Research On Variable Technology In Feature Modeling Based On Three Dimensional Geometry Constraint

Feature modeling is one of important techniques in integrated system of CAD or CAM, which is key to product modeling. Parameter technology and variable technology are two main methods for improving the efficiency of feature-based modeling system. Two methods has been very important in most CAID systems. HUST-CAID system (Computer Aided Industry Design System developed by Harbin University of Science and Technology), is also a feature-based modeling system which used parameter technology. In the system, we find the parameter technology has inherent disadvantage in 3D feature-based modling. Therefore, it's necessary to research the variable technology.We solve the 3D constraints by variable technology. We present a constraint model by extracting constraints from the boundary representation and the feature definition.Firstly, feature-based assembly method is elaborated. The feature position constraint is adopted by feature-based assembly method to determine the feature position, and the feature shape constraint is dopted to determine the feature shape. Then, dimension mapping method is presented. Based on topology entity naming technique that manages the relations between topology entities and sketch entities, 3D dimensions are mapped into 2D sketch constraints. By solving the 2D constraints, the 3D dimensions are satisfied indirectly. Consequently, a 3D constraint satisfaction problem is transformed into a 2D one that is much easier to handle.Since feature shape constraint can't express complicated features, the user-defined feature is introduced. Consequently, encapsulation of simple features is implemented and the complex modeling operation is simplified. It is necessary for a constraint solving module to solve constraint equations at variable leve. Therefore the process of solving engineering constraint is described in detail. Bipartite matching is adopted to adjust the relations between the constraint equations and variables, and then the matching result is mapped to a data structure named Engineering Constraint Graph(ECG). A solving sequence is achieved by topology sort and strong connected component condensation of ECG. Also the problem of consistency analysis for solving engineering constraint is discussed.Finally, a system model is set up, its structure and function are given. Therefore, correctness and effectiveness of research results is validated.