Unable to spawn xacro model on gazebo, what's the problem?

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Hi everybody,

I am trying to spawn a urdf.xacro model on gazebo for two weeks, but something is going wrong and I do not know what.

The model is perfectly working in Rviz as well as the "revolute" joints. Moreover, when I launch the model the output (below) doesn't show any error.

In the beginning, I thought it was a problem related to the syntax of my xacro model, but if I simplify the model to a few pieces it doesn't work anyway. Then, I tried to launch the xacro model in the tutorials (r2d2 tutorial) and it worked. Doing some experiments I noticed that if I remove one of the two legs of the robot the model spawn anyway, while if I remove both of the r2d2 legs (line 120, 121) the model doesn't spawn anymore, what's the meaning of this?

If you want to give a look at the code:

launch file

xacro model I'm trying to spawn

package.xml

If you want to give it a try just run this script (you need the environment installed): script.sh

I am using ROS melodic and Gazebo 9.0 and Ubuntu 18.04 bionic

output:

   when processing file: /home/[user]/catkin_ws/src/cricket_robot/cricket_description/urdf/fake_cricket_robot.urdf.xacro
xacro.py is deprecated; please use xacro instead
started roslaunch server http://[user]:36843/

SUMMARY
========

PARAMETERS
 * /gazebo/enable_ros_network: True
 * /robot_description: <?xml version="1....
 * /robot_state_publisher/publish_frequency: 30.0
 * /rosdistro: melodic
 * /rosversion: 1.14.10
 * /use_sim_time: True

NODES
  /
    gazebo (gazebo_ros/gzserver)
    gazebo_gui (gazebo_ros/gzclient)
    robot_state_publisher (robot_state_publisher/robot_state_publisher)
    urdf_spawner (gazebo_ros/spawn_model)

auto-starting new master
process[master]: started with pid [14571]
ROS_MASTER_URI=http://localhost:11311

setting /run_id to 627e6c46-9b57-11eb-854c-fcf8aece3038
process[rosout-1]: started with pid [14715]
started core service [/rosout]
process[gazebo-2]: started with pid [14718]
process[gazebo_gui-3]: started with pid [14729]
process[urdf_spawner-4]: started with pid [14734]
process[robot_state_publisher-5]: started with pid [14735]
[ INFO] [1618208520.194157846]: Finished loading Gazebo ROS API Plugin.
[ INFO] [1618208520.196876588]: waitForService: Service [/gazebo_gui/set_physics_properties] has not been advertised, waiting...
[ INFO] [1618208520.247758337]: Finished loading Gazebo ROS API Plugin.
[ INFO] [1618208520.249484144]: waitForService: Service [/gazebo/set_physics_properties] has not been advertised, waiting...
[ INFO] [1618208520.861148814]: waitForService: Service [/gazebo/set_physics_properties] is now available.
[ INFO] [1618208520.904280111, 0.030000000]: Physics dynamic reconfigure ready.
[INFO] [1618208521.021574, 0.000000]: Loading model XML from ros parameter robot_description
[INFO] [1618208521.036579, 0.000000]: Waiting for service /gazebo/spawn_urdf_model
[INFO] [1618208521.039967, 0.000000]: Calling service /gazebo/spawn_urdf_model
[INFO] [1618208521.311782, 0.317000]: Spawn status: SpawnModel: Successfully spawned entity
[INFO] [1618208521.313328, 0.317000]: Unpausing physics
[ INFO] [1618208521.406433616, 0.317000000]: Loading gazebo_ros_control plugin
[ INFO] [1618208521.406681629, 0.317000000]: Starting gazebo_ros_control plugin in namespace: /
[ INFO] [1618208521.408502441, 0.317000000]: gazebo_ros_control plugin is waiting for model URDF in parameter [robot_description] on the ROS param server.
[ INFO] [1618208521.554816939, 0.317000000]: Loaded gazebo_ros_control.
[urdf_spawner-4] process has finished cleanly
log file: /home/[user]/.ros/log/627e6c46-9b57-11eb-854c-fcf8aece3038/urdf_spawner-4*.log

the log file at the end shows no differences when gazebo spawns the model and when ... (more)