Vision Guided Laser Welding

The present invention relates to laser welding, and particularly to a method and apparatus for accurately positioning a laser welding beam precisely at a succession of weld sites to join a plurality of parts in a complex assembly.

There are numerous manufacturing operations calling for the welding of a multiplicity of parts together to form a complex assembly of exacting dimensional tolerances and physical integrity. One such assembly is a nuclear fuel bundle spacer of the construction disclosed in commonly assigned Matzner et al. U.S. Pat. No. 4,508,679. These spacers are comprised of a plurality of tubular cells or ferrules, each approximately 1.2 inches long and 0.64 inches in diameter, with a wall thickness of approximately 0.025 inches. The ferrules are arranged in a matrix array with contiguous peripheral points of adjacent ferrules conjoined by butt welds. The bore of each ferrule provides a passage for receipt of an elongated fuel rod of the bundle. The fuel rods are centered and laterally supported in the ferrule bores between resilient springs and opposed stops integrally formed near the upper and lower ferrule edges. If the laser beam is not precisely centered on the weld sites constituted by the points of virtual peripheral abutment between the upper and lower edges of the ferrule, an unacceptable butt weld is achieved due to insufficient weld penetration. Also, an off-center laser beam can impinge on the fuel rod centering springs located beneath the ferrule edges, inflicting damage thereon. In either case, the resulting spacer is unacceptable and must be scrapped.

It is accordingly a principle objective of the present invention to provide a method and apparatus for rapidly centering a laser welding beam on a succession of weld sites to achieve high quality welds joining multiple parts in assembly on a expedited, automated basis. To this end, the apparatus includes a laser welder having a head for focussing a highly concentrated laser welding beam on a workpiece resting on an X-Y positioning table and having a plurality of sites to be welded. A CNC controller activates the positioning table in accordance with a predetermined nominal weld site position map to successively locate each weld site at a nominal weld position relative to the laser beam. A source of back illumination casts a sharp image of each weld site in its nominal weld position, which is viewed by a camera through the laser beam optics. The weld site image is stored in a frame grabber as pixel image data for processing by a computer in accordance with a an optimal weld point location algorithm. Pursuant to this algorithm, the computer locates certain geometrical features of the workpiece immediately adjacent the weld site and, based on these located features, defines the position of the optimum weld point for the weld site in relation to the position of the laser beam centerline. The computer then fine positions the positioning table accordingly, via the CNC controller, to bring the optimum weld point into precise alignment with the centerline of the laser beam.

To calibrate the apparatus, a solid sheet metal coupon is placed on the positioning table in the same focal plane occupied by the weld sites. A hole is then burned in the coupon by the laser beam. The position of the burn hole centroid in the camera field of view is then located by the computer. The computer then instructs the CNC controller to reposition the coupon along one axis and then the other axis of the positioning table by a predetermined distance. The lengths of burn hole movement in the camera field of view along the X and Y axes (measured in pixels) are respectively ratioed with the predetermined distance of positioning table movement along the X and Y axes to derive X and Y calibration constants which are factored with the fine positioning X and Y inputs to the controller in indexing the weld site optimal weld point into precise alignment with the laser beam centerline.

The invention accordingly comprises the features of construction, combination of elements, arrangement of parts, and a combination of method steps, all as detailed hereinafter, and the scope of the invention will be indicated in the claims.

For a full understanding of the nature and objects of the present invention, reference may be had to the following Detailed Description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of vision guided laser welding apparatus constructed in accordance with the present invention; and

FIG. 2 is a plan view of a weld site imaged by the apparatus of FIG. 1 to illustrate a preferred algorithm for defining the position of the weld site optimal weld point.

The vision guided laser welding apparatus of the present invention includes, as seen in FIG. 1, a laser welder 10, such as a Coherent General Model S-51 CO.sub.2 laser, having a beam delivery tube 12 through which laser energy is directed to a focussing head 14 from which is emitted a concentrated, vertically oriented welding beam 16 for impingement on an underlying workpiece 18 fixtured on an X-Y positioning table, generally indicated at 20. A TV camera 22, such as a charge coupled device (CCD) camera, is affixed on the focussing head mounting bracket 24 to view the workpiece over the optical path of the laser beam and a pair of right angle reflecting prisms 26. The video signal output of the camera is digitized and stored by a conventional frame grabber 28 as pixel data representing the two-dimensional image of one of the many workpiece weld sites situated within the camera field of view (FOV) by the positioning table 20 in response to X-Y positioning inputs from a CNC controller 30 acting in response to coarse X-Y positioning instructions from a weld site nominal position map 32. As described below, a computer 34 processes the pixel data stored in the frame grabber to define the location of the optimum weld point on the weld site relative to the laser beam centerline and to generate fine X-Y positioning data inputs for bringing the optimum weld point into precise alignment with the centerline of laser welding beam 16.

For purposes of the following description, workpiece 18 is assumed to be a nuclear fuel bundle spacer of the construction disclosed in the above cited Matzner et al. U.S. patent. Thus, the workpiece is in the form of an assembly of ferrules fixtured in a tight matrix array. The objective of the present invention is to visually locate, as weld sites, the most contiguous tangent points on the outer edges of each adjacent pair of ferrules, define the optimum weld point for each weld site, and then guide each optimum weld point into precise alignment with the centerline of the laser welding beam preparation to butt welding the ferrules in conjoined assembly. To provide a sharp image of each weld site for viewing by camera 22, the spacer is back illuminated by a light panel 36, preferably of solid state fiber optic construction. Since the camera viewing angle is normal to the plane containing the multiple weld sites, this back illumination casts a precise image of each weld site at the appropriate focal point of the laser beam 16 established by focussing head 14. The substantial image contrast permits very accurate location of the upper edges of the ferrules at each weld site by computer 34 pursuant to defining the precise location of the optimum weld point therefor.

To ensure requisite accuracy, the position of the laser beam centerline must be referenced in the camera FOV, and the relationship between the dimensional units of the image data stored in the frame grabber memory and the dimensional units of the positioning table must be calibrated. Thus, in accordance with an important aspect of the present invention, a solid metal sheet or coupon (not shown) of approximately ten mils thickness is placed on the positioning occupy. The laser welder 10 then burns a small hole in the coupon, and the image of this hole in the camera FOV including the burn hole is stored in the frame grabber memory.

Computer 34 processes the stored pixel data to define the precise location of the hole centeroid in terms of pixel coordinate location (X.sub.1, Y.sub.1), which then locates the laser beam centerline in the camera FOV. The computer then instructs CNC controller 30 to move the coupon a predetermined number (N) of mils along the X axis, and the new X axis position coordinate X.sub.2 of the hole centroid is located. An X axis calibration constant KX is then calculated by the computer as ##EQU1## The computer then instructs the CNC controller to move the coupon N mils along the Y axis, and the new Y axis position coordinate Y.sub.2 of the hole centroid is located. The Y axis calibration constant KY is then calculated as ##EQU2##

As a result of this calibration algorithm, the location of the laser beam centerline in the camera FOV is defined, and the X-Y coordinate system of the positioning table is calibrated to the X-Y coordinate system of the stored FOV image. The apparatus is then also calibrated for the magnification of the camera optics and the aspect ratio of the FOV. Since the laser beam tends to drift over time, this calibration procedure may have to be repeated at frequent intervals to redefine its centerline position in the camera FOV.

To describe the operation of the apparatus in locating the optimum weld point for a weld site, reference is made to FIG. 2 which illustrates a FOV pixel image including a weld site 40 coarsely positioned into the camera FOV by the CNC controller in response to X-Y positioning inputs from weld site nominal position map 32 of FIG. 1. The size of the camera FOV may be in the order of 200 mils square, which, in the case of a nuclear fuel bundle spacer, is sufficient to cover only one weld site. As seen in FIG. 2, weld site 40 includes the region where the peripheral surfaces of adjacent ferrules 42 and 44 are abutting or in essentially abutting relation at their upper ends 42a and 44a, respectively, i.e., the region including the most contiguous tangent points on the outer edges of the two adjacent ferrules. Ideally, these contiguous tangent points are abutting, however satisfactory butt weld joints can be achieved even if they are in a gapped relation, as long as the gap is not excessive. Thus, the computer checks the dimension of any gap and, if it exceeds a predetermined limit, e.g. 2-3 mils, halts operation. The spacer is removed from inspection to see if the excessive gap can be cured by refixturing.

In accordance with the optimum weld point location algorithm of the present invention, the computer processes the stored pixel data to locate the lowest point 46 on the inner edge of upper ferrule 42 and the uppermost point 48 on the inner edge of the lower ferrule 44. The midpoints between the X axis pixel coordinates and the Y axis pixel coordinates of points 46 and 48 are calculated to define a location P1. This location is declared as a potential optimal laser weld point.

To confirm this point P1 as the true optimal weld point, the computer inspects the pixel image data along X coordinates a fixed distance M both to the left and to the right of point P1 to locate points 50 and 52 on the peripheral edge of ferrule 42 and points 54 and 56 on the peripheral edge of ferrule 44. The midpoint 58 between the Y coordinates of points 50 and 54 and the midpoint 60 between the Y coordinates of points 52 and 56 are located. A line connecting these midpoints 58 and 60 is bisected to locate a point P2. If points P1 and P2 are coincided within a pre-established manufacturing tolerance, e.g., lie within weld site 40, point P1 is declared as the optimum welding point. The optimum weld position in X-Y pixel coordinates is then correlated with the pixel coordinates of the actual position of the laser beam centerline, indicated for example at 16a in FIG. 2, to derive offset or fine X-Y positioning data defining the offset between the beam and optimum weld point locations. This : positioning data is factored with the calibration constants KX and KY and fed to the CNC controller, which, in response, fine positions the table to bring the optimum weld position into precise alignment with the centerline of the laser beam, indicated at 16 in FIG. 2. The laser welder then generates the laser welding beam to butt weld the ferrules together. After the completion of a weld, an image of the weld joint may be stored in the frame grabber memory, enabling the computer to inspect the weld joint and determine that it satisfies quality control (QC) standards 35 (FIG. 1).

It will be appreciated that the weld sites may be coarsely positioned with the adjacent ferrule pair in a different relationship in the camera FOV from that illustrated in FIG. 2, e.g., arranged in side-by-side relation along the X axis rather than along the Y axis, the optimum weld point location algorithm locates point P1 by finding the leftmost point on the inner edge of the left ferrule. The midpoints between the X coordinates and Y coordinate of these extreme points are calculated to define the position of point P1 in the FOV. The point P2 position is then defined by inspecting the area between the ferrules along the Y axis to locate the requisite outer or peripheral edge points corresponding to points 50, 52, 54 and 56 in FIG. 2. It will be appreciated that the characteristics of the optimum weld point location algorithm provided by the present invention compensate for any minor geometrical irregularities in the ferrules themselves and in the ferrule relative positions pursuant to locating the unique optimum weld point for each weld site.

Upon the completion of each weld joint and the optional QC inspection thereof, the next weld site in sequence is indexed into the camera FOV by the CNC controller in response to coarse positioning data inputs from weld site nominal position map 32 and then vision guided into precise , alignment with the laser beam for welding. If at anytime the optimum weld position can not be found or confirmed, welding is halted, and the spacer is removed from the positioning table for inspection. If the problem resides in correctable errors in fixturing or assembly, the spacer can be saved. Otherwise the spacer is scrapped or salvaged to the extent possible.

It is seen from the foregoing that the present invention provides apparatus for performing a plurality of laser welds at a multiplicity of weld sites on an automated, production line bases. The ability to optically guide the each weld site into a precise, optimum weld position with respect to the laser beam ensures weld joints of consistent premium quality. Throughput of acceptable welded assemblies is thus significantly enhanced. Accordingly, the objectives of the present invention set forth above, including those made apparent in the Detailed Description, are efficiently attained, and, since certain changes may be made in the disclosed embodiment without departing from the scope of the invention, it is intended that matters of detail be taken as illustrative and not in a limiting sense.