Device for monitoring a laser processing process

Abstract

The invention relates to a device (1) for monitoring a laser processing process with a first branching fiber (11), which has a first input part (32) and a first and a second output part (33, 34) and which is designed to split the measuring light (9) entering the first input part (32) into a first partial measuring light beam (12) and a second partial measuring light beam (13), wherein the first output part (33) is designed to guide and decouple the first partial measuring light beam (12), and the second output part (34) is designed to guide and decouple the second partial measuring light beam (13). The device (1) further comprises a first and a second filter element (14, 15), wherein the first filter element (14) is arranged and designed to filter at least parts of the first partial measuring light beam (12) decoupled from the first output part (33) in a first wavelength range.so that a first filtered partial measuring light beam (25) is created, and wherein the second filter element (15) is arranged and designed to filter at least parts of the second partial measuring light beam (13) coupled out from the second output part (34) in a second wavelength range, so that a second filtered partial measuring light beam (26) is created, and a first and a second detector (16, 17), wherein the first detector (16) is arranged and designed to at least partially detect the first filtered partial measuring light beam (25) and the second detector (17) is arranged and designed to at least partially detect the second filtered partial measuring light beam (26).

Description

The present invention relates to an apparatus for monitoring a laser machining process, in particular a welding process.Prior ArtMonitoring devices for monitoring a laser machining process, in particular a laser welding process, are known. The importance of such monitoring apparatus has increased in recent years as laser welding systems become more and more powerful and more and more welds are welded using such systems.DE 10 2017 002 922 discloses a monitoring device for a machining system for machining a workpiece by means of a laser beam. The monitoring device comprises a registration unit which is designed to record a reflected portion of a measurement beam. The registration unit can have at least one photodiode, or a plurality of redundant photodiodes, and can comprise at least one filter, preferably a color filter or a band pass filter, which allows a predetermined wavelength range of the reflected portion of the measurement beam to pass and which is connected upstream of the photodiode or photodiodes. The reflected portion of the measurement beam is guided as a free beam. In this way, the wavelength range of the reflected portion of the measurement beam can be analyzed in a targeted manner, which wavelength range is actually also of interest in the evaluation. Disruptive influences of ambient light or process light can be prevented in this case.With such a monitoring device, only a single wavelength range of the reflected portion of the measurement beam can be analyzed. If a further wavelength range is to be analyzed, the filters and photodiodes, as well as the optical elements necessary for guiding the free beam, must be exchanged. With such a device, it is not possible to separately and simultaneously detect signals in different wavelength ranges. In addition, the optical elements are complicated to adjust.In order to analyze a plurality of wavelength ranges of a signal, cascading of beam splitters is also customary. These are often special preparations, the respective elements of which can be exchanged only with considerable effort. Furthermore, so-called "sandwich diodes" are customary. This is a plurality of detectors arranged one behind the other, in particular a plurality of photodetectors arranged one behind the other. In this case, a first detector is transparent to the incident signal in a first wavelength range and sensitive in a second wavelength range, while a second detector, which is arranged downstream of the first detector as viewed in the direction of propagation of the signal, is sensitive in the second wavelength range. In this manner, different wavelength ranges of the signal can also be analyzed, but only a limited number of suitable combinations of detectors are available. Thus, for example, a silicon diode can be combined with a photodiode sensitive in the infrared. In this case, the silicon diode transmits the infrared component of the incident signal. This is then detected by a downstream photodiode sensitive in the infrared (for example an InGaAs diode).It is an object of the invention to provide an apparatus for monitoring a laser machining process which does not have the disadvantages of the prior art.Disclosure of the InventionThe present invention relates to a device for monitoring a laser machining process according to claim 1.The invention provides an apparatus for monitoring a laser processing process, the apparatus comprising a first branching fiber, a first and second filter element, and a first and second detector. The first branching fiber includes an input part and first and second output parts, and is configured to split measurement light entering the input part into a first partial measurement light beam and a second partial measurement light beam. The first output part is designed to guide and couple out the first partial measurement light beam. The first filter element is arranged and configured to filter at least parts of the first partial measurement light beam coupled out from the first output part in a first wavelength range, so that a first filtered partial measurement light beam is produced. The second filter element is arranged and configured to filter at least parts of the second partial measurement light beam coupled out of the second output part in a second wavelength range, so that a second filtered partial measurement light beam is produced. The first detector is arranged and designed to at least partially detect the first filtered partial measurement light beam. The second detector is arranged and designed to at least partially detect the second filtered partial measurement light beam. The device can have further branching fibers.The first branching fiber can be configured as an optical waveguide configured to conduct radiation in a wavelength range extending from the ultraviolet to the infrared, i.e. between wavelengths of approximately 1·10 -8 m to approximately 1·10 -5 m. The first branching fiber can be designed, for example, as a Y fiber, so that the measurement light coupled into its input part is divided into exactly two partial measurement light beams. Alternatively, the first branching fiber can also be designed to split the measurement light coupled into its input part into three or more partial measurement light beams. The first branching fiber can have a diameter between 5 μm and 1.1 μm, in particular between 50 μm and 500 μm.Branching fibers with large fiber diameters, in particular branching fibers with fiber diameters of more than 1 mm, have the disadvantage that their bending radii are very large and these fibers are therefore difficult to handle. The first branching fiber can be designed as a multimode fiber, in particular with a diameter between 50 μm and 500 μm.In this case, the fiber diameter, in particular of those output parts of the branching fibers which are arranged directly upstream of the filters assigned to them, can be smaller than or equal to the area of the detectors assigned to these filters. In particular, the fiber diameter of the first and / or second and / or of a further output part of the first branching fiber can be less than or equal to the area of the first and / or second and / or of a further detector. In this way, it is avoided that partial measurement light beams are guided past the respective detectors or have to be directed onto the respective detector by additional optical elements, so that signal intensity is lost.For example, in the case that an InGaAs diode is used as a detector, it may be advantageous if the diameter of the output part, which is arranged directly adjacent to the filter element adjoining the InGaAs diode, is at least approximately equal to or slightly less than the diameter of a detector surface of the InGaAs diode. In this way, a signal having a high bandwidth and a good signal-to-noise ratio is obtained.In the case that an Si photodiode is used as a detector, it may be advantageous to choose the diameter of the output part, which is arranged directly adjacent to the filter element adjoining the Si diode, to be significantly smaller than the diameter of the detector surface of the Si photodiode. The diameter of the detector surface of the Si photodiode is typically a few millimeters in size.The first and second filter elements are in particular configured to filter radiation in different wavelength ranges, so that signals in different wavelength ranges can be detected at the first and second detector. Thus, the first wavelength range can be, for example, in the visible spectral range (350 nm-700 nm). In this case, the first filter element filters the first partial measurement light beam in the visible wavelength range and the first detector receives a first signal in the visible wavelength range. The second wavelength range can be, for example. In some embodiments, the wavelengths may include near infrared wavelengths (NIR, 700 nm-3000 nm), in particular in a wavelength range between 700 nm and 1200 nm, referred to as IRA, or a wavelength range between 1200 nm and 3000 nm, referred to as IRB. In this case, the second filter element filters the second partial measurement light beam in the second wavelength range and the second detector receives a second signal in the second wavelength range. The detectors are in particular designed as photodiodes, in particular as silicon diodes and / or InGaAs diodes. Photodiodes, in particular silicon diodes and InGaAs diodes, are sensitive in a very broad wavelength range, at the same time being robust, sensitive and compact and having a low dark current.The use of the branching fiber makes it possible to very easily adjust the device, since no beam-deflecting or beam-shaping optical elements are required. Thus, the branching fiber has the advantage over a beam splitter that-apart from the step of coupling the measurement light into the branching fiber-no further adjustment steps are necessary. In some cases, pre-aligned modules may be used that include both the detector and the filter element. Tolerances can be compensated by the selection of the diameter of the branching fiber, for example the diameter and / or the NA of the branching fiber can be selected to be somewhat larger than necessary. The respective filter elements can also be mounted directly in front of the respective detectors. Such an arrangement is cost-effective, but may lead to losses and possibly to a shift of the wavelengths transmitted by the filter element to shorter wavelengths, which may be problematic in particular if the measurement light overall has only a narrow wavelength range.The branching fiber can be designed, for example, as a GLASS fiber, which are bent within certain limits, so that the measurement light and the first and second partial measurement light beams can be guided flexibly within the device. Alternatively, the branching fiber can also be designed as a plastic fiber. In addition, the individual filter elements and detectors can easily be exchanged and / or exchanged by filter elements and detectors which are suitable for a different wavelength range, so that the device can be easily and quickly adapted to the detection of a different wavelength range. In this way, the device according to the invention can be easily and quickly adapted to another system technology, for example when changing to a laser with a different wavelength or in the case that specific wavelength ranges are not available by further monitoring devices, for example on the basis of OCT (optical coherence tomography).According to a further development of the invention, the first branching fiber is designed to divide the measurement light with equal intensities into the first partial measurement light beam and the second partial measurement light beam. This enables a symmetrical construction of the device in which particularly many identical or similar components can be used.According to a further development of the invention, the first branching fiber is designed to divide the measurement light-in particular in its entire wavelength range-into the first partial measurement light beam and the second partial measurement light beam with different intensities. The intensity ratio between the first and the second partial measurement light beam is, for example. 90:10 or 75:25 In this way, partial measurement light beams of different intensity are produced at the detectors, so that different detector types are used for the detection of different partial measurement light beams, which differ, for example, in that the signals can detect in different wavelength ranges. In this way, it can be taken into account that, due to the emission characteristic of the laser process and the transmission properties of the optical elements which the measurement light passes before it impinges on the branching fiber, different wavelength ranges of the measurement light have different intensities. For example, a detector that is sensitive in a wavelength range that is present in the measurement light with only a low intensity can now be arranged in such a way that it detects the partial measurement light beam that has the higher intensity due to the division ratio of the branching fiber. In this way, the signal and / or the signal-to-noise ratio can be optimized for the respective wavelength ranges.Alternatively, the branching fiber may be configured to split the measurement light into partial measurement light beams having different wavelength ranges. The branching fiber is then in particular configured as a single-mode fiber. In this case, the filter elements may be dispensed with, since only partial measurement light beams in a specific wavelength range reach the detectors.In a development of the invention, the device comprises an optical arrangement which is designed to couple the measurement light which arises during the laser processing process into the first branching fiber. The laser processing process is in particular a laser welding process in which process radiation is produced during the welding process. In addition, the laser beam can be reflected on a workpiece which is being machined, so that a back reflection occurs.In particular, the optical arrangement is designed to conduct measurement light, which originates from a measurement spot, to the first branching fiber. In this case, the measurement spot is that region on a surface of the workpiece which is connected to the optical arrangement via a beam path, i.e. radiation which originates from the measurement spot can pass as measurement light via the beam path to the optical arrangement, from where it then reaches the detectors via the first branching fiber and the filter elements. The radiation emanating from the measurement spot can be process radiation and / or scattered and / or reflected portions of the laser beam.The optical arrangement can be designed as a fixed optical unit. In this case, the fixed optical unit is moved together with the laser beam, in particular during the laser processing process, and can be mechanically connected to a laser processing optical unit for this purpose. The optical elements of the fixed optics cannot be moved individually. Laser processing optics are all those optical elements that focus the laser beam onto the workpiece for processing the workpiece.In a development of the invention, the optical arrangement comprises at least one movable mirror. An optical arrangement with movable mirrors is usually referred to as scanner optics. The optical arrangement can have two movable mirrors, which can be moved in particular independently of one another. In addition, the optical arrangement can be focusing optical elements, such as, for example. The lenses may comprise lenses. Compared to fixed optics, scanner optics have the advantage that with their aid the position of the measurement spot can be set very much more quickly, so that it is advantageous to use a scanner optics, in particular in fast laser processing processes. The scanner optics is in particular connected to a control device, which in turn is connected to a laser controller. The control device is configured to move the mirrors of the scanner optics in such a way that the measurement spot at least partially overlaps a laser spot, i.e. a region of the surface of the workpiece on which the laser beam impinges. In this case, the diameter of the measurement spot is usually 1 to 10 times as large as the diameter of the laser spot.In a further development of the invention, the device has a second branching fiber with a second input part and a third and fourth output part, and wherein the first and the second branching fiber are arranged and aligned with respect to one another in such a way that the first or the second partial measurement light beam can be coupled into the second input part. The second branching fiber can be designed to split the first or second partial measurement light beam coupled into the second input part into a third and fourth partial measurement light beam. In this case, the third output part can be designed and arranged to guide and decouple the third partial measurement light beam, while the fourth output part can be designed to guide and decouple the fourth partial measurement light beam.The device can have a third filter element, wherein the third filter element is arranged and configured to filter the third or fourth partial measurement light beam coupled out of the third or fourth output part in a third wavelength range, so that a third or fourth filtered partial measurement light beam is produced. The apparatus can have a third detector which is arranged and designed to at least partially detect the third or fourth filtered partial measurement light beam. In this way, three signals can be generated with two branching fibers, which signals are detected separately from one another at three different detectors. It is understood that further branching fibers can be provided in order to further divide the respective partial measurement light beams, so that these can be detected at further detectors. In this way, it is possible to analyze even further wavelength ranges of the measurement light, wherein the intensity of the respective signals decreases and the price of the device increases the more frequently the measurement light is divided.This enables a flexible arrangement in which almost any number of signals can be recorded at a corresponding number of detectors. If, in addition, branching fibers are used which divide the intensity of the measurement light with different intensities into the respective partial measurement light beams, almost any desired intensity ratios at the respective detectors can be achieved with only a few branching fibers by a skillful cascading of the branching fibers. In this way, it can be ensured that a sufficient signal-to-noise ratio is present at each detector. In particular in the case that a plurality of branching fibers are arranged one behind the other, which divide the intensity of the measurement light into the various partial measurement light beams at an intensity ratio of either 50:50 or 75:25, all intensities relevant for a typical measurement can be achieved at the respective detectors.In this case, in a cascade of branching fibers, in particular the branching fibers which are arranged closer to the detectors have a larger diameter than the branching fibers which are arranged further away from the detectors. In particular, the diameter of the branching fibers in a cascade of branching fibers can increase from branching fiber to branching fiber, wherein here too it applies that the diameter of the starting parts of the last branching fiber is not larger than the surface of the detectors assigned to them in each case.According to a further development of the invention, the device comprises an evaluation unit which is designed to calculate the first signal detected by the first detector with the second signal detected by the second detector. Alternatively or additionally, the evaluation unit can also be designed to calculate the first or second signal detected by the first or second detector with the third signal detected by the third detector. In particular, a quotient is formed from the first signal and the second signal. Alternatively or additionally, the quotient can also be formed from the first or second signal and the third signal. In particular, the first signal can be detected in the first wavelength range, for example in the IRA, and the second signal can be detected in the second wavelength range, for example in the IRB. This makes it possible to eliminate or at least attenuate intensity-dependent effects. This is advantageous in particular when using scanner optics, since here the measurement light impinges on the first movable mirror of the scanner optics at different angles depending on the position of the measurement spot on the workpiece. In addition, the measurement spot is moved through the scanner optics. In this case, it may occur that the laser spot is not located in the center of the measurement spot. This can lead to measurement errors in which incorrect signal intensities are measured. In particular, the signal intensities may seem to vary with the direction in which the measurement spot moves. By forming the quotient from the first and second signals, these effects can be at least partially eliminated. In addition, with the aid of the Planck radiation spectra, the temperature at the location of the measurement spot can be determined at least approximately from the first signal measured in the first wavelength range and the second signal measured in the second wavelength range, which, however, presupposes a complicated calibration. By contrast, by forming the quotient, a temperature change can be established in a simple manner.Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawing.The invention is schematically illustrated in the drawing on the basis of exemplary embodiments and is described below with reference to the drawing.Brief Description of the DrawingsFIG. 1 is an overview of an apparatus for monitoring a laser processing process according to the invention, and a laser processing optical unit and a workpiece on which the laser processing process is carried out; FIG. 2 shows a schematic sketch of an embodiment of the invention; FIGS. 3 a) and b) show a first and a second signal, respectively, as can be detected with the device according to the invention, as well as a quotient calculated from the first and second signals.Embodiments of the InventionFIG. 1 shows an apparatus 1 for monitoring a laser machining process and a laser beam 3 which is directed onto a workpiece 4. The laser processing process can be a laser welding process in which two part workpieces are joined to one another along a weld seam 5. The laser beam 3 can be directed onto the workpiece 4 via an optical arrangement 8 designed as a scanner optical system with a first movable mirror 6 and a second movable mirror 7. The optical arrangement 8 can comprise further optical elements, such as, for example. These lenses and / or deflection mirrors are not shown in FIG. 1. The optical arrangement 8 is also designed to conduct measurement light 9, which originates from a measurement spot 10, to a first branching fiber 11. In this case, the measurement light 9 and the laser beam 3 are aligned, in particular, coaxially with respect to one another at the same position on the workpiece 4. The measurement light 9 has reflected and scattered portions of the laser radiation and also process radiation, for example as a result of heating of the material or plasma which is produced, the intensity and / or bandwidth of which changes depending on the state of the part workpieces. A beam splitter 39 can be arranged between the branching fiber 11 and the optical arrangement 8, at which the measurement light 9 is transmitted, so that it further reaches the branching fiber 11 while the laser beam 3 is reflected (or vice versa). The measurement light 9 is divided by the first branching fiber 11 into a first partial measurement light beam 12 and a second partial measurement light beam 13, which are schematically shown as arrows in FIG. 1. The first partial measurement light beam 12 passes through a first filter element 14, so that a first filtered partial measurement light beam is produced, which is then detected by a first detector 16. The second partial measurement light beam 13 passes through a second filter element 15, so that a second filtered partial measurement light beam is produced and is then detected by a second detector 17. An evaluation unit 35 is connected to the first and second detectors 16, 17, which is designed to calculate the first and second signals with one another.FIG. 2 schematically shows the mode of operation of an embodiment of the device according to the invention.The measurement light 9 is divided in the first branching fiber 11 into the first partial measurement light beam 12 having a first intensity and the second partial measurement light beam 13 having a second intensity. The first and the second intensity differ from one another--in FIG. 2, it is schematically indicated by the thickness of the arrows that the first intensity is greater than the second intensity. The first branching fiber 11 has a first input part 32 and a first and second output part 33, 34. The first partial measurement light beam 12 is coupled into a second input part 36 of a second branching fiber 24 and is divided there into a third partial measurement light beam 18 and a fourth partial measurement light beam 19. The second branching fiber 24 has third and fourth output parts 37, 38. The third and fourth output part 37, 38 of the second branching fiber 24 is designed to guide and couple out the third and fourth partial measurement light beams 18, 19. The third partial measurement light beam 18 then passes through the first filter element 14, which filters the third partial measurement light beam 18 in a first wavelength range, so that a first filtered partial measurement light beam 25 is produced. This is detected by a first detector 16. Similarly, the fourth partial measurement light beam 19 passes through a third filter element 20, so that a third filtered partial measurement light beam 27 is produced, which is detected by a third detector 23. The second partial measurement light beam 13 passes through the second filter element 15, so that a second filtered partial measurement light beam 26 is produced, which is detected at the second detector 17. It is understood that in this exemplary embodiment, the third partial measurement light beam 18 is part of the first partial measurement light beam 12.It is also possible to divide the measurement light 9 into four partial measurement light beams. In this case, for example, a third branching fiber can be arranged downstream of the second branching fiber 24, which is designed to split the third or fourth partial measurement light beam 18, 19 into two further partial measurement light beams. In principle, the intensity of the measurement light 9 is divided in such a way and the sensitivity of the detectors is designed in such a way that the optimum signal-to-noise ratio arrives at each detector, the total intensity available in the respective wavelength ranges being taken into account.FIGS. 3 a) and b) show a first signal 27 which was detected by the first detector 16, a second signal 28 which was detected, for example, by the second detector 17, a laser signal 30, and a quotient 29 from the first signal 27 and the second signal 28. The laser signal 30 reflects the intensity of the laser reflex as a function of time. By a suitable selection of the first filter element, the first signal 27 can image a first wavelength range, which is located, for example, in the IRA, of the measurement light, while the second signal 28 images a second wavelength range, for example, in the IRB. Alternatively, the two signals can also be recorded in different regions of the IRB. In FIG. 3 a, a variation of the signal intensity as well as of the laser back reflection intensity in the encircled region 31 is shown in all three signals 27, 28, 30 and in the quotient 29. In FIG. 3 b, variations are shown in all three signals 27, 28, 30 in the circled region 31. However, the quotient 29 has no or only slight fluctuations in the region 31. The machined workpiece does not show any defect. If, therefore, the quotient 29 is also evaluated in addition to the first signal 27, the second signal 28 and the laser signal 30, pseudo-errors can be excluded in many cases, while at the same time actual errors are detected with great accuracy.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedDE 10 2017 002 922

[0003]

Claims

Device (1) for monitoring a laser processing process, comprising - a first branching fiber (11), which has a first input part (32) and a first and a second output part (33, 34) and which is designed to split measurement light (9) entering the first input part (32) into a first partial measurement light beam (12) and into a second partial measurement light beam (13), wherein the first output part (33) is designed to guide and decouple the first partial measurement light beam (12) and the second output part (34) is designed to guide and decouple the second partial measurement light beam (13), - a first and a second filter element (14, 15), wherein the first filter element (14) is arranged and designed to filter at least parts of the first partial measurement light beam (12) decoupled from the first output part (33) in a first wavelength range, such that a first filtered partial measurement light beam (25) is produced, and the second filter element (15) is arranged and designed to filter at least parts of the second partial measurement light beam (13) coupled out of the second output part (34) in a second wavelength range, such that a second filtered partial measurement light beam (26) is produced, - a first and a second detector (16, 17), wherein the first detector (16) is arranged and designed to at least partially detect the first filtered partial measurement light beam (25) and the second detector (17) is arranged and designed to at least partially detect the second filtered partial measurement light beam (26).The device (1) according to claim 1, wherein the first branching fiber (11) is configured to split the measurement light (9) into the first partial measurement light beam (12) and the second partial measurement light beam (13) with equal intensitiesThe device (1) according to claim 1, wherein the first branching fiber (11) is configured to split the measurement light (9) with different intensities into the first partial measurement light beam (12) and the second partial measurement light beam (13).The device (1) according to any one of the preceding claims, wherein the device (1) comprises an optical arrangement (8) which is configured to couple the measurement light (9) into the first branching fiber (11).The apparatus (1) according to any of the preceding claims, wherein the optics arrangement (8) comprises at least one movable mirror (6, 7).Device (1) according to one of the preceding claims, wherein the detectors (14, 16) are designed as photodiodes, in particular as silicon diodes and / or InGaAs diodes.Device (1) according to one of the preceding claims, wherein the device (1) has a second branching fiber (24) with a second input part (36) and a third and fourth output part (37, 38), and wherein the first and second branching fibers (11, 24) are arranged and aligned with one another such that the first or the second partial measurement light beam (12, 13) can be coupled into the second input part (36), and wherein the second branching fiber (24) is designed to split the first or second partial measurement light beam (12, 13) coupled into the second input part (36) into a third and fourth partial measurement light beam (18, 19), and wherein the third output part (37) is designed and arranged to guide and decouple the third partial measurement light beam (18) and the fourth output part (38) is designed to guide and decouple the fourth partial measurement light beam (19).Device (1) according to Claim 7, having a third filter element (20), wherein the third filter element (20) is arranged and designed to filter the third or fourth partial measurement light beam (18, 19) coupled out from the third or fourth output part (37, 38) in a third wavelength range, so that a third filtered partial measurement light beam (27) is produced, and wherein the device (1) has a third detector (23) which is arranged and designed to at least partially detect the third filtered partial measurement light beam (27).Device (1) according to one of the preceding claims, having an evaluation unit (35) which is designed to calculate a first signal (27) detected by the first, second or third detector (16, 17, 23) with a second signal (28) detected by the first detector (16), the second detector (17) or the third detector (23), wherein the detector (16, 17, 23) which detects the first signal (27) is different from the detector (16, 17, 23) which detects the second signal (28).

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