PLASMA: a constraint based planning architecture

Until recently, planning research focused on solving problems of feasibility using models consisting of causal rules. Propositional logic is sufficient for representing such rules. However, many planning problems also contain time and resource constraints. It is often impractical to represent such planning domains with propositions. Large time horizons and possible resource states lead to enormous domain representations. Propositional representations can often obscure information that is useful during search. Finally, propositional representations can make it difficult for human modelers to express the domain in a convenient and natural way. The Constraint-based Planning paradigm employs constraints as the building blocks of both planning domain rules and plans. The building blocks of such plans are intervals of time over which some state holds or an action occurs in a plan. Each interval is represented by variables describing its properties (e.g. start, end, duration). At each step of the planning process, a mapping is maintained between the plan under construction and an underlying constraint network. As actions are added to the plan, constraints are posted on variables representing the action, its preconditions and its effects (a generalization of causal links). The domain rules contain directives for adding new intervals and for posting constraints over the variables on those intervals as plans are modified. Employing constraints in rules makes it easy to represent disjunctive preconditions, conditional effects, and mutual exclusions directly. The semantics of this mapping ensure that logical inference (e.g. propagation) on the constraint network can be used directly by search engines operating on plans.