Abstract:
The robot arm having a weight compensation mechanism has a first rotation member and a second rotation member which are respectively capable of making two-DOF rotation, a first rotation of the first rotation member is yaw rotation, and a second rotation of the first rotation member is pitch rotation perpendicular to the first rotation, a third rotation and a fourth rotation of the second rotation member are respectively pitch rotation and roll rotation, and the robot arm comprises a single-DOF gravity compensator connected to the first rotation member or the second rotation member and offsetting the gravity caused by weight of the first rotation member or the second rotation member by using an elastic force of an elastic member.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to Korean Patent Application No. 10-2012-0005264, filed on Jan. 17, 2012, and all the benefits accruing therefrom under 35 U.S.C. §119, the contents of which in its entirety are herein incorporated by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    The present disclosure relates to a robot arm having a weight compensation mechanism, and more particularly, to a robot arm having a weight compensation mechanism, which offsets the influence of gravity generated by a load of a multi-joint link mechanism. 
         [0004]    2. Description of the Related Art 
         [0005]    Recently, various kinds of robots are being developed for ensuring convenient life environments and helping works at industrial spots. In particular, robot arms utilized in various industrial fields such as painting and welding are being developed. For such an industrial multi-joint robot arm, it is important to generate so high torque to carry and support a heavy article. 
         [0006]    The multi-joint robot arm receives a load torque due to its weight or a weight of a handled article, and this load torque gives a direct influence when designing a capacity of a driver such as a driving motor. In particular, in regard of the load applied to a driving motor, a torque component due to the weight of the robot arm occupies a considerable portion. 
         [0007]    In case of determining a capacity of a driver of a conventional robot arm, since the gravity torque generated by the robot arm as well as the torque generated by a handled article should be considered, a power source for driving the robot arm should be designed to have a great capacity. 
         [0008]    In addition, even though a simple idea having the concept of theoretically compensating the gravity caused by the weight of the robot arm has been proposed, a mechanism which practically applies such a theory has not been developed. 
         [0009]    In this regard, the applicant has proposed a weight compensation mechanism in Korean Unexamined Patent No. 2011-0123012 and Korean Patent Application No. 2011-0045658, but the configuration and application of the weight compensation mechanism still have much room for improvement. 
       RELATED LITERATURES 
     Patent Literature 
       [0000]    
       
         Patent Literature 1: Korean Unexamined Patent No. 2011-0123012 
         Patent Literature 2: Korean Patent Application No. 2011-0045658 
       
     
       SUMMARY 
       [0012]    The present disclosure is directed to providing a robot arm having a weight compensation mechanism, which may offset the influence of gravity caused by the weight of a link mechanism composed of multi Degree Of Freedom (DOF) joints. 
         [0013]    In one aspect, there is provided a robot arm having a weight compensation mechanism, the weight compensation mechanism being installed at the robot arm having a first rotation member and a second rotation member which are respectively capable of making two-DOF rotation, wherein a first rotation of the first rotation member is yaw rotation, and a second rotation of the first rotation member is pitch rotation perpendicular to the first rotation, wherein a third rotation and a fourth rotation of the second rotation member are respectively pitch rotation and roll rotation, and wherein the robot arm comprises a single-DOF gravity compensator connected to the first rotation member or the second rotation member and offsetting the gravity caused by weight of the first rotation member or the second rotation member by using an elastic force of an elastic member. 
         [0014]    One end of the single-DOF gravity compensator connected to the first rotation member may be connected to an output link of the first rotation, and the other end of the single-DOF gravity compensator may be fixed by the first rotation member serving as an output member of the first rotation and the second rotation. 
         [0015]    The third rotation and the fourth rotation of the second rotation member may be restrained by a plurality of differential bevel gears. 
         [0016]    Among the plurality of differential bevel gears, a single fixed bevel gear may be provided on a third rotary shaft, and the other rotation bevel gears may be attached on a fourth rotary shaft to be freely rotatable. 
         [0017]    Among the differential bevel gears, a fixed bevel gear may be restricted by the second rotation of the first rotation member to move in parallel to the first rotation member serving as an output member of the first rotation and the second rotation. 
         [0018]    Among the differential bevel gears, a single fixed bevel gear may be fixed to a rotating pulley which rotates on a third rotary shaft. 
         [0019]    The robot arm may further include a rotating pulley rotating on a third rotary shaft and a fixed pulley located on a second rotary shaft and fixed to an output member of the first rotation. 
         [0020]    The robot arm may further include a device for identically rotating the rotating pulley and the fixed pulley. 
         [0021]    The device for identically rotating the rotating pulley and the fixed pulley may include timing belt pulleys respectively for the fixed pulley located at the second rotary shaft and the rotating pulley rotating on the third rotary shaft and connect the timing belt pulleys by a timing belt. 
         [0022]    The device for identically rotating the rotating pulley and the fixed pulley may include wire pulleys respectively for the fixed pulley located at the second rotary shaft and the rotating pulley rotating on the third rotary shaft and connect the wire pulleys by a wire. 
         [0023]    The device for identically rotating the rotating pulley and the fixed pulley may include rotation units respectively provided on the circumference of the fixed pulley located at the second rotary shaft and the circumference of the rotating pulley rotating on the third rotary shaft and connect the rotation units by a link. 
         [0024]    One end of the single-DOF gravity compensator connected to the second rotation member may be fixed to a cam plate disposed at an outer side of a rotation bevel gear, and the other end of the single-DOF gravity compensator may be disposed in the second rotation member serving as an output member of the third rotation and the fourth rotation. 
         [0025]    The single-DOF gravity compensator connected to the second rotation member may include: a spring having one end fixed to a spring fixing unit fixed to the second rotation member and the other end fixed to a sliding member moving along a guide bar attached to the spring fixing unit; and a wire having one end fixed to a rotatable connector provided at a cam plate and the other end fixed to the wire fixing unit fixed at the link member via an idle pulley fixed to the second member and a pulley provided in the sliding member, wherein the spring may be compressed when the sliding member moves toward the spring fixing unit. 
         [0026]    The single-DOF gravity compensator connected to the first rotation member may include: a spring having one end fixed to the first rotation member rotating on the first rotary shaft and the second rotary shaft and the other end fixed to a sliding member moving along a guide bar attached to the spring fixing unit; and a wire having one end connected to a rotatable connector provided at the first rotation output link and the other end connected to the wire fixing unit fixed at the link member via an idle pulley fixed to the first member and a pulley provided in the sliding member, wherein the spring may be compressed when the sliding member moves toward the spring fixing unit. 
         [0027]    The single-DOF gravity compensator connected to the first rotation member may include: a spring having one end fixed in a first rotation output link and the other end fixed to a sliding member moving along a guide bar attached to the spring fixing unit; and a wire having one end connected to a rotatable connector provided at the first rotation member rotating on the second rotary shaft and the other end connected to a wire fixing unit fixed to a link member via an idle pulley fixed at the first rotation output link and a pulley provided in the sliding member, wherein the spring may be compressed when the sliding member moves toward the spring fixing unit. 
         [0028]    The pulley may be provided at a pulley fixing unit having a screw, be connected to the sliding member by using a bolt, and adjust a tension of the wire by rotating the bolt. 
         [0029]    Four motors may be independently connected to make the first rotation, the second rotation, the third rotation and the fourth rotation of the first rotation member and the second rotation member. 
         [0030]    The plurality of differential bevel gears may include a rotation bevel gear and a fixed bevel gear provided in the second rotation member. 
         [0031]    The rotation bevel gear may be fixed on a third rotary shaft to be freely rotatable, and the other fixed bevel gear may be fixed on a fourth rotary shaft to the second rotation member which is an output of the third rotation and the fourth rotation. 
         [0032]    The rotation bevel gear may be connected to a plurality of second rotating pulleys rotating on a third rotary shaft, provided in the second rotation member. 
         [0033]    The robot arm may further include a plurality of first rotating pulleys rotating on a second rotary shaft and a plurality of second rotating pulleys rotating on a third rotary shaft. 
         [0034]    A plurality of single-DOF gravity compensators may be connected to a plurality of first rotating pulleys which rotate on the second rotary shaft. 
         [0035]    The robot arm may further include a device for identically rotating the first rotating pulley and the second rotating pulley. 
         [0036]    The device for identically rotating the first rotating pulley and the second rotating pulley may include timing belt pulleys respectively for the first rotating pulley and the second rotating pulley and connect the timing belt pulleys by a timing belt. 
         [0037]    The device for identically rotating the first rotating pulley and the second rotating pulley may include wire pulleys respectively for the first rotating pulley and the second rotating pulley and connect the wire pulleys by a wire. 
         [0038]    The device for identically rotating the first rotating pulley and the second rotating pulley may include rotation units respectively provided on the circumference of the first rotating pulley and the circumference of the second rotating pulley and connect the rotation units by a link. 
         [0039]    The robot arm having a weight compensation mechanism according to the present disclosure may greatly reduce the power of a power source used for driving the robot arm and various link members. Further, the reduced power brings the decrease of weight of the entire robot arm and the increase of driving efficiency, which results in energy saving. 
         [0040]    In addition, since the robot arm having a weight compensation mechanism according to the present disclosure requires a relatively small driving force, production costs are reduced, which allows the development of products with price competitiveness. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]    The above and other aspects, features and advantages of the disclosed exemplary embodiments will be more apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0042]      FIG. 1  is a schematic view showing a robot arm having a weight compensation mechanism according to an embodiment of the present disclosure; 
           [0043]      FIG. 2  is a schematic view showing a single-Degree Of Freedom (DOF) gravity compensator provided at the weight compensation mechanism according to an embodiment of the present disclosure; 
           [0044]      FIG. 3  is a diagram showing a spring unit employed in the single-DOF gravity compensator; 
           [0045]      FIG. 4  is a partial cross-sectional view showing a robot arm having a weight compensation mechanism according to an embodiment of the present disclosure; and 
           [0046]      FIG. 5  is a partial cross-sectional view showing a robot arm having a weight compensation mechanism according to another embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION OF MAIN ELEMENTS 
       [0000]    
       
         
           
               100 : robot arm 
               101 : first rotation member 
               102 : second rotation member 
               103 : base 
               110 : first rotary shaft 
               111 : second rotary shaft 
               112 : third rotary shaft 
               113 : fourth rotary shaft 
               201 ,  202 : single-DOF gravity compensator 
               210 ,  220 : rotation member 
               211 ,  221 : rotary shaft 
               212 ,  222 : wire connector 
               213 ,  223 : idle pulley 
               214 ,  224 : wire 
               215 ,  225 : spring 
               216 ,  226 : base 
               301 : spring fixing unit 
               302 : guide bar 
               303 : sliding member 
               304 : bolt 
               305 : spring 
               306 : pulley fixing unit 
               307 : pulley 
               308 : wire 
               410 : fixed pulley 
               411 : rotating pulley 
               412 ,  544 ,  545 : timing belt 
               420 ,  510 : differential bevel gear frame 
               430 ,  521 ,  531 : fixed bevel gear 
               431 ,  520 ,  530 : rotation bevel gear 
               432 : cam plate 
               440 : first rotation output member 
               540 ,  541 : second rotating pulley 
               542 ,  543 ,  546 : first rotating pulley 
           
         
       
     
       DETAILED DESCRIPTION 
       [0081]    Hereinafter, a robot arm having a weight compensation mechanism according to an embodiment of the present disclosure will be described in detail with reference to the accompanying drawings. 
         [0082]      FIG. 1  is a schematic view showing a robot arm having a weight compensation mechanism according to an embodiment of the present disclosure. 
         [0083]    Referring to  FIG. 1 , a robot arm  100  having a weight compensation mechanism according to an embodiment of the present disclosure includes a first rotation member  101  and a second rotation member  102  serving as a framework of the robot arm  100 , and the first rotation member  101  is connected to a fixed base  103 . 
         [0084]    A first rotary shaft  110  and a second rotary shaft  111  intersecting each other are formed between the base  103  and the first rotation member  101 , and a third rotary shaft  112  and a fourth rotary shaft  113  are formed between the first rotation member  101  and the second rotation member  102 , respectively. 
         [0085]    Therefore, in this embodiment, the first rotation member  101  of the robot arm  100  may make two-DOF rotation on the first rotary shaft  110  and the second rotary shaft  111  intersecting each other, and the second rotation member  102  of the robot arm  100  may make two-DOF rotation on the third rotary shaft  112  and the fourth rotary shaft  113  intersecting each other. 
         [0086]    Hereinafter, operations of the robot arm  100  will be described. A first motor (not shown) for making a first rotation on the first rotary shaft  110  is mounted to the base  103 , and a first output link (not shown) is connected to an output shaft of the first motor. In addition, a second motor is mounted to the first output link to make a second rotation on the second rotary shaft  111 , and the first rotation member  101  is connected to an output shaft of the second motor. Therefore, if the first motor or the second motor rotates, the first rotation member  101  rotates on the first rotary shaft  110  or the second rotary shaft  111 . 
         [0087]    In addition, a third motor (not shown) is mounted to the first rotation member  101  to make a third rotation on the third rotary shaft  112 , and a second output link (not shown) is connected to an output shaft of the third motor. Moreover, a fourth motor (not shown) is mounted to the second output link to make a fourth rotation on the fourth rotary shaft  113 , and the second rotation member  102  is connected to an output shaft of the fourth motor. Therefore, if the third motor and the fourth motor operate, the second rotation member  102  rotates on the third rotary shaft  112  and the fourth rotary shaft  113 . 
         [0088]      FIG. 2  is a schematic view showing a single-Degree Of Freedom (DOF) gravity compensator provided at the weight compensation mechanism according to an embodiment of the present disclosure. 
         [0089]    Referring to  FIG. 2 , the single-DOF gravity compensator  201 ,  202  includes a spring  215 ,  225  for storing elastic energy, a wire  214 ,  224 , an idle pulley  213 ,  223  and a wire connector  212 ,  222 . 
         [0090]    First, the single-DOF gravity compensator  201  where the spring  215  is provided at the rotation member  210  as shown in Portion (A) of  FIG. 2  will be described. One end of the spring  215  is fixed to the rotation member  210 , and the other end is connected to the wire  214 . The wire  214  is fixed via the idle pulley  213  fixed at the rotation member  210  to the wire connector  212  rotatably fixed at the base  216 . 
         [0091]    Subsequently, the single-DOF gravity compensator  202  where the spring  225  is provided at the base  226  as shown in Portion (B) of  FIG. 2  will be described. One end of the spring  225  is fixed to the base  226 , and the other end is connected to the wire  224 . The wire  224  is fixed via the idle pulley  223  fixed at the base  226  to the wire connector  222  rotatably fixed at the rotation member  220 . 
         [0092]    Operations of the single-DOF gravity compensators  201 ,  202  will be described. If the rotation member  210 ,  220  rotates on the rotary shaft  211 ,  221 , the wire  214 ,  224  is pulled, and accordingly the spring  215 ,  225  stretches to generate elastic energy. 
         [0093]    Even though a tension spring is applied in  FIG. 2 , a compression spring may also be used. 
         [0094]      FIG. 3  is a diagram showing a spring unit employed in the single-DOF gravity compensator. 
         [0095]    Referring to  FIG. 3 , one end of a plurality of springs  305  is fixed to a spring fixing unit  301  fixed to the base, and the other end of the plurality of springs  305  is fixed to a sliding member  303  moving along a guide bar  302  installed at the spring fixing unit  301 . Therefore, if the sliding member  303  moves toward the spring fixing unit  301 , the spring  305  is compressed. 
         [0096]    One end of the steel wire  308  is fixed to the wire connector  212 ,  222  shown in  FIG. 2  and fixed to the spring fixing unit  301  via the idle pulley  213 ,  223  and the pulley  307 . 
         [0097]    The pulley  307  is fixed to the pulley fixing unit  306 . The pulley fixing unit  306  has a screw, and the sliding member  303  has a through hole so that a bolt  304  is inserted into the sliding member  303  and couples the pulley fixing unit  306  and the pulley fixing unit  306 . Therefore, if the bolt  304  is fastened, the sliding member  303  moves toward the spring fixing unit  301 , and so the spring  305  is compressed and adjusts a tension. 
         [0098]    Even though this embodiment adopts a coil spring, the present disclosure is not limited thereto, and the coil spring may be modified into various elastic members such as a leaf spring. In addition, even though two guide bars  302  and two springs  305  are installed in this embodiment, the number of these components may be changed in various ways. Moreover, even though a steel wire  308  is used in this embodiment to make a spring displacement, a coil spring may be provided in a cylinder so that one end of the cylinder is connected to a rotatable connector  211 ,  221  and the other end is rotatably fixed to the idle pulley  213 ,  223 . 
         [0099]    Even though a spring, a wire and a pulley is used in this embodiment to compensate gravity, the present disclosure is not limited thereto, and various kinds of single-DOF gravity compensators may be alternatively used, for example a single-DOF gravity compensator having cam profiles at inner and outer sides thereof. In an alternative configuration, one end of the single-DOF gravity compensator may be connected to the rotation member  210 ,  220 , and the other end serving as an output may be fixed to the base  216 ,  226 . In other case, one end of the single-DOF gravity compensator may be fixed to the base  216 ,  226 , and the other end serving as an output may be fixed to the rotation member  210 ,  220 . 
         [0100]      FIG. 4  is a partial cross-sectional view showing a robot arm having a weight compensation mechanism according to an embodiment of the present disclosure, which is depicted based on an output link of the first rotation. In other words, components corresponding to the base  103  and the first rotary shaft  110  are not depicted in  FIG. 4 . 
         [0101]    Referring to  FIG. 4 , the robot arm includes a differential bevel gear composed of a fixed bevel gear  430  and a rotation bevel gear  431 , a differential bevel gear frame  420 , a cam plate  432 , a fixed pulley  410 , a rotating pulley  411 , and single-DOF gravity compensators  201 ,  202 . 
         [0102]    The differential bevel gear frame  420  is rotatably provided on the third rotary shaft  112 . The fixed bevel gear  430  is provided in the differential bevel gear frame  420  and is installed to be rotatable on the third rotary shaft  112 . In addition, the rotation bevel gear  431  is provided in the differential bevel gear frame  420  and is installed to be rotatable on the fourth rotary shaft  113 . 
         [0103]    Both cam plates  432  are connected and fixed to the rotation bevel gear  431  along a shaft formed through holes respectively formed in the second rotation member  102  and the differential bevel gear frame  420 . Therefore, the rotation bevel gear  431  and the cam plate  432  rotate identically and may freely rotate with respect to the second rotation member  102  and the differential bevel gear frame  420 . 
         [0104]    The wire connector  212  is provided at the side of the cam plate  432 , and the wire  214  of the single-DOF gravity compensator  201  is connected thereto. At this time, one end of the single-DOF gravity compensator  201  is fixed to the wire connector  212 , the other end is located in the second rotation member  102 , and the spring fixing unit  301  is provided at the second rotation member  102 . 
         [0105]    The fixed pulley  410  is provided to one end of the first rotation member  101 , and the rotating pulley  411  is provided to the other end of the first rotation member  101 , respectively. In addition, the fixed pulley  410  is disposed on the second rotary shaft  111  and fixed to an output member  440  of the first rotation, and the rotating pulley  411  is provided to be rotatable on the third rotary shaft  112 . Here, even though it is illustrated as if the fixed pulley  410  and the output member  440  are separated, the fixed pulley  410  is fixed to one surface of the output member  440 . 
         [0106]    The fixed pulley  410  and the rotating pulley  411  are configured to rotate identically. In other words, the rotating pulley  411  located at the other end of the first rotation member  101  may rotate by the fixed pulley  410  located at one end of the first rotation member  101 . In other case, the rotating pulley  411  located at the other end of the first rotation member  101  may rotate by the first rotation member  101  in a state where the fixed pulley  410  located at one end of the first rotation member  101  is fixed. For this, timing belt teeth are respectively provided on the circumferences of the fixed pulley  410  and the rotating pulley  411  located at both ends of the first rotation member  101 , and the fixed pulley  410  and the rotating pulley  411  provided at both sides of the first rotation member  101  as shown in  FIG. 4  are connected by a timing belt  412 . 
         [0107]    Even though the fixed pulley  410  and the rotating pulley  411  provided at both sides of the first rotation member  101  are connected by a timing belt in this embodiment, it is also possible that the pulleys  410 ,  411  have wire grooves to make a connection by using a steel wire. In addition, rotation units may be configured at sides of the pulleys  410 ,  411 , respectively, and connected by a link. 
         [0108]    The fixed bevel gear  430  of the differential bevel gear is fixed to the rotating pulley  411 . Therefore, the fixed bevel gear  430  rotates identical to the rotating pulley  411 . 
         [0109]    The wire connector  222  is provided at the side of the first rotation member  101 , and the wire  224  of the single-DOF gravity compensator  202  is connected thereto. At this time, one end of the single-DOF gravity compensator  202  is fixed to the wire connector  222 , the other end is located in the output member  440  of the first rotation, and the spring fixing unit is provided at the output member  440  of the first rotation. 
         [0110]    An example of a motor arrangement has been described with reference to  FIG. 1 , and another example of a motor arrangement will be described below even though it is not depicted in the figures. 
         [0111]    The first rotation and the second rotation of the first rotation member  101  on the first rotary shaft  110  and the second rotary shaft  111  are respectively driven by the first motor and the second motor. In addition, the third rotation and the fourth rotation of the second rotation member  102  on the third rotary shaft  112  and the fourth rotary shaft  113  are driven by the third motor and the fourth motor. Here, the fourth motor is provided between the differential bevel gear frame  420  and the second rotation member  102 . In addition, four motors may be independently provided to make the first rotation, the second rotation, the third rotation and the fourth rotation of the first rotation member  101  and the second rotation member  102 . 
         [0112]    Moreover, for the connection of the third motor and the fourth motor, it is possible that a gear is provided at the circumference of the cam plate  432  and is connected to a pinion gear, a timing belt pulley fixed to the pinion gear is installed, and then the third motor fixed to the second rotation member  102  is connected to a timing belt pulley fixed to the shaft of the fourth motor by means of a timing belt, thereby connecting the third motor and the fourth motor. The power transmission method for driving the cam plate  432  by using the third motor and the fourth motor as described in this embodiment is just an example, and the present disclosure is not limited thereto. 
         [0113]    Hereinafter, operations of the robot arm shown in  FIG. 4  will be described. 
         [0114]    First, if the first rotary shaft  110  of  FIG. 1  is parallel to the gravity direction, even though the first rotation member  101  makes the first rotation, the torque applied to the first rotary shaft  110  does not change. Therefore, the compensation of gravity with respect to the first rotation of the first rotation member  101  will not be considered. 
         [0115]    Meanwhile, if the first rotation member  101  rotates on the second rotary shaft  111 , the wire connector  222  moves to pull the wire  224 . The wire  224  pulls the sliding member  303  toward the output member  440  by means of the pulley  307  to compress the spring  305 . The compressed force of the spring  305  offsets the gravity caused by the weight of the robot arm  100 . Therefore, even though the robot arm  100  rotates by a predetermined angle on the second rotary shaft  111 , the robot arm  100  does not move downwards by gravity any more and may maintain its posture like a gravity-free state. Here, the rotating pulley  411  moves in parallel to the output member  440  of the first rotation while rotating in a direction opposite to the rotating direction of the first rotation member  101 . 
         [0116]    Next, the first rotation member  101  is fixed, and the second rotation member  102  rotates on the third rotary shaft  112 . In this case, the rotating pulley  411  is fixed, and the fixed bevel gear  430  is also fixed identically. Since the fourth rotary shaft  113  is formed through the second rotation member  102 , the differential bevel gear frame  420  rotates identical to the second rotation member  102 . In this state, the right and left rotation bevel gears  431  rotate in opposite directions, and relatively rotate with respect to the second rotation member  102 . In addition, the right and left cam plates  432  also make the same relative rotations with respect to the second rotation member  102 . Accordingly, the wire connector  212  also moves to compress the spring  305  while pulling and releasing the wire  214 . The compressed force of the spring  305  offsets the gravity caused by the weight of the robot arm  100 . Therefore, even though the robot arm  100  rotates by a predetermined angle on the third rotary shaft  112 , the robot arm  100  does not move downwards by gravity any more and may maintain its posture like a gravity-free state. 
         [0117]    Next, the first rotation member  101  is fixed, and the second rotation member  102  rotates on the fourth rotary shaft  113 . In this case, the rotating pulley  411  is fixed, and the fixed bevel gear  430  is also fixed identically. Since the fourth rotary shaft  113  is formed through the second rotation member  102 , the differential bevel gear frame  420  is fixed, and only the second rotation member  102  rotates. Therefore, the right and left rotation bevel gears  431  are fixed, and therefore, the right and left cam plates  432  are also fixed identically. This means that the wire connector  212  is fixed. The second rotation member  102  relatively rotates with respect to the rotation bevel gear  431  or the cam plate  432  by means of the driving of the fourth motor mounted between the differential bevel gear frame  420  and the second rotation member  102 . 
         [0118]    Accordingly, the idle pulley  213  fixed to the second rotation member  102  moves to pull the wire  214  and compress the spring  305 . The compressed force of the spring  305  offsets the gravity caused by the weight of the robot arm  100 . Therefore, even though the robot arm  100  rotates by a predetermined angle on the fourth rotary shaft  113 , the robot arm  100  does not move downwards by gravity any more and may maintain its posture like a gravity-free state. 
         [0119]    Meanwhile, the elastic modulus of the spring  305  may be appropriately designed in consideration of the weight, length or the like of the robot arm  100 . 
         [0120]      FIG. 5  is a partial cross-sectional view showing a robot arm having a weight compensation mechanism according to another embodiment of the present disclosure, depicted based on an output link of the first rotation. In other words, components corresponding to the base  103  and the first rotary shaft  110  are not depicted in  FIG. 5 . 
         [0121]    Referring to  FIG. 5 , the robot arm includes two differential bevel gears composed of fixed bevel gears  521 ,  531  and rotation bevel gears  520 ,  530 , a differential bevel gear frame  510 , first rotating pulleys  542 ,  543 ,  546 , second rotating pulleys  540 ,  541  and three single-DOF gravity compensators  202 . 
         [0122]    The differential bevel gear frame  510  is provided to be rotatable on the third rotary shaft  112 . The fixed bevel gears  521 ,  531  are provided in the differential bevel gear frame  510 , are installed along the fourth rotary shaft  113 , and are fixed to the second rotation member  102 . In addition, the rotation bevel gears  520 ,  530  are provided in the differential bevel gear frame  510  and are installed to be rotatable on the third rotary shaft  112 . The differential bevel gear frame  510  has a hole and a bearing in the direction of the third rotary shaft  112 , and the shaft of the first rotation bevel gear  520  is connected to the second rotating pulley  540  through the differential bevel gear frame  510 . In addition, the shaft of the first rotation bevel gear  520  has a hole and a bearing, and the shaft of the second rotation bevel gear  530  is connected to the second rotating pulley  541  through the shaft of the first rotation bevel gear  520 . Therefore, the first rotation bevel gear  520  and the second rotation bevel gear  530  may make free rotation with respect to the differential bevel gear frame  420  and the first rotation member  101 . 
         [0123]    The second rotating pulleys  540 ,  541  are respectively fixed to the shafts of the first rotation bevel gear  520  and the second rotation bevel gear  530 . Therefore, the second rotating pulleys  540 ,  541  rotate identical to the rotation bevel gears  520 ,  530 . 
         [0124]    The first rotating pulleys  542 ,  543 ,  546  are rotatably disposed on the second rotary shaft  111 . The first rotating pulley  542  and the first rotating pulley  546  are connected to each other through the shaft formed through the first rotation member  101 . Therefore, the first rotating pulley  542  and the first rotating pulley  546  make the same rotation. 
         [0125]    Timing belt teeth are respectively provided at the circumferences of the first rotating pulleys  542 ,  543  and the second rotating pulleys  540 ,  541 , and the first rotating pulleys  542 ,  543  and the second rotating pulleys  540 ,  541  are connected by using timing belts  544 ,  545  as shown in  FIG. 5 . 
         [0126]    Even though the first rotating pulleys  542 ,  543  and the second rotating pulleys  540 ,  541  provided at both sides of the first rotation member  101  are connected by using the timing belts  544 ,  545 , it is also possible that the first rotating pulleys  542 ,  543  and the second rotating pulleys  540 ,  541  have wire grooves to be connected by steel wires. In addition, it is also possible that rotation units are respectively provided at sides of the first rotating pulleys  542 ,  543  and the second rotating pulleys  540 ,  541  and connected by using links. 
         [0127]    The wire connector  222  is provided at the side of the first rotation member  101 , and the wire  224  of the single-DOF gravity compensator  202  is connected thereto. The wire  224  is fixed to the output member  440  of the first rotation through the idle pulley  223  fixed to the output member  440  of the first rotation and the pulley  307  provided at the sliding member  303  disposed in the output member  440 . 
         [0128]    The wire connector  222  is also provided at the sides of the first rotating pulleys  542 ,  546 , and the wire  224  of the single-DOF gravity compensator  202  is connected thereto. The wire  224  is fixed to the output member  440  of the first rotation through the idle pulley  223  fixed to the output member  440  of the first rotation and the pulley  307  provided at the sliding member  303  disposed in the output member  440 . 
         [0129]    Hereinafter, an embodiment of a motor arrangement, different from that of  FIG. 1  as described above, will be described. 
         [0130]    The first rotation member  101  rotates on the first rotary shaft  110  and the second rotary shaft  111  by the driving of the first motor and the second motor. In addition, for the third rotation and the fourth rotation, the third motor and the fourth motor are provided at the output member  440  of the first rotation. For the connection of the third motor and the fourth motor, it is possible that gears are provided at the circumferences of the first rotating pulleys  542 ,  543  and connected to a pinion gear, a timing belt pulley fixed to the pinion gear is installed, and then the third motor fixed to the output member  440  of the first rotation is connected to the timing belt pulley fixed to the shaft of the fourth motor by using a timing belt, so that the third motor and the fourth motor are connected. The power transmission method for driving the first rotating pulleys  542 ,  543  by using the third motor and the fourth motor as described in this embodiment is just an example, and the present disclosure is not limited thereto. 
         [0131]    Hereinafter, operations of the robot arm shown in  FIG. 5  will be described. 
         [0132]    First, if the first rotary shaft  110  of  FIG. 1  is parallel to the gravity direction, even though the first rotation member  101  makes the first rotation, the torque applied to the first rotary shaft  110  does not change. Therefore, the compensation of gravity with respect to the first rotation of the first rotation member  101  will not be considered. 
         [0133]    If the first rotation member  101  rotates on the second rotary shaft  111  in a state where the first rotating pulleys  542 ,  543 ,  546  are fixed, the wire connector  222  moves to pull the wire  224 . The wire  224  pulls the sliding member  303  toward the output member  440  by means of the pulley  307  to compress the spring  305 . The compressed force of the spring  305  offsets the gravity caused by the weight of the robot arm  100 . Therefore, even though the robot arm  100  rotates by a predetermined angle on the second rotary shaft  111 , the robot arm  100  does not move downwards by gravity any more and may maintain its posture like a gravity-free state. Here, the second rotating pulleys  540 ,  541  moves in parallel to the output member  440  of the first rotation while rotating in a direction opposite to the rotating direction of the first rotation member  101 . 
         [0134]    Next, the first rotation member  101  is fixed, and the second rotation member  102  rotates on the third rotary shaft  112 . Since the fourth rotary shaft  113  is formed through the second rotation member  102 , the differential bevel gear frame  510  rotates identical to the second rotation member  102 . Therefore, the rotation bevel gears  520 ,  530  engaged with the fixed bevel gears  521 ,  531  fixed to the second rotation member  102  rotate identical to the second rotation member  102 , and the second rotating pulleys  540 ,  541  connected to the rotation bevel gears  520 ,  530  also make the same rotation. In addition, due to the rotation of the second rotating pulleys  540 ,  541 , the first rotating pulleys  542 ,  543 ,  546  also make the same rotation. Accordingly, the wire connector  222  fixed to the first rotating pulleys  543 ,  546  also moves to pull or release the wire  224 , thereby compressing the spring  305 . The compressed force of the spring  305  offsets the gravity caused by the weight of the robot arm  100 . Therefore, even though the robot arm  100  rotates by a predetermined angle on the third rotary shaft  112 , the robot arm  100  does not move downwards by gravity any more and may maintain its posture like a gravity-free state. 
         [0135]    Next, the first rotation member  101  is fixed, and the second rotation member  102  rotates on the fourth rotary shaft  113 . Since the fourth rotary shaft  113  is formed through the second rotation member  102 , the differential bevel gear frame  510  is fixed, and only the second rotation member  102  rotates on the fourth rotary shaft  113 . In other words, the fixed bevel gears  521 ,  531  fixed to the second rotation member  102  rotate toward the fourth rotary shaft. Therefore, the rotation bevel gears  520 ,  530  engaged with the fixed bevel gears  521 ,  531  fixed to the second rotation member  102  rotate in opposite directions, and the second rotating pulleys  540 ,  541  connected to the rotation bevel gears  520 ,  530  also rotate in opposite directions. 
         [0136]    In addition, due to the rotation of the second rotating pulleys  540 ,  541 , the first rotating pulleys  543 ,  542 ,  546  rotate in opposite directions. Accordingly, the wire connector  222  fixed at the first rotating pulleys  543 ,  546  moves to pull or release the wire  224 , thereby compressing the spring  305 . The compressed force of the spring  305  offsets the gravity caused by the weight of the robot arm  100 . Therefore, even though the robot arm  100  rotates by a predetermined angle on the fourth rotary shaft  113 , the robot arm  100  does not move downwards by gravity any more and may maintain its posture like a gravity-free state. 
         [0137]    Even though the fixed bevel gears  521 ,  531  are disposed at right and left sides of the third rotary shaft  112  in this embodiment, the fixed bevel gears  521 ,  531  may also be disposed at only one side of the third rotary shaft  112 . 
         [0138]    Meanwhile, the elastic modulus of the spring  305  may be appropriately designed in consideration of the weight, length or the like of the robot arm  100 . 
         [0139]    While the exemplary embodiments have been shown and described, it will be understood by those skilled in the art that various changes in form and details may be made thereto without departing from the spirit and scope of the present disclosure as defined by the appended claims.