PDDL2.1: An Extension to PDDL for Expressing Temporal Planning Domains

1
PDDL2.1 An Extension to PDDL for Expressing
Temporal Planning Domains
By Maria Long and Derek Long

• The Third International Planning Competition
  Temporal and Metric
• Planning

2
History

• First Planning Systems Competition was in 1998
• Goals were meaningful comparisons and an
  indication of overall progress in the field
• Committee was formed in 1997 to create rules for
  competition
• One of the results was the PDDL

3
Tracks

• Tracks were to allow different categories of
  planner
• Proposed tracks were STRIPS, ADL, Hierarchical
  Planning, Reactive Planning
• The STRIPS track and ADL track were chosen
• Organizers focused on ADL as popular track
• Two planners entered in ADL track
• Eight planners entered in STRIPS track

4
Effects of Planning Competition

• One of the most important contributions has been
  the introduction of a widely accepted standard
  for domain description, Drew McDermott's PDDL
  language, leading to the sharing of domains and
  planning systems
• Many worry that competitions are focusing
  research efforts too closely on winning for its
  own sake and non-real world domains

5
New PDDL

• PDDL is an extension of PDDL intended to support
  representation of deterministic planning domains
  involving time and other numeric-valued resources
• Levels 1 to 4 of PDDL are referred to as PDDL2.1

6
Level 1

• The original PDDLs propositional and ADL (Action
  Description Language) levels
• Nothing changed to ensure backward compatibility
  with previous standard

7
Level 1 Example (ADL)

• (define (domain logisticsadl)
• (requirements adl)
• (types physobj object
• obj vehicle physobj
• truck airplane vehicle
• location city object
• airport location)
• (predicates (at ?x physobj ?l location)
• (in ?x obj ?t vehicle)
• (incity ?l location ?c city)
• (loaded ?x physobj)) ?x is
  loaded on a vehicle
• (action load
• parameters (?obj ?veh ?loc)
• precondition (and (vehicle ?veh)
• (location ?loc)
• (at ?obj ?loc)
• (at ?veh ?loc)
• (not (loaded
  ?obj)))
• effect (and (in ?obj ?veh)

8
Level 1 Example (STRIPS)

• (define (domain logisticsstrips)
• (requirements strips)
• (predicates (obj ?obj)
• (truck ?truck)
• (location ?loc)
• (airplane ?airplane)
• (city ?city)
• (airport ?airport)
• (at ?obj ?loc)
• (in ?obj1 ?obj2)
• (incity ?obj ?city))
• (action loadtruck
• parameters (?obj ?truck ?loc)
• precondition (and (obj ?obj)
• 
  (truck ?truck)
• 
  (location ?loc)
• (at
  ?truck ?loc)
• (at
  ?obj ?loc))
• effect (and (not (at ?obj ?loc))

9
Level 2

• Adds numeric variables and the ability to test
  and update their values instantaneously

10
Level 2 Example

• (define (domain jug-pouring)
• (requirements typing fluents)
• (types jug)
• (functions
• (capacity ?j - jug)
• (amount ?j - jug))
• (action pour
• parameters (?jug1 ?jug2 - jug)
• precondtion (gt (capacity ?jug2) ( (amount
  ?jug1) (amount ?jug2)))
• effect (and (assign (amount ?jug1) 0)
• (increases (amount ?jug2)
  (amount ?jug1)))
• )

11
Level 3 and 4

• 3 and 4 add durative actions actions the
  effects of which occur some time after the
  instant of their execution
• Level 3 durative actions discretize time
• Level 4 durative actions can have continuous
  effects

12
Level 3 Example

• (durative-action fly
• parameters (?p - plane ?t traveller
• ?a ?b - location)
• duration ( ?duration (flight-time ?a ?b))
• condition (and (at start (at ?p ?a)) (at start
  (at ?t ?a))
• (over all (aboard ?t
  ?p)) (over all (inflight ?p))
• (at start (gt
  (fuel-level ?p)
• ( (flight-time ?a
  ?b) (consumption-rate ?p)))))
• effect (and (at start (aboard ?t ?p))
• (at start (not (at ?t ?a)))
• (at start (not (at ?p ?a)))
• (at start (inflight ?p))
• (at end (at ?p ?b))
• (at end (at ?t ?b))
• (at end (not (inflight ?p)))
  
• (at end (not (aboard ?t
  ?p)))
• (at end (decrease
  (fuel-level ?p)
• ( (flight-time ?a ?b)
  (consumption-rate ?p))))
• )

13
Level 4 Example

• To express the fact that the fuel level of a
  plane, ?p, decreases continuously, as a function
  of the consumption rate of ?p, we write
• (decrease (fuel-level ?p) ( t (consumption-rate
  ?p)))
• This is importantly different from
• (at end (decrease (fuel-level ?p)
• ( (flight-time ?a ?b) (consumption-rate
  ?p))))
• because the latter is a single update happening
  at the end point of the action, whilst the
  former allows the correct calculation of the fuel
  level of the plane at any point in that interval.

14
Level 5

• Adds capability to represent arbitrary real-time,
  mixed/continuous domains
• Not used in planning competitions
• To encourage debate and widen horizons of the
  planning community to include domains with mixed
  discrete and continuous behavior

15
Other additions

• Plan Metric field in problem specification
• New domains (the Satellite, Mars Rover, Relief
  Mission) designed to be more application focus
• New domain, DriverLog, the classic Logistics
  domain with the need to allocate drivers.
  Modified for increased difficulty.