Case Studies of Design Methodologies: A Survey*

Design theory and methodology is an emerging area of research. It supports understanding of the design process and its automation in the form of Computer Aided Design. This paper is focused on design methodologies used in computer aided design systems. Several typical cases are reviewed. They are: Computational Model for Hierarchical Mechanical System Design, Optimal Directed Design Model, Knowledge-based Creative Design Model, Model-based System Design. Common trends in the area of design methodologies are also disc,ussed . 1. I N T R O D U C T I O N A design methodology is a very important basis for supporting automation of the engineering design process. This paper is a survey of design methodologies that employ model-based frameworks. A number of methodologies and design systems have been developed to aid the engineering design process in different domains. Although different systems use different models to represent design knowledge and different systems work in different domains, some common features can be found and comparisons can be made among them. In order to understand a design methodology for CAD, several concepts must be clarified. Engineering design can be considered as a technological activity in which knowledge about a specific domain is employed. I t is an activity that seeks all relevant knowledge and combines it to produce a design solution. One view of design is that it is a search process in which a satisfactory design solution is produced from a number of alternatives (Gero, John S. st al, 1989). Those alternatives come from knowledge of the relevant domain. The search is done in a design space which includes all knowledge and design decisions known so far. The design space can be expanded during the design process, i.e., new knowledge and design decisions can be added t o the space when the existing knowledge and available design decisions are not sufficient t o obtain the design solution. If a design solution can be achieved using only existing knowledge in the design space, the process is then termed routine design. In routine design, no new knowledge is added t o the design space and the design space is not expanded. If a design solution cannot be achieved using only the existing knowledge in the design space, new knowledge must be added. This kind of design This work was partially sponsored by Siemens Corporate Research and Support Division, Princeton is called creative design. In creative design, new knowledge is added into the design space, and thus the space is expanded. The new knowledge marks the creativity of design. The design, which is a search process in the design space, is guided by some principles. These principles are given by a design methodology. In other words, design methodology provides a search method in the design space. The first important feature of a design methodology is the creativity factor. If only the existing knowledge is used in the design process and there is no method t o add new knowledge, then the methodology is suitable for routine design and the creative factor of the methodology is limited. If a design methodology allows for new knowledge to be added when necessary, the design space can be expanded during the design process and the design methodology can be considered creative. The second feature is the representation method used in different stages of the design process. Some methodologies use a unique representation method throughout the whole design process. Some employ several different representation methods. The third characteristic is domain independence. Some methodologies can be used only in a specific domain. Some are domain independent. Engineering design problems are often quite large and complex. The decomposition of the large, complex design problem into subproblems is another characteristic of design methodologies. In the following sections, several case studies illustrating the discussed characteristics of design methodologies are presented (Rozenblit, J. W. and Zeigler, B. P., 1988). 2. ComDuta t iona l M o d e l for Hierarchical Mechanical S y s t e m Design This design methodology is based on a tree-like hierarchical model. It was proposed by J. R. Dixon, et.al. (1989,1988). It handles parameteric design problems in which the attributes of the design are known but the values of the attributes must be determined so as to satisfy a certain goal and contraints. The design process is directed by a tree-like structure. The root and interior nodes represent the subsystem manager. The terminal nodes represent the component designer. The subsystem manager is responsible for the problem specification and conflict resolution among lower level subsystem managers and component designers. The component designer is responsible for creating components that satisfy the constraints and design specifications. The design process is shown in Figure 1. TH0308-7/90/oooO/0136/$0.100