Top-down%20hierarchical%20planning

1
Top-down hierarchical planning

• A standard Artificial Intelligence mechanism used
  for simulating many aspects of intelligent
  behaviour

Main reference for planning Earl Sacerdoti
(1977) A Structure for Plans and Behavior,
Elsevier North-Holland. For text
planning Johanna Moore Cécile Paris (1993)
Planning text for advisory dialogues capturing
intentional and rhetorical information.
Computational Linguistics 19(4)651-694
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Requirements

• A representation of the state of the world
  also called domain knowledge
• Be-at (Ed, Washington)
• A library of plan operators appropriate for our
  planning tasks
• A planning mechanism, or algorithm, for
  constructing a plan to change the world from its
  current state to some goal state.

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Plan operators

• The job of a plan operator is to say how some
  effect can be achieved
• i.e., typically changing the world or someones
  beliefs about the world
• to do this it must also
• say how the world must be in order for the plan
  operator to work (the preconditions)
• say what must be done in order for the effect to
  come into operation

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Abstraction hierarchies

• If all possible actions were equal, then planning
  is made even more complex because there are very
  many more planning operators to be considered at
  each step.
• Solution
• hierarchical planning
• actions are divided up into more specific parts
• sequences of actions can be grouped together and
  given a more abstract label.
• e.g. dinner gt cooking dinner, eating dinner,
  washing up cooking dinner gt preparing
  the ingredients, cooking, serving

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Action hierarchies
have dinner
prepare dinner
wash up
eat dinner
wash dishes
clean ingredients
dry dishes
cut ingredients
stack dishes
cook ingredients
mix together
serve dinner
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Two kinds of operators

• Primitive operators
• actions that are directly executable
• no body
• Abstract operators
• actions that need to be decomposed into less
  abstract actions
• decomposition specified in the body

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A plan operator definition

• Operator names the operator and says what the
  main objects involved are
• Effect states what effect (i.e., what change in
  the world) the successful application of the
  planning operator would bring
• Preconditions states what has to be the case in
  the world for the plan to be used
• Body says what actions a more abstract action
  must be broken down to in order to apply the
  operator

Only when the actions in the body have been
applied is the effect achieved!!
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A plan operators role during the planning process
PLAN OPERATOR
effect
preconditions
body
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A complete plan

• is a hierarchically organized sequence of actions
• carrying out the actions in sequence will
  guarantee that the preconditions for achieving
  all subsequent effects are met in time
• when all the sub-actions of a more abstract
  action have been performed, the abstract action
  has also been achieved and the corresponding
  effect also