Axiomatic Design Theory for Systems Nam P. Suh The Ralph E. & Eloise F. Cross Professor, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA Abstract. A general theory for system design is presented based on axiomatic design. The theory is applicable to many different kinds of systems, including machines, large systems, software systems, organizations, and systems consisting of a combination of hardware and software. Systems are repre- sented by means of a system architecture, which takes the form of the {FR}/{DP}/{PV} hierarchies, a ‘junction-module’ diagram, and the ‘flow diagram’. The ‘flow diagram’ for system architecture concisely represents the system design, the relationship among modules, and the control sequence in operating systems. The flow diagram of the system architecture can be used for many different tasks: design, construction, operation, modification, and maintenance of the system. It should also be useful for distributed design and operation of systems, diagnosis of system failures, and for archival documentation. Keywords: Architecture; Axioms; Independence; Information; Design; Systems; Theory 1. Introduction What is a system? A system may be defined as an assemblage of sub-systems, hardware and software components, and people designed to perform a set of tasks to satisfy specified functional requirements and constraints. Engineers build systems. Machines, airplanes, soft- ware systems, and automobile assembly plants are systems – albeit systems of different kinds – and each has sub-systems and components. Systems often consist of hardware, software and people. Such human-made systems must be designed, fabricated and operated to achieve their intended functions. Each of these systems performs many functions. Some, like the automobile, perform a large number of different but dedicated functions. Others, such as a job-shops, perform a variety of different functions during their lifetimes and as such may be categorized as large flexible systems [1]. The design of effective systems is the ultimate goal of many fields, including engineering, business, and government. Yet system design has lacked a formal theoretical framework and thus, has been done heuristically or empirically [2]. Heuristic approaches emphasize qualitative guidelines – exemplified by use of the phrases ‘Murphy’s laws’, ‘make it simple’ and ‘ask five why’s’. After systems are designed, they are sometimes modeled and simulated. In many cases, they have to be constructed and tested. All these very expensive and unpredictable processes are done to debug and improve the design after heuristic design solutions are implemented in hardware and software. Such an approach to systems design entails both technical and business risks because of the uncertain- ties associated with the performance and the quality of a system that is created by means of empirical decisions. Some people use dimensional analysis, decision theory and others to check or optimize the system that has already been designed. There are three issues with this approach. First, they do not provide tools for coming up with a rational system design beginning from the definition of the design goals. Secondly, some of these methods simply confirm the result, if systems are correctly designed. For example, any physical system that is properly configured should satisfy the p-theorem – a correctly designed physical system should always satisfy the p-theorem. Thirdly, they are not general principles for system design since they cannot be applied to non-physical systems such as software and organizations. Systems with many functional requirements (FRs), physical components and many lines of computer codes can be complex, in the sense that the probability of satisfying the highest FRs decreases with increase in the number of FRs and design Research in Engineering Design (1998)10:189–209 ß 1998 Springer-Verlag London Limited Research in Engineering Design Correspondence and offprint requests to: N. P. Suh, The Ralph E. and Eloise F. Cross Professor, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.