How to define transmissions with ros2 control

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From my current understanding, if you want to use ROS2 Control with gazebo, you have to use one ros2_control tag with type systemfor all joints. (See https://github.com/ros-simulation/gaz...)

My question now: How do you define your transmissions?

I found this components_architecture_and_urdf_examples, but I can't find anything there.

Robot Description: I have 3 wheels that control the movement of the robot. Each of them has its own actuator. All of them have the same transmission ratio.

I would like to define three SimpleTansmissions, but how do I specify the corresponding joints? All examples I found define the transmission inside of <ros2_control name="..." type="actuator"> and not <ros2_control name="..." type="system">.

The only system transmission I could find, looks like this:

<transmission name="transmission1">
  <plugin>transmission_interface/SomeComplex2by2Transmission</plugin>
  <param name="joints">{joint1, joint2}</param>
  <param name="output">{output2, output2}</param>
  <param name="joint1_output1">1.5</param>
  <param name="joint1_output2">3.2</param>
  <param name="joint2_output1">3.1</param>
  <param name="joint2_output2">1.4</param>
</transmission>

And as far as I know, this is only a prototype. There is no SomeComplex2by2Transmission plugin.

Currently, I see two possible solutions:

1. It is somehow possible to use type="actuator" with gazebo_ros2_control.
2. There is some way to define simple transmissions in a system.
3. I have to write my own transmission plugin.

I would prefer the first option, because this is way easier to maintain with xacro.

Some links:

• ROS2 Control Documentation: https://ros-controls.github.io/contro...
• ROS2 Control Demo: https://github.com/ros-controls/ros2_...
• Gazebo ROS2 Control Plugin: https://github.com/ros-simulation/gaz...

My current ros2_control tag:

  <!-- ROS Control for the rims -->
  <ros2_control name="cassiopeia_rim_control" type="system">
    <xacro:if value="$(arg use_sim)">
      <hardware>
        <plugin>gazebo_ros2_control/GazeboSystem</plugin>
      </hardware>
    </xacro:if>
    <xacro:unless value="$(arg use_sim)">
      <hardware>
        <xacro:if value="$(arg use_fake_hardware)">
          <plugin>fake_components/GenericSystem</plugin>
          <param name="fake_sensor_commands">$(arg fake_sensor_commands)</param>
          <param name="state_following_offset">0.0</param>
        </xacro:if>
        <xacro:unless value="$(arg use_fake_hardware)">
          <!-- TODO: If we want this feature, we have to write an
                     hardware controller plugin in C++ for ROS2 Control.
            -->
        </xacro:unless>
      </hardware>
    </xacro:unless>

    <joint name="a_rim_joint">
      <command_interface name="velocity">
        <param name="min">-1</param>
        <param name="max">1</param>
      </command_interface>
      <state_interface name="position"/>
      <state_interface name="velocity"/>
    </joint>
    <joint name="b_rim_joint">
      <command_interface name="velocity">
        <param name="min">-1</param>
        <param name="max">1</param>
      </command_interface>
      <state_interface name="position"/>
      <state_interface name="velocity"/>
    </joint>
    <joint name="c_rim_joint">
      <command_interface name="velocity">
        <param name="min">-1</param>
        <param name="max">1</param>
      </command_interface>
      <state_interface name="position"/>
      <state_interface name="velocity"/>
    </joint>
  </ros2_control>

  <gazebo>
    <plugin filename="libgazebo_ros2_control.so" name="gazebo_ros2_control">
      <parameters>$(find pkg_name)/config/controllers.yaml</parameters>
    </plugin>
  </gazebo>