MAVLink

Fly autopilots over MAVLink through MAVROS, for when PX4's DDS bridge isn't an option.

The MAVLink platform talks to the autopilot over MAVLink through MAVROS, which turns MAVLink messages into ROS 2 topics and services. It builds on ROS 2 Humble and Jazzy.

It's written for PX4. It switches the autopilot to PX4's OFFBOARD mode and loads MAVROS's PX4 configuration, so ArduPilot, which uses GUIDED mode instead, isn't supported. For PX4 1.14 or newer, the Pixhawk platform talks to PX4 directly and is the usual choice. This one fits when MAVLink is what you already have, for example an older PX4 or a telemetry radio.

Install

Install MAVROS and the GeographicLib datasets it needs, then build the platform:

sudo apt install ros-$ROS_DISTRO-mavros ros-$ROS_DISTRO-mavros-extras
wget https://raw.githubusercontent.com/mavlink/mavros/ros2/mavros/scripts/install_geographiclib_datasets.sh
sudo bash install_geographiclib_datasets.sh

cd ~/aerostack2_ws/src
git clone https://github.com/aerostack2/as2_platform_mavlink.git
cd ~/aerostack2_ws
rosdep install --from-paths src --ignore-src -y
colcon build --symlink-install

Launch

Start MAVROS and the platform in the same namespace:

ros2 launch as2_platform_mavlink mavros_launch.py namespace:=drone0
ros2 launch as2_platform_mavlink as2_platform_mavlink_launch.py namespace:=drone0

The MAVROS configuration file says where each drone's autopilot is, keyed by namespace. The default one points drone0 to drone3 at PX4's simulator ports. For a real drone, pass your own file with the autopilot's address, such as a serial port:

config/mavros_config.yaml

drone0:
  mavros/mavros_node:
    ros__parameters:
      fcu_url: "/dev/ttyACM0:921600"
      tgt_system: 1

ros2 launch as2_platform_mavlink mavros_launch.py namespace:=drone0 \
  mavros_config_file:=config/mavros_config.yaml

The platform configuration file has the usual rates and the drone's thrust:

Parameter	Default	What it does
`cmd_freq`	100.0	How often commands are sent, in Hz
`info_freq`	10.0	How often `platform/info` is published, in Hz
`max_thrust`	15.0	The thrust in newtons that maps to full throttle
`min_thrust`	0.15	The lowest throttle sent, as a fraction
`external_odom`	false	Send Aerostack2's state estimate to the autopilot as vision input

Control modes

Control mode	Yaw
Position	Angle
Speed	Rate
Attitude	Angle
Body rates	Rate

There's no hover mode, so the motion controller holds the drone in place itself. Speed fits the PID speed controller, and body rates fit the differential flatness controller.

Topics

Topic	Contents
`sensor_measurements/odom`	The autopilot's state estimate, in the drone's `odom` frame
`sensor_measurements/imu`	The IMU, published by MAVROS
`sensor_measurements/gps`	GPS fixes, published by MAVROS
`sensor_measurements/battery`	The battery state, published by MAVROS
`platform/info`	The platform status

With the rest of the stack

Use the raw_odometry estimator plugin on the autopilot's odometry:

config/config.yaml

/**:
  state_estimator:
    ros__parameters:
      plugin_name: "raw_odometry"

When Aerostack2 knows more than the autopilot, set external_odom to true. The platform then sends the state estimate to MAVROS's vision pose and speed topics at 100 Hz, for the autopilot to fuse.

Things to know

Taking off and landing are done by Aerostack2's behaviors, not by the autopilot's own modes.
Switching to offboard also arms the drone if it isn't armed yet.
The platform never leaves offboard. Leave it from the remote control.
The kill switch doesn't work on this platform yet. Keep the remote control's kill switch at hand.