Simulation Launch File

The launch file will need two different Nodes in order to work.

Converting Robot Description to MJCF

MuJoCo, as mentioned before, cannot work with ROS2 topics directly. Instead, a conversion script creates a MJCF, which MuJoCo can interact with. This conversion script converts the URDF (or /robot_description topic) into an MJCF (or /mujoco_robot_description topic), and optionally combines it with a given scene. Below is an example:

mujoco_robot_description = Node(
  package="mujoco_ros2_control",
  executable="robot_description_to_mjcf.sh",
  output="both",
  arguments=[
    "--add_free_joint",
    "--scene", PathJoinSubstitution([FindPackageShare("sim"), "worlds", "my_mujoco_scene"]),
    "--publish_topic", "/mujoco_robot_description",
  ],
)

Here, we call the robot_description_to_mjcf.sh script, and give it some optional arguments:

  • --add_free_joint allows the robot to move independently from the rest of the MuJoCo scene. Otherwise, the robot will be stuck in place.
  • --scene appends a MuJoCo scene to the generated MJCF, allowing to test in different environments (such as the competition arenas).
  • --publish_topic will publish the MJCF to a topic with a given name (default is /mujoco_robot_description). Otherwise, the conversion script places the MJCF into a static file.

If a different topic is used, the <param name="mujoco_model_topic> tag will need to be updated in the MuJoCo ROS2 Control hardware interface.

All of these arguments are technically optional, but for a live session with a moveable robot, they are needed.

This script is also responsible for going through the optional mujoco_inputs.xacro file to add on the raw_inputs at the end of the MJCF, and substitutes parameters from the processed_inputs.

For more information, see the URDF to MJCF Conversion documentation.

Start the ROS2 Control Node

Once conversion completes, we can start the MuJoCo ROS2 Control Node. Below is an example:

control_node = Node(
  package="mujoco_ros2_control",
  executable="ros2_control_node",
  output="both",
  parameters=[
    {"use_sim_time": True},
    ParameterFile(PathJoinSubstitution([FindPackageShare("bringup"), "config", "controllers.yaml"])),
    ParameterFile(PathJoinSubstitution([FindPackageShare("sim"), "config", "mujoco_plugins.yaml"])),
  ],
)

Here, we execute the ros2_control_node from the mujoco_ros2_control package — in place of the normal one — with some arguments:

  • use_sim_time is set to True.
  • Specify the controllers.yaml file.
  • Optionally specify the mujoco_plugins.yaml file, which loads in any defined plugins.

This Node is also responsible for starting up the MuJoCo GUI application, once everything starts up and configures properly.

Now, if everything is configured properly thus far, it should be possible to launch the simulation file and see MuJoCo running!

Author: Aiden Kimmerling (https://github.com/TheKing349)