Device and method for monitoring a thermal cutting process

Description

[0001] The present invention relates to a device for monitoring, in particular for controlling, a thermal cutting process on a typically plate-like workpiece, and a method for monitoring, in particular for controlling, a cutting process on a workpiece, see claims 1 and 11.

[0002] WO 2015 / 036140 A1 describes a device for monitoring, and in particular controlling, a cutting process. This device includes an image acquisition unit for capturing an area on the workpiece to be monitored, wherein the area to be monitored comprises an interaction zone of a laser beam with the workpiece, and an evaluation unit configured to determine at least one characteristic parameter of a kerf formed during the cutting process, for example, a cutting front angle. The image acquisition unit described in WO 2015 / 036140 A1 is configured to form an observation beam for observing the interaction zone from an observation direction extending at an angle to the beam axis of the laser beam and includes imaging optics for generating an image of the interaction zone from the observation direction extending at an angle to the beam axis of the laser beam.If the observation direction does not run in or against a feed direction of the cutting process, the evaluation device can use the recorded image to determine burr formation, roughness and / or groove formation as characteristic parameters of the cutting edge.

[0003] WO 2012 / 107331 A1 (disclosing the preamble of claims 1 and 11) describes a device for monitoring and, in particular, for controlling a laser cutting process, which can be configured to determine the presence or absence of burr formation at a cutting gap based on an image of an interaction area between a laser beam and a workpiece. For example, in a fusion cutting process, the presence of burr formation (crumbly burr) can be inferred from the appearance of three luminous streaks emanating from the cutting front.

[0004] German patent DE 10 2014 000 330 B3 describes a method and a device for monitoring and, if necessary, controlling the focus position of a processing laser beam during laser cutting of a workpiece. In this method, at least one imaging camera captures the optical process emission caused by the laser processing and the advancing cutting front in the workpiece with spatial resolution in one or more images. The instantaneous focus position of the processing laser beam relative to the surface of the workpiece is then determined in each of the one or more images from the instantaneous extent of the process emission and the instantaneous distance of a point of maximum process emission to the uppermost cutting front vertex.

[0005] From WO 2012 / 037955 A1, a method and a device for monitoring or verifying (laser) processing of a workpiece are known, in which a first radiation component of thermal radiation with a first polarization and a second radiation component of thermal radiation with a second polarization different from the first are detected, emitted by at least one surface element in a processing area of ​​the workpiece. From the detected first and second radiation components, data values ​​of a surface structure of the workpiece at the at least one surface element are determined. In one variant of the laser processing, a relative movement occurs between the workpiece and a laser beam in a first direction of movement, wherein the first polarization is parallel and the second polarization is perpendicular to the first direction of movement.

[0006] The article "Numerical investigations on high-power laser cutting of metals", EH Amara et al., Appl. Phys. A (2015) 119:1245-1260, describes a theoretical approach to studying laser metal cutting based on numerical simulation. This approach uses a three-dimensional model of the cutting front and the kerf. The results of the numerical simulation are compared with experimental observations for six different feed rates. Aufgabe der Erfindung

[0007] The invention is based on the objective of providing a device and a method that enable a reliable determination of at least one measured variable for the cutting quality of the cutting process and, in particular, a control of the cutting process based on the measured variable. Gegenstand der Erfindung

[0008] A device for monitoring, in particular for controlling, a thermal cutting process on a workpiece according to a first aspect of the invention is defined in claim 1.

[0009] The image of the workpiece area to be monitored, typically captured from the top of the workpiece, is usually a thermal image. The image acquisition device uses a detector, typically an image sensor in the form of a camera, such as a CMOS or possibly a CCD camera, to generate this image. The monitored area may include the cutting edge of the kerf formed by the interaction of the processing beam with the workpiece, or it may simply encompass the cut flanks behind the cutting edge. However, the monitored area should not be too far from the cutting edge of the kerf or from the interaction zone between the processing beam and the workpiece, especially if the cutting process is to be controlled based on at least one measured parameter.

[0010] The inventors have recognized that there is a physical relationship between the geometric shape of the kerf width in the thickness direction of the workpiece from the top of the typically plate-like workpiece to the underside of the workpiece and the cutting quality: If the energy introduced into the workpiece per distance traveled along the feed direction (linear energy) decreases or other parameters of the cutting process change unfavorably, the width of the kerf decreases from the top of the workpiece towards the underside of the workpiece until, in extreme cases, the kerf is completely closed near the underside of the workpiece and a complete cut-off occurs.

[0011] The decrease in the kerf width from the top to the bottom of the workpiece (i.e., the formation of a V-shaped kerf in cross-section) can significantly affect the cut quality (roughness, scoring, burr formation, and dimensional accuracy) well before the cut is completed. By controlling the thermal cutting process to produce a kerf with a substantially constant width in the thickness direction, irregularities in the cutting process can be prevented, thus guaranteeing a high-quality cut. To this end, it is proposed to determine at least one measurable parameter that provides information about the kerf profile in the thickness direction, particularly about a reduction in the kerf width in this direction.One such measurement is the angle between the two cut faces of the kerf, which increases as the kerf width decreases towards the underside of the workpiece in the thickness direction. The angle between the two cut faces, which represents a measure of the V-shape of the kerf, is defined, for example, in the case of concave or convex curved cut faces, by the angle between the respective cut face on the top and underside of the workpiece. The measurement describing the kerf's path in the thickness direction can be the angle between the two cut faces, but it is also possible to determine the kerf width indirectly using another measurement, as described in more detail below.

[0012] According to the invention, the image acquisition device has a polarizer which transmits a first linear polarization component of an observation beam to a detector for observing the area to be monitored and which filters a second polarization component of the observation beam that is perpendicular to the first, wherein the evaluation device is configured to determine the measured quantity on the basis of two luminous strips running along the cutting edges of the cutting joint in the at least one image, and wherein the measured quantity is preferably a distance and / or an angle between the two luminous strips.

[0013] The inventors recognized that observing the luminous fringes, which allow the determination of the gap width in the thickness direction of the kerf, typically requires using a single linear polarization component of the observation beam to monitor the area under observation. The inventors further recognized that the two luminous fringes visible in the resulting image, running along the cutting front and the edges of the kerf, correspond to a portion of the process emissions where the cutting front or edges are oriented within an angular range relative to the observation direction in which the polarization-dependent emissions are particularly strong. In the region where the two luminous fringes are visible, the cutting front angle or the angle between the two edges of the kerf lies within a region of particularly strong emission.As the workpiece approaches its breaking point, where it is just barely completely cut through its entire thickness during the thermal cutting process, and also as it approaches its quality cutting point, where a good cut is still achieved but the width of the kerf is already decreasing in the thickness direction, the distance between the light strips also decreases. The angle between the two light strips in the image can also be used as a measure of cut quality, since the angle between the light strips, which are typically parallel in a quality cut, changes in a cut of poor quality or shortly before a cut break, where both light strips merge.

[0014] A decrease in the angle or distance between the cut edges indicates a deterioration in the melt extrusion in the lower region of the cut edges, leading to increased roughness. Therefore, the distance or angle between the light strips represents an (analogous) measurement of the process state, more precisely the cut quality (quality cut, separation cut, feed reserve, etc.) in a thermal cutting process, particularly a melt cutting process. This measurement is essentially proportional to the distance of the cutting process from the separation or quality cut boundary. For this reason, the distance and / or angle between the light strips can be used as input variables for controlling cutting process parameters (path energy, focus position, etc.).As described above, it is not strictly necessary to use the luminous strips to determine a measurement for the kerf width. For example, the angle between the two cut edges can serve as a (continuous) measurement for the cutting quality of the cutting process. It is understood that the measurement(s) can only be used for condition monitoring or quality control of the cutting process, without actually controlling the cutting process.

[0015] It is possible that in some process configurations of the cutting process, a continuous melt flows around the entire cut front. It can also occur that melt strands run along one cut flank as well as in the center of the cut front. However, such process conditions do not affect the fundamental measurement principle described here; that is, when observing with a linear polarization component (s-polarization), only two luminescent stripes are ever detected. Rather, by observing the luminescent stripes, conclusions can be drawn about the profile of the kerf width, in particular about the angle between the cut flanks in the thickness direction of the workpiece, so that a "quality cut" with low roughness of the cut flanks can be distinguished from a "separation cut" with higher roughness (and any intermediate values).As described above, a reduction in the width of the kerf from the top to the bottom of the workpiece indicates an impending miscut. Therefore, this measurement can also be used to determine the "robustness" of the process (i.e., feed reserve or power reserve of the (laser) beam source).

[0016] In a further training, the evaluation unit is designed to determine the distance between the light strips based on the positions of two intensity maxima in the image perpendicular to the feed direction of the thermal cutting process. The intensity profile or intensity distribution of the detected observation radiation is typically captured and evaluated in a section of the image perpendicular to the feed direction, which can, for example, run along an image line or pixel line of the image or camera. Particularly if the observation direction moves with the feed direction, the image line or pixel line being evaluated can be fixed in the image or on the camera chip; however, the position of the evaluated image area can also be changed relative to the processing beam or feed direction. Instead of evaluating a single image line or pixel line, it may be possible to...Multiple image lines can also be used for evaluation or for determining the intensity profile, over which averaging is performed in a suitable manner, for example, by using the arithmetic mean or the median of the intensity values ​​of the individual image lines. In the intensity profile, which may be averaged, the distance between the two intensity maxima is determined and can serve as a measure of the cutting quality of the cutting process, as described above. Methods from image processing can be used to calculate the positions of the intensity maxima from the intensity profile.

[0017] In an alternative or additional image analysis, the angle between the two light strips in a top view of the workpiece or the angle between the two cut edges (in the cross-sectional direction) is determined, and the angle or angle difference is used as the measured value. Image analysis algorithms described in the literature, such as the Hough transform, can also be used for angle measurement.

[0018] As described above, the angle between the cut faces or the reduction in the width of the kerf in the negative Z-direction or in the thickness direction of the workpiece can be used as a measurement parameter. The angle between the cut faces can be determined directly or, if necessary, by determining the kerf width on the top side of the workpiece and the kerf width on the bottom side of the workpiece (provided the workpiece thickness is known). Optical coherence tomography (OCT) or an OCT sensor can be used, for example, to determine the geometry of the kerf in the thickness direction.An interferometric method is used in which the depth profile of the kerf behind the machining beam is measured by means of an oscillating movement from the top of the workpiece. This involves measuring, for example, the angle between the cut edges of the kerf, the gap width of the kerf on the underside of the workpiece, and the gap width of the kerf on the top side of the workpiece. Structured illumination of the workpiece or a ratio delniometer can also be used for this purpose. In some cases, poor cutting quality can be inferred simply from determining the gap width on the underside of the workpiece if this gap width is (significantly) smaller than the focus diameter.

[0019] In a further image analysis, images of quality and separation cuts are clustered, and the current image of the monitored area of ​​the cutting process is assigned to these clusters via comparative image analysis. Well-known image analysis algorithms, such as the Haar algorithm, can also be used for this purpose. In this case, a measurement parameter for cutting quality can also be determined from the light bands in the clustered images of separation and quality cuts. This parameter can potentially be used for control purposes, provided it is a (nearly) continuous measurement parameter.

[0020] With all image evaluation algorithms, it may be necessary or advantageous to rotate the captured images beforehand with respect to the current feed direction and, if necessary, improve them using black and white image balancing, distortion correction, and other mechanisms known in image processing. Additionally, it can be advantageous to average several successive images pixel by pixel using the median, mean, or similar filters. Furthermore, it is advantageous to ignore images with process disturbances (sudden increases in image brightness due to spontaneous vaporization or plasma formation) during image evaluation. However, such process disturbances can serve as a further indicator of low feed reserve or as a sign that the process is close to being aborted.

[0021] According to the invention, the image acquisition device is configured to form an observation beam for observing the area to be monitored from an observation direction extending at an angle to the beam axis of the processing beam, wherein the observation beam preferably passes through the focusing device, and wherein the image acquisition device comprises imaging optics for generating the image of the area to be monitored from the observation direction extending at an angle to the beam axis of the processing beam. To enable such non-coaxial observation, the device or the image acquisition device can, for example, be configured as described in WO 2015 / 036140 A1 cited above, which is incorporated in its entirety into this application by reference. Non-coaxial observation of the light strips has proven advantageous, but it is not strictly necessary.In principle, it is also possible to observe the area to be monitored coaxially or parallel to the beam axis of the processing beam.

[0022] In further training, the angle of the observation direction to the beam axis of the processing beam is between 1° and 5°, preferably between approximately 1.5° and 4.5°. Observation at comparatively small angles to the beam axis of the processing beam has proven advantageous for the present application.

[0023] In a further development, the image acquisition device is designed to change the orientation of the observation direction of the observation beam and / or the direction of the first, transmitted linear polarization component of the observation beam in a plane perpendicular to the beam axis of the processing beam as a function of a feed direction of the thermal cutting process, and in particular to keep it constant relative to the feed direction. As described in the aforementioned WO 2015 / 036140 A1, it can be advantageous if the angle between the feed direction and the observation direction in the plane perpendicular to the beam axis has a constant value, even if the feed direction, i.e., the direction of the relative movement between the processing beam and the workpiece, changes.For example, for imaging purposes, it may be advantageous if the observation direction in the projection in the workpiece plane runs opposite to the feed direction, which is also referred to as trailing observation, or possibly in the feed direction, which is also referred to as piercing observation.

[0024] In many applications, it has also proven advantageous to keep the orientation of the polarization component filtered out by the polarization filter constant relative to the feed direction. In particular, the filtered polarization component can be aligned parallel or perpendicular to the feed direction to improve or simplify observation. To change or rotate the direction of the linear polarization component along with the feed direction, a motor-driven (typically rotatable) polarization filter or an electrically rotatable polarization filter (LCD polarizer) can be used, for example.If a rotatable aperture is used to change the direction of observation of the observation beam, which typically has at least one eccentric aperture opening, the polarizing filter can be mounted on the rotatable aperture and rotated together with it, without requiring a separate drive for rotating the polarizing filter.

[0025] In another embodiment, the observation direction is projected onto a plane perpendicular to the axis of the processing beam in the feed direction; that is, the feed direction and the observation direction are parallel and aligned in this plane. This so-called penetrating observation has proven particularly advantageous for detecting the luminous stripes.

[0026] In a further development, the direction of the first transmitted linear polarization component of the observation beam runs in a plane perpendicular to the beam axis of the processing beam (i.e., in the workpiece plane) at an angle between 55° and 125°, preferably between 80° and 100°, and particularly preferably at an angle of 90° to the feed direction. As described above, the luminous fringes can typically only be observed if the observation beam has a linear polarization component. This linear polarization component ideally runs perpendicular to the feed direction (s-polarization).

[0027] In a further development, the image acquisition device is designed to generate at least one image of the monitored area of ​​the workpiece at wavelengths between 700 nm and 2 µm. For iron-based workpiece materials, observation wavelengths between 700 nm and 1000 nm, preferably between 800 nm and 940 nm, have proven advantageous for observing the monitored area. These wavelengths can be captured with SiO₂-based image sensors or detectors. Observation at longer wavelengths (e.g., up to 1.6 µm) is also possible, but requires InGaAs-based image sensors or detectors, which are considerably more complex and generally have a lower frame rate. One advantage of observation at longer wavelengths is that the luminescent stripes can also be observed in workpieces made of materials with lower melting points (e.g., Al, Cu).Shorter wavelengths of less than 700 nm generally do not provide sufficiently intense temperature radiation to ensure reliable evaluation.

[0028] For the evaluation of the images typically captured or generated by a camera, or more precisely, the image data, the evaluation unit includes a standard image processing unit, which can be, for example, a computing unit or a microprocessor. The evaluation unit is also usually connected via an interface for data exchange with a control unit or a regulation unit for controlling the processing operation.

[0029] In a further embodiment, the device additionally comprises a control and / or regulating device for influencing at least one manipulated variable of the thermal cutting process depending on the at least one measured variable for the gap width in the thickness direction of the workpiece. In this case, the measured variable or the measured value, determined, for example, using the luminous strips, is used to ensure consistent cutting quality (especially edge roughness) and thus a reliable cut by influencing, adjusting, or regulating manipulated variables of the thermal cutting process (e.g., feed rate, laser power, focus position, gas pressure, etc.).

[0030] Furthermore, the measured value or parameter can be recorded and monitored to check the process status. In case of impermissible deviations, an operator can be informed and / or process improvement measures (adjustment, cleaning) can be initiated automatically or manually. The measured parameter can also indicate when maintenance of the device is due.

[0031] The device may include further components, such as a light source for illuminating the workpiece in the area to be monitored. The image acquisition device may be designed, in particular, to capture or generate an image through the nozzle opening of a laser processing nozzle for the laser beam to pass onto the workpiece. The device may, for example, be a laser cutting head or a laser cutting machine.

[0032] A method for monitoring, in particular for controlling, a thermal cutting process on a workpiece according to a second aspect of the invention is defined in claim 11.

[0033] As described above, the measured value determined in this way can, for example, be recorded and monitored to check the process status, and an operator can be informed if there are impermissible deviations.

[0034] As described above, the angle between the cutting edges or the reduction in the width of the kerf in the negative Z-direction or in the thickness direction of the workpiece can be used as a measurement parameter. The angle between the cutting edges can be determined directly or, for example, by measuring the kerf width on the top side of the workpiece and the kerf width on the underside (provided the workpiece thickness is known). Optical coherence tomography (OCT), structured illumination of the workpiece, or a ratio goniometer can be used to determine the geometry of the kerf in the thickness direction, as described above. If necessary, the kerf width on the underside of the workpiece can be a sufficient measurement parameter for the cutting quality, for example, when compared to the focus diameter of the machining beam.If the gap width on the underside of the workpiece is smaller than the focus diameter of the machining beam, this indicates poor cutting quality.

[0035] In one variant, a first linear polarization component of an observation beam is transmitted to a detector to generate the at least one image for observing the area to be monitored, and a second polarization component of the observation beam, perpendicular to the first, is filtered. The measured quantity is determined in the at least one image based on two luminous strips running along the cut edges of the kerf, preferably representing a distance or an angle between the two luminous strips. As described above, the luminous strips allow conclusions to be drawn about the kerf width in the thickness direction of the workpiece.

[0036] In another variant, the orientation of an observation direction of an observation beam for observing the area to be monitored and / or the direction of the first, transmitted linear polarization component of the observation beam in a plane perpendicular to the beam axis of the processing beam is changed depending on a feed direction of the thermal cutting process, in particular kept constant relative to the feed direction.

[0037] In one variant, the observation direction is aligned in a projection into a plane perpendicular to the beam axis of the processing beam in the feed direction, i.e., a piercing observation is made, which has proven particularly advantageous for observing the luminous stripes.

[0038] Preferably, the direction of the first transmitted linear polarization component of the observation beam runs in a plane perpendicular to the beam axis of the processing beam at an angle between 55° and 125°, preferably between 80° and 100°, and particularly preferably at an angle of 90° to the feed direction. In this case, the filtered linear polarization component is a polarization component oriented perpendicular to a plane containing the feed direction and the beam axis of the processing beam (s-polarization). As described above, filtering a linear polarization component may be necessary for imaging the luminous fringes.

[0039] In an advantageous variant, the method additionally includes: influencing at least one control variable of the thermal cutting process depending on the determined measured variable. By adjusting or controlling one or more control variables of the laser cutting process, consistent quality in laser cutting can be ensured.

[0040] Further advantages of the invention will become apparent from the description and the drawing. Likewise, the features mentioned above and those listed below can be used individually or in any combination. The embodiments shown and described are not to be understood as an exhaustive list, but rather serve as examples illustrating the invention.

[0041] They show: Fig. 1a a schematic representation of an embodiment of a device for monitoring and controlling a laser cutting process on a workpiece, Fig. 1b a representation of an aperture for forming an observation beam in such a device along a cutting line BB of Fig. 1a , Fig. 2a-c a three-dimensional representation of a cutting edge, a top view of a cutting edge and a thermal image of an area of ​​the workpiece to be monitored with two light strips during a quality cut, Fig. 3a-c representations analogous to Fig. 2a-c in a separation cut with low cutting quality, as well as Fig. 4 a representation of an intensity profile transverse to the feed direction along an image line of the thermal image of Fig. 2c .

[0042] In the following description of the drawings, identical reference symbols are used for identical or functionally equivalent components.

[0043] Fig. 1a Figure 1 shows an exemplary setup of a device 1 for monitoring and controlling a laser cutting process on a workpiece 3 using a processing beam in the form of a laser beam 2. In the example shown, the device 1 is designed as a laser processing head, which is part of a laser processing machine (not shown in detail). The laser beam 2 is generated by a CO₂ laser in this example. Alternatively, the laser beam 2 can be generated, for example, by a solid-state laser. For cutting operations on the workpiece 3, the laser beam 2 is focused onto the workpiece 3 by means of a focusing device in the form of a focusing lens 4.In the example shown, the focusing lens 4 is a zinc selenide lens that focuses the laser beam 2 through a laser processing nozzle 5, more precisely through its nozzle opening 5a, onto the workpiece 3, specifically onto a focus position F on the top surface of the workpiece 3. There, the laser beam 2 forms an interaction zone 18 with the workpiece 3, behind which a kerf 16 is created opposite a feed direction v or cutting direction of the laser cutting process. In the case of a laser beam 2 from a solid-state laser, a focusing lens made of, for example, quartz glass can be used.

[0044] In Fig. 1a Also visible is a partially transparent deflecting mirror 6, which reflects the incident laser beam 2 (e.g., with a wavelength of approximately 10.6 µm) and transmits observation radiation relevant for process monitoring to another partially transparent deflecting mirror 8. In the example shown, the deflecting mirror 6 is partially transparent to observation radiation in the form of thermal radiation with wavelengths λ of approximately 700 nm to 2000 nm. The second partially transparent deflecting mirror 8 reflects the observation radiation to an image acquisition device 9. An illumination source 10 serves to coaxially illuminate the workpiece 3 with illumination radiation 11. The illumination radiation 11 is transmitted by the second partially transparent deflecting mirror 8 and by the deflecting mirror 6 and directed through the nozzle opening 5a of the laser processing nozzle 5 onto the workpiece 3.

[0045] As an alternative to the partially transparent deflecting mirrors 6, 8, scraper mirrors or pinhole mirrors, which reflect incident radiation only from a peripheral region, can also be used to direct the observation radiation 7 to the image acquisition device 9 or to direct the illumination radiation 11 to the workpiece 3. At least one mirror inserted laterally into the beam path of the laser beam 2 can also be used to enable observation.

[0046] Diode lasers, LEDs, or flash lamps can be used as the light source 10, as described in Fig. 1a The light source 10 can be arranged coaxially or off-axis to the laser beam axis 13. For example, it can also be located outside (especially next to) the device 1 and directed towards the workpiece 3; alternatively, it can be located inside the device 1, but not coaxially to the laser beam 2 and directed towards the workpiece 3. The device 1 can also be operated without a light source 10.

[0047] Part of the image acquisition device 9 is a geometrically high-resolution camera 12 arranged in the observation beam path 7 behind the further partially transparent deflecting mirror 8. The camera 12 can be a high-speed camera that is arranged coaxially to the laser beam axis 13 or to the extension of the laser beam axis 13 and is therefore direction-independent. In the example shown, the camera 12 acquires images using reflected light in the NIR / IR wavelength range to record the process intrinsic luminescence or a thermal image of the cutting process. In the example shown Fig. 1a In the example shown, a filter can be placed in front of camera 12 if further radiation or wavelength components are to be excluded from detection by camera 12. The filter can, for example, be designed as a narrowband bandpass filter with a full width at half maximum (FWHM) of approximately 15 nm, which transmits wavelengths λ in the range of approximately 800 nm.

[0048] To generate images B1, B2 of a in Fig. 2a, b or in Fig. 3a, b In the area 15 of the workpiece 3 shown, which is to be monitored and contains the cutting edge 16 or a section of the cutting edge 16 with the cutting front 16a, the image acquisition device 9 has an imaging optic 14 on a detector surface 12a of the camera 12. In the example shown, the imaging optic 14 has an aperture 25 which is rotatably mounted about a central axis of rotation D, so that during rotation the position of an eccentrically arranged aperture opening 25a moves on a circular arc around the axis of rotation D.

[0049] Due to the arrangement of the aperture 25 in the beam path of the imaging optics 14, which is focused by means of a lens 17, only a portion of the observation beam path 7, which passes through an edge region of the focusing lens 4 and is aligned in the convergent beam path after the focusing lens 4 at an angle β to the beam axis 13 of the laser beam 2, passes through the aperture opening 25a, which is arranged eccentrically to the extension of the beam axis 13 of the laser beam 2, and forms an observation beam 7a, which is imaged onto the detector surface 12a. In the case of the Fig. 1a In the example shown, an observation direction R1 of the observation beam 7a runs in the projection onto the XY plane, or the workpiece plane, in the feed direction vR. Along this direction, the laser beam 2 and the workpiece 3 are moved relative to each other in the XY plane to form the desired cutting contour; that is, a penetrating observation is performed. The angle β at which the observation direction R1 is aligned with the beam axis 13 of the laser beam 2 lies between approximately 1° and approximately 5° in the example shown, for instance, at approximately 4°.

[0050] As in Fig. 1a As can be seen, a polarizing filter 21 is attached to the aperture 25, which rotates together with the aperture 25 about the axis of rotation D. The polarizing filter 21 is designed to filter a linear polarization component p, which in the example shown is aligned parallel to a plane (XZ plane) that contains the feed direction v as well as the Z direction or beam axis 13 of the laser beam 2. As shown in Fig. 1a As can be seen, the observation beam 7a exhibits only a polarization component s oriented perpendicular to the XZ plane after passing through the polarization filter 21. Filtering a linear polarization component of the observation beam 7a has proven advantageous for observing the interface 16 or the area 15 to be monitored. It is understood that, instead of the polarization component p oriented parallel to the XZ plane, the polarization component s oriented perpendicular to the XZ plane or a polarization component oriented differently can also be filtered using the polarization filter 21. The use of the s-polarized polarization component has proven particularly advantageous for observing the interface 16, since the Fig. 2a, 2b and in Fig. 3a, 3b The dashed lines shown essentially represent the Fig. 2c and in Fig. 3c The light strips shown, 22a and 22b, correspond to an optimal angle in order to emit a lot of s-polarized radiation.

[0051] Instead of a mechanically adjustable aperture 25, an electrically adjustable aperture, for example in the form of an LCD array, can also be used, in which individual pixels or pixel groups are electronically switched on or off to create the aperture effect. The mechanical aperture 25 can also be adjusted differently than in the Fig. 1a, 1b The aperture 25 can be moved or shifted perpendicular to the observation beam path 7, for example in the YZ plane, in order to shade or open different parts of the observation beam path 7 for observation. The aperture 25 can also be implemented as one or more hinged mechanical elements. Similarly, the polarizing filter 21 can also be designed as an LCD polarizer to suitably select the orientation of the filtered polarization component, in particular to rotate its orientation.

[0052] As in Fig. 2c and in Fig. 3c As can be seen, each image of the monitored area 15 of the workpiece 3, captured by the imaging optics 14, is bounded by a circular inner contour 5b of the laser cutting nozzle 5. The monitored area 15, bounded by the inner contour 5b of the laser cutting nozzle 5, contains an image B1, B2 of the interaction area 18, which is shown in the Fig. 2c und Fig. 3c The images shown were taken from the same observation direction R1. The projection of the observation direction R1 onto the XY plane is shown in Fig. 2a represented by an arrow. As described above, the observation direction R1 runs in the Fig. 1a The example shown is in or along the feed direction v R , i.e. in the positive X direction, which is referred to as a piercing observation with respect to a cutting front 16a on the workpiece 3, behind which the cutting joint 16 forms.

[0053] In the two recorded images B1, B2, two luminous strips 22a, 22b can be seen in each, which run essentially along the X-direction or the feed direction v R, along which the cutting flanks 23a, 23b of the cutting edge 16 also run, which in Fig. 2a and in Fig. 3a The images are shown on a partial area or section of the workpiece 3, on the upper surface 3a of which the circular outline of the area 15 to be monitored is also visible. Each of the two images B1, B2 was taken with a different feed rate v R, with the feed rate v R being shown in the Fig. 2c The image shown, B1, is smaller and in which it is in Fig. 3c The distance A shown in image B2 is larger. The distance A (cf. Fig. 4 ) between the light strips 22a, 22b in the Y-direction, i.e. perpendicular to the feed direction v R , decreases with increasing feed rate v R, until at the in Fig. 3c In the images shown in B2, the two light strips 22a and 22b partially overlap or merge into a single light strip. The two merging light strips 22a and 22b form an angle δ with each other.

[0054] It has been shown that in a cutting process where the in Fig. 2c As shown in image B1, the cut flanks 23a, 23b of the cutting edge 16 in the thickness direction of the workpiece 3 (i.e. in the Z direction) are essentially parallel to each other, i.e. at a very small angle γ, so that a quality cut is present (cf. Fig. 2a, 2b ). In the Fig. 3c In contrast, the cut shown in image B2, where the two light strips 22a, 22b partially merge, is of inferior quality, exhibiting burr formation and the formation of grooves along the cut edges 23a, 23b. In this case, these grooves are not essentially parallel, but run at a significantly larger angle γ to each other in the thickness direction (Z-direction), as shown in Fig. 3a is shown. In the Fig. 3a, 3b In the cut gap 16 shown, the width b(z) of the cutting edge 16 decreases continuously in the Z-direction from the top surface 3a to the bottom surface 3b of the workpiece 3, so that the cutting edge 16 at the point where Fig. 3b The cross-sectional view shown exhibits a clearly pronounced V-shaped profile. The in Fig. 3a, 3b The kerf 16 shown, which tapers sharply downwards in cross-section, is rather unfavorable for the melt cutting process; i.e., the kerf 16 should be the one shown in Fig. 2a, 2b The geometry shown has essentially parallel cutting edges 23a, 23b, i.e., a gap width b(z) that is essentially constant over the thickness d of the workpiece 3.

[0055] To regulate the cutting process so that a quality cut is always achieved, the following can be used in the Fig. 2c und Fig. 3c The light strips 22a and 22b shown in the thermal image are used, which essentially represent a top view of the Fig. 2a, 2b and in Fig. 3a, 3b The lines shown represent the areas along which polarization-dependent process emissions are particularly strong during the detection of s-polarized radiation. In the Fig. 2a, 2b The two lines along the cut 16 shown, along which strong process emissions occur and which correspond to the luminous strips 22a, 22b, run essentially parallel to each other, so that a distance A (cf. Fig. 4 ) between the two light strips 22a, 22b can be determined. In the case of the Fig. 3a, 3b In the example shown, the lines with strong polarization-dependent process emissions, corresponding to the luminous strips 22a, 22b, run along the oblique cut edges 23a, 23b of the cut joint 16, so that the distance A between the observed luminous strips 22a, 22b decreases in the negative X direction. In the Fig. 3a-c In the low-quality cut shown, the two light strips 22a, 22b converge behind the cut edge 16a, so that the two light strips 22a, 22b overlap and assume a V-shaped orientation towards each other when viewed from above on the top surface 3a of the workpiece 3, as shown in Fig. 3c can be seen.

[0056] The distance A between the two light strips 22a, 22b in the Y direction, i.e. perpendicular to the feed direction v, and the angle δ between the two light strips 22a, 22b can therefore serve as measurement parameters for the cutting quality of the cutting process.

[0057] The distance A between the two light strips 22a, 22b can be determined, for example, by the following: Fig. 2c The image B1 shown refers to the following in connection with Fig. 4 as described, for which a Fig. 1a The evaluation unit 19 shown serves the device 1. As shown in Fig. 2c As indicated, the radiation intensity I incident on the detector surface 12a is measured along an image or pixel line extending in the Y direction, which is essentially due to the thermal radiation of the workpiece 3. As in Fig. 4 As can be seen, the radiation intensity I exhibits two intensity maxima IM1, IM2, each corresponding to a position YM1, YM2 in the Y-direction. The difference YM2 - YM1 between the two positions YM1, YM2 corresponds to the distance A between the two light strips 22a, 22b. Instead of evaluating the radiation intensity I along a single image line extending in the Y-direction, the intensity values ​​of several image lines can also be used if they are appropriately averaged (e.g., mean or median). Suitable image processing methods can be used to locate the two intensity maxima IM1, IM2.

[0058] At the in Fig. 3c In the image B2 shown, image evaluation can be performed by determining the angle δ between the two converging light strips 22a, 22b and using the angle δ as a measure of the cutting quality. This can be done similarly to how it is done in Fig. 4 Based on the radiation intensity I in the recorded image B2, the essentially oval or elliptical outer contours of the two light strips 22a, 22b are first determined, for example, by considering the contour or geometry that corresponds to a given intensity value of the measured radiation intensity I in the recorded image B2. Once the outer contours of the two light strips 22a, 22b are known, their longitudinal axes can be determined. The angle δ is then measured between the two longitudinal axes of the light strips 22a, 22b. For the angle measurement, evaluation algorithms as described in the literature (e.g., Hough Transformation) can be used, either alternatively or additionally.

[0059] At the in Fig. 2c In the image B1 shown, determining the angle δ between the light strips 22a and 22b may be problematic, as they run almost parallel, whereas in the Fig. 3c In the image B2 shown, determining the distance A between the two converging light strips 22a, 22b may be problematic. However, it is understood that at a cutting speed that occurs between the two in Fig. 2a-c or Fig. 3a-c the extremes shown lie on which in Fig. 4 In the manner shown, the distance A between the two light strips 22a, 22b can be determined on the one hand using the two intensity maxima I M1 , I M2, and on the other hand the angle δ between the two light strips 22a, 22b can be determined in order to be able to assess the cutting quality on the basis of both measured quantities A, δ.

[0060] Alternatively, image analysis can be performed in the evaluation unit 19, in which images of quality cuts and separation cuts (i.e., cuts with lower cut quality), in which the light strips 22a and 22b are visible, are clustered, and the currently generated or recorded image can be assigned to these clusters via comparative image analysis. Algorithms known from image processing can also be used for this purpose, e.g., the hair algorithm. The assignment to a cluster also represents a measurement parameter for the cutting quality of the cutting process – although usually not an analogous or continuously changing one.

[0061] Depending on at least one measured quantity A, δ determined in the manner described above, a Fig. 1a The control device 20 shown has at least one manipulated variable, for example the feed rate, the power P of the in Fig. 1aThe laser (not shown in the image), the focus position F, the gas pressure of an auxiliary gas used in the cutting process, etc., are influenced or changed in such a way that the measured quantity A, δ has a predetermined value or does not deviate from a predetermined value range. Such control ensures that a consistently high cut quality is achieved in the (melt) cutting process, so that poor cuts and, in particular, cut breaks can ideally be completely avoided.

[0062] In particular, the feed rate or the laser power P can be set so that it is always just below the maximum permissible feed rate or laser power at which a quality cut is still possible, i.e., the feed reserve or power reserve can be almost completely utilized. For adjusting or influencing the feed rate, the control device 20 can, for example, act on motion devices (not shown) for moving the workpiece 3 and / or the laser cutting head 1.

[0063] To be able to observe the measured parameter for the cut quality, for example the distance A or the angle δ, even when the feed direction v R changes during cutting along curved cutting contours under a desired observation direction, in particular the observation direction R1 in the feed direction v R, it is necessary to change the orientation of the observation direction R1 in the XY plane as a function of the feed direction v R or the orientation of the feed direction v R in the XY plane. For this purpose, the evaluation device 19 (and optionally also the control device 20) can be used, which rotates the aperture 25 and thus the aperture opening 25a around the axis of rotation D when the feed direction v R changes, so that the observation direction R1 always remains aligned in the feed direction v R.By rotating the aperture 25, the orientation of the linear polarization component (typically s-polarization), which is filtered out by the also rotating polarizing filter 21, can be kept constant relative to the feed direction v R. It is understood that for this purpose, or rather for generating an observation beam 7a oriented at an angle β to the beam axis 13, an aperture 25 is not strictly necessary; instead, the image acquisition device 9 can be designed in a different way for this purpose, for example as described in WO 2015 / 036140 A1.

Claims

1. A device (1) for monitoring, in particular for feedback-controlling, a thermal cutting process carried out on a workpiece (3), the device comprising: a focusing unit (4) for focusing a machining beam, in particular a laser beam (2), onto the workpiece (3) to form a kerf (16) on the workpiece (3), as well as an image acquisition unit (9) which has a detector (12) for generating at least one image (B1, B2) of a region (15) to be monitored of the workpiece (3), characterized by an evaluation unit (19) designed to determine, based on the at least one image (B1, B2), at least one measured variable (A, δ) for the profile of the gap width (b(Z)) of the kerf (16) in the thickness direction (Z) of the workpiece (3), in particular for an angle (γ) between the two cut flanks (23a, 23b) of the kerf (16), and the evaluation unit (19) is designed to ascertain, in the at least one image (B1, B2), the measured variable (A, δ) based on two light stripes (22a,b) extending along the cut flanks (23a,b) of the kerf (16), wherein the image acquisition unit (9) is designed to form an observation beam (7a) for observing, from an observation direction (R1) which extends at an angle (β) relative to the beam axis (13) of the machining beam, the region (15) to be monitored, and wherein the image acquisition unit (9) comprises an image optic (14) for generating, from the observation direction (R1) which extends at the angle (β) relative to the beam axis (13) of the machining beam, the image (B1) of the region (15) to be monitored, wherein the image acquisition unit (9) has a polarizer (21) designed to transmit to the detector (12) a first linear polarization component (s) of the observation beam (7a) for observing the region (15) to be monitored, and to filter a second polarization component (p) of the observation beam (7a), the second polarization component being perpendicular to the first.

2. The device according to claim 1, wherein a distance (A) and / or an angle (δ) between the two light stripes (22a, 22b) is ascertained as a measured variable.

3. The device according to claim 2, in which the evaluation unit (19) is designed to determine the distance (A) between the light stripes (22a, 22b) based on the positions (YM1, YM2) of two intensity maxima (IM1, IM2) of the image (B1, B2) transverse to a feed direction (vR) of the thermal cutting process.

4. The device according to one of the preceding claims, wherein the observation beam (7a) passes through the focusing unit (4).

5. The device according to claim 4, in which the angle (β) of the observation direction (R1) relative to the beam axis (13) of the machining beam lies between 1° and 5°.

6. The device according to one of claims 4 or 5, in which the image acquisition unit (9) is designed to vary, as a function of a feed direction (vR) of the thermal cutting process, the orientation of the observation direction (R1) of the observation beam (7a) and / or the direction of the first transmitted linear polarization component (s) of the observation beam (7a) in a plane (X, Y) perpendicular to the beam axis (13) of the machining beam, in particular to keep said orientation and / or direction constant relative to the feed direction (vR).

7. The device according to claim 6, in which the observation direction (R1), in a projection into a plane (X, Y) perpendicular to the beam axis (13) of the machining beam (2), extends in the feed direction (vR).

8. The device according to claim 6 or 7, in which the direction of the first transmitted linear polarization component (s) of the observation beam (7a) extends, in a plane (X, Y) perpendicular to the beam axis (13) of the machining beam (2), at an angle (θ) of between 55° and 125°, preferably between 80° and 100°, relative to the feed direction (vR).

9. The device according to one of the preceding claims, in which the detector (12) of the image acquisition unit (9) is designed to generate the at least one image (B1 to B4) of the region (15) to be monitored of the workpiece (3) at wavelengths of between 700 nm and 2 µm.

10. The device according to one of the preceding claims, further comprising: an open-loop and / or a closed-loop control unit (20) for influencing at least one control variable (F, P) of the thermal cutting process as a function of the at least one ascertained measured variable (A, δ) for the profile of the gap width (b(Z)) of the kerf (16) in the thickness direction (Z) of the workpiece (3).

11. A method for monitoring, in particular for closed-loop controlling, a thermal cutting process on a workpiece (3), comprising: generating at least one image (B1, B2) of a region (15) to be monitored of the workpiece (3), the region containing the cut flanks (23a, 23b) of a kerf (16) formed on the workpiece (3) during the cutting process, characterized by ascertaining at least one measured variable (A, δ) for the profile of the gap width (b(Z)) of the kerf (16) in the thickness direction (Z) of the workpiece (3), in particular for an angle (γ) between the two cut flanks (23a, 23b) of the kerf (16), based on the at least one image (B1, B2).

12. The method according to claim 11, in which a first linear polarization component (s) of an observation beam (7a) for observing the region (15) to be monitored is transmitted to a detector (12) for generating the at least one image (B1 to B4), and a second polarization component (p) of the observation beam (7a) which is perpendicular to the first is filtered, and wherein the measured variable (A, δ) is ascertained in the at least one image (B1, B2) based on two light stripes (22a, 22b) extending along the cut flanks (23a, 23b) of the kerf (16), wherein the measured variable preferably forms a distance (A) or an angle (δ) between the two light stripes (22a, 22b).

13. The method according to one of claims 11 or 12, in which the orientation of an observation direction (R1) of an observation beam (7a) for observing the region (15) to be monitored, and / or the direction of the first transmitted linear polarization component (p) of the observation beam (7a) in a plane (X, Y) perpendicular to the beam axis (13) of the machining beam, is varied as a function of a feed direction (vR) of the thermal cutting process, in particular is kept constant relative to the feed direction (vR).

14. The method according to claim 13, in which the observation direction (R1), in a projection into a plane (X, Y) perpendicular to the beam axis (13) of the machining beam (2), is oriented in the feed direction (vR).

15. The method according to claim 13 or 14, in which the direction of the first transmitted linear polarization component (s) of the observation beam (7a) extends at an angle (θ) between 55° and 125°, preferably between 80° and 100°, relative to the feed direction (vR) in a plane (X, Y) perpendicular to the beam axis (13) of the machining beam (2).

16. The method according to one of claims 11 to 15, further comprising: influencing at least one control variable (F, P) of the thermal cutting process as a function of the ascertained measured variable (A, δ).