Abstract:
When a rotation shaft reaches a first standstill position, a contact piece is received in a first hooked groove. A grasping mechanism unit is forced to enjoy a change in the attitude around the rotation shaft by the rotation angle of 90 degrees. When the rotation shaft further reaches a first terminal position, the grasping mechanism unit is forced to restore the predetermined attitude around the rotation shaft by the rotation angle of 90 degrees. The linear motion of the rotation shaft is in this manner interconnected to the rotational movement of the grasping mechanism unit in a library apparatus. A pulley, a timing belt and an electric motor can be omitted from the driving mechanism for the grasping mechanism unit. This results in a simplified structure. The production cost is thus reduced. The transporting mechanism can have a longer service life.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a library apparatus such as a magnetic tape library apparatus. In particular, the present invention relates to a transporting mechanism employed in a library apparatus. The transporting mechanism includes a rail extending in the horizontal direction, and a grasping mechanism unit, namely a robot hand, related to the rail for relative rotation around a vertical shaft for grasping an object, namely a magnetic tape cartridge. 
     2. Description of the Prior Art 
     A magnetic tape library apparatus is well known. The magnetic tape library apparatus includes a pair of storage boxes opposed to each other. Each of the storage boxes includes cells. A magnetic tape cartridge is contained in the individual cell. 
     A predetermined space in the form of a parallelepiped is defined between the storage boxes. The slots of magnetic tape drives are arranged along the predetermined space. A transporting mechanism is placed within the predetermined space. The transporting mechanism includes a grasping mechanism, namely a robot hand. The robot hand is capable of transporting the magnetic tape cartridges between the cells and the slots of the magnetic tape drives. The robot hand swallows a select one of the magnetic tape cartridges from the slot when the robot hand receives the magnetic tape cartridge. 
     The robot hand is mounted on a rail. The robot hand is designed to move in the horizontal direction along the rail. The robot hand is in this manner positioned to a specific one of the cells or the slots of the magnetic tape drives. Simultaneously, the robot hand is designed to rotate around the vertical shaft on the rail. The robot hand is in this manner allowed to oppose its own slot to the specific one of the cells or the slots of the magnetic tape drives. 
     A linear motion mechanism is coupled to the robot hand on the rail for realization of the horizontal movement of the robot hand. A rotation mechanism is coupled to the robot hand for realization of the rotary movement of the robot hand. The rotation mechanism includes a pair of pulleys and a timing belt wound around the pulleys, for example. The linear motion mechanism and the rotation mechanism are individually coupled to the robot hand. The linear motion mechanism and the rotation mechanism are separately controlled. This results in a complicated structure. The production cost correspondingly increases. In addition, the timing belt inevitably suffers from deterioration due to aging. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the present invention to provide a transporting mechanism for a library apparatus, having a simplified structure and a long service life. 
     According to a first aspect of the present invention, there is provided a transporting mechanism for a library apparatus, comprising: a rail; a rotation shaft guided on the rail, the rotation shaft movable on a straight movement path in the opposite directions from a reference position to a first terminal position and a second terminal position; a grasping mechanism unit coupled to the rotation shaft for connection to the rail for relative rotation around the longitudinal axis of the rotation shaft, the grasping mechanism unit designed to grasp an object; a contact piece attached to the grasping mechanism unit, the contact piece extending in parallel with the longitudinal axis of the rotation shaft; a cam plate extending within a plane perpendicular to the longitudinal axis of the rotation shaft, the cam plate defining a cam groove for receiving the contact piece; a first driving mechanism designed to drive the contact piece toward the first terminal position around the rotation shaft when the rotation shaft moves from the reference position to the first terminal position; and a second driving mechanism designed to drive the contact piece toward the second terminal position around the rotation shaft when the rotation shaft moves from the reference position to the second terminal position, wherein the cam groove includes: a reference groove receiving the contact piece on a perpendicular line set perpendicular to the straight movement path at the longitudinal axis of the rotation shaft when the rotation shaft is positioned at the reference position; a first hooked groove receiving the contact piece on an extension of the straight movement path extending from the first terminal position when the rotation shaft is positioned at a first standstill position between the reference position and the first terminal position; a second hooked groove receiving the contact piece on an extension of the straight movement path extending from the second terminal position when the rotation shaft is positioned at a second standstill position between the reference position and the second terminal position; a first terminal groove receiving the contact piece on a perpendicular line set perpendicular to the straight movement path when the rotation shaft is positioned at the first terminal position; a second terminal groove receiving the contact piece on a perpendicular line set perpendicular to the straight movement path when the rotation shaft is positioned at the second terminal position; a first guide groove extending from the reference groove to the first hooked groove; a second guide groove extending from the reference groove to the second hooked groove; a third guide groove extending from the first hooked groove to the first terminal groove; and a fourth guide groove extending from the second hooked groove to the second terminal groove, wherein the first guide groove includes: a first guide wall staying outward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and inscribing the contact piece, when the rotation shaft moves from the reference position to the first standstill position, the tangent line extending from the contact piece in the direction of advancement of the contact piece; and a second guide wall staying inward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and circumscribing the contact piece, when the rotation shaft moves from the first standstill position to the reference position, the tangent line extending from the contact piece in the direction of advancement of the contact piece, wherein the second guide groove includes: a third guide wall staying outward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and inscribing the contact piece, when the rotation shaft moves from the reference position to the second standstill position, the tangent line extending from the contact piece in the direction of advancement of the contact piece; and a fourth guide wall staying inward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and circumscribing the contact piece, when the rotation shaft moves from the second standstill position to the reference position, the tangent line extending from the contact piece in the direction of advancement of the contact piece, wherein the third guide groove includes: a fifth guide wall staying outward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and inscribing the contact piece, when the rotation shaft moves from the first standstill position to the first terminal position, the tangent line extending from the contact piece in the direction of advancement of the contact piece; and a sixth guide wall staying inward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and circumscribing the contact piece, when the rotation shaft moves from the first terminal position to the first standstill position, the tangent line extending from the contact piece in the direction of advancement of the contact piece, wherein the fourth guide groove includes: a seventh guide wall staying outward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and inscribing the contact piece, when the rotation shaft moves from the second standstill position to the second terminal position, the tangent line extending from the contact piece in the direction of advancement of the contact piece; and an eighth guide wall staying inward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and circumscribing the contact piece, when the rotation shaft moves from the second terminal position to the second standstill position, the tangent line extending from the contact piece in the direction of advancement of the contact piece. 
     When the rotation shaft is positioned at the reference position, the transporting mechanism allows the grasping mechanism unit to take a predetermined attitude. When the rotation shaft reaches the first standstill position, the contact piece is received in the first hooked groove. The grasping mechanism unit is thus forced to enjoy a change in the attitude around the rotation shaft by the rotation angle of 90 degrees. When the rotation shaft further reaches the first terminal position, the grasping mechanism unit is allowed to restore the predetermined attitude around the rotation shaft by the rotation angle of 90 degrees. To the contrary, when the rotation shaft reaches the second standstill position, the contact piece is received in the second hooked groove. The grasping mechanism unit is thus forced to enjoy a change in the attitude around the rotation shaft by the rotation angle of 90 degrees in the direction opposite to the aforementioned direction. When the rotation shaft further reaches the second terminal position, the grasping mechanism unit is forced to restore the predetermined attitude around the rotation shaft by the rotation angle of 90 degrees. The linear motion of the rotation shaft is in this manner interconnected to the rotation of the grasping mechanism unit. A pulley, a timing belt and an electric motor can be omitted from the driving mechanism for the grasping mechanism unit. This results in a simplified structure. The production cost is thus reduced. The transporting mechanism can have a longer service life. 
     The transporting mechanism may allow the first driving mechanism to include: an auxiliary contact piece extending in parallel with the longitudinal axis of the rotation shaft on a straight line extending from the rotation shaft in the direction opposite to a straight line extending from the rotation shaft to the contact piece; and an auxiliary cam plate defined on a straight line extending from the rotation shaft in the direction opposite to a straight line extending from the rotation shaft to the reference groove, the auxiliary cam plate receiving the auxiliary contact piece in parallel with the straight movement path when the rotation shaft moves from the reference position to the first terminal position. The second driving mechanism may include: an auxiliary contact piece extending in parallel with the longitudinal axis of the rotation shaft on a straight line extending from the rotation shaft in the direction opposite to a straight line extending from the rotation shaft to the contact piece; and an auxiliary cam plate defined on a straight line extending from the rotation shaft in the direction opposite to a straight line extending from the rotation shaft to the reference groove, the auxiliary cam plate receiving the auxiliary contact piece in parallel with the straight movement path when the rotation shaft moves from the reference position to the second terminal position. 
     The transporting mechanism is utilized in a so-called library apparatus. The library apparatus may comprise: a rail; a rotation shaft guided on the rail, the rotation shaft movable on a straight movement path in the opposite directions from a reference position to a first terminal position and a second terminal position; a grasping mechanism unit coupled to the rotation shaft for connection to the rail for relative rotation around the longitudinal axis of the rotation shaft, the grasping mechanism unit designed to grasp an object; a contact piece attached to the grasping mechanism unit, the contact piece extending in parallel with the longitudinal axis of the rotation shaft; a cam plate extending within a plane perpendicular to the longitudinal axis of the rotation shaft, the cam plate defining a cam groove for receiving the contact piece; a first driving mechanism designed to drive the contact piece toward the first terminal position around the rotation shaft when the rotation shaft moves from the reference position to the first terminal position; and a second driving mechanism designed to drive the contact piece toward the second terminal position around the rotation shaft when the rotation shaft moves from the reference position to the second terminal position, wherein the cam groove includes: a reference groove receiving the contact piece on a perpendicular line set perpendicular to the straight movement path at the longitudinal axis of the rotation shaft when the rotation shaft is positioned at the reference position; a first hooked groove receiving the contact piece on an extension of the straight movement path extending from the first terminal position when the rotation shaft is positioned at a first standstill position between the reference position and the first terminal position; a second hooked groove receiving the contact piece on an extension of the straight movement path extending from the second terminal position when the rotation shaft is positioned at a second standstill position between the reference position and the second terminal position; a first terminal groove receiving the contact piece on a perpendicular line set perpendicular to the straight movement path when the rotation shaft is positioned at the first terminal position; a second terminal groove receiving the contact piece on a perpendicular line set perpendicular to the straight movement path when the rotation shaft is positioned at the second terminal position; a first guide groove extending from the reference groove to the first hooked groove; a second guide groove extending from the reference groove to the second hooked groove; a third guide groove extending from the first hooked groove to the first terminal groove; and a fourth guide groove extending from the second hooked groove to the second terminal groove, wherein the first guide groove includes: a first guide wall staying outward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and inscribing the contact piece, when the rotation shaft moves from the reference position to the first standstill position, the tangent line extending from the contact piece in the direction of advancement of the contact piece; and a second guide wall staying inward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and circumscribing the contact piece, when the rotation shaft moves from the first standstill position to the reference position, the tangent line extending from the contact piece in the direction of advancement of the contact piece, wherein the second guide groove includes: a third guide wall staying outward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and inscribing the contact piece, when the rotation shaft moves from the reference position to the second standstill position, the tangent line extending from the contact piece in the direction of advancement of the contact piece; and a fourth guide wall staying inward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and circumscribing the contact piece, when the rotation shaft moves from the second standstill position to the reference position, the tangent line extending from the contact piece in the direction of advancement of the contact piece, wherein the third guide groove includes: a fifth guide wall staying outward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and inscribing the contact piece, when the rotation shaft moves from the first standstill position to the first terminal position, the tangent line extending from the contact piece in the direction of advancement of the contact piece; and a sixth guide wall staying inward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and circumscribing the contact piece, when the rotation shaft moves from the first terminal position to the first standstill position, the tangent line extending from the contact piece in the direction of advancement of the contact piece, wherein the fourth guide groove includes: a seventh guide wall staying outward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and inscribing the contact piece, when the rotation shaft moves from the second standstill position to the second terminal position, the tangent line extending from the contact piece in the direction of advancement of the contact piece; and an eighth guide wall staying inward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and circumscribing the contact piece, when the rotation shaft moves from the second terminal position to the second standstill position, the tangent line extending from the contact piece in the direction of advancement of the contact piece. 
     According to a second aspect of the present invention, there is provided a transporting mechanism for a library apparatus, comprising: a rail; a rotation shaft guided on the rail for movement from a reference position to a terminal position on a straight movement path; a grasping mechanism unit coupled to the rotation shaft for connection to the rail for relative rotation around the longitudinal axis of the rotation shaft, the grasping mechanism unit designed to grasp an object; a contact piece attached to the grasping mechanism unit, the contact piece extending in parallel with the longitudinal axis of the rotation shaft; a cam plate extending within a plane perpendicular to the longitudinal axis of the rotation shaft, the cam plate defining a cam groove for receiving the contact piece; and a driving mechanism designed to drive the contact piece toward the terminal position around the rotation shaft when the rotation shaft moves from the reference position to the terminal position, wherein the cam groove includes: a reference groove receiving the contact piece on a perpendicular line set perpendicular to the straight movement path at the longitudinal axis of the rotation shaft when the rotation shaft is positioned at the reference position; a hooked groove receiving the contact piece on an extension of the straight movement path extending from the terminal position when the rotation shaft is positioned at a standstill position between the reference position and the terminal position; a terminal groove receiving the contact piece on a perpendicular line set perpendicular to the straight movement path when the rotation shaft is positioned at the terminal position; a first guide groove extending from the reference groove to the hooked groove; and a second guide groove extending from the hooked groove to the terminal groove, wherein the first guide groove includes: a first guide wall stays outward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and inscribing the contact piece, when the rotation shaft moves from the reference position to the standstill position, the tangent line extending from the contact piece in the direction of advancement of the contact piece; and a second guide wall staying inward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and circumscribing the contact piece, when the rotation shaft moves from the standstill position to the reference position, the tangent line extending from, the contact piece in the direction of advancement of the contact piece, wherein the second guide groove includes: a third guide wall staying outward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and inscribing the contact piece, when the rotation shaft moves from the standstill position to the terminal position, the tangent line extending from the contact piece in the direction of advancement of the contact piece; and a fourth guide wall staying inward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and circumscribing the contact piece, when the rotation shaft moves from the terminal position to the standstill position, the tangent line extending from the contact piece in the direction of advancement of the contact piece. 
     When the rotation shaft is positioned at the reference position, the transporting mechanism allows the grasping mechanism unit to take a predetermined attitude. When the rotation shaft reaches the standstill position, the contact piece is received in the hooked groove. The grasping mechanism unit is thus forced to enjoy a change in the attitude around the rotation shaft by the rotation angle of 90 degrees. When the rotation shaft further reaches the terminal position, the grasping mechanism unit is forced to restore the predetermined attitude around the rotation shaft by the rotation angle of 90 degrees. The linear motion of the rotation shaft is in this manner interconnected to the rotation of the grasping mechanism unit. A pulley, a timing belt and an electric motor can be omitted from the driving mechanism for the grasping mechanism unit. This results in a simplified structure. The production cost is thus reduced. The transporting mechanism can have a longer service life. 
     The driving mechanism may include: an auxiliary contact piece extending in parallel with the longitudinal axis of the rotation shaft on a straight line extending from the rotation shaft in the direction opposite to a straight line extending from the rotation shaft to the contact piece; and an auxiliary cam plate defined on a straight line extending from the rotation shaft in the direction opposite to a straight line extending from the rotation shaft to the reference groove, the auxiliary cam plate receiving the auxiliary contact piece in parallel with the straight movement path when the rotation shaft moves from the reference position to the terminal position. 
     According to a third aspect of the present invention, there is provided a transporting mechanism for a library apparatus, comprising: a rail; a rotation shaft guided on the rail for movement from a reference position to a standstill position on a straight movement path; a grasping mechanism unit coupled to the rotation shaft for connection to the rail for relative rotation around the longitudinal axis of the rotation shaft, the grasping mechanism unit designed to grasp an object; a contact piece attached to the grasping mechanism unit, the contact piece extending in parallel with the longitudinal axis of the rotation shaft; a cam plate extending within a plane perpendicular to the longitudinal axis of the rotation shaft, the cam plate defining a cam groove for receiving the contact piece; and a driving mechanism designed to drive the contact piece toward the standstill position around the rotation shaft when the rotation shaft moves from the reference position to the standstill position, wherein the cam groove includes: a reference groove receiving the contact piece on a perpendicular line set perpendicular to the straight movement path at the longitudinal axis of the rotation shaft when the rotation shaft is positioned at the reference position; a terminal groove receiving the contact piece on an extension of the straight movement path extending from the standstill position when the rotation shaft is positioned at the standstill position; and a guide groove extending from the reference groove to the terminal groove, wherein the guide groove includes: a first guide wall stays outward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and inscribing the contact piece, when the rotation shaft moves from the reference position to the standstill position, the tangent line extending from the contact piece in the direction of advancement of the contact piece; and a second guide wall staying inward of a tangent line tangent to a circle, having the center at the longitudinal axis of the rotation shaft and circumscribing the contact piece, when the rotation shaft moves from the standstill position to the reference position, the tangent line extending from the contact piece in the direction of advancement of the contact piece. 
     When the rotation shaft is positioned at the reference position, the transporting mechanism allows the grasping mechanism unit to take a predetermined attitude. When the rotation shaft reaches the standstill position, the contact piece is received in the terminal groove. The grasping mechanism unit is thus forced to enjoy a change in the attitude around the rotation shaft by the rotation angle of 90 degrees. The linear motion of the rotation shaft is in this manner interconnected to the rotation of the grasping mechanism unit. A pulley, a timing belt and an electric motor can be omitted from the driving mechanism for the grasping mechanism unit. This results in a simplified structure. The production cost is thus reduced. The transporting mechanism can have a longer service life. 
     The driving mechanism may include: an auxiliary contact piece extending in parallel with the longitudinal axis of the rotation shaft on a straight line extending from the rotation shaft in the direction opposite to a straight line extending from the rotation shaft to the contact piece; and an auxiliary cam plate defined on a straight line extending from the rotation shaft in the direction opposite to a straight line extending from the rotation shaft to the reference groove, the auxiliary cam plate receiving the auxiliary contact piece in parallel with the straight movement path when the rotation shaft moves from the reference position to the standstill position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiment in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a perspective view schematically illustrating a magnetic tape library apparatus; 
         FIG. 2  is a perspective view schematically illustrating the structure of the magnetic tape library apparatus; 
         FIG. 3  is a side view of the magnetic tape library apparatus for schematically illustrating the movement regions of first and second robot hands; 
         FIG. 4  is an enlarged perspective view schematically illustrating the first robot hand and a second rail base; 
         FIG. 5  is a plan view of the second rail base for schematically illustrating the reference position of a rotation shaft; 
         FIG. 6  is a plan view of the second rail base for schematically illustrating the terminal position of the rotation shaft; 
         FIG. 7  is a plan view of the second rail base for schematically illustrating the standstill position of the rotation shaft; 
         FIG. 8  is a partial enlarged plan view of the second rail base for schematically illustrating a first guide wall formed in a first guide groove; 
         FIG. 9  is a partial enlarged plan view of the second rail base for schematically illustrating a first guide wall formed in a second guide groove; 
         FIG. 10  is a partial enlarged plan view of the second rail base for schematically illustrating a second guide wall formed in the first guide groove; 
         FIG. 11  is a partial enlarged plan view of the second rail base for schematically illustrating a second guide wall formed in the second guide groove; 
         FIG. 12  is a partial enlarged plan view of the second rail base for schematically illustrating a third guide wall formed in a third guide groove; 
         FIG. 13  is a partial enlarged plan view of the second rail base for schematically illustrating a third guide wall formed in a fourth guide groove; 
         FIG. 14  is a partial enlarged plan view of the second rail base for schematically illustrating a fourth guide wall formed in the third guide groove; 
         FIG. 15  is a partial enlarged plan view of the second rail base for schematically illustrating a fourth guide wall formed in the fourth guide groove; 
         FIG. 16  is a plan view of the second rail base for schematically illustrating the first robot hand when the rotation shaft starts moving from the reference position; 
         FIG. 17  is a plan view of the second rail for schematically illustrating the first robot hand opposed to a specific one of cells; and 
         FIG. 18  is a plan view of the second rail base for schematically illustrating the first robot hand opposed to the slot of a specific one of magnetic tape drives. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  schematically illustrates the structure of a magnetic tape library apparatus  11  according to an embodiment of the present invention. The magnetic tape library apparatus  11  includes a box-shaped enclosure  12 . The enclosure  12  defines an inner space in the form of a parallelepiped standing upright from the floor, for example. As shown in  FIG. 2 , storage boxes  13   a ,  13   b  are placed in the inner space of the enclosure  12 . A pair of storage boxes  13   a , one of them not shown, is located at opposite sides of a predetermined central space in the form of a parallelepiped. Each of the storage boxes  13   a  includes cells  14 ,  4 , . . . . The openings of the cells  14 ,  14 , . . . are arranged along planes perpendicular to the floor, namely side surfaces of the central space. An object or recording medium such as a magnetic tape cartridge  15  is contained in the individual cell  14 . A linear tape-open (LTO) cartridge may be employed as the magnetic tape cartridge  15 , for example. 
     The storage box  13   b  is placed at a position adjacent to the central space between the storage boxes  13   a ,  13   a . Four, for example, recording medium drives such as magnetic tape drives  16  are placed in the storage box  13   b . The magnetic tape drives  16  respectively include slots arranged along a plane perpendicular to the floor, namely a side surface of the central space. The magnetic tape drive  16  is designed to write magnetic information data into a magnetic recording tape inside the magnetic tape cartridge  15 . The magnetic tape drive  16  is also designed to read magnetic information data out of the magnetic recording tape inside the magnetic tape cartridge  15 . The magnetic tape cartridge  15  is inserted into and withdrawn from the magnetic tape drive  16  through the slot. The magnetic recording tape is unwound from a reel within the magnetic tape cartridge  15  in the magnetic tape drive  16 . The unwound magnetic recording tape is then wound around a reel within the magnetic tape drive  16 . 
     Here, an xyz-coordinate system is defined in the central space. The y-axis of the xyz-coordinate system is set perpendicular to the floor. The cells  14  in the storage boxes  13   a  are arranged in rows in the vertical direction in parallel with the y-axis. The z-axis of the xyz-coordinate system is set to extend in the horizontal direction in parallel with the storage boxes  13   a . The z-axis thus extends across the rows of the cells  14  in the storage boxes  13   a  in the horizontal direction. The x-axis of the xyz-coordinate system is set to extend in the horizontal direction in parallel with the storage box  13   b . The x-axis thus extends across the magnetic tape drives  16  in the storage box  13   b  in the horizontal direction. 
     Two, for example, containers  17 ,  17  are placed in the inner space of the enclosure  12 . One of the containers  17  contains a library controller board and a first controller board. The other of the containers  17  likewise contains a second controller board. An external host computer, not shown, is connected to the library controller board. Various processings are effected at the library controller board as well as the first and second controller boards based on data and/or instructions supplied from the host computer. 
     First and second transporting robots  18 ,  19  as a transporting mechanism are placed in the central space in the enclosure  12 . The first and second transporting robots  18 ,  19  include first and second robot hands  21 ,  22  as grasping mechanism units, respectively. The first and second robot hands  21 ,  22  are individually designed to move relative to the first and second storage boxes  13   a ,  13   b . The first and second robot hands  21 ,  22  are capable of transporting the magnetic tape cartridges  15  between the cells  14  and the magnetic tape drives  16  for read/write operations of the information data. The first and second robot hands  21 ,  22  are designed to take the magnetic tape cartridge  15  out of the slot of the magnetic tape drive  16  for the transportation. The first and second robot hands  21 ,  22  are designed to oppose their own slots  23  to the opening of the individual cell  14  for giving and receiving the magnetic tape cartridge  15  to and from the select cell  14 . Likewise, the first and second robot hands  21 ,  22  are allowed to oppose the own slots  23  to the slot of the individual magnetic tape drive  16 . 
     A positioning mechanism  24  is connected to the first robot hand  21  in the first transporting robot  18 . The positioning mechanism  24  includes a support member or a first support column  25  standing upright from the floor. A first rail  26  is coupled to the first support column  25 . The first rail  26  extends in the vertical direction. A support body or a guide member  27  is coupled to the first rail  26 . A first rail base  28  is coupled to the guide member  27 . The guide member  27  and the first rail base  28  extend in the horizontal direction in parallel with the storage boxes  13   a . The first rail base  28  is positioned at an intermediate position equally spaced from the storage boxes  13   a ,  13   a.    
     The guide member  27  and the first rail base  28  are allowed to move upward and downward along the first rail  26  in parallel with the y-axis. A driving mechanism is connected to the guide member  27  for the upward and downward movement. The driving mechanism may include a belt coupled to the guide member  27  at the tip end, and a hoist designed to wind up the belt, for example. A power source such as an electric motor is incorporated in the hoist, for example. A stepping motor may be utilized as the electric motor, for example. The electric motor is referred to as “y-axis electric motor” hereinafter, for example. 
     Likewise, a positioning mechanism  29  is connected to the second robot hand  22  in the second transporting robot  19 . The positioning mechanism  29  includes a support member or a second support column  31  standing upright from the floor. A first rail  32  is coupled to the second support column  31 . The first rail  32  extends in the vertical direction. A support body or a guide member  33  is coupled to the first rail  32 . A first rail base  34  is coupled to the guide member  33 . The guide member  33  and the first rail base  34  extend in the horizontal direction in parallel with the storage boxes  13   a . The first rail base  34  is positioned at an intermediate position equally spaced from the storage boxes  13   a ,  13   a.    
     The guide member  33  and the first rail base  34  are allowed to move upward and downward along the first rail  32  in parallel with the y-axis in the same manner as the guide member  27  and the first rail base  28 . A driving mechanism is connected to the guide member  33  for the upward and downward movement. The driving mechanism may include a belt coupled to the guide member  33  at the tip end, and a hoist designed to wind up the belt, for example. A power source such as an electric motor is incorporated in the hoist, for example. A stepping motor may be utilized as the electric motor, for example. The electric motor is referred to as “y-axis electric motor” hereinafter, for example. The guide members  27 ,  33  and the first rail bases  28 ,  34  are arranged in the vertical direction along the y-axis. The first rail base  34  of the second transporting robot  19  moves in the vertical direction above the first rail base  28  of the first transporting robot  18 . 
     A second rail  35  is incorporated within each of the first rail bases  28 ,  34 . The second rail  35  extends in the horizontal direction in parallel with the storage boxes  13   a . A second rail base  36  is coupled to the second rail  35 . The second rail base  36  extends in the horizontal direction in parallel with the storage box  13   b . The second rail base  36  moves in the horizontal direction along the second rail  35  in parallel with the z-axis. A driving mechanism is connected to the second rail base  36  for the horizontal movement. The driving mechanism may include an endless belt wound around a pair of pulleys on the first rail base  28 ,  34 , and a power source establishing a driving force to drive one of the pulleys for rotation, for example. In this case, the endless belt is coupled to the second rail base  36 . An electric motor may be utilized as the power source. A stepping motor may be employed as the electric motor, for example. The electric motor is referred to as “z-axis electric motor” hereinafter, for example. 
     A pair of third rails  37 ,  37  is incorporated in the individual second rail base  36 . The third rails  37 ,  37   b  serve as rails according to the present invention. The third rails  37  extend in the horizontal direction in parallel with the storage box  13   b . The first and second robot hands  21 ,  22  are respectively coupled to the corresponding pair of the third rails  37 . The first and second robot hands  21 ,  22  are thus allowed to move in the horizontal direction along the third rails  37  in parallel with the x-axis. The first and second robot hands  21 ,  22  are also allowed to rotate on the third rails  37  around a rotation axis parallel to a vertical axis or the y-axis. The second rail bases  36  and the robot hands  21 ,  22  will be described later in detail. 
     The magnetic tape library apparatus  11  utilizes the coordinates in the xyz-coordinate system and the angle around the rotation axis to identify the position of the individual cell  14 . The first and second robot hands  21 ,  22  in the first and second transporting robots  18 ,  19  are positioned in accordance with the coordinates of the xyz-coordinate system. The attitude or orientation of the first and second robot hands  21 ,  22  is determined in accordance with the angle of rotation around the vertical axis. The first controller board determines the position of the first robot hand  21  in accordance with the coordinates set for the individual cell  14 . The positioning action generates a change in the attitude of the first robot hand  21  as described later. Likewise, the second controller board determines the position of the second robot hand  22  in accordance with the coordinates set for the individual cell  14 . The positioning action generates a change in the attitude of the second robot hand  22 . The first and second robot hands  21 ,  22  are in this manner allowed to oppose the slot  23  to the opening of the select cell  14  with a higher accuracy. 
     As shown in  FIG. 3 , the first and second robot hands  21 ,  22  are positioned at positions corresponding or opposed to the cells  14 ,  14 , . . . within an operating region  41 . The first or second robot hand  21 ,  22  is allowed to oppose the slot  23  to a select one of the cells  14 ,  14 , . . . within the operating region  41 . Standby regions  42 ,  43  are provided for the first and second robot hands  21 ,  22  in the magnetic tape library apparatus  11 , respectively. The second robot hand  22  is related to the standby region  43  located adjacent to the upper limit of the operating region  41 . In this case, the second transporting robot  19  allows the first rail base  34  to take the highest position on the first rail  32 . When the second robot hand  22  is in this manner positioned within the standby position  43 , the second robot hand  22  gets out of the operating region  41  so that the first robot hand  21  can be positioned at a position corresponding to any of the cells  14  within the operating region  41 . The movement region of the second robot hand  22  thus extends within the operating region  41  and the standby region  43 . 
     Likewise, the first robot hand  21  is related to the standby region  42  located adjacent to the lower limit of the operating region  41 . In this case, the first transporting robot  18  allows the first rail base  28  to take the lowest position on the first rail  26 . When the first robot hand  21  is in this manner positioned within the standby region  42 , the first robot hand  21  gets out of the operating region  41  so that the second robot hand  22  can be positioned at a position corresponding to any of the cells  14  within the operating region  41 . The movement region of the first robot hand  21  thus extends within the operating region  41  and the standby region  42 . It should be noted that the first and second robot hands  21 ,  22  may collide against the storage boxes  13   a ,  13   b , when the first and second robot hands  21 ,  22  takes a specific combination of the coordinates and angle of rotation, since the storage boxes  13   a ,  13   b  are located so closer to each other in the magnetic tape library apparatus  11 . Such a combination of the coordinates and angle of rotation is excluded from the movement regions of the first and second robot hands  21 ,  22 , respectively. 
     The magnetic tape library apparatus  11  normally allows the first transporting robot  18  to operate in accordance with instructions from the library controller board. The first robot hand  21  transports the magnetic tape cartridge  15  between the cells  14 ,  14 , . . . and the magnetic tape drives  16 ,  16 , . . . When the first robot hand  21  malfunctions, for example, the library controller board causes the second transporting robot  19  to start operating. The guide member  33  and the first rail base  34  of the second transporting robot  19  are driven downward along the first rail  32 . In this case, the first rail base  28  and the guide member  27  of the first transporting robot  18  are disengaged from the driving mechanism on the first rail  26 . When the guide member  33  contacts with the guide member  27  of the first transporting robot  18 , for example, the driving force of the guide member  33  serves to urge the first rail base  28  and the guide member  27  downward along the first rail  26 . When the guide member  33  and the first rail base  34  of the second transporting robot  19  reach the lower limit of the operating region  41 , the first rail base  28  of the first transporting robot  18  is positioned at the lower limit of the movement region. The first robot hand  21  is in this manner forced out into the standby region  42 . The second robot hand  22  thereafter serves to transfer the magnetic tape cartridge  15  between the cells  14 ,  14 , . . . and the magnetic tape drives  16 ,  16 , . . . in place of the first robot hand  21 . As long as the second robot hand  22  moves within the operating region  41 , the second transporting robot  19  is reliably prevented from interference with the first transporting robot  18 . 
     Repair can be effected on the first transporting robot  18  during the operation of the second transporting robot  19 . The first robot hand  21  may be replaced with a new one in the first transporting robot  18 , for example. The new first transporting robot  18  may take the place of the second transporting robot  19  immediately after the replacement of the first robot hand  21 . Alternatively, the second transporting robot  19  may be allowed to keep operating even after the replacement of the first robot hand  21 . If the second robot hand  22  malfunctions during the operation of the second transporting robot  19 , the first transporting robot  18  serves to drive the second robot hand  22  into the standby region  43  in the same manner as described above. The first robot hand  21  then takes the place of the second robot hand  22 . As long as the first robot hand  21  moves within the operating region  41 , the first transporting robot  18  is reliably prevented from interference with the second transporting robot  19 . 
     As shown in  FIG. 4 , a screw shaft  45  is mounted on the second rail base  36 . The screw shaft  45  extends in parallel with the third rails  37 ,  37 . A power source such as an electric motor  46  is coupled to the screw shaft  45 . A stepping motor may be employed as the electric motor  46 , for example. The electric motor  46  is referred to as “x-axis electric motor” hereinafter, for example. A predetermined transmission mechanism  47  couples a driving shaft  46   a  of the x-axis electric motor  46  and the screw shaft  45  to each other. The transmission mechanism  47  includes a pulley unit and a gear unit for a speed reducing mechanism, for example. The gear unit includes a small-sized gear  48  having a small diameter mounted on the driving shaft  46   a  of the x-axis electric motor  46 . A large-sized gear  49  having a large diameter is engaged with the small-sized gear  48 . The torque of the x-axis electric motor  46  is transmitted to the large gear  49  through the small gear  48 . The large gear  49  is integrally provided with a small-sized pulley  51  of the pulley unit. A large-sized pulley  52  of the pulley unit is mounted on the screw shaft  45 . An endless timing belt  53  is wound around the small-sized pulley  51  and the large-sized pulley  52 . The torque of the large-sized gear  49  is transmitted to the screw shaft  45  through the small-sized pulley  51  and the large-sized pulley  52 . 
     The first robot hand  21  includes a box-shaped enclosure  55 . The aforementioned slot  23  is defined in the front surface of the enclosure  55 . A support member  56  is placed in the enclosure  55 . The support member  56  is designed to move in the longitudinal direction of the first robot hand  21  between a front position and a rear position along an imaginary plane or a horizontal plane. 
     A pair of grasping fingers  57  is mounted on the support member  56 . A predetermined space is defined between the grasping fingers  57  along the horizontal plane in the lateral direction perpendicular to the longitudinal direction. When the support member  56  reaches the front position, the grasping fingers  57  protrude from the slot  23 . The magnetic tape cartridge  15  is allowed to get in/out of the space between the grasping fingers  57 . When the support member  56  starts receding from the front position, the grasping fingers  57  gets closer to each other. The magnetic tape cartridge  15  is thus held between the grasping fingers  57 . When the support member  56  completely recedes and reaches the rear position, the magnetic tape cartridge  15  is swallowed into the enclosure  55 . The magnetic tape cartridge  15  is in this manner enclosed in the enclosure  55 . When the support member  56  again moves forward to the front position, the magnetic tape cartridge  15  is received in a select one of the cells  14  or the slot of a select one of the magnetic tape drives  16  from the grasping fingers  57 . 
     As shown in  FIG. 5 , a pedestal  58  is mounted on the second rail base  36 . The pedestal  58  is coupled to the third rails  37 ,  37 . The pedestal  58  moves along the third rails  37 ,  37 . A rotation shaft  59  is fixed to the pedestal  58 . The rotation shaft  59  extends in the vertical direction. The rotation shaft  59  is allowed to move on a horizontal plane along a straight movement path  61  in this manner. 
     A rotating member  62  is mounted on the rotation shaft  59  for relative rotation around the longitudinal axis of the rotation shaft  59 . The first robot hand  21  is fixed to the rotating member  62 . The first robot hand  21  is in this manner related to the third rails  37  for relative rotation around the longitudinal axis of the rotation shaft  59 . 
     The pedestal  58  is engaged with the screw shaft  45 . A nut member, not shown, is fixed to the pedestal  58  for the engagement, for example. The rotation of the screw shaft  45  around its longitudinal axis allows the movement of the pedestal  58 . The rotation shaft  59  is designed to move between a first terminal position  63   a  and a second terminal position  63   b . The movement of the rotation shaft  59  is restrained between the first and second terminal positions  63   a ,  63   b . The rotation shaft  59  is positioned between the first and second terminal positions  63   a ,  63   b  in parallel with the x-axis of the magnetic tape library  11  depending on the rotation angle or amount of the screw shaft  45 . 
     A first cam plate  64  is mounted on the second rail base  36 . The first cam plate  64  extends along a plane perpendicular to the rotation shaft  59 , namely a horizontal plane. A cam groove  65  is defined in the first cam plate  64 . A detailed description will be made on the cam groove  65  later. Likewise, a second cam plate  66  is mounted on the second rail base  36 . The second cam plate  66  extends along a plane perpendicular to the rotation shaft  59 , namely a horizontal plane. A detailed description will be made on the second cam plate  66  later. 
     First and second contact pins  67 ,  68  are attached to the first robot hand  21 . The first and second contact pins  67 ,  68  are individually made of a columnar pin extending in parallel with the rotation shaft  59 , for example. The first contact pin  67  is received in the cam groove  65  in the first cam plate  64 . The second contact pin  68  is opposed to the edge of the second cam plate  66 . The longitudinal axis of the first contact pin  67  extends in parallel with the longitudinal axis of the rotation shaft  59 . The longitudinal axis of the second contact pin  68  extends in parallel with the longitudinal axis of the rotation shaft  59 . Imaginary vertical planes  69 ,  70  define the central angle of 180 degrees around the longitudinal axis of the rotation shaft  59 . The imaginary vertical plane  69  is defined to extend from the longitudinal axis of the rotation shaft  59  to the longitudinal axis of the first contact pin  67 . The imaginary vertical plane  70  is likewise defined to extend from the longitudinal axis of the rotation shaft  59  to the longitudinal axis of the second contact pin  68 . Specifically, the longitudinal axis of the second contact pin  68  is set to stand on a straight line extending within a horizontal plane from the longitudinal axis of the rotation shaft  59  in the direction opposite to a straight line extending within the horizontal plane from the longitudinal axis of the rotation shaft  59  to the longitudinal axis of the first contact pin  67 . The first and second cam plates  64 ,  66  serve to interconnect the linear movement of the first robot hand  21  with the rotary movement of the first robot hand  21  as described later. 
     The first cam plate  64  is symmetric with respect to a predetermined vertical symmetry plane  71 . The vertical symmetry plane  71  is set perpendicular to the third rails  37 . When the longitudinal axis of the rotation shaft  59  is positioned within the vertical symmetry plane  71 , a reference position  72  of the rotation shaft  59  is established. The longitudinal axis of the first contact pin  67  is also positioned within the vertical symmetry plane  71 . The first contact pin  67  is received in a reference groove  73  defined in the cam groove  65 . The longitudinal axis of the second contact pin  68  is also positioned within the vertical symmetry plane  71 . The second contact pin  68  is received in a first recess  74  formed in the second cam plate  66 . The first recess  74  is placed on a straight line extending within a horizontal plane from the longitudinal axis of the rotation shaft  59  in the direction opposite to a straight line extending within the horizontal plane from the longitudinal axis of the rotation shaft  59  to the reference groove  73 . The first recess  74  is defined between a pair of contact surfaces  75  extending upright to a horizontal plane in an opposed relation. The contact surfaces  75  are placed on a straight line extending within a horizontal plane across the longitudinal axis of the second contact pin  68  in parallel with the third rails  37  when the longitudinal axis of the second contact pin  68  is positioned within the vertical symmetry plane  71 . The first robot hand  21  is set in a reverse attitude to face the second storage box  13   b.    
     The first terminal position  63   a  is set symmetric to the second terminal position  63   b  with respect to the vertical symmetry plane  71 . In other words, the distance between the first terminal position  63   a  and the reference position  72  or the vertical symmetry plane  71  is set equal to the distance between the second terminal position  63   b  and the vertical symmetry plane  71 . When the longitudinal axis of the rotation shaft  59  is positioned at the first terminal position  63   a , the first contact pin  67  is received in a first terminal groove  76   a , as shown in  FIG. 6 , for example. The longitudinal axis of the first contact pin  67  in the first terminal groove  76   a  and the longitudinal axis of the rotation shaft  59  are positioned within an imaginary plane  77  set perpendicular to the third rails  37 . Specifically, the first terminal groove  76   a  receives the first contact pin  67  on a perpendicular line extending within a horizontal plane in a direction perpendicular to the straight movement path  61  when the rotation shaft  59  is positioned at the first terminal position  63   a . A second terminal groove  76   b  is set symmetric to the first terminal groove  76   a  with respect to the vertical symmetry plane  71 . When the rotation shaft  59  is positioned at either the first terminal position  63   a  or the second terminal position  63   b , the first robot hand  21  is set in the reverse attitude to face the second storage box  13   b . In this case, the second contact pin  68  is received in a second recess  78  formed in a second cam plate  66 . The second contact pin  68  is received on the edge of the second cam plate  66  at the outer end of the second recess  78 . 
     As shown in  FIG. 7 , a first hooked groove  79   a  is defined in the cam groove  65  at a position on an extension of the straight movement path  61  outward of the first terminal position  63   a . The first hooked groove  79   a  receives the first contact pin  67  when the rotation shaft  59  is positioned at a first standstill position  80   a  between the reference position  72  and the first terminal position  63   a . Accordingly, when the rotation shaft  59  is positioned at the first standstill position  80   a , the first robot hand  21  takes an attitude rotating around the rotation shaft  59  by the rotation angle equal to 90 degrees from the reverse attitude. The first robot hand  21  is thus set in a transverse (outward) attitude to face the first storage box  13   a . A second standstill position  80   b  is defined between the reference position  72  and the second terminal position  63   b . The second standstill position  80   b  is set symmetric to the first standstill position  80   a  with respect to the vertical symmetry plane  71 . The second hooked groove  79   b  is likewise set symmetric to the first hooked groove  79   a  with respect to the vertical symmetry plane  71 . The second contact pin  68  is completely separated from the second cam  66 . 
     As is apparent from  FIG. 7 , the reference groove  73  and the first hooked groove  79   a  are connected to each other through a first guide groove  81   a . The reference groove  73  and the second hooked groove  79   b  are connected to each other through a second guide groove  81   b . The first hooked groove  79   a  and the first terminal groove  76   a  are connected to each other through a third guide groove  82   a . The second hooked groove  79   b  and the second terminal groove  76   b  are connected to each other through a fourth guide groove  82   b.    
     As shown in  FIG. 8 , the first guide groove  81   a  serves to define a first guide wall  83 . The first guide wall  83  stays outward of a tangent line  84  tangent to a circle, having the center at the longitudinal axis of the rotation shaft  59  and inscribing the first guide pin  67 , when the rotation shaft  59  moves from the reference position  72  to the first standstill position  80   a . The tangent line  84  extends from the first contact pin  67  in the direction of the advancement of the first contact pin  67 . When the rotation shaft  59  moves along the straight movement path  61  from the reference position  72  to the first standstill position  80   a , the first contact pin  67  receives a pushing force  85  from the rotation shaft  59  within a plane including the longitudinal axes of the rotation shaft  59  and the first contact pin  67 . The pushing force  85  generates a driving force  86  for the first contact pin  67  along the first guide wall  83 . The first contact pin  67  is thus allowed to move in the first guide groove  81   a  in response to the movement of the rotation shaft  59 . Since the first contact pin  67  gradually rotates around the rotation shaft  59 , the first robot hand  21  is allowed to enjoy a change in the attitude around the rotation shaft  59 . The second guide groove  81   b  is set symmetric to the first guide groove  81   a  with respect to the vertical symmetry plane  71 , the second guide groove  81   b  likewise defines the first guide wall  83 , as shown in  FIG. 9 . 
     As shown in  FIG. 10 , the first guide groove  81   a  serves to define a second guide wall  87 . The second guide wall  87  stays inward of a tangent line  88  tangent to a circle, having the center at the longitudinal axis of the rotation shaft  59  and circumscribing the first contact pin  67 , when the rotation shaft  59  moves from the first standstill position  80   a  to the reference position  72 . The tangent line  88  extends from the first contact pin  67  in the direction of the advancement of the first contact pin  67 . When the rotation shaft  59  moves along the straight movement path  61  from the first standstill position  80   a  to the reference position  72 , the first contact pin  67  receives a pulling force  89  from the rotation shaft  59  within a plane including the longitudinal axes of the rotation shaft  59  and the first contact pin  67 . The pulling force  85  generates a driving force  91  for the first contact pin  67  along the second guide wall  87 . The first contact pin  67  is thus allowed to move in the first guide groove  81   a  in response to the movement of the rotation shaft  59 . Since the first contact pin  67  gradually rotates around the rotation shaft  59 , the first robot hand  21  is allowed to enjoy a change in the attitude around the rotation shaft  59 . The second guide groove  81   b  is set symmetric to the first guide groove  81   a  with respect to the vertical symmetry plane  71 , the second guide groove  81   b  likewise defines the second guide wall  87 , as shown in  FIG. 11 . 
     As shown in  FIG. 12 , a third guide groove  82   a  serves to define the third guide wall  92 . The third guide wall  92  stays outward of a tangent line  93  tangent to a circle, having the center at the longitudinal axis of the rotation shaft  59  and inscribing the first contact pin  67 , when the rotation shaft  59  moves from the first standstill position  80   a  to the first terminal position  63   a . The tangent line  93  extends from the first contact pin  67  in the direction of the advancement of the first contact pin  67 . When the rotation shaft  59  moves along the straight movement path  61  from the first standstill position  80   a  to the first terminal position  63   a , the first contact pin  67  receives a pushing force  94  from the rotation shaft  59  within a plane including the longitudinal axes of the rotation shaft  59  and the first contact pin  67 . The pushing force  94  generates a driving force  95  for the first contact pin  67  along the third guide wall  92 . The first contact pin  67  is thus allowed to move in the third guide groove  82   a  in response to the movement of the rotation shaft  59 . Since the first contact pin  67  gradually rotates around the rotation shaft  59 , the first robot hand  21  is allowed to enjoy a change in the attitude around the rotation shaft  59 . The fourth guide groove  82   b  is set symmetric to the third guide groove  82   a  with respect to the vertical symmetry plane  71 , the fourth guide groove  82   b  likewise defines the third guide wall  92 , as shown in  FIG. 13 . 
     As shown in  FIG. 14 , the third guide groove  82   a  serves to define a fourth guide wall  97 . The fourth guidewall  97  stays inward of a tangent line  98  tangent to a circle, having the center at the longitudinal axis of the rotation shaft  59  and circumscribing the first contact pin  67 , when the rotation shaft  59  moves from the first terminal position  63   a  to the first standstill position  80   a . The tangent line  98  extends from the first contact pin  67  in the direction of the advancement of the first contact pin  67 . When the rotation shaft  59  moves along the straight movement path  61  from the first terminal position  63   a  to the first standstill position  80   a , the first contact pin  67  receives a pulling force  99  from the rotation shaft  59  within a plane including the longitudinal axes of the rotation shaft  59  and the first contact pin  67 . The pulling force  99  generates a driving force  101  for the first contact pin  67  along the fourth guide wall  97 . The first contact pin  67  is thus allowed to move in the third guide groove  82   a  in response to the movement of the rotation shaft  59 . Since the first contact pin  67  gradually rotates around the rotation shaft  59 , the first robot hand  21  is allowed to enjoy a change in the attitude around the rotation shaft  59 . Since the fourth guide groove  82   b  is set symmetric to the third guide groove  82   a  with respect to the vertical symmetry plane  71 , the fourth guide groove  82   b  likewise defines the fourth guide wall  97 , as shown in  FIG. 15 . 
     Now, assume that the magnetic tape cartridge  15  is given and received between the first robot hand  21  and a specific one of the cells  14  of the first storage box  13   a . When the rotation shaft  59  moves from the reference position  72  to the first standstill position  80   a , for example, the second contact pin  68  is brought in contact with the edge of the second cam plate  66  within the recess  74 , namely the contact surface  75 , as shown in  FIG. 16 . The contact surface  75  receives the second contact pin  68  in parallel with the straight movement path  61 . The movement of the second contact pin  68  is restrained within the first recess  74 . The continued movement of the rotation shaft  59  thus serves to generate a driving force for the first robot hand  21  around the rotation shaft  59 . The driving force induces an advancement of the first contact pin  67  to the first hooked groove  79   a  along the first guide groove  81   a . The first contact pin  67  precedes an imaginary plane, including the longitudinal axis of the rotation shaft  59  and set perpendicular to the straight movement path  61 , toward the first hooked groove  79   a . The rotation shaft  59  thereafter serves to apply the pushing force  85  to the first contact pin  67  against the first guide wall  83  as mentioned above. The generated driving force  86  acts on the first contact pin  67  in response to the movement of the rotation shaft  59 . Here, the second contact pin  68  and the contact surface  75  serve as a first driving mechanism according to the present invention. 
     When the rotation shaft  59  reaches the first standstill position  80   a , the first contact pin  67  is received in the first hooked groove  79   a . The first robot hand  21  is set in a transverse (outward) attitude to face the first storage box  13   a , as shown in  FIG. 17 . The movement of the second rail base  36  on the first rail base  28  allows the first robot hand  21  to reach a specific position in the horizontal direction relative to the first storage box  13   a . The movement of the first rail base  28  along the first rail  26  allows the first robot hand  21  to reach a specific level in the vertical direction relative to the first storage box  13   a . The first robot hand  21  in this manner opposes the slot  23  to the specific one of the cells  14  of the first storage box  13   a . The magnetic tape cartridge  15  is given and received between the first robot hand  21  and the specific cell  14  based on the action of the grasping fingers  57 . 
     When the rotation shaft  59  moves from the reference position  72  to the second standstill position  80   b , the movement of the second contact pin  68  is restrained within the first recess  74  in the same manner as described above. The first robot hand  21  is allowed to enjoy a driving force around the rotation shaft  59  in the direction opposite to the aforementioned one. The driving force enables an advancement of the first contact pin  67  toward the second hooked groove  79   b  along the second guide groove  81   b . The first guide wall  83  serves to generate the driving force  86  for the first contact pin  67  in response to the movement of the rotation shaft  59 . When the rotation shaft  59  reaches the second standstill position  80   b , the first contact pin  67  is received in the second hooked groove  79   b . The first robot hand  21  is set in a transverse (outward) attitude to face the first storage box  13   a . The magnetic tape cartridge  15  is given and received between the first robot hand  21  and the specific cell  14  based on the action of the grasping fingers  57 . Here, the second contact pin  68  and the contact surface  75  serve as a second driving mechanism according to the present invention. 
     Next, assume that the magnetic tape cartridge  15  is given and received between the first robot hand  21  and the magnetic tape drive  16 . When the rotation shaft  59  moves from the first standstill position  80   a  to the first terminal position  63   a , for example, the rotation shaft  59  serves to apply the pushing force  94  to the first contact pin  67  against the third guide wall  92  as mentioned above. The first contact pin  67  is allowed to receive the driving force  95  in response to the movement of the rotation shaft  59 . When the rotation shaft  59  reaches the first terminal position  63   a , the first contact pin  67  is received in the first terminal groove  76   a . The first robot hand  21  is set in the reverse attitude to face the second storage box  13   b , as shown in  FIG. 18 . The movement of the second rail base  36  on the first rail base  28  allows the first robot hand  21  to get closest to the second storage box  13   b . The movement of the first rail base  28  along the first rail  26  allows the first robot hand  21  to reach a specific level in the vertical direction for the magnetic tape drive  16 . The first robot hand  21  in this manner opposes the slot  23  to the slot of a specific one of the magnetic tape drives  16 . The magnetic tape cartridge  15  is given and received between the first robot hand  21  and the magnetic tape drive  16  based on the action of the grasping fingers  57 . 
     When the rotation shaft  59  reaches the second terminal position  63   b , the first contact pin  67  is received in the second terminal groove  76   b . The first robot hand  21  is set in the reverse attitude to face the second storage box  13   b  in the same manner as described above. The first robot hand  21  is in this manner allowed to oppose the slot  23  to the slot of a specific one of the magnetic tape drives  16 . The magnetic tape cartridge  15  is given and received between the first robot hand  21  and the magnetic tape drive  16  based on the action of the grasping fingers  57 . 
     It should be noted that the second robot hand  22  and second rail base  36  may have the structure identical to that of the aforementioned first robot hand  21  and second rail base  36 .