Abstract:
A cable-driven wrist mechanism for a robot arm which executes a rolling motion and a pitching motion, the wrist mechanism comprising: first and second motors provided in the robot arm; a first drive body rotated by the first motor; a second drive body placed above the first drive body, the second drive body being rotated by the second motor about a same rotating axis as that of the first drive body and independently with respect to the first drive body; a first rotary body rotated about a rotating shaft which is perpendicular to the rotating axis of the first drive body and located in a same plane as the rotating axis of the first drive body; a second rotary body having a rotating shaft same as the first rotary body, the second rotary body being symmetric to the first rotary body with respect to the rotating axis of the first drive body; a third rotary body connected to a sub-shaft which perpendicularly branches from the rotating shaft of the first and second rotary bodies, the third rotary body being symmetric to the first and second drive bodies with respect to the rotating shaft of the first and second rotary bodies; a power transmitting unit for transmitting rotating forces from the first and second drive bodies to the first and second rotary bodies, respectively, the power transmitting unit including at least two cables which are connected between each of the first and second drive bodies and each of the first and second rotary bodies in a criss-cross manner to intersect between the drive body and the rotary body, with both ends of each of the two cables being fixed to the drive body and the rotary body, respectively; and rotating force transmitting devices for transmitting rotating forces of the first and second rotary bodies to the third rotary body.

Description:
PRIORITY CLAIM  
       [0001]     This application claims priority from Korean Patent Application No.10-2003-0046140 filed Jul. 8, 2003, which is herein incorporated by reference.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to wrist mechanisms for robot arms, and more particularly, to a cable-driven wrist mechanism for robot arms which smoothly executes wrist motions by use of cables.  
       DESCRIPTION OF THE RELATED ART  
       [0003]     A variety of techniques for wrist mechanisms of robot arms have been proposed. In particular, techniques to design general wrist shapes of robot arms and wrist mechanisms of robot arms for accomplishing specific objects have been proposed in Korean Utility Model Laid-Open Publication NOS. 1996-16644 and 1997-28012, and Korean Patent Laid-Open Publication NOS. 1998-0054687, 1998-0701448, and 1999-0070427.  
         [0004]      FIG. 1  is an exploded sectional view showing a wrist mechanism for robot arms, proposed in Korean Patent Laid-Open Publicatio NO. 1998-0701448. As shown in  FIG. 1 , a wrist unit  210  is coupled to an end of a robot arm  200 . A tool holder  220  is provided at the wrist unit  210  to connect an additional tool, such as a welding device to the wrist unit  210 .  
         [0005]     A first motor  230  is installed in the robot arm  200 , so as to transmit a rotating force of the first motor  230  to the wrist unit  210  via a bevel gear mechanism  250 . The wrist unit  210  thus rotates about a first rotating axis perpendicular to a direction of a motion of the robot arm  200 .  
         [0006]     A second motor  240  is mounted to the wrist unit  210 . The second motor  240  rotates the tool holder  220  which is provided at the wrist unit  210 . The second motor  240  is mounted to the wrist unit  210  in such a manner that the tool holder  220  rotates about a second rotating axis perpendicular to the first rotating axis of the wrist unit  210 . Accordingly, the wrist unit  210 , on which a tool is connected to the tool holder  220 , executes a pitching motion and a rolling motion according to operations of the first and second motors  230  and  240 .  
         [0007]     In general, in the conventional wrist mechanisms for robot arms, a gear mechanism has been used as a power transmitting unit to transmit a rotating force of a motor to a wrist of a robot arm to thereby accomplish a rotating motion of the wrist, such as the wrist unit  210  of  FIG. 1 . However, there are several disadvantageous in the conventional wrist mechanisms for robot arms using the gear mechanism as power transmitting means.  
         [0008]     First, backlash as well as friction is caused in the power transmitting unit using the gear mechanism. The backlash is clearance which a gear can be moved without moving a mating gear when two gears engage with each other. The backlash provides power transmission losses between gears. To reduce the backlash, the contact areas between the gears engaging with each other should be increased. However, if the contact areas between the gears increase to reduce the backlash, the power transmission efficiency of gears reduces, because the friction between the gears increases. Accordingly, problems of the backlash and friction are not easily solved in a power transmitting unit using the gears.  
         [0009]     Second, the power transmitting unit using gears is difficult to be adapted for use with small-sized machines. Generally, at least two drive motors should be provided in the wrist mechanism of the robot arm to secure a proper motion of the wrist. Moreover, the drive motors, which are installed in the wrist mechanism of the robot arm, must have respective rotating axes perpendicular to each other, so as to have different operational ranges. Such constraint in which two or more drive motors should be installed in the wrist mechanism of the robot arm makes it very difficult to apply drive motors to a small-sized wrist mechanism of the robot arm.  
         [0010]     Even though two or more drive motors may be installed in the small-sized wrist mechanism of the robot arm, a plurality of bevel gears must be installed therein, so as to secure the rolling and pitching motions of the wrist of the robot arm. Therefore, the rolling and pitching motions of the wrist of the robot arm may not be accurately executed, due to the large backlash and the large friction which are caused in the power transmitting unit using a number of gears.  
         [0011]     Recently, to solve such problem caused by backlash in the power transmitting unit using the gears, a cable-driven power transmitting unit has been proposed. As an example, a power transmitting unit using cables was disclosed in U.S. Pat. No. 5,046,375, entitled ‘COMPACT CABLE TRANSMISSION WITH CABLE DIFFERENTIAL’. In particular, the power transmitting unit disclosed in the above-mentioned &#39;375 patent executes two-degree of-freedom motion of rolling and pitching by use of the cables. However, the power transmitting unit proposed in the &#39;375 patent is problematic in that the application thereof in a small-sized structure, such as the wrist mechanism of a robot arm is very difficult in the viewpoint of outward appearance or functional operations, because the width of the mechanism must be increased by a horizontal arrangement of two motors in the power transmitting unit.  
       SUMMARY OF THE INVENTION  
       [0012]     Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a cable-driven wrist mechanism for robot arms which occupies a small space, and in which the pitching and rolling motions of the wrist for the robot arms are accurately and smoothly executed.  
         [0013]     Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.  
         [0014]     The above and/or other aspects are achieved by providing a cable-driven wrist mechanism for a robot arm including first and second motors provided in the robot arm, a first drive body rotated by the first motor, a second drive body placed above the first drive body. The second drive body is rotated by the second motor about a same rotating axis as that of the first drive body and independently with respect to the first drive body. The wrist mechanism further includes a first rotary body rotated about a rotating shaft which is perpendicular to the rotating axis of the first drive body and located in the same plane as the rotating axis of the first drive body, a second rotary body having a rotating shaft same as the first rotary body. The second rotary body is symmetric to the first rotary body with respect to the rotating axis of the first drive body. The wrist mechanism further includes a third rotary body connected to a sub-shaft which perpendicularly branches from the rotating shaft of the first and second rotary bodies. The third rotary body is symmetric to the first and second drive bodies with respect to the rotating shaft of the first and second rotary bodies. The wrist mechanism further includes a power transmitting unit for transmitting rotating forces from the first and second drive bodies to the first and second rotary bodies, respectively. The power transmitting unit includes at least two cables which are connected between each of the first and second drive bodies and each of the first and second rotary bodies in a criss-cross manner to intersect between the drive body and the rotary body, with both ends of each of the two cables being fixed to the drive body and the rotary body, respectively. The wrist mechanism further includes a rotating force transmitting devices for transmitting the rotating forces of the first and second rotary bodies to the third rotary body. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0016]      FIG. 1  is an exploded sectional view showing a construction of a conventional wrist mechanism for robot arms;  
         [0017]      FIG. 2  is a perspective view showing a cable-driven wrist mechanism for robot arms, according to an embodiment of the present invention;  
         [0018]      FIG. 3  is a view showing the construction of the cable-driven wrist mechanism of  FIG. 2 ;  
         [0019]      FIG. 4  is a partially enlarged view of the cable-driven wrist mechanism of  FIG. 3 ;  
         [0020]      FIG. 5  is a view showing an operation of the cable-driven wrist mechanism of  FIG. 3  when rolling and pitching motions are simultaneously executed;  
         [0021]      FIG. 6  is a view showing an operation of the cable-driven wrist mechanism of  FIG. 3  when only the rolling motion is executed; and  
         [0022]      FIG. 7  is a view showing an operation of the cable-driven wrist mechanism of  FIG. 3  when only the pitching motion is executed.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings.  
         [0024]     Reference now should be made to the drawings, in which the same reference numerals are used throughout the different drawings to designate the same or similar components.  
         [0025]      FIG. 2  is a perspective view showing a cable-driven wrist mechanism for robot arms, according to an embodiment of the present invention. A robot arm  100  has a pace at an end portion thereof such that a cable-driven wrist mechanism can be installed at the space. Support units  110  vertically installed at the space of the robot arm  100 .  
         [0026]     A first drive body  30  and a second drive body  40  are respectively placed on the end portion of the robot arm  100  to be layered on top of another. Each of the first and second drive bodies  30  and  40  has a rotating axis parallel to a longitudinal direction of the robot arm  100  and can rotate independently with respect to each other. The first and second drive bodies  30  and  40  preferably have the same rotating axis. A first rotary body  50  and a second rotary body  60  are provided on a second rotating shaft  91  between the two support units  110 . The first and second rotary bodies  50  and  60  rotate around the second rotating shaft  91 . The rotating axis of the fist and second drive bodies  30  and  40  are perpendicular to the second rotating shaft  91  of the first and second rotary bodies  50  and  60 .  
         [0027]     In the meantime, the second rotating shaft  91  of the first and second rotary bodies  50  and  60  has a sub-shaft  92  (see,  FIG. 3 ) which perpendicularly branches from a center of the second rotating shaft  91  in such a manner that the second rotating shaft  91  and the sub-shaft  92  define a T-shaped configuration. A third rotary body  70  is rotatably mounted to the sub-shaft  92  of the second rotating shaft  91 .  
         [0028]      FIG. 3  is a view showing an operation of the cable-driven wrist mechanism of  FIG. 2 .  
         [0029]     The robot arm  100  includes first and second motors  10  and  20  therein. The first and second motors  10  and  20  rotate the first and second drive bodies  30  and  40 , respectively.  
         [0030]     Each of the first and second drive bodies  30  and  40  comprises a concentrically stepped body with three drum parts having different diameters, respectively. The first drive body  30  is placed above the first and second motors  10  and  20 . The second drive body  40  is placed above the first drive body  30 . The first drive body  30  has a boss at the lower portion thereof and is hollowed along the rotating axis thereof to form a shaft hole. A first rotating shaft  90  passes through the shaft hole of the first drive body  30  and an end portion of the first rotating shaft  90  is connected to the second drive body  40 . Therefore, that the first body  30  has the same rotating axis as the second drive body  40 .  
         [0031]     The first and second motors  10  and  20  rotate the first and second drive bodies  30  and  40  by use of cables, respectively. To transmit the rotating forces of the first and second motors  10  and  20  to the first and second drive bodies  30  and  40 , respectively, two cables  11  and  12  or  21  and  22  are wound in opposite directions around the output shaft of the first and second motors  10  and  20  and bosses of the first drive body  30  and the first drive shaft  90 , respectively. The second motor  20 , which is provided under the first drive body  30 , cannot directly transmit its rotating force to the second drive body  40 , because the second drive body  40  is placed above the first drive body  30 . Accordingly, the rotating force of the second motor  20  is transmitted to the second drive body  40  via the first rotating shaft  90 . The first rotating shaft  90 , around which the two cables  21  and  21  are wound in opposite directions, preferably has a same diameter as the boss of the first drive body  30  around which the two cables  11  and  12  are wound in opposite directions, since it is preferable for the first drive body  30  to have a same rotating ratio as the second drive body  40  in order to efficiently control the motions of the wrist of the robot arm.  
         [0032]     When the first and second motors  10  and  20  respectively rotate in directions to wind the cables, the rotating forces of the first and second motors  10  and  20  are respectively transmitted to the first and second drive bodies  30  and  40  by the tensions of the cables. However, when the first and second motors  10  and  20  respectively rotate in directions to unwind the cables, the rotating forces of the first and second motors  10  and  20  are not transmitted to the first or second drive body  30  or  40 , since no tensile force is exerted to the cables. Therefore, two cables are required for each of the first and second motors  10  and  20 , so as to accurately transmit the rotating forces, which are generated by the reversible rotations of the first and second motors  10  and  20 , to the first and second drive bodies  30  and  40 , respectively.  
         [0033]     The first and second rotary bodies  50  and  60  are provided at the right and left sides above the first and second drive bodies  30  and  40 , respectively. Each of the first and second rotary bodies  50  and  60  rotate around the second rotating shaft  91 . Each of the first and second rotary bodies  50  and  60  comprises a concentrically stepped body with five drum parts having different diameters, thus having four steps.  
         [0034]     The second rotating shaft  91  of the first and second rotary bodies  30  and  40  has the sub-shaft  92  which branches perpendicularly from the center of the second rotating shaft  91 . The third rotary body  70  is rotatably connected to the sub-shaft  92  of the second rotating shaft  91 . The third rotary body  70  comprises s concentrically stepped body with five drum parts having different diameters, in the same manner as the first and second rotary bodies  50  and  60 . The third rotary body  70  is the body which executes the rolling and pitching motions of the wrist of the robot arm  100 . Additional tools or other equipments are held on an upper end of the third rotary body  70 .  
         [0035]     Two cables  80  and  81  are respectively connected between the first drive body  30  and the first rotary body  50  in a crisscross manner to intersect between the first drive body  30  and the first rotary body  50 . Another two cables  82  and  83  are respectively connected between the second drive body  40  and the second rotary body  60  in a criss-cross manner to intersect between the second drive body  40  and the second rotary body  60 . Thus, the rotating forces of the first and second drive bodies  30  and  40  are respectively transmitted to the first and second rotary bodies  50  and  60  via the cables  80  and  81 ,  82  and  83 .  
         [0036]     In the same manner, two cables  84  and  85 ,  86  and  87  are connected between each of the first and second rotary bodies  50  and  60  and the third rotary body  70 , so that the rotating force of each of the first and second rotary bodies  50  and  60  is transmitted to the third rotary body  70  via the cables  84  and  85 ,  86  and  87 . At this time, the cables are wound around the external surface of the drum parts of the first and second drive and rotary bodies  30 ,  40 ,  50  and  60  and does not interfere with each other, and to be securely fixed to the external surfaces of the drum parts of the first and second drive and rotary bodies  30 ,  40 ,  50  and  60 .  
         [0037]     In a detailed description,  FIG. 4  is a partially enlarged view of the cable-driven wrist mechanism of  FIG. 3 . As shown in  FIG. 4 , the two cables  80  and  81  are respectively connected between the first drive body  30  and the first rotary body  50 .  
         [0038]     The first drive body  30  and the first rotary body  50  are respectively placed, such that the rotating axis of the first drive body  30  is perpendicular to the rotating axis of the first rotary body  50 . Each of the first drive body  30  and the first rotary body  50  comprises a concentrically stepped body with a plurality of drum parts having different diameters. The edges of the drum parts of the first drive body  30  are contiguous to the edges of the drum parts of the first rotary body  50 , but do not contact with each other. Due to the above-mentioned configuration of the drum parts of the first drive and rotary bodies  30  and  50 , the two cables  80  and  81  are not separated from the junction of the first drive body  30  and the first rotary body  50  and, simultaneously, friction does not occur between the first drive body  30  and the first rotary body  50 . Fixing holes  31  and  51  are respectively provided on the first drive body  30  and the first rotary body  50 , so that the ends of the cables  80  and  81  are respectively fixed to the fixing holes  31  and  51 .  
         [0039]     Each of the two cables  80  and  81  is wound around the first rotary body  50  after being wound around the first drive body  30 . At this time, each of two the cables  80  and  81  is wound around the first rotary body  50  in a direction opposite to a direction along which each of the cables  80  and  81  is wound around the first drive body  30 , so that each of the cables  80  and  81  wound around the first drive body  30  and the first rotary body  50  defines a 8-shaped profile. Each of the two cables  80  and  81  is wound in opposite directions around the first drive body  30  and the first rotary body  50  in the same reason as that described for the winding directions of the two cables  11  and  12 ,  21  and  22  which are wound around the output shaft of each of the first and second motors  10  and  20  and each of the boss of the first drive body  30  and the first rotating shaft  90 .  
         [0040]     Due to the above-mentioned winding manner of the cables  80  and  81 , when the first drive body  30  rotates clockwise, the first rotary body  50  rotates clockwise by a tension of the cable  81  which is wound around an upper portion of the first drive body  30 . On the other hand, when the first drive body  30  rotates counterclockwise, the first rotary body  50  rotates counterclockwise by a tension of the cable  80  which is wound around a lower portion of the first drive body  30 .  
         [0041]     Accordingly, the rotating force of the first drive body  30  is transmitted via the two cables  80  and  81  to the first rotary body  50  of which the rotating axis is perpendicular to the rotating axis of the first drive body  30 . The transmission of the rotating forces between the second drive and rotary bodies  40  and  50 , between the first and third rotary bodies  50  and  70 , and between the second and third rotary bodies  50  and  70  is performed in the same principle as the above-mentioned transmission of the rotating force between the first and second drive and rotary bodies  30  and  50 . In the meantime, each of the cables  80  and  81  is not separated from the junction of the first drive body  30  and the first rotary body  50 , because the junction has a gap of which is smaller than a diameter of each of the cables  80  and  81 . (generally, the gap of the junction is formed to be twenty percentage or less than the diameter of each of the cables  80  and  81 .)  
         [0042]      FIG. 5  is a view showing an operation of the cable-driven wrist mechanism of  FIG. 3  when the rolling and pitching motions are simultaneously executed.  FIG. 6  is a view showing an operation of the cable-driven wrist mechanism of  FIG. 3  when only the rolling motion is executed.  FIG. 7  is a view of an operation of the cable-driven wrist mechanism of  FIG. 3  when only the pitching motion is executed.  
         [0043]     In the cable-driven wrist mechanism of the present invention, the wrist of the robot arm  100  executes a variety of motions according to rotating directions and operational states of each of the first and second motors  10  and  20 . As an example, in case that either of the first and second motors  10  and  20  is rotated, the rolling motion and the pitching motion of the wrist of the robot arm are simultaneously executed. As shown in  FIG. 5 , only the first motor  10  may be rotated. At this time, the second motor  20  is stopped, thus the second rotary body  60  is stopped. When the first motor  10  is rotated clockwise, the first drive body  30  is rotated by the tension of the cable  12  in the same direction as the rotation of the first motor  10 . Continuously, the third rotary body  70  is rotated counterclockwise by a tension of the cable  85  which is connected between the first rotary body  50  and the third rotary body  70 . At this time, because the second rotary body  60  is stopped, any rotating force is not transmitted from the second rotary body  60  to the third rotary body  70 . Accordingly, the cable  86  is wound around the external surface of the third rotary body  70 , and the cable  87  is unwound from the external surface of the third rotary body  70 , so that the wrist of the robot arm  100  executes the rolling motion in that the third rotary body  70  is rotated counterclockwise, and simultaneously, executes the pitching motion in that the third rotary body  70  is swung clockwise around the second rotating shaft  91  at a predetermined angle, in response to the rolling motion of the third rotary body  70 .  
         [0044]     In the above-mentioned consecutive motions, the wrist of the robot arm simultaneously executes the pitching motion in which the third rotary body  70  is swung around the second rotating shaft  91 , as well as the rolling motion. The above-mentioned motions of the third rotary body  70  are also executed by an operation of only the second motor  20 . When the second motor  20  is rotated clockwise, both the rolling and pitch motions of the third rotary body  70  are executed counterclockwise, opposite from the directions of the rolling and pitch motions of the third rotary body  70  when the first motor  10  is rotated clockwise.  
         [0045]     When the first and second motors  10  and  20  are simultaneously operated, the third rotary body  70  will execute the following motions.  
         [0046]     In case that the first motor  10  and the second motor  20  are simultaneously rotated at the same angular speed and in the same rotating direction (See.  FIG. 6 ).  
         [0047]     As shown in  FIG. 6 , both the first and second motors  10  and  20  are rotated counterclockwise. Each of the first and second drive bodies  30  and  40  is thus rotated in the same direction (counterclockwise) as the rotation of each of the first and second motors  10  and  20  via each of the cables  11  and  21  which is wound around the output shaft of each of the first and second motors  10  and  20 . At this time, only the cables  80  and  83  are tense by the rotations of the first and second drive bodies  30  and  40 . Accordingly, the first rotary body  50  is rotated counterclockwise, and the second rotary body  60  is rotated clockwise.  
         [0048]     Sequentially, the third rotary body  70  is rotated via the cables  84 ,  85 ,  86  and  87  by the rotations of the first and second rotary bodies  50  and  60 . At this time, the first rotary body  50  is rotated counterclockwise, so that only the cable  84  is tense, and the second rotary body  60  is rotated clockwise, so that only the cable  86  is tense. Therefore, the third rotary body  70  is rotated clockwise.  
         [0049]     After all, two pitching forces, in that the first and second rotary bodies  50  and  60  pitch the third rotary body  70 , are counterbalanced by each other, and two rolling forces, in that the first and second rotary bodies  50  and  60  rotate the third rotary body  70 , are superposed in the same direction. Therefore, the third rotary body  70  executes only the rolling motion when the first motor  10  and the second motor  20  are rotated at the same angular speed and in the same rotating direction.  
         [0050]     In case that the first motor  10  and the second motor  20  are rotated at the same angular speed, but the rotating direction of the first motor  10  is opposite to the rotating direction of the second motor  20  (See.  FIG. 7 ).  
         [0051]     As shown in  FIG. 7 , the first motor  10  is rotated counterclockwise, and the second motor  20  is rotated clockwise. The first drive body  30  is thus rotated counterclockwise via the cable  11 , and the second drive body  40  is thus rotated clockwise via the cable  22 . In the above state, the first and second rotary bodies  50  and  60  respectively rotates counterclockwise via the cables  80  and  82 .  
         [0052]     In the meantime, the rotating force, which is applied clockwise from the first rotary body  50  to the third rotary body  70  via the cable  84 , and the rotating force, which is applied counterclockwise from the second rotary body  60  to the third rotary body  70  via the cable  87 , are simultaneously transmitted to the third rotary body  70 . Therefore, the two rotating forces, which are oppositely applied to the third rotary body  70 , are counterbalanced by each other, so that the third rotary body  70  is stopped on the sub-shaft  92  of the second rotating shaft  91 , without executing any rolling motion.  
         [0053]     However, the cables  80  and  82  are respectively tense by the rotations of the first and second rotary bodies  50  and  60  in the same direction (counterclockwise). Therefore, the third rotary body  70  is swung counterclockwise around the second rotating shaft  91  by the tensions of the cables  80  and  82  which are caused by the rotations of the first and second rotary bodies  50  and  60 . That is, the third rotary body  70  executes only the pitching motion, if the first motor  10  and the second motor  20  are rotated at the same angular speed and in the rotating directions opposite to the second motor  20 .  
         [0054]     In a brief description, when the first motor  10  and the second motor  20  are rotated at the same angular speed and in the same rotating direction, the wrist of the robot arm  100  executes only the rolling motion. When the first motor  10  and the second motor  20  are rotated at the same angular speed and in the opposite rotating directions, the wrist of the robot arm  100  executes only the pitching motion.  
         [0055]     The above-mentioned motions of the wrist of the robot arm  100  are expressed by the following equations. The desired rolling and pitching motions of the wrist of the robot arm  100  can be designed by a linear combination of the following equations. 
 
rolling angle (θ r )=(θ1+θ2)/2 n  
 
pitching angle (θ p )=(θ1−θ2)/2 n  
 
         [0056]     In the equations, the rotating angles of the first and second motors  10  and  20  are respectively represented as variable factors θ1 and θ2. The reduction ratio between the first and second motors  10  and  20  and the third rotary body  70  is represented as a variable factor n. In the meantime, each of the cables, which is connected between each of the first and second drive bodies and each of the first and second rotary bodies or between each of the first and second rotary bodies and the third rotary body, is preferably wound around predetermined portions of the bodies which have the same diameter, such that the drive bodies and the rotary bodies are rotated at the same rotating ratio.  
         [0057]     In the embodiment of the present invention, each of the drive bodies and the rotary bodies comprises concentrically stepped body with three or five drum parts having different diameters. However, the number of the drum parts of the stepped bodies may reduce to simplify the manufacturing process of the drive and rotary bodies. That is, each of the drive bodies may comprise a concentrically stepped body with two drum parts having different diameters, and each of the rotary bodies may comprise a concentrically stepped body with four drum parts having different diameters. In addition, each of all the drive bodies and the rotary bodies may comprise a frusto-conical drum having a tapered surface.  
         [0058]     As described above, the present invention provides a cable-driven wrist mechanism for robot arms which accurately and smoothly executes the rolling and pitching motions of the wrist of the robot arm while solving the problems of a backlash in addition to friction which has been experienced in conventional wrist mechanisms for robot arms. Furthermore, the cable-driven wrist mechanism of the present invention reduces the production costs thereof by use of cables, in place of expensive gears.  
         [0059]     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.