Laser cutting of workpieces with protection from machining equipment

Abstract

The present invention relates to a method for monitoring a process of cutting a workpiece (12) with a laser beam (14), in which the workpiece (12) is cut from its upper surface (20a) to its lower surface (20b), which is located opposite the upper surface (20a) in the beam direction of the laser beam (14). One portion (30a) of the laser beam (14) is absorbed by the cutting front (14) of the workpiece (12), while another portion (30b) of the laser beam (14) passes through a kerf (28) and the lower surface (20b) of the workpiece (12), the kerf (28) being located behind the cutting front (28) in the feed direction (26) of the laser beam (14). Within the method, a characteristic value of the power of the portion (30b) of the laser beam (14) passing through the kerf (28) is determined.

Description

[Background technology]

[0001] The present invention relates to a method for monitoring a process for cutting a workpiece with a laser beam, and also to a machining device for carrying out such a method.

[0002] DE 102018218006 A1 provides a fusion cutting method in which a characteristic length of the cutting front is determined by an image sensor and adjusted to a predetermined target length in order to avoid errors during cutting, in particular cut interruptions. Similarly, WO 2012 / 107331 A1 deals with determining characteristic geometric parameters of the cutting front and kerf in a laser cutting method with the aid of a camera for controlling the cutting method.

[0003] The disclosed cutting method is primarily directed to producing a high quality cut on a work piece.

[0004] Object of the invention In contrast, it is an object of the present invention to provide a method for cutting through a workpiece with a laser beam in which the periphery of the workpiece, and in particular the support of the workpiece, is better protected from the laser beam, and it is also an object of the present invention to provide a machining apparatus for carrying out this method.

[0005] Summary of the Invention This object is achieved according to the invention by a method as defined in claim 1. The machining device according to the invention is characterized in claim 13. Advantageous configurations result from the dependent claims.

[0006] The method according to the invention comprises: a) irradiating a laser beam onto an upper surface of a workpiece; b) cutting through the workpiece with a laser beam from the top surface of the workpiece to the bottom surface of the workpiece, the bottom surface of the workpiece being opposite the top surface of the workpiece in the beam direction of the laser beam, and forming a cutting front and a kerf; e) limiting the power of the portion of the laser beam that has passed through the workpiece to a predetermined level, such that the portion of the laser beam that has passed through the workpiece exits the workpiece at the underside of the workpiece as it cuts through the workpiece.

[0007] This method is particularly useful for cutting methods using laser devices that emit high-power laser beams. As a result, in these methods, the part of the laser beam that is not absorbed by the workpiece but is emitted into the machine space of the machining device in question can also carry a large amount of energy. In particular, the part of the laser beam is emitted through the workpiece that is being processed by the cutting process. This creates a risk that the machine will be damaged by localized heating or will completely fail.

[0008] By adjusting this portion, the emitted power of the laser beam can be reduced, typically by increasing the cutting speed and / or reducing the power of the laser beam, thereby preventing damage to the machining equipment. The power of the portion of the laser beam that passes through the workpiece is determined, for example, as the difference between the power when the entire laser beam impinges on the surface of the workpiece and the power when the laser beam strikes the cutting front. The predetermined value to which the power of the laser beam is limited can be determined, inter alia, by previous tests on the same workpiece. In particular, method parameters can be determined to achieve a predetermined power of the portion of the laser beam that has passed through the workpiece in a predetermined time interval.

[0009] The cutting front is preferably understood to mean the material of the workpiece or the geometric shape of the material of the workpiece, which is irradiated and melts or vaporizes 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 created by removing material from the workpiece during the cutting process. The kerf is usually located behind the cutting front in the feed direction of the laser beam. The cutting front and the kerf act in particular like an opening on the laser beam. The power of the portion of the laser beam that passes through the workpiece is preferably referred to as the power surplus, in other words the portion or proportion of the power of the laser beam that is not used to cut the workpiece.

[0010] The method according to the present invention can also be applied when the contour cutting of the workpiece is interrupted and the workpiece is further cut after the cutting method is resumed. To do this, the cutting head is typically moved backward a relatively small distance along the contour to be cut and then continues cutting across the contour that has already been cut. Most of the power of the laser beam can pass through the kerf that has already been cut. The determination of the excess power can be used to control the power of the portion of the laser beam that passes through the workpiece.

[0011] A preferred embodiment of the method includes 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 when the entire laser beam impinges on the surface of the workpiece and the power when the laser beam strikes the cutting front. In particular, the difference between the power when the entire laser beam impinges on the surface of the workpiece and the power when the laser beam impinges on the cutting front can be used as the 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 transmitted portion of the laser beam to the power of the portion of the laser beam absorbed by the workpiece and / or the power of the entire laser beam can be used as the characteristic value. The portion of the laser beam power that is reflected from the cutting front typically has a relatively small effect on the machining device used and is therefore preferably ignored. Advantageously, the characteristic value provides a specific indicator of the power of the portion of the laser beam that has passed through the workpiece, which can be used in the control process. By determining the power of the portion of the laser beam that radiates through the workpiece, it can be determined whether the machining device used to perform the cutting operation is being supplied with too much energy by that portion of the laser beam during the cutting operation, causing damage to the machining device, thereby allowing the cutting method to be monitored so that damage to the machining device is prevented.

[0012] In some embodiments of the method, the absorbed power portion of the laser beam can be determined by integrating the laser power distribution over the area of ​​the cut front and can be subtracted from the total laser beam power to determine the power surplus. Alternatively or additionally, a characteristic value of the power surplus can be determined by integrating the laser power distribution over the area of ​​the kerf.

[0013] According to an advantageous embodiment of the method, the power of the transmitted laser beam portion is determined using the output power of the laser source for generating the laser beam, the caustic surface of the laser beam, the Rayleigh length of the laser beam, the position of the focal point of the laser beam, the focal point position, the focal point diameter, the thickness of the workpiece, and / or the distance of the cutting nozzle at which the laser beam exits the workpiece. The specified parameters determine, in particular, the position, shape, and power density of the laser beam. Therefore, the parameters significantly affect the shape of the cutting front and the kerf during the cutting process. In particular, the curvature of the cutting front is primarily determined by the specified parameters and the feed rate of the laser beam. These laser parameters can be used to control which portions of the laser beam are absorbed and which portions pass through the workpiece.

[0014] In a preferred embodiment, the method comprises: c) recording a process image of the entire partial volume of the workpiece with the cutting front penetrated by the laser beam by projecting this partial volume onto a recording surface of an image sensor, the image sensor being arranged above the upper surface of the workpiece; d) determining the length of the cutting front in the feed direction of the laser beam in the plane of the beam axis of the laser beam and the feed direction of the laser beam based on the process image.

[0015] During the cutting process, a partial volume of the workpiece, 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 irradiation time, includes the cutting front and the part of the kerf through which the laser beam radiates.

[0016] In this embodiment, the portion of the laser beam that has passed through the workpiece is determined or estimated by an imaging method using an imaging sensor for cutting process control during the cutting method. For this purpose, a partial volume of the workpiece having a three-dimensional process zone in which the cutting process is performed is imaged on a two-dimensional recording surface of the imaging sensor. The length of the cutting front can be determined by taking into account the position and orientation of the sensor (and possibly other optical elements used for imaging) relative to the process zone of the workpiece or the partial volume of the workpiece having the cutting zone in which the cutting method is performed. The process image is recorded, in particular, by a nozzle from which the laser beam emerges. The length of the cutting front makes it easy to estimate which portion of the laser beam is absorbed by the cutting front.

[0017] A further development of the above-described embodiment includes determining further parameters of the entire partial volume of the workpiece penetrated by the laser beam based on the process image, in particular the geometric parameters and / or the intensity distribution of the radiation emitted by the partial volume. The radiation emitted by the partial volume is mainly thermal radiation. The process image of the partial volume can be used to determine not only the length of the cutting front, but also its width and shape. Furthermore, the intensity curve of the cutting front and / or the intensity curve of the entire imaged partial volume can be determined. Determining such parameters improves the estimation of the size and shape of the cutting front and therefore of the portion of the laser beam absorbed by the cutting front.

[0018] Preferably, the beam axis of the radiation used to record the process image, starting from the partial volume of the workpiece, has an angle of 0° to 15°, particularly 0° to 5°, relative to the beam axis of the laser beam. In particular, the beam axis of the thermal radiation emitted by the partial volume impinging on the imaging sensor is inclined relative to the beam axis of the laser beam. Advantageously, in this embodiment, the imaging sensor and, if applicable, the optical element for directing the monitoring beam can be positioned outside the spatial area illuminated by the laser beam, so that they are not irradiated and are not damaged. In particular, the process image can be recorded as part of a vertical observation of simultaneous dragging, drilling, and / or cutting processes. Different observation directions can advantageously be used to calculate the laser power surplus.

[0019] In a further embodiment of the method, the 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 top surface of the workpiece, the ratio of the length of the cutting front to the diameter of the laser beam providing a simple geometric estimate of the portion of the surface of the cutting front that covers the surface of the laser beam in the feed direction, and therefore an estimate of the portion of the laser beam that is absorbed by the cutting front.

[0020] A further development of the above-described embodiment includes determining the power of the portion of the laser beam that has passed through the workpiece by means of a surface and / or intensity distribution of the laser beam on the upper surface of the workpiece as a further parameter. By correlating the parameters of the cutting front determined by the process image with predetermined or measured parameters of the laser beam, it is possible to calculate the power of the laser beam that has been absorbed or not absorbed by the workpiece. The parameters of the laser beam include, inter alia, its caustic surface, the position of its focus, and above all its intensity distribution along the diameter of the cross-sectional area of ​​the laser beam. For this purpose, the overlap of the area of ​​the cutting front with the cross-sectional area of ​​the laser beam on the surface of the workpiece is preferably taken into account, which cross-sectional area is essentially determined by the caustic surface of the laser beam.

[0021] In a further variant of the method, at least a partial volume of the workpiece from which the process image is captured is illuminated with incident light illumination, and image data obtained from the reflected incident light illumination is integrated into the process image.

[0022] Based on the characteristics of the reflection behavior of the reflected radiation of the incident light illumination, it is possible to determine how much material the laser beam hits or will hit for its current feed rate when illuminating a partial volume to record a process image. This information can be used to adjust the laser power surplus compensation. For example, the kerf of an existing cut can be measured using the incident light illumination. The incident light illumination is preferably integrated into the machining device, in particular its sensor.

[0023] Preferably, the method includes comparing the power of the passing portion of the laser beam with a first control value to control the cutting method. If the first control value is exceeded, the cutting method is preferably automatically adjusted to protect the laser processing machine. The excess laser power is preferably limited to a defined level. In particular, this is done by completely switching off the laser beam, reducing the power of the laser beam, and / or increasing the feed speed of the laser beam.

[0024] In a further embodiment of the method, the energy emitted in a predetermined area over a predetermined time interval is determined from the portion of the laser beam that has passed through. The degree of damage that can occur to the machining device used essentially depends on the local and temporal influence of the power of the portion of the laser beam that has passed through the workpiece. This effect preferably results from the determination of the time-integrated radiation intensity in the area under consideration, preferably belonging to the machining device. The determination of the radiation intensity takes into account parameters of the cutting process, such as the cutting direction, the cutting speed, the focal position of the laser beam, the diameter of the focal point, the gas pressure for generating the gas jet, in particular used to eject the melt, and / or the position of the cutting head in the machining device.

[0025] Advantageously, the method comprises comparing the emitted energy with a second control value for controlling the cutting method. In particular, the above-mentioned radiation intensity can be compared with a limit value, and if the limit value is exceeded, the cutting method is readjusted, for example by adjusting the laser power or switching off the laser beam.

[0026] A machining apparatus for carrying out a method according to one of the preceding embodiments comprises a laser source for emitting a laser beam, an image sensor for recording process images of the entire partial volume of the workpiece with the cutting front penetrated by the laser beam, and a controller for controlling the method by means of 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 apparatus can be effectively protected against damage caused by the laser beam.

[0027] Further advantages of the invention will become apparent from the description and the drawings. Likewise, the above-mentioned and further proposed features can in each case be used individually or together in any desired combination. The illustrated and described embodiments should not be understood as an exhaustive list, but rather as exemplary features for explaining the invention. [Brief explanation of the drawings]

[0028] [Figure 1] 1 shows a schematic cross section through a machining device for controlling the cutting of a workpiece by a laser beam; [Figure 2] 1 shows a schematic cross section through an irradiated workpiece on a support; [Figure 3] 5A to 5C are process images schematically showing a cutting process in the first embodiment; [Figure 4] 10A and 10B are schematic process diagrams of a cutting process in a second embodiment; DETAILED DESCRIPTION OF THE INVENTION

[0029] 1 shows a schematic cross section through a machining device 10 for controlling the cutting of a workpiece 12 with a laser beam 14. A laser beam source 16 having a focusing device 18 emits the laser beam 14 through a cutting nozzle that generates a gas jet 17 to an upper surface 20a of the workpiece 12, which is cut by the laser beam 14 and the gas jet. The cut is made from the upper surface 20a of the workpiece 12 to its lower surface 20b, whereby the workpiece 12 is placed on a support 22. The lower surface 20b of the workpiece 12 is opposite the upper surface 20a in the direction of the laser beam 14. The laser beam 14, together with the gas jet, generates a cutting front 24 in the workpiece 12, which extends from the upper surface 20a of the workpiece 12 to its lower surface 20b and moves with the laser beam 14 in a feed direction 26 of the laser beam 14. The cutting front 24 is created by the portion 30a of the laser beam 14, and material of the workpiece 12 at the cutting front 24 is melted and / or vaporized by heating and then expelled from the kerf 28 by a gas jet. This creates an open kerf 28 behind the cutting front 24 in the feed direction 26 of the laser beam 14, through which the portion 30b of the laser beam 14 emits unimpeded. This portion 30b of the laser beam 14 exits the lower surface 20b of the workpiece 12 and impinges on the support 22 below the workpiece 12, thereby heating and possibly damaging the support 22. The portion 30b of the laser beam 14 is also referred to as the passing portion 30b.

[0030] The heated material in the partial volume 32 of the workpiece 12 penetrated by the laser beam 14, which has the cutting front 24, emits thermal radiation, which at least partially serves as a monitoring beam 34 that impinges on the recording surface 36 of the image sensor 38. A controller 40 or control device with an evaluation unit 42 uses the data of the image sensor 38 to generate process images 46a, 46b (see FIGS. 3 and 4) of the partial volume 32 of the workpiece 12 penetrated by the laser beam 14, which has the cutting front 24. For this purpose, the controller 40, the evaluation unit 42, and the image sensor 38 are connected to each other via signals, as shown in FIG.

[0031] FIG. 2 shows a schematic cross section through the irradiated workpiece 12 on the support 22. It shows both the portion 30a of the laser beam 14 cutting through the workpiece 12, impinging on the cutting front 24, and the passing portion 30b of the laser beam 14, which is emitted through the workpiece 12 through the kerf 28 and impinges on the support 22 on which the workpiece 12 rests. The diameter D1 of the laser beam 14 on the upper surface 20a of the workpiece 12 is indicated by a double-headed arrow on the upper surface 20a of the workpiece 12 in FIG. 2 and therefore consists of two subsections. The first subsection is the process length L of the cutting front 24 in the feed direction 26 of the laser beam 14, in the plane of the beam axis 44a of the laser beam 14 and the feed direction 26. The feed direction 26 of the laser beam 14 is from right to left in FIG. 2. The second subsection is the diameter D2 of the passing portion 30b of the laser beam 14 emitted through the kerf 28. The diameter D2 is determined in the horizontal direction according to Figure 2, which extends perpendicular to the laser beam 14. Using the process images 46a, 46b recorded by the image sensor 38 (see Figures 3 and 4, described below), these subsections L, D2 can be determined and related to the diameter D1 of the entire laser beam 14 on the top surface 20a of the workpiece 12, to estimate the passing portion 30b of the laser beam 14 passing through the kerf 28.

[0032] The angle of the beam axis 44b of the monitoring beam 34 relative to the vertical beam axis 44a of the laser beam 14 is typically between 0° and 5°.

[0033] 3 shows a schematic process image 46a of a partial volume 32 (see FIG. 1) of the workpiece 12 irradiated during an irradiation time in a first embodiment of the cutting process. The process image 46a is recorded by the cutting nozzle 17 (see FIG. 1) from which the laser beam 14 emerges, the nozzle orifice 48 of which surrounds the process image 46a. The process image 46a includes, in particular, a projected representation of the cutting front 24, the kerf 28 and the non-cut area 50 of the upper surface 20a of the workpiece 12. In the process image 46a, the cutting front 24 is shown with a projected process length L , which indicates the length of the cutting front 24 in the horizontal direction according to FIG. 3. (P) The projected process length L of the cutting front 24 (P) From the process image 46a having the (P) and / or geometric parameters of the cutting front 24, such as the width of the cutting front 24, can be determined by imaging methods based on the position and orientation of the image sensor 38 (see FIG. 1 ) and, if necessary, other optical elements for directing the monitoring beam 34. The intensity distribution of the monitoring beam 34 can also be determined from the process image 46 a.

[0034] 4 shows a schematic representation of another process image 46b of the partial volume 32 (see FIG. 1) of the workpiece 12 irradiated during an irradiation time in a second embodiment of the cutting process, where a cut is first performed with a first kerf 52a of smaller width. This is followed by a cut with a second kerf 52b of larger width, which is guided at least partially along the first kerf 52a. The first kerf 52a divides the cutting front 24 into two partial cutting fronts 54a, 54b generated on either side of the first kerf 52a by the laser beam 14 (see FIG. 1), both partial cutting fronts 54a, 54b having a projected process length L (P)When cutting the second kerf 52b, a portion 30b of the laser beam 14 (see FIG. 1) radiates unimpeded through the first kerf 52a. This passing portion 30b of the laser beam 14 can be controlled by the method according to the invention in order to avoid damage to the machining device 10. In addition to the two kerfs 52a, 52b and the partial cutting fronts 54a, 54b, the uncut, i.e. not yet cut, periphery 50 of the top surface 20a of the workpiece 12 and the nozzle opening 48 of the cutting nozzle 17, which surrounds a further process image 46b, are also shown in the projection.

[0035] Summarizing all the figures of the drawings, the present invention relates to a method for monitoring the process of cutting a workpiece 12 with a laser beam 14, in which the workpiece 12 is cut through in the beam direction of the laser beam 14 from an upper surface 20a to a lower surface 20b opposite the upper surface 20a. One portion 30a of the laser beam 14 is absorbed by a cutting front 24 in the workpiece 12, while another portion 30b of the laser beam 14 emits through a kerf 28 and the lower surface 20b of the workpiece 12, the kerf 28 being behind the cutting front 28 in the feed direction 26 of the laser beam 14. Within the scope of the method, a characteristic value of the exit of the passing portion 30b of the laser beam 14 is determined by a machining device and the power of the passing portion 30b of the laser beam 14 emitted through the kerf 28 is limited.

Claims

1. 1. A method for monitoring a process of cutting a workpiece (12) with a laser beam (14), the method comprising: a) irradiating a laser beam (14) onto the upper surface (20a) of the workpiece (12); b) cutting through the workpiece (12) using the laser beam (14) from the top surface (20a) of the workpiece (12) to the bottom surface (20b) of the workpiece (12), the bottom surface (20b) of the workpiece (12) being opposite the top surface (20a) of the workpiece (12) in the beam direction of the laser beam (14), and forming a cutting front (24) and a kerf (28); e) limiting the power of a portion (30b) of the laser beam (14) that has passed through the workpiece (12) to a predetermined level, such that the portion (30b) of the passed laser beam (14) exits the workpiece (12) at the lower surface (20b) of the workpiece (12) as it cuts the workpiece (12).

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

3. 3. The method according to claim 1, further comprising determining the power of the portion (30b) of the laser beam (14) with the aid of an output power of a laser source (16) for generating the laser beam (14), a caustic surface of the laser beam (14), a Rayleigh length of the laser beam (14), a position of the focus of the laser beam (14), a focus position, a focus diameter, a thickness of the workpiece, and / or a distance of a cutting nozzle (17) from where the laser beam (14) exits to the workpiece (12).

4. c) recording a process image (46a, 46b) of the entire sub-volume (32) of the workpiece (12) with the cutting front (24) penetrated by the laser beam (14) by projecting this sub-volume (32) onto a recording surface (36) of an image sensor (38), the image sensor (38) being arranged above the top surface (20a) of the workpiece (12); d) determining 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) based on the process images (46a, 46b).

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

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

7. The length (L) of the cutting front (24) and the diameter (D) of the laser beam (14) at the top surface (20a) of the workpiece (12) １ 7. The method of claim 2, further 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 characteristic value of the power of the portion (30b) of the laser beam (14).

8. 8. The method of claim 7, comprising determining the power of the portion (30b) of the laser beam (14) that has passed through the workpiece (12) by means of an area and / or intensity distribution of the laser beam (14) on the top surface (20a) of the workpiece (12) as a further parameter.

9. 10. The method according to claim 1, wherein at least the partial volume (32) of the workpiece (12), from which a process image (46a, 46b) is recorded, is illuminated with incident light illumination, and image data obtained from the reflected incident light illumination are integrated into the process image (46a, 46b).

10. The method of any one of claims 1 to 9, comprising comparing the power of the portion (30b) of the laser beam (14) with a first control value to control a cutting operation.

11. The method of any one of claims 1 to 10, comprising determining the energy emitted in a predetermined area from the portion (30b) of the laser beam (14) over a predetermined time interval.

12. 12. The method of claim 11, comprising comparing the applied energy to a second control value for controlling a cutting operation.

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

14. 14. A machining apparatus (10) configured to perform the method according to any one of claims 1 to 13, the machining apparatus (10) comprising: a laser source (16) for emitting the laser beam (14); an image sensor (38) for recording process images (46a, 46b) of the entire partial volume (32) of the workpiece (12) with the cutting front (24) penetrated by the laser beam (14); and a controller (40) for controlling the method by means of an evaluation unit (42) for determining the power of the portion (30b) of the laser beam (14) that has passed through the workpiece (12).

Images