A representation of time for automatic planning

This paper describes a representation of time and its associated inference rules that is being applied to process planning. The representation associates a time interval and a duration with every proposition in a world model, so that particular time relationships can be reasoned about explicitly. These time intervals are defined by their beginning and end instants, so interval relationships, such as "before" and "overlaps", are defined indirectly by the corresponding relations between the end instants. This new representation allows simpler rules for deducing the usual planning interval relationships. Similarly, interval durations are associated with each proposition by asserting the maximum and minimum durations, thus allowing the discovery of critical paths and allowing partial plans to be optimized. A planner has been designed to use the above time representation to produce plans for work stations with multiple robots, for example. A method is given for the detection and correction of interactions in the current partial plan that avoids any conflicts and deadlocks that could arise.