Separating control from structural knowledge in construction expert systems

In most expert systems for constructional tasks the knowledge base consists of a set of facts or object definitions and a set of rules. These rules contain knowledge about correct or ideal solutions as well as knowledge on how to control the construction process. In this paper we present an approach that avoids this type of rules and thus the disadvantages caused by them. We propose a static knowledge base consisting of a set of object definitions interconnected by is-a and part-of links. This conceptual hierarchy declaratively defines a taxonomy of domain objects and the aggregation of components to composite objects. Thus, the conceptual hierarchy describes the set of all admissible solutions to a constructional problem. Interdependencies between objects are represented by constraints. A solution is a syntactically complete and correct partial instantiation of the conceptual hierarchy. No control knowledge is included in the conceptual hierarchy. Instead, the control mechanism will use it as a guideline. It is thus possible to determine in which respects a current partial solution is incomplete, simply by comparing it with the conceptual hierarchy syntactically. The most important advantage of this approach is the ability to represent control knowledge and structural knowledge separately.