External parameter calibration method of weld tracker and three-axis welding platform in laser welding

Abstract

The present invention relates to a method for calibrating external parameters of a weld tracker and a three-axis welding platform in laser welding. The steps are: controlling a laser to move on a welding platform so that it strikes a light spot on the upper surface of a calibration block, and recording the three-dimensional coordinates of the weld tracker in the welding platform coordinate system when striking the light spot; using a weld tracker to collect a light spot image and process and calculate the two-dimensional image coordinates of the center of the light spot; using a weld tracker to shoot an image of a calibration plate placed on the upper surface of the calibration block, identifying the position of a mark point on the calibration plate, and thereby calculating the three-dimensional coordinates of the light spot on the calibration block; according to the coordinates of the center of the light spot in the weld tracker coordinate system and the corresponding coordinates in the welding platform coordinate system, calculating the rotation R and translation matrix T between the two coordinate systems, and completing the calibration. The method of the present invention can effectively calibrate the external parameters between the weld tracker and the three-axis welding platform, the calibration method is simple, easy to operate, and has high calibration accuracy, and can achieve accurate correction of the three degrees of freedom of the welding platform.

Description

Technical Field

[0001] The present invention relates to the field of laser welding technology and computer vision technology, in particular to a method for calibrating external parameters of a weld tracker and a three-axis welding platform in laser welding. Background Art

[0002] During the welding process of the three-axis welding platform, the weld position information measured by the weld tracker is in the tracker coordinate system. When correcting the motion parameters of the three-axis welding platform, the weld position coordinates measured in the tracker coordinate system need to be converted to the welding platform coordinate system. Therefore, it is necessary to first calibrate the external parameters of the tracker and the welding platform to obtain the position conversion relationship between the two coordinate systems.

[0003] Laser welding is different from other welding methods: first, the weld point is at the focus of the laser, and the coordinate value of the weld point cannot be directly measured; second, when the laser is focused, the area of ​​the weld point is very small, so the error allowed by the correction parameters is very small, otherwise the weld point will deviate from the weld and cause welding failure.

[0004] At present, the following methods are used to calibrate the parameters of weld trackers and welding platforms: 1) Generally, the distance between the tracking point of the tracker and the welding point is measured manually to obtain the welding advance; 2) Only the deviation perpendicular to the weld direction on the welding plane is corrected for the motion parameters. However, this method has low measurement accuracy and is only suitable for tracking planar straight welds. It is not applicable to situations where height adjustment is required in the direction perpendicular to the welding plane. Summary of the invention

[0005] In view of the above shortcomings in the prior art, the technical problem to be solved by the present invention is to provide a method for calibrating the external parameters of the weld tracker and the three-axis welding platform in laser welding, which can realize the calibration of the external parameters between the tracker and the laser welding platform, so that the weld position obtained by the tracker can be converted to the welding platform coordinate system, thereby realizing real-time adjustment of the three degrees of freedom of the weld position parameters x, y, and z during the welding process, and realizing precise control of laser welding. The method is easy to implement and has high calibration accuracy.

[0006] The technical solution adopted by the present invention to achieve the above-mentioned purpose is:

[0007] The method for calibrating the external parameters of a weld tracker and a three-axis welding platform in laser welding comprises the following steps:

[0008] Control the laser to move on the welding platform so that it hits the light spot on the upper surface of the calibration block, and record the three-dimensional position coordinates of the light spot in the welding platform coordinate system {P li};

[0009] The weld tracker is used to collect the spot image and process and calculate the two-dimensional image coordinate position of the spot center;

[0010] The weld tracker is used to capture the calibration plate image, and the position coordinates of the calibration plate plane in the tracker coordinate system are determined by identifying the positions of the marking points on the calibration plate;

[0011] According to the positional relationship between the calibration plate and the calibration block, the three-dimensional coordinates of the center of the light spot in the weld tracker coordinate system {P ti};

[0012] According to the coordinates of the center of the light spot in the weld tracker coordinate system {P ti}, and the corresponding three-dimensional coordinate value {P li}, calculate the rotation R and translation matrix T between the tracker coordinate system and the three-axis welding platform coordinate system to complete the calibration of external parameters.

[0013] The method is implemented based on a calibration system, which includes: a welding platform, a laser, a weld tracker, a calibration block, and a calibration plate;

[0014] The welding platform is a three-axis linear module, the weld tracker and the laser are fixedly mounted at the ends of the three-axis linear module, and the relative positions of the two remain unchanged. The laser focus is adjusted to coincide with the origin of the welding platform coordinates, and the coordinates of the welding platform movement are the coordinates of the light spot under the welding platform;

[0015] The calibration block has two stepped upper surfaces with a height difference of h, and is placed within the motion range of the welding platform. When the welding platform is at the origin position, the weld tracker can capture the spot image of the calibration block being hit;

[0016] The surface of the calibration plate is provided with a plurality of circular marking points, and the centers of adjacent circular points are equidistant. The marking points are used to determine the direction of the coordinate axis of the calibration plate.

[0017] The step of the laser striking a light spot on the upper surface of the calibration block comprises:

[0018] Control the weld tracker and the laser to move on the welding platform; when they move above the calibration block, turn on the laser to emit the laser, strike a light spot on each step surface of the calibration block, and keep the laser focused during the striking of the light spot.

[0019] The upper surfaces of the two steps of the calibration block are struck with at least three light spots in total.

[0020] Maintaining the focus of the laser during the striking of the light spot includes: controlling the laser to rise or fall to a height h on the welding platform so that the laser remains focused on the surface to be processed.

[0021] The steps of performing image processing and calculating the two-dimensional coordinate position of the center of the light spot include:

[0022] The light spot contour is obtained through image enhancement and edge extraction processing; the geometric center of the light spot is calculated through ellipse fitting.

[0023] The step of determining the position coordinates of the calibration plate plane in the tracker coordinate system by identifying the position of the marking point on the calibration plate comprises:

[0024] The collected calibration plate image is enhanced and edge extracted to obtain the contour of the marker point, and the center position of the marker point is calculated by ellipse fitting to determine the three-dimensional position coordinates of the calibration plate plane in the tracker coordinate system.

[0025] The steps of image enhancement and edge extraction processing include:

[0026] The image enhancement is to denoise the acquired image to reduce the influence of noise;

[0027] The edge extraction is as follows: firstly, setting a threshold parameter to perform pixel-level edge extraction, filtering all edges to remove non-target edge pixels; secondly, performing sub-pixel edge extraction to obtain edge information for edge graphic fitting.

[0028] The step of further determining the three-dimensional coordinates of the surface where the light spot on the calibration block is located according to the positional relationship between the calibration plate and the calibration block comprises:

[0029] According to the thickness value of the calibration plate and its position coordinates in the weld tracker coordinate system, the plane equation of the surface where the light spot on the calibration block is located is calculated, and the three-dimensional position coordinates of the center of the light spot in the weld tracker coordinate system are calculated accordingly.

[0030] The external parameters refer to the transformation matrix from the tracker coordinate system to the welding platform coordinate system, which is represented by the rotation matrix R and the translation matrix T.

[0031] The present invention has the following beneficial effects and advantages:

[0032] 1. The method for calibrating external parameters of a weld tracker and a three-axis welding platform in laser welding described in the present invention can effectively realize the calibration of position parameters between the weld tracker and the welding platform coordinate system.

[0033] 2. The method for calibrating external parameters of a weld tracker and a three-axis welding platform in laser welding described in the present invention can realize the direct conversion of the tracker coordinate value to the welding platform coordinate value, thereby achieving accurate correction of the motion parameters of the three degrees of freedom in the x, y, and z directions of the welding platform.

[0034] 3. The calibration block used in the external parameter calibration method of the weld tracker and the three-axis welding platform in laser welding described in the present invention is simple to process and low in cost, and the calibration method is easy to operate and implement.

[0035] 4. The calibration method for external parameters of a weld tracker and a three-axis welding platform in laser welding described in the present invention has high calibration accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 is a flow chart of the method of the present invention;

[0037] Figure 2 A schematic diagram of a three-dimensional calibration block used in the method of the present invention;

[0038] Figure 3 Schematic diagram of a plane calibration plate used in the method of the present invention;

[0039] Figure 4 It is a schematic diagram of the three-axis welding platform calibrated in the method of the present invention. DETAILED DESCRIPTION

[0040] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation method of the present invention is described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the invention, so the present invention is not limited by the specific implementation disclosed below.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0042] The present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0043] The external parameter calibration process of the weld tracker and the three-axis welding platform in laser welding is based on the following system: welding platform, laser, weld tracker, calibration block, calibration plate. The weld tracker consists of an industrial CCD camera and a linear structured light laser.

[0044] like Figure 2 As shown, the three-dimensional calibration block used in this embodiment has a high processing accuracy, the upper surface has two stepped planes, and the height difference h between the two upper surfaces is known. The calibration block is generally made of metal, such as aluminum alloy, and the laser can leave a circular spot on its surface for easy image detection.

[0045] like Figure 3 As shown, the two-dimensional calibration plate used in this embodiment has a known thickness, a surface with dots, and the distances between the centers of all dots are equal. Five large dots (as marking points) are used to determine the coordinate system of the calibration plate and to mark and sort all other dots.

[0046] like Figure 4 As shown, in this embodiment, a three-dimensional welding platform is used, the weld tracker is fixed at the end of the three-axis welding platform, and the laser is also fixed at the end of the three-axis welding platform, and the position relationship with the end of the laser remains unchanged.

[0047] When calibrating, the laser must be focused first, and it is defined that when each motion axis is at the initial position, the laser focus coincides with the origin of the three-axis platform coordinate system, and the coordinate axis direction is consistent with the motion direction of the three axes. At this time, the motion parameters of the three-axis platform are consistent with the motion parameters of the laser focus.

[0048] See attached Figure 2-4 The specific implementation steps of the external parameter calibration method of the weld tracker and the three-axis welding platform in laser welding described in the present invention are as follows:

[0049] Step 1: Figure 4 As shown, the weld tracker is installed at a fixed position at the end of the three-axis welding platform, and the relative position relationship between the tracker and the end of the welding platform (the end of the laser) remains unchanged. When the welding platform moves, the tracker and the end of the laser can move simultaneously and remain relatively still. At this time, the laser should be adjusted to the focusing state. When the three moving axes are in the initial position, the laser focus is defined to coincide with the origin of the welding platform coordinates, and the x, y, and z directions of the coordinate system are consistent with the movement direction of the axis. The purpose of defining the laser focus to coincide with the origin of the welding platform coordinates is that the motion parameters of the welding platform are consistent with the motion parameters of the laser focus. At this time, the xy value of the welding platform movement can be considered as the coordinate value of the laser focus (that is, the light spot) in the platform coordinate system.

[0050] Step 2: Place the 3D calibration block within the motion range of the three-axis welding platform, and when the tracker is at the origin of the welding platform, the tracker can capture the upper surface image of the calibration block. Figure 2 The surface of the three-dimensional calibration block shown has two steps, and the height difference h is known.

[0051] Step 3: Move the welding motion platform to the top of the calibration block, turn on the laser and emit the laser at a lower output power (which will not damage the surface of the calibration block but can leave a spot on it, such as 500 watts), leaving a circular spot on the surface of the calibration block. At this time, the laser needs to be focused, that is, when the laser moves from one step of the calibration block to another, the distance moved in the Z-axis direction should be the same as h;

[0052] Step 4: Repeat step 3, process at least one spot on each step surface of the calibration block, and process at least three spots on the two surfaces in total, and record the three-dimensional coordinate value (x li ,y li , z li ); for example, Figure 2 The coordinates of the three-axis welding platform during the three spot processing are: (x l1 ,y l1 , z l1 )、(x l2 ,y l2 , z l2 )、(x l3 ,y l3 , z 13 ), z l1 =z l2 , z l3 =z l1 +h.

[0053] Step 5: After the motion platform hits the light spot, it returns to the origin position, and uses the tracker to obtain the image of all the light spots on the surface of the calibration block. The geometric center coordinate value of the light spot in the tracker coordinate system is calculated through image enhancement, edge extraction, ellipse fitting, etc., that is, the two-dimensional coordinate value of the center of the light spot in the tracker image coordinate system.

[0054] The image enhancement includes image smoothing to remove noise and enhance the edge of the light spot. When extracting the edge, the Canny operator is first used to extract the pixel-level edge, and then the sub-pixel edge detection algorithm of the pseudo-Zernike moment is used to extract the sub-pixel edge, so as to obtain the high-precision coordinates of the center of the circular spot image. This algorithm is also applicable to the identification of the center of the calibration plate dots in step 6.

[0055] Step 6: Place the plane calibration plate shown in the figure on the surface of the calibration block, use the tracker to obtain the image data of the calibration plate (the tracker is still at the origin at this time), identify the calibration plate, obtain the coordinates of each mark point in the tracker coordinate system, and then use the known tracker internal parameters to calculate the plane equation of the calibration plate.

[0056] Step 7: According to the plane equation of the calibration plate and the thickness of the calibration plate, the plane equation of the upper surface of the two steps of the calibration block is calculated, and the three-dimensional coordinate value of the light spot in the tracker coordinate system is calculated accordingly {P ti}.

[0057] Step 8: According to the three-dimensional coordinate value of the center of the light spot in the tracker coordinate system {P ti} and the corresponding three-dimensional coordinate value of the three-axis welding platform when hitting each spot position {P li}, calculate the rotation R and translation matrix T between the tracker coordinate system and the three-axis welding platform coordinate system, and complete the calibration of external parameters. The coordinate value of the spot under the welding platform is {P li}(i=0,1,2,3...,n), the coordinate value in the tracker coordinate system is {P ti}(i=0, 1, 2, 3..., n), where i is the light spot sequence number and n is the total number of light spots. The specific calculation process is as follows:

[0058] (1) First, calculate the point set {P li The geometric center of gravity of Then calculate the point set {P ti}Geometric center of gravity

[0059] (2) Calculate the coordinate difference Q between each point and the geometric center li =P li -P lo , Q ti =P ti -P to ;

[0060] (3) Calculation matrix

[0061] (4) Perform SVD decomposition on the matrix M: M = UΛV T

[0062] (5) Calculated value X = VU T , if det(X)=1, then R=X; if det(X)=-1, then R=V′U T , where V′ is equal to V with the sign of the third column reversed.

[0063] (6) Calculate the translation matrix T = P lo -RP to .

[0064] The present invention designs a three-dimensional calibration block and a calibration method to calibrate the external parameters of the weld tracker and the three-axis welding platform in laser welding to obtain the conversion matrix between the tracker coordinate system and the three-axis platform coordinate system. The method is simple to operate, the calibration accuracy can reach 0.08mm, and the precise motion parameter correction of the welding platform under three degrees of freedom can be realized.

[0065] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should be regarded as within the scope of protection of the present invention.

Claims

1. A method for calibrating external parameters of a weld tracker and a three-axis welding platform in laser welding, characterized in that: The following steps are involved: Control the laser to move on the welding platform so that it hits the light spot on the upper surface of the calibration block, and record the three-dimensional position coordinates of the light spot in the welding platform coordinate system {P li }; The weld tracker is used to collect the spot image and process and calculate the two-dimensional image coordinate position of the spot center; The weld tracker is used to capture the calibration plate image, and the position coordinates of the calibration plate plane in the tracker coordinate system are determined by identifying the positions of the marking points on the calibration plate; According to the positional relationship between the calibration plate and the calibration block, the three-dimensional coordinates of the center of the light spot in the weld tracker coordinate system {P ti }; According to the coordinates of the center of the light spot in the weld tracker coordinate system {P ti }, and the corresponding three-dimensional coordinate value {P li }, calculate the rotation R and translation matrix T between the tracker coordinate system and the three-axis welding platform coordinate system to complete the calibration of external parameters; The method is implemented based on a calibration system, which includes: a welding platform, a laser, a weld tracker, a calibration block, and a calibration plate; The welding platform is a three-axis linear module, the weld tracker and the laser are fixedly mounted at the ends of the three-axis linear module, and the relative positions of the two remain unchanged. The laser focus is adjusted to coincide with the origin of the welding platform coordinates, and the coordinates of the welding platform movement are the coordinates of the light spot under the welding platform; The calibration block has two stepped upper surfaces with a height difference of h, and is placed within the motion range of the welding platform. When the welding platform is at the origin position, the weld tracker can capture the spot image of the calibration block being hit; The surface of the calibration plate is provided with a plurality of circular marking points, and the centers of adjacent circular points are equidistant. The marking points are used to determine the direction of the coordinate axis of the calibration plate.

2. The method for calibrating external parameters of a weld tracker and a three-axis welding platform in laser welding according to claim 1, characterized in that: The step of the laser striking a light spot on the upper surface of the calibration block comprises: Control the weld tracker and the laser to move on the welding platform; when they move above the calibration block, turn on the laser to emit the laser, strike a light spot on each step surface of the calibration block, and keep the laser focused during the striking of the light spot.

3. The method for calibrating external parameters of a weld tracker and a three-axis welding platform in laser welding according to claim 2, characterized in that: The upper surfaces of the two steps of the calibration block are struck with at least three light spots in total.

4. The method for calibrating external parameters of a weld tracker and a three-axis welding platform in laser welding according to claim 2, characterized in that: Maintaining the focus of the laser during the striking of the light spot includes: controlling the laser to rise or fall to a height h on the welding platform so that the laser remains focused on the surface to be processed.

5. The method for calibrating external parameters of a weld tracker and a three-axis welding platform in laser welding according to claim 2, characterized in that: The steps of performing image processing and calculating the two-dimensional coordinate position of the center of the light spot include: The light spot contour is obtained through image enhancement and edge extraction processing; the geometric center of the light spot is calculated through ellipse fitting.

6. The method for calibrating external parameters of a weld tracker and a three-axis welding platform in laser welding according to claim 1, characterized in that: The step of determining the position coordinates of the calibration plate plane in the tracker coordinate system by identifying the position of the marking point on the calibration plate comprises: The collected calibration plate image is enhanced and edge extracted to obtain the contour of the marker point, and the center position of the marker point is calculated by ellipse fitting to determine the three-dimensional position coordinates of the calibration plate plane in the tracker coordinate system.

7. The method for calibrating external parameters of a weld tracker and a three-axis welding platform in laser welding according to claim 5 or 6, characterized in that: The steps of image enhancement and edge extraction processing include: The image enhancement is to denoise the acquired image to reduce the influence of noise; The edge extraction is as follows: firstly, setting a threshold parameter to perform pixel-level edge extraction, filtering all edges to remove non-target edge pixels; secondly, performing sub-pixel edge extraction to obtain edge information for edge graphic fitting.

8. The method for calibrating external parameters of a weld tracker and a three-axis welding platform in laser welding according to claim 1, characterized in that: The step of further determining the three-dimensional coordinates of the surface where the light spot on the calibration block is located according to the positional relationship between the calibration plate and the calibration block comprises: According to the thickness value of the calibration plate and its position coordinates in the weld tracker coordinate system, the plane equation of the surface where the light spot on the calibration block is located is calculated, and the three-dimensional position coordinates of the center of the light spot in the weld tracker coordinate system are calculated accordingly.

9. The method for calibrating external parameters of a weld tracker and a three-axis welding platform in laser welding according to claim 1, characterized in that: The external parameters refer to the transformation matrix from the tracker coordinate system to the welding platform coordinate system, which is represented by the rotation matrix R and the translation matrix T.

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