An Introduction to
Artificial Intelligence
CE 40417
Chapter 12 – Planning and Acting in Real World
Ramin Halavati (halavati@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.
Outline
• Time, Schedules, and Resources
• Hierarchical Task Network Planning
• Planning and Acting in Nondeterministic
Domains
• Multi Agent Planning
Time, Schedules, & Resources
• Basic Planning:
– What to do and in which order?
• Real World:
– What an When to do? + Limited Resources.
– JOB SHOP SCHEDULING
Job Shop Scheduling
Job Shop Scheduling
• How to assign time to a partial order
plan?
Critical Path Method (CPM)
• Forward March:
– Set Earliest Start (ES)
Critical Path Method (CPM)
• Backward March:
– Set Latest Start (LS)
Critical Path Method (CPM)
Limited Resources
• Resources:
– Consumable vs. Reusable.
• Notation:
– Aggregation
– Immediate Effect
– Resource:R(k)
• Requirement / Temporary Effect
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.
Job Shop Scheduling, One Last Word.
• Separated / Integrated Planning and
Scheduling.
• Semi Automatic
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…
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
•…
HTN Sample (cont)
• Macro Action in Library:
– Build House:
HTN Sample (cont)
HTN Sample (cont)
HTN Cons and Pros
• What’s 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.
HTN Cons and Pros
• What’s Good:
– Almost all real applications are HTN + some
thing else.
– It’s a heuristic to decrease the branching
factor by a great level.
NonDeterministic Domains
• What if we don’t 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.
Sensorless Planning
• We don’t know all beforehand and we
can’t find it out, even when it is done.
– Plan so that to reach the goal state,
regardless of everything. (Coercion)
– Not always possible.
Conditional Planning
• We can check the state ahead, then
perform the pre-planned program.
– Sense Actions
– Conditional Branches
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
Notation Expantion:
• Expanding Plan Notation:
– If (state) Then (…) else (…)
– If (AtLeftCleanLeft CleanRight) Then {}
else Suck.
State Space:
Conditional Planner:
Unavoidable Loops in
Conditional Planner
• New Notation:
– Instead of just Left : while (AtRight) Left
Partially Observable Domains
Partially Observable Domains
• Easiest Approach:
– Assume set of current states and the next
state sets are created, quite similar to nondeterministic actions case.
Execution Monitoring and
Replanning
• Check if the plan is going on is predecided? If not, replan based on current
situation.
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).
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.
Execution Monitoring in
Partially Observable Domains
• Things may fail and we don’t 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.
Continues Planner
• Keep planning, sensing and executing…
– Which is not unlikely, such as maintenance
planning, auto-pilot, plant control, …
Continues Planner
Continues Planner
Continues Planner
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.
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.
Uncertainty is Over.
Multi Agent Planning
• When there is more than one agent in the
scene.
– Competitive
– Cooperative
• Coordination
– Communication
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…
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.
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!
Cooperation – Emergence
•
Evolutionary Emergent Behavior
– Birds Flocking:
•
Separation
•
Cohesion
•
Alignment
– Ants.
Coop. - Communication
• A short message expressing
– the plan / next step.
• A message expressing the next step.
• Plan Recognition!
Competition
• Minimax + Conditional Planning
Essey & Project Proposals
• To Do.