Patent Publication Number: US-2022228852-A1

Title: Apparatus and method for edge detection when machining workpieces

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is continuation of U.S. patent application Ser. No. 16/710,700 filed Dec. 11, 2019, which claims the benefit of German Application No. DE 10 2019 133 695.3 filed Dec. 10, 2019, both of which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and a method for detection of edges or seam tracking when machining workpieces by means of a laser beam, e.g. joining or cutting. The machining can be a laser material processing of workpieces in three-dimensional space. 
     2. BRIEF DESCRIPTION OF THE RELATED ART 
     During laser material processing of workpieces, seam tracking is used for process monitoring, regulation or control and for quality management. For this purpose, mostly image processing systems are used. 
     Edge detection is an essential step in seam tracking. From the prior art methods and systems are known in which the camera image of an optical camera is evaluated by means of special algorithms. The evaluation is based on a monochrome image, from which the in-tensity of the light reflected into the camera is utilized as image information. With optically difficult materials, which have only a small or very diffuse reflection, there are difficulties in the evaluation, since often only very poor and low-contrast images are available. Such materials may have, for example, matte or dark surfaces. 
     Remote laser welding is a special variant of laser beam welding. In this method, welding can be performed in any direction in three-dimensional space, and the direction may also change during the welding operation. This is a difference in comparison to other welding processes, which usually only weld in one direction. In such devices, detectors or guide elements are used, which run ahead of the joining process. 
     Furthermore, in the methods known from the prior art several recordings with different optical filters of the machining process of the workpieces are needed to allow reliable control. For this purpose, the required information is calculated by means of special algorithms. 
     Furthermore, in methods known from the prior art, it is usually also necessary to use a plurality of cameras in order to obtain the required image data. 
     There is a need for an apparatus and a method which allow even with optically difficult materials to ensure reliable seam tracking. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a novel apparatus and a novel method for seam tracking. 
     The present invention provides an apparatus for seam tracking of the process in laser material processing, comprising at least two separate illumination elements, each emitting differently polarized light, and a polarization camera. 
     In a further aspect of the invention, it is provided that the at least two separate illumination elements each emit light having polarization rotated by 90°. 
     Furthermore, an embodiment is provided, comprising four separate illumination elements. 
     In a further aspect of the invention, an apparatus may also comprise an even multiple number of illumination elements. 
     If more than two illumination elements are used, it is provided that in each case two of the separate illumination elements emit identically polarized light. 
     Furthermore, it is provided that then the at least two separate illumination elements, which emit identically polarized light, are arranged opposite to each other. 
     In a further aspect of the invention it is provided that the polarization camera is arranged coaxially to a laser beam for material processing. 
     Another object of the present invention is a method for seam tracking in laser material processing, comprising the steps: 
     a. Use at least two separate illumination elements to illuminate the area of interest with polarized light;
 
b. Capturing at least one image of the illuminated area of interest with a polarization camera;
 
c. Transferring the at least one image to an evaluation unit;
 
d. Evaluation of the at least one image.
 
     In the method according to the invention, the two separate illumination elements may emit light having polarization rotated by 90°. 
     In a further aspect of the invention, it is provided for the method that the evaluation unit is a device for data processing. 
     The method according to the invention can also be designed such that four separate illumination elements are used. 
     Furthermore, in each case two of the four illumination elements can emit identically polarized light when carrying out the method. 
     The method may, in another aspect of the present invention, comprise the arrangement of the illumination elements with identically polarized light opposite each other. 
     Another object of the present invention relates to a laser material processing head comprising an apparatus for seam tracking in laser material processing as described above. 
     A method of using an apparatus as described above for seam tracing in a remote laser welding process is a further object of the present invention. 
     Still other aspects, features, and advantages of the present invention are readily apparent from the following detailed description, simply by illustrating a preferable embodiments and implementations. The present invention is also capable of other and different embodiments and its several details can be modified in various obvious respects, all without de-parting from the spirit and scope of the present invention. Accordingly, the drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive. Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description or may be learned by practice of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described on the basis of figures. It will be understood that the embodiments and aspects of the invention described in the figures are only examples and do not limit the protective scope of the claims in any way. The invention is defined by the claims and their equivalents. It will be understood that features of one aspect or embodiment of the invention can be combined with a feature of a different aspect or aspects of other embodiments of the invention, in which: 
         FIGS. 1 and 2  show an optic of a remote laser welding head from below with different polarization directions of the illumination elements. 
         FIG. 3  shows an optic of a remote laser welding head from below with an alternative arrangement of the illumination elements. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The above-mentioned object of the invention is solved by the features of the independent claims. The dependent claims cover further specific embodiments of the invention. 
     The invention of  FIGS. 1-3  provides an apparatus ( 20 ) and a method in which an image evaluation is possible even with only slight or diffuse contrasts of the materials or even with highly reflective materials. When reflecting on metallic surfaces, or reflective surfaces in general, there may be a change in the polarization properties of the reflected light compared to the incident light. 
     The present invention is based on the use of a polarization camera ( 40 ). Thus, it is possible in addition to the pure optical image data also additionally to determine the polarization properties of the reflected light. For reflections on differently inclined surfaces, the polarization of the light also changes. 
     Furthermore, the invention provides that the camera ( 40 ) is arranged coaxially to the laser beam, with which a workpiece (not shown) is processed. 
     Furthermore, it has been found in experiments using a polarization camera ( 40 ) that the use of polarized light to illuminate the workpiece is advantageous. In this respect, polarized light sources ( 21 ,  22 ,  23 ,  24 ) or the use of polarizing filters (not shown) for the light sources ( 21 ,  22 ,  23 ,  24 ) are an essential element of the present invention. 
     Experiments have shown that the use of at least two different polarized light sources ( 21 ,  22 ,  23 ,  24 ) is advantageous. It is furthermore advantageous if the at least two light sources ( 21 ,  22 ,  23 ,  24 ) have a polarization direction rotated by 90° with respect to another. 
     Four light sources ( 21 ,  22 ,  23 ,  24 ) can also be used. In this case, two light sources (e.g.,  21  and  23 ,  22  and  24 ) each have an identical polarization direction and the two pairs (e.g.,  21  and  23 ,  22  and  24 ) with identical polarization direction are arranged opposite to one another. In principle, an odd number of lighting elements or light sources ( 21 ,  22 ,  23 ,  24 ) is within the scope of the invention. 
     According to the invention polarization cameras ( 40 ) will be used, in which only one frame of a recording or captured image ( 42 ) is sufficient for evaluation. 
     The inventive method is advantageous because not only the purely optical properties of the image ( 42 ) are taken as a basis, but due to the inclusion of the polarization of the reflected light contrast-sharp images arise, which can then be evaluated by means of a suitable algorithm. The stronger contrast leads to a better recognition. 
       FIG. 1  shows the optics of a remote laser welding head  10  from below. Four illumination elements  21 ,  22 ,  23 ,  24  are arranged laterally around the outlet opening of the laser processing optics  15 . The arrows indicate the polarization direction of the individual illumination elements  21 ,  22 ,  23 ,  24 . Opposite illumination elements (i.e.,  21  and  23 ,  22  and  24 ) each have the same direction of polarization, respectively emitted polarized light. In  FIG. 1  is the polarization direction of the lighting elements  21 ,  22 ,  23 ,  24  along their longitudinal direction. The polarization is achieved by polarizing filters (not shown separately) in front of the lighting elements  21 ,  22 ,  23 ,  24 . 
     In  FIG. 2  are also four illumination elements  21 ,  22 ,  23 ,  24  arranged around the laser processing optics  15 . A polarization camera  40  is arranged coaxially to a laser beam for material processing. Here, the polarization filters (not shown separately) lead to a polarization of the light of the illumination elements  21 ,  22 ,  23 ,  24  transversely or perpendicular to the longitudinal extent of the illumination elements. 
       FIG. 3  shows an alternative arrangement of the illumination elements  21 ,  22 ,  23 ,  24 . These are arranged in the corners of the housing of the laser welding head  10 . Opposing illumination elements in the diagonal, i.e.  21 ,  23  and  22 ,  24  have an identical polarization direction of the emitted light. 
     It can be seen from  FIGS. 1 to 3  that the polarization direction of the two pairs is in each case rotated by 90° with respect to one another. 
     In principle, two, four, six, eight or any other even multiple of lighting elements  21 ,  22 ,  23 ,  24  are conceivable. Importantly, the lighting elements  21 ,  22 ,  23 ,  24  are present in pairs, and the lighting elements  21 ,  22 ,  23 ,  24  of a pair face each other, with both lighting elements  21 ,  22 ,  23 ,  24  having the same polarization direction of the emitted light. 
     The present invention provides an apparatus ( 20 ) for seam tracking of the process in laser material processing, comprising at least two separate illumination elements ( 21 ,  22 ,  23 ,  24 ), each emitting differently polarized light, and a polarization camera ( 40 ). 
     In a further aspect of the invention, it is provided that the at least two separate illumination elements ( 21 ,  22 ,  23 ,  24 ) each emit light having polarization rotated by 90°. 
     Furthermore, an embodiment is provided, comprising four separate illumination elements ( 21 ,  22 ,  23 ,  24 ). 
     In a further aspect of the invention, an apparatus ( 20 ) may also comprise an even multiple number of illumination elements ( 21 ,  22 ,  23 ,  24 ). 
     If more than two illumination elements ( 21 ,  22 ,  23 ,  24 ) are used, it is provided that in each case two of the separate illumination elements (e.g.,  21  and  23 ,  22  and  24 ) emit identically polarized light. 
     Furthermore, it is provided that then the at least two separate illumination elements ( 21 ,  22 ,  23 ,  24 ), which emit identically polarized light, are arranged opposite to each other. 
     In a further aspect of the invention, it is provided that the polarization camera ( 40 ) is arranged coaxially to a laser beam for material processing. 
     Another object of the present invention is a method for seam tracking in laser material processing, comprising the steps: (a) using at least two separate illumination elements ( 21 ,  22 ,  23 ,  24 ) to illuminate the area of interest with polarized light; (b) capturing at least one image ( 42 ) of the illuminated area of interest with a polarization camera ( 40 ); (c) transferring the at least one image ( 42 ) to an evaluation unit ( 30 ); and (d) evaluating the at least one image ( 42 ). 
     In the method according to the invention, the two separate illumination elements ( 21 ,  22 ,  23 ,  24 ) may emit light having polarization rotated by 90°. 
     In a further aspect of the invention, it is provided for the method that the evaluation unit ( 30 ) is a device for data processing. 
     The method according to the invention can also be designed such that four separate illumination elements ( 21 ,  22 ,  23 ,  24 ) are used. 
     Furthermore, in each case two of the four illumination elements (e.g.,  21  and  23 ,  22  and  24 ) can emit identically polarized light when carrying out the method. 
     The method may, in another aspect of the present invention, comprise the arrangement of the illumination elements ( 21 ,  22 ,  23 ,  24 ) with identically polarized light opposite each other. 
     Another object of the present invention relates to a laser material processing head ( 10 ) comprising an apparatus ( 20 ) for seam tracking in laser material processing as described above. 
     A method of using an apparatus ( 20 ) as described above for seam tracking in a remote laser welding process is a further object of the present invention. 
     The foregoing description of the preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiment was chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. The entirety of each of the afore-mentioned documents is incorporated by reference herein. 
     REFERENCE NUMERALS 
     
         
         
           
               20  Apparatus 
               21 - 24  illumination elements 
               30  Camera 
               40  Evaluation Unit 
               15  Beam extraction of the laser processing optics, which also acts as imaging optics for the camera  30   
               10  Housing laser welding head