Method for OCT weld seam monitoring, and associated laser processing machine and computer program product

Abstract

The present application document relates to a method for monitoring a curved weld seam by means of a measuring beam of an optical coherence tomography device when welding workpieces with a processing laser beam. The method has the following method steps: During welding, by means of the measuring beam, distance measurements are carried out at least at one front measuring point (M Pre ) that is observed along the welding direction in front of the current welding position (22) and at least at one rear measuring point (M Pos t ) that is observed along the welding direction behind the current welding position (22) respectively by deflecting the measuring beam onto the workpiece; and - monitoring the curved hardened weld seam (21a) based on the rear distance measurement, wherein the rear measurement line (24) formed by a plurality of rear measuring points (M Post ) is positioned such that the rear measurement line (24) is offset along the direction of the front measurement line (23) towards the curved hardened weld seam (21a) with respect to the front measurement line (23) formed by a plurality of front measuring points (M Pre ) and / or is twisted with respect to the front measurement line (23) along the direction of the normal towards the curved hardened weld seam (21a).

Description

Field of Technology

[0001] The present invention relates to a method for monitoring a curved weld seam by means of a measuring beam of an optical coherence tomography device (OCT for short) when welding workpieces by means of a processing laser beam, the method having the following method steps:

[0002] - During the welding, distance measurements are respectively carried out by deflecting the measuring beam onto the workpiece at at least one pre-measurement point before the current welding position and at at least one post-measurement point after the current welding position along the welding direction, and

[0003] - The curved weld seam is monitored based on the post-distance measurement. Background Art

[0004] Such a method for OCT weld seam monitoring is disclosed, for example, by DE 10 2016 014 564 A1.

[0005] During laser beam welding, the precise positioning of the laser beam relative to the workpiece is particularly important. Due to the limited accuracy of the positioning system and the usual component tolerances, a system for detecting the position of the workpiece and correspondingly adjusting the position of the laser beam is indispensable. Typically, the position of geometric features is previously detected relative to the laser beam. After further processing of the geometric features, the position of the laser beam is adjusted relative to the position of the geometric features. In laser beam welding of fillet welds at lap joints, the edge of the upper metal plate is mainly used as a geometric feature for positioning the laser beam. After the process, the geometry of the hardened weld seam can be measured. The geometric parameters thus obtained are used for the external inspection of the weld seam and provide information about the quality of the welded joint.

[0006] Commercially available seam tracking control systems are based on the imaging light section method or the incident illumination method. Recently, methods based on OCT (optical coherence tomography) have also been used. OCT-based systems employ an OCT (small field of view) scanner that rapidly moves the OCT measuring beam through the component. Then, an OCT distance measurement image is calculated from the individual measurement points, and the measured OCT distances are plotted along the measurement points in the image. Compared with the widely used light section method, the advantage of OCT-based systems is that the scan pattern of the OCT (small field of view) scanner can be changed during processing.

[0007] Image processing algorithms for determining the position of geometric features or geometric measurement parameters play an important role in seam position adjustment. When two metal plates are overlapped, the position of the edge of the upper metal plate is previously determined relative to the laser beam (so-called pre-measurement), and then seam features are determined for evaluating the hardened seam (so-called post-measurement). The reliability of the algorithm mainly depends on the position of the regions of interest (pre-region: edge of the upper metal plate, post-region: hardened weld seam) in the OCT distance measurement image. In the image generated by means of OCT, it is suitable to determine the surface of the metal plate by means of an image processing algorithm. Here, an important step is to interpolate the lines of the metal plate surface from the existing image data. If the interpolation length next to the geometric feature is too short, the interpolation becomes unreliable. For example, if the region provided for interpolation in the OCT distance measurement image is too small, the result becomes inaccurate or cannot be obtained. If the trajectory of the laser beam is described as a curved path, it leads to mispositioning of the post-measurement line. In the OCT distance measurement image, it causes the seam geometry to shift in a direction away from the curved weld seam. Therefore, not enough information can be provided to the image processing algorithm to calculate the seam geometric features. Summary of the Invention

[0008] In contrast, the object of the present invention is to further improve a method of the type described at the beginning such that a curved weld seam can be detected as optimally as possible in the OCT distance measurement image.

[0009] This object is solved according to the invention in such a way that, in the case of a post-measurement line formed by a plurality of post-measurement points, the post-measurement line is positioned such that the post-measurement line is offset relative to a pre-measurement line formed by a plurality of pre-measurement points in the direction of the pre-measurement line towards the curved hardened weld seam and / or is twisted relative to the pre-measurement line in the direction of the normal towards the curved hardened weld seam, and in the case of a single post-measurement point, the single post-measurement point is positioned such that, compared to each pre-measurement point, the single post-measurement point is at a greater distance in the direction of the curved hardened weld seam from the line traversed along the welding direction by the current welding position.

[0010] The inventive dynamic positioning of the post-measurement line or the single post-measurement point enables a significantly more stable and precise evaluation of the post-measurement data. Then, based on the post-distance measurement, the hardened weld seam can be geometrically measured and monitored. Therefore, the seam geometric features can be measured significantly more stably and accurately.

[0011] Particularly preferably, the post-measurement line is positioned such that the line center point of the post-measurement line lies on the curved hardened weld seam. In this case, the OCT distance measurement image can be evaluated optimally.

[0012] The rear measurement line and the front measurement line can be, for example, straight measurement lines or curved, closed or open measurement lines. Particularly advantageously, the rear measurement line intersects the curved hardened weld seam at an angle of 90° ± 10°, particularly 90°. In these cases, the OCT distance measurement images can be optimally evaluated. In a preferred embodiment, the front measurement line and the rear measurement line are the same, i.e., of equal length in the case of a straight measurement line.

[0013] In a particularly advantageous variant, the rear measurement line is shifted by an offset and / or rotated by a rotation angle such that the rear measurement line is moved from an initial position where it is not twisted relative to the front measurement line and is at an equal distance from the line to the measurement position of the rear measurement line. The offset or rotation angle required for this of the rear measurement line can be derived from the following position of the curved hardened weld seam, which is calculated, for example, from the trajectory of the processing laser beam.

[0014] For the case of a single rear measurement point, the measurement position of the single rear measurement point is preferably selected such that it lies on the curved hardened weld seam.

[0015] The invention also relates to a laser processing machine having: a laser beam generator for generating a processing laser beam; a laser scanner for two-dimensionally deflecting the processing laser beam onto a workpiece; an optical coherence tomography device for generating an OCT measurement beam, which is directed by the laser scanner onto the workpiece; an OCT scanner, which is arranged between the coherence tomography device and the laser scanner and which is used for two-dimensionally deflecting the OCT measurement beam onto the workpiece; and a machine control device for controlling the laser scanner and the OCT scanner, wherein, according to the invention, the machine control device is programmed to position the rear measurement line or the single rear measurement point in accordance with the method according to the invention.

[0016] Finally, the invention also relates to a computer program product having code which is suitable for performing all steps of the method according to the invention when the program runs on the machine control device of a laser processing machine. Description of the Drawings

[0017] Further advantages and advantageous configurations of the subject matter of the invention can be learned from the description, the drawings and the claims. The features mentioned above and those further listed can equally be used individually or in any combination of several. The embodiments shown and described should not be understood as an exhaustive listing, but rather have exemplary features for describing the invention.

[0018] In the drawings:

[0019] Figure 1Schematically shows a laser processing machine for performing the method according to the present invention;

[0020] Figure 2A and Figure 2B shows a method for monitoring straight welds ( Figure 2A ) and curved welds ( Figure 2B ) by means of an OCT measurement beam respectively through the respective OCT distance measurement images according to the prior art; and

[0021] Figures 3A to 3C shows a method for monitoring curved welds according to the present invention by means of an OCT measurement beam respectively through the respective OCT distance measurement images. Detailed description

[0022] Figure 1 The laser processing machine 1 schematically shown in the detailed description includes a laser beam generator 2 for generating a processing laser beam 3, a laser scanner 4 for two-dimensionally deflecting the processing laser beam 3 in the x-direction and y-direction onto a workpiece 5, and an optical coherence tomography device (OCT) 6 for optically scanning a region of the surface 7 of the workpiece 5. The laser scanner 4 can have, for example, a scanner mirror that can be deflected around two axes, or two scanner mirrors that can be deflected around axes respectively.

[0023] The OCT 6 has, in a known manner, an OCT light source (such as a superluminescent diode) 8 for generating a light beam 9, and a beam splitter 10 for splitting the light beam 9 into an OCT measurement beam 11 and a reference beam 12. The OCT measurement beam 11 is transferred to the measurement arm 13 and impinges on the workpiece surface 7. The OCT measurement beam 11 is at least partially reflected on the workpiece surface and is guided back to the beam splitter 10 that is not passable or partially passable in that direction. The reference beam 12 is transferred to the reference arm 14 and is reflected by a mirror 15 at the end of the reference arm 14. The reflected reference beam is also guided back to the beam splitter 10. The superposition of the two reflected beams is finally detected by a detector (OCT sensor) 16 in order to obtain height information about the workpiece surface 7 and / or the current penetration depth of the processing laser beam 3 into the workpiece 5 taking into account the length of the reference arm 14. This method is based on the basic principle of light wave interference and enables the detection of height differences in the micrometer range along the measurement beam axis.

[0024] The OCT (small field of view) scanner 17 is connected to the measuring arm 13 so that the OCT measuring beam 11 is deflected two-dimensionally (i.e., along the x and y directions) onto the workpiece surface 7 and thereby scans, for example by line scanning, an area of the workpiece surface 7. The OCT scanner 17 can have, for example, a scanner mirror that can be deflected about two axes, or two scanner mirrors that can be deflected about axes respectively. The mirror 18 is arranged obliquely in the optical path of the machining laser beam 3 and is transmissive for the machining laser beam 3 and reflective for the OCT measuring beam 11. The OCT measuring beam 11 is coupled into the laser scanner 4 through this mirror so that the OCT measuring beam 11 points to the workpiece 5. The sensor data of the OCT sensor 16 is transmitted to the machine control device 19, which also controls the movement of the scanners 4 and 17.

[0025] Figure 1 Shows welding of two superposed workpiece parts 5a, 5b at the lap joint by means of the machining laser beam 3, which is guided along the joint edge (welding direction 20) of the two workpiece parts 5a, 5b to weld the two workpiece parts 5a, 5b to each other by weld seams 21a, 21b extending along the joint edge. The hardened weld seam is marked as 21a, and the weld seam yet to be produced is marked as 21b. The current welding position, i.e., the point of incidence of the machining laser beam 3 on the workpiece 5, is marked as 22.

[0026] During welding, by means of the OCT measuring beam 11, a plurality of front measurement points M are observed not only on the workpiece surface 7 along the welding direction 20 in front of the current welding position 22 Pre but also a plurality of rear measurement points M are observed on the workpiece surface 7 along the welding direction 20 behind the current welding position 22 Post to perform distance measurement. For this purpose, the OCT measuring beam 11 is deflected onto the workpiece surface 7 by means of the OCT scanner 17 accordingly. As Figure 1 shown, the plurality of front measurement points M Pre are arranged along a front measurement line 23 extending transversely on the weld seam 21b yet to be produced, and the plurality of rear measurement points M Post are arranged along a rear measurement line 24 extending transversely on the hardened weld seam 21a. Then, the hardened weld seam 21a can be geometrically measured and monitored based on the rear distance measurement.

[0027] In the case of monitoring a straight hardened weld seam 21a ( Figure 2A ) and a curved hardened weld seam 21a ( Figure 2B) In known methods, the same scan pattern is used for the front measurement line 23 and the rear measurement line 24, that is, for example, the front measurement line 23 and the rear measurement line 24 of equal length, and the front measurement line and the rear measurement line extend parallel to each other and without offset in the y direction, and are oriented at right angles and centered with respect to the welding direction 20, which extends in the x direction at the current welding position 22. More precisely, first the front measurement line 23 is determined, and then this scan pattern is also used for the rear measurement line 24.

[0028] As Figure 2A shown, in the case of straight weld seams 21a, 21b, the line centers of the front measurement line 23 and the rear measurement line 24 are respectively positioned on the weld seams 21a, 21b. Therefore, the corresponding regions of interest of the workpiece surface 7 are optimally detected in the OCT distance measurement image 25, that is, on the one hand, the steps of the lap joint in the front region and, on the other hand, the hardened weld seam 21a in the rear region, and the measured distances (heights in the z direction) are plotted along the measurement lines 23, 24 in the OCT distance measurement image. However, if the hardened weld seam 21a is curved as Figure 2B shown, then due to the curvature, there is a mispositioning of the rear measurement line 24 and thus a shift of the region of interest (hardened weld seam 21a) in the OCT distance measurement image 25 in a direction away from the curved weld seam 21a. Therefore, less image information is provided, which, for example, results in an insufficient interpolation length 26.

[0029] Figures 3A-3C shows three variant embodiments of a method according to the invention for monitoring a curved hardened weld seam 21a by means of an OCT measurement beam 21 via the respective OCT distance measurement image 25, specifically, examples of straight front measurement lines 23 and straight rear measurement lines 24 of equal length. Alternatively, the front measurement line 23 and the rear measurement line 24 can also be curved, closed or open measurement lines.

[0030] In Figure 3A , the rear measurement line 24 is offset by an offset A in the direction of the front measurement line 23 with respect to the front measurement line 23 towards the curved hardened weld seam 21a. For this purpose, the rear measurement line 24 can, for example, be shifted with respect to an initial position that is non-offset and parallel to the front measurement line 23 and as shown in Figure 2A , Figure 2B towards the curved hardened weld seam 21a to the rear measurement line's position in Figure 3AAmong the measurement positions shown, specifically, preferably shifted to such an extent that the line center point of the rear measurement line 24 lies on the curved hardened weld 21a. Thus, the hardened weld 21a is optimally detected in the OCT distance measurement image 25. The offset A required for this can be determined, for example, based on the position of the curved hardened weld 21a, which is calculated, for example, based on the trajectory of the processing laser beam 3.

[0031] In Figure 3B the rear measurement line 24 is rotated relative to the front measurement line 23 by a rotation angle B in the direction of the normal to the curved hardened weld 21a. For this purpose, the rear measurement line 24 can be rotated relative to the front measurement line 23 which is non-offset and parallel and in the Figure 2A , Figure 2B initial position shown in Figure 3B around an arbitrary point of the non-offset rear measurement line 24, in particular around the line point (e.g., the line center point) to its measurement position shown, specifically, preferably rotated to such an extent that the rear measurement line 24 intersects the curved hardened weld 21a at an angle of 90°. Thus, the hardened weld 21a is optimally detected in the OCT distance measurement image 25. The rotation angle B required for this can be determined, for example, based on the position of the curved hardened weld 21a, which is calculated, for example, based on the trajectory of the processing laser beam 3.

[0032] In Figure 3C the rear measurement line 24 is not only offset by the offset A relative to the front measurement line 23 but also rotated by the rotation angle B. Preferably, the line center point of the rear measurement line 24 lies on the curved hardened weld 21a, and the rear measurement line 24 intersects the curved hardened weld 21a at an angle of 90°.

[0033] That is to say, the position of the rear measurement line 24 is adjusted translationally ( Figure 3A ), rotationally ( Figure 3B ) or translationally and rotationally ( Figure 3C ) according to the present invention so that the region of interest is optimally positioned in the image section. Thereby, the determination of the seam geometric features becomes significantly more reliable and accurate. The offset A and the rotation angle B of the rear measurement line 24 are calculated based on input parameters which are transmitted by another system part or a control device (motion vector) or come from the system itself. Examples of system measurements are the previously measured lateral positioning angle, the length of the metal plate in the previous image, and the position of the upper metal plate edge. The closed-loop control algorithm uses the measured or estimated position (post-measurement value) of the hardened weld 21a as an input parameter.

[0034] Instead of forming a plurality of rear measurement points M of the rear measurement line 24 Post , it is also possible to use only a single unique rear measurement point MPost , the only post-measurement point is positioned such that, compared with each pre-measurement point M Pre , the only post-measurement point is at a greater distance from the line L( Figures 3A-3C ) traveled through the current welding position 22 along the welding direction 20 in the direction towards the curved hardened weld 21a. Preferably, the measurement position of the only post-measurement point M Post is chosen such that it lies on the curved hardened weld 21a.

Claims

1. A method for monitoring a curved weld seam (21a) by means of a measuring beam (11) of an optical coherence tomography device (6) when welding a workpiece (5) with a processing laser beam (3), the method having the following method steps: - during the welding, by means of the measuring beam (11) not only observes in the welding direction (20) at least one preceding measuring point (M) located before the current welding position (22) Pre ) and at least one subsequent measuring point (M) located after the current welding position (22) as viewed along the welding direction (20). Post ) respectively by deflecting the measuring beam (11) to the workpiece (5), and - Monitoring the curved hardened weld seam (21a) according to a back distance measurement, characterized in that For the case of a post-measurement line (24) formed by a plurality of post-measurement points (M Post ), the post-measurement line (24) is positioned such that the post-measurement line (24) is offset along the direction of the pre-measurement line (23) formed by a plurality of pre-measurement points (M Pre ) towards the curved hardened weld seam (21a) and / or twisted relative to the pre-measurement line (23) along the direction of the normal to the curved hardened weld seam (21a), and for the case of a single post-measurement point (M Post ), the single post-measurement point (M Post ) is positioned such that, compared to each pre-measurement point (M Pre ), the single post-measurement point is at a greater distance from the line (L) traversed by the current welding position (22) along the welding direction (20) in the direction towards the curved hardened weld seam (21a).

2. The method according to claim 1, characterized in that the back measurement line (24) is positioned such that the line center point of the back measurement line (24) lies on the curved hardened weld seam (21a).

3. The method according to claim 1 or 2, characterized in that the back measurement line (24) and the front measurement line (23) extend straight, or the back measurement line (24) and the front measurement line (23) extend curved.

4. The method according to claim 1 or 2, characterized in that the back measurement line (24) is positioned such that the back measurement line (24) intersects the curved hardened weld seam (21a) at an angle of 90° ± 10°.

5. The method according to claim 1 or 2, characterized in that the front measurement line (23) and the back measurement line (24) are identical.

6. The method according to claim 5, characterized in that by shifting the back measurement line (24) by an offset (A) and / or by rotating the back measurement line (24) by a rotation angle (B) such that the back measurement line (24) is moved from an initial position that is not twisted relative to the front measurement line (23) and is at an equal distance from the line (L) to the measurement position of the back measurement line.

7. The method according to claim 6, characterized in that the offset (A) and / or the rotation angle (B) are obtained according to the calculated position of the curved hardened weld seam (21a).

8. The method according to claim 6 or 7, characterized in that the back measurement line (24) is rotated relative to an initial position parallel and non-offset to the front measurement line (23) about an arbitrary point of the non-offset back measurement line (24) into the measurement position of the back measurement line.

9. The method according to claim 1, characterized in that The measurement position of the only post-measurement point (M Post ) is selected such that it lies on the curved hardened weld seam (21a).

10. The method according to claim 4, characterized in that the back measurement line (24) is positioned such that the back measurement line (24) intersects the curved hardened weld seam (21a) at an angle of 90°.

11. The method according to claim 8, characterized in that the back measurement line (24) is rotated relative to an initial position parallel and non-offset to the front measurement line (23) about the line point of the non-offset back measurement line (24) into the measurement position of the back measurement line.

12. A laser processing machine (1), having: a laser beam generator (2) for generating a processing laser beam (3); a laser scanner (4) for two-dimensionally deflecting the processing laser beam (3) onto the workpiece; an optical coherence tomography device (6) for generating an OCT measurement beam, which OCT measurement beam is directed by the laser scanner (4) onto the workpiece; An OCT scanner (17), which is arranged between a coherence tomography device (6) and a laser scanner (4), and which is used to deflect the OCT measurement beam two-dimensionally onto the workpiece; and a machine control device (19) for controlling the laser scanner (4) and the OCT scanner (17), characterized in that The machine control device (19) is programmed to position the post-measurement line (24) or the only post-measurement point (M Post ) in accordance with the method according to any one of claims 1 to 11.

13. A computer program product having code which is adapted to perform all steps of the method according to any one of claims 1 to 11 when the program runs on a machine control device (19) of a laser processing machine (1).

Images