State interface `velocity` is wrong with `position` command interfaces

[christophfroehlich]

State interface velocity is wrong with position command interfaces. Sometimes the value remains zero, sometimes it is a random value.

I guess that the physics engine just does not report a valid velocity, and we should warn users with this configuration, manually set the velocity zero, or even disallow it.

Steps to reproduce

Run a position-only demo

ros2 launch gazebo_ros2_control_demos cart_example_position.launch.py

launch a GUI, e.g, plotjuggler with this layout gazebo.pj.zip

ros2 run plotjuggler plotjuggler -l gazebo.xml

And send a command

ros2 topic pub --once /joint_trajectory_controller/joint_trajectory trajectory_msgs/msg/JointTrajectory "{joint_names: ['slider_to_cart'], points: [{positions: [-0.1], time_from_start: {sec: 5, nanosec: 0}}]}"

Versions

gazebo11 + tested on humble as well as rolling built from source.