JP2016002580A - Method for detecting focal point shift of laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for detecting focal point shift of a laser capable of detecting and correcting focal point shift due to thermal effect at a low cost.SOLUTION: A processing head (1) is positioned with respect to a reference measurement surface (11) having a small opening (10) so that a laser beam axis (5) passes through the small opening. A laser beam (4) is emitted focusing on the reference measurement surface or the front or rear of the surface. The level of radiant light (14) radiated from at least a part of the periphery of the small opening on the reference measurement surface is measured by a radiant light measurement sensor (12), and a focal point shift amount is obtained using the measured value to correct the focal point of the laser beam.

Description

  The present invention relates to a laser defocus inspection method, and more particularly to a method of detecting a defocus due to a thermal effect and correcting a focus position based on the defocus.

  In order to perform laser welding and laser processing with stable quality, the focal length, focal diameter, and spot diameter of the laser must be strictly controlled. However, the focal length changes, that is, defocusing occurs due to various causes. It has been known.

  For example, an optical lens is used in the laser processing head, and when the laser beam passes through the lens, a part of the laser beam is absorbed, and the refractive index changes to produce a temperature distribution. Becomes shorter. Further, the protective glass provided between the focus lens and the object to be processed for the purpose of protecting the lens itself is not affected by heat, but the same as described above by heating the fumes attached to the protective glass. Defocus occurs in principle. Since the refractive index distribution of the lens converges to a steady value with a predetermined time constant depending on the lens material, etc., the thermal lens effect saturates, but the dirt on the protective glass changes depending on the processing state. The lens effect does not saturate to a predetermined value. When defocusing occurs, the machining spot diameter on the object to be processed changes, and there is a problem in that machining variations and machining defects occur.

  Processing defects can be confirmed by visual inspection or inspection by image processing, but can be detected to some extent by observing the processing state with a camera incorporated in the processing head even during laser processing. However, processing defects do not occur only due to defocusing, but occur due to various factors such as changes in the state of the workpiece and the positioning state on the processing jig. Accordingly, it is impossible to determine the presence or absence and degree of defocus by the above inspection method.

  Patent documents 1 to 4 measure the temperature change of a laser beam or a lens by a temperature sensor installed between a processing lens in a processing head and a workpiece, and adjust the position of the processing lens based on the measurement result. Disclosure. Japanese Patent Application Laid-Open No. H10-228707 discloses that an imaging deviation is detected using a camera and an imaging optical system incorporated in a processing head, and the laser optical system is corrected based on the detection. However, all of these require a dedicated laser processing head, which cannot be used in existing systems, and the construction of the system is very expensive, making it difficult to select the optimal processing head according to the processing content. There was a problem.

Japanese Examined Patent Publication No. 5-85276 Japanese Patent No. 2627205 JP 2000-94173 A JP 2013-173176 A Special table 2012-533434 gazette

  The present invention has been made in view of such a situation of the prior art, and an object of the present invention is to provide a laser defocus inspection method capable of detecting and correcting defocus due to thermal effects at low cost. is there.

  In order to solve the above-described problem, the laser defocus inspection method according to the present invention is configured such that a processing head is positioned so that a laser optical axis passes through the small opening with respect to a measurement reference plane having the small opening. A laser is focused on the measurement reference plane or its front side or rear side, and the level of radiation emitted from at least a part of the measurement reference plane around the small aperture is measured. The shift amount is obtained, and the focal position of the laser is corrected.

  When the laser is irradiated so that the laser optical axis passes through the small aperture, if the laser is defocused, the laser irradiation diameter increases according to the magnitude of the deviation, and the radiation emitted from the periphery of the small aperture Therefore, the defocus can be detected by measuring the radiation light level.

  Therefore, according to the above method, it is possible to accurately detect the defocus due to the thermal lens effect or the contamination of the protective glass simply by adding or using a measurement reference surface having a small aperture and a means for measuring the emitted light level in the laser processing equipment. Therefore, the machining can always be performed with the same machining spot diameter, and the occurrence of defects can be prevented. Moreover, it is not necessary to use a dedicated processing head, and it can be introduced into existing laser processing equipment at a low cost, and it is possible to cope with selection of a processing head according to the processing content.

  In the present invention, correlation data between a synchrotron radiation level and a focus shift amount is acquired in advance, a focus shift amount that matches the measurement value is designated with reference to the correlation data, and the focal position of the laser is corrected. Is preferred. Thereby, a correction amount can be acquired by one measurement between processing steps, and the processing step can be returned in a short time.

  In the present invention, it is preferable that the correlation data is acquired at a measurement position where a focal point is shifted by a predetermined amount toward the front side or the rear side of the measurement reference plane. In the vicinity of the focal point (beam waist), the laser spot diameter change is small and the spot diameter itself is small, but at a position far away from the focal point, the laser is in a convergent or diffuse state, and the spot diameter change is relative. Therefore, defocus can be detected easily and with high accuracy.

  In the present invention, instead of acquiring the correlation data between the synchrotron radiation level and the focus shift amount in advance, the synchrotron radiation level is measured at least twice, and the intensity gradient of the synchrotron radiation level is obtained thereby to determine the focus shift amount. It is also possible to calculate and correct the focal position of the laser. In this case, there is an advantage that a preparation step for acquiring data in advance is not necessary.

  In the present invention, it is preferable that the small opening is a circular hole or slit penetrating the measurement reference plane, or a U-shaped or V-shaped cutout cut out from one side of the measurement reference plane. . Since the circular hole emits radiated light from the entire periphery, the radiated light level change can be detected with high accuracy. In the case of V-shape, the distance between the opposing edges is not uniform, so the position of the laser optical axis can be determined. By changing, there is an advantage that it is possible to cope with a plurality of different spot diameters on one measurement reference plane.

  As described above, the laser defocus inspection method according to the present invention can detect and correct defocus due to the thermal lens effect, dirt on the protective glass, and the like with low cost and simple operation.

It is a perspective view which shows the implementation condition of the defocus inspection method of the laser which concerns on this invention. It is a typical perspective view which shows laser welding. (A) is a graph showing the relationship between the focal shift amount and the spot diameter, (b) when the lower part of the beam waist is near the measurement reference plane, and (c) when the upper part of the beam waist is near the measurement reference plane. It is a schematic perspective view shown. It is a schematic side view which shows the case where a focus exists in the upper direction (near side) and the downward direction (rear side) of a measurement reference plane. It is a flowchart which shows the defocusing inspection method of the laser which concerns on 1st Embodiment of this invention. (A) is a flowchart which shows the defocusing inspection method of the laser which concerns on 2nd Embodiment of this invention, (b) is a graph which shows the relationship between a radiated light level and a focus shift amount.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
When carrying out the laser defocusing inspection method according to the present invention, as shown in FIG. 1, a measurement reference surface 11 having a small opening 10 at an inspection position 100 and an upper side of the measurement reference surface 11 on the laser irradiation side. A synchrotron radiation measuring sensor 12 arranged to face the small opening 10 is prepared.

  The measurement reference surface 11 is an upper surface of a measurement plate made of, for example, metal or ceramic. In the illustrated example, a circular hole is provided as the small opening 10 and is fixed to a laser processing facility such as a laser welding facility with a jig not illustrated. Has been placed. The small opening 10 is a circular hole having a size slightly wider than the focal diameter at which the laser beam is most narrowed for reasons described later, and the size including such a circular hole is also used when the small opening 10 has another shape. Formed.

  The synchrotron radiation measuring sensor 12 is preferably a sensor capable of measuring the synchrotron radiation 14 from the measurement reference plane 11 accompanying laser irradiation, for example, a sensor capable of detecting a wavelength band of 400 to 1100 nm, but is not limited thereto. is not. If the laser processing equipment is equipped with a synchrotron radiation measuring device, it can also be used.

  At the time of inspection and at the time of data acquisition prior to the inspection, the laser processing head 1 is vertically positioned with a predetermined distance from the measurement reference plane 11 so that the optical axis 5 passes through the center of the small opening 10. . In the illustrated example, the measurement reference plane 11 is disposed horizontally, but the present invention is not limited to this.

  When the small aperture 10 is irradiated with the laser beam at the inspection position 100 in a steady state, all the laser beam passes through the small aperture 10, so there is no reflection upward on the measurement reference plane 11, and the synchrotron radiation measuring sensor 12 The detected radiation is at a very low level. On the other hand, when a focus shift occurs due to the thermal lens effect or the like, the light is focused once on the focal diameter above the small aperture 10 and then spreads. Therefore, it is measured as the radiated light 14 by the radiated light measurement sensor 12, thereby detecting the occurrence of the focus shift.

  Since the level of the radiation light 14 increases as the focus shift amount increases, the focus shift amount can be specified by measuring the radiation light level. Further, if the focal point of the laser processing head 1 is adjusted from that state and the radiation light level is corrected to the minimum, the defocus can be corrected immediately. As described above, there are several embodiments for detecting and correcting defocus, and typical ones will be described below.

(First embodiment)
When focus adjustment is performed little by little and an attempt is made to detect a state in which the level of emitted light is minimized when focus is shifted, it is necessary to repeat focus adjustment, laser irradiation, and emitted light measurement several times. If the correction amount for each time is small, the number of measurements increases, and if it is large, the minimum state may be exceeded. Therefore, correlation data between the synchrotron radiation level and the focus shift amount is acquired in advance, and at the time of actual inspection, the correlation shift data is referred to and a focus shift amount that matches the measurement value of the synchrotron radiation level is designated. Thus, the focal position of the laser can be corrected by only one measurement.

  When acquiring the correlation data between the synchrotron radiation level and the focus shift amount, the laser optical axis 5 passes through the small aperture 10 at the inspection position 100 shown in FIG. 1 in a steady state where the laser is not defocused. The laser processing head 1 is positioned at a predetermined position above the measurement reference plane 11, laser irradiation is performed by intentionally changing the focal position above and below the measurement reference plane 11, and the emitted light is emitted by the synchrotron radiation measuring sensor 12. 14 levels are measured, and correlation data with the focus shift amount is prepared as a function or a lookup table.

  FIG. 3A is a graph showing the relationship between the focus shift amount and the spot diameter on the measurement reference plane 11, where the y axis shows the focus shift amount, the x axis shows the spot diameter, and the y axis direction. The origin is the measurement reference plane 11. For this reason, the positive direction of the y-axis is the direction in which the lens 2 is separated from the measurement reference plane 11. The correlation data between the synchrotron radiation level and the focus shift amount acquired as described above are almost consistent with the relationship between the focus shift amount and the spot diameter, and it has been experimentally known that the synchrotron radiation level increases with the spot diameter.

  However, in the beam waist 40 between the reference numerals 4a and 4b in FIG. 3A, the change in the spot diameter is small and the radiated light level does not change. In contrast, when the focal point is above the measurement reference plane 11 (FIGS. 4 and 4c) and below the measurement reference surface 11 (FIGS. 4 and 4d), changes in the spot diameter and the emitted light level become large. Therefore, the size of the small aperture 10 is set with reference to the spot diameters on both sides (4a, 4b) of the beam waist 40, and the measurement is performed at the focal position (4c, 4d) at which the difference in emitted light intensity is easily detected. .

  After performing the above-mentioned pre-preparation, the defocus inspection during normal processing is performed as follows. For example, in the welding process (200) of the workpieces 21 and 22 (steel plates) as shown in FIG. 2, if the laser irradiation spot 24 has an appropriate diameter, a weld bead 23 having a certain width is formed. Since the lens 2 is heated as (200) overlaps and defocusing occurs due to the thermal lens effect, the welding process (200) at the end of the predetermined number of welding steps (200) or after a predetermined time has elapsed from the start of processing. At the end, the defocus inspection step (100) is performed according to the procedure shown in FIG.

  First, the laser processing head 1 is moved to the inspection position 100, laser irradiation is performed by positioning the laser processing head 1 at a predetermined position above the measurement reference plane 11 so that the laser optical axis 5 passes through the small aperture 10, and the radiation measurement sensor 12 to measure the emitted light level. If the emitted light level is below the normal threshold, the inspection process (100) is immediately terminated, and the laser processing head 1 returns to the normal welding process (200).

  On the other hand, when the radiated light level is larger than the normal threshold, it is determined that a defocus has occurred, and the measured radiated light level is determined by referring to the correlation data between the radiated light level and the focus shift amount acquired in advance. Then, a focal shift amount is designated, and a focal length correction value is calculated based on the focal shift amount. The distance between the laser processing head 1 and the workpiece 21 (or the measurement reference surface 11) is corrected by this correction value, and the inspection process (100) is completed with this, and the laser processing head 1 is subjected to a normal welding process ( 200).

  As shown by a broken line in FIG. 5, after calculating the correction value, the distance between the laser processing head 1 and the measurement reference plane 11 is corrected, laser irradiation is performed again, and the emitted light measurement sensor 12 emits the emitted light. After the level is measured and it is confirmed that the emitted light level is not more than the normal threshold value, the inspection process (100) is terminated, and the laser processing head 1 can be returned to the normal welding process (200).

  The calculated correction value is fed back to the machining system. However, in the case of a movable laser machining head, the focal length is corrected by preparing a program in which the machining position has been changed in advance, and suitable for the calculated correction value. You may be made to select the program. On the other hand, in the case of a fixed laser processing head, the position on the workpiece side may be adjusted. If the irradiation position of the laser beam can be changed by the head itself, the function can be used. . All of these can be implemented by automatic control.

(Second Embodiment)
In the above embodiment, the case where the predetermined position of the laser processing head 1 in the defocus inspection is set with the measurement reference plane 11 as the starting point has been described. However, as described above, the change in the spot diameter is small at the beam waist 40. Moreover, since the processing spot diameter used in automobile steel plate welding is usually very thin, 1 mm or less, it is possible to manage the hole diameter of 1 mm or less and measure the emitted light at an appropriate intensity to detect defocusing. However, it is disadvantageous in terms of equipment costs. Therefore, the measurement of the steady state is not a focal point (beam waist), but a portion whose diameter is expanded to several mm or more after condensing as shown by reference numeral 4c in FIG. In addition, since the amount of change can be increased, defocus detection can be performed easily and at low cost.

  That is, the measurement reference plane 11 is installed at a position deviated from the focal position by a predetermined amount (yc). In other words, a position deviated by a predetermined amount (yc) above the measurement reference plane 11 is set as the focal position, and a small aperture 10 is created in an appropriate size smaller than the spot diameter when the predetermined amount (yc) is set as the defocus amount, and in this state, correlation data between the radiation light level and the focus shift amount is acquired in advance. Also at the time of inspection, laser irradiation and radiation light level measurement are performed with a position shifted by a predetermined amount (yc) above the measurement reference plane 11 as a focal position.

  For example, as shown in FIG. 6 (b), the correlation data acquired in a state shifted by a predetermined amount (yc) above the measurement reference plane 11 when the radiation level is at the position 4e on the measurement plane. By referencing, it is possible to recognize the focus shift amount for focusing on the measurement surface with reference to the radiation level at the position 4c on the measurement surface. Thereafter, as described above, a correction value of the focal length is calculated based on the focal shift amount, and the distance between the laser processing head 1 and the workpiece 21 (or the measurement reference plane 11) is corrected, and the inspection step ( 100) is finished, or laser irradiation and radiated light level measurement are performed again, and after confirming that the radiated light level is below the normal threshold, the inspection step (100) is finished, and the laser processing head 1 is moved to the normal state. Return to the welding process (200). As shown in FIG. 3, the diameter increases on the near side (4d) of the light collection, but since the focal shift usually occurs in the direction of shortening the focal length, measurement is performed in the vicinity of the position 4c on the measurement surface. It is often advantageous to do so.

(Third embodiment)
In each of the above embodiments, by referring to the correlation data between the radiated light level and the focus shift amount acquired in advance, the focus position is specified by designating a focus shift amount suitable for the measurement from the radiated light level in one laser irradiation. Although the case where correction is performed has been described, the number of times of measurement can be reduced by calculating the focus shift amount as described below even when the radiation light level is directly measured without preparing correlation data.

  First, in the same manner as described above, the laser processing head 1 is positioned above the measurement reference plane 11, and the first laser irradiation is performed to measure the emitted light level. Next, the focal position is moved upward by a predetermined pitch, the second laser irradiation is performed, the emitted light level is measured, and the amount of change in the emitted light level per predetermined pitch is compared with the first emitted light level. That is, an intensity gradient of the radiated light level is obtained, and based on this intensity gradient, a focal position where the radiated light level becomes zero can be obtained, and a focus shift amount at the first laser irradiation position can be calculated based on this.

  Also in this case, as described above, it is preferable that the position shifted by a predetermined amount (yc) above the measurement reference plane 11 is the focal position (4c). Further, after the second laser irradiation and the measurement of the emitted light level, the focus position is further moved upward by the same pitch (or an arbitrary pitch), and the third laser irradiation is performed, and the emitted light level becomes zero. It is also possible to calculate the focus shift amount at the first laser irradiation position with the focus position approximated by a curve and using it as a reference.

  Further, when the second laser irradiation is performed by moving the focal position downward after the first laser irradiation, the radiation light level in the first laser irradiation is relatively small, that is, the focus shift amount is relatively small. Can be considered to relatively reduce the movement pitch of the focal position of the second laser irradiation. This is because it is estimated that the focal point substantially coincides with the measurement surface if the second level of emitted light is as low as the first level. On the contrary, when the level of the emitted light in the first laser irradiation is relatively high, it is also effective to perform the second laser irradiation by moving the focal position downward.

  In each of the above embodiments, the case where the small opening 10 is a circular hole penetrating the measurement reference surface 11 is shown. In addition, a parallel slit, a V-shaped slit, or a notch is formed from one side of the measurement reference surface 11. It may be a U-shaped or V-shaped cutout or slit. In the case of a V-shaped slit, the distance between the opposing edges changes in proportion to the position. Therefore, by changing the position of the laser optical axis, it is possible to cope with a plurality of different spot diameters on one measurement reference plane. .

  In addition, a plurality of small openings (circular holes, slits, notches) having different sizes may be provided on the measurement reference surface 11 so as to correspond to a plurality of different spot diameters. The measurement reference plane 11 may have a block shape other than a flat plate shape. In that case, the small aperture does not necessarily pass through, but it is necessary to prevent the emitted light from being affected by the reflected light from the inside of the aperture.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications and changes can be made based on the technical idea of the present invention.

DESCRIPTION OF SYMBOLS 1 Laser processing head 2 Lens 3 Protective glass 4 Laser light 5 Laser optical axis 10 Small opening (circular hole)
11 Measurement reference plane (measurement plate)
12 Synchrotron radiation measurement sensor 14 Synchrotron radiation 21, 22 Workpiece 23 Weld bead 24 Laser irradiation spot 40 Beam waist 100 Inspection position (inspection process)
200 Processing position (welding process)

Claims (5)

  With respect to the measurement reference plane having a small aperture, a processing head is positioned so that the laser optical axis passes through the small aperture, and the measurement reference plane or its front side or rear side is focused and irradiated with laser, A level of radiation emitted from at least a part of the measurement reference plane around the small aperture is measured, a focus shift amount is obtained from the measured value, and the focal position of the laser is corrected. Defocus inspection method.   Correlation data between a synchrotron radiation level and a focus shift amount is acquired in advance, a focus shift amount that matches the measurement value is designated with reference to the correlation data, and the focus position of the laser is corrected. The laser defocusing inspection method according to claim 1.   3. The laser defocus inspection method according to claim 2, wherein the correlation data is acquired at a measurement position in which a focus is shifted by a predetermined amount toward the front side or the rear side of the measurement reference plane.   2. The laser according to claim 1, wherein the measurement of the emitted light level is performed at least twice, an intensity gradient of the emitted light level is obtained therefrom, a focus shift amount is calculated, and a focal position of the laser is corrected. Defocus inspection method.   The said small opening consists of a circular hole or a slit which penetrates the said measurement reference plane, or a U-shaped or V-shaped notch cut out from one side of the said measurement reference plane. 5. The method of inspecting a laser defocus according to claim 4.

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