Patent Publication Number: US-11049268-B2

Title: Superimposing position correction device and superimposing position correction method

Description:
TECHNICAL FIELD 
     The present invention relates to a superimposing position correction device to be used for a processing system. 
     BACKGROUND ART 
     Correction for position shift of a processing position is generally employed in laser processing systems using a laser processing machine. For example, there is proposed a method of correcting a start position of processing in laser processing machine by performing processing on a tabular workpiece for correction and comparing the processed position with the previously specified processing position (see Patent Reference 1, for example). 
     Further, in recent years, there is proposed a method of superimposing an image or information indicating a processing position (processing plan locus) or the like on a workpiece that is a processing object displayed on a display device such as a monitor by means of AR (Augmented Reality). In laser processing systems employing AR, it is necessary to associate positions with each other between a machine coordinate system as a plane on which the workpiece is placed and an image coordinate system in a camera image. 
     PRIOR ART REFERENCE 
     Patent Reference 
     Patent Reference 1: Japanese Patent Application Publication No. 2010-99674 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     However, there is a case where the whole of the laser processing machine vibrates during the processing and a change occurs in the position of the workpiece or the position and posture of the camera. In this case, a method of correcting the positions of the workpiece and the camera by using a checker pattern or markers is used, for example. However, the conventional technology has a problem of lacking simplicity of maintenance such as position adjustment for displaying. 
     An object of the present invention, which has been made to resolve the above-described problem, is to correct the processing plan locus to be superimposed on the workpiece with a simple method. 
     Means for Solving the Problem 
     A superimposing position correction device according to the present invention includes: a processor to execute a program; and 
     a memory to store the program which, when executed by the processor, performs processes of, 
     acquiring a first image frame including an image of a workpiece that is processing object and a second image frame including an image of the workpiece after being processed; 
     acquiring the first image frame and the second image frame and generating a difference image, the difference image being an image including a process region that is difference between the first image frame and the second image frame; 
     generating a processing plan image based on a predetermined processing plan locus to be superimposed on the workpiece; 
     generating at least one subregion including a processing plan region of the workpiece, the processing plan region being determined based on the predetermined processing plan locus; 
     searching the difference image for a region similar to the subregion and to acquire the region similar to the subregion as an identified region; 
     extracting barycenter coordinates of the processing plan region included in the subregion in a machine coordinate system and barycenter coordinates of the processed region included in the identified region in an image coordinate system; 
     calculating a projection matrix for performing projection transformation between the machine coordinate system and the image coordinate systme by using the barycenter coordinates in the machine coordinate system and the barycenter coordinates in the image coordinate system; and 
     transforming the predetermined processing plan locus in the machine coordinate system into a new processing plan locus in the image coordinate system by using the projection matrix. 
     Effect of the Invention 
     According to the present invention, the processing plan locus to be superimposed on the workpiece can be corrected with a simple method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram schematically showing a configuration of a laser processing system including a superimposing position correction device according to an embodiment of the present invention. 
         FIG. 2  is a diagram showing an example of an image frame acquired by an image acquisition unit. 
         FIG. 3  is a diagram showing processing plan loci superimposed on a workpiece in the image frame. 
         FIG. 4  is a diagram showing the image frame as a camera image before processing is performed. 
         FIG. 5  is a diagram showing an image frame as a camera image after the processing is performed. 
         FIG. 6  is a diagram showing a difference image generated by a difference image generation unit. 
         FIG. 7  is a diagram showing an image frame as a comparative example. 
         FIG. 8  is a flowchart showing an example of a superimposing position correction method in the laser processing system. 
         FIG. 9  is a diagram showing a difference image normalized by the difference image generation unit. 
         FIG. 10  is a diagram showing a processing plan image in an image coordinate system after projective transformation is performed. 
         FIG. 11  is a diagram showing subregions generated in an image frame. 
         FIG. 12  is a flowchart showing an example of the flow of a similar shape search process. 
         FIG. 13  is a diagram showing identified regions in the difference image. 
         FIG. 14  is a flowchart showing an example of a representative point extraction process. 
         FIG. 15  is a diagram schematically showing sets of representative points. 
         FIG. 16  is a diagram showing corrected processing plan loci superimposed on the workpiece in an image frame. 
         FIG. 17  is a block diagram showing a concrete example of a hardware configuration of the superimposing position correction device. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Embodiment 
       FIG. 1  is a block diagram schematically showing a configuration of a laser processing system  100  including a superimposing position correction device  1  according to an embodiment of the present invention. 
     The laser processing system  100  includes the superimposing position correction device  1 , a laser processing machine  2  and a storage device  3 . While the storage device  3  in this embodiment is a storage medium provided outside the superimposing position correction device  1  and the laser processing machine  2 , the storage device  3  may be provided inside the superimposing position correction device  1  or the laser processing machine  2 . 
     The superimposing position correction device  1  includes a camera  11 , an image acquisition unit  12 , a projective transformation unit  13 , a superimposition display unit  14 , a display device  15 , a difference image generation unit  16 , a processing plan image generation unit  17 , a shift judgment unit  18 , a subregion generation unit  19 , a similar shape search unit  20 , a representative point extraction unit  21  and a projection matrix calculation unit  22 . 
     The camera  11  captures a workpiece w 1  that is a material to be processed (i.e., processing object) and a region around the workpiece w 1 . 
     The image acquisition unit  12  acquires images captured by the camera  11  as respective still images (e.g., image frame f 1  which will be explained below). 
       FIG. 2  is a diagram showing an example of the image frame f 1  (first image frame) acquired by the image acquisition unit  12 . The image frame f 1  shown in  FIG. 2  is an image including an image of the workpiece w 1  that is the processing object. The image frame f 1  is a camera image acquired by the camera  11 . 
       FIG. 3  is a diagram showing processing plan loci  31  superimposed on the workpiece w 1  in the image frame f 1 . The processing plan loci  31  are data indicating processing positions that are processing plans in a machine coordinate system. In the image frame f 1 , each of the processing plan loci  31  is indicated by graphics. The processing plan locus  31  indicated in the image frame f 1  is referred to also as a processing plan image. 
     The projective transformation unit  13  transforms the processing plan locus  31  into data corresponding to an image coordinate system by using a projection matrix  32 . The image coordinate system is an orthogonal coordinate system (i.e., xy plane) in the image frame f 1 . 
     The superimposition display unit  14  draws the processing plan locus  31  as graphics in the image frame f 1  acquired by the image acquisition unit  12 . 
     The image frame f 1  is displayed on the display device  15 . In this embodiment, the display device  15  is a display. For example, on the display device  15 , the processing plan locus  31  transformed into data corresponding to the image coordinate system is superimposed on the workpiece w 1  in the image frame f 1 . 
       FIG. 4  is a diagram showing the image frame f 1  as a camera image before the processing is performed. Each region surrounded by the processing plan locus  31  is a processing plan region  33   a.    
       FIG. 5  is a diagram showing an image frame f 2  (second image frame) as a camera image after the processing is performed. The image frame f 2  shown in  FIG. 5  is an image including an image of a workpiece w 2  processed by the laser processing machine  2  (i.e., workpiece w 2  after the processing) and processed regions  33   b  surrounded by processing tracks  33 . 
       FIG. 6  is a diagram showing a difference image f 3  generated by the difference image generation unit  16 . 
     The difference image generation unit  16  generates the difference image f 3  based on difference between the image frame f 1  ( FIG. 2 ) as the camera image before the processing is performed and the image frame f 2  that is the camera image after the processing is performed. 
     The processing plan image generation unit  17  generates the processing plan image based on the predetermined processing plan locus  31  to be superimposed on the workpiece w 1 . The processing plan image is an image including the processing plan locus  31 . 
       FIG. 7  is a diagram showing an image frame as a comparative example. 
     As shown in  FIG. 7 , there is a case where the processing plan locus  31  and the processing track  33  are shifted from each other when the position of the workpiece w 2  shifted during the processing. The shift judgment unit  18  judges whether or not the processing plan locus  31  and the processing track  33  are shifted from each other by comparing the processing plan image and the difference image (difference image f 4  which will be explained later). 
     The subregion generation unit  19  generates at least one subregion  34 . The subregion  34  is a region including a processing plan region  33   a  determined based on the processing plan locus  31  (specifically, a region of the workpiece w 1  surrounded by the processing plan locus  31 ). 
     The similar shape search unit  20  searches the difference image (the difference image f 4  which will be explained later) for a region similar to the subregion  34  generated by the subregion generation unit  19 . 
     The representative point extraction unit  21  extracts a representative point as barycenter coordinates of the processing plan region  33   a  (processing shape in this embodiment) included in the subregion  34  generated by the subregion generation unit  19  in the machine coordinate system and a representative point as barycenter coordinates of the processed region  33   b  included in an identified region  35  in the image coordinate system. 
     The projection matrix calculation unit  22  calculates a projection matrix (projection matrix H which will be explained later) for making the superimposing position correction in the laser processing system  100 . 
     The laser processing machine  2  includes a processing head  2   a  that emits a laser beam and a processing control unit  2   b  that controls the processing head  2   a . The processing head  2   a  is an emission port of a laser. The processing control unit  2   b  is capable of moving the processing head  2   a  according to the processing plan locus  31 . 
     The storage device  3  stores the processing plan locus  31  and the projection matrix  32 . 
     The processing plan locus  31  is predetermined data and indicates processing positions that is a processing plan in the machine coordinate system. 
     The projection matrix  32  is data (matrix) for performing projective transformation between the machine coordinate system and the image coordinate system and indicates correlation between the machine coordinate system in the laser processing machine  2  and the image coordinate system in the camera  11  (i.e., camera image) previously determined by using an index such as a checkerboard. 
     Next, a superimposing position correction method in the laser processing system  100  will be described. 
       FIG. 8  is a flowchart showing an example of the superimposing position correction method in the laser processing system  100 . 
     In step S 1 , the camera  11  captures the workpiece w 1  before the processing is performed and the image acquisition unit  12  acquires the image frame f 1 . 
     In step S 2 , the laser processing machine  2  processes the workpiece w 1  according to the processing plan locus  31 . Specifically, the processing control unit  2   b  controls the processing head  2   a  according to the processing plan locus  31  and thereby processes the workpiece w 1 . For example, when the workpiece w 1  is processed into four processing shapes (referred to also as processing geometries) of an ellipse, a pentagram, a cross, and a hexagon, the workpiece w 2  is obtained as shown in  FIG. 5 . 
     In step S 3 , the camera  11  captures the workpiece w 2  and the image acquisition unit  12  acquires the image frame f 2  from the camera image. 
     In step S 4 , the difference image generation unit  16  acquires the image frame f 1  and the image frame f 2  from the image acquisition unit  12  and generates the difference image f 3  including the processed regions  33   b  that are the difference between the image frame f 1  and the image frame f 2 . In the difference image f 3 , only the processing tracks  33  and the processed regions  33   b  (four processing shapes in the example shown in  FIG. 6 ) are shown. 
       FIG. 9  is a diagram showing a difference image f 4  normalized by the difference image generation unit  16 . 
     In the difference image f 3  shown in  FIG. 6 , pixel values in a region other than the processed regions  33   b  are 0 and pixel values in the processed regions  33   b  are indefinite values other than 0, and thus normalization is performed so that the pixel values in the processed regions  33   b  become 1. As a result, a binary image in which the pixel values in the region other than the processed regions  33   b  are 0 and the pixel values in the processed regions  33   b  are 1 is obtained as shown in  FIG. 9 . 
     In step S 5 , the projective transformation unit  13  performs the projective transformation on the processing plan locus  31 . Namely, the projective transformation unit  13  transforms the processing plan locus  31  based on the machine coordinate system into data based on the image coordinate system by using the following expression 1: 
     
       
         
           
             
               
                 
                   
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     In the expression 1, x c  and y c  represent a position on the xy plane in the image coordinate system in the camera image. In the expression 1, x m  and y m  represent a two-dimensional position in the machine coordinate system as the coordinate system for controlling the processing head  2   a . In the expression, H represents the projection matrix  32  and A represents any real number. In this embodiment, the projection matrix  32  is a 3×3 matrix and is previously calculated. 
       FIG. 10  is a diagram showing the processing plan image in the image coordinate system after the projective transformation is performed. 
     In step S 6 , the processing plan image generation unit  17  generates the processing plan image. Specifically, the processing plan image is generated based on the processing plan locus  31   a  after undergoing the projective transformation in the step S 5  so that the pixel values in the processing plan regions  33   a  are 1 and the pixel values in the region other than the processing plan regions  33   a  are 0. As a result, the processing plan image after performing the projective transformation is obtained in an image frame f 5  shown in  FIG. 10 . 
     In step S 7 , the shift judgment unit  18  compares the difference image f 4  and the processing plan image in the image frame f 5  and thereby judges whether or not the processing plan image (specifically, the processing plan locus  31   a  in the image frame f 5 ) is shifted from the difference image f 4  (specifically, the processing track  33  in the difference image f 4 ). In this embodiment, the shift judgment unit  18  compares the pixel value of each pixel in the processing plan image in the image frame f 5  and the pixel value of each pixel in the difference image f 4  in regard to coordinates equal to each other and judges whether the processing plan image (f 5 ) is shifted from the difference image (f 4 ) or not by counting the total number of pixels having different pixel values (pixel total number). 
     The shift judgment unit  18  judges that “there is no shift” when the pixel total number is smaller than a predetermined shift judgment threshold value, or judges that “there is a shift” when the pixel total number is larger than or equal to the shift judgment threshold value. The shift judgment threshold value may be set at 1% of the total number of pixels in the image frame f 5 , for example. 
     When the shift judgment unit  18  judges that the processing plan locus  31   a  is not shifted from the processing track  33  (NO in the step S 7 ), the process advances to step S 13 . 
     When the shift judgment unit  18  judges that the processing plan locus  31   a  is shifted from the processing track  33  (YES in the step S 7 ), the process advances to step S 8 . 
       FIG. 11  is a diagram showing subregions  34  generated in an image frame f 6 . 
     In the step S 8 , the subregion generation unit  19  generates at least one subregion  34  including at least one processing plan region  33   a  (at least one processing shape in this embodiment). For example, the subregion generation unit  19  determines a circumscribed rectangle of each processing shape in the image frame f 5 . By this method, each subregion  34  including one processing shape can be generated in the image frame f 6  as shown in  FIG. 11 . Namely, four subregions  34  are generated in this embodiment. 
     In the step S 8 , a plurality of subregions  34  externally contacting each other may be formed. In this case, two or more processing shapes are included in the plurality of subregions  34 . 
     In step S 9 , a similar shape search process is executed. Specifically, the similar shape search unit  20  searches the difference image f 4  for a region similar to a subregion  34  in the processing plan image and acquires the region similar to a subregion  34  in the processing plan image as an identified region  35 . 
       FIG. 12  is a flowchart showing an example of the flow of the similar shape search process. 
     In step S 91 , the similar shape search unit  20  calculates an Hu moment invariant of the image in the subregion  34  of the processing plan image. For example, the Hu moment invariant is calculated by using a method described in the following document as a non-patent reference: 
     Ming-Kuei HU. “Visual Pattern Recognition by Moment Invariants” IRE TRANSACTIONS ON INFORMATION THEORY, vol. IT-8, pp 179-187, 1962 
     The Hu moment invariant can be calculated for any region in an image and has a characteristic of being invariable between an image in which a certain shape is shown and an image in which the shape is changed in scale, rotated, or translated. 
     In step S 92 , the similar shape search unit  20  identifies and acquires a region in the difference image f 4  having the same size as the subregion  34  and having an Hu moment invariant closest to the Hu moment invariant obtained in the step S 91 . Specifically, a region having the same size as the subregion  34  is moved in the difference image f 4  pixel by pixel, the Hu moment invariant is obtained upon each movement of the region, and a region minimizing the norm of the difference between the Hu moment invariant of the subregion  34  in the processing plan image and the Hu moment invariant obtained in the difference image f 4  is identified. 
     In step S 93 , the region in the difference image f 4  obtained in the step S 92  is acquired and stored as the identified region  35 . 
       FIG. 13  is a diagram showing the identified regions  35  in the difference image f 4 . 
     The processing from the step S 91  to the step S 93  is performed for each of the subregions  34  in the processing plan image. Accordingly, in this embodiment, four identified regions  35  are acquired as shown in  FIG. 13 . 
     In step S 10 , a representative point extraction process is executed. Specifically, the representative point extraction unit  21  extracts the representative point as the barycenter coordinates of the processing plan region  33   a  (processing shape in this embodiment) included in the subregion  34  in the machine coordinate system and the representative point as the barycenter coordinates of the processed region  33   b  included in the identified region  35  in the image coordinate system. 
       FIG. 14  is a flowchart showing an example of the representative point extraction process. 
     In step S 101 , the representative point extraction unit  21  calculates the barycenter coordinates of the processing plan region  33   a  included in a subregion  34  corresponding to one identified region  35  in the difference image f 4 . Specifically, the representative point extraction unit  21  is capable of extracting the barycenter coordinates by calculating the average of all machine coordinates (i.e., coordinates in the machine coordinate system) in the processing plan region  33   a  included in the subregion  34  corresponding to one identified region  35  (i.e., a subregion  34  similar to the identified region  35 ) in the difference image f 4 . 
     In step S 102 , the representative point extraction unit  21  calculates the barycenter coordinates of the processed region  33   b  included in the identified region  35  (i.e., the identified region  35  used in the step S 101 ) in the difference image f 4 . Specifically, the representative point extraction unit  21  is capable of extracting the barycenter coordinates by calculating the average of all image coordinates (i.e., coordinates in the image coordinate system) in the processed region  33   b  where the pixel value equals 1 in the identified region  35 . 
     In step S 103 , the barycenter coordinates obtained in the steps S 101  and S 102 , namely, two pairs of barycenter coordinates, are stored as the representative points. 
       FIG. 15  is a diagram schematically showing sets of representative points. 
     The processing from the step S 101  to the step S 103  is performed for each identified region  35  and each subregion  34  corresponding to the identified region  35 . Accordingly, as many representative point sets g 1 , g 2 , g 3  and g 4  as the identified regions  35  are acquired. In this embodiment, four representative point sets g 1 , g 2 , g 3  and g 4  are acquired as shown in  FIG. 15 . 
     In step S 11 , the projection matrix calculation unit  22  calculates the projection matrix H for performing the projective transformation between the machine coordinate system and the image coordinate system (in other words, the projection matrix H for making the superimposing position correction) by using the barycenter coordinates in the machine coordinate system and the barycenter coordinates in the image coordinate system. Specifically, the projection matrix H can be obtained by substituting the representative point sets g 1 , g 2 , g 3  and g 4  acquired in the step S 10  into the aforementioned expression 1. Since the degree of freedom of the projection matrix  32  is 8, at least four representative point sets are necessary for obtaining the projection matrix H. 
     In step S 12 , the projection matrix H obtained in the step S 11  is stored in the storage device  3 . Specifically, the projection matrix  32  is updated to the new projection matrix (i.e., the projection matrix H obtained in the step S 11 ). Accordingly, a corrected projection matrix for making the superimposing position correction can be obtained. 
       FIG. 16  is a diagram showing corrected processing plan loci  31   b  superimposed on the workpiece w 1  in an image frame f 7 . 
     In step S 13 , the superimposition display unit  14  transforms the processing plan locus  31  in the machine coordinate system into the processing plan locus  31   b  in the image coordinate system by using the projection matrix obtained in the step S 12  and makes the display device  15  display the new processing plan locus  31   b  so as to superimpose the new processing plan locus  31   b  on the workpiece w 1  in the image frame f 7 . Accordingly, the processing plan locus  31  used before making the superimposing position correction is corrected and the new processing plan locus  31   b  is superimposed at the position where the new processing plan locus  31   b  should be superimposed on the workpiece w 1 . Further, the superimposition display unit  14  is capable of making the display device  15  display the image frame f 7 . 
     The laser processing machine  2  (specifically, the processing control unit  2   b ) is capable of performing the processing according to the new processing plan locus  31   b . Further, the user can check the new processing plan locus  31   b  displayed on the display device  15 . Thus, the user can monitor and control the operation of the laser processing machine  2  while viewing the new processing plan locus  31   b  displayed on the display device  15 . 
     As described above, according to this embodiment, the position of the processing plan locus to be superimposed on the workpiece w 1  is corrected by using the processing plan locus  31  and the processing track  33 , and thus the superimposing position correction can be made with a simple method without the need of making a correction by using a checker pattern or the like every time the processing is performed. 
     Further, according to this embodiment, the superimposing position correction is made not by using a local feature value such as a circle but by using a shape feature value such as the processing track  33  and the processed region  33   b , and thus it is possible to increase the accuracy of the shift judgment (step S 7 ) between the processing plan locus (specifically, the processing plan locus  31   a  used in the step S 7 ) and the processing track  33  and to increase the accuracy of the superimposing position correction even when the processing track  33  and the processed region  33   b  are hard to detect. 
       FIG. 17  is a block diagram showing a concrete example of a hardware configuration of the superimposing position correction device  1 . 
     The functions of the image acquisition unit  12 , the projective transformation unit  13 , the superimposition display unit  14 , the difference image generation unit  16 , the processing plan image generation unit  17 , the shift judgment unit  18 , the subregion generation unit  19 , the similar shape search unit  20 , the representative point extraction unit  21  and the projection matrix calculation unit  22  described in the above embodiment can be implemented by a processor  1   a  such as a CPU (Central Processing Unit). Various data described in the above embodiment such as the subregions  34 , the identified regions  35  and the barycenter coordinates can be stored in a memory l b . The storage device  3  shown in  FIG. 1  can be the memory l b  in the superimposing position correction device  1 . In this case, data such as the processing plan locus  31  and the projection matrix  32  are stored in the memory l b . A network interface  1   c  is communicatively connected with the laser processing machine  2  and the storage device  3 . A camera  1   f  corresponds to the camera  1   l  shown in  FIG. 1 , and a display device  1   e  corresponds to the display device  15  shown in  FIG. 1 . The camera  1   f  is connected to the display device  1   e  via a display device interface  1   d  and the camera image is displayed on the display device  1   e.    
     DESCRIPTION OF REFERENCE CHARACTERS 
       1 : superimposing position correction device,  1   a : processor,  1   b : memory,  1   c : network interface,  1   d : display device interface,  1   e : display device,  1   f : camera,  2 : laser processing machine,  2   a : processing head,  2   b : processing control unit,  3 : storage device,  11 : camera,  12 : image acquisition unit,  13 : projective transformation unit,  14 : superimposition display unit,  15 : display device,  16 : difference image generation unit,  17 : processing plan image generation unit,  18 : shift judgment unit,  19 : subregion generation unit,  20 : similar shape search unit,  21 : representative point extraction unit,  22 : projection matrix calculation unit,  31 ,  31   a ,  31   b : processing plan locus,  32 : projection matrix,  33 : processing track,  33   a : processing plan region,  33   b : processed region, f 1 : image frame (first image frame), f 2 : image frame (second image frame), f 3 , f 4 : difference image, w 1 , w 2 : workpiece.