introduction to artificial intelligence lecture 13 : advanced planning

Intro to AI Fall 2002 © L. Joskowicz 1

Introduction to Artificial Intelligence LECTURE 13: Advanced Planning

• Motivation: least commitment principle

• Partial-order planning (POP)

• Planning with partially instantiated operators

• Hierarchical decomposition

• Other extensions “An Introduction to Least Commitment Planning” D. Weld,

Artificial Intelligence Magazine, Winter 1994, pp 27-61.

Least commitment principle• Make choices only when necessary, leaving

the decision for the time it is required– variable binding: most-general unifier is a least

commitment strategy

Prefer buy(Store,drill) to buy(store55,drill)– partial ordering: assume operators can be

performed simultaneously unless there is a requirement to do otherwise

if S1 deletes precondition c and c is needed by S2, perform S2 before S1

Example: putting on shoes• Start: {}• Goal: {RightShoeOn, LeftShoeOn} • Operators:

Op(Action: RightShoeOn, Precond: RightSockOn, Effect: RightShoeOn)Op(Action: LeftShoeOn Precond: LeftSockOn, Effect: LeftShoeOn)Op(Action: RightSockOn

Effect: RightSockOn) Op(Action: LeftSockOn,

Effect: LeftSockOn)

Partial vs. total order plans

Operator representation

• Operator name, precondition, effect (both add and delete lists)Op(Action: action-name,

Precond: conjunction of literals (positive)

Effect: conjuction of literals (positive and negative)

• Graphically

action-name

p1 p2 .... pn

e1 e2 .... em

preconditions

effects

Plan representation (1)• Plan steps: a sequence of operators

<S1, S2, …., Sn>

• Step ordering constraints: indicate step precedence relations Si

< Sj “Si must be executed sometime before Sj”

• Variable binding constraints: indicate variable assignments X = a, Y b, etc

• Causal links: record the purpose of the step Si

-- c --> Sj “Si achieves precondition c for Sj”

Plan representation (2)• Initially, the plan consists of two steps, Start and

Finish, with null actions associated to them, with ordering Start < Finish and with the desired goal (g1 /\ g2 /\ … /\ gn) as precondition

Plan(Steps:{S1: Op(Action: Start), S2: Op(Action: Finish, Precond: (g1 /\ g2 … /\gn))}Orderings: {Si < Sj },Bindings: {},

Links: {})

Example of plan representation

Finish

LeftShoeOn /\ RightShoeOn

Ordering:Left Sock < Left ShoeRight Sock < Right ShoeStart < all, Finish > “all”

Complete plans

A plan is complete iff each precondition of each step is achieved by some other step. A step achieves a precondition if the condition is one of the effects of the step and if no other step can cancel out the condition:

Si achieves precondition c of Sj iff

1. (Si < Sj) /\ (c in Effects(Si))

2. ~Sk (~c in Effects(Sk)) /\ (Si < Sk < Sj)

Consistent plans

A plan is consistent iff there are no contradictions in the ordering or binding constraints. A contradiction occurs when:

1. (Si < Sj) and (Sj < Si)

2. (X = a) /\ (X = b) or (X=a) /\ (X a)

Solutions as plans

• A solution is a complete and consistent plan that achieves the desired goal.

• Any linearization of a partial plan is also a solution

• Partially ordered plans are better solutions than totally ordered plans because:– no arbitrary choice of ordering– parallel execution of branches– easier to combine plan fragments

Partial Order Planner: Overview• Regression planning: work from goal to start

• Start from the initial plan, add one step (operator) in each iteration

• Add only steps that serve to achieve a precondition that has not been achieved yet.

• Keep track of interactions with causal links. When a conflict occurs, resolve it by imposing an order between steps

• Keep track of all choice points and backtrack as necessary

Example: shopping for groceriesSM = SupermarketHWS = Hardware Store

Steps: {Start: Op(Action: Start, Effect: At(Home) /\ Sells(HWS,Drill)/\ Sells(SM,Milk) /\ Sells(SM,Banana),

Finish: Op(Action: Finish, Precond: At(Home) /\ Have(Drill)/\ Have(Milk) /\ Have(Banana)}

Actions: Go and Buy• Op(Action: Go(there) Precond: At(here)

Effect: At(there) /\ ~At(here))

• Op(Action: Buy(x)

Precond: At(store) /\ Sells(store,x)

Effect: Have(x)

Go(there)

At(here)

At(there) ~At(here)

Buy(x)

At(store) Sells(store(x)

Have(x)

Plan to achieve three preconditions

Have(Milk) Have(Ban.)Have(Drill)

At(Home) Sells(HWS,Drill) Sells(SM,Milk) Sells(SM,Ban.)

Bold links are causal linksLight links are ordering links

Instantiation and causal links

{s/SM} {s/SM}{s/HWS}

Causal links can be added because there is no conflict! No ordering is necessary

Next step: get to the store

At(HWS) ~At(x) At(SM) ~At(x)

Instantiation and causal links

{x/Home}{x/Home}

At(HWS) ~At(Home) At(SM) ~At(Home)

Flawed plan! Causal links conflict: cannot be in two places simultaneously ! Re-ordering is necessary

Soving causal link conflicts

Promotion and demotion sequentialize actions

After threat resolution (demotion)

At(HWS) At(SM)

Have(Milk) Have(Ban.) At(Home)Have(Drill)

FinalSolution

Have(Milk) Have(Ban.)

At(SM)

At(Home)

Have(Drill)

At(HWS)

POP algorithm (1)function POP(initial,goal,operators) returns plan

plan := Make-Minimal-Plan(initial,goal)

loop do if Solution?(plan) then return plan (S-need,c) := Select-Sub-Goal(plan) Choose-Operator(plan,operators,S-need,c) Resolve-Threats(plan) end

function Select-Subgoal(plan) returns (S-need,c) pick a plan step S-need from STEPS(plan) with a precondition c that has not been achievedreturns (S-need,c)

POP algorithm (2)procedure Choose-Operator(plan,operators,S-need,c)

choose (a step S-add from operators) or

(STEPS(plan) that has c as an effect) if there is no such step then fail add causal link (S-add -- c --> S-need) to LINKS(plan) add ordering constraint S-add < S-need to ORDERINGS(plan) if S-add is a newly added step from operators then

add S-add to STEPS(plan) add Start < S-add < Finish to ORDERINGS(plan)

procedure Resolve-Threats(plan) for each S-threat that threatens a link (Si -- c --> Sj) in

LINKS(plan) do choose either Promotion: add S-threat < Si to ORDERINGS(plan) Demotion: add Sj < S-threat to ORDERINGS(plan) if not Consistent(plan) then fail

POP is sound and complete• POP constructs a proof that each

precondition of the goal step is achieved:– Choose-Operator selects an action to get subgoal– Resolve-Threats sequentializes to ensure no

interference between operations

• POP is sound and complete: every plan it returns is a solution, and if there is a solution, it will be found (assuming complete search -- BFS or iterative deepening search)

• It is also sound and complete with partially instantiated operators (see next slides)

• Resolving conflicts with partially instantiated operators: is At(x) a threat to ~At(Home)? It is a possible threat, which can be dealt with by1. resolve now with an equality constraint

add binding x = HWS

2. resolve now with an inequality constraint

add the clause x Home

3. resolve later: do nothing. It is not a threat until x becomes instantiated. When it does, use promotion and demotion to resolve the conflict.

Partially instantiated operators

Extended notion of achievingA step achieves a precondition if the condition is

one of the effects of the step, and if no other step can cancel out the condition for all instantiations.

Si achieves precondition c of Sj iff

1. (Si < Sj) and Si has an effect c’ that necessarily unifies with c

2. ~Sk (Si < Sk < Sj) in some linearization of the plan and Sk has an effect c’ that possibly unifies with ~c.

Modified Choose-Operator*

procedure Choose-Operator(plan,operators,S-need,c)

(STEPS(plan) that has c-add as an effect)

such that u = Unify(c,c-add,Bindings(plan)) if there is no such step then fail add u to Bindings(plan) add causal link (S-add -- c --> S-need) to LINKS(plan) add ordering constraint S-add < S-need to ORDERINGS(plan) if S-add is a newly added step from operators then

* for resolving later -- least commitment strategy

Modified Resolve-Threats*procedure Resolve-Threats(plan) for each (Si -- c --> Sj) in LINKS(plan) do for each S-threat in STEPS(plan) do for each c’ in EFFECTS(S-threat) do if Subst(Bindings(plan),c) = Subst(Bindings(plan),~c’)

then choose either

Promotion: add S-threat < Si to ORDERINGS(plan) Demotion: add Sj < S-threat to ORDERINGS(plan) if not Consistent(plan) then fail end end end

* for resolving later -- least commitment strategy

Blocks world revisited

Follow POP on blocks world examples!

Advanced planning topics

• Hierarchical planssteps at different levels of resolution

• More complex conditionsuniversal quantification, conditionals

• Dealing with time constraintsincorporate time intervals an deadlines

• Resources and costschoose the plan that satisfies resource

and cost constraints

Hierarchical decomposition

• POP does not distinguish between different levels of abstraction of operators:

go(home,airport) vs. go(bed,living_room)

Typical plans usually have many steps!

Figure out first how to get to the airport, then find out how to exit the house!

• Operators should describe actions at different levels of abstraction, so “big” goals get solved first

Example

Abstract operators

• Decompose operators into a group of more detailed operators that form a plan to implement it.

• The decomposition ends with primitive operators which are not decomposedBuild(House) is decomposed into Build(Foundation), Build(Floor), Build(Walls), Build(Roof), …..

Decomposition methods (1)• Specify that a nonprimitive operator that unifies

with it can be decomposed into a plan

• Decompose(operation,p) is a new structure akin to a subroutine or a macro for operators:Decompose(Construction,

Plan(Steps:{S1: Build(Foundation), S2: Build(Frame), S3: Build(Roof), S4: Build(Walls), S5: Build(Interior)}

Orderings:{S1<S2; S2<S3; S2<S4; S3<S5 ; S4<S5, ..... }, Bindings: {},

Foundation Frame Frame Roof Walls

Links: {S1 --->S2 , S2 ---->S3, S2 --->S4, S3 --->S5, S4 --->S5 }))

Decomposition methods (2)• Plan p correctly implements an operator o if it

is a complete and consistent plan for the problem of achieving the effects of o given the preconditions of o:– p must be consistent (no ordering or assignment

contradiction).– every effect of o must be asserted by at least one

step of p and not denied by a later step.– every precondition of steps in p must be achieved

by a step in p or be one of the preconditions of o.

Hierarchical POP algorithm

function HD-POP(plan,operators,methods) returns plan

loop do if Solution?(plan) then return plan (S-need,c) := Select-Sub-Goal(plan) Choose-Operator(plan,operators,S-need,c) S-nonprimitive := Select-Nonprimitive(plan) Choose-Decomposition(plan,methods,S-nonprimitive) Resolve-Threats(plan) end

HD-POP subroutines• Solution? must check that every step of the plan

is primitive • Select-Nonprimitive arbitrarily selects a non-

primitive step of the plan (no backtracking)• Choose-Decomposition: when a method is

chosen:1. Steps: add all method steps, remove S-nonprimitive2. Bindings: add all bindings of method3. Orderings: place new constraints latest or earliest4. Links: explicitly add all links

Fail if 1 or 2 introduce a contradiction!

Detailed decomposition of a step

Analysis of hierarchical decomposition• HD helps prune branches in the search tree.

Two useful properties of solutions are: – Downward solution property: if p is an abstract

solution, then there is a primitive solution of which p is an abstraction.

Once an abstract solution is found, all other branches can be pruned!

– Upward solution property: if p is an inconsistent abstract plan, then there is no primitive solution of which it is an abstraction

Prune away all descendants of inconsistent plans!

Solution space properties

Bold boxes are solutionsDotted boxes are inconsistentBoxes marked with “X” can be pruned

abstract

primitive

Complexity of hierarchical decomp.• For a plan with n steps and an average of b

choices at each step (branching factor), the complexity of search is O(bn)

• Let d be the depth of the hierarchical plan, and s average number of decomposition steps. When only searching for abstract solutions, one of every b decompositions is a solution. If each decomposition has s steps, the planner looks at bsi steps at depth d =i. The complexity is O(bsd) << O(bn)

Quantitative example

abstract

primitive

Is completeness preserved?• The upward and downward solution properties

are not necessary correctness conditions for decompositions!

• To avoid loosing completeness, no pruning can take place -- still can be used to guide search

• There is an abstract solution that is inconsistent, but the decomposition solves the problem.

“a couple has two possesions: he a gold watch and her beautiful hair. They each plan to buy presents to make each other happy. He wants to trade the watch for a comb, she wants to trade her hair for a watch chain. Can they execute their plans?

Ex: no upward solution property

Cannot be ordered!

Solution: unique main subaction

• To guarantee the upward solution property, require that there is one step of the decomposed plan to which all preconditions and effects of the abstract operator are attached

• In the previous example, the unique main subaction condition does not hold!

Approximation• Another way of guiding the search is to rank

goals by order of importance (criticality level).

Op(Action: Buy(x)Effect: Have(x) /\ Have(MoneyAmount)

Precond: 1. Sells(store,x) /\ 2. At(store) /\ 3. Have(MoneyAmount)

• Solve the problem by considering ONLY preconditions with criticality less or equal than 1, than 2, etc.

Other extensions

• More expressive operator descriptions– conditional effects: add when conditions– universal quantification: preconditions with

“forall” quantifier

• Resource constraints: consider costs of each action -- leads to optimization problems

• Time constraints: can be handled as resources

Conditional effects• Previous scheme sometimes forces premature

commitment that can lead to inefficiencies• Solution: extend the operator language to include

conditional effects: “condition c must hold when p holds”. Such type of clauses will be added to the effects of an action.

• If later p appears, the condition c will be added and handled.

• Extend Select-SubGoal and Resolve-Threats to deal with this new type of conditionals

Conditionals: example• Two different actions for picking a block:

Op(Action: move(B,X,Y),

Precond: on(B,X) /\ clear(B) /\ clear(Y)

Effect: on(B,Y) /\ ~on(B,X) /\ clear(X) /\ clear(B) /\

~clear(Y))

Op(Action: movetotable(B,X),

Precond: on(B,X) /\ clear(B)

Effect: on(B,table) /\ ~on(B,X) /\ clear(X) /\ clear(B)

• Start: on(a,b)

• Goal: clear(b)

• Problem: two operators for the same type of action!

Conditionals: solution • One operator with conditional effect:

Op(Action: move(B,X,Y),Precond: on(B,X) /\ clear(X) /\ clear(Y)

Effect: on(B,Y) /\ ~on(B,X) /\ clear(X) /\ clear(B)

/\ (~clear(Y)) when Y table)

• When Y gets instantiated, the condition ~clear(Y) will be added if appropriate.

• To resolve threats: if a step has the effect (~c’ when p) is a possible threat to the causal link Si

-- c --> Sj when c’ and c unify. Resolve threat by

confrontation: ensuring that p does not hold.

Resolve-Threats with conditionalsprocedure Resolve-Threats(plan) for each (Si -- c --> Sj) in LINKS(plan) do for each S-threat in STEPS(plan) do for each c’ in EFFECTS(S-threat) do if Subst(Bindings(plan),c) = Subst(Bindings(plan),~c’)

then choose either

Promotion: add S-threat < Si to ORDERINGS(plan) Demotion: add Sj < S-threat to ORDERINGS(plan)

Confrontation: if c’ is of the form (c’ when p) thenChoose-Operator(plan,operators,S-threat,~p)Resolve-Threats(plan)

if not Consistent(plan) then fail end end end~

Negated and disjuctive preconditions• Choose-Operator introduced a negated literal• Can be handled by checking for effects that

match the goal, and ensure that unification between p and ~~p is possible. Also, must deal with special “closed-world assumption” requirements for start, where no negative literals are present.

• Disjunctive preconditions p \/ q introduce nondeterministic choices.

• Disjunctive effects are harder to deal with... Ex: Flip(coin)

Universal quantification (1)• Extend the language and algorithms to handle

general statements

• In preconditions: instead of clear(b), writeX block(X) => ~on(X,B) “no block is on top of b”

• In effects:Op(Action: carry(Bag,X,Y),

Precond: bag(Bag) /\ at(Bag,X), Effect: at(Bag,Y) /\ ~at(Bag,X) /\

I item(I) => (at(I,Y) /\ ~at(I,X)) when in(Y,Bag))

“all objects that are in a bag are in location Y after the bag has been carried from location X to location Y”

Universal quantification (2)

• Note that adding universal quantification does NOT turn the language into FOL. The restrictions are:– worlds with finite, static, typed universe– universally quantified conditions satisfied by simple

enumeration

X t(X) => c(X) is c(x1) when t(x1) /\

c(x2) when t(x2) /\

c(xn) when t(xn)

Universal quantification (3)• The planner must expand universally quantified

preconditions to eliminate the quantifier (possibly inefficient, but no better solution…)

• Universally quantified effects need not be expanded, since it might be that many literals are irrelevant. Instead, leave as is but make sure that Resolve-Threats and Choose-Operator are properly modified.

• Modified routines for POP-DUNC (POP with disjunction, universal quantification, negation, and conditionals). It is sound and complete.

Extended-POP Select-Subgoal

function Select-Subgoal(plan) returns (plan,precondition conjunct)

pick a plan step S-need from STEPS(plan) with a precondition c that has not been achieved if c is a universally quantified expression then

return (S-need, Expansion(c)) else if c is a disjunction c1 \/ c2 .. \/ cn then

return (S-need, choose(c1,c2,…,cn)) else returns (S-need,c)

Extended-POP Choose-Operatorprocedure Choose-Operator(plan,operators,S-need,c)

(STEPS(plan) that has c-add as an effect

such that u = Unify(c,c-add,Bindings(plan)) if there is no such step then fail u’ := u without universally quantified variables of c-add add u’ to Bindings(plan) add causal link (S-add -- c --> S-need) to LINKS(plan) add ordering constraint S-add < S-need to ORDERINGS(plan) if S-add is a newly added step from operators then

Extended-POP Resolve-Threatsprocedure Resolve-Threats(plan) for each (Si -- c --> Sj) in LINKS(plan) do for each S-threat in STEPS(plan) do for each c’ in EFFECTS(S-threat) do if Subst(Bindings(plan),c) = Subst(Bindings(plan),~c’)

then choose either

Promotion: add S-threat < Si to ORDERINGS(plan) Demotion: add Sj < S-threat to ORDERINGS(plan)

Confrontation: if c’ is of the form (c’ when p) thenChoose-Operator(plan,operators,S-threat,~p)Resolve-Threats(plan)

if not Consistent(plan) then fail end end end

Resource constraints• Need to deal with quantities: cost, time, etc.

• Ex: buying an object decreases the amount of cash we have:

• New construct: measures, which are global variables that can be compared and updated

• Check inequality constrains each time an operator is chosen ==> a CSP problem!

buy(X,Store)

have(X) /\ Cash := Cash - price(X,Store)

at(Store) /\ sells(Store,X) /\ Cash > price(X,Store)

Planning and acting• Up to now, we assumed that we first plan,

and then execute the plan. All the necessary knowledge is available to do the plan.

• Sometimes, we need gather additional information: to see if the bus station is open, we need to go there first!

• Conditional or contingency planning: generate plan alternatives that account for each possible outcome of a contingency. Include sensing operators (see Chapter 13).

Conditional plan for fixing flat tireFixing a flat tire by either inflating it or replacing withthe spare tire. Since we don’t know why the tire is flat, we need to generate two contingency plans

Planning in practice• Job shop scheduling: Plan

production and assembly schedule Hitachi’s Tosca: 350 products, 35 assembly machines, 2,000

operations. Plan 30-day schedule for three 8-hour shifts. Follows a partial-order, least commitment approach

• Space mission scheduling: plans order of experiments and resource use

used for Hubble space telescope, Voyager, etc.

• SIPE planner: planning maintenance and materials logistics military operations for US Air Force.

introduction to artificial intelligence lecture 13 : advanced planning

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