ROS 2 Lyrical Luth · Lesson 61 of 86

Global planners: NavFn and Smac

Choosing a global planner, Dijkstra/A*, hybrid-A* and lattice planners.

Estimated time · 60–90 min

Learning goals

What you will understand

Explain global planners: navfn and smac in your own words
Use it in a small robotics example
Verify the result instead of assuming it worked

Understand the idea first

The mental model

The global planner computes a collision-free path from the robot to the goal on the global costmap. NavFn uses Dijkstra or A* on a grid and is simple and fast. Smac has three planners: Smac 2D (grid A*), Smac Hybrid-A* (respects a minimum turning radius, for car-like robots) and Smac Lattice (any motion primitive set).

Differential-drive robots usually start with NavFn or Smac 2D; Ackermann or large robots need Hybrid-A*. Planner plugin names and parameters differ between releases, so read the Nav2 documentation for your distribution. Plans are computed on demand, not continuously.

Guided lesson

Work through these steps

1.
Source the ROS 2 Lyrical environment in a fresh terminal.
2.
Read the global planners: navfn and smac example and identify what each line does.
3.
Build only the affected package, source the workspace overlay, and run the example.
4.
Inspect the result from a second terminal with the ros2 CLI or RViz2 and compare it with the expected result.

Worked example

Read it before you run it

planner_server:
# Node section.
  ros__parameters:
# ROS parameters.
    planner_plugins: ["GridBased"]
# Names of the planners to load.
    GridBased:
# Settings for that planner.
      plugin: "nav2_navfn_planner::NavfnPlanner"
# Use NavFn (check the exact plugin string for your release).
      tolerance: 0.5
# Accept a goal within 0.5 m if the exact one is blocked.
      use_astar: true
# Use A* instead of Dijkstra for speed.

What the example is doing

1.The ros2 commands query the running graph; they do not prove that a node's internal behavior is correct.
2.Build from the workspace root, then source install/setup.bash so the shell can discover the new package.
3.Use a second sourced terminal to inspect nodes and interfaces while the example runs.

Watch it run · Mobile robot

See what this code does

The terminal types this lesson's practice solution and runs it. The scene shows the kind of robot behaviour that code drives. It plays by itself and loops.

Practice code

Now type it yourself

Copy the starter into your ROS 2 package, replace every TODO, then build with colcon build and run it from a sourced terminal.

planner_server:
  ros__parameters:
    planner_plugins: ["GridBased"]
    GridBased:
# TODO: use the NavFn planner plugin with tolerance 0.5 and A* enabled

Expected output

The planner server loads the GridBased plugin

Stuck? Show the full solution (try it yourself first)

planner_server:
  ros__parameters:
    planner_plugins: ["GridBased"]
    GridBased:
      plugin: "nav2_navfn_planner::NavfnPlanner"
      tolerance: 0.5
      use_astar: true

Expected result

How you know it worked

ROS_DISTRO is lyrical in every terminal used for the exercise.
The complete package from the exercise builds and the expected graph entity appears in a second terminal.

Practical exercise

Apply the lesson

Plan the same route with NavFn and Smac 2D in a simulated warehouse, compare path length, smoothness and computation time from the planner's logs, and explain which suits your robot.

Before you continue

ROS_DISTRO reports lyrical in every terminal you used
The example builds and runs without errors
You checked the result with a ros2 command, RViz2 or a test, not just by reading the code

If something goes wrong

ros2 cannot find a package or executable.

Source /opt/ros/lyrical/setup.bash, build from the workspace root, then source install/setup.bash in that terminal.

Nodes run but cannot discover each other.

Confirm both terminals use Lyrical, the same ROS_DOMAIN_ID, and compatible network/discovery settings.