How do I use the velocity controllers with joint command interfaces in the gz simulator?

[SpaceSoarer90]

More specifically, I noticed in auv_control_demos the chained_controller example has multiple joints named the "Cartesian Joints" that seem to be where the state interface and command interfaces are set up in the <ros2_control> tag in the example's corresponding ros2_control.xacro file.

I attempted to create my own .xacro model with this kind of setup, but more complete (such that it includes the collisions, inertials, visuals, as well as the thrusters). Adding the cartesian_joints as the demos have suggested. However, it seems that the gz simulator does not support multiple joints connecting the same links to one another.

here's the specific error and warning that the simulator spits out when i attempt to do the cartesian joints:

zebo-1] Warning [parser_urdf.cc:2774] Error Code 19: Msg: urdf2sdf: link[base_link] has no <inertial> block defined. Please ensure this link has a valid mass to prevent any missing links or joints in the resulting SDF model.
[gazebo-1] Error Code 19: Msg: urdf2sdf: allowing joint lumping by removing any <disableFixedJointLumping> or <preserveFixedJoint> gazebo tag on fixed child joint[main_body_joint], as well as ensuring that ParserConfig::URDFPreserveFixedJoint is false, could help resolve this warning. See http://sdformat.org/tutorials?tut=sdformat_urdf_extensions for more information about this behavior.
[gazebo-1] Error Code 19: Msg: urdf2sdf: [2] child joints ignored.
[gazebo-1] Error Code 19: Msg: urdf2sdf: [2] child links ignored.
[gazebo-1] Warning [parser_urdf.cc:2777] Error Code 19: Msg: urdf2sdf: link[base_link] is not modeled in sdf.[Err] [UserCommands.cc:1152] Error Code 17: Msg: A model must have at least one link.
[gazebo-1] [Err] [UserCommands.cc:1152] Error Code 17: Msg: A model must have at least one link.
[gazebo-1] [Err] [UserCommands.cc:1152] Error Code 25: Msg: FrameAttachedToGraph error: scope context name[] does not match __model__ or world.

I'm not sure how I'll be able to pass these commands without defining a joint, since command interfaces in ros2_control seem to always require a joint to be parented to. But if there's another way of passing the commands please let me know!

Oh, I almost forgot, how I structured the controllers is:
[ISMC] -> {cartesian_joints command interfaces} -> [TAMC] -> {thrusters vector} -> [Gazebo Passthrough Controller] -> {gz cmd_thrust topic}

[SpaceSoarer90]

is it possible to just send a command interface of all the efforts on each cartesian axes through just one single joint instead? so instead of doing

  <ros2_control name="demo_hardware" type="system">
    
     ...

    <joint name="x">
      <state_interface name="velocity">
        <param name="initial_value">0.0</param>
      </state_interface>
      <command_interface name="effort" />
    </joint>

    <joint nae="y">
      <state_interface name="velocity">
        <param name="initial_value">0.0</param>
      </state_interface>
      <command_interface name="effort" />
    </joint>

    ...
    
  </ros2_control>

it would be

  <ros2_control name="demo_hardware" type="system">
    
     ...

    <joint name="base_link_main_body_joint">
      <state_interface name="velocity_x">
        <param name="initial_value">0.0</param>
      </state_interface>
      <command_interface name="effort_x" />
      <state_interface name="velocity_y">
        <param name="initial_value">0.0</param>
      </state_interface>
      <command_interface name="effort_y" />
    </joint>

    ...
    
  </ros2_control>

or is there an underlying reason behind why it was structured that way?
whatever the reason, please let me know too!

[evan-palmer]

I'll caveat this message by saying that it has been a while since I made that design decision, so there may be something that has changed since I implemented that or something that I missed... from what I remember, ros2_control only loads one command interface per joint, so you need to define multiple joints.

To help you get started, I would recommend going through the UVMS control demo that I recently wrote here. The demo has been tested and works. It uses the architecture that you described above with a complete setup that you can reference (you can ignore the whole body control)

[evan-palmer]

Regardless, with the control architecture that you described in your first message, you shouldn't need to define those joints in your URDF. The TAMC defines those command interfaces for you in "chained mode". The only extra joints that you should need to add are the thruster joints. The examples folder that I linked in the previous post should provide some complete examples demonstrating how to setup your system.

[SpaceSoarer90]

okay! that's actually really helpful, i'll look more into how the TAMC handles "chained mode".

thanks for clearing it up!