Abstract:
The present invention features a method whereby the vacuum nozzle of each extendable vacuum spindle in a multi-head component placement machine may be calibrated during component pick/place cycles. Calibration of each vacuum nozzle ensures accurate location of the vacuum nozzle over a component at a component pick station. This is particularly important with extremely small components where a slight misalignment of a vacuum nozzle with a component to be picked could result in a missed pick. Because of the vacuum nozzle inspection and calibration on the fly during the placement cycle, there is no slowdown of the placement machine cycle rate.

Description:
RELATED APPLICATIONS 
   This application is a Continuation-in-part of U.S. patent application Ser. Nos. 10/307,848, filed Dec. 2, 2002 and 09/947,099, filed Sep. 5, 2001 and now U.S. Pat. No. 6,625,878. This application is also related to United States Reissued Pat. No. RE 35,027, which is hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   This invention relates to component placement machines and, more particularly, to a method for calibrating vacuum nozzle positions to ensure accurate and reliable component pick during the machine&#39;s placement cycle. 
   BACKGROUND OF THE INVENTION 
   The use of sophisticated component placement machines in manufacturing printed circuit or similar cards, boards, panels, etc. is well known. The term printed circuit board (PCB) as used herein refers to any such electronic packaging structure. Typically, reels of tape-mounted circuit components are supplied to the placement machine by multiple feeders, each of which holds a reel of components. A pick station by each feeder assembly provides components. A rotatable frame is equipped with multiple pick/place heads, each having an extendable vacuum spindle that may be moved in the Z-axis (i.e., in and out) between and extended and a retracted position. The rotatable frame itself is mounted in a housing that may be moved along both the X and Y axes in a plane above a PCB being populated. Each vacuum spindle is equipped with a vacuum nozzle at its tip. The vacuum nozzles are sized and otherwise configured for use with each different size and style of component to be placed by the machine. 
   In operation, the housing with the rotatable frame holding the pick/place heads is moved to the pick station and the vacuum nozzle is positioned over a tape-mounted component. The vacuum spindle is lowered (i.e., extended) to a point where, upon application of vacuum, the component is removed from its backing tape, centered, and held tightly against the vacuum nozzle orifice. The rotatable frame is then moved to a point over the printed circuit board being assembled, the vacuum spindle is lowered, and the component is deposited on the printed circuit board at a predetermined location. 
   As component sizes have shrunk, the accuracy of placement of the vacuum nozzle over the component being picked has become more critical. Typically, calibration routines are performed upon machine setup or periodically as required for operation of the machine. However, with micro-miniature components, small variations occurring over time can quickly lead to miss-picks, as well as recognition and orientation problems of these components. 
   Currently, component placement assembly machines utilize multi-head frames to improve assembly speed. Each frame contains multiple pick/place heads with vacuum spindles, each vacuum spindle having its own vacuum nozzle. With multi-head machines, the need for real-time monitoring of the vacuum nozzle positions becomes even more critical. A vacuum nozzle&#39;s position may vary over time due to mechanical binding, build-up of debris, damage to the vacuum nozzle, thermal drift, etc. A changed vacuum nozzle position can be difficult to determine, manifesting its presence only through intermittent pick problems from the various feeders being used to supply the components for placement on the printed circuit boards and through component recognition and orientation errors. If the component itself is misaligned on the PCB, such an error can obviously affect proper operation of other components and larger assemblies. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method whereby each vacuum nozzle position is calibrated during each placement cycle. Any deviation in a vacuum nozzle&#39;s position from an expected position may cause pick errors and component rejections that may degrade machine performance and cause defects in the PCBs being populated. The inventive process allows for accurate picking of components, especially very small ones. When the calibration process is applied to a multi-head frame, each vacuum nozzle can be calibrated during each placement cycle. Therefore, the calibration process in multi-head frames causes no slowdown of the placement machine cycle rate (i.e., placement machine throughput). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent detailed description, in which: 
       FIG. 1  is a side, schematic view of a multi-head frame showing a plurality of pick/place heads having vacuum spindles disposed thereupon; 
       FIG. 2  is a simplified schematic diagram of a portion of a component placement machine adapted to practice the method of the invention; 
       FIG. 3  is a timing diagram of vacuum nozzle image acquisition and processing during the placement cycle time in accordance with the inventive method; and 
       FIG. 4  is a simplified flow chart of the method of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention pertains to calibration of vacuum nozzle positions on the multi-head rotatable frame of a component placement machine used for assembling printed circuit boards. 
   Referring to  FIG. 1 , a schematic view of a housing  102  having a multi-pick/place head frame  104  rotatably mounted therein is shown. A plurality of pick/place heads  106  having vacuum spindles  108  are disposed radially around the frame  104 . It will be recognized by those skilled in the component placement machine arts that a frame  104  may carry any number of pick/place heads  106  disposed radially about its perimeter. For purposes of this disclosure, a general case of n pick/place heads  106  is discussed. 
   Multi-pick/place head frames  104  are known and the concept forms no part of the present invention. Exemplary multi-pick/place head frames are described in U.S. Pat. No. RE. 35,027 and European Patent No. EP 0 315 799. 
     FIG. 2  is a schematic block diagram  200  of a typical image acquisition arrangement for use in a component placement machine. At least one camera  202  is used to capture images, often at different magnifications or with different lighting conditions. The output of the camera  202  is connected to electronic signal processing and control circuitry  210  (i.e., the machine controller). The circuitry  210  controls the camera  202  and provides image capture functions. 
   The output of electronic signal processing and control circuitry  210  is connected to a vision system  212 . It is also known to use a vision system at a process station in component placement machines. Typically, vision systems of the prior art are used to process images of components to facilitate identifying, positioning, and manipulating or orienting the components held against a vacuum nozzle  110  of a vacuum spindle  108 . The present invention expands the use of such vision systems  212  to perform calibration of the vacuum nozzle  110  positions during each placement cycle performed by each vacuum nozzle  110  of the component placement machine. The inventive method is operative with any number of pick/place heads  106  and is not considered limited to any particular number thereof. It will also be recognized that the timing data ( FIG. 3 ) used for purposes of disclosure may vary depending on the actual design of the rotatable frame  104 . 
   In the embodiment chosen for purposes of disclosure, each active vacuum nozzle  110  on each vacuum spindle  108  of each pick/place head  106  on the rotatable frame  104  is imaged during each placement cycle. It will be recognized that in alternate embodiments, other methods including, but not limited to, imaging only a subset of the active vacuum nozzles  110  during a particular placement cycle, and imaging only a particular vacuum nozzle  110  every nth placement cycle could be implemented; therefore, the invention is not limited to the particular sequence chosen for purposes of disclosure. 
   When a vacuum nozzle  110  is picking up a component  214 , its associated vacuum spindle  108  is in an extended position. At calibration time (i.e., when the vacuum nozzle  110  is adjacent the camera  202 ), the exact position of the vacuum nozzle  110  may be recorded, thereby re-calibrating the position of the vacuum nozzle  110 . It is important that the vacuum spindle  108  likewise be in an extended position at calibration time. This is accomplished by an actuator, not shown, typically affixed to the housing  102 , which extends the vacuum spindles  108  as they reach the camera  202 . Such an actuator is described in detail in the included-by-reference applications cited hereinabove. It is preferable to calibrate the vacuum nozzles  110  while their associated vacuum spindles  108  are in their extended positions because a vacuum spindle&#39;s  108  eccentricity or other factors may cause inaccuracies in the vacuum nozzle calibration data if the calibration process is performed with the vacuum spindles  108  in their retracted position. 
   The position calibration data is typically placed in a lookup table, not shown, so that the most recent position data may be utilized by the placement machine for the next pick or place cycle involving that particular vacuum nozzle  110 . While methods other than lookup tables could be used for storing vacuum nozzle calibration information, a fixed table of vacuum nozzles  110  associated with their position on the rotatable frame  104  has been found to be satisfactory. It will be recognized that other suitable data storage formats could also be used. Utilization of the calibration data by the inventive process is described in more detail hereinbelow. 
   Referring now to  FIG. 3 , a timing diagram for image acquisition and processing within a pick or place cycle is shown. As may be seen, image acquisition and processing (calibration) for each vacuum nozzle are always completed within a single pick or place cycle. 
   Referring again to  FIG. 2  and also now to  FIG. 4 , a flow chart  400  of the steps of the inventive method are shown. It is assumed that initial placement machine setup, vacuum nozzle installation, and calibration have been previously performed. The rotatable frame  104  is moved to a pick station  204 , a particular vacuum spindle  108  is lowered, and a component  214  is picked, step  402 . This calibration step  406  is repeated for the number of active pick/place heads  106  in the multi-head frame  104  (i.e., each active vacuum nozzle has picked-up a component). It is possible, however, that one or more pick/place heads  106  may be out of service and therefore may be ignored during any particular pick or place cycle. 
   The frame  104  is then moved under program control to the desired X-Y coordinates over the printed circuit board being assembled at the place station  208 . The first vacuum spindle  108  is lowered and the component  214 , picked in step  402 , is placed onto the printed circuit board, step  404 . After component placement begins, the empty vacuum nozzles  110  proceed to the camera  202  where they are recalibrated (i.e., their exact location is determined), step  406 . 
   As the components  214  continue to be placed or when the frame  104  returns to the component pick station  204  and the previously calibrated vacuum nozzles  110  begin to acquire new components  214 , the remaining vacuum nozzles  110  requiring calibration move to the camera  202 . Therefore, during either the placement cycle or the pick cycle, this step is repeated for all vacuum nozzles  110 . Details of the vacuum nozzle calibration process, step  406 , are provided hereinbelow. 
   After image acquisition is complete, the image or images are processed, step  406 . Processing involves locating the vacuum nozzle position within the acquired image. Assuming that the calibration process, step  406 , is able to locate the vacuum nozzle  110  (i.e., the exact position/location of the vacuum nozzle  110  may be ascertained from one or more images from the camera  202 ), the new position is recorded, step  412 , and the placement cycle continues, step  402 . If a calibration problem occurs, step  408  (i.e., the exact vacuum nozzle  110  location cannot be determined or the location exceeds a predetermined tolerance range), the operator is alerted, step  410 . Depending on the severity of the problem, the component placement machine could be automatically stopped or the pick/place head  106  with the problem vacuum nozzle  110  could be removed from active service until the problem is resolved. 
   It will be recognized that calibration of a particular vacuum nozzle  110  associated with a particular vacuum spindle  108  need not necessarily be performed. For example, if a vacuum nozzle  110  is not currently in active service, calibration is skipped. It will also be possible to define algorithms for periodically skipping calibration of a vacuum nozzle  110  if placement machine speed places an undue burden on the vision system  212 , particularly image processing. Therefore, the invention is not considered limited to a method wherein each vacuum nozzle  110  is calibrated during each placement cycle. 
   Although the present invention has been described in connection with exemplary embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without departing from the spirit and scope of the invention as described and defined in the appended claims. 
   Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.