Patent Publication Number: US-7581465-B2

Title: Joint structure of robot

Description:
TECHNICAL FIELD 
   This invention relates to a robot joint structure. 
   BACKGROUND ART 
   The prior art teaches a joint structure of an industrial robot in which an arm is interconnected through a parallel linkage having four or more links (see for example Japanese Laid-Open Patent Application No. Hei 10-296680 (paragraphs 0002, 0008 and 0012, and FIGS. 2, 3, 5 and 6). 
   A robot joint structure that interconnects links by a single axis is also known. From the viewpoint of safety and protection against dust, it is ordinarily preferable for a robot to have the outside of the links covered by covers so as not to expose the internal structure. A joint structure has therefore been proposed for a legged mobile robot, for example, in which links (e.g., a thigh link and a shank link) are interconnected by a single axis (i.e., the thigh link and shank link are directly connected without use of an intermediate linkage), the edge of the cover covering one link is formed to have a spherical surface centered on the single axis and the edge of the cover covering the other link is given a concave shape corresponding to the spherical surface, whereby no gap arises between the covers when the joint is moved (see Japanese Laid-Open Patent Application No. 2002-210682 (FIG. 4)). 
   In stationary industrial robots used for various tasks in plants and the like, the need to expand the range of motion of a task-performing hand, so as to increase the size of the reachable space, and the need to increase the critical or limit value of the driven speed can be met by appropriately defining the number of joints between the main unit and the task-performing hand, the arm (linkage) length and the driving power of the actuators. In contrast, in a legged mobile robot, particularly a humanoid robot or the like modeled after the form of the human body, design factors such as the number of joints and the link lengths are subject to greater restriction than in the case of an industrial robot owing to appearance and functional considerations. Moreover, autonomous robots are also limited with regard to usable actuators owing to power consumption, available mounting space and other considerations. In an autonomous legged mobile robot or the like therefore, expansion of the range of motion of the arms and legs, for example, and increase of the critical value of the driven speed have to be achieved by increasing the range of motion (angle of rotation) and increasing the critical value of the driven speed of the individual joints of the arms and legs. 
   As shown in  FIG. 23 , in a legged mobile robot equipped with ordinary single-axis joints, in order to avoid physical interference in the links  102  and  104  or covers covering them, their rotation axis  100  is sometimes offset outward from the center of links. This outward offsetting of the rotation axis lowers the likelihood of interference in the links and covers on the offset side and increases the range of movement. 
   In an articulated robot, however, as shown in  FIG. 24 , a posture in which their rotation axes (joints)  110 ,  112  and  114  are positioned on the same line constitutes a singularity posture. Since control diverges or oscillates when a robot assumes a singularity posture, the angle of rotation of the joints has to be constricted so that the singularity does not occur. In the elbow joint of the arm of a humanoid robot, for example, the elbow joint (corresponding to the rotation axis  112 ) has a range of motion between a slightly bending angle and the maximum bending angle. 
   Here, if, as shown in  FIG. 23 , the rotation axis  100  is offset outward for avoiding physical interference in the links and covers, it is necessary, as shown in  FIG. 25 , to establish an angle of rotation θos between the state in which the links  102  and  104  are fully extended (state of the joint being driven as far in the extending direction as the mechanism permits) and the state in which the rotation axes  100 ,  106  and  108  are located on the same line, i.e., the singularity posture. The range of motion (angle of rotation) of the rotation axis  100  that can be utilized in control is therefore the range of motion determined by the mechanism, minus the angle of rotation θos. The angle of rotation θos increases with increasing amount of offset of the rotation axis  100 . The prior art therefore involves the inconvenience that when the rotation axis is offset in order to expand the range of motion in the bending direction, the range of motion in the direction of extension is markedly constricted and reduced. 
   DISCLOSURE OF THE INVENTION 
   The object of this invention is therefore to overcome the foregoing problem by providing a robot joint structure which enables the range of motion (angle of rotation) of the joint in the bending direction to be expanded without giving rise to physical interference with links or covers that cover them, minimizes reduction of the movable range in the direction of extension attributable to a singularity, and increases the critical value of the rotational speed (driven speed). 
   In order to achieve the object, as recited in claim  1  mentioned below, this invention is configured to have a robot joint structure having a first main link and a second main link connected through a first movable link and a second movable link, and an actuator installed on the first main link and driving the first movable link such that the first main link and the second main link are displaced relative to each other; characterized in that: rotation axes A and B each provided at the first main link; and rotation axes C and D each provided at the second main link; wherein in a quadrangle whose apices are formed by the rotation axes A, B, C and D, when assuming that rotation axes that are diagonally opposed to each other are A and C, while those that are diagonally opposed to each other are B and D, the rotation axes A and C are connected through the first movable link and the rotation axes B and D are connected through the second movable link in such a manner that the first movable link and the second movable link are disposed to cross and that the rotation axis A is driven by the actuator to drive the first movable link, such that the first main link and the second main link are displaced relative to each other. 
   Thus, since it is configured such that the first main link (e.g., an upper arm link) and the second main link (e.g., a forearm link) are connected through the first movable link and the second movable link, and the two movable links are arranged to cross. This structure makes it possible to increase the overall driven angle of the joint (e.g., an elbow joint) relative to the input, expand the range of motion of the joint in the bending direction, and also raise the critical value of the driven speed (rotational speed). 
   In addition, the amount of outward projection of the two movable links is small, so that there is little risk of the movable links and covers covering them coming into physical interference. Further, interference with a cover covering the first main link and a cover covering the second main link becomes unlikely because the joint bends over two stages with the two rotation axes acting as fulcrums. As a result, the range of motion of the joint in the bending direction can be further expanded. Moreover, the rotation axes of the joint do not have to be offset outward of the joint, so that reduction of the movable range of the joint in the direction of extension attributable to the singularity can be minimized. 
   As recited in claim  2  mentioned below, the invention is configured such that, the rotation axis A and the rotation axis B are provided on or near a same straight line lying perpendicular to a longitudinal direction of the first main link. 
   Thus, since it is configured such that, the rotation axis A and the rotation axis B are provided on or near the same straight line lying perpendicular to the longitudinal direction of the first main link, it becomes possible to further expand that range of motion of the joint in the bending direction and also to increase the driven speed of the joint. 
   As recited in claim  3  mentioned below, this invention is configured such that, the rotation axis C and the rotation axis D are provided on or near a same straight line lying perpendicular to a longitudinal direction of the second main link. 
   Thus, since it is configured such that, the rotation axis C and the rotation axis D are provided on or near the same straight line lying perpendicular to the longitudinal direction of the second main link, it becomes possible to further expand that range of motion of the joint in the bending direction and also to increase the driven speed of the joint. 
   As recited in claim  4  mentioned below, this invention is configured such that, at least one of the first movable link and the second movable link is given a curved shape, so as not to interfere with the rotation axes of the other of the first movable link and the second movable link. 
   Thus, since it is configured such that, at least one of the first movable link and the second movable link is given the curved shape, so as not to interfere with the rotation axes of the other of the first movable link and the second movable link, the first movable link and the second movable link do not interfere with the rotation axes of the others, so that the range of motion of the joint in the bending direction can be further expanded. 
   As recited in claim  5  mentioned below, this invention is configured such that, at least one of the first movable link and the second movable link is provided with an over-rotation prevention mechanism that prevents the joint from over-rotating beyond predetermined angles. 
   Thus, since it is configured such that, at least one of the first movable link and the second movable link is provided with the over-rotation prevention mechanism that prevents the joint from over-rotating beyond predetermined angles, it becomes possible to prevent the control from diverging or oscillating attributed to the singularity and to preclude covers from being damaged due to excessive joint bending. 
   As recited in claim  6  mentioned below, this invention is configured such that, the joint is provided with covers covering the first main link, the first movable link, the second movable link, the second main link and the actuator from outside, the covers comprising: a first cover covering the first main link and the actuator; a second cover covering the second main link; and a third cover slidably interposed between the first cover and the second cover, and covering the first and second movable links and a gap formed between the first and second covers occurring with rotation of the joint. 
   Thus, since it is configured such that, covers covering the joint comprises the first cover, the second cover and the third cover slidably interposed between the first cover and the second cover, and covering the first and second movable links and a gap formed between the first and second covers occurring with rotation of the joint, the joint can be kept from exposing the internal structure and in addition to the foregoing effects or advantages, it become further possible to improve the dust resistance and safety of the joint. In addition, the appearance of the robot can be enhanced. 

   
     BRIEF EXPLANATION OF THE DRAWINGS 
       FIG. 1  is a front view of a robot for explaining a robot joint structure according to an embodiment of this invention when taking a legged mobile robot as an example. 
       FIG. 2  is a right side view of the robot shown in  FIG. 1 . 
       FIG. 3  is an explanatory view showing a skeletonized view of the robot of  FIG. 1 . 
       FIG. 4  is a perspective view of the vicinity of an elbow joint shown in  FIG. 3 . 
       FIG. 5  is a perspective view of the elbow joint shown in  FIG. 4  with an upper arm first plate and a forearm first plate removed. 
       FIG. 6  is an enlarged plan view of the elbow joint shown in  FIG. 5 . 
       FIG. 7  is a plan view of the elbow joint shown in  FIG. 6  together with covers. 
       FIG. 8  is a plan view showing a driven state of the elbow joint shown in  FIG. 7 . 
       FIG. 9  is a plan view similar to  FIG. 8  showing a driven state of the elbow joint shown in  FIG. 7 . 
       FIG. 10  is an explanatory view schematically illustrating an arm linkage in the fully extended state of the elbow joint of  FIG. 6 . 
       FIG. 11  is an explanatory view schematically illustrating an arm linkage in the bent state of the elbow joint of  FIG. 6 . 
       FIG. 12  is a graph showing how the relationship between θ 1  and θ 2  changes at different values of θAB when θCD is made 45 degrees in the elbow joint of  FIG. 6 . 
       FIG. 13  is a graph showing the relationship of the straight-line distance R from the shoulder joint to the wrist joint of the robot shown in  FIG. 3  and the driven angle θ 1 +θ 2  of the elbow joint. 
       FIG. 14  is a set of views for explaining, inter alia, the maximum value Rmax of the straight-line distance R from the shoulder joint to the wrist joint of the robot shown in  FIG. 3 . 
       FIG. 15  is a plan view, similar to  FIG. 8 , but showing a driven state of the elbow joint shown in  FIG. 7 . 
       FIG. 16  is a plan view, similar to  FIG. 7 , but showing a robot joint structure according to a second embodiment of this invention. 
       FIG. 17  is a plan view showing a driven state of the elbow joint shown in  FIG. 16 . 
       FIG. 18  is a plan view similar to  FIG. 17  showing a driven state of the elbow joint shown in  FIG. 16 . 
       FIG. 19  is a plan view similar to  FIG. 17  showing a driven state of the elbow joint shown in  FIG. 16 . 
       FIG. 20  is a graph showing how the relationship between θ 1  and θ 2  changes at different values of θCD when θAB is made −15.5 degrees in the elbow joint of FIG.  16 . 
       FIG. 21  is a graph, similar to that of  FIG. 13 , but relating to the second embodiment. 
       FIG. 22  is a plan view, similar to  FIG. 16 , but showing a robot joint structure according to a third embodiment of this invention. 
       FIG. 23  is an explanatory view showing a conventional robot joint structure. 
       FIG. 24  is an explanatory view showing a singularity of an articulated robot. 
       FIG. 25  is an explanatory view showing a problem of the joint structure in a conventional robot. 
   

   BEST MODE OF CARRYING OUT THE INVENTION 
   A robot joint structure according to embodiments of the present invention will be explained with reference to the attached drawings in the following, taking as an example a legged mobile robot, more specifically a humanoid robot (mobile robot modeled after the form of the human body). 
     FIG. 1  is a front view of the robot according to a first embodiment and  FIG. 2  is right side view thereof. 
   As shown in  FIG. 1 , the robot  1  is equipped with a pair of leg linkages  2  and an upper body (main unit)  3  above the leg linkages  2 . A head  4  is formed on the upper end of the upper body  3  and two arm linkages  5  are connected to opposite sides of the upper body  3 . As shown in  FIG. 2 , a housing unit  6  is mounted on the back of the upper body  3  for accommodating therein, inter alia, an electronic control unit and a power supply battery. The robot  1  shown in  FIGS. 1 and 2  is equipped with covers for protecting its internal structures. 
     FIG. 3  is an explanatory view showing a skeletonized view of the robot  1 . The number and locations of the joints will be explained with reference to this drawing. As illustrated, the left and right leg linkages  2  of the robot  1  are each equipped with six joints and the arm linkages  5  are equipped with five joints. 
   In the leg linkages  2 , the six joints comprises, from top to bottom in the gravity direction, joints  10 R,  10 L (R and L indicating the right and left sides; hereinafter the same) (Z axis) for leg swiveling around a crotch (hips), pitch direction (around Y axis) joints  12 R,  12 L of the crotch (hips), roll direction (around X axis) joints  14 R,  14 L of the crotch (hips), pitch direction joints  16 R,  16 L of knees, pitch direction joints  18 R,  18 L of ankles, and roll direction joints  20 R,  20 L of the ankles. In other words, the crotch joints (or hip joints) comprises the joints  10 R(L),  12 R(L),  14 R(L), the knee joints of the joints  16 R(L), and the ankle joints of the joints  18 R(L),  20 R(L). 
   Feet  22 R, L are attached to the lower ends of the ankle joints  18 R(L),  20 R(L). The crotch joints  10 R(L),  12 R(L),  14 R(L) are linked to the knee joints  16 R(L) by thigh links  24 R, L, and the knee joints  16 R(L) are linked to the ankle joints  18 R(L),  20 R(L) by shank links  26 R, L. 
   On the other hand, in the arm linkages  5 , the five joints comprises from top to bottom in the gravity direction, pitch direction joints  30 R,  30 L of the shoulders, roll direction joints  32 R,  32 L of the shoulders, joints  34 R,  34 L for arm swiveling, pitch direction joints  36 R,  36 L of elbows, and joints  38 R,  38 L for wrist swiveling. In other words, the shoulder joints comprises the joints  30 R (L),  32 R(L),  34 R(L), the elbow joints of the joints  36 R(L), and the wrist joints of the joints  38 R(L). 
   Hands (end effectors)  40 R,  40 L are attached to the distal ends of the wrist joints  38 R(L). The shoulder joints  30 R(L),  32 R(L),  34 R(L) are linked to the elbow joints  36 R(L) by upper arm links  42 R, L, and the elbow joints  36 R(L) are linked to the wrist joints  38 R(L) by forearm links  44 R, L. 
   The head  4  is linked to the upper body  3  through a neck joint  46  around a vertical axis and a head swivel mechanism  48  for rotating the head  4  around an axis perpendicular thereto. Inside the head  4  are mounted a visual sensor  50  composed of a CCD camera for taking images and outputting image signals, and an audio input/output device  52  composed of a receiver and a microphone. 
   As shown in the drawing, conventional six-axis force sensors (floor reaction force detectors)  56 R(L) are attached between the ankle joints  18 ,  20 R(L) and the ground contact ends of the feet  22 R(L) for outputting signals representing force components Fx, Fy and Fz of three directions and moment components Mx, My and Mz of three directions. 
   Similar six-axis force sensors  58 R(L) are also attached between the wrist joints  38 R(L) and the hands  40 R(L) for outputting signals representing force components Fx, Fy and Fz of three directions and moment components Mx, My and Mz of three directions of external forces other than floor reaction forces acting on the robot  1 , specifically external forces acting on the hands  40 R(L) from objects (object reaction forces). 
   In addition, an inclination sensor  62  is installed on an upper body link  60  for outputting a signal representing inclination relative to the Z axis (vertical direction (gravity direction)) and the angular velocity thereof. Moreover, the electric motors (actuators; not shown) for driving the respective joints are provided with rotary encoders (not shown) for outputting signals representing amount of rotation. 
   The outputs of the six-axis force sensors  56 R(L),  58 R(L), inclination sensor  62  etc. are input to an electronic control unit  64  provided in the housing unit  6 . The control unit  64  comprises a microcomputer. Based on data stored in a memory (not shown) and the input detection values, the control unit  64  computes the control values for the electric motors (not shown in the drawing) that drive the joints. 
   The left and right leg linkages  2 R(L) of the robot  1  are thus each imparted with six degrees of freedom, so that the legs as a whole can be imparted with desired movements by operating the electric motors for driving these 6×2=12 joints based on the control values computed by the control unit  64 , thereby enabling the robot  1  to move freely within three-dimensional space. Further, the left and right arm linkages  5 R(L) are each imparted with five degrees of freedom (not including the hands  40 R(L)), so that the arms as a whole can be imparted with desired movements by operating the electric motors for driving these 5×2=10 joints based on the control values calculated by the control unit  64 , thereby enabling execution of desired operations (tasks). 
   The joint structure of the robot  1  will now be explained with reference to  FIG. 4  and the ensuing figures. Although the explanation will be made in the following taking the elbow joints  36 R(L) as an example, the symbols R and L will be omitted because the structures of the elbow joints  36 R(L) are laterally symmetrical. 
     FIG. 4  is a perspective view of the vicinity of the elbow joint  36 . The drawing shows the elbow joint  36  with covers for protecting its internal structure removed. 
   As illustrated, the upper arm link  42  is constituted by a first plate (hereinafter called the “upper arm first plate)  42   a  and a second plate (hereinafter called the “upper arm second plate)  42   b . The upper arm first plate  42   a  and upper arm second plate  42   b  are fastened by bolts (not shown). The forearm link  44  is similarly constituted by a first plate (hereinafter called the “forearm first plate”)  44   a  and a second plate (hereinafter called the “forearm second plate)  44   b , which are also fastened by bolts (not shown). 
     FIG. 5  is a perspective view of the elbow joint  36  shown in  FIG. 4  with the upper arm first plate  42   a  and forearm first plate  44   a  removed. 
   As shown in the drawing, the upper arm link  42  is provided with an rotation axis A and an rotation axis B. The rotation axis A comprises projections provided on the upper arm first plate  42   a  and upper arm second plate  42   b  and a bearing disposed around the outer periphery thereof (none of which are shown). The rotation axis B comprises a projection provided on the upper arm first plate  42   a  and a bearing disposed around the outer periphery thereof (neither shown). The centers of the rotation axes A and B are designated Ac and Bc. 
   The forearm link  44  is provided with a rotation axis C and a rotation axis D. The rotation axis C comprises projections provided on the forearm first plate  44   a  and forearm second plate  44   b  and a bearing disposed around the outer periphery thereof. The rotation axis D comprises similar members. The centers of the rotation axes C and D are designated Cc and Dc. 
   The elbow joint  36  is provided with a first movable link  70  and second movable link  72 . One end of the first movable link  70  is fastened to the rotation axis A and the other end thereof is rotatably connected to the rotation axis C. On the other hand, one end of the second movable link  72  is rotatably connected to the rotation axis B and the other end thereof is rotatably connected to the rotation axis D. In other words, the upper arm link  42  and forearm link  44  are connected through the first movable link  70  and second movable link  72  to be displaceable relative to each other. 
   In the upper arm link  42 , an electric motor (actuator)  76  is installed above (toward the shoulder from) the rotation axis A and rotation axis B. The output of the electric motor  76  is transmitted to the rotation axis A through a speed reducer (not shown) to drive the first movable link  70  fastened to the rotation axis A. As a result, the upper arm link  42  and forearm link  44  are displaced relative to each other. 
     FIG. 6  is an enlarged plan view of the elbow joint  36  shown in  FIG. 5 . Symbol  78  in this drawing designates a member for supporting the rotation axis B on the upper arm first plate  42   a  and symbol  80  designates a member for supporting the rotation axis D on the forearm first plate  44   a . The members  78 ,  80  are omitted in  FIGS. 4 and 5 . 
   As shown in  FIG. 6 , in the quadrangle having the centers of rotation Ac, Bc, Cc and Dc of the rotation axes A, B, C and D as its apices, the diagonally opposed rotation axes are the rotation axis A and rotation axis C, and the rotation axis B and rotation axis D. Thus, owing to the connection of the rotation axis A and rotation axis C by the first movable link  70  and the connection of the rotation axis B and rotation axis D by the second movable link  72 , the first movable link  70  and second movable link  72  are disposed so as to cross. The second movable link  72  is given a curved shape, specifically an S-like shape, detouring the rotation axis A so as to avoid interference with the rotation axis A. 
     FIG. 7  is a plan view of the elbow joint  36  shown in  FIG. 6  together with covers. 
   As shown in the drawing, the elbow joint  36  is provided with covers (casings) for covering the links from the outside. The covers include a first cover  84  covering the upper arm link  42  and electric motor  76 , a second cover  86  covering the forearm link  44 , and a third cover  88  slidably interposed between the first cover  84  and second cover  86  for covering the first movable link  70  and second movable link  72 . 
     FIGS. 8 and 9  are plan views showing the elbow joint  36  shown in  FIG. 7  in driven states. 
   As shown in  FIGS. 8 and 9 , when the rotation axis A is rotated to drive the first movable link  70  in the bending direction, the straight-line distance between the rotation axis B and rotation axis C is shortened. Since the straight-line distance between the rotation axis B and rotation axis D and the relative positional relationship between the rotation axis C and rotation axis D remain unchanged at this time, the forearm link  44  continues to be driven in the bending direction around the rotation axis C as the fulcrum. 
     FIG. 10  is an explanatory view schematically illustrating the arm linkage  5  in the fully extended state of the elbow joint  36 . Here, the fully extended state of the elbow joint means the state in which the elbow joint  36  has been driven in the extending direction to the movement limit determined by the mechanism (and is not necessarily the same as the state in which the straight-line distance between the shoulder joint and wrist joint is longest). 
   In this drawing, EL 1  means the rotation axis A of the elbow joint  36 , and E 12  means the rotation axis C of the elbow joint  36 . As shown in  FIG. 6 , angle α is the angle formed between the longitudinal direction of the upper arm link  42  in the fully extended state of the elbow joint  36  (designated  42   z ) and the longitudinal direction of the first movable link  70  (designated  70   z ), and angle β is the angle formed between the longitudinal direction of the first movable link  70  and the longitudinal direction of the forearm link  44  (designated  44   z ). 
   In the arm linkage  5  shown in  FIG. 10 , when the electric motor operates to drive EL 1 , i.e., the rotation axis A, by θ 1 , then, as shown in  FIG. 11 , the angle formed between the longitudinal direction of the upper arm link  42  and the longitudinal direction of the first movable link  70  becomes α+θ 1 . Further, when EL 2  is driven, i.e., when the forearm link  44  is driven around the rotation axis C as the fulcrum, the angle formed between longitudinal direction of the first movable link  70  and the longitudinal direction of the forearm link  44  becomes θ 2 −β. The overall driven angle of the elbow joint  36  therefore becomes θ 1 +θ 2 . Here, θ 2  can be represented by the following Equation 1. 
   
     
       
         
           
             
               
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   As shown in  FIG. 6 , θAB in this equation is the angle formed between the direction perpendicular (designated  42   x ) to the longitudinal direction ( 42   z ) of the upper arm link  42  and the line connecting the centers of rotation Ac and Bc of the rotation axes A and B. In this embodiment, it is 30 degrees (the angle being defined as a positive angle in the case where the rotation axis B is located upward in the gravity direction (toward the shoulder) from the rotation axis A and as a negative angle in the opposite case). Further, θCD is the angle formed between the direction perpendicular (designated  44   x ) to the longitudinal direction ( 44   z ) of the forearm link  44  and the line connecting the centers of rotation Cc, Dc of the rotation axes C and D. In this embodiment, it is 45 degrees (the angle being defined as a positive angle in the case where the rotation axis D is located upward in the gravity direction (toward the shoulder) from the rotation axis C and as a negative angle in the opposite case). 
   The values a, b, b′, c, c′ and d can be represented by the following Equations 2, when the center distance between the rotation axis A and rotation axis C (specifically, the straight-line distance between the centers of rotation Ac and Cc, i.e., the axis-to-axis distance of the first movable link  70 ) is defined as rAC, the center distance between the rotation axis B and rotation axis D (specifically, the straight-line distance between the centers of rotation Bc and Dc, i.e., the axis-to-axis distance of the second movable link  72 ) is defined as rBD, the center distance between the rotation axis A and rotation axis B is defined as rAB, and the center distance between the rotation axis C and rotation axis D is defined as rCD.
 
 a=rAC   2   +rAB   2   +rCD   2   −rBD   2  
 
 b= 2 rAC×rAB  
 
 b′= 2 rAC×rCD  
 
 c= 2 rAB×rCD  
 
 c′= 2 rCD   2  
 
 d= 2 rBD   2   Eqs. 2
 
   When the center distance rAB between the rotation axis A and rotation axis B and the center distance rCD between the rotation axis C and rotation axis D are equal, the values b and b′ are equal, and the values c and c′ are equal, so that θ 2  can be represented by the following Equation 3. 
   
     
       
         
           
             
               
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   Here, the values a, b, c and d in Equation 3 can be represented by the following Equations 4.
 
 a=rAC   2 +2 rAB   2   −rBD=rAC   2 +2 rCD   2   −rBD   2  
 
 b= 2 rAC×rAB= 2 rAC×rCD  
 
 c= 2 rAB   2 =2 rCD   2  
 
 d= 2 rBD   2   Eqs. 4
 
     FIG. 12  is a graph showing how the relationship between θ 1  and θ 2  changes at different values of θAB when θCD is made 45 degrees. 
   From the graph it can be seen that when θCD is made 45 degrees and θAB is made 30 degrees, θ 2  increases in a substantially one-to-one relationship approximately in proportion to increase in θ 1 . In other words, the output angle of rotation (overall angle of rotation of the elbow joint  36 , i.e., θ 1 +θ 2 ) becomes double the angle of rotation input through the speed reducer by the electric motor  76  (the angle of rotation of the rotation axis A, i.e., θ 1 ). Therefore, the range of motion of the elbow joint  36  in the bending direction increases and the critical value of the driven speed (rotational speed) of the elbow joint  36  for the same input can be increased. 
     FIG. 8  referred to earlier shows the elbow joint  36  when θ 1  is 30 degrees and θ 2  is 19.2 degrees, so that θ 1 +θ 2  is 49.2 degrees.  FIG. 9  shows the elbow joint  36  when θ 1  is 55 degrees and θ 2  is 44 degrees, so that θ 1 +θ 2  is 99 degrees. 
   The explanation of  FIGS. 8 and 9  will be continued. Owing to the crossing arrangement of the first movable link  70  and second movable link  72 , the amount of projection of the two movable links outward of the joint is small even when the amount of driving of the elbow joint  36  is large, so that there is little risk of the movable links  70 ,  72  coming into physical interference with the covers. Moreover, the likelihood of interference with the first cover  84  and second cover  86  can be reduced because the elbow joint  36  bends in two stages (due to the presence of two rotation axes (i.e., the rotation axis A and rotation axis C) acting as fulcrums). 
   Moreover, the S-like shape of the second movable link  72  prevents interference of the second movable link  72  with the rotation axis A and the rotation axis C to which the first movable link  70  is connected, when the elbow joint  36  is driven, so that the range of motion of the elbow joint  36  in the bending direction can be further expanded. 
   When the elbow joint  36  is driven (rotated), the first cover  84  and second cover  86  are displaced relative to each other to produce a gap therebetween (change the size of the gap). As explained above, however, the third cover  88  is slidably interposed between the first cover  84  and second cover  86 , so that the gap can be covered by the third cover  88  to make the elbow joint  36  more resistant to dust and to improve safety of the elbow joint  36 . Another effect or advantage is improved appearance of the robot  1 . 
   The singularity of the arm linkage  5  will be explained next.  FIG. 13  is a graph showing the relationship of the straight-line distance (designated by R) from the shoulder joint  30 ,  32 ,  34  to the wrist joint  38  and the driven angle θ 1 +θ 2  of the elbow joint  36 . Note that θ 1 +θ 2  being 0 degree in the graph means the state of the arm linkage  5  being fully extended (state of the elbow joint  36  being driven as far in the extending direction as the mechanism permits; this is designated by R 1   mt  in the upper diagram of  FIG. 14 ). 
   When the elbow joint  36  is driven in the bending direction from the state shown by the upper diagram of  FIG. 14  (state of the elbow joint  36  being driven as far in the extending direction as the mechanism permits), the arm linkage  5  assumes the singularity posture in which the joints of the arm linkage  5  (shoulder joint, elbow joint, and wrist joint) are positioned on the same straight line. The straight-line distance R from the shoulder joint  30 ,  32 ,  34  to the wrist joint  38  is maximum at this time (designated Rmax in the middle diagram of  FIG. 14 ). 
   When the robot assumes the singularity posture, the controlled variable of the position and posture control of the robot diverges or oscillates, so that of the total range within which the mechanism permits the angle of rotation to be determined (designated by θELm in the lower diagram of  FIG. 14 ), only the range of angles of rotation that does not pass beyond the singularity posture (designated by θELc) can be utilized in control. In other words, of θ 1 +θ 2 , angles from 0 degree (R=R 1   mt ) to the angle at which R becomes the maximum value Rmax are angles of rotation that cannot be utilized in control (designated by θELcerr). 
   As shown in  FIG. 13 , in this embodiment, R becomes the maximum value Rmax when θ 1 +θ 2  is about 2.8 degrees, so that angles of rotation from 0 degree to 2.8 degrees cannot be utilized in control. This range is only about ¼ of that in the case of the ordinary conventional single-axis joints and is smaller. This is ascribable to the fact that the upper arm link  42  and forearm link  44  are connected through the two movable links, i.e., the first movable link  70  and the second movable link  72 , and these two movable links  70  and  72  are arranged to cross, so that interference between the first cover  84  and second cover  86  is less likely to occur and no need to offset the rotation axes of the elbow joint  36  outward arises. 
   Thus, in the joint structure of the robot  1  according to this embodiment, the upper arm link  42  and forearm link  44  are connected through the two movable links (the first movable link  70  and second movable link  72 ) and the two movable links  70 , and  72  are arranged to cross. As a result, the overall angle of rotation of the elbow joint  36  for a given input can be made larger, the range of motion of the elbow joint  36  in the bending direction increases, and the critical value of the driven speed (rotational speed) of the elbow joint  36  for the same input can be increased. 
   Moreover, the amount of outward projection of the two movable links  70 ,  72  can be made small, so that there is little risk of the movable links  70  and  72  coming into physical interference with the third cover  88  that covers them. Further, interference between the first cover  84  and the second cover  86  becomes unlikely because the elbow joint  36  bends over two stages with the two rotation axes A, C acting as fulcrums. As a result, the range of motion of the elbow joint  36  in the bending direction can be further expanded. In addition, the rotational axes of the elbow joint  36  do not have to be offset outward of the joint, so that reduction of the movable range of the elbow joint  36  in the direction of extension can be minimized. 
   Further, the second movable link  72  is given a curved shape, specifically the S-like shape, detouring the rotation axis A and rotation axis C, when the elbow joint  36  is driven, so that the second movable link  72  does not interfere with the rotation axis A and rotation axis C to which first movable link  70  is connected. As a result, the range of motion of the elbow joint  36  can be further expanded. 
   Moreover, since the third cover  88  is slidably interposed between the first cover  84  and second cover  86 , the gap formed between the first cover  84  and second cover  86  occurring with rotation of the elbow joint  36  can be covered by the third cover  88 . The elbow joint  36  can therefore be driven without exposing the internal structure of the joint, so that, in addition to realizing the foregoing effects or advantages, it is further possible to improve the dust resistance and safety of the elbow joint  36 . In addition, the appearance of the robot  1  can be enhanced. 
   Although the foregoing explanation has been made taking the elbow joint  36  as an example joint, the robot joint structure according to this embodiment is also likewise suitable for the knee joints  16  and the like. 
   As shown in  FIG. 12 , the amount of increase in θ 2  with increase in θ 1  is maximum when θAB is set at or near 0 degree. Therefore, by setting θAB at or near 0 degree, i.e., by providing both the rotation axis A and the rotation axis B on or near the straight line ( 42   x ) lying perpendicular to the longitudinal direction of the upper arm link  42  ( 42   z ), it is possible to further expand that range of motion of the elbow joint  36  in the bending direction and also to increase the driven speed of the elbow joint  36  for the same input. However, owing to the fact that driving torque of the elbow joint  36  decreases in proportion as the amount of increase in θ 2  with increase in θ 1  becomes larger, θAB of course needs to be appropriately defined in accordance with the actuator used and/or the intended operation (task). In this embodiment, θAB is set at 30 degrees in order to achieve optimum balance among expansion of range of motion, enhancement of driven speed, and realization of driving torque. 
   On the other hand, as shown in  FIG. 15 , owing to the aforesaid S-like shape of the second movable link  72  in this embodiment, when the amount of driving of the elbow joint  36  is large, the convex of the S-shaped second movable link  72  interferes with the first cover  84  (see encircled area ( 1 ) in the drawing). Although this interference can be eliminated by cutting away the edge of the first cover  84 , doing so would make it necessary extend the edge of the third cover  88  in the upper arm direction so as to cover the resulting gap. If the edge of the third cover  88  should be extended in the upper arm direction, however, interference would arise with the electric motor  76  mounted on the upper arm when the amount of driving of the elbow joint  36  is large (shown by encircled area ( 2 ) in the drawing). 
   Although interference between the edge of the third cover  88  and the electric motor  76  can be eliminated by shifting the electric motor  76  downward, doing so would reduce the sliding distance of the third cover  88  because it would become impossible to form a concavity at the lower portion of the first cover  84  (shown by encircled area ( 3 )). As a result, a gap would arise between the first cover  84  and third cover  88  when the amount of driving of the elbow joint  36  is large (shown by encircled area ( 4 )). Note that  FIG. 15  shows the elbow joint  36  when θ 1  is 75 degrees and θ 2  is 75 degrees, i.e., when θ 1 +θ 2  is 150 degrees. 
   A second embodiment of the invention to be explained in the following is therefore configured to increase the range of motion of the joints while preventing interference between the links and covers. 
   A robot joint structure according to the second embodiment of the invention will now be explained. 
     FIG. 16  is a plan view, similar to  FIG. 7 , but showing a robot joint structure according to the second embodiment of the invention. 
   An explanation will be made focusing on the points of difference from the first embodiment. As illustrated, in the second embodiment a second movable link  72   a  is curved in the shape of an archery bow, θAB is defined as −15.5 degrees, and θCD is defined as 30 degrees. 
     FIGS. 17 to 19  are plan views showing driven states of the elbow joint  36  shown in  FIG. 16 . Note that  FIG. 17  shows the elbow joint  36  when θ 1  is 24 degrees and θ 2  is 25.7 degrees, i.e., when θ 1 +θ 2  is 49.7 degrees, and  FIG. 18  shows the elbow joint  36  when θ 1  is 45 degrees and θ 2  is 55 degrees, i.e., when θ 1 +θ 2  is 100 degrees.  FIG. 19  shows the elbow joint  36  when 01 is 68 degrees and θ 2  is 82.2 degrees, i.e., when θ 1 +θ 2  is 150.2 degrees. 
   As mentioned above, when θAB is made −15.5 degrees, in other words, when the rotation axis B is provided toward the side of the forearm link  44  from the rotation axis A, the second movable link  72   a  does not interfere with the rotation axis A. Therefore, unlike in the first embodiment, there is no need to shape the second movable link  72   a  so as to project toward the first cover  84  (so as to detour the rotation axis A). As a result, as shown in  FIGS. 17 to 19 , the second movable link  72   a  does not interfere with the first cover  84  even when the amount of driving of the elbow joint  36  is large. Moreover, the second movable link  72   a  does not interfere with the rotation axis C even when the amount of driving of the elbow joint  36  is large, because the second movable link  72   a  is curved in the shape of the archery bow. Therefore, the range of motion of the elbow joint  36  in the bending direction can be expanded beyond that in the first embodiment. 
     FIG. 20  is a graph showing how the relationship between θ 1  and θ 2  changes at different values of θCD when θAB is made −15.5 degrees. From the graph it can be seen that when θAB is made −15.5 degrees and θCD is made 30 degrees, θ 2  increases at the rate of one-to-one substantially in proportion to increase in θ 1 . In other words, the output angle of rotation (overall angle of rotation of the elbow joint  36 , i.e., θ 1 +θ 2 ) becomes double the angle of rotation inputted through the speed reducer by the electric motor  76  (the angle of rotation of the rotation axis A, i.e., θ 1 ) or more. Therefore, the range of motion of the elbow joint  36  in the bending direction can be increased beyond that in the first embodiment and the critical value of the driven speed (rotational speed) of the elbow joint  36  for the same input can be increased. 
   As shown by the graph, the amount of increase in θ 2  with increase in θ 1  increases in proportion as θCD is made smaller. Therefore, by setting θCD at or near 0 degree, i.e., by providing both the rotation axis C and the rotation axis D on or near the straight line ( 44   x ) lying perpendicular to the longitudinal direction of the forearm link  44  ( 44   z ), it is possible to further expand that range of motion of the elbow joint  36  in the bending direction and also to increase the driven speed of the elbow joint  36  for the same input. However, owing to the fact that the driving torque of the elbow joint  36  decreases in proportion as the amount of increase in θ 2  with increase in θ 1  becomes larger, θCD needs to be appropriately defined in accordance with the actuator used and/or the intended operation (task), similarly to what has been explained earlier. In this embodiment, θCD is set at 30 degrees in order to achieve optimum balance among expansion of range of motion, enhancement of driven speed, and realization of driving torque. 
   Thus, in the robot joint structure according to the second embodiment, the second movable link  72   a  is formed in the shape of the archery bow, so that when the elbow joint  36  is driven, the second movable link  72   a  does not interfere with the first cover  84  and rotation axis C. As a result, the range of motion of the elbow joint  36  can be further expanded. 
   Moreover, owing to the fact that θAB is set at −15.5 degrees and θCD is set at 30 degrees, the range of motion of the elbow joint  36  can be expanded beyond that in the first embodiment and the critical value of the driven speed (rotational speed) of the elbow joint  36  for the same input can be increased. 
   Further, as shown in  FIG. 21 , in this embodiment, R becomes the maximum value Rmax when θ 1 +θ 2  is about 3.9 degrees, so that angles of rotation from 0 degree to 3.9 degrees cannot be utilized in control. This range is only about ⅓ of that in the case of the ordinary conventional single-axial joint and is smaller. Therefore, as in the first embodiment, reduction of the movable range of the elbow joint  36  in the direction of extension attributable to the singularity can be minimized. 
   Other structural features of the second embodiment are similar to those of the first embodiment and will not be explained again. 
   A robot joint structure according to a third embodiment of the invention will be explained next. 
     FIG. 22  is a plan view similar to  FIG. 16  showing a robot joint structure according to the third embodiment of the invention. 
   An explanation will be made focusing on the points of difference from the first and second embodiments. In the third embodiment, the joint structure is provided with a mechanical over-rotation prevention mechanism for prevention over-rotation (excessive driving) of the elbow joint  36 . 
   As shown in  FIG. 22 , a pin  90  is provided on the first movable link  70  such that the pin  90  is inserted into an arcuate hole  94  formed in a stopper  92 . The stopper  92  is fastened to the upper arm first plate  42   a  (not shown). 
   The hole  94  is formed to have the same arcuate shape as the locus the pin  90  describes when the first movable link  70  is driven. One terminal end  94   a  of the hole  94  is defined so that, when the elbow joint  36  is driven, the rotation of the first movable link  70  terminates at a location where interference with the first cover  84  and second cover  86  does not occur. The other terminal end  94   b  thereof is defined so that θ 1 +θ 2  does not become an angle at which the arm linkage  5  assumes the singularity posture. That is, it is defined in the first embodiment so that θ 1 +θ 2  does not become 2.8 degrees or less and is defined in the second embodiment so that θ 1 +θ 2  does not become 3.9 degrees or less. 
   This arrangement prevents over-rotation of the first movable link  70 , thereby preventing over-rotation of the elbow joint  36  to preclude interference with and damage to the first cover  84  and second cover  86  and also preventing the arm linkage  5  from assuming the singularity posture. 
   Other structural features and effects of the third embodiment are similar to those of the first and second embodiments and will not be explained again. 
   As stated above, the first to third embodiments are configured to have a joint structure of robot (elbow joint  36 ) having a first main link (upper arm link  42 ) and a second main link (forearm link  44 ) connected through a first movable link ( 70 ) and a second movable link ( 72 ,  72   a ), and an actuator (electric motor  76 ) installed on the first main link and driving the first movable link ( 70 ) such that the first main link and the second main link are displaced relative to each other; characterized in that: rotation axes A and B each provided at the first main link; and rotation axes C and D each provided at the second main link; wherein in a quadrangle whose apices are formed by the rotation axes A, B, C and D, when assuming that rotation axes that are diagonally opposed to each other are A and C, while those that are diagonally opposed to each other are B and D, the rotation axes A and C are connected through the first movable link ( 70 ) and the rotation axes B and D are connected through the second movable link ( 72 ,  72   a ) in such a manner that the first movable link ( 70 ) and the second movable link ( 72 ,  72   a ) are disposed to cross and that the rotation axis A is driven by the actuator to drive the first movable link, such that the first main link and the second main link are displaced relative to each other. 
   In the joint structure, the rotation axis A and the rotation axis B are provided on or near a same straight line ( 42   x ) lying perpendicular to a longitudinal direction ( 42   z ) of the first main link. 
   In the joint structure, the rotation axis C and the rotation axis D are provided on or near a same straight line ( 44   x ) lying perpendicular to a longitudinal direction ( 44   z ) of the second main link. 
   In the joint structure, at least one of the first main link ( 70 ) and the second main link ( 72 ,  72   a ) is given a curved shape (S-like shape or shape of an archery bow), so as not to interfere with the rotation axes of the other of the first main link and the second main link. 
   In the joint structure, at least one of the first movable link ( 70 ) and the second movable link ( 72 ,  72   a ) is provided with an over-rotation prevention mechanism (pin  90 , stopper  92 , hole  94 ) that prevents the joint from over-rotating beyond predetermined angles. 
   In the joint structure, the joint is provided with covers covering the first main link, the first movable link ( 70 ), the second movable link ( 72 ,  72   a ), the second main link and the actuator from outside, the covers comprising: a first cover ( 84 ) covering the first main link and the actuator; a second cover ( 86 ) covering the second main link; and a third cover ( 88 ) slidably interposed between the first cover ( 84 ) and the second cover ( 86 ), and covering the first and second movable links ( 70 ,  72 ,  72   a ) and a gap formed between the first and second covers ( 84 ,  86 ) occurring with rotation of the joint. 
   Although the robot joint structure according to this invention has been explained in the foregoing taking a legged mobile robot, more specifically a humanoid robot, as an example, this invention is also suitable for application to other types of mobile robots and industrial robots. 
   Moreover, the actuator used is not limited to the electric motor but can be another type of actuator. 
   Further, θAB and θCD are not limited to the aforesaid concrete examples but should of course be set at appropriate values in accordance with the intended operation (task) to be performed by the robot. 
   Further, although it has been explained that, of the first movable link  70  and second movable link  72  ( 72   a ), only the second movable link  72  ( 72   a ) is curved so as not to interfere with the rotation axis A and rotation axis C, the first movable link  70  can also be curved so as not to interfere with the rotation axis B and rotation axis D. Needless to say, the shape of the curve is not limited to the illustrated examples. 
   INDUSTRIAL APPLICABILITY 
   In this invention, a robot joint structure is provided wherein the first main link (e.g., the upper arm link) and the second main link (e.g., the forearm link) are connected through the first movable link and the second movable link, and the two movable links are arranged to cross. This structure makes it possible to increase the overall driven angle of the joint (e.g., an elbow joint) relative to the input, expand the range of motion of the joint in the bending direction, and also raise the critical value of the driven speed (rotational speed). 
   In addition, the amount of outward projection of the two movable links is small, so that there is little risk of the movable links and the covers coming into physical interference. Further, interference with the cover covering the first main link and the cover covering the second main link becomes unlikely because the joint bends over two stages with the two rotation axes acting as fulcrums. As a result, the range of motion of the joint in the bending direction can be further expanded. Moreover, the rotational axes of the elbow joint do not have to be offset outward of the joint, so that reduction of the movable range of the joint in the direction of extension attributable to the singularity can be minimized.