Axis calibration method of linear structured light system

Abstract

The invention relates to an axis calibration method for a line structured light system, and belongs to the field of machine vision measurement, and the method comprises the steps: S1, a three-jaw chuck clamps a cylindrical calibration body with an ideal Lambert surface, and the axis of the cylindrical calibration body and the axis of the three-jaw chuck are kept coaxial; s2, completing the installation and calibration of the line structured light system; s3, scanning and recording each frame of contour point cloud and rotation angle of the cylindrical calibration body which rotates around the axis at a constant speed, and checking a coaxial relationship; s4, fitting the contour point cloud into an ellipse by using a structured light plane normal vector, and converting the ellipse into a perfect circle to obtain an initial value of an axis; and S5, splicing each frame of contour point cloud by using a Rodrigues formula, constructing a target function by combining the cylindrical geometric constraint and the variance of the distance distribution from the overall contour point cloud of the cylindrical calibration body to the axis, and iteratively optimizing the axis equation. The axis initial value acquisition method provides a reliable initial value for a subsequent optimization algorithm, and the objective function is iteratively solved through the optimization algorithm, so that the accuracy is improved.

Description

Technical Field

[0001] The present invention relates to the technical field related to computer vision, and in particular to an axis calibration method for a line structured light system. Background Art

[0002] With the increasing application of computer-aided design (CAD) in the field of mechanical manufacturing, the three-dimensional digital model of parts has become an indispensable part of the manufacturing and quality inspection process. Non-contact measurement has gradually replaced the contact measurement method based on three-dimensional coordinate measuring machine (CMM) with its low cost and high shape measurement efficiency. Line structured light, as a non-contact active visual measurement method, is widely used in dimension measurement and quality inspection with high efficiency and high precision. At present, line structured light is usually used in conjunction with a motion mechanism, relying on a pre-calibrated moving direction to realize the splicing of contour point clouds, that is, integrating the point cloud data obtained by scanning at different positions to form a complete contour point cloud. However, there is a problem with this scanning method. The back side of the object is blocked, resulting in the inability to scan the contour information of this part, which is lost. Based on this, it is very important for line structured light to develop an efficient and accurate axis calibration method. Only by achieving accurate axis calibration can complete point cloud data be obtained when scanning cylindrical parts, and the shape and other information of cylindrical parts can be fully presented.

[0003] There are two main methods for obtaining the complete circumferential contour point cloud of the measured cylindrical parts. One is the point cloud registration method based on ICP and its variants, and the other is the rotation splicing method of the known axis equation. ICP has limitations in point cloud registration. It has high requirements on the spatial pose of the point cloud to be registered. When the initial spatial position and pose are poor, it is easy to fall into the local optimal solution during the solution process, that is, the global optimal result cannot be obtained. In order to solve this problem, corresponding variants of ICP are produced. The role of these variants is to make it easier for the algorithm to converge to the correct result during iterative calculation. Parts with repeated features on the surface contours such as shafts, stepped shafts, ball screws, and screws often have incorrect matching when performing point matching operations. This incorrect matching will further lead to registration errors in the parts during the registration process, or even complete registration failure, making related assembly or positioning work unable to proceed normally. There are currently two main methods for axis calibration for the rotation stitching method of known axis equations. The first method is to estimate the camera extrinsic parameter matrix based on the pinhole imaging model for axis calibration. This method has limitations. In the camera field of view, the angle between the calibration plate normal vector and the camera optical axis cannot be too large. If the angle is too large, it will become difficult to detect the corner points of the calibration plate, which will have an adverse effect on the calibration accuracy. This limits the calibration application scenarios of this method.

[0004] The second method is to calibrate the axis based on a standard calibration body. Its advantage is that it avoids multiple camera extrinsic parameter estimations and reduces the introduction of errors. However, this method also has disadvantages. Standard calibration bodies are usually not easy to obtain, and the accuracy of the calibration body itself is extremely high.

[0005] Currently, there is an urgent need for an axis calibration method for a line structured light system with a simple operation process and easy to obtain calibration objects. Summary of the invention

[0006] The present invention provides an axis calibration method for a line structured light system, which starts from a cylindrical calibration body and an axis optimization algorithm based on contour point cloud features, fundamentally reducing the problems of large axis calibration error, difficulty in obtaining the calibration body, and complex operation procedures of the line structured light system.

[0007] A method for calibrating an axis of a line structured light system, preferably, an axis calibration system of the line structured light system used comprises: Line structured light system: including camera and line laser; Mechanism to be calibrated: three-jaw chuck, rotating belt, turntable to be calibrated, motor wheel, chuck wheel, rotating motor; A cylindrical calibration body and an ejector pin mechanism to be calibrated; The axis calibration method of the line structured light system comprises: S1: The three-jaw chuck at the front end of the turntable to be calibrated clamps a cylindrical calibration body with an ideal Lambertian surface, and the axis of the cylindrical calibration body is kept coaxial with the axis of the three-jaw chuck; S2: completing the installation of the line structured light system, and calibrating the camera and structured light plane of the line structured light system; S3: the cylindrical calibration body rotates at a uniform speed around the axis, and the line structured light system scans and records the contour point cloud and rotation angle of each frame of the cylindrical calibration body; the coaxial relationship between the cylindrical calibration body and the axis of the turntable to be calibrated is verified. If the coaxial relationship is satisfied, continue to execute S4, otherwise return to S1 to recalibrate; S4: transforming an ellipse fitted by the normal vector of the structured light plane and the contour point cloud into a perfect circle through projection of the ellipse, and acquiring an initial value of the axis based on the projection of the perfect circle in the axis direction; S5: Based on the initial value of the axis, the Rodriguez formula is used to splice the contour point clouds of each frame to obtain the overall contour point cloud. The objective function is constructed by combining the cylindrical geometric constraints and the variance of the distance distribution from the overall contour point cloud of the cylindrical calibration body to the axis, and the axis equation is iteratively optimized.

[0008] Preferably, the cylindrical calibration body is made of alumina ceramics and has a cross-sectional diameter of 18 mm.

[0009] Preferably, the camera parameter calibration adopts Zhang's two-step calibration method to determine the camera's internal and external parameters and the lens's distortion coefficient; the structured light plane calibration uses a coplanar target method to obtain the spatial equation of the structured light plane, and the specific steps of S2 are as follows; The working surface of the calibration plate is coplanar with the Lambertian plate to establish a coplanar target. The structured light plane is irradiated on the Lambertian plate, and the spatial pose of the coplanar target is changed multiple times. The camera extrinsic parameters are estimated using the extracted corner points of the calibration plate to obtain the plane equations of each calibration plate in the camera coordinate system. The plane equations of the calibration plate under each pose are obtained by multiple joint calculations. The center point of the light strip extracted at the current pose , using the extracted light strip center point The corresponding dedistorted normalized coordinates Construction Rays , the coordinates in the camera coordinate system are obtained by combining the following equation (1): , by least squares fitting coordinates The final light plane equation is obtained; (1); in , , , are the parameters of the equation, Normalized coordinates for dedistortion.

[0010] Preferably, the specific steps of S3 are as follows: S3.1: Setting the rotation speed of the turntable to be calibrated, where the rotation speed matches the frame rate captured by the camera; S3.2: The three-jaw chuck rotates at a constant speed to drive the cylindrical calibration body to rotate, and the line structured light system scans the cylindrical calibration body and records the contour point cloud and rotation angle of each frame; S3.3: Compare the offsets of the corresponding points of the contour point clouds of each frame in the camera coordinate system, and use the method based on pixel size mapping to verify the coaxial relationship between the cylindrical calibration body and the turntable to be calibrated. The camera pixel size , Object distance and camera focal length It is known that when the rotation axis of the three-jaw chuck coincides with the rotation axis of the cylindrical calibration body, the corresponding distances of the point pairs in the contour point cloud in the three-dimensional space are Less than tolerance , satisfying the following formula (2): (2); in , are the points corresponding to image i and image j in the contour point cloud, is the regulating factor; If the corresponding distance That is, the offset is less than the tolerance , then the coaxial relationship is satisfied; the corresponding distance Greater than tolerance , return to S1, calibrate the three-jaw chuck with the aid of a micrometer, and re-clamp the cylindrical calibration body until the coaxial relationship is met.

[0011] Preferably, the specific steps of S4 are as follows: S4.1: Based on the normal vector of the structured light plane The initial point in the single frame contour point cloud Constructing an orthogonal 2D coordinate system ; S4.2: Projecting the contour point cloud generated by the intersection of the structured light plane and the cylindrical calibration body to the orthogonal two-dimensional coordinate system The ellipse fitting is completed and the center point of the ellipse is obtained after fitting. , the major axis of the ellipse , minor axis of ellipse and ellipse inclination ; S4.3: Based on the normal vector of the structured light plane, find the direction vector that makes the contour point cloud project into a perfect circle, and the direction vector is the initial value of the axis direction vector , the center point of the ellipse The three-dimensional coordinates obtained by mapping The initial value of the spatial point passing through the axis to be calibrated is ; The derivation process of obtaining the initial value of the axis is as follows: The orthogonal two-dimensional coordinate system Orthogonal unit vectors The formula is shown in (3)(4)(5): (3); (4); (5); To derive intermediate variables, used to construct orthogonal vectors and decide The vector direction is: Three-dimensional coordinate system Each 3D point in the contour point cloud In the two-dimensional coordinate system The coordinates below As shown in formula (6): (6); Fitting the contour point cloud in a two-dimensional coordinate system can obtain the coordinates of the ellipse center , which is mapped to the three-dimensional coordinate system and becomes the initial value of the spatial point through the axis As shown in (7): (7); Ellipse major axis direction As shown in (8): (8); One of the projection directions is the initial value of the axis direction vector As shown in (9): (9); in , .

[0012] Preferably, the specific steps in S5 are as follows: S5.1: Based on the initial value of the axis direction vector and the initial value of the spatial point through the axis Using the Rodriguez formula, the contour point clouds of each frame are rotated and spliced ​​around the iterative intermediate value or the iterative initial value to obtain the overall contour point cloud. The contour point cloud splicing process around the rotation axis is shown in formula (10): (10); in is the rotation matrix, is the identity matrix, is the rotation angle, Represented as the sine value of the rotation angle and the antisymmetric matrix The product of It is expressed as the difference between 1 and the cosine of the rotation angle and the antisymmetric matrix Multiply the square of : in It is an antisymmetric matrix composed of axis direction vectors, as shown in formula (11): (11); Before rotating, the contour point cloud is placed at the origin, as shown in formula (12): (12); The contour point cloud Rotate around the axis of rotation, and when a new point cloud is added during the stitching process, the original point cloud To rotate, is the contour point cloud obtained after rotation, Point cloud for each contour The spatial coordinates of the spatial point passing through the axis The coordinates are transposed after vector subtraction. Multiply the above result by the rotation matrix Then transpose; S5.2: Establish an objective function based on the geometric characteristics of the cylindrical calibration body, and make the obtained rotation axis equation approximate the theoretical value after multiple iterations. The objective function expression is shown in (13): (13); The objective function consists of two parts. The first part depends on the geometric dimensions of the cylindrical calibration body. , the second is the distribution of the distance from the overall contour point cloud to the axis of the current iteration Variance To constrain, is the variance of the distance distribution from the point cloud to the axis, where is the weight factor, The number of the overall contour point cloud, the initial value of the iteration depends on the S4, It is the sum of the square roots of the difference between the distance from each point to the axis and the size of the cylindrical calibration body. The weight is multiplied by the inverse of the total number of contour point clouds. It means to get the minimum value of the function in brackets; After calibrating the rotation axis, the rotation angle of the i-th contour point cloud and the camera sampling frequency and rotation angular velocity The relationship is shown in (14): (14); In the iterative process, the contour point cloud is spliced ​​around the iterative intermediate quantities of the spatial axis in each iteration, and the overall contour point cloud obtained after splicing is constrained by the objective function until the objective function obtained by the rotation axis equation is minimized, thus completing the acquisition and calibration process of the spatial axis.

[0013] Preferably, the line structured light system is inclined at an angle of 20-45 degrees and is arranged on the side of the axis calibration platform parallel to the axis direction. The mechanism to be calibrated is arranged at one end of the axis calibration platform, and the ejector mechanism to be calibrated is located at the other end of the axis calibration platform. The three-jaw chuck includes three jaws. The three-jaw chuck is used to clamp and tightly fix the cylindrical calibration body. The motor wheel and the chuck wheel are connected by a rotating belt and are arranged at an end away from the three-jaw chuck. The motor wheel is connected to the rotating motor. The rotating motor drives the motor wheel to rotate, and then drives the rotating belt outside the motor wheel to rotate. The rotating belt drives the chuck wheel to rotate, and the chuck wheel drives the three-jaw chuck to rotate. When the center of gravity of the clamped cylindrical calibration body is unstable, the ejector mechanism to be calibrated is used to support the cylindrical calibration body.

[0014] The beneficial effects of the present invention are: 1. The cylindrical calibration body designed in the present invention reduces the difficulty of obtaining the calibration body compared with the existing axis calibration method.

[0015] 2. Based on the geometric features formed by the intersection of the cylindrical calibration body and the structured light plane, the present invention proposes a method for obtaining the initial value of the axis using the calibration body contour point cloud, which provides a reliable initial value for the subsequent optimization algorithm and accelerates the convergence of the iteration.

[0016] 3. The present invention utilizes the size and geometric features of the cylindrical calibration body to construct an objective function for optimizing the parameters of the axis equation. The objective function can be iteratively solved by an optimization algorithm to obtain an accurate axis equation.

[0017] 4. Compared with the method of using a calibration plate to calibrate the axis, the present invention can calibrate the axis of a line structured light system when the camera internal parameters are unknown, which greatly reduces the conditions for applying the method. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The drawings described herein are used to provide further understanding of the present invention and constitute a part of this application. The illustrative examples of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0019] Figure 1 It is a schematic flow chart of the axis calibration method of the line structured light system of the present invention; Figure 2 This is a schematic diagram of the placement of the cylindrical calibration body and the line structured light system when the axis calibration is performed in the present invention; Figure 3 The left side diagram is a schematic diagram of the placement of the cylindrical calibration body and the line structured light system when the axis calibration is performed in the present invention; Figure 4 A top view of a schematic diagram of the placement of a cylindrical calibration body and a line structured light system when performing axis calibration in the present invention; Figure 5 Two frames of contour point clouds of the cylindrical calibration body obtained by the line structured light system of the present invention, and the deviation of corresponding points when the cylindrical calibration body is not coaxial with the axis to be calibrated; Figure 6 It is a schematic diagram of the method for obtaining the initial value of the axis proposed in the present invention; Figure 7 This is a rendering of the effect of rotating and splicing the contour point cloud of some cylindrical calibration bodies in the present invention around a known axis; Figure 8 The overall contour point cloud of the stepped axis reconstructed by the axis calibration method of the line structured light system of the present invention; Fig. 9 The overall outline point cloud of the rectangular spline shaft is reconstructed using the axis calibration method of the line structured light system of the present invention.

[0020] Figure numerals: 1 line structured light system; 2 mechanism to be calibrated; 2-1 three-jaw chuck; 2-2 rotating belt; 2-3 turntable to be calibrated; 2-4 motor wheel; 2-5 chuck wheel; 2-6 rotating motor; 3 cylindrical calibration body; 4 ejector pin mechanism to be calibrated; 5 axis calibration platform. DETAILED DESCRIPTION

[0021] In order to solve the above problems, the present invention proposes an axis calibration method for a line structured light system, which can simplify the axis calibration process of the system and improve reconstruction efficiency.

[0022] See also Figures 1 to 6 As shown, the axis calibration device of a line structured light system in an axis calibration method of a line structured light system proposed by the present invention comprises: a line structured light system 1, a mechanism to be calibrated 2, a cylindrical calibration body 3, a pin mechanism to be calibrated 4, and an axis calibration platform 5.

[0023] The structured light system 1 includes a camera and a line laser. The line laser of the line structured light system 1 is used to emit line structured light to the surface of the cylindrical calibration body 3. The line structured light system 1 is inclined at a certain angle, the angle range is 20-45 degrees, and is arranged on the side of the axis calibration platform 5 parallel to the axis direction. The line structured light system 1 is inclined to ensure that the structured light plane intersects with the cylindrical calibration body 3 to produce an elliptical contour point cloud. The inclination angle only needs to ensure that it can intersect to produce an elliptical contour point cloud. During the experiment, it was found that the ellipse fitting effect was not good when the angle was too large, and the ellipse trend was not obvious when the angle was too small. Therefore, the ellipse fitting effect of the line structured light system 1 with an angle of 20-45 degrees is the best.

[0024] Due to the height change and shape difference of the object surface, the intersection line between the structured light plane and the measured object will be deformed. The camera is used to collect images of the deformed intersection line. The line structured light system 1 analyzes the deformation of the intersection line in the image collected by the camera, combines the triangulation principle and related algorithms, calculates the three-dimensional coordinates of each point on the object surface, and thus obtains the three-dimensional information of the object. In this process, it is necessary to ensure that the camera parameters such as focal length, exposure time, etc. are set reasonably to obtain clear and accurate images. The line structured light system 1 is a prior art, so it is not described in detail.

[0025] The mechanism to be calibrated 2 includes a three-jaw chuck 2-1, a rotating belt 2-2, a turntable to be calibrated 2-3, a motor wheel 2-4, a chuck wheel 2-5, and a rotating motor 2-6. The mechanism to be calibrated 2 is located at one end of the axis calibration platform 5, and the ejector mechanism to be calibrated 4 is located at the other end of the axis calibration platform 5; The three-jaw chuck 2-1 includes three jaws, and the three-jaw chuck 2-1 is used to clamp and tightly fix the cylindrical calibration body 3 so that the cylindrical calibration body 3 remains stable during subsequent operations; The motor wheel 2-4 and the chuck wheel 2-5 are rotatably connected by a rotating belt 2-2 and are arranged at an end away from the three-jaw chuck 2-1. The motor wheel 2-4 is connected to the rotating motor 2-6. The rotating motor 2-6 drives the motor wheel 2-4 to rotate, and then drives the rotating belt 2-2 outside the motor wheel 2-4 to rotate. The rotating belt 2-2 drives the chuck wheel 2-5 to rotate, and the chuck wheel 2-5 drives the three-jaw chuck 2-1 to rotate.

[0026] The cylindrical calibration body 3 is a standard object in the shape of a cylinder. The cylindrical calibration body 3 is used to calibrate and calibrate instruments and equipment as a standard reference to ensure the accuracy and reliability of measurement or detection. The diameter of the cylindrical calibration body 3 is 18 mm, and the diameter is not limited to a range of 10 mm to 100 mm. In this embodiment, 18 mm is preferred. When the length is too long and the center of gravity is unstable, the calibrated ejector mechanism 4 is used to support the cylindrical calibration body 3 to ensure that the center of gravity does not shift downward.

[0027] The cylindrical calibration body 3 is made of a material with Lambertian surface optical properties such as alumina ceramics. The energy distribution of the light strip normal vector obtained by alumina ceramics in the structured light system is closer to the Gaussian distribution, avoiding the error introduced by the error in the extraction of the light strip center point algorithm, avoiding the occurrence of high reflection, and facilitating the camera to obtain high-quality light strip images.

[0028] A flowchart of an axis calibration method for a line structured light system is shown in Figure 1 The specific process includes the following steps: S1: clamping cylindrical calibration body; The three-jaw chuck 2-1 clamps and firmly fixes the cylindrical calibration body 3. During installation, it should be ensured that the cylindrical calibration body 3 coincides with the rotation axis of the turntable 2 to be calibrated, and the coaxiality error is less than or equal to 0.01mm. The diameter of the cylindrical calibration body 3 is 18mm and the length is 200mm.

[0029] S2: completing the installation of the line structured light system, and calibrating the camera and structured light plane of the line structured light system; The camera parameters were calibrated using Zhang's two-step calibration method to determine the camera's internal and external parameters and the lens's distortion coefficient. The structured light plane calibration used the coplanar target method to obtain the spatial equation of the structured light plane.

[0030] The specific process is as follows: The designed fixture ensures that the working surface of the calibration plate is coplanar with the Lambertian material plate to establish a coplanar target. The spatial position of the coplanar target is changed multiple times while ensuring the clarity of the calibration plate corners and the structured light plane irradiating the Lambertian plate. The camera extrinsic parameters are estimated using the extracted calibration plate corners to obtain the plane equations of each calibration plate in the camera coordinate system.

[0031] The plane equation of the plate is calibrated by multiple joint calculations at different poses The center point of the light strip extracted at the current pose , the corresponding dedistorted normalized coordinates Construction Rays , the coordinates in the camera coordinate system are obtained by combining the following formulas , by least squares fitting these coordinates The final light plane equation is obtained; (1); in , , , are the parameters of the equation, Normalized coordinates for dedistortion.

[0032] The schematic diagram of the arrangement space of the cylindrical calibration body 3 clamping and axis calibration device is as follows Figure 2 shown.

[0033] S3: Complete the contour scanning of the cylindrical calibration body 3 through the line structured light system. During the scanning process, the calibration body rotates at a constant speed and the rotation angle is recorded; S3.1: Set the rotation speed of the turntable 2 to be calibrated. The rotation speed should match the camera acquisition frame rate. In this embodiment, the camera shooting frame rate is 30, and the turntable is fixed to rotate 30° per second, that is, the angular interval between the contour point clouds is 1°; S3.2: During the rotation of the turntable 2 to be calibrated, the linear structured light system records the rotation angle of the contour point cloud in each frame; In this embodiment, after completing the camera calibration and the light plane calibration, the pixel coordinates of the center point of the light strip in the image can correspond to the world coordinates one by one. The contour point cloud can be obtained by rotating and scanning the cylindrical calibration body 3 by the three-claw chuck 2-1. In order to avoid the calibration error of the rotation axis caused by the different axes of the three-claw chuck, the present invention uses a method based on pixel size mapping. The camera pixel size used in the line structured light system for size measurement , Object distance and camera focal length are all known. When the chuck axis coincides with the axis of rotation of the cylindrical calibration body, it is intuitively reflected as the distance between the corresponding point pairs in the contour point cloud in three-dimensional space. Should be less than the tolerance , the schematic diagram is as follows Figure 3 , which satisfies the following formula (2): (2); in , is the corresponding point between image i and image j, is the regulating factor; When formula (2) is satisfied, the clamping of the cylindrical calibration body meets the calibration requirements; If formula (2) is not satisfied, re-clamp the cylindrical calibration body with the aid of tools. The specific steps are as follows: Calibrate the three-jaw chuck 2 - 1 : Use a tool such as a dial gauge to assist in calibrating the three-jaw chuck 2 - 1 . Install the dial gauge at a suitable position so that its measuring head contacts the surface of the cylindrical calibration body 3 .

[0034] Slowly rotate the three-jaw chuck 2-1, observe the changes in the dial indicator pointer, and fine-tune the three-jaw chuck 2-1 according to the offset of the pointer, so that the change in the reading of the dial indicator of the cylindrical calibration body 3 during the rotation process is controlled within a very small range, so as to ensure that the axis of the cylindrical calibration body 3 is as coaxial as possible with the axis to be calibrated, and the axis to be calibrated is the axis of the turntable to be calibrated. During the calibration process, it is necessary to measure the offset of the contour point cloud multiple times, and repeatedly adjust the clamping three-jaw chuck 2-1 until the coaxial relationship requirements are met.

[0035] S4: Based on the acquired cylindrical calibration body contour point cloud, the initial value of the axis is obtained: S4.1: Based on the normal vector of the structured light plane The initial point in the single frame contour point cloud Constructing an orthogonal 2D coordinate system ; S4.2: Projecting the contour point cloud generated by the intersection of the structured light plane and the cylindrical calibration body to the orthogonal two-dimensional coordinate system The ellipse fitting is completed and the center point of the ellipse is obtained after fitting. , the major axis of the ellipse , minor axis of ellipse and ellipse inclination ; S4.3: Based on the normal vector of the structured light plane, find the direction vector that makes the corresponding contour point cloud project into a perfect circle. This vector is the initial value of the axis direction vector. , the center point of the ellipse The three-dimensional coordinates obtained by mapping The initial value of the spatial point passing through the axis to be calibrated is ; The derivation process of obtaining the initial value of the axis is as follows: The orthogonal two-dimensional coordinate system Orthogonal unit vectors The formula is shown in (3)(4)(5): (3); (4); (5); To derive intermediate variables, used to construct orthogonal vectors and decide The vector direction is: Three-dimensional coordinate system Each 3D point in the contour point cloud In the two-dimensional coordinate system The coordinates below As shown in formula (6): (6); Fitting the contour point cloud in a two-dimensional coordinate system can obtain the coordinates of the ellipse center , which is mapped to the three-dimensional coordinate system and becomes the initial value of the spatial point through the axis As shown in (7): (7); Ellipse major axis direction As shown in (8): (8); One of the projection directions is the initial value of the axis direction vector As shown in (9): (9); in , , so far the optimization initial value has been obtained.

[0036] S5: Based on the obtained initial value of the axis, the optimization solution of the axis equation is completed by relying on the Rodriguez formula. The specific steps are as follows: S5.1: Based on the initial axis values ​​obtained in S4.3, the initial rotation and splicing of the cylindrical calibration body 3 is performed using the Rodriguez formula; In this embodiment, based on the Rodriguez formula, in the process of subsequent iterative solution of the objective function, the overall contour point cloud can be obtained by rotating and splicing around the iterative intermediate value or the iterative initial value.

[0037] The process of splicing the contour point cloud around the rotation axis is shown in formula (10): (10); in is the rotation matrix, is the identity matrix, is the rotation angle, Represented as the sine value of the rotation angle and the antisymmetric matrix The product of It is expressed as the difference between 1 and the cosine of the rotation angle and the antisymmetric matrix Multiply the square of in is an antisymmetric matrix composed of direction vectors, as shown in formula (11): (11); Before rotating, the contour point cloud is placed at the origin, as shown in formula (12): (12); The contour point cloud is now complete During the process of rotation around the axis of rotation, the addition of new point clouds during the splicing process requires the addition of the original point clouds To rotate, is the contour point cloud obtained after rotation, Point cloud for each contour The spatial coordinates of the spatial point passing through the axis The coordinates are transposed after vector subtraction. Multiply the above result by the rotation matrix Then transpose; S5.2; Establish an optimization objective function based on the geometric characteristics of the cylindrical calibration body, which should make the obtained axis equation approximate the theoretical value after multiple iterations; In this embodiment, the objective function consists of two parts, which depends on the geometric dimensions of the standard cylindrical calibration body 3. On the other hand, the distance distribution from the overall contour point cloud to the rotation axis under the current iteration Variance To constrain, is the variance of the distance distribution from the point cloud to the axis, is the number of overall contour point clouds. The expression of the objective function is shown in (13): (13); in is the weight factor, the initial value of the iteration depends on S4, It is the sum of the square roots of the difference between the distance from each point to the axis and the size of the cylindrical calibration body. The weight is multiplied by the inverse of the total number of contour point clouds. To find the average and weight of the sum of the square roots obtained above, It means to get the minimum value of the function in the brackets.

[0038] After calibrating the rotation axis, the rotation angle of the i-th contour point cloud and the camera sampling frequency and rotation angular velocity The relationship is shown in (14): (14); In the iterative process, the contour point cloud is spliced ​​around the iterative intermediate quantities of the spatial axis in each iteration, and the overall contour point cloud obtained after splicing is constrained by the objective function until the objective function obtained by the rotation axis equation is minimized, thus completing the acquisition and calibration process of the spatial axis.

[0039] Figure 7 This is a diagram showing the effect of rotating and splicing the contour point cloud of some cylindrical calibration bodies in the present invention around a known axis.

[0040] Figure 8-Figure 9 The overall contour point cloud of the stepped shaft and the overall contour point cloud of the rectangular spline shaft are reconstructed by the axis calibration method of the line structured light system of the present invention. It can be seen from the spliced ​​overall contour point cloud that the rotary motion axis calibrated by the present invention is reconstructed based on the principle of laser triangulation, combined with camera calibration, structured light plane calibration, axis calibration, contour point cloud processing and other technologies. The line structured light system can complete the reconstruction of cylindrical parts with high precision based on the obtained axis equation, which intuitively demonstrates the practicality of the present invention in the field of rotary reconstruction.

[0041] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made to the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for calibrating an axis of a line structured light system, characterized in that: The axis calibration system of the applied line structured light system includes: Line structured light system: including camera and line laser; Mechanism to be calibrated: three-jaw chuck, rotating belt, turntable to be calibrated, motor wheel, chuck wheel, rotating motor; A cylindrical calibration body and an ejector mechanism for the body to be calibrated; The axis calibration method of the line structured light system comprises: S1: The three-jaw chuck at the front end of the turntable to be calibrated clamps a cylindrical calibration body with an ideal Lambertian surface, and the axis of the cylindrical calibration body is kept coaxial with the axis of the three-jaw chuck; S2: completing the installation of the line structured light system, and calibrating the camera and structured light plane of the line structured light system; S3: the cylindrical calibration body rotates at a uniform speed around the axis, and the line structured light system scans and records the contour point cloud and rotation angle of each frame of the cylindrical calibration body; the coaxial relationship between the cylindrical calibration body and the axis of the turntable to be calibrated is verified. If the coaxial relationship is satisfied, continue to execute S4, otherwise return to S1 to recalibrate; S4: transforming an ellipse fitted by the normal vector of the structured light plane and the contour point cloud into a perfect circle through projection of the ellipse, and acquiring an initial value of the axis based on the projection of the perfect circle in the axis direction; S5: Based on the initial value of the axis, the Rodriguez formula is used to splice the contour point clouds of each frame to obtain the overall contour point cloud. The objective function is constructed by combining the cylindrical geometric constraints and the variance of the distance distribution from the overall contour point cloud of the cylindrical calibration body to the axis, and the axis equation is iteratively optimized.

2. The axis calibration method of a line structured light system according to claim 1, characterized in that: The cylindrical calibration body is made of alumina ceramics and has a cross-sectional diameter of 18 mm.

3. The axis calibration method of a line structured light system according to claim 1, characterized in that: The camera parameter calibration uses Zhang's two-step calibration method to determine the camera's internal and external parameters and the lens's distortion coefficient; the structured light plane calibration uses the coplanar target method to obtain the spatial equation of the structured light plane. The specific steps of S2 are as follows: The working surface of the calibration plate is coplanar with the Lambertian plate to establish a coplanar target. The structured light plane is irradiated on the Lambertian plate, and the spatial pose of the coplanar target is changed multiple times. The camera extrinsic parameters are estimated using the extracted corner points of the calibration plate to obtain the plane equations of each calibration plate in the camera coordinate system. The plane equations of the calibration plate under each pose are obtained by multiple joint calculations. The center point of the light strip extracted at the current pose , using the extracted light strip center point The corresponding dedistorted normalized coordinates Construction Rays , the coordinates in the camera coordinate system are obtained by combining the following equation (1): , by least squares fitting coordinates The final light plane equation is obtained; （1）； in , , , are the parameters of the equation, Normalized coordinates for dedistortion.

4. The axis calibration method of a line structured light system according to claim 1, characterized in that: The specific steps of S3 are as follows: S3.1: Setting the rotation speed of the turntable to be calibrated, where the rotation speed matches the frame rate captured by the camera; S3.2: The rotating motor is started, the three-jaw chuck rotates at a constant speed to drive the cylindrical calibration body to rotate, and the line structured light system scans the cylindrical calibration body and records the contour point cloud and rotation angle of each frame; S3.3: Compare the offsets of the corresponding points of the contour point clouds of each frame in the camera coordinate system, and use the method based on pixel size mapping to verify the coaxial relationship between the cylindrical calibration body and the turntable to be calibrated. The camera pixel size , Object distance and camera focal length It is known that when the rotation axis of the three-jaw chuck coincides with the rotation axis of the cylindrical calibration body, the corresponding distances of the point pairs in the contour point cloud in the three-dimensional space are Less than tolerance , satisfying the following formula (2): （2）； in , are the points corresponding to image i and image j in the contour point cloud, is the regulating factor; Corresponding distance That is, the offset is less than the tolerance , then the coaxial relationship is satisfied; the corresponding distance Greater than tolerance , return to S1, calibrate the three-jaw chuck with the aid of a micrometer, and re-clamp the cylindrical calibration body until the coaxial relationship is met.

5. The axis calibration method of a line structured light system according to claim 1, characterized in that: The specific steps of S4 are as follows: S4.1: Based on the normal vector of the structured light plane The initial point in the single frame contour point cloud Constructing an orthogonal 2D coordinate system ; S4.2: Projecting the contour point cloud generated by the intersection of the structured light plane and the cylindrical calibration body to the orthogonal two-dimensional coordinate system The ellipse fitting is completed and the center point of the ellipse is obtained after fitting. , the major axis of the ellipse , minor axis of ellipse and ellipse inclination ; S4.3: Based on the normal vector of the structured light plane, find the direction vector that makes the contour point cloud project into a perfect circle, and the direction vector is the initial value of the axis direction vector , the center point of the ellipse The three-dimensional coordinates obtained by mapping The initial value of the spatial point passing through the axis to be calibrated is ; The derivation process of obtaining the initial value of the axis is as follows: The orthogonal two-dimensional coordinate system Orthogonal unit vectors The formula is shown in (3)(4)(5): （3）； （4）； （5）； To derive intermediate variables, used to construct orthogonal vectors and decide The vector direction is: 3D Coordinate System Each 3D point in the contour point cloud In the two-dimensional coordinate system The coordinates below As shown in formula (6): （6）； Fitting the contour point cloud in a two-dimensional coordinate system can obtain the coordinates of the ellipse center , which is mapped to the three-dimensional coordinate system and becomes the initial value of the spatial point through the axis As shown in (7): （7）； Ellipse major axis direction As shown in (8): （8）； One of the projection directions is the initial value of the axis direction vector As shown in (9): （9）； in , .

6. The axis calibration method of a line structured light system according to claim 1, characterized in that: The specific steps in S5 are as follows: S5.1: Based on the initial value of the axis direction vector and the initial value of the spatial point through the axis Using the Rodriguez formula, the contour point clouds of each frame are rotated and spliced ​​around the iterative intermediate value or the iterative initial value to obtain the overall contour point cloud. The contour point cloud splicing process around the rotation axis is shown in formula (10): （10）； in is the rotation matrix, is the identity matrix, is the rotation angle, Represented as the sine value of the rotation angle and the antisymmetric matrix The product of It is expressed as the difference between 1 and the cosine of the rotation angle and the antisymmetric matrix Multiply the square of : It is an antisymmetric matrix composed of axis direction vectors, as shown in formula (11): （11）； Before rotating, the contour point cloud is placed at the origin, as shown in formula (12): （12）； The contour point cloud Rotate around the axis of rotation, and when new point clouds are added during the stitching process, the original point clouds are To rotate, is the contour point cloud obtained after rotation, Point cloud for each contour The spatial coordinates of the spatial point passing through the axis The coordinates are transposed after vector subtraction. Multiply the above result by the rotation matrix Then transpose; S5.2: Establish an objective function based on the geometric characteristics of the cylindrical calibration body, and make the obtained rotation axis equation approximate the theoretical value after multiple iterations. The objective function expression is shown in (13): （13）； The objective function consists of two parts. The first part depends on the geometric dimensions of the cylindrical calibration body. , the second is the distribution of the distance from the overall contour point cloud to the axis of the current iteration Variance To constrain, is the variance of the distance distribution from the point cloud to the axis, where is the weight factor, The number of the overall contour point cloud, the initial value of the iteration depends on the S4, It is the sum of the square roots of the difference between the distance from each point to the axis and the size of the cylindrical calibration body. The weight is multiplied by the inverse of the total number of contour point clouds. To average and weight the sum of the square roots obtained above, It means to get the minimum value of the function in brackets; After calibrating the rotation axis, the rotation angle of the i-th contour point cloud and the camera sampling frequency and rotation angular velocity The relationship is shown in (14): （14）； In each iteration of the contour point cloud, the iterative intermediate quantities around the spatial axis are spliced, and the overall contour point cloud obtained after splicing is constrained by the objective function until the objective function obtained by the rotation axis equation is minimized, thus completing the acquisition and calibration process of the spatial axis.

7. The axis calibration method of a line structured light system according to claim 1, characterized in that: The line structured light system is inclined at an angle of 20-45 degrees and is arranged on the side of the axis calibration platform in parallel with the axis direction; The mechanism to be calibrated is arranged at one end of the axis calibration platform, and the ejector mechanism to be calibrated is located at the other end of the axis calibration platform; The three-jaw chuck includes three clamping jaws, and the three-jaw chuck is used to clamp and tightly fix the cylindrical calibration body; The motor wheel and the chuck wheel are connected by a rotating belt and are arranged at an end away from the three-jaw chuck. The motor wheel is connected to the rotating motor. The rotating motor drives the motor wheel to rotate, which in turn drives the rotating belt outside the motor wheel to rotate. The rotating belt drives the chuck wheel to rotate, and the chuck wheel drives the three-jaw chuck to rotate. When the center of gravity of the clamped cylindrical calibration body is unstable, the calibrated ejector pin mechanism is used to support the cylindrical calibration body.

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