before the description of the present invention proceeds , it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings . referring now to the drawings , there is shown in fig1 a robot according to the first embodiment of the present invention which includes a first prime mover 1 , a second prime mover 2 , an arm 3 , a load 4 , a movement mechanism 5 , an extension mechanism 6 , an extension portion 7 , a grasping mechanism 8 , a force sensor 9 , an object 10 to be grasped , a movement - extension control apparatus 11 , and a robot control apparatus 12 . one end of the first arm 3a is mounted on the driving shaft of the first prime mover 1 . also , the second prime mover 2 is mounted on a shaft coaxial to the rotary shaft of the first prime mover 1 . one end of this second arm 3b is mounted on the driving shaft of the second prime mover 2 . one end of the third arm 3c which is equal in length to the first arm 3a is coupled to the other end of the second arm 3b . also , the fourth arm 3d is coupled to the other end of the third arm 3c and the other end of the first arm 3a . the four arms 3a - 3d constitute a parallelogram link construction . the load 4a is provided on the third arm 3c . the load 4a is connected with the movement portion of the movement mechanism 5a disposed on the third arm 3c , and composed of an air cylinder or motor and ball screw or the like , so that the load 4a is adapted to move along the third arm 3c by the movement mechanism 5a . one portion 7a of the one end of the third arm 3c is separate from and adapted to move in an extension movement along the remainder of the third arm 3c as an extension portion 7a by an extension mechanism 6a composed of an air cylinder disposed within the third arm 3c or a motor and ball screw or the like . the third arm 3c is thus adapted to extend and retract so as to vary the position of the center of gravity of the third arm 3c . the load 4b is provided on the fourth arm 3d , is connected with the movement portion of the movement mechanism 5b composed of an air cylinder or a motor and ball screw of the like . the load 4b is adapted to move along the fourth arm 3d by the movement mechanism 5b . also , one portion 7b of one end of the fourth arm 3d is separate from and is adapted to move along the remainder of the fourth arm 3d as an extension portion 7b by the extension mechanism 6b composed of an air cylinder or a motor and ball screw or the like disposed within the fourth arm 3d . the fourth arm 3d is extended and retracted by the movement of the extension portion 7b so as to change the position of the center of gravity of the fourth arm 3d . a grasping mechanism 8 is provided on the other end of the fourth arm 3d , and the object 10 is grasped by the grasping mechanism 8 . the force sensor 9 is built in the grasping mechanism 8 . the signal from the force sensor 9 is adapted to be inputted into the movement - extension control apparatus 11 . the movement - extension control apparatus detects the mass of the grasped object 10 and gives the proper operation instructions to the movement mechanism 5 and the extension mechanism 6 , and also transmits the contents of the operation instructions to the robot control apparatus 12 . the robot control apparatus 12 receives the signal from the movement - extension control apparatus 11 to change the operation parameter of the center of gravity position and so on so as to be in conformity with the present condition of the robot , and thereafter , to control the operation of the first prime mover 1 , the second prime mover 2 and the grasping mechanism 8 . the operation of the robot constructed as described hereinabove will be described hereinafter . in the above - described embodiment , in accordance with the operation data stored in advance within the memory ( not shown ) of the robot control apparatus 12 , the operation instructions are given to the first prime mover 1 , the second prime mover 2 and the grasping mechanism 8 to move the arm 3a to move the arm 3b to effect the desired operations so as to effect the picking up and placing operation with respect to the object 10 . at this time , the movement - extension control apparatus 11 normally watches the signal output from the force sensor 9 . the moment , for example , the grasping mechanism 8 grasps the object 10 , the movement - extension control apparatus 11 reads the amount of weight change and gives the operation instructions to the movement mechanism 5 and the extension mechanism 6 in accordance with a predetermined simple calculation rule to be described later and to cause them to effect the proper operations , so that the already described calculation results are transmitted to the robot control apparatus 12 . the robot control apparatus 12 also effects the operation control for the arm 3 or the like as described , and also normally watches the signal input from the movement - extension control apparatus 11 . immediately after the signal input has been received , the predetermined parameters in the necessary operation torque production formula with respect to the first prime mover 1 and the second prime mover 2 to be described later are adjusted in accordance with the contents of the input signal so as to continue the operation control of the above described robot in accordance with the formula . fig2 shows the torques t1 , t2 caused in the first and second prime movers 1 and 2 when the above described picking up and placing operation is effected . in fig2 the center of rotation of the first and second prime movers 1 and 2 conforms to the origin on the absolute coordinates axis ( x - y ), an angle to be formed with the first arm 3a and the y positive axis is θ1 , an angle to be formed by the second arm 3b and the x negative axis is θ2 , the length of the first arm 3a , the mass , the inertial moment around the center of gravity , the force such as frictional force or the like in the bearing portion are respectively l1 , m1 , i1 , f1 , the length of the second arm 3b , the mass , the inertial moment around the center of gravity , the force such as frictional force or the like in the bearing portion are respectively l2 , m2 , i2 , f2 , the mass of the third arm 3c ( length is l1 ), the inertial moment around the center of gravity are respectively m3 , i3 , the length of the fourth arm 3d , the mass , the inertial moment around the center of gravity are respectively ( l2 + l4 ), m4 , ir , the inertial moments of the first prime mover 1 and the second prime mover 2 are respectively is1 , is2 , the distance from the center of rotation of the prime mover 1 to the center of gravity ( shown by the x mark in the drawing ) of the second arm 3b are respectively h1 , h2 , the distance from the tip end of the second arm 3b to the center of gravity ( shown by the x mark in the drawing ) of the third arm 3c from the tip end of the second arm 3b is h3 , the distance from the tip end of the first arm 3a to the center of gravity ( shown by the x mark in the drawing ) of the fourth arm 3d is h4 , the mass of the grasping mechanism 8 , the object 10 and so on which are at the tip end portion of the fourth arm 3d is considered to be at the material point ( shown by the • mark in the drawing ) as w2 . the mass of the load 4a which is disposed on the third arm 3c , and is moved along the third arm 3c by the movement mechanism 5a is considered to be at the material point ( shown by the • mark in the drawing ) as w6 , the distance from the tip end of the second arm 3b to the mass point position of the load 4a is h6 , the mass of the load 4b point ( shown by the • mark in the drawing ) which is disposed on the fourth arm 3d , and is moved along the fourth arm 3d by the movement mechanism 5b is w5 , the distance from the tip end of the first arm 3a to the material point position of the load 4b is h5 . the torques t1 and t2 are given by the following dynamic equation of lagrange . ## equ3 ## as θ1 and θ2 are included in respective formulas j12 , j21 , e12 , e21 , the coefficients of θ1 and θ2 in the formula ( 3 ) becomes and the values of t1 and t2 change every moment during the operation of the arm 3 if formula ( 4 ) is not &# 34 ; 0 &# 34 ; so as to cause dynamic interferences between the first and second prime movers 1 and 2 . but three minus terms are included in formula ( 4 ), so that formula ( 4 ) can be made &# 34 ; 0 &# 34 ; by proper design of the arm . the terms j12 , j21 , e11 , e22 , e12 , e21 all become &# 34 ; 0 &# 34 ; and j11 , j22 which do not include the θ1 and θ2 have only constant values , so that dynamic interferences between the first and second prime movers 1 and 2 during operation of the arm 3 can be prevented . when the grasping mechanism 8 moves and places the object 10 which is comparatively larger in mass , the value of w2 in formula ( 4 ) changes greatly between a case where the object 10 is grasped and a case where no object is grasped . suppose the design of the arm 3 is such that formula ( 4 ) has a value other than &# 34 ; 0 &# 34 ; when an object 10 is grasped , and dynamic interference is caused between the first and second prime movers 1 and 2 . the movement - extension control apparatus 11 recognizes the variation in the amount of w2 sensed by the force sensor 9 so as to cause the value w2l1l4 to be changed in formula ( 4 ) by the amount of variation . thereafter , one value or a plurality of values h5 or h6 which can be changed by the operation of the movement mechanisms 5a and 5b and h3 , h4 are properly changed by the operation of the extension mechanisms 6a and 6bby an amount which is capable of offsetting the changed value of w2l1l4 by giving proper operation instructions to the movement mechanisms 5a and 5b and the extension mechanisms 6a and 6b for getting them to effect the operation . also , at the same time , the values of h3 , h4 , h5 , h6 after the variation are transmitted to the robot control apparatus 12 . the robot control apparatus 12 is adapted to change h3 , h4 , h5 , h6 and so on from the parameter values of formula ( 3 ) stored in advance within the memory ( not shown ) so as to change them into the received values for continuing the operations of the first and second prime movers 1 and 2 . according to the robot of the first embodiment of the present invention as described hereinabove , the movement mechanisms 5a and 5b and the extension mechanisms 6a and 6b are properly moved so that dynamic interference terms in the torque formula ( 3 ) for interference between the first and second prime movers 1 and 2 become &# 34 ; 0 &# 34 ; in response to the amount of weight changes of the object 10 , so that dynamic interference caused between the first and second prime movers 1 and 2 is prevented in spite of the operation track of the arm 3 and the operation speed so as to achieve the stable robot operation . fig3 shows the construction of a robot according to a second embodiment of the present invention . the robot in fig3 includes a first prime mover 13 , a second prime mover 14 , an arm 15 , a belting 16 , a load 17 , a movement mechanism 18 , an extension mechanism 19 , an extension portion 20 , a grasping mechanism 21 , a force sensor 22 , an object 23 to be grasped , a movement - extension control apparatus 24 , and a robot control apparatus 25 . one end of the first arm 15a is mounted on the driving shaft of the first prime mover 13 . one end of the second arm 15b is coupled to the other end of the first arm 15a , and the driving shaft of the second prime mover 14 , which is disposed coaxially with the center rotation of the driving shaft of the first prime mover 13 , and the second arm 15b are coupled to each other by the belt 16 , so that the revolution of the driving shaft of the second primer mover 14 is transmitted to the second arm 15b . the load 17 is movably mounted on the second arm 15b , and is connected to a movement mechanism 18 which can be an air cylinder as shown or a motor and ball screw or the like , so that the load 17 is adapted to move along the second arm 15b by operation of the movement mechanism 18 . one portion 20 of one end of the second arm 15b is separate from and is adapted to move along the remainder of the second arm 15b as an extension portion by the extension mechanism 19 which can be an air cylinder as shown or a motor and ball screw or the like disposed within the second arm 15b . the second arm 15b is extended or retracted by the movement of the extension portion 20 to change the position of the center of gravity of the second arm 15b . a grasping mechanism 21 is provided on the other end of the second arm 15b , and the object 23 is grasped by the grasping mechanism 21 . the force sensor 22 is built into the grasping mechanism 21 . the signal from the force sensor 22 is adapted to be inputted into the movement - extension control apparatus 24 so as to detect the mass of the grasped object 23 and to give the proper operation instructions to the movement mechanism 18 and the extension mechanism 19 , and also , to transmit the contents of the operation instructions to the robot control apparatus 25 . the robot control apparatus 25 receives the signal from the movement - extension control apparatus 24 to change the operation parameters of the center of gravity position of the arm in conformity with the present condition of the robot and , thereafter , to control the operations of the first and second prime movers 13 and 14 and the grasping mechanism 21 . the operation of the robot constructed as described above will be described hereinafter . in the above described embodiment , in accordance with the operation data stored in advance within the memory ( not shown ) of the robot control apparatus 25 , the operaton instructions are given to the first and second prime movers 13 and 14 and the grasping mechanism 21 from the robot control apparatus 25 to move the arm 15 to effect the desired operations so as to effect the picking up and placing operation with respect to the object 23 . at this time , the movement - extension control apparatus 24 normally watches the signal output from the force sensor 22 . the moment , for example , the grasping mechanism 21 grasps the object 23 , the movement - extension control apparatus 24 reads the amount of weight change and gives operation instructions to the movement mechanism 18 and the extension mechanism 19 in accordance with a predetermined simple calculation rule to be described later and to cause them to effect the proper operations , so that the already described calculation results are transmitted to the robot control apparatus 25 . the robot control apparatus 25 also effects the operation control for arm 15 or the like as described hereinabove , and also normally watches the signal input from the movement - extension control apparatus 24 . immediately after the signal input has been received , the predetermined parameters in the necessary operation torque production formula with respect to the first and second prime movers 13 and 14 to be described later are adjusted in accordance with the contents of the input signal so as to continue the operation control of the above described robot in accordance with the formula . fig4 shows the torques t1 and t2 caused in the first and second prime movers 13 and 14 when the above described picking up and placing operation is effected . in fig4 the center of rotation of the first and second prime movers 13 and 14 conforms to the origin on the absolute coordinates axis ( x - y ), an angle to be formed with the first arm 15a and the y positive axis is θ1 , an angle to be formed by the second arm 15b and the y positive axis is θ2 , the length of the first arm 15a , the mass , the inertial moment around the center of gravity , the force such as frictional force or the like in the bearing portion are respectively l1 , m1 , i1 , f1 , the length of the second arm 15b , the mass , the inertial moment around the center of gravity , the force such as frictional force in the bearing portion are respectively l2 , m2 , i2 , f2 , the inertial moments of the first and second prime movers 13 and 14 are respectively is1 , is2 , the distance from the center of rotation of the first prime mover 13 to the center of gravity ( shown by the x mark in the drawing ) of the first arm 15a is h1 , and the distance to the center of gravity ( shown by the x mark in the drawing ) of the second arm 15b from the tip end of the first arm 15a is h2 , the mass of the grasping mechanism 21 , the object 23 on which are provided at the tip end portion of the second arm 15b is considered to be at a material point ( shown by the • mark in the drawing ) as w2 . the load 17 which is disposed on the second arm 15b , and is moved along the second arm 15b by the movement mechanism 18 is considered to be at the material point ( shown by the mark • in the drawing ) as w3 , the distance from the tip end of the first arm 15a to the material point position of the load 17 is h3 . the torques t1 and t2 are given by the following dynamic equation of lagrange . ## equ4 ## as θ1 and θ2 are included in respective formulas for j12 , j21 , e12 , e21 in the above described formula , and the coefficients of θ1 and θ2 become values of t1 and t2 change every moment during the operation of the arm 15 if formula ( 6 ) is not &# 34 ; 0 &# 34 ; so as to cause dynamic interferences between the first and second prime movers 13 and 14 . but minus terms are included in formula ( 6 ), so that formula ( 6 ) can be made &# 34 ; 0 &# 34 ; by proper design of the arm . the terms j12 , j21 , e11 , e22 , e12 , e21 all become &# 34 ; 0 &# 34 ; and j11 , j22 which do not include the θ1 and θ2 become constant values , so that dynamic interferences between the first and second prime movers 13 and 14 during the operation of the arm 15 can be prevented . when the grasping mechanism 21 moves , and places the object which is comparatively larger in mass , the value of w2 in formula ( 6 ) changes greatly between a case where the object 23 is grasped and a case where no object is grasped . if the design of the arm 15 is effected such that formula ( 6 ) becomes &# 34 ; 0 &# 34 ; when no object 23 is grasped , and formula ( 6 ) has a value other than &# 34 ; 0 &# 34 ; when an object 23 is grasped , and dynamic interference is caused between the first and second prime movers 13 and 14 . the movement - extension control apparatus 24 recognizes the variation in the amount of w2 sensed by the force sensor 22 so as to cause the value w2l1l2 to be changed in formula ( 6 ) by the amount of rotation . thereafter , one value or two values h3 which can be changed by the operation of the movement mechanism 18 or h2 which can be changed by the operation of the extension mechanism 19 are properly changed by an amount which is capable of offsetting the changed value of w2l1l2 , by giving operation instructions to the movement mechanism 18 and the extension mechanism 19 to cause them to effect the operation . also , at the same time , the values h2 , h3 after the variations are transmitted to the robot control apparatus 25 . the robot control apparatus 25 is adapted to change h2 , h3 from the parameter values of formula ( 5 ) stored in advance within the memory ( not shown ), to match the received values for continuing the operations of the first and second prime movers 13 and 14 and so on . according to the robot of the second embodiment of the present invention as described above , the movement mechanism 18 and the extension mechanism 19 are properly moved so that dynamic interference terms in the torque formula ( 5 ) for interference between the first and second prime movers 13 and 14 become &# 34 ; 0 &# 34 ; in response to the amount of variation of each of the weight changes of the object 23 , so that dynamic interferences caused between the first and second prime movers 13 and 14 are prevented in spite of the operation track of the arm 15 and the operation speed so as to achieve a stable robot operation . fig5 shows the construction of a robot according to a third embodiment of the present invention . the robot in fig5 includes a first prime mover 26 , a second prime mover 27 , first and second arms 28a and 28b , a load 29 , a movement mechanism 30 , an extension mechanism 31 , and extension portion 32 , a grasping mechanism 33 , a force sensor 34 , an object 35 to be grasped , a movement - extension control apparatus 36 , and a robot control apparatus 37 . one end of the first arm 28a is mounted on the driving shaft of the first prime mover 26 . the second prime mover 27 is disposed on the other end of the first arm 28a , and the second arm 28b is mounted on the driving shaft of the second prime mover 27 . the load 29 is movably amounted on the second arm 28b . the load 29 is connected to a movement portion of the movement mechanism 30 which can be an air cylinder as shown or a motor and ball screw or the like , so that the load 29 can be moved along the second arm 28b by the movement mechanism 30 . one portion 32 of one end of the second arm 28b is separate from and adapted to move along the remainder of the second arm 28b as an extension portion by the extension mechanism 31 which can be an air cylinder as shown or a motor and ball screw or the like , disposed within the second arm 28b . the second arm 28b is extended and retracted so as to vary the position of the center of gravity of the second arm 28b . a grasping mechanism 33 is provided on the other end of the second arm 28b , and the object 35 is grasped by the grasping mechanism 33 . the force sensor 34 is built into the grasping mechanism 33 . the signal from the force sensor 34 is adapted to be inputted into the movement - extension control apparatus 36 so as to detect the mass of the grasped object 35 and to give proper operation instructions to the robot control apparatus 37 . the robot control apparatus 37 receives the signal from the movement - extension control apparatus 36 to change the operation parameters of the center of gravity position of the arm in conformity with the present condition of the robot , and thereafter to control the operation of the first and second prime movers 26 and 27 and the grasping mechanism 33 . the operation of the robot constructed as described above is described below . in the above described embodiment , in accordance with the operation data stored in advance within the memory ( not shown ) of the robot control apparatus 37 , the operation instructions are given to the first and second prime movers 26 and 27 and the grasping mechanism 33 from the robot control apparatus 37 to move the arm 28 to effect the desired operations so as to effect the picking up and placing operation with respect to the object 35 . at this time , the movement - extension control apparatus 36 normally watches the output signal from the force sensor 34 . the moment , for example , the grasping mechanism 33 grasps the object 35 , the movement - extension control apparatus 36 reads the amount of weight change and gives operation instructions to the movement mechanism 18 and the extension mechanism 31 in accordance with a predetermined simple calculation rule to be described later and to cause them to effect proper operation , so that the already described calculation results are transmitted to the robot control apparatus 37 . the robot control apparatus 37 also effects the operation control for the arm 28 as described above , and also normally watches the input signal from the movement - extension control apparatus 36 . immediately after the input signal has been received , the predetermined parameters in the necessary operation torque production formula with respect to the first and second prime movers 26 and 27 to be described later are adjusted in accordance with the contents of the input signal so as to continue the operation control of the above described robot in accordance with the formula . fig6 shows the torques t1 and t2 caused in the first and second prime movers 26 and 27 when the above described picking up and placing operation is effected . in fig6 the center of rotation of the first and second prime movers 26 and 27 conforms to the origin on the absolute coordinates axis ( x - y ), an angle to be formed with the first arm 28a and the y positive axis is θ1 , an angle to be formed by the first arm 28a and the second arm 28b is θ2 , the length of the first arm 28a , the mass , the inertial moment around the center of gravity , the force such as frictional force in the bearing portion are respectively l1 , m1 , i1 , f1 , the length of the second arm 28b , the mass , the inertial moment around the center of gravity , the force such as frictional force in the bearing portion are respectively l2 , m2 , i2 , f2 , the inertial moments of the first and second prime movers 26 and 27 are respectively is2 , is2 , the distance from the center of rotation of the first prime mover 26 to the center of gravity ( shown by the x mark in the drawing ) of the first arm 28a is h1 , and the distance to the center of gravity ( shown by an x in the drawing ) of the second arm 28b from the tip end of the first arm 28a is h2 , the mass of the second prime mover 27 which is located on the tip end of the first arm 28a is considered to be at a material point ( shown by the • in the drawing ) as w1 , the mass of the grasping mechanism 33 , the object 35 which is provided at the tip end of the second arm 28b is considered to be at the material point ( shown by the mark in the drawing ) as w2 . the load 29 which is disposed on the second arm 28b , and is moved along the second arm 28b by the movement mechanism 30 is considered to be at the material point ( shown by • mark in the drawing ) as w3 , the distance from the tip end of the first arm 28a to the material point position of the load 29 is h3 . the torques t1 and t2 are given by the following dynamic equation of lagrange . ## equ5 ## as θ2 is included in respective formulas of j11 , j12 , j21 , e12 , k32 , k1 in formula ( 7 ), the common coefficient of θ2 become the values of t1 and t2 change every moment during the operations of the first and second arms 28a and 28b if formula ( 8 ) is not &# 34 ; 0 &# 34 ;, and dynamic interference components between the first and second prime movers change every moment because of the positive posture change in the second arm 28b with respect to the first arm 28a . in the case of the third robot , j12 and j21 which are the interference components of t1 and t2 cannot be offset completely as compared with those in the first and second robots . but as j12 and j21 become minimum values and e12 , e21 , k1 become &# 34 ; 0 &# 34 ; so at least formula ( 8 ) can be made &# 34 ; 0 &# 34 ;, the interference components of t1 and t2 can be made minimum and simple . as a minus term is contained in formula ( 8 ), it can be made &# 34 ; 0 &# 34 ; by proper design of the arm . when the grasping mechanism 33 moves and places the object 35 which is comparatively large in mass , the value of w2 in formula ( 8 ) changes greatly between the case where the object 35 is grasped and the case where no object is grasped . if the design of the arm 28 is effected such that formula ( 8 ) will become &# 34 ; 0 &# 34 ; when no object is grasped , and formula ( 8 ) is adapted to have a value other than &# 34 ; 0 &# 34 ; when an object 35 is grasped , the components of dynamic interferences caused between the first and second prime movers 26 and 27 are increased and complicated . the movement - extension control apparatus 36 recognizes the variation in the amount w2 sensed by the force sensor 34 so as to cause the value w2l1l2 to be changed in formula ( 8 ) by the amount of variation . thereafter , by the proper change of one or two values h3 changed by the operation of the movement mechanism 30 or h2 , the amount of variation capable of offsetting the value of w1l1l2 is calculated , operation instructions are given to the movement mechanism 30 and extension mechanism 31 so as to cause them to effect the operations . at the same time , the changed values of h2 , h3 are transmitted to the robot control apparatus 37 . the robot control apparatus 37 changes h2 , h3 from the parameter values of formula ( 7 ) stored in advance within the memory ( not shown ), so as to match the received values for continuing the operations of the first and second prime movers 26 and 27 . according to the robot of the third embodiment of the present invention , the movement mechanism 30 and the extension mechanism 31 are properly operated so that dynamic interference terms in the torque formula ( 7 ) for interference between the first and second prime movers 26 and 27 are made minimum and simple in response to the amount of variation at each of the weight changes of the object 35 , so that dynamic interferences caused between the first and second prime movers 26 and 27 become minimum and the amount becomes easy to calculate with respect to the change in the operation track and the operation speed of the arm 28 , thus achieving a comparatively stable robot operation . although only a driving mechanism in the form of an air cylinder as shown or a motor and ball screw has been described as the movement mechanism and the extension mechanism in the robot of the above described third embodiment , any other driving mechanism which can achieve the same results may , needless to say , be used . although the unimportant things especially among the material points which can exist in the arm are omitted as much as possible for the simple modeling of the robot in fig2 and 6 , the influences are not applied in theory if the contents in the torque formula of the t1 and t2 are different somewhat by the fixed material points not described hereinabove . also , the driving portions and the like which effect the vertical operations and the rotation portions which the above described grasping mechanism are conventional . further , although an example where belting is used as the rotation transmission means to the second arm from the second prime mover is provided in the robot of the second embodiment , other transmission means such as gears and the like may be used . as is clear from the foregoing description , according to the arrangement of the present invention , the load movement mechanism and the arm extension mechanism , which are provided on the robot arms can be operated in response to a changed condition between a case where a weight is picked up and placed and a case where no weight is grasped by the sensing of gravity force changes by a force sensor so that the mutual dynamic interferences of the respective driving portions of the robot are not caused or can be kept to a minimum , and may be made simple in shape by a comparatively simple control apparatus , with the effect that a superior robot which is capable of stable operations can be achieved . although the present invention has been fully described by way of example with reference to the accompanying drawings , it is to be noted here that various changes and modifications will be apparent to those skilled in the art . therefore , unless such changes and modifications depart from the scope of the present invention , they should be construed as included therein .