METHOD AND DEVICE FOR MONITORING A CUTTING PROCESS

Abstract

A method for monitoring, in particular for controlling, a cutting process on a workpiece, includes focusing a machining beam, in particular a laser beam, on the workpiece, detecting a region of the workpiece to be monitored, the region including an interaction region in which the machining beam interacts with the workpiece, and determining at least one characteristic variable of the cutting process, in particular of a kerf formed during the cutting process, on the basis of the detected interaction region. In a fusion cutting process, a cutting front length of a cutting front formed at the kerf is determined as a characteristic variable on the basis of the detected interaction region. A corresponding device for monitoring, in particular for controlling, a cutting process on a workpiece, is also provided.

Claims

1-12. (canceled)

13. A method for monitoring or controlling a cutting process on a workpiece, the method comprising: focusing a machining beam or a laser beam on the workpiece; detecting a region of the workpiece to be monitored, the region including an interaction region in which the machining beam interacts with the workpiece; and in a fusion cutting process, determining, based on the detected interaction region, a cutting front length of a cutting front formed at a kerf during the cutting process, as a characteristic variable of the cutting process.

14. The method according to claim 13, which further comprises using an observation beam path extending coaxially to a beam axis of the machining beam for detecting the region to be monitored.

15. The method according to claim 13, which further comprises providing a nozzle opening of a machining nozzle for passage of a cutting gas jet with a maximum extension of at least 7 mm.

16. The method according to claim 15, which further comprises providing the maximum extension to be between 7 mm and 12 mm.

17. The method according to claim 13, which further comprises carrying out the fusion cutting process at a cutting gas pressure of less than 10 bar.

18. The method according to claim 13, which further comprises carrying out the fusion cutting process at a cutting gas pressure of greater than 1 bar and less than 10 bar.

19. The method according to claim 13, which further comprises carrying out the fusion cutting process at a cutting gas pressure of at least 2 bar and less than 6 bar.

20. The method according to claim 13, which further comprises carrying out the fusion cutting process at a cutting speed which is at least 80% of an incomplete cut speed.

21. The method according to claim 13, which further comprises carrying out the fusion cutting process at a cutting speed which is at least 90% of an incomplete cut speed.

22. The method according to claim 13, which further comprises determining the cutting front length from an image of the interaction region as a length between two points along a profile section of the interaction region extending in a cutting direction.

23. The method according to claim 22, which further comprises defining the two points as points at which a brightness falls below a brightness threshold value.

24. The method according to claim 13, which further comprises controlling the cutting front length to a predetermined target length by influencing at least one adjustment parameter of the cutting process.

25. The method according to claim 24, which further comprises influencing at least one of a cutting speed between the machining beam and the workpiece or a power of the machining beam, as adjustment parameters for controlling the cutting front length.

26. A device for monitoring or controlling a cutting process on a workpiece, the device comprising: a focusing unit for focusing a machining beam or a laser beam on the workpiece; an image acquisition unit for detecting a region to be monitored on the workpiece, the region to be monitored including an interaction region of the machining beam with the workpiece; and an evaluation unit configured to determine, based on the detected interaction region, a cutting front length of a cutting front formed at a kerf during the cutting process, as a characteristic variable of the cutting process.

27. The device according to claim 26, which further comprises a control unit for controlling the cutting front length to a predetermined target length by influencing at least one adjustment parameter of the cutting process.

28. The device according to claim 27, wherein said control unit is configured to control the cutting front length to the target length, at which a cutting speed (V) is at least 80% of an incomplete cut speed.

29. The device according to claim 27, wherein said control unit is configured to control the cutting front length to the target length, at which a cutting speed is at least 90% of an incomplete cut speed.

Description

IN THE FIGURES

[0027] FIG. 1 shows a schematic illustration of an exemplary embodiment of a device for monitoring and controlling a laser cutting process,

[0028] FIG. 2 shows an illustration of an image recorded using an image detection unit of a region to be monitored of the workpiece, on the basis of which a cutting front length is determined as a characteristic variable of the cutting process, and

[0029] FIG. 3 shows an illustration of the cutting front length as a function of the ratio of the cutting speed to an incomplete cut speed.

[0030] In the following description of the drawings, identical reference signs are used for equivalent or functionally equivalent components.

[0031] FIG. 1 shows an exemplary structure of a device 1 for process monitoring and control of a laser fusion melting process on a plate-shaped workpiece 2 by means of a laser machining system, of which only a machining unit 3 (part of a laser machining head) having a focusing lens 4 for focusing a 002, solid-state, or diode laser beam 5 of the laser machining system, a machining nozzle 6, and having a deflection mirror 7 is shown in FIG. 1. In the present case, the deflection mirror 7 is made partially transmissive and therefore forms an entry-side component of the device 1 for process monitoring. The device 1 for process monitoring is, like the machining unit 3, part of the laser machining head.

[0032] The deflection mirror 7 reflects the incident laser beam 5 and transmits the process radiation, which is relevant for the process monitoring and reflected from the workpiece 2 and which is emitted from the interaction zone, in a wavelength range which in the present example is between approximately 550 nm and 2000 nm. Alternatively to the partially transmissive deflection mirror 7, a scraper mirror or a perforated mirror can also be used to supply the process radiation to an observation beam path 8. However, the use of a scraper mirror typically results in suppression of a part of the process radiation and limiting of the raw beam diameter. The use of a perforated mirror generally results in diffraction effects of the process radiation and a strong influence of the laser radiation.

[0033] In the device 1, a further deflection mirror 9 is arranged behind the partially transmissive mirror 7, which deflects the process radiation onto a geometrically high-resolution camera 10 as the image acquisition unit. The camera 10 can be a high-speed camera, which is arranged coaxially to the laser beam axis 11 or to the extension 11a of the laser beam axis 11 and thus directionally independent. The observation beam path 8 in the example shown accordingly also extends coaxially to the laser beam axis 11 or to its extension 11a. In principle, there is the option of recording the image by way of the camera 10 in the incident light method, i.e., in the VIS wavelength range, possibly also in the NIR wavelength range, if an additional illumination source 15 is provided, which emits in the NIR range and couples illumination radiation 17 into the beam path coaxially to the laser beam axis 11 via a further partially transmissive mirror 16. As an additional illumination source 15, laser diodes, for example having a wavelength of 658 nm, or diode laser, for example having a wavelength of 808 nm, can be provided, which can be arranged coaxially as shown in FIG. 1, or also off-axis to the laser beam axis 11. Alternatively, recording the process intrinsic light in the wavelength ranges UV and NIR/IR without additional illumination is possible.

[0034] For improved imaging, an imaging focusing optical system 12, which is shown as a lens in FIG. 1 and which focuses the radiation relevant for the process monitoring on the camera 10, is provided in the present example between the partially transmissive mirror 7 and the camera 10. Spherical aberrations in the imaging can be prevented or at least reduced by an aspheric design of the imaging optical system or the lens 12 for focusing.

[0035] In the example shown in FIG. 1, a filter 13 in front of the camera 10 is advantageous if further radiation or wavelength components are to be excluded from the detection by the camera 10. The filter 13 can be designed, for example, as a narrowband bandpass filter having low full width at half maximum, to avoid or reduce chromatic aberrations. The location of the camera 10 and of the imaging optical element 12 provided in the present example and/or of the filter 13 along the laser beam axis 11 is settable and changeable if needed via a positioning system known to a person skilled in the art, which is illustrated by a double arrow for simplification.

[0036] The camera 10 is operated in the present example without the additional illumination source 15, i.e., the intrinsic light of the process zone in the N IR/IR wavelength is detected. As shown in FIG. 2, the camera 10 records on its sensor surface 10a a high-resolution image 20 of a region 21 to be monitored (detail) of the workpiece 2. The image 20 is delimited by the circular inner contour of the nozzle opening 6a (cf. FIG. 1) of the nozzle 6, the diameter D or the maximum extension of which at the exit-side end of the nozzle 6 is between 7 mm and 12 mm in the example shown. The cutting process shown in FIG. 1 is a fusion cutting process using nitrogen as the cutting gas. The nitrogen exits as the cutting gas jet 14 from the nozzle opening 6a of the machining nozzle 6 at a comparatively low cutting gas pressure ps of less than approximately 10 bar, preferably of greater than 1 bar and less than 10 bar, ideally of greater than 2 bar and less than approximately 6 bar.

[0037] Alternatively to the example shown in FIG. 2, the nozzle 6 can also be formed as a ring flow nozzle having two (typically concentric) nozzle openings: The laser beam 5 then exits through the opening of the inner nozzle and the cutting gas jet 14 exits through the outer nozzle opening or through the inner and outer nozzle openings. In this case, the outer nozzle opening has a diameter or a maximum extension of at least 7 mm. The image recording of the camera 10 takes place through the inner nozzle opening, so that the image 20 is delimited by the circular inner contour of the inner nozzle opening, which has a diameter of, for example, 3 mm.

[0038] An evaluation unit 18 shown in FIG. 1 is used to evaluate the image 20 and in particular to detect an interaction region 22 within the region 21 to be monitored of the workpiece 2. The evaluation unit 18 has a signaling connection to a control unit 19 (also shown in FIG. 1), which controls or regulates the laser cutting process, and does so as a function of a characteristic variable of the laser cutting process determined by the evaluation unit 18, which variable is a cutting front length L of a cutting front 23 (cf. FIG. 1) formed during the cutting machining, on which a kerf 24 adjoins against a feed or cutting direction (i.e., in the negative X direction). As can be seen in FIG. 2, the cutting front length L is measured between a point P1 at the front end of the interaction region 22 and a point P2 at the rear end of the interaction region 22 along the feed or cutting direction, along which the laser beam 5 is guided over the workpiece 2 at a cutting or feed speed V (cf. FIG. 1). In the example shown, the feed direction corresponds to the X direction.

[0039] To determine the cutting front length L, rapid image recording can take place during the cutting process with the aid of the image acquisition unit 10, for example at a frequency of 100-1000 Hz. The individual images 20 are evaluated, for example, by a threshold value method, i.e., binarization of a respective image 20 is carried out by comparing the intensity values of the recorded light appearance at the individual pixels to a threshold value. The length of the light appearance in the cutting direction (X direction) is determined from the binarized image 20, which corresponds to the cutting front length L. The cutting front length L can thus be determined from the image 20, for example, via brightness threshold values Is of a profile section 25 of the light appearance extending in the cutting direction (X direction), i.e., the cutting front length can be determined as the length L between two points P1, P2 of the profile section 25, at which the brightness falls below a predetermined brightness threshold value Is or predetermined brightness threshold values. A calibration of the measured values of the intensity I to a reference value within the image 20, for example to a maximum intensity value of the image 20, can be carried out. Moreover, a calibration of the image acquisition unit 10 can be carried out in a reference cutting process to reference cutting parameters and by comparison of the measured values to those of a reference image acquisition unit.

[0040] Further relevant process parameters in addition to the cutting gas pressure ps, the diameter D of the machining nozzle 6, and the cutting speed V are the laser power P of the laser beam 5 or the laser source (not shown in the figures), the material of the workpiece 2, and the thickness d of the plate-shaped workpiece between an upper side 2a and a lower side 2b of the workpiece 2.

[0041] The further above-described fusion cutting process can be carried out, for example, using the following process parameters:

[0042] Structural steel:

d=4 mm, P=10 kW, V=20 m/min, ps=7 bar

d=10 mm, P=10 kW, V=5 m/min, ps=9 bar

[0043] Stainless steel:

d=4 mm, P=10 kW, V=21 m/min, ps=6 bar

d=10 mm, P=10 kW, V=5.5 m/min, ps=4 bar

[0044] Aluminum:

d=4 mm, P=10 kW, V=35 m/min, ps=8 bar

d=10 mm, P=10 kW, V=8 m/min, ps=9 bar

[0045] In a fusion cutting process, which is carried out under the above-described conditions, i.e., with a comparatively low cutting gas pressure ps and a large diameter D of the machining nozzle 6, a good edge quality of the kerf 24 can be achieved even at high cutting speeds V. The good cutting quality is maintained in particular even at cutting speeds V which are close to the incomplete cut speed V.sub.s, i.e., the fusion cutting method can also be carried out at high cutting speeds V which are at least 80%, preferably at least 90% of an incomplete cut speed V.sub.s. The incomplete cut speed Vs can be determined beforehand in measurement series for a respective workpiece material, a respective workpiece thickness d, a predetermined laser power P, and a predetermined cutting gas pressure p.sub.s. The corresponding values for the incomplete cut speed Vs can be stored, for example, in technology tables or the like in a storage unit, which can be arranged in the evaluation unit 18 or at another location.

[0046] At high cutting speeds V close to the incomplete cut speed V.sub.s, an incomplete cut is more likely to occur than in previous standard processes, which are carried out at higher cutting gas pressure ps and lower cutting speeds V. In the event of an imminent incomplete cut, the cutting front length L increases strongly, so that it is favorable to control the cutting front length L with the aid of the control unit 19 to a predetermined, constant target length L.sub.s. To achieve this, the control unit 19 influences or changes at least one adjustment parameter of the cutting process, which influences the introduction of energy into the workpiece 2.

[0047] FIG. 3 shows the dependence of the cutting front length L determined with the aid of the evaluation unit 18 on the cutting speed V, more precisely on the ratio of the cutting speed V to the incomplete cut speed V.sub.s, for the example of structural steel having a thickness d of the workpiece of 2 to 8 mm. As can be seen in FIG. 3, the rise of the cutting front length L becomes more and more pronounced with rising cutting speed V, so that a control of the cutting front length L with the aid of the cutting speed V or the feed as adjustment parameter is possible at high cutting speeds V, which are typically greater than 80% or greater than 90% of the incomplete cut speed V.sub.s.

[0048] In the example shown in FIG. 3, the target length L.sub.s of the cutting front length L is approximately 0.6 mm, which corresponds to a ratio of the cutting speed V to the incomplete cut speed Vs of approximately 95%. A control of the cutting front length L to a predetermined target length L.sub.s can alternatively or additionally also be carried out with the aid of the laser power P of the laser beam 5 as the adjustment parameter. In both cases, the fusion cutting process can be guided with a sufficient distance from the incomplete cut by the influencing of the introduction of energy, which ensures the robustness of the fusion cutting process under interfering influences.

[0049] If the cutting speed V or the feed is used as the adjustment parameter for the control, the feed specification or the feed adjustment ΔV (change of the cutting speed V) can take place in a regular cycle (for example 200 Hz). The feed adjustment ΔV can be formed, for example, from the present speed V, which is stored in the control unit 19 or in the evaluation unit 18, the target length L.sub.s, the difference ΔL between the cutting front length L presently measured by the evaluation unit 18 and the target length L.sub.s, and a (constant) proportionality factor f according to the following formula:

ΔV/V=f*ΔL/L.sub.s.

[0050] The control of the cutting front length L to the target length L.sub.s can be carried out on the basis of the individual images, if it takes place slowly enough (for example at a clock rate of 200 Hz), so that a good control behavior without overshoots is obtained. Averaging of the individual images 20 recorded by means of the image acquisition unit 10 can make the image processing, i.e., the determination of the cutting front length L, more robust. A sliding, possibly weighted mean value can be determined for the averaging. For example, the averaging can be carried out in that a current image and the last mean value image are combined using a predetermined weighting to form a new mean value image: for example, 30% current image +70% old mean value image=new mean value image.

[0051] In the above-described way, the fusion cutting process can be carried out close to the incomplete cut speed V.sub.s, i.e., the feed range up to the incomplete cut speed V.sub.s can be nearly fully utilized, without the quality of the cut edges of the kerf 24 worsening or an incomplete cut occurring.