An Introduction to Artificial Intelligence CE 40417

1
An Introduction to Artificial Intelligence CE
40417

• Chapter 12 Planning and Acting in Real World
• Ramin Halavati (halavati_at_ce.sharif.edu)

In which we see how more expressive
representations and more interactive agent
architectures lead to planners that are useful in
real world.
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Outline

• Time, Schedules, and Resources
• Hierarchical Task Network Planning
• Planning and Acting in Nondeterministic Domains
• Multi Agent Planning

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Time, Schedules, Resources

• Basic Planning
• What to do and in which order?
• Real World
• What an When to do? Limited Resources.
• JOB SHOP SCHEDULING

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Job Shop Scheduling
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Job Shop Scheduling

• How to assign time to a partial order plan?

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Critical Path Method (CPM)

• Forward March
• Set Earliest Start (ES)

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Critical Path Method (CPM)

• Backward March
• Set Latest Start (LS)

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Critical Path Method (CPM)
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Limited Resources

• Resources
• Consumable vs. Reusable.
• Notation
• Aggregation
• Immediate Effect
• ResourceR(k)
• Requirement / Temporary Effect

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Limited Resources

• No General Approach (NP-Hard)
• Just Order the task so that the requirements are
  met.
• Heuristic
• Minimum Slack Algorithm
• Give more priority to the task with least
  remaining slack.

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Job Shop Scheduling, One Last Word.

• Separated / Integrated Planning and Scheduling.
• Semi Automatic

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Hierarchical Planning

• Hierarchical Task Network
• At each level, only a small number of
  individual planning actions, then descend to
  lower levels to solve these for real.
• At higher levels, the planner ignores internal
  effects of decompositions. But these have to be
  resolved at some level

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HTN Sample

• Construction Domain
• Actions
• Buy Land Money ? Land
• Get Load Good Credit ? Money
• Get Permit Land ? Permit
• Hire Builder ? Contract
• Construction Permit ? Contract ? House Built
• Pay Builder Money ? House Built ? House

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HTN Sample (cont)

• Macro Action in Library
• Build House

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HTN Sample (cont)
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HTN Sample (cont)
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HTN Cons and Pros

• Whats Bad?
• Recursion?
• Sub Task Sharing
• Enjoy honey moon in Hawaii and raise a family.
• Library
• Enjoy Honey moon in Hawaaii Get Married , Go to
  Hawaii.
• Raise Family Get Married, Have two children.

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HTN Cons and Pros

• Whats Good
• Almost all real applications are HTN some thing
  else.
• Its a heuristic to decrease the branching factor
  by a great level.

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NonDeterministic Domains

• What if we dont know all about situations and
  effects.
• E.g.
• Init A table and a chair of unknown colors.
• Goal A table and a chair of the same colors.
• Condition Painting may have flaws.

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Sensorless Planning

• We dont know all beforehand and we cant find it
  out, even when it is done.
• Plan so that to reach the goal state, regardless
  of everything. (Coercion)
• Not always possible.

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Conditional Planning

• We can check the state ahead, then perform the
  pre-planned program.
• Sense Actions
• Conditional Branches

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Conditional Planning in Fully Observable Domains

• Vacuum World
• Left AtRight ? AtLeft ? ?AtRight
• Left AtRight ?
• (AtLeft ? ?AtRight)? (?AtLeft ? AtRight)
• Suck when AtLeft?CleanLeft
• when AtRight?CleanRight
• Left when AtLeft? ?CleanLeft
• when AtRight?AtLeft ?AtRight

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Notation Expantion

• Expanding Plan Notation
• If (state) Then () else ()
• If (AtLeft?CleanLeft ?CleanRight) Then
• else Suck.

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State Space
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Conditional Planner
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Unavoidable Loops in Conditional Planner

• New Notation
• Instead of just Left while (AtRight) Left

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Partially Observable Domains
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Partially Observable Domains

• Easiest Approach
• Assume set of current states and the next state
  sets are created, quite similar to
  non-deterministic actions case.

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Execution Monitoring and Replanning

• Check if the plan is going on is pre-decided? If
  not, replan based on current situation.

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Execution Monitoring Replanning

• Action Monitoring
• See if current state is as it was supposed, if
  not, find a solution to return it to what it was
  (repair).

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Execution Monitoring Replanning

• Plan Monitoring
• See if the previous plan is still wise?
• Serendipity!
• A precondition of future actions has failed and
  can not be recovered.

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Execution Monitoring in Partially Observable
Domains

• Things may fail and we dont know.
• Sensing actions may be required
• And they may need extra-planning.
• We may stuck in futile attempts
• The electronic key is incorrect, but we think it
  might be due to incorrect pushing in.

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Continues Planner

• Keep planning, sensing and executing
• Which is not unlikely, such as maintenance
  planning, auto-pilot, plant control,

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Continues Planner
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Continues Planner
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Continues Planner
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Continuous Planner

• POP
• Missing Goal
• A new goal has erupted. Just add it.
• Open precondition
• An action has lost its support links. Add a new
  causal link.
• Causal Conflicts
• A causal link is suddenly threatened. Choose an
  appropriate ordering.

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Continuous Planner

• POP
• Unsupported Link
• A link from start to something has suddenly last
  its true value. Remove it.
• Redundant Action
• An action no more produces something needed.
  Remove it.

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Uncertainty is Over.
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Multi Agent Planning

• When there is more than one agent in the scene.
• Competitive
• Cooperative
• Coordination
• Communication

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Cooperation

• Multi Body Planning
• One is in charge of all decisions
• Having the agent as one of parameters
• Go(R2D3, Right) Go(C3PO,Left).
• Synchronization and Timing

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Cooperation Multi Body

• Joint Planning
• Planning using action pairs
• Exponentially Many Actions Actions Agents
• Having Concurrent Actions List
• Which actions happen together and which not, such
  as orders in POP.

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Cooperation - Coordination

• Accepting a prior Convention.
• Everyone drive on his/her right side of the road.
• Domain Independent
• Choosing the first feasible action.
• Producing all possible feasible actions and
  choosing the one which stands first in alphabetic
  order!

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Cooperation Emergence

• Evolutionary Emergent Behavior
• Birds Flocking
• Separation
• Cohesion
• Alignment
• Ants.

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Coop. - Communication

• A short message expressing
• the plan / next step.
• A message expressing the next step.
• Plan Recognition!

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Competition

• Minimax Conditional Planning

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Essey Project Proposals

• To Do.