Abstract:
A method and installation for precision imaging of the geometry of workpieces provides relative movement of a CCD detector ( 13 ) and the object workpiece ( 4 ) in a guided motion. In the process, the CCD detector ( 13 ) images the workpiece ( 4 ) segment by segment, and the positions of the CCD detector ( 13 ) relative to the workpiece ( 4 ) and the positions of the imaged workpiece segments are determined. Prior to functional motion of the CCD detector relative to the workpiece ( 4 ) to determine its geometry, a rectilinearity and/or angularity calibration is/are performed and, if necessary, a correction value is defined. In at least one subsequent functional motion of the installation, the correction value is factored in for the determination of the positions of the imaged workpiece segments.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to a method and apparatus for imaging the geometry of workpieces in which an object-detecting unit and the workpiece are moved relative to each other in a guided functional motion during which the object-detecting unit images the workpiece in segmental fashion.  
           [0002]    The functional motion of the object-detecting unit and of the workpiece takes place along at least one track in one guided direction, or along two tracks extending at an angle relative to each other. In the former case, the respective positions of the imaged workpiece segments are determined by the position of the object-detecting unit opposite the workpiece along the path of the directional movement and by the position of the object-detecting unit opposite the workpiece in a transverse direction perpendicular to the direction of guided travel. The transverse position is calculated on the basis of the nominal, i.e., specified, track direction.  
           [0003]    In the second case, the geometric determination of the imaged workpiece segments is based on the respective position of the object-detecting unit opposite the workpiece in the direction of the first axis of guided travel as well as on the position of the object-detecting unit opposite the workpiece in the direction of the second axis of guided travel. The angle between the axes of guided travel has a nominal value.  
           [0004]    These methods and systems serve the purpose of imaging in composite fashion workpieces or parts thereof which, for instance, due to their dimensions cannot be captured in one single image with adequate resolution. The desired composite image is a mosaic of individual representations of segments of the object, i.e. from subsections of the overall object to be imaged. To that effect it is necessary to define the location of the individually imaged object segments relative to one another. Such positional determination of the object segments is accomplished by way of the detection of the positions occupied by the object-detecting unit at the time the image of the object segment is acquired. A specific position of the object-detecting unit is mapped for each object segment. A precise image of the object or of the object subregion concerned reflecting the actual, true situation necessarily presupposes an exact determination of the positions occupied by the object-detecting unit at the time the object segments are imaged.  
           [0005]    Methods and systems of the type mentioned above are described in U.S. Pat. No. 5,184,217 granted Feb. 2, 1993 to J. W. Doering. In this prior art concept, the object-detecting unit in the form of a CCD (charge coupled device) array is mounted on a sliding stage so as to be capable of moving in the direction of a first coordinate axis. The stage on its part, is positioned on a table that holds the workpieces to be scanned, and can move in the direction of a second coordinate axis. The CCD array traveling in the direction of the first coordinate axis and the stage supporting the CCD array and traveling in the direction of the second coordinate axis are each driven by a dedicated DC motor.  
           [0006]    For imaging the geometry of a workpiece, the CCD array is moved across the workpiece starting from a reference position. As the CCD array occupies successive positions in the direction of the first coordinate axis, the stage carrying the CCD array is moved in the direction of the second coordinate axis. During these scanning motions, the CCD array, fixed in its position relative to the stage, captures consecutive segments of the workpiece being imaged. The positions of these workpiece segments are defined as a function of the respective positions of the CCD array.  
           [0007]    The proviso for each movement of the CCD array is that, as it travels, the CCD array changes direction only in relation to the second coordinate axis while in the direction of the first coordinate axis, an unchanging position of the CCD array is assumed. In other words, the assumption is that the CCD array travels along a perfectly straight path in the direction of the first coordinate axis extending in ideal alignment with the second coordinate axis.  
           [0008]    A concurrent assumption is that the second track, i.e., the axis of guided travel of the CCD array in the direction of the second coordinate axis, extends in a direction perfectly perpendicular to the first track, i.e., to the path of the guided, stepwise movement of the CCD array in the direction of the first coordinate axis. On that premise alone could one be assured that, as the CCD array is shifted relative to the second coordinate axis, it is indeed only its coordinates in the direction of the second coordinate axis and not the coordinates of the CCD array in the direction of the first coordinate axis that change.  
           [0009]    However, given the fact that, even in highly precise systems of the type discussed, certain variations are caused due to, e.g., manufacturing tolerances or to environmental conditions such as temperature fluctuations. As a result, neither ideal rectilinearity nor perfect squareness of the guide tracks for the CCD array can be assured, and prior art designs encounter deviations between specified and actual positions of the CCD array as it scans the workpiece segments. It follows that the positions of the imaged workpiece segments, the composite of which is supposed to represent the desired total image, are equally flawed. In other words, the resulting total image does not precisely reflect actual conditions.  
           [0010]    It is the object of this invention to provide a novel imaging method and apparatus in which there is greater precision in the imaging of the workpiece.  
         SUMMARY OF THE INVENTION  
         [0011]    It has now been found that the foregoing and related objects may be readily attained in a method for imaging the geometry of workpieces in an installation in which an object-detecting unit ( 13 ) and the object workpiece ( 4 ) are moved in a guided motion relative to each other and the workpiece ( 4 ) is scanned by the object-detecting unit ( 13 ) section-by-section. The guided motion of the object-detecting unit ( 13 ) and the workpiece ( 4 ) takes place along at least one track in the direction of one axis of guided travel (Y-axis), and the respective positions of the scanned workpiece segments are determined by the position of the object-detecting unit ( 13 ) adjacent one face of the workpiece ( 4 ) along the axis of guided travel (Y-axis). The position of the object-detecting unit ( 13 ) relative to the workpiece ( 4 ) and along the track in a transverse direction perpendicular to the axis of guided travel (Y-axis) follows a nominal path.  
           [0012]    The installation is calibrated prior to functional use for imaging a workpiece by relative movement between the object-detecting unit ( 13 ) and a calibration line ( 14 ) that extends in a transverse direction in the axis of guided travel (Y-axis) to determine the actual orientation as contrasted to the nominal path of the track. The object-detecting unit ( 13 ) scans sections of the calibration line ( 14 ) and positional data are determined for scanned sections of the calibration line ( 14 ) and the orientation of the calibration line perpendicular to the axis of guided travel (Y-axis). The scanned orientation of the calibration line ( 14 ) is compared with its actual path, and at least one correction value is defined on the basis of any detected deviations. After such calibration, in the course of at least one subsequent functional relative motion of a workpiece and object-detecting unit to image the workpiece, the correction value is applied to the determined positions of the imaged workpiece segments.  
           [0013]    In one embodiment, the relative motion of the object-detecting unit ( 13 ) and the workpiece ( 4 ) takes place along a rectilinear track and, correspondingly, calibration is effected by relative movement of the object-detecting unit ( 13 ) and a rectilinear calibration line ( 14 ).  
           [0014]    In a preferred method of imaging, relative to each other the workpiece ( 4 ) is scanned by the object-detecting unit ( 13 ) section-by-section, by guided bi-directional relative motion of the object-detecting unit ( 13 ) and the workpiece ( 4 ) along two axes of guided travel (X and Y axes) which extend at an angle (a) relative to each other. The respective positions of imaged workpiece segments are determined by the position of the object-detecting unit ( 13 ) adjacent one surface of the workpiece ( 4 ) in the direction of one of the axes of guided travel (X-axis) and by the position of the object-detecting unit ( 13 ) relative to the workpiece ( 4 ) in the direction of the other axis of guided travel (Y-axis), in connection with which a nominal value is calculated for the angle (α) between the axes of guided travel (X and Y axes). Prior to imaging, a calibration procedure uses a calibration span with a first angular position which is aligned in a plane extending parallel to the plane defined by the axes of guided travel (X and Y axes). The calibration span is situated in its first angular position, and the calibration span and the object-detecting unit ( 13 ) are moved relative to each other, in the process of which, the object-detecting unit ( 13 ) images sections of the calibration span to determine the actual orientation as contrasted with the nominal path. On the basis of the imaged sections of the calibration span, a length (l) of the calibration span is determined. Thereupon the calibration span is realigned at least once more with another angular position in a plane extending parallel to the plane defined by the axes of guided travel (X and Y axes. The calibration span in its additional angular position and the object-detecting unit ( 13 ) are moved relative to each other in the process of which the object-detecting unit ( 13 ) images sections of the calibration span. On the basis of the imaged sections of the calibration span, a length (l*) of the calibration span is determined, and the lengths (l, l*) of the calibration span thus determined are compared. If necessary, a correction value is defined on the basis of any deviations detected. The correction value is applied to the positions of imaged workpiece segments after factoring in an angle (α) in the course of at least one subsequent functional motion relative to a workiece being scanned.  
           [0015]    As can be seen, the improved precision results from the scanning of the geometry of the object workpiece only after a verification of whether the track along which the object-detecting unit and the workpiece to be scanned are moved relative to each other actually extends in the ideal case alignment required for determining the positions of the object-detecting unit relative to the workpiece. If the actual direction of the track deviates from the ideal or specified orientation, correction values are defined which in the subsequent directional movements of the object-detecting unit and the workpiece are factored into the determination of the respective positions occupied by the object-detecting unit relative to the workpiece during the imaging of the workpiece segments. This ensures that each acquired position of the object-detecting unit opposite the workpiece is in fact the correct, actual position. Correspondingly, it also ensures that the workpiece segments imaged by the object-detecting unit are correctly mapped in their actual positions. When the individual workpiece segments are assembled in a mosaic, the result will be a composite image that most accurately reflects the actual geometric conditions.  
           [0016]    For appropriate implementation, axial angle verification and calibration technique is also employed. In the event that a deviation of the actual axial angle from the nominal angle is detected, corresponding correction values are factored into the determination of the positions of the object-detecting unit relative to the workpiece as the workpiece segments are imaged and thus into the determination of the positions of the workpiece segments themselves.  
           [0017]    In one embodiment the object-detecting unit and the object workpiece are moved relative to each other along only a straight guide track. A stretched wire or comparable string or the like is a simple means for establishing a calibration line that extends in a specified direction. 
       
    
    
     BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS  
       [0018]    The following description will explain this invention in more detail with the aid of the attached schematic illustrations in which:  
         [0019]    [0019]FIG. 1 is a partially diagrammatical side elevational view of an apparatus embodying the present invention for the geometric imaging of workpieces;  
         [0020]    [0020]FIG. 2 is a top view of the apparatus of FIG. 1 with an element for verification of rectilinearity and calibration;  
         [0021]    [0021]FIGS. 3 and 4 are enlarged sections of FIG. 2 containing an added device for verification of rectilinearity and calibration; and  
         [0022]    [0022]FIG. 5 is a graphic illustration for visualization of the mathematical and angular relationships. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    As shown in FIGS. 1 and 2, an installation for imaging the geometry of workpieces in accordance with the present invention is generally designated by the numeral  1 , and it includes a coordinate table  2  with a transparent support plate  3  on which a workpiece  4  has been placed. Also included is a CCD camera  5  which, by means of a coordinate guide track assembly  6 , can be moved in guided fashion relative to the workpiece  4 . As indicated in FIGS.  2 - 4 , the axes of guided travel are referred to as the X-axis and the Y-axis, respectively, covering a horizontal plane across which the CCD camera  5  travels.  
         [0024]    The coordinate guide track assembly  6  for the CCD camera  5  consists of an X-rail  7  extending in the direction of the X-axis along the coordinate table  2 , and a cantilevered arm  8  that projects from the X-rail  7 , in the direction of the Y-axis. For driving the cantilevered arm  8  on the X-rail  7 , an X-motor  9  is provided. Similarly, a Y-motor  10  moves the CCD camera  5  along the cantilevered arm  8 . The Y-axis defines the direction in which the CCD camera  5  is guided along the cantilevered arm  8 .  
         [0025]    A flash unit  11 , located on the far side of the support plate  3  opposite from the CCD camera  5 , travels in synchronization with the CCD camera  5 . The flash unit  11  is positioned directly opposite the lens assembly  12  of the CCD camera  5 . Located inside the CCD camera  5  behind the lens assembly  12  is an object-detecting unit  13  in the form of an array of CCD elements. Both the CCD camera  5  and the flash unit  11  are of a conventional design.  
         [0026]    As seen in FIG. 2, along one side of the support plate  3  of the coordinate table  2  is provided a calibration line  14  is provided. In the example shown, it consists of a stretched wire extending underneath the transparent support plate  3 . Since the wire is stretched taut, the calibration line  14  is straight, and extends in the direction of the Y-axis.  
         [0027]    As shown in FIGS. 3 and 4 a reference element  15  is bar-shaped and has several perforations  16  that are variably spaced from one another in the longitudinal direction. If and as needed, that reference bar  15  can be placed and suitably aligned on the support plate  3  of the coordinate table  2 . FIGS. 3 and 4 show the reference bar  15  in two different angular positions.  
         [0028]    A central processor  17  monitors and controls all of the functions of the installation  1 .  
         [0029]    Before the irregular outer contour of the workpiece  4  is scanned, the system  1  undergoes a check for rectilinearity and rectangularity and is calibrated.  
         [0030]    For verification of rectilinearity and calibration, the cantilevered arm  8  is first moved along the X-axis into a position in which the CCD camera  5  with the object-detecting unit  13  is situated above the calibration line  14 . From there, the object-detecting unit  13  moving along the cantilevered arm  8  is guided along the calibration line  14  in the direction of the Y-axis. In the process, the object-detecting unit  13  scans consecutive sections of the calibration line along the Y-axis. That sectional scan is performed in conventional fashion with the flash unit  11  appropriately flashing stroboscopically.  
         [0031]    The central processor  17  assembles the scanned sections of the calibration line  14  and the result reflects the course of the calibration line  14  in the form the object-detecting unit  13  has seen it during the scan. This is indicated in exaggerated fashion by the broken line in FIG. 3. The central processor  17  compares that detected course of the calibration line  14  with the stored actual orientation of the calibration line  14 . The deviations detected indicate that, unlike the actual orientation of the calibration line  14 , the track of the object detecting unit  13  in the direction of the Y-axis does not follow a straight line. The central processor  17  then defines correction values that compensate for the linear deviations of the track of the object-detecting unit  13 .  
         [0032]    For the verification of rectangularity and calibration, the reference bar  15  is first placed on the support plate  3  of the coordinate table  2  and oriented as shown in FIG. 3 which is essentially parallel to the X-axis. Next, the CCD camera  5  is moved across the reference bar  15 . In the process, the object-detecting unit  13  images sections of the reference bar  15 . Based on the sections scanned, the central processor  17  determines the distance l between two perforations  16  in the reference bar  15 . The spacing between the two perforations  16  defines a calibration span. The distance l between the perforations  16  constitutes the length of the calibration span.  
         [0033]    After that length has been determined as described, the reference bar  15  is rotated around any desired pivotal point and aligned in the angular position shown in FIG. 4. In the example illustrated, the reference bar  15  is inclined relative to the X-axis by an angle of about 45°. In this position, the reference bar  15  is scanned by the CCD camera  5 . The object-detecting unit  13  again images sections of the reference bar  15 . Based on the imaged sections of the reference bar  15 , the central processor  17  determines the distance, i.e. the length l*, between the same perforations  16 , which distance was determined previously when the reference bar  15  extended in the direction of the X-axis.  
         [0034]    The assumption is that the object-detecting unit  13  is guided in precise and error-free fashion in the direction of the X-axis and that no measurement errors occur in the longitudinal measurement of the distance l and l*, respectively. The angular error δ will then be detected when the distance l* is measured.  
         [0035]    According to FIG. 5, the result will be: 
         δ=α−90°,  
         [0036]    where α is the angle between the X-axis and the Y-axis, i.e. between the axes of guided travel of the object-detecting unit  13 .  
         [0037]    Hence, according to the law of cosine:  
         cos                 α     =           l   2     -     l   x     *   2       -     l   y     *   2           2        l   x   *          l   y   *         .                           
 
         [0038]    To attain highest possible precision, the above-described determination of the lengths, i.e., distances l, l* is made consecutively for different pairs of perforations  16 . In each case, an angle α is defined. For the axial angles thus obtained the geometric mean value is determined via  
       a   =         ∑   i            a   i          l   I             ∑   i          l   I                               
 
         [0039]    This correlates: (i) the sum of all products from the various angles and, with the reference bar  15  aligned in the direction of the X-axis, the associated individual distances between perforations  16 , and, (ii), the sum of all distances between perforations measured with the reference bar  15  aligned in the direction of the X-axis. In other words, the individual angles are weighted as a function of the associated distance between perforations as determined when the reference bar  15  is aligned in the direction of the X-axis.  
         [0040]    Based on the angle a determined by averaging, the central processor  17  defines the correction values.  
         [0041]    Following the checks for rectilinearity and rectangularity and calibration, the CCD camera  5 , and with it, the object-detecting unit  13  are moved in a directional pass across the workpiece  4 . The imaging is done along the usual swath pattern.  
         [0042]    To that effect the cantilevered arm  8  is moved in the direction of the X-axis into successively different positions. In each position of the cantilevered arm  8  in the direction of the X-axis the object-detecting unit  13  travels along the cantilevered arm  8  in the direction of the Y-axis, imaging the scanned swath of the workpiece  4  in segmental fashion. In the process the X-motor  9  and the Y-motor  10  provide feedback information to the central processor  17  as to the travel position of the cantilevered arm  8  in the direction of the X-axis and the progressive positions of the CCD camera  5 , i.e. its object-detecting unit  13 , in the direction of the Y-axis. The X-motor  9  also serves to define the starting point of the cantilevered arm  8  on the X-rail in the direction of the X-axis. The Y-motor  10  covers the positions of the object-detecting unit  13  in the direction of the Y-axis.  
         [0043]    If the track along which the object-detecting unit  13  is moved in the direction of the Y-axis were perfectly straight and if the axial angle a between the X-axis and the Y-axis were exactly 90°, the feedback information provided by the X-motor  9  and the Y-motor  10  would permit the error-free determination of the positions occupied by the object-detecting unit  13  as it scans the segments of the individual strips of the workpiece  4 . However, if the installation  1  does not meet that prerequisite due to, e.g., manufacturing related tolerances, the precise determination of the positions of the object-detecting unit  6  requires a correction of the positions established alone on the basis of the feedback information provided by the X-motor  9  and the Y-motor  10 . That correction is made with the aid of the correction values acquired in the process of the above described check for rectilinearity and rectangularity and calibration.  
         [0044]    As a result, the positions of the object-detecting unit  13  as it scans the segments of the workpiece  4  are determined with a high level of precision. It follows that the positions of the workpiece segments relative to one another are defined with equally high precision. By means of the central processor  17 , it is now possible to assemble the workpiece segments into a mosaic which is a complete composite image of the entire workpiece  4  with a highly accurate representation of the actual geometric conditions.  
         [0045]    The composite image thus obtained of the workpiece  4  is used in traditional fashion for quality control comparison with production parts or for programming CNC-controlled production machines as a template for parts to be manufactured.  
         [0046]    Thus, it can be seen that the apparatus and method of the present invention provides precision imaging of a workpiece as required for other operations.