JP2010279993A - Method for evaluating laser beam weld zone - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for evaluating a laser beam weld zone by which the irregularity of the hue in the weld zone is determined accurately. <P>SOLUTION: First, the image data of the weld zone which is formed on a workpiece by the irradiation with the laser beam is obtained (S31). Then, an RGB value for determination is calculated by performing RGB separation of the image data (S32). A gray scale value is calculated by performing the gray scale conversion of the RGB value for determination (S33). The divided value for determination is calculated by dividing the RGB value for determination by the gray scale value (S34). Mahalanobis distance is calculated from the divided value for determination (S35). When the Mahalanobis distance is smaller than a predetermined value, it is determined that the irregularity of the hue in the weld zone is normal (S36→S37). <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a laser weld evaluation method.

  As a conventional method for evaluating a laser welded portion, a method for evaluating a welded portion formed on a base material by laser beam irradiation is known (for example, see Patent Document 1). In such a laser welded part evaluation method, image quality (image data) of a welded part is captured by a CCD camera, and the welding quality is determined by determining the width of the heat-affected discoloration part based on this image information.

JP-A-10-58170

  By the way, in general, in laser welding, when a welded portion appears in the appearance of a base material, discoloration such as scorching generated in the welded portion may be removed by post-welding processing (for example, electrolytic polishing). In this case, if the hue of the welded portion is composed of the same color system, the discoloration can be suitably removed by post-welding processing. Therefore, it is desired that the welded portion has less hue unevenness (that is, there are various hues, not density variations within the same color system). Therefore, a laser welded part evaluation method is required that can accurately determine the hue unevenness of the welded part.

  Here, in the laser welded part evaluation method described above, since the influence of the unevenness existing in the welded part is not taken into consideration, when judging the hue unevenness of the welded part, for example, the color caused by the position of the light source, etc. The effect of shading (shadows due to unevenness) will be affected. That is, the same-color density variation (brightness and darkness) in the welded portion is also determined as hue unevenness. Therefore, in the laser welded part evaluation method, it may be difficult to accurately determine the hue unevenness of the welded part.

  Then, this invention makes it a subject to provide the laser welded part evaluation method which can determine the hue nonuniformity of a welded part accurately.

  In order to achieve the above object, a laser welded portion evaluation method according to the present invention is a laser welded portion evaluating method for evaluating a welded portion formed on a base material by irradiation with a laser beam. A hue determination step for determining unevenness; in the hue determination step, the image information of the welded portion is acquired, the RGB value of the image information is calculated, and the value obtained by converting the RGB value to grayscale is the grayscale value. And the hue unevenness is determined based on the division value obtained by dividing the RGB value by the gray scale value.

  In the laser weld evaluation method of the present invention, the division value is obtained by dividing the RGB value by the gray scale value. That is, calculation is performed so that the RGB value is normalized by a gray scale value representing the brightness of the color, and a division value is obtained. Therefore, in the division value, the variation in density of the same color system in the image information is removed. Therefore, it is possible to accurately determine the hue unevenness by determining the hue unevenness of the welded portion based on the division value.

  In the hue determination step, it is preferable to calculate the Mahalanobis distance based on the division value and determine that the hue unevenness is normal when the Mahalanobis distance is smaller than the first threshold. In this case, it is possible to qualitatively determine the hue unevenness of the welded portion using an MT (Mahalanobis-Taguchi) system, and to determine the hue unevenness of the welded portion with higher accuracy.

  Here, before the hue determination step, there may be a determination unit space generation step for generating in advance a determination unit space used when calculating the Mahalanobis distance in the hue determination step.

  At this time, the determination unit space generation step acquires image information of the base material, calculates the RGB value of the image information as the base material RGB value, and converts the base material RGB value to a gray scale value to obtain a value obtained from the base material. A first step of generating a reference unit space based on a base material division value obtained by dividing the base material RGB value by the base material gray scale value, and calculating image information of the reference weld The RGB value of the image information is calculated as a reference RGB value, the value obtained by converting the reference RGB value to grayscale is calculated as a reference grayscale value, and the reference RGB value is calculated as a reference grayscale value. Based on the reference division value obtained by dividing by the scale value, the Mahalanobis distance is calculated using the reference unit space generated in the first step, and when the Mahalanobis distance is smaller than the second threshold, Base A second step of generating a determination unit space based on the use division value, it is preferable to include a. In this case, the above-described effect of qualitatively determining the hue unevenness of the welded portion using the MT system can be suitably exhibited.

  According to the present invention, it is possible to accurately determine the hue unevenness of the welded portion.

It is the schematic which shows the structure of the laser welding system which enforces the laser welding part evaluation method which concerns on one Embodiment of this invention. It is a flowchart which shows the process process by the laser welding system of FIG. It is a flowchart which shows the unit space generation process for determination in the process before welding of FIG. It is a flowchart which shows the hue determination process in the post-welding process of FIG. (A) is a figure which shows the sample hue of red, (b) is a graph which shows R value of the sample hue of FIG. 5 (a), (c) is a gray scale R value of FIG. 5 (a). It is a graph which shows the value divided by the value. (A) is a figure which shows a green sample hue, (b) is a graph which shows G value of the sample hue of FIG. 6 (a), (c) is a gray scale for G value of FIG. 6 (a). It is a graph which shows the value divided by the value. (A) is a figure which shows the sample hue of blue, (b) is a graph which shows B value of the sample hue of FIG. 7 (a), (c) is B scale of FIG. It is a graph which shows the value divided by the value. It is a graph which shows the Mahalanobis distance of the welding part for reference | standard. (A) is a graph which shows the RGB value for determination of the welding part of normal hue unevenness, (b) is a graph which shows the division value for determination of the welding part of normal hue unevenness. (A) is a graph which shows the RGB value for determination of the welding part of abnormal hue unevenness, (b) is a graph which shows the division value for determination of the welding part of abnormal hue unevenness.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the following description, the same or equivalent elements will be denoted by the same reference numerals, and redundant description will be omitted.

  FIG. 1 is a schematic diagram showing the configuration of a laser welding system for implementing a laser weld evaluation method according to an embodiment of the present invention. The laser welded part evaluation method of this embodiment digitizes the color of a welded part formed by irradiating a laser beam, and evaluates it using numerical analysis and an MT system. That is, after acquiring the image of the welded portion and separating it into RGB values, the influence of the light and darkness of the image is removed, and then the evaluation is performed based on the Mahalanobis distance (MD).

  The “RGB value” means a value by RGB which is a color system, and an R (RED) value for “red” which is the three primary colors of light, a G (GREEN) value for “green”, and “blue And a component with a B (BLUE) value. Each of these components is expressed in 256 gradations from 0 to 255. The RGB values here are associated with positions in the image, that is, set for each pixel of the image data. Incidentally, RGB is also referred to as an RGB color model.

  The “MT system” is a multivariate analysis method that distinguishes state changes using the basic concept of Mahalanobis distance, and expresses multiple measured quantities (multivariables) by Mahalanobis distance (single variable). I handle it. For details of the Mahalanobis distance, refer to Japanese Unexamined Patent Application Publication No. 2006-160153.

  As shown in FIG. 1, the laser welding system 1 forms a linear welded portion W at a substantially central portion of workpieces (base materials) 10A and 10B, and lap welds the workpieces 10A and 10B. As the workpieces 10A and 10B, plate-like outer plate panels and bone members that are formed of a metal such as stainless steel and are used for a railway vehicle structure are used. The laser welding system 1 includes a laser welding apparatus 2 that forms a welded portion W by irradiating a laser beam.

  The laser welding apparatus 2 includes a feeding device 21, a workpiece fixing device 22, a laser irradiation device 23, and a gas supply device 24. Each of these devices 21 to 24 is connected to a host control device (not shown), and automatically executes each operation in accordance with operation instruction information output from this control device.

  The feeding device 21 scans the irradiation position of the laser beam on the workpieces 10A and 10B. Specifically, the feeding device 21 moves the workpieces 10 </ b> A and 10 </ b> B placed on the movable stage 25 relative to the laser beam by the laser irradiation device 23 along the planned welding region R.

  The workpiece fixing device 22 fixes the workpieces 10A and 10B to the movable stage 25. In the workpiece fixing device 22, both end portions of the workpieces 10 </ b> A and 10 </ b> B sandwiching the planned welding region R are pressed against the movable stage 25 by a long pressing plate 26 a.

The laser irradiation device 23 irradiates a laser beam toward the planned welding region R of the workpieces 10A and 10B. Specifically, the laser irradiation device 23 emits a laser beam such as a YAG laser or a CO ₂ laser for a predetermined time from the tip of the laser head 27 above the workpieces 10A and 10B. The laser irradiation device 23 includes an output switching mechanism (not shown) inside, and can be switched between continuous irradiation with the laser beam and irradiation with the laser beam in pulses. .

  The gas supply device 24 supplies assist gas (argon gas or the like) to the planned welding region R of the workpieces 10A and 10B. In the gas supply device 24 here, the supply nozzle 28 is disposed so as to be inclined by about 30 degrees with respect to the thickness direction of the workpieces 10A and 10B. The gas supply device 24 supplies assist gas to the laser beam irradiation positions of the workpieces 10A and 10B with a predetermined supply amount.

  The laser welding system 1 also includes a determination device 3 that determines whether or not the hue unevenness of the welded portion W is normal. The determination device 3 is physically a computer system including a CPU, a memory, a communication interface, a storage unit such as a hard disk, a display unit such as a display, and the like. An image acquisition sensor 4 is connected to the determination device 3.

  The image acquisition sensor 4 is for observing the welded portion W, and acquires image data (image information) of the welded portion W. As the image acquisition sensor 4, a digital camera, an image scanner, or the like is used. The image acquisition sensor 4 outputs the acquired image data to the determination device 3.

  The determination apparatus 3 includes a calculation unit 31, a comparison determination unit 32, and a database 33 as functional components. The calculation unit 31 performs a calculation for separating the image data into RBG values and a calculation for removing the contrast of the image data. The comparison determination unit 32 determines the hue unevenness of the welded portion W based on the Mahalanobis distance of the MT system. The database 33 stores a unit space for determination which is a unit space of the MT system generated from data of a normal welded portion W and is used when the comparison determination unit 32 determines the welded portion W. Details of processing relating to the calculation unit 31, the comparison determination unit 32, and the database 33 will be described later.

  Next, the processing steps of the laser welding system 1 described above will be described with reference to the flowcharts shown in FIGS.

  In the laser welding system 1, as a pre-welding process, the workpieces 10 </ b> A and 10 </ b> B are first placed on the movable stage 25, and the pressing jig 26 is lowered from above to fix the reference workpieces 10 </ b> A and 10 </ b> B to the movable stage 25. . And operation check of laser welding system 1 is performed (S1).

  Subsequently, as a welding process, a laser beam is irradiated from the laser head 27 and the movable stage 25 is moved to scan the workpieces 10A and 10B in the direction of arrow A (see FIG. 1). At the same time, assist gas is supplied by the gas supply device 24. Thereby, the welding part W is formed along the welding planned area | region R of workpiece | work 10A, 10B (S2). Finally, as a post-welding process, the quality of the welded portion W is determined, the workpiece fixing device 22 is released, and the workpieces 10A and 10B welded to each other are carried out (S3).

  By the way, in the present embodiment, the determination unit space is generated in advance in the pre-welding process (determination unit space generation process), and the hue unevenness of the welded portion W is determined in the post-welding process (hue determination process). First, the determination unit space generation step will be described in detail.

[Unit space generation process for determination]
The determination unit space is generated in advance before the hue determination step based on the data of the normal welded portion W. Therefore, first, in order to identify the normal welded portion W by the Mahalanobis distance, the reference unit space of the MT system used for such identification is determined from the image data of the workpiece 10A (or workpiece 10B) as the base material. Generate.

  Specifically, the surface of the workpiece 10A (or workpiece 10B) before laser welding is imaged (digitized) by the image acquisition sensor 4, and image data of the workpiece 10A is acquired (S11). Subsequently, the calculation unit 31 separates the image data into RGB and calculates a base material RGB value (S12).

Subsequently, the base material RBG value is subjected to gray scale conversion (S13). Here, gray scale conversion is performed according to the following equation (1). Thereby, the gray scale value Y representing the brightness (luminance) of the color is calculated. The gray scale value Y is an R value, a B value, or a G value that are equal to each other after the gray scale conversion.
Y = 0.2126 × R + 0.7152 × G + 0.0722 × B (1)
However, R: R value, G: G value, B: B value.

  Subsequently, the base material RGB value is divided by the gray scale value Y calculated in S13 to derive a base material division value (S14). Thereby, the contrast of the image is removed from the base material RGB values. Based on the base material division value, the unit space of the MT system is generated as a reference unit space (S15). Here, the principle of the removal of the brightness of the image will be described below.

  FIG. 5A shows a red sample hue, FIG. 6A shows a green sample hue, and FIG. 7A shows a blue sample hue. In the sample hues 40R, 40G, and 40B in each of these drawings, one end (the left end in the drawing) is set to 0 pixel, and the coordinate axis of the pixel is set from one end to the other end (the right end in the drawing).

  As shown in each figure, in the hues of the sample hues 40R, 40G, and 40B, both ends are the darkest and the center is the brightest, and the intervals are gradually changed. Therefore, when the image data of these sample hues 40R, 40G, and 40B are separated into RGB values, the R value, G value, and B value are respectively shown in FIGS. 5B, 6B, and 7B. As shown in (), on the graph with the horizontal axis as pixels, it is a mountain-shaped data graph.

  As shown in FIG. 5C, FIG. 6C, and FIG. 7C, when each of the R value, the G value, and the B value is divided by the gray scale value Y, these are normalized by the brightness of the color. The calculation is performed so that the value becomes constant, regardless of the brightness of the color. Therefore, the influence regarding the contrast of an image is excluded from RGB value.

  Next, image data is acquired for the reference welded portion W that has already been subjected to laser welding (S16), and this image data is separated into RGB to calculate reference RGB values (S17). Then, after the grayscale value Y is calculated by performing grayscale conversion on the reference RBG value (S18), the reference RGB value is divided by the grayscale value Y to derive a reference divided value (S19).

  Subsequently, based on the reference division value, the Mahalanobis distance is calculated using the base material unit space formed in S15 (S20). When the Mahalanobis distance is smaller than a predetermined value (second threshold), it is determined that the reference welded portion W is normal (healthy), and the determination unit space is based on the reference division value derived in S19. Is generated (S21 → S22). Thereafter, the generated determination unit space is stored in the database 33. On the other hand, if the Mahalanobis distance is greater than or equal to a predetermined value, it is determined that the reference weld W is abnormal, and the process of S16 is performed again for another reference weld W.

  FIG. 8 is a graph showing the Mahalanobis distance of the reference weld. In the example illustrated in FIG. 8, the Mahalanobis distance calculated using the base material unit space for the plurality of reference welds WA to WG based on the reference division value is shown. Here, the predetermined value is set to 400 from the viewpoint of experience and statistics. As shown in FIG. 8, the reference welded portion WA has a Mahalanobis distance greater than a predetermined value and is determined to be abnormal, while the reference welded portions WB to WG have the Mahalanobis distance set to a predetermined value. The following is determined to be normal.

[Hue determination process]
Next, the hue determination process will be described. First, the welded portion W formed in S2 is imaged by the image acquisition sensor 4, and image data of the welded portion W is acquired (S31). Subsequently, the calculation unit 31 separates the image data into RGB and calculates a determination RGB value (S32).

  Subsequently, the RBG value for determination is subjected to gray scale conversion, and a gray scale value Y representing the brightness of the color is calculated (S33). Subsequently, the determination RGB value is divided by the gray scale value Y calculated in S33 to derive a determination division value (S34). Then, based on the division value for determination, the Mahalanobis distance is calculated using the unit space for determination generated in S22 (S35).

  Then, in the comparison / determination unit 32, the hue unevenness of the welded portion W is determined based on the Mahalanobis distance calculated in S35. Specifically, when the Mahalanobis distance is smaller than a predetermined value (first threshold), it is determined that the hue unevenness of the welded portion W is normal (sound) (S36 → S37). That is, it is determined that the degree of uneven hue of the welded portion W is within the allowable value. The predetermined value here is set to about 4 to 10 from the viewpoint of experience and statistics. If the Mahalanobis distance is 1, the hue quality is equivalent to that of the reference weld W used to generate the determination unit space.

  On the other hand, if the Mahalanobis distance calculated in S35 is equal to or greater than a predetermined value, it is determined that the hue unevenness of the welded portion W is abnormal (S36 → S38). By the way, “hue unevenness” here means that there are various hues instead of variations in lightness and darkness (density) in each of the red, green, and blue hues.

  As described above, according to the present embodiment, the division value for determination is obtained by dividing the RGB value for determination by the gray scale value Y (S34). In other words, the R value, the G value, and the B value of the RGB value are calculated so as to be normalized by the gray scale value Y as the luminance, and the determination division value is obtained. Therefore, the variation (brightness and darkness) of the same color system in the image of the welded portion W can be removed in the division value for determination. Therefore, it is possible to accurately determine the hue unevenness by determining the hue unevenness of the welded portion W based on the determination division value (S36).

  As a result, in the post-welding process (S3 described above), for example, a ring-shaped scorch or the like formed in the welded portion W can be accurately discolored by electrolytic polishing, and the product appearance can be further enhanced. Further, it is possible to eliminate the filtering process of a complicated algorithm, and it is possible to simplify the determination of the hue unevenness.

  In the present embodiment, as described above, when determining the hue unevenness (S36), the Mahalanobis distance is calculated based on the division value for determination, and when the Mahalanobis distance is smaller than a predetermined value, the hue is calculated. It is determined that the unevenness is normal. Therefore, it is possible to qualitatively determine the hue unevenness of the welded part W using the MT system, and to determine the hue unevenness of the welded part W with higher accuracy.

  FIG. 9A is a graph showing RGB values for determination of welds with normal hue unevenness, FIG. 9B is a graph showing divided values for determination, and FIG. 10A is a welded portion with abnormal hue unevenness. FIG. 10B is a graph showing the division value for determination. 9 and 10, a SUS301L material is used as the workpieces 10A and 10B to be laser welded, and the surface state is a so-called 2B finish (conforming to JIS standards). Each of the workpieces 10A and 10B has a thickness of 2 mm, a width of 100 mm, and a length of 100 mm. In the example of FIG. 9, the output of the laser beam is 3.4 kW, and the moving speed of the movable stage 25 is about 3.5 m / min. On the other hand, in the example of FIG. 10, the output of the laser beam is 3.4 kW, and the moving speed of the movable stage 25 is about 5 m / min.

  As shown in FIGS. 9 and 10, when the welded portion W with normal hue unevenness is compared with the welded portion W with abnormal hue unevenness, the determination RGB values both have a large amplitude, and the difference between them is determined. Have difficulty. On the other hand, in the division value for determination, the amplitude of the waveform is suppressed to such an extent that a normal hue unevenness weld can be distinguished from an abnormal hue unevenness weld. Thereby, in the division value, it can be confirmed that the adverse effect of light and darkness in the image of the welded portion W is removed, and the component related to the hue unevenness becomes remarkable.

  The preferred embodiment of the present invention has been described above, but the present invention is not limited to the above embodiment.

  For example, in the above embodiment, the hue unevenness of the welded portion W is determined based on the Mahalanobis distance calculated from the division value, but the sample line (for example, μ ± nσ: where μ is the average of the division value) A value, n is an integer, and σ is a standard deviation of the division value), and when there is a division value in the sample line, the hue unevenness of the weld W may be determined to be normal. In some cases, the division value graph may be visually determined.

  Further, the mode of laser welding is not limited to the lap welding as in the above embodiment, and may be various modes such as butt welding. The “RGB value (RGB)” includes a value by a similar color specification method such as sRGB or RGBA.

  10A, 10B ... Workpiece (base material), W ... Welded part.

Claims (4)

A laser weld evaluation method for evaluating a weld formed on a base material by laser beam irradiation,
Having a hue determination step of determining hue unevenness of the weld,
In the hue determination step,
Obtain image information of the weld, calculate the RGB value of the image information,
A value obtained by performing gray scale conversion on the RGB value is calculated as a gray scale value,
The method for evaluating a laser welded portion, wherein the hue unevenness is determined based on a division value obtained by dividing the RGB value by the gray scale value. In the hue determination step,
2. The laser weld evaluation according to claim 1, wherein a Mahalanobis distance is calculated based on the division value, and the hue unevenness is determined to be normal when the Mahalanobis distance is smaller than a first threshold. Method.   3. The laser according to claim 2, further comprising: a determination unit space generation step for generating in advance a determination unit space used when calculating the Mahalanobis distance in the hue determination step before the hue determination step. Weld evaluation method. The determination unit space generation step includes:
Obtaining image information of the base material, calculating the RGB value of the image information as a base material RGB value, calculating a value obtained by performing gray scale conversion of the base material RGB value as a base material gray scale value, A first step of generating a reference unit space based on a base material division value obtained by dividing a base material RGB value by the base material grayscale value;
Obtain image information of a reference weld, calculate the RGB value of the image information as a reference RGB value, calculate a value obtained by performing gray scale conversion of the reference RGB value as a reference gray scale value, Based on the reference division value obtained by dividing the reference RGB value by the reference grayscale value, the Mahalanobis distance is calculated using the reference unit space generated in the first step, 4. The laser weld evaluation method according to claim 3, further comprising: a second step of generating the determination unit space based on the reference division value when the Mahalanobis distance is smaller than a second threshold value. 5. .