ã€Key words】 Solid modeling Feature modeling
1 Feature Modeling Method
Mechanical product design is a multi-factor and multi-cycle complex decision-making process. In order to meet the needs of social development and market competition, people are constantly seeking for modern product design methods that use high-tech as a support platform. The product design method based on feature modeling is produced with CAD/CAM integration requirements. It is based on solid modeling methods and is more suitable for product design methods of computer integrated manufacturing systems. It is closely related to the physical modeling in terms of principles and methods, and has the following differences.
(1) Three-dimensional wireframes, curved surfaces, and solid models focus on perfecting the product's geometric description, while ignoring the significance of product engineering, making design and manufacturing information inconsistent. Feature modeling focuses on the complete technical and production management information of the product, and preserves the original definition and interdependence of each functional element on the final product, so that the unified product model can be used to replace the set of drawings and technical documents in the traditional design, so that product design And the preparations for the preparation of all aspects can be carried out in parallel.
(2) In the product feature model, reference is made to the reference point, the center line, the local coordinate system, etc., and the role of the protruding surface (such as mating surface, supporting surface, positioning surface, datum surface, etc.). These surfaces are different from general geometric surfaces and must be It is easy to search for its shaping and positioning dimensions. This requires the existence of isolated points, lines, and faces outside the three-dimensional object, the introduction of non-manifolds and non-rule sets, and extended Euler's operating range.
(3) The feature modeling product design is performed at a higher level. The operation object is no longer the original lines and voxels used in the solid modeling, but the functional elements of the product, such as threaded holes, positioning holes, keyways, and bosses. Ear pads, etc., which require the use of local operation and size drive technology, and require the use of new data structures and new feature combination algorithms. The citation of features directly reflects the design intent, making the product model easy to understand and saving product design time.
(4) The geometric and non-geometric information in the features implements the product design intent to all subsequent stages and receives feedback in a timely manner, strengthening the links between product design, analysis, process preparation, processing, and inspection departments, helping to promote product design. And process design standardization, standardization and serialization.
It can be seen that the product design method based on feature modeling is the core of product virtual design, providing services for design, manufacturing and production management, and creating conditions for the progressive realization of intelligent CAD systems and intelligent manufacturing systems. Therefore, it is very necessary to study the feature-based modeling of mechanical products.
2 Feature Model Information Description
From the perspective of CIMS and virtual design, feature classification is shown in Figure 1.
Figure 1 Feature Classification
The generation of part models does not rely on voxel consolidation, but instead highlights the role of various surfaces, such as datum planes, work planes, and connection planes. It is necessary to process and record the inheritance, adjacency, subordination, and citation links between different features. According to the connection between features, the instance of the feature class is defined as an object, and the feature-oriented connection diagram shown in FIG. 2 is obtained.
Figure 2 Feature Contact
There are three attributes in the attribute set of the feature: 1 parameter attribute, description feature shape composition and other definition attributes of non-geometric information; 2 constraint attributes, constraints describing feature members themselves and constraint relationships between feature members; 3 associations An attribute that describes the property of the mutual constraint or mutual reference relationship between this feature and other features, the shape feature and the low-level geometry element or other non-geometric information description.
According to the definition of features and feature connections, a hierarchical structure based on the feature part information model is established, which is divided into three layers: component layer, feature layer, and geometric layer. The geometric information of the part is expanded in layers so as to extract information according to different needs. The part layer mainly reflects the overall part information, which is the index pointer or address of the part submodel; the feature layer contains the combination of each submodel of the feature and the relationship among each model, and forms the characteristic diagram or tree structure. The feature layer is the part information. The core of the model and the relationships between the feature sub-models reflect the semantic relationship between features, making the features a basic unit for constructing parts and having high-level engineering implications. The geometric/topological information of the B-REP structure expression is the basis of the entire model, and is also the object of attention of application systems such as part drawing, finite element analysis, and assembly analysis.
3 parametric feature modeling method
The use of the variable geometry method of parametric feature modeling and the two modeling methods based on the generation process method enable the construction and editing of features.
A geometric model is defined as a series of feature points, and a non-linear constraint equation group is formed with the feature coordinates as variables, and dimensional constraints such as length, radius, and intersection angle constraints on the elements are determined, and azimuth or relative position relationship of the limit elements is set. Geometric constraints. When the constraints change, the iterative method is used to solve the system of equations to obtain a series of new feature points, thereby generating a new geometric model. Generate a 3D geometric model by using several simple models to generate 3D geometric models, record all information in the model generation process, and use the recorded quantitative information as variable parameters. When assigning parameters with different values, update the model generation process to obtain different sizes or shapes. The geometric model. The model can be very complex and is often used for parametric modeling of 3D solids. Because the 3D model is generated by other simple so-called sub-models through multiple operations, the model generation process is a tree, the leaves represent the basic sub-models, the branch nodes represent the intermediate models generated by the operations, and the tree root represents the model itself.
When parametric modeling is performed based on the model's generation history, the objects that can be parameterized are the basic model data and various operation parameters contained in the history tree. The parameterized dimensions and imposed constraints are retained in the model generation process. The parameterizable basic model data is the geometric dimensions of various voxel feature sizes and plane shapes. The intermediate model or the final model is generated by operations, so the process This type of model in the tree contains various types of operation parameters, which are related to Boolean operation, scan conversion, rounding and chamfering, and various types of positioning operations.
Since the processing environment, production scale, degree of product similarity, standardization, and degree of serialization are all different, a parametric design method is used for feature design. For standardized serialized products, the change of each part family can be controlled by a set of parameters, or Describe with some variation law. Therefore, the geometric features and processing of each part family can be expressed by parameters and variation laws.
4 Feature Modeling System Structure and Function
Based on the existing geometric modeling system, feature recognition and feature modeling methods are used interactively to realize the development and research of a feature-based mechanical product modeling system. The system structure diagram is shown in FIG. 3 .
Figure 3 Overall system architecture
4.1 Feature Definition and Editing Subsystems define the parameterized entities, attribute information patterns, and processing knowledge rules of the subsystem. It includes the following features: First, it is a relatively general and relatively fixed design feature definition of components, such as cylindrical holes and concaves. Pits and simple slots; second, instantiated compound features or components that are dynamically generated for a particular application during the design process; third, custom features that serve as feature sets for a particular application, based on defining new features The geometric complexity of the features is achieved by means of solid modeling using parametric methods.
A Boolean processor with Boolean operations such as "combination, intersection, and difference" is set up, and its calculation accuracy meets design and manufacturing accuracy requirements. Provides feature editing functions, including feature modification, addition and subtraction, feature size drive, feature recognition, feature replacement, and feature movement, allowing designers to generate part models in a convenient, fast, and flexible manner for the downstream design process Provide product basic information.
4.2 Feature Library Management Subsystem From the practical application of the project, the mechanical parts are divided into five categories: shafts, discs, curved bodies, cases and brackets. Feature classification is based on part families. The feature database of the product is divided into a feature library corresponding to the component family and managed according to the group division method of the group technology.
Each type of feature library is used to express certain types of component clusters. The user can analyze the characteristics of each type of component cluster according to the classification of the components, thereby extracting a set of features. This set of features constitutes the feature library of the components.
4.3 database operation and management subsystem <br> for concurrent design engineering design management system database for effective management and maintenance, include: initdb (database generated central area), extendb (to establish a database of basic data area), centdb (for The database center area performs a variety of operations, progdb (operations on the engineering entities in the database), sosdb (restoring the damaged database) and other functions. It can perform many important management tasks such as retrieval, backup, editing, restoration, pointer management, decoding, sorting, and space compression of engineering entities.
4.4 Subsystem <br> product engineering drawing function using a two-dimensional sketch modeling and engineering drawings generated directly from three-dimensional modeling, there is provided a set of drawing commands and functionality to users, complete graphic editing functions such as cut, extending, translation, rotation, The ability to copy, round, cross-hatched, parametrically dimensioned, complex 2D graphs, intersections, and difference calculations enables designers to quickly and easily draw 2D drawings.
5 Application Examples
Low speed and high torque reducer is one of the mechanical transmission equipments with large carrying capacity. According to the technical requirements and working conditions put forward by the user, the gear box is used as the object for analysis and research, so as to achieve the goal of small size, light weight, and large carrying capacity. It is proposed to use rigid box and welded structure. The front and rear walls of the box adopt a double wall structure type. In the lower part of the housing where the box has maximum stress and deformation, box-shaped stiffeners are used to increase the rigidity of the box. In order to reduce the distortion of the box body, two positioning beams are added in the box body, and reinforcing ribs are added on the bottom plate. The method and steps of gearbox feature design are as follows:
(1) Construction of the basic body According to the preliminary design scheme, a three-dimensional modeling function is used to carry out the basic body configuration to generate a prototype of a reduction box. The configuration of the matrix can be unlimited and can express extremely complex shapes.
(2) Establish the part information model call features (such as adding bosses, digging holes, grooving, chamfering, etc.) acting on the base body to form the required part information model. The system uses a combination of various forms to represent the parts. B-REPs and CSGs are used to represent the bases, B-REPs are used to record the boundary information of the bodies, and the image processing is facilitated. The CSG tree records the creation history of the bodies and the original data. The description of the features is dominated by parameters while retaining their B-REP representation and CSG tree. The feature tree is used to record the entire process of forming parts from the matrix and features, and described by CSG tree, B-REP and parametric methods.
(3) Build the box's shape feature model Select the desired design feature from the feature library, input the design parameters, generate the feature's geometric model through the solid modeling, and combine the multiple related features to form the box's Shape feature model.
(4) Create a shape feature Activate the voxel generation module in the corresponding feature library to create a shape feature and generate a value for its representation frame. The generation of feature voxels is mostly based on scanning body generation rules. For example, the generation of holes is generated by rotating a cross-section around an axis; considering the design requirements, some voxels are compounded by multiple simple voxels, making the processing more efficient. Convenience.
(5) Generate the geometric model of the box using the CSG tree hybrid B-REP method. For each shape feature voxel, a voxel boundary representation is created at the time of creation, and a product model is created using the CSG tree record during the construction of the product geometry model from the shape feature voxel. After the model is constructed, the information recorded by the CSG tree is searched. The B-REP model of the product is automatically generated so that the two models fuse to express the geometric features and structure of the product.
(6) Edit box features Edit the feature shaping parameters and positioning parameters. Select the features and modification types to be modified, change the corresponding parameters, and the system automatically transfers the parameter changes to the feature geometry representation according to the parameter constraint relationship and feeds back on the graphic display, and at the same time, changes the parameters of the relevant features. And automatically check whether the feature meets the definition constraint of the feature itself. The feature editing functions mainly include feature editing, deletion, addition, feature replacement, feature movement, feature tilt, feature position adjustment, and feature restoration.
(7) Build a digital simulation cabinet to carry out color design and material texture processing on the cabinet, set up multiple perspective observation points and multiple light sources, generate a colored illumination model with shadow and reflection, and build a digital simulation box with strong sense of reality body.
6 Conclusion
　　 Feature modeling is a key technology for product virtual design and a new milestone in CAD/CAM technology. The research starting point of the feature-based modeling system of mechanical products is high. It is necessary to comprehensively absorb and apply various modern design theories and high technologies. The system function needs to be further strengthened and improved to meet the requirements of modern mechanical product design.
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