A Method for Generating a Tool Path Parallel to the Machining Profile Based on Point Cloud Data

Abstract

The present invention belongs to the technical field of reverse engineering, and discloses a method for generating a contour parallel tool path for numerical control machining based on point cloud data. First, boundary points are extracted, and after sorting, the nested relationship between the generated boundaries is distinguished to generate an initial tool path; then, the offset points are calculated in turn by means of the point projection method to obtain offset lines, and after eliminating the self-intersection interference therein, the next contour parallel tool path is generated; if this tool path intersects with the offset curve of the island boundary, the tool path is updated by using the fusion process; subsequently, it is judged whether this tool path meets the stop criterion, and only the valid tool paths are saved to the stack; a tool path is taken out from the stack as input, and the generation of the next valid tool path is repeated until the stack is empty. The method of the present invention bypasses the complex and time-consuming surface reconstruction process, realizes the automatic generation of the contour parallel tool paths based on the point cloud model, is particularly suitable for the model with islands, and solves the problem that the reverse engineering technology is seriously dependent on the professional experience of the process personnel and it is difficult to achieve automation.

Description

Technical Field

[0001] The present invention relates to the technical field of reverse engineering, and particularly relates to a method for generating a numerically controlled machining contour parallel tool path based on point cloud data. Background Technique

[0002] At present, due to its function of quickly obtaining replicas of existing products, reverse engineering technology has advantages such as shortening the development cycle and saving costs, and is widely used in fields such as aviation, automobiles, and medicine. General reverse engineering mainly includes three processes: first, obtaining the point cloud data of the existing product with the help of contact or non-contact three-dimensional measurement equipment, then obtaining a format acceptable to the CAD / CAM system from the point cloud data through surface reconstruction technology, and finally completing the machining tool path planning in the CAD / CAM system and performing post-processing, outputting the final tool path and performing actual machining, so as to realize the imitation of the existing product.

[0003] However, the surface reconstruction process is very complex and time-consuming. Although some commercial software has provided the function of reconstructing from point cloud to parametric surface, operations such as complex point cloud partitioning, trimming, and splicing still inevitably rely on the experience of technicians and are difficult to automate. Therefore, some scholars have carried out a number of technical researches on the direct generation technology of tool paths based on point cloud, so as to avoid the surface reconstruction process and realize the automation of reverse engineering from point cloud measurement to tool path planning. The literature "Lin A C, Liu H. Automatic generation of NC cutter path from massive data points[J]. Computer-Aided Design, 1998, 30(1): 77-90." resampled the point cloud with the z-map model and calculated the tool positions at equal cross-sections by linear interpolation. The literature "Feng H, Teng Z. Iso-planar piecewise linear NC tool path generation from discrete measured data points[J]. Computer-Aided Design, 2005, 37(1): 55-64." first constructed a tool position grid according to the machining step and row spacing, and determined the tool positions by calculating the weighted average of the grid nodes, so as to generate an iso-planar tool path considering machining accuracy. The literature "Yingjie Z, Liling G. Adaptive tool-path generation on point-sampled surfaces[J]. Precision Engineering, 2011, 35(4): 591-601." approximated the local point cloud data with a moving least squares surface, and generated a curvature adaptive tool path with higher machining accuracy after calculating the curvature and normal vector.

[0004] However, so far, most of the researches have focused on the generation methods of tool paths in the iso-planar mode. When applying the tool paths in this mode to models with complex boundaries and islands, there will be too many short paths and tool lift times, which seriously limits the machining efficiency. Compared with the iso-planar mode tool paths, the contour parallel tool paths are smooth and continuous, have good boundary adaptability, and are more suitable for the machining of models with complex boundaries and islands. So far, the method of directly generating contour parallel tool paths based on point cloud considering island models has not appeared in relevant literatures and patents. To solve the above technical problems, the present invention proposes a method for generating a contour parallel tool path for numerical control machining based on point cloud data that can handle models with island models. Summary of the Invention

[0005] The objective of the present invention is to generate a tool path directly based on point clouds, bypassing the complex and time-consuming surface reconstruction process in reverse engineering; to solve the problem that in existing methods for directly generating tool paths based on point clouds, most of them focus on generating tool paths in an equal-plane mode, resulting in low machining efficiency due to excessive short paths and tool retraction times when dealing with models with complex boundaries and islands. The present invention provides a method for generating a contour parallel tool path for numerical control machining based on point cloud data, especially considering the case of models with islands, and automatically generating a tool path from the point cloud model without manual intervention.

[0006] The technical solution of the present invention is as follows: A method for generating a contour parallel tool path for numerical control machining based on point cloud data, comprising the following steps:

[0007] Step (a), extract the inner boundary and outer boundary of the point cloud model; offset the inner boundary outward once to obtain an inner offset curve, and use the outer boundary as the initial input tool path; each point in the input tool path is a tool contact point.

[0008] Step a1. Extract boundary feature points:

[0009] Fit a local point set composed of a point P in the point cloud i and its k nearest neighbor points to obtain a local tangent plane; after projecting the local point set onto the local tangent plane, connect the projection point of point P i to the projection points of its k nearest neighbor points respectively to obtain k vectors; when the maximum value of the included angle between two adjacent vectors is greater than a preset threshold, it is determined that P i is a boundary feature point.

[0010] Step a2. Sort the boundary feature points to obtain multiple boundaries:

[0011] Take any boundary feature point P n as the initial point P0, and search for its nearest point as the second-order point P1; establish a search direction and search for the next nearest point P m from this, and judge whether this nearest point meets the vector included angle θ m requirement:

[0012]

[0013] where θ th is a preset angle threshold; if it is met, then P m is considered as the next-order point P2; update the search direction to Repeat the above search process until P n is found again to obtain a closed boundary curve; input the unvisited boundary feature points again and repeat the sorting process to obtain multiple boundaries.

[0014] Step a3. Determine the nested relationship between the boundaries to obtain the inner boundary and the outer boundary: Select any point on one boundary and determine whether it is inside the other boundary. If it is inside, then this boundary is considered the inner boundary; otherwise, it is the outer boundary. After sequentially judging any two boundaries, finally obtain the inner boundary and the outer boundary; adjust the rotation direction of the inner boundary to counterclockwise and the outer boundary to clockwise.

[0015] Step (b). Sequentially offset each tool contact point in the input tool path inward to generate offset lines; eliminate self-interference in the offset lines and perform curve smoothing to obtain the next contour parallel tool path.

[0016] Step (c). Determine whether the contour parallel tool path intersects with the inner offset curve. If it does not intersect, then this contour parallel tool path is a valid tool path; otherwise, update the contour parallel tool path through fusion processing to obtain a valid tool path:

[0017] Step c1. Traverse the tool contact points to determine whether they are inside the inner offset curve; when an internal point appears, record the corresponding index value until the next tool contact point is outside the inner offset curve to obtain the internal point index set A in {j, j + 1, j + 2..., j + m};

[0018] Step c2. Calculate the intersection points of the line segments P j-1 P j and P j+m P j+m+1 with the inner offset curve respectively. The inner offset curve is divided into two parts at the intersection points; retain the part of the original contour parallel tool path that is inside the inner offset curve as the valid tool path and delete the other part.

[0019] Step c3. Delete the points corresponding to the index values in the internal point index set A in from the original contour parallel tool path, and insert the valid tool path retained in Step c2 after P j-1 ;

[0020] Step (d). Determine whether the valid tool path obtained in Step c3 meets the offset stop criterion. If it meets, then this tool path is considered invalid; otherwise, it is considered a valid tool path.

[0021] Step (e). Save the valid tool path to the stack, take out the tool path at the top of the stack as the input tool path, and repeat Steps b - d to generate the next contour parallel tool path; when the stack is empty, complete the generation of all contour parallel tool paths.

[0022] The step of generating the offset line includes:

[0023] Step b1. Initial offset of the tool contact point: The coordinates of the tool contact point C and its k nearest neighbor points form a covariance matrix, and calculate the normal vector V of this tool contact point through the eigenvector corresponding to the minimum eigenvalue in the covariance matrixn ; The tool contact point forms vectors with the two sequential points before and after it respectively, and the average value of the two vectors is used as the tool contact point cutting vector V t , and the initial offset direction V is obtained as follows:

[0024] V = V n × V t (2)

[0025] The offset distance L is a preset fixed value, and the initial offset point I of the tool contact point C is obtained:

[0026] I = C + LV (3)

[0027] Step b2. Project the initial offset point back to the surface of the point cloud model: For each point P i (x i , y i , z i ) in the point cloud model, the corresponding weight value is:

[0028]

[0029] Among them, represents the distance from the point P i to the ray formed by the initial offset point I and the tool contact point normal vector V n ; From this, the projection distance is obtained as:

[0030]

[0031] The normal vector V n coordinates are (n x , n y , n z ), where:

[0032]

[0033] The projection direction is set as the normal vector V of the tool contact point C n , thus obtaining the initial projection point I * :

[0034] I * = I + tV n (7)

[0035] Step b3. Iteratively adjust the projection point to obtain the next path tool contact point: Calculate the distance d * between the initial projection point I CC and the tool contact point C, and adjust the offset distance according to the distance d CC :

[0036]

[0037] Among them, δL is the adjusted distance, and ε is the allowable machining error value; repeat the offset-projection process until d CC is within the range of [L - ε, L + ε], and the finally obtained projection point is used as the point on the equidistant offset line.

[0038] The offset stop criterion is specifically as follows: when there are tool contact points in the effective tool paths retained in step c3 outside the input tool path or no contour parallel tool paths are retained after offset line interference elimination, it means not to continue the next offset.

[0039] Compared with the prior art, the beneficial effects of the method of the present invention are as follows: The present invention can automatically generate numerically controlled machining contour parallel tool paths with good boundary adaptability and smoothness for models containing complex boundaries and islands, bypassing the complex and time-consuming surface reconstruction process in reverse engineering technology, realizing the automation from the measured point cloud model to the generation of tool paths, and solving the problem that reverse engineering technology is seriously dependent on the professional experience of process personnel and difficult to achieve automation. Brief Description of the Drawings

[0040] Figure 1 is the flow chart of the present invention;

[0041] Figure 2 is a schematic diagram of the method for extracting boundary feature points of the present invention; Figure 2(a) is a schematic diagram of the point cloud model, Figure 2(b) is a schematic diagram of the local features of internal points, Figure 2(c) is a schematic diagram of the local features of internal boundary points, and Figure 2(d) is a schematic diagram of the local features of external boundary points.

[0042] Figure 3 is a schematic diagram of the method for sorting boundary feature points and determining the nesting relationship of the present invention;

[0043] Figure 4 is a schematic diagram of the offset line of the tool path of the present invention;

[0044] Figure 5 is a schematic diagram of determining the initial offset point of the present invention;

[0045] Figure 6 is a schematic diagram of iteratively adjusting the projection point of the present invention;

[0046] Figure 7 is a schematic diagram of self-intersection interference elimination of the present invention, Figure 7(a) is a schematic diagram of the established face-edge set, Figure 7(b) is a schematic diagram of the intersection formed by two consecutive edges, and Figure 7(c) is a schematic diagram of the effective intersection at the interference location;

[0047] Figure 8 is a schematic diagram of the fusion processing of the tool path and the inner offset curve of the present invention;

[0048] Figure 9 is a schematic diagram of the tool path offset stop criterion of the present invention. Detailed implementation manners

[0049] To make the objectives, technical solutions and advantages of the present invention clearer, the following further describes the detailed implementation manners of the present invention in conjunction with the accompanying drawings and embodiments.

[0050] Embodiment 1

[0051] As Figure 1 shown in the flowchart, a method for generating a numerically controlled machining contour parallel tool path based on point cloud data includes the following steps:

[0052] Step 1: Extract the inner and outer boundaries of the point cloud model and generate an initial input tool path: Extract the boundary feature points in the point cloud model, sort the boundary feature points to obtain multiple boundary contours, and obtain the outer boundary contour and the island boundary contour after judging the nesting relationship; Adjust the rotation direction of the island boundary contour to counterclockwise, and offset each point outward to obtain an inner offset curve; Adjust the rotation direction of the outer boundary contour to clockwise and use it as the initial input tool path;

[0053] The extraction of the boundary feature points, the sorting of the boundary feature points, and the judgment of the nesting relationship include the following specific steps:

[0054] a1. Extract the boundary feature points in the point cloud model: As shown in Figure 2, find the k nearest neighbor points of each point P i in the point cloud to form a local point set. Fit the local point set by the least square method to obtain a local tangent plane, and project the local point set onto the local tangent plane. As Figure 2(b) - Figure 2(d) shown, on the local tangent plane, connect the projection point of P i to the projection points of its k nearest neighbor points to form k vectors respectively. It can be observed from Figure 2(b) that when the maximum value of the included angle between any two adjacent vectors is relatively small, it is considered that the point P i is an internal point; while as shown in Figures 2(c) and 2(d), when the maximum value of the included angle between any two adjacent vectors is greater than 120 degrees, it is considered that the point P i is a boundary feature point; This method can extract both the external and internal boundary points of the model at the same time;

[0055] a2. Sort the extracted boundary feature points: As Figure 3 shown, take any point in the extracted boundary feature points as the initial point P0, and search for its nearest point as the second-order point P1. Establish a search direction and continue to search in this direction to obtain the next nearest point and judge whether this point satisfies the following formula:

[0056]

[0057] where θ this the angle threshold; if the above formula is satisfied, it is considered that is the next sequential point P2, and the search direction is updated to Repeat the above search process until the nearest point found is exactly the initial point;

[0058] a3. Determine the nesting relationship between boundaries and generate the initial tool path: As Figure 3 shown, sort all the extracted boundary points. After several sorts, several boundary contours B1, B2, and B3 of the point cloud model are obtained. Select a point in B1 and determine whether it is inside B2. Since it is inside, B1 is considered the inner island boundary; after judging the nesting relationship between any two boundaries using the above method, B1 and B3 are obtained as the inner island boundary contours, and B2 is the outer boundary contour;

[0059] Step 2: As Figure 4 shown, offset each tool contact point in the input tool path inward to generate an equidistant offset line; eliminate the self-intersection interference in the equidistant offset line to obtain the next contour parallel tool path; note that since the self-intersection interference is divided into global interference and local interference, the number of contour parallel tool paths obtained may be more than one;

[0060] The offset of the tool contact point includes the following specific steps:

[0061] b1. Perform an initial offset on the tool contact point: As Figure 5 shown, C B is the input tool path, where the covariance matrix formed by the tool contact point P i and the coordinates of its k-nearest neighbor points is used to estimate the eigenvector corresponding to its minimum eigenvalue as the normal vector V i of the point P n ; the tool contact point P i forms vectors i-1 and i+1 with the sequential points P before and after it respectively. Take the average of these two vectors as the tangent vector V i of the tool contact point P t , then the initial offset direction V is set as:

[0062] V = V n × V t (10)

[0063] The initial offset distance is set to a fixed value L. As Figure 5 shown, the initial offset point I can be obtained as follows:

[0064] I = C + LV (11)

[0065] b2. Project the initial offset point back to the point cloud surface to obtain the initial projection point: Calculate each point P i(x i , y i , z i ) has a weight value of:

[0066]

[0067] Wherein, represents the distance from point P i to the ray formed by the initial offset point I and the normal vector V of the tool contact point n ; From this, the projection distance is obtained as:

[0068]

[0069] Where:

[0070]

[0071]

[0072] As Figure 6 shown, the initial projection point can be obtained, and thus the initial projection point I * :

[0073] I * = I + tV n (15)

[0074] b3. Iteratively adjust the projection point to obtain the next path tool contact point: As Figure 6 shown, find the distance d * between the initial projection point I CC and the tool contact point C, and adjust the initial offset distance according to the following formula:

[0075]

[0076] where δ L is the adjusted distance, determined according to the actual situation, and recommended to be set to 0.1L; ε is the machining allowable error value; repeat the offset - projection process according to the adjusted offset distance until d CC is within the range of [L - ε, L + ε], and take the finally obtained projection point I * as the point on the equidistant offset line;

[0077] Eliminate the self - intersection interference in the equidistant offset line to obtain the next contour parallel tool path, including the following specific steps:

[0078] c1. As shown in Figure 7, project the equally-spaced offset lines onto the XY plane, establish a quadrilateral mesh, number the mesh and the offset edges respectively, and record which offset edges pass through a mesh; as shown in Figure 7(b) and Figure 7(c), find the intersection points of two offset edges passing through the same mesh respectively, and determine whether they are inside the mesh; it is found that only the intersection point of offset edges 4 and 21 is inside mesh 19, so self-intersection interference occurs on the equally-spaced offset lines here, and this intersection point is taken as the valid intersection point;

[0079] c2. At the valid intersection point, split out a closed loop, and judge whether the rotation direction of the closed loop is consistent with that of the equally-spaced offset line; if they are consistent, this loop is a valid loop and is retained as the tool path, otherwise this loop is an invalid interference loop and should be deleted;

[0080] Step 3: Judge whether there is an intersection point between the tool path parallel to the contour and the inner offset curve. If there is no intersection, this tool path parallel to the contour is a valid tool path; otherwise, update the original tool path by using the fusion process to obtain a valid tool path;

[0081] The fusion process of the tool path and the inner offset curve includes the following steps:

[0082] d1. As Figure 8 shown, traverse each tool contact point in the tool path, judge whether it is inside the inner offset curve. When an internal point appears, start recording its index value. The next several points will also be inside the inner offset curve, and continue to record the index value until the tool contact point is outside the inner offset curve, obtaining the internal point index set A in {j,j + 1,j + 2...,j + m};

[0083] d2. Calculate the intersection points of two line segments P j-1 P j and P j+m P j+m+1 with the inner offset curve respectively. The inner offset curve is divided into two parts at these two intersection points. Among them, the part inside the original tool path is retained as the valid tool path, and the other part is deleted;

[0084] d3. Delete the points corresponding to the index values in A in in the original tool path, and insert the valid tool path retained by the inner offset curve behind the P j-1 point;

[0085] Step 4: Judge whether each tool path meets the given offset stop criterion. If it meets the stop criterion, it is considered that the inward offset cannot continue and this tool path is an invalid tool path, otherwise it is considered to be a valid tool path parallel to the contour.

[0086] As Figure 9 shown, the offset stop criterion includes the following specific contents:

[0087] e1. If there is a tool contact point outside the input tool path in the tool path, it means that the continuous offset should stop;

[0088] e2. If no tool path is retained after interference elimination, it means that the continuous offset should stop.

[0089] Step 5: Save the obtained valid contour parallel tool paths to a result stack, take out the tool path at the top of the stack as the input tool path, and repeat steps 2-4 to generate the next contour parallel tool path; when the stack is empty, the tool path covers the entire surface and no continuous offset is performed, and the generation of all contour parallel tool paths is completed.

[0090] In summary, the present invention directly generates numerically controlled machining contour parallel tool paths based on point cloud data, has good applicability for cases containing complex boundaries and island models, bypasses the complex and time-consuming surface reconstruction process in traditional reverse engineering technology, and solves the problem that reverse engineering technology is highly dependent on the professional experience of process personnel and is difficult to achieve automation.

Claims

1. A method for generating a tool path parallel to the contour of numerical control machining based on point cloud data, characterized in that It includes the following steps: Step (a), extract the inner boundary and outer boundary of the point cloud model; the inner boundary is offset outward once to obtain the inner offset curve, and the outer boundary is used as the initial input tool path; each point in the input tool path is a tool contact point; Step a1. Extract the boundary feature points: Fitting a point P in the point cloud i and its local point set composed of k nearest neighbor points to obtain a local tangent plane; after projecting the local point set onto the local tangent plane, the projection points of point P i are respectively connected to the projection points of its k nearest neighbor points to obtain k vectors; when the maximum value of the angles between two adjacent vectors is greater than a preset threshold, it is determined that P i is a boundary feature point; Step a2. Sort the boundary feature points to obtain multiple boundaries: Take any boundary feature point P n as the initial point P0, and search for its nearest point as the second-order point P1; establish the search direction and search for the next nearest point P therefrom m , and determine whether the nearest point satisfies the vector included angle θ m requirement: where θ th is a preset angle threshold; if satisfied, it is considered that P m is the next sequential point P2; update the search direction to Repeat the above search process until P n is found again, obtaining a closed boundary curve; input the unvisited boundary feature points again and repeat the sorting process to obtain multiple boundaries; Step a3. Judge the nesting relationship between the boundaries to obtain the inner boundary and the outer boundary: select any point on one boundary and judge whether it is inside another boundary. If it is inside, then this boundary is considered the inner boundary, otherwise it is the outer boundary; after judging any two boundaries in sequence, finally obtain the inner boundary and the outer boundary; adjust the rotation direction of the inner boundary to counterclockwise and the outer boundary to clockwise; Step (b), offset each tool contact point in the input tool path inward in sequence to generate an offset line; eliminate the self-interference in the offset line and perform curve smoothing to obtain the next contour parallel tool path; Step (c), judge whether the contour parallel tool path intersects with the inner offset curve. If it does not intersect, then this contour parallel tool path is a valid tool path; otherwise, update the contour parallel tool path through fusion processing to obtain a valid tool path: Step c1. Traverse the cutter contact points and determine whether they are inside the inner offset curve; when an interior point appears, record the corresponding index value until the next cutter contact point is outside the inner offset curve, obtaining the interior point index set A in {j, j + 1, j + 2..., j + m}; Step c2. Calculate the intersections of the line segments P j-1 P j and P j+m P j+m+1 with the inner offset curve respectively. The inner offset curve is divided into two parts at the intersection points. Retain the part of the original contour parallel tool path that is inside the inner offset curve as the effective tool path, and delete the other part; Step c3. Delete the points corresponding to the index values in the internal point index set A from the original contour parallel tool path, and insert the effective tool paths retained in step c2 after P in ; j-1 ​ Step (d), judge whether the valid tool path obtained in step c3 meets the offset stop criterion. If it meets, then this tool path is considered invalid, otherwise it is considered a valid tool path; Step (e), save the valid tool path to the stack, take out the tool path at the top of the stack as the input tool path, and repeat steps b - d to generate the next contour parallel tool path; when the stack is empty, complete the generation of all contour parallel tool paths.

2. The method for generating a tool path parallel to a numerical control machining profile based on point cloud data according to claim 1, wherein The step of generating the offset line includes: Step b1. Initial offset of the tool contact point: The coordinates of the tool contact point C and its k nearest neighbor points form a covariance matrix, and the normal vector V of the tool contact point is calculated through the eigenvector corresponding to the minimum eigenvalue in the covariance matrix n ; Vectors are respectively formed by the tool contact point and its two adjacent sequential points, and the average value of the two vectors is used as the tangential vector V of the tool contact point t , and the initial offset direction V is obtained as follows: V = V n × V t (2) The offset distance L is a preset fixed value to obtain the initial offset point I of the tool contact point C: I = C + LV (3) Step b2. Project the initial bias point back to the surface of the point cloud model: For each point P in the point cloud model i (x i , y i , z i ), the corresponding weight value is: Among them, represents the point P i to the initial offset point I and the ray distance formed by the tool contact point normal vector V n The projection distance is obtained as follows: Normal vector V n The coordinates are (n x , n y , n z ), where: The projection direction is set to the normal vector V of the tool contact point C n , thereby obtaining the initial projection point I * : I * = I + tV n (7) Step b3. Iteratively adjust the projection point to obtain the next path tool contact point: calculate the initial projection point I * The distance d from the tool contact point C CC , and based on the distance d CC Adjust the offset distance: where δ L is the adjusted distance, and ε is the machining allowable error value; repeat the offset-projection process until d CC is within the range of [L - ε, L + ε], and take the finally obtained projection point as the point on the equidistant offset line.

3. The method for generating a numerically controlled machining contour parallel tool path based on point cloud data according to claim 1 or 2, characterized in that The offset stop criterion is specifically: when there is a tool contact point in the valid tool path retained in step c3 outside the input tool path or no contour parallel tool path is retained after the interference of the offset line is eliminated, it means not to continue the next offset.

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