Abstract:
A system for accurately determining whether a work piece such as a plastic molded bottle cap is defective from quality norms and removing those defective pieces from a stream of work pieces includes a feed conveyor for serially delivering work pieces where adjacent work pieces are in contact with each other. The feed conveyor delivers these work pieces onto an inspection conveyor with a plurality of air holes. Air is forced through the air holes to impinge upon the work piece and thus rapidly accelerate them away from the adjacent work piece it was in contact with and move through an inspection station. An air blower provides pressurized air through the air holes to continue accelerating and separating the work pieces as they move. The inspection station includes a camera to image each work piece. The camera communicates these images to a processing unit such as a computer that can rapidly compare the image to a set of quality control standards each work piece should meet.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to article inspection systems and particularly to vision systems for inspecting work pieces such as plastic molded closure caps for containers and the like. 
       BACKGROUND 
       [0002]    During the manufacture of many parts such as plastic molded closure caps, a number of defects in the closure cap may exist which should cause the cap to be rejected. Commonly, closure caps of this type have a liner inserted therein against the inner surface of an end wall of the cap. Typically, the cap has a skirt projecting annularly from the peripheral rim of the end wall and the skirt may include a closure or sealing mechanism such as threads for cooperation with mating threads around the neck of a bottle, container, or the like. Examples of defects in such closure caps include a liner which is positioned off-center within the closure, a missing liner, a malformed liner (commonly referred to as a “moon-cut liner”), a cap which is asymmetric or off-round, a cap having an edge broken or flashing on the edge from extraneous plastic material, a pull tab defect on the liner caps with excessively large “gates” resulting from the molding process, or other similar problems. Such flaws or defects are sometimes produced during the manufacturing process, or as a result of contamination or damages after manufacture but prior to the filling of the container. 
         [0003]    Machine vision systems represent one technology for acquiring or sensing an image of at least a selected portion of a work piece, such as a cap as previously described, through an electronic sensor or camera. The image generated by the camera is then analyzed by a computer program for one or more of the above-described defects. Vision systems are commonly used to determine the existence of any marks or defects in the image of the cap and the acceptability of any such marks or defects by use of a vision computer as described. 
         [0004]    While human vision may outperform its automatic equivalent in the ability to analyze very complex, everyday scenes, when it comes to repeated tasks, such as the inspection of plastic molded caps over and over again, a human observer understandably tires, loses concentration, and makes mistakes. Machine vision inspection of such articles is known to provide some important advantages, including sophisticated image processing/analysis, repeatable performance, image acquisition for diagnosis and set up, ability to inspect a variety of articles in large tolerance and required part placement. Moreover, at inspection rates of up to 1600 parts per minute or more, each part or cap spends on the order of 33 milliseconds at an inspection station. At such speeds, only a machine vision system is fast enough to reliably and repeatedly inspect such articles. 
         [0005]    While known vision systems have the above described advantages for inspecting articles such as plastic molded caps and the like, they do have specific and significant limitations. Vision systems typically rely on video cameras to image the article to be inspected and detect any flaws. The resolution of the camera, or its ability to detect a flaw, is directly related to its ability to capture an accurate and reliable image of each individual cap, article, or similar item. Typically, plastic molded caps are manufactured by the hundreds of thousands and each individual cap must be inspected by the vision system for quality control purposes. The caps are typically gathered in an accumulated mass and are, at best, similarly oriented on a flat surface. For accurate vision inspection and detection of flaws, the vision system must be able to precisely and accurately produce an image of each individual cap without interference from the surrounding environment or other caps. Furthermore, inspection rates required of such systems mandate that the individual images be serially produced, analyzed, and acted upon accordingly for each individual cap, once again without interference, for accurate detection of relatively small flaws or problems. 
         [0006]    One problem in efficiently creating photographic records of each plastic molded cap is providing enough separation between adjacent caps on a conveyor leading to the vision system. One solution is disclosed in U.S. Pat. No. 6,384,421, assigned to the assignee of this invention and hereby incorporated by reference entirely. Vision systems of this type separate caps by funneling a mass of caps into a single-file conveyor belt and then providing a vertical incline or ramp to accelerate and separate each cap from adjacent caps. This process is effective, but often requires a relatively large space and floor space in most industrial settings is limited. Therefore, an improved vision system that allows high-precision and high-speed inspection while occupying a small foot-print is needed. 
       SUMMARY OF THE INVENTION 
       [0007]    This invention provides a machine and method for inspecting masses of industrial work pieces such as plastic molded bottle caps and other articles. This invention overcomes the limitations and problems associated with known inspection systems. 
         [0008]    In a first embodiment, this invention is a system for accurately determining whether a work piece such as a plastic molded bottle cap is defective from quality norms and removing those defective pieces from a stream of work pieces. The system includes a feed conveyor for serially delivering work pieces where adjacent work pieces are in contact with each other. The feed conveyor delivers these work pieces onto an inspection conveyor that is generally horizontal. Adjustable guide rails and a guide rail cover create a closed passage just large enough for the work pieces to flow through without impediment. The inspection conveyor may also have one or more machined grooves to allow slightly defective work pieces to flow through the passage without dragging or getting stuck. 
         [0009]    The inspection conveyor is provided with a number of angled air holes there through and the system also contains a separate air jet near the feed conveyor. The air jet receives a signal from a trigger such as a photoelectric switch that a work piece has entered the opening of the passage, and a control mechanism opens a high speed valve, allowing high pressure air to blow through the air jet and onto the rear surface of the work piece. The work piece is thus rapidly accelerated away from the adjacent work piece it was in contact with and moves through the passage. An air blower provides pressurized air through the air holes to continue accelerating and separating the work pieces as they move from the air jet through the passage. This air blower is controlled using a pressure sensor and a variable speed drive mechanism for the air blower&#39;s motor. This system is customizable for a variety of shapes and sizes of work pieces that need to be inspected for flaws as well as other operating parameters. 
         [0010]    The inspection conveyor in one embodiment is generally made with a translucent plastic material that transmits some light. In the middle of the passage, the guide rails and the guide rail cover each have an opening where an inspection station is located. This inspection station includes at least one camera and one light source. The light source can be any kind of light that illuminates the work piece such as a bottle cap, and one embodiment uses an infrared light. The light source is located under the inspection conveyor and lights the underside of each work piece. The camera is located above the inspection conveyor and light source and is oriented perpendicular to the inspection conveyor so that a full image of each work piece can be recorded as the work piece moves through the inspection station. The camera is timely activated by a trigger such a photoelectric switch that determines exactly when a work piece is in position to be photographed. 
         [0011]    The camera communicates these images to a processing unit such as a computer that can rapidly compare the image to a set of quality control standards each work piece should meet. The processing unit analyzes each image for defects and controls a rejection mechanism located just after the camera and light source in the inspection station. This rejection mechanism can be another air jet or mechanical arm that pushes work pieces determined to be defective out of the passage through the opening at the inspection station. 
         [0012]    A second embodiment of the invention includes all the same elements as the first embodiment, but in this embodiment a portion of the inspection conveyor is replaced with a glass window at the inspection station. In this embodiment the light source and the camera can be switched so that the camera takes images from beneath the work pieces sitting on the inspection conveyor, and the light source illuminates the work pieces from above the inspection conveyor. Alternatively, this second embodiment could allow for two cameras positioned one above and one below the inspection conveyor instead of one camera and a light source. With two cameras, more work pieces that do not transmit light well can be analyzed and inspected. This second embodiment will allow for different kinds of work pieces to be inspected and sorted by this invention. 
         [0013]    This invention overcomes the above-described disadvantages of known vision inspection systems by providing a precision-controlled separation distance and time between each of a series of work pieces such as plastic molded bottle caps. The level of air pressure delivered by the air jet and the air blower as well as the angle of the air holes disposed through the inspection conveyor can all be altered to fit the needs of a specific set of work pieces, thereby giving the current invention much more precision than known gravity-driven inspection systems in a smaller work space. The processing and analyzing rates are not affected by the changes from known systems, as the current invention can handle the inspection of up to 1600 work pieces per minute, depending on the work piece and the air pressure configuration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the principles of the invention. 
           [0015]      FIG. 1  is a front view of a first embodiment of the air separator conveyor and inspection system according to this invention. 
           [0016]      FIG. 2  is a top view of the first embodiment of this invention showing work pieces flowing through the system&#39;s passage and inspection station. 
           [0017]      FIG. 3  is a top view of the first embodiment of this invention with the guide rail cover removed from the top of the device. 
           [0018]      FIG. 4  is a cross-sectional side view of the first embodiment of this invention illustrating the flow of work pieces and pressurized air. 
           [0019]      FIG. 5  is a top view of an inspection conveyor according to the first embodiment of this invention. 
           [0020]      FIG. 6  is a cross-sectional side view of the center of the inspection conveyor taken along line  6 - 6  of  FIG. 5  with arrows depicting how pressurized air flows on both sides of the inspection conveyor. 
           [0021]      FIG. 7  is an enlarged detailed top view of a the first end of the inspection system where a feed conveyor delivers work pieces to the passage and an air jet separates these work pieces. 
           [0022]      FIG. 8  is a cross-sectional view taken along line  8 - 8  of  FIG. 4  looking toward the first end of the inspection conveyor from the inspection station. 
           [0023]      FIG. 9  is a schematic representation of the first embodiment where a defective work piece is being removed from the passage. 
           [0024]      FIG. 10  is a cross-sectional front view of a second embodiment of this invention where the light source and the camera are reversed from the first embodiment. 
           [0025]      FIG. 11  is a cross-sectional front view of an alternative second embodiment of this invention where two cameras are used to capture images of each work piece. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Referring to  FIGS. 1-9 , a first embodiment of this invention of an air conveyor vision inspection system and associated method for industrial work pieces is shown. The inspection system  10  is used to individually inspect each of a number of work pieces  12  and determine if each work piece  12  meets quality control standards. These work pieces  12  can be any variety of industrially produced items, such as plastic molded bottle caps as shown in  FIGS. 1-11 . For bottle caps  12 , a batch  14  of the caps  12  is created using an injection molding process and then delivered by a feed conveyor  16  or other means to the inspection system  10 . Bottle caps  12  made in this manner typically include a peripheral skirt  18  projecting from a base or end wall  20  and a liner  22  may also be inserted into the cap  12 . 
         [0027]    As shown in  FIG. 1 , the inspection system  10  processes work pieces or caps  12  from a feed conveyor  16 , which comprises a belt  24  trained to travel around at least two rotating rollers  26  (of which one is shown in  FIG. 1 ), one of which drives the belt  24 . The feed conveyor  16  delivers caps  12  serially in the direction of arrow  28  to the system  10 . 
         [0028]    The structural features of this first embodiment of the invention are shown in detail in  FIGS. 1-4 . Referring to  FIGS. 1-2 , the system  10  includes an inspection conveyor  30 , at least two guide rails  32 , and a guide rail cover  34 . The inspection conveyor  30  is generally horizontal, but may also be set at an incline in other embodiments. The inspection conveyor  30  receives work pieces  12  onto a top surface  36  at a first end  40  and the work pieces  12  continue moving toward a second end  42  of the inspection conveyor  30 . The inspection conveyor  30  is generally planar on the top surface  36  as well as a bottom surface  38 . As shown in more detail in  FIGS. 5-6 , the inspection conveyor  30  further includes a shallow groove  44  running lengthwise from the first end  40  to the second end  42 . This shallow groove  44  is designed to allow work pieces or caps  12  with minor acceptable defects such as “high gate” to move down the inspection conveyor  30  without dragging or getting stuck. Air holes  46  extend through the inspection conveyor  30  from the bottom surface  38  to the top surface  36  and run lengthwise down the inspection conveyor  30 . These air holes  46  may be vertical or angled and may have any appropriate inner diameter for delivering pressurized air as shown by arrows  48  in  FIG. 6 . 
         [0029]    The guide rails  32  are coupled to the inspection conveyor  30  on the top surface  36  by placing the guide rails a certain distance  50  apart from each other, this distance  50  being at least the width of work pieces  12  being inspected plus an allowable tolerance for the passage of irregularly shaped work pieces  12 . The guide rails  32  have projections  52  such as pegs extending away from the inspection conveyor  30 . The guide rail cover  34  is shown in  FIG. 2  as a generally flat and horizontal body with slots  54  designed to accept the projections  52 . The guide rail cover  34  can then be coupled to the guide rails  32  by tightening a nut  56  on a projection  52  if the projection  52  is threaded, or by any alternative appropriate means depending on the projections  52 . Thus connected, the inspection conveyor  30 , guide rails  32 , and guide rail cover  34  create an air tunnel or passage  58  adapted to accept work pieces  12  of a given shape and size (see  FIG. 8  for detail). 
         [0030]    In the first embodiment shown in  FIGS. 1 and 4 , the inspection conveyor  30  rests on top of and is coupled to the side walls  62  of an air delivery chamber  60 . This chamber  60  may be any size or shape, but is shown in  FIGS. 1-4  as a rectangular box with side walls  62 , a bottom wall  64 , and an inlet  66  connected an air blower  80 . The inspection conveyor  30  can be coupled to the side walls  62  using a plurality of screws  68  or other connectors. The entire system  10  is further stabilized and coupled together with mounting brackets  70  that are attached to the guide rail cover  34  and the side walls  62  by screws  68  or by other acceptable means such as T-nuts  72 . 
         [0031]    The delivery rate or speed of the feed conveyor  16  depends upon the required inspection rate for the work pieces  12 , the size of the work pieces  12 , and other relevant factors. For example, if the work pieces  12  are plastic molded caps  12  with a diameter of 1.1 inches and an inspection rate of 1600 caps per minute is desired, the feed conveyor  16  will operate at about 147 feet per minute maximum speed for the delivery of the caps  12  to the first end  40  of the inspection conveyor  30 . The corners of the guide rails  32  and guide rail cover  34  may be chamfered or beveled as shown in  FIGS. 1-4  to facilitate work pieces  12  entering the passage  58 . The inspection conveyor  30 , guide rails  32 , and guide rail cover  34  are all made of a smooth and translucent material such translucent UHMW plastic which is very wear resistant in one embodiment of this invention. 
         [0032]    In  FIG. 4  the cross-section of the system  10  is shown to emphasize the air flow driving the work pieces  12 . The air blower  80  takes ambient air as shown by arrows  82  through a filter go (not shown in detail) to keep dust particles out of the inspection system  10 . The air blower  80  then pumps pressurized air into the air delivery chamber  60  through an inlet  66  as depicted by arrows  84 . A pressure sensor  86  inside the air delivery chamber  60  measures the air pressure being delivered by the air blower  80  and sends this information to a controller  88 . This controller  88  maintains the desired air pressure by adjusting a variable-speed drive mechanism (not shown) of the air blower  80  to increase or decrease the pressure as necessary. The controller  88  can also monitor if the air blower filter go becomes dirty and ineffective and give users a signal to change that filter go. The controller  88  can be any appropriate computer or device for the tasks described, including a PLC to create the control algorithm. Also, the passage  58  created by the inspection conveyor  30 , guide rails  32 , and guide rail cover  34  creates a “wind tunnel effect,” which amplifies the effect the pressurized air has on each work piece  12  and reduces the need for large volumes of pressurized air. The airflow produced by the air blower  80  and delivered to the inspection conveyor  30  can be adjusted depending upon the operating parameters of the system  10  and the work pieces  12 . One adjustment may include the removal of portions of the guide rails  32  proximate a downstream portion of the passage  58  to alleviate congestion of the caps in the passage by allowing for induced airflow for more volume of air through the system. 
         [0033]    Positioned intermediate the first end  40  and the second end  42  of the inspection conveyor  30  is an inspection station  100 , which includes a camera  102  mounted generally perpendicular to and above the top surface  36  of the inspection conveyor  30 . The inspection station  100  further includes a light source  104  mounted opposite the camera  102  and underneath the inspection conveyor  30 , which is made of translucent material as described above. In the current embodiment, the light source  104  may be an infrared or other color LED strobe light which is preferably adjustable to provide a frequency that is appropriate for the inspection rate of the work pieces  12 , typically as high as 1600-2000 work pieces per minute. Backlighting of the work pieces  12  by the light source  104  in many instances allows for better contrast and image quality by the camera  102 . An infrared light source  104  provides increased imaging capabilities for particular colors of work pieces  12  such as white. However, other light sources and imaging arrangements are contemplated within this invention. 
         [0034]    A trigger  106  such as a photoelectric switch shown in  FIGS. 1-4  activates the camera  102  and light source  104 . When the leading edge of a work piece  12  crosses the plane (depicted by line  108 ) of the trigger  106 , the trigger  106  sends a signal to a processing unit such as a computer (not shown). The processing unit coordinates the operation of the camera  102  and the light source  104  and the camera  102  captures an image of the work piece  12  illuminated by the light source  104 . The camera is operably coupled to the processing unit and sends the images immediately to the processing unit for analyzing. The processing unit then compares the image to certain quality control standards set by the user. In the case of bottle caps  12 , defects such as off-center or missing liners, a moon-cut liner, or a cap that is not properly shaped can all be detected. If any undesirable defect is detected by the processing unit, the processing unit sends another signal to a rejection mechanism  110  that is operably coupled to the processing unit. The rejection mechanism  110  may be any one of a number of items designed to remove a work piece  12  from the passage  58  or stream of work pieces. For example, an air jet  110  as shown in  FIGS. 1-4  and  9  may be positioned immediately following the camera  102  and light source  104  locations in order to blow a puff of pressurized air in the direction of arrow  112  ( FIG. 9 ) to remove a defective work piece  114 . The guide rails  32  and guide rail cover  34  may have an opening as shown in  FIGS. 1-9  to allow the inspection station  100  to capture images and remove selected defective work pieces  114  from the passage  58 . Again at the end of the inspection station  100 , the corners of the guide rails  32  and guide rail cover  34  may be chamfered or beveled as shown in  FIGS. 1-4  to facilitate work pieces  12  entering the passage  58 . 
         [0035]    The cross-section of the inspection conveyor  30  is shown in  FIG. 6 . The air holes  46  may be configured in any appropriate manner, but the first embodiment depicted in  FIG. 6  provides a typical arrangement. Most of the air holes  46  are angled approximately 45 degrees from vertical, but this angle may be changed depending on the work pieces  12  being moved and other parameters. One vertical air hole  120  is located at the inspection station  100 , and this vertical hole  120  keeps work pieces  12  moving at a steady rate. As shown in  FIG. 6 , the air holes  46  disposed between the first end  40  and the inspection station  100  can have a larger diameter and be spaced closer together than the air holes  46  disposed between the inspection station  100  and the second end  42 . The air holes  46  disposed after the inspection station  100  may also have a shallower angle than the air holes  46  before the inspection station  100 . This allows for work pieces  12  to be accelerated at a higher rate when the work pieces  12  need to be separated. Separation between the work pieces  12  is important to provide an accurate image and subsequent analysis of each work piece  12  without interference from adjacent work pieces  12 . For example, the air holes  46  upstream from the inspection station  100  may be spaced 0.50 inches apart and have inner diameters of 0.078 inches while the air holes  46  downstream from the inspection station  100  may be spaced 1.00 inches apart and have inner diameters of 0.062 inches. One other feature of the air hole arrangement shown in  FIG. 6  is the air holes  122  near the first end  40  may start vertical and progressively gain more angle for each adjacent hole  122  until the desired 45 degree or similar angle is achieved, but this arrangement is optional. 
         [0036]    As depicted by  FIGS. 3 and 7 , the front end of the system  10  has additional elements. An air jet  130  is located through at least one of the guide rails  32  to initially accelerate and separate entering work pieces  12  from adjacent work pieces  12 . The air jet  130  includes a trigger  106  such as a photoelectric switch, an air valve (not shown), and a control mechanism (not shown) such as the PLC controller  88  for the air blower  80 . In fact, the PLC controller  88  can be configured with a high speed module that can control and provide pulses of pressurized air in one millisecond increments. Once an entering work piece  12  crosses the plane (line  108 ) of trigger  106 , the trigger  106  sends a signal to the control mechanism, which in turn opens the air valve and sends a blast of pressurized air into the rear side of the work piece  12 . The air valve must be able to cycle at very high rates of speed for this application, and a MAC air valve that can open and close in less than five milliseconds is an appropriate example. Just like the other air holes  46 , the pressure to the air jet  130  can be controlled and modified for various sizes and weights of work pieces  12 . The position of the trigger  106  may be modified in a similar fashion. Additional air jets  130  can be included one on or both of the guide rails  32  at downstream locations proximate the inspection station  100  as needed. 
         [0037]    A second embodiment of this invention is shown in  FIGS. 10-11 . The second embodiment contains all the elements as the first embodiment described in detail above and in  FIGS. 1-9 , as well as one additional feature. Instead of the inspection conveyor  30  being made completely out of translucent plastic material, an inspection window  200  is placed in the inspection conveyor  30  instead of plastic material at the inspection station  100 . This window  200  can be used where the inspection conveyor  30 , guide rails  32 , and guide rail cover  34  are each made of a different material than transparent or translucent plastic. The window  200  preferably produces a high diffusion of light and may be created out of glass such as opal glass. The glass segment of the inspection conveyor  30  in this embodiment does not interfere with or change the orientation of air holes  46  extending throughout the inspection conveyor  30 . 
         [0038]    The second embodiment also enables a user to set up the inspection station elements in different ways. In  FIG. 10  the light source  104  and the camera  102  have been reversed so that the camera captures images of the work pieces  12  from the lower side of the inspection conveyor  30 . Another alternative setup is shown in  FIG. 11 , where two cameras  102  are located perpendicular to the stream of work pieces  12  with one below the inspection conveyor  30  and one above the inspection conveyor  30 . Therefore the second embodiment allows for a wide variety of work pieces  12  to be inspected by the system  10 , including work pieces  12  more solid and less translucent than plastic molded bottle caps. All the advantages of the first embodiment are present in the second embodiment, and the invention overcomes the difficulties of known industrial part visual inspection systems. 
         [0039]    One skilled in the art will appreciate that both embodiments can be used by a similar method to analyze and inspect a quantity of work pieces. This method includes feeding work pieces to an inspection conveyor, separating and moving adjacent work pieces by funneling pressurized air to the rear surfaces of the work pieces, inspecting the work pieces by illuminating them with a light source and capturing an image with a camera, analyzing these images against quality control standards, and rejecting defective work pieces from the flow of non-defective work pieces, which are discharged at the end of the inspection conveyor. 
         [0040]    This invention is not to be limited by what has been particularly shown and described, except as indicated by the following claims.