Patent Publication Number: US-2022226983-A1

Title: Externally-driven joint structure

Description:
BACKGROUND OF THE INVENTION 
     Technical Field 
     The present invention relates to a technique of an externally-driven joint structure. 
     Background Art 
     Patent Literature 1 proposes a joint structure of a module-type manipulator. Specifically, the joint structure disclosed in Patent Literature 1 has a built-in motor, and includes attachment faces that can be coupled with another joint structure respectively at two points consisting of a circumferential face and an end face of a rotatable member that rotates in accordance with the rotation of the motor. Accordingly, a plurality of joint structures can be coupled with each other, and thus it is possible to form a manipulator having a multi-joint structure. 
     Furthermore, Patent Literature 2 proposes a multiaxial joint in which a distal joint part and a proximal joint part are connected such that they can axially pivot and swivel via a rotary pivot joint and a rotary swivel joint that are connected to each other in series. According to this multiaxial joint, it is possible to realize a joint having a high degree of freedom in two directions in a link mechanism of a robot. 
     CITATION LIST—PATENT LITERATURES 
     Patent Literature 1: JP 562-282886A 
     Patent Literature 2: JP 2010-255852A 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     Joint structures that can be used for link mechanisms of robots such as exoskeletal robots or robot arms include joint structures with a built-in actuator (e.g., Patent Literature 1) that are directly coupled with a drive source or that have a built-in drive source, and externally-driven joint structures (e.g., Patent Literature 2) that are disconnected from a drive source and that are driven by an external force transmitted from an external device such as a link member coupled thereto. 
     Joint structures with a built-in actuator are internally provided with a housing for accommodating an actuator such as a motor for directly driving the joint structures, and thus the size becomes relatively large. Furthermore, their shape, structure, drive direction, and the like are limited due to the built-in actuator. Thus, use situations of the joint structures with a built-in actuator are limited. 
     Meanwhile, there is no such a limitation on externally-driven joint structures, and thus they can be relatively freely designed according to a link mechanism that is to be formed. Thus, externally-driven joint structures can be used in various situations, and various link mechanisms can be formed using the externally-driven joint structures. 
     However, conventionally, externally-driven joint structures are in many cases individually designed so as to be optimal for each use situation, and they are seldom modularized. That is to say, externally-driven joint structures that can be used for general purposes have rarely been developed. 
     An aspect of the present invention has been made in view of these circumstances, and it is an object thereof to provide a modularized externally-driven joint structure that can be used for general purposes. 
     Solution to Problem 
     In order to solve the above-described problems, the present invention employs the following configurations. 
     That is to say, an aspect of the present invention is directed to an externally-driven joint structure including: a shaft member that extends in an axial direction; and a plurality of rotatable members that are arranged along the axial direction, and are coupled with each other by the shaft member in an axially rotatable manner, wherein each of the rotatable members includes a pair of face portions that face each other in the axial direction, a side wall portion that is arranged along outer circumferential edges of the pair of face portions, and at least one coupling portion that is arranged at the face portions or the side wall portion, and is coupled with a link member constituting a link of a robot. 
     With this configuration, a plurality of rotatable members are coupled with each other in an axially rotatable manner. Moreover, each of the rotatable members includes at least one coupling portion for coupling a link member constituting a link of a robot. 
     Thus, it is possible to couple a plurality of link members via the joint structure according to the above-described configuration, by coupling different link members with different rotatable members. Furthermore, when the link members are moved by an external force acting from actuators or the like, the rotatable members coupled with the link members can axially rotate in accordance with the rotation of the link members. 
     That is to say, the joint structure according to the above-described configuration can be driven by an external force transmitted from the link members, and thus it is possible to change a positional relationship between the link members coupled with different rotatable members. Accordingly, with this configuration, it is possible to provide a modularized externally-driven joint structure that can be used for general purposes. 
     Furthermore, as another mode of the externally-driven joint structure according to the above-described aspect, it is possible that at least one rotatable member of the plurality of rotatable members includes a plurality of the coupling portions arranged at the side wall portion. With this configuration, a plurality of link members can be coupled with a side wall portion of at least one rotatable member, and thus it is possible to realize a complex link mechanism such as a parallel-linked Scott Russell mechanism, which will be described later. 
     Furthermore, as another mode of the externally-driven joint structure according to the above-described aspect, it is possible that at least one rotatable member of the plurality of rotatable members includes at least one coupling portion arranged at either one of the pair of face portions, and other rotatable members of the plurality of rotatable members include at least one coupling portion arranged at the side wall portion. With this configuration, the link connecting direction can be changed between a link member coupled with a face portion of at least one rotatable member and a link member coupled with a side wall portion of another rotatable member. Accordingly, the link connecting direction can be changed without a special structure, and thus the link mechanism that is to be constructed can be made compact on the whole. 
     Furthermore, as another mode of the externally-driven joint structure according to the above-described aspect, it is possible that the face portions of the rotatable members are provided with a recess portion with a shape that allows a bearing in the shape of a ring that receives a force that acts in the axial direction to be accommodated between rotatable members that are adjacent to each other in the axial direction. With this configuration, it is possible to provide a modularized joint structure that can be reinforced in the axial direction by a bearing. 
     Furthermore, as another mode of the externally-driven joint structure according to the above-described aspect, it is possible that an encoder for detecting a relative rotational angle between the rotatable members that are adjacent to each other in the axial direction is further accommodated between the recess portions of the adjacent rotatable members. With this configuration, an encoder for detecting a rotational angle is built in the joint structure. Thus, it is possible to provide a compact and modularized joint structure that can detect an angle. 
     Furthermore, as another mode of the externally-driven joint structure according to the above-described aspect, it is possible that the recess portions are formed in the shape of a circular ring, bases of inner circumferential faces of the recess portions are provided with a step portion in the shape of a circular ring extending inward in a radial direction from the inner circumferential faces, a face portion of a rotatable member that faces the recess portions, the rotatable member being adjacent to the rotatable members, is provided with a projecting portion in the shape of a circular ring with a diameter smaller than that of the recess portions, a base of an outer circumferential face of the projecting portion is provided with a step portion in the shape of a circular ring extending outward in the radial direction from the outer circumferential face of the projecting portion, and a cross roller bearing as the bearing in the shape of a ring is arranged so as to be supported by the inner circumferential face of the recess portion, a face along the axial direction of the step portion of the recess portion, the outer circumferential face of the projecting portion, and a face along the axial direction of the step portion of the projecting portion. With this configuration, since a cross roller bearing is used, it is possible to increase the outer diameter of the shaft member compared with the case in which a thrust bearing is used. Accordingly, the rigidity of the shaft member can be improved. 
     Furthermore, as another mode of the externally-driven joint structure according to the above-described aspect, it is possible that the joint structure includes two rotatable members, the coupling portions of the rotatable members are arranged symmetric about the axial direction such that, even when the joint structure is reversed about an axis that is perpendicular to the axial direction, the joint structure can be used while a positional relationship between the link members is maintained, one of the two rotatable members is formed in one piece with the shaft member, the other rotatable member of the two rotatable members has a through hole into which the shaft member is allowed to be inserted, and a radial bearing is arranged so as to be interference-fitted to the shaft member and clearance-fitted to an inner circumferential wall of the through hole, or so as to be clearance-fitted to the shaft member and interference-fitted to the inner circumferential wall of the through hole. With this configuration, it is possible to provide a joint structure that can be applied to situations with different load conditions such as unbalanced loads and stationary loads, because the structure is symmetric about the axial direction. In this case, the coupling portions can be coupled with the same type of link members. Furthermore, following link mechanisms can be constructed using the joint structure according to this embodiment. That is to say, an aspect of the present invention is directed to a link mechanism including: two or more joint structures according to this embodiment; and a link member that is coupled with the coupling portions of the joint structures, wherein two joint structures that are adjacent to each other via the link member are arranged such that one of the joint structures is used in a state of being reversed about an axis that is perpendicular to the axial direction with respect to the other joint structure so that the rotatable members face each other in the direction that is perpendicular to the axial direction. With this configuration, it is possible to construct a link mechanism that is compact in the width direction. 
     Furthermore, as another mode of the externally-driven joint structure according to the above-described aspect, it is possible that the joint structure includes two rotatable members, the coupling portions of the rotatable members are arranged symmetric about the axial direction such that, even when the joint structure is reversed about an axis that is perpendicular to the axial direction, the joint structure can be used while a positional relationship between the link members is maintained, the shaft member is formed separately from the two rotatable members, the rotatable members each have a through hole into which the shaft member is allowed to be inserted, and a radial bearing is arranged between the shaft member and the rotatable members so as to be interference-fitted to the shaft member and clearance-fitted to an inner circumferential wall of the through hole, or so as to be clearance-fitted to the shaft member and interference-fitted to the inner circumferential wall of the through hole. With this configuration, it is possible to provide a joint structure that is symmetric about the axial direction. 
     Furthermore, as another mode of the externally-driven joint structure according to the above-described aspect, it is possible that the joint structure includes three or more rotatable members, and the coupling portions of at least two rotatable members of the three or more rotatable members are coupled with a same link member. With this configuration, one link member is supported by a plurality of rotatable members, and thus an external force that acts from the link member can be dispersed between the rotatable members. Accordingly, with this configuration, even when a relatively large force acts from a link member, deformation of the shaft member of the joint structure can be suppressed. 
     Furthermore, as another mode of the externally-driven joint structure according to the above-described aspect, it is possible that coupling between the coupling portions and the link member is constituted by a magnet. With this configuration, it is easy to couple the rotatable members and the link member with each other, and thus it is easy to produce a link mechanism. 
     Furthermore, as another mode of the externally-driven joint structure according to the above-described aspect, it is possible that the rotatable members have at least one coupling portion arranged at the side wall portions, the side wall portions of the rotatable members are formed in the shape of a cylinder, and the coupling portions arranged at the side wall portions have a shape obtained by cutting, in a tangential direction, arc portions of the side wall portions. With this configuration, it is possible to provide a joint structure that can be more easily produced through lathe machining or the like. Note that a side wall portion being in the shape of a cylinder refers to a state in which the outer shape of the side wall portion is cylindrical, except for the portion obtained by cutting for forming the coupling portion. 
     Furthermore, as another mode of the externally-driven joint structure according to the above-described aspect, it is possible that the side wall portions of the rotatable members have a height that matches a thickness of the link member. With this configuration, it is possible to provide a joint structure that can form a compact link mechanism. 
     Furthermore, as another mode of the externally-driven joint structure according to the above-described aspect, it is possible that the coupling portions arranged at the side wall portions have a projecting portion projecting outward in the radial direction at a center in the tangential direction, in conformity with a recess portion provided at a center of an end face of the link member. With this configuration, the portion obtained by cutting as a coupling portion in each rotatable member can be arranged on the outer side in the radial direction, and thus it is possible to provide a joint structure in which a bearing with a relatively large diameter can be arranged. 
     Furthermore, as another mode of the externally-driven joint structure according to the above-described aspect, it is possible that a reinforcing plate for supporting a coupling region of the coupling portion arranged at the side wall portion of the rotatable member and the link member is provided on at least one of both sides in the axial direction of the coupling region. With this configuration, it is possible to provide a joint structure that is unlikely to be broken by twisting. 
     Furthermore, as another mode of the externally-driven joint structure according to the above-described aspect, it is possible that each of the rotatable members has a plurality of the coupling portions at the side wall portion, and the plurality of coupling portions are arranged symmetric about an axis in each of the rotatable members. With this configuration, it is possible to provide a joint structure that can be used while a positional relationship between the link members is maintained even when the joint structure is axially rotated. 
     Furthermore, an aspect of the present invention is directed to a link mechanism including: the joint structure according to any one of above-described aspects; and a link member that is coupled with the coupling portion arranged at the side wall portions of the rotatable members of the joint structure, wherein the side wall portions of the rotatable members of the joint structure include a wire-driving groove portion, a fixture is attached to the link member, and a wire that is driven by an external drive source is arranged along the wire-driving groove portion, and the end portion of the wire is fixed to the fixture. 
     Advantageous Effects of Invention 
     According to the present invention, it is possible to provide a modularized externally-driven joint structure that can be used for general purposes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view schematically showing an example of a joint structure according to an embodiment; 
         FIG. 2  is a cross-sectional view schematically showing an example of the joint structure according to the embodiment; 
         FIG. 3  schematically shows an example of a state in which the joint structure according to the embodiment is exploded; 
         FIG. 4  is a partially enlarged view schematically showing an example of a coupling portion of the joint structure according to the embodiment; 
         FIG. 5A  is a cross-sectional view schematically showing an example of a state before a link member is coupled with the coupling portion of a rotatable member according to the embodiment; 
         FIG. 5B  is a cross-sectional view schematically showing an example of a state after the link member is coupled with the coupling portion of the rotatable member according to the embodiment; 
         FIG. 6  schematically shows an example of a coupling state between the coupling portion of the rotatable member according to the embodiment and an end face of the link member; 
         FIG. 7A  is a perspective view schematically showing an example of a robot (Scott Russell mechanism) using the joint structure according to the embodiment; 
         FIG. 7B  is a side view schematically showing an example of the robot (Scott Russell mechanism) using the joint structure according to the embodiment; 
         FIG. 8  is a perspective view schematically showing an example of a robot (wire driving mechanism) using the joint structure according to the embodiment; 
         FIG. 9A  is a perspective view schematically showing an example of a robot (delta robot) using the joint structure according to the embodiment; 
         FIG. 9B  is a perspective view schematically showing an example of the robot (delta robot) using the joint structure according to the embodiment; 
         FIG. 10  is a cross-sectional view schematically showing an example of a joint structure according to another embodiment; 
         FIG. 11  is a perspective view schematically showing an example of a joint structure according to another embodiment; 
         FIG. 12  is a perspective view schematically showing an example of a robot (Scott Russell mechanism) using a joint structure according to another embodiment; 
         FIG. 13  is a cross-sectional view schematically showing an example of a joint structure according to another embodiment; 
         FIG. 14  is a perspective view schematically showing an example of a joint structure according to another embodiment; 
         FIG. 15  is a cross-sectional view schematically showing an example of a joint structure according to another embodiment; 
         FIG. 16A  schematically shows an example of a joint structure according to another embodiment; 
         FIG. 16B  is a partial cross-sectional view (a cross-section taken along the line C-C in  FIG. 16A ) schematically showing an example of a joint structure according to another embodiment; 
         FIG. 17  is a perspective view schematically showing an example of a rotatable member according to another embodiment; and 
         FIG. 18  is a cross-sectional view schematically showing an example of a joint structure according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment according to an aspect of the present invention (hereinafter, also described as “the present embodiment”) will be described based on the drawings. The present embodiment described below is, however, to be considered in all respects as illustrative of the present invention. It is to be understood that various improvements and modifications can be made without departing from the scope of the present invention. In other words, in implementing the present invention, specific configurations that depend on the embodiment may be employed as appropriate. 
     § 1 Configuration Example 
     First, an externally-driven joint structure  1  according to the present embodiment will be described with reference to  FIGS. 1 and 2 .  FIG. 1  is a perspective view schematically showing an example of the joint structure  1  according to the present embodiment.  FIG. 2  is a cross-sectional view schematically showing an example of the joint structure  1  according to the present embodiment. In  FIG. 2 , hatching is used in order to identify each constituent element. This hatching is for the sake of ease of description, and is not for specifying the material or the like of each constituent element. The same applies to other cross-sectional views using hatching. 
     As shown as an example in  FIGS. 1 and 2 , the joint structure  1  according to the present embodiment includes a shaft member  13  that extends in an axial direction (the left-right direction in  FIG. 2 ) and two rotatable members ( 11  and  12 ) that are arranged along the axial direction and are rotatably coupled with each other via the shaft member  13 . As described later, the joint structure  1  according to the present embodiment does not include a housing for accommodating an actuator such as a motor, and is driven by a separate drive source. Hereinafter, each constituent element will be described. In the description below, for the sake of ease of description, the two rotatable members ( 11  and  12 ) are also referred to as a first rotatable member  11  and a second rotatable member  12  respectively. Furthermore, in  FIG. 1 , for the sake of ease of description, each direction is indicated using an x axis, a y axis, and a z axis. The x axis refers to the axial direction of the shaft member  13 , and the y axis and the z axis each refer to an example of a direction that is perpendicular to the axial direction of the shaft member  13 . 
     Shaft Member 
     First, the shaft member  13  will be described. As shown as an example in  FIG. 2 , the shaft member  13  according to the present embodiment is formed in one piece with the first rotatable member  11 . Specifically, the shaft member  13  is coupled in one piece with the center of a second face portion  112 , which will be described later, of the first rotatable member  11 , and extends in a direction that is away from the second face portion  112 . Accordingly, in the present embodiment, the second rotatable member  12  is coupled with the second face portion  112  side of the first rotatable member  11 . 
     Furthermore, the shaft member  13  according to the present embodiment is formed in the shape of a cylinder, and includes a hollow portion  132  in the shape of a column that extends through the axial direction. The hollow portion  132  is arranged at the center in a direction that is along the radius of the shaft member  13  (hereinafter, it is also referred to as a “radial direction”), and extends in the axial direction through the shaft member  13  and the first rotatable member  11 . A male thread (not shown) is formed on the outer circumferential wall of the upper end portion (the end portion on the left side in  FIG. 2 ) of the shaft member  13  such that a fastener  131  (e.g., a nut) in the shape of a circular ring whose inner circumferential wall is provided with a female thread can be attached to the male thread. 
     Rotatable Members 
     Next, the rotatable members ( 11  and  12 ) will be described. First, the first rotatable member  11  will be described. The first rotatable member  11  according to the present embodiment includes a pair of face portions ( 111  and  112 ) that face each other in the axial direction and a side wall portion  113  that is arranged along the outer circumferential edges of the pair of face portions ( 111  and  112 ). Hereinafter, for the sake of ease of description, the pair of face portions ( 111  and  112 ) are also referred to as a first face portion  111  and a second face portion  112 . 
     In the present embodiment, the face portions ( 111  and  112 ) are formed in the shape of a circle, and the height (the length in the left-right direction in  FIG. 2 ) of the side wall portion  113  is slightly shorter than the diameter of each of the face portions ( 111  and  112 ). Thus, the first rotatable member  11  is formed in the shape of a cylinder with a low height (length in the left-right direction in  FIG. 2 ). The first face portion  111  arranged on the outer side is formed as a flat face. Meanwhile, the second face portion  112  arranged on the second rotatable member  12  side has a circular ring-like recess portion  115  around the shaft member  13 . 
     Furthermore, in the present embodiment, the side wall portion  113  is provided with two coupling portions  21 . Specifically, the two coupling portions  21  are arranged at positions at 180 degrees about the center in the surface direction of the face portions ( 111  and  112 ). A Link member  31  constituting a link of a robot such as an exoskeletal robot or a robot arm is coupled with the coupling portions  21 . The robot has a link mechanism and includes a machine that is driven at a degree of freedom of 1 or more. 
     The method for coupling each coupling portion  21  and the link member  31  according to the present embodiment will be described later in detail. Schematically, as shown as an example in  FIG. 1 , each coupling portion  21  has a groove portion  211  in the shape of an inverted T extending throughout a tangential direction that is perpendicular to the radial direction, and a wedge (a wedge member  32 , which will be described later) is attached to the groove portion  211 . The link member  31  with a substantially H-shaped cross-section is coupled via the wedges to the coupling portion  21 . 
     Next, the second rotatable member  12  will be described. The second rotatable member  12  according to the present embodiment has substantially the same shape as the first rotatable member  11  excluding the shaft member  13 . That is to say, the second rotatable member  12  according to the present embodiment includes a pair of circular face portions ( 121  and  122 ) that face each other in the axial direction and a side wall portion  123  that is arranged along the outer circumferential edges of the pair of face portions ( 121  and  122 ). The face portions ( 121  and  122 ) have the same diameter as the face portions ( 111  and  112 ) of the first rotatable member  11 , and the side wall portion  123  has the same height (the same length in the axial direction) as the side wall portion  113  of the first rotatable member  11 . Furthermore, the side wall portion  123  of the second rotatable member  12  is provided with two coupling portions  21  are arranged at positions at 180 degrees about the center in the surface direction of the face portions ( 121  and  122 ). 
     Contrary to the first rotatable member  11 , the second rotatable member  12  has a through hole  124  in the shape of a column that extends through the axial direction, at the center in the surface direction of each of the face portions ( 121  and  122 ). The through hole  124  has a diameter that is larger than the outer diameter of the shaft member  13  such that the second rotatable member  12  can be attached to the shaft member  13 . Accordingly, the second rotatable member  12  is configured such that, in a state where radial bearings  15  are arranged between the inner circumferential wall of the second rotatable member  12  and the outer circumferential wall of the shaft member  13 , the shaft member  13  can be inserted into the through hole  124 . The second rotatable member  12  and the first rotatable member  11  are coupled with each other in an axially rotatable manner, by inserting the shaft member  13  into the through hole  124  of the second rotatable member  12 , and then attaching the fastener  131  to the upper end portion of the shaft member  13 . The side face portions ( 113  and  123 ) of the rotatable members ( 11  and  12 ) have a shape that is symmetric about a plane perpendicular to the axial direction of the shaft member  13  such that the outer shape of the rotatable members ( 11  and  12 ) is bilaterally symmetric. The radial bearings  15  may be interference-fitted to the shaft member  13  and clearance-fitted to the inner circumferential wall of the through hole  124 , or may be clearance-fitted to the shaft member  13  and interference-fitted to the inner circumferential wall of the through hole  124 . 
     The radial bearings  15  can receive a force that acts in the radial direction. As shown as an example in  FIG. 2 , in the present embodiment, two radial bearings  15  are arranged in a line in the axial direction between the inner circumferential wall of the second rotatable member  12  and the outer circumferential wall of the shaft member  13 . The inner circumferential wall of the second rotatable member  12  is provided with an interlock projecting portion  125  projecting inward in the radial direction, and the radial bearings  15  are positioned by being interlocked with the interlock projecting portion  125  in the axial direction. 
     At this time, the inner diameter of the radial bearings  15  is substantially the same as the outer diameter of the shaft member  13 , and the fastener  131  prevents the radial bearing  15  arranged on the outer side (the left side in  FIG. 2 ) from being detached from the shaft member  13 . Accordingly, the second rotatable member  12  is prevented from being detached from the shaft member  13 , via the interlock projecting portion  125  by the radial bearings  15  and the fastener  131 . Thus, even when the outer diameter of the fastener  131  is smaller than the diameter of the through hole  124 , the second rotatable member  12  can be prevented from being detached. Accordingly, the outer diameter of the fastener  131  may be larger than or smaller than the diameter of the through hole  124 . 
     Furthermore, in the present embodiment, the first face portion  121  arranged on the first rotatable member  11  side is provided with a circular first recess portion  126  corresponding to the recess portion  115  of the second face portion  112  of the first rotatable member  11  that faces the first face portion  121 . That is to say, the first recess portion  126  has the same diameter as the recess portion  115 , and the first recess portion  126  and the recess portion  115  are positioned adjacent to each other in the axial direction to form a circular ring-like internal space. The first recess portion  126  of the second rotatable member  12  and the recess portion  115  of the first rotatable member  11  respectively correspond to “recess portions” of the present invention. The internal space defined by the first recess portion  126  and the recess portion  115  accommodates a thrust bearing  14  and an encoder  16 . The constituent elements accommodated in the internal space will be described later. 
     Meanwhile, the second face portion  122  is provided with a second recess portion  127  with a diameter that is smaller than the diameter of the first recess portion  126 . The diameter of the second recess portion  127  is larger than the outer diameter of the fastener  131 . Thus, when using the fastener  131  to prevent detachment of the second rotatable member  12  that has been attached to the shaft member  13 , the fastener  131  is prevented from projecting significantly outward (leftward in the drawing) from the second face portion  122  of the second rotatable member  12 , by the height (the length in the left-right direction in  FIG. 2 ) of the second recess portion  127 . 
     Furthermore, in the present embodiment, the side wall portion  123  is provided with two wire-driving groove portions  129  that are arranged in a line in the axial direction and each extend in the circumferential direction. Wires for pulling and driving the joint structure  1  are arranged respectively along the wire-driving groove portions  129 . The driving by pulling wires will be described later. 
     The shaft member  13  and the rotatable members ( 11  and  12 ) can be produced using a known method such as cutting or injection molding. Furthermore, the shaft member  13  and the rotatable members ( 11  and  12 ) can be produced as appropriate using a 3D printer. The material of the shaft member  13  and the rotatable members ( 11  and  12 ) may be selected as appropriate according to an embodiment, and examples thereof include metals such as aluminum and titanium and resins such as engineering plastic. 
     Thrust Bearing and Encoder 
     Next, the constituent elements accommodated in the internal space defined by the recess portions ( 115  and  126 ) of the rotatable members ( 11  and  12 ) that are adjacent to each other in the axial direction will be described with reference to  FIG. 3  as well.  FIG. 3  schematically shows an example of a state in which the joint structure  1  according to the present embodiment is exploded. The recess portions ( 115  and  126 ) are formed to define a shape that allows the thrust bearing  14  and the encoder  16  to be accommodated, and thus, as described above, the thrust bearing  14  and the encoder  16  are accommodated in the internal space defined by the recess portions ( 115  and  126 ). 
     The thrust bearing  14  can receive a force that acts in the axial direction (the thrust direction). The thrust bearing  14  is generally configured such that a holding unit holding a plurality of rotating components is held between a housing washer and a shaft washer. The type of rotating components of the thrust bearing  14  may be selected as appropriate according to an embodiment, and examples thereof include balls and rollers. However, the type of the thrust bearing  14  is not limited to those including rotating components, and may be those not including rotating components, such as oilless bushes or oilless bearings. The same applies to the radial bearings  15  described above. 
     As shown as an example in  FIG. 2 , the thrust bearing  14  is formed in the shape of a ring, and the outer diameter of the thrust bearing  14  is substantially the same as the diameter of each of the recess portions ( 115  and  126 ). Meanwhile, the inner diameter of the thrust bearing  14  is larger than the outer diameter of the shaft member  13 , and thus a circular ring-like gap portion  116  is formed so as to surround the shaft member  13 , between the inner circumferential wall of the thrust bearing  14  and the outer circumferential wall of the shaft member  13 . 
     In the present embodiment, as shown as an example in  FIGS. 2 and 3 , the gap portion  116  accommodates the encoder  16  capable of detecting a relative rotational angle between the adjacent rotatable members ( 11  and  12 ). Specifically, the encoder  16  of the optical reflection type including a scale  161  and a detecting element  162  is accommodated in the gap portion  116 . The scale  161  and the detecting element  162  are arranged in the gap portion  116  as follows. 
     That is to say, as shown as an example in  FIG. 2 , a circular ring-like plate  142  with the same outer diameter as the thrust bearing  14  is arranged on the second rotatable member  12  side of the thrust bearing  14 . The bottom face of the first recess portion  126  of the second rotatable member  12  is provided with a projecting portion  128  projecting toward the first rotatable member  11  side (to the right side in  FIG. 2 ), and the bottom face of the plate  142  is provided with a hole portion  143  corresponding to the projecting portion  128 . Thus, the plate  142  is positioned by the projecting portion  128 . 
     As shown as an example in  FIG. 2 , the inner portion in the radial direction of the plate  142  projects in the shape of a circular ring toward the first rotatable member  11 . The outer diameter of the projecting portion is the same as the inner diameter of the thrust bearing  14 , and the inner diameter of the projecting portion is substantially the same as or slightly larger than the outer diameter of the shaft member  13 . Accordingly, the projecting portion is fitted into the hollow portion of the thrust bearing  14 . The circular ring-like scale  161  is attached to the end face of the projecting portion on the first rotatable member  11  side. 
     Meanwhile, a circular ring-like washer  141  with the same outer diameter and inner diameter as the thrust bearing  14  is arranged on the first rotatable member  11  side of the thrust bearing  14 . As shown as an example in  FIG. 3 , the detecting element  162  of the encoder  16  is arranged between the inner circumferential wall of the washer  141  and the outer circumferential wall of the shaft member  13 . Specifically, the detecting element  162  is attached to the bottom face of the recess portion  115  of the first rotatable member  11  that faces the scale  161  in the axial direction. 
     The scale  161  is concentric with the shaft member  13 , and has a surface provided with divisions on which the optical reflectance periodically changes in the circumferential direction. It is possible to detect a relative rotational angle between the adjacent rotatable members ( 11  and  12 ), by reading the divisions using the detecting element  162 . That is to say, the detecting element  162  is configured as appropriate to be capable of emitting light to the scale  161  and receiving light reflected from the scale  161 . 
     The detecting element  162  outputs an electrical signal according to the received reflected light via a wiring board  163  to the outside. The wiring board  163  is constituted, for example, by a flexible printed circuit (FPC). The wiring board  163  is formed in an L-shape, and has a straight-line portion and a projecting portion  164  projecting from the straight-line portion. As shown as an example in  FIG. 3 , the end face of the projecting portion  164  is provided with a connector portion  165 . 
     Furthermore, as shown as an example in  FIGS. 1 and 3 , the first rotatable member  11  is provided with a wiring groove portion  114  with a shape that conforms to the shape of the wiring board  163  such that the wiring board  163  can be extended from the internal space to the outside. The wiring groove portion  114  linearly extends from the second face portion  112  including the recess portion  115  to the side wall portion  113 , and has substantially the same length as the straight-line portion of the wiring board  163 . Moreover, the portion of the wiring groove portion  114  positioned at the side wall portion  113  is adjacent to the coupling portion  21 . 
     Thus, as indicated by the arrows in  FIG. 3 , when the straight-line portion of the wiring board  163  is positioned along the wiring groove portion  114 , and the projecting portion  164  is bent toward the coupling portion  21 , the projecting portion  164  can be arranged at a bottom portion  214  of the groove portion  211  of the coupling portion  21 . Accordingly, in the present embodiment, the projecting portion  164  of the wiring board  163  is bonded to the bottom portion  214 . That is to say, the connector portion  165  of the wiring board  163  is arranged inside the groove portion  211  of the coupling portion  21 . 
     Accordingly, in the present embodiment, a cable  17  extending from an apparatus that uses data of the rotational angle detected by the detecting element  162  (e.g., a control apparatus for controlling an actuator) can be arranged along groove portions  314  of the link member  31  so as to be coupled with the wiring board  163 . That is to say, as shown as an example in  FIG. 3 , in a state where a cord portion of the cable  17  is fitted into the groove portion  314  of the link member  31 , a connector portion  171  of the cable  17  can be coupled with the connector portion  165  of the wiring board  163  in the groove portion  211  of the coupling portion  21 . 
     With this configuration, the encoder  16  can detect a relative rotational angle between the adjacent rotatable members ( 11  and  12 ). That is to say, since the plate  142  is positioned by causing the projecting portion  128  of the second rotatable member  12  to be fitted into the hole portion  143 , when the second rotatable member  12  axially rotates, the scale  161  axially rotates by the same angle as the axial rotation of the second rotatable member  12 . In a similar manner, since the detecting element  162  is attached to the bottom face of the recess portion  115 , when the first rotatable member  11  axially rotates, the detecting element  162  axially rotates by the same angle as the axial rotation of the first rotatable member  11 . That is to say, the scale  161  and the detecting element  162  relatively rotate axially by the angle of the relative rotation between the first rotatable member  11  and the second rotatable member  12 . The end face of the scale  161  is provided with divisions on which the optical reflectance periodically changes in the circumferential direction, and the detecting element  162  can read the divisions (reflected light). Thus, it is possible to specify a relative rotational angle between the adjacent rotatable members ( 11  and  12 ), from the output (an electrical signal according to reflected light) of the detecting element  162 . 
     Coupling Portion 
     Next, the method for coupling the link member  31  with the coupling portion  21  will be described with reference to  FIGS. 4, 5A, 5B, and 6  as well.  FIG. 4  is a partially enlarged view schematically showing an example of the coupling portion  21  of the joint structure  1  according to the present embodiment.  FIG. 5A  is a cross-sectional view schematically showing an example of a state before the link member  31  is coupled with the coupling portion  21 .  FIG. 5B  is a cross-sectional view schematically showing an example of a state after the link member  31  is coupled with the coupling portion  21 .  FIG. 6  schematically shows an example of a coupling state between an end face  210  of the coupling portion  21  and an end face  310  of the link member  31 . 
     As shown as an example in  FIGS. 1 and 4 , the coupling portions  21  according to the present embodiment each have a shape obtained by cutting, in the tangential direction, an arc portion of the side wall portion ( 113  or  123 ) of the rotatable member. Specifically, the coupling portions  21  of the rotatable members ( 11  and  12 ) each have an end face  210  that is flat and perpendicular to the radial direction, and the groove portion  211  is formed inward in the radial direction from the end face  210 . The groove portion  211  extends through a tangential direction that is perpendicular to the radial direction, and thick-wall portions  212  projecting inward are respectively provided at the upper ends of a pair of groove walls of the groove portion  211 . Accordingly, the groove portion  211  is formed to have a substantially inverted T-shaped cross-section. Note that the end face  210  is provided with four rectangular protruding portions  213  projecting outward in the radial direction. 
     Meanwhile, as shown as an example in  FIGS. 1 and 6 , the link member  31  according to the present embodiment is provided with the groove portions  314  that are respectively along both side face portions in the longitudinal direction. Accordingly, the link member  31  is formed to have a substantially H-shaped cross-section. Since edge portions  315  of a pair of groove walls constituting each groove portion  314  both project inward, the groove portion  314  is formed to have a substantially inverted T-shaped cross-section. Furthermore, as shown as an example in  FIGS. 5A and 5B , the link member  31  has, at the center on its flat end face  310 , a hole portion  311  extending in the longitudinal direction from the end face  310 . The link member  31  is a frame member made of, for example, a metal such as aluminum or titanium or a resin such as engineering plastic. However, the material of the link member  31  does not have to be limited to these, and may be selected as appropriate according to an embodiment. 
     In the present embodiment, as shown as an example in  FIGS. 5A and 5B , the coupling portion  21  and the link member  31  are coupled to each other via the wedge member  32  as follows. That is to say, the wedge member  32  includes a rectangular head portion  321  with substantially the same size as the wide-width portion of the groove portion  211  of the coupling portion  21 , and a rectangular body portion  322  with substantially the same size as the narrow-width portion of the groove portion  211 . Accordingly, the wedge member  32  is formed to have a substantially T-shaped cross-section. 
     The wedge member  32  is arranged such that the head portion  321  is fitted into the groove portion  211  of the coupling portion  21 . Accordingly, as shown as an example in  FIG. 5A , the wedge member  32  is interlocked with the thick-wall portions  212  of the groove portion  211  of the head portion  321 , and the body portion  322  project out of the groove portion  211 . The portion projecting out of the groove portion  211  of the body portion  322  is provided with a through hole  323  in the shape of a column with a diameter that is slightly larger than the diameter of a male thread portion  333  of a screw  33  such that the screw  33  can be inserted thereinto. Furthermore, the side of the through hole  323  for receiving the screw  33  is provided with a tapered portion  324  that conforms to a tapered portion  332  of the screw  33 . 
     In conformity with the through hole  323 , the link member  31  is provided with a through hole  312  that extends in the width direction (the upper-lower direction in  FIGS. 5A and 5B ) from the upper face in the drawings to the hole portion  311 , and a through hole  313  that extends in the width direction from the hole portion  311  to the lower face in the drawings. In the present embodiment, the through hole  312  has a diameter that is substantially the same as the outer diameter of a head portion  331  of the screw  33 , in order to allow the screw  33  to be inserted from the through hole  312  side. Furthermore, the through hole  313  has a diameter that is substantially the same as the outer diameter of the male thread portion  333  of the screw  33 , and the inner circumferential wall thereof is provided with a female thread into which the male thread portion  333  is to be screwed. Accordingly, as shown as an example in  FIG. 5B , the coupling portion  21  and the link members  31  can be coupled with each other, by fitting the head portion  321  of the wedge member  32  into the groove portion  211  of the coupling portion  21 , inserting the body portion  322  into the hole portion  311  of the link member  31 , and fastening the screw  33 . 
     Here, a distance WA from the end face  210  of the coupling portion  21  to the through hole  323  in a state where the head portion  321  of the wedge member  32  is fitted into the groove portion  211  is slightly shorter than a distance WB from the end face  310  of the link member  31  to the through hole  313  into which the male thread portion  333  of the screw  33  is screwed. Thus, when screwing the male thread portion  333  of the screw  33  into the through hole  313 , the tapered portion  332  of the screw  33  comes into contact with the tapered portion  324  of through hole  323  of the wedge member  32  and pulls the wedge member  32  toward the link member  31 . 
     Accordingly, the wedge member  32  is tensioned in the radial direction (the left-right direction in the drawing), and, due to this tension, the coupling portion  21  and the link member  31  are firmly coupled with each other. Specifically, the coupling portion  21  and the link member  31  are coupled with each other in the radial direction due to a force that acts from the head portion  321  of the wedge member  32  to the thick-wall portions  212  of the coupling portion  21  and a force that acts from the screw  33  via the through hole  323  of the wedge member  32  to the inner circumferential walls of the through holes ( 312  and  312 ) of the link member  31 . At this time, the link member  31  is coupled with the coupling portion  21  such that the link member  31  extends along the radial direction of the rotatable members ( 11  and  12 ), that is, such that the radial direction of the rotatable members ( 11  and  12 ) matches the longitudinal direction of the link member  31 . In the present embodiment, the link member  31  can be coupled with the coupling portion  21  of each of the rotatable members ( 11  and  12 ) through such simple fastening using the wedge member  32  and the screw  33 . 
     However, in the present embodiment, since the groove portion  211  of the coupling portion  21  extends throughout a tangential direction (direction that is perpendicular to the section of the diagram in  FIGS. 5A and 5B ) that is perpendicular to the radial direction, the wedge member  32  may move in the tangential direction, and the head portion  321  may be detached from the groove portion  211  in the tangential direction. Thus, in the present embodiment, the end face  210  of the coupling portion  21  is provided with the four protruding portions  213 . 
     Specifically, as shown as an example in  FIG. 6 , the four protruding portions  213  are arranged at four corners of a rectangle so as to be interlocked with the edge portions  315  of the link member  31 . Accordingly, in a state where the coupling portion  21  and the link member  31  are coupled with each other, the edge portions  315  of the link member  31  are interlocked with the protruding portions  213 , and thus movement in the tangential direction (the upper-lower direction in  FIG. 6 ) of the wedge member  32  for coupling the coupling portion  21  and the link members  31  can be suppressed. Furthermore, the protruding portions  213  are in contact with the link member  31  also in the axial direction, wobbling of the link member  31  in the axial direction can be suppressed. Furthermore, in the present embodiment, the protruding portions  213  conform to the shape near the edge portions  315  of the link member  31 , and thus the protruding portions  213  can be used for positioning of the link member  31 . 
     As shown as an example in  FIGS. 2, 5A, and 5B , the thickness of each of the rotatable members ( 11  and  12 ) according to the present embodiment, in other words, the height of each of the side wall portions ( 113  and  123 ) is the same as the thickness (the length in the left-right direction in  FIG. 2 ) of each of the link members  31 . Accordingly, when the rotatable members ( 11  and  12 ) rotate, the link members  31  coupled with the rotatable members ( 11  and  12 ) do not interfere with each other. Note that “the same” refers not only to a state in which the thickness of each of the rotatable members ( 11  and  12 ) is completely the same as the thickness of each of the link members  31  but also to a state in which the thickness of each of the rotatable members ( 11  and  12 ) is larger than the thickness of each of the link members  31  to the extent that they do not interfere with each other or later-described reinforcing plates  51  can be arranged. 
     § 2 Usage Example 
     Various link mechanisms can be constructed using the joint structure  1  according to the present embodiment. Hereinafter, three examples are shown. 
     Scott Russell Mechanism 
     First, an example of constructing a robot  400  having parallel-linked Scott Russell mechanism using six joint structures  408   a  to  408   f  will be described with reference to  FIGS. 7A and 7B .  FIGS. 7A and 7B  are a perspective view and a side view schematically showing an example of the robot  400  according to this usage example. Note that the joint structures are denoted by reference numerals  408   a  to  408   f  merely for the sake of ease of description, and the joint structures  408   a  to  408   f  correspond to the joint structure  1  described above. The joint structures  408   a  to  408   f  are arranged such that the second rotatable member is on the front side in the section of the diagrams. In a similar manner, the link members are denoted by reference numerals  407   a  to  407   h  merely for the sake of ease of description, and the link members  407   a  to  407   h  correspond to the link members  31  described above. 
     The robot  400  according to this usage example includes a rectangular base  401  that is placed on the ground, and a support  402  in the shape of a rectangular column extending in the vertical direction from the upper face of the base  401 . A pair of actuators ( 403  and  404 ) are attached to the support  402  so as to be spaced from each other in the upper-lower direction. Furthermore, two movable portions ( 405  and  406 ) are attached so as to be movable (slidable) in the upper-lower direction between the pair of actuators ( 403  and  404 ). 
     The actuators ( 403  and  404 ) drive output rods in the vertical direction, thereby moving the movable portions ( 405  and  406 ) in the upper-lower direction. Specifically, the actuator  403  arranged on the upper side moves the movable portion  405  in the upper-lower direction, and the actuator  404  arranged on the lower side moves the movable portion  406  in the upper-lower direction. That is to say, the movable portions ( 405  and  406 ) can move in the upper-lower direction independently of each other. 
     Note that the type of the actuators ( 403  and  404 ) may be selected as appropriate according to an embodiment as long as the output rods can be moved in the vertical direction. For example, linear actuators, electric actuators, hydraulic actuators, pneumatic actuators, hybrid actuators, or the like may be used as the actuators ( 403  and  404 ). Furthermore, the type of the movable portions ( 405  and  406 ) may be selected as appropriate according to an embodiment as long as they can move in the upper-lower direction. For example, linear bearings may be used as the movable portions ( 405  and  406 ). 
     A link member  407   a  extending in the horizontal direction is attached to the movable portion  405 . In a similar manner, two link members ( 407   b  and  407   c ) extending in the horizontal direction are attached to the movable portion  406  so as to be spaced from each other in the upper-lower direction. The link members  407   a  to  407   c  are formed as short members. 
     A joint structure  408   a  is attached to the end portion of the link member  407   a  on the side opposite to the movable portion  405 . Specifically, the link member  407   a  is coupled with a coupling portion of the first rotatable member of the joint structure  408   a . Furthermore, a link member  407   d  that is longer in the longitudinal direction than the link member  407   a  is coupled with a coupling portion of the second rotatable member of the joint structure  408   a.    
     Meanwhile, the end portion of the link member  407   b  on the side opposite to the movable portion  406  is coupled with a coupling portion of the second rotatable member of the joint structure  408   b . Furthermore, a link member  407   e  that is longer in the longitudinal direction than the link member  407   b  is coupled with a coupling portion of the first rotatable member of the joint structure  408   b.    
     The link members ( 407   d  and  407   e ) are coupled with a joint structure  408   d . Specifically, the link member  407   e  is coupled with a coupling portion of the first rotatable member of the joint structure  408   d , and the link member  407   d  is coupled with a coupling portion of the second rotatable member. Furthermore, a link member  407   g  with a length similar to that of the link member  407   e  is coupled with another coupling portion of the first rotatable member of the joint structure  408   d.    
     Accordingly, the Scott Russell mechanism is constituted by the three joint structures ( 408   a ,  408   b , and  408   d ) and the five link members ( 407   a ,  407   b ,  407   d ,  407   e , and  407   g ). The end portion of the link member  407   g  on the side opposite to the joint structure  408   d  is coupled with a coupling portion of the first rotatable member of a joint structure  408   e . Furthermore, a link member  407   h  with a length that is the same as the distance in the upper-lower direction between the two joint structures ( 408   b  and  408   c ) adjacent to the movable portion  406  is coupled with a coupling portion of the second rotatable member of the joint structure  408   e . The end portion of the link member  407   h  on the side opposite to the joint structure  408   e  is coupled with a coupling portion of the second rotatable member of the joint structure  408   f . A front end portion  409  such as an end effector is attached to the link member  407   h.    
     Meanwhile, the end portion of the link member  407   c  arranged below the link member  407   b , on the side opposite to the movable portion  406 , is coupled with a coupling portion of the second rotatable member of the joint structure  408   c . Furthermore, a link member  407   f  with a length that is the same as the total length of the two link members ( 407   e  and  407   g ) and the joint structure  408   d  arranged above is coupled with a coupling portion of the first rotatable member of the joint structure  408   c . The end portion of the link member  407   f  on the side opposite to the joint structure  408   c  is coupled with a coupling portion of the first rotatable member of the joint structure  408   f.    
     That is to say, in the robot  400 , the pair of link members ( 407   e  and  407   g ) are parallel to the link member  407   f , the links connecting the four joint structures ( 408   b ,  408   c ,  408   f , and  408   e ) form a parallelogram (parallel link). Thus, due to the characteristics of the Scott Russell mechanism, the robot  400  can move the front end portion  409  in the upper-lower and front-rear directions (the arrow directions in  FIG. 7B ), by driving the actuators ( 403  and  404 ) and moving the movable portions ( 405  and  406 ) in the upper-lower direction. Moreover, due to the characteristics of the parallel link, the robot  400  can keep the front end portion  409  horizontal even when driving the actuators ( 403  and  404 ) and moving the front end portion  409  attached to the link member  407   h  in the upper-lower and front-rear directions. 
     Note that, in the robot  400 , two coupling portions of the same rotatable member are simultaneously used only in the joint structure  408   d . That is to say, two coupling portions of the first rotatable member of the joint structure  408   d  are used to linearly couple the two link members ( 407   e  and  408   g ). On the other hand, only one coupling portion is used to couple the link members in the rotatable members of the other joint structures. Thus, it is sufficient that the rotatable members of the other joint structures each have at least one coupling portion, and the other coupling portions may be omitted. Furthermore, the rotatable members that are coupled by the link members do not have to be limited to the examples described above, and may be selected as appropriate according to an embodiment. 
     As described above, if two or more coupling portions are provided on a side wall portion of at least one rotatable member, the robot  400  having parallel-linked Scott Russell mechanism can be constructed. Thus, it is possible to realize a complex link mechanism such as a parallel-linked Scott Russell mechanism as described above by arranging a plurality of coupling portions on a side wall portion of at least one rotatable member among the plurality of rotatable members. 
     Wire Driving Mechanism 
     Next, an example of driving the joint structure  1  by wires using the wire-driving groove portions  129  will be described with reference to  FIG. 8 .  FIG. 8  is a perspective view schematically showing an example of a robot  410  having three joint structures  412  that are driven by pulling wires. As in the foregoing example, the joint structures are denoted by reference numerals  412 A and  412 B merely for the sake of ease of description, and the joint structures ( 412 A and  412 B) correspond to the joint structure  1  described above. Specifically, the joint structure using the first rotatable member  11  on the front side in the section of the diagram is denoted by “ 412 A”, and the joint structure using the second rotatable member  12  on the front side in the section of the diagram is denoted by “ 412 B”. Hereinafter, they will be simply referred to as “joint structures  412 ” if they are not to be distinguished from each other. In a similar manner, the link members are denoted by a reference numeral  411  merely for the sake of ease of description, and the link members  411  correspond to the link members  31  described above. 
     In the robot  410  according to this usage example, four link members  411  are coupled by the three joint structures  412 . The link members  411  are as appropriate coupled with coupling portions of the joint structures  412 . Specifically, the first rotatable member of the joint structure  412 B and the second rotatable member of the joint structure  412 A arranged below the joint structure  412 B are coupled with each other via the link member  411 . Furthermore, the second rotatable member of the joint structure  412 B and the first rotatable member of the joint structure  412 A arranged above the joint structure  412 B are coupled with each other via the link member  411 . Above each of the joint structures  412 , a pair of fixtures ( 413  and  414 ) are fixed to the groove portions of the link member  411 . 
     End portions of the wires ( 415  and  416 ) are fixed to the fixtures ( 413  and  414 ). Specifically, an end portion of the wire  415  is fixed to the fixture  413  and an end portion of the wire  416  is fixed to the fixture  414 . The wires ( 415  and  416 ) are Bowden cables that are arranged along the wire-driving groove portions  129  and are then allowed to pass through binding members  417  arranged below the respective joint structures  412  so as to be coupled with a drive source provided outside. The drive source is, for example, a pneumatic actuator, a motor, or the like. 
     The thus configured robot  410  according to this usage example operates as follows. That is to say, when the wire  415  is pulled by the external drive source, the force acts on the fixture  413 , and the link member  411  above the joint structure  412  that is to be driven is pulled in the arrow A 1  direction. Accordingly, the rotatable member coupled with the link member  411  rotates. In a similar manner, when the wire  416  is pulled by the external drive source, the force acts on the fixture  414 , and the link member  411  above the joint structure  412  that is to be driven is pulled in the arrow A 2  direction. Accordingly, the rotatable member coupled with the link member  411  rotates. The robot  410  according to this usage example can drive the joint structures  412  by pulling wires in this manner. 
     In this usage example, each joint structure  412 A is used in a state of being reversed about an axis perpendicular to the axial direction with respect to the joint structure  412 B. The first rotatable member of the joint structure  412 B and the second rotatable member of the joint structure  412 A arranged below the joint structure  412 B are coupled with each other via the link member  411 . The second rotatable member of the joint structure  412 B and the first rotatable member of the joint structure  412 A arranged above the joint structure  412 B are coupled with each other via the link member  411 . Accordingly, two joint structures ( 412 A and  412 B) adjacent to each other via the link member  411  are arranged such that the rotatable members face each other in the direction that is perpendicular to the axial direction. Thus, the robot  410  according to this usage example is compact in the axial direction. However, the use state of the joint structures for making the link mechanism compact in the axial direction is not limited to the foregoing example. For example, as in the robot  400  described above, adjacent two joint structures may be used in a state of being oriented in the same direction. In this case, the radial bearings may be transition-fitted with slight gap provided at the shaft members and the inner walls of the through holes instead of being interference-fitted to any of the shaft members and the inner walls of the through holes. With this configuration, since adjacent two joint structures are used in a state of being oriented in the same direction, fasteners and the like can be arranged in one direction. Accordingly, it is possible to construct a link mechanism in which the joint structures can be subjected to maintenance operations from one direction even when grease up of bearings or adjustment of pressurization is needed. 
     Delta Robot 
     Next, an example of constructing a delta robot  420  having parallel link mechanisms at three points through  18  joint structures  424  will be described with reference to  FIG. 9A .  FIG. 9A  is a perspective view schematically showing an example of the delta robot  420  according to this usage example. As in the foregoing example, the joint structures denoted by a reference numeral  424  merely for the sake of ease of description, and the joint structures  424  correspond to the joint structure  1  described above. 
     The delta robot  420  according to this usage example has a base portion  421  in the shape of a triangular frame. A rotary motor  422  is attached to the center of each side of the base portion  421 , and the rotary motor  422  is coupled with a link member  423   a . The link member  423   a  corresponds to the link member  31  described above. 
     The joint structure  424  is coupled with the other end portion of the link member  423   a . A T-shaped link member  423   b  is coupled with the joint structure  424 . A parallel link is constituted by the link member  423   b  together with four joint structures  424 , two link members ( 423   c  and  423   d ) with the same length, and a T-shaped link member  423   e.    
     The link members ( 423   c  and  423   d ) correspond to the link members  31  described above. Furthermore, the end portions of the T-shaped link members ( 423   b  and  423   e ) have a configuration similar to that of the end portions of the link members  31 . The T-shaped link members ( 423   b  and  423   e ) can be each produced, for example, by welding or bonding two link members  31  as appropriate. The link members  423   a  to  423   e  are coupled with the coupling portions of the joint structures  424  as appropriate. 
     Furthermore, the remaining end portion of the T-shaped link member  423   e  is also coupled with joint structures  424 , and a front end portion  425  in the shape of a triangular frame is attached to the three joint structures  424  in total arranged lowermost. Specifically, the corners of the front end portion  425  are respectively provided with link portions  426  with a configuration similar to that of the end portions of the link members  31 , and the front end portion  425  are coupled with the joint structures  424  respectively via the link portions  426 . 
     The thus configured delta robot  420  according to this usage example operates as follows. That is to say, in the delta robot  420  according to this usage example, parallel link mechanisms are respectively coupled with the three rotary motors  422  arranged at the base portion  421 . Thus, if all or a part of the three rotary motors  422  are driven, the parallel link mechanisms coupled with the driven rotary motors  422  can be moved in the upper-lower direction, and thus it is possible to move the front end portion  425  in each direction while maintaining the horizontal posture. 
     As shown as an example in  FIG. 9B , the actuators used for the delta robot  420  do not have to be limited to the rotary motors  422 .  FIG. 9B  is a perspective view schematically showing an example of a delta robot  420 A in which linear motors  427  that drive output rods in the vertical direction are used as actuators. 
     As shown as an example in  FIG. 9B , the delta robot  420 A according to this usage example has the same configuration as that of the delta robot  420  described above, except that the rotary motors  422  are replaced by the linear motors  427  for linear movement. The delta robot  420 A can operate in a manner similar to that of the delta robot  420  by moving the output rods of the linear motors  427  in the upper-lower direction. 
     Characteristics 
     As described above, in the joint structure  1  according to the present embodiment, the two rotatable members ( 11  and  12 ) are coupled with each other in an axially rotatable manner. Moreover, each of the rotatable members ( 11  and  12 ) includes two coupling portions  21  for coupling the link members  31  constituting a link of a robot. Thus, as shown in the foregoing usage example, it is possible to couple the plurality of link members  31  via the joint structures  1 , by coupling the different link members  31  with the different rotatable members ( 11  and  12 ). Furthermore, when the link members  31  are moved by an external force acting from actuators or the like, the rotatable members ( 11  and  12 ) can axially rotate in accordance with the rotation of the link members  31 . 
     That is to say, the joint structures  1  according to the present embodiment can be driven by an external force transmitted from the link members  31 , and thus it is possible to change a positional relationship between the link members  31  coupled with the different rotatable members ( 11  and  12 ). Moreover, it is possible to construct various link mechanisms as described in the usage examples using the joint structures  1 . Accordingly, the joint structures  1  according to the present embodiment are modularized and can be used for general purposes. 
     Furthermore, in the present embodiment, the face portions ( 112  and  121 ) that face each other, of the rotatable members ( 11  and  12 ) that are adjacent to each other in the axial direction, are respectively provided with the recess portions ( 115  and  126 ), and the thrust bearing  14  is arranged in the internal space defined by the recess portions ( 115  and  126 ). Thus, the strength in the axial direction of the joint structure  1  according to the present embodiment is ensured by the thrust bearing  14 . 
     Furthermore, in the present embodiment, the internal space defined by the recess portions ( 115  and  126 ) further accommodates the encoder  16  capable of detecting a relative rotational angle between the rotatable members ( 11  and  12 ). Thus, in the present embodiment, the encoder  16  can be prevented from being coming into contact with the outside without using extra constituent elements such as casings, and thus the possibility that the encoder  16  will be out of order due to an external force can be significantly lowered. 
     Furthermore, since the encoder  16  is arranged in the internal space, it is less likely to be affected by the deformation of the joint structure  1  compared with the case in which it is arranged outside. That is to say, even when the outer shape of the joint structure  1  is deformed by an external force, the internal space defined by the recess portions ( 115  and  126 ) is less likely to be deformed. Thus, even when the outer shape of the joint structure  1  is deformed, a positional relationship between the scale  161  and the detecting element  162  constituting the encoder  16  hardly changes. Accordingly, even when used in a situation where an external force is applied, the joint structure  1  can stably detect a relative rotational angle between the rotatable members ( 11  and  12 ). 
     Moreover, in the present embodiment, the scale  161  rotates in one piece with the second rotatable member  12 , and the detecting element  162  rotates in one piece with the first rotatable member  11 . Thus, in the joint structure  1  according to the present embodiment, errors are not caused by backlash or slippage compared with a method in which rotation of the rotatable members ( 11  and  12 ) is measured using an external encoder via transmission components such as belts, gears, or couplings. Accordingly, the joint structure  1  according to the present embodiment can accurately detect a relative rotational angle between the rotatable members ( 11  and  12 ). 
     In the present embodiment, the rotatable members ( 11  and  12 ) each have a columnar basic shape, and the coupling portions  21  are formed by cutting, in the tangential direction, an arc portion of the basic shape. That is to say, the rotatable members ( 11  and  12 ) do not have a shape having a portion projecting from a circle, and thus the rotatable members ( 11  and  12 ) can be produced through lathe machining. Thus, even when producing the rotatable members ( 11  and  12 ) through processing, it is very easy to produce the rotatable members ( 11  and  12 ). 
     Furthermore, the joint structure  1  according to the present embodiment is of an externally-driven type, and constituent components that are essential for a joint structure with a built-in actuator, such as a housing for accommodating the actuator, are not necessary. Thus, the joint structure  1  can be made compact and light. Moreover, the joint structure  1  according to the present embodiment does not have a complex structure, and thus it can be easily produced with a simple design. 
     Furthermore, in the joint structure  1  according to the present embodiment, the first rotatable member  11  excluding the shaft member  13  has substantially the same shape as the second rotatable member, and thus the rotatable members ( 11  and  12 ) are bilaterally symmetric about the axial direction. Specifically, the side face portions ( 113  and  123 ) have a shape that is symmetric about a plane perpendicular to the axial direction of the shaft member  13 . Thus, with the joint structure  1  according to the present embodiment, it is easy to construct a closed link mechanism. Furthermore, if the rotatable members ( 11  and  12 ) of the plurality of joint structures  1  are alternately coupled via the link members  31 , it is possible to construct a link mechanism without increasing the volume. 
     Furthermore, in the present embodiment, the two coupling portions  21  provided in each of the rotatable members ( 11  and  12 ) are arranged at positions at 180 degrees about an axis, on the side wall portions ( 113  and  123 ). Accordingly, the coupling portions  21  of each of the rotatable members ( 11  and  12 ) are arranged symmetric about the axial direction, and thus the link members  31  coupled with the rotatable members ( 11  and  12 ) can be used symmetrically about the axial direction. For example, even when a link mechanism including the joint structure  1  in which the link member  31  on the first rotatable member  11  side is fixed is changed by reversing the joint structure  1  so that the link member  31  on the second rotatable member  12  side is fixed, the same link mechanism can be constructed. Furthermore, in the present embodiment, the rotatable member  11 , which is one of the two rotatable members ( 11  and  12 ), is formed in one piece with the shaft member  13 , and the rotatable member  12 , which is the other rotatable member, has the through hole  124  into which the shaft member  13  can be inserted. Then, the radial bearings  15  can be arranged so as to be interference-fitted to the shaft member  13  and clearance-fitted to the inner circumferential wall of the through hole  124 , or so as to be clearance-fitted to the shaft member  13  and clearance-fitted to the inner circumferential wall of the through hole  124 . Accordingly, for example, the following effects can be expected. That is to say, in the joint structure  1  according to the present embodiment, when the link member  31  on the first rotatable member  11  side is fixed, the second rotatable member  12  rotates, and radial loads of the rotating outer ring and the stationary inner ring act inside. Thus, the diameter of the through hole  124  into which the radial bearings  15  are to be inserted is determined assuming the radial loads. For example, if the second rotatable member  12  is driven at unbalanced load, the fitting of the radial bearings  15  is set such that the inner ring is interference-fitted and the outer ring is clearance-fitted. That is to say, the radial bearings  15  with an inner diameter that is slightly smaller than the outer diameter of the shaft member  13  and an outer diameter that is slightly smaller than the diameter of the through hole  124  are arranged so as to be interference-fitted to the shaft member  13  and clearance-fitted to the inner circumferential wall of the through hole  124 . Thus, the diameter of the through hole  124  is determined so as to be larger than the outer diameter of the radial bearings  15 . Meanwhile, when the second rotatable member  12  is driven at a stationary load, setting of the radial bearings  15  needs to be changed such that the inner ring is clearance-fitted and the outer ring is interference-fitted. That is to say, the radial bearings  15  with an inner diameter that is slightly larger than the outer diameter of the shaft member  13  and an outer diameter that is slightly larger than the diameter of the through hole  124  are arranged so as to be clearance-fitted to the shaft member  13  and interference-fitted to the inner circumferential wall of the through hole  124 . At this time, if the radial bearings  15  are not changed, the diameter of the through hole  124  needs to be changed. On the other hand, the joint structure  1  according to the present embodiment has a shape that allows the link members  31  to be used in a bilaterally symmetric manner. 
     Thus, if the link member  31  on the second rotatable member  12  side is fixed and a stationary load is caused to act on the first rotatable member  11 , the joint structure  1  according to the present embodiment can be used as it is for link mechanisms without changing the diameter of the through hole  124  or the diameter of the shaft member  13 , even when load conditions applied thereto vary. If a link mechanism is constructed using a plurality of joint structures  1 , the joint structures  1  in both forms in which the inner rings of the radial bearings  15  are interference-fitted and in which the inner rings of the radial bearings  15  are clearance-fitted may be used. In this case, it is possible to construct a link mechanism that is compact in the axial direction, by using, symmetrically about the axial direction, the joint structure  1  in which the inner rings of the radial bearings  15  are interference-fitted and the joint structure  1  in which the inner rings of the radial bearings  15  are clearance-fitted. 
     Here, “the coupling portions are arranged symmetric about the axial direction” refers to a state in which the connecting relationship of the rotatable members ( 11  and  12 ) can be switched by reversing the joint structure  1  about an axis (an axis SA or an axis SB in  FIG. 1 ) perpendicular to the axial direction, while maintaining the positional relationship between the plurality of link members  31  coupled with the rotatable members ( 11  and  12 ). That is to say, if it is assumed that the joint structure  1  given as an example in  FIG. 1  is reversed about the axis SA (y axis) or the axis SB (z axis), the link member  31  coupled with the first rotatable member  11  before reversing can be coupled with the coupling portion  21  of the second rotatable member  12  without changing the position after reversing. Furthermore, the link member  31  coupled with the second rotatable member  12  before reversing can be coupled with the coupling portion  21  of the first rotatable member  11  without changing the position after reversing. Accordingly, it is possible to use the joint structure  1  as a joint of inner ring rotation in which the first rotatable member  11  rotates or as a joint of outer ring rotation in which the second rotatable member  12  rotates, by changing the orientation of the joint structure  1  that is used for the link mechanism, without changing the structure of the link mechanism. Furthermore, it is also possible to change the positions of the wiring groove portions  114  as appropriate. Note that “the coupling portions are arranged symmetric about the axial direction” is not limited to the example in which two coupling portions provided in each of the rotatable members ( 11  and  12 ) are arranged at positions at 180 degrees about an axis, but may be designed as appropriate according to an embodiment. 
     Furthermore, the two coupling portions  21  provided on each of the side wall portions ( 113  and  123 ) of the rotatable members ( 11  and  12 ) are arranged at positions at 180 degrees about an axis, and thus the coupling portions  21  are symmetric about an axis in each of the rotatable members ( 11  and  12 ). Thus, even when the joint structure  1  according to the present embodiment is axially rotated, it can be used while maintaining the positional relationship between the link members. Accordingly, it is also possible to change the positions of the wiring groove portions  114  as appropriate. Note that the state in which the coupling portions are symmetric about an axis in each rotatable member is not limited to the example in which two coupling portions are arranged at positions at 180 degrees about an axis, and may be designed as appropriate according to an embodiment. 
     Moreover, in the joint structure  1  according to the present embodiment, it is also possible to make not only the outer shape but also the entire weight balance bilaterally symmetric, by selecting as appropriate the weights of the constituent elements. Thus, the following effects can be expected. That is to say, since a conventional joint structure with a built-in actuator is driven by an electric motor, use of an electric motor alone leads to driving at high speed and low torque, which is not suitable to drive a robot. Thus, an electric motor is used in combination with a reduction drive. 
     Accordingly, the conventional joint structure with a built-in actuator cannot make the entire weight balance bilaterally symmetric due to a difference between the flow rates of the electric motor and the reduction drive. Thus, in a link mechanism using such a joint structure, typically, the weights on the left and right sides cannot be balanced. Accordingly, a force that twists the links occurs, and thus the links before and after the joint structure may come into contact with each other. This aspect is problematic especially in the case of robots such as the robot  400  in which the link mechanisms are arranged in a vertical face. 
     Furthermore, for example, it is assumed that joint structures whose weight balance is not bilaterally symmetric are alternatively arranged to make the weight balance of the entire link mechanism bilaterally symmetric. Also in this case, since the joint structures are alternatively arranged, wires extending from the joint structures become alternatively located, and thus the arrangement of wires in the entire link mechanism becomes poor. 
     Meanwhile, the joint structure  1  according to the present embodiment is of an externally-driven type, and does not have a built-in electric motor and reduction drive, and thus it is possible to make the entire weight balance bilaterally symmetric, by selecting as appropriate the weights of the constituent elements. Thus, in the link mechanism (e.g., the robot  400  described above) using the joint structure  1 , the entire weight balance can be made substantially bilaterally symmetric, and the link members  31  before and after the joint structure  1  can be prevented from coming into contact with each other. Moreover, the joint structures  1  do not have to be alternatively arranged in order to make the weight balance of the link mechanism bilaterally symmetric, and thus the arrangement of wires in the entire link mechanism can be prevented from becoming poor. 
     § 3 Modified Example 
     Above, an embodiment of the present invention was described in detail, but the description above is in all aspects merely an example of the present invention. It will be appreciated that various improvements and modifications can be made without departing from the scope of the present invention. Furthermore, the constituent elements of the joint structure  1  may be omitted, replaced or added, as appropriate, according to an embodiment. The shape and the size of the constituent elements of the joint structure  1  may be set as appropriate according to an embodiment. For example, the following changes may be made. Note that in modified examples described below, the same constituent elements as in the foregoing embodiment are denoted by the same reference numerals, and a description thereof has been omitted as appropriate. The following modified example may be combined as appropriate. 
     3.1 
     For example, the joint structure  1  according to the foregoing embodiment includes two rotatable members ( 11  and  12 ). However, the number of rotatable members included in the joint structure of the present invention does not have to be that in the examples of the foregoing embodiment, and may be three or more. 
     Hereinafter, an example thereof will be described with reference to  FIG. 10 .  FIG. 10  is a cross-sectional view schematically showing an example of a joint structure  1 A including three rotatable members ( 11 ,  12 , and  18 ). As shown in  FIG. 10 , the joint structure  1 A according to this modified example is formed so as to be substantially similar to in the joint structure  1  described above. That is to say, a third rotatable member  18  has the same configuration as that of the second rotatable member  12 . The internal space between the first rotatable member  11  and the second rotatable member  12  and the internal space between the second rotatable member  12  and the third rotatable member  18  accommodate the thrust bearing  14  and an encoder as appropriate as in the foregoing embodiment. Furthermore, a shaft member  13 A has the same configuration as that of the shaft member  13  according to the foregoing embodiment, except that the length in the axial direction is made longer by the length for allowing the third rotatable member  18  to be attached. Furthermore, two radial bearings  15  are arranged between the second rotatable member  12  and the shaft member  13 A and between the third rotatable member  18  and the shaft member  13 A, as in the foregoing embodiment. Accordingly, the joint structure  1 A includes three rotatable members ( 11 ,  12 , and  18 ) coupled with each other in an axially rotatable manner. 
     That is to say, in the foregoing embodiment, it is possible to adjust the number of second rotatable members  12  that are attached to the shaft member  13  by adjusting the length in the axial direction of the shaft member as appropriate. Accordingly, a joint structure including three or more rotatable members can be produced as appropriate. Note that the method for producing a joint structure including three or more rotatable members is not limited to the example described above, and may be selected as appropriate according to an embodiment, as in modified examples, which will be described later. 
     3.2 
     For example, in the foregoing embodiment, the face portions ( 111 ,  112 ,  121 , and  122 ) of the rotatable members ( 11  and  12 ) are formed in the shape of a circle. However, the shape of the rotatable members ( 11  and  12 ) does not have to be limited to this example, and may be selected as appropriate according to an embodiment. For example, the shape of the face portions ( 111 ,  112 ,  121 , and  122 ) of the rotatable members ( 11  and  12 ) may be a polygon such as a hexagon, or may be an oval. In the case of these shapes, the outer shape of the rotatable members ( 11  and  12 ) may be formed symmetric about the axial direction. 
     3.3 
     Furthermore, for example, in the foregoing embodiment, each of the rotatable members ( 11  and  12 ) includes two coupling portions  21 . However, the number of coupling portions  21  included in each of the rotatable members ( 11  and  12 ) is not limited to two, and may be selected as appropriate according to an embodiment. For example, the number of coupling portions  21  included in each of the rotatable members ( 11  and  12 ) may be one, or may be three or more. At this time, the coupling portions  21  may be arranged at the side wall portions ( 113  and  123 ) as in the foregoing embodiment, or may be arranged at the face portions ( 111 ,  112 ,  121 , and  122 ) as in the foregoing modified example. 
     3.4 
     Furthermore, for example, in the foregoing embodiment, the coupling portions  21  are arranged at the side wall portions ( 113  and  123 ) of the rotatable members ( 11  and  12 ). However, the arrangement of the coupling portions  21  does not have to be limited to this example, and the coupling portions  21  may be arranged at the face portions ( 111 ,  112 ,  121 , and  122 ) of the rotatable members ( 11  and  12 ). 
     Hereinafter, an example thereof will be described with reference to  FIG. 11 .  FIG. 11  schematically shows an example of a joint structure  1 B in which a first face portion  111 B of a first rotatable member  11 B has a coupling portion  21 B. As shown as an example in  FIG. 11B , the first rotatable member  11 B has the same configuration as that of the first rotatable member  11  described above, except that the first face portion  111 B is provided with the coupling portion  21 B. The coupling portion  21 B has the same configuration as that of the coupling portion  21 . Thus, it is possible to couple the link members  31  with the coupling portion  21 B, using the above-described coupling method. 
     If at least one coupling portion is provided at one of a pair of face portions of at least one of a plurality of rotatable members, and at least one coupling portion is provided at a side wall portion of another rotatable member of the plurality of rotatable members as shown as an example in  FIG. 11 , the following effects can be expected. That is to say, the link connecting direction can be changed between the coupling portion provided at the face portion (e.g., the coupling portion  21 B of the first rotatable member  11 B) and the coupling portion provided at the side wall portion (e.g., the coupling portion  21  of the second rotatable member  12 ). Thus, the link connecting direction can be changed without a special structure, and thus the link mechanism that is to be constructed can be made compact on the whole. 
     Note that the face portion at which a coupling portion can be provided is not limited to the first face portion of the first rotatable member. For example, a coupling portion may be provided on the second face portion side of the second rotatable member. Furthermore, if a coupling portion is provided at a face portion of each rotatable member, the coupling portion may be provided via an attachment or the like. 
     3.5 
     Furthermore, for example, in the foregoing embodiment, the two coupling portions  21  are arranged at positions at 180 degrees about the center in the surface direction of the rotatable members ( 11  and  12 ) (hereinafter, this angle is referred to as “angle between adjacent coupling portions”). However, if a plurality of coupling portions  21  are provided at the side wall portions ( 113  and  123 ), the positional relationship between the coupling portions  21  does not have to be limited to this example, and may be selected as appropriate according to an embodiment. 
     For example, the angle between adjacent coupling portions may be set at an obtuse angle or an acute angle. Furthermore, for example, if a polygonal link is constructed using link members, the angle between adjacent coupling portions may be set to be the same as the angle of one corner of the polygon such that the joint structures can be arranged at the respective corners. If the joint structures in which the angle between adjacent coupling portions is an obtuse angle are used, for example, a boomerang-shaped parallel link mechanism as shown in  FIG. 12  can be constructed. 
       FIG. 12  is a perspective view schematically showing an example of a robot  400 C using joint structures  1 C in which an angle between adjacent coupling portions is an obtuse angle. In the robot  400 C given as an example in  FIG. 12 , the joint structure  408   d  arranged at the middle in the Scott Russell mechanism portion of the robot  400  described above is replaced by a joint structure  1 C in which the angle between adjacent coupling portions is an obtuse angle. 
     Thus, two link members ( 407   e  and  407   g ) coupled with the joint structure  1 C constitute a link that is bent in a boomerang shape. Accordingly, in this modified example, the link member  407   f  of the robot  400  is replaced by the joint structure  1 C and two link members  31   c.    
     Each link member  31   c  has the same configuration as that of the link member  31 , and has the same length as each of the link members ( 407   e  and  407   g ). Accordingly, the link constituted by the two link members  31   c  and the joint structure  1 C has the same shape as the link constituted by the two link members ( 407   e  and  407   g ) and the joint structure  1 C. That is to say, a boomerang-shaped parallel link is formed. 
     As described above, if the joint structure  1 C in which the angle between adjacent coupling portions is an obtuse angle is used, the robot  400 C in which the parallel link is in a boomerang shape can be constructed. Note that the link on the lower side may use link members with the same shape as the boomerang-shaped link constituted by two link members ( 407   e  and  407   g ) without using the joint structure  1 C. 
     Furthermore, for example, in the foregoing embodiment and modified examples, the link member  31  is coupled with a rotatable member so as to extend in the radial direction or the axial direction (direction that is perpendicular to a face). However, the orientation in which the link member  31  is coupled does not have to be limited to this example, and may be selected as appropriate according to an embodiment. For example, the end face  210  of the coupling portion  21  ( 21 B) may be at an angle with respect to the radial direction (axial direction). Accordingly, the link member  31  can be coupled with the coupling portion  21  ( 21 B) so as to be inclined from the radial direction or the axial direction with respect to the rotatable member. 
     3.6 
     Furthermore, for example, in the foregoing embodiment and modified examples, the shaft member  13  ( 13 A) is formed in one piece with the first rotatable member  11 . However, the shaft member  13  ( 13 A) may be formed in one piece with a rotatable member other than the first rotatable member  11 . If a joint structure includes three or more rotatable members as in the foregoing modified example, the shaft member  13  ( 13 A) may be formed in one piece with either of the two rotatable members arranged on the outermost side. Furthermore, the shaft member  13  ( 13 A) may be formed in one piece with any of those arranged between the two rotatable members arranged on the outermost side. In this case, the shaft member  13  ( 13 A) is formed so as to extend in the axial direction from face portions on both sides of the rotatable member. Furthermore, the shaft members  13  ( 13 A) may be formed in one piece respectively with the two rotatable members arranged on the outermost side. In this case, the shaft members  13  ( 13 A) extending from the rotatable members may be configured such that they can be coupled with each other through screwing or the like. 
     Furthermore, as shown as an example in  FIG. 13 , the shaft member  13  ( 13 A) may be formed separately from the rotatable members.  FIG. 13  is a perspective view schematically showing an example of a joint structure  1 D including a shaft member  13 D formed separately from a first rotatable member  11 D. As shown as an example in  FIG. 13 , the rotatable members ( 11 D,  12 D, and  18 D) each have substantially the same configuration as that of the second rotatable member  12  described above. 
     The shaft member  13 D according to this modified example includes a circular ring-like base portion  133  and a cylindrical portion  134  extending in the axial direction from the base portion  133 . The cylindrical portion  134  is formed so as to be substantially similar to in the shaft member  13  ( 13 A) described above. That is to say, the length in the axial direction of the cylindrical portion  134  matches the total of the widths of the rotatable members ( 11 D,  12 D, and  18 D). Furthermore, the outer diameter of the cylindrical portion  134  is smaller than the inner diameter of the rotatable members ( 11 D,  12 D, and  18 D) to the extent that the radial bearings  15  can be arranged. Accordingly, the radial bearings  15  are arranged between the shaft member  13 D and the rotatable members ( 11 D,  12 D, and  18 D). 
     Meanwhile, the outer diameter of the base portion  133  is larger than the outer diameter of the cylindrical portion  134 . Accordingly, the base portion  133  is not allowed to be inserted into the through holes of the rotatable members ( 11 D,  12 D, and  18 D). In this modified example, a recess portion  117  provided at a face portion of the first rotatable member  11 D has the same diameter as the outer diameter of the base portion  133 , and the base portion  133  is fitted into the recess portion  117  of the first rotatable member  11 D. The base portion  133  may be fixed to the face portion of the first rotatable member  11 D through screwing or the like. Note that the inner diameter of the base portion  133  is the same as the inner diameter of the cylindrical portion  134 . 
     As described above, the shaft member may be formed separately from the rotatable members. In this case, as shown as an example in the foregoing modified example, all the rotatable members ( 11 D,  12 D, and  18 D) may be formed in the same shape. Thus, the production cost of the joint structures can be reduced, which makes it possible to construct a link mechanism of a robot at a lower cost. 
     As in the foregoing embodiment, the face portions of the rotatable members ( 11 D,  12 D, and  18 D) may include recess portions with a shape that allows the thrust bearing  14  and the encoder  16  to be accommodated when the face portions are positioned facing each other. Furthermore, in the foregoing embodiment and modified examples, the shaft member  13  ( 13 A,  13 D) is formed so as to be hollow. However, the shaft member  13  ( 13 A,  13 D) may be formed so as to be solid. 
     Furthermore, in the joint structure  1 D according to this modified example, the shaft member  13 D is coupled with the first rotatable member  11 , and thus the radial bearings  15  arranged inside the through hole of the first rotatable member  11  may be omitted. The joint structure  1 D on the first rotatable member  11  side may be heavier by the weight corresponding to the base portion  133  of the shaft member  13 D being arranged on the first rotatable member  11  side. Meanwhile, if the radial bearings  15  arranged inside the through hole of the first rotatable member  11  are omitted, it is easy to make the entire weight balance of the joint structure  1 D bilaterally symmetric. 
     3.7 
     Furthermore, in the foregoing embodiment and modified examples, a link member is individually coupled with a rotatable member. However, the corresponding relationship between a link member and a rotatable member does not have to be limited to this example. As in the modified examples given as an example in  FIGS. 10 and 13  above, if a joint structure includes three or more rotatable members, coupling portions of at least two rotatable members may be coupled with the same link member. 
     Hereinafter, an example thereof will be described with reference to  FIG. 14 .  FIG. 14  is a perspective view schematically showing an example of a state in which a coupling portion of the first rotatable member  11  ( 11 D) and a coupling portion of the third rotatable member  18  ( 18 D) are coupled with the same the link member  34 . “Same link member” may be formed in one piece, or may be formed by combining a plurality of members, as long as a plurality of rotatable members can be simultaneously driven. 
     As shown as an example in  FIG. 14 , a link member  34  according to this modified example is formed in the shape of a U with square corners, and has end portions with the same configuration as that of the link member  31 . The link member  34  can be produced by coupling two link members  31  as appropriate through welding or the like. The end portions of the link member  34  are coupled with the coupling portions of the first rotatable member  11  ( 11 D) and the third rotatable member  18  ( 18 D). Accordingly, the first rotatable member  11  ( 11 D) and the third rotatable member  18  ( 18 D) can be coupled with the same the link member  34 . Note that the number of rotatable members that are coupled with the same link member does not have to be limited to this example. Coupling portions of three or more rotatable members may be coupled with the same link member. 
     In this manner, if coupling portions of at least two rotatable members are coupled with the same link member, even when a relatively large force acts from the link member on the joint structure, the force can be divided between and received by the plurality of rotatable members. Accordingly, deformation of the shaft member of the joint structure due to an external force can be suppressed. 
     Note that the rotatable members that are coupled with the same link member may be selected as appropriate according to an embodiment. For example, as shown as an example in  FIG. 14 , coupling portions of the two rotatable members arranged on the outermost side may be coupled with the same link member. Furthermore, coupling portions of a pair of rotatable members with one or a plurality of rotatable members interposed therebetween may be coupled with the same link member. Accordingly, a force that acts from a rotatable member arranged between a pair of rotatable members coupled with the same link member can be received by the pair of rotatable members arranged on both sides. Thus, a force that acts on the joint structure can be prevented from being locally concentrated and be dispersed. Accordingly, it is possible to properly suppress deformation of the shaft member of the joint structure due to an external force, by selecting the rotatable members that are coupled with the same link member in this manner. 
     3.8 
     Furthermore, as shown as an example in  FIG. 15 , coupling between a coupling portion of a rotatable member and a link member may be reinforced using a reinforcing plate.  FIG. 15  is a cross-sectional view schematically showing an example of a joint structure  1 E including reinforcing plates  51  for reinforcing coupling between the coupling portion  21  and the link member  31 . As shown as an example in  FIG. 15E , the joint structure  1 E according to this modified example includes two rotatable members ( 11 E and  12 E) as in the foregoing embodiment. 
     The face portions ( 111 ,  112 ,  121 , and  122 ) of the rotatable members ( 11 E and  12 E) include reinforcing plate-corresponding recess portions  511  to  514  in the shape of a circular ring extending inward in the radial direction from the outer circumferential face such that the reinforcing plates  51  substantially in the shape of a circular ring can be arranged. Except for this aspect, the rotatable members ( 11 E and  12 E) have the same configuration as that of the rotatable members ( 11  and  12 ) described above. 
     The inner diameter of the reinforcing plate-corresponding recess portions  511  to  514  is the same as the inner diameter of the reinforcing plates  51 . The inner diameter of the reinforcing plate-corresponding recess portions ( 512  and  513 ) is larger than the outer diameter of the recess portions ( 115  and  126 ) such that a partition wall is provided between the reinforcing plate-corresponding recess portions ( 512  and  513 ) and the recess portions ( 115  and  126 ). Accordingly, the inner circumferential walls of the reinforcing plates  51  and the thrust bearing  14  are prevented from being coming into contact with each other. The inner diameter of the reinforcing plate-corresponding recess portion  514  is also larger than the outer diameter of the second recess portion  127 . 
     With this configuration, the reinforcing plates  51  are arranged adjacent in the axial direction to the coupling portions  21  respectively. Furthermore, the reinforcing plates  51  have an outer diameter that is larger than the outer diameter of the face portions ( 111 ,  112 ,  121 , and  122 ) of the rotatable members ( 11 E and  12 E). Thus, as shown as an example in  FIG. 15 , a pair of reinforcing plates  51  are arranged so as to hold the coupling region of the coupling portion  21  and the link member  31  from both sides in the axial direction and support the coupling portion. Accordingly, the reinforcing plates  51  can reinforce coupling between the coupling portion  21  and the link member  31 . 
     That is to say, it is assumed that a moment in the axial direction (tangential direction) starting from the coupling region of the coupling portion  21  and the link member  31  acts on the link member  31 . In this case, there is a possibility that a large force will act on the end face  210  of the coupling portion  21 , break the thick-wall portions  212 , and cancel the coupling between the coupling portion  21  and the link member  31 . On the other hand, as in this modified example, if the reinforcing plates  51  are arranged adjacent in the axial direction to the coupling region of the coupling portion  21  and the link member  31 , such a force can be received by the reinforcing plates  51 . Accordingly, a large force can be prevented from acting on the end face  210  of the coupling portion  21 , and thus the thick-wall portions  212  can be prevented from being broken. Accordingly, according to this modified example, it is possible to produce a joint structure that is unlikely to be broken by twisting. 
     Note that the method for arranging the reinforcing plates  51  does not have to be limited to this example, and may be selected as appropriate according to an embodiment. For example, it is also possible that the reinforcing plate-corresponding recess portions  511  to  514  as described above are not provided and the reinforcing plates  51  are directly arranged along the face portions ( 111 ,  112 ,  121 , and  122 ) of the rotatable members ( 11 E and  12 E). The reinforcing plates  51  may be formed respectively in one piece with the rotatable members ( 11 E and  12 E). Furthermore, in the foregoing modified example, the pair of reinforcing plates  51  are arranged on both sides in the axial direction of the coupling portion  21 . However, the reinforcing method using the reinforcing plates  51  does not have to be limited to this example, and, for example, either one of the reinforcing plates  51  may be omitted. That is to say, it is possible to produce a joint structure that is unlikely to be broken by twisting, by providing the reinforcing plate  51  for supporting the coupling region of the coupling portion  21  arranged at a side wall portion of the rotatable members ( 11 E and  12 E) and the link member  31 , on at least one of both sides in the axial direction of the coupling region. 
     Furthermore, in the joint structure  1 E according to this modified example, the joint structure  1 E on the second rotatable member  12 E side may be heavier by the weight corresponding to the radial bearings  15  being arranged on the second rotatable member  12 E side. Meanwhile, if the reinforcing plates  51  on the second rotatable member  12 E side (on the left side in  FIG. 15 ) attached to the joint structures ( 11 E and  12 E) are omitted, it is easy to make the entire weight balance of the joint structure  1 E bilaterally symmetric. 
     3.9 
     Furthermore, in the foregoing embodiment, the coupling portion  21  and the link member  31  are coupled with each other via the wedge member  32 . However, the method for coupling the coupling portion  21  and the link member  31  does not have to be limited to this example, and may be selected as appropriate according to an embodiment. For example, as shown as an example in  FIGS. 16A and 16B , the coupling between the coupling portion  21  and the link member  31  may be constituted by a magnet. 
       FIG. 16A  schematically shows an example of a rotatable member  19  in which the coupling between a link member  31 F and a coupling portion  21 F is constituted by a magnet.  FIG. 16B  is a partial cross-sectional view taken along the line C-C in  FIG. 16A . The rotatable member is denoted by a reference numeral  19  for the sake of ease of description, and the rotatable member  19  corresponds to, for example, the second rotatable member  12  described above. 
     As shown as an example in  FIG. 16B , the coupling portion  21 F of the rotatable member  19  has the same shape as the coupling portion  21 , and a rectangular soft magnetic plate  61  is attached to the groove portion  211  of the coupling portion  21 F. Meanwhile, the link member  31 F has the same shape as the link member  31 , and a columnar permanent magnet  62  is arranged spanning between the groove portions  314 . At this time, the permanent magnet  62  is arranged such that its N pole is positioned toward either one of the groove portions  314 . The groove portions  314  are provided with rectangular soft magnetic pins ( 63  and  64 ) that are arranged in contact with the permanent magnet  62 . The soft magnetic pins ( 63  and  64 ) are fixed by a non-magnetic bolt  65 . 
     Note that the material of the soft magnetic plate  61  and the soft magnetic pins ( 63  and  64 ) may be electromagnetic soft iron. The material of the soft magnetic plate  61  and the soft magnetic pins ( 63  and  64 ) may be selected as appropriate from among soft magnetic materials. Furthermore, the material of the non-magnetic bolt  65  may be selected as appropriate from among non-magnetic materials. 
     The soft magnetic pins ( 63  and  64 ) and the soft magnetic plate  61  are arranged such that they can be brought into contact with each other. For example, the soft magnetic plate  61  is arranged such that the end face of the soft magnetic plate  61  is positioned close to the end face of the coupling portion  21 F. In a similar manner, the soft magnetic pins ( 63  and  64 ) are arranged such that the end faces of the soft magnetic pins ( 63  and  64 ) are positioned close to the end face of the link member  31 F. 
     Accordingly, when the soft magnetic pins ( 63  and  64 ) are brought into contact with the soft magnetic plate  61 , a loop of a magnetic force is formed by the permanent magnet  62 , the soft magnetic pins ( 63  and  64 ), and the soft magnetic plate  61 . Thus, the soft magnetic pins ( 63  and  64 ) and the soft magnetic plate  61  can be coupled with each other at an appropriate intensity. In this modified example, the coupling between the coupling portion  21 F and the link member  31 F is constituted by a magnet in this manner. 
     According to this modified example, the coupling between the coupling portion  21 F and the link member  31 F is constituted by a magnet, and thus a link mechanism of a robot can be constructed without using tools. Accordingly, it is very easy to produce a robot. 
     Furthermore, when an excessive load is applied, coupling using a magnet is likely to be canceled. Thus, for example, if the coupling method using a magnet is used in a joint structure in which a force directly acts from an actuator, such as the joint structures ( 408   a ,  408   b , and  408   c ) of the robot  400 , the link mechanism can be disconnected from the actuator when an excessive load is applied. Accordingly, accidents that occur when an excessive load is applied can be suppressed. In a similar manner, in the case where the joint structure is used in an exoskeletal robot, if the coupling method using a magnet is used in a joint structure that acts on a human body, an excessive load that may damage the human body can be prevented. 
     Note that, in this modified example, the permanent magnet  62  is arranged on the link member  31 F side. However, the arrangement of the permanent magnet  62  does not have to be limited to this example, and the permanent magnet  62  may be arranged on the rotatable member  19  side. Furthermore, as long as the loop of a magnetic force can be formed, the soft magnetic pins ( 63  and  64 ) and the soft magnetic plate  61  may be partially made of a non-magnetic material. Furthermore, the shape of each constituent element may be selected as appropriate according to an embodiment. For example, the permanent magnet  62  may be formed in a rectangular shape. 
     Furthermore, a coupling portion and a link member may be coupled with each other using a method other than a magnet. For example, in the foregoing embodiment, in a state where the end face  210  of the coupling portion  21  and the end face  310  of the link member  31  are arranged facing each other, the coupling portion  21  and the link member  31  are coupled with each other. However, depending on the thickness of the link member  31  and the width of the groove portion  211 , the coupling portion  21  and the link member  31  may be coupled with each other in a state where the link member  31  is inserted into the groove portion  211 . Furthermore, if each of the rotatable members includes a plurality of coupling portions, the coupling portions may be coupled with link members using different methods. 
     3.10 
     Furthermore, in the foregoing embodiment, the thrust bearing  14  are accommodated between the rotatable members ( 11  and  12 ) that are adjacent to each other in the axial direction, in order to receive a force that acts in the axial direction from the rotatable members ( 11  and  12 ). However, the bearing that can be arranged between the rotatable members ( 11  and  12 ) that are adjacent to each other in the axial direction do not have to be limited to this example as long as they are ring-like bearing for receiving a force that acts in the axial direction, and may be selected as appropriate according to an embodiment. For example, angular contact ball bearings capable of receiving a force in both of the thrust direction and the radial direction may be accommodated between the adjacent rotatable members ( 11  and  12 ). 
     If a joint structure includes three or more rotatable members, a recess portion with a shape that allows the bearing to be accommodated (e.g., the recess portion  115  and the first recess portion  126  of the foregoing embodiment) is provided between rotatable members that are adjacent to each other in the axial direction. The recess portion for accommodating the bearing may be arranged on both sides or one side of faces that face each other (e.g., the second face portion  112  and the first face portion  121  of the foregoing embodiment) of the adjacent rotatable members. If the recess portion is provided on both sides of the faces that face each other of the adjacent rotatable members, the heights (the lengths in the left-right direction in  FIG. 2 ) of the recess portions may be the same or different from each other as long as they match the thickness of the bearing. 
     3.11 
     Furthermore, in the foregoing embodiment, the scale  161  is arranged on the second rotatable member  12  side, and the detecting element  162  is arranged on the first rotatable member  11  side. However, the arrangement of the scale  161  and the detecting element  162  does not have to be limited to this example, and they may be switched. That is to say, the scale  161  may be arranged on the first rotatable member  11  side, and the detecting element  162  may be arranged on the second rotatable member  12  side. In this case, the wiring groove portion  114  is provided on the second rotatable member  12  side, and the output of the detecting element  162  is taken out on the second rotatable member  12  side. 
     Furthermore, in the foregoing embodiment, the encoder  16  of the optical reflection type is used. However, the type of encoder that can be built in the joint structure  1  according to the present embodiment does not have to be limited to this example, and may be selected as appropriate according to an embodiment. For example, the joint structure  1  may have a built-in encoder of the optical transmissive type. 
     The encoder of the optical transmissive type can be constituted by, for example, a transmissive scale on which the optical transmittance periodically changes in the circumferential direction, and a detecting element including a light-emitting portion and a light-receiving portion. In this case, it is possible to detect a relative rotational angle between the adjacent rotatable members ( 11  and  12 ), by arranging the light-emitting portion and the light-receiving portion of the detecting element such that light emitted from one face side of the transmissive scale is received by the other face side. 
     Moreover, the joint structure  1  may include a magnetic-type or electrical resistance-type encoder. For example, the magnetic-type encoder can be constituted by a scale on which the magnetic force changes in the circumferential direction, and a detecting element such as a Hall element for detecting the magnetic force. For example, as the magnetic-type encoder, a magnetic encoder (model No.: AEAT-6600-T16, etc.) manufactured by AVAGO can be used. Furthermore, as the magnetic-type encoder, a resolver (e.g., Singlsyn (registered trademark) manufactured by Tamagawa Seiki Co., Ltd., etc.) can be also used. 
     Furthermore, in the foregoing embodiment, the scale  161  is attached to the plate  142  separately from the thrust bearing  14 . However, the position at which the scale  161  is attached does not have to be limited to this example, and the scale  161  may be attached to the thrust bearing  14 . For example, if the housing washer (not shown) of the thrust bearing  14  has a shape similar to that of the plate  142 , the scale  161  may be attached to the end face of the housing washer. At this time, the washer  141  may be omitted, and the shaft washer of the thrust bearing  14  may be allows to be directly in contact with the bottom face of the recess portion  115 . Accordingly, the encoder  16  can be constituted using a part of the thrust bearing  14 , and thus the number of parts and the number of assembly steps can be reduced, and, moreover, the constituent elements accommodated in the internal space of the joint structure  1  can be made compact. 
     Furthermore, in the foregoing embodiment, the scale  161  and the detecting element  162  constituting the encoder  16  are arranged facing each other in the axial direction. However, the arrangement of the encoder  16  does not have to be limited to this example, and, for example, the scale  161  and the detecting element  162  may be arranged at the outer circumferential wall of the shaft member  13  and the inner circumferential wall of the thrust bearing  14  such that they face each other in the axial direction. 
     Furthermore, in the foregoing embodiment, the gap portion  116  in the shape of a circular ring is ensured such that the scale  161  and the detecting element  162  face each other in the axial direction. However, the shape of the gap portion  116  does not have to be limited to this example as long as the scale  161  and the detecting element  162  can face each other in the axial direction, and may be selected as appropriate according to an embodiment. For example, the gap portion  116  may have a sector-shaped cross-section. 
     Furthermore, if an optical encoder of the reflection type or the transmissive type is used as the encoder accommodated in the joint structure  1 , an optical fiber may be arranged in the internal space of the joint structure  1 , and the optical fiber may be used to emit and receive light to and from the scale. In this case, an electrical signal can be output via the optical fiber to the outside of the joint structure  1 , and thus the detecting element and the board may be arranged outside the joint structure  1 . In this case, metal materials can be eliminated from the constituent elements of the encoder built in the joint structure  1 . Furthermore, if the other constituent elements are made of a resin, the joint structure  1  can be produced without using a metal material. 
     Furthermore, in the foregoing embodiment, the detecting element  162  transmits and receives an electrical signal by wire via the wiring board  163 . However, the method of the detecting element  162  for transmitting and receiving an electrical signal does not have to be limited to this example. For example, the detecting element  162  may transmit and receive an electrical signal wirelessly using a wireless module. In this case, the wiring board  163  may be omitted. In the foregoing embodiment, the wiring board  163  is extended from the internal space to the outside via the wiring groove portion  114 . However, the route of the wiring board  163  does not have to be limited to this example, and the wiring board  163  may be extended to the outside via the hollow portion  132  of the shaft member  13 . 
     3.12 
     Furthermore, in the foregoing embodiment, the end face  210  of the coupling portion  21  is provided with four protruding portions  213 , so that firm coupling between the coupling portion  21  and the link member  31  is realized. However, the number and the shape of protruding portions do not have to be those in the examples of the foregoing embodiment, and may be selected as appropriate according to an embodiment. The protruding portions provided at the end face of the coupling portion may be designed as appropriate according to the end face shape of the link member that is coupled with the coupling portion. 
     Furthermore, in the foregoing embodiment, the protruding portions  213  are formed in one piece with the end face  210 . However, the configuration of the protruding portions  213  does not have to be limited to this example, and may be selected as appropriate according to an embodiment. For example, the protruding portions  213  may be provided by forming holes in the end face  210  and inserting pins into the holes. 
     Furthermore, in the foregoing embodiment, the protruding portions  213  are provided at the coupling portions  21 . However, the positions at which the protruding portions  213  are provided do not have to be limited to this example, and the protruding portions  213  may be provided at the end face  310  of the link member  31 . In this case, if the end face  210  of the coupling portion  21  is provided with holes for receiving the protruding portions  213 , the coupling portion  21  and the link member  31  can be coupled with each other as described above. 
     3.13 
     Furthermore, in the foregoing embodiment, the wiring board  163  of the encoder  16  is extended to the outside from the first rotatable member  11  side. However, the direction in which the wiring board  163  is extended to the outside does not have to be limited to this example. For example, if the detecting element  162  is arranged on the second rotatable member  12  side, a wiring groove portion similar to the wiring groove portion  114  may be provided in the second rotatable member  12 , and the wiring board  163  may be arranged so as to be extended to the outside from the second rotatable member  12  side. Furthermore, both of the rotatable members ( 11  and  12 ) may be provided with the wiring groove portions  114 , and the direction in which the wiring board  163  is extended to the outside may be selected according to a situation in which the joint structure  1  is to be used. 
     3.14 
     Furthermore, in the foregoing embodiment, the end faces of the coupling portions  21  arranged at the side wall portions ( 113  and  123 ) of the rotatable members ( 11  and  12 ) are formed as flat faces, except for the protruding portions  213 . However, the shape of the coupling portions  21  does not have to be limited to this example. 
       FIG. 17  shows a modified example of the shape of each coupling portion  21 . A rotatable member  11 G shown in  FIG. 17  is formed so as to be similar to the first rotatable member  11 , except for a coupling portion  21 G. Furthermore, the coupling portion  21 G is formed so as to be similar to the coupling portion  21 , except for the shape of its end face. Furthermore, a link member  31 G is formed so as to be similar to the link member  31 , except for the shape of its end face. 
     In this modified example, a recess portion recessed in the longitudinal direction is provided at the center of the end face of the link member  31 . In conformity with this aspect, the coupling portion  21 G arranged at a side wall portion of the rotatable member  11 G is configured so as to have a projecting portion  2101  projecting outward in the radial direction at the center in the tangential direction. Furthermore, at the end face of the coupling portion  21 G, the positions on both sides of the projecting portion  2101  are formed as flat faces that are slightly lower than the projecting portion  2101 , and the protruding portions  213  are arranged at these portions. Note that this coupling portion  21 G can be applied not only to the first rotatable member  11  but also to the second rotatable member  12 . 
     According to this modified example, in each coupling portion  21 G, the length in the radial direction of the thick-wall portions can be made shorter by the length of the projecting portion  2101  being provided. Thus, in this modified example, the position of the groove portion into which the head portion  321  of the wedge member  32  is inserted can be changed slightly to the outer side in the radial direction, compared with the foregoing embodiment. Accordingly, the internal space of the joint structure can be increased, and thus a bearing with a large diameter can be arranged inside and the strength of the joint structure can be improved. Furthermore, since the diameter of the hollow portion of the shaft member can be increased, the joint structure can be made lighter. Furthermore, since the outer diameter of the shaft member can be increased, the rigidity of the shaft member can be improved. 
     3.15 
     Furthermore, in the foregoing embodiment, the thrust bearing  14  and the radial bearings  15  are used as bearings that are arranged inside the joint structure  1 . However, the bearings that can be used do not have to be limited to these, and may be selected as appropriate according to an embodiment. 
       FIG. 18  shows a modified example using a bearing different from that in the foregoing embodiment. A joint structure  1 H given as an example in  FIG. 18  includes a first rotatable member  11 H and a second rotatable member  12 H. The first rotatable member  11 H is formed so as to be substantially similar to the first rotatable member  11 , and the second rotatable member  12 H is formed so as to be substantially similar to the second rotatable member  12 . 
     The recess portion  115  of the first rotatable member  11 H is formed in the shape of a circular ring, and the base of an inner circumferential face  1151  of the recess portion  115  is provided with a step portion  1152  in the shape of a circular ring extending inward in the radial direction from the inner circumferential face  1151 . Meanwhile, the first face portion  121  of the second rotatable member  12  that faces the recess portion  115 , the second rotatable member  12  being adjacent to the first rotatable member  11 H, is provided with a projecting portion  1211  in the shape of a circular ring with a diameter smaller than that of the recess portion  115  and the step portion  1152 . Furthermore, the base of an outer circumferential face  1212  of the projecting portion  1211  is provided with a step portion  1213  in the shape of a circular ring extending outward in the radial direction from the outer circumferential face  1212  of the projecting portion  1211 . 
     In this internal structure of the joint structure  1 H, a cross roller bearing  71  in the shape of a ring is arranged so as to be supported by the inner circumferential face  1151  of the recess portion  115 , a face along the axial direction of the step portion  1152  of the recess portion  115 , the outer circumferential face  1212  of the projecting portion  1211 , and a face along the axial direction of the step portion  1213  of the projecting portion  1211 . The cross roller bearing  71  can receive a load that acts in the axial direction and the radial direction on the joint structure  1 H. Note that, if three or more rotatable members are provided, this structure having such a built-in bearing may be provided in each gap between two adjacent rotatable members. 
     According to this modified example, the following effects can be achieved. That is to say, typically, the inner diameter of the cross roller bearing  71  is larger than the inner diameter of the thrust bearing. Thus, in this modified example, an available region inside the bearing is wide. Accordingly, for example, the outer diameter of the shaft member  13  can be increased, and thus the rigidity of the shaft member  13  can be improved. Moreover, the radial bearings  15  with a large diameter can be used, and thus a load that is received by the radial bearings  15  can be increased. Furthermore, the diameter of the hollow portion of the shaft member  13  can be increased, and thus the joint structure can be made lighter. 
     LIST OF REFERENCE NUMERALS 
     
         
         
           
               1  Joint structure 
               11  First rotatable member 
               111  First face portion 
               112  Second face portion 
               113  Side wall portion, 
               114  Wiring groove portion 
               115  Recess portion 
               116  Gap portion 
               12  Second rotatable member 
               121  First face portion 
               122  Second face portion 
               123  Side wall portion 
               124  Through hole 
               125  Interlock projecting portion 
               126  First recess portion 
               127  Second recess portion 
               128  Projecting portion 
               129  Wire-driving groove portion 
               13  Shaft member 
               131  Fastener 
               132  Hollow portion 
               133  Base portion 
               134  Cylindrical portion 
               14  Thrust bearing 
               141  Washer 
               142  Plate 
               143  Hole portion 
               15  Radial bearing 
               16  Encoder 
               161  Scale 
               162  Detecting element 
               163  Wiring board 
               164  Projecting portion 
               165  Connector portion 
               17  Cable 
               171  Connector portion 
             Coupling portion 
               210  End face 
               211  Groove portion 
               212  Thick-wall portion 
               213  Protruding portion 
               214  Bottom portion 
               31  Link member 
               310  End face 
               311  Hole portion 
               312 - 313  Through hole 
               314  Groove portion 
               315  Edge portion 
               32  Wedge member 
               321  Head portion 
               322  Body portion 
               323  Through hole 
               324  Tapered portion 
               33  Screw 
               331  Head portion 
               332  Tapered portion 
               333  Male thread portion 
               400  Robot 
               401  Base 
               402  Support 
               403 - 404  Actuator 
               405 - 406  Movable portion 
               407   a - 407   h  Link member 
               408   a - 408   f  Joint structure 
               409  Front end portion 
               410  Robot 
               411  Link member 
               412  Joint structure 
               413 - 414  Fixture 
               415 - 416  (Bowden) wire 
               417  Binding member 
               420  Delta robot 
               421  Base portion 
               422  Rotary motor 
               423   a - 423   e  Link member 
               424  Joint structure 
               425  Front end portion 
               426  Link portion 
               420 A Delta robot 
               427  Linear motor 
               1 A Joint structure 
               13 A Shaft member 
               18  Third rotatable member 
               1 B Joint structure 
               11 B First rotatable member 
               111 B First face portion 
               21 B Coupling portion 
               400 C Robot 
               1 C Joint structure 
               31   c  Link member 
               1 D Joint structure 
               11 D First rotatable member 
               117  Recess portion 
               12 D Second rotatable member 
               13 D Shaft member 
               133  Base portion 
               134  Cylindrical portion 
               18 D Third rotatable member 
               34  Link member 
               1 E Joint structure 
               11 E First rotatable member 
               12 E Second rotatable member 
               51  Reinforcing plate 
               511 - 514  Reinforcing plate-corresponding recess portion 
               19  Rotatable member 
               21 F Coupling portion 
               31 F Link member 
               61  Soft magnetic plate 
               62  Permanent magnet 
               63 - 64  Soft magnetic pin 
               65  Non-magnetic bolt