JPH11226766A - Method for controlling laser welding position - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control a welding position by optically detecting an object to be welded, in butt welding by a laser beam. SOLUTION: A keyhole 4 or a molten pool 5 formed in a weld zone and a butt groove 2 are irradiated with an ultraviolet laser as a measuring laser beam, detecting an image consisting of wavelength components in the ultraviolet region containing the wavelength of the ultraviolet laser, determining from this image the position of the butt groove 2 and the keyhole 4 or the molten pool 5, and controlling the welding position. By the irradiation of the ultraviolet laser, not only the welding laser beam (infrared region) but also the reflected light from the weld zone (melting part), for which a visible ray region is the main body, and the reflected light of the ultraviolet laser beam are separated by a filter. In the reflected light beams, by detecting the components in the ultraviolet region, an image in the part irradiated with the welding laser beam and in its neighborhood can be directly obtained, enabling the welding position to be measured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser welding position control method for controlling a welding position by optically detecting an object to be welded in butt welding using a laser beam.

[0002]

2. Description of the Related Art Laser welding is more efficient than gas welding.
High-speed welding with deep penetration is possible, and the total heat input required for welding can be reduced because the width of the welded portion (referred to as “penetration width”) is narrow. Therefore, application of this laser welding method has been attempted also in the production of steel pipes. In that case, in the laser welding method, since the penetration width is narrower than in the gas welding method, precise control of the welding position is required. Therefore, several techniques have been proposed for controlling the laser welding position.

Japanese Patent Application Laid-Open No. Hei 8-57667 discloses a seam shift amount detection method using a surface thermometer and a CCD camera (hereinafter referred to as “prior art 1”). In Prior Art 1, a metal strip is formed into an open pipe, the end face of which is heated by high frequency, and the temperature distribution near the abutment point is measured by a surface thermometer. Further, the end face of the metal band is
The position of the collision point is calculated based on the results detected by the surface thermometer and the CCD camera.

Japanese Patent Application Laid-Open No. Hei 8-323377 discloses a method in which a light source is provided inside an open pipe, light coming out of a gap between two end faces is detected by a CCD camera, and the measurement result is image-processed to obtain a collision point. An invention for calculating a position is disclosed (hereinafter, referred to as “prior art 2”). In Prior Art 2, the light used for the light source preferably has a high-intensity luminescent line in a wavelength range of 0.5 to 0.7 μm, whereby the seam edge that is glowed by the reflected light of the welding laser or preheating. It is described that disturbance such as radiation from a part can be removed.

[0005]

As described above, the prior arts all use a method of estimating an abutting point (welding position) of an end face of a metal strip from an image of the end face at a position away from the end face by image processing. is there. These methods are based on the premise that the shape of the end face of the metal strip draws a straight line or a constant curve. However, actually, the end face of the metal band is somewhat wavy, and accordingly, the estimated position of the abutment point is also wavy. Therefore, the welding position deviates from the actual abutment point.

In the prior art, in order to eliminate the error of the estimated position of the abutting point, it is necessary to detect the end face of the metal strip near the abutting point (welding position). However, in the prior art 1, when the end face of the metal band is detected near the welding position, the influence of welding heat increases, and the temperature distribution due to high-frequency heating changes. As a result, it becomes impossible to detect the end face of the metal strip based on the temperature distribution, and data for estimating the position of the collision point cannot be obtained.

In the prior art 2, in order to detect the end face of the metal strip near the welding position, it is necessary to install a light source near the welding position. However, near the welding position, the light source is stained by welding spatter or the like, and measurement cannot be performed.

The wavelength range (0.5 to 0.7 μm) of light used as a light source in this technique is a visible light range. As can be seen from the fact that light from a high-temperature welded portion (melted portion) cannot be directly seen with the naked eye, the wavelength component in the visible light region is strong. Since the light thermally radiated from the welding portion includes the wavelength range of the light source for measurement, when the light is reflected and enters the measuring instrument, it becomes disturbance in detecting the end face of the metal band. Further, since the light from the welded portion enters the wavelength range for measurement, it was impossible to directly observe the welded portion.

Accordingly, an object of the present invention is to solve the above-mentioned problems and to provide a laser welding position control method capable of controlling a welding position with high accuracy irrespective of the shape of the end face of a metal strip. .

[0010]

According to the first aspect of the present invention,
In a butt welding by a laser beam, in a welding position control method for optically detecting a welding target and controlling a welding position, a keyhole or a weld pool formed in a welded portion by irradiation of a laser beam, and the vicinity thereof. The butt groove is irradiated with an ultraviolet laser as a laser beam for measurement, and the butt groove, and of the reflected light from the keyhole or the molten pool, wavelength components in the ultraviolet region including the wavelength of the ultraviolet laser. The welding position is controlled by detecting an image consisting of: and obtaining the butt groove and the position of the keyhole or the molten pool from the image.

According to the present invention, an ultraviolet laser is applied to a keyhole or the like formed in a welded portion and a butt groove.
By irradiating an ultraviolet laser as a measuring laser beam, not only a welding laser beam (infrared region) but also a reflected light from a welding portion (fused portion) mainly in a visible light region and a measuring laser beam. The reflected light can be separated. Separating the ultraviolet region can be easily realized by a filter.

By detecting a component in the ultraviolet region of the reflected light, an image of the portion irradiated with the welding laser beam and the vicinity thereof can be directly obtained. As described above, since the welding position can be directly observed and measured, the welding position can be accurately obtained even if the shape of the end face of the metal strip is defective, and the accuracy of the welding position control is improved.

According to a second aspect of the present invention, in the first aspect of the invention, a brightness profile is created by scanning each pixel with respect to an image including a keyhole or a molten pool and a butt groove in the vicinity thereof. The boundary between the butt groove and the keyhole and the boundary between the keyhole and the molten pool are determined from the change in the shape of the luminance profile, and the butt groove and the keyhole or the molten pool are determined. The feature is that the welding position is controlled by calculating each position from the profile.

According to the present invention, the shape of the brightness profile changes as follows depending on the location of the welding target. First,
In the butt groove, the image of the groove-processed surface has lower brightness than the surface of the metal strip. This is because the grooved surface (chamfered surface) is inclined at a steep angle (45 ° or more) with respect to the surface of the metal strip to form a V-shaped groove, and the amount of reflected light on this surface Is significantly reduced. On the other hand, the surface of the metal band has reflected light in all directions due to irregular reflection by the surface oxide, and exhibits a certain level of luminance.

There is a molten metal at the boundary between the butt groove and the keyhole, and the brightness is equal to or higher than the brightness of the surface of the metal strip. This is because the molten metal fills the V-shaped groove formed by the butt groove and approaches a flat surface to obtain reflected light.

The portion of the keyhole is an opening surrounded by a wall perpendicular to the surface of the metal strip, and the wall has high luminance in the visible light region. However, in general, the laser light has high straightness, and the ultraviolet laser light for measurement is reflected on the wall surface of the keyhole and passes through the back side of the metal band. Therefore, the reflected light from this portion is hardly detected in the image, so that the brightness of the keyhole portion is low.

In the molten pool portion, the ultraviolet laser beam for measurement is reflected on the surface of the molten metal, so that the luminance is equal to or higher than the luminance of the surface of the metal band. In particular, when an image is detected at the position of specular reflection of ultraviolet laser light, the brightness of the molten pool increases. This is because the surface of the molten pool, which is a molten metal, has higher reflectivity and stronger directivity of reflection (particularly the direction of specular reflection) than the surface of the metal band covered with oxide or the like. It is.

As described above, since the brightness profile from each portion of the metal strip changes sequentially, it is possible to determine to which portion the obtained brightness profile corresponds. From these brightness profiles, the butt groove and the position of each keyhole or weld pool are calculated. Next, control of the laser beam device, that is, control of the welding position, is performed so that the calculated position difference becomes zero.

According to a third aspect of the present invention, in accordance with the first aspect of the present invention, the brightness profile of an image including a butt groove and a keyhole or a molten pool is measured for each of a plurality of scanning lines, and the plurality of obtained profiles are obtained. Is calculated, and the respective positions are calculated from the average value of the luminance profiles.

According to the present invention, in the average value (average luminance profile) of the obtained luminance profiles, the noise of the luminance profile for each scanning line is canceled out by the averaging process. Therefore, by using this average luminance profile for position detection, the detection accuracy of the position detection target is improved.

[0021]

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a front view showing an arrangement of detectors in the method for detecting a welding position according to the present invention. A laser beam 10 for welding is applied to the butted portion of the end face of the metal strip 1. An ultraviolet laser light source 7 and a camera 9 are arranged before and after the irradiation position 3 of the laser beam 10. The ultraviolet laser light emitted from the ultraviolet laser light source 7 is
The light is reflected by the surface of the metal strip 1 and enters the camera 9. An optical filter 8 that transmits only ultraviolet light is attached in front of the camera 9 to remove components in an infrared region and a visible light region.

The positional relationship between the ultraviolet laser light source 7 and the camera 9 is not particularly limited, and may be determined from the arrangement of equipment such as parts of a welding device. However, the ultraviolet laser light source 7
Alternatively, if the position of the camera 9 is too low with respect to the surface of the metal strip 1, it is difficult to detect the welding position. This is because the irradiation position 3 of the laser beam 10, that is, the welding position may be hidden behind the welding bead. Therefore, it is necessary that the position of the ultraviolet laser light source 7 or the camera 9 is set to a position at which an angle for the welding position with respect to the surface of the metal strip 1 can be secured to some extent.

FIG. 2 is a diagram schematically showing an image obtained by the camera 9 using wavelength components in the ultraviolet region. Groove 2
Is lower in brightness than the portions on both sides (the surface of the metal strip 1), and the portion of the keyhole 4 is further lower in brightness.
The portion of the molten pool 5 has a luminance equal to or higher than the surface of the metal band 1, and the portion of the weld bead 6 has a luminance close to the surface of the metal band 1.

The lines (scanning lines) aa 'to-shown in FIG.
When the image is scanned along the line dd ′, FIG.
The luminance profile shown in (d) is obtained. FIG. 3 (a)
Has a low luminance portion at the center of the figure, which corresponds to groove 2 (slope). Scan line a
Even if −a ′ is moved to the keyhole 4 side, the shape of the luminance profile does not change much as long as it passes through the groove 2, and each has a low-luminance part with a constant width.

In the scanning line bb ', the luminance profile is as shown in FIG. 3 (b). The low-luminance portion disappears and has a relatively flat shape with some irregularities. Thus, it is determined that the position of the scanning line bb ′ passes through the boundary between the butt groove 2 and the keyhole 4.

Next, in the luminance profile of FIG. 3C, a low luminance portion appears again in the center of the figure. When the scanning line cc ′ is further moved (downward in the figure), the low-luminance portion increases in width but becomes narrower after reaching the maximum width. Eventually, the low luminance portion disappears from the luminance profile. From this, it is determined that the portion with low luminance corresponds to the keyhole 4.

The luminance profile corresponding to the groove 2 and the keyhole 4 obtained in this manner (FIG. 3A,
Each position is determined using (c). Regarding the determination of the position, the position of the center of the low luminance portion may be obtained by setting a threshold value or the like.

To further improve the accuracy of position determination, the number of scanning lines may be increased and the luminance profile may be added (averaged). The number of scanning lines passing through the groove 2 or the keyhole 4 is M, the number of scanning lines is j, and the number of pixels in the scanning direction is i.
And the luminance at coordinates (i, j) is Iij,
The added value (histogram) Pi of the luminance profile is represented by the following equation (1).

Pi = ΣIij (1) Here, the symbol Σ represents the sum of 1 to M for the subscript j. It is needless to say that dividing the histogram Pi by M results in an average value Pi / M (average luminance profile).

In the obtained histogram Pi, the noise in the luminance profile due to the individual scanning lines cancels each other between the scanning lines, so that the noise is reduced. Therefore, by using the histogram Pi for position detection, detection accuracy is improved. This histogram P
The method of determining the position of the target portion from i is Pi
A threshold value (however, M times the threshold value of each luminance profile) is provided for the value of, and the positions of both ends are obtained and set as the center thereof,
A method of determining the position of the center of gravity by integration, or a method of finding a position having a minimum value as a simple method can be used as appropriate.

[0031]

As described above, according to the present invention, an ultraviolet laser is used as a measuring laser beam for optically detecting an object to be welded such as a metal band, and the wavelength component in the ultraviolet region including the wavelength is used. Since the image composed of is detected, the wavelength components in the infrared region and the visible light region can be removed from the image. Therefore, a laser beam for welding, and
Since the welding position itself can be obtained without being disturbed by the heat radiation from the keyhole (welding position) or the molten pool, the welding position can be accurately controlled irrespective of the shape of the end face such as a metal band. Postcards and thus useful effects are provided.

[Brief description of the drawings]

FIG. 1 is a front view showing an arrangement of detectors in a method for detecting a welding position according to the present invention.

FIG. 2 is a diagram schematically illustrating an image based on wavelength components in an ultraviolet region.

FIG. 3 is a diagram illustrating a luminance profile along a scanning line. (A) to (d) are luminance profiles along the scanning lines aa 'to dd'.

[Explanation of symbols]

 1: metal strip 2: groove 3: irradiation position 4: keyhole 5: weld pool 6: welding bead 7: ultraviolet laser light source 8: filter 9: camera 10: laser beam

Claims (3)

[Claims] 1. A welding position control method for controlling a welding position by optically detecting an object to be welded in a butt welding by a laser beam, wherein a keyhole or a weld pool formed in a welding portion by irradiation of a laser beam is provided. And, in the vicinity of the butt groove, irradiate an ultraviolet laser as a laser beam for measurement, the butt groove, and, of the reflected light from the keyhole or the molten pool, including the wavelength of the ultraviolet laser A laser welding position characterized by controlling a welding position by detecting an image composed of wavelength components in an ultraviolet region and obtaining a position of the butt groove and the keyhole or the molten pool from the image; Control method. 2. An image including a keyhole or a weld pool and a butt groove in the vicinity thereof is scanned with each pixel to create a luminance profile. By determining the boundary between the keyhole and the boundary between the keyhole and the weld pool, calculating the respective positions from the butt groove, and the profile of the keyhole or the weld pool, the welding position is determined. The laser welding position control method according to claim 1, wherein the control is performed. 3. A brightness profile of an image including a butt groove and a keyhole or a weld pool is measured for each of a plurality of scanning lines, and an average value of the obtained brightness profiles is calculated. 2. The method according to claim 1, wherein each position is calculated from an average value.
The laser welding position control method according to the above.