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MIT16_410F10_lec08b - Activity Planning II Plan Extraction...

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1 Activity Planning II: Plan Extraction and Analysis Brian C. Williams 16.410-13 October 4 th , 2010 Slides draw upon material from: Prof. Maria Fox Univ Strathclyde, Scotland Brian Williams, Fall 10 2 Assignments • Remember: Problem Set #5: Constraint Satisfaction and Activity Planning, out Wed. Sep. 29 th , due Wed, Oct. 6 th , 2010. • Reading: Today: Advanced Planning [AIMA] Ch. 11 " GraphPlan, # by Blum & Furst. Wednesday: Wednesday: Dechter, R., I. Meiri, J. Pearl, “Temporal Constraint Networks,” Artificial Intelligence, 49, pp. 61-95,1991 posted on Stellar . • Exam: Mid-Term - October 20 th .
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2 Example Problem: Dinner Date Initial Conditions : (:init (cleanHands) (quiet)) Goal : (:goal (noGarbage) (dinner) (present)) Actions : (:operator carry :precondition :effect (and (noGarbage) (not (cleanHands))) (:operator dolly :precondition :effect (and (noGarbage) (not (quiet))) (:operator cook :precondition (cleanHands) :effect (dinner)) (:operator wrap :precondition (quiet) :effect (present)) + noops Solution : (Cook, Wrap, Carry) Approach: Graph Plan 1. Construct compact encoding of candidates that prunes many invalid plans Plan Graph. 2. Generate plan by searching for a consistent subgraph that achieves the goals . Proposition Init State Action Time 1 Proposition Time 1 Action Time 2
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3 Plan Graph Compactly encodes the space of consistent plans, while pruning . . . 1. partial states and actions at each time i that are not reachable from the initial state. 2. pairs of propositions and actions that are mutually inconsistent at time i. 3. plans that cannot reach the goals. Graph Plan Create plan graph level 1 from initial state Loop 1. If goal propositions of the highest level ( nonmutex ), 2. Then search graph for solution If solution found, then return and terminate. 3. Extend graph one more level. 6 A kind of double search: forward direction checks necessary (but insufficient) conditions for a solution, ... Backward search verifies...
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4 Layer 1: Add Proposition Mutexs noGarb cleanH quiet dinner present carry dolly cook wrap cleanH quiet 0 Prop 0 Action 1 Prop 1 Action 2 Prop Do all goal propositions appear non-mutex? No Proposition Mutexs Round 2: Extend the Plan Graph noGarb cleanH quiet dinner present carry dolly cook wrap carry dolly cook wrap cleanH quiet noGarb cleanH quiet dinner present 0 Prop 0 Action 1 Prop 1 Action 2 Prop
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5 Search Graph for Solution noGarb cleanH quiet dinner present carry dolly cook wrap carry dolly cook wrap cleanH quiet noGarb cleanH quiet dinner present 0 Prop 0 Action 1 Prop 1 Action 2 Prop Outline Graph Plan – Solution Extraction – Memos – Properties – Termination with Failure
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6 2. Search for a Solution Recursively find consistent actions that achieve all goals at time t, t-1 ... : Find actions to achieve each goal G i at time t: For each action A i that makes G i true at t: • If A i isn ! t mutex with a previously chosen action at t, Then select it.
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