Management of Temporal Constraints for Factory Scheduling

In this paper, we present constraint propagation techniques used in the OPIS scheduling system to update schedule descriptions and detect introduced inconsistencies. Our approach is summarized as follows: • A hierarchical model is used to represent resources and operations to be performed. Schedules are developed and maintained at different levels of precision which are explicitly associated with resources and operations; constraint propagation is correspondingly performed at different levels. • Various scheduling constraints are attached to resources and operations, and combined to derive time bound constraints. A description of the original constraints that collectively impose a bound is explicitly recorded. • Time bound constraints are maintained by an object-oriented propagation process: through messages, resources and operations communicate constraints and cooperate to compute time bounds. • When time bounds are inconsistent, their origins provide the information required to construct an appropriate description of the conflicting situation. This description provides OPIS with information needed to make reactive decisions.