Abstract:
Apparatus and method for the formation and application of small drops of liquid to a workpiece at a predetermined location thereon. The apparatus includes a machine vision system including a computer operable to view the drop as it is being formed and calculate a value indicative of the drop volume and control formation of the drop using the value. Application of the drop to the workpiece is also monitored and controlled by the machine vision system. The method and apparatus are particularly suited for use in the application of very small drops having actual volumes of less than about 30 nanoliters.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    This invention relates to the field of automated assembly systems and more specifically to an adhesive dispensing and vision system for an automatic assembly system.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the manufacture of devices such as electronic circuit boards, electronic components and other miniature or subminiature devices it is often necessary to deposit liquid (including semi-liquids), like glue, in small amounts, for example less than 30 nanoliters (nl), at very precise locations on a workpiece. In applying liquid to such components several problems have been encountered. Controlling the volume of the drop as it is being formed has been difficult because of the extremely small quantity of liquid in the drop. These quantities can be 30 nl and smaller and often are less than 5 nl. A 20 nl drop, if spherical, would have a diameter of approximately 0.016 centimeters or 0.006 inches. In addition to dealing with very small volumes, in many applications, it is extremely important for dispensing systems to deposit drops that have very tight tolerances on volume onto a workpiece. Another important factor in the application of such small drops is that in many instances the placement of a drop must be accurately controlled. The inability to accurately control drop volume and location leads to increased scrap and the time consuming re-work of workpieces.  
           [0003]    Numerous dispensing devices are available for depositing small quantities of liquid. However, those devices fail to adequately control one or more of the above factors. In addition to the foregoing, current apparatus for applying small drops cannot verify that the drop was applied on the correct location or verify that the correct amount of liquid was applied. Incorrect location of the liquid or correct location of the liquid with inadequate volume can lead to the processing of a bad workpiece. The accomplishment of proper drop application is also complicated in the case of applying liquid by building up the liquid on a dispensing nozzle. This approach, for the current apparatus could adversely affect the application of a drop to a workpiece.  
           [0004]    There is thus a need for an improved apparatus and method for applying small drops of liquid to a workpiece.  
         SUMMARY OF THE INVENTION  
         [0005]    Among the several objects and features of the present invention may be noted the provision of an apparatus and method that can be utilized to apply small drops of liquid, such as glue, to workpieces for such things as subsequent attachment of components thereto. The apparatus utilizes a machine vision system including a video camera connected to a computer to view a drop of liquid being formed at a dispensing nozzle and to determine an approximate value for the volume of the drop. The computer provides a signal to a liquid pump to control its operation and hence the volume of the drop as it is being formed. The vision system can also be utilized to control the volume of liquid applied to the workpiece. After application of a drop, the apparatus is capable of verifying the location of the applied drop prior to further processing of the workpiece. If a drop of liquid on the nozzle is too large or there is a build-up on the applicator of excess liquid, for example when glue is the liquid, mechanism is provided to remove the excess or build-up of the liquid. A workpiece manipulator is also provided for moving a workpiece to a position adjacent a drop on the dispensing nozzle (applicator) and is operable to precisely place the workpiece relative to the drop in at least X, Y, and Z axes and when properly positioned move the workpiece into the drop a predetermined amount (Z axis movement) to apply at least a portion of the drop to the workpiece. After application of one or more drops, the workpiece is moved such that it can be viewed by a second video camera to determine the location(s) of the applied drop(s) to confirm that they have been properly applied. The apparatus is particularly applicable for the application of small drops of liquid for example, 30 nl or less.  
           [0006]    Other objects and features will be in part apparent and in part pointed out hereinafter.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    For a more complete understanding of the device and advantages thereof, reference is now made to the following descriptions in which like reference numerals represent like parts:  
         [0008]    [0008]FIG. 1 a  is a front elevation view of an apparatus used for robotically moving and assembling parts;  
         [0009]    [0009]FIG. 1 b  is an elevation view of the apparatus of FIG. 1;  
         [0010]    [0010]FIG. 1 c  is a cutaway overhead view of the apparatus;  
         [0011]    [0011]FIG. 2 is plan view of an apparatus for applying drops of liquid to a workpiece.  
         [0012]    [0012]FIG. 3 is a side view of the apparatus of FIG. 2 as viewed from the direction indicated by arrow A of FIG. 2.  
         [0013]    [0013]FIG. 4 is a side view of the apparatus of FIG. 2 as viewed from the direction indicated by the arrow B of FIG. 2.  
         [0014]    [0014]FIG. 5 is an enlarged view of an applicator nozzle and camera.  
         [0015]    [0015]FIGS. 6 a - 6   d  are enlarged views of an applicator nozzle with a drop of liquid formed.  
         [0016]    [0016]FIGS. 7 a - 7   c  illustrate the application of a drop to a work piece.  
         [0017]    [0017]FIG. 8 is a view of a work piece from an upward viewing camera.  
         [0018]    [0018]FIG. 9 is a flow chart of the operation of the apparatus.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    The term “liquid” as used herein includes those substances that are technically liquid but also includes substances that would be more accurately classed as pressure flowable semi-liquids such as plastics, solids or semi-solids for example, grease, paste, gums and the like which are not technically liquids but can be forced through a nozzle for application to a workpiece, for adhesion thereto. The liquid can be oils, glues, adhesives, lubricants, solvents, cleaners, dyes, paints, etc. The liquid may also contain solid materials therein as well as dissolved and entrained gases.  
         [0020]    [0020]FIG. 1 a  is a front view of an assembly system  100 , FIG. 1 b  is a side view of assembly system  100  and FIG. 1 c  is a cutaway overhead view of the assembly system  100 . Illustrated in these drawings are an assembly system  100 . Assembly system  100  includes a top portion  102  coupled to a base portion  104  using isolation pad  106 . Top portion  102  is preferably manufactured from granite. Top portion includes a top surface  102   a  and a bottom surface  102   b . Base portion  104  is preferably manufactured using a welded structural steel. Isolation pad  106  is manufactured from urethane. Top portion  102 , base portion  104  and isolation pad  106  together form an assembly system that is extremely rigid and vibration free.  
         [0021]    Inside top portion  102  and coupled to a top surface  102   a  is a robot platen  108 . Coupled to robot platen  108  is a manipulator device  110 . Robot platen  108  in one embodiment is a magnetic plate. Manipulator device  110 , discussed in further detail below, is able to move about the magnetic plate. This is accomplished by injecting compressed air between the manipulator device  110  and the robot platen  108 . This forms what is commonly known as an air bearing between the manipulator device  110  and robot platen  108 .  
         [0022]    Inside top portion  102  and coupled to a base plate  102   b  are an adhesive dispense system  114 , part assembly station  112 , and a part pick up station  116 . Adhesive dispense system  114  is described in further detail below.  
         [0023]    Part assembly station  112  is an area where an object may be assembled with another. Part pickup system  116  is an area where manipulator device  110  can pick up a part have it operated on at a station such as the adhesive system and then move the part at the assembly station.  
         [0024]    Bottom portion  104  provides a rigid support base for top portion  102 . Bottom portion  102  also provides an area to place an AC distribution enclosure as well as mount controls and provide various storage areas.  
         [0025]    A computer  150  including is provided to control the manipulator device  110 , part processing station  114  and other parts of the system  100 . Computer  150  can be any general purpose computer, such as a small office computer running the WINDOWS operating system, as sold by Microsoft, Corp. of Redmond , Wash. Computer  150  will typically include a display screen, keyboard, sensor inputs and other input output connections.  
         [0026]    In operation, under computer control or, optionally under manual control, manipulator device  110  utilizing the air bearing formed between manipulator device  110  and robot platen  108 , will move over to part pickup station  116  where it will acquire a work piece. Manipulator device  110  will then move the work piece to the part assembly station  112  such as an adhesive dispensing system  114 . There, the adhesive dispensing system  114  applies adhesive to the work piece. The manipulator device  110  will then move the work piece to part assembly station  112  where the manipulator device  110  will place the work piece on a second work piece while applying force to connect the two work pieces.  
         [0027]    The manipulator  110  is operable to move a workpiece in at least X, Y, and Z directions for positioning over the nozzle  202  for the application of liquid to the workpiece at predetermined locations thereon. The manipulator  110  is also operable to position the workpiece over an upward viewing camera to confirm the locations of the drops applied to the workpiece.  
         [0028]    The details of construction and operation of the manipulator  110  are disclosed in co-pending U.S. patent application Ser. No. ______, entitled “MANIPULATOR/END EFFECTOR HEAD FOR ROBOTIC ASSEMBLY”, and filed May 25, 2001. The disclosure of that application is incorporated herein by reference.  
         [0029]    [0029]FIGS. 2, 3 and  4  are views of an adhesive dispensing station  114  in accordance with the teaching of the present inventions. Illustrated is a dispenser  201  including a nozzle  202  for the application of one or more drops of liquid. A first vision system designated generally  204  is positioned to view drops that are formed from time to time on the end of the nozzle  202 . The vision system  204  is operable to provide information to a computer to add in calculating the volume of a drop prior to its application to a workpiece. Vision system  204  is also operable to provide a signal indicative of the location of a workpiece relative to the drop to control the application of the drop to the workpiece. A second vision system  212  may be provided to view the surface of a workpiece where one or more drops have been applied and provide signals indicative of the locations of the applied drops. A blotting device may be to remove drops from the nozzle if the drops are too large and/or to remove unwanted build-up of liquid, for example, glue, from the nozzle.  
         [0030]    The blotter mechanism  303  is also mounted on the base  214  and is operable for selectively, and on command, to remove a drop or build up of liquid from the nozzle  202 . The blotter  303  includes a cassette  304  that is moveably mounted on the base  214  via an X-Y movement mechanism  306 . The mechanism  306  is operable to move the cassette  304  into selective engagement with the nozzle  202  to remove a drop therefrom or a build up of liquid on the nozzle  202 . If the drop  602  is large, as calculated by computer  150 , the blotter will blot the drop  602 . Also, if too much time elapse after the formation of a drop, the drop is blotted off. In one embodiment, blotting is done on a regular basis to ensure fresh drops.  
         [0031]    The dispenser  201 , nozzle  202  and blotter  303  are mounted on a base  214 . A computer  150 , such as a PC, is seen schematically in FIG. 1, the computer  150  is connected to various components of the apparatus  114  for controlling operations of the apparatus  114 .  
         [0032]    The operation of the various components of the apparatus  114  are controlled by the computer  150  which is connected to the cameras  205  and  207  to form the vision systems. The computer  150  is also connected to controller elements of the X-Y blotter movement mechanism  306  and the manipulator  110  to provide signals thereto to control their operations. Likewise, the computer  150  is connected to the dispenser  201  to provide signals thereto for controlling its operation. The computer  150  is also operable to receive information from the cameras  208  and  209 , process that information and generate control signals for controlling operation of the dispenser  201 , manipulator  110 , and blotter  303 .  
         [0033]    The dispenser  201  includes a means for supplying the liquid such as a positive displacement pump  402 , or any other pumping means such as a rotary pump and the like. Turning to FIG. 5, in the embodiment illustrated, the pump is a syringe like device having a body  302  forming a reservoir  502  with a piston  504  therein which is selectively driven forward and backward in the body  302  by a servo motor  506  connected to the piston  504 . Alternatively, a separate source of liquid may be connected in flow communication with the dispenser  201  and the nozzle  202  and would be operable to supply liquid thereto. As shown in FIG. 5, the nozzle  202  is upwardly opening having a free end  508  that is at the top of the nozzle  202 . A discharge orifice  510  opens onto the free end  508  for the formation of a drop thereon. The nozzle  202  can be in the form of a small diameter needle with a flow passage therein communicating between the reservoir  502  and the exit orifice  510 . When dispensing small quantities of liquid, the nozzle  202  can have an outside diameter in the range of about 0.008 inches through 0.030 inches and the orifice  510  can have a diameter in the range of about 0.004 inches through about 0.020 inches. When formed, the drop will be on and project upwardly from the free end  508  of the nozzle. Such dispensers  201  are commercially available of which the DISPENSE  2000  available from IVER Corp. of North Springfield, Vt. is an example. In one embodiment of the present invention, the nozzle  202  is stationary while the workpiece is moved by an overhead robotic manipulator  110  to and into the drop as described below. In an alternate embodiment the workpiece is stationary and the nozzle  202  is moved to the workpiece or, both the workpiece and nozzle  202  could be moved. Further, the nozzle  202  could be oriented in a downward direction whereby the drop would form on the bottom end and be applied to an upwardly facing surface of the workpiece. Other orientations of the nozzle  202  could also be utilized.  
         [0034]    The first and second vision systems  204  and  210  can be the same general type vision systems but mounted at different orientations and locations for viewing the nozzle and/or the workpiece. The first vision system  204  includes a video camera and lens assembly and the first vision system  204  is adjustably mounted on the base  214  by an adjustable bracket  216 . The bracket  216  permits adjustment of first vision system  204  in two axes i.e. horizontally and vertically. The adjustable bracket  216  can also permit rotation of the lens portion of the first vision system  204  for proper viewing of the free end  508  of the nozzle  202  along with the drop thereon and a portion of the workpiece as seen in FIGS.  5 - 7 . The first vision system  205  is designed to be spaced far enough from the nozzle  202  to allow access to the nozzle  202  by a workpiece for the application of liquid thereto while permitting adequate viewing of the drop and workpiece.  
         [0035]    In the illustrated embodiment of FIGS.  2 - 4  the first vision system  204  is allowed to provide generally horizontally viewing of the side of the nozzle  202  as well as the drop. A back light  206  is mounted on the base  114  and, as the lens assembly  207  views the back light  206  in its position on the back side of the nozzle  202 , the back light  206  illuminates and silhouettes the drop and nozzle. The back light  206  can be any suitable light source as for example, a IT  9420  by Illuminations Technology of Syracuse N.Y., type light.  
         [0036]    The second vision system  210  also includes a video camera with a lens assembly. The second vision system  210  is positionally adjustably mounted on the base  114  by a bracket  220 . The bracket  220  permits adjustment of the position of the lens system  211  in X, Y, and Z axes. A suitable light  212  is provided to illuminate the object to be viewed by the second vision system  210 . As seen in FIG. 2, the light produces a ring light  222  that surrounds objective lens end of the lens assembly  211  of second vision  210 . In a preferred embodiment, the lens assembly of second vision system  210  directs it field of view generally upwardly to view the bottom surface of a workpiece attached to a manipulator device  110 .  
         [0037]    The cameras for the first vision system  204  and the second vision system  210  can be any suitable video camera such as an XC- 75  available from Sony Corp. The focus distance, field of view, depth of field and focal length of the cameras for the first vision system  204  and the second vision system  210  are such as to permit their positioning relative to the nozzle and workpiece so as to not interfere with the operation of the apparatus.  
         [0038]    [0038]FIGS. 6 a - 6   d  illustrate a view of nozzle  202  as it dispenses adhesive in order to determine the volume dispensed. FIG. 6 a  is a view of nozzle  202  with opening exit orifice  510 . FIG. 6 b  and FIG. 6 c  shows the build up of a drop  602  on nozzle  202 . FIG. 6 c  shows the final size drop  602  before application to a work place. These figures shows first vision system  204  image. The information gathered by first vision system  204  is sent to computer  150  where the volume of the drop is calculated. As camera  205  takes an image of the drop, computer  150  is operable to view slices of the drop and calculate the volume of the slices as individual cylinders and then sum the cylinders together to get the overall volume. FIG. 6 d  illustrates a drop  602  with lines  604  showing the divisions where the cylinder calculations are made.  
         [0039]    [0039]FIG. 7 illustrates the application of an adhesive to a workpiece  702 . FIG. 7 a  illustrates a workpiece  702  attached via a vacuum to a gripper  704  which is attached to manipulator  110 . Also illustrated is needle  202  with a drop  602  formed thereupon. Both the workpiece  702  and needle  202  with drop  602  are shown within the field of view  706  first vision system  204  of the camera  205 . At this point, the volume of drop  602  on nozzle  202  can be calculated.  
         [0040]    [0040]FIG. 7 b  shows the work piece moved down (z-axis direction) to the glue drop to get the glue on to workpiece  702 . Then, in FIG. 7 c , the work piece is moved up from the nozzle  202 . Now, a second glue drop  708  is attached to work piece  702 . Computer  150  is operable to calculate the volume of the glue drop  602  remaining on nozzle  202 . By taking the difference between the original drop in FIG. 7 a  and the reduced drop in FIG. 7 c , the amount applied to work piece  702  can be determined to ensure proper coverage. While FIGS. 7 a - 7   c  show the application of one dot of glue  708 , any number can be applied by moving the workpiece and applying more glue.  
         [0041]    [0041]FIG. 8 illustrates a view of workpiece  702  as seen from second camera  213 . Illustrated is workpiece  702  with glue drop  708  applied. This view allows for a qualitative indication of glue placement.  
         [0042]    A flow chart of the operation of the adhesive assembly system  150  is seen in FIG. 9. In step  902 , a pump is pressurizing liquid for dispensing from the nozzle  202  and a camera  205  views the formation of the drop  602 . In step  904 , information regarding the drop  602  is transmitted to the computer  150  which processes the information to provide a signal indicative of the volume of the drop as it is being formed and the final volume of the drop.  
         [0043]    As discussed in conjunction with FIG. 6,the volume calculation is preferably accomplished by viewing the drop in slices, measuring the width of each slice within the drop and using the measured dimensions to calculate an approximate volume of each slice of the drop and thereafter adding (integrating) the calculated volumes to provide an estimate of the volume of the drop. It is assumed for the calculations that the drop  602  is spherical. If the liquid to be dispensed has a generally constant viscosity, the volume could be estimated by knowing the height or height and width of the drop thereby simplifying the calculation of an estimated volume. The computer provides a signal to the servo motor attached to the pump to control operation of the pump and thus the size of the drop formed on nozzle  202 . The size of the drop is monitored throughout its formation and when the desired volume is reached, operation of the pump is terminated and the drop is subsequently applied to a workpiece  23 . The size of the drop is re-evaluated after its formation is stopped in step  906 . Should the drop  602  be too large, in step  908 , the computer sends a signal to the blotting mechanism to remove the oversized drop after which a subsequent drop is formed for application to the workpiece  23 . Should the drop be too small, in step  910 ,the pump can be reactivated to enlarge the drop or the drop may be removed by the blotter and a new one formed for subsequent application.  
         [0044]    When the drop  602  reaches a target volume or a volume within a target range of volumes, in step  912 , the computer  150  terminates operation of the pump and sends a signal to the manipulator  110  to lower the workpiece into the drop  602 . The vision system  205  also views the movement of the workpiece into the drop  602 . When the workpiece has moved into the drop a predetermined amount, downward movement of the workpiece is terminated by control of the manipulator  110 .  
         [0045]    The computer  150  is also connected to the manipulator  110  to provide control signals thereto for controlling operation of the manipulator which moves a workpiece in at least X, Y, and Z axes directions as preprogrammed for a particular workpiece. Rotational movement in a θ direction may also be provided. The manipulator  110  first moves a workpiece into the appropriate X-Y location over the nozzle  212 . When in the appropriate X-Y location, the manipulator  110  will lower (Z direction movement) the workpiece into the drop  602  a predetermined distance. When the workpiece is in the appropriate X-Y location and prior to Z direction movement, the vision system  205  views the space between the top of the drop and the bottom surface of the workpiece onto which the drop is to be applied. Data is gathered by the computer  150  from the vision system  205  regarding the spacing. The computer then calculates the amount of Z direction movement that is required for the appropriate movement (to achieve application of the correct amount of liquid) of the workpiece into the drop  602 . A signal is then provided to the manipulator  110  by the computer  150  to effect the appropriate Z direction movement.  
         [0046]    After application of a drop to the workpiece the vision system  205  continues to view the remaining portion of the drop on the nozzle  202 . In step  914 , the volume of the remainder of the drop is calculated as described above. The remaining volume is subtracted from the beginning volume to confirm that the target value for volume of applied drop size was achieved. If the target value of drop size was achieved then, in step  916 , the workpiece will continue to be processed for example by the application of additional drops of liquid and the subsequent formation of an assembly using the workpiece. If the target value was not achieved then, in step  918 , the part may be rejected, or if the drop size is not large enough additional liquid may be applied. If the workpiece can be reworked and is reworked, it can then continue on for additional processing for completion.  
         [0047]    After application of the drop  602 , the workpiece is then raised in the Z direction and advanced to the next location on the workpiece needing a drop of liquid or if the workpiece is completed with the application of the drop, then the workpiece can be moved to a position over the second vision system  213  for verification of the locations and/or sizes of the applied drops. The camera  213  provides data to the computer  150  for analysis to determine if the drops have been applied in the appropriate X-Y locations. If not, the part may be rejected or returned to the nozzle for correction by the application of drop(s) in the appropriate location(s).  
         [0048]    In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.  
         [0049]    When introducing elements of the present invention, or the preferred embodiment(s) thereof, the articles “a,” “an,” and “the” are intended to mean there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the specific listed elements.  
         [0050]    As various changes could be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.