Enable setting default position of the simulated robot using ros2_control URDF tag.

[destogl]

Adds support for initial robot's position through ros2_control URDF tag.
This is also compatible with "FakeHardware" and can be used within the driver if needed.

Related PRs are:

• Adding support for 'initial_value' parameter. ros2_control#593
• Support of "initial_value" for the 'FakeSystem' ros2_control#598

The example joint definition looks like (example from UR driver):

     <joint name="${prefix}shoulder_pan_joint">
        <command_interface name="position"/>
        <state_interface name="position">
          <param name="initial_value">${initial_positions['shoulder_pan_joint']}</param>  <!-- initial position for Simulators and FakeHardware -->
        </state_interface>
        <state_interface name="velocity"/>
        <state_interface name="effort"/>
      </joint>

This feature is already released in ros2_control, but not synced yet.

I am not sure about one thing: Do we need a separate example for this?

[bmagyar]

Sounds like a good idea and would save some headache with artificial homing, etc.

What do you think if all the defaults were collected under a separate tag within the gazebo component?

hand-wavy example

<system name="gazebosystem">
  <initial_state>
    <joint name="shoulder_pan_joint" position="1.0 "/>
    <joint name="shoulder_tilt_joint" position="0.5 "/>
    <joint name="gripper_joint" position="1.0" effort="0.0 "/>
  </initial_state>
</system>

alternatively, extending your example

     <joint name="${prefix}shoulder_pan_joint">
        <command_interface name="position"/>
        <state_interface name="position" initial_value="${initial_positions['shoulder_pan_joint']}"/>
        <state_interface name="velocity"/>
        <state_interface name="effort"/>
      </joint>

[destogl]

I am more on the second approach. Because then we can reuse this in other cases like FakeSystem. Moreover, the information is available for hardware drivers if needed. (I could imagine it in some cases at least.)

[destogl]

@ahcorde I removed the new files as discussed. Can you please do one more check? After Friday's sync this should be buildable.

[ahcorde]

@destogl can you merge with master?

[destogl]

@destogl can you merge with master?

@ahcorde I just rebased it. can you please enable workflows again.

[ahcorde]

Just one last thing @destogl , linters! https://github.com/ros-simulation/gazebo_ros2_control/runs/4966401765?check_suite_focus=true

[destogl]

@ahcorde done