Multi-Vehicle Sim

This topic explains how to simulate multiple UAV vehicles using Gazebo Classic and SITL (Linux only). A different approach is used for simulation with and without ROS.

Multiple Vehicle with Gazebo Classic

To simulate multiple iris or plane vehicles in Gazebo Classic use the following commands in the terminal (from the root of the Firmware tree):

Tools/simulation/gazebo-classic/sitl_multiple_run.sh [-m <model>] [-n <number_of_vehicles>] [-w <world>] [-s <script>] [-t <target>] [-l <label>]

<model>: The vehicle type/model to spawn, e.g.: iris (default), plane, standard_vtol, rover, r1_rover typhoon_h480.
<number_of_vehicles>: The number of vehicles to spawn. Default is 3. Maximum is 254.
<world>: The world that the vehicle should be spawned into, e.g.: empty (default)
<script>: Spawn multiple vehicles of different types (overriding the values in -m and -n). For example:
```
-s "iris:3,plane:2,standard_vtol:3"
```
- Supported vehicle types are: iris, plane, standard_vtol, rover, r1_rover typhoon_h480.
- The number after the colon indicates the number of vehicles (of that type) to spawn.
- Maximum number of vehicles is 254.
<target>: build target, e.g: px4_sitl_default (default), px4_sitl_nolockstep
<label> : specific label for model, e.g: rplidar

Each vehicle instance is allocated a unique MAVLink system id (2, 3, 4, etc.). MAVLink system id 1 is skipped in order to have consistency among namespaces. Vehicle instances are accessed from sequentially allocated PX4 remote UDP ports: 14541 - 14548 (additional instances are all accessed using the same remote UDP port: 14549).

:::note The 254-vehicle limitation occurs because mavlink MAV_SYS_ID only supports 255 vehicles in the same network (and the first one is skipped). The MAV_SYS_ID is allocated in the SITL rcS: init.d-posix/rcS :::

Build and Test (XRCE-DDS)

Tools/simulation/gazebo-classic/sitl_multiple_run.sh can be used to simulate multiple vehicles connected via XRCE-DDS in Gazebo Classic.

:::note You will need to have installed the XRCE-DDS dependencies. For more information see: ROS 2 User Guide (PX4-ROS 2 Bridge), for interfacing with ROS 2 nodes. :::

To build an example setup, follow the steps below:

Clone the PX4/Firmware code, then build the SITL code:

cd Firmware_clone
git submodule update --init --recursive
DONT_RUN=1 make px4_sitl gazebo-classic

Build the micro xrce-dds agent and the interface package following the instructions here.
Run Tools/simulation/gazebo-classic/sitl_multiple_run.sh. For example, to spawn 4 vehicles, run:
```
./Tools/simulation/gazebo-classic/sitl_multiple_run.sh -m iris -n 4
```
:::note Each vehicle instance is allocated a unique MAVLink system id (2, 3, 4, etc.). MAVLink system id 1 is skipped. :::
Run MicroXRCEAgent. It will automatically connect to all four vehicles:
```
MicroXRCEAgent udp4 -p 8888
```
:::note The simulator startup script automatically assigns a unique namespace to each vehicle. :::

Multiple Vehicles with MAVROS and Gazebo Classic

This example demonstrates a setup that opens the Gazebo Classic 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.

Required

Current PX4 ROS/Gazebo development environment
:::note At time of writing this is Ubuntu 18.04 with ROS Melodic/Gazebo 9. See also Gazebo Classic Simulation. :::
MAVROS package
a clone of latest PX4/PX4-Autopilot

Build and Test

To build an example setup, follow the step below:

Clone the PX4/PX4-Autopilot code, then build the SITL code

cd Firmware_clone
git submodule update --init --recursive
DONT_RUN=1 make px4_sitl_default gazebo-classic

Source your environment:

source Tools/simulation/gazebo-classic/setup_gazebo.bash $(pwd) $(pwd)/build/px4_sitl_default
export ROS_PACKAGE_PATH=$ROS_PACKAGE_PATH:$(pwd):$(pwd)/Tools/simulation/gazebo-classic/sitl_gazebo

Run launch file:
```
roslaunch px4 multi_uav_mavros_sitl.launch
```
:::note You can specify gui:=false in the above roslaunch to launch Gazebo Classic without its UI. :::

The tutorial example opens the Gazebo Classic 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 Classic model: This is defined as xacro file in PX4-Autopilot/Tools/simulation/gazebo-classic/sitl_gazebo-classic/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 Classic 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.
:::note 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 Classic, through the same UDP port defined in the Gazebo Classic 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 separate 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 Classic,

  <!-- 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 classic 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_tcp_port" value="4560"/>
      <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>

:::note 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 Classic
- 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
  :::note Be aware of which port is src and dst for the different endpoints. :::

Multiple Vehicles using SDF Models

This section shows how developers can simulate multiple vehicles using vehicle models defined in Gazebo Classic SDF files (instead of using models defined in the ROS Xacro file, as discussed in the rest of this topic).

The steps are:

Install xmlstarlet from your Linux terminal:
```
sudo apt install xmlstarlet
```

Use roslaunch with the multi_uav_mavros_sitl_sdf.launch launch file:

roslaunch multi_uav_mavros_sitl_sdf.launch vehicle:=<model_file_name>
```

:::note
Note that the vehicle model file name argument is optional (`vehicle:=<model_file_name>`); if omitted the [plane model](https://github.com/PX4/PX4-SITL_gazebo/tree/master/models/plane) will be used by default.
:::

This method is similar to using the xacro except that the SITL/Gazebo Classic port number is automatically inserted by xmstarlet for each spawned vehicle, and does not need to be specified in the SDF file.

To add a new vehicle, you need to make sure the model can be found (in order to spawn it in Gazebo Classic), and PX4 needs to have an appropriate corresponding startup script.

You can choose to do either of:
- modify the single_vehicle_spawn_sdf.launch file to point to the location of your model by changing the line below to point to your model:
  $(find px4)/Tools/simulation/gazebo/sitl_gazebo-classic/models/$(arg vehicle)/$(arg vehicle).sdf
  :::note Ensure you set the vehicle argument even if you hardcode the path to your model. :::
- copy your model into the folder indicated above (following the same path convention).
The vehicle argument is used to set the PX4_SIM_MODEL environment variable, which is used by the default rcS (startup script) to find the corresponding startup settings file for the model. Within PX4 these startup files can be found in the PX4-Autopilot/ROMFS/px4fmu_common/init.d-posix/ directory. For example, here is the plane model's startup script. For this to work, the PX4 node in the launch file is passed arguments that specify the rcS file (etc/init.d/rcS) and the location of the rootfs etc directory ($(find px4)/build_px4_sitl_default/etc). For simplicity, it is suggested that the startup file for the model be placed alongside PX4's in PX4-Autopilot/ROMFS/px4fmu_common/init.d-posix/.

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.

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