Abstract:
The present invention discloses a weight compensation mechanism installed at a rotatable three-degree-of-freedom link member, wherein a first rotation of the link member is a yaw rotation aligned with the direction of the gravity and second and third rotations of the link member are respectively a roll rotation and a pitch rotation, wherein the second and third rotations are restrained by a plurality of differential bevel gears, and wherein a pair of cam plates is fixed to shafts of a pair of rotary bevel gears in the plurality of differential bevel gears, and a one-degree-of-freedom weight compensator is provided to be connected to the cam plates.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Korean Patent Application No. 10-2011-0045658, filed on May 16, 2011, 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 
     1. Field 
     The present disclosure relates to a weight compensation mechanism and a robot arm using the same, and more particularly, to a weight compensation mechanism removing an influence generated in a multi-joint link mechanism such as a robot arm due to weight such as gravity and a robot arm using the same. 
     2. Description of the Related Art 
     In recent years, various robots have been developed in order to facilitate the human living environment or assist a work in the industrial field. Especially, many types of robot arms have been developed which are utilized in various industrial fields including painting, welding, and the like. Since such an industrial multi-joint robot arm needs to transfer and support a heavy working object, it is very important to design the robot arm capable of generating high torque. 
     Since such a multi-joint robot arm receives load torque due to the own weight or the weight of the working object, this load torque directly affects a design in capacity of a driving machine such as a driving motor. In particular, a torque component generated by the own weight of the robot arm occupies a large portion in the load acting on the driving motor. 
     In the case of the existing robot arm, when the capacity of the driving machine of the robot arm is determined, not only the torque generated by the working object, but also the gravity torque generated by the own weight of the robot arm need to be considered. For this reason, there is a problem in that the capacity of the power source for driving the robot arm increases. 
     Furthermore, simple ideas have been theoretically suggested to compensate the weight caused by the own weight of the robot arm and the like, but mechanisms practically adopting the ideas have not been developed. 
     SUMMARY 
     The present disclosure is directed to providing a weight compensation mechanism removing an influence generated in a link mechanism such as a robot arm configured as a multi-degree-of-freedom joint due to gravity caused by own weight and a robot arm using the same. 
     In one aspect, there is provided a weight compensation mechanism installed at a rotatable three-degree-of-freedom link member, wherein a first rotation of the link member is a yaw rotation aligned with the direction of the gravity and second and third rotations of the link member are respectively a roll rotation and a pitch rotation, wherein the second and third rotations are restrained by a plurality of differential bevel gears, and wherein a pair of cam plates is fixed to shafts of a pair of rotary bevel gears in the plurality of differential bevel gears, and a one-degree-of-freedom weight compensator is provided to be connected to the cam plates. 
     One fixed bevel gear of the plurality of differential bevel gears may be fixed onto a second rotary shaft, and the other rotary bevel gears may be rotatably attached onto a third rotary shaft. 
     One end of the one-degree-of-freedom weight compensator may be fixed to the link member, and the other end of the one-degree-of-freedom weight compensator may be fixed to the rotary bevel gear or a cam plate connected to the rotary bevel gear. 
     One end of the one-degree-of-freedom weight compensator may be fixed to the link member, and the other end of the one-degree-of-freedom weight compensator may be fixed to the rotary bevel gear rotating about the third rotary shaft connected to the fixed bevel gear fixed onto the second rotary shaft. 
     The one-degree-of-freedom weight compensator may include: a spring of which one end is fixed to a spring fixing portion fixed to the link member and the other end is fixed to a sliding member moving along a guide bar attached to the spring fixing portion; and a wire of which one end is fixed to a rotatable coupler provided in a side surface of a cam plate and the other end is connected to a wire fixing portion fixed to the link member through an idle pulley fixed to the link member and a pulley provided inside the sliding member, and wherein when the sliding member moves toward the spring fixing portion, the spring may be compressed. 
     In one aspect, there is provided a robot arm including the weight compensation mechanism. 
     In another aspect, there is provided a robot arm capable of performing a weight compensation with three degrees of freedom by connecting the plurality of weight compensation mechanisms to each other. 
     In another aspect, there is provided a weight compensation mechanism installed at a robot arm of which one end is rotatable with three digress of freedom and the other end is rotatable with two degrees of freedom, wherein a first rotation of one end of the robot arm is a yaw rotation aligned with the direction of the gravity, and second and third rotations of one end of the robot arm are respectively a roll rotation and a pitch rotation, wherein the second and third rotations of one end of the robot arm are restrained by a plurality of differential bevel gears, wherein a pair of first cam plates is fixed to shafts of a pair of first rotary bevel gears in the plurality of differential bevel gears, and a one-degree-of-freedom weight compensator is provided to be connected to the first cam plates, and wherein a second rotary link as an output link of the two-degree-of-freedom rotation (fourth and fifth rotations) of the other end of the robot arm is restrained by the second and third rotations of one end of the robot arm to move in parallel to one end of the robot arm. 
     One fixed bevel gear of the plurality of differential bevel gears may be fixed onto a second rotary shaft, and the other rotary bevel gears may be rotatably attached onto a third rotary shaft. 
     One end of the one-degree-of-freedom weight compensator may be fixed to a second cam plate connected to a second rotary bevel gear or the second rotary bevel gear rotating about a fourth rotary shaft or the first cam plate disposed outside the first rotary bevel gear, and the other end of the one-degree-of-freedom weight compensator may be fixed to a first rotary link. 
     The two-degree-of-freedom rotation of the other end of the robot arm may be restrained by a differential bevel gear. 
     One fixed bevel gear the plurality of differential bevel gears may be fixed to the second rotary link disposed on a fifth rotary shaft, and the other rotary bevel gears may be rotatably fixed to the first rotary link. 
     The weight compensation mechanism may further include a synchronization device synchronizing the rotation of a second rotary bevel gear rotating about the fourth rotary shaft and the rotation of the first rotary bevel gear rotating about the third rotary shaft. 
     The synchronization device may have a structure in which timing belt pulleys are respectively provided on the second rotary bevel gear rotating about the fourth rotary shaft and the first rotary bevel gear rotating about the third rotary shaft and the timing belt pulleys are connected to each other through a timing belt. 
     The synchronization device may have a structure in which wire pulleys are respectively provided on the second rotary bevel gear rotating about the fourth rotary shaft and the first rotary bevel gear rotating about the third rotary shaft and the wire pulleys are connected to each other through a wire. 
     The synchronization device may have a structure in which rotary portions are respectively provided on the circumference of the second rotary bevel gear rotating about the fourth rotary shaft and the circumference of the first rotary bevel gear rotating about the third rotary shaft and the rotary portions are connected to each other through a link. 
     The one-degree-of-freedom weight compensator may include: a spring of which one end is fixed to a spring fixing portion fixed to the link member and the other end is fixed to a sliding member moving along a guide bar attached to the spring fixing portion; and a wire of which one end is fixed to a rotatable coupler provided in a side surface of a cam plate and the other end is connected to a wire fixing portion fixed to the link member through an idle pulley fixed to the link member and a pulley provided inside the sliding member, and wherein when the sliding member moves toward the spring fixing portion, the spring may be compressed. 
     Three motors may be independently connected to generate the first, second, and third rotations of one end of the robot arm. 
     In still another aspect, there is provided a robot arm including the weight compensation mechanism. 
     In still another aspect, there is provided a robot arm capable of performing weight compensation with more than three degrees of freedom by connecting the plurality of weight compensation mechanisms to each other. 
     In still another aspect, there is provided a robot arm including: a rotatable three-degree-of-freedom link member; and a weight compensation mechanism, wherein a first rotation of the link member is a yaw rotation aligned with the direction of the gravity and second and third rotations of the link member are respectively a roll rotation and a pitch rotation, wherein the second and third rotations are restrained by a plurality of differential bevel gears, wherein a pair of cam plates is fixed to shafts of a pair of rotary bevel gears in the plurality of differential bevel gears, and a one-degree-of-freedom weight compensator is provided to be connected to the cam plates, and wherein one motor is independently disposed for the first rotation, and two motors are connected to the differential bevel gear for the second and third rotations. 
     The two motors may be connected to a rotary bevel gear rotating about a third rotary shaft. 
     In still another aspect, there is provided a weight compensation mechanism installed at a robot arm of which one end is rotatable with three digress of freedom and the other end is rotatable with one degree of freedom, wherein a first rotation of one end of the robot arm is a yaw rotation aligned with the direction of the gravity, and second and third rotations of one end of the robot arm are respectively a roll rotation and a pitch rotation, wherein the second and third rotations of one end of the robot arm are restrained by a plurality of differential bevel gears, wherein a pair of cam plates is fixed to shafts of the pair of rotary bevel gears in the plurality of differential bevel gear, wherein a plurality of pulleys and a second link member are rotatably disposed on a fourth rotary shaft of the other end of the robot arm, wherein the plurality of pulleys rotates by being restrained by the second and third rotations of one end of the robot arm, and wherein a plurality of one-degree-of-freedom weight compensators is provided between the plurality of pulleys and the second link member. 
     One fixed bevel gear of the plurality of differential bevel gears may be fixed onto a second rotary shaft, and the other rotary bevel gears may be rotatably attached onto a third rotary shaft. 
     The one-degree-of-freedom weight compensator may include: a spring of which one end is fixed to a spring fixing portion fixed to the link member and the other end is fixed to a sliding member moving along a guide bar attached to the spring fixing portion; and a wire of which one end is fixed to a rotatable coupler provided in a side surface of a cam plate and the other end is connected to a wire fixing portion fixed to the link member through an idle pulley fixed to the link member and a pulley provided inside the sliding member, and wherein when the sliding member moves toward the spring fixing portion, the spring may be compressed. 
     The weight compensation mechanism may further include a synchronization device synchronizing the rotation of the plurality of pulleys and the rotation of a rotary bevel gear rotating about a third rotary shaft. 
     The synchronization device may have a structure in which timing belt pulleys are respectively provided on the pulley rotating about the fourth rotary shaft and the rotary bevel gears rotating about the third rotary shaft and the timing belt pulleys are connected to each other through a timing belt. 
     The synchronization device may have a structure in which wire pulleys are respectively provided on the pulley rotating about the fourth rotary shaft and the rotary bevel gears rotating about the third rotary shaft and the wire pulleys are connected to each other through a wire. 
     The synchronization device may have a structure in which rotary portions are respectively provided on the circumference of the pulley rotating about the fourth rotary shaft and the circumference of the rotary bevel gears rotating about the third rotary shaft and the rotary portions are connected to each other through a link. 
     One end of the one-degree-of-freedom weight compensator provided between the plurality of pulleys and the second link member may be fixed to the second link member, and the other end of the one-degree-of-freedom weight compensator may be fixed to the plurality of pulleys. 
     In still another aspect, there is provided a robot arm including: the weight compensation mechanism; and four motors independently connected for first, second, third, and fourth rotations. 
     In still another aspect, there is provided a robot arm including: the weight compensation mechanism; one motor independently disposed for first and fourth rotations; and two motors respectively connected to differential bevel gears for second and third rotations. 
     The two motors may be connected to rotary bevel gears rotating about a third rotary shaft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a perspective view illustrating a robot arm equipped with a weight compensation mechanism according to an embodiment of the disclosure; 
         FIG. 2  is a bottom view illustrating the robot arm shown in  FIG. 1 ; 
         FIG. 3  is a side view illustrating the robot arm shown in  FIG. 1 ; 
         FIG. 4  is a diagram illustrating a structure of a one-degree-of-freedom weight compensator; 
         FIG. 5  illustrates a structure in which the robot arms shown in  FIG. 1  are connected to each other in series; and 
         FIGS. 6 and 7  are diagrams illustrating a four-degree-of-freedom robot arm according to another embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments now will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. The present disclosure may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth therein. Rather, these exemplary embodiments are provided so that the present disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art. In the description, details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the presented embodiments. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, the use of the terms a, an, etc. does not denote a limitation of quantity, but rather denotes the presence of at least one of the referenced item. The use of the terms “first”, “second”, and the like does not imply any particular order, but they are included to identify individual elements. Moreover, the use of the terms first, second, etc. does not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. It will be further understood that the terms “comprises” and/or “comprising”, or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof. 
     Unless otherwise defined, all terms (including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     In the drawings, like reference numerals denote like elements. The shape, size and regions, and the like, of the drawing may be exaggerated for clarity. 
     A weight compensation mechanism and a robot arm using the same according to the exemplary embodiments of the disclosure will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a perspective view illustrating a robot arm equipped with a weight compensation mechanism according to an embodiment of the disclosure.  FIG. 2  is a bottom view illustrating the robot arm shown in  FIG. 1 .  FIG. 3  is a side view illustrating the robot arm shown in  FIG. 1 . 
     Referring to  FIGS. 1 to 3 , a robot arm  100  with the weight compensation mechanism according to one embodiment of the disclosure includes a link member  103  that is a base frame of the robot arm, first and second rotary members  101  and  102  that are installed at the joints of the link member  103  and are operable to move with the rotation of the link member  103 , and a one-degree-of-freedom weight compensator  150  that absorbs the gravity caused by the own weight with the movement of the robot arm  100 . 
     In the embodiment, the robot arm  100  is configured as a three-degree-of-freedom robot arm that is operable to rotate about a first rotary shaft  105 , a second rotary shaft  106 , and a third rotary shaft  107  intersecting each other. 
     First, the rotation of the robot arm  100  will be described. A motor  140  for a first rotation is attached to the base frame, and the output shaft of the motor  140  is connected with the first rotary member  101 . Therefore, the first rotary member  101  rotates with the rotation of the motor  140 . A connection shaft  113  corresponding to the second rotary shaft  106  is fixed to the first rotary member  101 . 
     A bearing is attached to the side surface of the second rotary member  102 , so that the second rotary member  102  is rotatable about the connection shaft  113 . The second rotary member  102  has a differential bevel gear. Among these, a fixed bevel gear  110  is fixed to the connection shaft  113 , and a pair of first rotary bevel gears  111  is connected to both side surfaces of the second rotary member  102  through bearings. That is, the fixed bevel gear  110  is disposed on the second rotary shaft  106 , and the pair of first rotary bevel gears  111  is disposed on the third rotary shaft  107 . 
     The link member  103  includes a pair of side wall links  114  that is disposed with a predetermined gap therebetween and a plurality of spacer links  115  that maintains the gap between the side wall links  114 . Both ends of each side wall link  114  are provided with bearings, and one end of the link member  103  is connected with the shaft of the first rotary bevel gear  111 . That is, the link member  103  is rotatable about the third rotary shaft  107  with respect to the second rotary member  102 . 
     The outside of the link member  103  is provided with a pair of first cam plates  112  fixed to the shafts of the pair of first rotary bevel gears  111 . Therefore, the pair of first rotary bevel gears  111  and the pair of first cam plates  112  rotate in a synchronized manner. Furthermore, a relative rotation is generated between the pair of first cam plates  112  and the link members  103  during the rotation of the link member  103 . 
     The other end of the link member  103  is also provided with a differential bevel gear. The bearing provided at the other end of the link member  103  is connected with the shafts of the pair of second rotary bevel gears  121 . The first rotary link  160  located at the other end of the link member  103  is rotatably connected to the pair of second rotary bevel gears  121  through a bearing. That is, the first rotary link  160  is rotatable about the fourth rotary shaft  108  located at the other end of the link member  103 . 
     The fixed bevel gear  120  located inside the first rotary link  160  is attached to a fixed shaft  161  fixed to the second rotary link  104  located at the other end of the link member  103 . The first rotary link  160  is provided with a bearing, and is connected to the fixed shaft  161  through the bearing. Therefore, the second rotary link  104  located at the other end of the link member  103  is rotatable about the fourth rotary shaft  108  and the fifth rotary shaft  109  with the rotation of the second rotary bevel gear  121 . 
     Like the structure of one end of the link member  103 , the other end of the link member  103  is provided with a pair of second cam plates  122  fixed to the shaft of the second rotary bevel gear  121 . Therefore, the second rotary bevel gear  121  and the second cam plate  122  rotate in a synchronized manner. Furthermore, a relative rotation is generated between the second cam plate  122  and the link member  103  during the rotation of the second rotary link  104 . 
     The second cam plate  122  located at the other end of the link member  103  is rotatable by the first cam plate  112  located at one end of the link member  103 . For this structure, a timing belt gear  130  is provided on each of the circumference of two pairs of cam plates  112  and  122  located at both ends of the link member  103 , and as shown in  FIG. 3 , the cam plates  112  and  122  provided at both sides of the link member  103  are connected to each other through a timing belt  131 . 
     In the embodiment, the cam plates  112  and  122  provided at both sides of the link member  103  are connected to each other through the timing belt. However, a structure may be adopted in which the cam plates  112  and  122  are provided with wire grooves and are connected to each other through a steel wire. Furthermore, a structure may be adopted in which the side surfaces of the cam plates  112  and  122  are provided with rotary portions and are connected to each other through a link. 
       FIG. 4  is a diagram illustrating the structure of the one-degree-of-freedom weight compensator. 
     Referring to  FIG. 4 , a structure for storing energy generated by the gravity is shown which includes a spring  212 , a wire  210 , and a pulley  214 . One end of the spring  212  is fixed to a spring fixing portion  219  fixed to the link member  103 , and the other end of the spring  212  is fixed to a sliding member  213  moving along a guide bar  211  attached to the spring fixing portion  219 . Therefore, when the sliding member  213  moves toward the spring fixing portion  219 , the spring  212  is compressed. 
     One end of the steel wire  210  is fixed to a rotatable coupler  217  provided at the side surface of the first cam plate  112 . The steel wire  210  is connected to a wire fixing portion  218  fixed to the link member  103  through idle pulleys  216  and  215  provided at the link member  103  and a pulley  214  provided inside the sliding member  213 . Although it is not shown in the drawings, the spring  212 , the wire  210 , and the pulley  214  of  FIG. 4  are disposed to be symmetrical about the center line A-A. 
     In the embodiment, the coil spring is adopted, but the disclosure is not limited thereto. For example, various elastic members such as a plate spring may be adopted. Furthermore, in the embodiment, two guide bars  211  and two springs  212  are installed, but the number thereof may increase or decrease. Furthermore, in the embodiment, the steel wire is used to make the displacement of the spring, but a structure may be adopted in which a coil spring is provided inside a cylinder, one end of the cylinder is fixed to a rotatable coupler  217 , and the other end thereof is rotatably fixed to the idle pulley  216 . 
     In the embodiment, the spring, the wire, and the pulley are used to compensate the weight, but the disclosure is not limited thereto. For example, various one-degree-of-freedom weight compensators such as a one-degree-of-freedom weight compensator having cam profiles provided at the inside and outside thereof may be used. In this structure, one end of the one-degree-of-freedom weight compensator may be fixed to the link member  103 , and the other end serving as the output portion may be fixed to the first cam plate  112 . Alternatively, one end of the one-degree-of-freedom weight compensator may be fixed to the first cam plate  112 , and the other end serving as the output portion may be fixed to the link member  103 . Alternatively, one end of the one-degree-of-freedom weight compensator may be fixed to the link member  103 , and the other end serving as the output portion may be fixed to the second cam plate  122 . Alternatively, one end of the one-degree-of-freedom weight compensator may be fixed to the second cam plate  122 , and the other end serving as the output portion may be fixed to the link member  103 . 
     The rotation at the first rotary shaft  105  is performed by the motor  140 . Furthermore, for the rotation at the second rotary shaft  106  and the third rotary shaft  107 , the pair of first cam plates  112  has a gear provided on the circumferential surfaces thereof to be connected to a pinion gear. Then, by using the timing belt pulley fixed to the pinion gear, the pair of first cam plates  12  may be connected to the timing belt pulley fixed to the shafts of the motors  141  and  142  fixed to the link member  103  through the timing belt. The power transmission method of driving the first cam plates  112  using the motors  141  and  142  described in the embodiment is merely an example, and the disclosure is not limited thereto. 
     Furthermore, although it is not described in the embodiment, for the rotation at the second rotary shaft, one end of the motor may be fixed to the first rotary member  101 , and the other end serving as the output portion may be fixed to the second rotary member  102  through a gear pulley or the like. Similarly, for the rotation at the third rotary shaft, the motor may be directly connected between the link member  103  and the second rotary member  102 . 
     So far, the structure of the robot arm  100  with the weight compensation mechanism has been described. The operation of the robot arm  100  with the weight compensation mechanism will be described below. 
     When the first rotary shaft  105  is parallel to the direction of the gravity, there is no variation in torque acting on the first rotary shaft  105  of the link member  103 . Therefore, the weight compensation in the first rotation direction will not be considered. 
     First, the operation of the robot arm  100  rotating about the second rotary shaft  106  will be examined. When the motor  141  and the motor  142  facing each other rotate in the same direction (which looks like the same direction, but the actual rotation directions are opposite to each other), the link member  103  rotates about the second rotary shaft  106 . 
     The second rotary member  102  rotates about the connection shaft  113  of the fixed bevel gear  110  in accordance with the rotation of the link member  103 . At this time, the pair of first rotary bevel gears  111  rotates while meshing with the fixed bevel gear  110  in accordance with the rotation of the second rotary member  102 . Accordingly, the pair of first cam plates  112  integrally connected to the rotary bevel gear  111  rotates together. 
     When the first cam plate  112  rotates, the coupler  217  attached to the first cam plate  112  moves to pull the steel wire  210 , so that the sliding member  213  moves toward the spring fixing portion  219  to compress the spring  212  due to the tension of the steel wire  210 . The force of the compressed spring  212  removes the gravity caused by the own weight of the robot arm  100 . Therefore, even when the robot arm  100  rotates about the second rotary shaft  106  by a constant angle, the robot arm does  100  not move downward due to the gravity, and maintains the posture thereof as in the non-gravitation state. 
     Next, the operation of the robot arm  100  rotating about the third rotary shaft  107  will be examined. When the link member  103  rotates about the third rotary shaft  107 , the link member  103  rotates about the second rotary shaft  106 . At this time, the second rotary member  102  and the bevel gears  110  and  111  do not rotate, and maintain the original posture thereof. 
     Therefore, only the link member  103  equipped with the spring  212  rotates, and tension is generated in the steel wire  210  to pull the sliding member  213 , so that the spring  210  is compressed. The force of the compressed spring  212  removes the gravity caused by the own weight of the robot arm  100 . Therefore, even when the robot arm  100  rotates about the third rotary shaft  107  by a constant angle, the robot arm  100  does not move downward due to the gravity, and maintains the posture thereof as in the non-gravitation state. 
     The elastic coefficient of the spring  212  may be appropriately designed in consideration of the own weight, the length, and the like of the robot arm  100 . 
     Next, the operation of the second rotary link  104  located around the second cam plate  122  will be examined. The first cam plate  112  and the second cam plate  122  are connected to each other through the timing belt  131 . Therefore, the second cam plate  122  rotates with respect to the link member  103  by the same angle as that of the first cam plate  112 . 
     When the robot arm  100  rotates about the third rotary shaft  107  of the link member  103 , the first cam plate  112  relatively rotates in the direction opposite to the rotation direction of the link member  103 . Furthermore, the second cam plate  122  and the pair of second rotary bevel gears  121  connected to the second cam plate  122  also rotate in the direction opposite to the rotation direction of the link member  103 . At this time, the first rotary link  160  connected to the shaft of the fixed bevel gear  120  also rotates in the direction opposite to the rotation direction of the link member  103 . Therefore, the first rotary link  160  rotates in the direction opposite to the rotation direction of the third rotation  107  of the robot arm  100 . 
     When the robot arm  100  rotates about the second rotary shaft  106  of the link member  103 , the pair of second rotary bevel gears  121  connected to the pair of second cam plates  122  rotate in the opposite direction each other. The fixed bevel gear  120  rotates in the direction opposite to the rotation direction of the robot arm  100 . Furthermore, the second rotary link  104  fixed to the fixed bevel gear  120  also rotates in the direction opposite to the rotation direction of the robot arm  100 . 
     Therefore, the second rotary link  104  is operated in parallel to the first rotary member  101  with respect to the second rotation and the third rotation of the robot arm  100 . 
     In the multi-degree-of-freedom robot arm, when only the weight compensation for the three-degree-of-freedom rotation is considered, all components  104 ,  122 ,  131 ,  160 , and  161  relating to the differential bevel gears  120  and  121  may be excluded in the robot arm  100 . That is, when multiple degrees of freedom equal to or more than three degrees of freedom need to be handled, the components relating to the differential bevel gears  120  and  121  are attached. 
       FIG. 5  illustrates a configuration of a multi-degree-of-freedom robot arm with three degrees or more of freedom adopting the robot arm  100  of  FIG. 1 . A second robot arm  200  having the same structure as that of the robot arm  100  is connected to the second rotary link  104  of the robot arm  100 . That is, the second robot arm  200  is rotatable in three degrees of freedom with respect to the second rotary link  104  of the robot arm  100 . Furthermore, since the second rotary link  104  of the robot arm  100  is operated in parallel to the first rotary member  101  of the robot arm  100 , the weight compensation may be also perfectly guaranteed in the second robot arm  200  as in the robot arm  100 . 
       FIGS. 6 and 7  are diagrams illustrating a four-degree-of-freedom robot arm according to another embodiment of the disclosure. 
     Referring to  FIGS. 6 and 7 , as in the robot arm  100  of  FIG. 1 , a first link member  321  rotates about a first rotary shaft  310 , a second rotary shaft  311 , and a third rotary shaft  312 , and further a second link member  322  rotates about a fourth rotary shaft  313 . 
     The differential bevel gear of  FIG. 1  is provided in the second rotary link  320 , and a pair of first cam plates  314  and  315  is fixed to the bevel gear rotating about the second rotary shaft  311 . One ends of one-degree-of-freedom weight compensators  330  and  331  are rotatably fixed to the first cam plates  314  and  315 , and the other ends of the one-degree-of-freedom weight compensators  330  and  331  are fixed to the first link member  321 . 
     For the weight compensation of the second link member  322 , second cam plates  316  and  317  are adapted to be rotatable about the fourth rotary shaft  313 . Furthermore, one ends of one-degree-of-freedom weight compensators  332  and  333  are rotatably fixed to the second cam plates  316  and  317 , and the other ends of the one-degree-of-freedom weight compensators  332  and  333  are fixed to the second link member  322 . 
     Wire grooves are respectively provided on the circumferential surfaces of the first cam plates  314  and  315  and the second cam plates  316  and  317 , and both first cam plates  314  and  316  are respectively connected to both second cam plates  315  and  317  through wires  340 . Therefore, when the first cam plate  314  rotates, the second cam plate  316  also rotates by the same angle. 
     Since the four-degree-of-freedom robot arm of  FIG. 7  further has a structure for the fourth rotation compared to the three-degree-of-freedom structure of  FIG. 1 , the weight compensation may be perfectly performed in all postures. Furthermore, the cam plates  314 ,  315 ,  316 , and  317  respectively includes the timing belt pulleys as in the embodiment of  FIG. 1 , and may be connected to each other through the timing belts. Furthermore, a structure may be adopted in which a rotary portion is provided on each of the side surfaces of the cam plates  314 ,  315 ,  316 , and  317  and the rotary portions are connected to each other through links. 
     For the rotation of the first link member  321 , motors  350  and  351  are provided in the first link member  321 , and the first link member  321  is connected to the bevel gear rotating about the second rotary shaft  311 . Therefore, the third rotation and the second rotation may be realized by the motors  350  and  351 . Although the motor for the first rotation is not shown in  FIG. 7 , the motor may be disposed similarly to the motor of  FIG. 1 . 
     A motor  352  for the fourth rotation is provided in the second link member  322 , and is connected to the fourth rotary shaft  313  through the belt. The arrangement of the motors of  FIG. 7  is merely an example, and various structures may be adopted. 
     The weight compensation mechanism according to the disclosure can remarkably reduce power of a power source used for driving a robot arm and various link members. Further, since such power reduction leads to a decrease in weight of the entire robot arm and an increase in power efficiency, there is an advantage in that much energy is saved. 
     Furthermore, since the weight compensation mechanism according to the disclosure needs a relatively small driving force, manufacturing cost can be reduced, and hence there is an advantage in that a product having a competitive price can be developed if the weight compensation mechanism is practically used some day. 
     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. 
     In addition, many modifications can be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular exemplary embodiments disclosed as the best mode contemplated for carrying out the present disclosure, but that the present disclosure will include all embodiments falling within the scope of the appended claims.