plansys2_pddlplus_example

A robot looks after a greenhouse. It is the example with the most of PDDL in it: PDDL+ (time, processes, events), PDDL 2.1 continuous effects, PDDL3 constraints and preferences, and the ADL features, each one with a part to play. PlanSys2 plans it with Kairos and executes it to the end, and the controller checks, in the final state, that each part did what it does.

ros2 launch plansys2_pddlplus_example plansys2_pddlplus_example.launch.yaml
ros2 run plansys2_pddlplus_example greenhouse_controller_node

The task

The rover starts at the dock with little battery. It has to tend the orchid and the fern in the nursery, fertilize the orchid, fill the cistern and open the window before the nursery gets too hot. The feeder it needs to fertilize is delivered at the dock after a while, and the nursery has no power until the rover plugs the cable in the corridor. It should come back to the dock, if it is worth it.

Kairos plans it in about 100 ms. Its plan waits for the feeder while it recharges, plugs the cable, and opens the valve and the window with the nursery at about 32 degrees; it gives up the preference rather than add a trip back.

How PlanSys2 executes it

• The problem expert runs the dynamics: each step, the processes whose precondition holds and the continuous effects of the running actions act for the time since the last one, and then the events whose precondition holds happen. It applies the timed literals and fluents at their time, and keeps the derived predicates up to date. A function that perception measures goes in its measured_functions parameter: the dynamics leave it to perception.
• With processes, events, continuous effects or constraints with times, the executor starts each action at its planned time, holds a durative one until its planned duration, and ends the plan at the end Kairos gives it, when the goal must hold. An action that starts late shifts the rest of the plan, so the planned intervals hold.
• The executor follows the hard constraints while the plan runs: if one is violated, the plan fails and says which one. Preferences count only for the planner.
• The controller loads watering.pddl each time it runs, once PlanSys2 is up: the nursery warms up from the moment the problem is there.
• timed_action_node performs any action: it lasts what the plan gives it (get_planned_duration()). With fail_once_at, its first execution fails at that fraction of its duration.
• A plan that fails is made again from where the world is: the executor stops the continuous effects of what was running and takes back what had started, and the controller plans from there, up to three times. With the first recharge failing,

  ros2 launch plansys2_pddlplus_example plansys2_pddlplus_example.launch.yaml recharge_fails_at:=0.3
  

the second plan goes on with the charge it got, if it is enough. A plan replaced while it runs keeps the running actions it still has: their continuous effects go on, for the time of the new plan, which their performers are told (get_planned_duration() changes).

colcon test --packages-select plansys2_pddlplus_example runs the whole example, and again with the failing recharge, and checks the goal is reached and every part did its job.

Changelog for package plansys2_pddlplus_example

Forthcoming

• PDDL+ example: a greenhouse watered by a robot, with continuous effects, processes and events