Method for correcting optical path length measurement errors of a measuring scanner on a laser processing optical unit

Abstract

The invention relates to a method for correcting optical path length measurement errors of a measuring scanner (15) on a laser processing optical unit (10), wherein the measurement beam (18) of the measuring scanner (15), for measuring distance, is coaxially coupled into the processing laser beam (14) and is moved laterally in an x-y plane across a workpiece in the vicinity of the processing laser beam (14) and the distance values measured by the measuring scanner (15) at various measurement points (M1-M5) of the workpiece are corrected in the z direction by change values, wherein the change values are obtained from calculated or already known optical path lengths (I) of the measurement beam (18) at various selection points in the x-y plane.

Description

[0001]Method for correcting optical path length measurement errors of a measuring scanner on laser processing optics Description: In laser material processing, measuring scanners, and in particular OCT measuring scanners, are increasingly being used as distance-measuring sensors on laser processing devices. These sensors can be used to ensure that laser welding or marking occurs at the desired location. The measuring scanner scans the workpiece almost coaxially with the processing laser beam. The measuring beam can be deflected on the workpiece independently of the processing laser beam. The measured values ​​are evaluated by an image processing device, which enables optical monitoring of the processing process. When the measuring beam is deflected laterally relative to the beam axis of the processing laser beam, the optical path length of the measuring beam changes.One reason for the change in the optical path length is the geometric change in the path length due to the adjustment of the deflection angles of the measuring scanner mirrors and / or the mirrors of the laser processing optics. Another reason for the change in the optical path length of the measuring beam is the different glass passage lengths of the measuring beam when the measuring beam is moved through optical elements, such as an F-theta lens. Both causes overlap and lead to measurement errors. For example, if the measuring beam is moved along a line across a flat surface, this line appears curved in the image generated by the measuring scanner, even though it is straight. When the measuring beam is deflected by deflection mirrors of the laser processing optics throughout the entire workspace of the laser processing device, this is referred to as a global change in the optical path length. This change is usually several millimeters.If, however, the measuring beam is deflected only by the deflection mirror of the measuring scanner in the vicinity of the processing laser beam, the change in the optical path length is referred to as a local change. The local change is in the range of less than 100 m. For measuring systems that are intended to have a resolution of 50 m, a change in this magnitude represents a problem. The invention is based on the object of eliminating the influence of a local optical path length change on the measurement result of a measuring scanner on a laser processing optics.The problem is solved by a method for correcting optical path length measurement errors of a measuring scanner on laser processing optics, wherein the measuring beam of the measuring scanner is coaxially coupled into the processing laser beam for distance measurement and is moved laterally in an xy plane over a workpiece, which is characterized in that the distance values ​​measured by the measuring scanner at various scanning points on the workpiece are corrected in the z direction by change values, wherein the change values ​​are obtained from calculated or previously known optical path lengths of the measuring beam at various selection points in the xy plane. Furthermore, the problem is solved by an image processing device of a laser processing apparatus according to independent claim 9. By correcting the measured values ​​of the measuring scanner, distortions of the image of the workpiece generated by the measuring scanner can be eliminated.After correction, the workpiece's distance data is accurate to within a few micrometers, even in the z-direction. The image is corrected immediately after the measured values ​​have been recorded. The selection points correspond to various deflection angles of the measuring beam in the measuring scanner and / or in the laser processing optics. They are thus directly correlated to the positions of deflection mirrors in the measuring scanner and / or in the laser processing optics. In a first process variant, the change values ​​for the selection points can be stored in an image processing device of a laser processing apparatus. The image processing device can contain various optics-specific correction data sets for this purpose. With knowledge of the laser processing optics and the measuring scanner used, the corresponding correction data set can be selected, and the change values ​​can be taken from it and applied to the measured values.Each correction data set contains change values ​​for the optical path length for a variety of angular positions of the deflection mirrors of the laser processing optics and the measuring scanner, whereby these values ​​can be stored in the form of a table. In an alternative process variant, the change values ​​can be obtained from a simulation of the optical system formed by the laser processing optics and the measuring scanner. This is done by calculating the local change in the optical path length of the measuring beam at several selected points and using this to determine a polynomial that contains the coordinates of the laser processing optics and the measuring scanner and with which the optical path length of the measuring beam at the measuring points of the measuring scanner is calculated. Here, the actual optical path length of the measuring beam at the respective measuring points is calculated during the measurement, and from this the change values ​​for the measured values ​​are determined.In all process variants, distance data in the z-direction at measuring points of the measuring beam that do not coincide with selection points can be corrected using change values ​​calculated by interpolation from the change values ​​of the nearest selection points. It is therefore not necessary to determine these values ​​for a very large number of points in the process variant with stored change values, or to calculate the local change in the optical path length at a large number of selection points to determine the polynomial in the second process variant in order to achieve sufficient accuracy of the measured value correction. To obtain an undistorted image of the workpiece, an image of the workpiece generated from the measurement data of the measuring scanner can be corrected by the image processing device using a geometric shearing method using the change values ​​in the z-direction.The shearing of the image can preferably be carried out column by column, with each column being shifted in the z-direction by the change value of the optical path length. To achieve the most accurate correction result possible, the optical path length calculation is preferably carried out taking into account the geometric structure of the laser processing optics, the optical elements used in the laser processing optics and the measuring scanner, and their materials, i.e. taking into account all parameters relevant to the optical path length. An exemplary embodiment of the method according to the invention is explained in more detail below with reference to the drawings. They show: Fig. 1 a, b schematic representations of a laser processing optics with a measuring scanner to explain a global and local change in the optical path length of a measuring beam; Fig. 2 a block diagram of a correction of the local change in the optical path length of measured values; Fig.3 is a schematic representation of the correction of the local change in the optical path length using the example of a line scan. Fig. 1 schematically shows a laser processing optics 10 with three deflection mirrors 11, 12 and 13 for a processing laser beam 14 and a measuring scanner 15 with two deflection mirrors 16, 17 for a measuring beam 18. The deflection mirror 11 is fixed, while the deflection mirrors 12, 13 are adjustable. The measuring beam 18 is coaxially coupled into the processing laser beam 14 via the deflection mirror 11. In Fig. 1a, the measuring beam 18 is deflected together with the processing laser beam 14 by the deflection mirror 13 by an angle ^ relative to the vertical by adjusting the deflection mirror 13 by an angle ^1. This results in the optical path length ^. 1,0 of the measuring beam 18 to a longer path length ^ 1,^ The change value is ∆^ 1,^ = ^ 1,^ - ^ 1,0and is referred to as a global change in the optical path length. It is usually a few millimeters. In Fig. 1b, however, the measuring beam 18 is deflected by the deflection mirror 16 in the measuring scanner 15 by an angle ^ relative to the vertical by adjusting the deflection mirror 16 by an angle ^1. This deflection results in the optical path length ^ 1,0 of the measuring beam 18 ^ 1, ^ changes where- at the change value ^ = ^ - ^ 1,0 This change is called local Change in the optical path length and is significantly smaller than the global change in the optical path length and is in the range of 100 μm. With the method shown in Fig. 2, the influence of the local change in the optical path length of the measuring beam 18 on the measured values ​​of the measuring scanner 15 is eliminated. The block diagram schematically shows the measuring scanner 15 with a control device 19 for the deflection mirrors 16, 17 for the measuring beam 18 and an image processing device 20. The image processing device 20 has a correction data set 21 for the optical path length, a computing unit 22 for calculating the corrected measured values ​​and a memory 23 for the corrected measured values. Using the known angular positions ^1, ^2 of the deflection mirrors 13 and 12 of the laser processing optics 10 and the angular positions ^1, ^2 of the deflection mirrors 16 and 17 of the measuring scanner 15, the change values ​​∆^1, ^ , ∆^ 2, ^ , ∆^ 3, ^..... the optical path length is read out and applied in the computing unit to the measured values ​​from a memory of a control device 19 of the measuring scanner 15. The corrected measured values ​​resulting from this calculation are then stored in the memory 23 of the image processing device. From the corrected measured values, an undistorted image of a workpiece to be processed by the processing laser beam 14 can be generated. Fig. 3 illustrates this using the example of a line scan by the measuring scanner 15 in the y-direction. Fig. 3a shows the image of line 30 without correction of the measured values ​​of the measuring scanner 15. The actually straight line appears curved in the z-direction because the change in the optical path length of the measuring beam 18 is not taken into account. The curvature is greatest at the end points of line 30 because this is where the measuring beam 18 experiences its greatest deflection. Fig.Figure 3b illustrates the correction of the measured values ​​acquired by the measuring scanner 15, which in the example shown were determined at five measuring points M1 to M5 on line 30 by the measuring beam 18. The measuring points M1 to M5 are selection points on line 30. At the measuring point M3, the measuring beam 18 experiences no deflection. The measured value at the measuring point M3 is correct, with no change in the optical path length, and is not corrected. At the other measuring points M1, M2, M4, and M5, however, the measured values ​​of line 30 are corrected in the z-direction by the change values ​​∆^. 1, ^ , ∆^ 2, ^ , ∆^ 3, ^ , ∆^ 4, ^ the optical path length of the measuring beam 18. The result is shown in Fig. 3c: after correcting the measured values ​​of line 30, it takes on a straight line 30', as shown in Fig. 3c. The z-coordinates of all measuring points M1 to M5 are identical after the correction.

Claims

Claims:

1. Method for correcting optical path length measurement errors of a measuring scanner (15) on a laser processing optics (10), wherein the measuring beam (18) of the measuring scanner (15) is coaxially coupled into the processing laser beam (14) for distance measurement and is moved laterally in an xy plane over a workpiece in the vicinity of the processing laser beam (14), characterized in that the distance values ​​measured by the measuring scanner (15) at different measuring points (M1 - M5) of the workpiece in the z-direction by change values ​​(∆^ 1, ^ , ∆^ 2, ^ , ∆^ 3, ^ , ∆^ 4, ^ ) should be corrected, whereby the calculated or previously known optical path lengths (I) of the measuring beam (18) at different selection points in the xy plane.

2. Method according to claim 1, characterized in that the selection points correspond to different deflection angles (11, 12; 11, 12) of the measuring beam (18) in the measuring scanner (15) and / or in the (10).

3. Method according to claim 1 or 2, characterized in that the change values ​​(∆^ 1, ^ , ∆^ 2, ^ , ∆^ 3, ^ , ∆^ 4, ^ ) for the selection points are stored in an image processing device (20) of a laser processing device.

4. Method according to claim 1 or 2, characterized in that the change values ​​(∆^ 1, ^ , ∆^ 2, ^ , ∆^ 3, ^ , ∆^ 4, ^) can be obtained from a simulation of the optical system formed by the laser processing optics (10) and the measuring scanner (15) by calculating the local change in the optical path length of the measuring beam (18) at several selection points and determining a polynomial therefrom which contains the coordinates of the laser processing optics (10) and the measuring scanner (15) and with which the optical path length of the measuring beam (18) at the measuring points (M1-M5) of the measuring scanner (15) is calculated.

5. Method according to one of the preceding claims, characterized in that distance data in the z-direction at measuring points (M1 - M5) of the measuring beam (18) that do not correspond to selection points are replaced by change values ​​(Δ^ 1, ^ , ∆^ 2, ^ , ∆^ 3, ^ , ∆^ 4, ^ ) which are calculated by interpolation from the change values ​​(∆^ 1, ^ , ∆^ 2, ^ , ∆^ 3, ^ , ∆^ 4, ^) of the nearest selection points are calculated.

6. Method according to one of the preceding claims, characterized in that an image of the workpiece generated from the measurement data (19) of the measuring scanner (15) is processed by the image processing device (20) by a shearing method using the change values ​​(Δ^ 1, ^ , ∆^ 2, ^ , ∆^ 3, ^ , ∆^ 4, ^ ) in the z-direction.

7. Method according to claim 6, characterized in that the shearing process of the image is carried out column by column, in that each column in the z-direction is corrected by the change value of the optical path length (∆^ 1, ^ , ∆^ 2, ^ , ∆^ 3, ^ , ∆^ 4, ^) is shifted.

8. The method according to one of the preceding claims, characterized in that the optical path length calculation is carried out taking into account the geometric structure of the laser processing optics (10), the optical elements used in the laser processing optics (10) and the measuring scanner (15), and their materials.

9. An image processing device (20) of a laser processing apparatus for carrying out the method according to claim 1.

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