Comparative Study of Representation Structures for Modeling Function and Behavior of Mechanical Devices

ing current approaches to functional modeling, it appears that two basic approaches have been used to varying degrees of granularity and success: the flow of inputs and outputs of systems and syntactic languages. The models of flow inputs and outputs are generally based upon systems engineering approaches to model energy, signal, and material passing through a system component under a select set of functions: change, vary, connect, channel, and store. At a high level of functional modeling, this approach is useful, but at lower levels the functional modeling becomes unintuitive and cumbersome. Shapiro and Voelcker (1989) work with energy exchange across specific geometric boundaries of a part based upon three basic principles. First, function is determined by the energy ports (subsets of the physical boundary) and the energy regimes acting on these ports. Both the ports and the energy should be fully defined. The geometry of the design must provide for at least one instance of each energy port and satisfy other functional constraints (size, weight, etc.). Second, energy exchanges may be represented independent of the geometry. Finally, both the first and second principles hold for all subsystems defined on combinatorial decompositions of a system. Identifying the energy and the associated geometries is, at present, too cumbersome for practical application. Syntactic grammars are also used for functional modeling. Grammars may consist of words and rules for combining the words. In functional modeling, the words may be functions, principles, or components. Descriptive words may be used to refine the meanings of the functions, principles, and components. Some illustrations of these grammars include the functional description language of Lai and Wilson (1989) and the catalog of function structures provided by Johnson and Thornton (1991). Bond graphs are also useful in representing gearboxes (Sthanusubramonian, et al., 1992). A limitation is that no additional information from other phases of the product life cycle is included. Some grammars that have been developed for specific design domains include: lathe grammar (Brown, et al., 1994), truss grammar (Shea and Cagan, 1997), vehicle grammar (Schmidt, et al., 1999), and gear grammar (Li and Schmidt, 2000). Several researchers have developed languages to describe function in terms of flow (McAdams, et al., 1998; Collins, et al., 1976; Altshuller, 1984). In developing a standard taxonomy for function, Szykman, et al., (1999) have identified three benefits: reduction of ambiguity between function definitions, clarification of uniqueness from minimal vocabulary, and uniformity of information exchange between systems. Szykman, et al., (1999) developed their representation based upon a minimalist description of function and flow. Some examples of functions include: usage, combination, and transformation. Likewise, some examples of flow include: material, signal, and energy; much like the flow types proposed by Pahl and Beitz (1996). Baxter, et al. (1994) have developed a functional data model based upon the structure of the product data model defined using EXPRESS-G. In this model, the product representation is combined with the function representation. Functions have the attributes: named, inputs, outputs, has_need_of, and performed_by. The attribute named provides a naming mechanism for the function. Thus, possible associated names may be cataloged to provide appropriate connections for the function. Similar to systems engineering, the function is composed of inputs and outputs. These connect a performer and an action. Performers are associated with sub-systems, components, or features. Actions associate the function with parametrics. The function satisfies function (product) requirements through the attribute has_need_of. Finally, the function is performed_by a sub-system, component, or feature.