Function is an important aspect of artifacts in engineering design. Although many definitions of function have been proposed in the extensive research mainly in engineering design and philosophy, the relationship among them remains unclear. Aiming at a contribution to this problem, this paper investigates some ontological issues based on the role concept in ontological engineering. We discuss some ontological distinctions of function such as essentiality and actuality and then propose some fundamental kinds of function such as essential function and capacity function. Based on them, we categorize some existing definitions in the literature and clarify the relationship among them. Then, a model of function in a product life-cycle is proposed. It represents the changes of existence of the individuals of each kind of function, which are caused by designing, manufacturing and use. That model enables us to give answers to some ontological questions such as when and where a function exists and what a function depends on. The consideration on these issues provides engineers with some differentiated viewpoints for capturing functions and thus contributes to consistent functional modeling from a specific viewpoint. The clarified relationships among the kinds of function including the existing definitions in the literature will contribute to interoperability among functional models based on the different kinds and/or definitions.

In order to facilitate sharing of functional models, some functional taxonomies each of which provides a set of verbs for representing generic functions (called functional terms here) have been developed. Their examples include some sets of generally valid functions in the book written by Pahl and Beitz, Functional Basis (FB) developed by Hirtz et al. and FOCUS/Tx developed by the authors of this paper. The issue addressed in this paper is the implicitness of the criteria of classification of functional terms in those taxonomies and thus unclearness of their definitions. This paper proposes an ontology of logical criteria for classification of functional terms (called FOCUS/View). Using the classes defined in FOCUS/View, the classification criteria of a functional taxonomy can be explicitly represented. These classes have been conceptualized based on deep investigation on FB and FOCUS/Tx. The benefits of the proposed FOCUS/View ontology include: (1) users of a taxonomy can easily understand differences of similar terms and select an appropriate term out of them, (2) a developer of a taxonomy can check its logical classification structure and then improve its logical clearness, and (3) we can compare different taxonomies and establish more reliable mappings between their terms for interoperability of functional models. In this paper, as a demonstration of the benefit (1), the classification criteria of FB, FOCUS/Tx, Krumhauer’s and Roth’s generally valid functions are explicitly presented using FOCUS/View. As a demonstration of the benefit (2), some logically problematic classification structures of FB and the Krumhauer’s functions are discussed and modified for the logical clearness. For the benefit (3), this paper demonstrates the mappings between FB and FOCUS/Tx and a semantic interoperable document search system based on these mappings.