EP2244856A1

EP2244856A1 - Method and device for automatic laser focusing

Info

Publication number: EP2244856A1
Application number: EP09714756A
Authority: EP
Inventors: Mark Geisel
Original assignee: MTU Aero Engines GmbH
Current assignee: MTU Aero Engines AG
Priority date: 2008-02-25
Filing date: 2009-02-18
Publication date: 2010-11-03
Also published as: WO2009106043A1; DE102008010981A1; US20110007304A1; CA2714599A1; US8379194B2

Abstract

The invention relates to a method and to a device for automatic laser focusing. The basic concepts of the method are the automatable, and particularly preferred the automated photography of the focal points generated during a focus series by means of a camera and a determination of the respective focal point diameters, and an evaluation of said data using an interpolated or approximated function, which allows a conclusion of those focal points having the smallest focal point diameters and consequently the associated optimum focal distance. By using the method according to the invention, a number of the disadvantages known from the prior art can be avoided, in particular the high time expenditure for the determination of the optimal focal distance, and the uncertainty connected with a user in the selection of the optimal focal point, are eliminated as part of the photography of a focus series.

Description

Method and device for automatic laser focusing

The invention relates to a method and a device for automatic laser focusing.

Lasers are used today to process a variety of engineering materials, such as of metals, plastics, but also ceramics. Due to the high and concentrated in a very small focal spot energy, which is provided in the form of normally coherent and monochromatic light, melts or evaporates the material to be processed. Particularly advantageous in the use of lasers as tools is the fact that the laser beam as such is subject to no wear and a direct physical contact between the tool and the workpiece is eliminated.

In order to obtain the best possible machining result, it is necessary that the laser beam is optimally focused on the workpiece to be machined. Otherwise, the scattered light energy is distributed over too large an area so that the temperatures achieved during the irradiation may not be sufficient for effective processing. In addition, it is often desirable that the machined by the tool area in the form of a cutting edge as narrow as possible, which is also achieved by an optimally focused laser beam.

In order to set the least possible focal spot, the optics present on the laser systems have corresponding adjustment possibilities for the focal distance. Although these optics may also be operated automatically under certain circumstances, it is nevertheless necessary to make the desired setting of the focal distance manually insofar as that the result is selected from a series of experiments with respectively different optical settings in which the focal spot is the smallest is. The parameters associated with this result are then forwarded to the optics of the laser system. Usually, this is done a plurality of experiments, each with different focus settings. Such a series of experiments is also called focus series. The evaluation of the results is done manually by an operator. Since the focal spots to be examined are naturally very small in nature, this examination normally takes place with the additional aid of optical aids such as loupes or microscopes.

This approach has a number of disadvantages. First of all, the quality of the result depends on the experience of the operator performing the examination and the selection of the optimal focal spot. Furthermore, the implementation of the experiments and the subsequent evaluation is time consuming. Particularly in the case of frequently changing workpiece shapes, where each time a new determination of the optimum focus distance is required, it may happen that the respective preparation takes longer than the actual machining of the workpiece. In addition, the selection of the optimal focal spot is often difficult, since the usually only slight changes in the focal spot near the optimum are difficult to recognize with the human eye. However, in order to achieve an optimal machining result, it is necessary to reliably detect even the smallest changes in order to find the optimal parameters and forward them to the laser optics. In particular, it may happen that the quality of a focal spot deteriorates only because, for example, the underlying material has an inhomogeneity, although the change in the focal distance should actually have led to improve the quality of the focal spot. In such cases, it may happen that only a local, but not a global, minimum of the focal spot diameter is found and the corresponding suboptimal parameters are used in the later processing.

The object of the invention is therefore to provide a method and a device for avoiding the disadvantages existing in the prior art, in particular the high expenditure of time in determining the optimum Focus distance and the uncertainty associated with an operator when choosing the optimal focal spot.

The object is solved by the features of the method proposed in claim 1 and the features of the device according to claim 7. Accordingly, a method is provided which enables fully automatic execution of all steps necessary for determining the optimum focus distance.

Further preferred embodiments can be found in the dependent claims and the following detailed description and the figures.

The inventive method is used for automatic laser focusing, in order to avoid in this way known from the prior art, the disadvantages described above.

The core idea of the method is the automatable and particularly preferably the automated recording of the focal spots generated during a focus row by means of a camera and a determination of the respective focal spot diameter, and an evaluation of this data by means of an interpolated or approximated function, which draws an inference to that focal spot with the smallest Focal spot diameter and thus the associated optimal focus distance allowed.

Accordingly, the method according to the invention for automatic laser focusing for achieving the smallest possible focal spot in laser processing initially comprises the following steps:

(a) adjusting the focal distance f of a laser to a starting value A;

(b) resetting a run index i;

(c) forming a focal spot on a workpiece to be machined;

(d) storing the respective running index i together with the respective value of the focus distance f; (e) moving the workpiece about a path increment Dx along a predetermined path;

(f) adjusting the focal distance f in the direction of a final value B by a delivery amount Df; (g) repeating steps (c) to (f), successively increasing the running index i until a predetermined number i _max of focal spots 2 is reached or until the final value B is reached or until the end of the predetermined path 4 is reached;

(h) locating the focal spot with the smallest focal spot diameter d _opt ; (i) determining the optimal running index i _op t associated with this focal tab; (j) adjusting the belonging to this running index i _opt optimum focus distance f _op t-

The adjustment of the focus distance f can be done manually or preferably automatically. An automatic adjustability has the advantage that it is faster and safer than a manual operation possible.

For example, the running index i starts at 1 and incrementally increases by 1 as it passes through appropriate loops.

For storing the obtained data such as the running index i and the focal distance fi, a computer is preferably used; Alternatively, however, it is also possible to manually write the data accordingly.

Preferably, the workpiece on which the focus row is performed, moved. However, according to an alternative embodiment, the laser can also be moved, or the laser and workpiece can be moved relative to one another. The predetermined path may be straight, curved, angular or otherwise shaped. It may have a separate start and end point, or alternatively a common (superimposed) start and end point, such as a circle. The start and end values A and B are preferably selected to cover a meaningful range in terms of the focal distance. This means that focus distances that are unnecessarily far away from the expected optimum should be excluded from the outset in order to avoid unnecessarily long focus rows.

The Weginkement Dx is preferably such that on the one hand the generated focal spots are not too close to each other, on the other hand, the length of the predetermined path is minimized.

Also, the delivery amount Df is to be such that the expected variations between the individual Brennfieckdurchmessern fail to be too large, otherwise finding the optimal focus distance will be limited possible.

The production of focal spots is preferably stopped _{according to the} invention when a predetermined number i of focal spots has been generated. Alternatively, the generation can be stopped even if the final value B, which represents the limit of the focus distance to be adjusted, has been reached or exceeded. Finally, according to the invention, the production can also be stopped when the end of the predetermined path has been reached, since otherwise the focal spots could possibly overlap or at least be too close to each other.

Locating the focal spot with the smallest focal spot diameter d _opt is done manually, possibly with the aid of appropriate optical aids, according to one embodiment.

The optimum focus distance f _{opt associated} with the focal spot having the smallest focal spot diameter is thus found and can now be set at the corresponding laser optics.

According to the invention, the focal spot is searched out with the smallest focal spot diameter d _opt or the determination of the associated optimum Running indexes iopt _opt as follows, whereby previously the respective focal spot positions (posi) must have been stored:

After completing the focus row, the count index i is reset first.

Subsequently, the respective stored focal spot positions pos; approached.

The respective focal spot diameter dj is determined and stored.

Now, with successive increase of the running index i, all subsequent stored focal spot positions pos; approached and their respective focal spot diameter determined until finally reaches the predetermined number i _max of focal positions posj and the respective focal spot diameter dj has been determined.

In a next step, an interpolating or approximating function is determined which determines the focal spot diameter d; as a function of the running index i.

By means of graphic or computer-aided analysis of the course of this interpolating function, its minimum can now be determined. This point also identifies the number of the focal spot which has the searched minimum focal spot diameter. For the frequent case that the minimum of the interpolating function does not coincide exactly with a particular focal spot, then that focal spot is selected which is closest to the determined minimum of the interpolating or approximating function.

Likewise, the running index i _opt closest to the minimum is now also known.

With these allocations, the optimum focus distance f _opt can also be determined in a last step, which was previously stored together with the run index. This can then be transmitted to the laser system or its focusing optics for setting the optimum and desired focus distance. By analogy with the above procedure, it is of course also possible to determine optimal focus systems between two interpolation points with the aid of the interpolating or approximating function.

According to a preferred embodiment of the method according to the invention this is characterized in that the determination of the respective focal spot diameter, the interpolating or approximating function whose minimum, the associated Laufmdexes, the associated focal spot not only after completion of the generation of the focus row happens, but continuously following the generation of each focal spot takes place. Although in such a case it can generally be assumed that the form of the interpolating or approximating function still changes as the number of data sets increases; however, this change is less pronounced if the number of nodes is high and the mathematical type of the interpolating or approximating function is known in advance.

According to a particularly preferred form of the method according to the invention, the search for the minimum focal spot diameter d _opt is discontinued as soon as the continuous evaluation of the interpolating or approximating function and / or a comparison of the previously determined focal spot diameter with one another increases or increases again for the following focal spot diameter d; expect. In other words, if it is to be assumed that the minimum is either already found or will be outside the investigated range, then it makes sense to end the production of the respective focal spots. This results in the advantage that under certain circumstances, the focus row does not have to be performed until its predetermined end, but that it can be canceled beforehand, thus saving time and material and, if necessary, an adjustment of the parameters of the focus row (eg start and / or or final value) to repeat the focus row. The continuous evaluation according to the invention of the interpolating or approximating function and / or the comparison of the previously determined focal spot diameter with one another can be carried out manually according to an embodiment, but automatically according to a preferred embodiment.

According to a further embodiment of the method according to the invention, the determination of the respective focal spot diameter is carried out with a digital camera and optionally with the aid of robust algorithms. Particularly preferred as a digital camera, a CCD camera is used. The mentioned robust algorithms have the purpose of working up the images of the focal spots, which are often difficult for a human viewer to evaluate, in order to simplify the corresponding evaluation.

According to a particularly preferred form of the method according to the invention, the interpolating function has the mathematical form y = const- (l + x ² ) ^1/2 , where y is the value of the focal spot diameter dj and x is the value for the running index i. Investigations have shown that a function of the described form is in principle very well suited to describing the changes of the focal spot as a function of the focal distance.

Of course, according to alternative embodiments, other interpolating functions are conceivable, such as Fourier interpolation, spline interpolation, trigonometric interpolation, and the like. Of course, the same also applies to the corresponding approximating function variants.

The invention further provides an apparatus for automatic laser focusing for achieving a smallest possible focal spot in the laser machining of a workpiece. This comprises at least the following components:

a tool laser, 5 a focus position adjustment device,

a tensioning device for clamping the workpiece, a movement device for changing the relative position of the workpiece and the tool laser,

a recording device for recording the image data of focal spots,

a memory and arithmetic unit for storing the recorded image data and mathematical analysis thereof,

- A device for data transmission of the analysis results to the Fokuslagenverstelleinrichtung.

The tool laser can be, for example, a diode laser, a fiber laser, or another solid-state or gas laser.

The focus position adjustment device can be configured manually or automatically operable. Alternatively to an adjustment of the focus position by changing a corresponding optics, the relative distance between the tool laser and the workpiece can also be changed, whereby both the tool laser and / or the workpiece can be made movable.

By means of the displacement device for changing the relative position of the workpiece and the tool laser, either the workpiece, the tool laser, or both devices can be moved together relative to each other.

The recording device for recording the image data of focal spots is preferably a digitally operating recording device.

Preferably, a commercially available PC is used as the storage and arithmetic unit for storing the recorded image data and mathematical analysis thereof.

According to a particularly preferred embodiment of the device according to the invention, this is characterized in that the receiving device is a CCD camera. According to a further, particularly preferred embodiment, the device according to the invention comprises a focus position adjustment device, which is automatically adjustable. In this way, necessary for carrying out the method according to the invention adjustment of the focus distance can be done fully automatically and without the intervention of an operator.

The present invention will be described below with reference to a preferred embodiment together with the accompanying drawings. Show:

Figure 1: a preferred embodiment of the device according to the invention

FIG. 2 shows an exemplary focus row

FIG. 1 shows a preferred embodiment of the device according to the invention. The tool laser 1 is arranged above the workpiece 3. In the illustrated embodiment, the distance between the tool laser 1 and workpiece 3 is fixed. To change the focal distance, a focus position adjustment device 5 is provided according to the invention, for example in the form of a lens optic. The adjustment is indicated in the figure 1 by the small vertical double arrow. Where the laser beam touches the workpiece 3, a focal spot 2 is generated. By means of a traversing device 6, the workpiece 3 can be moved in the illustrated embodiment such that the laser beam travels a predetermined path 4. This predetermined path 4 has a start value A and an end value B.

The device according to the invention further comprises a receiving device 7 for receiving the focal spots produced by the tool laser. In the illustrated variant, a mirror 10 indicated by a diagonal line is located in the beam path, which deflects a part of the light reflected by the focal spot 2 and supplies it to the recording device 7. This makes it possible to take a picture of the same directly after the creation of the focal spot 2, without having to start this position again at a later time for the purpose of a time-delayed recording. The image data recorded by the recording device 7 are transmitted via a Device for transmitting data 9 to a storage and processing unit 8 transmitted. In this memory and arithmetic unit 8, the storage of the recorded data and the analysis of the same takes place. After completion of the analysis, the result in the form of a control value via the device for data transmission 9 is forwarded to the Fokuslagenverstelleinrichtung 5. The recording of the image data and the analysis of the same can either be delayed, ie after completion of a complete focus series, or preferably at the same time, directly after the generation of each focal spot 2, take place.

FIG. 2 shows by way of example the result of a focus series. The number of the respective image of a focal spot is plotted on the x axis, and the respective generated hole radius is plotted on the y axis. The discrete measured values are marked by corresponding asterisks. As can be seen from the position of the asterisks, the hole radius initially decreases as the image number increases up to image number 7. Subsequently, the hole radius increases again, and then drops slightly again (Fig. 10). Even with the picture number 12, an unexpected slight drop in the hole radius' recognizable. The diagram also shows the curve of an approximating function that approximates the measuring points shown as well as possible. The minimum of the curve is approximately at picture number 9 and not, as can be expected from the discrete measured values, at picture number 7. The correct focus distance found by the method according to the invention is therefore that which belongs to the focal spot with picture number 9. This value can now be transmitted by means of the device according to the invention for data transmission 9 to the focus position adjustment device 5.

Claims

claims

A method for automatic laser focusing in the laser machining of a workpiece (3), comprising the following steps:

(a) adjusting the focal distance (f) of a tool laser (1) to a starting value (A);

(b) resetting a run index (i);

(c) producing a focal spot (2) on a workpiece (3) to be machined;

(d) storing the respective run index (i) together with the corresponding value of the focus distance (fi); (e) moving the workpiece (3) by one path increment (Dx) along a predetermined path (4);

(f) adjusting the focal distance (f) in the direction of a final value (B) by a delivery amount (Df);

(g) repeating steps (c) to (f) by successively increasing the index of run (i) until a predetermined number (i _max ) of

Focal spots (2) or until reaching the final value (B) or until reaching the end of the predetermined path (4);

(h) determining the focal spot with the smallest focal spot diameter (d _opt ); (i) determining the index of run associated with that focal spot (i _opt ); Q) Setting the optimum belonging to this optimal running index (i _op t)

Focus distance (f _op t); characterized in that the respective focal spot positions (pos;) are stored and the determination of the focal spot with the smallest focal spot diameter (d _opt ) or the determination of the associated running index (i _opt ) _{comprises the} following steps:

(1) resetting the counting index i; (2) approaching the respective stored focal spot position (pos,);

(3) determining and storing the respective focal spot diameter (dj);

(4) Approaching the next stored focal position (posi) and repeating steps (2) to (4) while successively increasing the running time (i) until the predetermined number (i _m ax) of focal spot positions (posO;

(5) determining an interpolating or approximating function representing the focal spot diameter (dj) as a function of the index of travel (i);

(6) determining the minimum of the interpolating or approximating function;

(7) determining the closest running index (i _O pt) to the minimum of the interpolating or approximating function;

(8) possibly assigning the running index (i _opt ) to the corresponding focal spot.

2. The method according to claim 1, characterized in that the determination of the respective focal spot diameter (d;) according to step (3), the interpolating or approximating function according to step (5), the minimum according to step (6) of the associated running index (i _opt ) according to step (7), the associated focal spot according to step (8) not only after the completion of steps (g) or (4) happens, but continuously takes place at each passing through the step (4) in - - connection to the same.

3. The method according to claim 2, characterized in that the search for the minimum focal spot diameter d _{opt is} then terminated as soon as the continuous evaluation of the interpolating or approximating function and / or a comparison of previously determined focal spot diameter (dj) with each other increasing or rising values for the following focal spot diameter di expect.

4. The method according to any one of the preceding claims, characterized in that the determination of the respective focal spot diameter is done with a digital camera and optionally with the aid of robust algorithms.

5. The method according to any one of the preceding claims, characterized in that the interpolating function has the mathematical form y = const- (l + x ² ) ^1/2 , where y is the value of the focal spot diameter (di) and x is the value for the running index (i) is.

6. Computer program for execution on a computer, which allows the implementation of the method according to claim 1-5.

7. A data carrier comprising a computer program according to claim 6.

8. An apparatus for automatic laser focusing for achieving a smallest possible focal spot in the laser machining of a workpiece (3), comprising

a tool laser (1), a focus position adjustment device (5),

a tensioning device for clamping the workpiece (3),

- A traversing device (6) for changing the relative position of the workpiece (3) and tool laser (1),

- A recording device (7) for receiving the image data of focal spots, - A memory and arithmetic unit (8) for storing the recorded

Image data and mathematical analysis of the same,

- A device for data transmission (9) of the analysis results to the Fokuslagenverstelleinrichtung (5).

9. Apparatus according to claim 8, characterized in that the Aufnahmevorrich- device (7) is a CCD camera.

10. Apparatus according to claim 8 or 9, characterized in that the Fokuslagenverstelleinrichtung (5) is automatically adjustable.