Method for calibrating a measurement coordinate system of a coherence tomograph to the machining coordinate system of a laser machining arrangement

Abstract

The invention relates to a method for calibrating a measurement coordinate system of a coherence tomograph (24) to the machining coordinate system of a laser machining arrangement (12), having the method steps of: a. adjusting a camera coordinate system (42) of an observation camera (26) to the machining coordinate system b. adjusting the measurement coordinate system to the camera coordinate system (42) by i. scanning a predetermined scan figure (52) using a scanner of the coherence tomograph (24) ii. recording the scan figure (52) using the observation camera (26), in particular with an exposure duration adapted to the scan of the scan figure (52), iii. capturing the scan figure (52) in the camera image iv. determining an offset and a rotation deviation v. adjusting the measurement coordinate system taking into consideration the offset and rotation deviation.

Description

[0001] Method for calibrating a measurement coordinate system of a coherence tomograph to the processing coordinate system of a laser processing arrangement

[0002] The invention relates to a method for calibrating a measurement coordinate system of a coherence tomograph to the processing coordinate system of a laser processing arrangement.

[0003] For example, when welding fillet welds, it is necessary to control the relative position between the laser focal spot and the workpiece. This weld position control can be achieved using optical coherence tomography (OCT). This method is based on the fundamental principle of light wave interference and makes it possible to detect height differences along a measurement beam axis in the micrometer range.

[0004] Calibrating a scanner, especially an OCT scanner or coherence tomograph, relative to the laser processing optics requires either consumables (sheet metal, film, etc.) or a separate sensor. Consumables cannot be used in fully automated processes. Separate sensors are expensive and sometimes cannot be integrated into a machine design.

[0005] WO 2021 / 211 960 A1 discloses a method for calibrating a coherence imaging measurement system. This method comprises providing a material processing system configured to generate and direct a processing beam to a target, and providing a coherence imaging measurement system configured to generate a measurement beam and provide a measurement system output for controlling and / or monitoring the material processing system, wherein the calibration measurement output is provided by the measurement system and / or by an auxiliary sensor. The object of the present invention is to provide an improved method for calibrating a measurement coordinate system to a processing coordinate system.

[0006] According to a first aspect of the invention, this problem is solved by a method for calibrating a measurement coordinate system of a coherence tomograph to the processing coordinate system of a laser processing arrangement, comprising the following process steps: a. Adjusting a camera coordinate system of an observation camera to the processing coordinate system, b. Adjusting the measurement coordinate system to the camera coordinate system by i. scanning a predetermined scan figure (or simply "figure") with a scanner of the coherence tomograph, ii. capturing the scan figure with the observation camera, in particular with an exposure time adapted to the scan of the scan figure, iii. capturing the scan figure in the camera image, iv. determining an offset and a rotational deviation, and v. adjusting the measurement coordinate system taking into account the offset and rotational deviation.

[0007] The term "measuring coordinate system" can be understood within the meaning of the present invention as follows: A reference system used by a coherence tomograph to scan a scan figure on a workpiece.

[0008] The term "camera coordinate system" can be understood as the reference system used to determine the position of a structure shown in the camera image.

[0009] According to the invention, it is therefore proposed to calibrate the

[0010] The camera coordinate system can be used to align the measurement coordinate system with the processing coordinate system in order to calibrate or adjust the measurement coordinate system to the processing coordinate system. This can be done without consumables and fully automatically.

[0011] Such a consumable-free and fully automated calibration can be performed using a coaxial camera. This camera, or rather its camera coordinate system, is first calibrated to the laser processing optics or the processing coordinate system. The camera calibration serves as a reference for adjusting the measurement coordinate system. To calibrate the OCT scanner, a scan pattern, such as a cross geometry, a circle, or one or more individual lines, can be scanned and captured by the observation camera using long-exposure photography. A measurement beam can be emitted by a light source of the coherence tomograph, such as an SLED (superluminescent diode), for example, at a wavelength in the range of 820–860 nm.

[0012] It may be possible to record the scanned figure with the observation camera using an exposure time greater than 100 ms, in particular approximately 200 ms. Such an exposure time is significantly longer than that of a static measurement, where the exposure time is in the range of 1 ms. The exposure time can be tailored to the observation camera used.

[0013] By using a suitable exposure time of the observation camera, improved image recordings of the scanned figure can be made possible, especially when considering different ambient light conditions.

[0014] Another advantage can be increased reliability of the detection under fluctuating lighting conditions, which in turn allows for a more stable and robust calibration of the system.

[0015] It can be provided that the adjustment of the measuring coordinate system to the camera coordinate system is carried out without consumables. The term "without consumables" can be understood, within the meaning of the present invention, to mean that no additional materials, substances, or components are required, particularly for calibrating the measuring coordinate system to the camera coordinate system.

[0016] A significant technical advantage of this design lies in the minimization of resource consumption and the associated cost savings, as no consumables are required for the calibration process.

[0017] Furthermore, avoiding consumables can simplify operational processes and increase efficiency.

[0018] It may be planned that the coherence tomograph will be scaled.

[0019] A key technical advantage of this measure can lie in the increased accuracy of the measurement result. Scaling corrects errors caused by system-related deviations or environmental influences, thus enabling higher precision in the measurement.

[0020] It may be provided that the scanner of the coherence tomograph scans the specified scan figure with an offset compared to a previous scan, the scan figure is captured in the camera image, an offset is determined, and a scaling is determined from the ratio of the set offset and the captured offset.

[0021] The term "offset" can be understood, within the meaning of the present invention, as referring to a deliberately introduced displacement of the scan of the scan figure, which serves to check and fine-tune the agreement between the measurement coordinate system and the camera coordinate system. The term "offset" can be understood as a measurable difference between the expected and the actually detected position of a scan figure in the camera image, which is used to assess the measurement accuracy of the system and, if necessary, to make adjustments.

[0022] A significant technical advantage of this feature is that the deliberate introduction of an offset and its subsequent determination allows for a higher degree of fine-tuning of the coherence tomograph's scaling. This contributes to improved measurement accuracy and calibration reliability.

[0023] It may be provided that the scan of the scan figure with offset, the capture of the scan figure in the camera image and the determination of the offset are repeated at least once and that an average value for scaling is determined from the repeated measurements.

[0024] This leads to an increase in the accuracy and reliability of the coherence tomograph's scaling. By repeating the measurements and calculating a mean value, random errors are compensated for, resulting in a more precise calibration.

[0025] Furthermore, repeated application of the process steps and the resulting averaging can improve the consistency and stability of the calibration procedure, which is particularly advantageous in industrial applications with high precision requirements.

[0026] A laser processing optic with a focal length in the range of 200 to 600 millimeters can be used. This makes it possible to scan a large figure.

[0027] The camera coordinate system can be adjusted to the machining coordinate system by: a. creating a pattern, in particular a dot pattern, on the workpiece by laser machining; b. taking an image with the observation camera; c. capturing the pattern in the camera image; d. determining an offset, a rotational deviation and / or a scaling; and e. adjusting the camera coordinate system based on the values ​​determined in the previous step.

[0028] The term “pattern” can be understood, within the meaning of the present invention, as a precise arrangement of visually distinguishable markings that are applied to a workpiece and serve as reference points or lines for the adjustment of coordinate systems.

[0029] In this context, the term "dot pattern" can refer to a specific type of pattern consisting of a group of points that are in a known, especially predetermined, geometric relationship to each other.

[0030] By using a pattern created through laser processing, an accurate reference for adjusting the camera coordinate system can be established.

[0031] By capturing the point pattern in the camera image and subsequently fine-tuning the camera coordinate system, precise alignment relative to the processing coordinate system can be achieved.

[0032] The position of the pattern, especially the points of the dot pattern, can be detected. This makes it possible to align the camera coordinate system with the pattern. For this purpose, the camera coordinate system can be focused on the pattern, specifically the points. A workpiece with a black surface, especially an anodized sheet metal, can be used as the sample.

[0033] This enables the creation of precise and easily recognizable patterns and markings through laser processing. This can contribute to improved visibility and detectability when adjusting the camera coordinate system.

[0034] A beam splitter can be used between the observation camera and the coherence tomograph, which has a transmission of more than 1% for the wavelength of the measuring beam, or the measuring beam reflected from the workpiece can be converted into a different wavelength range.

[0035] This ensures that the scan of the scan figure is reliably captured by the observation camera.

[0036] According to another aspect of the invention, it can be provided that a laser processing system comprising a coherence tomograph, a laser beam source, a laser processing optics, an observation camera and a control system is provided, which is configured to carry out the method according to the invention.

[0037] Suitable laser beam sources include, for example, a solid-state laser, in particular a fiber laser or a disk laser, a diode laser or a CO2 laser.

[0038] The term "laser processing optics" describes the components that serve to focus, guide and direct the laser processing beam onto the workpiece.

[0039] The term "observation camera" refers to a camera unit capable of observing both the processing process and reference patterns for calibration purposes, particularly with spatial resolution. The control unit can be connected to the observation camera, the coherence tomograph, the laser beam source, and the laser processing optics, especially via signal transmission, and can receive data from and send data, particularly commands, to these components. The control unit can be connected to the observation camera, the coherence tomograph, the laser beam source, and / or the laser processing optics via a wired or wireless connection. The control unit can include a processing unit and a memory in which control instructions are stored as software.

[0040] With such a laser processing system, the calibration of the coherence tomograph's measurement coordinate system to the camera coordinate system can be carried out automatically and without consumables.

[0041] In another aspect, the invention relates to a computer program product containing machine-readable control instructions which, when loaded into a control system of a laser processing system according to the invention, cause the laser processing system to carry out the method according to the invention.

[0042] Further features and advantages of the invention will become apparent from the following detailed description of exemplary embodiments of the invention with reference to the figures in the drawing, which show essential details of the invention, as well as from the claims. The features shown therein are not necessarily to scale and are depicted in such a way that the inventive features are clearly visible. The various features can be implemented individually or in any combination in variants of the invention.

[0043] The schematic drawing shows exemplary embodiments of the invention in various stages of use, which are explained in more detail in the following description.

[0044] Figure 1 shows a highly schematic representation of a laser processing system;

[0045] Fig. 2a shows a processing coordinate system of a laser processing arrangement and a measurement coordinate system of a coherence tomograph before calibration;

[0046] Fig. 2b the coordinate systems after calibration;

[0047] Fig. 3 is a schematic representation of the laser processing system to illustrate the adjustment of the measuring coordinate system to the camera coordinate system;

[0048] Fig. 4a shows a scanned figure obtained by a scanner of the coherence tomograph in relation to the camera coordinate system;

[0049] Fig. 4b shows the scanned figure after calibration to the camera coordinate system by the coherence tomograph scanner;

[0050] Fig. 5a shows the scanning of several scan figures by the coherence tomograph scanner to determine a scaling; and

[0051] Fig. 5b shows the scanned figure after scaling.

[0052] Figure 1 shows a laser processing system 10 with a laser processing arrangement 12, which includes a laser beam source 14 and a laser processing optic 16, comprising a focusing optic 18, for focusing a processing beam 20 onto a workpiece 22. The workpiece 22 can, for example, be a black sheet, in particular an anodized sheet. The laser processing system 10 also includes a coherence tomograph 24, in particular an OCT tomograph. Furthermore, the laser processing system 10 includes a viewing camera 26. The viewing camera 26 can capture a spatially resolved image of the workpiece 22, which is indicated by the viewing beam 28. The viewing beam 28 is directed onto the viewing camera 26 by a mirror 30.

[0053] The processing beam 20 can be moved in the xy direction relative to the workpiece 22. This can be achieved, for example, by the laser processing optics 16 having a processing scanner that can deflect the processing beam 20 in the xy direction. Alternatively, the workpiece 22 can be arranged on a workpiece support that is movable in the xy direction, or a laser processing head can be movable relative to the workpiece 22.

[0054] The positioning of the machining beam 20 relative to the workpiece 22 is carried out taking into account a machining coordinate system. The evaluation of the image recorded by the observation camera 26 is carried out taking into account a camera coordinate system.

[0055] A control unit 31 can be set up to carry out or trigger the steps of the method according to the invention and to evaluate the images taken by the observation camera 26.

[0056] Figure 2a shows that a pattern 32 was generated on the workpiece 22 by means of the machining beam 20, controlled by the controller 31. In particular, a dot pattern with points 34 and 36 was generated, where point 34 lies on the x-axis 38 and point 36 on the y-axis 40 of the machining coordinate system. The camera coordinate system 42, on the other hand, has an offset in both the x- and y-directions. Furthermore, it is rotated relative to the machining coordinate system, i.e., it exhibits a rotational deviation. The lateral offset, i.e., the offset in the x- and y-directions, as well as the rotation of the camera coordinate system 42, are detected. Corresponding correction values ​​are applied, in particular by the controller 31, so that the camera coordinate system 42 is brought into alignment with the machining coordinate system, as shown in Figure 2b.

[0057] The camera coordinate system 42 is thus calibrated to the processing coordinate system. The next calibration step is explained with reference to Figure 3. Figure 3 shows the laser processing system 10 according to Figure 1, with the processing beam switched off, which is why it is not shown in Figure 3. A measuring beam 46 is emitted by a light source 44, in particular an SLED, of the coherence tomograph 24. The measuring beam 46 is directed onto the workpiece 22 by a scanner optic 48, hereinafter also referred to as scanner. In particular, a scan pattern on the workpiece 22 is scanned by the scanner 48. The scan pattern can be, for example, a cross or a single line. The measuring beam 46 is reflected by the workpiece 22 and directed by a beam splitter 50, so that an observation beam 28 is captured by the observation camera 26.The observation camera 26 can therefore capture the scanned figure scanned by the scanner 48 on the workpiece 22.

[0058] Figure 4a shows that the scan figure 52, which is designed here as a cross and which was generated taking into account a measurement coordinate system, has a lateral offset in both the x and y directions, as well as a rotation (rotational deviation) compared to the camera coordinate system 42.

[0059] Figure 4b shows that the offset, i.e., lateral displacement, can be determined, for example, using the controller 31. The rotation relative to the camera coordinate system 42 can also be detected. By applying appropriate correction values, the measurement coordinate system can be adjusted to the camera coordinate system 42 so that the camera coordinate system 42 and the scan figure 52, and thus the measurement coordinate system, coincide, as shown in Figure 4b. Figure 5a shows that the scan figure 52 can be scanned multiple times with an offset, see reference numerals 52', 52". The center point 54, 54', 54" of the scan figures 52, 52', 52" can be determined – in the example shown, this is the intersection of the lines forming the respective cross. Furthermore, the offset of the center points 54, 54', 54" from the center point or origin of the camera coordinate system 42 can be determined.A scale can be determined from the relationship between the set offset and the measured offset. By scanning multiple scan figures 52, 52', 52" and determining the corresponding offset, an average can be calculated, thereby increasing the accuracy. The result of the scaling is shown in Figure 5b.

Claims

Patent claims 1. Method for calibrating a measurement coordinate system of a Coherence tomograph (24) to the processing coordinate system of a laser processing arrangement (12) with the following process steps: a. Adjustment of a camera coordinate system (42) of an observation camera (26) to the processing coordinate system, b. Adjustment of the measurement coordinate system to the camera coordinate system (42) by i. scanning a predetermined scan figure (52) with a scanner (48) of the coherence tomograph (24), ii. recording the scan figure (52) with the observation camera (26), in particular with an exposure time adapted to the scan of the scan figure (52), iii. capturing the scan figure (52) in the camera image, iv. determining an offset and a rotational deviation, v. adjusting the measurement coordinate system taking into account the offset and rotational deviation.

2. Method according to claim 1, characterized in that the scan figure (52) is recorded by the observation camera (26) with an exposure time greater than 100 ms, in particular of approximately 200 ms.

3. Method according to one of the preceding claims, characterized in that step 1.b. is carried out without consumables.

4. Method according to one of the preceding claims, characterized in that a scaling of the coherence tomograph (24) is performed.

5. Method according to claim 4, characterized in that the predefined scan figure (52, 52', 52") is scanned by the scanner (48) of the coherence tomograph (24) with an offset compared to step l .bi, the scan figure (52, 52', 52") is captured in the camera image, an offset is determined and a scaling is determined from the ratio of the set offset and the captured offset.

6. Method according to claim 5, characterized in that the method steps of claim 5 are repeated at least once and an average value for scaling is determined.

7. Method according to one of the preceding claims, characterized in that a laser processing optic (16) with a focal length in the range of 200-600mm is used.

8. Method according to one of the preceding claims, characterized in that the camera coordinate system (42) is adjusted to the processing coordinate system by: a. generating a pattern (32), in particular a dot pattern, on the workpiece (22) by laser processing of a workpiece (22); b. taking an image with the observation camera (26); c. capturing the pattern (32) in the camera image; d. determining an offset, a rotational deviation and / or a scaling; e. adjusting the camera coordinate system (42) based on the values ​​determined in the previous step.

9. Method according to claim 8, characterized in that the position of the pattern (32), in particular the points of the dot pattern, is detected.

10. Method according to one of the preceding claims, characterized in that a black, in particular anodized, sheet metal is used as the workpiece (22).

11. Method according to one of the preceding claims, characterized in that a beam splitter (50) with a transmission > 1 % for the wavelength of the measuring beam (46) is provided between the observation camera (26) and the coherence tomograph (24), or that the measuring beam (46) reflected at the workpiece (22) is converted into a different wavelength range.

12. Laser processing system (10) comprising a coherence tomograph (24), a laser beam source (14), a laser processing optics (16), an observation camera (26) and a control system (31) configured to perform the method according to one of the preceding claims.

13. Computer program product containing machine-readable control instructions which, when loaded into a controller (31) of a laser processing system (10) according to claim 12, cause the laser processing system (10) to perform the method according to any one of claims 1 to 11.

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