Next: The Interpretation of Concurrent Up: PDDL2.1 : An Extension Previous: Plan Metrics

# Durative Actions

Most recent work on temporal planning [Smith WeldSmith Weld1999,Bacchus KabanzaBacchus Kabanza2000,Do KambhampatiDo Kambhampati2001] has been based on various forms of durative action. In order to facilitate participation in the competition we therefore developed two forms of durative action allowing the specification only of restricted forms of timed conditions and effects in their description. Although constrained in certain ways, these durative actions are, nevertheless, more expressive than many of the proposals previously explored, particularly in the way that they allow concurrency to be exploited. The two forms are discretised durative actions and continuous durative actions.

Both forms rely on a basic durative action structure consisting of the logical changes caused by application of the action. We always consider logical change to be instantaneous, therefore the continuous aspects of a continuous durative action refer only to how numeric values change over the interval of the action. Figure 6 depicts a basic durative action, load-truck, in which there is no numeric change.

The modelling of temporal relationships in a discretised durative action is done by means of temporally annotated conditions and effects. All conditions and effects of durative actions must be temporally annotated. The annotation of a condition makes explicit whether the associated proposition must hold at the start of the interval (the point at which the action is applied), the end of the interval (the point at which the final effects of the action are asserted) or over the interval from the start to the end (invariant over the duration of the action). The annotation of an effect makes explicit whether the effect is immediate (it happens at the start of the interval) or delayed (it happens at the end of the interval). No other time points are accessible, so all discrete activity takes place at the identified start and end points of the actions in the plan.

Invariant conditions in a durative action are required to hold over an interval that is open at both ends (starting and ending at the end points of the action). These are expressed using the over all construct seen in Figures 6 and 8. If one wants to specify that a fact holds in the closed interval over the duration of a durative action, then three conditions are required: (at start p), (over all p) and (at end p).