Working Paper Alfred P. Sloan School of Management Identifying Controlling Features of Engineering Design Iteration Identifying Controlling Features of En^neering Design Iteration

Engineering design generally involves a very complex set of relationships among a large number of coupled elements. It is this complex coupling that leads to iteration among the various engineering tasks in a large project. The Design Structure Matrix (DSM) is useful in identifying where iteration is necessary. The Work Transformation Model developed in this paper is a powerful extension of the DSM method which can predict slow and rapid iteration within a project, and predict those features of the design problem which will require many iterations to reach a technical solution. This model is applied to an automotive brake system development process in order to illustrate the model's utility in describing the main features of an actual design process. Introduction The goal of this work is to develop a modeling framework which is useful for describing engineering design iteration. The framework is applied to brake system design to illustrate its utility in understanding the engineering design process. Engineering design is the process whereby a technical solution is created to solve a given problem. There have been several attempts to give formal structure to the design process, such as those of Sub [1990], Pahl and Beitz [1988] and Alexander [1964]. This stream of research characterizes good design practice in general terms, but does not describe what makes some design problems more difficult than others. We intend to further the development of design process modeling by providing richness to the descriptions of design procedures and strategies which will enable a design organization to identify the difficult portions of their particular design problem. Strategies can then be developed to facilitate the effective execution of these difficult aspects. The Design Structure Matrix (DSM) serves as the basis for our formal analysis and will be briefly reviewed in this section. (For a more detailed overview of the DSM method the reader is referred to Steward [1981] and Eppinger et al. [1991].) The work herein extends the analytical method, and demonstrates the utility of our framework for the management of engineering projects. The philosophy of the DSM method is that the design project is divided into individual tasks, and the relationships among these tasks can be analyzed to identify the underlying structure of the project. It has been suggested that studying the relationships between individual design tasks can improve the overall design process, and is a powerful way to analyze alternative design strategies [von Hippel 1990]. Earlier work developed a modeling formalism which shows how different aspects of a design problem are related [Alexander 1964]. Alexander describes a graphical technique where the functional needs of the technology are nodes, and interactions between tlie needs are arcs. His idea is to segment the graph into subsections which have relatively few interactions which cross boundaries. These graph segmentations give rise to technical subsystems which should separate the technical needs into independently solvable problems. The DSM method is similar to Alexander's technique, but the nodes are now specific design tasks and the arcs are directed and indicate information flows between tasks. The nodes in the graph are arranged in a square matrix where each row and its corresponding colunm are identified with one of the tasks. Along each row, the marks indicate from which other tasks the given task requires input. Reading down each column indicates which other tasks receive its output. Diagonal elements do not convey any meaning at this point, since a task cannot depend upon its own completion. For example, in Figure 1 (based on a simplified view of camera body design), task C requires input from tasks B, D, E and F, task B requires input only fi"om task A, and task A needs no input to begin. A Set Specifications B Design Concept C Design Shutter Mechanism D Design Viewfinder E Design Camera Body F Design Film Mechanism G Design Lens Optics H Design Lens Housing A B reason, iteration is a typical feature of engineering design projects [Hubka 1980]. The sub-matrix in Figiare 1 depicts a design problem defined such that the tasks are sufficiently complex and interrelated so that iteration will be necessary to complete the tasks. There is an established set of models which allow looping within a PERT modeling framework. This set of models is known as GERT, for General Evaluation and Review Technique. Direct analysis of any but a simple GERT network rapidly becomes unwieldy, so simulation is typically used to evaluate a project. (Taylor and Moore [1980] discuss the application of GERT to R&D projects.) It is the intention of this modeling effort to provide an analytically tractable model of the design iteration process, even for large projects. It is hoped that by preserving tractability it will be possible to observe the relationship between the structiu-e of the problem and the development time of the project. Because GERT relies on simulation for large projects, it is difficult to discern this