As described in the Creating your own Gazebo ROS Package, it is common in ROS to create a package that contains all the world files and launch files used with Gazebo. These files are located in a ROS package named /MYROBOT_gazebo. For DiffBot the package is named diffbot_gazebo. Another example that follows best pratices is rrbot which can be found in the gazebo_ros_demos repository.
fjp@ubuntu:~/git/diffbot/ros/src$ catkin create pkg diffbot_gazebo
Creating package "diffbot_gazebo" in "/home/fjp/git/diffbot/ros/src"...Created file diffbot_gazebo/package.xmlCreated file diffbot_gazebo/CMakeLists.txtSuccessfully created package files in /home/fjp/git/diffbot/ros/src/diffbot_gazebo.
The diffbot_gazebo package contains a launch file to lauch a world in Gazebo and spawn the robot model,
which is defined in the previously created diffbot_description package.
For the launch files the convention is to have a folder named launch and for Gazebo world files a folder named world inside a package.
fjp@ubuntu:~/git/diffbot/ros/src/diffbot_gazebo$ mkdir launch world
Inside the launch folder is the diffbot.launch.
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<launch><!-- these are the arguments you can pass this launch file, for example paused:=true --><argname="paused"default="false"/><argname="use_sim_time"default="true"/><argname="gui"default="true"/><argname="headless"default="false"/><argname="debug"default="false"/><!-- We resume the logic in empty_world.launch, changing only the name of the world to be launched --><includefile="$(find gazebo_ros)/launch/empty_world.launch"><argname="world_name"value="$(find diffbot_gazebo)/worlds/diffbot.world"/><argname="debug"value="$(arg debug)"/><argname="gui"value="$(arg gui)"/><argname="paused"value="$(arg paused)"/><argname="use_sim_time"value="$(arg use_sim_time)"/><argname="headless"value="$(arg headless)"/></include></launch>
In the world folder of the diffbot_gazebo package is the diffbot.world file:
roslaunch diffbot_gazebo diffbot.launch ... logging to /home/fjp/.ros/log/6be90ef2-fdd8-11ea-9cb3-317fd602d5f2/roslaunch-tensorbook-393333.logChecking log directory for disk usage. This may take a while.Press Ctrl-C to interruptDone checking log file disk usage. Usage is <1GB.started roslaunch server http://tensorbook:32837/SUMMARY========PARAMETERS * /gazebo/enable_ros_network: True * /rosdistro: noetic * /rosversion: 1.15.8 * /use_sim_time: TrueNODES / gazebo (gazebo_ros/gzserver) gazebo_gui (gazebo_ros/gzclient)ROS_MASTER_URI=http://localhost:11311process[gazebo-1]: started with pid process[gazebo_gui-2]: started with pid [ INFO] [1600950165.494721382]: Finished loading Gazebo ROS API Plugin.[ INFO] [1600950165.495515766]: waitForService: Service [/gazebo/set_physics_properties] has not been advertised, waiting...[ INFO] [1600950165.649461740]: Finished loading Gazebo ROS API Plugin.[ INFO] [1600950165.650277038]: waitForService: Service [/gazebo_gui/set_physics_properties] has not been advertised, waiting...[ INFO] [1600950166.640929113]: waitForService: Service [/gazebo/set_physics_properties] is now available.[ INFO] [1600950166.659917502, 0.007000000]: Physics dynamic reconfigure ready.
Also, the Gazebo simulator will open a new window with the objects defined in diffbot.world except for the Gas station because it is a model
that has to be downloaded first, which is happening in the background. TODO: gas station is not showing this way.
To get the Gas station or other available models it is possible to clone the gazebo models repository into your /home/your_username/.gazebo folder, e.g.:
/home/fjp/.gazeb$ git clone osrf/gazebo_models
Then add this path inside Gazebo to insert these models into your world file.
Using ROS launch to Spawn URDF Robots
According to the Gazebo roslaunch tutorial the recommended way
to spawn a robot into Gazebo is to use a launch file. Therefore, edit the diffbot.launch inside the diffbot_gazebo package by adding the following inside the <launch> </launch tag:
<!-- Load the URDF into the ROS Parameter Server --> <param name="robot_description" command="$(find xacro)/xacro --inorder '$(find diffbot_description)/urdf/diffbot.xacro'" /> <!-- Run a python script to the send a service call to gazebo_ros to spawn a URDF robot --> <node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model" respawn="false" output="screen" args="-urdf -model diffbot -param robot_description"/>
To add sensors to a robot model make use of link and joint tags to define the desired location and shape, possibly using meshes.
For the simulation of these sensor there exist common Gazebo plugins that can be used. See, the [Tutorial: Using Gazebo plugins with ROS]
for existing plugins and more details how to use them. For a full list of plugins see also gazebo_ros_pkgs which is a package or interface
for using ROS with the Gazebo simulator.
In case the output looks like the following, there are most certainly missing <inertial> tags in the <link> tag.
For the Gazebo simulator the <inertial> must be present, in order to simulate the dynamics of the robot.
See also http://wiki.ros.org/urdf/XML/link and the Gazebo tutorials on URDF.