The Extraction And Reconstruction Of STEP-based Geometrical Information

With the rapid development of computer., the technology of CAD/CAE/CAM has been applied widely. But because of the differences among the use of CAD/CAE/CAM, the share of the information and exchange of data among them becomes the most important problem. The only way to solve the problem is to build a complete standard for the Exchange of Product Model Data which can be used by all the CAX systems. STEP is a international standard for the expression and exchange of product data. The STEP provides a data model which can express the data in the whole period of the product,and the data model is not depend on the neutral way. Now, STEP has be used extensively in the data exchange between CAD/CAE/CAM,which is better than the other standards for the Exchange of Product Model Data. The geometrical information of parts is the basement of the design of CAE/CAPP/CAM. Making the extraction of geometrical information as the import information of CAX system is the premise of the integration of CAX. The research on STEP-based 3D solid feature extraction and model regeneration technology provides the technolgy support for integration of CAD/CAPP.First,this paper designs a interpreter of STEP data model. It can extract the 3D informations. Second, rebuilding the 3D model with the extracted information to make the result on screen. The purpose of the article is to develop an interface by which can acquire a 3D model based on STEP standard for general CAX system.1.The information model of STEP AP203AP203 is an application protocol of STEP. AP203 is most about the product configuration information and 3D geometrical information. Now,the popular CAD softwares all support the AP203, they can output AP203 directly and preserve the protocol as STEP neutral file.The object of information extraction in this paper is the STEP Neutral File which is generated by the model in CATIA. The information model of AP203 uses B-rep entity modeling method, and also it describes the entity by the sequence:Shell—Face—Loop—Edge—Point. The STEP Neutral File is the declaration of AP203. It is a kind of ASCII file which has a special format. It has two parts:HEAD part and DATA part. HEAD part is about the elemental information. DATA part is about the entity instances.Each entity instance has the format as:Entity ID=Entity Keyword('Entity name','Attribute 1','Attribute 2'…..)2.The map between EXPRESS and C++The EXPRESS is a powerful formalize-describing language in STEP. The EXPRESS is not a program language,so the data model which is described by it can not be used by program. Map the EXPRESS to C/C++ which is an Object Oriented program language.First, the map between them is the mapping of the data types. Second, Map the entity in AP203 to the C++ class. The attribute can be mapped to the variable of C++. The limit rule can be mapped to the function of C++. The subtype and supertype of the entity can be mapped to the subtype and supertype of C++ class. The C++ class should invole the geometrical and topology relationship which is in accordance with the relationship in AP203. 3.The Design of STEP Model InterpreterThe STEP model interpreter is to identify and extract the STEP 3D information. It can accomplish the read and exchange of the information in STEP Nutral File from the general CAD system. The interpreter is designed by tool of VC++6.0. The STEP model interpreter is composed of four parts:Data Pretreament Module, C++ Object Structure Module, Feature Identity and Extration Module, Data Exchange Module.Data Pretreament Module:Analyze the fixed format of the STEP Neutral File. Use the particular sign as the division to divide the each line of the STEP Neutral File. Then extract the information to preserve as a Struct. C++ Object Structure Module: Judge the Keyword of the struct to find the corresponding C++ entit class. Transmission the other information of the stuct to the member variable to accomplish the Object-instantiated in C++.Feature Identity and Extration Module: Identify and extract the information of the STEP Neutral File according to the identification rule. It is only for the feature of simple entity (cuboid, cylinder, cone, sphere, spherical ring,regular polyhedron) in this paper.Data Exchange Module: Exchange the information which is identified and extracted before into the format which is according with the model rebuilding data format. It is using memory trasfermation to exchange the data format in this paper. That is exchange the feature data which has been Object-instantiated to the data which is according with the next step's need by the way of calling the different funtion in the computer.The process of identification and extraction in STEP model interpreter is :4. Model ReconstructionIt is using the Three-dimensional Animation Technology OpenGL as tool to restruct the 3D model in the paper. OpenGL3D dynamic link library is be used in the restructing module. It is developed according to the system needs. OpenGL3D use OpenGL as the bottom of the graphical display support, and it is on the basis of Intra3D. The STEP 3D model is reconstructed when the body node is created by the way of adding the basic graphics node in itIt is designed a module to introduce external data for the CNCSimu1.0 (Process Simulation System of Three-Dimension Numerical Control Turning) as an example which can verify the theory of the STEP model parser that we have design before.