Research On The Theory, Method Of Assembly Sequence Generation, Evaluation And Optimization In Digital Product Pre-assembly

In Concurrent Engineering (CE),digital product pre-assembly planning is one of the NP-Hard problems and becomes a research focus in industrial automation area.There are three key issues in the planning of digital product pre-assembly:definition of assembly modeling,strategy of computing assembly sequences and evaluation/optimization of assembly sequences. Based on the current researches in these issues,this thesis proposes a series method to generate,evaluate and optimize the digital product pre-assembly sequence in large scale. Contributions of this thesis are as following:Product assembly model is the basis and main content of the study of assembly sequence planning. In this thesis,a hierarchical assembly model for assembly planning is brought forward firstly,which is established by accessing the basic product data obtained from commercial CAD systems. The data is then analyzed and classified into two layers. The low layer stores the basic geometric information like part's shape and position;the high layer stores the abstract information like contact and connection information. The model consists of full information of product,assembly relationship and assembly tree.Based on knowledge reasoning,fuzzy theory,extraction of sub-assembly and cluster,and hierarchical connection relation graph,an assembly sequence generation algorithm is studied in this paper,which breaks down the original complex assembly sequence generation problem into several small scale assembly sequence planning problems to reduce the computing complexity caused by cut-set theory. A hierarchical AND/OR graph to store all the generated sequences is also employed for this purpose. This algorithm can be used to generate not only all feasible assembly sequences,but also those optimized or near-optimized assembly sequences. It enhances the efficiency and reliability of sequence computing method,promotes the capability of handling the assembly that has a large number of parts.In the process of generate assembly sequence by using the cut-set method,the Matrix of Local Separate Freedom of Motion and the Matrix of Internal Freedom of Motion of subassemblies are used to judge the feasibility of assembly cut-sets,which may prune all the non-suitable assembly cut-sets that do not satisfy local geometrical feasibility and stability of subassemblies in the early stage. And Sub-Super-Set Rules,mechanical feasibility and technology feasibility judgements are also introduced step by step to comfirm the feasibility of all assembly cut-setsFuzzy theory is employed in the evaluation of assembly sequence in this thesis.It is one of the creative works of this paper. This paper analyzes and summarizes all the factors that affect the quality of assembly sequences. To provide a quantitative evaluation result,a special assembly synthetic fuzzy evaluation mathematical model based on the fuzzy theory is established. The evaluating result could be used to select the best sequence from all feasible solutions and verify the result of assembly sequence optimization. Compared with other techniques,this method has the characteristics of impersonality and operation convenience.The assembly sequence optimization is divided into two categories based on the different sources and treated respectively,which is proposed for the first time and is another creative work in this paper. As for the first category,i.e. for the optimization of the non-optimized assembly sequences,expert system is employed for those expressed by binary tree and the algorithm of searching of AND/OR graph with cost for those by AND/OR graph. As for the second category,which is caused by the disturbance of manufacturing environment,network-planning theory of production management is now utilized to assistant the optimization of the assembly sequence. The solutions of these two types of optimization problems are very important for the practical application of digital product assembly sequence planning.The selection of proper basic part may have great impact on the choosing of assembly fixt...