Method for checking an optical element of a laser processing device for contaminants

Abstract

The invention relates to a method for checking an optical element (4) of a laser processing device (10) for contaminants (7), wherein a laser beam (1) passes through the optical element (4), in particular is caused to radiate through the latter, and scattered light emanating from the optical element (4) is measured by an optical sensor (6), wherein N individual measurements are carried out, where N≥3; wherein during each individual measurement i the laser beam (1) passes through the optical element (4) and scattered light emanating from the optical element (4) is measured by the optical sensor (6), and a signal strength Si is determined at the optical sensor (6); for the individual measurements i different diameters Di of the laser beam (1) at the location of the optical element (4) are set, and information about a location-dependent contaminant of the optical element (4) is ascertained from the signal strengths Si of the N individual measurements, where i=1...N and i: measurement index. The method according to the invention enables more information about contaminants on an optical element to be obtained in a simple manner, in particular in order to make possible a higher availability of the laser processing device.

Description

[0001] Method for checking an optical element of a laser processing device for contamination

[0002] The invention relates to a method for checking an optical element of a laser processing device for contamination, wherein the optical element is passed by a laser beam, in particular irradiated, and scattered light emanating from the optical element is measured with an optical sensor.

[0003] Laser processing is a widely used and efficient method for machining workpieces. Laser cutting, for example, allows workpieces to be cut out of sheet metal and other materials simply and efficiently, without the need for a workpiece-specific cutting tool. Laser welding, for example, allows partial workpieces to be joined together quickly and reliably to form a single workpiece.

[0004] In laser processing, a laser beam from a laser source is typically directed onto the workpiece using a laser processing head. The laser beam is typically focused onto the surface of the workpiece or into a specific plane near the surface. The laser processing head contains optical elements, particularly lenses and protective glass, through which the laser beam passes.

[0005] Laser processing of the workpiece itself or other processes can lead to contamination of the optical elements. For example, molten workpiece material can splash onto the optical element, or dust particles can be introduced into the laser processing head during maintenance work. If the laser beam passes through a contaminated optical element, especially if it shines through it, the contamination can locally shadow the laser beam, which can impair the processing result on the workpiece. In addition, the optical element heats up significantly at the location of the contamination. Such local heating can lead to the optical imaging properties of the optical element being changed, at least locally, due to thermal expansion. The position and shape of a focus of the laser beam can be changed or distorted.

[0006] Contamination can also occur globally on an optical element, for example, due to oil particles contained in a cooling gas, which can be introduced into the cooling gas by pumping. Such global contamination leads to an overall attenuation of a passing laser beam and to a general heating of the optical element during operation. For example, DE 202010 006 047 U1 discloses the use of temperature measurement to determine increased absorption of a protective glass caused by contamination.

[0007] To determine the degree of contamination of an optical element in a laser processing device, an optical sensor can be directed at the optical element to measure the scattered light emitted by the optical element, which is caused by the passing laser beam. The greater the contamination of the optical element, the more scattered light is generated, and the higher the signal registered by the sensor. If the signal at the sensor is too high, workpiece processing can be aborted, and the optical element can be cleaned or replaced.

[0008] This procedure provides integral information about the degree of contamination of the optical element within the cross-section occupied by the laser beam. If, for example, local contamination is located in a radial edge region of the optical element, laser processing, in particular laser cutting, could in many cases still be continued without having to accept a significant loss of quality in the laser processing of the workpiece. It may be possible to switch the laser processing of workpieces to a process in which the detected contamination would be acceptable due to its location, for example by using a narrower laser beam that does not illuminate the detected contamination. Accordingly, in many cases, laser processing with a laser processing device is aborted even though further laser processing of workpieces would still be possible.

[0009] Object of the invention

[0010] It is an object of the invention to provide a method for checking an optical element of a laser processing device, with which more information about contamination on an optical element can be obtained in a simple manner, in particular in order to enable a higher availability of the laser processing device.

[0011] Description of the invention

[0012] This object is achieved according to the invention by a method of the type mentioned at the outset, wherein N individual measurements are carried out, with N>3; wherein during each individual measurement i the laser beam passes through the optical element, and scattered light emanating from the optical element is measured with the optical sensor, and a signal strength Si is determined at the optical sensor; for the individual measurements i different diameters Di of the laser beam are set at the location of the optical element; and from the signal strengths Si of the N individual measurements information about location-dependent contamination of the optical element is determined, with i=1 ... N and i: measurement index.

[0013] Within the scope of the present invention, it is provided to carry out at least three individual measurements when measuring the scattered light, which take place with different diameters of the laser beam at the location of the optical element (where the location of the optical element is related to the beam propagation direction). To change the diameter at the location of the optical element, the beam divergence of the laser beam is preferably changed, typically whereby the focus diameter of the laser beam is changed. For a given beam parameter product, a smaller focus diameter is associated with a larger beam divergence and vice versa. Depending on the position and size of a local contamination, this is illuminated by the laser beam in all or only some of the individual measurements and thereby completely, only partially, or not at all, and accordingly contributes to the generation of scattered light, which is measured with the sensor.Accordingly, the signal strengths Si of the N individual measurements provide information about the location-dependent contamination of the optical element.

[0014] Throughout the individual measurements, the laser beam typically remains aligned and centered along an optical axis. This makes it easy to obtain information about the radial distribution of contamination on the optical element around the optical axis. Furthermore, the laser power typically remains unchanged across the individual measurements. This is particularly easy to control and improves the comparability of the individual measurements.

[0015] For example, if there is local contamination (such as a dust particle) in a radial edge area of ​​the optical element, it will only enter the cross-section of the laser beam at large laser beam diameters and only then contribute to the scattered light. Conversely, local contamination near the laser beam axis will be illuminated at all laser beam diameters and will therefore contribute to the scattered light in all individual measurements.

[0016] With the spatially resolved information about the contamination of the optical element, a decision can be made on an improved basis as to whether a planned laser processing process with the laser processing device (also called a laser processing machine) is still feasible in the current state of contamination. Likewise, with the spatially resolved information about the contamination of the optical element, a process for further laser processing of workpieces can be selected that is still feasible with the laser processing device in the current state of contamination. Thus, with the aid of the measurement method according to the invention, the necessary information for improved availability (improved utilization) of the laser processing device can be provided.

[0017] Typical optical elements that can be checked for contamination using the method according to the invention are lenses and protective glasses, optionally also mirrors, including curved mirrors and semi-transparent mirrors, apertures, beam splitters, diffractive optical elements, or filters. A typical optical sensor for measuring scattered light is a photodetector, in particular a zero-dimensional photodetector, which is directed essentially laterally onto the optical element. During scattered light measurement, the laser beam passes through the optical element, typically by passing the laser beam through the optical element; however, it is also possible for the laser beam to be reflected off the optical element and thus pass through the optical element.

[0018] Preferred variants of the invention

[0019] In a preferred variant of the method according to the invention, an increased degree of contamination of the optical element within the smaller diameter Di is inferred from signal strengths Si, which increase from a larger diameter Di to a smaller diameter Di. In this way, initial, qualitative information about the location-dependent contamination can be obtained in a simple way. If local contamination is present (only or predominantly) in the region of the smaller diameter, reducing the diameter of the laser beam from the larger to the smaller diameter (with constant total laser power) leads to a higher radiation density in the region of the contamination, and thus to more scattered light. The increase in signal strengths (relative or absolute) can also be used to approximately quantitatively determine the degree of contamination within the smaller diameter compared to the larger diameter (more on this below).

[0020] In an equally preferred variant, signal strengths Si, which increase from a smaller diameter Di to a larger diameter Di, are used to conclude that there is an increased degree of contamination of the optical element outside the smaller diameter Di. In this way, initial, qualitative information about the location-dependent contamination can be obtained in a simple way. If local contamination is present (only or predominantly) in the area of ​​the larger diameter, it does not enter the cross-section of the laser beam at the smaller diameter of the laser beam, but only at the larger diameter of the laser beam, and only then does it contribute to the scattered light. This effect generally outweighs any attenuation of the local radiation density due to an increase in the beam diameter (at constant total laser power).From the increase in signal strength (relative or absolute) one can approximately quantitatively conclude the degree of contamination inside the larger diameter (and outside the smaller diameter) compared to inside the smaller diameter (more on this below).

[0021] A preferred variant provides that a degree of contamination Gi of the optical element within a smallest diameter Di and respective degrees of contamination Gj of the optical element in the range between the diameter Dj and the diameter Dj-i are determined from the signal strengths Si, with j=2... N and j: counting index of the remaining larger diameters. Using the degrees of contamination Gi, Gj within the smallest diameter Di and the respective surrounding rings within Dj-i to Dj, an intuitively interpretable assessment of the location-dependent contamination can be made in a simple manner and, if necessary, the utilization of the laser processing machine can be easily optimized in a targeted manner. It should be noted that, alternatively, integral degrees of contamination within the respective diameters Di can also be determined.

[0022] A particularly preferred further development of this variant is one in which the determination of the degrees of contamination Gi, Gj is carried out iteratively from a smallest diameter Di to a largest diameter DN. This allows the determination of the degrees of contamination Gi, Gj in a simple manner. The contamination Gi can be determined directly from Si. The contamination within the outermost rings Gj can then, for example, be determined from the inside outwards with the values ​​Sj and Gi up to Gj-i (which are compared to a raw contamination Gj determined from Sj). roh for the entire inner area of ​​the diameter Dj can be deducted). Alternatively, the contamination levels Gj can also be determined using the values ​​Sj and Sj-i, or alternatively using the values ​​Sj and Si to Sj-i , see also below.

[0023] A further development of the above variant is also preferred, which provides that in a step 1) a degree of contamination Gi of the optical element within the smallest diameter Di is inferred from a signal strength Si for a smallest diameter D1, and from the signal strength Si an expected signal contribution C2 in the signal S2 of the next larger diameter D2 due to contamination of the optical element within the smallest diameter Di is determined, and that in further steps j) a degree of contamination Gj of the optical element in a range between the diameter Dj and the diameter Dj-1 is inferred from a respective corrected signal strength KSj=Sj - Cj for the diameter Dj,and from the signal strength Sj or alternatively from the signal strengths Si to Sj, an expected signal contribution Cj+1 in the signal Sj+1 of the next larger diameter Dj+1 due to contamination of the optical element within the diameter Dj is determined. This procedure is simple and efficient for determining the contamination levels Gi to GN. It is understood that in the last step N), a determination of CN+I is no longer necessary.

[0024] An advantageous sub-variant of this further development provides that the signal contributions Cj are determined at least approximately according to

[0025] Cj=Sj-i*[Dj_i / Dj] 2

[0026] This simple estimate can contribute to a very accurate estimation of location-dependent contamination. This estimate assumes that the signal contribution Cj in the larger diameter Cj due to contamination within the next smaller diameter Dj-1 is essentially proportional to the signal strength Sj-1 of the next smaller diameter and also proportional to the ratio of the area of ​​the smaller diameter Dj-1 to the area of ​​the larger diameter Dj. The latter takes into account a redistribution of the total beam power of the laser beam (assumed to be constant) when changing the diameter, which is assumed to be the key factor for the scattered light intensity due to contamination in the smaller diameter. If desired, the accuracy of the determination of Cj can be increased by a correction factor Fj specific for j in the above formula, which can be determined by calibration.For an even more precise determination of the signal contributions Cj, these can be expressed as a sum of m summands, each specific to the G. m assigned areas are determined, with m=1 and m: index of the smaller diameters.

[0027] A sub-variant of the above development is also preferred, in which the degrees of contamination Gi to GN are determined at least approximately according to and (DJ-1) 2], where F: proportionality constant. This simple estimate allows for easy comparison of the contamination levels. The area of ​​a ring region further out is (at the same ring width) comparatively large compared to ring regions further in. Accordingly, the laser power is then distributed over a larger area, and signal strengths from the generated scattered light become weaker (for similarly sized and similarly composed local contamination). By multiplying by a factor proportional to the respective ring area, which in Gj is then [(Dj) 2 - (Dj-i ) 2 ], the area-related signal strength attenuation can be compensated. Note that the contamination levels Gi and Gj can also be determined (without weighting by area) using Gi=F'*Si and Gj=F'*KSj, where F' is the alternative proportionality constant.

[0028] A further preferred development of the above variant is that the degrees of contamination Gi, Gj are converted into remaining availabilities Vi, Vj within the smallest diameter Di for Vi or within the range between Dj and Dj-i for a respective Vj, in particular wherein the degrees of contamination Gi, Gj are assigned maximum values ​​Mi, Mj, upon reaching which the laser processing is no longer usable and the remaining availabilities Vi, Vj are calculated as Vi=1-Gi / Mi and

[0029] Vj=1-Gj / Mj, and in particular, wherein the remaining availabilities Vi, Vj are displayed on the laser processing device. The determination of remaining availabilities, in particular via Mi, Mj and Vi, Vj, enables users of the laser processing device to gain an intuitive and application-related understanding of the degree of contamination and facilitates further planning of workpiece production or the selection of further workpiece processing processes to be performed.

[0030] A further preferred variant provides that, prior to further evaluation of the signal strengths Si, the signal strengths Si are corrected for a bias after their measurement by subtracting a base signal strength Bi from the respective signal strength Si, which was obtained with the optical element in a contamination-free state using the corresponding laser beam with the diameter Di. This makes the determination of the location-dependent contamination even more precise. Bias correction taking into account an offset is particularly recommended if noticeable stray light occurs in the laser processing head regardless of the contamination of the optical element being tested, for example due to surface roughness of the optical element or scattering at locations distant from the optical element.

[0031] Alternatively or additionally, for further evaluation of the signal strengths Si, the signal strengths Si can be set in relation to a bias after their measurement in order to determine a degree of contamination, which corresponds to a contamination state in relation to a clean state. This makes the determination of location-dependent contamination even more precise. Bias correction taking into account a factor is particularly recommended if noticeable stray light occurs in the laser processing head independent of the contamination of the optical element being inspected, for example, due to surface roughness of the optical element or scattering at locations distant from the optical element.

[0032] Operating methods according to the invention

[0033] The present invention also includes a method for operating a laser processing device, wherein an inspection of an optical element of the laser processing device for contamination is carried out, characterized in that the inspection of the optical element of the laser processing device for contamination is carried out according to a method according to the invention described above, that a decision is made on the basis of a result of the inspection as to whether a notified process of laser processing a workpiece can be carried out with the laser processing device or not, and that in the decision as to whether the notified process can be carried out or not,At least the determined location-dependent information about the contamination of the optical element and information about the diameter(s) of the laser beam at the optical element to be used in the intended process must be taken into account. If the current location-dependent contamination of the optical element and the diameter(s) of the laser beam at the optical element to be used for the intended process are taken into account when deciding whether or not a notified process can still be carried out, unnecessary downtime of the laser processing device can be minimized.

[0034] Likewise within the scope of the present invention is a method for operating a laser processing device, wherein an optical element of the laser processing device is checked for contamination, characterized in that the optical element of the laser processing device is checked for contamination according to a method according to the invention described above, and that in the event that the determined information about the location-dependent contamination of the optical element shows that relevant contamination is present only in a radial edge region of the optical element, but not in a central region of the optical element, the laser processing device remains ready for operation with the proviso that until the optical element is cleaned or replaced, only processes for laser processing of workpieces are carried out,in which the diameter of the laser beam at the optical element remains within the central area. This procedure ensures that, in the event of significant contamination (precluding continued operation) only in the radial edge area, the laser processing device remains available at least for certain processes, which in particular only require the laser beam to be guided in the central area of ​​the optical element. With appropriate planning or replanning of the processes, downtimes of the laser processing device can be minimized.

[0035] In an advantageous variant of the two above methods for operating a laser processing device, the optical element of the laser processing device is checked for contamination:

[0036] After any maintenance or repair to a laser processing head of the laser processing device; and / or

[0037] After a specified operating time of the laser processing device; and / or

[0038] Each time the laser processing device is started up; and / or

[0039] Before starting any new process of laser machining of workpieces; and / or

[0040] With manual triggering. This ensures high processing quality of the workpieces. At the same time, good availability of the laser processing device can be achieved.

[0041] Laser processing devices according to the invention

[0042] The scope of the present invention also includes a laser processing device, comprising a laser source for providing a laser beam, an optical element through which the laser beam passes, in particular through which it is irradiated, an adjusting device for adjusting a diameter of the laser beam at the location of an optical element, an optical sensor for measuring scattered light emanating from the optical element, and an electronic control device, characterized in that the electronic control device is designed to carry out, in an automated sequence, a method for checking the optical element of the laser processing device for contamination according to a method according to the invention described above, wherein the electronic control device is designed toFor the N individual measurements, the adjustment device is used to successively set different diameters Di of the laser beam at the location of the optical element, and to determine a corresponding signal strength Si at the optical sensor using the respective diameter Di. This laser processing device enables a location-triggered determination of contamination on the optical element, thus achieving high availability for workpiece processing.

[0043] A preferred embodiment of the laser processing device according to the invention is one in which the laser processing device is a laser cutting device. In laser cutting, the cutting processes can often be varied with respect to the diameter of the laser beam at the location of an optical element with little effort and without any significant loss of quality, thus achieving particularly high availability in the event of contamination only in the radial edge region of the optical element.

[0044] Further advantages of the invention will become apparent from the description and the drawings. Likewise, the above-mentioned and further-described features can be used individually or in combination in any desired manner. The embodiments shown and described are not intended to be exhaustive, but rather are exemplary in nature for describing the invention.

[0045] Detailed description of the invention and drawing

[0046] Fig. 1a schematically illustrates, for an exemplary variant of a method according to the invention for checking the contamination of an optical element of a laser processing device, a first individual measurement with a smallest diameter Di of the laser beam at the location of an optical element, in longitudinal section along the beam propagation direction;

[0047] Fig. 1 b illustrates a second single measurement with a mean diameter D2 for the variant of Fig. 1 a;

[0048] Fig. 1c illustrates a third individual measurement with a largest diameter D3 for the variant of Fig. 1a;

[0049] Fig. 2a illustrates schematically for the variant of Fig. 1a the first individual measurement, with a cross-section at the location of the optical element;

[0050] Fig. 2b illustrates the second individual measurement for the variant of Fig. 2a;

[0051] Fig. 2c illustrates the third individual measurement for the variant of Fig. 2a; Fig. 3 schematically illustrates the measurement situation for the three individual measurements for the variant of Fig. 1a, with a cross-section at the location of the optical element (left), with contamination in the area between D2 and D3, and with an associated diagram of the measured signal strengths (right);

[0052] Fig. 4 schematically illustrates a measurement situation for three individual measurements similar to the variant of Fig. 1a, with a cross-section at the location of the optical element (left), but with contamination in the area between Di and D2, and with an associated diagram of the measured signal strengths (right);

[0053] Fig. 5 schematically illustrates a measurement situation for three individual measurements similar to the variant of Fig. 1a, with a cross-section at the location of the optical element (left), but with contamination in the area within Di, and with an associated diagram of the measured signal strengths (right);

[0054] Fig. 6 schematically illustrates the measurement situation for the three individual measurements similar to the variant of Fig. 1a, with a cross-section at the location of the optical element, for a bias measurement without contamination;

[0055] Fig. 7a schematically illustrates an exemplary embodiment of a laser processing device according to the invention, wherein a smallest diameter Di of the laser beam is set on the optical element by means of the adjusting device;

[0056] Fig. 7b illustrates the embodiment of Fig. 7a, wherein the adjustment device is used to set a mean diameter D2 of the laser beam at the optical element; Fig. 7c illustrates the embodiment of Fig. 7a, wherein the adjustment device is used to set a maximum diameter D3 of the laser beam at the optical element;

[0057] Fig. 8 schematically illustrates an exemplary design of a laser processing head of a laser processing device for the invention, wherein a protective glass is monitored with a sensor as the optical element;

[0058] Fig. 9 schematically illustrates an exemplary design of a laser processing head of a laser processing device for the invention, wherein a beam splitter is monitored with a sensor as the optical element;

[0059] Fig. 10 schematically illustrates an exemplary design of a laser processing head of a laser processing device for the invention, wherein a lens with a sensor is monitored as the optical element.

[0060] Figures 1a to 1c and Figures 2a to 2c illustrate an exemplary variant of a method according to the invention for checking the contamination of an optical element of a laser processing device based on three individual measurements. Figures 1a to 1c each show longitudinal sections along the beam propagation direction through a portion of a laser processing head of the laser processing device near the workpiece to be processed, and Figures 2a to 2c each show cross-sections at the location of the optical element.

[0061] As can be seen in Fig. 1a, the laser processing device directs a laser beam 1 onto a workpiece (not shown, but see Fig. 7a for this), with a focus 2 of the laser beam 1 typically lying on the workpiece surface. In the illustrated variant, the laser beam 1 is focused by a lens 3, with the laser beam 1 passing through an optical element 4, here a protective glass 5, located between the lens 3 and the focus 2. The protective glass 5 may contain contamination, for example as a result of splashes of molten workpiece material, or simply dust particles. By way of example, local contamination 7 in the form of a dust particle is illustrated here. An optical sensor 6 is directed onto the optical element 4 from the side and measures scattered light emanating from the optical element 4. Scattered light is primarily generated by contamination 7 on the optical element 4 when the laser beam 1 is scattered by the contamination 7.

[0062] Within the scope of the invention, several individual measurements of the scattered light are performed, each using a different diameter Di of the laser beam 1 at the location of the optical element 4. In all cases, the laser beam 1 remains centered on a common optical axis OA, and the laser power is kept constant. However, the beam divergence is changed to change the diameter Di at the location of the optical element (more on this in Figs. 7a-7c).

[0063] In the first individual measurement of Fig. 1a and Fig. 2a, the smallest diameter Di is used. Since the contamination 7 lies outside the diameter Di of the laser beam 1, it does not contribute to the generation of scattered light.

[0064] The second individual measurement of Fig. 1b and Fig. 2b is performed with an average diameter D2 of the laser beam 1 at the location of the optical element 4. The contamination 7 is also located outside D2 in this case and therefore does not contribute to the generation of scattered light in the second individual measurement.

[0065] The third individual measurement of Fig. 1c and Fig. 2c is performed with the largest diameter D3 of the laser beam 1 at the location of the optical element 4. Now, the contamination 7 lies within D3 and therefore contributes to the generation of scattered light.

[0066] In the contamination situation illustrated in Figures 2a to 2c, a noticeable signal strength due to scattered light is only achieved in the third individual measurement with D3. This makes it easy to conclude that noticeable contamination is only present in the area between D2 and D3. It is understood that the individual measurements illustrated in Figures 1a to 1c and 2a to 2c can be performed in any order. Furthermore, it should be noted that, according to the invention, four, five, or even more individual measurements (with additional diameters D4, D5, etc.) can also be performed.

[0067] In the following, typical contamination situations on an optical element, which are checked using a method according to the invention, will be discussed qualitatively and quantitatively using examples. It is assumed that three individual measurements as shown in Fig. 1 a-1 c and Fig. 2a-2c are carried out with the diameters Di, D2, D3 of the laser beam at the location of the optical element, where Di = 1.0 mm, D2 = 1.5 mm and D3 = 2.0 mm were selected. The diameter of the laser beam can be determined using the 86% criterion (86% of the laser power lies within a circle with the specified diameter). During these measurements, the signal strengths Si, S2 and S3 are measured at the sensor, for example using a photocurrent (unit milliampere, mA).

[0068] Fig. 3 shows the contamination situation as already discussed in Figs. 1 a-1 c and 2a-2c. As can be seen in the left-hand cross-section of Fig. 3, a single local contamination 7 is located in the area between D2 and D3. Typical corresponding measured values ​​of signal strengths S and Si at the corresponding diameters D and Di are given in the right-hand diagram of Fig. 3 and in the following Table 1 (along with other values):

[0069] Table 1: Example measured values ​​for the situation in Fig. 3

[0070] Since the local contamination 7 (the only one in the example) is located outside of D2, as shown in Fig. 3, a significant signal strength S3 is obtained at the scattered light sensor only in the third individual measurement, which takes place with diameter D3. The signal strengths Si and S2, in contrast, are much smaller.

[0071] In the situation of Fig. 4, the only local contamination 7 is located within the innermost diameter Di, cf. the left-hand cross-section in Fig. 4.

[0072] As can be seen in the right-hand diagram of Fig. 4 and in Table 2, a high signal strength S1 is obtained in the first individual measurement i=1. Since this local contamination 7 also lies within D2 and within D3, noticeable signal strengths S2 and S3 are also measured. Since the radiant power of laser beam 1 is distributed over a larger area in the individual measurements i=2 and i=3, the scattered light intensity (and thus the measured signal strength S) decreases with increasing diameter D of laser beam 1.

[0073] Table 2: Example measured values ​​for the situation in Fig. 4

[0074] In the situation of Fig. 5, the only local contamination 7 is located in the area between Di and D2, cf. the left-hand cross section in Fig. 5.

[0075] Accordingly, the signal strength S2 of the scattered light sensor is greatest in the second individual measurement. Contamination 7 also affects the third individual measurement i=3, and a noticeable signal strength S3 is obtained, albeit lower than S2, because the laser power is distributed over a larger area from D2 to D3. In the first individual measurement at Di, only a minimal signal strength S1 is obtained, because contaminant 7 is not within D1. All of this can be seen in the right-hand diagram in Fig. 5 or in Table 3.

[0076] Table 3: Example measured values ​​for the situation in Fig. 5

[0077] Now, the degree of contamination Gi for the area within Di , G2 for the annular area between Di and D2, and G3 for the annular area between D2 and D3 are to be determined.

[0078] According to the invention, the procedure can be "from the inside out". Assuming an approximately uniform illumination of the cross-section of the laser beam for all beam diameters Di, D2, D3, an expected signal contribution Cj due to impurities in this inner diameter for the signal strength Sj at the next larger beam diameter Dj (index value j) can be determined from the signal strength Sj-1 for an inner diameter Dj-1 (index value j-1) according to

[0079] Cj=Sj-i*[Dj_i / Dj] 2 with j: index of the (larger) diameters, starting at j=2 and running up to N, here with N=3. Note that here [DI / D2] 2 = 0.4444 and [D2 / D3] 2=0.5625. Using this expected signal contribution Cj, a corrected signal strength KSj=Sj-Cj can then be determined for the outermost (annular) regions; note that the innermost region (within Di) does not require such a correction. The corrected signal strengths KSj therefore describe the respective signal strength attributable to scattered light from impurities in the respective annular region from Dj-1 to Dj, i.e., excluding a respective signal strength attributable to scattered light from impurities within the diameter Dj-1. The corresponding values ​​of Cj and KSj for the various situations in Fig. 3, 4 and 5 are entered in Tables 1, 2 and 3, respectively. In order to obtain a comparable degree of contamination of the respective regions, the degree of contamination Gi, Gj in a respective region can now be determined according to

[0080] Gi=F*Si*[Di] 2 and Gj=F*KSj*[(Dj) 2- (Dj.i ) 2 ], with F: proportionality constant, in the above tables 1, 2, 3 simply chosen as 1 / mA. The factor [(Dj) 2 - (Dj.i ) 2 ] compensates for the lower power density of the laser beam as the beam diameter increases. Note that here [(D2) 2 - (Tue) 2 ]=1 ,25 and [(D3) 2 - (D2) 2 ]=1.75. The respective values ​​G1, Gj then roughly correspond to the absolute quantity (size / area) of scattering contaminants in the respective corresponding area. These values ​​are also entered in Tables 1, 2, and 3.

[0081] In the cases of Figures 3, 4, 5 and Tables 1, 2, 3, it is clear that in each of the examples, similarly large impurities are present in the respective area. In Fig. 3 / Table 1, in the area between D2 and D3, G3=7.212, and G1, G2 are each <0.1. In Fig. 4 / Table 2, in the area within Di, Gi=7.134, and G2, G3 are each <0.1. In Fig. 5 / Table 3, in the area Di to D2, G2=7.005, and G1, G3 are each <0.1. Thus, based on the measurements and the evaluations based on Gi, it is possible to very precisely determine where the respective impurity is located.

[0082] In practice, a large number of individual, smaller contaminants (e.g. dust particles, splashes of workpiece material) typically contribute to the overall contamination of the optical element 4, which are distributed across the various regions of the optical element 4. In this case, a maximum degree of contamination can be determined experimentally for each region, referred to here as Mi, at which laser processing of a workpiece is no longer possible (with sufficient quality). In the present case, the values ​​Mi=7.5, M2=15 and Ms=30 were determined or specified for the laser processing device. In other words, more contamination can be accepted in the outer regions than in the inner regions, which can generally be regarded as a preferred specification according to the invention and is suitable for many laser cutting processes.

[0083] About the formula

[0084] Using Vi=1-Gi / Mi, the remaining availability of the optical element in the respective range associated with the index value i can be determined. In the case of Gi>Mi, V can then be set to 0. This can also be expressed as a percentage.

[0085] As can be seen from Tables 1, 2, and 3, the impurities in the situation in Fig. 3, 4, and 5, which are of comparable size, mean that in the case of impurity 7 in the innermost region of Fig. 4, this region within Di is already almost exhausted, with a remaining availability of only Vi=0.049 or 4.9%. In contrast, in the case of Fig. 3, with impurity 7 in the radially outermost region, between D2 and D3, with a remaining availability V3=0.760 or 76%, this region is only slightly exhausted. In the case of Fig. 5, with impurity 7 in the middle ring region between Di and D2, the remaining availability V2 is 0.533 or 53.3%.

[0086] The area-specific degrees of contamination Gi or the area-specific remaining availabilities represent typical information about location-dependent contamination (also called spatially resolved contamination information) about the optical element for the invention.

[0087] Based on the area-specific remaining availability, an electronic control device can estimate whether the laser processing device is still generally operational. For such a check, minimum values ​​(here also called MINi) of remaining availability can be specified for the individual areas. A simple general operational readiness check could, for example, stipulate that at least 10% remaining availability must be available in each area before the next process on a workpiece can begin (i.e., MINi>10% for all i, i=1 to N). For more stringent quality requirements, higher minimum values ​​MINi, e.g., at least 50%, can also be specified.

[0088] Preferably, the invention provides that specific minimum values ​​of remaining availability for process execution are assigned to individual notified processes of workpiece machining (also called MIN(p)i, with p: process indicator, and i: index of the individual measurements or the measured diameters). These individual specific minimum values ​​MIN(p)i of remaining availability then contain information about the diameter of the laser beam to be used in the notified process (specified by p). An electronic control device can then judge, based on a comparison of the remaining availability with the specific minimum values ​​MIN(p)i, whether a certain notified process (corresponding to p) may still be started or not. A start is possible if MIN(p)i>V for all i.

[0089] A designated process with p=X, for example, which only uses laser beams with diameters of D2 or smaller (in a "central region"), specifies MIN(X)i=25%, MIN(X)2=25%, and MIN(X)3=0%; the availability in the third region from D2 to D3 is irrelevant for this process, since this process X does not use any laser power in the region greater than D2 (in a "radial edge region") and is therefore "zero." If, in this example, the final remaining availabilities for the laser processing device were experimentally determined to be, for example, Vi=77%, V2=88%, and Vb=0%, then this process X can be approved without any problems, even though the optical element in the third region from D2 to D3 (in the "radial edge region") is completely contaminated, as evidenced by V3=0%.

[0090] In an analogous manner, the electronic control device can select from workpiece processing processes suitable for an upcoming processing task whose respective sets of minimum values ​​MIN(p)i are met by the remaining availabilities determined experimentally as shown above. Processes suitable for the upcoming processing task but whose respective sets of minimum values ​​MIN(p)i are no longer met by the remaining availabilities are blocked as long as the optical element has not been cleaned or replaced. The laser processing device remains available for workpiece processing as long as one or more processes remain whose sets of minimum values ​​MIN(p)i are still met by the currently remaining availabilities V.The latter is usually the case when the relevant contamination detected primarily affects the radial edge area of ​​the optical element, but not the central area. In this case, a laser beam that is (radially) narrow at the location of the optical element can still be used effectively.

[0091] To enable an even more precise estimation of the contamination levels in individual areas, bias correction can be performed. Figure 6 shows, as an example, a schematic cross-section of optical element 4, which is measured without contamination in three individual measurements i=1, 2, 3 with laser beam diameters Di, D2, D3 (at the location of the optical element) by the scattered light sensor. This yields baseline signal strengths Bi; see the entries in Table 4 for this. The respective baseline signal strength can be based, for example, on scattering due to the roughness of the contamination-free surface of the optical element, or scattering in front of or behind the optical element, and possibly multiple scattering.

[0092] These basic signal strengths Bi can be derived from the “raw” (directly measured, not yet bias-corrected) signal strengths of the scattered light sensor, shown in Table 4 Si rohare subtracted from the individual measurements of the contamination check. As an example, the signal strengths Si from Table 3 were taken in Table 4 as unadjusted signal strengths. With the thus adjusted signal strengths Si=Si roh - Bi the calculations shown above can then be carried out with improved accuracy (not further elaborated).

[0093] Table 4: Example bias correction of signal strengths Si

[0094] In the example shown here, the baseline signal strengths are quite low compared to the signal strengths with scattered light due to contamination, which is also a common occurrence in practice. In this case, bias correction only makes a minor difference in the calculation of contamination levels or the spatially resolved contamination determination. However, if contamination-independent scattered light (e.g., due to surface roughness) plays a significant role, bias correction can significantly improve the accuracy of the spatially resolved contamination determination.

[0095] Fig. 7a shows an example of an embodiment of a laser processing device 10 according to the invention in the region of a laser processing head in a schematic longitudinal section.

[0096] The laser beam 1 emerges divergently from a laser source 11, for example the end of an optical fiber connected to a laser oscillator (not shown in detail). The laser beam 1 is collimated using the two lenses 14 and 15 of a collimation system, with lens 14 imaging an exit pupil of the laser source 11 into an intermediate focus 16, and with the intermediate focus 16 lying in front of the lens 15 at the distance of its focal length. The laser beam 1 is then focused onto the surface of a workpiece 12 using lens 3. The laser beam 1 passes through the optical element 4, which is designed here as a protective glass 5. An optical sensor 6 is directed onto the optical element 4 and registers scattered light from the optical element 4. At the location of the optical element 4, the laser beam 1 in Fig. 7a has a diameter Di for a first individual measurement.To adjust this diameter for further individual measurements of the scattered light from the optical element 4, the laser processing device 10 has an adjustment device 13. This adjustment device 13 allows the position of the lens 14 and the position of the lens 15 to be changed along the beam propagation direction. The sensor 6 and the adjustment device 13 are connected to an electronic control device 17, with which the individual measurements of the scattered light can be performed automatically.

[0097] The laser processing device 10 here is a laser cutting device with which cuts can be made into the workpiece 12. The workpiece 12 can be, for example, a metal sheet.

[0098] In Fig. 7b, the lens 14 was moved slightly away from the laser source 11 using the adjustment device (not shown in detail for simplicity), and the lens 15 was moved slightly away from the lens 14 according to the imaging requirements. This resulted in a slightly larger diameter D2 at the location of the optical element 4.

[0099] In Fig. 7c, the lens 14 was moved a little further away from the laser source 11 using the adjustment device (not shown in detail for simplicity), and the lens 15 was moved a little further away from the lens 14 according to the imaging requirements. This resulted in an even larger diameter D3 at the location of the optical element 4.

[0100] Figures 8, 9, and 10 illustrate typical optical elements and their positions in a laser processing device 10, which can be examined for contamination within the scope of the invention. The laser processing device 10 largely corresponds to the laser processing device shown in Fig. 7a (see there). In the design of Fig. 8, the optical sensor 6 monitors a protective glass 5 as the optical element 4 for contamination and measures scattered light emanating from the protective glass 5 at different laser beam diameters. In the design of Fig. 9, the sensor 6 monitors a beam splitter 20 or a semi-transparent mirror as the optical element 4 for contamination. The semi-transparent mirror can, for example, couple out thermal radiation emanating from the processing location on the workpiece for process monitoring in order to analyze this radiation.

[0101] In the design of Fig. 10, the optical element 4 monitored by the sensor 6 is the lens 3, with which the laser beam 1 is focused onto the workpiece (not shown).

[0102] List of reference symbols

[0103] 1 laser beam

[0104] 2 Focus

[0105] 3 Lens (focusing lens)

[0106] 4 optical element

[0107] 5 protective glass

[0108] 6 optical sensor

[0109] 7 (local) pollution

[0110] 10 Laser processing device

[0111] 11 Laser source

[0112] 12 Workpiece

[0113] 13 Adjustment device

[0114] 14 Lens (the collimation system)

[0115] 15 Lens (the collimation system)

[0116] 16 Intermediate focus

[0117] 17 electronic control device

[0118] 20 beam splitters / semi-transparent mirrors

[0119] D Diameter of the laser beam (general)

[0120] Di, D2, D3 diameter of the laser beam

[0121] OA optical axis

[0122] S Signal strength (general)

Claims

Patent claims 1. A method for checking an optical element (4) of a laser processing device (10) for contamination (7), wherein a laser beam (1) passes through the optical element (4), in particular irradiates it, and scattered light emanating from the optical element (4) is measured with an optical sensor (6), characterized in that N individual measurements are carried out, with N>3, wherein during each individual measurement i, the laser beam (1) passes through the optical element (4), and scattered light emanating from the optical element (4) is measured with the optical sensor (6), and a signal strength Si is determined at the optical sensor (6), that different diameters Di of the laser beam (1) are set at the location of the optical element (4) for the individual measurements i, and that information about location-dependent contamination of the optical element (4) is determined from the signal strengths Si of the N individual measurements, with i=1 ...N and i: measurement index.

2. Method according to claim 1, characterized in that from signal strengths Si, which increase from a larger diameter Di to smaller diameter Di, it is concluded that there is an increased degree of contamination of the optical element (4) within the smaller diameter Di.

3. Method according to one of the preceding claims, characterized in that from signal strengths Si, which increase from a smaller diameter Di to a larger diameter Di, it is concluded that there is an increased degree of contamination of the optical element (4) outside the smaller diameter Di.

4. Method according to one of the preceding claims, characterized in that from the signal strengths Si a degree of contamination Gi of the optical element (4) within a smallest diameter Di and respective degrees of contamination Gj of the optical element (4) in the range between the diameter Dj and the diameter Dj-i are determined, with j=2... N and j: counting index of the remaining larger diameters.

5. Method according to claim 4, characterized in that the determination of the degrees of contamination Gi, Gj is carried out iteratively from a smallest diameter Di to a largest diameter DN.

6. Method according to claim 4 or 5, characterized in that in a step 1) a degree of contamination Gi of the optical element (4) within the smallest diameter Di is inferred from a signal strength Si for a smallest diameter Di, and an expected signal contribution C2 in the signal S2 of the next larger diameter D2 due to contamination (7) of the optical element (4) within the smallest diameter Di is determined from the signal strength Si, and in that in further steps j) a degree of contamination Gj of the optical element (4) in a range between the diameter Dj and the diameter Dj-1 is inferred from a respective corrected signal strength KSj=Sj - Cj for the diameter Dj,and from the signal strength Sj or alternatively from the signal strengths Si to Sj an expected signal contribution Cj+1 in the signal Sj+1 of the next larger diameter Dj+1 is determined by a contamination (7) of the optical element (4) within the diameter Dj., 7. Method according to claim 6, characterized in that the signal contributions Cj are determined at least approximately according to Cj=Sj-i*[Dj_i / Dj] 2 8. Method according to claim 6 or 7, characterized in that the degrees of contamination Gi to GN are determined at least approximately according to Gi=F*Si*[Di] 2 and Gj=F*KSj*[(Dj) 2 - (DJ-1) 2 ], with F: proportionality constant.

9. Method according to one of claims 4 to 8, characterized in that the degrees of contamination Gi, Gj are converted into remaining availabilities Vi, Vj within the smallest diameter Di for Vi or within the range between Dj and Dj-i for a respective Vj, in particular wherein the degrees of contamination Gi, Gj are each assigned maximum values ​​Mi, Mj, upon reaching which the laser processing is just no longer usable and the remaining availabilities Vi, Vj are calculated as Vi=1 -Gi / Mi and Vj=1-Gj / Mj, and in particular wherein the remaining availabilities Vi , Vj are displayed on the laser processing device (10).

10. Method according to one of the preceding claims, characterized in that before a further evaluation of the signal strengths Si, the signal strengths Si are corrected for a bias after their measurement by subtracting from the respective signal strength Si a basic signal strength Bi which was obtained with the optical element (4) in a contamination-free state with the corresponding laser beam (1) with the diameter Di. 11 . Method for operating a laser processing device (10), wherein a check of an optical element (4) of the laser processing device (10) for contamination (7) is carried out, characterized in that the check of the optical element (4) of the laser processing device (10) for contamination (7) is carried out according to one of claims 1 to 10, that a decision is made on the basis of a result of the check whether a notified process of laser processing a workpiece (12) with the Laser processing device (10) can be carried out or not, and that in the decision as to whether the indicated process can be carried out or not, at least the determined location-dependent information about the contamination of the optical element (4) and information about the diameter(s) (D) of the laser beam (1) on the optical element (4) to be used in the context of the indicated process are taken into account.

12. A method for operating a laser processing device (10), wherein an inspection of an optical element (4) of the laser processing device (10) for contamination (7) is carried out, characterized in that the inspection of the optical element (4) of the laser processing device (10) for contamination is carried out according to one of claims 1 to 10, and in that in the event that the determined information about the location-dependent contamination of the optical element (4) shows that relevant contamination (7) is present only in a radial edge region of the optical element (4), but not in a central region of the optical element (4), the laser processing device (10) remains ready for operation with the proviso that until the optical element (4) is cleaned or replaced, only processes for laser processing of workpieces (12) are carried out,in which the diameter (D) of the laser beam (1) at the optical element (4) remains within the central area., 13. Method according to claim 11 or 12, characterized in that the optical element (4) of the laser processing device (10) is checked for contamination (7): - after each maintenance or repair on a laser processing head of the laser processing device (10); and / or - after a predetermined operating time of the laser processing device (10); and / or - each time the laser processing device (10) is started up; and / or - before each start of a new laser processing process (10) of workpieces (10); and / or - when triggered manually.

14. Laser processing device (10), comprising a laser source (11) for providing a laser beam (1), an optical element (4) through which the laser beam (1) passes, in particular through which it is irradiated, an adjusting device (13) for adjusting a diameter (D) of the laser beam (1) at the location of an optical element (4), an optical sensor (6) for measuring scattered light emanating from the optical element (4), and an electronic control device (17), characterized in that the electronic control device (17) is configured to carry out, in an automated sequence, a method for checking the optical element (4) of the laser processing device (10) for contamination (7) according to one of claims 1 to 10, wherein the electronic control device (17) is configured tofor the N individual measurements, using the adjustment device (13) to successively set different diameters Di of the laser beam (1) at the location of the optical element (4) and to determine an associated signal strength Si at the optical sensor (6) using the respective diameter Di.

15. Laser processing device (10) according to claim 14, characterized in that the laser processing device (10) is a laser cutting device.

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