Patent Publication Number: US-2011063426-A1

Title: Vision system and method for inspecting solar cell strings

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/243,333, filed Sep. 17, 2009, hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to solar cell strings. In particular, the invention is directed to a vision system and a method for inspecting a solar cell string. 
     BACKGROUND OF THE INVENTION 
     To make automatic placement of strings onto glass in a Glass Layup System, the robot placing the strings must be accurate and properly guided to pick and place the string. Vision inspection and alignment of strings in a lay-up system is critical to the system&#39;s success. Existing systems place individual strings to a vision table and a camera is moved under servo control along the length of the string, inspecting and gathering position data. The string is then picked from the vision table and placed into a module. The existing systems have not accounted for ambient light levels and variations. The result is a vision system that is difficult to set-up and susceptible to changes in ambient light that may occur at different times of day in a production facility with windows. 
     Vision inspection and guidance is most reliable when the lighting of the object is carefully controlled and shielded from the effects of ambient light. It is the design of these systems and the management of the light in the environment that makes the system less susceptible to those changes in light throughout the working day. 
     It would be desirable to develop a vision system and a method for inspecting objects, wherein the system and method minimize a risk of damage and susceptibility to a surrounding environment. 
     SUMMARY OF THE INVENTION 
     Concordant and consistent with the present invention, a vision system and a method for inspecting objects, wherein the system and method minimize a risk of damage and susceptibility to a surrounding environment, has surprisingly been discovered. 
     The key to success of any vision system is proper optics on the camera and lighting of the work piece. Proper lighting is frequently difficult to achieve since handling systems and cycle times may make it undesirable to place the work piece in a “dark box” for imaging. The present invention provides a small, localized area of lighting control (lighting zone) instead of a large box of darkness. 
     In addition, the vision system and methods of the present invention provide vision inspection and string location without placing the string onto an inspection table, thereby minimizing cycle time (higher throughput). The vision system and method of the present invention also minimize the risk for cell or string damage because the solar cell string is only picked up once (at a stringer) and placed once (onto the ethylene vinylacetate copolymer (EVA) on the glass panel or onto a matrix assembly fixture). 
     In one embodiment, a vision tunnel comprises: a housing having a plurality of panels to define a cavity therebetween; a lighting zone disposed adjacent the housing, the lighting zone including a light source and a light shield which cooperate to illuminate the object disposed in the lighting zone, while blocking at least a portion of an ambient light; and a sensor disposed in the cavity of the housing to scan the object disposed in the lighting zone. 
     In another embodiment, a vision system for inspecting an object comprises: a housing having a plurality of panels to define a cavity therebetween; a lighting zone disposed adjacent the housing, the lighting zone including a light source and a plurality of first light shields, wherein the first light shields are positioned to define a channel therebetween; a pick bar for moving the object through the lighting zone, the pick bar including an engaging device coupled to a main body thereof; and a sensor disposed in the cavity of the housing to scan the object. 
     The invention also provides methods for inspecting an object. 
     One method comprises the steps of: providing a lighting zone including a light source and a plurality of first light shields, wherein the first light shields are positioned to define a channel therebetween; providing a pick bar for moving the object through the lighting zone, the pick bar including an engaging device coupled to a main body thereof; scanning the object as the object moves through the lighting zone to gather a data relating to at least one of a position and an orientation of the object; and controlling a movement of the pick bar based upon the data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiment when considered in the light of the accompanying drawings in which: 
         FIG. 1  is a perspective view of a vision tunnel according to an embodiment of the present invention, shown with a panel in a closed position; 
         FIG. 2  is a perspective view of the vision tunnel of  FIG. 1 , shown with a panel in an opened position; 
         FIG. 3  is an enlarged fragmentary perspective view of the vision tunnel of  FIG. 1 ; 
         FIG. 4  is a fragmentary front elevational view of a vision system according to an embodiment of the present invention; and 
         FIG. 5  is an enlarged fragmentary perspective view of the vision system of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
     The following detailed description and appended drawings describe and illustrate various embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. In respect of the methods disclosed, the steps presented are exemplary in nature, and thus, the order of the steps is not necessary or critical. 
       FIGS. 1-3  illustrate a vision tunnel  10  according to an embodiment of the present invention. The vision tunnel  10  includes a housing  12  having a first end  14  and a second end  16 . The housing  12  includes a plurality of panels  18  which cooperate to define a cavity  20  therebetween. As shown, at least one of the panels  18  is slidably coupled to a plurality of frame rails  22  to allow movement of the at least one of the panels  18  for accessing the cavity  20 . As shown, a light sensor  24  is disposed within the cavity  20 . As a non-limiting example, the sensor  24  is a camera such as an Insight® camera manufactured by Cognex Corporation. 
     In certain embodiments, a base  26  is disposed at the first end  14  of the housing  12  to provide stability to the vision tunnel  10 . A lighting zone  28  is disposed at the second end  16  of the housing  12 . 
     As more clearly shown in  FIG. 3 , the lighting zone  28  includes a plurality of enclosure elements  30 , a plurality of first light shields  32 , and a light source  34 . As a non-limiting example, the lighting zone  28  may include a plurality of bearing rails (not shown) for aligning and guiding an object through the lighting zone  28 . The enclosure elements  30  are mounted to the second end  16  of the housing  12 . The enclosure elements  30  include a plurality of flanges  36 , which are bent to enclose a portion of the cavity  20 . 
     The first light shields  32  are formed from a “backlighting” material having light reflective qualities. In certain embodiments, the backlighting material is white in color. Each of the first light shields  32  is coupled to at least one of the enclosure elements  30  and the housing  12 . As a non-limiting example, each of a pair of the first light shields  32  is coupled on opposite sides of the housing  12  to enclose a portion of the cavity  20  and define an unenclosed channel  38  therebetween. 
     The light source  34  may be any device for emitting light such as an array of light emitting diodes, for example. The light source  34  is disposed adjacent the housing  12  and at least one of the enclosure elements  30  and adapted to illuminate the lighting zone  28 . 
       FIGS. 4-5  illustrate a robotic pick bar  40  disposed adjacent the vision tunnel  10 , which are collectively referred to as a vision system, according to an embodiment of the present invention. The pick bar  40  includes a main body  42 , a plurality of engaging devices  44 , and a plurality of second light shields  46 . In certain embodiments, the pick bar  40  includes a plurality of bearing rails (not shown) to cooperate with bearings on the vision tunnel  10  for aligning and guiding the pick bar  40  through the light zone  28 . 
     The main body  42  is an elongate member coupled to a robotic controller  41  for moving and rotating the pick bar  40 . The engaging devices  44  are coupled to the main body  42  and adapted to engage an object such as a solar cell string  48  to securely move the solar cell string  48 . As a non-limiting example, the engaging devices  44  are suction cups. As a further example, the engaging devices  44  are formed from a transparent or translucent material. In certain embodiments, a fixed light source is integrated into each of the engaging devices  44  to maximize illumination of the solar cell string  48 . 
     The second light shields  46  are formed from a “backlighting” material having light reflective qualities. In certain embodiments, the backlighting material is a white colored material. The second light shields  46  are disposed between the engaging devices  44  and the main body  42 . As shown, the second light shields  46  are coupled to the main body  42  adjacent each of the engaging devices  44 . It is understood that any number of second light shields  46  may be used. 
     In use, the pick bar  40  guides an object, such as the solar cell string  48 , through the lighting zone  28  to be illuminated by the light source  34  and thereafter scanned by the sensor  24 . As more clearly shown in  FIGS. 4-5 , the pick bar  40  is guided through the channel  38 . The second light shields  46  mounted on the pick bar  40  cooperate with the first light shields  32  to effectively enclose a portion of the cavity  20  to shelter the cavity  20  from ambient light. The light shields  32 ,  46  also provide a reflective surface for light emitted by the light source  34  to backlight the solar cell string  48 . The light shields  32 ,  46 , the enclosure elements  30 , and the panels  18  cooperate to minimize an amount of ambient light entering the lighting zone  28  and cavity  20 . As the sensor  24  scans the solar cell string  48 , a light outside of the lighting zone  28  does not affect the data gathered by the sensor  24 . 
     In one embodiment, the sensor  24  gathers positional data of the solar cell string  48  in the form of captured images. The positional data gathered from the sensor  24  is processed along with a position of the robotic controller  41  (e.g. encoder) at the time of the image capture for alignment correction. Using this data, the solar cell string  48  is placed into a module properly oriented and aligned. Any strings found to contain damaged cells will be rejected. 
     Specifically, the sensor  24  captures an image of the individual cells of the solar cell string  48 . The captured images are analyzed to determine a transverse center point between a top edge and a bottom edge of each of the individual cells of the solar cell string  48  along the longitudinal axis thereof. As a non-limiting example, the transverse center point of each of the outer most individual cells represent an end point on an “orientation line” of the solar cell string  48  and provide a reference orientation thereof. As a further example, a longitudinal center point along the orientation line can be determined. In certain embodiments, a rotation of the solar cell string  48  is calculated as an angular offset between the orientation line and a calibrated zero-degree position of the robotic controller  41 . A relative position is calculated as a rectangular offset between the calculated longitudinal center point and a calibrated center of a wrist joint of the robotic controller  41 . 
     The robotic controller  41  relies upon the data gathered and calculated by the vision system to control the pick bar  40  and to properly place, move, rotate, and orient the solar cell string  48 . 
     The vision system and methods according to the present invention minimize a susceptibility to changes in ambient light throughout the working day. In addition, the vision system and methods of the present invention provide vision inspection and string location without placing the solar cell string  48  onto an inspection table, thereby minimizing cycle time (higher throughput). The vision system and method of the present invention also minimize the risk for damage to the solar cell string  48  or individual cells because the solar cell string  48  is only picked up once (e.g. at the stringer) and placed once (e.g. onto the EVA on the glass panel). 
     From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, make various changes and modifications to the invention to adapt it to various usages and conditions.