Method for monitoring laser beam welding process

Abstract

A method for monitoring a laser beam welding process. The invention relates to a method for monitoring a laser beam welding process, in which at least one measured value of a welding point or the surroundings thereof is detected and compared to a reference value, and in which an error message is output if the deviation from the reference value exceeds a predefined tolerance range, characterized in that the error message is sent to the welding point or the surroundings thereof. The reference value is selected as a function of the trajectory of the movement of the laser beam (12) on the material to be welded in the region of the welding point (14).

Description

Technical Field

[0001] The invention relates to a method for monitoring a laser beam welding process. Background Art

[0002] A method for monitoring a manufacturing process is already known from DE 19962967 A1, in which measured values ​​of a manufacturing process, for example a laser welding process, are recorded and compared with limit values. When the limit values ​​are exceeded, a corresponding error signal is signaled. The standard deviation of the recorded measured values ​​is used to define the limit values. The limit values ​​are calculated automatically in this case. Summary of the invention

[0003] Advantages of the invention

[0004] The method for monitoring a laser beam welding process according to the invention has the following advantages in contrast: the reference values ​​for the measured values ​​of the welding points during the laser beam welding process are specified according to the trajectory of the movement of the laser at the respective welding point. It is thus possible to reliably assess whether the laser beam welding process is carried out incorrectly or correctly. In this case, for the laser beam welding process to be carried out, it is not necessary to calibrate the respective process individually according to calibration measurements that may have been carried out several times, because the corresponding reference values ​​are already stored and automatically provided for the trajectory of the performed process and, if necessary, also for other trajectory processes. In this case, it is utilized that the ideal measurement signal, for example, the reflection signal of the wall in a certain wavelength range in the so-called hole or so-called steam capillary of the welding process, changes according to the trajectory of the laser beam on the workpiece. If, for example, the laser beam is guided horizontally from left to right above the workpiece from the perspective of the observer, this may result in different measurement signals compared to a vertical processing direction guided at an angle of 90° relative to the horizontal weld seam. These ideal measurement values ​​can be determined in advance for different types of trajectory, so that when one of these trajectory is reproduced later, a measurement value is provided without the user having to carry out one or more additional calibration processes for the complex trajectory of the laser welding process. The automated statement as to whether the laser beam welding process has been performed correctly can thereby be improved.

[0005] Further advantages are derived from the dependent claims. It is therefore particularly advantageous if additional tolerance ranges are also selected for a plurality of reference values. Experiments have shown that not only the absolute measured value can depend on the trajectory of the laser on the workpiece, but also the tolerance range around this reference value for a successful laser welding process can depend on the trajectory of the laser on the workpiece. By adapting the tolerance range, it can be ensured that no unnecessary false alarms occur, and on the other hand it can be ensured that singularities during the welding process can be reliably detected not only by adapting the reference value but also by appropriately adapting the tolerance around this reference value. If the tolerance range has the same size for all reference values, a particularly efficient regulation of this tolerance range can be achieved. As a result, a tolerance band around the reference value is generated, which has a constant width over the entire trajectory and can therefore be stipulated in a simple manner starting from the reference value.

[0006] It is also advantageous that the actual trajectory course to be executed at a position is respectively assigned to an idealized trajectory course appropriately selected from a plurality of provided idealized trajectory courses. It is thus possible to provide reference values ​​for almost any trajectory course by providing a limited number of different idealized trajectory courses, wherein these arbitrary trajectory courses are compiled from different sub-blocks of the idealized trajectory course. Thus, by providing a limited number of idealized trajectory courses, reference values ​​can be provided for a very large number of arbitrary trajectory courses. In another embodiment, it is also possible, if necessary, to automatically calculate intermediate values ​​for intermediate trajectory courses by interpolating between two reference values ​​for similar trajectory courses.

[0007] It is particularly advantageous to provide such a selection of the trajectory course by means of a coding, in particular an n-bit coding, where n is a natural number. It is particularly advantageous to use an 8-bit coding, since this already provides 256 different idealized trajectory courses, and only an 8-bit value has to be transmitted to transmit a trajectory segment. Control with such a small amount of data is advantageous in particular for programming industrial laser welding installations, since the assignment of the trajectory course by means of a sequence of different 8-bit values ​​can be easily transmitted to such an installation via a conventional data connection, for example a CAN bus.

[0008] It is also advantageous if the location position is determined based on an evaluation of the time that has been completed for the laser welding process, wherein the position reached is selected as the position that should have been reached after this duration when the process is carried out ideally. The location determination is thus possible only by time measurement of the duration of the laser welding process that has been completed so far. The determination of the trajectory course at the location and thus the determination of the corresponding reference value are thus possible again, so that an absolute location determination by a corresponding location sensor system of the laser position is not necessary.

[0009] It is also advantageous if error messages or success messages about a successfully performed laser welding process are output via the interface after the laser welding process has been completed, so that either a user or, in the case of automated processing, a processing device can record this information and preferably assign it in an automated manner to the manufactured component or its digital twin. This ensures that only successfully welded components are processed further.

[0010] It is also advantageous to store reference values ​​for a plurality of locations or location regions along the trajectory of the laser light, respectively. By storing such regions, the evaluation is facilitated, since only the stored values ​​for the individual regions need to be accessed, and the trajectory consists of only a limited number of location regions and therefore only a limited number of reference values ​​need to be considered.

[0011] Corresponding advantages result for the method according to the invention and for a computer program product for carrying out the method and a corresponding computer-readable storage medium having instructions for carrying out the method by a computer. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Exemplary embodiments are shown in the drawings and are explained in more detail in the following description.

[0013] Figure 1 An exemplary embodiment of a laser welding device for carrying out the method according to the invention is shown.

[0014] Figure 2 An example showing a laser welding track on a workpiece,

[0015] Figure 3 shows the assignment of idealized trajectories to corresponding reference and tolerance values,

[0016] Figure 4 Show according to Figure 2 Flow of the welding process for a welding track in which reference values ​​and tolerance ranges are plotted relative to the location coordinates of the welding track,

[0017] Figure 5 An exemplary embodiment of the method according to the invention for monitoring a laser beam welding process is shown. DETAILED DESCRIPTION

[0018] exist Figure 11 shows a laser 10, from which a laser beam 12 is directed via an optical device 11 onto a workpiece 13. A laser beam welding process is to be performed on the workpiece 13. Due to the action of the laser 12, a melt is formed at the bottom of the vapor capillary at the welding point 14, wherein by moving the laser 10 over the surface of the workpiece 13 or by controlling the optical device 11, the impact point of the laser on the workpiece 13 is displaced so that a weld seam is formed on the workpiece 13 when the laser 10 is guided accordingly.

[0019] To change the position, either the workpiece 13 or the laser 10 can be moved. The laser 10 is controlled by a control device 15 according to a program stored in a memory 16. According to the program stored in the memory 16, the movement of the laser 10 is defined such that the laser beam 12 describes a trajectory on the workpiece 13, along which the welding process is performed. The location information can be communicated to the control device 15, for example, via a location determination unit 17.

[0020] The laser welding process is tracked by a measuring device 20, whose field of view 21 includes at least the surroundings of the current impact point of the laser beam 12. A computing unit 22 evaluates the optical information detected by the measuring device 20 and transmits it to the control device 15 via a data connection 23. A coordination unit 24 ensures that the laser 10 and in particular the control device 15 and the computing unit 22 refer to the same time base.

[0021] The measuring device 20 can be implemented, for example, as a camera, wherein the computing unit 22 evaluates the image information in this case and derives therefrom, for example, a measured value of the temperature of the wall of the steam capillary. In another embodiment, the measuring device 20 can also have a photodiode, which is sensitive, for example, in the infrared range and detects the temperature in the region of the impact region of the laser beam 12. Thus, thermal information from the steam capillary, for example, its wall, can be detected. The trajectory to be performed by the laser 10 on the workpiece 13 and reference values ​​of the measured values ​​of the measuring device 20 are stored in the memory 16. Now, taking into account the corresponding position of the laser beam 12 on the workpiece 13, the control device 15 compares the measured values ​​received from the measuring device 20 with the stored reference values. In one embodiment, a tolerance range is also stored for the reference value in order to describe the permissible variance of the reference value. In another embodiment, a tolerance value related to the absolute value of the measured value, for example a percentage tolerance value, can be pre-specified. In a further embodiment having a particularly simple configuration, the tolerance value is constant, ie the magnitude of the tolerance range around the respective reference value is constant for all reference values.

[0022] The detected data can hereby be preprocessed in a conventional manner, possibly already in the measuring device 20 , while a decision is made as to whether a singularity or a tolerance violation exists, depending on which sub-block of the beam path of the welding process is actually processed by the laser 10 .

[0023] In another embodiment, it is also possible to transmit information about the current trajectory course from the laser 10 to the computing unit 22 via the data connection 23 and to carry out the evaluation in the computing unit 25, ie the comparison between the measured value and the reference value, in the measuring device 20. Such a computing unit 25 is Figure 1 Shown by dotted lines.

[0024] If the control device 15 determines an error, i.e. a deviation of the measured value from the reference value that exceeds a permissible tolerance value upward or downward, the control device 15 can output this error via the interface 26. The interface 26 can be implemented, for example, as a display for a user. However, the interface can also be implemented as an interface to a production system that automatically controls the laser welding process. In this case, in one embodiment, the measurement results, for example errors or successfully performed welding processes, can be automatically assigned to the digital twin of the manufactured component and stored.

[0025] The reference values ​​stored in the memory 16 are determined, for example, by measurements, by evaluating a neural network or by interpolation between stored measurement points. The corresponding reference values ​​can be loaded into the memory 16, for example, via a connection 27 to a data network, such as the Internet.

[0026] exist Figure 2 3 shows an exemplary trajectory 31 of a welding process on a workpiece 30 in a top view of the workpiece 30. Here, the trajectory 31 describes a "U" that is open to the left in this illustration, wherein the welding trajectory is divided into different sections. In the first section 32, up to a fictitious first mark 33 introduced only for the sake of overview, the laser beam is guided from left to right along a straight line in the direction of the arrow 43. From the first fictitious mark 33, the trajectory now describes a right curve 34 up to a second fictitious mark 35. The laser beam is now guided along a straight line 36 in the direction of the arrow 37 up to a third mark 38. Now follows a second right curve 39, which is, however, rotated 90° relative to the first right curve 34 and is guided up to a fourth fictitious mark 40. Then follows again a straight-line region 41, which is again guided in a straight line in the direction of the arrow 42, but in the opposite direction to the first section 32 of the trajectory 31.

[0027] The respective segments 32, 34, 36, 39, 41 are similar in their individual points, so that during optical measurement with a camera designed as a measuring device 20, calculations can be performed for all points of the respective sub-block using similar measured values ​​or, in some areas, also measured values ​​that can be derived linearly from the position. Therefore, in the described embodiment, reference values ​​or reference functions are assigned to the respective sub-block or to all points of the respective sub-block. For this purpose, a corresponding table is advantageously stored in the memory 16.

[0028] exist Figure 3 An example of such a table 49 is shown in FIG. 5 . The corresponding reference values ​​or reference functions are stored in a first column 50 and, in one embodiment, the corresponding tolerance ranges for the reference values ​​are stored in a second column 51. These have preferably already been determined in a preliminary phase and can be assigned to all configurations of the trajectory course stored in a third column 53. In a first row 52, ​​the first reference values ​​are assigned to the straight lines from left to right. Alternatively, the tolerance ranges can also be calculated automatically by adding and subtracting constant values ​​that are uniformly specified for all reference values ​​to the corresponding reference values.

[0029] For the corresponding Figure 2 For the right curve of the right curve 34 in FIG. 1 , there is an entry 54 in the second row, for the downward straight line guidance there is a third reference value in the third row 56, for the curve rotated further by 90° according to the fourth sub-block there is a reference value in the fourth row and for the straight section 41 there is a reference value in the fifth row 61. Reference values ​​can also be stored for other corresponding trajectories in the table 49 stored in the memory 16. The stored reference values ​​are preferably idealized trajectories, by arranging the trajectories in a row, it is possible to describe the real trajectory to be executed by the laser.

[0030] Thus, for example, the control device 28 of the manufacturing control device can be used to inform the laser 16 of the corresponding idealized trajectory sequence that should be driven away by the laser 10 in sequence. In this case, the trajectory can be assigned to a code, for example an 8-bit code, so that, for example, Figure 3 Each row in is assigned an 8-bit value of such an 8-bit code. In one embodiment, for each trajectory, the control device 28 transmits the sequence of 8-bit codes to the laser 10 together with the duration of the corresponding operation time for the laser along the trajectory at a constant propulsion speed, which the laser should follow for an ideally constant speed movement. This describes that the laser 10 should execute the corresponding encoded trajectory in a predetermined sequence within this duration. Accordingly, reference values ​​and, if necessary, corresponding tolerances are also assigned to the trajectory segments.

[0031] In another embodiment, it is also possible that specific trajectory information is transmitted via the control device 28, and in order to monitor the laser beam welding process, the control device 15 computationally maps the transmitted trajectory onto various idealized trajectory paths in the memory 16 so as to be as close as possible to the actual trajectory path that should be executed.

[0032] For example, according to Figure 2 , a reference value can be assigned to a specific point of the welding trajectory in such a way that the location determination unit 17 determines the absolute position of the laser along the trajectory to be executed. In another embodiment, it is also possible to determine, from the executed time of the start of the welding process, for example by evaluation by the coordination unit 24, how much time has elapsed since the start of the welding process and therefore at what position the laser must be expected. A corresponding reference value is then determined for this point of the trajectory determined in this way. The location determination unit 17 can also be implemented, for example, as a current detection of the scanner coordinates of the laser relative to the workpiece. The trajectory of the laser can here be derived from the X and Y coordinates. The idealized trajectory course can also be derived from the course of these values.

[0033] exist Figure 4 2 shows an example of a measured value curve, for example an infrared intensity curve, detected by the measuring device 20 as a measurement signal. Here, the measured intensity value 71 is plotted on a Y axis 70 relative to an X axis 72, on which a curve corresponding to a prescribed trajectory (here according to Figure 2 The values ​​of the past sections of the trajectory direction are plotted for the positions along the trajectory. Figure 2 With the exception of two outlier measured values ​​74 and 75, the measurement curve 71 follows the assigned reference values ​​or only insignificantly Figure 4 oscillates around the reference value below the threshold value in . It is shown that not only the reference value but also the corresponding tolerance value changes along the trajectory in a location-dependent manner, the tolerance values ​​each describing a dashed area 73 around the reference value. As can be seen from the course of the changes corresponding to the two curve areas, the reference value does not necessarily have to be a constant, but in this case can also be formed by a linear progression between the beginning and the end of the corresponding assigned trajectory section of the welding trajectory. The first abnormal measured value 74 lies within the tolerance range and does not result in an error message. The second abnormal measured value 75 leaves the tolerance range and results in an error message.

[0034] exist Figure 5An embodiment of the method according to the invention is shown in FIG. Starting from an initialization step 80, a reference value for the measured value is determined for the corresponding point along the trajectory in a determination step 81. In a subsequent first comparison step 86, it is checked whether the specific reference value still corresponds to the corresponding point along the trajectory. If this is no longer the case, a branch is returned to the determination step 81. If the trajectory has been completely traversed, a branch is made to an end step 82 and the monitoring method is terminated. Otherwise, the measured value is recorded in a recording step 83 and then further branched to a second comparison step 84. In the second comparison step 84, it is checked whether the measured value corresponds to the reference value including the tolerance range. If this is not the case, an error message 85 is output and the laser beam welding method is at least interrupted or terminated, otherwise a branch is returned to the first comparison step 86.

Claims

1. A method for monitoring a laser beam welding process, wherein at least one measured value of a welding point or its surroundings is detected and compared with a reference value, and wherein an error message is output when a deviation from the reference value exceeds a predefined tolerance range, characterized in that The reference value is selected as a function of the path course of the movement of the laser beam (12) in the region of the welding point (14) on the material to be welded.

2. The method according to any one of the preceding claims, characterized in that Reference values ​​are stored for a plurality of location regions along the trajectory of the laser.

3. The method according to claim 2, characterized in that Select a tolerance range that belongs to multiple reference values.

4. The method according to claim 3, characterized in that The numerical size of the tolerance range around the respective reference value is constant for all reference values.

5. The method according to any one of the preceding claims, characterized in that An actual trajectory course at a location is assigned to an idealized trajectory course selected from a plurality of idealized trajectory courses (53), and the assigned idealized trajectory course (53) is taken into account for determining a reference value.

6. The method according to claim 5, characterized in that The information about the idealized trajectory course is provided in an n-bit code, preferably in an 8-bit code.

7. The method according to any one of the preceding claims, characterized in that The current location position is determined based on an evaluation of the duration of the laser welding process from the start until the current time.

8. The method according to any one of the preceding claims, characterized in that An error message or, after completion of the laser welding process, a message about successful welding if no error message occurred, is output from the interface to the control device of the laser welding process.

9. An apparatus for performing the method according to any one of the preceding claims.

10. A computer program product comprising instructions which, when the program is executed by a computer, cause the computer to perform the method according to any one of claims 1 to 8.

11. A computer-readable storage medium comprising instructions which, when executed by a computer, cause the computer to perform the method according to any one of claims 1 to 8.

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