Abstract:
To provide an improved cable guide for an articulated joint of a robot or the like that allows a compact design of the articulated joint while minimizing the length of the cable, a substantially circular outer peripheral wall ( 16 ) of a hollow center drum ( 13 ) fixedly attached to the first link member is formed with a hole ( 28 ) at a first angular position substantially aligning with the first link and a slot ( 17 ) formed at a second angular position and elongated circumferentially over an angle substantially corresponding to a range of angular movement of the second link member relative to the first link member in such a manner that the cable member is passed into the center drum from the hole and out of the center drum from the slot. Thereby, the cable member can pass through the interior of the articulated joint along the shortest path, and the necessary length of the cable can be minimized.

Description:
TECHNICAL FIELD 
   The present invention relates to a cable guide for guiding a cable member for conducting a signal and/or power in an articulated joint of a robot or the like. 
   BACKGROUND OF THE INVENTION 
   In an industrial robot or the like equipped with a multi-joint arm, an electric motor and sensors provided in each of the articulated joints corresponding to the shoulder, elbow and wrist are required to be fed with power and control signals from a control circuit incorporated in the main body of the robot. The cables for conducting the power to the electric motor and the control signals to the sensors extend along the length of the arm, and are typically given with a slack at each of the articulated joints to accommodate for the flexing movement of the articulated joint. 
   As methods for passing a cable through an articulated joint, it is known to provide a cable passage that bypasses the bearing portion of the articulated joint (as disclosed in Japanese patent laid open publication No. 63-288692) and to use a hollow shaft in the articulated joint to pass the cable therethrough (as disclosed in Japanese patent laid open publication No. 61-168478). 
   However, according to the technique disclosed in Japanese patent laid open publication No. 63-288692, because the wiring duct for protecting the cable is provided on the exterior of the articulated joint and there is no difference in that the cable extends along the exterior of the articulated joint, an increase in the external dimensions of the articulated joint cannot be avoided. 
   However, according to the technique disclosed in Japanese patent laid open publication No. 61-168478, because the cable is passed through the inner bore of the hollow shaft, a length of the cable corresponding to the length of the shaft must be spent at the articulated joint so that the total length of the cable becomes relatively large for the given length of the arm and this prevents a compact design of the articulated joint. 
   BRIEF SUMMARY OF THE INVENTION 
   In view of such problems of the prior art, a primary object of the present invention is to provide an improved cable guide for-an articulated joint of a robot or the like that allows a compact design of the articulated joint while minimizing the length of the cable. 
   To achieve such an object, the present invention provides a cable guide for guiding a cable member ( 27 ) for conducting a signal and/or power in an articulated joint that connects a first link member ( 2 ) and a second link member ( 2 ) to each other in a relatively rotatable manner, comprising: a hollow center drum ( 13 ) including a substantially circular outer peripheral wall ( 16 ) and fixedly attached to the first link member, the outer peripheral wall being provided with a hole ( 28 ) formed at a first angular position substantially aligning with the first link and a slot ( 17 ) formed at a second angular position and elongated circumferentially over an angle substantially corresponding to a range of angular movement of the second link member relative to the first link member in such a manner that the cable member is passed into the center drum from the hole and out of the center drum from the slot. 
   Preferably, the cable guide further comprises a substantially annular drum holder ( 14 ) coaxially surrounding the center drum and integrally attached to the second link member, the drum holder being provided with a cutout for preventing interference with a part of the cable member extending into the hole of the center drum. Thereby, the cable member can pass through the interior of the articulated joint along the shortest path, and the necessary length of the cable can be minimized. 
   If the cable guide further comprises a cable support member ( 21 ) integrally attached to the center drum and extending inside the center drum to a point adjacent to a center of a rotational movement between the first link member and second link member, the minimum radius of curvature of the cable member can be favorably controlled. If the center drum is provided with an end wall, the cable support member can be integrally formed with the end wall. 
   If the cable guide further comprises a shutter plate ( 23 L) slidably guided by the center drum for selectively closing the slot, dust and other foreign matter are prevented from getting into the cable support member, and the interior of the cable support member can be concealed from view. 
   The shutter plate may be actuated by a part of the cable member when opening the slot and by a part of the second link member when closing the slot. 
   The cable member may be retained by a part of the center drum at a point adjacent to the hole whereby the flexing of the articulated joint would not cause any unstable movement of the cable member. 
   The first link member and second link member may be relatively rotatably joined to each other on either side the center drum. Thereby, the center drum is entirely received in the articulated joint so that the articulated joint can be designed in a highly compact manner and the center drum is favorably protected. Also, the center drum is protected from excessive forces. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Now the present invention is described in the following with reference to the appended drawings, in which: 
       FIG. 1  is a fragmentary sectional side view of a robot arm embodying the present invention taken along line I-I in  FIG. 2 ; 
       FIG. 2  is a sectional plan view of the articulated joint in a straightened state; and 
       FIG. 3  is a sectional plan view of the articulated joint in a flexed state. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2  show the outline of the structure of the cable guide for an articulated joint according to the present invention that can be applied to the elbow joint of an arm of an industrial robot or the knee joint of a bipedal robot. The illustrated embodiment is described in the following assuming for the convenience of description that the embodiment is applied to an articulated joint of a revolving arm (first link) which is joined to a fixed arm (second link) so as to be rotatable around a vertical or horizontal axis with respect to the-stationary fixed arm. 
   The revolving arm  1  is connected to the fixed arm  2  via a knuckle joint  3  in a relatively rotatable manner. The knuckle joint  3  that forms the articulated joint comprises two pairs of extensions  4   a,    4   b,    5   a,    5   b,  one pair extending from the revolving arm  1  in a mutually spaced relationship along the rotational axial line and the other pair extending from the fixed arm in a similar manner. The pair of extensions  5   a,    5   b  extending from the fixed arm  2  are each fitted with a ball bearing  6 . The pair of extensions  4   a,    4   b  extending from the revolving arm  1  are interposed between the pair of extensions  5   a,    5   b  extending from the fixed arm  2 . A shaft member  7  is fitted into the inner race of each of the ball bearings  6  fitted to the extensions  5   a,    5   b  of the fixed arm  2 , and is fixedly attached to a corresponding one of the extensions  4   a,    4   b  of the revolving arm  1  by using threaded bolts B so that the revolving arm  1  is rotatably supported by the fixed arm  2 . 
   The fixed arm  2  may consist of a tubular member having a rectangular cross section, and an electric motor  8  equipped with a reduction gear is mounted within the fixed arm  2  adjacent to the articulated joint. The output shaft of the electric motor  8  extends out of the fixed arm  2 , and the projecting end thereof is fixedly provided with a cogged drive pulley  9 . The shaft member  7  fixedly attached to the corresponding extension  4   a  of the revolving arm  1  is centrally and fixedly provided with a cogged driven pulley  10 , and an endless cogged belt  11  is passed around the two cogged pulleys  9  and  10 . Thereby, by rotating the electric motor  8  fixedly attached to the fixed arm  2 , the shaft members  7  fixedly attached of the extensions  5   a,    5   b  of the fixed arm  2  is made to turn, and the revolving arm  1  integrally connected to the extensions  4   a,    4   b  is made to rotate in a horizontal plane. The reduction gear may be provided on the side of the knuckle joint  3  depending on the need of the design. 
   Between the extensions  4   a,    4   b  of the revolving arm  1  is interposed a cable guide  12  which includes a center drum  13  and a drum holder  14  that are disposed so as to be rotatable relative to each other. 
   The center drum  13  consists of a drum-shaped member having a relatively small axial dimension and closed at the both axial ends and substantially around the periphery, and is provided with a funnel-like extension tube  15  extending from a part of the circumferential wall of the center drum  13  and having an axial line extending in a tangential direction. An opposite side of the circumferential wall  16  of the center drum  13  is formed with a slot  17  extending over an angular range (of about half a circumference in the case of the illustrated embodiment) corresponding to the moveable angle between the revolving arm  1  and fixed arm  2 . 
   The inner surface of each of the axial end walls  18  of the center drum  13  is provided with a rib  20  that defines a circumferential groove  19  in cooperation with the outer circumferential wall  16 . The circumferential groove  19  and rib  20  extending substantially over the entire circumference except for the part adjacent to the root portion of the extension tube  15 . One end of the rib  20  connects to a snail-like portion  21  formed as a spiral rib extending from the inner surface of the outer peripheral wall  16  toward the center, and the corresponding end of the circumferential groove  19  connects to a guide groove  22  formed as a recess in the snail-like portion  21 . 
   The circumferential groove  19  slidably receives a long shutter plate  23 L and a short shutter plate  23 S which extend from a middle part of the slot  17  in the two opposite directions. The shutter plates  23 L,  23 S are curved along the circumferential groove  19 . In particular, the long shutter plate  23 L is made of thin plastic plate which is flexible enough to freely slide into the guide groove  22  having a smaller radius of curvature than the circumferential groove  19 . 
   The outer peripheral part of each axial end wall  18  of the center drum  13  is provided with an annular shoulder  24  which is reduced in diameter from the outer circumferential wall  16 . This annular shoulder  24  is engaged the central part of the drum holder  14  over an angular range of more than half the entire circumference so that the center drum  13  can rotate around a certain point by being retained by the drum holder  14 . 
   The drum holder  14  is engaged by engagement pieces  25   a,    25   b  formed at an end of the revolving arm  1  adjacent to the knuckle joint and is thereby integrally joined to the revolving arm  1 . The drum holder  14  is provided with a clearance groove  26  in an axially middle part thereof over an angular range of about half the entire circumference to stay clear from the extension tube  15  of the center drum  13 . 
   A cable  27  is drawn from the fixed arm  2  into the center drum  13 . The cable  27  is passed into a circular opening  28  of the extension tube  15  that faces the fixed arm  2 , and extends along the outer circumferential surface of the snail-like portion  21  in the center drum  13 . The cable  27  further extends through a gap defined between the ends  30 ,  31  of the two shutter plates  23 L,  23 S opposing each other in the middle part of the slot  17 , and then into the revolving arm  1 , which consists of a tubular member having a rectangular cross section similarly as the fixed arm  2 . 
   When the knuckle joint  3  is flexed in the direction indicated by arrow A in  FIG. 2  by actuating the electric motor  8 , the cable  27  is allowed to curve by being guided by the outer circumferential surface of the snail-like portion  27 . Because the part of the cable  27  that has been introduced into the circular opening  28  of the extension tube  15  is substantially immobile in a same way as the fixed arm  2 , the center drum  13  which is engaged by the part of the cable  27  extending in the fixed arm  2  by way of the circular opening  28  of the extension tube  15  is therefore integral with the fixed arm  2 , and remains immobile at all times. On the other hand, the drum holder  14  that is engaged by the revolving arm  1  rotates integrally with the revolving arm  1 . 
   The part of the cable  27  extending in the revolving arm  1  is secured to appropriate parts of the revolving arm  1 , for instance by using saddle members so as to maintain a fixed relationship to the revolving arm  1 . Therefore, as the revolving arm  1  rotates, the cable  27  therein moves in the direction indicated by arrow A in  FIG. 2 , and at the same time pushes the end portion  31  of the long shutter plate  23 L. As a result, the long shutter plate  23 L moves into the guide groove  22  formed in the snail-like portion  21  in continuation with the circumferential groove  19 , and progressively opens the surface of the slot  17  opposing the end portion of the revolving arm  1 . At this time, the short shutter plate  23 S on the opposite side maintains a fixed relationship with the circumferential groove  19  until the flexing angle reaches a certain value. 
   In this manner, the cable  27  can freely bend over the angular range defined by the slot  17  formed in the outer circumferential wall  16  of the center drum  13 . Owing to the clearance groove  26  formed in the axially middle part of the drum holder  14 , the extension tube  15  would not interfere with the rotational movement of the drum holder  14 . 
   When the revolving arm  1  has rotated by about 90 degrees, a large part of the clearance groove  26  of the drum holder  14  is received in the space defined between the two extensions  5   a,    5   b  of the fixed arm  2  so that only the outer circumferential wall  16  of the center drum  13  is visible from the exterior of the knuckle joint  3  (in the direction indicated by arrow C in  FIG. 3 ). The part of the cable  27  extending inside the center drum  13  curves along-the outer circumferential surface of the snail-like portion  21 , and the minimum radius of curvature is determined by the curvature of radius of the snail-like portion  21  so that the cable  27  is prevented from being bent at an undesirably sharp angle. Furthermore, because the long shutter plate  23 L is inside the guide groove  22  of the snail-like portion  21 , the long shutter plate  23 L would not interfere with the cable  27  drawn into the center drum  13 . 
   When the revolving arm  1  is flexed even further from the state illustrated in  FIG. 3 , one of the circumferential walls  29  of the clearance groove  26  engages the corresponding end portion  30  of the short shutter plate  23 S and closes the slot  17  so that the interior of the center drum  13  is prevented from becoming visible between the clearance grooves  26  from the direction indicated by arrow C in  FIG. 3 . 
   When the revolving arm  1  is straightened, one of the engagement pieces  25   a  of the revolving arm  1  engaging the drum holder  14  pushes the end portion  31  of the long shutter plate  23 L and closes the slot  17 . When the knuckle joint  3  is straightened, the slot  17  is completely closed by the long shutter plate  23 L. Thereby, dust and other foreign matter are prevented from getting into the center drum  13 , and the interior of the center drum  13  is prevented from becoming visible between the clearance grooves  26  from the direction indicated by arrow B in  FIG. 2 . 
   The cable guide of the present invention described above is equally applicable to other articulated joints such as shoulders and wrists. The relationship between the revolving arm  1  and fixed arm  2  is only relative, and the present invention can also be implemented in a reversed relationship. As for the relationship between each arm  1 ,  2  and cable  27 , as long as one of the arms and cable move in a substantially integral manner, it is possible to allow a relative rotation between the center drum  13  and drum holder  14  without any problem. The present invention is applicable not only to guide electric cables as described above but also to guide flexible tubes for supplying hydraulic and pneumatic media. 
   INDUSTRIAL APPLICABILITY 
   As can be appreciated from the foregoing description, according to the present invention, the dimensions of the part of an articulated joint for passing a cable can be minimized, and the weight of the robot can be reduced while the loss of power transmission can be reduced at the same time. Furthermore, because the cable is favorably concealed, not only the external appearance can be enhanced but also the cable is protected from damages. In particular, because the curvature of the cable at the time of flexing the articulated joint is controlled, the damage to the cable due to repeated bending can be minimized, and the reliability of the robot can be improved. Furthermore, because the relative rotation between the center drum and drum holder is effected without causing any excessive bending of the cable, the need for a powered actuator is eliminated while the structure is simplified at the same time. 
   Although the present invention has been described in terms of a preferred embodiment thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims.