TLP-GP: Solving Temporally-Expressive Planning Problems

This article describes an algorithm which solves temporally-expressive planning problems, that is problems for which all possible solutions require concurrency of actions. The planner TLP-GP which implements this algorithm constructs a simplified planning graph until the goals are attained, as in classic atemporal planners. It then establishes temporal constraints between actions and searches backward for a solution-plan in the planning graph using a disjunctive temporal constraint solver. If the search fails, the graph is extended to the next level and the search is restarted. This method can solve problems in a language whose expressivity is greater than PDDL 2.1. Preconditions can be required and effects can take place on any temporal interval relative to the start-time of an action. This algorithm can also take into account, in a very natural way, exogenous events as well as temporally extended goals. We also propose several different means of extending expressivity even further. TLP-GP is complete for the temporally-expressive sublanguages of PDDL 2.1. We compared our planner with two state-of-the-art temporally-expressive planners such as LPGP and VHPOP. These experimental trials not only show the efficiency of our approach but also demonstrate the practical possibility of solving temporally expressive problems which up until now were unsolvable by existing techniques.