Patent Publication Number: US-9851200-B2

Title: Non-contact gaging system and method for contoured panels having specular surfaces

Description:
TECHNICAL FIELD 
     This invention relates to a method and apparatus for gaging the conformance of the surface of a contoured panel having a specular surface to a pre-defined specification. 
     BACKGROUND 
     Manufacturers of panels having specular surfaces, particularly panels formed into various curved shapes, are interested in measuring the shape of the formed panel to assess conformity of the actual formed shape to design specification. 
     It is therefore desirous to develop a system and method for comparing the formed surface of a particular panel to a pre-defined surface specification. 
     It is also desirous to develop a panel gaging system which may be integrated in, and operating in-line with, panel forming and processing systems. 
     It is thus also desirous for at least the above purposes to develop a system and method for quickly inspecting a curved panel to acquire data corresponding to the surface the panel, particularly as the panel is being transported on a conveyor between or after other processing operations. 
     SUMMARY 
     The disclosed system and method for inspecting and gaging the shape of a contoured panel includes, as components, (1) a system and method for acquiring three-dimensional surface data corresponding to the panel, and (2) a system and method for receiving the acquired surface data, comparing the acquired surface to a pre-defined surface description, and developing indicia of the level of conformance of the contoured panel to the pre-defined specification. 
     The surface data acquisition system may include a conveyor for conveying the panel in a first direction generally parallel to the first dimension of the panel, at least one display projecting a preselected contrasting pattern, and at least one camera, each one of the cameras uniquely paired with one of the displays, wherein each display and camera pair are mounted in a spaced-apart relationship a known distance and angle from the surface of the panel such that the camera detects the reflected image of the pattern projected on the surface of the panel from its associated display. 
     The surface data acquisition system may, in one embodiment, include two or more cameras, each one of the cameras being uniquely paired with one of the displays as described above, wherein each of the display and camera pairs are spaced apart from each other at least in a second direction across the second dimension of the panel such that each camera detects the reflected image of the pattern projected on the surface of the panel from only its associated display, and wherein the patterns detected by the two or more cameras together cover the entire surface in the direction of the second dimension of the panel. 
     The surface data acquisition system may also include a programmable control including at least one processor programmed to execute logic for controlling each of the cameras to acquire at least one image of the reflected pattern of the associated display on the panel as the panel is conveyed across the path of the projected pattern in the first direction, and logic for analyzing and combining the data acquired by the two or more cameras to construct surface data representative of the surface of the panel. 
     The disclosed gaging system may include at least one processor including logic for receiving the surface data, comparing the data to a pre-defined exemplary surface, and displaying or otherwise reporting selected information associated with the comparison. 
     The gaging system may utilize a single computer which controls the conveyor and the operation of the cameras, and includes the previously described surface data acquisition logic, as well as the optical distortion processing logic. Alternatively, the conveyor control, camera controls, surface data acquisition and optical processing may be integrated but implemented on separate or multiple processors or computers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic partial end view of one embodiment of the disclosed panel inspection system; 
         FIG. 2  is a schematic top view of the disclosed panel inspection system of  FIG. 1 ; 
         FIG. 3  is a schematic view of a three-frequency pattern which may be employed in one embodiment of the inspection system; 
         FIG. 4  is a schematic view of a two-frequency pattern which may be employed in another embodiment of the inspection system; 
         FIG. 5  is a top diagrammatic view of the arrangement of multiple display/camera pairs shown in  FIG. 2  including the angular orientation of the displays/cameras for a particular glass part; 
         FIG. 6  is a top view of a particular curved panel illustrating the areas of the panel surface analyzed by each of the display/camera pairs utilized in the system of  FIG. 1 ; 
         FIG. 7  is a flow chart describing the logic employed in one embodiment of the disclosed inspection system; 
         FIG. 8  is a schematic illustration of the pertinent geometric parameters that may be utilized to determine the elevation of a single point on the surface of the panel according to the steps depicted in  FIG. 9 ; 
         FIG. 9  is a flow chart describing the surface point resolution logic employed in one embodiment of the surface data acquisition portion of the system; 
         FIG. 10  is a schematic diagram of one embodiment of the disclosed inspection system installed in-line in a typical automotive glass sheet forming and tempering line; and 
         FIG. 11  is a schematic diagram of another embodiment of the disclosed inspection system installed in-line in a typical automotive windshield forming line. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
     Referring to  FIG. 1 , a panel inspection system, generally indicated as  10 , is disclosed and includes a conveyor  12  which conveys the panel, which in the illustrated embodiment is a glass sheet, G, in a first direction generally parallel to a first dimension of the panel. In the depicted embodiment, the contoured panel G is a generally rectangular vehicle windshield or backlight, having a first dimension which is the relatively smaller dimension (and which may alternatively be referred to as the height) and a second, relatively larger dimension (which may alternatively be referred to as the width). The panel G is curved about one or more axes of curvature that are generally parallel to the first direction. The conveyor  12  may be a single conveyor dedicated solely to transporting the panel G through the inspection system  10  which may be configured and/or operated as a stand-alone inspection system. Alternatively, the conveyor  12  may be one of a series of conveyors which convey the panel through a variety of process stations, such as, for example, heating, forming, and annealing or tempering stations found in a typical automotive, architectural and/or solar panel fabrication systems. 
     The inspection system  10  depicted in  FIGS. 1 and 2  also includes two or more displays  14 - 24 . Each display projects a contrasting pattern, such as, for example, those patterns shown in  FIGS. 4 and 5 , which pattern is projected onto the surface of the panel as it is conveyed beneath the screens. The depicted system  10  also includes two or more cameras  28 - 40 . Each one of the cameras  28 - 40  is uniquely paired with one of each of the corresponding number of displays  14 - 26 . In the depicted embodiment of the system  10 , an aperture is formed in the center of each of the displays  14 - 26 . The camera associated with a particular display is mounted such that the viewing aperture of the camera extends through the aperture in its associated display such that the principal viewing access of the camera is perpendicular to the surface the screen. It will, of course, be appreciated by those skilled in the art that each camera may be alternatively arranged at other locations with respect to its associated display, so long as the camera is positioned to detect the reflected image of the pattern projected on the surface of the panel from that display, and not detect reflected images of patterns projected from other displays in its field of view. 
     Referring still to  FIGS. 1 and 2 , the number and placement of the displays is dependent upon the size of the displays, as well as the width and the curvature of the panel. In the depicted embodiment of the system  10 , the camera/display pairs are positioned such that the principal viewing axis of each camera is generally perpendicular to the surface of the panel. The total number of camera/display pairs must be sufficient such that the total number of projected patterns span the entire width of the surface of the panel part to be analyzed. 
     Referring again to  FIG. 1 , the inspection system  10  also includes a programmable control, depicted in this embodiment as a computer  42 , which includes at least one processor programmed to detect the panel as it advances on the conveyor, control each of the cameras  28 - 40  to acquire one or more images of the pattern reflected off the surface of the panel as it is conveyed below the cameras/displays, construct the definition of the panel surface, and perform the desired optical distortion analysis (as depicted in  FIGS. 7-9 , and as further described hereinafter). In any embodiments of the system  10  (such as the embodiment depicted in the Figures) where the field of view of a camera in any width zone is smaller than the first dimension (height) of the panel, the system control may be programmed to acquire multiple images as the glass is conveyed in the first direction. It will be appreciated that the number of images acquire by each camera should be sufficient that the surface information developed from each image (as hereinafter described) can be combined to form a description of the entire surface across the height (i.e., in the direction of conveyance) of the panel. 
     Where, as in the depicted embodiment, the cameras are mounted with their viewing aperture extending through an aperture in the display, the system control  42  may be programmed to acquire multiple images as the glass is conveyed in the first direction to insure that an image of the reflected pattern is obtained in a previous or subsequent image of the moving sheet for that portion of the surface of the panel that, in any one of the captured images, is in the area of reflection of the display aperture. Again, it will be appreciated that the number of images acquire by each camera should also be sufficient that the surface information developed from each image (as hereinafter described) can be combined to form a description of the entire surface across the height in the area in which a single image might include an image of the display aperture rather than the reflected pattern. 
     The surface descriptions for each of the cameras are similarly combined to form a description of the entire surface across the width (or across the area of interest in the direction of the width) of the panel. 
     Referring to  FIG. 3 , in one embodiment the screen pattern is a three-frequency pattern constructed by superimposition of three different frequency sinusoidal patterns in each of the x and y directions of the coordinate system employed by the system logic. It should be noted that, in the illustrated embodiment, the x-y coordinate system axes are chosen to be oriented such that they are coincident with the x and y axes of the display (and, as well, the y axis is parallel to the direction of travel of the conveyor and the x axis is orthogonal to the direction of travel of the conveyor). 
     The sinusoidal patterns are chosen and combined to insure that the portion of the resultant pattern appearing on the display is non-repetitive, thereby ensuring that, for the image data collected, each pixel in the camera&#39;s field of view will correspond uniquely to a single point on the display. Each of the three frequencies may be relatively prime values, and are selected such that they are spaced apart within the envelope of frequencies bound by the minimum and maximum frequency limits of the camera&#39;s optics. 
     The image of this three-frequency pattern reflected from the surface of the panel may then be mathematically deconstructed into three single frequency images in each of the x and y direction. Phase information corresponding to each of the three frequencies can then be isolated and utilized as hereinafter described to develop an accurate description of the panel surface. 
     In another embodiment, illustrated in  FIG. 4 , a two-frequency pattern may be utilized. This two-frequency pattern may be constructed by superimposition of two different frequency sinusoidal patterns in each of two orthogonal directions which are rotated (or skewed) about the axes that are used to separate the analysis into orthogonal components, such that each of the sinusoidal components of the pattern yields phase information in both the x and y directions. In the illustrated embodiment, the x, y coordinate system axes that are used by the system logic to separate the analysis into orthogonal components are coincident with the x and y axes of the display (and the y axis is as well, coincident with the direction of conveyance). 
     Thus, in the illustrated embodiment, the orthogonal directions of the sinusoidal patterns are skewed from the x and y axes of the display. It will be appreciated, however, that any other convenient orientation may be chosen for the axes that are used by the system to separate the analysis into orthogonal components, so long as the sinusoidal patterns are rotated about the axes that are used to separate the analysis into orthogonal components to yield phase information in both the x and y directions. 
     Again, the sinusoidal patterns are chosen (relatively prime frequencies and spaced apart as described above) and combined to insure that the portion of the resultant pattern appearing on the display is non-repetitive, thereby ensuring that the image data collected that each pixel in the camera&#39;s field of view will correspond uniquely to a single point on the display. 
     The image of this two-frequency pattern reflected from the surface of the panel may then be similarly mathematically deconstructed. Again, phase information corresponding to each of the two frequencies can be isolated and utilized as hereinafter described to develop an accurate description of the panel surface. 
     It will be appreciated by those skilled in the art that, by employing a multi-frequency, non-repeating pattern and employing the deflectometry techniques hereinafter described, an accurate mathematical description of the panel surface may be obtained from a single image for each point on the surface of the panel from which the camera detects the reflected pattern. It is thus unnecessary to capture utilize multiple patterns, and/or multiple images, except as described herein where multiple images are acquired as the panel is moved on the conveyor to construct a surface for that portion of the panel that does not reflect the projected pattern in any single acquired image (e.g., (1) that portion of the panel directly below the aperture in the screen, or (2) for that portion of the panel that is not in the viewing area of the camera due to the fact that the height of the panel is greater that the projected pattern from the screen in the direction of conveyance). 
     Referring now to  FIGS. 5 and 6 , in the illustrated embodiment, and for the depicted panel, glass sheet part G, the second dimension (width) of the panel was divided into seven zones. These zones were identified as required due to the dimension of display pattern viewed by the camera, and the width dimension and curvature of the particular panel part. In this example, first zone display  20  is oriented at an angle of about 25° counterclockwise from horizontal (when viewed as in  FIG. 1 ), display  18  is angled at about 15° counterclockwise, display  16  is angled at about 7.5° counterclockwise, display  14  is approximately horizontal, display  22  is angled at approximately 7.5° clockwise from horizontal, display  24  is angled at about 15° clockwise, and display  26  is angled at about 25° clockwise. In the illustrated embodiment, the seven displays  14 - 26  are arranged in in the direction of conveyance of the panel G. However, as will be appreciated by those skilled in the art, other arrangements may be optimal for panel parts of different widths and curvatures, provided that the screens are arranged such that each associated camera detects only the reflected pattern from its associated display in its field of view, and the surface areas detected by all cameras together comprise the surface across the entire width of the panel part. 
     The panel inspection system  10  includes a surface data acquisition system which employs the above-described camera and display pairs and acquired images, as well as logic for developing an accurate three-dimensional description of the surface from the reflected patterns from each image, and logic for combining the surface descriptions developed from the images as hereinafter described to obtain an accurate mathematical description of the entire surface of the panel. 
     The panel inspection system  10  may also, in addition to the surface data acquisition system, include one or more computers and/or programmable controls including logic for processing the acquired surface data to determine the conformance of the curved sheet with a pre-defined shape specification. 
     The inspection system  10  may, in turn, be incorporated into a system for fabricating panels including one or more processing stations and one or more conveyors for conveying the panels from station to station during processing, such as fabrication systems  200  and  300  schematically shown in  FIGS. 10 and 11 . 
       FIG. 7  describes the method  80  performed by the control logic of the disclosed inspection system  10 . When the system  10  determines that a panel is in the appropriate position on the conveyor, the system activates the appropriate camera(s), at  82 , to acquire an image of the pattern reflected from the surface of the panel. The position of the panels can be determined using conventional sensors. 
     As indicated at  84 , one or more additional images may be obtained from each camera, as required, as the panel moves on the conveyor. As previously described, the number of images acquired by each camera is determined by at least two considerations. First, in embodiments of the system wherein the cameras are mounted within an aperture of their associated displays, a sufficient number of images must be acquired to ensure that the system acquires a reflected image of the pattern for all of the points in the viewing area, including those points from which the display pattern is not reflected in a particular image due to the fact that it is located within the area that includes a reflection of the aperture. Second, multiple images may be required as the glass is conveyed across the viewing area of the camera in embodiments of the system where the field of view of the camera is not large enough to acquire a reflection of the display pattern from the surface of the panel across its entire first dimension (i.e., the entire height) in one image. 
     For each of the acquired images, the system, at  86 , must determine the precise location in three-space of each point on the surface of the panel based upon the reflected pattern in the image. As previously described, the use of a pattern which is non-repeating in the camera&#39;s viewing area ensures that each point on the display screen that is reflected within the viewing area of the camera will be uniquely associated with a pixel that detects the reflected pattern. Conventional image processing techniques may be employed to determine the x and y locations (i.e., in the focal plane of the camera) for each point on the surface of the panel that is in the viewing area of the camera for that image. Other known processing techniques may be employed to determine the z location (a.k.a. the elevation) of each point. In the disclosed embodiment, a mapping vector technique is employed (as depicted in  FIGS. 8 and 9 , and as more fully described hereinafter) to determine the elevation of each single point on the surface of the glass from the image of the reflected projected pattern. 
     In one embodiment, the x, y, and z values developed for each point in the viewing area of a particular camera are typically developed in a coordinate system associated with that camera. In one embodiment, for example, the origin of the coordinate system for each camera is set at that camera&#39;s origin  98  (as shown in  FIG. 8 ). The resulting collection of points associated with the surface in the viewing area of each camera (“the point cloud”) may then be combined for each image collected by that camera. 
     The system, at  88 , then combines the developed surface data for each of the images acquired from all of the cameras to obtain the surface definition which identifies the location of each point in three-space for the entire surface of the panel. In one embodiment, the point clouds for each camera are converted to a common (“global”) coordinate system, and the point clouds are then combined to form the entire surface. 
     It will be appreciated that one or more other coordinate systems/origins may be selected and employed based upon a particular system&#39;s camera/display architecture and/or for computational convenience. Similarly, the combination of the surface developed from the individual acquired images may be performed using other conventional image data processing techniques. 
     The system then, at  90 , performs one or more known processing techniques to compare the surface to a pre-defined exemplary surface contour. In one embodiment, a statistical correlation of the points in the surface data of the panel with the points in the predefined surface shape is conducted. The variance in locations of the correlated points may be examined, for the entire surface, and/or for selected areas of concern, such as, for example, around the perimeter of the panel. Selected indicia of conformance of the panel to the pre-defined surface, including, for example, color coding or numerical values, may then be developed and displayed or otherwise reported. 
       FIGS. 8 and 9  illustrate, respectively, the theoretical basis and the method performed by the control logic for determining the elevation (z value) of each point on the surface of the panel from the image of the reflected projected pattern for each acquired image.  FIG. 8  illustrates the pertinent geometrical relationships between the camera  28 , the display screen  14 , and the surface of the panel (which is depicted in  FIG. 8  as a glass sheet, G). The three principles used to determine the elevation of a single point on the surface of the panel from a reflected projected image are (1) the surface of any object can be defined by the normal vector  92  for each discrete point of the surface; (2) the law of reflection defines the normal vector  92  at each point by bisecting the angle between the incident ray  94  and the reflected ray  96  of light (also referred to herein as the “geometric optical” or “reflection angle” equation); and (3) the normal vector can also be defined by the differential geometry which describes each point on the surface of the panel (also referred to herein as the “differential geometry” equation). 
     Referring still to  FIG. 8 , based upon the law of reflection, the incident ray is defined entirely by the camera intrinsics. Thus each pixel in the cameras receptor cell at the cameras origin  98  sees a point in space at varying distances through the lens. Continuing with the law of reflection, the reflected ray  96  is defined by a screen position and a surface point on the panel. The distance is constrained only where it intersects the incident ray  94 . There are two mathematical expressions which define the normal vector  92 . One is derived from the law of reflection. The second differential equation is derived from the geometric partial differentiation at any point on the surface. To solve the corresponding differential equations, a mapping vector  100  needs to be established, which defines, for each pixel, where the reflected ray will hit the projected pattern on the display (viewed from the camera origin). Once the mapping field (i.e., the set of mapping vectors for each pixel in the camera&#39;s field of view) is established, the distances from the camera&#39;s origin and each discrete point on the surface can be calculated. 
     The geometric optical equation is: 
     
       
         
           
             
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     Where n is the surface normal, v is the camera pixel vector, m is the mapping vector, and s is the distance from the camera to the surface (along the camera vector so that the surface point 
     
       
         
           
             
               
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       FIG. 9  illustrates how a suitably programmed computer could implement this mapping vector technique  102 . At  104  the system develops a mapping vector that defines where the reflected ray projects to the display (again, viewed mathematically from the camera origin). A first expression, the geometric optical equation, at  106 , defines the surface normal vector for each point on the panel surface within the camera&#39;s viewing area based upon the law of reflection. A second equation, the differential geometry equation, at  108 , defines the surface normal vector-based on the multiplication of the partial differentials which describe the point on the surface in the x, y directions. These two equations may be solved simultaneously using the mapping vector to obtain the elevation, s (that is, the z distance—the distance between the panel surface and the camera origin) for each point on the panel surface that is within the viewing area of the camera. This information, coupled with the previously developed x and y locations of each surface point, yields a specific description, in x, y, and z, for each point on the surface. 
     It will be appreciated by those skilled in the art that other known methods may be utilized to develop unambiguous locations in three dimensions for each of the points on the surface of the panel based upon the unambiguous x and y locations of the reflected patterns at each pixel location of the image, and the geometrical relationship between the focal plane of the camera, the display screen, and the panel. However, it has been determined that the elevation of each point on the surface of the panel can be quickly determined using the principles described above and illustrated in  FIG. 8 , and the technique described above and illustrated in  FIG. 9 , without resorting to projecting multiple, varying patterns and/or analyzing multiple images of the same viewing area to thereby determine a three-dimensional definition of the panel surface. 
     Referring again to  FIGS. 1 and 2 , the disclosed panel inspection system  10  may be mounted in-line to inspect panels as they are transported on a conveyor associated with a panel processing system which performs multiple fabricating operations on the panels. The disclosed system  10  includes a surface data acquisition system and a computer including logic for receiving the captured image data, developing a three-dimension description of the panel surface from the image data, performing one or more optical processing operations to analyze the optical characteristics of the panel and displaying or otherwise reporting selected information associated with the optical analysis. As previously described, computer  42  may be operably connected to the conveyor and cameras to perform the image acquisition, the surface development, and the gaging described herein. Alternatively, computer  42  may be combined with one or more other computers and/or programmable controls to perform these functions. 
     The system  10  may also be programmed by the user to graphically and numerically display various indicia of conformance to the desired shape, including those indicia most relevant to industry standards, or other indicia considered relevant in the industry to the analysis of the shape of the formed panels. 
     The digital cameras  28 - 40  are each connected via a conventional data line to one or more computers, such as computer  42 , which may be suitably programmed to acquire the digital image data from the camera, process the image data to obtain the desired surface definition for the panel, and analyze the data to develop various indicia of shape conformance. The computer  42  may also be programmed to present the derived information in both graphical (e.g., color-coded images) and statistical forms. If desired, various other statistical data which may be of interest can be derived and reported for predefined areas of the panel. 
     As will be appreciated by those skilled in the art, the inspection system  10  may additionally or alternatively employ other known image processing techniques to collect and analyze the acquired image data, develop a definition of the surface, and provide various indicia of the shape conformance for each panel. 
     In one embodiment, the displays  14 - 26  are light boxes that utilize conventional lighting (such as fluorescent lights) behind a translucent panel upon which the contrasting pattern is printed, painted, or otherwise applied using conventional methods. The digital cameras  28 - 40  are connected to the computer  60  using known methods, preferably so that the acquisition of the image by the camera may be controlled by the computer  42 . 
       FIG. 11  illustrates a typical glass sheet heating, bending, and tempering system  200  which includes the in-line inspection system  10 , as well as the surface data acquisition system, of the present invention. In this installation, the glass sheets (indicated as G) enter a heating zone  202  where the glass is softened to a temperature suitable for forming the glass into the desired shape. The heated glass sheet is then conveyed to a bending station  204  where the softened sheet is formed to the desired shape, and thereafter further conveyed to a cooling station  206  where the glass sheet is cooled in a controlled manner to achieve the appropriate physical characteristics. In this embodiment, the glass sheet would then be conveyed out of the cooling station onto a conveyor from which the sheet is conveyed for image acquisition and analysis by the disclosed inspection system  10 . Following the measurement, the glass sheet would be moved on the conveyor  12  for further processing. It will be appreciated that the transport and conveyance of the glass can be achieved by using known techniques such as by roller, air-float, or belt conveyors, positioners, and robotic arms, in order to handle the glass in the manner described. It will also be appreciated that a plurality of conveyors, each of which may be independently controlled, may be employed to move the glass sheets through the different processing stations at speeds to efficiently govern the flow and processing of the glass sheets throughout the system  200 . 
       FIG. 12  similarly schematically illustrates an in-line inspection system  10  and the associated surface data acquisition system of the present invention in a typical automotive windshield fabrication system  300 , which may include a heating station  302 , a bending station  304 , a cooling station  306 , and a lamination station  308 , upstream of the inspection system  10 . 
     Selected data output by the disclosed in-line inspection system  10  may also be provided as input to the control logic for the associated glass sheet heating, bending, and tempering system  200  (or automotive windshield fabrication system  300 , or other panel fabrication/processing system) to allow the control(s) associated with one or more of the stations the system to modify its (their) operating parameters as a function of the gaging data developed from previously processed glass sheets (or panels). 
     It will be appreciated that the inspection system  10  of the present invention could alternatively be mounted in-line at various other points in the above-described and other panel fabrication systems as desired to maximize the production rate of the system, so long as the shape conformance measurements are taken after the panel has been formed to its final shape. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.