Abstract:
A centering device for a plate-shaped workpiece comprises: a table ( 21 ) having a conveyor mechanism ( 23 ) for conveying a plate-shaped workpiece ( 10 ) in the horizontal direction; a camera ( 56 ) for taking an image of the plate-shaped workpiece ( 10 ) placed on the table ( 21 ); a table movement mechanism ( 28 ) for moving the table ( 21 ) horizontally in the direction orthogonal to the conveyance direction; a table rotation mechanism ( 60 ) for rotating the table ( 21 ) about a vertical axis; a computation unit ( 58 ) for comparing target central position information and information acquired by the camera ( 56 ) and computing the movement amount of the conveyor mechanism ( 23 ), the movement amount of the table movement mechanism ( 28 ), and the movement amount of the table rotation mechanism ( 60 ); and a control unit ( 59 ) for controlling the conveyor mechanism ( 23 ), the table movement mechanism ( 28 ), and the table rotation mechanism ( 60 ) on the basis of the movement amounts calculated by the computation unit ( 58 ).

Description:
TECHNICAL FIELD 
     The present invention relates to a centering device for positioning a plate-shaped workpiece in a target centering position for a subsequent step. 
     BACKGROUND ART 
     Steps for obtaining a press-molded article using a band plate as a starting material are performed in the following sequence. 
     A blank is cut from a band plate by a blanking press or another device. The blank is conveyed near to a molding press machine, loaded into the molding press machine by a robot, and press-molded by the molding press machine. 
     The molding press machine is provided with a molding die. The blank must be placed properly in the molding die. However, the blank may not always be placed in the desired position. As a countermeasure, a centering device is installed to carry out the positioning of the blank. 
     A variety of centering devices have been proposed (e.g., see Patent Literature 1). 
     The centering device disclosed in Patent Literature 1 uses implements to grasp blanks. When the blanks have different shapes, numerous implements must be prepared. Manufacturing the implements then incurs high costs, and space for storing the implements is needed. Scratches may also be left in the blanks. When the implements are moved more quickly to increase the conveying speed, the scratches become more prominent. 
     Implements are replaced with each different blank shape, but the molding press machine is stopped during this replacement. The rate of operation of the molding press machine decreases. 
     Furthermore, when an attempt is made to simultaneously convey two blanks, controlling the implements is becomes complicated, and as a result it is difficult to convey the two blanks. 
     It is possible for the blanks to be transported and correctly positioned by a robot without the use of implements, but in this case, a single robot takes too much time and is too slow for the cycle time. When two robots are used in order to make the process fast enough, interference between the robots becomes a problem, control becomes complicated, and more space is needed to install the robots. 
     Specifically, the centering device of Patent Literature 1 presents problems in that it is difficult to increase the speed, the rate of operation is low, and the device cannot be adapted to conveying a plurality of workpieces. 
     PRIOR ART LITERATURE 
     Patent Literature 
     Patent Literature 1: Japanese Application Laid-Open Publication No. H07-001059 
     SUMMARY OF INVENTION 
     Technical Problem 
     The present invention addresses the problem of providing a centering device in which the speed can be increased, the rate of operation can be increased, and a plurality of workpieces can be conveyed. 
     Solution to Problem 
     According to a first aspect of the present invention, there is provided a centering device for a plate-shaped workpiece, which centering device comprises: a table having a conveyor mechanism for conveying a plate-shaped workpiece in the horizontal direction; a camera for capturing an image of the plate-shaped workpiece placed on the table; a table movement mechanism for moving the table horizontally in a direction orthogonal to the conveying direction; a table rotation mechanism for rotating the table about a vertical axis; a computation unit for comparing target centering position information and position information acquired by the camera, and computing the amount of movement of the conveyor mechanism, the amount of movement of the table movement mechanism, and the amount of movement of the table rotation mechanism; and a control unit for controlling the conveyor mechanism, the table movement mechanism, and the table rotation mechanism on the basis of the movement amounts calculated by the computation unit. 
     Preferably, the table movement mechanism includes a first movement mechanism and a second movement mechanism arranged parallel to each other. 
     In a preferred form, the computation unit perceives the characteristics of the plate-shaped workpiece from the position information acquired by the camera, and computes the amount of movement of the conveyor mechanism, the amount of movement of the table movement mechanism, and the amount of movement of the table rotation mechanism. 
     It is preferred that a plurality of the centering devices for a plate-shaped workpiece of the third aspect be provided in series. 
     The plate-shaped workpiece may be a long piece of sufficient length to span over a plurality of tables. 
     One plate-shaped workpiece may be placed on each of the tables. 
     Preferably, the plate-shaped workpiece is an irregularly shaped piece. 
     According to a second aspect of the present invention, there is provided a centering device for a plate-shaped workpiece, which centering device comprises: a table having a conveyor mechanism for conveying a plate-shaped workpiece in the horizontal direction; a camera for capturing an image of the plate-shaped workpiece placed on the table; a table movement mechanism for moving the table horizontally in the direction orthogonal to the conveying direction; a table rotation mechanism for rotating the table about a vertical axis; a computation unit for comparing target centering position information and position information acquired by the camera, and computing the amount of movement of the conveyor mechanism, the amount of movement of the table movement mechanism, and the amount of movement of the table rotation mechanism; and a control unit for controlling the conveyor mechanism, the table movement mechanism, and the table rotation mechanism on the basis of the movement amounts calculated by the computation unit; wherein the centering device for a plate-shaped workpiece is also provided with a first robot for transferring a plate-shaped workpiece conveyed from a previous step into a subsequent step, and a second robot for transferring the movement-controlled plate-shaped workpiece to a subsequent step; the table movement mechanism includes a first movement mechanism and a second movement mechanism arranged parallel to each other; and the computation unit perceives the characteristics of the plate-shaped workpiece from the position information acquired by the camera, and computing the amount of movement of the conveyor mechanism, the amount of movement of the table movement mechanism, and the amount of movement of the table rotation mechanism. 
     Advantageous Effects of Invention 
     In the first aspect of the present invention, the plate-shaped workpiece is conveyed in the horizontal direction by the conveyor mechanism. The workpiece is conveyed together with the table in a direction orthogonal to the conveying direction by the table movement mechanism. Furthermore, the workpiece is turned together with the table around a vertical axis by the table rotation mechanism. The plate-shaped workpiece is positioned by the process described above. The position of the workpiece is adjusted and the workpiece is positioned on the table. With a table, the speed can be increased more easily and the rate of operation can be increased to a greater extent than with implements or robots. 
     In the invention, the table can be rotated by the first movement mechanism and the second movement mechanism. Specifically, the table movement mechanism is used as a rotation mechanism as well, whereby the table rotation mechanism can be omitted. The funding cost of the centering device can be lowered because an expensive table rotation mechanism is not needed. 
     In the invention, the computation unit perceives the characteristics (e.g., the edges, holes, etc.) of the plate-shaped workpiece from the position information acquired by the camera. Because there are few elements for which computations are performed, the computations are simple, the load on the computation unit is lessened, and the computation time can be shortened. 
     In the invention, a plurality of centering devices for a plate-shaped workpiece is provided in series, and it is therefore possible to center a long piece capable of spanning over a plurality of tables. Because a plurality of centering devices for a plate-shaped workpiece is provided in series, one plate-shaped workpiece can be centered on each table. As a result, the centering device can be used for a greater range of purposes. 
     In the invention, the work of centering a long piece that exceeds the length of each of the tables can be performed. 
     In the invention, one plate-shaped workpiece is placed on each of the plurality of tables, and a plurality of plate-shaped workpieces can be simultaneously centered. 
     In the invention, the plate-shaped workpiece is irregularly shaped. The invention is not limited to rectangular plate-shaped workpieces; pieces of non-rectangular, irregular shapes can be centered. 
     In the second aspect of the invention, the plate-shaped workpiece is conveyed in the horizontal direction by the conveyor mechanism. The workpiece is conveyed together with the table in a direction orthogonal to the conveying direction by the table movement mechanism. Furthermore, the workpiece is turned together with the table around a vertical axis by the table rotation mechanism. The plate-shaped workpiece is positioned by the process described above. The position of the workpiece is adjusted and the workpiece is positioned on the table. With a table, the speed can be increased more easily and the rate of operation can be increased to a greater extent than with implements or robots. Additionally, the computation unit perceives the characteristics (e.g., the edges, holes, etc.) of the plate-shaped workpiece from the position information acquired by the camera. Because there are few elements for which computations are performed, the computations are simple, the load on the computation unit is lessened, and the computation time can be shortened. The present invention provides a centering device in which speed can be increased and the rate of operation can be easily increased. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a top plan view of a molding press line, including a centering device according to the present invention, with one plate-shaped workpiece placed thereon; 
         FIG. 2  is a top plan view of a molding press line including the centering device according to the present invention, with two plate-shaped workpieces placed thereon; 
         FIG. 3  is a top plan view of a molding press line including the centering device according to the present invention, with two plate-shaped workpieces of irregular shapes placed thereon; 
         FIG. 4  is an exploded view of the centering device; 
         FIG. 5  is a front view of the centering device; 
         FIG. 6  is a view illustrating an operation of the centering device; 
         FIG. 7  is a graph illustrating peak coordinates and shift angles of the plate-shaped workpiece; 
         FIG. 8  is a supplemental view for deriving a computation formula; 
         FIG. 9  is a supplemental view for deriving a computation formula; 
         FIG. 10  is a view illustrating centering performed on a long workpiece; 
         FIG. 11  is a view illustrating a separate embodiment of a centering device; and 
         FIG. 12  illustrates an operation of the centering device according to the separate embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Certain preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings. 
     EMBODIMENTS 
     As shown in  FIG. 1 , one plate-shaped workpiece  10  that has been conveyed in from a previous step is placed on a receiving tray  11 . A first robot  12  transfers the plate-shaped workpiece  10  from the receiving tray  11  to an entry table  13 . The plate-shaped workpiece  10  is loaded from the entry table  13  onto a washing device  14  and is washed. The washed plate-shaped workpiece  10  is then conveyed to a first centering device  20 A and a second centering device  20 B. 
     In this case, the plate-shaped workpiece  10  may be a long piece so as to span over both the first centering device  20 A and the second centering device  20 B. 
     The plate-shaped workpieces  10  are hereinafter distinguished by appending the symbol  10 B to a plate-shaped workpiece that has been washed but not yet centered, and appending the symbol  10 C to a plate-shaped workpiece that has been centered. 
     The first centering device  20 A and the second centering device  20 B have the same configuration, and are therefore simply denoted as the centering device  20  when there is no need to make a distinction. 
     When a non-rectangular plate-shaped workpiece passes through the orthogonal washing roll, the workpiece readily becomes slanted due to the effects of frictional force, high-pressure washing liquid, and the like. 
     As shown by the solid lines, the plate-shaped workpiece  10 B transferred from the washing device  14  to the centering device  20  is slanted. 
     The role of the centering device  20  is to center the plate-shaped workpiece  10 B as shown by the imaginary lines, after the workpiece has been slanted by several causes. 
     The centered plate-shaped workpiece  10 C is loaded onto a molding press device  16  by a second robot  15 , and mold-pressing is performed. 
     It is not impossible for a slanted plate-shaped workpiece  10 B to be loaded onto the molding press device  16  while being centered by the second robot  15 . However, the act of centering the plate-shaped workpiece using robots must be carried out by the robots in at least two steps (one step for position adjustment, one step for parallel movement). As a result, the operating time of the second robot  15  is longer, and the cycle time of the pressing step is affected. Productivity decreases when the cycle time is longer. 
     In this respect, the present invention has a shorter operating time for the second robot  15  because the robot merely moves the centered plate-shaped workpiece  10 C in parallel. As a result, the cycle time can be reduced and productivity can be increased. 
     Because the centering device  20  of the present invention is composed of the first centering device  20 A and the second centering device  20 B, it is also possible to center two plate-shaped workpieces  10 ,  10 , as shown in  FIG. 2 . The same symbols from  FIG. 1  are used, and no detailed description is given. 
     In this case, each plate-shaped workpiece  10  has a length (size) suitable to be accommodated on the first centering device  20 A (or the second centering device  20 B). 
     Furthermore, the centering device  20  of the present invention can also center non-rectangular plate-shaped workpieces  10 ,  10 , e.g., trapezoidal workpieces  10 ,  10 . A non-rectangular plate-shaped workpiece  10  may be a single long sheet-shaped object. The same symbols from  FIG. 1  are used, and a detailed description is omitted. 
     As shown in  FIG. 4 , the centering device  20  is composed of a table  21  and a table support section  22  for supporting the table  21 . 
     The table  21  is composed of a long conveyor mechanism  23  extending along an x-axis, and free rolls  24  arranged on both sides of the conveyor mechanism  23 . The main elements of the free rolls  24  are flat rollers. Flat rollers are much less expensive round rollers, and can therefore lower the manufacturing cost of the table  21 . 
     The conveyor mechanism  23  may be any type of conveyor as long as long as it moves back and forth along the x-axis, but is preferably a rubber belt conveyor that does not scuff the bottom surface of the plate-shaped workpiece. Because the type of conveyor does not matter, the entire surface of the table  21  may be made into a belt conveyor. 
     The main element of the table support section  22  is a table movement mechanism  28  that moves horizontally while supporting the table  21 . In this example, the table movement mechanism  28  is composed of a first movement mechanism  30  and a second movement mechanism  40 . 
     The first movement mechanism  30  is composed of two rails  31 ,  31  arranged along a y-axis on a base  26 , a first slider  32  moveably mounted on the rails  31 ,  31 , a first threaded shaft  33  that extends along the y-axis and moves the first slider  32 , and a first servo motor  34  that rotates the first threaded shaft  33 . 
     The first threaded shaft  33  is preferably a ball screw. A ball screw has very little friction loss and enables the first servo motor  34  to be reduced in size. 
     The second movement mechanism  40  is composed of two rails  41 ,  41  arranged along the y-axis on the base  26 , a second slider  42  moveably mounted on the rails  41 ,  41 , a second threaded shaft  43  that extends along the y-axis and moves the second slider  42 , a second servo motor  44  that rotates the second threaded shaft  43 , sub rails  45 ,  45  provided on the second slider  42  and extending along the x-axis, and a sub slider  46  moveably mounted on the sub rails  45 ,  45 . 
     As shown in  FIG. 5 , the conveyor mechanism  23  is composed of, for example, pulleys  52 ,  53  rotatably mounted on a table frame  51 , a conveyor motor  54  for rotating one pulley  52 , and a rubber belt  55  wound on the pulleys  52 ,  53 . 
     A first support shaft  35  extends upward from the first slider  32  which moves in a direction orthogonal to the image plane, the first support shaft  35  supporting the table frame  51 . 
     A second support shaft  47  extends upward from the sub slider  46  which moves left and right in the drawing, the second support shaft  47  supporting the table frame  51 . The first support shaft  35  and the second support shaft  47  are rotatably fitted into the table frame  51 . 
     A camera  56  is arranged above the conveyor mechanism  23 . The camera  56  fulfills the role of capturing an image of the entire table ( FIG. 4 , symbol  21 ). The camera  56  is preferably a CCD camera, but may be of any type as long as the camera can output image information as electronic information. 
     A computation unit  58  and a control unit  59  are connected to the camera  56 . 
     As shown in  FIG. 6( a ) , the plate-shaped workpiece  10 B shown by the solid lines can be moved a distance Xx along the x-axis to the position of the imaginary lines by the rubber belt  55  being moved by the conveyor motor  54 . This distance Xx is equivalent to the amount of movement of the conveyor mechanism  23  needed to move the plate-shaped workpiece  10 B. Xx is abbreviated below to the amount of movement of the conveyor mechanism. 
     As shown in  FIG. 6( b ) , the table  21  can be moved by a distance Yy along the y-axis to the position of the imaginary lines by the first threaded shaft  33  and the second threaded shaft  43  being turned synchronously by the first servo motor  34  and the second servo motor  44 . This distance Yy is equivalent to the amount of movement of the table movement mechanism  28  need to move the plate-shaped workpiece  10 B. Yy is abbreviated below to the amount of movement of the table movement mechanism. 
     As shown in  FIG. 6( c ) , the first threaded shaft  33  is stopped and only the second threaded shaft  43  is turned. The table  21  is then rotated (pivoted) by an angle α about the first support shaft  35  to the position of the imaginary lines. Therefore, the table rotation mechanism  60  is configured from the first movement mechanism  30  and the second movement mechanism  40 . The angle α is equivalent to the amount of movement of the table rotation mechanism  60  needed to rotate the plate-shaped workpiece  10 B. α (the θ described hereinafter) is abbreviated below to the amount of movement of the table rotation mechanism. 
     Because the table rotation mechanism  60  is included in the table movement mechanism  28  in this invention, there is no need to install special equipment for the table rotation mechanism  60 . 
     Conversely, the action of the table movement mechanism  28  is complicated because the table is pivoted not with the use of a rotation mechanism, but with the use of a linear motion mechanism. The operating principle of the centering device  20  of this structure is described in geometrical terms. 
     For the sake of convenience in the description,  FIG. 7  shows the first threaded shaft  33  overlaid on the y-axis, and the second threaded shaft  43  to the right thereof. The rubber belt  55  is overlaid on the x-axis. 
     As shown in  FIG. 7 , an image is captured of the plate-shaped workpiece  10 C centered in the target centering position on the x-axis. The plate-shaped workpiece  10 B which has not yet been centered is in the first quadrant, and the camera recognizes the image of the plate-shaped workpiece  10 B. The plate-shaped workpiece  10 B is a non-rectangular plate, having a substantially trapezoidal shape. 
     The top left peak coordinates of the plate-shaped workpiece  10 C in the target centering position are denoted as (x0, y0). The shape and dimension of each of the plate-shaped workpieces  10 C is managed by a computer. This information is inputted in advance from the computer into a separate computation unit ( FIG. 5 , symbol  58 ). Specifically, the computation unit has the peak coordinates (x0, y0) in advance. 
     The top left peak coordinates of the plate-shaped workpiece  10 B, of which an image is taken by the camera, are denoted as (xs, ys). These coordinates (xs, ys) are specified by the computation unit ( FIG. 5 , symbol  58 ) by analyzing the camera image. Additionally, with the plate-shaped workpiece  10 C as a reference, the computation unit detects from the taken image that the plate-shaped workpiece  10 B is shifted counterclockwise by an angle θ. 
     The coordinates (xs, ys) are moved along the x-axis to the coordinates (x1, y1) as indicated by the arrow ( 1 ). This movement is carried out as shown in  FIG. 6( a ) . 
     The coordinates (x1, y1) are turned clockwise by an angle θ about the point of origin (0, 0) as indicated by the arrow ( 2 ). The coordinates after this rotation are (x2, y2). This rotation is carried out as shown in  FIG. 6( c ) . 
     The coordinates (x2, y2) are moved along the y-axis to the coordinates (x0, y0) as indicated by the arrow ( 3 ). This movement is carried out as shown in  FIG. 6( b ) . 
     In  FIG. 7 , the distance L is the gap between the first threaded shaft  33  and the second threaded shaft  43 , and is known. The coordinates (x0, y0), the coordinates (xs, ys), and the angle θ are also known. 
     In view of this, the following is an examination of how the amount of movement of the conveyor mechanism, the amount of movement of the first movement mechanism, and the amount of movement of the second movement mechanism are established using the known values θ, x0, y0, xs, and ys.
 
[Eq. 1]
         Section of Arrow ( 1 ):
 
movement amount of conveyor mechanism= x   1   −x   s   (1)
 
 y   1   =y   s   (2)
       

     In the section of the arrow ( 2 ) in  FIG. 7 , turning only the second threaded shaft  43  causes the table to pivot. 
     As shown in  FIG. 8 , the second threaded shaft  43  is set apart from the point of origin by a distance L. The table can be pivoted clockwise an angle θ by the second movement mechanism, and the amount of movement of the second movement mechanism at this time is determined geometrically.
 
[Eq. 2]
 
movement amount of second movement mechanism only=− L  tan θ  (3)
 
     As shown in  FIG. 9 , the coordinates (x1, y1) and the coordinates (x2, y2) are points on the radius R, and therefore can be computed as follows. 
     
       
         
           
             
               
                 
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     In the section of the arrow ( 3 ) in  FIG. 7 , the movement is a simple movement along the y-axis.
 
[Eq. 4]
 
 x   2   =x   0   (6)
 
movement amount of first movement mechanism= y   0   −y   2   (7)
 
movement amount of second movement mechanism=movement amount of first movement mechanism  (8)
 
[Eq. 5]
 
From (4),  x   1   =x   2  cos θ− y   2  sin θ
 
From (6),  x   2   =x   0  
 
 x   1   =x   0  cos θ− y   2  sin θ  (9)
 
From (5),  y   1   =x   2  sin θ+ y   2  cos θ
 
From (2),  y   1   =y   s  and from (6),  x   2   =x   0 ; therefore
 
 y   s   =x   0  sin θ+ y   2  cos θ  (10)
 
Modifying (10),  y   2 =( y   s   −x   0  sin θ)/cos θ  (11)
 
Substituting (9) for (11),  x   1   =x   0  cos θ−( y   s   −x   0  sin θ)tan θ  (12)
 
[Eq. 6]
 
From (1), movement amount of conveyor mechanism= x   1   −x   s  
 
Substituting (12),
 
movement amount of conveyor mechanism= x   0  cos θ−( y   s   −x   0  sin θ)tan θ− x   s  
 
[Eq. 7]
 
From (7), movement amount of first movement mechanism= y   0   −y   2  
 
Substituting (11),
 
movement amount of first movement mechanism= y   0 −( y   s   −x   0  sin θ)/cos θ
 
[Eq. 8]
 
Combining (3) and (8), movement amount of second movement mechanism=− L  tan θ+(movement amount of first movement mechanism)
 
     From the above equations, the computation formulas shown in the following table are determined. 
     
       
         
               
             
               
               
             
               
               
             
           
               
                   
               
               
                 [Eq. 9] 
               
             
          
           
               
                   
                 Computation Formulas 
               
               
                   
                   
               
             
          
           
               
                 movement amount of 
                 x 0  cos θ − (y s  − x 0  sin θ) tan θ − x s   
               
               
                 conveyor mechanism 
               
               
                 movement amount of 
                 y 0  − (y s  − x 0  sin θ)/cos θ 
               
               
                 first movement 
               
               
                 mechanism 
               
               
                 movement amount of 
                 −L tan θ + (movement amount of first movement 
               
               
                 second movement 
                 mechanism) 
               
               
                 mechanism 
               
               
                   
               
             
          
         
       
     
     On the basis of the computation formulas given above, the control unit  59  shown in  FIG. 6  controls the conveyor motor  54  to move the plate-shaped workpiece  10 B in  FIG. 6( a ) . The control unit  59  also controls the first servo motor  34  to turn the first threaded shaft  33 , and controls the second servo motor  44  to turn the second threaded shaft  43 , in  FIGS. 6( b ) and ( c ) . As a result, the plate-shaped workpiece  10 B shown in  FIG. 7  overlaps the plate-shaped workpiece  10 C in the target centering position. 
     The conveyor motor  54  and the first and second servo motors  34 ,  44  are operated collectively and simultaneously. Specifically, the motors are started simultaneously, and stopped sequentially upon reaching the computed movement amounts. 
     Carrying out the arrows ( 1 ), ( 2 ), and ( 3 ) shown in  FIG. 7  in the stated order can greatly shorten the movement time. Instead of being started simultaneously, the motors may be started in any order. 
     The computation formulas described above are examined here. 
     The formulas are computed with the following values: θ=5°, L=700 (mm, the same hereinafter), x0=350, y0=250, xs=500, ys=400. 
     Movement amount of conveyor, −183.6, 
     movement amount of first movement mechanism, −120.9, 
     movement amount of second movement mechanism, −186.5, satisfactorily consistent with  FIG. 7 . 
     In  FIG. 1 , the plate-shaped workpieces  10 B,  10 B can be centered because the first centering device  20 A and the second centering device  20 B both include a camera. 
     Next, the process for centering a long workpiece will be described. 
     As shown in  FIG. 10 , a long workpiece  61 B is placed so as to span over both the first centering device  20 A and the second centering device  20 B. A long workpiece  61 C set in the target centering position is shown by imaginary lines. 
     The coordinates (xs, ys) and the shift angle θ can be detected from the image taken by the camera of the second centering device  20 B. The coordinates (x0, y0) are inputted in advance. 
     On the basis of the computations by the computation unit, the control unit synchronously operates a right conveyor motor  54 A and a left conveyor motor  54 B 
     With a first threaded shaft  33 A of the first centering device  20 A kept still, a second threaded shaft  43 A of the first centering device  20 A and first and second threaded shafts  33 B,  43 B of the second centering device  20 B are turned about the first support shaft  35  of the first centering device  20 A, the first support shaft  35  being an overall pivotal center. Both tables are pivoted by an angle θ about the first threaded shaft  33 A of the first centering device  20 A, the first threaded shaft  33 A being an overall pivotal center. 
     For the movement along the y-axis, four servo motors  34 A,  44 A,  34 B, and  44 B are preferably turned simultaneously. 
     Specifically, the control unit operates a first servo motor  34 A and a second servo motor  44 A of the first centering device  20 A, and a first servo motor  34 B and a second servo motor  44 B of the second centering device  20 B. As a result, the plate-shaped workpiece  61 B is centered and overlaid on the plate-shaped workpiece  61 C. 
     In this example, two centering devices  20 A,  20 B are arranged in series, but three or more may also be arranged. Doing so makes it possible to center workpieces ranging from extremely short plate-shaped pieces to heavy and long pieces with one set of equipment. 
     Modifications of the centering device according to the present invention are described next. 
     As shown in  FIG. 11 , rails  62 ,  62  are laid on the base  26 , a slider  63  is provided on the rails  62 ,  62  so as to be capable of moving along the y-axis, and the slider  63  is moved in the y-axis direction by a threaded shaft  68  turned by a movement servo motor  67 . Specifically, the table movement mechanism  28  is configured from the threaded shaft  68  and the movement servo motor  67  in this example. Therefore, the configuration of the table movement mechanism  28  can be modified as appropriate. 
     A rotation servo motor  64  is placed on the slider  63 , a reduction gear mechanism  65  is placed on the rotation servo motor  64 , and the table  21  is supported by a reduction gear mechanism shaft  65   a  of the reduction gear mechanism  65 . Four adjustable wheels  66  are provided to the table  21 . The adjustable wheels  66  travel over the base  26 , suppressing upward and downward movement of the table  21 . The bending load on the reduction gear mechanism shaft  65   a  can be greatly reduced by the adjustable wheels  66 . 
     As shown in  FIG. 12( a ) , the reduction gear mechanism shaft  65   a  is located at the point of origin (0, 0). The coordinates (x0, y0), the coordinates (xs, ys), and the angle θ are known. 
     The plate-shaped workpiece  10 B is turned clockwise by the angle θ about the point of origin (0, 0). The peak coordinates (x4, y4) after the turning are geometrically determined, similar to the range ( 2 ) in  FIG. 7 . 
     As shown in  FIG. 12( b ) , the orientation of the plate-shaped workpiece  10 B is consistent with that of the plate-shaped workpiece  10 C. The plate-shaped workpiece  10 B is moved a distance Xx along the x-axis, and is also moved a distance Yy along the y-axis. Xx is unambiguously determined to be (x0-x4), and Yy to be (y0-y4). As a result, the plate-shaped workpiece  10 B is overlaid on the plate-shaped workpiece  10 C. 
     In  FIG. 11 , it is preferable that the Xx movement be performed by the conveyor mechanism  23 , the shift angle θ be corrected by the rotation servo motor  64 , and the Yy movement be performed by the reduction gear mechanism  65  and the threaded shaft  68 . 
     Specifically, in this example, the reduction gear mechanism  65  and the threaded shaft  68  are equivalent to the table movement mechanism  28 , and the rotation servo motor  64  and the reduction gear mechanism  65  are equivalent to the table rotation mechanism  60 . 
     A mechanism that yields a high reduction ratio, such as a planetary gear mechanism, is employed for the reduction gear mechanism  65 . Because such a precise rotational angle is required, a reduction gear mechanism having a high reduction ratio is needed. 
     The threaded shaft  43  shown in  FIG. 4  also serves alone as a high-reduction-ratio reduction gear mechanism. Consequently, costs can easily be reduced with the configuration of  FIG. 4 . 
     As illustrated in  FIG. 1 , the centering device of the present invention is preferably installed in the entrance of a molding press machine, but may also be used in other applications. 
     As shown in  FIG. 1 , when a plate-shaped workpiece is of sufficient length to span over two tables, the centering device  20  of the present invention can carry out the centering of the long piece by simultaneously rotating and moving the long piece. In the present invention, rotation and movement are performed simultaneously and in parallel, and therefore the working time for centering a long piece can be reduced and productivity can be increased to a greater extent than in a case in which rotation and movement are performed in series (sequentially). 
     As shown in  FIG. 2 , when a plate-shaped workpiece is placed on each of two tables, the centering device  20  of the present invention centers the plate-shaped workpieces independently of each other by simultaneously rotating and moving the two plate-shaped workpieces. Because rotation and movement are performed simultaneously and in parallel, the working time for centering two plate-shaped workpieces can be reduced and productivity can be increased. Productivity can be further improved because two plate-shaped workpieces are centered simultaneously. 
     As shown in  FIG. 3 , workpieces of irregular shapes, such as trapezoids, can be centered. Irregularly shaped workpieces can be centered quickly and easily, no different from rectangular pieces. 
     Two tables were aligned in series in the embodiments, but three or more tables may be aligned in series as well. 
     INDUSTRIAL APPLICABILITY 
     In the present invention, it is preferable that the centering device of the present invention be installed in the entrance of a molding press machine. 
     LIST OF REFERENCE SIGNS 
       10 : plate-shaped workpiece,  10 B: plate-shaped workpiece before being centered,  10 C: plate-shaped workpiece centered in target centering position,  12 : first robot,  15 : second robot,  20 : centering device,  21 : table,  23 : conveyor mechanism,  28 : table movement mechanism,  30 : first movement mechanism,  40 : second movement mechanism,  56 : camera,  58 : computation unit,  59 : control unit,  60 : table rotation mechanism.