Patent Publication Number: US-8109171-B2

Title: Parallel mechanism

Description:
This application is a national phase entry under 35 U.S.C. §371 of PCT Patent Application No. PCT/JP2007/067937, filed on Sep. 14, 2007, which claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2006-308546, filed Nov. 15, 2006, both of which are incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a parallel mechanism having a plurality of arms aligned in parallel. 
     BACKGROUND ART 
     A parallel mechanism is a manipulator that is capable of operating at high speed with high precision and is lightweight and highly rigid, as compared to a serial mechanism employing serial arms. Related art for such a parallel mechanism is described in Patent documents 1 and 2 shown below. Parallel mechanisms described in these patent documents have three arms in total. Each arm has a pair of rods arranged parallel to one another. A motor is connected to a base end portion of the arm in such a manner that power can be transmitted, and a bracket is attached to a free end portion of the arm. An end effecter can be attached to the bracket. For example, a hand can be used as the end effecter. 
     The motor rotates to operate each arm, and thereby move the end effecter to a given position. Here, the rods and the bracket of each arm are connected via ball joints, and therefore the bracket can move in three dimensions with respect to the arm. Also, as shown in FIG. 5 of Patent document 1, a pair of rods is configured so as to be pulled closer to each other by a spring, and thus a ball portion and a socket portion of the ball joint are connected so as not to be disengaged from each other. In this manner, when the motor rotates to operate the arm, the bracket is not disengaged from each rod. 
     In the foregoing related art, each hook provided at opposite ends of the spring is simply hooked on each rod. The spring hook has poor durability, and therefore when the arm is continually operated, the hook might be broken due to fatigue. Also, in this configuration, since the spring hook is simply hooked on the rod, there is scope for the rod to rotate about an axis that is parallel to its longitudinal direction. Accordingly, when operating the arm, the rods are inevitably caused to rotate about the axis, resulting in an increased amount of movement of the socket portion with respect to the ball portion at the ball joint. Therefore, the ball joints, which are made of synthetic resin, deteriorate quickly due to abrasion of the ball portion and the socket portion, and such abrasion of the ball joints leads to unstable attitude control of the end effecter (bracket). This results in a problem in that the end effecter is not positioned with high precision. Patent document 3 also describes a manipulator structure. This structure addresses the problem of the spring hook being broken due to fatigue, but it does not address any problem caused by rotation of the rod about the axis that is parallel to a longitudinal direction of the rod.
     Patent document 1: Japanese Utility Model Application Laid-Open Publication No. H8-403   Patent document 2: Japanese Patent No. 3050992   Patent document 3: Japanese Unexamined Patent Application Publication No. 2002-529258   

     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     The problem to be solved by the present invention is to position an end effecter with high precision by stabilizing its attitude. 
     Solution to the Problems 
     To solve the above problem, the present invention provides a parallel mechanism, including: 
     arms each being capable of swingably pivoting on a base end portion and including a pair of rods aligned in parallel in a longitudinal direction of the rod; 
     a bracket having an end effecter attached thereto, the bracket being retained between the pair of rods at a free end portion of the arms; 
     a pair of ball joints for displaceably connecting the bracket with the arms, the ball joints each including a first joint element having a ball and provided to one of the rod and the bracket, and a second joint element having a socket for retaining the ball and provided to the other of the rod and the bracket; 
     a connecting member for connecting the pair of parallel rods, and restricting rotation of each rod about an axis that is parallel to the longitudinal direction of the rod; and 
     a biasing member for providing a biasing force to retain the balls in the sockets at the ball joints. 
     Preferably, the biasing force of the biasing member is limitedly effected only in one direction. 
     Preferably, the connecting member includes:
         a first connecting member attached to one of the pair of rods such that the first connecting member can swingably pivot on a first axis that is perpendicular to the longitudinal direction of the rod;   a second connecting member attached to the other of the pair of rods such that the second connecting member can swingably pivot on a second axis that is parallel to the first axis;   a piston rod portion fixed to the first connecting member; and   a cylinder portion fixed to the second connecting member, the cylinder portion having the piston rod portion inserted thereinto movabley in the axial direction of the piston rod portion.       

     The biasing member is a coil spring provided to wrap around the circumference of the piston rod portion, the coil spring being compressed to pull the pair of rods closer to each other, thereby retaining the ball in the socket at each of the ball joints. 
     Preferably, the connecting member includes:
         a portion attached to one of the pair of rods such that the portion can swingably pivot on the first axis that is perpendicular to the longitudinal direction of the rod; and   a portion attached to the other of the pair of rods such that the portion can swingably pivot on the second axis that is parallel to the first axis.       

     Provided at the ball joints are:
         the second joint elements each being fixed to each rod, and   the first joint elements each including a cylinder portion fixed to the bracket, and a piston rod portion inserted into the cylinder portion movably in its axial direction.       

     The biasing member is a coil spring contained in a cylinder chamber of the cylinder portion, the coil spring expanding to bias the first joint elements away from each other, thereby retaining the ball in the socket at each of the ball joints. 
     Preferably, the connecting member includes:
         a portion attached to one of the pair of rods such that the portion can swingably pivot on the first axis that is perpendicular to the longitudinal direction of the rod; and   a portion attached to the other of the pair of rods such that the portion can swingably pivot on a second axis that is parallel to the first axis.       

     The first and second joint elements are provided at the ball joints, the first joint elements each being fixed to the bracket, the second joint elements each being fixed to the rod, and 
     at least one of the second joint elements fixed to the pair of rods is provided with the socket in a movable manner such that the socket is biased by the coil spring serving as a biasing member, the coil spring expanding to push the movable socket upon the ball, thereby retaining the ball in the socket at each of the ball joints. 
     Preferably, the connecting member includes:
         a first connecting member attached to one of the pair of rods such that the first connecting member can swingably pivot on the first axis that is perpendicular to the longitudinal direction of the rod;   a second connecting member attached to the other of the pair of rods such that the second connecting member can swingably pivot on the second axis that is parallel to the first axis;   a piston rod portion fixed to the first connecting member; and   a cylinder portion fixed to the second connecting member, the cylinder portion having the piston rod portion inserted thereinto movably in its axial direction.       

     The biasing member is a coil spring provided to wrap around the circumference of the piston rod portion, the coil spring expanding to pull the pair of rods closer to each other, thereby retaining the ball in the socket at each of the ball joints. 
     Preferably, the connecting member includes a pair of connecting members supported such that the pair of connecting members can swingably pivot relative to each other in a direction perpendicular to the longitudinal direction of the pair of rods. 
     A first one of the pair of connecting members is attached to one of the pair of rods such that the first connecting member can swingably pivot on the first axis that is parallel to a direction that is perpendicular to the longitudinal direction of the rod. 
     A second one of the pair of connecting members is attached to the other of the pair of rods such that the second connecting member can swingably pivot on the second axis that is parallel to the first axis. 
     The biasing member is a coil spring having one end attached to the first connecting member such that the spring can swing around a third axis that is parallel to the first axis and having the other end attached to the second connecting member such that the spring can swing around a fourth axis that is parallel to the second axis. 
     The coil spring is compressed to place all of the first, second, third, and fourth axes on a straight line, and thereby pulls the pair of rods closer to each other, so that the ball is retained in the socket at each of the ball joints. 
     Preferably, the connecting member includes a pair of connecting members supported such that the connecting members can swingably pivot relative to each other in a direction perpendicular to the longitudinal direction of the pair of rods. 
     A first one of the pair of connecting members is attached to one of pair of rods such that the first connecting member can swingably pivot on the first axis that is parallel to a direction that is perpendicular to the longitudinal direction of the rod. 
     A second one of the pair of connecting members is attached to the other of the pair of rods such that the second connecting member can swingably pivot on the second axis that is parallel to the first axis. 
     The biasing member is a torsion coil spring having one end attached to the first connecting member such that the spring can swing around a third axis that is parallel to the first axis and having the other end attached to the second connecting member such that the spring can swing around a fourth axis that is parallel to the second axis. 
     Both ends of the torsion coil spring draw each other to place all of the first, second, third, and fourth axes on a straight line, thereby pulling the pair of rods closer to each other, so that the ball is retained in the socket at each of the ball joints. 
     EFFECT OF THE INVENTION 
     A parallel mechanism according to the present invention includes a connecting member for connecting together a pair of rods arranged in parallel. The connecting member restricts rotation of the rods about an axis that is parallel to a longitudinal direction of the rod. Thus, the rotational swing of the rods in their longitudinal directions can be restricted and inhibited. Accordingly, when operating the arms, rotation of the rods about the axis is restricted, resulting in an extremely reduced amount of movement of the socket portion with respect to the ball portion at each ball joint. Therefore, even if the ball joints are made of synthetic resin, deterioration due to abrasion of the ball portion and the socket portion is significantly reduced, and attitude control of the end effecter (bracket) can be kept stable, making it possible to position the end effecter with high precision. Furthermore, the parallel mechanism according to the present invention has no hook provided at portions for connecting the arms with the biasing members. Accordingly, hook breakage due to fatigue is eliminated, resulting in significantly improved durability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrating a parallel mechanism  100  according to an embodiment. 
         FIG. 2  is a view illustrating the parallel mechanism  100  as seen from arrow  101  in  FIG. 1 . 
         FIG. 3A  is an enlarged view of a connecting structure according to a first embodiment. 
         FIG. 3B  is a cross-sectional view taken along line I-I in  FIG. 3A . 
         FIG. 4A  is an enlarged view of a connecting structure according to a second embodiment. 
         FIG. 4B  is a cross-sectional view taken along line I-I in  FIG. 4A . 
         FIG. 4C  is a cross-sectional view taken along line II-II in  FIG. 4A . 
         FIG. 5  is an enlarged view of a connecting structure according to a third embodiment. 
         FIG. 6A  is an enlarged view of a connecting structure according to a fourth embodiment. 
         FIG. 6B  is a cross-sectional view taken along line I-I in  FIG. 6A . 
         FIG. 7A  is an enlarged view of a connecting structure according to a fifth embodiment. 
         FIG. 7B  is a cross-sectional view taken along line I-I in  FIG. 7A . 
         FIG. 7C  is a view in which ball joints are detached. 
         FIG. 8  is an enlarged view of a connecting structure according to a sixth embodiment. 
     
    
    
     DESCRIPTION OF THE REFERENCE CHARACTERS 
       100  parallel mechanism 
       101  arm unit 
       11  first arm 
       12  second arm 
       121  rod 
       41 ,  42  ball joint 
       2 ,  2 ′ connecting member 
       3  bracket 
       301  attachment piece 
       302  attachment surface 
       413  ball 
       411  first joint element 
       414  socket 
       412  second joint element 
       201  holding member 
       23  rod cylinder mechanism (telescopic mechanism) 
       211  first connecting piece 
       212  second connecting piece 
       213  coupling piece 
       10  pin 
       231  piston rod portion 
       232  cylinder portion 
       24  coil spring 
       21  first plate member 
       22  second plate member 
       420  movable socket 
       421  movable portion 
       422  coil spring 
       220  first plate member 
       221  second plate member 
     C 10  axis 
     C 7  axis 
       27  coil spring 
       28  torsion coil spring 
     C 1 , C 2 , C 3 , C 4 , C 5 , C 6  axis 
     C 8  straight line 
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments of a parallel mechanism according to the present invention will be described in detail. Note that the embodiments as described below are illustrative examples preferred for carrying out the present invention, and therefore, although various technical limitations are made, the present invention is not limited to these embodiments unless otherwise specified in the following description. 
     &lt;Overall Configuration of the Parallel Mechanism&gt; 
       FIG. 1  is a perspective view illustrating a parallel mechanism  100  according to an embodiment.  FIG. 2  is a view illustrating the parallel mechanism  100  as seen from arrow  101  in  FIG. 1 . As shown in  FIGS. 1 and 2 , the parallel mechanism  100  has a base portion  6  provided in its upper portion. The parallel mechanism  100  is supported by fixing the base portion  6  at its attachment surface  608  (the top surface in  FIG. 1 ) to, for example, a horizontal ceiling. The base portion  6  has three support members  601  provided on its lower surface side. Each support member  601  supports its drive motor  60 . The drive motor  60  is supported in such an attitude that an axis C 2  of a motor shaft is parallel (horizontally) to the attachment surface  608  of the base portion  6 . Each support member  601  is positioned an equal angle (120°) apart from one another, centering around a vertical axis C 1  of the base portion  6 , and thus, each drive motor  60  is also positioned an equal angle (120°) apart from one another, centering around the vertical axis C 1  of the base portion  6  (see  FIG. 2 ). A substantially hexagonal column-shaped arm support member  607  concentric with the axis C 2  is fixed to an output shaft of each drive motor  60 , and the arm support member  607  rotates about the axis C 2  via rotation of the output shaft of the drive motor  60 . Note that the drive motor  60  has a motor driver connected to an unillustrated control device, which controls the rotation of the output shaft of the drive motor  60 . 
     The parallel mechanism  100  has three arm units  101 , each consisting of a first arm  11  and a second arm  12 . The first arm  11  has an elongated, substantially columnar shape. The first arm  11  has a base end portion fixed to a side surface of the arm support member  607 . The first arm  11  extends from the base end portion such that its longitudinal direction is in parallel to a line that is perpendicular to the axis C 2 . The first arm  11  has a free end portion connected to the base end portion of the second arm  12  such that the second arm  12  swingably pivots on the free end portion of the first arm  11 . The second arm  12  consists of a pair of elongated rods  121 , which are positioned in parallel across their entire lengths in their longitudinal directions. The base end portion of each rod  121  is connected to the free end portion of the first arm  11  via a pair of ball joints  4 . Note that an axis C 3  extending between the ball joints  4  at the base end portions of the rods  121  is parallel to the axis C 2  of the drive motor  60 , and therefore the second arm  12  also swingably pivots on the axis C 3 . Also, one rod  121  is connected to the other rod  121  by a connecting member  2 ′ at the base end portion of the second arm  12  and by a connecting member  2  at the free end portion of the second arm  12 . The connecting members  2  and  2 ′ may be structured differently, but from the perspective of cost reduction, it is preferable that they have the same structure. The connecting members  2  and  2 ′ both have a function of preventing each rod  121  from rotating about an axis C 5  (see  FIG. 3 ) that is parallel to the longitudinal direction of the rod  121 . 
     Also, the parallel mechanism  100  has a plate-like bracket  3  for attaching an end effecter (not shown) as typified by a hand. The bracket  3  has a substantially triangular shape (see  FIG. 2 ). This bracket  3  is retained by three arm units  101  such that an end effecter attachment surface  302  of the bracket  3  (a lower surface of the bracket  3  in  FIG. 1 ) is parallel (horizontally) to the attachment surface  608  of the base portion  6 . An attachment piece  301  is formed on each side of the equilateral-triangular bracket  3 . Each attachment piece  301  is connected to the free end portion of its arm unit  101  (the free ends of the pair of rods  121  included in the second arm  12 ), so that the bracket  3  pivots on the free ends of the arm units  101  with respect to the arm units  101 . More specifically, each attachment piece  301  of the bracket  3  is connected at its end portion to the free end of its corresponding rod  121  via the ball joints  41  and  42 . Note that an axis C 4  (see  FIG. 2 ) extending between the pair of ball joints  41  and  42  is also parallel to the axis C 2  of the drive motor  60 . Therefore, the bracket  3  is capable of swingably pivoting on the horizontal axes C 4  with respect to the arm units  101 . In addition, the equilateral-triangular bracket  3  is supported by the three arm units  101  such that the bracket  3  can swingably pivot on the horizontal axes C 4  at all sides thereof. 
     The distance between the pair of ball joints  4  at the connection between the first and second arms  11  and  12  is set to be equal to the distance between the pair of ball joints  41  and  42  at the connection between the rods  121  of the second arm  12  and the bracket  3 . In this manner, as described above, the pair of rods  121  included in the second arm  12  are positioned in parallel across their entire lengths. The axes C 2 , C 3 , and C 4  are all parallel to the attachment surface  608  of the base portion  6 , and therefore however the first arm  11 , the second arm  12 , and the bracket  3  pivot on their respective axes C 2 , C 3 , and C 4 , the parallel relationship between the end effecter attachment surface  302  of the bracket  3  and the attachment surface  608  of the base portion  6  is maintained. In addition, in response to an instruction from the unillustrated control device, the motor drivers of the drive motors  60  control the rotational position of the arm support members  607  fixed to the output shafts of the drive motors  60 , thereby controlling the position of the free end portion of each first arm  11 . The position of the free end portion of the second arm  12  follows the position of the free end portion of the first arm  11  thus controlled, thereby positioning the end effecter attachment surface  302  of the bracket  3 . At this time, as described above, the bracket  3  moves while maintaining its horizontal attitude. 
     Also, the parallel mechanism  100  has provided in its center a swivel axis rod  13  extending in the vertical direction (the top-bottom direction in  FIG. 1 ) and a motor  61  for rotating the swivel axis rod  13 . The motor  61  is fixed to the base portion  6  with its output shaft pointing downward in the vertical direction. The swivel axis rod  13  has one end attached to the output shaft of the motor  61  and the other end attached at the median point of the bracket  3 . The swivel axis rod  13  may be configured such that an end effecter can be coupled to the aforementioned other end. The swivel axis rod  13  is telescopically realized by a rod and a cylinder. In addition, universal joints are employed at one and the other end of the swivel axis rod  13 , and therefore even when the three drive motors  60  drive the bracket  3  to move forward, backward, rightward, or leftward to a given position, the swivel axis rod  13  follows that given position. Note that the motor driver of the motor  61  is connected to an unillustrated control device, which controls the rotation of the output shaft of the motor  61 , thereby rotating the end effecter attached to the bracket  3  about the axis C 1 . 
     First Embodiment 
     Referring to  FIGS. 3A and 3B , a first embodiment of the structure for connecting the free end portion of the second arm  12  with the attachment piece  301  at one side of the bracket  3  will be described.  FIG. 3A  is an enlarged view of the connecting structure according to the first embodiment, and  FIG. 3B  is a cross-sectional view taken along line I-I in  FIG. 3A . In this embodiment, the ball joints  41  and  42  employed for connecting the free end portions of the rods  121  with the end portions of the attachment piece  301  of the bracket  3  are configured in the same manner. The ball joints  41  and  42  each include a first joint element  411 , which is provided with a spherical ball  413 , and a second joint element  412  on which is formed a socket  414  for retaining the ball  413 . The first joint element  411  of one of the ball joints  41  is fixed to one side surface of the attachment piece  301  of the bracket  3 , whereas the first joint element  411  of the other ball joint  42  is fixed to the other side surface of the attachment piece  301  of the bracket  3 . 
     The second joint element  412  is composed of a rectangular tube portion  415  into which a tip of the rod  121  is inserted, and the socket  414  that has a semicircular column-like shape and concatenated with the rectangular tube portion  415 . Provided at the concatenation between the rectangular tube portion  415  and the socket  414  is a hole  418  penetrating diametrically. The socket  414  has a recess  417  caved in hemispherically from its planar surface  416 , and the aforementioned ball  413  of the first joint element  411  is loosely fitted in the recess  417 . The tip of the rod  121  is inserted into the rectangular tube portion  415  of the second joint element  412  thus configured, thereby fixing the second joint element  412  and the rod  121  so as not to rotate with respect to each other. 
     The connecting member  2  for connecting the second joint element  412  of one rod  121  with the second joint element  412  of the other rod  121  includes a pair of holding members (first and second connecting members)  201  and a telescopic mechanism  23 . The pair of holding members  201  have the same shape, and each holding member  201  externally holds the second joint element  412  (the rectangular tube portion  415 ) through clipping. The holding member  201  is a substantially rectangular plate being bent approximately at a right angle at two places, and composed of mutually opposing first and second connecting pieces  211  and  212 , and a coupling piece  213  for coupling the connecting pieces  211  and  212  at their ends. In addition, each of the first and second connecting pieces  211  and  212  has provided therein a hole penetrating therethrough in its thickness direction. With the holding member  201  clipping the second joint element  412 , a pin  10  having the same diameter as the hole in each connecting piece  211 ,  212 , is inserted through each of the hole in the first connecting piece  211 , the hole  418  in the second joint element  412 , and the hole in the second connecting piece  212 , so that the holding member  201  can pivot only about the pin  10  with respect to the second joint element  412 . The pin  10  extends in the direction of an axis C 6  that is perpendicular to the longitudinal direction C 5  of the rod  121 . In this manner, the holding member  201  is attached to the rod  121  connected to the second joint element  412  such that the holding member  201  can swingably pivot only about the pin  10  (axis C 6 ). Note that the pins  10  (axes C 6 ) are parallel to each other. 
     The pair of holding members  201  are joined together by the telescopic mechanism  23 . The rod cylinder mechanism (telescopic mechanism)  23  includes a piston rod portion  231  fixed to the coupling piece  213  of one holding member  201 , and a cylinder portion  232  fixed to the coupling piece  213  of the other holding member  201 . The elongated rod of the piston rod portion  231  is removably inserted into a through hole axially passing through the cylinder portion  232 . The piston rod portion  231  includes a seat portion  233  having a larger diameter than the rod, and the cylinder portion  232  includes a seat portion  234  having a larger diameter than the tube portion. A tension coil spring  24  serving as a biasing member is provided to wrap around the circumference of the rod cylinder mechanism  23 . The tension coil spring  24  is fixed at one end  241  to the seat portion  233  of the piston rod portion  231  and at the other end  242  to the seat portion  234  of the cylinder portion  232 , so that the tension coil spring  24  pulls the piston rod portion  231  and the cylinder portion  232  in a mutually drawing direction. With such a structure having the connecting member  2  and the tension coil spring  24 , the free end portion of one rod  121  and the free end portion of the other rod  121  are pulled in a mutually approximating direction. 
     In this manner, with the balls  413  of the first joint elements  411 , which are fixed at the ends of the attachment piece  301  of the bracket  3 , being retained in the recesses  417  of the second joint elements  412  at the free end portions of the pair of rods  121 , the bracket  3  is held by the second arm  12  such that the bracket  3  can swingably pivot on the axis C 4 . In addition, when replacing or inspecting the bracket  3  or the second joint element  412 , the operator forces the free end portions of the pair of rods  121  away from each other in opposition to the spring force of the tension coil spring  24 , thereby releasing the connection between the first and second joint elements  411  and  412 , and thus the bracket  3  can be detached from the second arm  12 . 
     With the above structure for connecting the free end portion of the second arm  12  with the attachment piece  301  on one side of the bracket  3 , the biasing force of the tension coil spring  24  serving as a biasing member is limitedly effected in only the axial direction of the piston rod portion  231  of the telescopic mechanism  23 , so that the free end portion of the rod  121  and the holding member  201  of the connecting member  2  can swingably pivot only about the pin  10  (axis C 6 ). Accordingly, even when the drive motors  60  (see  FIG. 1 ) are driven to operate the arm units  101  and thereby move the bracket  3  to a given spatial position, each rod  121  does not rotate on the axis C 5 . Therefore, deterioration of the ball joints  41  and  42 , which are made of synthetic resin, due to abrasion of the ball  413  and the socket  414  is retarded, and the attitude control of the end effecter (bracket  3 ) is stabilized. As a result, the end effecter is positioned with high precision. Also, hook breakage due to fatigue does not occur because the tension coil spring  24  serving as a biasing member has no hook. 
     Note that the aforementioned connecting structure of the first embodiment is also employed for the connection between the free end portion of the first arm  11  and the base end portion of the second arm  12 , i.e., the connection between the free end portion of the first arm  11  and the base end portions of the pair of rods  121 . In addition, the connecting structure is also employed for the connection between the free end portion of the first arm  11  and the base end portion of the second arm  12 , so that the rotation of each rod  121  about the axis C 5  is limited during operation of the arm unit  101 , minimizing deterioration of the ball joints  4 . As a result, the attitude control of the end effecter (bracket  3 ) is stabilized. 
     Second Embodiment 
     Referring to  FIGS. 4A ,  4 B, and  4 C, a second embodiment of the structure for connecting the free end portion of the second arm  12  with the attachment piece  301  on one side of the bracket  3  will be described.  FIG. 4A  is an enlarged view of the connecting structure according to the second embodiment,  FIG. 4B  is a cross-sectional view taken along line I-I in  FIG. 4A , and  FIG. 4C  is a cross-sectional view taken along line II-II in  FIG. 4A . Note that only differences from the first embodiment will be described in detail. As in the first embodiment, the free end portions of the rods  121  are connected to the end of the attachment piece  301  of the bracket  3  by the ball joints  41  and  42 . The ball joints  41  and  42  each include a first joint element  411  and a second joint element  412 . The second joint element  412  includes a rectangular tube portion  415  and a socket  414 . Provided at the concatenation between the rectangular tube portion  415  and the socket  414  is a hole  418  penetrating diametrically. 
     In this embodiment, the connecting member  2  for connecting the second joint element  412  of one rod  121  with the second joint element  412  of the other rod  121  includes a pair of elongated plate members  21  and  22 . The pair of plate members  21  and  22  have the same shape, and externally hold the second joint elements  412  (the rectangular tube portions  415 ) through clipping. Provided at both ends of each plate member  21 ,  22  is a hole penetrating therethrough in its thickness direction. With the plate members  21  and  22  clipping the second joint elements  412 , a pin  10  having approximately the same diameter as the hole in each plate member  21 ,  22  is inserted through the hole in the first plate member  21 , the hole  418  in the second joint element  412 , and the hole in the second plate member  22 , so that the plate member  21 ,  22  can swingably pivot only about the pin  10  with respect to the second joint element  412 . The pin  10  extends in the direction of the axis C 6  that is perpendicular to the longitudinal direction C 5  of the rod  121 . In this manner, each plate member  21 ,  22  is attached to the rod  121  such that the plate members  21 ,  22  can swingably pivot only about the pin  10  (axis C 6 ). 
     The telescopic mechanism  23  is fixed to the attachment piece  301  of the bracket  3 . The rod cylinder mechanism (telescopic mechanism)  23  includes a piston rod portion  231  and a cylinder portion  232 . The cylinder portion  232  is fixed to the attachment piece  301  of the bracket  3 . The cylinder portion  232  is closed at one end to which one first joint element  411  is fixed. Through the other end of the cylinder portion  232 , the piston rod portion  231  is removably inserted in the direction of the axis C 4 . The piston rod portion  231  has the other first joint element  411  fixed at one end (outside the cylinder portion  232 ). In addition, the cylinder portion  232  has contained in its cylinder chamber a compression coil spring  24  serving as a biasing member. The compression coil spring  24  is fixed at one end  241  within the cylinder chamber of the cylinder portion  232  and at the other end  242  to the other end (inside the cylinder portion  232 ) of the piston rod portion  231 . When the compression coil spring  24  expands, the first joint elements  411  are biased away from each other, so that the first joint elements  411  are held by the second joint elements  412  at the ball joints  41  and  42 . 
     In this manner, with the first joint elements  411  fixed to the attachment piece  301  of the bracket  3  being retained by the second joint elements  412  at the free end portions of the pair of rods  121 , the bracket  3  is retained by the second arm  12  such that the bracket  3  can swingably pivot on the axis C 4 . In addition, when replacing or inspecting the bracket  3  or the second joint element  412 , the operator forces the first joint elements  411  away from each other in opposition to the spring force of the compression coil spring  24 , thereby releasing the connections between the first joint elements  411  and the second joint elements  412 , so that the bracket  3  can be detached from the second arm  12 . 
     With the above structure for connecting the free end portion of the second arm  12  with the attachment piece  301  on one end of the bracket  3 , the free end portion of the rod  121  and the plate member  21 ,  22 , of the connecting member can swingably pivot only about the pin  10  (axis C 6 ). Thus, even when the bracket  3  is moved to a given spatial position, each rod  121  does not rotate on the axis C 5 . 
     Third Embodiment 
     Referring to  FIG. 5 , a third embodiment of the structure for connecting the free end portion of the second arm  12  with the attachment piece  301  on one side of the bracket  3  will be described.  FIG. 5  is a partial cutaway view illustrating in enlargement the connecting structure according to the third embodiment. Note that only differences from the above embodiments will be described in detail. The connecting member  2  for connecting the second joint element  412  of one rod  121  with the second joint element  412  of the other rod  121  has the same configuration as in the second embodiment. Accordingly, even when the bracket  3  is moved to a given spatial position, each rod  121  does not rotate on the axis C 5 . Also, as in the first embodiment, the first joint elements  411  of the ball joints  41  and  42  are fixed to opposite ends of the attachment piece  301  of the bracket  3 . In addition, as in the first embodiment, the second joint element  412  of one ball joint  41  includes a rectangular tube portion  415  and a socket  414  in which a recess  417  is provided. A ball  413  of the first joint element  411  is fitted in the recess  417 . 
     In this embodiment, the second joint element  412  of the other ball joint  42  is composed of a rectangular tube portion  415  and a movable socket  420 . The movable socket  420  includes a movable portion  421  with a recess  417  in which the ball  413  of the first joint element  411  is fitted. Also, the movable socket  420  has a housing portion  419  protruding away from a planar surface  416 , and the housing portion  419  has contained therein a compression coil spring  422  serving as a biasing member. Also, the housing portion  419  has the movable portion  421  contained therein. Thus, the biasing force of the compression coil spring  422  is effected in the direction of the axis C 4 , so that the movable portion  421  is biased in such a direction that its recess  417  approximates and is pressed upon the ball  413 . Note that the movable socket  420  may be provided to both the second joint elements  412  of the ball joints  41  and  42 . 
     In this manner, with the balls  413  of the first joint elements  411 , which are fixed at the end portions of the attachment piece  301  of the bracket  3 , being retained in the recesses  417  of the second joint elements  412  at the free end portions of the pair of rods  121 , the bracket  3  is retained by the second arm  12  such that the bracket  3  can swingably pivot on the axis C 4 . In addition, when replacing or inspecting the bracket  3  or the second joint element  412 , the operator forces the movable portion  421  of the movable socket  420  of the second joint element  412  away from the ball  413  of the first joint element  411  in opposition to the spring force of the compression coil spring  422 , thereby releasing the connection between the first and second joint elements  411  and  412 , so that the bracket  3  can be detached from the second arm  12 . 
     With the above structure for connecting the free end portion of the second arm  12  with the attachment piece  301  on one side of the bracket  3 , the free end portion of the rod  121  and the plate members  21 ,  22  of the connecting member  2  can swingably pivot only about the pin  10  (axis C 6 ). Thus, even when the bracket  3  is moved to a given spatial position, each rod  121  does not rotate about the axis C 5 . 
     Fourth Embodiment 
     Referring to  FIGS. 6A and 6B , a fourth embodiment of the structure for connecting the free end portion of the second arm  12  with the attachment piece  301  on one side of the bracket  3  will be described.  FIG. 6A  is an enlarged view of the connecting structure according to the fourth embodiment, and  FIG. 6B  is a cross-sectional view taken along line I-I in  FIG. 6A . Note that only differences from the above embodiments will be described in detail. In this embodiment, the ball joints  41  and  42  employed for connecting the free end portions of the rods  121  with the end portions of the attachment piece  301  of the bracket  3  are configured in the same manner as in the first embodiment. The ball joints  41  and  42  each include a first joint element  411  and a second joint element  412 . The second joint element  412  is composed of a rectangular tube portion  415  and a socket  414 . 
     In this embodiment, the connecting member  2  for connecting the second joint element  412  of one rod  121  with the second joint element  412  of the other rod  121  includes a pair of holding members (first and second connecting members)  201 , and a telescopic mechanism  23 . The pair of holding members  201  externally hold the second joint elements  412  through clipping. As in the first embodiment, the holding member  201  is composed of mutually opposing first and second connecting pieces  211  and  212 , and a coupling piece  213  for coupling the connecting pieces  211  and  212  at their ends. In addition, each of the first and second connecting pieces  211  and  212  has provided therein a hole penetrating therethrough in its thickness direction. With the holding member  201  clipping the second joint element  412 , a pin  10  having the same diameter as the hole in each connecting piece  211 ,  212  is inserted into the hole in the first connecting piece  211 , the hole  418  in the second joint element  412 , and the hole in the second connecting piece  212 , so that the holding member  201  can swingably pivot only about the pin  10  with respect to the second joint element  412 . In this manner, the holding member  201  is attached to the rod  121  such that the holding member  201  can swingably pivot only about the pin  10  (the axis C 6  that is perpendicular to the longitudinal direction C 5  of the rod  121 ). 
     The pair of holding members  201  are joined together by the telescopic mechanism  23 . The rod cylinder mechanism (telescopic mechanism)  23  includes a piston rod portion  231  fixed to the coupling piece  213  of one holding member  201 , and a cylinder portion  232  fixed to the coupling piece  213  of the other holding member  201 . The coupling piece  213  at which the aforementioned other holding member  201  is provided, and the cylinder portion  232  each have provided therein a through hole axially passing therethrough, and the piston rod portion  231  is removably inserted into the through hole. A compression coil spring  24  serving as a biasing member is provided within a cylinder chamber of the cylinder portion  232  so as to wrap around the circumference of the piston rod portion  231 . The compression coil spring  24  is fixed at one end  241  within the cylinder chamber of the cylinder portion  232  and fixed at the other end  242  to a brim portion having a slightly larger diameter provided at the other end of the piston rod portion  231 . When the compression coil spring  24  expands, the piston rod portion  231  and the cylinder portion  232  are pulled in a mutually drawing direction. With such a structure having the connecting member  2  and the compression coil spring  24 , the free end portion of one rod  121  and the free end portion of the other rod  121  are pulled in a mutually approximating direction. 
     In this manner, the bracket  3  is held by the second arm  12  such that the bracket  3  can swingably pivot on the axis C 4 . In addition, when replacing or inspecting the bracket  3  or the second joint element  412 , the operator forces the free end portions of the pair of rods  121  away from each other in opposition to the spring force of the compression coil spring  24 , thereby releasing the connection between the first and second joint elements  411  and  412 , so that the bracket  3  can be detached from the second arm  12 . 
     With the above structure for connecting the free end portion of the second arm  12  with the attachment piece  301  on one side of the bracket  3 , the biasing force of the compression coil spring  24  serving as a biasing member is limitedly effected in only the axial direction of the piston rod portion  231  of the telescopic mechanism  23 , so that the free end portion of the rod  121  and the holding member  201  of the connecting member  2  can swingably pivot only about the pin  10  (axis C 6 ). Thus, even when the drive motors  60  (see  FIG. 1 ) are driven to operate the arm units  101  and thereby move the bracket  3  to a given spatial position, each rod  121  does not rotate about the axis C 5 . 
     Fifth Embodiment 
     Referring to  FIGS. 7A ,  7 B, and  7 C, a fifth embodiment of the structure for connecting the free end portion of the second arm  12  with the attachment piece  301  on one side of the bracket  3  will be described.  FIG. 7A  is an enlarged view of the connecting structure according to the fifth embodiment,  FIG. 7B  is a cross-sectional view taken along line I-I in  FIG. 7A , and  FIG. 7C  is a view in which ball joints are detached. Note that only differences from the above embodiments will be described in detail. In this embodiment, the ball joints  41  and  42  employed for connecting the free end portions of the rods  121  with the end portions of the attachment piece  301  of the bracket  3  have the same configuration as in the first embodiment. The ball joints  41  and  42  each include a first joint element  411  and a second joint element  412 . The second joint element  412  is composed of a rectangular tube portion  415  and a socket  414 . 
     In this embodiment, the second joint element  412  of one rod  121  and the second joint element  412  of the other rod  121  are connected together by a pair of connecting members  2 . The first connecting member  2 , one of the pair of connecting members  2 , includes a pair of first plate members  220 . The second connecting member  2 , the other of the pair of connecting members  2 , includes a pair of second plate members  221 . The first and second plate members  220  and  221  each externally hold the second joint element  412  (the rectangular tube portion  415 ) through clipping. The first and second plate members  220  and  221  are each an elongated plate member bent at angle θ (e.g., 130°). In addition, the first and second plate members  220  and  221  each have provided at one end a hole penetrating therethrough in its thickness direction. With the first plate members  220  clipping the second joint element  412 , a pin  10  having the same diameter as the hole in each first plate member  220  is inserted into the hole in one first plate member  220 , the hole  418  in the second joint element  412 , and the hole in the other first plate member  220 , so that each first plate member  220  can swingably pivot only about the pin  10  (the axis C 6  that is perpendicular to the length C 5  of the rod  121 ) with respect to the second joint element  412 . Also, with the second plate members  221  clipping the second joint element  412 , a pin  10  having the same diameter as the hole in each second plate member  221  is inserted into the hole in one second plate member  221 , the hole  418  in the second joint element  412 , and the hole in the other second plate member  221 , so that each second plate member  221  can swingably pivot only about the pin  10  (axis C 6 ) with respect to the second joint element  412 . Moreover, the first and second plate members  220  and  221  are rotatably connected together at the other end by a connecting shaft  2   a . The connecting shaft  2   a  has as its center an axis C 10  that is perpendicular to the longitudinal direction C 5  of the rod  121 , and therefore the first and second plate members  220  and  221  can swingably pivot only about the connecting shaft  2   a  (axis C 10 ) relative to each other. In this manner, the connecting members  2  are each attached to the rod  121  such that the connecting members  2  can swingably pivot only about the pin  10  (axis C 6 ) and the connecting shaft  2   a  (axis C 10 ). 
     The pair of mutually opposing first plate members  220  are pivotably connected together at their bending points by a support shaft  275 . Also, the pair of mutually opposing second plate members  221  are pivotably connected together at their bending points by a support shaft  276 . The support shafts  275  and  276  each have as its center an axis C 7  that is parallel to the axis C 6 . In this manner, the connecting members  2  are each attached to the rod  121  such that the connecting members  2  can swingably pivot only about the support shafts  275 ,  276  (axis C 7 ). 
     The pair of connecting members  2  are further connected by a tension coil spring  27  serving as a biasing member. The tension coil spring  27  is pivotably attached at one end  271  to the support shaft  275  and at the other end  272  to the support shaft  276 . In this manner, the tension coil spring  27  is attached such that the spring  27  can pivot only about the support shafts  275  and  276  (axes C 7 ). Moreover, when the tension coil spring  27  is compressed, the support shafts  275  and  276  are pulled in a mutually drawing direction. In addition, the biasing force of the tension coil spring  27  is limitedly effected in only one direction. Accordingly, each first plate member  220  (first connecting member  2 ) and each second plate member  221  (second connecting member  2 ) swingably pivot on the pin  10 , the connecting shaft  2   a , and the support shafts  275 ,  276 . In addition, each pin  10  (axis C 6 ) and each support shaft  275 ,  276 , (axis C 7 ) are aligned on a straight line C 8 , and the free end portion of one rod  121  and the free end portion of the other rod  121  are pulled in a mutually approximating direction. Note that in this state, the connecting shaft  2   a  is placed away from the straight line C 8 . 
     In this manner, the bracket  3  is held by the second arm  12  such that the bracket  3  can swingably pivot on the axis C 4 . The second plate member  221  is provided with a stopper  210  in the vicinity of the connecting shaft  2   a . When replacing or inspecting the bracket  3  or the second joint element  412 , the operator moves the connecting shaft  2   a  toward the compression coil spring  27  in opposition to the spring force of the compression coil spring  27  (see  FIG. 7C ). Then, when the connecting shaft  2   a  passes a line C 9  extending between the support shafts  275  and  276 , the stopper  210  is brought into contact or is engaged with the first plate member  220 , thereby retaining the positions of the first and second plate members  220  and  221 . As a result, the compression coil spring  27  expands to push the free end portions of the pair of rods  121  away from each other, thereby releasing the connection between the first and second joint elements  411  and  412 , so that the bracket  3  can be detached from the second arm  12 . 
     With the above structure for connecting the free end portion of the second arm  12  with the attachment piece  301  on one side of the bracket  3 , the direction in which the biasing force of the compression coil spring  27  serving as a biasing member is effected is limited, and thus the free end portions of the rods  121  and the connecting members  2  can swingably pivot only about the pin  10  (axis C 6 ), the connecting shaft  2   a  (axis C 10 ), and the support shafts  275 ,  276  (axis C 7 ). Thus, even when the drive motors  60  (see  FIG. 1 ) are driven to operate the arm units  101  and thereby move the bracket  3  to a given spatial position, each rod  121  does not rotate about the axis C 5 . 
     Sixth Embodiment 
     Referring to  FIG. 8 , a sixth embodiment of the structure for connecting the free end portion of the second arm  12  with the attachment piece  301  on one side of the bracket  3  will be described.  FIG. 8  is an enlarged view of the connecting structure according to the sixth embodiment. Note that only differences from the fifth embodiment will be described in detail. In this embodiment, a torsion coil spring  28  is provided instead of the compression coil spring  27  of the fifth embodiment. The torsion coil spring  28  is pivotably attached at one end  281  to the support shaft  275  and at the other end  282  to the support shaft  276 . Other features are the same as in the fifth embodiment. Accordingly, the connecting member  2  is attached to the rod  121  such that the connecting member  2  can swingably pivot only about the pin  10  (axis C 6 ), the connecting shaft  2   a  (axis C 10 ), and the support shafts  275 ,  276  (axis C 7 ). In addition, the spring force of the torsion coil spring  28  causes the two ends  281  and  282  to draw each other, so that each pin  10  (axis C 6 ) and each support shaft  275 ,  276 , (axis C 7 ) are aligned on the straight line C 8 , pulling the free end portion of one rod  121  and the free end portion of the other rod  121  in a mutually approximating direction. In addition, the biasing force of the torsion coil spring  28  is limitedly effected only in one direction. Thus, even when the drive motors  60  (see  FIG. 1 ) are driven to operate the arm units  101  and thereby move the bracket  3  to a given spatial position, each rod  121  does not rotate about the axis C 5 . 
     &lt;Variants&gt; 
     In any of the above embodiments, the connecting member  2  retains the second joint element  412  (the rectangular tube portion  415 ) through clipping, but it may directly retain the rod  121  through clipping, thereby limiting rotation of the rod  121  about the axis C 5 . Also, for example, in the second embodiment, the effect does not change even if the second plate member  22  is replaced with an anti-fall-off member for the pin  10  (i.e., the second plate member  22  does not connect the second joint elements  412  together, and only the first plate member  21  connects the second joint elements  412  together).