Research On Freeform Feature And Family Of Object Feature Modeling

Feature semantics consist of shape information and function information in semantic feature model,it is not only to define completely feature semantic, but also to maintain model’s validity in total life cycle in semantic feature modeling system, therefore, users design model which is right in grammar and consistent in semantics in all of development stages. With the development of CAD technology, industrial design model has been more and more complexity, and its accuracy has also been higher and higher, feature modeling technology has been driven by these requirements to develop. At present, semantic feature modeling technology is immature, and has lots of technique difficulties to be conquered. To semantic feature modeling system, key technologies about freeform feature and family of object feature are researched in this thesis, and the main contents are as follows:A parameterization algorithm of freeform feature was proposed in this thesis, parameters and constraints were encapsulated to create freeform shape volume feature in this method. Several freeform feature definition points were specified on a base surface, and interpolation algorithms were computed on these points to construct cross-section located on and orthogonal with the base surface, then these cross-sections were computed with skinning algorithm. The algorithm’s example indicated that G2continuity between freeform feature created by this method and base surface can be guaranteed. To improve efficiencies of freeform feature modeling, a geometric constraint solving algorithm of semantic feature modeling was presented in this thesis. In this algorithm, feature dependency graph was analyzed to recognize tetrahedron and triangle sub-problems. Distance constraints and angle constraints on freeform feature definition points were mapped to a constraint graph of tetrahedron and triangle by a kind of graph analysis algorithms. Then each sub-problem was solved by local propagation method, triangle constraint was solved, and tetrahedron constraint was solved by hand-direction rules. Finally, solves of every sub-problem were combined into a global solve. A new kind of declarative semantic feature model was proposed in this thesis, and geometric representation, topological structure and geometric structure were provided. Basic elements, geometric information and topological information were defined, different properties of different features and family of object were specified. Based on cluster rewrite approach and degree of freedom analyzing method, geometric constraint solving algorithm of declarative family of object feature model was proposed. Feature models were divided three types, namely, rigid cluster, scalable cluster and radial cluster in this algorithm. Rewrite rules were described, rules were to exhaustively applied a small set of rewrite rules to a system of rigid and non-rigid clusters, and the set of clusters remaining when no more rewrite rules can be applied represent the generic solution of the system. From the generic solution, modeling system could compute particular solutions, and determine whether the system is well-constrained, under-constrained or over-constrained.A topological constraint solving method of declarative family of object model was presented in this thesis, all topological constraints were mapped to boolean constraints by mapping functions, then constraint problems were solved in solver. The algorithm for finding all solutions assumes that a solution has already been found by the solver, and is given as an argument to the algorithm, along with the solver state, and a third argument representing the number of fixed variables. The algorithm attempts to fix the first free variable in the system to the complementary value of the value in the given solution. It does this by adding a unit-clause to the system. If this results in an un-satisfiable problem, then instead the variable is fixed to the value found in the solution. As long as no new solution is found, the next free variable in the system is fixed, until no free variables remain.To topological change problem in declaring family of object models, a tracking topological change algorithm was proposed in this thesis. Relations between parameter and topology were built, and created a system of geometric constraints to relate the parameters of a model to topological entities in the geometric representation. From the decomposition of this system into its rigid subsystems, we determine which entities are dependent on the variant parameter. For each dependent entity, degenerate cases were constructed by adding geometric constraints to the system. The degenerate cases were solved using a geometric constraint solver, and from the solutions the corresponding values of the variant parameter were computed, i.e. the critical values. This process was repeated for different topological variants of the model, until all critical values had been found.Geometric constraint and topological constraint are two important representations of validity constraint in declarative family of object model. Invalid models are produced because of un-available parameters when users are maintaining feature models, an approach for automatically determining the allowable range for parameters of geometric constraints was presented in this thesis, and each parameter value is available in this range. Firstly, topological constraint graph was analyzed to find angle constraint and distance constraint, and add these constraints to geometric constraints. Then structure graph created by geometric constraint solver was analyzed to find one or more critical parameter value of degenerate sub-problems. Finally, parameter ranges in all ranges were tested to determine accurate parameter value.