Contingency control in expert planning systems

A prime function of expert planning systems is to provide interactive planning facilities for use by experienced engineers. One of the strengths of a system such as IDEAS described by Eklund et al. (1987) and Marshall et al. (1987) is its ability to construct a project plan interactively from a hierarchy of possible actions and products. The system´s knowledge base contains rules which describe relationships within the hierarchy and determine the necessary actions to be included in a plan. It is often possible to perform a plan task by more than one method. The rules need to provide advice and explanation to an engineer on which method or activity to select under various conditions. It is suggested that the control of these contingency activities could be improved by the use of a formal modelling technique which is understandable by planning engineers and knowledge engineers. This paper considers the use of Petri net graphs for modelling contingencies and their implementation in an expert system