Fix issue #641

[caguero]

See issue #641.

The problem was our well known Gazebo "feature" that eliminates fixed joints when converting from URDF to SDF. Then, the pinger link disappeared. I converted the fixed joint into a revolute joint with no movement to bypass the problem.

How to test it?

• Launch the simulation as usual:

  ros2 launch vrx_gz competition.launch.py world:=sydney_regatta

• Look at the WAMV in the entity tree and verify that there is a "pinger" listed. If you select the pinger link, you should observe a tiny sphere between the two pontoons as I enabled a small visual for reference.

• Additionally, if you comment out the line 221 in wamv_gazebo.urdf.xacro:

  <xacro:wamv_pinger name="pinger" position="-528 191 -2.0" />

  You shouldn't see the pinger link in the WAMV entity tree and the functionality of the pinger won't be available (gz topic -l shouldn't report the topic /wamv/pingers/pinger/range_bearing).

[M1chaelM]

@caguero This seems to be working but there is a little quirk where even though gz topic -l doesn't show /wamv/pingers/ping/range_bearing, ros2 topic list still includes it:

/clock
/parameter_events
/rosout
/tf
/tf_static
/vrx/contacts
/vrx/debug/wind/direction
/vrx/debug/wind/speed
/vrx/task/info
/wamv/pingers/pinger/range_bearing
/wamv/pingers/pinger/set_pinger_position
/wamv/pose
...

If I try to echo it I get nothing, so maybe it's ok?

[caguero]

@caguero This seems to be working but there is a little quirk where even though gz topic -l doesn't show /wamv/pingers/ping/range_bearing, ros2 topic list still includes it

I just tweaked the code to not advertise the ROS 2 topic if the pinger isn't present. 6bfcbbc