Patent Publication Number: US-2021170587-A1

Title: Robot and Grasping System

Description:
FIELD OF THE INVENTION 
     The invention relates to a robot and a grasping system. 
     BACKGROUND OF THE INVENTION 
     In recent years, the need for removal of space debris has been increasing more and more. Space debris with particularly high removal priority are the upper stage units of launch vehicles, and the upper stage units of launch vehicles have various shapes and dimensions of diameters according to the standards of respective countries although rough shapes of the upper stage units which are cylindrical are equal. In order to remove the rockets that have become space debris, it is necessary to capture the rockets. A PAR (Payload Adapter Ring) that connects a rocket and a satellite is the most predominant as a grasping connection unit for rocket capture for the reason that the PAR is accessible and always present on each rocket. Further, a nozzle of a rocket is also predominant as the grasping connection unit for rocket capture. 
     Meanwhile, the launched satellite side also has a PAR having the same diameter as the PAR of the rocket. Even in maintenance service or the like of a broken satellite, grasping connection of a PAR is needed. 
     In this respect, as the technique for grasping a PAR, there is an MDA method as shown in Non Patent Literature 1 described below. This is a method in which a hand  901  pinches a cylinder end portion  902   a  of a PAR  902  (refer to  FIG. 13 ). 
     Further, as the technique for grasping a nozzle, there is a technique of grasping a thruster for satellite attitude control with three claws from outside as shown in Non Patent Literature 2 described below. 
     CITATION LIST 
     [Non Patent Literature] 
     
         
         Non Patent Literature 1: John Ratti et al, “Spacecraft Robotic Capture Tool”, i-SAIRAS 2014, 2014 
         Non Patent Literature 2: Haidong Hu, et al., “A Vision System for Autonomous Satellite Grapple with Attitude Thruster”, ICINCO 2014, 2014, pp. 333-337 
       
    
     SUMMARY OF INVENTION 
     Problem to be Solved by the Invention 
     However, in the MDA method as shown in Non Patent Literature 1, as illustrated in  FIG. 13 , in the case where a positional error or an attitude error occurs, there is a high probability of causing a grasping failure, and flicking off a space debris. 
     Further, in the technique as shown in Non Patent Literature 2, the grasping mechanism needs to have a width of a size of a grasping target part or more. The grasping mechanism becomes huge for a large grasping target part like a nozzle of a rocket instead of a small grasping target part like a thruster which is handled by Non Patent Literature 2. 
     Thus, the present invention has an object to provide a small robot and a grasping system that are excellent in robustness with respect to a position error and an attitude error of the robot with versatility even for grasping targets having different shapes and dimensions of diameters. 
     Means for Solving Problem 
     One aspect of the present invention provides a robot including an extendable device capable of extending and contracting in at least one direction, at least two end effectors that are respectively connected to at least two end portions of the extendable device, and a control unit capable of causing the extendable device to extend and contract, wherein each of the at least two end effectors includes at least one grasping unit that extends outward, and the robot is operable to grasp a target by the at least two end effectors by the control unit causing the extendable device to extend or contract. 
     The end effectors may be rotatably connected to at least two end portions of the extendable device to bend to a direction in which the extendable device extends and contracts, and the robot is operable to grasp the target by the at least two end effectors by the control unit causing to extend the extendable device and rotate the end effectors, or to extend the extendable device, rotate the end effectors, and contract the extendable device. 
     The extendable device is capable of extending and contracting in at least two directions, and may include a first to an n th  (n is an integer of two or more) extendable arms that are respectively capable of extending and contracting in a first to an n th  directions different from each other. 
     The extendable device may further include a first to an n th  base units, each of the first to n th  base units may include a first drive unit, and the first to the n th  extendable arms may respectively have base end portions connected to the first to the n th  base units, and extend and contract along the first to n th  directions by power from the first drive unit. 
     The first to n th  extendable arms may respectively include a plurality of movable bodies that advance and retract along an axial direction with them interlocked with each other. 
     The end effector is rotatable with respect to the extendable arms by power from a second drive unit provided at the base units. 
     At least one of the plurality of movable bodies may include a frame, a base end side pulley that is rotatably provided at the frame, and rotates by the frame moving, a tip end side pulley that is rotatably provided at the frame, and a first transmission member that is provided in a tense state between the base end side pulley and the tip end side pulley, and transmits rotation of the base end side pulley to the tip end side pulley, wherein the first transmission member may be fixed to a movable body adjacent to a tip end side. 
     Each of the base units may include a base frame, the first drive unit may include a first motor, a first motor pulley that is directly connected to a rotation shaft of the first motor, and rotatably provided at the base frame of the base unit, a first driven pulley that is rotatably provided at the base frame, and a second transmission member that is provided in a tension state between the first motor pulley and the first driven pulley, and transmits rotation of the first motor to the first driven pulley, the second transmission member being fixed to the movable body adjacent to a tip end side. 
     The first transmission member may include a third wire in which one end of the third wire is wound on the base end side pulley, and the other end of the third wire is wound on the tip end side pulley respectively from a first side, and a fourth wire in which one end of the fourth wire is wound on the base end side pulley, and the other end of the fourth wire is wound on the tip end side pulley respectively from a second side. 
     The end effector may be connected to the second drive unit by a second transmission member through pulleys that are provided on at least one of the base units and the plurality of movable bodies, and the second transmission member may be laid on an opposite side to the end effector, of at least one of the pulleys provided on at least one of the base units and the plurality of movable bodies, and a length of the second transmission member may be configured to be constant regardless of an extension or contraction state of the first to the n th  extendable arms. 
     Another aspect of the present invention provides a grasping system including the robot, and a grasping control device that positions the robot to a grasping target part included by a grasping target, and instructs extension or contraction of the extendable device to the control unit so as to grasp the grasping target part. 
     The grasping control device may instruct extension of the extendable device and rotation of the end effector to the control unit. 
     The grasping control device may instruct extension of the extendable device, rotation of the end effector, and contraction of the extendable device to the control unit. 
     The grasping control device may determine whether or not the grasping system succeeds in grasping the grasping target, and when it is determined that the grasping system does not succeed in grasping of the grasping target, the grasping control device may contract or extend the extendable device in accordance with a shape of the grasping target part. 
     A shape of the grasping target part may be a tapered cylindrical shape or an inversely tapered cylindrical shape. 
     The grasping target may be a space debris, and the grasping target part may be a PAR or a nozzle. 
     Effect of the Invention 
     According to the present invention having the above described configuration, the compact robot and grasping system are provided which are excellent in robustness with respect to a position error and an attitude error of the robot with versatility even for grasping targets having different shapes and dimensions of diameters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall configuration diagram of a grasping system according to an embodiment of the present invention. 
         FIG. 2  is a perspective view (contraction state) of a robot according to the embodiment of the present invention. 
         FIG. 3  is a partial perspective view (extension state) seen from a tip end side of the robot according to the embodiment of the present invention. 
         FIG. 4  is a partial perspective view (extension state) seen from a base end side of the robot according to the embodiment of the present invention. 
         FIG. 5  is a front view (extension state) of the robot according to the embodiment of the present invention. 
         FIG. 6  is a partial front view (extension state) of the robot according to the embodiment of the present invention. 
         FIG. 7  is a sectional view taken along line VII-VI (extension state) in  FIG. 6 . 
         FIG. 8  is a view illustrating a principle of a rotation transmission mechanism by wire. 
         FIG. 9  is a diagram illustrating a principle that a length of a fifth wire  35  becomes a constant length regardless of an extension/contraction state of a first extendable arm  40 . 
         FIG. 10  is a diagram illustrating an operation of a removal satellite disposing a robot on a PAR of a space debris. 
         FIG. 11  is a diagram illustrating a grasping operation to the PAR. 
         FIG. 12  is a diagram illustrating a grasping operation to a nozzle. 
         FIG. 13  is a diagram illustrating grasping according to a conventional MDA method. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. 
       FIG. 1  is an overall configuration diagram of a grasping system according to the embodiment of the present invention.  FIG. 2  is a perspective view (contraction state) of a robot according to the embodiment of the present invention.  FIG. 3  is a partial perspective view (extension state) seen from a tip end side of the robot according to the embodiment of the present invention.  FIG. 4  is a partial perspective view (extension state) seen from a base end side of the robot according to the embodiment of the present invention.  FIG. 5  is a front view (extension state) of the robot according to the embodiment of the present invention.  FIG. 6  is a partial front view (extension state) of the robot according to the embodiment of the present invention.  FIG. 7  is a sectional view taken along line VII-VII in  FIG. 6  (extension state). 
     A grasping system  1000  includes a robot  1  and a removal satellite  2  that is a grasping control device. The removal satellite  2  instructs extension and contraction of an extendable device  100 , and rotation of an end effector  90  to a control unit  95  so as to grasp a PAR  401  or a nozzle  402  of a space debris  400 , which is a grasping target part, and determines a success and a failure of grasping, as will be described later. The robot  1  is connected to a tip end of an extension spring  3  which is contained in the removal satellite  2 . 
     The robot  1  includes the extendable device  100 , the end effector  90 , and the control unit  95 . 
     The extendable device  100  includes a base unit  10 , a first extendable arm  40 , and a second extendable arm  80 . 
     The base unit  10  includes a base frame  11 , a first drive unit  20 , and a second drive unit  30 . In the present embodiment, a base unit for the first extendable arm  40  and a base unit for the second extendable arm  80  are formed as the common base unit  10 . 
     The base frame  11  is formed in a rectangular plate shape. The first drive unit  20  and the second drive unit  30  are mounted to the base frame  11 . 
     The first drive unit  20  includes a first motor (not illustrated), a first drive gear  21 , a first driven gear  22 , a first motor pulley  23 , a first drive pulley  24 , a first driven pulley  25 , a first wire  26 , and a second wire  27 . 
     To a rotation shaft of the first motor, the first drive gear  21  and the first motor pulley  23  are directly connected. The first wire  26  and the second wire  27  which are a second transmission member are provided in a tense state on the first motor pulley  23  and the first driven pulley  25 , and a rotational force of the first motor is transmitted to the first driven pulley  25  through the first motor pulley  23 , the first wire  26  and the second wire  27  to rotate the first driven pulley  25 . Hereinafter, this point will be described in detail. 
       FIG. 8  is a view illustrating a principle of a rotation transmission mechanism by wire. Note that in an upper view of  FIG. 8 , the second wire  27  is described as separating from the first motor pulley  23  and the first driven pulley  25  to facilitate understanding, but the second wire  27  is actually wound around the first motor pulley  23  and the first driven pulley as in a lower drawing in  FIG. 8  and as described later. When one side with respect to a line that connects a center of rotation of the first motor pulley  23  and a center of rotation of the first driven pulley  25  is referred to as a first side, and the other side with respect to the line that connects the center of rotation of the first motor pulley  23  and the center of rotation of the first driven pulley  25  is referred to as a second side, one end of the first wire  26  is wound on the first motor pulley  23 , whereas the other end is wound on the first driven pulley  25  from an upper side that is the first side, and the one and other ends are respectively fixed by fixing members. One end of the second wire  27  is wound on the first motor pulley  23 , whereas the other end is wound on the first driven pulley  25 , from a lower side that is the second side, and the one and other ends are respectively fixed by fixing members. 
     When the first motor pulley  23  is rotated clockwise in such configuration, a portion of the first wire  26  which is between the pulleys is moved in a −z direction, and thereby the first driven pulley  25  rotates clockwise. On the other hand, when the first motor pulley  23  is rotated counterclockwise, a portion of the second wire  27  which is between the pulleys is moved in the −z direction, and thereby the first driven pulley  25  rotates counterclockwise. Note that in a state where the first extendable arm  40  is completely contracted, the first wire  26  of a substantially same length as a length between the pulleys is wound on the first motor pulley  23 , and the second wire  27  of a substantially same length as the length between the pulleys is wound on the first driven pulley  25 . On the other hand, in a state where the first extendable arm  40  is completely extended, the second wire  27  of the substantially same length as the length between the pulleys is wound on the first motor pulley  23 , and the first wire  26  of the substantially same length as the length between the pulleys is wound on the first driven pulley  25 . In order to prevent an error of the length of the wires which are wound, grooves for passing the wire are provided in the respective pulleys to be adjacent to one another, so that the wires are not laid on each other. 
     Consequently, according to such configuration, the first driven pulley can be rotated in both directions in response to a rotation direction of the first motor pulley  23 . Note that when the first driven pulley  25  is a drive pulley, and the first motor pulley  23  is a driven pulley, rotation in both the directions is enabled based on the same principle. 
     According to such configuration, the transmission mechanism can be realized without using a belt that is difficult to use in a cosmic space exposed to radiation or the like, but the transmission member is not limited to this, and any suitable transmission member such as a belt may be used. 
     A long first guide projection  12  that extends in a longitudinal direction (+z direction) of the base frame  11  is mounted to the base frame  11 . 
     Further, to the base frame  11 , pulleys  13  and  14  for winding a fifth wire  35  for rotation control of an end effector  90  described later are rotatably mounted. 
     The first extendable arm  40  includes a first movable body  50  and a second movable body  70  that are interlocked with each other and advance and retract along the first direction (+z direction). 
     The first movable body  50  includes a first frame  51 , a first guide groove portion  52 , a second guide projection  53 , a base end side pulley  54 , a base end side pulley support unit  55 , a tip end side pulley  56 , a tip end side pulley support unit  57 , a third wire  58 , and a fourth wire  59 . 
     The first frame  51  is formed in a rectangular plate shape, and the long first guide groove portion  52  that extends in the first direction (+z direction) is mounted to an upper side of a surface that faces a base unit, of the first frame  51 . The first guide groove portion  52  receives the first guide projection  12 , and holds the first guide projection  12 . By engaging the base unit  10  and the first movable body  50  with each other via the guide mechanism (the guide groove portion, the guide projection), the first movable body  50  can be moved in the first direction (+z direction) with respect to the base unit  10 . 
     Further, at a lower side of a surface at opposite side to the surface where the first guide groove portion  52  is mounted, of the first frame  51 , the long second guide projection  53  that extends in the first direction (+z direction) is mounted. 
     The base end side pulley support unit  55  and the tip end side pulley support unit.  57  are mounted at a predetermined space in a longitudinal direction (first direction) in the first frame  51 . By the base end side pulley support unit  55  and the tip end side pulley support unit  57 , the base end side pulley  54  and the tip end side pulley  56  are respectively rotatably supported. In other words, the base end side pulley  54  and the tip end side pulley  56  are mounted at the predetermined space in the longitudinal direction (first direction) in the first frame  51 . As in the first drive unit  20 , the third wire  58  and the fourth wire  59  which are the first transmission member are provided in a tense state on the base end side pulley  54  and the tip end side pulley  56 . In other words, one end of the third wire  58  is wound on and fixed to the base end side pulley  54 , whereas the other end is wound on and fixed to the tip end side pulley  56  from the upper side (first side), and one end of the fourth wire  59  is wound on and fixed to the base end side pulley, whereas the other end is wound on and fixed to the tip end side pulley  56  from the lower side (second side). By such configuration, the base end side pulley  54  and the tip end side pulley  56  can be rotated together in both clockwise and counterclockwise directions. 
     Further, to a rotation shaft of the tip end side pulley  56 , pulleys  61  and  62  for winding wires for rotation control of the end effector  90  described later are rotatably mounted. 
     The second movable body  70  includes a second frame  71 , a second guide groove portion  72 , an end effector support unit  73 , and pulleys  74  and  75 . 
     The second frame  71  is formed in a rectangular plate shape, and the long second guide groove portion  72  that extends in the first direction (+z direction) is mounted to a surface facing the first frame  51 , of the second frame  71 . The second guide groove portion  72  receives the second guide projection  53 , and holds the second guide projection  53 . The first movable body  50  and the second movable body  70  are engaged with each other via the guide mechanism (the guide groove portion, the guide projection unit), and thereby the second movable body  70  can be moved in the first direction (+z direction) with respect to the first movable body  50 . 
     The bracket-shaped end effector support unit  73  that rotatably supports the end effector  90  is provided at a tip end portion of the second frame  71 . 
     Further, the pulleys  74  and  75  for winding a fifth wire for rotation control of the end effector  90  described later are rotatably mounted to the second frame  71 . 
     Between the first motor pulley  23  and the first driven pulley  25 , the first wire  26  and the first frame  51  are fixed via a second fixing member  63 . Further, between the base end side pulley  54  and the tip end side pulley  56 , the third wire  58  and the base frame  11  are fixed via an L-shaped first fixing member  15 . Further, between the base end side pulley  54  and the tip end side pulley  56 , the fourth wire  59  and the second frame  71  are fixed via a third fixing member  76 . 
     The end effector  90  includes grasping units  90   b  and  90   c  that extend in V-shapes outward in planes perpendicular to a longitudinal direction of a base unit  90   a  in both ends of the long base unit  90   a , and a cylindrical shaft support unit  90   d  that is mounted to a center of the base unit. At both ends in the longitudinal direction of the shaft support unit  90   d , protrusions  90   e  are formed, the protrusions  90   e  are rotatably fitted in bores  73   a  provided in the end effector support unit  73 , and the end effector  90  is supported so that a longitudinal direction of the shaft support unit  90   d  is in a direction parallel with a principal plane of the second frame  71  and perpendicular to a direction in which the first extendable arm  40  extends and contracts. In other words, the end effector  90  is rotatably connected to a tip end portion of the first extendable arm with a direction parallel with the principal plane of the second frame  71  and perpendicular to the direction in which the first extendable arm  40  extends and contracts as an axis. A groove  90   f  for laying the fifth wire is provided in a circumferential direction, in a center in the longitudinal direction of the shaft support unit  90   d . The configuration of connection of the end effector to the extendable arm is not limited to the above described configuration, but may be any suitable configuration as long as the configuration is such that the end effector is rotatably connected to the end portion of the extendable arm in such a manner as to be folded to a direction in which the extendable arm extends and contracts. 
     Here, since the grasping units  90   b  and  90   c  are disposed with a predetermined space left, the grasping units  90   b  and  90   c  can grasp a grasping target stably at a time of grasping the grasping target. A shape of the grasping unit may be any suitable shape such as a U-shape, and a y shape as long as the shape of the grasping unit extends outward. 
     The second drive unit  30  includes a second motor (not illustrated), a second drive gear  31 , a second driven gear  32 , a second motor pulley  33 , and a second drive pulley  34 . 
     To a rotation shaft of the second motor, the second drive gear  31 , and the second motor pulley  33  are directly connected. 
     In the second motor pulley  33 , two grooves  33   a  and  33   b  are formed in a circumferential direction. One end of a fifth wire  35  is fixed to the second motor pulley  33  by a fixing member  33   c , and is led out from an end effector  90  side, the fifth wire  35  is laid on the groove  33   a  of the second motor pulley  33 , the pulley  13  which is mounted to the base unit  10 , the pulley  13  which is mounted to the rotation shaft of the tip end side pulley  56 , the pulley  74  which is mounted to the second frame  71 , the groove  90   f  of the shaft support unit  90   d  of the end effector  90 , the pulley  75  which is mounted to the second frame  71 , the pulley  14  which is mounted to the rotation shaft of the tip end side pulley  56 , the pulley  14  which is mounted to the base unit  10 , and the groove  33   b  of the second motor pulley  33  in this order, and the other end of the fifth wire  35  is fixed to the second motor pulley  33  by a fixing member  33   d  from a second drive pulley  34  side. According to such configuration, the control unit  95  may control a rotation angle of the end effector  90  by controlling a rotation angle of the second motor. 
     In such configuration, a length of the fifth wire  35  is constant regardless of an extension/contraction state of the first extendable arm  40 . Hereinafter, this point will be described in detail. 
       FIG. 9  is a diagram illustrating a principle that the length of the fifth wire  35  is constant regardless of the extension/contraction state of the first extendable arm  40 . When L is the length of the fifth wire  35 , L 1  is a length from the fixing member  33   c  to a lower end of the pulley  13 , L 2  is a length half a circumference of the pulley  61 , L 3  is a length half a circumference of the pulley  74 , L 4  is a length from a lower end of the pulley  14  to the fixing member  33   d , z a  is a z coordinate of a center of the pulley  13 , z 1  is a z coordinate of a center of the pulley  61 , z 10  is a z coordinate of the center of the pulley  61  at a time of the first extendable arm  40  being in a contraction state (initial state), z 2  is a z coordinate of a center of the pulley  74 , z 20  is a z coordinate of the center of the pulley  74  at a time of the first extendable arm  40  being in the contraction state, z 3  is a z coordinate of the shaft support unit  90   d  of the end effector  90 , and z 30  is a z coordinate of the shaft support unit  90   d  of the end effector  90 , L=L 1 +2{(z 1 −z a )+L 2 +(z 1 −z 2 )+L 3 +(z a −z 2 )}+L 4  is established. 
     Further, since a distance between the shaft support unit  90   d  of the end effector  90  and the pulley  74  is constant, z 3 −z 2 =z 30 −z 20  is established. 
     As described later, a moving velocity of the pulley  74  is twice as high as a moving velocity v of the pulley  61 , so that a moving distance of the pulley  74  is twice as large as a moving distance of the pulley  61 , and z 2 −z 20 =2(z 1 −z 10 ) is established. 
     Therefore, from the above equations, L=2{(z 10 −z a )+(z 10 −z 20 )+(z 3 −z 20 )}+L 1 +L 2 +L 3 +L 4  is established, and it is found that the length of the fifth wire  35  is constant regardless of the extension/contraction state of the first extendable arm  40 . 
     Accordingly, drive units of the respective end effectors may be disposed at the base end side of the respective extendable arms, that is, a center portion of the robot  1 , instead of the tip end sides of the respective extendable arms. Consequently, an inertial moment of the entire robot  1  can be decreased. Further, since the drive units of the respective end effectors can be disposed at the base end side of the respective extendable arms, the respective end effectors can be driven by the one common drive unit, and the number of actuators can be decreased. 
     Here, the configuration in which the length of the fifth wire is made constant regardless of the extension or contraction state of the extendable arm is not limited to the above described configuration, but may be any suitable configuration. That is, the end effector may be connected to the second drive unit by the fifth wire through the pulleys that are provided on at least one of the base unit and a plurality of movable bodies, the fifth wire may be laid on the opposite side to the end effector, of at least one of the pulleys provided on at least one of the base unit and the plurality of movable bodies (in the above described configuration, the fifth wire  35  is laid on the right side of the pulley  74 ), and thereby, a portion of the wire (portion of the wire between the pulley  61  and the pulley  74  in the above described configuration) that can be fed out when the extendable arm extends may be prepared. Accordingly, such configuration can be implemented by moving the pulley with the fifth wire laid on the opposite side to the end effector so that the length of the fifth wire is constant regardless of the extension/contraction state of the extendable arm, and configuring other elements in accordance with necessity. 
     The second extendable arm  80  has a similar configuration to the first extendable arm  40 . Rotation of the first motor is transmitted to the first drive pulley  24  as rotation in an opposite direction through the first drive gear  21  and the first driven gear  22 . Further, rotation of the second motor is transmitted to the second drive pulley  34  as rotation in an opposite direction through the second drive gear  31  and the second driven gear  32 . 
     The control unit  95  receives an instruction from inside or outside of the robot, and controls the rotations of the first motor and the second motor. 
     (Operation) 
     Based on the above system configuration, operations of the robot and the grasping system according to one embodiment of the present invention will be described.  FIG. 10  is a diagram illustrating an operation of a removal satellite disposing the robot on a PAR of a space debris.  FIG. 11  is a diagram illustrating a grasping operation to the PAR.  FIG. 12  is a diagram illustrating a grasping operation to a nozzle. 
     The removal satellite  2  including the robot  1  approaches the space debris  400  while carrying out relative position measurement to the space debris  400 . Finally, the removal satellite  2  moves to a region corresponding to an extension length of the extension spring  3  so that the robot  1  faces the PAR  401  or the nozzle  402  of the space debris  400 , and extends the extension spring  3  having the robot  1  connected to a tip end ( FIG. 10 ). Such method for disposing the removal device included by the removal satellite for the purpose of removing the space debris in a vicinity of the space debris is known as disclosed in Japanese Patent Laid-Open No. 2014-226974, for example, and therefore detailed explanation is omitted. 
     Subsequently, the removal satellite  2  starts a grasping operation of the robot  1 . First, a case of performing a grasping operation to the PAR  401  having a tapered cylindrical shape will described. 
     The removal satellite  2  changes the position and attitude by a propulsive device included by the removal satellite  2 , and thereby the robot  1  which is connected to the removal satellite  2  through the extension spring  3  is positioned to an opening surface  401   a  of the PAR  401  which is a grasping target part. 
     Subsequently, based on an instruction to the control unit  95  from the removal satellite  2 , the first extendable arm  40  and the second extendable arm  80  are extended. Specifically, this is as follows. 
     By the control unit  95 , the rotation shaft of the first motor is rotated counterclockwise. Thereby, the power from the first motor is transmitted to the first driven pulley  25  through the first motor pulley  23 , the first wire  26  and the second wire  27 , and the first driven pulley  25  rotates counterclockwise. At this time, the first wire  26  moves at the velocity v in the first direction (+z direction), that is, from the base end side to the tip end side, and with this, the first frame  51  which is fixed to the first wire  26  by the second fixing member  63  also moves in the first direction (+z direction) at the velocity v. 
     The first frame  51  moves in the first direction (+z direction), and thereby the third wire  58  which is fixed to the base unit  10  by the first fixing member  15  moves in an opposite direction (−z direction) to the first direction, whereby the base end side pulley  54  and the tip end side pulley  56  rotate clockwise. At this time, the third wire  58  moves in the −z direction at the relative velocity v with respect to the first frame  51  which moves in the +z direction at the velocity v, and therefore moves in the +z direction at a velocity  2   v.    
     The base end side pulley  54  and the tip end side pulley  56  rotate clockwise, and therefore, the fourth wire  59  moves in the first direction (+z direction) at the velocity  2   v . Thereby, the second frame  71  which is fixed to the fourth wire  59  by the third fixing member  76  and the end effector  90  which is mounted to the second frame  71  move in the first direction (+z direction) at the velocity  2   v.    
     By the control unit  95 , the rotation shaft of the first motor is rotated counterclockwise, but the first drive pulley  24  rotates clockwise through the first drive gear  21  and the first driven gear  22 . Thereby, the second extendable arm  80  having the similar configuration as the first extendable arm  40  extends symmetrically to the first extendable arm  40  in the second direction (−z direction) by the similar operation to what is described above. When the rotation shaft of the second motor is rotated by the control unit  95 , the second drive pulley  34  rotates in an opposite direction through the second drive gear  31  and the second driven gear  32 . Thereby, the end effector  90  which is connected to the tip end of the second extendable arm  80  having the similar configuration to the first extendable arm  40  rotates the same rotation angle in the opposite direction to the rotation direction of the end effector  90  which is connected to the tip end of the first extendable arm  40 , by a similar operation to what is described above. 
     When both of the end effector  90  which is mounted to the first extendable arm  40  and the end effector  90  which is mounted to the second extendable arm  80  contact opening end portions  401   b  of the PAR  401 , the respective extendable arms  40  and  80  are positionally guided to the opening end portions of the PAR  401  since the grasping units  90   b  and  90   c  have V-shapes, and the respective extendable arms  40  and  80  extend so that valley portions of the grasping units  90   b  and  90   c  reach the opening end portions  401   b  of the PAR  401 . Subsequently, when the valley portions of the grasping units  90   b  and  90   c  reach the opening end portions  401   b  of the PAR  401 , the grasping operation is completed. At this time, a value of an encoder of the first motor or a value of a current that is supplied to the first motor is monitored by the removal satellite  2 , and when the respective extendable arms  40  and  80  extend, the value of the encoder increases, whereas the value of the current which is supplied to the motor is small. When extensions of the respective extendable arms  40  and  80  stop by completion of the grasping operation, increase in the value of the encoder stops, or the value of the current which is supplied to the motor increases. Accordingly, when the removal satellite  2  detects change in such encoder value or current value, the removal satellite  2  determines that the removal satellite  2  succeed in grasping. 
     On the other hand, when one or both of the end effector  90  which is mounted to the first extendable arm  40  and the end effector  90  which is mounted to the second extendable arm  80  does not or do not contact the opening end portion  401   b  of the PAR  401  even after a fixed time period elapses, grasping fails. In this case, the first extendable arm  40  and the second extendable arm  80  continue to extend, so that increase in the value of the encoder does not stop, or the value of the current which is supplied to the motor remains to be small. Accordingly, when such changes in the encoder value or the current value is detected after a fixed time period elapses, it is determined that grasping fails. 
     When it is determined that grasping fails, the removal satellite  2  gives an instruction to the control unit  95 , and causes the first extendable arm  40  and the second extendable arm  80  to contract. The removal satellite  2  readjusts the position of the robot  1  and redisposes the robot  1  on the opening surface  401   a  of the PAR  401  by changing the position and the attitude by the propulsive device included by the removal satellite  2 , and repeats the above described operation until the removal satellite  2  succeeds in grasping. 
     Note that the end effector  90  may be rotated so that angles to the longitudinal directions of the respective extendable arms  40  and  80 , of the respective end effectors  90  become substantially equal to a taper angle of the PAR  401  by an instruction to the control unit  95  from the removal satellite  2 , before the extension operations of the respective extendable arms  40  and  80 , or simultaneously with the extension operations. The taper angle of the PAR  401  can be obtained based on data of specifications of the space debris  400  which are stored in advance, analysis on an image of the space debris  400  photographed by the removal satellite  2  and the like. According to such configuration, the end effector and the PAR  401  may be in surface contact instead of point contact, and firmer grasping is enabled. 
     Next, a case of performing a grasping operation to the nozzle  402  having an inversely tapered cylindrical shape will be described. 
     First, by an instruction to the control unit  95  from the removal satellite  2 , the respective extendable arms  40  and  80  are extended in a same way as described above until the respective extendable arms  40  and  80  become larger than a diameter of the nozzle  402 , and the end effectors  90  are rotated so that the angles of the respective end effectors  90  to the longitudinal directions of the respective extendable arms  40  and  80  become substantially equal to a taper angle of the nozzle. The taper angle of the nozzle can be obtained based on the data of specifications of the space debris  400  which are stored in advance, analysis on the image of the space debris  400  photographed by the removal satellite  2  and the like. Note that the extension operation and the rotation operation may be performed in reverse order, or simultaneously. According to such configuration, the end effector and the PAR  401  may be in surface contact instead of point contact, and firmer grasping, and prevention of breakage of the nozzle that is easily broken are enabled. 
     Subsequently, the removal satellite  2  changes the position and the attitude by the propulsive device included by the removal satellite  2 , and thereby the robot  1  which is connected to the removal satellite  2  through the extension spring  3  is positioned to the opening surface  402   a  of the nozzle  402  which is the grasping target part. 
     Subsequently, by an instruction to the control unit  95  from the removal satellite  2 , the respective extendable arms  40  and  80  are contracted. When both of the end effector  90  which is mounted to the first extendable arm  40  and the end effector  90  which is mounted to the second extendable arm  80  contact side surfaces  402   b  of the nozzle  402 , the respective extendable arms  40  and  80  contract so that the side surfaces  402   c  of the nozzle  402  are held between respective extendable arm sides of the V-shaped grasping units  90   b  and  90   c  and the respective extendable arms  40  and  80 , and respective valley portions reach the opening end portions  302   b  of the nozzle  402 . When the valley portions reach the opening end portions  402   b  of the nozzle  402 , the grasping operation is completed. At this time, the value of the encoder of the first motor, or the value of the current that is supplied to the first motor is monitored by the removal satellite  2 , or when the respective extendable arms  40  and  80  are contracted, the value of the encoder decreases, or the value of the current which is supplied to the motor is small. When contraction of the respective extendable arms  40  and  80  stops by completion of the grasping operation, decrease in the value of the encoder stops, or the value of the current which is supplied to the motor increases. Accordingly, when the removal satellite  2  detects change in the encoder value or the current value as above, the removal satellite  2  determines that the removal satellite  2  succeeds in grasping. 
     On the other hand, when one or both of the end effector  90  which is mounted to the first extendable arm  40  and the end effector  90  which is mounted to the second extendable arm  80  does not or do not contact the side surface  402   c  of the nozzle  402  even after a fixed time period elapses, grasping fails. In this case, the first extendable arm  40  and the second extendable arm  80  continue to contract, so that decrease in the value of the encoder does not stop, or the value of the current which is supplied to the motor remains to be small. Accordingly, when the change in the encoder value or the current value as above is detected after the fixed time period elapses, it is determined that grasping fails. 
     When it is determined that grasping fails, the removal satellite  2  instructs the control unit  95  and the control unit  95  extends the respective extendable arms  40  and  80 . The removal satellite  2  readjusts the position of the robot  1 , and redisposes the robot  1  on the opening surface  402   a  of the nozzle  402  by changing the position and the attitude by the propulsive device included by the removal satellite  2 , and the above described operation is repeated until the removal satellite  2  succeeds in grasping. 
     According to the above described embodiment, the grasping system may be made robust and compact to the positional error and the attitude error of the robot with versatility to grasping targets having different shapes and dimensions of diameters. 
     In the above described embodiment, explanation is made on the space debris as an example of the grasping target, but the grasping target is not limited to this, and may be any suitable grasping target. 
     In the above described embodiment, positioning of the robot is performed by the external device of the robot, but, for example, the robot itself may be adapted to include a position/attitude control mechanism and perform positioning, and positioning of the robot may be performed by any suitable device inside or outside of the robot. 
     In the above described embodiment, the number of extendable arms is two, but the number of extendable arms may be any suitable number of three or more. If the number of extendable arms is increased, the grasping target can be grasped more stably. 
     In the above described embodiment, the extendable device includes the two extendable arms, but the extendable device may include one extendable arm in which end effectors are mounted to a base end and a tip end. Further, the extendable device may include an extendable arm and an arm of a fixed length. Further, the extendable device may include any suitable extendable mechanism other than the extendable arm. 
     In the above described embodiment, a number of movable bodies is two, but the number of movable bodies may be any suitable number of three or more. 
     Concerning the present invention, several embodiments are described for illustration thus far, but the present invention is not limited to the embodiments, and it would be obvious to a person skilled in the art that various modifications and corrections can be made for modes and details without departing from the scope and spirit of the present invention. 
     EXPLANATIONS OF LETTERS OR NUMERALS 
     
         
           1  Robot 
           2  Removal satellite 
           3  Extension spring 
           400  Space debris 
           401  PAR 
           420  Nozzle 
           10  Base unit 
           11  Base frame 
           13 ,  14  Pulley 
           20  First drive unit 
           21  First drive gear 
           22  First driven gear 
           23  First motor pulley 
           24  First drive pulley 
           25  First driven pulley 
           26  First wire 
           27  Second wire 
           30  Second drive unit 
           31  Second drive gear 
           32  Second driven gear 
           33  Second motor pulley 
           34  Second drive pulley 
           35  Fifth wire 
           40  First extendable arm 
           50  First movable body 
           51  First frame 
           54  Base end side pulley 
           55  Base end side pulley support unit 
           56  Tip end side pulley 
           57  Tip end side pulley support unit 
           58  Third wire 
           59  Fourth wire 
           61 ,  62  Pulley 
           63  Second fixing member 
           70  Second movable body 
           71  Second frame 
           74 ,  75  Pulley 
           76  Third fixing member 
           80  Second extendable arm 
           90  End effector 
           90   b ,  90   c  Grasping unit 
           90   d  Shaft support unit 
           95  Control unit 
           100  Telescoping device 
           1000  Grasping system