Design Margins as a Key to Understanding Design Iteration

Abstract

Design processes are subject to many uncertainties. Changes resulting from the need to respond to external uncertainties are one of the main drivers of engineering change and therefore for iteration in design processes. Another important cause of iteration in design processes arises from the dependencies in design information which is being generated as part of the design process itself. At the beginning of the design process engineers need to make an informed guess about the values of parameters that they need and can achieve. These values are passed on to others, who base their decisions on them. Design decisions are distributed and iterative among design teams, customers and suppliers. Communicated parameter values are uncertain in two different but related ways. First, there is the confidence, precision and commitment that the designers have in the values they specify. Second there are uncertainties in the values that can be achieved with the technology the new design employs. These issues become particularly challenging when they span design teams, customers and suppliers as they iterate to converge on a mutually effective solution. This paper looks at this type of convergent iteration through an example from the aerospace industry, which illustrates how uncertainty in operating temperature at the beginning of the design process requires a thorough understanding of the temperature ranges that solution alternatives, at different degrees of maturity, can operate under. This paper argues that the key to managing convergent iterations lies in communicating the available ranges of parameter values and in understanding how design margins have arisen in existing technologies. These margins on product parameters provide potential performance which exceeds immediate functional requirements. The paper develops and formalizes the concept of design margins and argues that margins are included into products for a variety of reasons that are not always transparent to different team members. Analysis of margins enables design companies to reason in terms of ranges of values describing the scope for design change in meeting customer and supplier requirements without being forced into unplanned iteration loops.