ros2_control SimpleTransmission implementation

[AlePuglisi]

Hello!
I'm working on ros2 Humble, Ubuntu 22.04, Gazebo classic 11.0.
I'm investigating how SimpleTransmission is implemented in ros2_control, because I need it for a bigger project.
( In the big project I work on a 4-limbed robot with 7 DOF per limb, and I want to control each joint using the /effort_controller/commands topic, created by the effort_controllers/JointGroupEffortController. All the control input works, but I want to simulate the internal gearbox of the motors, which I need for independent joint control. Because having a high gear ratio allows for disturbance reduction in the closed-loop motor control)

To understand it, I wrote a simplified system composed of just a box and cylinder connected by a revolute joint.
I want to make it possible to properly simulate a simple gearbox transmission ratio, as the one implemented by a Dynamixel Motor.
Here is the simple_system_ros2_control.urdf:

  <hardware>
    <plugin>gazebo_ros2_control/GazeboSystem</plugin>
  </hardware>

  <joint name="motor_joint">
    <command_interface name="position">
      <param name="min">-10</param>
      <param name="max">10</param>
    </command_interface>
    <command_interface name="velocity">
      <param name="min">-3</param>
      <param name="max">3</param>
    </command_interface>
    <command_interface name="effort">
      <param name="min">-10</param>
      <param name="max">10</param>
    </command_interface>
    <state_interface name="position">  <!-- Set initial joint position -->
      <param name="initial_value"> 0.0 </param>
    </state_interface>
    <state_interface name="velocity"/>
    <state_interface name="effort"/>

    <!-- TENTATIVE N.1-->
    <!-- <transmission name="motor_joint_transmission">
      <plugin>transmission_interface/SimpleTransmission</plugin>
      <param name="joint_to_actuator">250.0.0</param>
    </transmission>
     -->
    
  </joint>

  <!-- TENTATIVE N.2-->
  <!-- <transmission name="motor_joint_transmission">
    <plugin>transmission_interface/SimpleTransmission</plugin>
    <joint name="motor_joint" role="motor_joint">
      <mechanical_reduction>250.0</mechanical_reduction>
    </joint>
  </transmission> -->


  <!-- TENTATIVE N.3 -->
  <!-- <transmission name="motor_joint_transmission">
    <type>transmission_interface/SimpleTransmission</type>
      <joint name="motor_joint">
        <hardwareInterface>EffortJointInterface</hardwareInterface>
      </joint>
      <actuator name="motor_joint_motor">
        <mechanicalReduction>250.0</mechanicalReduction>
        <hardwareInterface>EffortJointInterface</hardwareInterface>
      </actuator>
  </transmission> -->


</ros2_control>

-------------------------------------------------------------------------------------------------- I commented out the different tentative here, none of them seems to work. Tentatives N.1 and N.2 are completely ignored by the parser I think. In fact I also tried to set a reduction ratio of 0.0 and no ERROR was reported... even if as I saw from simple_transmission.hpp there is a proper handling of 0 reduction ratio setting. While tentative N.3 give me the error: [ERROR] [gzserver-1]: process has died [pid 41504, exit code 255, cmd 'gzserver -slibgazebo_ros_init.so -slibgazebo_ros_factory.so -slibgazebo_ros_force_system.so']. ( so maybe is a bad plugin import...)

Someone has some clear example code on how to use simple transmission that works properly??
Also, if there is an already written plug-in to create a topic that publish the internal motor angle and torque?
Ideally I want to work at the motor level.. But I don't know how to do it in an intuitive way without implementing low level plug-ins on my own.
Anyway, for now my main issue is related to a working implementation of Transmission interface for gearbox reduction ratio simulation in Gazebo.

Thanks all!