WO2024223412A1 - Method for monitoring, controlling and regulating laser cutting processes - Google Patents

Abstract

The invention relates to a method for controlling laser cutting processes of a laser cutting machine using an imaging device for imaging a process zone of a laser cutting machine onto an image capture device, the method comprising the steps of: - capturing the radiation emitted from the process zone from a first and at least one second observation direction, the first and second observation directions being different from one another, and imaging, in a spatially separated manner, the radiation captured from the two observation directions in an image capture device; - based on the images, measuring the vertical position (Z) of elements (11, 12, 13, 14, 15, 22) in the process zone and/or measuring the vertical distance between elements (13, 14; 10, 11; 14, 15) using triangulation; - comparing the measured distances with target values and, in the event of deviations, initiating corrective measures and/or safety measures.

Description

Methods for monitoring, controlling and regulating laser cutting processes

The invention relates to a method for monitoring, controlling and regulating laser cutting processes of a laser cutting machine with an imaging device for imaging a process zone of a laser cutting machine onto an image recording device.

Such methods are known in various embodiments from the prior art and are used to monitor the laser cutting process. As a rule, an imaging device displays the three-dimensional structure of the process zone on an image sensor as a two-dimensional image, and this image is then evaluated. If the image is sufficiently accurate, important functional variables and control variables for the laser cutting machine can be derived from this, which can, for example, support feed control or incorrect cut detection during the cutting process. However, with such two-dimensional images, the depth information disappears. This can result in support webs or slugs, for example, being imaged in such a way that the feed control is incorrectly adjusted or an incorrect cut is detected and a machine stop is triggered, since the illumination of a support web can be imaged in a similar way to a cut break.

EP 3 159 093 A1 describes a method for controlling laser cutting processes in which the cutting process is interrupted after a partial machining process and the cutting gap created during the partial machining process is then scanned. Certain process parameters are recorded and compared with target values. If the measured values deviate from the target values, an error message can be issued, the cutting process can be interrupted or continued with changed cutting parameters. Re-machining of the workpiece is also possible. A disadvantage of the known method is the requirement to interrupt the cutting process for the measurements.

The invention is based on the object of enabling a reliable determination of process variables for monitoring, controlling and regulating a laser cutting process even during the processing of a workpiece.

The object is achieved by a method having the features of claim 1.

The detection of the vertical position or the distance of elements in the process zone and thus the detection of the three-dimensional structure of the process zone significantly improves, for example, feed control or miscut detection of a laser cutting machine during the processing process. This is achieved by the process zone preferably being imaged simultaneously under at least two different observation directions. The different directions can preferably run along the plane spanned by the direction of propagation of the processing laser beam and the current feed direction. In this case, an angle for the propagation of the processing laser beam with positive values can be defined as an observation angle for a piercing observation in which the observation is inclined in the direction of the feed. Likewise, an angle for the propagation of the processing laser beam with negative values can be defined as an observation angle for a dragging observation in which the observation is inclined against the feed. The images recorded by the image recording device at the different angles are compared with one another and the vertical position or the mutual distance of the elements emitting radiation is also measured by triangulation. The measurement accuracy in the vertical direction depends on the choice of the different observation directions as well as on the size, shape, topology and radiation characteristics of the radiating elements. In addition, the optical Resolution and the size of the aperture of the image recording device influence the measurement accuracy.

In addition to the cutting gap, metal vapor lamps, objects or structures on the workpiece surface as well as radiating elements below the workpiece can also be detected. Contaminants in the field of view of the processing nozzle can also be detected.

The method can preferably be carried out during, but also before or after, the processing of a workpiece. In particular, when measuring before or after the processing of a workpiece, it is advantageous if the process zone is illuminated and the radiation reflected back from elements in the process zone is imaged.

The measurements are conveniently carried out through the opening of the processing nozzle of the laser cutting machine. The edge of the nozzle opening can preferably be used as a reference for the measurements. This can be clearly seen in the images taken from different viewing directions with appropriately designed nozzles.

The angle between the two observation directions can generally be chosen arbitrarily, advantageously between 1° and 10° and preferably between 3° and 4°. One of the angles can also be 0°, whereby the angles are measured relative to the laser beam direction.

In a first preferred embodiment of the method, the thickness of the workpiece is determined by measuring the vertical position of the start of a cutting front of a workpiece and the vertical position of the end of the cutting front. By determining the start and end of the cutting front under two observation directions, the vertical position of the start and end of the cutting front can be calculated from the lateral offset of these points in the two images. The distance between these positions corresponds to the thickness of the workpiece. By measuring the workpiece thickness in this way, it is possible to check whether an incorrect workpiece, which is usually a sheet, has been placed on the machine. The placement of a double sheet is also detected. In these cases, an error message is issued so that the machine operator can stop production and place the correct workpiece. Production can also be stopped automatically. When cutting corrugated sheets or teardrop sheets, local increases in sheet thickness typically point downwards. If such variations in sheet thickness are detected, the feed rate can be set or regulated so that the sheet is separated safely.

If there are only minor deviations between the target value for the workpiece thickness and the measured value, at least one parameter of the cutting process, the monitoring process or the control process can be adjusted to the measured value or an error message can be issued and/or the cutting process can be stopped. Thicker workpieces, for example, require a slower laser feed than thinner workpieces. If the workpiece thickness only increases or decreases slightly and locally, a poor quality cutting result or even a cut break can be avoided by adjusting the feed rate.

Local increases in the thickness of a sheet can also be caused by support webs for the sheet or material residues on the webs resulting from the processing of the previous sheet. In these cases, the processing of the sheet can be omitted locally and an appropriate error message issued, or the area can be cut with a sufficiently small feed rate. This avoids unnecessary machine stops. It is also possible to provide the user with a selection of possible reactions to a local deviation in the sheet thickness from a target value. The machine's reaction to a sheet thickness deviation can also be made dependent on the machine's operating mode. In a further embodiment of the method, the distance of the processing nozzle to the workpiece surface can be calculated by triangulation by measuring the lateral offset of an element on the workpiece surface in the two images taken under the different observation directions.

The distance between the processing nozzle and a workpiece is usually measured capacitively and the distance is regulated to a setpoint. However, errors in the distance control can occur at the edge of a workpiece, if there are recesses in the workpiece or if there are contaminations on the processing nozzle. If the distance between the nozzle and the workpiece deviates from the setpoint, a poor quality cut can result or the cut can break.

If the distance between the processing nozzle and the workpiece is measured using a method according to the invention, if the measured distance between the workpiece surface and the processing nozzle deviates from the target value, the position of the processing nozzle can be readjusted or the cutting process can be interrupted and the processing nozzle inspected and cleaned or replaced if necessary. The inspection and, if necessary, replacement of the processing nozzle can be carried out automatically. Alternatively, if the distance deviates from a target value, the cutting process can be stopped and an error message can be issued.

Further advantages arise when the distance of elements in the cutting gap to the image recording device or to the processing nozzle or to the top or bottom of the workpiece is measured.

When cutting a workpiece over support bars or other material that is located under the workpiece, whether desired or undesired, an additional luminous phenomenon caused by the bars or the material can occur in the images of the image recording device in the area of the cutting gap. By measuring the luminous phenomenon under different Observation directions, the distance between the image recording device and the luminous phenomenon can be measured. By comparing it with the vertical position of another element, in particular the beginning or end of a cutting front, it can be determined whether the luminous elements are below or above the workpiece or inside the workpiece. If it is determined that the luminous elements are below the workpiece, they come from the webs or previously cut sheet metal residues and are not caused by the workpiece processing.

However, if the distance between the light elements is smaller than the distance between the underside of the workpiece, a cut break can be detected and the cutting process interrupted. The light element is then located in the cutting gap, i.e. it is caused by the machining of the workpiece.

The method can also be used to measure the vertical position of contaminants protruding into the nozzle opening and, for example, to issue an error message with the vertical position of the contaminant.

If an interfering contour is detected in the opening of the processing nozzle on the images taken from different directions, its vertical position can be determined by triangulation. In this way, it is possible to determine whether the interfering contamination is on the underside of the nozzle or inside the nozzle opening, and a corresponding message can be issued. This makes it possible to distinguish, for example, whether the contamination is a chip from a ceramic part that was created when the processing nozzle was screwed into the ceramic part, or whether the contamination is an adhesion to the front surface of the nozzle.

If the vertical position determination shows that the contamination is located within the nozzle opening, a message describing the problem may be displayed along with a suggestion for a corrective action or such a corrective action, such as replacing the nozzle, may be initiated automatically. Preferably, the images taken under the different observation directions can be unpolarized or identically polarization-filtered and in the same spectral range in order to obtain only variations between the images resulting from the different observation directions.

Alternatively, the images can be taken from different observation directions, in different polarization directions relative to the feed direction of the laser or in different spectral ranges. This allows further information to be obtained regarding the cutting front geometry or the temperature in the process zone.

All measurement results determined by the method and the associated observation directions can be made available to a self-learning Kl system, which classifies the process states that can be used for further processing.

In the following, examples of the method according to the invention are described in more detail with reference to the drawing.

The following show:

Fig. 1 shows a schematic cross-section through a process zone of a laser cutting machine;

Fig. 2a shows a cross-section through the process zone corresponding to Fig. 1 with different observation directions along the feed direction marked;

Fig. 2b is a schematic representation of the images of the process zone taken under the different observation angles from Fig. 2a; Fig. 3a-c Images of a process zone of a laser cutting machine under three different observation angles with observation directions inclined along the feed direction.

Fig. 1 shows a schematic representation of a processing nozzle 10 of a laser cutting machine (not shown here) and a workpiece, which here is a sheet 11 and rests on support webs 15. The nozzle has a nozzle opening 10.1 and guides a laser beam (not shown here) in the x direction over the sheet 11. This has an upper side 11.1 and a lower side 11.2. The method according to the invention is intended in particular to examine a cutting front 12 of the sheet 11, which extends from a cutting front start 13 on the sheet upper side 11.1 to a cutting front end 14 on the sheet lower side 11.2. Of particular interest here are the positions in the z direction ZBO of the sheet upper side 11.1 and ZBU of the sheet lower side 11.2, from the difference of which the sheet thickness d can be calculated. Furthermore, the method can be used to determine the vertical positions ZD of the nozzle opening 10.1 and Zs of the tips 16 of the support webs 15 as well as the position of further luminous phenomena below the sheet 11, as is explained in more detail with reference to Fig. 2. The distance D of the nozzle 10 to the sheet 11 can be calculated from the difference between the vertical position ZBO of the sheet top 11.1 and the position ZD of the nozzle opening 10.1. The zero point in the z direction can be defined, for example, in an image recording direction of an imaging device of the laser cutting machine, which is not shown here and which is located above the nozzle 10. This means that the various vertical positions ZD, ZBO, ZBU, ZS correspond to the distance of the individual elements 10.1, 11.1, 11.2 and 16 from the image recording device. However, the zero point can also be set elsewhere, for example on the lower edge of the nozzle.

Fig. 2a shows the representation of the processing nozzle 10, the sheet 11 and the support webs 15 corresponding to Fig. 1 together with the observation angles ai, «2, tilted against the processing direction or feed direction x, under which these elements 10, 11 and 15 after the The observation angles ai, o2 are measured relative to the laser direction, ie the z-direction, and the tilt direction is opposite to the feed direction x of the laser. In the example shown, the first observation angle ai = 0°, ie the radiation emanating perpendicularly from the elements 10, 11 and 15 is recorded. The second observation angle oc2 corresponds to a piercing observation, ie an observation direction inclined opposite to the processing direction x of the laser. Of course, one of the angles ai, oc2 or both could correspond to trailing observations, ie tilted in the feed direction x. Observation directions to the side and in any other direction are also possible.

Fig. 2b shows the images 20, 21 of the image recording device resulting from the two observation angles ai, oc2, wherein image 20 was recorded at the angle ai to the laser beam direction and image 21 at the angle «2. Both images show the nozzle opening 10.1, the top surface of the sheet 11.1, the cutting front 12 and a cutting gap 22 through which a web tip 16 can be seen. In addition, an element or a marking 23 can be seen on the top surface of the sheet 11.1. Since the angle oc2^ is ai, an offset XBO in the x-direction arises in the positions of the elements 23, 13 on the top surface of the sheet 11.1, an offset XBU of the cutting front end 14 on the bottom surface of the sheet 11.2 and an offset Xs of the web tip 16 between the two images 20, 21. The nozzle opening 10.1 serves as a reference for the measurement, i.e. XD = 0.

From the size of the difference between the positions of the cutting front start 13 and the cutting front end 14 in the x-direction, the thickness d of the sheet 11 can be calculated: d = (XBU - XBO) / (tana2- tana-i), with oc2> ai.

The distance D between the nozzle opening 10.1 and the top of the sheet 11.1 is: D = XBO / (tana2 - tana-i)

The depth position Zs of a luminous phenomenon in the cutting gap 22 - here originating from a web tip 16 - can be compared with the depth position ZBU of the underside of the sheet:

Zs — ZBU ⁼ (Xs — XBU) / (tana2 - tana-i).

If this value is positive, it can be concluded that the light element is located below the sheet 11. If the value is negative, however, the light element is located either in the area of the cutting front 12 of the sheet 11 or between the sheet 11 and the processing nozzle 10. This allows the cause of the light phenomenon to be determined. If the value is positive, the light phenomenon must come from a web tip 16 or other material that is located below the sheet 11. Otherwise, it is caused either by the cutting front 12 or, for example, by a metal vapor lamp above the sheet 11.

Fig. 3 shows three different images of a process zone of a laser cutting machine as examples. The top side of the sheet metal 11.1, the cutting front 12 and the cutting gap 22 of the process zone can be seen in each case. The image in Fig. 3a was taken at an observation angle of 3.6° to the laser beam direction z. The observation angle of the image in Fig. 3b is 1.8° and the observation angle in Fig. 3c is 0°.

Claims

patent claims 1 . Method for monitoring, controlling and regulating laser cutting processes of a laser cutting machine with an imaging device for imaging a process zone of a laser cutting machine on an image recording device, characterized by the steps: Detecting the radiation emanating from the process zone under a first and at least one second observation direction, wherein the first and second observation directions are different from one another, and spatially separated imaging (20, 21) of the radiation detected under the two observation directions in an image recording device; from the images, measuring the vertical position (Z) of elements (11, 12, 13, 14, 15, 22, 23) in the process zone and/or measuring the vertical distance (d, D, Zs - ZBU) between elements (13, 14; 10, 11; 14, 15) by triangulation; Comparison of the measured distances (d, D, Zs - ZBU) with target values or target ranges; if the measured values (d, D, Zs - ZBU) deviate from the target values outside a tolerance range, carry out at least one of the following measures depending on the size of the deviation: a) issuing an error message or warning message; b) aborting the cutting process; c) adjusting at least one cutting parameter; d) adjusting the processing nozzle of the laser cutting machine; e) excluding an area of the workpiece from the cutting process; f) reworking one or more defects. 2. Method according to claim 1, characterized in that the radiation emanating from the process zone is recorded simultaneously under the two observation directions. 3. Method according to claim 1 or 2, characterized in that the thickness (d) of the workpiece (11) is determined from the measurement of the vertical position (ZBO) of the start (13) of a cutting front (12) of a workpiece (11) and the vertical position (ZBU) of the end (14) of the cutting front (12). 4. Method according to claim 3, characterized in that in the case of predetermined deviations between the target value for the workpiece thickness (d) and the measured value, at least one parameter of the cutting process, the monitoring process or the control process is adapted to the measured value or an error message is issued and/or the cutting process is stopped. 5. Method according to one of the preceding claims, characterized in that by measuring the lateral offset (XBO) of an element (13) on the workpiece top side (11.1) in the two images recorded under different observation directions, the distance (D) of the processing nozzle (10) to the workpiece top side (11.1) is calculated by triangulation. 6. Method according to claim 5, characterized in that if the measured distance (D) between the workpiece top (11.1) and the processing nozzle (10) deviates from a target value, a readjustment of the position of the processing nozzle (10) and/or an adjustment of at least one cutting parameter is carried out or the cutting process is interrupted and the processing nozzle (10) is inspected and, if necessary, cleaned or replaced. 7. Method according to one of the preceding claims, characterized in that the distance (Zs) of elements (16) in the cutting gap (22) to the image recording device, the processing nozzle or the upper side or lower side of the workpiece is measured. 8. Method according to claim 3 and 7, characterized in that if the distance (Zs) between the light elements is smaller than the distance (ZBU) of the workpiece underside (14), a cut break is detected and the cutting process is interrupted. 9. Method according to one of the preceding claims, characterized in that the vertical position of contaminants protruding into the nozzle opening (10.1) is measured and an error message is issued with the vertical position of the contaminant. 10. Method according to one of the preceding claims, characterized in that the recordings (20, 21) are made under the different observation directions unpolarized or identically polarization-filtered and in the same spectral range. 11. Method according to one of claims 1 to 9, characterized in that the recordings (20, 21) are made under the different observation directions in different polarization directions relative to the feed direction (x) of the laser or in different spectral ranges. 12. Method according to one of the preceding claims, characterized in that the measurement results and the associated observation directions (ai, «2) are made available to a self-learning Kl system which carries out a classification of the process states. 13. Method according to one of the preceding claims, characterized in that the measurements are carried out through the opening (10.1) of the processing nozzle (10). 14. Method according to one of the preceding claims, characterized in that the edge of the nozzle opening (10.1) is defined as a reference for the measurement. 15. Method according to one of the preceding claims, characterized in that it is carried out before, during or after the machining of a workpiece (11). 16. Method according to one of the preceding claims, characterized in that the angle difference between two observation directions is chosen between 1° and 10°, preferably between 3° and 4°.