During the design process, a designer transforms an abstract functional description for a device into a physical description that satisfies the functional requirements. In this sense, design is a transformation from the functional domain to the physical domain; however, this transformation process is not well characterized nor understood for mechanical systems. The difficulty arises, at least in part, because mechanical designs are often composed of highly-integrated, tightly-coupled components where the interactions among the components are essential to the behavior and economic execution of the design. This assertion runs counter to design methodologies in other engineering fields, such as software design and circuit design, that result in designs in which each component fulfills a single function with minimal interaction. Because of the geometry, weight, and cost of mechanical components, converting a single behavioral requirement into a single component is often both impractical and infeasible. Each component may contribute to several required behaviors, and a single required system behavior may involve many components. In fact, most mechanical components perform not only the desired behavior, but also many additional, unintended behaviors. In good mechanical designs, these additional behaviors often are exploited.
The long term goal of our research is to create a transformational strategy in which the design specifications for a mechanical system can be transformed into a description of a collection of mechanical components. To realize this goal requires formal representations for the behavioral and the physical specifications of mechanical systems as well as formal representations for the behaviors and the physical characteristics of mechanical components. Because the interactions of components are important in our synthesis strategy, the representation of the behaviors of mechanical components must be linked to the representation of their physical characteristics; that is, we are concerned with modeling the relationship between form and function of components. Finally, we need a strategy that enables us to transform an abstract description of the desired behavior of a device into a description that corresponds to a collection of available physical components.
In this paper, we present a graph-based language to describe both the behavioral specifications of a design as well as the behavior of the available physical components. We also briefly discuss a graph-based grammar for the representation of the physical characteristics of the components that enables us to guide the translation from specifications to components [Pinilla 89]. The transformation strategy is discussed in a companion paper [Hoover 89].