Patent Publication Number: US-6986200-B2

Title: Method for mounting parts

Description:
This is a division of Application No. 09/617,125, filed Jul. 14, 2000, now U.S. Pat. No. 6,634,091 which is currently pending and is incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a part mounter and a method for mounting parts, and more particularly, to a part mounter and a method for mounting parts by which the required time and the distance equipment must move for mounting parts is minimized and the position of the parts is precisely recognized. 
   2. Description of the Related Art 
   In general, electronic parts such as semiconductor packages are mounted on a printed circuit board by using a part mounter. The part mounter picks up electronic parts supplied by a part supply portion such as a tray feeder or a tape feeder using vacuum suction and mounts the electronic parts at predetermined positions on the printed circuit board. Here, a head unit moves the electronic part to an image sensor to obtain information on the posture of the part held by a suction is nozzle. As the image sensor, a vision camera (a line CCD or an area CCD) is used. The image sensor detects state of the part held by the part suction portion so that information on the posture of the part is obtained. The part suction portion corrects the posture of the held part according to the image information and then mounts the part on the printed circuit board. 
     FIG. 1  is a perspective view showing the structure of a typical part mounter. Referring to the drawing, the part mounter includes a first Y-axis  12  and a second Y-axis  13 , an X-axis  11  moving along the first and second Y-axes  12  and  13 , and a head unit  14  installed to be capable of moving along the X-axis  11 . A suction nozzle  15  is installed at the head unit  14  to be capable of rotating and moving up and down. A printed circuit board (PCB)  25  is transferred by a conveyer  19  to the position of a part  20  to be mounted. The head unit  14  moves between part feeders  18   a ,  18   b ,  18   c ,  18   d  and  18   e , and the PCB  25 . The suction nozzle  15  moves up and down and/or rotates to hold a part and mounts it on the PCB  25 . At least one among the part feeders  18   a ,  18   b ,  18   c ,  18   d  and  18   e  may be a tape feeder or a tray. During a mounting process, the head unit  14  passes above an image sensor  16 . The image sensor  16  is fixed at one side of the part mounter and detects the part held by the suction nozzle  15  so that it corrects a positional error generated when a part is held by the suction nozzle  15 . The image sensor  16 , for example, may have an image detecting device such as a vision camera (a line CCD or an area CCD). 
     FIG. 2  is a view schematically showing the path along which the head unit  14  shown in  FIG. 1  moves. Referring to the drawing, the suction nozzle  15  of the head unit  14  picks up a part from the part feeders  18   a ,  18   b ,  18   c ,  18   d  and  18   e  and then moves to the position directly above the image sensor  16 . The image sensor  16  senses the part held by the suction nozzle  15 . Through a detecting process performed by the image sensor  16 , an error generated when the part is held by the suction nozzle  15  can be recognized. Next, the head unit moves above a mounting position P on the PCB  25 . The suction nozzle  15  of the head unit  14  descends and mounts the part at the mounting position P of the PCB  25 . Here, to correct an error recognized through the sensing process, the head unit  14  changes its position in the coordinate system or the suction nozzle  15  rotates and then the part is mounted. 
   In the part mounter having the above structure, since the head unit  14  must travel from the part feeders  18   a ,  18   b ,  18   c ,  18   d  and  18   e  to the part mounting position P on the PCB  25  through the image sensor  16 , the path of movement is long and complicated. That is, since the image sensor  16  is fixed at a particular position, the head unit  14  must move to the position directly above the image sensor  16  so as to sense the state of the held part. 
     FIG. 3  is a view showing an image of a state in which a part is held by the suction nozzle  15 , which is taken by an image sensing apparatus. Referring to the drawing, the center of the suction nozzle  15  matches the central point A of an image  121 . That is, when the suction nozzle  15  picks up a part  111  and moves just above the image sensor  16 , the center of the suction nozzle  15  is designed to match the center of the image sensor  16 . Thus, when a sensing process is performed by the image sensor  16 , the center point A of the image  121  substantially matches the center of the suction nozzle  15 . This is because a typical image recognition apparatus is fixed at one side of a part supply portion of a part mounter and the head unit  14  moves to a particular position after moving along a set path, that is, the center of the suction nozzle  15  matches the center of the image  121 . 
   When the image sensing is performed after the suction nozzle  15  has picked up a part, the positional information of the central point O of a part is recognized with information on the size and shape of the part. Next, information on the tilt of the part is recognized by the information on the shape of the part. As shown in  FIG. 3 , recognizing the distance separated in the X direction and Y direction between the central point A of the image  121  and the central point O of the part  111 , and a rotational degree θ is recognized and the separated distance and the rotational degree are compensated for. 
   However, to correct the error generated when the part is held, the image sensor  16  must be maintained in a fixed state. This is because the central point of the suction nozzle  14  must match the central point A of the monitor  121  to recognize the error when suctioning the part. Thus, if the central point of the suction nozzle  14  does not match the central point of the image  121  during the sensing process, performing correction an error is not possible. The positional information with respect to the nozzle is indirectly obtained and it is assumed that the above preconditions are met. Also, at least one of the image sensor  16  and the head unit  14  must be fixed. 
   SUMMARY OF THE INVENTION 
   To solve the above problems, it is an object of the present invention to provide a part mounter and a method for mounting parts so that part mounting work is performed quickly while correcting an error generated. 
   Accordingly, to achieve the above objects, there is provided a part mounter comprising a plurality of part feeders for storing and supplying a plurality of parts, a conveyer for transferring a printed circuit board, a head unit capable of moving and having a suction nozzle for picking up a part from the part feeders and mounting the part on the printed circuit board on the conveyer, and an image sensor installed to move along a path crossing a movement path of the head unit, for detecting an image of the part held by the suction nozzle. 
   It is preferred in the present invention that at least one reference portion, which is detected if not covered by the part when the image sensor detects an image of the part held by the suction nozzle, is provided at the head unit. 
   Also, it is preferred in the present invention that the image sensor moves close to the part feeder where a part is picked up. 
   Also, it is preferred in the present invention that the image sensor moves to a position where a path along which the head unit moves, between a position where a part is picked up from the part feeder and a position where the part is mounted on the printed circuit board, and a path along which the image sensor moves cross. 
   Also, it is preferred in the present invention that the image sensor is moved by a motor for providing a driving force, a ball screw rotated by the motor, and a bushing and a linear guide member installed at one side of the image sensor and coupled to the ball screw. 
   Also, it is preferred in the present invention that the image sensor is capable of moving by a linear motor. 
   Also, it is preferred in the present invention that the head unit is installed to be capable of moving along an X-axis which is installed to be capable of moving along a first Y-axis and a second Y-axis installed parallel to each other. 
   Also, it is preferred in the present invention that a pair of X-axes are installed to be capable of moving along a first Y-axis and a second Y-axis installed parallel to each other, and two head units are provided so that one can move along each of the X-axes, and two image sensors are provided close to both sides of the conveyer. 
   To achieve the above objects, there is provided a method for mounting parts which is achieved by recognizing a particular part feeder where a part to be mounted is contained among a plurality of part feeders, moving a head unit to a position for picking up a part on the part feeder, moving an image sensor installed to be capable of moving to a position close to the particular part feeder, picking up a part from the particular part feeder with a suction nozzle installed at the head unit, moving the head unit to a position above the image sensor, detecting an image of the part held by the suction nozzle, moving the suction nozzle to a printed circuit board from the image sensor, and aligning the part with the printed circuit board while determining and compensating for an error generated when the part is picked up, and mounting the aligned part on the printed circuit board. 
   It is preferred in the present invention that the method for mounting parts further comprises a step of recognizing whether the image sensor has moved to a position close to the particular part feeder after the image sensor was supposed to have moved to the position close to the particular part feeder. 
   Also, it is preferred in the present invention that the determining and compensating is performed by determining the coordinates of the center of the suction nozzle by detecting a position of a reference portion which is not covered by the part when the image is obtained, determining the coordinates of the center of the part from an image obtained by the image sensor, calculating a value of correction of the part position by determining an offset of the part from the displacement of the center of the part held by the suction nozzle from the center of the suction nozzle, and controlling a position for mounting the part with the suction nozzle according to the value of correction. 
   According to another aspect of the present invention, there is provided a method for mounting a part which is achieved by calculating an optimal movement path between a position for picking up a part from a part feeder and a position for mounting the part on a printed circuit board, moving a head unit to the position for picking up a part from a part feeder, calculating a position where a movement path of an image sensor crosses a path formed between the part feeder and the position for mounting the part along which the head unit moves, moving the image sensor to the crossing position, picking up a part with a suction nozzle of the head unit, moving the head unit to a position above the image sensor, detecting the part held by the suction nozzle by the image sensor, moving the head unit to the position for mounting the part on the printed circuit board, and aligning the part while determining and compensating for an error generated when the part is picked up, and mounting the aligned part. 
   It is preferred in the present invention that the method further comprises a step of recognizing whether the image sensor has moved to the cross position after the image sensor is moved to the cross position. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is a perspective view showing the structure of the typical part mounter; 
       FIG. 2  is a view schematically showing the path along which the head unit of  FIG. 1  moves; 
       FIG. 3  is a view showing an image produced by an image sensing apparatus of a state in which a part is held by the suction nozzle; 
       FIG. 4A  is a perspective view showing the structure of a part mounter according to the present invention; 
       FIG. 4B  is a perspective view showing part of the part mounter shown in  FIG. 4A ; 
       FIG. 5  is a view schematically showing a method for mounting parts according to a first preferred embodiment of the present invention; 
       FIG. 6  is a flow chart for explaining the method for mounting parts according to the first preferred embodiment of the present invention; 
       FIG. 7  is a view schematically showing a method for mounting parts according to a second preferred embodiment of the present invention; 
       FIG. 8  is a flow chart for explaining the method for mounting parts according to the second preferred embodiment of the present invention; 
       FIGS. 9 and 10  are views showing states in which an error in holding a part is recognized on a monitor of the part mounter according to the present invention; 
       FIG. 11  is a flow chart for explaining a method of determining and correcting an error in holding a part according to the present invention; and 
       FIG. 12  is a plan view showing a part mounter of a double gantry type to which the present invention is applied. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 4A  shows the structure of a part mounter for performing a method for mounting parts according to the present invention. Here, the same reference numerals as these appearing in  FIG. 1  denote the same elements having the same functions. Referring to  FIG. 4A , a part mounter according to the present invention includes the first and second Y-axes  12  and  13  installed parallel to each other, the X-axis  11  installed to be capable of moving along the first and second Y-axes  12  and  13 , the head unit  14  installed to be capable of moving along the X-axis  11 , the suction nozzle  15  installed at the head unit  14  to be capable of rotating and moving up and down, and the conveyor  19  for transferring the printed circuit board (PCB)  25  to positions where parts are mounted. The head unit  14  moves between the part feeders  18   a ,  18   b ,  18   c ,  18   d  and  18   e  and the PCB  25 . The suction nozzle  15  moves up and down and rotates to hold parts and mount the held parts on the PCB  25 . 
   According to one characteristic feature of the present invention, the image sensor  46  for detecting a part held by the suction nozzle  15  of the head unit  14  is installed to be capable of moving. The image sensor  46  is installed to be capable of moving while being close to a particular part feeder for holding a part among the part feeders  18   a ,  18   b ,  18   c ,  18   d  and  18   e  of which one or more are provided. After the suction nozzle  15  is lowered to hold a part, the head unit  14  moves just above the image sensor  46 , which is moved in advance near a particular part feeder from which the part is picked up, and performs sensing. Thereafter, the part is mounted on the printed circuit board  25 . 
   According to another characteristic feature of the present invention, the movement path of the image sensor  46  for detecting the part held by the suction nozzle  15  of the head unit  14  may be set to cross linear paths along which the head unit  14  moves to part mounting positions from the respective part feeders  18   a ,  18   b ,  18   c ,  18   d  and  18   e.    
   Furthermore, the head unit  14  moves along a substantially shortened path between the part mounting positions and the respective part feeders  18   a ,  18   b ,  18   c ,  18   d  and  18   e , and the image sensor  46  moves to the position where the movement path of the image sensor  46  and the movement path of the head unit  14  meet, so that the part held by the suction nozzle  15  can be detected. 
   In  FIG. 4A , a servo motor  33  and a ball screw  35  are applied to move the image sensor  46 . When the servo motor  33  rotates, the ball screw  35  which is rotatably supported by a bearing  34  rotates. Since the ball screw  35  is coupled to a bushing  32  installed at the image sensor  46 , the image sensor  46  can move close to any of the part feeders  18   a ,  18   b ,  18   c ,  18   d  and  18   e . As a controller (not shown) controls driving of the servo motor  33 , while the suction nozzle  15  of the head unit  14  picks up a part from one of the part feeders  18   a ,  18   b ,  18   c ,  18   d  and  18   e , or before holding a part, the image sensor  46  moves in advance to the meeting position close to the corresponding part feeder and waits there. In another example, a linear motor is used instead of the ball screw  35 . 
     FIG. 4B  is a perspective view showing part of the head unit  14  provided at the apparatus of  FIG. 4A . Referring to the drawing, at least one of reference portions  132   a  and  132   b  is installed at one side of the head unit  14 . The reference portions  132   a  and  132   b  are installed such that, when the image sensor  46  senses a part  135  held by the suction nozzle  15 , at least one reference portion can be viewed, that is, the reference portions  132   a  and  132   b  are not covered by the part  135 . 
   Providing the reference portions  132   a  and  132   b  is important in that the moving image sensor  46  (see  FIG. 4A ) recognizes an error generated during picking up a part. Since the position of the reference portions  132   a  and  132   b  and the center of the nozzle  15  are values which are known, when the image sensor  46  senses at least one of the reference portions  132   a  and  132   b , the position of the center of the suction nozzle  15  can be determined from the position of the reference portions  132   a  and the  132   b  and the relative positions of the center of the suction nozzle  15  and the center of the part can be obtained. Thus, an error during picking up a part can be recognized from these values. A method of recognizing an error during picking up a part will be described later. 
     FIG. 5  is a view schematically showing a method for mounting parts according to a first preferred embodiment of the present invention. The same reference numerals as those in  FIG. 4A  denote the same elements having the same functions. Referring to the drawing, a part feeder is a tray feeder, a stick feeder or a general feeder in which parts are wound around a reel. Five part feeders  18   a ,  18   b ,  18   c ,  18   d  and  18   e  are provided like the preferred embodiment shown in the drawing. Also, the image sensor  46  (see  FIG. 4A ) can move to a position close to each of the part feeders  18   a ,  18   b ,  18   c ,  18   d  and  18   e , as shown in the drawing, and the positions of the image sensor  46  at that time are indicated by reference numerals  31   a ,  31   b ,  31   c ,  31   d  and  31   e . When the ball screw  35  is driven by the servo motor  33  to rotate, the image sensor  46  linearly reciprocates along a typical guide member (a linear motion guide in the example shown in the drawing). 
   If the suction nozzle  15  picks up a part from the part feeder  18   b , for example, and mounts the part at a position indicated by P 2  on the PCB  25 , the image sensor  46  moves to the position indicated by reference numeral  31   b  so as to be positioned as close as it can get to the part feeder  18   b . Thus, the movement path of the head unit  14  can be reduced to the shortest distance from the part feeder  18   b  to the position  31   b  of the image sensor  46 . Also, the movement path of the head unit  14  from the position  31   b  of the image sensor  46  to the position P 2  for mounting the part becomes the shortest distance. Likewise, when a part is picked up from the part feeder  18   e  and mounted on a position P 5  on the PCB  25 , the image sensor  46  moves to the position indicated by the reference numeral  31   e . Next, the suction nozzle  15  of the head unit  14  holding the part moves to the position  31   e  of the image sensor  46  so that detecting the part is performed. Then, the head unit  14  moves to the position P 5  for mounting the part along the shortest route. The above operation applies in the same manner to the other positions P 1 , P 2  and P 3  and the other part feeders  18   a ,  18   c  and  18   d.    
   In an actual embodiment, a plurality of suction nozzles can be provided to a head unit. Accordingly, the head unit moves between a plurality of part feeders so that each suction nozzle can pick up a part, before the head unit moves to the position directly above an image sensor. In this case, the image sensor moves close to a part feeder where the last part is picked up. As each suction nozzle of the head unit passes above the image sensor, detecting is performed. Next, correcting positional errors of the parts is performed according to information obtained from the detecting of parts held by the respective suction nozzles of the head unit. Then the head unit moves above a PCB and the parts are mounted thereon. 
     FIG. 6  is a flow chart for explaining the method for mounting parts which was described with reference to  FIG. 5 . Referring to  FIG. 6 , a particular part feeder containing a part to be mounted, among a plurality of part feeders, is recognized prior to each performance of mounting a part (step  51 ). Next, the head unit  14  is moved to the position for picking up the part from the part feeder (step  52 ). The image sensor  46  is moved to the position close to the part feeder (step  53 ). It is determined whether the image sensor  46  is at the correct position (step  54 ). If it is determined that the image sensor  46  is not at the position close to the part feeder, the step of recognizing a particular part feeder begins again. If it is determined that the image sensor  46  is at the position close to the part feeder, the suction nozzle  15  picks up a part from the particular part feeder (step  55 ). The part feeder holding the part moves just above the image sensor  46  (step  56 ). Detecting an image of the part is performed (step  57 ). A signal for the detected image of the part is used through data processing as data for determining and correcting to an error in picking up the part and for determining the position for mounting the part. The head unit  14  is moved to the PCB  25  from the image sensor  46  (step  58 ). While determination of the part suction error is performed, the position for mounting a part is determined and the part is mounted on the PCB  25  (step  59 ). 
     FIG. 7  is a view showing a method for mounting parts according to a second preferred embodiment of the present invention. The reference numerals in the drawing which are the same as those in  FIG. 4A  denote the same elements having the same functions. Referring to the drawing, part feeders are indicated by reference numerals  18   a ,  18   b ,  18   c ,  18   d  and  18   e . Also, an image sensor (not shown) can reciprocate along a typical guide member (not shown) when the ball screw  35  driven by the servo motor  33  rotates. 
   When a head unit (not shown) picks up a part from the part feeder  18   b  to mount the part at the position indicated by reference numeral  61   b  on the PCB  25 , a linear route connecting a position for picking up a part on the part feeder  18   b  and a position  61   b  for mounting the part on the PCB  25  is chosen as the movement path of the head unit. Here, since the image sensor moves along the ball screw  35 , the movement path of the image sensor matches the lengthwise direction of the ball screw  35 . While the head unit moves to the position  61   b  for mounting a part on the PCB  25  from the part feeder  18   b , the image sensor is positioned just above the head unit at the time when the head unit and the image sensor cross. Here, the position of the image sensor is indicated by reference numeral  31   b ′ in  FIG. 7 . 
   Also, when parts are picked up from the other part feeders  18   a ,  18   c  and  18   d  to be mounted at other positions  61   a ,  61   c  and  61   d  on the PCB  25 , the image sensor is moved to a position where a line connecting the relevant part feeder and mounting position and the movement path of the image sensor  31  cross. The positions are indicated by reference numerals  31   a ′,  31   c ′,  31   d ′ and  31   e′.    
   When the part held by the suction nozzle is sensed, a correction may need to be made considering an angle made by the image sensor  31  and the suction nozzle. Such a case occurs in a part mounter using a multi-joint robot, not in the gantry type part mounter shown in  FIG. 4A . For example, when the head unit moves along a linear path to the part mounting position  61   a  on the PCB  25  from the part feeder  18   a , and if the part is detected at the position  31 ′, an angle θ is formed between a rectangular coordinate of the head unit and a rectangular coordinate of the image sensor. Thus, considering the above angle, calculation of the angle is needed in order to determine an error in picking up a part. The angle θ can be automatically calculated by constructing a data base with respect to the positions for mounting a part on the PCB, the position of the part feeder, and the current position of the image sensor on the movement path. Alternatively, as the suction nozzle holding a part passes above the image sensor, image information is received so that an angle can be calculated. 
   To compensate for the calculated angle θ, the suction nozzle  15  is rotated or the image sensor  46  is rotated. For example, as the suction nozzle  15  is rotated by the power from the servo motor (not shown), the angle θ is compensated for. In another example, the image sensor  46  is configured not only to reciprocate but also to rotate to compensate for the angle θ. 
     FIG. 8  is a flow chart for explaining the method for mounting parts described with reference to  FIG. 7 . Referring to  FIG. 8 , an optimal movement path between a position for picking up a part from the part feeder and a position for mounting a part on the PCB is calculated by the controller of the part mounter (step  71 ). The head unit is moved to the position for picking up a part on the PCB (step  72 ). The crossing point of the movement path of the image sensor and the movement path of the head unit is calculated (step  73 ). The image sensor is moved to the crossing point (step  74 ). It is determined whether the image sensor is moved to the crossing point (step  75 ). Here, if the image sensor is not precisely moved to the crossing point, the crossing point is recalculated and the step of moving the image sensor to the crossing point is repeated. 
   If the image sensor is positioned at the crossing point, the suction nozzle of the head unit picks up a part (step  76 ) and the head unit is moved just above the image sensor (step  77 ). The image of the picked part is detected (step  78 ). The detected image of the part is processed in a predetermined method so as to be used as information for determination of the part mounting position after an error in picking up a part is determined. Then, the head unit is moved to the part mounting position on the PCB (step  79 ). The part is aligned and mounted at a predetermined position on the PCB with error correction (step  80 ). 
   In the case of a head unit having a plurality of suction nozzles, the image sensor is positioned at the point where the movement path of the image sensor and the shortest distance between the part picking position of the part feeder where the last part is picked up by one of the suction nozzles of the head unit and the part mounting position, cross. The head unit moves along the shortest linear path from the position where the last part is picked up to the part mounting position. While the head unit moves just above the image sensor positioned on the above movement path, information on position of the part is obtained by detecting the image of the part. Each suction nozzle of the head unit rotates the part according to the position information obtained by detecting while moving, and mounts the part on the PCB. 
     FIG. 9  shows the image of a part detected by the image sensor. Referring to the drawing, when the center of a monitor  97  is B, the center B of the monitor  97  matches the center of a suction nozzle  94  holding a part  91 . Elements indicated by reference numerals  92  and  93  on the monitor  97  are the reference portions  132   a  and  132   b  of  FIG. 4B  detected and displayed on the monitor  97 . Since the center of the suction nozzle  94  of the part  91  can be determined by the reference portions  92  and  93 , the displacement between the center of the suction nozzle  94  and the center O of the part  91  can be calculated. 
   That is, as shown in  FIG. 4B , as at least one of the reference portions  132   a  and  132   b  which are installed at the head unit  14  where the suction nozzle  94  is installed is not covered by the part when the part is picked up, the displacement between the reference portions  132   a  and  132   b  and the centers of the suction nozzle  15  can be determined. Thus, the position of the center of the suction nozzle  13  is obtained from the image displayed on the monitor  97  by detecting the reference portions  132   a  and  132   b , so that the displacement of the center O of the part can be easily obtained. 
     FIG. 10  shows another example of an image detected by the image sensor and displayed on the monitor. Referring to the drawing, when the center of the monitor  97  is B, the center B of the monitor  97  and the center of the suction nozzle  94  holding a part  95  do not match. Even when the center B of the monitor  97  and the center of the suction nozzle  94  do not match, a positional error of the part  95  can be measured. In this case, even if the fixed coordinates of the image sensor are not set, an accurate position for mounting a part can be obtained by calculating the coordinates of the center of the part  95  and the value of a relative position of the suction nozzle  94 . 
   That is, the center of the suction nozzle  94  holding the part  95  being covered by the part  95  is not shown on the monitor  97 . However, the position of the center of the suction nozzle  94  is obtained from the reference portions  92  and  93  displayed on the monitor  97  by detecting the reference portions  132   a  and  132   b  in  FIG. 4B . The position of the center of the part is calculated from the detected image and the displacement between the two centers is obtained. The displacement can be treated in rectangular coordinates or in angular coordinates. 
     FIG. 11  is a flow chart for explaining a method of determining an error generated while picking up a part, by the image sensor described with reference to  FIGS. 9 and 10 . Referring to  FIG. 11 , the part held by the suction nozzle  94  is moved, passing just above the image sensor, and the image of the part is detected (step  210 ). To detect the amount of deviation of the parts  91  and  95  from the center of the suction nozzle  94 , the position of the center of the suction nozzle  94  is calculated by using the position of the reference portions  92  and  93  which are not covered by the parts  91  and  95 , and a value of a relative position is obtained (step  220 ). The center of each of the parts  91  and  95  is obtained from the image of the part (step  230 ). An offset value of the part is obtained by comparing the value of the relative position and the value of the position of the center of the part, and a value of the error in the position of the part is obtained (step  240 ). Finally, as the controller is operated according to the error correction value, controlling the suction nozzle  94  to align the parts  91  and  95  is performed (step  250 ). Then, the parts are mounted. 
   Obtaining the relative position of the nozzle with at least one reference portion as above is possible because the head unit itself does not rotate in a gantry type part mounter and the relative position initially set between the reference portion of the head unit and the center of the suction nozzle is not changed. Thus, the value can be calculated with at least one reference portion. However, when the measurement is performed with two reference portions, even if a change occurs to a relative angle between axes for moving the head unit in X and Y directions in a gantry type part mounter, the center of the currently held part and the relative position of the suction nozzle can be calculated. Also, even when the head unit itself rotates, the value can be calculated. 
     FIG. 12  is a plan view showing a part mounter of a double gantry type to which the present invention is applied. Referring to the drawing, a first X-axis  310  and a second X-axis  320  are movably installed on a first Y-axis  330  and a second Y-axis  340 . A first head unit  380  is installed on the first X-axis  310  to be capable of moving along the first X-axis  310 . A second head unit  390  is installed on the second X-axis  320  to be capable of moving along the second X-axis  320 . A PCB  400  is moved by a conveyor  370  and part feeders  410  and  420  for the respective head units are installed close to both sides of the conveyor  370 . 
   In a double gantry type part mounter as shown in  FIG. 12 , two image sensors are installed. That is, as shown in the drawing, a first image sensor  350  and a second image sensor  360  are installed close to both sides of the conveyor  370 . When each of the head units  380  and  390  picks up a part from the part feeders  410  and  420 , each of the image sensors  350  and  360  moves close to each of the part feeders  410  and  420  to detect an image of the picked up part. Thus, the speed of mounting parts increases. 
   As described above, in the part mounter and the method for mounting parts according to the present invention, since the image sensor moves close to a particular part feeder or crosses the movement path of the part picking portion, the movement path of the part picking portion is shortened and thus the speed of mounting a part increases and the efficiency of mounting is improved. Also, an error generated during picking up a part can be easily determined and corrected with the moving image sensor. 
   It is noted that the present invention is not limited to the preferred embodiments described above, and it is apparent that variations and modifications by those skilled in the art can be effected within the spirit and scope of the present invention defined in the appended claims.