Multi-Vehicle Simulation

This tutorial explains how to simulate multiple UAV vehicles using Gazebo and SITL.

It demonstrates an example setup that opens the Gazebo client GUI showing two Iris vehicles in an empty world. You can then control the vehicles with QGroundControl and MAVROS in a similar way to how you would manage a single vehicle.

This is tested only in Linux.

Required

• ROS indigo or higher
• MAVROS package
• Gazebo 7 (see Gazebo Simulation)
• a clone of latest PX4/Firmware

Build and test

To build an example setup, follow the step below:

1. Clone the PX4/Firmware code, then build the SITL code

   cd Firmware_clone
   git submodule update --init --recursive
   make posix_sitl_default
   make posix_sitl_default sitl_gazebo
   

2. Source your environment:

   source Tools/setup_gazebo.bash $(pwd) $(pwd)/build/posix_sitl_default
   export ROS_PACKAGE_PATH=$ROS_PACKAGE_PATH:$(pwd):$(pwd)/Tools/sitl_gazebo
   

3. Run launch file:

   roslaunch px4 multi_uav_mavros_sitl.launch
   

   You can specify gui:=false in the above roslaunch to launch Gazebo without its UI.

The tutorial example opens the Gazebo client GUI showing two Iris vehicles in an empty world.

You can control the vehicles with QGroundControl or MAVROS in a similar way to how you would manage a single vehicle:

• QGroundControl will have a drop-down to select the vehicle that is "in focus"
• MAVROS requires that you include the proper namespace before the topic/service path (e.g. for <group ns="uav1"> you'll use /uav1/mavros/mission/push).

What's happening?

For each simulated vehicle, the following is required:

• Gazebo model: This is defined as xacro file in Firmware/Tools/sitl_gazebo/models/rotors_description/urdf/<model>_base.xacro see here. Currently, the model xacro file is assumed to end with base.xacro. This model should have an argument called mavlink_udp_port which defines the UDP port on which gazebo will communicate with PX4 node. The model's xacro file will be used to generate an urdf model that contains UDP port that you select. To define the UDP port, set the mavlink_udp_port in the launch file for each vehicle, see here as an example.

  If you are using the same vehicle model, you don't need a separate xacro file for each vehicle. The same xacro file is adequate.

• PX4 node: This is the SITL PX4 app. It communicates with the simulator, Gazebo, through the same UDP port defined in the Gazebo vehicle model, i.e. mavlink_udp_port. To set the UDP port on the PX4 SITL app side, you need to set the SITL_UDP_PRT parameter in the startup file to match the mavlink_udp_port discussed previously, see here. The path of the startup file in the launch file is generated based on the vehicle and ID arguments, see here. The MAV_SYS_ID for each vehicle in the startup file, see here, should match the ID for that vehicle in the launch file here. This will help make sure you keep the configurations consistent between the launch file and the startup file.

• MAVROS node (optional): A seperate MAVROS node can be run in the launch file, see here, in order to connect to PX4 SITL app, if you want to control your vehicle through ROS. You need to start a MAVLink stream on a unique set of ports in the startup file, see here. Those unique set of ports need to match those in the launch file for the MAVROS node, see here.

The launch file multi_uav_mavros_sitl.launchdoes the following,

• loads a world in gazebo,

    <!-- Gazebo sim -->
    <include file="$(find gazebo_ros)/launch/empty_world.launch">
        <arg name="gui" value="$(arg gui)"/>
        <arg name="world_name" value="$(arg world)"/>
        <arg name="debug" value="$(arg debug)"/>
        <arg name="verbose" value="$(arg verbose)"/>
        <arg name="paused" value="$(arg paused)"/>
    </include>
  

• for each vehicle,

  • creates urdf model from xacro, loads gazebo model and runs PX4 SITL app instance

      <!-- PX4 SITL and vehicle spawn -->
      <include file="$(find px4)/launch/single_vehicle_spawn.launch">
          <arg name="x" value="0"/>
          <arg name="y" value="0"/>
          <arg name="z" value="0"/>
          <arg name="R" value="0"/>
          <arg name="P" value="0"/>
          <arg name="Y" value="0"/>
          <arg name="vehicle" value="$(arg vehicle)"/>
          <arg name="rcS" value="$(find px4)/posix-configs/SITL/init/$(arg est)/$(arg vehicle)_$(arg ID)"/>
          <arg name="mavlink_udp_port" value="14560"/>
          <arg name="ID" value="$(arg ID)"/>
      </include>
    

  • runs a mavros node

      <!-- MAVROS -->
      <include file="$(find mavros)/launch/px4.launch">
          <arg name="fcu_url" value="$(arg fcu_url)"/>
          <arg name="gcs_url" value=""/>
          <arg name="tgt_system" value="$(arg ID)"/>
          <arg name="tgt_component" value="1"/>
      </include>
    

  The complete block for each vehicle is enclosed in a set of <group> tags to separate the ROS namespaces of the vehicles.

To add a third iris to this simulation there are two main components to consider:

• add UAV3 to multi_uav_mavros_sitl.launch
  • duplicate the group of either existing vehicle (UAV1 or UAV2)
  • increment the ID arg to 3
  • select a different port for mavlink_udp_port arg for communication with Gazebo
  • selects ports for MAVROS communication by modifying both port numbers in the fcu_url arg

• create a startup file, and change the file as follows:

  • make a copy of an existing iris rcS startup file (iris_1 or iris_2) and rename it iris_3
  • MAV_SYS_ID value to 3
  • SITL_UDP_PRT value to match that of the mavlink_udp_port launch file arg
  • the first mavlink start port and the mavlink stream port values to the same values, which is to be used for QGC communication

  • the second mavlink start ports need to match those used in the launch file fcu_url arg

    Be aware of which port is src and dst for the different endpoints.

Additional Resources

• See Simulation for a description of the UDP port configuration.
• See URDF in Gazebo for more information about spawning the model with xacro.
• See RotorS for more xacro models.