Method for calibrating a measuring system of a laser processing head

Abstract

The invention relates to a method for calibrating a measuring system (16) of a laser processing head (10) with a mirror (13) rotatable about two axes, wherein the laser processing head (10) has a nozzle (14) through which a working laser beam is directed onto a workpiece, comprising the steps: - directing several measuring laser beams (12) one after the other over the mirror (13) and at different angular positions of the mirror (13) through the nozzle (14); - determining the center point of the outlet opening of the nozzle (14) by determining the distance of the measuring laser beams (12) to the edge of the outlet opening of the nozzle (14); - Directing a measuring laser beam (12) through the determined center of the nozzle (14) and using the associated mirror position as the zero point of the rotational movements of the mirror (13) about its two axes of rotation in the measuring system (16).

Description

[0001] The invention relates to a method for calibrating a measuring system of a laser processing head with a mirror rotatable about two axes, wherein the laser processing head has a nozzle through which a working laser beam is directed onto a workpiece.

[0002] When laser machining a workpiece, the precise alignment of the working laser beam is crucial for the quality of the machining. In addition to the focusing plane of the working laser beam, the lateral position of the laser beam's impact point in the xy direction must also correspond to a target position, which is usually located at the center of the exit opening of the nozzle of the machining head. The measuring system integrated into the laser machining head can precisely determine the relative position of the deflection mirror with respect to its rotational axes, for example, using capacitive position measurement or optical sensors.However, such integrated measuring systems alone cannot determine the absolute position of the mirror's rotation axes in space and the resulting center of gravity of the laser beam in the nozzle with sufficient precision. This is because long tolerance chains exist between the measuring system and the nozzle across a multitude of mechanical and optical elements, and large optical translation levers act between the deflection of the laser beam at the mirror and the laser spot on the workpiece. The mirror's rotation axes must therefore be precisely and reproducibly referenced, at least when starting up the laser processing process. This referencing has usually been done manually to date by sealing the nozzle with an adhesive strip and burning a small hole into the adhesive strip with a laser.The position of the hole relative to the edge of the nozzle opening is checked using a magnifying glass, and the position of the deflection mirror is manually adjusted until the laser beam creates a hole in the adhesive strip at the center of the nozzle. However, this does not allow for automated calibration of the measuring system.

[0003] From WO2009 / 000356 A1, it is known to center a working laser beam within the nozzle of a laser cutting head by tilting a deflecting mirror or by moving a focusing lens in the cutting head. When the laser beam hits the edge of the nozzle's exit opening, this is detected by a photoacoustic sensor. The nozzle center is then calculated from the contact points of the laser beam with the nozzle edge determined in this way.

[0004] JP 2015009263 A and WO 2012 / 101533 A1 describe the centering of the laser beam within the cutting nozzle by contacting the nozzle edge with a laser beam. Contact with the nozzle edge by the laser beam is detected by optical or thermal sensors based on the resulting heat radiation.

[0005] The invention is based on the object of specifying a method for the automated calibration of a measuring system of a laser processing head.

[0006] This object is achieved according to the invention by a method for calibrating a measuring system of a laser processing head with a mirror rotatable about two axes, wherein the laser processing head has a nozzle through which a working laser beam is directed onto a workpiece, comprising the steps: - Directing several measuring laser beams one after the other over the mirror and at different angular positions of the mirror through the nozzle; - Determining the center point of the nozzle outlet opening by determining the distance of the measuring laser beams to the edge of the nozzle outlet opening; - Directing a measuring laser beam through the specific center of the nozzle and using the corresponding mirror position as the zero point of the rotational movements of the mirror around its two axes of rotation in the measuring system.

[0007] The subclaims relate to preferred embodiments.

[0008] According to the method according to the invention, the nozzle of the machining head itself is used to calibrate the measuring system. The center of the nozzle's outlet opening defines the zero point of the mirror's rotational movements about its two axes of rotation. The mirror's rotational movements, starting from this zero point, are precisely recorded by the measuring system integrated into the machining head. The measuring laser beams can originate from a pilot laser integrated into the machining head. Alternatively, the working laser itself can be used for calibration. The measuring laser beams can be working laser beams with a low power, for example, <200 watts. A positioning laser integrated directly into the laser system can also be used preferably.

[0009] In a first embodiment of the method, scanning movements can be performed with the measuring laser beams in different directions perpendicular to the direction of propagation of the measuring laser beams through the nozzle outlet opening. The scattered light reflected when the measuring laser beams hit the edge of the nozzle outlet opening is recorded in each case and the center of the outlet opening can be calculated from the various impact points of the measuring laser beams on the opening edge. Preferably, a measuring laser beam can be guided through the nozzle starting from a zero position of the mirror known from previous processing. This measuring laser beam is then deflected in different directions by the measuring system. As soon as the measuring laser beam hits the edge of the nozzle outlet opening, part of its light is reflected as scattered light, which is easily detected.The center point of the nozzle opening can then be calculated from the coordinates of the impact points of the measuring laser beams on the nozzle opening edge.

[0010] Preferably, these scanning movements can be carried out as linear movements in different radial directions of the nozzle outlet opening and the center point between the contact points of the scans with the edge of the nozzle outlet opening can be calculated.

[0011] To calculate the nozzle opening center point with sufficient accuracy, at least three radial scans can be performed, with the angular separation between the scans being 120°. The accuracy of the calculation can be further increased by performing additional star-shaped scans.

[0012] The scattered light of the measuring laser beams striking the nozzle opening edge can be detected by at least one photodiode or a camera.

[0013] In an alternative embodiment of the process, the measuring laser beams can be used to create markings on a workpiece or on a welding wire. Their positions relative to the edge of the nozzle outlet are recorded by a camera or photodiode. Using image analysis, the center of the nozzle outlet can be determined.

[0014] If a camera is used for image capture, a crosshair can be displayed on the camera. The center of the crosshair is positioned at the center of the nozzle outlet for calibration of the measuring system. If a measuring laser beam is positioned through the center of the crosshair, the corresponding position of the mirror corresponds to the zero position of its rotational movement. The working laser beam can then be positioned and visually controlled using the crosshair. The center of the crosshair marks the tool center point (TCP).

[0015] If, to calibrate a measuring system for a laser welding processing head, the end of a welding wire is marked with a measuring laser beam and the center of the nozzle outlet opening is determined from its position in a camera image, the wire tip and the working laser beam can be positioned on the center of the nozzle outlet opening. This positioning is also possible within a workpiece's machining program.

[0016] The measuring system in the machining head can have various configurations. For example, the measuring system can measure the relative angular positions of the mirror around its rotational axes using capacitive position measurement or optical sensors.

[0017] After determining the center of the nozzle's exit opening, the working laser beam can be directed to this point to enable high-quality machining of a workpiece. This procedure can be performed before and / or during workpiece machining. Before workpiece machining begins, it is used to calibrate the measuring system. During a machining process, it can be performed to check the position of the working laser beam.

[0018] The invention also relates to a laser processing head with a measuring system having a mirror pivotable about two axes and a nozzle through which a working laser beam can be directed onto a workpiece. The measuring system is configured to carry out a method according to the invention. In particular, the processing head can have devices for detecting scattered light from measuring laser beams guided through the nozzle. The processing head can be a laser cutting head or a laser welding head.

[0019] Further features and advantages of the invention will become apparent from the description, the claims, and the drawings. According to the invention, the features mentioned above and those further described can be used individually or in combination in any convenient way. The embodiments shown and described are not intended to be exhaustive, but rather are exemplary in nature for describing the invention. Detailed description of the invention and drawing Fig. 1 shows a schematic representation of a laser processing head with a measuring system; Fig. Figure 2 shows a diagram of the scattered light level detected by a photodiode during a scan with a measuring laser beam through a nozzle of the laser processing head of Fig. 1.

[0020] In Fig. 1 shows a laser processing head 10 according to the invention, which has an entrance opening 11 for a laser beam 12, in this case a measuring laser beam. The measuring laser beam 12 is directed inside the laser processing head 10 by a mirror 13 that can be rotated about two axes to a nozzle 14, on the outside of which a workpiece (not shown here) can be placed. The axes of rotation of the mirror 13 are not shown. The first axis of rotation extends perpendicular to the plane of the drawing, and the second axis of rotation extends parallel to the plane of the drawing. This allows the measuring laser beam 12 to be adjusted in the x-direction and y-direction relative to the nozzle 14 for calibrating the measuring system 16 arranged in a machine control 15, in order to perform scanning movements or to apply measuring markings to a workpiece.

[0021] If scanning movements are performed with the measuring laser beam 12 and the measuring laser beam 12 touches the edge of the opening of the nozzle 14, a portion of its light is scattered, which can be detected by a photodiode 17. The signals from the photodiode 17 are evaluated by an intensity measuring device 18 within the measuring system 16 of the machine control 15. This determines the contact points of the measuring laser beam 12 with the edge of the nozzle 14 and calculates the midpoint between the contact points in an evaluation unit 19. The calculation result is forwarded to a control unit 20 for the rotational movements of the mirror 13, which initiates further measurement scans or, if the center point of the nozzle 14 is calculated with sufficient accuracy, stores the corresponding angular positions of the mirror 13 as the zero point for the rotational movements of the mirror 13 about its rotational axes.

[0022] Fig.Figure 2 illustrates the measurement level curve I of scattered light detected by the photodiode 17 during a scanning movement of the measuring laser beam 12 through the nozzle 14 in the negative and positive radial direction r. The measurement diagram shows a sharp increase in the measurement level I of the scattered light as soon as the measuring laser beam 12 hits the edge of the nozzle 14. The midpoint between the contact points of the measuring laser beam 12 with the nozzle edge provides an approximate value for the nozzle center sought for the calibration of the measuring system 16. To ensure sufficient accuracy in determining the nozzle center, at least three scanning movements with the measuring laser beam 12 in the radial direction r through the nozzle 14 should be performed, with the angular distance between each scanning movement being 120°. The accuracy of the center point calculation can be further increased by further star-shaped scans with the measuring laser beam 12.

[0023] In an alternative approach, markings can be applied to a workpiece or welding wire using the measuring laser beam 12, and their relative position to the nozzle edge can be recorded using a camera. The center point of the nozzle 14 can also be calculated from the distance of the markings from the nozzle edge, and the measuring system 16 can be calibrated to this point. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] WO 2009 / 000356 A1

[0003] JP 2015009263 A

[0004] WO 2012 / 101533 A1

[0004]

Claims

[1] Method for calibrating a measuring system (16) of a laser processing head (10) with a mirror (13) rotatable about two axes, wherein the laser processing head (10) has a nozzle (14) through which a working laser beam is directed onto a workpiece, characterized by the steps: - directing several measuring laser beams (12) one after the other over the mirror (13) and at different angular positions of the mirror (13) through the nozzle (14); - determining the center point of the outlet opening of the nozzle (14) by determining the distance of the measuring laser beams (12) to the edge of the outlet opening of the nozzle (14); - Directing a measuring laser beam (12) through the determined center of the nozzle (14) and using the associated mirror position as the zero point of the rotational movements of the mirror (13) about its two axes of rotation in the measuring system (16). [2] Method according to claim 1, characterized bythat scanning movements are carried out with the measuring laser beams (12) in different directions perpendicular to the direction of propagation of the measuring laser beams (12) through the outlet opening of the nozzle (14) and the scattered light reflected when the measuring laser beams (12) strike the edge of the outlet opening of the nozzle (14) is recorded and the center point of the outlet opening is calculated from the various points of impact of the measuring laser beams (12) on the opening edge. [3] Method according to claim 2, characterized by that the scanning movements are carried out as linear movements in different radial directions of the outlet opening of the nozzle (14) and the center point between the contact points of the scans with the edge of the outlet opening of the nozzle (14) is calculated. [4] Method according to claim 3, characterized bythat at least three radial scanning movements are performed, with the angular distance between the scanning movements being 120 degrees. [5] Method according to one of claims 2 to 4, characterized by that the scattered light is detected by means of at least one photodiode (17) or a camera. [6] Method according to claim 1, characterized by that the measuring laser beams (12) are used to produce markings on a workpiece or on a welding wire, the positions of which relative to the edge of the nozzle outlet opening are detected by a camera or by a photodiode (17). [7] Method according to claim 6, characterized by that a crosshair is displayed in the camera, the center of which is placed on the specific center of the outlet opening of the nozzle (14) for the purpose of calibrating the measuring system (16). [8] Method according to one of the preceding claims, characterized bythat in the measuring system (16) the relative angular positions of the mirror (13) about its axes of rotation are detected by means of a capacitive position measurement or by means of optical sensors. [9] Method according to one of the preceding claims, characterized by that after determining the center of the outlet opening of the nozzle (14), a working laser beam is directed to this point. [10] Method according to one of the preceding claims, characterized by that it is carried out before the start and / or during workpiece machining. [11] Laser processing head with a measuring system (16) and a nozzle (14) through which a working laser beam can be directed onto a workpiece, and with a mirror (13) pivotable about two axes, wherein the measuring system (16) is configured such that it can carry out a method according to one of claims 1 to 10.

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