Complex casting installation pose error identification method based on scanning point cloud data

Abstract

The invention discloses a complex casting installation pose error identification method based on scanning point cloud data. The method comprises the steps of standard block position setting and pose calibration, casting installation, point cloud data scanning, matrix conversion between a casting coordinate system and a standard block coordinate system, matrix rotation and translation vector solving, and solving of the rotation angle of a workpiece coordinate system relative to each axis of the standard block coordinate system according to the rotation matrix. According to the method, the casting in the clamping state and the standard block are scanned at the same time through the 3D camera, related coordinate point data are obtained in combination with point cloud analysis software, a rotation matrix and an offset vector of a coordinate system of the casting in the clamping state relative to a coordinate system of the standard block are obtained through a data analysis algorithm, and then the actual pose of the casting in the clamping state is obtained. Compared with traditional methods of multi-point tool setting, on-machine detection and the like, the method is suitable for workpiece pose automatic identification of small-allowance formed blanks and complex castings without reference measurement faces, and method support is provided for improving the cutting and grinding efficiency of the multi-process characteristics of the complex castings.

Description

Technical Field

[0001] This invention relates to the field of cutting and grinding technology for the gating and riser of complex castings, and more specifically to a method for identifying the installation posture error of complex castings based on scanned point cloud data. Background Technology

[0002] Riser gates play a crucial role in casting production by compensating for shrinkage and venting and removing slag, effectively preventing casting defects such as porosity and shrinkage cavities. After casting is formed, the remaining riser gate structure needs to be cut. Castings come in various types, with complex riser gate shapes and large dimensional variations. Furthermore, due to factors such as casting process errors and the lack of consistent positioning benchmarks, most companies still use manual or semi-automated cutting and grinding methods. This not only results in low efficiency but also poses safety hazards and makes it difficult to guarantee consistent quality.

[0003] CNC machine tools are crucial for automating the cutting of risers and gating points in complex castings. The cutting process is achieved by executing NC code, where the position coordinates are the coordinates of a point on the workpiece within the machine tool's reference coordinate system. First, the coordinates of the points on the workpiece in the workpiece coordinate system (CAM toolpath points) are obtained. Then, a coordinate transformation model from the workpiece coordinate system to the machine tool's reference coordinate system is established, ultimately obtaining the coordinates of the points on the workpiece within the machine tool's reference coordinate system. This requires that the actual installation posture of the workpiece be consistent with the workpiece posture in the coordinate transformation model. In traditional machining, the shape of blocky blanks is regular, and the workpiece's coordinate origin position can be obtained through multi-point tool setting and on-machine inspection. Its posture can be ensured by the fixture's positioning. However, castings have complex shapes, and riser cutting is the first machining process, lacking reference features for clamping. Furthermore, considering the allowance deviation of the casting blank, the actual installation posture of the workpiece deviates significantly from the digital model. Therefore, accurate identification of the installation posture of complex castings is key to automated cutting and grinding.

[0004] To this end, this invention proposes a casting installation posture recognition method based on in-situ casting and reference datum scanned point cloud data, providing support for the automated cutting and grinding of complex casting risers and gating gates. Summary of the Invention

[0005] In view of this, the present invention provides a method for identifying the installation pose error of complex castings based on scanned point cloud data. The method uses a 3D camera to simultaneously scan the casting and the standard block in the clamping state, combines point cloud analysis software to obtain relevant coordinate point data, and uses data analysis algorithms to obtain the rotation matrix and offset vector of the coordinate system of the casting in the clamping state relative to the coordinate system of the standard block, thereby obtaining the actual pose of the casting in the clamping state.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for identifying the installation pose error of complex castings based on scanned point cloud data includes the following steps: Step 1, Standard block position setting and pose calibration: Based on the tooling design of the casting to be processed, a rectangular standard block is set inside or beside the tooling. A laser interferometer is used to detect and adjust the position and orientation of the standard block, ensuring that the surface of the standard block is parallel to the coordinate axes of the machine tool's reference coordinate system. A coordinate system for the standard block is established. O C The origin is set at the centroid of the standard block, and the coordinates of the standard block relative to the reference coordinate system are measured. O R offset vector C P R =[ x 1, y 1, z [1, 1], if the orientation of the standard block coordinate system is consistent with that of the machine tool reference coordinate system, then the homogeneous coordinate transformation matrix of the standard block coordinate system relative to the machine tool reference coordinate system is... C T R It can be written as: (1) Step 2, Casting Installation: Position and clamp the casting to be processed; Step 3, Point Cloud Data Scanning: A 3D camera was used to scan the full process features of the casting and the full morphology of the standard block to obtain point cloud data of the casting and the standard block in the clamping state. Step 4: Transformation matrix between casting coordinate system and standard block coordinate system: Based on the rigid body transformation description method, using the RPY angle method around the fixed axes x, y, and z, the attitude matrix between the workpiece coordinate system and the standard block coordinate system is: (2) In the formula, θ x θ y θ z The workpiece coordinate system O W Relative to the standard block coordinate system O C of x , y , z The rotation angle of the shaft; Workpiece coordinate system O w Relative to the standard block coordinate system O C Translation vector WP C for: (3) Then the workpiece coordinate system O w Relative to the standard block coordinate system O C homogeneous coordinate transformation matrix W T C for:

[0007] (4) (5) Step 5, Solve for the rotation matrix and translation vector: Select a set of points from the scanned casting point cloud data. N Non-collinear points P ,in N is An integer not less than 3, read the coordinates of this set of points in the standard block coordinate system. P C Read the coordinate values ​​of this set of points in the workpiece coordinate system. P W , P C and P W Given 3×N matrices, the rotation matrix and translation vector are solved using Singular Value Decomposition (SVD). The specific steps are as follows: ①. Calculate two point sets P C and P W The centroids are denoted as Center_C and Center_W, respectively, both of which are 3×1 vectors; ②. Decentralize the point set, i.e., point set P C and P W Subtracting the coordinates of the centroids Center_C and Center_W from the coordinates of each point in the set, the decentralized point set is denoted as follows: P C_center and P W_center ; ③. Calculate the covariance matrix H = P C_center* P C_center T ; ④. For the covariance matrixH Singular Value Decomposition (SVD) is performed using the built-in svd function in Matlab software. The solution statement is as follows, which returns the U and V matrices. [U, ~, V] = svd ( H ) ⑤. Solving for the rotation matrix W R C = VU T ; ⑥. Calculate the translation vector W P C = Center_C - W R C * P C_center ; Step 6: Solve for the rotation angles of the workpiece coordinate system relative to each axis of the standard block coordinate system using the rotation matrix. Equation (1) can be expressed as: (6) The rotation angles of the workpiece coordinate system relative to each coordinate axis of the gauge block coordinate system can be calculated as follows: (7) Then, the position and orientation of the workpiece coordinate system relative to the machine tool reference coordinate system in the clamping state are obtained.

[0008] The method for recognizing the installation posture of complex castings based on point cloud data of the present invention, compared with traditional methods such as multi-point tool setting and in-machine inspection, can be applied to the automated recognition of workpiece posture of complex castings with small allowance forming blanks and no reference measurement surface, and provides method support for improving the cutting and grinding efficiency of complex castings with multiple process features. Attached Figure Description

[0009] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0010] Figure 1 The overall flowchart provided for this invention.

[0011] Figure 2 This is a schematic diagram showing the positional relationship and coordinate system between the casting and the standard block. Detailed Implementation

[0012] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention. In the description of this invention, it should be understood that the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified. In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0013] This invention proposes a workpiece pose recognition method based on synchronous scanning using a known reference. Specifically, a standard block is installed at a fixed position on the worktable (e.g., a CNC rotary table), and the standard block is scanned simultaneously with the 3D camera scanning the casting. The coordinate system of the standard block... O C With machine tool reference coordinate system O R The relative positional relationship between them is determined and remains unchanged. Therefore, as long as the workpiece coordinate system is obtained... O w The attitude matrix relative to the standard block coordinate system R 3×3 Translation vector P 3×1， This allows us to obtain the coordinate description of points on the workpiece in the machine tool's reference coordinate system.

[0014] The specific implementation steps are as follows: Step 1, setting the position and calibrating the pose of the standard block (or standard gauge block): like Figure 2 As shown, based on the tooling design of the casting (workpiece) to be processed, a rectangular standard block is set inside or beside the tooling. A laser interferometer and other detection equipment are used to detect and adjust the position and orientation of the standard block, ensuring that the surface of the standard block is parallel to the coordinate axes of the machine tool's reference coordinate system. The coordinate system of the standard block is set as follows: O C The origin is set at the centroid of the standard block, and the coordinates of the standard block relative to the reference coordinate system are measured. O R offset vector C P R =[ x 1, y 1, z [1, 1], if the orientation of the standard block coordinate system is consistent with that of the machine tool reference coordinate system, then the homogeneous coordinate transformation matrix of the standard block coordinate system relative to the machine tool reference coordinate system is... C T R It can be written as: (1) Step 2, Casting Installation: Position and clamp the casting to be processed; Step 3, Point Cloud Data Scanning: A 3D camera was used to scan the full process features of the casting and the full morphology of the standard block to obtain point cloud data of the casting and the standard block in the clamping state. Step 4: Transformation matrix between casting coordinate system and standard block coordinate system Based on the rigid body transformation description method, using the RPY angle method around the fixed axis xyz (reference coordinate system), the attitude matrix (from left to right) between the workpiece coordinate system and the standard block coordinate system is: (2) In the formula, θ x θ y θ z The workpiece coordinate system O W Relative to the standard block coordinate system O C of x , y , z The rotation angle of the shaft; Workpiece coordinate system O w Relative to the standard block coordinate system O C Translation vector W PC for: (3) Then the workpiece coordinate system O w Relative to the standard block coordinate system O C homogeneous coordinate transformation matrix W T C for:

[0015] (4) (5) Step 5, Solve for the rotation matrix and translation vector: Select a set of points from the scanned casting point cloud data. N Non-collinear points P ( N (Integers not less than 3), read the coordinates of this group of points in the standard block coordinate system. P C Read the coordinate values ​​of this set of points in the workpiece coordinate system. P W , P C and P W Given 3×N matrices, the rotation matrix and translation vector are solved using Singular Value Decomposition (SVD). The specific steps are as follows: ①. Calculate two point sets P C and P W The centroids are denoted as Center_C and Center_W, respectively, both of which are 3×1 vectors; ②. Decentralize the point set, i.e., point set P C and P W Subtracting the coordinates of the centroids Center_C and Center_W from the coordinates of each point in the set, the decentralized point set is denoted as follows: P C_center and P W_center ; ③. Calculate the covariance matrix H = P C_center* P C_center T ; ④. For the covariance matrix HSingular Value Decomposition (SVD) is performed using the built-in svd function in Matlab software. The solution statement is as follows, which returns the U and V matrices. [U, ~, V] = svd ( H ) ⑤. Solving for the rotation matrix W R C = VU T ; ⑥. Calculate the translation vector W P C = Center_C - W R C * P C_center ; Step 6: Solve for the rotation angles of the workpiece coordinate system relative to each axis of the standard block coordinate system using the rotation matrix: Equation (1) can be expressed as: (6) The rotation angles of the workpiece coordinate system relative to each coordinate axis of the gauge block coordinate system can be calculated as follows: (7) Then, the position and orientation of the workpiece coordinate system relative to the machine tool reference coordinate system in the clamping state are obtained.

[0016] Experimental verification: Taking a certain type of valve body casting as the object, six non-collinear coordinate points were selected from the point cloud data of the valve body. The coordinate values ​​of this group of points in the workpiece coordinate system are shown in Table 1, and the coordinate values ​​in the standard block coordinate system are shown in Table 2.

[0017] Table 1 Point Set P W coordinates

[0018] Table 2 Point Set P C coordinates

[0019] Find the centroids of two point sets, and the results are as follows: Table 3 Point Sets P W and P C The coordinates of the centroids Center_W and Center_C

[0020] Table 4 Point Sets P W Decentralized coordinates P W_Center

[0021] Table 5 Point Sets P C Decentralized coordinates P C_Center

[0022] The result of calculating the covariance matrix H is:

[0023] The U and V matrices were obtained by SVD decomposition, and the results are as follows:

[0024]

[0025] Find the rotation matrix of the workpiece coordinate system relative to the standard block coordinate system. W R C :

[0026] According to the rotation matrix W R C Using formula (7), the rotation angles of the workpiece coordinate system relative to each coordinate axis of the gauge block coordinate system are calculated, and the result is θ. x =7.7397e-09°≈0°,θ y =9.1189e-10°≈0°,θ z =10°.

[0027] Translation vector W P C for:

[0028] Table 6 shows a comparison between the set values ​​and the actual calculated values.

[0029] Table 6 Comparison and Analysis of Calculation Results and Set Values

[0030] This invention proposes a method for casting installation pose recognition based on point cloud data from simultaneous scanning of standard blocks and complex castings. The key points are: 1. Set a fixed standard block on the casting mounting turntable. Each plane of the standard block is parallel to each axis of the reference coordinate system, and define the transformation matrix between the standard block coordinate system and the machine tool reference coordinate system.

[0031] 2. A high-precision 3D camera is used to scan the casting and the standard block simultaneously, that is, the casting and the standard block are scanned in the same field of view.

[0032] 3. Based on the point cloud data from synchronous scanning, the standard block coordinate system and the workpiece coordinate system are set in the point cloud processing software.

[0033] 4. Select a set of N non-coplanar data points (N not less than 3) from the casting point cloud data, and obtain the coordinate values ​​of the points in the set standard block coordinate system and workpiece coordinate system.

[0034] 5. Solve for the rotation matrix and translation vector of the workpiece coordinate system relative to the standard block coordinate system using the SVD method; 6. Based on the rotation matrix transformation formula, calculate the rotation angle of each axis of the workpiece coordinate system relative to the standard block coordinate system, and then obtain the pose of the workpiece coordinate system relative to the machine tool reference coordinate system.

[0035] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0036] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A complex casting installation position error identification method based on scanning point cloud data, characterized in that, Comprising the following steps: Step 1, standard block position setting and pose calibration: According to the jig design of the casting to be processed, a rectangular standard block is arranged in or beside the jig, and the position and posture of the standard block are detected and adjusted by a laser interferometer detection device, so that the face of the standard block is parallel to the coordinate axis of the machine tool reference coordinate system, and the standard block coordinate system is established O C The origin is set at the mass center of the standard block, and the offset vector of the standard block relative to the reference coordinate system is measured O R C P R [ x 1, y 1, z 1, 1],standard block coordinate system and machine tool reference coordinate system The posture of the coordinate system is consistent, and the homogeneous coordinate transformation matrix of the standard block coordinate system relative to the machine tool reference coordinate system C T R can be written as:​ （1） Step 2, casting installation: Position and clamp the casting to be processed; Step 3, point cloud data scanning: Full process feature and standard block full appearance scanning of the casting are performed by using a 3D camera to obtain point cloud data of the clamped casting and the standard block; Step 4: Casting coordinate system and standard block coordinate system conversion matrix: According to the rigid body transformation description method, the pose matrix between the workpiece coordinate system and the standard block coordinate system is obtained by using the RPY angle method around the fixed axis x-y-z: （2） In the formula, θ x θ y θ z The workpiece coordinate system O W Relative to the standard block coordinate system O C of x , y , z The rotation angle of the shaft; Workpiece coordinate system O w Translation vector relative to the standard block coordinate system O C of the standard block coordinate system W P C is: （3） The workpiece coordinate system O w The homogeneous coordinate transformation matrix of the standard block coordinate system O C W T C is:​ （4） （5） Step 5, rotation matrix and translation vector solving: Select a group of non-collinear points on the scanned casting point cloud data N P Wherein N is An integer not less than 3, the coordinate values of the group of points read in the standard block coordinate system are P C The coordinate values of the group of points read in the workpiece coordinate system are P W , P C And P W Both are 3xN matrices, and the singular value decomposition method SVD is used to solve the rotation matrix and the translation vector, and the specific steps are as follows:​ ①. Calculate the centroids of two point sets P C and P W The results are Center_C and Center_W, both 3 x 1 vectors. ②. Decenter the point set, i.e., the point set P C and P W The coordinate values of each point in the decentered point set are P C_center and P W_center ; iii. Compute the covariance matrix H = P C_center* P C_center T ; (4) Singular value SVD decomposition is performed on the covariance matrix H The svd function of Matlab software is used to solve the singular value SVD decomposition, and the solving statement is as follows, and the U and V matrices are returned; [U, ~, V] = svd ( H ) 5. Solving rotation matrix W R C = VU T ; 6. Compute translation vector W P C = Center_C - W R C * P C_center ; Step 6, solving the rotation angle of the workpiece coordinate system relative to the standard block coordinate system according to the rotation matrix Express formula (1) as: （6） Then the rotation angle of the workpiece coordinate system relative to the standard block coordinate system can be calculated as: （7） Further, the position and pose of the clamped workpiece coordinate system relative to the machine tool reference coordinate system are obtained.

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