Research On The Selection Method Of Tolerance And Fit Based On Ontology

Automatic generation of assembly dimension chain is an important foundation of computer-aided tolerance design.At present,most assembly dimension chains depend on manual interaction.This method is not only difficult to transmit and share the relevant tolerance information in the product representation model in the computer design system,but also generally does not consider the influence of the fit deviation under different fit types on the size chain generation,so it is difficult to ensure the accuracy and efficiency of the size chain generation.Based on the advantages of ontology in concept description,knowledge expression and data sharing,this paper studies an intelligent method to realize automatic generation of assembly dimension chain considering fit deviation,improves the accuracy of dimension chain generation,and realizes the sharing and expansibility of tolerance information.The main elements are as follows:1.Build the geometric representation model automatically generated by the dimension chain.This paper analyzes and extracts the systematic information of assembly constraint relation,fit type and geometric feature in the field of assembly dimension chain.For the existing assembly,it is divided into part layer,geometric feature layer,assembly constraint layer and dimension and tolerance element layer by top-down method.In order to reduce the influence of matching deviation on dimension chain generation,according to whether there is a gap between feature surfaces,the amount of gap between feature surfaces is mapped to drift fit.each layer is used to extract and represent the corresponding tolerance information of the layer,and then the tolerance information between the elements is used to represent the tolerance information between the parts,which provides a prerequisite for dimension chain to generate inference knowledge base.2.Construct the ontology representation model automatically generated by the dimension chain.On the basis of geometric representation model,the ontology model of assembly dimension chain is constructed,the technical terms involved in each fit relation hierarchy are divided,and the important terms such as fit deviation,assembly constraint and fit type are listed.3.Build the inference rule base automatically generated by dimension chain.according to the constructed dimension chain,the ontology model is automatically generated,and the relevant tolerance information is converted into JESS inference rules.Using ontology language rules to understand the dimension chain to generate the corresponding tolerance relationship and tolerance information,using inference rules to describe the classification of fit types,drift fit processing,dimension extraction and the selection of shortest path,etc.to establish the inference rule base,which lays the knowledge foundation for prototype system development.A prototype system for size chain generation based on ontology is 4.developed.The prototype system structure is constructed based on ontology representation model and inference rule base.The knowledge base system generated by assembly dimension chain is developed using ontology editing tool Protege and JESS inference engine.Finally,the feasibility of the prototype system is verified by the example of roller.