Theory And Method Research On Product Conceptual Design Based On Function-structure Model

Conceptual design of mechanical products is the extremely important and most creative stage in the process of product design, the scheme's quality of which determines final value of products over 80%. The modelling technology of traditional product design mainly focused on the physical modelling and geometric representation aspect, which resulted in products conceptual design phase lacked of strong technical support. In the guidance of design theory and method, how to carry out effective sorting task, feature extraction and information modelling to conceptual design and conceptual design process knowledge, and then build computer aided conceptual design system through reasonable man-machine coordinate strategy, is one of the research hotspots of today's scholars. Based on the research status and progress of overview conceptual design, in order to solve the problems existing in the process of conceptual design of mechanical products, this paper centers on the key problems of conceptual design, such as the modelling of conceptual design process, and description of product information, and carries out in-depth study in the design modell and solution strategy of conceptual design scheme process, description modell of conceptual design products and the configuration design of qualitative space etc. This paper mainly focuses on the following aspects:1) Theoretical basis of conceptual design, such as decomposition and reconstruction principle, general positioning principle and growth-type design theory, is represented firstly, which is based on the function-structure model. At the philosophical level, decomposition-reconstruction principle reveals the essence of invention-creation. At the theoretical level, general positioning principle illustrates the constraint law that each part of mechanical products must follow; At the method level, growth-type design theory expounds the general law of product evolution design on the basis of mirroring the biological growth mechanism. On this basis, combined the design modell-Zigzag modell of xiomatic concept process design, this paper presents the conceptual process design modell based on function-function-structure model-structure. The conceptual process design modell is top-down oriented, and takes the function-structure model as mapping media, what's more, function decomposition and reconstruction is done synchronously. Design problem is a cognitive process, and design information increases gradually along with the expand of design process under the support of design knowledge, which reflect the discipline of human cognition and thinking; and also show the completion from the product prototype to conceptual design scheme, every stage of design all reflects the structural growth process of design object, which is a structural refining process. This modell is divided into product special layer, product general layer (i.e. mechanism layer), product base layer (i.e. standard parts layer), and the deeper the design level is, the lower of the product correlation of design knowledge and the worse of innovation ability are. At different design process levers, different design strategies need to be adopted.2) Function modelling is one of the key problems of conceptual design, and characteristics of functional information itself determine the expression difficulty in computer. Modelling of user requirements, characteristics of user requirements, and collection, organization and processing of demand are discussed, which make the modell of user function possible. Simultaneously, the paper discusses some concepts and definitions corresponding with the functional modelling, and redefines these concepts and terms, such as function, quality and performance etc., on the basis of learning and mirroring some scholars'ideas. An innovative functional information modell based on the analysis of functional expression modell proposed in current documents, is probed, which includes functional behavior expression set, functional behavior parameters set and functional constraint feature set, and it applies to the functional description of three-layer construction as product-level, transmission chain-level, transmission-level, also, which is quite flexible and can rerun concept expand in the course of reuse. Functional behavior set is the main motive power of functional decomposition; functional constraint feature set is the knowledge foundation of structure reconstruction. The decomposition-reconstruction process of functional information expression modell in design is illustrated; correlation factor modell of functional constraint is put forward and the effect, decomposition and reuse method of performance factor on decomposition-reconstruction is also analyzed. The concept provides possibility for raising each performance of products in the process of automatic design, which not only has great significance to the innovative design and adaptive design, also provides new ideas for the reverse conceptual design.3) Based on the decomposition-reconstruction and generalized positioning principle, the minimum information unit (i.e. function surface) of the whole lifecycle of conceptual design product is described in detail, and the function surface classification is introduced, and the relationship among function surface, structure and positioning is analyzed. On this basis, the paper expounds the basic information unit (i.e. function-structure model) of conceptual design product systematically, and discusses the effect that function-structure model has on conceptual design product is, meanwhile, realize the information transfer from function to structure in deeper levers using the function-structure model as physical carrier. Establishing the mapping model of function-function-structure model-structure, which makes the product information expression more complete, organization more flexible,and more rigorous, scientific in concept and theory. At the same time, the evolutionary growth process of function-structure model in the move of function is analyzed, and the general reasoning rule of the transmission chain design of function-structure model is introduced. Data structure of function-structure model uses object-oriented technology, and function-structure model coding uses variable-length, hierarchical scheme, which belongs to the classification coding system of characteristics of things. Function-structure model classification and coding is divided into three parts: identifier, SID code and functional characteristic code. SID code is the classification code of components and is an active type, when is driven by external news, SID code calls himself to carry out a retrieval. Based on the reverse engineering theory, the acquire method of parts function factor and conceptual structure factor on the basis of function-structure model is proposed, and the reconstruction method of function-structure model based on reverse engineering product design process is analyzed.4) On the basis of growth-type and axiomatic design theory, the conceptual design reasoning modell of mechanical products is built, which solves the mapping from function to function-structure model effectively. Aimed to the problems that design information of conceptual design phase is unable to quantify and design solution is unsure, the three-layer conceptual design principle to solve fuzzy evaluation system based on Delphi analytic hierarchy process (AHP), minimum information analysis method and sigma method is proposed. Optimizing the product conceptual design scheme on the whole when solve the morbid problem of function-structure mapping. PART-NET assembly modell based on virtual transmission chain is established, which conveys the assembly relation to part and part, surface to surface, which realizes the full association of parts. The evolutionary retrieval mechanism based on examples was put forward, matching degree weighted coefficient is determined by using the analytic hierarchy process to create movement and structural matching degree function, and retrieval process based on examples is illustrated, which solves the problems of design knowledge representation and entity structure transformation effectively and is helpful for the development of computer aided conceptual design tools.5) On the basis of studying these design theories and key technologies mentioned above, assembly-oriented computer aided conceptual design system-FM-BDARFAD is developed, creating process of product structure, which is product structure-oriented and supports the parallel design, is established, and the create and design process of product structure realizes the process from scratch and from small to large of parts, which really reflects the top-down design process. This idea based on growth-type design theory of the system starts from product design requirements, which creates product prototypes interactively, and goes through functional decomposition-reconstruction to obtain product conceptual structure scheme taking the transmission chain design as the core, and reflects product design lifecycle of design information from small to large, construction from disorder to order and from low to high. This system includes demand management module, function management module, scheme reasoning module, scheme assessment module, virtual simulation module,entity matching module, products reverse solving module, tolerance solution module, information management module, safety management module,and so on. Finally, taking the multi-functional lumbar spine traction bed as an example, this paper describes the functional decomposition and product evolution process in detail. Case study shows that establish the product process design modell supporting product conceptual design on the basis of function-function-structure model-structure mapping, which provides a new idea for the computer aided product conceptual design.