ROS 2 Lyrical Luth · Lesson 33 of 86

Simulated differential drive

The diff-drive system plugin, cmd_vel and odometry in simulation.

Estimated time · 60–90 min

Learning goals

What you will understand

Explain simulated differential drive 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

Gazebo has a DiffDrive system plugin that turns a Twist on cmd_vel into wheel motion and publishes odometry and TF. Configure the wheel joints, wheel separation and radius. Bridge cmd_vel (ROS→Gazebo) and odom (Gazebo→ROS).

The numbers in the plugin must match your URDF, otherwise the simulated robot will not travel the distance you command. Test by driving 1 m at 0.2 m/s for 5 s and comparing odometry with the ruler in the world.

Guided lesson

Work through these steps

1.
Source the ROS 2 Lyrical environment in a fresh terminal.
2.
Read the simulated differential drive 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

<plugin filename="gz-sim-diff-drive-system" name="gz::sim::systems::DiffDrive">
<!-- Gazebo's built-in differential drive. -->
  <left_joint>left_joint</left_joint>
<!-- Joint that drives the left wheel. -->
  <right_joint>right_joint</right_joint>
<!-- Joint that drives the right wheel. -->
  <wheel_separation>0.30</wheel_separation>
<!-- Distance between wheels in metres. -->
  <wheel_radius>0.05</wheel_radius>
<!-- Wheel radius in metres. -->
  <topic>cmd_vel</topic>
<!-- Gazebo topic for commands. -->
</plugin>
<!-- End of the plugin. -->

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.

# TODO: bridge /cmd_vel from ROS 2 (geometry_msgs/msg/Twist) into Gazebo (gz.msgs.Twist) using the ] direction
ros2 topic pub -r 10 /cmd_vel geometry_msgs/msg/Twist "{linear: {x: 0.2}}"

Expected output

The robot moves forward about 1 metre

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

ros2 run ros_gz_bridge parameter_bridge /cmd_vel@geometry_msgs/msg/Twist]gz.msgs.Twist
ros2 topic pub -r 10 /cmd_vel geometry_msgs/msg/Twist "{linear: {x: 0.2}}"

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

Add the DiffDrive plugin, bridge /cmd_vel and /odom, drive at 0.2 m/s for 5 s, and compare the odometry position with the true position in Gazebo. Explain any difference.

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.