Abstract:
An apparatus for measuring the visual characteristics of the surface of a workpiece includes a light source which produces a beam of light. One or more mirrors are positioned so as to direct the light from the source onto the workpiece for reflection therefrom, and to direct the reflected beam onto a photodetector. At least one of the mirrors is supported on a movable mirror carriage which is coupled to a member which contacts the workpiece surface and adjusts the position of the mirror or mirrors supported by the carriage so as to assure that the reflected beam falls on the photodetector. The detector may be mechanically scanned across the reflected beam to provide a spatial profile of that beam.

Description:
RELATED APPLICATION  
       [0001]    This patent application claims priority of U.S. Provisional Patent Application Serial No. 60/292,746 filed May 21, 2001, also entitled “Compact Surface Quality Meter.” 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to instrumentation for measuring the surface quality of the finish of workpieces. More specifically, the invention relates to a low cost, compact, simple to use device which is capable of measuring the surface quality of curved and other non-planar workpiece surfaces.  
         BACKGROUND OF THE INVENTION  
         [0003]    The surface appearance of many articles of manufacture such as motor vehicles, appliances, and furniture is very important. Surface finishing of such items generally involves polishing, painting, plating and the like; and, it is desirable to be able to quantify the quality of finish of such surfaces for evaluation and quality control. As a consequence, the industry has developed a number of parameters such as gloss, distinctness of image, orange peel, and the like which can be utilized to characterize and quantify surface quality. Such parameters are generally measured by reflecting a beam of light from the surface of the workpiece being measured, and correlating the characteristics of the light reflected from the surface with the particular parameter. There are a number of methods and algorithms well known to those of skill in the art for accomplishing such measurements. Typically, surface quality measurements are carried out in a production environment; and as a consequence, self-contained metering devices are employed to direct a beam of light onto a workpiece, detect the reflected light, and generate an electrical signal which corresponds to one or more attributes of the reflected light. In some instances, such systems further process the signal within the measuring device to provide a direct readout of surface quality parameters. In other instances, the signal is then conveyed to another unit for processing.  
           [0004]    Heretofore, such measuring apparatus has generally been utilized in connection with the high volume production of motor vehicles, appliances and the like, and prior art surface quality monitoring systems are generally very large and expensive. Also, prior art apparatus has generally been configured for measurements of relatively planar surfaces such as motor vehicle panels and the like; and hence, is not capable of accurately measuring the surface quality of items having a relatively small radius of curvature.  
           [0005]    There is a need for a surface quality measuring apparatus which is compact, low in cost, easy to use and which can accommodate relatively small, fairly high curved, workpieces. As will be explained hereinbelow, the present invention provides a small, simple, low cost surface quality measuring system which includes a uniquely configured optical system which reduces the system&#39;s size and cost, and which enables it to accommodate highly curved surfaces. In addition, the present invention also includes a low cost detector assembly which provides performance equivalent to that achieved through the use of expensive detector arrays.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0006]    Disclosed herein is an apparatus for measuring the visual characteristics of the surface of a workpiece. The apparatus includes a housing having a light source supported therein and operable to provide a beam of light. The apparatus further includes a photodetector supported by the housing. The photodetector is operable to provide an electrical signal in response to the illumination thereof. A first mirror is supported by the housing so as to direct the beam of light from the light source onto the surface of a workpiece for reflection therefrom so as to produce a reflected beam. A second mirror is supported by the housing so as to receive the reflected beam and direct it onto the photodetector. At least one of the first and second mirrors is supported by a mirror carriage so as to be movable relative to the housing, along an axis which is generally normal to the surface of the workpiece at a point on the workpiece at which the beam of light impinges. A workpiece contact member is in mechanical communication with a mirror carriage. The workpiece contact member projects from the housing and is operable, when the housing is in contact with the workpiece, to engage the workpiece proximate to the location thereupon at which the beam of light impinges onto it, and to move the mirror carriage to a position along the aforementioned axis so that the reflected beam of light falls on the detector.  
           [0007]    In further embodiments of the invention, a second and a third mirror may be supported by the housing and disposed so as to direct the beam of light from the light source to the workpiece and to the detector. One or more of these third and fourth mirrors may be mounted on the mirror carriage.  
           [0008]    In further embodiments of the invention, the photodetector has an active area which is less than the cross-sectional area of the reflected beam of light, and the apparatus includes a scanner for scanning the active area of the photodetector across at least a portion of the cross-sectional area of the reflected beam so that the electrical signal produced by the detector is a time varying signal corresponding to the intensity profile of the scanned cross-sectional area of the reflected beam. Scanning of the detector may be accomplished by mechanical means such as a pushbutton, or electrical means such as a solenoid or other actuator. The scanning system may include a position indicator associated therewith for providing an output signal indicative of the positional relationship of the active area of the photodetector relative to the cross-sectional area of the reflected beam. This position indicator may comprise an electronic linear position sensor. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a schematic, cutaway, depiction of one embodiment of the present invention;  
         [0010]    [0010]FIG. 2 is a schematic depiction of another embodiment of the present invention;  
         [0011]    [0011]FIG. 3 is a schematic depiction of a detector assembly of the present invention;  
         [0012]    [0012]FIG. 4 is a graph of the spatial distribution of the intensity of a reflected beam of light as being scanned by the detector of the assembly of FIG. 3;  
         [0013]    [0013]FIG. 5 is a drawing of a contact pad which may be utilized in the present invention;  
         [0014]    [0014]FIG. 6 a  is a drawing of a portion of the mirror carriage of the present invention showing the action of the contact member thereof; and  
         [0015]    [0015]FIG. 6 b  is a perspective view of a portion of the contact member of FIG. 6 a.   
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    The present invention is directed to an apparatus for measuring the visual characteristics of the surface of a workpiece. The system of the present invention is compact, low in cost, and employs a unique optical system and detector assembly. Referring now to FIG. 1, there is shown one embodiment of apparatus  10  structured in accord with the principles of the present invention. The apparatus  10  of FIG. 1 is enclosed within a housing  12  which contains and supports various elements of the apparatus. The housing  12  may be fabricated from metal, polymers, and the like. While the housing  12  is shown in FIG. 1 as enclosing the elements of the apparatus  10 , it is to be understood that the housing may be otherwise configured, and within the context of this disclosure it should not be interpreted as being restricted to structures which enclose the remaining components of the system. But rather, the term “housing” is meant to include support structures, both open and closed, which support and retain the elements described and claimed herein. As such, the housing may comprise a supporting framework, an optical bench or the like. Disposed within the housing  12  is a light source  14  which in the preferred embodiment comprises a solid state laser, most preferably operating at near infrared wavelengths. Other light sources such as a discharge lamp, an incandescent lamp and the like may be readily substituted herein by one of skill in the art.  
         [0017]    In the illustrated embodiment, the light source  14  emits a beam of light  16 , and as illustrated, the apparatus may include a field stop or other aperture for shaping or collimating, or otherwise controlling the size or character of the beam  16 . Likewise, filters or polarizers may be included in the optical path. In the illustrated embodiment, the beam of light  16  is directed onto a workpiece  20  by a first pair of mirrors  22   a ,  22   b . In the illustrated embodiment, the incident beam  16  strikes the workpiece  20  at an angle “a” which is 20° from normal. This is an industry standard for measuring gloss, and is referred to as a 20° gloss measurement.  
         [0018]    The workpiece  20  reflects the incident beam of light  16  so as to produce a reflected beam  24  which is directed by a second pair of mirrors  26   a ,  26   b  onto a photodetector  28 . As illustrated, the reflected beam  24  passes through a lens and slit  32  prior to striking the detector  28 ; although, it is to be understood that these elements may be eliminated; alternatively, other elements may be substituted therefor.  
         [0019]    The photodetector receives the reflected beam  24  and generates an electrical signal proportional thereto. The detector  28  may comprise any photoresponsive device known in the art. For example, the detector  28  may be a linear array detector which provides a position responsive signal indicating the spatial distribution of the reflected beam  24 . However, and as will be explained in detail hereinbelow, the detector  28  may preferably comprise a simple, low cost photodiode which is scanned across the reflected beam  24 , as indicated by double arrow “B” so as to provide information about the spatial distribution of the reflected beam.  
         [0020]    The output of the photodetector  28  may be processed using methods and algorithms known in the art to provide measurements such as gloss, distinctness of image and the like, indicative of the quality of the surface of the workpiece  20 . Such processing may be accomplished within the apparatus  10  by a microprocessor (not shown), or processing may be carried out at another location.  
         [0021]    In accord with the present invention, the mirrors  22   a ,  22   b ,  26   a ,  26   b  are mounted onto a mirror carriage  34  which is supported in the housing  12  so as to be movable along an axis which is generally normal to the surface of the workpiece  20  at the point thereupon at which the beam of light  16  impinges. As illustrated, the mirror carriage  34  is supported in the housing  12  by a support bracket  35 , and is biased by means of a spring  36 , and operates in cooperation with a contact member  42  (best shown in FIGS. 6 a  and  6   b ) which engages the surface of the workpiece  20  and moves the carriage along the axis so as to reposition the mirrors so that the reflected beam  24  will fall onto the detector  28 . As illustrated, all four of the mirrors are mounted on the carriage in a fixed relationship relative to one another, and are translated in a group, along the axis. In other embodiments of the invention, only a portion of the mirrors are mounted on the carriage.  
         [0022]    Referring now to FIG. 2, there is shown another embodiment of the present invention, generally similar to the FIG. 1 embodiment, but configured to measure 60° gloss. In the FIG. 2 embodiment, the housing is not shown, and it is to be understood that the various elements are all appropriately supported.  
         [0023]    The apparatus of FIG. 2 includes a light source  14  and field stop  18  generally similar to those discussed hereinabove. The light source  14  produces an incident beam  16  which is directed onto the workpiece  20  by a mirror  22 . (It will be noted that in this embodiment, only a single mirror  22  directs the incident beam  16  onto the workpiece  20 .) The incident light  16  is reflected from the workpiece to produce a reflected beam  24  which is directed onto a photodetector  28  by means of a mirror  26 . As in the previous embodiment, a lens  30  and detector slit  32  are employed to direct and shape the reflected beam  24 . In this embodiment, an electrically powered servo  40  operates to scan the photodetector  28 , which is most preferably a photo diode, across the reflected beam  24  in the direction “B” as indicated.  
         [0024]    As in the FIG. 1 embodiment, the mirrors  22 ,  26  of the FIG. 2 embodiment are mounted onto a mirror carriage  34  (which in this illustration is not in cross section) in a fixed relationship to one another. The carriage is supported by the housing by a bracket  35 , so as to be movable along an axis generally normal to the surface of the workpiece  20 . The FIG. 2 embodiment better depicts the contact member  42  which engages the workpiece  20  and also moves the carriage  34  along its axis of motion. In the FIG. 2 embodiment, the carriage includes two springs  36   a ,  36   b  for biasing the motion of the carriage.  
         [0025]    Referring now to FIG. 3, there is shown another version of detector assembly for scanning a photodetector across the reflected beam of light. As depicted in FIG. 3, a photodetector  28  is mounted onto a carriage  44  which includes a pushbutton  46  or other such mechanical actuator which moves the carriage  44  and detector  28  along a back and forth path of travel as indicated by arrow B. Associated with the carriage  44  is a linear position sensor  48 . Such devices are known in the art, and operate to provide an electrical signal indicative of linear displacement. In the assembly of FIG. 3, a reflected beam of light  24  is directed onto the photodetector  28 , in this instance by a mirror  26 . The photodetector  28  has an active area which is less than the cross-sectional area of the beam  24 , and as the detector is scanned across the beam  24 , it produces a varying output signal. The linear position sensor  48  produces a corresponding signal which is indicative of the detector&#39;s position and which may be correlated with the signal from the photodetector so as to provide a composite signal indicative of the spatial profile of the reflected light. As shown, the outputs of the photodetector  28  and position sensor  48  may be directed to a signal processor  49  which generates an output signal in response thereto and activates an appropriate display  51 .  
         [0026]    Referring now to FIG. 4, there is shown a graph  50  depicting the intensity (I) of a reflected beam of light as a function of the cross-sectional dimension (D) of the beam. As shown herein, this distribution is approximately gaussian; although, depending upon the nature of the surface quality of the workpiece, other distributions may be encountered. FIG. 4 also depicts, in schematic form, a photodetector  28  which has an active area smaller than the cross-sectional dimension of the reflected beam. As shown in the figure, the detector  28  is scanned across the width of the cross section of the beam, and this will cause the detector  28  to produce a positionally varying signal which embodies information describing the spatial distribution of the intensity of the reflected beam. As is known in the art, this information can be processed to provide measurements of parameters such as distinctness of image, gloss, orange peel and the like. It is a notable feature of the present invention that by the use of a relatively low cost small active area photo diode in connection with a scanning system, the need to employ expensive positionally sensitive detectors, such as linear arrays, is avoided.  
         [0027]    Referring now to FIG. 5, there is shown a depiction of a contact pad which may be employed in the practice of the present invention. The contact pad  50  is a portion of, or is affixed to, the housing and provides an aperture through which the incident and reflected beams pass. In addition, the contact pad establishes contact with the workpiece. It has been found that a contact pad having a circular center cutout portion superimposed over a narrower cutout slot is particularly advantageous insofar as it functions to align spherical, cylindrical, or otherwise curved workpieces  20  with the optical center of the apparatus.  
         [0028]    Referring now to FIG. 6 a  and FIG. 6 b , there is shown an enlarged view of a portion of an apparatus generally similar to that of FIG. 1 or FIG. 2. FIG. 6 a  shows a workpiece  20  in engagement with a contact pad  50 . As further shown in FIG. 6 a , the workpiece  20  is highly curved; consequently, the portion of the workpiece being measured actually projects a small distance into the housing of the apparatus. As shown, the contact member  42  of the mirror carriage engages the workpiece surface and correctly positions the mirrors (not shown) so that the incident beam  16  strikes the optical center of the workpiece  20  so that the reflected beam  24  is correctly aligned with the detector system. FIG. 6 b  shows a perspective view of the contact member  42 . As will be noted, a cutout center portion permits the incident  16  and reflected beam  24  to pass therethrough. The contact member may be otherwise configured provided it does not interfere with the incident and reflected beams.  
         [0029]    It is to be understood that in view of the teaching presented herein, other modifications and variations of the present invention may be readily implemented by one of skill in the art. The foregoing drawings, discussion and description are illustrative of specific embodiments of the invention, but are not meant to be limitations upon the practice thereof. It is the following claims, including all equivalents, which define the scope of the invention.