[ignition::gazebo::systems::MecanumDrive] No cmd_vel gazebo topic is generated by the mecanum drive example

[TZECHIN6]

Environment

• OS Version: Ubuntu 22.04.5 LTS
• Source or binary build? binary, Ignition Fortress, Gazebo Sim 6.16.0
• ROS Version: Humble (ROS2)

thomasluk@inmsm-fractal-1:~$ ign gazebo --version
Gazebo Sim, version 6.16.0
Copyright (C) 2018 Open Source Robotics Foundation.
Released under the Apache 2.0 License.

thomasluk@inmsm-fractal-1:~$ ign gazebo --version
Gazebo Sim, version 6.16.0
Copyright (C) 2018 Open Source Robotics Foundation.
Released under the Apache 2.0 License.

Description

• Expected behavior:
  What I am trying to do is to create a mecanum robot simulation and integrate with ros2 Nav2 stack. Currently is building a sim robot in gazebo ignition. Reason why I am using new gazebo is that first the old one is EoL, second is that Mecanum Drive is available in the new gazebo, not the old one.
  I have used the example from here. And also refereed to this API call document. I expect it will just work like the turtlebot3 diff drive example, with the "tf", "odom" and most importantly "cmd_vel" available in gazebo side.

• Actual behavior:
  However, after launching the example sdf, the topic only gives "tf" and "odom", but not the "cmd_vel". Even through the sdf file do not come with topic tag, according to the API, its optional, and by default it will generate a topic with suffix ../../cmd_vel.

Steps to reproduce

1. create a mecanum_drive.sdf exactly same to the reference example
2. launch it by this cmd: ign gazebo mecanum_drive.sdf
3. The simulation started, and check the topic by ign topic -l
4. No any "cmd_vel" related topic is given by gazebo

Output

Terminal ouput:

thomasluk@inmsm-fractal-1:~$ ign topic -l
/clock
/gazebo/resource_paths
/gui/camera/pose
/model/vehicle_blue/odometry
/model/vehicle_blue/tf
/stats
/world/mecanum_drive/clock
/world/mecanum_drive/dynamic_pose/info
/world/mecanum_drive/pose/info
/world/mecanum_drive/scene/deletion
/world/mecanum_drive/scene/info
/world/mecanum_drive/state
/world/mecanum_drive/stats

[TZECHIN6]

actually the topic can receive cmd... there are also few issue showing this issue (not showing the cmd_vel, but actually working).

Hopefully this will be solved, or mentioned in the document.