4 is a diagram for illustrating the engagement of gears of the planetary-gear speed reducer.  
           [0007]    A center gear  53  includes a gear wheel  53   a  and a pinion  53   b.  An input gear  52  that is connected directly to an output shaft of a motor  51  is in mesh with the gear wheel  53   a  of the center gear  53 . As shown in FIG. 4, moreover, three spur gears  54  that are arranged at equal spaces in the circumferential direction are in mesh with the pinion  53   b  of the center gear  53 .  
           [0008]    The spur gears  54  are fixed to a crankshaft  55 . The crankshaft  55  is rotatably mounted on a rotating member  57  through a bearing  56 . An external gear  58 , which is rockably mounted on an eccentric portion of the crankshaft  55  through a bearing, meshes with an internal gear  60  inside a casing  59 . The rotating member  57  is attached to the casing  59  for relative rotation through a bearing.  
           [0009]    When the motor  51  is driven to rotate the input gear  52 , the gear wheel  53   a  that is in mesh with the gear  52  is driven so that the center gear  53  rotates, whereupon the rotating speed is reduced in accordance with the gear ratio. The input gear  52  and the gear wheel  53   a  of the center gear  53  constitute a first speed reducing unit.  
           [0010]    When the center gear  53  rotates, the three spur gears  54  that are in mesh with the pinion  53   b  of the gear  53  rotate. The spur gears  54  constitute an input portion of a second speed reducing unit. When the three spur gears  54  rotate, the crankshaft  55  that is connected to the spur gears  54  moves eccentrically, so that the external gear  58  makes an eccentric motion. The number of teeth of the internal gear  60  which are formed inside the casing  59  and are in mesh with the external gear  58  is greater than that of the gear  58  by one. When the crankshaft  55  makes one revolution, therefore, the external gear  58  rotates for one tooth in the direction opposite to the rotating direction of the crankshaft  55 . If the casing  59  is fixed, the rotating member  57  is made to rotate correspondingly for one tooth through the crankshaft  55 . A portion that causes the rotating member  57  finally to rotate at reduced speed as the spur gears  54  rotate constitutes a second speed reducing unit.  
           [0011]    The speed reducer has a through hole in its central portion. More specifically, a through hole is formed penetrating the respective central portions of the rotating member  57  and the center gear  53 , and this through hole serves as a passage for wiring and/or piping. Accordingly, the motor that is connected to the speed reducer is mounted in a position eccentric to the central through hole portion.  
           [0012]    As described above, the joint structure of the industrial robot that uses the planetary-gear speed reducer of the eccentric rocking type requires use of the center gear  53  between the input gear  52 , which is connected directly to the motor  51 , and the spur gears  54  of the speed reducer. The center gear  53  has a double-gear structure including the gear wheel  53   a  that is in mesh with the input gear  52  and the pinion  53   b  that is in mesh with the spur gears  54 . Inevitably, therefore, the center gear  53  has a complicated construction, which entails high manufacturing cost. Further, supporting the center gear  53  requires two bearings. The use of the center gear and the two bearings results in increase in cost of the hollow speed reducer, and constitutes a hindrance to the reduction of the manufacturing cost of the robot joint structure.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0013]    The object of the present invention is to provide a joint structure of a robot, of which the number of components is reduced without lowering the reliability of a joint drive system.  
           [0014]    The present invention relates to a robot joint structure between a first member and a second member that are connected to each other for relative rotation by means of a speed reducer. The first and second members include a base, arms, turning trunk, etc. of a robot. The speed reducer is composed of a first-stage speed reducing mechanism and a second-stage speed reducing mechanism.  
           [0015]    The first-stage speed reducing mechanism includes an input gear connected directly to the output shaft of a motor and a single spur gear in mesh with the input gear. The second-stage speed reducing mechanism includes a crankshaft connected directly to the spur gear, an external gear which engages the crankshaft to be rocked eccentrically, a casing of the speed reducer, an internal gear which is formed inside the casing and is in mesh with the external gear, and a rotating member which supports the crankshaft for rotation and can rotate around the central axis of the internal gear with respect to the casing.  
           [0016]    A joint is constructed in a manner such that the casing is fitted with the first member, the rotating member is fitted with the second member, and the motor is attached to the second member so that the input gear is in mesh with the spur gear.  
           [0017]    Further, the second member is provided with a mounting portion for mounting the motor in a given position and is attached to the rotating member by fitting in order to align the axis of the second member with the axis of the output of the speed reducer. Further, a positioning pin is used for settling a rotational phase of the second member with respect to the rotating member when attaching the second member to the rotating member, thereby securing the engagement between the input gear and the spur gear. Furthermore, the first and second members of the robot have a hollow structure inside, and the casing and the rotating member are provided with through holes around their axes such that a space for wiring and/or piping is secured inside the joint.  
           [0018]    The joint structure according to the present invention, compared with a joint structure that uses a conventional speed reducer, requires no center gear, so that bearings for supporting the center gear can be omitted, that is, the number of essential components can be reduced. Since such center gear has a complicated shape, in particular, its manufacturing cost is high. Thus, the omission of the center gear and the bearings that support it results in a corresponding reduction in cost of a drive system for robot joint portions. Since the number of essential components is reduced, moreover, the reliability and operating efficiency of the joint drive system can be improved. Since no center gear is used, furthermore, the number of spots for gear engagement is reduced, so that the noise level can be lowered. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    The foregoing and other objects and features of the invention will become apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings, in which:  
         [0020]    [0020]FIG. 1 is a sectional view showing a joint structure of a robot according to a first embodiment of the present invention;  
         [0021]    [0021]FIG. 2 is a sectional view showing a joint structure of a robot according to a second embodiment of the invention;  
         [0022]    [0022]FIG. 3 is a sectional view of a conventional planetary-gear speed reducer of the eccentric rocking type used in a joint mechanism of a robot; and  
         [0023]    [0023]FIG. 4 is a diagram for illustrating the engagement of gears of the planetary-gear speed reducer of FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    A robot joint structure according to a first embodiment of the present invention will now be described with reference to FIG. 1.  
         [0025]    This robot joint structure includes a first member  11  and a second member  12 , which are connected to each other for relative rotation by means of a speed reducer. The first and second members  11  and  12  include a base, arms, turning trunk, etc. of a robot. A conventional hollow planetary-gear speed reducer (but having no center gear) of the eccentric rocking type is used as the speed reducer.  
         [0026]    A casing  18  of the speed reducer is fixed to the first member  11 . On the other hand, a rotating member  21  of the speed reducer is fixed to the second member  12 . A motor  13  is fastened to a mounting portion  22  that is located in a given position on the second member  12 , and causes the speed reducer to rock the second member  12  relatively to the first member  11 .  
         [0027]    A mounting portion  23  (in the form of a circumferential groove) is formed in the second member  12 , whereby the rotating member  21  of the speed reducer is fixed to the second member  12 . An end portion of the rotating member  21  is fitted in the mounting portion  23 . As this is done, the central axis of the second member  12  is in alignment with the axis of the rotating member  21  (or the axis of the output shaft of the speed reducer).  
         [0028]    In order to settle the rotational phase of the second member  12  with respect to the rotating member  21 , moreover, the rotating member  21  is fixed to the second member  12  by means of a positioning pin  24 . Thus, a given distance is secured between the central axis of an input gear  14  (mentioned later) and the central axis of a spur gear  15  of the speed reducer, so that the input gear  14  and the spur gear  15  can mesh with each other.  
         [0029]    Further, the rotating member  21  is rotatably supported on the casing  18  of the speed reducer through a bearing  25 .  
         [0030]    The input gear  14  is connected directly to the output shaft of the motor  13 . The input gear  14 , which can mesh with the spur gear  15  of the speed reducer, as mentioned before, constitutes a first-stage speed reducing mechanism of the speed reducer.  
         [0031]    The present invention is characterized in that the input gear  14  of the motor  13  is directly in mesh with the spur gear  15  of the speed reducer. In this arrangement, the power of the motor  13  is transmitted directly to the spur gear  15 .  
         [0032]    In the conventional planetary-gear speed reducer of the eccentric rocking type shown in FIG. 3, on the other hand, the input gear  52  meshes with the gear wheel  53   a  of the center gear  53 , while the pinion  53   b  of the center gear  53  meshes with the spur gears  54 . In this arrangement, the power of the motor is transmitted from the input gear  52  to the spur gears  54  via the center gear  53 . Although the spur gears  54  shown in FIG. 3 are three in number, the spur gear  15  used in the present invention is one.  
         [0033]    Returning to FIG. 1, there is shown a crankshaft  16  that is fixed to the spur gear  15 . The crankshaft  16  is rotatably mounted on the rotating member  21  through a bearing  20 . As in the case of the conventional planetary-gear speed reducer of the eccentric rocking type, an external gear  17  is rockably mounted on an eccentric portion of the crankshaft  16  through a bearing. The external gear  17  rocks eccentrically as the crankshaft  16  rotates. This arrangement is shared by the conventional planetary-gear speed reducer.  
         [0034]    An internal gear  19  that meshes with the external gear  17  is provided on the inner peripheral surface of the casing  18  of the speed reducer. The number of teeth of the internal gear  19  is greater than that of the external gear  17  by one. When the crankshaft  16  makes one revolution, therefore, the external gear  17  rotates for one tooth with respect to the internal gear  19  in the direction opposite to the rotating direction of the crankshaft  16 . Corresponding to the rotation of the external gear  17 , the rotating member  21  rotate with respect to the casing  18  through the crankshaft  16 .  
         [0035]    Thus, the second member  12  that is fixed to the rotating member  21  rotates relatively to the first member  11  to which the casing  18  is fixed. The crankshaft  16 , external gear  17  and internal gear  19  constitute a second-stage speed reducing mechanism of the speed reducer.  
         [0036]    The first and second members  11  and  12  that constitute the robot joint structure have a hollow structure. The speed reducer of this joint structure has a through hole  50  in its central portion. The motor  13  is attached eccentrically to the second member  12 . Thus, the through hole  50  in the central portion of the speed reducer serves as a passage for cables and hoses for wiring and piping.  
         [0037]    If the motor  13  is driven to rotate the input gear  14 , the rotary power of the motor  13  is transmitted, with the rotary speed reduced, to the crankshaft  16  through the input gear  14  and the spur gear  15  that constitute the first-stage speed reducing mechanism. As the crankshaft  16  rotates, the external gear  17  rocks eccentrically and rotates for one tooth with respect to the internal gear  19  of the casing  18  which is in mesh with the external gear  17 , whereupon the rotating member  21  rotates with respect to the casing  18 . In consequence, the second member  12  that is fixed to the rotating member  21  rotates relatively to the first member  11  to which casing  18  is fixed.  
         [0038]    The motor  13  that is fixed to the second member  12  also rotates together with the second member  12  and the rotating member  21  that rotates integrally with the second member. Since the spur gear  15  that is mounted on the rotating member  21  through the crank shaft  16  also rotates together with the rotating member  21 , on the other hand, the input gear  14  that is connected directly to the output shaft of the motor  13  cannot be disengaged from the spur gear  15  if the second member  12  rotates relatively to the first member  11 .  
         [0039]    A robot joint structure according to a second embodiment of the invention will now be described with reference to FIG. 2.  
         [0040]    According to the first embodiment, as mentioned before, the casing of the speed reducer is fixed to the first member that constitutes the base, and the rotating member of the speed reducer is fixed to the second member, so that the second member is rotated relatively to the first member (or the base). The motor is mounted on the second member. According to the second embodiment, on the other hand, a rotating member of a speed reducer is fixed to a first member that constitutes a base, and a casing of the speed reducer is fixed to a second member, so that the second member is rotated relatively to the first member (or base). In this case, a motor is mounted on the first member.  
         [0041]    The first and second embodiments are based on common technical designs in which one of the rotating member and the casing that constitute the speed reducer is provided on the first member (or the base) while the other is provided on the second member, thereby allowing the second member to be rotated with respect to the first member through the speed reducer, and the motor is mounted on the first or second member to which the rotating member of the speed reducer is attached.  
         [0042]    The following is a brief description of the second embodiment. As shown in FIG. 2, a first member  11  that constitutes a base is provided with a motor mounting portion  22  for a motor  13 . Further, the first member  11  is formed with a mounting portion  23  (in the form of a circumferential groove) for fixing a rotating member  21  of the speed reducer. An end portion of the rotating member  21  of the speed reducer is fitted into the mounting portion  23 , and the rotational phase of the first member  11  with respect to the rotating member  21  is settled by means of a positioning pin  24 .  
         [0043]    When the rotating member  21  of the speed reducer is attached to the first member  11  in this manner, the distance between the respective axes of an input gear  14  that is connected directly to the output shaft of the motor  13  and a spur gear  15  of the speed reducer is settled, and the input gear  14  and the spur gear  15  are positioned so that they can mesh with each other.  
         [0044]    Further, a second member  12  that rotates relatively to the first member  11  serving as the base is fixed to a casing  18  of the speed reducer.  
         [0045]    The second embodiment shares other configurations with the first embodiment. More specifically, the spur gear  15  is fitted with a crankshaft  16 , which is rotatably mounted on the rotating member  21  through a bearing  20 . An external gear  17  is rockably mounted on an eccentric portion of the crankshaft  16  through a bearing, and meshes with an internal gear  19  that is provided inside the casing  18 .  
         [0046]    When the motor  13  is driven, the rotation of the input gear  14  is transmitted, with the rotational speed reduced, to the crankshaft  16  through the spur gear  15 . When the crankshaft  16  makes one revolution, the external gear  17  rocks eccentrically and rotates for one tooth with respect to the internal gear  19 . Corresponding to the rotation of the external gear  17 , the rotating member  21  rotates with respect to the casing  18 . In consequence, the second member  12  that is attached to the casing  18  rotates relatively to the first member  11 , since the rotating member  21  is fixed to the first member  11 .  
         [0047]    According to the present invention, as described above, the input gear that is connected directly to the output shaft of the motor is caused directly to mesh with the spur gear of the speed reducer, so that the center gear, which is essential to the conventional speed reducer, can be omitted.  
         [0048]    To attain this, a motor is mounted on a robot member attached to the rotating member of the speed reducer on which a spur gear is mounted so that the motor rotationally moves integrally with the rotating member to maintain its relative position to the rotating member, thus preventing the spur gear from disengaging from the input gear. Besides, as the rotating member is connected to the robot member on which the motor is mounted by fitting and the rotational phase is settled through the positioning pin or the like, the distance between the respective axes of the spur gear and the input gear is settled to ensure the engagement of the spur gear with the input gear.