Abstract:
A balancer is provided on robot including a first arm and an arm support for the base end of the first arm to rotate freely via bearing members that have a first axis line extending in the horizontal direction as a central axis; and a gas spring mechanism for causing the first arm to generate torque centered on the first axis line by elastic expansion or elastic contraction in a direction opposite the torque centered on the first axis line that is generated by gravity acting on the first arm between a first angular position and a second angular position in which the slope with respect to the vertical direction is greater than the first angular position.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a balancer device for use in a robot. 
       BACKGROUND ART 
       [0002]    Conventionally, there is known a balancer device which cancels a variation in gravitational torque generated by the rotation of a robot arm (see e.g., Patent Literature 1). 
         [0003]    This balancer device couples a first arm to a base body. A rod end of a balancer is coupled to a fixed shaft protruding laterally of the first arm. The other end of the balancer which is closer to a cylinder is coupled to a fixed shaft provided at the lower portion of the base body. With this configuration, the rotation of the first arm is assisted. 
       CITATION LIST 
     Patent Literature 
       [0004]    Patent Literature 1: Japanese Laid-Open Patent Application Publication No. 2005-319550 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    However, in the balancer device disclosed in Patent Literature 1, the first arm and the rod end of the balancer are connected to each other at a location that is outward relative to a bearing member provided between the first arm and the base body, in the axial direction of the bearing member. In this configuration, a great bending moment is applied to a bearing. For this reason, it is necessary to use the bearing member with a great permissible bending moment. As a result, the weight of a robot arm increases, and it becomes necessary to use a motor which is capable of generating a high output. This brings about an increase in electric power consumption. 
       Solution to Problem 
       [0006]    To achieve the above-described object, according to an aspect of the present invention, there is provided a balancer device which is provided at a robot including a robot arm, and an arm support section to which a base end portion of the robot arm is mounted via a bearing member having a first axis extending horizontally as a center axis in such a manner that the robot arm is rotatable, the balancer device including an elastic structure unit which is elastically extended or elastically contracted to cause the robot arm to generate torque (hereinafter this torque will be referred to as balance torque) around the first axis in a direction opposite to torque around the first axis which is generated by a gravitational force applied to the robot arm, between a first angular position and a second angular position which is greater in inclination with respect to a vertical direction than the first angular position is, wherein a first end portion of the balancer device is coupled to a first fastening section which is independent of the robot arm in such a manner that the balancer device is rotatable around a second axis extending horizontally, wherein a second end portion of the balancer device is coupled to a second fastening section provided at the robot arm in such a manner that the balancer device is rotatable around a third axis extending horizontally, and wherein the balancer device is placed in such a manner that the first end portion and the second end portion are located in a region in which the bearing member is located in a direction in which the first axis extends. 
         [0007]    In accordance with this configuration, a bending moment acting on the bearing member can be reduced, and hence the bearing member with a small permissible bending moment can be used. Therefore, the weight of the robot can be reduced, and the output of the drive section of the robot can be reduced. As a result, the electric power consumption in the robot can be reduced. 
         [0008]    The elastic structure unit may be a gas spring mechanism. 
         [0009]    In accordance with this configuration, the gas spring mechanism with a small size is able to generate a great spring force. 
         [0010]    A spacing formed between the second axis and the third axis in a state in which the robot arm is at the second angular position may be greater than a spacing formed between the second axis and the third axis in a state in which the robot arm is at the first angular position. 
         [0011]    In accordance with this configuration, the balance torque can be generated by extending the balancer device. 
         [0012]    The gas spring mechanism may be extended to expand a working fluid, and the balance torque may be generated by a reactive force generated by expansion of the working fluid. 
         [0013]    In accordance with this configuration, the balance torque can be generated by using the gas spring mechanism which is configured to be extendable. 
         [0014]    The gas spring mechanism may be extended to compress a working fluid, and the balance torque may be generated by a reactive force generated by compression of the working fluid. 
         [0015]    In accordance with this configuration, great balance torque can be generated by using the gas spring mechanism which is configured to be contractible. 
         [0016]    The gas spring mechanism may be contracted to compress a working fluid, the balancer device may comprise a conversion mechanism which contracts the gas spring mechanism, according to a rotation of the robot arm from the first angular position toward the second angular position, and the balance torque may be generated by a reactive force generated by compression of the working fluid. 
         [0017]    In accordance with this configuration, great balance torque can be generated using the gas spring mechanism which is configured to be contractible. In addition, the durability of the gas spring mechanism can be improved. 
         [0018]    The gas spring mechanism may include: a cylinder extending in a direction from a first end portion of the elastic structure unit toward a second end portion of the elastic structure unit, the cylinder having a first end portion formed with an opening and a second end portion closed; a piston which is relatively slidable with respect to an inner wall surface of the cylinder; a working fluid provided in a space formed between the cylinder and the piston; and a piston rod having a first end portion coupled to the piston, the piston rod extending from a spatial position at which the piston rod is coupled to the piston to a location that is outside the cylinder through the opening of the cylinder, the conversion mechanism may include: a first coupling member which couples the cylinder to one of the first fastening section and the second fastening section which is located in a direction from the second end portion of the cylinder toward the first end portion of the cylinder; and a second coupling member which couples a second end portion of the piston rod to the other of the first fastening section and the second fastening section, while preventing interference with the cylinder and the first coupling member. 
         [0019]    In accordance with this configuration, the gas spring mechanism which is configured to be contractible can be suitably mounted to the robot arm. 
         [0020]    The gas spring mechanism may further include seal oil provided in a space formed between the cylinder and the piston, and the gas spring mechanism may be placed in such a manner that the first fastening section is located in the direction from the second end portion of the cylinder toward the first end portion of the cylinder. 
         [0021]    In accordance with this configuration, he durability of the gas spring mechanism can be further improved. 
         [0022]    Third axis may extend through the cylinder. 
         [0023]    In accordance with this configuration, the operation range of the robot arm can be increased. 
       Advantageous Effects of Invention 
       [0024]    The present invention can obtain an advantage that electric power consumption in a robot can be reduced. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0025]      FIG. 1  is a rear view showing the exemplary configuration of a robot including a balancer device according to Embodiment  1  of the present invention, which is partly cut away. 
           [0026]      FIG. 2  is a rear view showing the exemplary configuration of a joint section of the robot of  FIG. 1 , which is partly cut away. 
           [0027]      FIG. 3  is a rear view showing the exemplary configuration of the balancer device of the robot of  FIG. 1 , which is partly cut away. 
           [0028]      FIG. 4  is a side view showing the exemplary configuration of the joint section of the robot of  FIG. 1 , which is partly cut away. 
           [0029]      FIG. 5  is a side view showing the exemplary configuration of the joint section of the robot of  FIG. 1 , which is partly cut away. 
           [0030]      FIG. 6  is a side view showing the exemplary configuration of a robot including a balancer device according to Embodiment  2  of the present invention, which is partly cut away. 
           [0031]      FIG. 7  is a side view showing the exemplary configuration of a robot including a balancer device according to Embodiment  3  of the present invention, which is partly cut away. 
           [0032]      FIG. 8  is a side view showing the exemplary configuration of a robot including a balancer device according to Embodiment  4  of the present invention, which is partly cut away. 
           [0033]      FIG. 9  is a side view showing the exemplary configuration of a robot including a balancer device according to Embodiment  5  of the present invention, which is partly cut away. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0034]    Hereinafter, the embodiments of the present invention will be described with reference to the drawings. The present invention is not limited by the embodiments. Throughout the drawings, the same or corresponding components are designated by the same reference symbols, and will not be described repeatedly. 
       Embodiment 1 
       [0035]      FIG. 1  is a rear view showing the exemplary configuration of a robot  100  including a balancer (balancer device)  1  according to Embodiment 1 of the present invention, which is partly cut away.  FIG. 2  is a rear view showing the exemplary configuration of a first joint section  5  (described later) of the robot  100 , which is partly cut away.  FIG. 3  is a rear view showing the exemplary configuration of the balancer  1  of the robot  100 , which is partly cut away. 
         [0036]    As shown in  FIGS. 1 and 2 , the robot  100  is a multi joint type robot, and includes the balancer  1 , a base body  2 , a rotary section  3 , a first arm (robot arm)  4 , and a first joint section  5 . However, the robot  100  is not limited to the multi-joint type robot. 
         [0037]    [Overall Configuration of Robot] 
         [0038]    As shown in  FIGS. 1 and 2 , the base body  2  includes is fixed on, for example, a placement surface, and supports the upper structure of the base body  2 . 
         [0039]    The rotary section  3  is coupled to the base body  2  in such a manner that the rotary section  3  is rotatable around a rotational axis (not shown) extending vertically. The rotary section  3  includes a rotary plate  31  extending in parallel with the base body  2 , an arm support section  32  provided above the rotary plate  31  and fixed on the rotary plate  31 , and a balancer coupling section  33 . 
         [0040]    The rotary plate  31  is coupled to the base body  2  in such a manner that the rotary plate  31  is rotatable around the rotational axis. The rotary plate  31  is driven to be rotated around the rotational axis by a drive section (not shown). 
         [0041]    The arm support section  32  includes a cylindrical portion  34  having a cylindrical shape extending horizontally. A first end portion of the cylindrical portion  34  opens to form an opening  34   a.  A second end portion of the cylindrical portion  34  is closed to form a bottom wall  34   b.  The center portion of the bottom wall  34   b,  namely, a portion through which the axis of the cylindrical portion  34  extends, is formed with a through-hole  34   c.  The through-hole  34   c  is a hole into which an output shaft  51  a of a first arm drive section  51  which will be described later is inserted. The axis of the cylindrical portion  34  constitutes a first axis L 1 . 
         [0042]    The balancer coupling section  33  protrudes upward from the upper portion of the arm support section  32 . This structure makes it possible to prevent interference between the balancer  1  and the arm support section  32 . The balancer coupling section  33  includes a shaft (first fastening section)  35  extending in parallel with the first axis L 1 . The axis of the shaft  35  constitutes a second axis L 2 . A location at which the shaft  35  is provided in a direction in which the first axis L 1  extends includes a location range R which will be described later. 
         [0043]    The first arm  4  has an arm shape and includes a rotary shaft  41  extending horizontally at a base end portion of the first arm  4 , a body section  42  extending from the rotary shaft  41  toward a tip end portion of the first arm  4 , and a balancer coupling section  43 . 
         [0044]    The rotary shaft  41  has a substantially cylindrical shape with a bottom. The rotary shaft  41  is located inside the cylindrical portion  34  in such a manner that the axis of the rotary shaft  41  conforms to the axis of the cylindrical portion  34  of the arm support section  32 , namely, the first axis L 1 . 
         [0045]    The body section  42  is curved to extend in a direction perpendicular to the first axis L 1  as the body section  42  extends from the rotary shaft  41  toward the tip end portion of the first arm  4 . 
         [0046]    As shown in  FIG. 3 , the balancer coupling section  43  includes a pair of shafts (second fastening section)  44 , and a support section  46 . The pair of shafts  44  are located on a third axis L 3  extending in parallel with the first axis Li and the second axis L 2 . The pair of shafts  44  are arranged in a direction in which the third axis L 3  extends. A first shaft  44  of the pair of shafts  44  protrudes from the side surface of a portion of the body section  42  which is between the base end portion and the tip end portion of the body section  42 . A second shaft  44  of the pair of shafts  44  is supported by the support section  46  and is located to be apart from the first shaft  44 . The base end portion of the support section  46  is attached to the side surface of the body section  42  of the first arm  4 , while the tip end portion of the support section  46  supports the second shaft  44 . The support section  46  extends to surround the outer side of a second coupling member  72  (which will be described later) of the balancer  1 , while preventing interference with the balancer  1 . A region where the pair of shafts  44  are placed in the direction in which the first axis L 1  extends includes the location range R which will be described later. 
         [0047]    As shown in  FIG. 2 , the first joint section  5  includes the first arm drive section  51 , a reduction gear mechanism  52 , and bearing members  53 . 
         [0048]    The first arm drive section  51  is, for example, a servo motor, and includes the output shaft  51  a which outputs a driving force. A casing of the first arm drive section  51  is fixed on the bottom wall  34   b  of the cylindrical portion  34  in such a manner that the axis of the output shaft  51   a  is coaxial with the first axis L 1 . The output shaft  51   a  is inserted into the through-hole  34   c  of the cylindrical portion  34 . The output shaft  51   a  is located inside the rotary shaft  41  through the opening  41   a  of the rotary shaft  41 . 
         [0049]    The reduction gear mechanism  52  is located inside the rotary shaft  41 . The reduction gear mechanism  52  is a mechanism which transmits the driving force of the first arm drive section  51  to the first arm  4 , and is, for example, an epicyclic gear mechanism. Specifically, the reduction gear mechanism  52  includes, for example, a sun gear  52   a  fixed to the tip end portion of the output shaft  51   a,  an epicyclic gear unit  52   b  including a plurality of epicyclic gears which are arranged at equal intervals around the first axis L 1  and are in mesh with the sun gear  52   a,  and an inner gear  52   c  which is provided on the inner peripheral surface of the cylindrical portion  34  of the arm support section  32  and is in mesh with the epicyclic gear unit  52   b.  The reduction gear mechanism  52  is configured by a known reduction gear mechanism of an industrial robot, and therefore, further description will not be given. 
         [0050]    The bearing members  53  are placed between the inner peripheral surface of the cylindrical portion  34  of the rotary section  3  and the outer peripheral surface of the rotary shaft  41  of the first arm  4  in such a manner that the center axes of the bearing members  53  conform to the first axis L 1 . In this configuration, the first arm  4  is rotatable around the first axis L 1  with respect to the arm support section  32  of the rotary section  3 . In other words, the arm support section  32  of the rotary section  3  supports the rotary shaft  41  of the first arm  4  in such a manner that the rotary shaft  41  is rotatable. In the present embodiment, two bearing members  53  are provided, and are arranged in the direction in which the first axis L 1  extends. A region from a location at which a first bearing member  53  of the two bearing members  53  is provided to a location at which a second bearing member  53  of the two bearing members  53  is provided, constitutes the location range R. The number of the bearing members  53  provided in the location range R is not limited to this, and may be one, or three or more. 
         [0051]    As shown in  FIG. 5 , for example, the first arm  4  is rotatable within a range including a range between a first angular position P 1  and a second angular position P 2  which is greater than the first angular position P 1  in inclination with respect to a vertical direction of  FIG. 3 . For example, the first angular position P 1  is an angular position at which the first arm  4  is upright. At the first angular position P 1  at which the first arm  4  is upright, torque (this will be hereinafter referred to as gravitational torque around the first axis L 1 ) around the first axis L 1  which is generated by a gravitational force applied to the first arm  4  is substantially zero. As the first arm  4  rotates from the first angular position P 1  toward the second angular position P 2 , and the inclination of the first arm  4  increases, the gravitational torque around the first axis L 1  increases. 
         [0052]    In the present embodiment, as shown in  FIG. 5 , the shaft  35  of the rotary section  3  and the pair of shafts  44  of the first arm  4  are positioned so that a spacing S formed between the second axis L 2  and the third axis L 3  in the state in which the first arm  4  is at the second angular position P 2  becomes greater than a spacing S fanned between the second axis L 2  and the third axis L 3  in the state in which the first arm  4  is at the first angular position P 1 . In other words, the robot  100  is configured in such a manner that the spacing S between the second axis L 2  and the third axis L 3  gradually increases, as the first arm  4  rotates from the first angular position P 1  toward the second angular position P 2 . 
         [0053]    [Balancer] 
         [0054]    As shown in  FIGS. 1 and 3 , the balancer  1  includes a gas spring mechanism (elastic structure unit)  11 , and a conversion mechanism  12 . The balancer  11  extends in a direction in which a fourth axis L 4  extends, the fourth axis L 4  connecting the second axis L 2  to the third axis L 3  and extending in the direction perpendicular to the second axis L 2  and the third axis L 3 . The balancer  1  is mounted to the robot  100  in such a manner that the fourth axis L 4  is located within the location range R, to be precise, a first end portion  1   a  which is one end portion of the balancer  1  and a second end portion lb which is the other end portion of the balancer  1  are located within the location range R. 
         [0055]    The gas spring mechanism  11  is a mechanism which uses a reactive force of a high-pressure gas filled therein, as a spring force. The gas spring mechanism  11  is smaller in size and is able to generate a greater spring force, compared to a coil spring. For this reason, by use of the gas spring mechanism  11 , the size of the robot  100  can be reduced. Thereby, the output of the drive section of the robot  100  can be reduced, and hence electric power consumption in the robot  100  is not increased. 
         [0056]    The gas spring mechanism  11  includes a cylinder  61 , a piston  62 , a working fluid  63 , and a piston rod  64 . In addition, in the present embodiment, the gas spring mechanism  11  includes seal oil  65 . 
         [0057]    The cylinder  61  is a bottomed cylindrical body extending in the direction in which the fourth axis L 4  extends. The cylinder  61  is formed with an opening at a first end portion  61   a  thereof. A second end portion  61   b  of the cylinder  61  is closed, to form a bottom wall of the cylinder  61 . 
         [0058]    The piston  62  is a cylindrical block. The piston  62  is relatively slidable with respect to the inner wall surface of the cylinder  61 . The piston  62  comparts the inner space of the cylinder  61  into a first space  68  and a second space  69  in the direction in which the fourth axis L 4  extends. The first space  68  is a space formed to be closer to the second end portion  61   b  (closed end portion) of the cylinder  61  than the piston  62  is. The second space  69  is a space formed to be closer to the first end portion  61   a  (end portion formed with the opening) of the cylinder  61  than the piston  62  is. As will be described later in detail, the gas spring mechanism  11  is mounted to the robot  100  in such a manner that the first space  68  is located above the second space  69 . 
         [0059]    The working fluid  63  is a fluid provided in a space formed between the cylinder  61  and the piston  62 , namely, the first space  68 . The working fluid  63  is, for example, high-pressure nitrogen. As described above, since the second end portion  61   b  of the cylinder  61  is closed, the working fluid  63  does not leak from the second end portion  61   b  of the cylinder  61 . This makes it possible to improve the durability of the gas spring mechanism  11 . 
         [0060]    The piston rod  64  is a rod-like member extending in the direction in which the fourth axis L 4  extends. A first end portion  64   a  of the piston  64  is coupled to the piston  62 . The piston rod  64  extends from the piston  62  to a region that is outside the cylinder  61  through the second space  69  and then the opening formed in the first end portion  61  a of the cylinder  61 . A second end portion  64   b  of the piston rod  64  is located outside the cylinder  61 . 
         [0061]    The gas spring mechanism  11  is configured to compress the working fluid  63  and thereby generate a reactive force in such a manner that the gas spring mechanism  11  moves the piston rod  64  in the direction in which the fourth axis L 4  extends, to push the piston rod  64  into the cylinder  61  and thereby contract (reduce the length of) the gas spring mechanism  11 . 
         [0062]    The seal oil  65  is oil which prevents a leakage of the working fluid  63  from the first space  68 , and is filled into a portion of the first space  68  of the cylinder  61 . As described above, the gas spring mechanism  11  is placed in such a manner that the first space  68  is located above the second space  69 . For this reason, the seal oil  65  is reserved on the piston  62 , while the working fluid  63  which is a gas with a specific weight which is smaller than that of the seal oil  65  is present above the seal oil  65 . Therefore, the seal oil  65  is present in a region between the working fluid  63  which is the gas and a portion of the cylinder  61  and a portion of the piston  62 , which contact each other, to prevent a leakage of the working fluid  63  from the first space  68 . As a result, the durability of the gas spring mechanism  11  can be improved. 
         [0063]    The conversion mechanism  12  is a mechanism which contracts (reduces the length of) the gas spring mechanism  11 , as the spacing S formed between the second axis L 2  and the third axis L 3  expands. 
         [0064]    In the present embodiment, the conversion mechanism  12  includes a first coupling member  71  and a second coupling member  72 . 
         [0065]    The first coupling member  71  couples the first end portion  61   a  of the cylinder  61  to the shaft  35  of the rotary section  3 . More specifically, the first coupling member  71  couples to the cylinder  61 , the shaft  35  of the rotary section  3 , which is located in a direction from the second end portion  61   b  (closed end portion) of the cylinder  61  toward the first end portion  61   a  (end portion formed with the opening) of the cylinder  61 . Alternatively, the first coupling member  71  may be mounted to a location different from the first end portion  61   a  of the cylinder  61 . 
         [0066]    The first coupling member  71  includes a pair of plate sections  73  extending in parallel with each other, and a coupling section  74 . First end portions  73   a  of the plate sections  73  are coupled to the first end portion  61   a  of the cylinder  61  and extend in the direction in which the fourth axis L 4  extends. A first plate section  73  of the pair of plate sections  73  is attached to the first end portion  61   a  on a first side in the direction in which the second axis L 2  and the third axis L 3  extend. A second plate section  73  of the pair of plate sections  73  is attached to the first end portion  61   a  on a second side in the direction in which the second axis L 2  and the third axis L 3  extend. The coupling section  74  couples the second end portions  73   b  of the pair of plate sections  73  to each other, and also couples the second end portions  73   b  of the pair of plate sections  73  to the shaft  35 . The coupling section  74  is mounted to the shaft  35  via a bearing member in such a manner that the coupling section  74  is rotatable around the second axis L 2 . A portion of the coupling section  74 , which is mounted to the shaft  35 , constitutes the first end portion  1   a  which is one end portion of the balancer  1 . As described above, the region at which the shaft  35  is provided in the direction in which the first axis L 1  extends, includes the location range R. Therefore, the first end portion  1   a  is located on the location range R. 
         [0067]    The second coupling member  72  couples the second end portion  64   b  of the piston rod  64  to the shafts  44  of the first arm  4 . More specifically, the second coupling member  72  couples to the piston rod  64 , the shafts  44  which are located in the direction from the first end portion  61   a  (end portion formed with the opening) of the cylinder  61  toward the second end portion  61   b  (closed end portion) of the cylinder  61 . Therefore, the gas spring mechanism  11  is mounted to the robot  100  by the conversion mechanism  12  in such a manner that the first space  68  is located above the second space  69 . 
         [0068]    The second coupling member  72  includes a tubular section  75  and a coupling section  76 . 
         [0069]    The tubular section  75  is a tubular body extending in the direction in which the fourth axis L 4  extends. The tubular section  75  is placed to surround the outer periphery of the gas spring mechanism  11  and the outer periphery of the first coupling member  71 . The second coupling member  72  is configured such that a first end portion  75   a  of the tubular section  75  is mounted to the pair of shafts  44  via bearing members in the location range R. In this configuration, the tubular section  75  is coupled to the shafts  44  in such a manner that the tubular section  75  is rotatable around the third axis L 3 . Portions of the tubular section  75 , which are mounted to the shafts  44 , constitute the second end portion lb which is the other end portion of the balancer  1 . As described above, the region in which the pair of shafts  44  are provided in the direction in which the first axis L 1  extends includes the location range R. Therefore, the second end portion lb is located on the position range R. 
         [0070]    The pair of shafts  44  are positioned in such a manner that the third axis L 3  extends through the cylinder  61 . The cylinder  61  protrudes in the direction in which the fourth axis L 4  extends, from a location at which the second coupling member  72  and the first arm  4  are coupled to each other. This makes it possible to increase the stroke of the gas spring mechanism  11 . As a result, the operation range of the first arm  4  can be expanded. 
         [0071]    The coupling section  76  is mounted to the second end portion  64   b  of the piston rod  64  and is located between the pair of plate sections  73  of the first coupling member  71 . The coupling section  76  is provided to connect the edge portions of the second end portion  75   b  of the tubular section  75 . The both end portions of the coupling section  76  are coupled to the edge of the tubular section  75  at a location that is closer to the second end portion  75   b.  The coupling section  76  is located between the pair of plate sections  73  extending in the direction in which the fourth axis L 4  extends. Therefore, when the piston rod  64  is moved in the direction in which the fourth axis L 4  extends, the coupling section  76  does not interfere with the pair of plate sections  73 . 
         [0072]    In the conversion mechanism  12  configured as described above, when the first arm  4  rotates from the first angular position P 1  toward the second angular position P 2 , and thereby the spacing S formed between the second axis L 2  and the third axis L 3  expands, the first end portions  73   a  of the pair of plate sections  73  of the first coupling member  71  and the second end portion  75   b  of the tubular section  75  of the second coupling member  72  become close to each other. Thereby, the first end portion  61   a  of the cylinder  61  which is mounted to the first end portions  73   a  of the pair of plate sections  73  of the first coupling member  71  and the second end portion  64   b  of the piston rod  64  move so as to become close to each other, so that the piston rod  64  and the piston  62  are pushed into the cylinder  61 . Therefore, the working fluid  63  in the space formed between the cylinder  61  and the piston  62  is compressed, and hence the reactive force is generated. 
         [0073]    [Exemplary Operation] 
         [0074]    Next, the exemplary operation of the robot  100  including the balancer  1  will be described. 
         [0075]    As the first arm  4  rotates from the first angular position P 1  toward the second angular position P 2 , and the inclination of the first arm  4  increases, the gravitational torque around the first axis L 1  increases. When the spacing S formed between the second axis L 2  and the third axis L 3  expands, according to the rotation of the first arm  4  from the first angular position P 1  toward the second angular position P 2 , the conversion mechanism  12  contracts (reduces the length of) the gas spring mechanism  11 . Thereby, the working fluid  63  is compressed, and hence the gas spring mechanism  11  generates the reactive force applied in a direction to push back the piston  62 , namely, a direction to extend (increase the length of) the gas spring mechanism  11 , in the direction in which the fourth axis L 4  extends. The conversion mechanism  12  converts the reactive force applied in the direction to extend the gas spring mechanism  11  into a force applied in a direction to reduce the length of the spacing S formed between the second axis L 2  and the third axis L 3 . As a result, torque (balance torque) around the first axis L 1  which is applied in a direction opposite to the direction of the gravitational force around the first axis L 1  is generated. This balance torque cancels a part or all of the gravitational torque. This makes it possible to reduce the load of the first arm drive section  51 , and prevent an increase in the electric power consumption in the robot  100 . 
         [0076]    As described above, the gas spring mechanism  11  is mounted to the robot  100  in such a manner that the fourth axis L 4  is included in the location range R in which the bearing members  53  are located in the direction in which the first axis Li extends, to be precise, the first end portion  1   a  which is one end portion of the balancer  1  and the second end portion lb which is the other end portion of the balancer  1 , are located in the location range R. In this structure, the bending moment acting on the bearing members  53  can be reduced, and hence the bearing members  53  with a small permissible bending moment can be used. Therefore, the weight of the robot  100  can be reduced, and a motor which generates a low output can be used as the drive section of the robot  100 , in particular, the drive section which rotates the rotary section  3 . As a result, the electric power consumption in the robot  100  can be reduced. 
       Embodiment 2 
       [0077]      FIG. 6  is a side view showing the exemplary configuration of a robot  200  including a balancer  201  according to the embodiment of the present invention. 
         [0078]    In the above-described Embodiment 1, the balancer  1  includes the gas spring mechanism  11  which is contracted to compress the working fluid  63  and thereby generate the reaction force applied in the direction to extend (increase the length of) the gas spring mechanism  11 , and the conversion mechanism  12  which converts the reaction force applied in the direction to extend the gas spring mechanism  11  into the force applied in the direction to reduce the length of the spacing S formed between the second axis L 2  and the third axis L 3 . In contrast, in the present embodiment, as shown in  FIG. 6 , the balancer  201  includes a gas spring mechanism  211  which is extended to expand a working fluid  263  and thereby generate a reactive force applied in a direction to contract the gas spring mechanism  211 , and coupling members  271 ,  272  for coupling the gas spring mechanism  211  to the robot  200 . 
         [0079]    The gas spring mechanism  211  is mounted to the robot  200  in such a manner that a first space  268  in which the working fluid  263  is provided, is located below a second space  269 . The working fluid  263  is, for example, a normal-pressure gas. A piston rod  264  is moved in the direction in which the fourth axis L 4  extends, and drawn out of a cylinder  261  to extend the gas spring mechanism  11 . Thereby, the working fluid  263  is expanded, and hence the force applied in the direction to contract the gas spring mechanism  11  is generated. 
         [0080]    The coupling member  271  couples a second end portion  261   b  of the cylinder  261  to the shaft  35  of the rotary section  3 . The coupling member  271  is coupled to the shaft  35  via a bearing member in such a manner that the coupling member  271  is rotatable around the second axis L 2 . 
         [0081]    The coupling member  272  couples a second end portion  264   b  of the piston rod  264  to the shafts  44  of the first arm  4 . The coupling member  272  is coupled to the shafts  44  via bearing members in such a manner that the coupling member  272  is rotatable around the third axis L 3 . In Embodiment 1, the shafts  44  are positioned so that the third axis L 3  extends through the cylinder  61 . In contrast, in the present embodiment, the shafts  44  are positioned so that the third axis L 3  extends through the tip end portion of the first arm  4 . 
         [0082]    Except the above, the configuration of Embodiment 2 is the same as that of Embodiment 1. 
         [0083]    [Exemplary Operation] 
         [0084]    Next, the exemplary operation of the robot  200  including the balancer  201  will be described. 
         [0085]    As the first arm  4  rotates from the first angular position P 1  toward the second angular position P 2 , and the inclination of the first awl  4  increases, the gravitational torque around the first axis L 1  increases. When the spacing S formed between the second axis L 2  and the third axis L 3  expands, according to the rotation of the first arm  4  from the first angular position P 1  toward the second angular position P 2 , the gas spring mechanism  211  is extended. Thereby, the working fluid  263  is expanded, and the gas spring mechanism  211  generates the reactive force applied in a direction to draw back the piston  62 , namely, a direction to contract the gas spring mechanism  211 . The reactive force applied in the direction to contract the gas spring mechanism  211  works as the force applied in the direction to reduce the length of the spacing S formed between the second axis L 2  and the third axis L 3 . As a result, torque (balance torque) around the first axis L 1  which is applied in a direction opposite to the direction of the gravitational force around the first axis L 1  is generated. This balance torque cancels a part or all of the gravitational torque. 
       Embodiment 3 
       [0086]      FIG. 7  is a side view showing the exemplary configuration of a robot  300  including a balancer  301  according to Embodiment 3 of the present invention. 
         [0087]    In the above-described Embodiment 1, the balancer  1  includes the gas spring mechanism  11  which is contracted to compress the working fluid  63  and thereby generate the reaction force applied in the direction to extend the gas spring mechanism  11 , and the conversion mechanism  12  which converts the reaction force applied in the direction to extend the gas spring mechanism  11  into the force applied in the direction to reduce the length of the spacing S formed between the second axis L 2  and the third axis L 3 . In contrast, in the present embodiment, as shown in  FIG. 7 , the balancer  301  includes a gas spring mechanism  311  which is extended to compress the working fluid  63  and thereby generate a reactive force applied in a direction to contract the gas spring mechanism  311 , and coupling members  371 ,  372  for coupling the gas spring mechanism  311  to the robot  300 . 
         [0088]    A cylinder  361  of the gas spring mechanism  311  has both end portions closed. A first end portion  361   a  of the cylinder  361  is formed with an insertion hole  361   c  into which a piston rod  364  is inserted. A second end portion  361   b  of the cylinder  361  is formed with a through-hole  361   d.  In this structure, an air pressure in a second space  369  of the cylinder  361  is equal to an atmospheric pressure. The piston rod  364  extends from the piston  62  to a location that is outside the cylinder  361  through a first space  368  and the insertion hole  361   c  of the cylinder  361 . A second end portion  364   b  of the piston rod  364  is located outside the cylinder  361 . The piston rod  364  is moved in the direction in which the fourth axis L 4  extends, and drawn out of the cylinder  361  to extend the gas spring mechanism  311 . Thereby, the working fluid  63  is compressed and hence the force applied in the direction to contract the gas spring mechanism  311  is generated. 
         [0089]    The coupling member  371  couples the second end portion  361   b  of the cylinder  361  to the shaft  35  of the rotary section  3 . The coupling member  371  is coupled to the shaft  35  via a bearing member in such a manner that the coupling member  371  is rotatable around the second axis L 2 . 
         [0090]    The coupling member  372  also couples the second end portion  364   b  of the piston rod  364  to the shafts  44  of the first arm  4 . The coupling member  372  is coupled to the shafts  44  via bearing members in such a manner that the coupling member  372  is rotatable around the third axis L 3 . 
         [0091]    The gas spring mechanism  311  is mounted to the robot  300  in such a manner that the first space  368  in which the working fluid  63  is provided, is located above the second space  369 . 
         [0092]    In Embodiment  1 , the shafts  44  are positioned so that the third axis L 3  extends through the cylinder  61 . In contrast, in the present embodiment, the shafts  44  are positioned so that the third axis L 3  extends through the tip end portion of the first arm  4 . 
         [0093]    Except the above, the configuration of Embodiment 3 is the same as that of Embodiment 1. 
         [0094]    [Exemplary Operation] 
         [0095]    Next, the exemplary operation of the robot  300  including the balancer  301  will be described. 
         [0096]    As the first arm  4  rotates from the first angular position P 1  toward the second angular position P 2 , and the inclination of the first arm  4  increases, the gravitational torque around the first axis L 1  increases. When the spacing S formed between the second axis L 2  and the third axis L 3  expands, according to the rotation of the first arm  4  from the first angular position P 1  toward the second angular position P 2 , the gas spring mechanism  311  is extended. Thereby, the working fluid  63  is compressed, and the gas spring mechanism  311  generates a reactive force applied in a direction to push back the piston  62 , namely, a direction to contract (reduce the length of) the gas spring mechanism  311 , in the direction in which the fourth axis L 4  extends. The reactive force applied in the direction to contract the gas spring mechanism  311  works as a force applied in a direction to reduce the length of the spacing formed between the second axis L 2  and the third axis L 3 . As a result, torque (balance torque) around the first axis L 1  which is applied in a direction opposite to the direction of the gravitational force around the first axis L 1  is generated. This balance torque cancels a part or all of the gravitational torque. 
       Embodiment 4 
       [0097]      FIG. 8  is a side view showing the exemplary configuration of a robot  400  including a balancer  401  according to Embodiment 4 of the present invention. 
         [0098]    In the above-described Embodiment 1, the first coupling member  71  couples the first end portion  61   a  of the cylinder  61  to the shaft  35  of the rotary section  3 , while the second coupling member  72  couples the second end portion  64   b  of the piston rod  64  to the shafts  44  of the first arm  4 . In contrast, as shown in  FIG. 8 , in the present embodiment, the gas spring mechanism  11  is mounted to the robot  100  in such a manner that the first space  68  is located below the second space  69 . A first coupling member  471  couples to the cylinder  61 , the shafts  44  of the first arm  4 , which are located in a direction from the second end portion  61   b  (closed end portion) of the cylinder  61  toward the first end portion  61   a  (end portion formed with the opening) of the cylinder  61 . A second coupling member  472  couples to the piston rod  64  the shaft  35  located in a direction from the first end portion  61   a  (end portion formed with the opening) of the cylinder  61  toward the second end portion  61   b  (closed end portion) of the cylinder  61 . 
         [0099]    In Embodiment 1, the shafts  44  are positioned so that the third axis L 3  extends through the cylinder  61 , while in the present embodiment, the shafts  44  are positioned so that the third axis L 3  extends through the tip end portion of the first arm  4 . Except the above, the configuration of Embodiment 4 is the same as that of Embodiment 1. 
       Embodiment 5 
       [0100]      FIG. 9  is a side view showing the exemplary configuration of a robot  500  including a balancer  501  according to Embodiment 5 of the present invention. 
         [0101]    In the above-described Embodiment 1, the shafts  44  are positioned so that the third axis L 3  extends through the cylinder  61 , while in the present embodiment, the shafts  44  are positioned so that the third axis L 3  extends through the tip end portion of the first arm  4 . Except the above, the configuration of Embodiment 5 is the same as that of Embodiment 1. 
         [0102]    &lt;Modified Example&gt; 
         [0103]    Although in the above-described embodiments, the gas spring mechanism is used, the present invention is not limited to this. Instead of this, a coil spring may be used. 
         [0104]    Numerous improvements and alternative embodiments of the present invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, the description is to be construed as illustrative only, and is provided for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details of the structure and/or function may be varied substantially without departing from the spirit of the invention. 
       INDUSTRIAL APPLICABILITY 
       [0105]    The present invention is applicable to industrial robots. 
       Reference Signs List 
       [0106]    L 1  first axis 
         [0107]    L 2  second axis 
         [0108]    L 3  third axis 
         [0109]    L 4  fourth axis 
         [0110]    P 1  first angular position 
         [0111]    P 2  second angular position 
         [0112]      1  balancer 
         [0113]      1   a  first end portion 
         [0114]      1   b  second end portion 
         [0115]      2  base body 
         [0116]      3  rotary section 
         [0117]      4  first arm 
         [0118]      5  first joint section 
         [0119]      11  gas spring mechanism 
         [0120]      12  conversion mechanism 
         [0121]      31  rotary plate 
         [0122]      32  arm support section 
         [0123]      33  balancer coupling section 
         [0124]      34  cylindrical portion 
         [0125]      35  shaft 
         [0126]      41  rotary shaft 
         [0127]      42  body section 
         [0128]      43  balancer coupling section 
         [0129]      44  shaft 
         [0130]      46  support section 
         [0131]      51  first arm drive section 
         [0132]      52  reduction gear mechanism 
         [0133]      53  bearing member 
         [0134]      61  cylinder 
         [0135]      61   a  first end portion 
         [0136]      61   b  second end portion 
         [0137]      62  piston 
         [0138]      63  working fluid 
         [0139]      64  piston rod 
         [0140]      65  seal oil 
         [0141]      68  first space 
         [0142]      69  second space 
         [0143]      71  first coupling member 
         [0144]      72  second coupling member 
         [0145]      73  plate section 
         [0146]      74  coupling section 
         [0147]      75  tubular section 
         [0148]      76  coupling section 
         [0149]      100  robot