Laser welding apparatus for LNG cargo tank and laser welding system for LNG cargo tank, comprising same

Abstract

A laser welding apparatus for an LNG cargo tank and a laser welding system for an LNG cargo tank, comprising same are disclosed. The laser welding apparatus for the LNG cargo tank, according to the present invention, comprises: a laser receiving unit for receiving a laser beam; a light homogenization unit for increasing the cross-sectional area of an effective beam area of the laser beam received by the laser receiving unit; and a mask unit allowing the effective beam area of the laser beam that has passed through the light homogenization unit to pass therethrough in a specific cross-sectional shape.

Description

Laser welding device for LNG cargo tanks and laser welding system for LNG cargo tanks including the same

[0001] The present invention relates to a laser welding device for an LNG cargo tank and a laser welding system for an LNG cargo tank including the same, and more specifically, to a laser welding device for an LNG cargo tank and a laser welding system for an LNG cargo tank including the same that can perform laser welding without applying beam wobbling motion by increasing the effective cross-sectional area of ​​the laser beam.

[0002] Due to its characteristics, liquefied natural gas (LNG) must be stored and transported at cryogenic temperatures; to this end, ships or storage tanks used for storing or transporting LNG require highly reliable cargo tanks. The cargo tanks of LNG carriers are critical structures for safely storing and transporting cryogenic LNG. LNG carrier cargo tanks are classified into MOSS and membrane types. Inside membrane-type cargo tanks, membrane-shaped components are manufactured from metal materials such as Invar or stainless steel, and multiple panels are connected by welding.

[0003] Conventional membrane welding methods have mainly applied traditional arc welding technologies such as TIG (Tungsten Inert Gas) or plasma welding. However, while TIG and plasma welding have the advantages of good weld quality and aesthetics, the welding speed is slow, resulting in low welding work efficiency.

[0004] To address these issues, the introduction of laser welding technology has been considered. Laser welding offers advantages such as high precision and consistency, fast welding speeds, a small heat-affected zone, and low deformation. Automated laser systems can ensure uniform welding quality, and the use of high-energy-density laser beams can significantly improve welding speeds.

[0005] However, there are still several technical challenges in laser welding of LNG cargo tank membranes. Major challenges remain, such as the difficulty of laser welding on metal surfaces with high reflectivity, precise laser beam control and positioning for large structures, ensuring welding quality for curved and corner sections, and real-time monitoring and quality control during the welding process.

[0006] Specifically, LNG cargo tank membranes consist of very thin metal panels, so the welding depth and energy control must be very precise during laser welding. Otherwise, excessive welding may damage the membrane or reduce airtightness. In addition, optimization of the size and shape of the laser welding device is required, and due to the structural characteristics of the vessel, welding operations must be possible at various positions and angles.

[0007] Therefore, developing a laser welding device specialized for welding the membranes of LNG cargo tanks and achieving high-quality welding through this is an important task in this technology field to increase the efficiency of the manufacturing process and ensure the stability of the cargo tanks.

[0008] Meanwhile, since the laser beam of a conventional fiber laser welding device has a diameter of several hundred micrometers, welding must be performed through 'beam wobbling' when the gap size increases during membrane welding. Here, 'beam wobbling' refers to a method of rapidly moving the laser beam in a circular or straight line perpendicular to the weld seam.

[0009] However, applying a wobble head to a laser welding device to implement 'beam wobbling' has disadvantages. The addition of motors and other components for wobbling increases the system's weight, making installation and relocation difficult and reducing productivity. Furthermore, the increased complexity of the system can affect maintenance and durability. Additionally, 'beam wobbling' increases welding time and can cause sagging during side and overhead welding of cargo tanks, potentially leading to quality issues.

[0010] Therefore, there is a need to develop a laser welding system capable of responding to gap increases by increasing the effective cross-sectional area of ​​the laser beam without configuring additional devices for 'beam wobbling' such as wobble heads and motors.

[0011] Embodiments of the present invention aim to enable laser welding without applying beam wobbling motion by increasing the effective cross-sectional area of ​​the laser beam, provide convenience for installation and movement by reducing the weight of the system, and improve productivity.

[0012] In addition, we aim to ensure good quality and reliability by drastically reducing welding time and preventing sagging during side and overhead welding of LNG cargo tanks.

[0013] In addition, by implementing a rectangular cross-sectional shape for the laser beam, we aim to minimize the occurrence of incomplete welds at the start and end points of the weld.

[0014] In addition, the goal is to operate the equipment without colliding with or interfering with surrounding structures due to the rotation of the laser welding device and the cables connected thereto in a confined area, while ensuring a sufficient focal length in a limited space.

[0015] In addition, by acquiring image information on the same axis as the laser beam, we aim to minimize sensing distortion and errors and accurately track the weld line.

[0016] In addition, when welding the membrane corrugations, the rotation angle of the laser welding device is reduced so that it can smoothly pass over the membrane corrugations, thereby preventing a decrease in welding speed and minimizing interference with surrounding structures.

[0017] In addition, by applying a bandpass filter to filter out only the light necessary for sensing, we aim to minimize sensing errors and increase the accuracy of sim tracking and profile tracking.

[0018] According to one aspect of the present invention, a laser welding device for an LNG cargo tank may be provided, comprising: a laser receiver for receiving a laser beam; a light equalization unit for increasing the cross-sectional area of ​​the effective beam region of the laser beam received from the laser receiver; and a mask unit for passing the effective beam region of the laser beam that has passed through the light equalization unit through a specific cross-sectional shape.

[0019] The laser beam received by the above laser receiver may have a wavelength range of 800 nm to 1100 nm.

[0020] The above light homogenization unit can perform light splitting, resynthesis, and homogenization of the laser beam.

[0021] The light equalization unit may comprise a lens array unit that divides the laser beam into a plurality of small beams to form a uniform distribution, and a focusing lens unit that increases the cross-sectional area of ​​the effective beam region by focusing and advancing the plurality of small beams that have formed the uniform distribution at a desired point.

[0022] The above lens array section includes a first lens array section and a second lens array section, and the first lens array section and the second lens array section may be arranged side by side.

[0023] The laser beam passing through the above mask portion may have a square cross-section.

[0024] The laser beam passing through the above mask portion may have a cross-sectional side length of 3 mm to 6 mm.

[0025] According to another aspect of the present invention, a laser welding system for an LNG cargo tank may be provided, comprising: a main body; a moving part that moves the main body along a certain trajectory; and a laser welding device provided on one side of the main body and moving together with the main body to perform laser welding, wherein the laser welding device performs welding by increasing the cross-sectional area of ​​the effective beam region of the laser beam.

[0026] The laser welding device described above may comprise a laser receiver for receiving a laser beam, a light equalization unit for increasing the cross-sectional area of ​​the effective beam region of the laser beam received from the laser receiver, and a mask unit for passing the effective beam region of the laser beam that has passed through the light equalization unit through a specific cross-sectional shape.

[0027] The light equalization unit may comprise a lens array unit that divides the laser beam into a plurality of small beams to form a uniform distribution, and a focusing lens unit that increases the cross-sectional area of ​​the effective beam region by focusing and advancing the plurality of small beams that have formed the uniform distribution at a desired point.

[0028] The above lens array section includes a first lens array section and a second lens array section, and the first lens array section and the second lens array section may be arranged side by side.

[0029] The laser beam passing through the above mask portion may have a square cross-section.

[0030] The laser beam passing through the above mask portion may have a cross-sectional side length of 3 mm to 6 mm.

[0031] The laser beam received by the above laser receiver may have a wavelength range of 800 nm to 1100 nm.

[0032] The above laser welding device can use the flat-top region of the laser beam as an effective beam region.

[0033] Embodiments of the present invention can perform laser welding without applying beam wobbling motion by increasing the effective cross-sectional area of ​​the laser beam, reduce the weight of the system to provide convenience for installation and movement, and improve productivity.

[0034] In addition, good quality and reliability can be ensured by drastically reducing welding time and preventing sagging even during side and overhead welding of LNG cargo tanks.

[0035] In addition, by implementing a rectangular cross-sectional shape for the laser beam, the occurrence of incomplete welds at the start and end points of the weld can be minimized.

[0036] In addition, the equipment can be operated in a confined space without colliding with or interfering with surrounding structures due to the rotation of the laser welding device and the cables connected thereto in a narrow area while securing a sufficient focal length.

[0037] In addition, by acquiring image information on the same axis as the laser beam, the occurrence of sensing distortion and errors can be minimized, and the welding line can be accurately tracked.

[0038] In addition, by reducing the rotation angle of the laser welding device when welding the membrane corrugations so that it can smoothly pass over the membrane corrugations, the welding speed can be prevented and interference with surrounding structures can be minimized.

[0039] In addition, by applying a bandpass filter to filter out only the light necessary for sensing, sensing errors can be minimized and the accuracy of sim tracking and profile tracking can be increased.

[0040] FIG. 1 is a perspective view of a laser welding system for an LNG cargo tank according to one embodiment of the present invention.

[0041] FIG. 2 is a front view of a laser welding device for an LNG cargo tank according to an embodiment of the present invention.

[0042] FIG. 3 is a perspective view of a laser welding device for an LNG cargo tank according to an embodiment of the present invention.

[0043] FIG. 4 is a perspective view of a laser welding device for an LNG cargo tank according to one embodiment of the present invention, viewed from another side.

[0044] FIG. 5 is a configuration diagram illustrating a light homogenization section of a laser welding device for an LNG cargo tank according to an embodiment of the present invention.

[0045] Figures 6a to 6c are images comparing the cross-sectional profile of a typical laser beam with a profile in which the cross-sectional area of ​​the effective beam region is increased by the optical equalization unit.

[0046] FIG. 7 is a configuration diagram illustrating the state of performing membrane welding with the cross-sectional shape of the laser beam being rectangular.

[0047] Figure 8 is a conceptual diagram comparing whether an incomplete weld occurs at the end of the weld line according to the laser beam cross-sectional shape.

[0048] FIG. 9 is a flowchart illustrating a laser welding control method for an LNG cargo tank according to an embodiment of the present invention.

[0049] Figure 10 is a flowchart showing the detailed steps performed in the coordinate extraction process.

[0050] FIG. 11 is a block diagram illustrating the configuration of a laser welding control system for an LNG cargo tank according to an embodiment of the present invention.

[0051] FIGS. 12a to 12e are images illustrating the process of image processing performed by a laser welding control method for an LNG cargo tank according to an embodiment of the present invention.

[0052] Fig. 13 is a conceptual diagram illustrating the state in which a weld line image has been converted to grayscale.

[0053] Figure 14 is a conceptual diagram illustrating an example of distinguishing the boundary lines of a weld line and extracting coordinates in a grayscale converted state.

[0054] FIG. 15 is a configuration diagram showing the change in irradiation angle of a laser welding device performing welding on a membrane corrugated portion.

[0055] FIG. 16 is a configuration diagram indicating the points where control parameters change during welding of the membrane corrugated portion.

[0056] FIG. 17 is a configuration diagram illustrating an example of sensing performed by applying a bandpass filter.

[0057] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings. However, the present invention is not limited to the embodiments described herein and may be embodied in other forms. Rather, the embodiments introduced herein are provided to ensure that the disclosed content is thorough and complete, and to ensure that the spirit of the present invention is sufficiently conveyed to those skilled in the art. Throughout the specification, the same reference numerals indicate the same components.

[0058] FIG. 1 is a perspective view of a laser welding system for an LNG cargo tank according to an embodiment of the present invention, FIG. 2 is a front view of a laser welding device for an LNG cargo tank according to an embodiment of the present invention, and FIG. 3 is a perspective view of a laser welding device for an LNG cargo tank according to an embodiment of the present invention. FIG. 4 is a perspective view of a laser welding device for an LNG cargo tank according to an embodiment of the present invention viewed from another side, FIG. 5 is a configuration diagram illustrating a light homogenization unit of a laser welding device for an LNG cargo tank according to an embodiment of the present invention, and FIG. 6a to 6c are images comparing a cross-sectional profile of a general laser beam with a profile in which the cross-sectional area of ​​the effective beam region is increased by the light homogenization unit. FIG. 7 is a configuration diagram illustrating a state in which membrane welding is performed while the cross-sectional shape of the laser beam is rectangular, and FIG. 8 is a conceptual diagram comparing whether an unwelded portion occurs at the end of the weld line according to the cross-sectional shape of the laser beam.

[0059] Referring to FIGS. 1 to 8, a laser welding system (1000) for an LNG cargo tank according to one embodiment of the present invention may largely comprise: a main body (100); a moving part that moves the main body (100) along a certain trajectory; and a laser welding device (200) provided on one side of the main body (100) and moving together with the main body (100) to perform laser welding.

[0060] The above main body (100) has a certain shape so that various equipment for laser welding can be combined. The above main body (100) is configured to move along a certain track by means of a moving part, and in this embodiment, a guide rail (10) is provided, and the main body (100) moves along the guide rail (10) through the moving part.

[0061] Any driving means capable of moving along the guide rail (10) may be applied to the moving part. For example, the moving part may be composed of an LM guide that moves along the guide rail (10) by receiving driving force from a motor. In addition, a wheel or roller may be configured to move along the guide rail (10) through an electric motor, and various other driving methods may be applied, such as a gear and rack drive method, a chain and belt drive method, a magnetic drive method, a hydraulic or pneumatic drive method.

[0062] The above main body (100) is equipped with a laser welding device (200). The laser welding device (200) moves along the guide rail (10) together with the main body (100) and performs laser welding on the LNG cargo tank membrane (20).

[0063] A laser welding device (200) according to one embodiment of the present invention can perform welding by increasing the cross-sectional area of ​​the effective beam region of a laser beam (201). To this end, the laser welding device (200) may comprise a laser receiving unit (210) for receiving a laser beam (201), a light equalization unit (230) for increasing the cross-sectional area of ​​the effective beam region of the laser beam (201) received from the laser receiving unit (210), and a mask unit (240) for passing the effective beam region of the laser beam (201) that has passed through the light equalization unit (230) through a specific cross-sectional shape.

[0064] The laser receiver (210) performs the role of receiving a laser beam generated by amplifying energy from a laser oscillator provided in the main body (100) to produce strong light. A laser suitable for welding an LNG cargo tank membrane (20) must possess high output, precision, and stable quality.

[0065] In particular, the membrane (20) of the LNG cargo tank uses thin and complex metal materials (e.g., stainless steel, Invar alloy, etc.) to withstand a cryogenic environment, so the welding quality is very important. Therefore, a fiber laser, disk laser, CO2 laser, diode laser, etc. capable of high output and precision welding can be transmitted to the laser receiver (210).

[0066] The laser beam (201) received by the laser receiver (210) is composed of a wavelength range of 900 nm to 1100 nm, so high output can be achieved.

[0067] In this embodiment, a diode laser having the advantages of high electric-to-optical conversion efficiency and wide welding width may be applied, but is not limited thereto.

[0068] Generally, since lasers used for welding have a diameter of several hundred micrometers, a wobble head configuration capable of implementing 'beam wobbling' is additionally required, as mentioned above.

[0069] Accordingly, in an embodiment of the present invention, a light equalization unit (230) is provided to increase the cross-sectional area of ​​the effective beam region of the laser beam (201) received from the laser receiver (210). The light equalization unit (230) is configured to perform light splitting, re-synthesis, and equalization of the laser beam (201).

[0070] Specifically, the light equalization unit (230) may comprise a lens array unit (232, 234) that divides the laser beam (201) into a plurality of small beams to form a uniform distribution, and a focusing lens unit (235) that increases the cross-sectional area of ​​the effective beam region by focusing the plurality of small beams that have formed the uniform distribution to a desired point.

[0071] The lens array (232, 234) serves to divide the laser beam (201) into a plurality of small beams. The lens array (232, 234) can be manufactured in various sizes and shapes, and the distribution and intensity pattern of the laser beam (201) can be controlled according to the position and size of each lens.

[0072] As shown in FIG. 5, the lens array section (232, 234) is composed of a first lens array section (232) and a second lens array section (234) arranged side by side. The first lens array section (232) divides the laser beam (201) into small parts, and the second lens array section (234) combines the divided beams to form a uniform beam profile.

[0073] The first lens array section (232) and the second lens array section (234) are composed of a plurality of small micro-lenses arranged in a grid. Here, each micro-lens may be identical or, if necessary, designed to have different sizes and shapes. They are mainly made of glass or plastic, and an anti-reflective (AR) coating may be applied to provide high transmittance and low distortion.

[0074] The focusing lens unit (235) is used to make the laser beam (201) uniform and then focus it at a desired point. Typically, an aspherical lens is used to minimize aberrations and form a uniform focal plane.

[0075] The focusing lens unit (235) is also called a Fourier lens and serves to make the spatial frequency distribution of the laser beam (201) passing through the lens array unit (232, 234) uniform. Behind the lens array unit (232, 234), the focusing lens unit (235) collects each spatial frequency component of the incident beam into a Fourier plane, and as a result, a flat-top pattern with uniformly distributed energy can be formed.

[0076] A flat-top distribution is formed at the moment when the phase and intensity distribution of the laser beam (201) becomes uniform, which is the point where small beams passing through the lens array (232, 234) combine at the focal plane of the focusing lens (235). Therefore, without the focusing lens (235), a uniform flat-top distribution cannot be created because the small beams separated by the lens array (232, 234) are scattered.

[0077] The light uniformization unit (230) configured in this way consequently performs the role of increasing the effective beam cross-sectional area that can be used for welding.

[0078] In fact, the laser beam initially received by the laser receiver (210) forms a Gaussian distribution (Gausian Profile) with a small effective beam cross-sectional area in the center as seen in FIG. 6a, but as it passes through the light equalization unit (230), it can be seen that it changes to a flat-top distribution with an increased effective beam cross-sectional area as shown in FIG. 6b.

[0079] Meanwhile, the laser beam (201) that has passed through the light uniformization unit (230) passes through the mask unit (240). The mask unit (240) allows the effective beam area of ​​the laser beam (201) to pass through in a specific cross-sectional shape.

[0080] As illustrated in FIG. 6c, the mask portion (240) filters the area other than the effective beam and passes the effective beam area through a rectangular cross-sectional shape. Through this, in an embodiment of the present invention, the laser welding device (200) can perform welding with a laser beam (201) formed in a large rectangular shape.

[0081] As shown in FIG. 7, a laser welding device (200) according to one embodiment of the present invention can apply a large-area rectangular beam to welding, in which the cross-section of the laser beam (201) is formed into a rectangular shape through the light uniformization unit (230) and the mask unit (240), and the length (a) of each side of the cross-section is formed to be 3 mm to 6 mm.

[0082] At this time, the laser welding device (200) uses the flat-top area of ​​the laser beam (201) as the effective beam area, and accordingly, can perform welding along the welding line (21) of the membrane (20) in one go without a 'beam wobbling' function.

[0083] In addition, as shown in FIG. 8, when the laser beam (201) is circular, there may be incomplete welds at the starting and ending points of the weld line (21). However, if the cross-sectional shape of the laser beam (201) is implemented as a square as in the embodiment of the present invention, the occurrence of incomplete welds at the starting and ending points of the weld can be minimized.

[0084] Meanwhile, as described above, in order to implement a large-area laser beam (201), it is necessary to secure a sufficient focal length, but as the length of the laser welding device (200) increases, difficulties arise in operating the equipment.

[0085] In particular, when welding the corrugation shape of the membrane (20), the radius of rotation increases, so there is a problem that the work cannot be performed due to interference caused by the rotation of the laser welding device (200) and the cable connected thereto in a narrow area.

[0086] Therefore, it is an important task to operate the equipment without colliding with or interfering with surrounding structures by rotating the laser welding device (200) and the cable connected thereto in a narrow area while securing a sufficient focal length of 250 mm to 350 mm in a limited space.

[0087] To this end, a laser welding device (200) for an LNG cargo tank according to one embodiment of the present invention may comprise a first extension part (222) that extends a certain length from the laser receiving part (210), a bending part (224) where the path of the received laser beam (201) is changed, and a second extension part (226) that forms a certain angle with the first extension part (222) and extends a certain length from the bending part (224) in the direction of the path of the changed laser beam (201).

[0088] As illustrated in FIGS. 2 to 4, the laser welding device (200) has a bending section (224) where the path of the laser beam (201) bends. The body section (220) of the laser welding device (200) is composed of a first extension section (222), a bending section (224), and a second extension section (226), and the body section (220) itself forms the path along which the laser beam (201) travels.

[0089] One end of the first extension part (222) is connected to the laser receiver (210), and the laser beam (201) received from the laser receiver (210) travels along the inside of the first extension part (222).

[0090] A bending section (224) is provided at the other end of the first extension section (222). The laser beam (201) traveling along the first extension section (222) has its path changed at the bending section (224). Then, a second extension section (226) is extended a certain length in the direction of the changed path of the laser beam (201).

[0091] Here, the second extension (226) is extended at a certain angle with respect to the first extension (222) at the same angle as the path of the laser beam (201) is changed. In this embodiment, as shown in FIGS. 2 to 4, the second extension (226) may be extended at a substantially perpendicular angle with respect to the first extension (222).

[0092] In this way, the first extension part (222), the bending part (224), and the second extension part (226) constitute the body part (220) of the laser welding device (200).

[0093] Meanwhile, the above-mentioned bending section (224) may be equipped with a first reflective mirror (2241) that reflects the laser beam (201) to change its path. The first reflective mirror (2241) performs the function of reflecting the laser beam (201) that has traveled along the first extension section (222) to change its direction so that it can travel along the second extension section (226). Additionally, the light equalization section (230) may be provided on the second extension section (226).

[0094] As such, a laser welding device (200) according to one embodiment of the present invention includes a curved portion (224) and a first reflective mirror (2241) to secure a laser beam (201) travel path and a sufficient focal distance, and can also be configured to enable welding in a narrow area by reducing the length or height of the laser welding device (200) to make it compact.

[0095] In addition, since the optical cable is connected horizontally, there is an advantage that welding can be done without interference even when the laser welding device (200) rotates.

[0096] Meanwhile, the welding line (21) tracking method applied to conventional laser welding is generally to recognize the welding line (21) by reading cross-sectional information using a 2D profile sensor and to track it in the laser welding device (200).

[0097] This conventional welding line (21) tracking method tracks the welding line (21) through a 2D profile sensor attached separately from the laser welding device (200), so there is a problem in that it is difficult to accurately track the welding line (21) due to distortion and error caused by the sensing angle being misaligned.

[0098] In an embodiment of the present invention, a coaxial camera unit (250) is provided to acquire image information on the same axis as the laser beam (201) and to track the welding line. The coaxial camera unit (250) is provided on one side of the body part (220), more specifically on the second extension part (226), and captures image information on the same axis as the laser beam (201) that performs welding.

[0099] As seen in FIG. 2, the coaxial camera unit (250) is connected to the lower part of the light equalization unit (230) of the second extension unit (226) to form another light path. Specifically, a dichroic mirror (2261) is provided on the laser beam (201) path of the second extension unit, which transmits the laser beam (201) and reflects visible light and light of a specific wavelength range to transmit to the coaxial camera unit (250).

[0100] The dichroic mirror (2261) is positioned in the path of the laser beam (201) as shown in FIG. 2 and guides only visible light and light of a specific wavelength that enters coaxially with the laser beam (201) performing welding to the coaxial camera unit (250).

[0101] In addition, a second reflective mirror (252) is provided to reflect visible light transmitted to the coaxial camera unit (250) to the internal image sensor side. By configuring it in this way, real-time welding control is possible by tracking the welding line (21) without distortion or error through an image acquired on the same axis as the laser beam (201).

[0102] Additionally, a configuration such as an aperture (not shown) that controls the amount of light entering the coaxial camera unit (250) from the second reflective mirror (252) may be further included.

[0103] In addition, if it is necessary to additionally measure light of a specific wavelength range, a corresponding measuring device may be provided on one side of the body part (220). Furthermore, if it is necessary to measure various scattered wavelengths in addition to the visible light range, a dichroic mirror (2261) or a coaxial camera part (250) may be replaced or added.

[0104] FIG. 9 is a flowchart illustrating a laser welding control method for an LNG cargo tank according to an embodiment of the present invention, FIG. 10 is a flowchart illustrating detailed steps performed during the coordinate extraction process, and FIG. 11 is a block diagram illustrating the configuration of a laser welding control system for an LNG cargo tank according to an embodiment of the present invention. FIG. 12a to 12e are images illustrating the process of image processing performed by the laser welding control method for an LNG cargo tank according to an embodiment of the present invention, FIG. 13 is a conceptual diagram illustrating the state in which a weld line image is converted to grayscale, and FIG. 14 is a conceptual diagram illustrating an example of distinguishing the boundary line of a weld line and extracting coordinates in the state converted to grayscale.

[0105] Referring to FIGS. 9 to 14, a laser welding control method for an LNG cargo tank according to one embodiment of the present invention may largely comprise: a step of excluding the laser light source wavelength by applying a filter to a captured image (S110); a step of obtaining an HDR image by applying an HDR (High Dynamic Range) method to the image from which the laser light source wavelength has been excluded (S120); a step of setting a reference line after preprocessing the obtained HDR image by removing the remaining parts while leaving only the ROI (Region of Interest) (S130); a step of extracting coordinates through calibration from the image in which the preprocessing and reference line have been set (S140); and a step of controlling a laser welding device (200) to follow a welding line (21) through the extracted coordinates (S150).

[0106] As described above, in an embodiment of the present invention, a coaxial camera unit (250) is applied to obtain image information coaxially with the laser beam (201) and track the welding line in order to minimize distortion and errors caused by the path for obtaining image information being different from the axis of the laser beam (201). However, in this case, image processing technology capable of minimizing the influence of the laser light source generated in the welding area is required.

[0107] Therefore, a laser welding control method for an LNG cargo tank and a laser welding control system (300) for an LNG cargo tank that implements the same for achieving these technical challenges are described in more detail as follows.

[0108] First, the laser light source wavelength is filtered and excluded from the captured image in the image filter unit (310) (S110). The image captured before filtering is in a state where it is difficult to recognize the weld line (21) as shown in FIG. 12a, but after filtering in the image filter unit (310), the weld line (21) becomes recognizable as shown in FIG. 12b.

[0109] Afterward, the HDR image acquisition unit (320) acquires an image in the HDR (High Dynamic Range) method from the image from which the laser light source wavelength has been excluded (S120). HDR image processing is a process of combining images with various exposure values ​​to clearly express both bright and dark areas. The HDR data created in this way undergoes tone mapping to adjust brightness and color, and can provide richer colors with a wide color gamut.

[0110] The reason for performing HDR image processing in this manner is to accurately detect changes in illumination and complex contrast differences to improve welding quality. In a laser welding environment, it is difficult to clearly identify the welding line (21) and the surrounding environment in a normal image due to the strong laser beam (201), shadows, reflections, etc.

[0111] However, as in the present embodiment, when acquired as an HDR image, dark and bright areas are simultaneously and clearly expressed, thereby reducing the problem of overexposure caused by strong light and allowing dark weld lines (21) to be accurately recognized. Through this, the laser welding control system (300) for LNG cargo tanks can precisely track the weld lines (21) and detect irregular paths or minute deviations in real time.

[0112] Fig. 12c is an image obtained as an HDR video, and it can be seen that it is expressed much more clearly than Fig. 12a or Fig. 12b.

[0113] Next, the image preprocessing unit (330) preprocesses the acquired HDR image by removing the remaining parts while leaving only the ROI (Region of Interest), and then sets a reference line (S130).

[0114] ROI (Region of Interest) refers to an area of ​​interest and is a concept of selecting only the important parts among the subjects of analysis or processing. In weld line (21) seam tracking, the ROI refers to the weld line (21) and the important sections around it.

[0115] That is, at this stage, unnecessary backgrounds or obstructing elements (reflected light, surrounding objects, etc.) are removed from the entire HDR image, leaving only the core area containing the weld line (21). By using the ROI in this way, processing speed is increased and noise is reduced, allowing the weld line (21) to be tracked more accurately. FIGS. 12d and FIGS. 12e are images showing the image preprocessed as described above.

[0116] Next, the coordinate extraction unit (340) extracts coordinates through calibration from the image where the preprocessing and baseline are set (S140). This step is carried out through detailed steps as shown in FIG. 10.

[0117] First, as shown in FIG. 13, the preprocessed image is converted to grayscale (S141). The reason for converting the HDR image to grayscale in this way is to increase image processing efficiency and to clearly extract necessary information. Since the RGB channels of the color image have a large capacity and color information is unnecessary for tracking the weld line (21), the amount of data is reduced by converting it to grayscale. This speeds up the computation, enabling real-time processing, and makes the contrast clearer, making it easier to distinguish between the weld line (21) and the background. This process also helps to increase the accuracy of boundary detection and path tracking, which will be described later.

[0118] Next, the boundary line of the weld line (21) is distinguished in the image converted to grayscale (S142). A composite threshold detection algorithm is applied to distinguish the boundary line of the weld line (21).

[0119] The composite threshold detection algorithm is a method that identifies boundary lines by determining which range each pixel value in an image falls into based on a specific threshold.

[0120] While the general single threshold method uses only one threshold, the composite threshold combines multiple thresholds to enable more accurate distinction of boundaries even in environments with varying brightness or contrast differences.

[0121] Since the area around the weld line (21) has complex changes in brightness due to arc light, reflections, shadows, etc., it is difficult to detect the boundary line accurately with a simple threshold. A complex threshold algorithm reflects various brightness conditions to clearly recognize the boundary between the weld line and the background, and this process is essential for the laser welding control system (300) for LNG cargo tanks to accurately track the path.

[0122] Next, pixels are extracted from the above baseline to the boundary line and the pixels are coordinated (S143). The process of distinguishing the boundary line, extracting pixels, and coordinated the pixels is illustrated as an example in FIG. 14.

[0123] Specifically, as described above, in the data boundary distinction process, the welding line (21) and the background are distinguished by utilizing the difference in brightness and contrast of the image. At this time, a composite threshold detection algorithm is applied to find accurate boundary lines even under various brightness and brightness conditions.

[0124] Next, pixels from the baseline to the boundary line are extracted, and the corresponding values ​​are coordinated based on the position information of these pixels. The coordinated data is used so that the laser welding control system (300) for the LNG cargo tank can track the path and move accurately along the welding line.

[0125] After the coordinate extraction is completed in this manner, the laser welding device control unit (350) controls the laser welding device (200) to follow the welding line (21) using the extracted coordinates (S150).

[0126] For example, if there is a 40-pixel difference in coordinates from the reference position, the relationship '1mm : 4 pixels = Y : 40 pixels' is established, and since Y is 10mm, the laser welding device (200) is moved 10mm along the Y-axis. Here, the coordinate system can be defined such that the welding line (21) is the X-axis, the axis perpendicular to the welding line (21) is the Y-axis, and the height direction is the Z-axis.

[0127] In this process, by extending the position-velocity 2D model for boundary tracking in real-time video, the path can be predicted by simultaneously considering the position and movement speed of the weld line (21). The laser welding device control unit (350) calculates the estimated position of the next moment using parameters after setting the system model.

[0128] The predicted position is cross-referenced with the image acquired in real time to correct errors, thereby confirming the accurate position of the actual weld line (21). Since welding operations are performed continuously and the path does not change abruptly, generating a virtual path line based on the predicted position enables faster and more stable position tracking. This contributes to improving processing speed and welding quality.

[0129] Furthermore, in the step (S140) of extracting the coordinates, the present invention may additionally apply complex algorithms such as deep learning (artificial neural network), deep learning techniques using pattern detection, and machine learning.

[0130] FIG. 15 is a configuration diagram showing the change in irradiation angle of a laser welding device performing welding on a membrane corrugated portion, and FIG. 16 is a configuration diagram showing the points where changes in control parameters occur when welding on a membrane corrugated portion.

[0131] Referring to Figures 15 and 16, the laser welding control method for the corrugated portion of the LNG cargo tank membrane is explained as follows.

[0132] In conventional laser welding or plasma welding, when welding the corrugated portion of the membrane (20), the welding is performed with the head aligned in the normal direction, so the rotation angle increases and the Z-axis movement also increases, which causes a problem of reduced welding speed.

[0133] Embodiments of the present invention provide a laser welding control method for a corrugated portion in which the laser welding device (200) can smoothly pass over the corrugated portion of the membrane (20) as shown in FIG. 15, even at a high speed of laser welding.

[0134] First, in a laser welding control method for a corrugated portion of an LNG cargo tank membrane according to one embodiment of the present invention, the laser welding device may perform welding on the corrugated portion, and the laser irradiation angle may be in the range of -45° to 45° relative to a vertical line as shown in FIG. 16.

[0135] At this time, as shown in FIG. 16, the section for performing welding on the corrugated section can be divided based on the entry point, the curved entry point, the first inflection point, the second inflection point, the third inflection point, the fourth inflection point, the curved exit point, and the exit point.

[0136] Throughout the above entire section, the driving speed of the laser welding device (200) is in the range of 800 mm / min to 2500 mm / min.

[0137] Here, rapid welding of the corrugated section can be ensured by optimizing the welding distance, the range of variation in the irradiation angle, and the range of rotational speed of the irradiation angle for each section. Laser welding of the corrugated section of the membrane (20) requires precise welding control according to each section and curvature of the corrugated section. In this process, the welding distance, the range of variation in the irradiation angle, and the rotational speed for each section are adjusted to minimize irregularities that may occur at curved surfaces and inflection points, and to maintain consistent welding quality.

[0138] Looking at each section, laser welding control between the entry point and the curve entry point can be performed with a welding distance of 10.0 mm to 50.0 mm, a beam angle variation range of 10° to 45°, and a beam angle rotation speed range of 0.6 rpm to 31.3 rpm.

[0139] In this section, the welding distance is maintained between 10.0 mm and 50.0 mm, and the irradiation angle varies between 10° and 45°. The laser welding device (200) rotates at a speed between 0.6 rpm and 31.3 rpm and performs an initial approach to the corrugated section. This step helps the laser beam (201) smoothly enter the surface of the corrugated section, and it is important to prevent excessive heat accumulation at the start of welding and to secure a smooth path.

[0140] And the laser welding control between the above-mentioned curve entry point and the first inflection point can be performed such that the welding distance is 11.6 mm to 20.0 mm, the irradiation angle variation range is 10° to 35°, and the irradiation angle rotation speed range is 0 rpm to 21.1 rpm.

[0141] In this section, the wrinkles deepen and the curves continue, so more precise welding is required. Reduce the rotation speed or stop it to improve welding quality and allow the laser beam (201) to stably track along the curved surface.

[0142] Laser welding control between the first inflection point and the second inflection point can be performed such that the welding distance is 31.3 mm to 62.9 mm, the irradiation angle variation range is 35° to 45°, and the irradiation angle rotation speed range is 1.9 rpm to 10.0 rpm.

[0143] The inflection point section is an important part where the direction of the wrinkles changes, so welding is performed according to the curvature by making a large change in the angle of inspection. At this stage, the rotation speed is maintained stably to ensure consistency of the weld line (21).

[0144] Laser welding control between the second inflection point and the third inflection point can be performed such that the welding distance is 10.0 mm to 13.5 mm, the irradiation angle variation range is -20° to 20°, and the irradiation angle rotation speed range is 1.2 rpm to 13.9 rpm.

[0145] In this section, precise control is required to prevent residual stress or non-uniformity of the weld line (21) that may occur at small inflection point intervals.

[0146] Laser welding control between the third and fourth inflection points can be performed such that the welding distance is 31.3 mm to 62.9 mm, the irradiation angle variation range is -45° to -35°, and the irradiation angle rotation speed range is 1.9 rpm to 10.0 rpm.

[0147] In this section, it is important to smoothly change direction through significant variations in the beam angle and ensure the continuity of the corrugated weld.

[0148] Laser welding control between the above-mentioned fourth inflection point and the curve exit section can be performed such that the welding distance is 11.6 mm to 20.0 mm, the irradiation angle variation range is -35° to -10°, and the irradiation angle rotation speed range is 0 rpm to 21.1 rpm.

[0149] As this is the finishing section of the curve, the work is completed by maintaining consistent welding to the end of the corrugated section through precise adjustments.

[0150] Laser welding control between the above-mentioned curved exit section and the exit section transition point can be performed such that the welding distance is 10.0 mm to 50.0 mm, the irradiation angle variation range is -45° to -10°, and the irradiation angle rotation speed range is 0.6 rpm to 31.3 rpm.

[0151] Since this is the section where corrugated welding is completed and transitions back to flat welding, the finish is smooth without abrupt adjustments to maintain the welding quality of the entire corrugated section.

[0152] Figure 17 is a configuration diagram illustrating an example of sensing being performed by applying a bandpass filter.

[0153] Referring to FIG. 17, a laser welding system (1000) for an LNG cargo tank according to one embodiment of the present invention may comprise: a sensor unit (110) that is provided on one side of the main body (100) and senses changes in the height of the corrugated part of the welding target and changes in the welding line; and a first bandpass filter unit (112) that is provided on the sensor unit (110) and filters interference caused by a laser beam (201).

[0154] In welding the membrane (20) for the LNG cargo tank, precise control is required simultaneously in two planes, namely the XY plane and the XZ plane based on the aforementioned coordinate system.

[0155] In conventional plasma welding systems, contact and non-contact sensors are used in combination, but when using a high-speed laser welding system (1000), it is impossible to apply contact sensors. In addition, in the case of a laser welding system (1000), a laser beam (201) is irradiated from a position separated by several centimeters, and the laser light interference caused by this causes a problem in that it causes errors in the non-contact sensors.

[0156] In an embodiment of the present invention, to solve this problem, a bandpass filter is applied to detect only the necessary optical signal even during laser beam (201) irradiation. Specifically, the sensor unit (110) is composed of a non-contact sensor and is equipped with a first bandpass filter unit (112) that filters interference caused by the laser beam (201).

[0157] Here, the position sensed by the sensor unit (110) may be spaced at least 10 mm apart from the position where the laser beam (201) is irradiated, and preferably spaced 10 mm to 20 mm apart.

[0158] The aforementioned coaxial camera unit (250) may also be equipped with a second bandpass filter unit (254) that filters interference caused by the laser beam (201).

[0159] Here, the first bandpass filter and the second bandpass filter may have a wavelength range of 400 nm to 600 nm.

[0160] The present invention can increase the utility of a non-contact sensor and maximize the performance of a high-speed laser welding system (1000) by solving the optical interference problem through the first bandpass filter section (112) and the second bandpass filter section (254).

[0161] According to the laser welding device for an LNG cargo tank and the laser welding system for an LNG cargo tank including the same according to the embodiments of the present invention described so far, by increasing the effective cross-sectional area of ​​the laser beam, laser welding can be performed without applying beam wobbling motion, the weight of the system is reduced to provide convenience for installation and movement, and productivity can be improved.

[0162] In addition, good quality and reliability can be ensured by drastically reducing welding time and preventing sagging even during side and overhead welding of LNG cargo tanks, and by implementing a rectangular cross-sectional shape of the laser beam, the occurrence of unwelded areas where welding is not completed at the starting and ending points of the weld can be minimized.

[0163] In addition, the equipment can be operated without colliding with or interfering with surrounding structures due to the rotation of the laser welding device and the connected cable in a narrow area while securing a sufficient focal length in a limited space, and by acquiring image information on the same axis as the laser beam, the occurrence of sensing distortion and error can be minimized and the welding line can be accurately tracked.

[0164] Furthermore, by reducing the rotation angle of the laser welding device during membrane corrugation welding so that it can smoothly pass over the membrane corrugations, the welding speed can be prevented and interference with surrounding structures can be minimized. Additionally, by applying a bandpass filter to filter out only the light necessary for sensing, sensing errors can be minimized and the accuracy of seam tracking and profile tracking can be increased.

[0165] Although the present invention has been described above with reference to an embodiment thereof, those skilled in the art may modify and change the present invention in various ways without departing from the spirit and scope of the invention as described in the claims below. Therefore, if a modified embodiment basically includes the components of the claims of the present invention, it should be considered to be included within the technical scope of the present invention.

[0166] [Explanation of the symbol]

[0167] 10: Guide rail 20: Membrane

[0168] 21: Welding line 100: Main body

[0169] 110: Sensor section 112: First bandpass filter section

[0170] 200: Laser welding device 201: Laser beam

[0171] 210: Laser receiver 220: Body

[0172] 222: First extension 224: Bent

[0173] 2241: First reflective mirror 226: Second extension

[0174] 2261: Dichroic mirror 230: Light homogenizer

[0175] 232: 1st lens array section 234: 2nd lens array section

[0176] 235: Focusing lens part 240: Mask part

[0177] 250: Coaxial camera section 252: Second reflective mirror

[0178] 254: Second bandpass filter section 1000: Laser welding system

Claims

1. A laser receiver that receives a laser beam; A light equalization unit that increases the cross-sectional area of ​​the effective beam region of the laser beam received from the laser receiver; and, A laser welding device for an LNG cargo tank comprising: a mask portion that passes the effective beam region of a laser beam passing through the light uniformization portion above through a specific cross-sectional shape.

2. In Paragraph 1, A laser welding device for an LNG cargo tank, characterized in that the laser beam received by the laser receiver has a wavelength range of 800 nm to 1100 nm.

3. In Paragraph 1, A laser welding device for an LNG cargo tank characterized by the above-mentioned light homogenizing unit performing light splitting, re-synthesis, and homogenization of the above-mentioned laser beam.

4. In Paragraph 1, The above light homogenization unit is, A lens array section that divides the above laser beam into a plurality of small beams to form a uniform distribution, and A laser welding device for an LNG cargo tank characterized by including a focusing lens unit that increases the cross-sectional area of ​​the effective beam region by focusing and advancing a plurality of small beams forming the above uniform distribution at a desired point.

5. In Paragraph 4, A laser welding device for an LNG cargo tank, characterized in that the lens array section includes a first lens array section and a second lens array section, and the first lens array section and the second lens array section are arranged side by side.

6. In Paragraph 1, A laser welding device for an LNG cargo tank characterized in that the laser beam passing through the above mask portion has a square cross-section.

7. In Paragraph 6, A laser welding device for an LNG cargo tank, characterized in that the laser beam passing through the above mask portion has a cross-sectional side length of 3mm to 6mm.

8. Main body; A moving part that moves the above main body along a certain trajectory; and, A laser welding device provided on one side of the main body and moving together with the main body to perform laser welding; is included. A laser welding system for an LNG cargo tank characterized by the above-mentioned laser welding device performing welding by increasing the cross-sectional area of ​​the effective beam region of the laser beam.

9. In Paragraph 8, The above laser welding device is, A laser receiver that receives a laser beam, and A light equalization unit that increases the cross-sectional area of ​​the effective beam region of the laser beam received from the above-mentioned laser receiver, and, A laser welding system for an LNG cargo tank, characterized by comprising a mask portion that passes the effective beam area of ​​the laser beam passing through the light uniformization portion above through a specific cross-sectional shape.

10. In Paragraph 9, The above light homogenization unit is, A lens array section that divides the above laser beam into a plurality of small beams to form a uniform distribution, and A laser welding system for an LNG cargo tank characterized by including a focusing lens unit that increases the cross-sectional area of ​​the effective beam region by focusing and advancing a plurality of small beams forming the above uniform distribution at a desired point.

11. In Paragraph 10, A laser welding system for an LNG cargo tank, characterized in that the lens array section includes a first lens array section and a second lens array section, and the first lens array section and the second lens array section are arranged side by side.

12. In Paragraph 9, A laser welding system for an LNG cargo tank characterized in that the laser beam passing through the above mask portion has a square cross-section.

13. In Paragraph 12, A laser welding system for an LNG cargo tank characterized in that the laser beam passing through the above mask portion has a cross-sectional side length of 3mm to 6mm.

14. In Paragraph 9, A laser welding system for an LNG cargo tank, characterized in that the laser beam received by the laser receiver has a wavelength range of 800 nm to 1100 nm.

15. In Paragraph 8, A laser welding system for an LNG cargo tank characterized by the above-mentioned laser welding device using the flat-top region of the laser beam as the effective beam region.

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