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 AtLeftCleanLeft

when AtRightCleanRight

– Left: when AtLeft CleanLeft

when AtRightAtLeft AtRight

Notation Expantion:

• Expanding Plan Notation:

– If (state) Then (…) else (…)

– If (AtLeftCleanLeft 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 non-

deterministic actions case.

Execution Monitoring and Replanning

• Check if the plan is going on is pre-

decided? 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).

• 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 & Replanning

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.