Laser cutting of workpiece under protection of machining device

Abstract

The invention relates to a method for monitoring a cutting process of a workpiece (12) by means of a laser beam (14), the workpiece (12) being cut through from an upper side (20a) to a lower side (20b) of the workpiece lying opposite the upper side (20a) in the beam direction of the laser beam (14). A part (30a) of the laser beam (14) is absorbed by a cutting front (14) in the workpiece (12), and another part (30b) of the laser beam (14) passes through a slit (28) and an underside (20b) of the workpiece (12), the slit (28) being located behind the cutting front (28) in a feed direction (26) of the laser beam (14). Within the scope of the method, a characteristic value of the power of the laser beam (14) passing through the portion (30b) of the slit (28) is determined.

Description

Background Art

[0001] The invention relates to a method for monitoring a cutting process of a workpiece using a laser beam and also to a processing device for carrying out such a method. Background Art

[0003] DE 10 2018 218 006 A1 discloses a melting cutting method, wherein the characteristic length of the cutting front is determined by an imaging sensor and adjusted to a predetermined target length to avoid faults during cutting, in particular interruptions of cutting. Similarly, WO 2012 / 107331 A1 relates to determining characteristic geometric parameters of the cutting front and the cutting seam in a laser cutting method by means of a camera device for controlling the cutting method.

[0004] The disclosed cutting method is generally directed to the production of high quality cuts of a workpiece. Summary of the invention

[0005] In contrast, the object of the present invention is to provide a method for cutting through a workpiece using a laser beam, in which the environment of the workpiece, in particular a support of the workpiece, is better protected. The object of the present invention is also to provide a processing device for carrying out the method.

[0006] According to the invention, this object is achieved by a method according to claim 1. The processing device according to the invention is characterized by what is stated in claim 13. Advantageous embodiments are disclosed in the dependent claims.

[0007] The method according to the invention comprises the following steps:

[0008] a) irradiating the upper side of the workpiece with a laser beam;

[0009] b) cutting through the workpiece by means of a laser beam from the upper side of the workpiece to the lower side of the workpiece, wherein the lower side of the workpiece is opposite the upper side of the workpiece in the beam direction of the laser beam while forming a cutting front and a cutting seam;

[0010] e) limiting the power of the portion of the laser beam that has passed through the workpiece to a predetermined level, wherein

[0011] When cutting through the workpiece, the portion of the laser beam that has passed through the workpiece exits the workpiece on the underside of the workpiece.

[0012] This method is particularly effective for cutting methods using laser devices that emit a laser beam at high power. Therefore, in these methods, the part of the laser beam that is not absorbed by the workpiece but emitted into the machine space of the processing device involved also transmits a large amount of energy. In particular, parts of the laser beam irradiate through the workpiece to be processed by the cutting process. This creates the risk of damage to the machine due to local heating or even complete failure.

[0013] By adjusting this portion, the emission power of the laser beam can be reduced, usually by increasing the cutting speed and / or reducing the power of the laser beam to prevent damage to the processing device. The power of the portion of the laser beam that has passed through the workpiece is determined, for example, as the difference between the power of the entire laser beam acting on the surface of the workpiece and the power of the laser beam that hits the cutting front. In particular, the predetermined value to which the power of the laser beam is limited can be determined by previous tests on structurally identical workpieces. In particular, method parameters can be determined for reaching a predetermined power of the portion of the laser beam that has passed through the workpiece within a predetermined time interval.

[0014] The cutting front is preferably understood to be the material of the workpiece or the geometry of the material of the workpiece, wherein the material is irradiated and melted or evaporated at the time when the workpiece is irradiated by the laser beam. The cutting front moves with the laser beam. The kerf is preferably understood to be the gap produced by the cutting process by removing material from the workpiece. The kerf is usually located behind the cutting front in the feed direction of the laser beam. The cutting front and the kerf act on the laser beam in particular like an aperture. The power of the part of the laser beam that passes through the workpiece is preferably referred to as the power surplus, in other words, the part or proportion of the power of the laser beam that is not used for cutting the workpiece.

[0015] The method according to the invention can also be applied if the contour cutting of the workpiece has been interrupted and the workpiece is to be cut further after resuming the cutting method. For this purpose, the cutting head is usually moved back a relatively small distance along the cutting contour and then continues cutting on the already cut contour. Most of the power of the laser beam can pass through the already cut kerf. The determination of the power surplus can be used to control the power of the part of the laser beam that passes through the workpiece.

[0016] A preferred embodiment of the method has: determining a characteristic value of the power of the portion of the laser beam that has passed through the workpiece. The power of the portion of the laser beam that has passed through the workpiece is determined, for example, as the difference between the power of the entire laser beam acting on the surface of the workpiece and the power of the laser beam hitting the cutting front. In particular, the difference between the power of the entire laser beam acting on the surface of the workpiece and the power of the laser beam acting on the cutting front can be used as a characteristic value of the power of the portion of the laser beam that has passed through the workpiece. Alternatively or additionally, the ratio of the power of the portion of the laser beam that has passed through to the power of the portion of the laser beam absorbed by the workpiece and / or to the power of the entire laser beam can be used as a characteristic value. The power of the portion of the laser beam reflected from the cutting front usually has a relatively small effect on the processing device used and is therefore preferably ignored. Advantageously, the characteristic value provides a specific indication of the power of the portion of the laser beam that has passed through the workpiece, which indication can be used to adjust the process. By determining the power of the portion of the laser beam radiating through the workpiece, it can be determined whether the processing device used to perform the cutting operation is supplied with too much energy by this portion of the laser beam during the cutting process, thereby causing damage to the processing device. It is thus possible to monitor the cutting method to prevent damage to the processing device.

[0017] In some embodiments of the method, the absorbed power portion of the laser beam can be determined by integrating the laser power distribution on the face of the cutting front, and the absorbed power portion can be subtracted from the power of the total laser beam to determine the power surplus. Alternatively or additionally, a characteristic value of the power surplus can be determined by integrating the laser power distribution on the face of the kerf.

[0018] According to an advantageous embodiment of the method, the power of the part of the laser beam that has been traversed is determined by means of the output power of the laser source for generating the laser beam, the divergence of the laser beam, the Rayleigh length of the laser beam, the position of the focus of the laser beam, the focal position, the focal diameter, the thickness of the workpiece and / or the distance from the cutting nozzle from which the laser beam emerges to the workpiece. The given parameters determine in particular the position, shape and power density of the laser beam. These parameters therefore have a significant influence on the shape of the cutting front and the kerf during the cutting process. In particular, the curvature of the cutting front is largely determined by the given parameters and the feed speed of the laser beam. These laser parameters can be used to control which part of the laser beam is absorbed and which part passes through the workpiece.

[0019] In a preferred embodiment, the method has the following steps:

[0020] c) recording a processed image of the partial volume of the workpiece penetrated by the laser beam and having the cutting front by projecting the entire partial volume onto a recording surface of an image sensor, wherein the image sensor is arranged above the upper side of the workpiece;

[0021] d) determining, based on the processed image, the length of the cutting front in the feed direction of the laser beam in a plane of the beam axis of the laser beam and the feed direction of the laser beam.

[0022] During the cutting process, a partial volume of the workpiece and in particular the cutting front usually emits a monitoring beam, in particular thermal radiation, which can be used to record a process image. The partial volume of the workpiece (in particular at the time of irradiation) includes the cutting front and the part of the kerf through which the laser beam irradiates.

[0023] Within the framework of this configuration, during the cutting method, the portion of the laser beam that has passed through the workpiece is determined or estimated by an imaging method with the aid of an imaging sensor for the purpose of adjusting the cutting process. For this purpose, a partial volume of the workpiece having a three-dimensional processing zone in which the cutting process takes place is imaged onto a two-dimensional recording surface of the imaging sensor. By taking into account the position and orientation of the sensor (and possibly other optical elements for imaging) relative to the partial volume of the workpiece having the processing zone or cutting zone in which the cutting process takes place, the length of the cutting front can be determined. The processing image is recorded in particular by the nozzle from which the laser beam emerges. The length of the cutting front makes it possible to easily estimate which part of the laser beam is absorbed by the cutting front.

[0024] An extension of the aforementioned configuration has: based on the processed image, further parameters of the entire partial volume of the workpiece penetrated by the laser beam, in particular geometric parameters and / or intensity distribution of the radiation emitted by the partial volume, are determined. The radiation emitted by the partial volume is in particular formed as thermal radiation. The processed image of the partial volume can be used not only to determine the length of the cutting front, but also in particular to determine its width and shape. In addition, the intensity curve of the cutting front and / or the intensity curve of the entire imaged partial volume can be determined. The determination of such parameters improves the estimation of the size and shape of the cutting front, and therefore improves the estimation of the part of the laser beam absorbed by the cutting front.

[0025] The beam axis of the radiation for recording the processing image preferably has an angle of 0° to 15°, in particular 0° to 5°, with the beam axis of the laser beam, starting from the partial volume of the workpiece. In particular, the beam axis of the thermal radiation radiated by the partial volume and impinging on the imaging sensor is inclined relative to the beam axis of the laser beam. Advantageously, in this configuration, the image sensor and, if applicable, the optical element for guiding the monitoring beam can be arranged outside the spatial region radiated by the laser beam, so that the image sensor and the optical element are not irradiated and damaged. In particular, the processing image can be recorded as part of a simultaneous dragging, penetration and / or vertical observation of the cutting process. Different observation directions can advantageously be used for the calculation of the laser power surplus.

[0026] In a further embodiment of the method, a characteristic value of the power of the portion of the laser beam that has passed through the workpiece is determined based on the length of the cutting front and the diameter of the laser beam at the upper side of the workpiece. The ratio of the length of the cutting front to the diameter of the laser beam provides a simple geometric estimate of the portion of the face of the cutting front that covers the face of the laser beam in the feed direction and therefore provides an estimate of the portion of the laser beam that is absorbed by the cutting front.

[0027] An extension of the aforementioned configuration has the following features: the power of the portion of the laser beam that has passed through the workpiece is determined as a further parameter by means of the area and / or intensity distribution of the laser beam on the upper side of the workpiece. By associating the parameters of the cutting front determined by the processed image with predetermined or measured parameters of the laser beam, the power of the laser beam absorbed by the workpiece or the power of the laser beam that is not absorbed is calculated. The parameters of the laser beam include in particular its divergence, the position of its focus and its intensity distribution, in particular the intensity distribution along the diameter of the cross section of the laser beam. For this purpose, the overlap of the surface of the cutting front with the cross section of the laser beam on the surface of the workpiece is preferably taken into account, wherein the cross section is essentially determined by the divergence of the laser beam.

[0028] In a further variant of the method, at least a partial volume of the workpiece is irradiated with incident light illumination, from which a process image is acquired, wherein image data obtained from the reflected incident light illumination are integrated into the process image.

[0029] Based on the properties of the reflection behavior of the reflected radiation of the incident light illumination, it is possible to measure how much material the laser beam hits or will hit relative to its current feed rate when irradiating the partial volume to record the processing image. This information can be used to adjust the laser power surplus compensation. For example, the incident light illumination can be used to measure the kerf of an existing cut. The incident light illumination is preferably integrated into the processing device, in particular into its sensor.

[0030] Preferably, the method comprises comparing the power of the traversed portion of the laser beam with a first control value for controlling the cutting process. If the first control value is exceeded, the cutting process is compensated, preferably automatically, to protect the laser processing machine. The laser power surplus is preferably limited to a defined level. In particular, this is achieved by completely switching off the laser beam, reducing the power of the laser beam and / or increasing the feed rate of the laser beam.

[0031] In a further embodiment of the method, the energy radiated by the part of the laser beam that has passed through in a predetermined area within a predetermined time interval is determined. The extent of possible damage to the processing device used depends essentially on the local and temporal influence of the power of the part of the laser beam that passes through the workpiece. This influence is preferably generated by the determination of the time-integrated radiation intensity in the area under consideration, which preferably belongs to the processing device. The determination of the radiation intensity takes into account the parameters of the cutting process, such as the cutting direction, the cutting speed, the focus position of the laser beam, the diameter of the focus, the gas pressure for generating the gas jet and / or the position of the cutting head in the processing device, wherein the gas jet is used in particular to discharge the melt.

[0032] Advantageously, the method comprises comparing the radiated energy with a second control value for controlling the cutting process. In particular, the aforementioned radiation intensity can be compared with a limit value, wherein, if the limit value is exceeded, the cutting process is compensated, for example by adjusting the laser power or switching off the laser beam.

[0033] A machining device for carrying out a method according to one of the aforementioned embodiments, the machining device having a laser source for emitting a laser beam, an image sensor for recording a processed image of the entire partial volume of a workpiece having a cutting front penetrated by the laser beam, and a controller for controlling the method using an evaluation unit for determining the power of the portion of the laser beam that has passed through the workpiece. By determining the power of the portion of the laser beam that has passed through the workpiece, such a machining device can be effectively protected from damage caused by the laser beam.

[0034] Further advantages of the present invention are derived from the description and the accompanying drawings. Similarly, the features mentioned above and those still to be presented further can be used separately or together in any desired combination. The embodiments shown and described should not be understood as an exhaustive enumeration, but should be understood as having exemplary features for describing the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 schematically shows a section through a processing device for controlled cutting of a workpiece using a laser beam;

[0036] Figure 2 schematically shows a cross section through an irradiated workpiece on a support;

[0037] Figure 3 schematically illustrates a processing image of a cutting process in a first configuration;

[0038] Figure 4 A processing image of a cutting process in the second configuration is schematically shown. DETAILED DESCRIPTION

[0039] Figure 1 Schematically, a section through a machining device 10 for controlled cutting of a workpiece 12 with a laser beam 14 is shown. A laser beam source 16 with a focusing device 18 radiates the laser beam 14 via a cutting nozzle 17 generating a gas jet onto an upper side 20a of the workpiece 12, which is cut with the laser beam 14 and the gas jet. The cutting is carried out from the upper side 20a of the workpiece 12 to its lower side 20b, with which the workpiece 12 rests on a support 22. The lower side 20b of the workpiece 12 is opposite to the upper side 20a in the direction of the laser beam 14. The laser beam 14, together with the gas jet, produces a cutting front 24 in the workpiece 12, which extends from the upper side 20a of the workpiece 12 to the lower side 20b of the workpiece and moves together with the laser beam 14 in the feed direction 26 of the laser beam 14. The cutting front 24 is irradiated by a portion 30a of the laser beam 14, wherein the material of the workpiece 12 is melted and / or evaporated at the cutting front 24 by heating and then discharged from the kerf 28 by the gas jet. This produces an open kerf 28 behind the cutting front 24 in the feed direction 26 of the laser beam 14, through which a portion 30b of the laser beam 14 radiates unhindered. This portion 30b of the laser beam 14 exits from the underside 20b of the workpiece 12 and impinges on the support 22 below the workpiece 12, whereby the support is heated and possibly damaged. The portion 30b of the laser beam 14 is also referred to as a through portion 30b.

[0040] The heated material in the partial volume 32 of the workpiece 12 having the cutting front 24 penetrated by the laser beam 14 emits thermal radiation which at least partially strikes the recording surface 36 of the image sensor 38 as monitoring beam 34. The controller 40 or control device having an evaluation unit 42 creates processed images 46a, 46b of the partial volume 32 of the workpiece 12 having the cutting front 24 penetrated by the laser beam 14 from the data of the image sensor 38 (see Figure 3 and Figure 4 For this purpose, the control unit 40, the evaluation unit 42 and the image sensor 38 are connected to one another in terms of signal technology, such as Figure 1 shown.

[0041] Figure 2 A cross section through an irradiated workpiece 12 on a support 22 is schematically shown. The figure shows both the portion 30a of the laser beam 14 that is used to cut through the workpiece 12 and that strikes the cutting front 24, and the through portion 30b of the laser beam 14 that has been irradiated through the workpiece 12 via the kerf 28 and strikes the support 22 on which the workpiece 12 rests. The diameter D1 of the laser beam 14 on the upper side 20a of the workpiece 12 thus consists of two partial sections, the diameter D2 being 20a and 20b being 20b. Figure 214 is indicated by a double arrow on the upper side 20a of the workpiece 12. The first subsection is the processing length L of the cutting front 24 in the plane of the beam axis 44a of the laser beam 14 and the feed direction 26 in the feed direction 26 of the laser beam 14. Figure 2 In the embodiment, the feeding direction 26 of the laser beam 14 is from right to left. The second partial section is the diameter D2 of the passing portion 30b of the laser beam 14 through the slit 28. The diameter D2 is based on Figure 2 Determined in the horizontal direction, the diameter runs perpendicular to the laser beam 14. By processing the images 46a, 46b recorded by the image sensor 38 (see description below) Figure 3 and Figure 4 ), these partial sections L, D2 can be determined and correlated with the diameter D1 of the entire laser beam 14 on the upper side 20a of the workpiece 12 to estimate the penetration portion 30b of the laser beam 14 that passes through the cut 28.

[0042] The angle of the beam axis 44 b of the monitoring beam 34 to the vertical beam axis 44 a of the laser beam 14 is typically 0° to 5°.

[0043] Figure 3 The partial volume 32 of the workpiece 12 irradiated during the irradiation time is schematically shown in a first configuration of the cutting process (see Figure 1 ) of the processed image 46a. The processed image 46a is formed by the cutting nozzle 17 (see FIG. 1 ) from which the laser beam 14 is emitted. Figure 1 ) is recorded, whereby the nozzle opening 48 of the cutting nozzle surrounds the processing image 46a. The processing image 46a includes in particular the cutting front 24, the cutting slit 28 and the non-cutting area 50 of the upper side 20a of the workpiece 12 in the projection representation. In the processing image 46a, the cutting front 24 has a projected processing length L (P) (Based on Figure 3 Indicates the length of the cutting front 24 in the horizontal direction). According to the projection processing length L with the cutting front 24 (P) The processed image 46a of the laser beam 14, the geometric parameters of the cutting front 24 (for example, the processing length L of the cutting front 24 in the feeding direction 26 of the laser beam 14) (P) and / or the width of the cutting front 24) can be determined by an imaging method based on an image sensor 38 (see Figure 1 ) and, if necessary, further optical elements for guiding the monitoring beam 34. The intensity distribution of the monitoring beam 34 can also be determined from the processed image 46a.

[0044] Figure 4 The partial volume 32 of the workpiece 12 irradiated during the irradiation time is schematically shown in the second configuration of the cutting process (see Figure 1), wherein first a first slit 52a with a smaller width is cut. This is followed by a second slit 52b with a larger width, which is guided at least partially along the first slit 52a. Due to the first slit 52a, the cutting front 24 is divided into Figure 1 ) Two partial cutting fronts 54a and 54b are generated on both sides of the first slit 52a, wherein the two partial cutting fronts 54a and 54b have a projected processing length L (P) When cutting the second slit 52b, the portion 30b of the laser beam 14 (see Figure 1 ) radiates unhindered through the first slit 52a. This passing portion 30b of the laser beam 14 can be controlled by the method according to the invention to avoid damage to the processing device 10. In addition to the two slits 52a, 52b and the partial cutting fronts 54a, 54b, the uncut or not yet cut environment 50 of the upper side 20a of the workpiece 12 and the nozzle opening 48 of the cutting nozzle 17 surrounding the further processing image 46b are also shown in the projection view.

[0045] Summarizing all the figures of the drawings, the invention relates to a method for monitoring a cutting process of a workpiece 12 with a laser beam 14, wherein the workpiece 12 is cut through in the beam direction of the laser beam 14 from an upper side 20a of the workpiece to an underside 20b opposite the upper side 20a. A portion 30a of the laser beam 14 is absorbed by the cutting front 24 in the workpiece 12, while another portion 30b of the laser beam 14 radiates through a kerf 28 and the underside 20b of the workpiece 12, wherein the kerf 28 is located behind the cutting front 28 in the feed direction 26 of the laser beam 14. Within the scope of the method and with the aid of a processing device, a characteristic value of the power of the penetrated portion 30b of the laser beam 14 is ascertained, and the power of the laser beam 14 radiating through said penetrated portion 30b of the kerf 28 is limited.

Claims

1. A method for monitoring a cutting process of a workpiece (12) using a laser beam (14), wherein: The method comprises the following steps: a) irradiating the upper side (20a) of the workpiece (12) with a laser beam (14); b) cutting through the workpiece (12) by means of the laser beam (14) from the upper side (20a) of the workpiece (12) to the lower side (20b) of the workpiece (12), wherein the lower side (20b) of the workpiece (12) is opposite the upper side (20a) of the workpiece (12) in the beam direction of the laser beam (14) while forming a cutting front (24) and a cutting seam (28); e) limiting the power of a portion (30b) of the laser beam (14) that has passed through the workpiece to a predetermined level, wherein, when cutting through the workpiece (12), the portion (30b) of the laser beam (14) that has passed through emerges from the workpiece (12) on the underside (20b) of the workpiece (12).

2. The method according to claim 1, comprising determining a characteristic value of the power of a portion (30b) of the laser beam (14) that has passed through the workpiece (12).

3. A method according to any one of the preceding claims, comprising determining the power of the traversing portion (30b) of the laser beam (14) by means of the output power of a laser source (16) for generating the laser beam (14), the divergence of the laser beam (14), the Rayleigh length of the laser beam (14), the position of the focus of the laser beam (14), the focus position, the focus diameter, the workpiece thickness and / or the distance from a cutting nozzle (17) from which the laser beam (14) is emitted to the workpiece (12).

4. The method according to any one of the preceding claims, comprising: c) recording a processed image (46a, 46b) of the entire partial volume (32) of the workpiece (12) penetrated by the laser beam (14) and having the cutting front (24) by projecting the partial volume onto a recording surface (36) of an image sensor (38), wherein: The image sensor (38) is arranged above the upper side (20a) of the workpiece (12); d) determining, based on the processed images (46a, 46b), a length (L) of the cutting front (24) in the feed direction (26) of the laser beam (14) in a plane of the beam axis (44a) of the laser beam (14) and the feed direction (26) of the laser beam (14).

5. The method according to claim 4 comprises determining further parameters of the entire partial volume (32) of the workpiece (12) penetrated by the laser beam (14) based on the processed image (46a, 46b), in particular geometric parameters and / or intensity distribution of the radiation (34) emitted by the partial volume (32).

6. The method according to claim 4 or 5, wherein: The beam axis (44b) of the radiation (34) for recording the processing image (46a, 46b) starting from the partial volume (32) of the workpiece (12) forms an angle of 0° to 15°, in particular 0° to 5°, with the beam axis (44a) of the laser beam (14).

7. A method according to any one of claims 4 to 6 and claim 2, comprising determining a characteristic value of the power of the portion (30b) of the laser beam (14) that has passed through the workpiece (12) based on the length (L) of the cutting front (24) and the diameter (D1) of the laser beam (14) on the upper side (20a) of the workpiece (12).

8. The method according to claim 7 has the step of determining the power of the portion (30b) of the laser beam (14) that has passed through the workpiece (12) by means of the area and / or intensity distribution of the laser beam (14) on the upper side (20a) of the workpiece (12) as a further parameter.

9. The method according to any one of the preceding claims and claim 4, wherein: At least the partial volume (32) of the workpiece (12) is irradiated with incident light illumination, and a processed image (46a, 46b) is recorded from at least the partial volume of the workpiece, wherein image data obtained from the reflected incident light illumination are integrated into the processed image (46a, 46b).

10. The method according to any of the preceding claims, comprising comparing the power of the passed portion (30b) of the laser beam (14) with a first control value for controlling the cutting process.

11. Method according to any of the preceding claims, having the step of determining the energy of the passing portion (30b) of the laser beam (14) radiated in a predetermined area within a predetermined time interval. 12 . The method according to claim 11 , comprising comparing the radiated energy with a second control value for controlling the cutting process.

13. The method according to any one of claims 9 to 12, wherein: The image data obtained from the reflected incident light illumination is used to determine the volume of the workpiece (12).

14. A processing device (10) configured to carry out a method according to any one of the preceding claims, the processing device comprising a laser source (16) for emitting a laser beam (14), an image sensor (38) for recording a processed image (46a, 46b) of the entire partial volume (32) of a workpiece (12) having a cutting front (24) penetrated by the laser beam (14), and a controller (40) for controlling the method using an evaluation unit (42), the evaluation unit being used to determine the power of the portion (30b) of the laser beam (14) that has passed through the workpiece (12).

Images