WO2013113479A1

WO2013113479A1 - Method for regulating a laser cutting process and laser cutting machine

Info

Publication number: WO2013113479A1
Application number: PCT/EP2013/000193
Authority: WO
Inventors: Tim Hesse; David Schindhelm
Original assignee: Trumpf Werkzeugmaschinen Gmbh + Co. Kg
Priority date: 2012-01-30
Filing date: 2013-01-23
Publication date: 2013-08-08
Also published as: CN104271307B; DE102012100721B3; CN104271307A

Abstract

The invention relates to a method for regulating a laser cutting process, comprising the steps: laser cutting a workpiece by means of a focused laser beam, forming a cutting gap on the workpiece; detecting the laser power of laser radiation reflected back during the laser cutting of the surface of the workpiece adjacent to the cutting gap; and regulating the detected laser power (P_L,R) to a target value (P_L,R,SOLL) at which the laser power (P_L,R) takes on a minimum value (P_L,R,MIN) or has a predetermined difference (ΔP_L,R) from the minimum value (P_L,R,MIN). The invention also relates to a laser cutting machine that is designed to perform said method.

Description

Method for controlling a laser cutting process and laser cutting machine

The present invention relates to a method for controlling a

Laser cutting processes and a laser cutting machine for laser cutting of workpieces, in particular sheets.

CONFIRMATION COPY In a laser beam cutting process, in addition to other cutting parameters, the focal position of the laser beam has a great influence on the machining quality of the workpiece. So far, the position of the focus position of the laser beam, in particular the position of the beam waist on the workpiece to be machined, usually determined offline. For this purpose, for example, lines can be cut into a test workpiece before the actual laser cutting task. The focus position is varied in discrete steps, so that a comb with different kerf widths is created. Subsequently, the smallest kerf width is determined either manually by the machine operator or by means of an optical sensor. The smallest kerf width is formed where the

Focus position on the workpiece top or possibly in the middle of the workpiece is.

From EP 1750891 B1 a method for focus position control is known in which it is determined offline for different focal positions of the laser beam, whether the edge region of the laser beam comes into contact with the workpiece. For this purpose, a cutting gap is first cut into a workpiece, where appropriate, the focus position is tuned. Then, the focus adjustment is changed and the beam is redirected (at the same location) into the kerf, and the focus position can also be tuned. In order to detect whether the laser beam comes into contact with the workpiece, radiation emitted by the workpiece or a plasma or process light can be detected.

From DE102009059245B4 it is known to determine the focus position during laser processing by the detection of laser radiation reflected and / or scattered on the workpiece around a processing point and by radiation emitted by at least two alignment light sources and reflected back at the workpiece to be machined. In DE102009059245B4 reference is made inter alia to DE10248458B4, which discloses a method in which a focusing optics arranged in a machining head is displaced such that a portion of radiation coming from the region of the interaction zone between the laser beam and the workpiece assumes a maximum value. This maximum value is reached when the focus position of the laser beam is optimal relative to the workpiece for processing. Object of the invention

The object of the present invention is to provide a method for controlling a laser cutting process and a laser cutting machine, in which the cutting quality can be optimized during a laser cutting process.

OBJECT OF THE INVENTION According to a first aspect, this object is achieved by a method comprising the following steps: laser cutting a workpiece by means of a focused laser beam, forming a cutting gap on the workpiece, detecting the power of laser cutting from the surface of the workpiece

Workpiece adjacent to the cutting gap back reflected laser radiation, as well as regulating the detected laser power to a target value at which the laser power assumes a minimum value or has a predetermined difference to the minimum value.

During a laser cutting process always a certain amount of

Laser radiation from the workpiece top reflected back into the beam path of the laser beam, since the geometric propagation of the laser beam on the

Workpiece top is usually larger than the resulting kerf width. The inventors have recognized that in (particularly coaxial) detection of the workpiece top side adjacent to the kerf, i. In the region of the flanks of the laser beam, back-reflected laser radiation is then measured a minimum of laser power, if the laser beam cuts the workpiece with a minimum kerf width. The more defocused the laser beam the

Workpiece top side lights, the more laser radiation is reflected in the edge region of the laser beam from the workpiece top back to the detector, the greater the measured radiation power.

The regulation of the detected laser power can be carried out to a desired value at which the kerf width is minimal. This is typically equivalent to the fact that the focal position (in beam propagation direction) at the top of the Workpiece is located. Depending on the application but may also be cut defocused in order to obtain an optimum cutting result, ie it may be beneficial if the target value of the laser power does not match the minimum value, so that the focus position of the laser beam at the

Workpiece top is located. This is for example when burning or

Melting thicker workpieces is the case.

In a variant of the method, a relationship between the detected laser power and the distance of the focus position of the laser beam to the top of the workpiece is determined in a preliminary step, and the reference is set based on the relationship that the focal position of the laser beam is a desired distance to the top of the workpiece having. In this variant, based on test measurements, e.g. can be performed on a test workpiece, determines a relationship (characteristic) between the measured laser power and the focal position of the laser cutting beam. For this purpose, for example, the distance between a focusing optics for focusing the laser beam and the workpiece and / or the radius of curvature of a possibly

existing Fokussierspiegels changed and the change in the detected radiation power associated with this change in distance are detected in order to obtain the desired characteristic.

In a further development of this variant, the relationship for different feed rates of a relative movement between the laser beam and the workpiece in the formation of the cutting gap is determined on the workpiece. As a rule, the intensity of the laser power reflected back on the workpiece depends on the feed rate during laser cutting. These

Feed dependency can be determined during calibration of the laser cutting machine and taken into account during process control. For this purpose, for example, a plurality of characteristic curves can be measured on a test workpiece, which at each different feed rate the

Map relationship between the back reflected laser power and the focus position of the laser beam.

In a further variant takes place in a previous step Laser cutting of a workpiece (test cut) to form a cutting gap, wherein the distance of the focal position of the laser beam to the top of the workpiece varies and the intensity of the reflected laser power is continuously measured or detected. Subsequently, the burr formation at the kerf or along the cut edges is examined to a range of values of the detected

To determine the laser power at which the cut edge or the cutting gap is burr-free. The cut quality or burr formation can be assessed automatically by means of a suitable optics and evaluation or, if necessary, manually by a machine operator. The setpoint for the reflected laser power is a value that lies within a range of values at which the kerf has a burr-free cut edge (more precisely burr-free cut edges) in the test cut. The test cut (for example in the form of a line as a cut contour) can be made on the workpiece on which the subsequent laser cutting process also takes place. Alternatively, the test cut can be made on another workpiece (test piece) which is the same

Material properties (and typically the same thickness) as that

having cutting workpiece.

The setpoint value is preferably the mean value of the value range of the reflected

Laser power determined in which the kerf has a burr-free edge.

Typically, the cut quality is good in a certain range of laser power (corresponding to a range of values of the distance between focus position and workpiece top), i. the cut is (essentially) burr-free. A desired value of the laser power, which forms the mean value of this power range, enables process-reliable control of the laser cutting process, in which any overshoot that may occur does not lead to negative cutting results.

In a further variant, at least one parameter of the laser cutting process, which influences the focal position of the laser beam, is changed to regulate the detected laser power to the target value. For example, the distance between a focusing optic and the workpiece, or the optical properties (e.g., focal length) of an adaptive focusing optic may be appropriately adjusted. Additionally or alternatively, other

Process parameters of the laser cutting process, eg the feed rate or the beam source power of the laser source, used as manipulated variables and adapted appropriately.

In another variant, the reflected back from the workpiece

Laser radiation for the purpose of detection by means of a decoupling element coupled out of the beam path of the laser beam. In this type of detection, which is also referred to as coaxial, the portion of the laser radiation power reflected back from the workpiece into the beam path of the laser beam is detected by decoupling it from the beam path. If necessary, a partially transmissive mirror serving only as an outcoupling element in an area at the edge of the

Beam path transmits a portion of the incident laser radiation. Alternatively, a deflecting mirror can be dimensioned so small that the back-reflected laser radiation is not deflected by this and hits a arranged behind the deflection mirror detector surface.

In a particularly favorable embodiment serves as a decoupling a aligned at an angle to the laser beam axis scraper mirror. The scraper mirror is a hole mirror whose centric

Passage opening is typically arranged centrally on the optical axis of the laser beam. If the diameter of the passage opening is greater than the maximum diameter of the machining beam (with an arrangement at about 45 ° to the beam axis, the diameter is typically more than 1.5 times the maximum diameter of the machining beam), the laser beam can pass through unhindered propagate the hole in the scraper mirror. The reflected back laser radiation hits the reflective surface of the scraper mirror outside the passage opening and is reflected by this to the detector. The angle at which the scraper mirror is aligned to the laser beam axis, for example, be at 45 °, so that the back-reflected laser radiation is coupled out at an angle of 90 ° from the beam path.

The regulation of the laser cutting process can be done with a high time resolution, since it is sufficient for the power measurement, a single integral reading for the incident on the detector surface laser power capture, ie there is usually no spatially resolved detection of the laser power necessary. A high time resolution can in this case with a fast

Power detector or power measuring head are achieved, whose measuring principle based on the thermoelectric effect (Seebeck effect). A suitable one

Power measuring head, which is designed as an atomic layer detector, for example, for the laser wavelength of a C0 ₂ laser from the company HTS ForTech GmbH offered (see www.fortech-hts.com).

Another aspect of the invention is realized in a laser cutting machine for laser cutting workpieces, comprising: a laser cutting head for

Aligning a laser beam on the workpiece, a focusing device for focusing the laser beam at a focus position at a distance from

Top of the workpiece, at least one drive means for generating a relative movement between the laser cutting head and the workpiece for

Forming a cutting gap on the workpiece, a detector for detecting the power of laser radiation when laser cutting from the surface of the workpiece adjacent to the cutting gap back reflected laser radiation, and a control device, which is configured to control the detected laser power to a desired value or programmed, wherein the Setpoint the laser power takes a minimum value or has a predetermined difference to the minimum value.

With the help of the control device, the cutting quality of the laser cutting process can be optimized. Thus, the desired value of the detected laser power (and thus the focus position) can be set or regulated in such a way that a (substantially) burr-free cut takes place.

In one embodiment, the control device is designed or programmed, based on a predetermined relationship between the detected laser power and the distance between the focus position of the laser beam to the top of the workpiece set the desired value so that the focal position of the laser beam has a desired, predetermined distance from the top of the workpiece , Depending on the application, the optimum focus position, ie the optimum distance of the focus position from the top of the workpiece, can vary. On the basis of the predetermined relationship (characteristic), which can be determined eg in previous test cutting processes, the Control to a defined, different from the top of the workpiece focus position done. The predetermined relationship (characteristic curve) is typically stored in a memory device associated with the control device (eg in the form of a table or the like). It goes without saying that, if necessary, a plurality of characteristic curves can be stored in the memory device, which were recorded at respectively different cutting parameters. In this case, the control device can select a suitable characteristic curve for the relationship between the detected laser power and the focus position on the basis of the cutting parameters assigned to a respective laser cutting process.

In a further development, the control device is designed or programmed, the desired value as a function of a speed of the means of

Drive device defined relative movement between the laser cutting head and the workpiece set. The proportion of the laser power reflected back depends on the feed rate in the laser cutting, so that it is favorable for the relationship between the detected laser power and the

Focus position several characteristics with different ones

Feed rates are determined and stored in the control device or at another point at which access can be made by the control device.

In a further embodiment, the control device is designed or

programmed to control the detected laser power to the target value to change at least one parameter of the laser cutting process, which affects the focus position of the laser beam relative to the workpiece. Since the laser beam emerges focused from the laser cutting head, the focus position can be changed, for example, by changing the distance between the laser cutting head or the focusing optics in the cutting head and the workpiece. But other parameters of the laser cutting process can be used as manipulated variables, for example, the feed rate or the beam source power.

In a further embodiment, the laser cutting machine additionally comprises a decoupling device for decoupling the back-reflected laser radiation from the beam path of the laser beam. In this way, a so-called coaxial detection can take place, ie, the laser radiation reflected back into the beam path of the laser beam can be decoupled and detected. However, it is understood that the radiation power reflected back can also be detected in another way, for example by observing the instantaneous machining location at which the laser beam strikes the workpiece by means of a detector arranged coaxially annularly around the machining head.

In an advantageous development, the decoupling device is designed as a scraper mirror aligned at an angle to the laser beam axis. Of the

Scraper mirror allows in a particularly advantageous manner a decoupling of the workpiece adjacent to the cutting gap back reflected laser radiation, since the reflected back laser radiation from the edges or from the edge of the

Intensity distribution of the laser beam on the workpiece originates from the

Beam axis is spaced and can be coupled out of the beam path in a simple manner by means of a suitably positioned scraper mirror.

In another embodiment, the (power) detector is on

thermoelectric detector, i. a detector which uses the thermoelectric effect (Seebeck effect) to measure power. In particular, fast power detectors can be realized by the use of thin layers, e.g. in the form of nuclear detectors. Such detectors enable a detection of the detected laser power with a high time resolution and thus a particularly fast control of the laser cutting process.

Further advantages of the invention will become apparent from the description and the drawings. Likewise, the features mentioned above and the features listed further can be used individually or in combination in any combination. The embodiments shown and described are not to be understood as exhaustive enumeration, but rather have exemplary character for the description of the invention.

Show it: a schematic representation of an embodiment of a laser cutting machine according to the invention in the cutting machining of a workpiece, Fig. 2a, b are schematic representations of a beam profile and a

Intensity distribution of the laser beam at the surface of the workpiece at two different focal positions, a schematic representation of a laser cutting head having a detector for laser radiation reflected back from the workpiece, a graph of the laser power measured by the detector as a function of the focus position of the laser beam, and a schematic Representation of a control loop for controlling the focus position of the laser beam to a target value.

1 shows a machine tool in the form of a laser cutting machine 1 for laser cutting with a CCV laser or a solid-state laser as a beam generator 2, a laser processing head 4 and a workpiece support 5. A beam path 3 of the laser cutting beam 6 via a beam guide by means of (not shown) deflection of the C0 ₂ laser or guided by a light guide cable from the solid-state laser to the laser processing head 4. Of the

Cutting beam 6 is arranged by means of a machining head 4

Focusing optics focused and aligned in the present example perpendicular to the surface 8a of a workpiece 8 in the form of a sheet, i. the beam axis (optical axis) of the laser beam 6 emerging from the laser processing head 4 extends in the Z direction perpendicular to the workpiece 8, which is mounted on the workpiece support 5 for the cutting operation, which forms a working plane (XY plane).

For laser cutting of the workpiece 8 is first pierced with the laser beam 6, ie, the workpiece 8 is melted point-shaped or oxidized at one point and the resulting melt is blown out. following the laser beam 6 and the workpiece 8 are moved relative to each other, so that a two-dimensional processing path is formed in the form of a continuous cutting gap 9, along which the laser beam 6, the workpiece 8 is severed. Both grooving and laser cutting can be assisted by adding a gas. As cutting gases 10 oxygen, nitrogen, compressed air and / or application-specific gases can be used. Which gas is ultimately used depends on which materials are cut and what quality requirements are placed on the workpiece 8. Resulting particles and gases can be sucked by means of a suction device 11, which is connected to a suction chamber, which is located below the workpiece support 5.

In order to process the workpiece 8 along a two-dimensional

Machining path (corresponding to a kerf 9) in the XY plane

to perform, the machining head 4 is at a in the Y direction

extending portal 12 by means of a conventional drive unit 7b guided linearly displaceable, wherein the machining head 4 typically over the entire width of the workpiece support 5 and the workpiece 8 can be moved. The portal 12 is displaceable in the X direction by means of a conventional drive unit 7a (for example a linear drive), so that the laser processing head 4 can also be moved over the entire length of the workpiece support 5. Of the

Processing head 4 is in the present example additionally by means of another drive unit 7c in the Z direction, i. perpendicular to the workpiece 8 movable to change the Fokusiage of the focused laser beam 6 during laser cutting can.

Fig. 2a, b show a detail of the laser processing head 4 and the workpiece 8 at two different focus positions of the laser beam 6. The focus position is defined here by the minimum extent (beam waist) of the laser beam in a plane perpendicular to the beam propagation direction. In Fig. 2a, the focus of the laser beam 6 is located in the laser beam direction exactly at the top 8a of the workpiece 8, which is defined below (arbitrarily) as a zero position, ie in the focal position shown in Fig. 2a Z _F = 0 applies. In Fig. 2b is the beam waist and thus the focus position at a distance d from the workpiece 8, that is Z _F = d. If the focus position does not coincide with the zero position shown in FIG. 2a, the laser beam 6 does not impinge on the workpiece 8 in the region of its smallest extent, but with a correspondingly enlarged diameter (see FIG. Accordingly, the width of the

Cut gap ai in Fig. 2a minimal and thus less than the kerf width a ₂ in the case shown in Fig. 2b (ai <a ₂ ). As can also be seen in FIGS. _{2 a} , b, the laser beam 6 has a (for example Gaussian) intensity distribution, l _2, on the upper side 8 a of the workpiece 8, the geometric extent of which is greater than the respective kerf width ai, a ₂ , The laser radiation or laser power at the edge of the intensity distribution \, l ₂ (from the hatched areas) thus does not contribute to the formation of the

Cutting gap 9 at is at least partially reflected back into the beam path of the laser beam 6 and can be used to determine the focus position of the laser beam 6.

A possibility for detecting the laser radiation 13a, 13b reflected back from the upper side 8a of the workpiece 8 (from the areas hatched in FIG. 2a, b), which are reflected within the laser beam 13a, 13b, will be described below with reference to FIG

Laser cutting head 4 is done. The laser cutting head 4 has a deflection mirror 15 for deflecting the laser beam 6 fed from the beam source 2 along a beam axis 20 in the direction of the workpiece 8. A focusing lens 16 arranged downstream of the deflecting mirror 15 serves to focus the laser beam 6 on the workpiece 8. The focusing can not be seen in FIG. 3, since only the beam axis 20 is shown. A conventional driving device 17 (for example, a linear motor or the like) allows the focusing lens 16 to be displaced and thus a change in the focal position ZF of the laser beam 6 in the Z direction.

The laser radiation 13a, 13b reflected back from the two edges of the cutting gap 9 passes through the focusing lens 16 and the deflecting mirror 15 and impinges on a plane scraper mirror 19 arranged in the beam path 3 which is aligned (tilted) at an angle α of 45 ° to the laser beam axis 20 ). The scraper Mirror 19 has a centric passage opening whose diameter in the present example is dimensioned such that it is at least approximately 1.5 times the maximum diameter of the laser beam 6. In this way it is ensured that the laser beam 6 can propagate unhindered through the passage opening of the scraper mirror 19 therethrough. By contrast, the laser radiation 13a, 13b reflected back from the workpiece 8 is coupled out of the beam path of the laser beam 6 at an angle of 90 ° and impinges on a detector 18. The detector 18 in the present example is a fast power detector (power Measuring head, eg in the form of an atomic layer detector), which detects the laser power P _L , R with high temporal resolution. The

Messp nzip the power detector 18 is based in this example on the thermoelectric effect.

FIG. 4 shows the dependence of the laser power P _L , R (integrally) measured by the detector 18 as a function of the focal position Z. FIG. As can be clearly seen in FIG. 4, the detected laser power P _L , R has a minimum PL, R, MIN at a focal position Z _F = 0 at which the laser beam focus is located on the upper side 8a of the workpiece 8 (cf. Fig. 2a). In this case, both the kerf width ai and the back surface 8a of the workpiece 8 are reflected

Laser radiation 13a, 13b minimal. In the case shown in Fig. 2b, in which the focal position Z _F is at a non-zero distance d from the top 8a of the workpiece 8, the back-reflected laser power P _L , R is greater, namely by an amount AP _L , R. Likewise, the back is reflected

Laser power P _L, R larger when the focus position Z _{F is} below the top 8a of the workpiece 8.

The relationship between the back-reflected laser power P _L , R and the focus position Z shown in FIG. 4 can be determined before carrying out the laser cutting process on the basis of test measurements in which the focus position Z _{F is} shifted by moving the focusing lens 16 in the Z direction by means of this associated drive means 17 and / or by displacement of the laser cutting head 4 in the Z direction by means of the associated drive 7c is varied. In this case, the minimum of the detected laser power is assigned to that focal position Z _F = 0 at which the laser beam 6 is focused on the upper side 8 a of the workpiece 8. Since the relationship shown in Fig. 4 depends on the (instantaneous) feed rate v (see Fig. 1) with which the laser beam 6 is moved over the workpiece 8, a plurality of characteristic curves can be produced at a test workpiece

Different feed rates v are measured to reflect the respective relationship between the reflected back laser power P _L , R and the focus position Z _F. It is understood that a corresponding recording of several characteristic curves can also be carried out for further cutting parameters which have a (significant) influence on the relationship between reflected laser power P _L , R and focus position Z _F.

The respective characteristic curves can be stored in a memory device which is part of a control device 14 shown in FIG. 1, which performs control and control tasks for the laser cutting machine 1. The

Control device 14 can be embodied as a hardware component (computer) with suitably adapted software or in another manner (for example as a programmable hardware component in the form of a "Field Programmable Gate Array", FPGA, etc.) Execution of a machining program designed which the movement of

Laser processing head 4 for forming a desired cutting contour (a cutting gap 9 with desired geometry) on the workpiece 8 controls or regulates.

The regulating device 14 can also be used to control the laser cutting process to achieve an improved cutting result (eg to obtain burr-free cut edges at the kerf 9), as will be described below with reference to a control loop shown in FIG. 5 whose components form parts of the laser cutting machine 1 , The control variable used is the laser power P _L , R reflected back (see FIG. 4). For this controlled variable, first of all a setpoint value PL.R.SOLL is set which, in the present example, coincides with the minimum P _L , R MIN of the laser power P _L , R reflected back, ie it is intended to control to a minimum kerf width or to a focus position at the top (Z _F = 0) of the workpiece 8 done. With the aid of the detector 18 as a measuring element, the instantaneously reflected back laser power P _LI R, IST is detected and fed to a controller 21, which may be implemented in the control device 14, for example. Of the Controller 21 is designed to minimize the control difference, ie the deviation PL, R, IST - PL,, SOLL.

For this purpose, different manipulated variables of the laser cutting process can be changed, in particular those manipulated variables which have a (direct or indirect) influence on the focal position Z _{F of} the laser beam 6. For example, the controller 21 can drive the drive device 17 of the focusing lens 16 or the drive device 7c for moving the laser cutting head 4 in the Z direction in order to influence the laser cutting process P (as a controlled system) such that the control deviation, ie the difference between measured back reflected laser power P _L , R, IST and the setpoint PL.R.SOLL is changed towards a minimum. It is understood that also parameters of

Laser cutting process, which affect the focus position Z _F only indirectly, for example, the feed rate v and the power P _L , Q of the radiation source (see Fig. 1) can be included in the scheme.

The setpoint PL.R.SOLL may vary depending on the application

(Sheet thickness, geometry of the contours to be cut, type of material to be cut, etc.) are set so that a burr-free cut takes place. It is understood that the scheme is not necessarily reflected to the minimum

Laser power PL.R.MIN must be done: If defined defocused cut should be able to control to a defined setpoint of the back-reflected

Laser power PL.R.SOLL be greater than the minimum value PL.R.MIN by a predetermined amount AP _L , R is greater. If the focal position Z _F required for a burr-free cut or a high quality of cut is known in a particular application, then the definition of the desired value PL, R.SOLL can take place based on the relationship shown in FIG.

It should be understood that the relationship shown in FIG. 4 need not necessarily be resorted to when a direct relationship between optimal

(burr-free) cutting result and setpoint PL.R.SOLL which reflected back

Laser power P _L , R is known. For example, in the event that a minimum kerf width or a focal position on the upper side 8a of the workpiece 8 (Z _F = 0) is desired, the controller 21 does not specify the desired value PL, R, SOLL from the outside but the controller 21, the setpoint specification can be made that the actual value of the reflected laser power PL, R.IST should be minimized.

It is not necessary to resort to the relationship shown in FIG. 4 if the value range of the back-reflected laser power P _L , R in which the cutting process leads to a burr-free cut edge is first determined in a test cut. For this purpose, in the test section, the distance between the focal position Z _L to the top 8a of the (test) workpiece 8 varies and the burr formation on

Cutting gap can be determined for example by an operator or an (optical) sensor. In this case, for example, the mean value of the value determined during the test cut can be used as desired value PL, R, SET for the reflected laser power P _L , R

Value range at which no burring occurs.

It is understood that control of the laser cutting process in the above-described manner can not be performed only on a laser cutting machine as shown in FIG. 1. Rather, the control described above can also be carried out on a laser cutting machine in which the

Workpiece does not rest, but is moved in at least one spatial direction. Also, the control process described above is not limited to laser cutting machines for machining substantially plate-shaped workpieces.

Rather, a detection of the laser radiation reflected at the top of a substantially tubular or a three-dimensionally variably shaped workpiece can also be carried out.

Claims

claims

A method of controlling a laser cutting process, comprising the steps of:

Laser cutting a workpiece (8) by means of a focused laser beam (6), forming a cutting gap (9) on the workpiece (8),

Detecting the laser power (PL.R) of laser radiation (13a, 13b) reflected back in laser cutting from the surface (8a) of the workpiece (8) adjacent to the kerf (9), and

Controlling the detected laser power (PL.R.IST) to a setpoint (PL, R, SOLL). wherein the laser power (PL, R) assumes a minimum value (PL.R.SOLL = PL, R, MIN) or has a predetermined difference (AP _L , R) to the minimum value (PL, R, MIN).

A method according to claim 1, wherein in a preceding step, a relationship between the detected laser power (PR.L) and the distance (d) of the focus position (Z _L ) of the laser beam (6) to the top surface (8a) of the workpiece (8) is determined and based on the relationship of the target value (PL.R.SOLL) SO, that the focal position (Z _L ) of the laser beam (6) has a desired distance (d) to the top (8a) of the workpiece (8).

The method of claim 2, wherein the relationship is different

Feed speeds (v) between the laser beam (6) and the workpiece (8) in the formation of the cutting gap (9) on the workpiece (8) is determined. 4. The method of claim 1, wherein in a preceding step

Test cut on a workpiece (8) to form a cutting gap (9) is performed, wherein the distance (d) of the focal position (Z _L ) of the laser beam (6) to the top (8a) of the workpiece (8) is varied, and wherein setpoint

(PL, R, SOLL) a value of the reflected laser power (P _{L, R} ) is set, in which the kerf (9) has a burr-free edge.

5. The method of claim 4, wherein the setpoint value (PL, R, SOLL) is an average of a value range of the reflected laser power (P _L, R) is determined, wherein the cutting gap formed in the test cut (9) has a burr-free edge.

6. The method according to any one of the preceding claims, wherein for controlling the detected laser power (PL.R.IST) to the target value (PL.R.SOLL) at least one parameter of the laser cutting process is changed, which the focus position (Z _F ) of the laser beam (6) influenced.

7. The method according to any one of the preceding claims, wherein the from

Workpiece (8) back reflected laser radiation (13a, 13b) for detection by means of a decoupling element (9) from the beam path (3) of the laser beam (6) is decoupled.

8. The method according to claim 7, wherein as a decoupling element under a

Angle (a) to the laser beam axis (20) aligned scraper mirror (19) is used. 9. A laser cutting machine (1) for laser cutting workpieces (8), comprising: a laser cutting head (4) for aligning a laser beam (6) on the

Workpiece (8),

a focusing device (16) for focusing the laser beam (6) at a focus position (Z _F ) at a distance (d) to the upper side (8a) of the workpiece (8), at least one drive device (7a, 7b) for generating a

Relative movement between the laser cutting head (4) and the workpiece (8) for forming a cutting gap (9) on the workpiece (8),

a detector (18) for detecting the laser power (P _L , R) of the

Laser cutting from the surface (8a) of the workpiece (8) adjacent to the cutting gap (9) of the laser radiation (13a, 13b) reflected back, as well as

a control device (14) for controlling the detected laser power (PL, R, IST) to a desired value (PL, R, SET), in which the laser power (PL.R) assumes a minimum value (PL.R.IN) or a predetermined difference (AP _L , R) to the minimum value (PL, R, IN).

10. Laser cutting machine according to claim 9, wherein the control device (14) is formed, based on a predetermined relationship between the detected laser power (P _R , _L ) and the distance (d) of the focal position (Z _L ) of the laser beam (6) to the top (8a) of the workpiece (8) set the target value (PL.R.SOLL) SO such that the focal position (Z _L ) of the laser beam (6) has a desired distance (d) from the upper side (8a) of the workpiece (8) ,

11. Laser cutting machine according to claim 9 or 10, wherein the

Control device (14) is adapted to set the desired value (PL, R, SET) in response to a speed (v) of the relative movement between the laser cutting head (4) and the workpiece (8) generated by the at least one drive means (7a, 7b) ,

12. Laser cutting machine according to one of claims 9 to 11, wherein the

Control device (14) is designed to control the detected laser power (PL.R.IST) to the desired value (PL.R.SOLL) at least one parameter of

Laser cutting process to change the focal position (Z _F ) of the

Laser beam (6) influenced. 13. Laser cutting machine according to one of claims 9 to 12, further comprising: a decoupling device (19) for decoupling the back reflected

Laser radiation (13a, 13b) from the beam path (3) of the laser beam (6).

14. Laser cutting machine according to claim 13, in which the decoupling device is designed as a scraper aligned at an angle (a) to the laser beam axis (20).

Mirror (19) is formed.

15. Laser cutting machine according to one of claims 9 to 14, wherein the detector (18) is a thermoelectric detector.