CN114386118A - CAD model multi-resolution gridding method for self-adaptive maintenance of mechanism semantics - Google Patents
CAD model multi-resolution gridding method for self-adaptive maintenance of mechanism semantics Download PDFInfo
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Abstract
The invention discloses a CAD model multi-resolution gridding method for self-adaptive maintenance of mechanism semantics, which comprises the steps of firstly extracting mechanism semantic information in a CAD model and organizing the mechanism semantic information into multi-level representation; then, a multi-resolution technology is adopted, and the CAD model is gridded in a mode of reducing the number of part models, geometrically compressing and eliminating hidden contents; finally, a flat structure is designed for standardly and uniformly representing and storing the multi-resolution gridding CAD model; the final output may be a multi-resolution gridded CAD model that provides support for the VR/AR environment. The invention carries out multi-resolution gridding on the CAD model, and simultaneously maintains necessary mechanism semantic information in the CAD model in a self-adaptive manner, so that the gridded CAD model not only can carry out efficient interaction and rendering in a VR/AR environment, but also can accurately and effectively develop application meeting industrial standards.
Description
Technical Field
The invention relates to the field of computer aided design, in particular to a CAD model multi-resolution gridding method for mechanism semantic self-adaptive maintenance.
Background
Due to the difference of model formats, a CAD model (a boundary representation model as a bottom layer) designed on mainstream industrial software generally needs to be converted to be used in a VR/AR environment, that is, the model is subjected to gridding processing. For a CAD model (especially a complex mechanical assembly model), directly performing gridding operation on the CAD model usually loses rich mechanism semantic information such as assembly constraint in the original CAD model. The information is usually generated in the product design stage (such as conceptual design and concrete design), and has very important guiding significance for the simulation and manufacture of the product at the later stage. For example, the information of assembly constraint, kinematic pair and the like in the CAD model directly determines the motion mode and relative degree of freedom between parts (or sub-assemblies) in the simulation of the product model; determine the connection mode between the parts (or sub-assemblies) when the product is manufactured and assembled, and the like. In order to enable the gridded CAD model to accurately and effectively develop applications (simulation, assembly, etc.) meeting industrial standards in a VR/AR environment, the necessary mechanical semantics in the CAD model need to be adaptively maintained during the gridding process.
Meanwhile, in order to accurately and efficiently display a CAD model in a VR/AR environment, it is necessary to perform a multi-resolution gridding process on the CAD model. The above processing often requires discretization, simplification, compression, and even culling of precise geometric information in CAD models, particularly complex mechanical assembly models. Therefore, improper gridding will not only lose the original accuracy of the CAD model, but also lose the original engineering application value of the transformed CAD model. For example, due to excessive loss of precision, assembly process plans designed by a gridded CAD model cannot be effectively used to guide the accurate assembly of the corresponding product.
In conclusion, the problem that the gridding CAD model can simultaneously take the requirements of VR/AR environment and the effectiveness of engineering application into consideration is solved, and the problem is a very key and basic problem for effectively developing the engineering application based on VR/AR.
At present, there are three main methods for CAD model gridding:
(1) and (4) integrally gridding the CAD model. The CAD model is exported into a grid model integrally through the existing mainstream CAD software. The integrally derived CAD model cannot be split, and each part in the CAD model cannot be operated and moved independently, so that the value of engineering application is lost.
(2) And (5) carrying out secondary assembly after the parts are singly meshed. Each part in the CAD model is independently exported into a part grid model through the existing mainstream CAD software. And then, importing the grid models corresponding to all the parts into a virtual reality scene for secondary assembly, and adjusting the positions of the grid models corresponding to all the parts to be consistent with the positions of all the parts in the original CAD model. The method needs manual interaction to carry out secondary assembly on the parts in the virtual reality scene, wastes time and labor, and is more difficult to carry out correct assembly on some complex CAD models.
(3) Developed separately for a particular CAD model format. And carrying out secondary development independently for the CAD model format corresponding to each different industrial software. The method is characterized in that secondary development is carried out on common industrial software (such as common SolidWorks, Catia and the like), CAD model formats supported by the industrial software are read and analyzed, each part in a CAD model is separated, position information is read, then each part is gridded and placed at a designated position, and gridding of the CAD model is achieved. However, this type of method is only directed to a certain type of CAD software and its corresponding model format, and is not universal.
Disclosure of Invention
The invention mainly aims to provide a CAD model multi-resolution gridding method for mechanism semantic self-adaptive maintenance, which aims to solve the problems that the requirements of VR/AR environment and the effectiveness of engineering application are difficult to meet simultaneously after the CAD model is gridded in the background technology. In particular, the CAD models referred to in the present invention each refer to an assembly body model, each of which is composed of a plurality of parts.
The invention leads out the CAD model after gridding into a multi-resolution gridding CAD model. The multi-resolution gridding CAD model contains information such as multi-level mechanism semantic information and multi-resolution gridding part models among parts, and can accurately and effectively develop applications (simulation, assembly and the like) meeting industrial standards in a VR/AR environment. Compared with the prior art, the method overcomes the format difference among different CAD systems, does not need to independently carry out secondary development on each different CAD system, and has more universality; the independence of each part after the CAD model is gridded is kept, and manual assembly in a virtual reality environment is not needed again; necessary mechanism semantic information in the CAD model is maintained in a self-adaptive mode, and applications (simulation, assembly and the like) meeting industrial standards can be developed more effectively; a multi-resolution gridded rigging model is provided that can interact and render more efficiently in a VR/AR environment.
The CAD model multi-resolution gridding method for self-adaptive maintenance of mechanism semantics comprises the following steps:
step 1, extracting multilayer mechanism semantics of a CAD model;
and extracting mechanism semantic information in the assembly body model, and adopting multi-level representation.
Step 2, multi-resolution gridding of the CAD model;
and performing multi-resolution gridding processing on the CAD model by adopting a multi-resolution technology and in a mode of reducing the number of part models, geometrically compressing and removing hidden contents.
Step 3, flattening storage of the multi-resolution gridding CAD model;
and designing a flat structure of the multi-resolution gridding CAD model, and storing all parts of the gridded CAD model and multi-level mechanism semantic information among the parts in the same layer. The final output may be a multi-resolution gridded CAD model that provides support for the VR/AR environment.
Further, the specific method of step 1 is as follows;
before gridding of the CAD model, the mechanism semantic information in the CAD model is extracted and organized into multi-level representation. The mechanism semantic information comprises geometric constraint, freedom degree and kinematic pairs among all parts of the CAD model. Specifically, the method comprises the following steps:
and 1-1, extracting mechanism semantic information in the CAD model on the constraint layer and the semantic layer respectively.
1-2, calling an API (application programming interface) for reading geometric constraints among parts in industrial CAD software corresponding to the CAD model on the constraint layer, and extracting the geometric constraints among the parts in the CAD model, wherein the API specifically comprises the following geometric constraints: coplane, coaxiality, distance, tangency, verticality, parallelism, point-line coincidence, point-plane coincidence, and edge-plane coincidence.
1-3, on the semantic layer, the degrees of freedom and the kinematic pairs are deduced from the extracted geometric constraints through the existing method based on geometric constraint deduction (automatic kinematic pair recognition method).
The extracted degrees of freedom between the parts in the CAD model specifically include: 1) translating; 2) rotating; 3) and (4) compounding.
The extracted kinematic pair types among the parts in the CAD model specifically include: the device comprises a rotary pair, a moving pair, a cylindrical pair, a spiral pair, a spherical pair, a plane pair, a gear pair, a universal joint, a point-surface pair, a point-line pair, a curved surface pair and a fixed pair.
And 1-4, adopting multi-level representation for the extracted mechanism semantic information in the CAD model. The multi-level representation is specifically divided into a constraint layer and a semantic layer, wherein the constraint layer is used for representing the extracted geometric constraint, and the semantic layer is used for representing the extracted freedom degree and the kinematic pair.
Further, the specific method in step 2 is as follows;
firstly, selecting mesh generation precision in a self-adaptive manner, and carrying out meshing processing on a CAD model under a plurality of resolutions; and then, further lightening the gridded CAD model in a mode of reducing the number of part models, geometrically compressing and eliminating hidden contents. Specifically, the method comprises the following steps:
and 2-1, carrying out secondary development on the OpenCascade open source library.
And reading the CAD model in the general model format by utilizing the related function of the CAD model in the general model format read from the OpenCasCade open source library. For a special model format generated by industrial CAD software, the special model format is firstly converted into a general model format by using a relevant function of model format conversion in an OpenCascade open source library.
And 2-2, improving the original mesh generation function in the OpenCascade open source library, and meshing the CAD model in the general model format. In the gridding process, the part to which the grid surface belongs is judged, and the grid surfaces belonging to different parts of the CAD model are recombined to generate a group of different part grid models. Specifically, the method comprises the following steps:
and 2-2-1, automatically generating by a program or interactively appointing the precision of the CAD model mesh generation by a user.
And 2-2-2, traversing each surface on the CAD model in sequence, and calling the original mesh generation function in the OpenCascade open source library to carry out mesh generation on the surface according to the set precision. The mesh generation function returns position information for each mesh surface, the position information is used for recording the position of the mesh surface in the space, and the returned position information of the mesh surfaces positioned in the same part is the same.
And 2-2-3, after the mesh subdivision is completed on all the surfaces on the CAD model, distinguishing and recombining the mesh surfaces according to the parts according to the position information returned by each mesh surface. And obtaining a plurality of part grid models under the grid subdivision precision, wherein the part grid models jointly form a gridding CAD model under the grid subdivision precision.
And 2-3, repeating the gridding process in the step 2-2 on the same CAD model by adopting different grid subdivision accuracies according to actual requirements, and obtaining a plurality of gridded CAD models of the CAD model under different grid subdivision accuracies and a plurality of groups of corresponding part grid models.
And 2-4, performing further lightweight processing on the gridding CAD model by reducing the number of part models, geometrically compressing and eliminating hidden contents. And finally obtaining a plurality of gridding CAD models and a plurality of groups of corresponding part grid models under different resolutions. Specifically, the method comprises the following steps:
and 2-4-1, reducing the number of real part grid models in the gridding CAD model by using a shared geometric node technology in a VR/AR environment. For the same part in the CAD model, only one part mesh model is stored to reduce the number of part mesh models in the CAD model.
And 2-4-2, for each topological face in each part grid model, if the geometric area of the topological face is smaller than a given threshold value, removing all grid patches corresponding to the topological face.
And 2-4-3, removing the hidden contents. And finding out mesh patches which are invisible under all visual angles in the part mesh model by rotating the model or the viewpoint based on a viewpoint removing mode, and removing the mesh patches from the part mesh model.
Further, the specific method in step 3 is as follows;
and 3-1, designing a flattening structure of the multi-resolution gridding assembly model.
And designing a flattening structure of the multi-resolution gridding assembly body model, wherein the flattening structure is divided into 5 layers.
And 3-2, the first layer of the flat structure of the multi-resolution gridding CAD model is a root node, and the root node represents a multi-resolution gridding CAD model. And binding a transformation matrix for the root node so that the multi-resolution gridding CAD model can be transformed integrally in the VR/AR environment.
And 3-3, the second layer of the flat structure of the multi-resolution gridding CAD model comprises a multi-level mechanism semantic node and a plurality of multi-resolution gridding part model nodes which are all hung below the root node of the first layer. The multi-level mechanism semantic nodes store multi-level mechanism semantic information among CAD model parts. Each multi-resolution gridding part model node represents a multi-resolution gridding part model, and a transformation matrix is bound to each multi-resolution gridding part model node, so that the multi-resolution gridding part models can be interacted and transformed independently in a VR/AR environment.
And 3-4, the third layer of the flat structure of the multi-resolution gridding CAD model is part grid model nodes with different resolutions, the part grid model nodes are hung under the multi-resolution gridding part model nodes corresponding to the second layer, and one multi-resolution gridding part model node comprises a plurality of part grid model nodes with different resolutions. Each of the part mesh model nodes at the different resolutions represents the part mesh model at a particular resolution.
And 3-5, hanging a fourth layer grid patch node and a non-geometric information node of a flattened structure of the multi-resolution gridding CAD model under a part grid model node with a certain resolution of a third layer, wherein one part grid model node comprises one grid patch node and one non-geometric information node. The mesh patch nodes are used for storing all mesh patches of the part mesh model under the resolution. The non-geometric information node is used for storing non-geometric information of the part mesh model at the resolution.
And 3-6, the fifth layer of the flat structure of the multi-resolution gridding CAD model is a vertex set node. And the vertex set nodes are hung under the mesh patch nodes of the fourth layer and are used for storing vertices corresponding to the mesh patches, and one mesh patch node comprises one vertex set node.
The invention has the following beneficial effects:
the invention designs a CAD model multi-resolution gridding method with self-adaptive maintenance of mechanism semantics. Firstly, extracting mechanism semantic information in a CAD model and organizing the mechanism semantic information into multi-level representation; then, a multi-resolution technology is adopted, and the CAD model is gridded in a mode of reducing the number of part models, geometrically compressing and eliminating hidden contents; finally, a flat structure is designed for standardly and uniformly representing and storing the multi-resolution gridding CAD model; the final output may be a multi-resolution gridded CAD model that provides support for the VR/AR environment.
The invention carries out multi-resolution gridding on the CAD model, and simultaneously self-adaptively maintains necessary mechanism semantic information in the CAD model, so that the gridded CAD model not only can carry out efficient interaction and rendering in a VR/AR environment, but also can accurately and effectively develop applications (simulation, assembly and the like) meeting industrial standards.
Drawings
FIG. 1 is a flow chart of the present invention.
FIG. 2 is an example CAD model undergoing gridding.
FIG. 3 is an exemplary CAD model gridding preview effect.
FIG. 4 is a comparison of the effects of part mesh models at different resolutions.
5-6 are display effects of the multi-resolution gridding CAD model obtained based on the present invention imported into a virtual reality scene.
FIG. 7 is a flattened structure of a multi-resolution gridded CAD model.
Detailed Description
The invention is further illustrated by the following figures and examples.
As shown in fig. 1, the multi-resolution gridding method for a CAD model with self-adaptive maintenance of mechanism semantics mainly includes the following steps:
step 1, extracting multilayer mechanism semantics of a CAD model;
CAD models contain rich mechanistic information, such as assembly constraints. The information is usually generated in the product design stage (such as conceptual design and concrete design), and has very important guiding significance for the simulation and manufacture of the product at the later stage. In order to enable the gridded CAD model to accurately and effectively develop applications meeting industrial standards in VR/AR environment, such as product simulation and assembly process planning, before gridding of the CAD model, the mechanism semantic information in the CAD model is extracted and organized into multi-level representation. And storing the multi-level mechanism semantic information as one node in a flattened structure of the multi-resolution gridding CAD model and the part model after gridding together. The mechanism semantic information comprises geometric constraint, freedom degree and kinematic pairs among all parts of the CAD model. Specifically, the method comprises the following steps:
and 1-1, extracting mechanism semantic information in the CAD model on the constraint layer and the semantic layer respectively.
1-2, calling an API (application programming interface) for reading geometric constraints among parts in industrial CAD software corresponding to the CAD model on the constraint layer, and extracting the geometric constraints among the parts in the CAD model, wherein the API specifically comprises the following geometric constraints: coplane, coaxiality, distance, tangency, verticality, parallelism, point-line coincidence, point-plane coincidence, and edge-plane coincidence.
1-3, on the semantic layer, the degrees of freedom and the kinematic pairs are deduced from the extracted geometric constraints through the existing method based on geometric constraint deduction (automatic kinematic pair recognition method).
The extracted degrees of freedom between the parts in the CAD model specifically include: 1) translating; 2) rotating; 3) and (4) compounding.
The extracted kinematic pair types among the parts in the CAD model specifically include: the device comprises a rotary pair, a moving pair, a cylindrical pair, a spiral pair, a spherical pair, a plane pair, a gear pair, a universal joint, a point-surface pair, a point-line pair, a curved surface pair and a fixed pair.
And 1-4, adopting multi-level representation for the extracted mechanism semantic information in the CAD model. The multi-level representation is specifically divided into a constraint layer and a semantic layer, wherein the constraint layer is used for representing the extracted geometric constraint, and the semantic layer is used for representing the extracted freedom degree and the kinematic pair. Specifically, the results are shown in Table 1.
TABLE 1 Multi-level mechanism semantics between part models
Step 2, multi-resolution gridding of the CAD model;
in order to effectively display the CAD model in the VR/AR environment and support the development of efficient interaction and rendering, the invention adopts a multi-resolution technology to grid the CAD model. Firstly, selecting mesh generation precision in a self-adaptive manner, and carrying out meshing processing on a CAD model under a plurality of resolutions; and then, further lightening the gridded CAD model in a mode of reducing the number of part models, geometrically compressing and eliminating hidden contents. Specifically, the method comprises the following steps:
and 2-1, carrying out secondary development on the OpenCascade open source library.
Reading the CAD model in the general model format (STEP format) by utilizing the related functions of the CAD model in the OpenCasCade open source library read in the general model format (STEP format). For a proprietary model format generated by industrial CAD software, it is first converted to a generic model format (STEP format) using the correlation function of the model format conversion in the OpenCascade open source library. An example assembly model read is shown in fig. 2.
And 2-2, improving the original grid subdivision function in the OpenCascade open source library, and meshing the CAD model in the general model format (STEP format). After the original grid division function in the OpenCascade open source library divides the CAD model, the whole CAD model can be integrally divided into a grid model which cannot be divided, and each part in the CAD model cannot be independently operated and moved, so that the value of engineering application is lost. Therefore, the original grid subdivision function in the OpenCascade open source library is improved, in the gridding process, the part of the CAD model to which the grid surface belongs is judged, and the grid surfaces belonging to different parts of the CAD model are recombined to generate a group of different part grid models. Specifically, the method comprises the following steps:
and 2-2-1, automatically generating by a program or interactively appointing the precision of the CAD model mesh generation by a user.
And 2-2-2, traversing each surface on the CAD model in sequence, and calling the original mesh generation function in the OpenCascade open source library to carry out mesh generation on the surface according to the set precision. The accuracy of the mesh generation is given in 2-2-1. The mesh generation function returns position information for each mesh surface, the position information is used for recording the position of the mesh surface in the space, and the returned position information of the mesh surfaces positioned in the same part is the same.
And 2-2-3, after the mesh subdivision is completed on all the surfaces on the CAD model, distinguishing and recombining the mesh surfaces according to the parts according to the position information returned by each mesh surface. And obtaining a plurality of part grid models under the grid subdivision precision, wherein the part grid models jointly form a gridding CAD model under the grid subdivision precision. Each finally obtained part mesh model specifically comprises a mesh patch forming the mesh model, a normal direction of the mesh patch, a vertex forming the mesh patch and vertex colors, and all the parts can be extracted in the mesh subdivision process. The gridding effect of the example CAD model is shown in FIG. 3.
And 2-3, repeating the gridding process in the step 2-2 on the same CAD model by adopting different grid subdivision accuracies according to actual requirements, and obtaining a plurality of gridded CAD models of the CAD model under different grid subdivision accuracies and a plurality of groups of corresponding part grid models.
And 2-4, performing further lightweight processing on the gridding CAD model by reducing the number of part models, geometrically compressing and eliminating hidden contents. And finally obtaining a plurality of gridding CAD models and a plurality of groups of corresponding part grid models under different resolutions. The effect pairs of part mesh models at different resolutions are shown in fig. 4. Specifically, the method comprises the following steps:
and 2-4-1, reducing the number of real part grid models in the gridding CAD model by using a shared geometric node technology in a VR/AR environment. There are typically many identical parts in a CAD model, such as standard connectors, symmetrical fittings, etc. The same part mesh model can be reused in a VR/AR environment by sharing only geometric nodes. Thus, for the same part in the CAD model, only one part mesh model is stored to reduce the number of part mesh models in the CAD model.
2-4-2, for each topological face in each part grid model, if the geometric area of the topological face is smaller than a given threshold (usually corresponding to the detail features in some models), eliminating all grid patches corresponding to the topological face.
And 2-4-3, removing the hidden contents. The method mainly comprises the steps of mainly performing outer surface display in a VR/AR environment, and therefore, finding out mesh patches which are invisible in a part mesh model under all visual angles by rotating the model or rotating the viewpoint in a viewpoint removing mode commonly used in computer graphics, and removing the mesh patches from the part mesh model.
Step 3, flattening storage of the multi-resolution gridding CAD model;
in order to enable the gridded CAD model to have a standard and uniform expression and storage mode in a VR/AR environment, the invention designs a flat structure of the multi-resolution gridded CAD model, and stores all part grid models of the gridded CAD model and multi-level mechanism semantic information among all parts of the CAD model in the same layer. The final output may be a multi-resolution gridded CAD model that provides support for the VR/AR environment. The display effect of importing the multi-resolution gridding CAD model obtained based on the method into the virtual reality scene is shown in FIGS. 5-6. Specifically, the method comprises the following steps:
and 3-1, designing a flattening structure of the multi-resolution gridding assembly model.
An assembly model of a product (generally expressed by a hierarchical assembly tree) has various hierarchical assembly trees along with differences of specific requirements, design styles of designers and the like. In order to enable the gridded CAD model to have a standard and uniform representation and storage mode in a VR/AR environment, the invention designs a flat structure of the multi-resolution gridded assembly body model. As shown in fig. 7, the flattened structure is divided into 5 layers in total. Specifically, the method comprises the following steps:
and 3-2, the first layer of the flat structure of the multi-resolution gridding CAD model is a root node, and the root node represents a multi-resolution gridding CAD model. And binding a transformation matrix (initial value is an identity matrix) for the root node so that the multi-resolution gridding CAD model can be transformed integrally in the VR/AR environment.
And 3-3, the second layer of the flat structure of the multi-resolution gridding CAD model comprises a multi-level mechanism semantic node and a plurality of multi-resolution gridding part model nodes which are all hung below the root node of the first layer. The multi-level mechanism semantic node stores multi-level mechanism semantic information among the CAD model parts acquired in 1-4. Each multi-resolution gridding part model node represents a multi-resolution gridding part model, and a transformation matrix (the initial value is an identity matrix) is bound to each multi-resolution gridding part model node, so that the multi-resolution gridding part models can be interacted and transformed independently in a VR/AR environment.
And 3-4, the third layer of the flat structure of the multi-resolution gridding CAD model is part grid model nodes with different resolutions, the part grid model nodes are hung under the multi-resolution gridding part model nodes corresponding to the second layer, and one multi-resolution gridding part model node comprises a plurality of part grid model nodes with different resolutions. Each of the part mesh model nodes at the different resolutions represents the part mesh model at a particular resolution.
And 3-5, hanging a fourth layer grid patch node and a non-geometric information node of a flattened structure of the multi-resolution gridding CAD model under a part grid model node with a certain resolution of a third layer, wherein one part grid model node comprises one grid patch node and one non-geometric information node. And the mesh patch nodes are used for storing all mesh patches of the part mesh model under the resolution and are obtained from 2-4. The non-geometric information node is used for storing non-geometric information of the part mesh model under the resolution, wherein the non-geometric information comprises colors and normal directions corresponding to mesh patches, and the colors and the normal directions are obtained from 2-4.
And 3-6, the fifth layer of the flat structure of the multi-resolution gridding CAD model is a vertex set node. And the vertex set nodes are hung under the mesh patch nodes of the fourth layer and are used for storing vertices corresponding to the mesh patches, and one mesh patch node comprises one vertex set node and is obtained from 2-4.
Claims (6)
1. The CAD model multi-resolution gridding method for self-adaptive maintenance of mechanism semantics is characterized by comprising the following steps of:
step 1, extracting multilayer mechanism semantics of a CAD model;
extracting mechanism semantic information in the assembly body model, and adopting multi-level representation;
step 2, multi-resolution gridding of the CAD model;
performing multi-resolution gridding processing on the CAD model by adopting a multi-resolution technology in a mode of reducing the number of part models, geometrically compressing and removing hidden contents;
step 3, flattening storage of the multi-resolution gridding CAD model;
designing a flat structure of a multi-resolution gridding CAD model, and storing all parts of the gridded CAD model and multi-level mechanism semantic information among the parts in the same layer; the final output may be a multi-resolution gridded CAD model that provides support for the VR/AR environment.
2. The method for multi-resolution gridding of a CAD model with self-adaptive maintenance of mechanism semantics according to claim 1, characterized in that the concrete method of step 1 is as follows;
before gridding of the CAD model, extracting mechanism semantic information in the CAD model and organizing the mechanism semantic information into multi-level representation; the mechanism semantic information comprises geometric constraint, freedom degree and kinematic pairs among parts of the CAD model; specifically, the method comprises the following steps:
1-1, extracting mechanism semantic information in the CAD model on a constraint layer and a semantic layer respectively;
1-2, calling an API (application programming interface) for reading geometric constraints among parts in industrial CAD software corresponding to the CAD model on the constraint layer, and extracting the geometric constraints among the parts in the CAD model, wherein the API specifically comprises the following geometric constraints: coplane, coaxiality, distance, tangency, verticality, parallelism, point-line superposition, point-plane superposition and edge-plane superposition;
1-3, on a semantic layer, deducing a degree of freedom and a kinematic pair from the extracted geometric constraint by using an existing method based on geometric constraint derivation (a kinematic pair automatic identification method);
the extracted degrees of freedom between the parts in the CAD model specifically include: 1) translating; 2) rotating; 3) compounding;
the extracted kinematic pair types among the parts in the CAD model specifically include: the device comprises a rotating pair, a moving pair, a cylindrical pair, a spiral pair, a spherical pair, a plane pair, a gear pair, a universal joint, a point-surface pair, a point-line pair, a curved surface pair and a fixed pair;
1-4, adopting multi-level representation for the extracted mechanism semantic information in the CAD model; the multi-level representation is specifically divided into a constraint layer and a semantic layer, wherein the constraint layer is used for representing the extracted geometric constraint, and the semantic layer is used for representing the extracted freedom degree and the kinematic pair.
3. The method for multi-resolution gridding of the CAD model with self-adaptive maintenance of mechanism semantics according to claim 1, wherein the specific method in step 2 is as follows;
firstly, selecting mesh generation precision in a self-adaptive manner, and carrying out meshing processing on a CAD model under a plurality of resolutions; then, further light weight processing is carried out on the CAD model after gridding in a mode of reducing the number of part models, geometrically compressing and eliminating hidden contents; specifically, the method comprises the following steps:
2-1, carrying out secondary development on the OpenCascade open source library;
reading a related function of a CAD model in a general model format from an OpenCasCade open source library, and reading the CAD model in the general model format; for a special model format generated by industrial CAD software, firstly, a related function of model format conversion in an OpenCascade open source library is utilized to convert the special model format into a general model format;
2-2, improving the original mesh generation function in the OpenCascade open source library, and meshing the CAD model in the general model format; in the gridding process, judging which part of the CAD model the grid surface belongs to, and recombining the grid surfaces of different parts belonging to the CAD model to generate a group of different part grid models;
2-3, repeating the gridding process in the step 2-2 on the same CAD model by adopting different grid subdivision accuracies according to actual requirements to obtain a plurality of gridded CAD models of the CAD model under different grid subdivision accuracies and a plurality of groups of corresponding part grid models;
2-4, further carrying out lightweight processing on the gridding CAD model by reducing the number of part models, geometrically compressing and eliminating hidden contents; and finally obtaining a plurality of gridding CAD models and a plurality of groups of corresponding part grid models under different resolutions.
4. The method for multi-resolution gridding of CAD (computer-aided design) model with self-adaptive maintenance of mechanism semantics according to claim 3, wherein the concrete method in step 2-2 is as follows:
2-2-1, automatically generating by a program or interactively appointing the precision of the CAD model mesh generation by a user;
2-2-2, traversing each surface on the CAD model in sequence, calling an original mesh generation function in an OpenCascade open source library, and carrying out mesh generation on the surface according to set precision; the mesh generation function returns position information for each mesh surface, the position information is used for recording the position of the mesh surface in the space, and the returned position information of the mesh surfaces positioned in the same part is the same;
2-2-3, after the mesh subdivision is completed on all the surfaces on the CAD model, the mesh surfaces are distinguished and recombined according to the parts according to the position information returned by each mesh surface; and obtaining a plurality of part grid models under the grid subdivision precision, wherein the part grid models jointly form a gridding CAD model under the grid subdivision precision.
5. The method for multi-resolution gridding of CAD (computer-aided design) model with self-adaptive maintenance of mechanism semantics according to claim 4, characterized in that the concrete method in the step 2-4 is as follows:
2-4-1, reducing the number of real part grid models in the gridding CAD model by using a shared geometric node technology in a VR/AR environment; for the same part in the CAD model, only one part grid model is stored to reduce the number of part grid models in the CAD model;
2-4-2, for each topological face in each part grid model, if the geometric area of the topological face is smaller than a given threshold value, removing all grid patches corresponding to the topological face;
2-4-3, removing hidden contents; and finding out mesh patches which are invisible under all visual angles in the part mesh model by rotating the model or the viewpoint based on a viewpoint removing mode, and removing the mesh patches from the part mesh model.
6. The method for multi-resolution gridding of the CAD model with self-adaptive maintenance of mechanism semantics according to claim 1, wherein the specific method in step 3 is as follows;
3-1, designing a flat structure of the multi-resolution gridding assembly model;
designing a flat structure of the multi-resolution gridding assembly body model, wherein the flat structure is divided into 5 layers;
3-2, the first layer of the flat structure of the multi-resolution gridding CAD model is a root node, and the root node represents a multi-resolution gridding CAD model; binding a transformation matrix for the root node, so that the multi-resolution gridding CAD model can be subjected to overall transformation in a VR/AR environment;
3-3, a second layer of the flat structure of the multi-resolution gridding CAD model comprises a multi-level mechanism semantic node and a plurality of multi-resolution gridding part model nodes, and the multi-level mechanism semantic node and the multi-resolution gridding part model nodes are all hung below a root node of the first layer; the multi-level mechanism semantic nodes store multi-level mechanism semantic information among CAD model parts; each multi-resolution gridding part model node represents a multi-resolution gridding part model, and a transformation matrix is bound to each multi-resolution gridding part model node, so that the multi-resolution gridding part models can be interacted and transformed independently in a VR/AR environment;
3-4, a third layer of the flat structure of the multi-resolution gridding CAD model is part grid model nodes with different resolutions, and the part grid model nodes are hung under the multi-resolution gridding part model nodes corresponding to the second layer, and one multi-resolution gridding part model node comprises a plurality of part grid model nodes with different resolutions; each part grid model node under different resolutions represents a part grid model under a specific resolution;
3-5, a fourth layer grid surface patch node and a non-geometric information node of a flat structure of the multi-resolution gridding CAD model are hung under a part grid model node with a certain resolution of a third layer, and one part grid model node comprises one grid surface patch node and one non-geometric information node; the mesh patch nodes are used for storing all mesh patches of the part mesh model under the resolution; the non-geometric information node is used for storing non-geometric information of the part mesh model under the resolution;
3-6, taking the fifth layer of the flat structure of the multi-resolution gridding CAD model as a vertex set node; and the vertex set nodes are hung under the mesh patch nodes of the fourth layer and are used for storing vertices corresponding to the mesh patches, and one mesh patch node comprises one vertex set node.
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