Pixhawk

The Pixhawk platform flies any drone running the PX4 autopilot, such as a Holybro PX4 Vision or an X500. It talks to PX4 through the uXRCE-DDS bridge, which puts PX4's own topics on ROS 2 as px4_msgs messages, so no MAVLink or MAVROS is involved. It builds on ROS 2 Humble and Jazzy.

PX4 keeps flying the drone and Aerostack2 sends it setpoints in offboard mode. The platform reads PX4's estimate of the drone's state and passes it on as odometry.

Install

The platform needs PX4's messages at the version of the firmware on the drone. The repository pins them to PX4 1.14 in its dependencies.repos, so import them along with the platform:

cd ~/aerostack2_ws/src
git clone https://github.com/aerostack2/as2_platform_pixhawk.git
vcs import < as2_platform_pixhawk/dependencies.repos
cd ~/aerostack2_ws
rosdep install --from-paths src --ignore-src -y
colcon build --symlink-install

If your drone runs a different PX4 release, check out the matching release/ branch of px4_msgs instead. Messages that don't match the firmware won't reach the drone.

You also need the Micro XRCE-DDS Agent, the program on the companion computer that PX4 connects to. It builds in the same workspace:

cd ~/aerostack2_ws/src
git clone https://github.com/eProsima/Micro-XRCE-DDS-Agent.git
cd ~/aerostack2_ws
colcon build --packages-select microxrcedds_agent

Set up PX4

1. Flash the PX4 firmware on the flight controller with QGroundControl. For the PX4 Vision, the build target is px4_fmu-v6c_default.
2. Set XRCE_DDS_0_CFG to the telemetry port the companion computer is wired to, for example TELEM 2.
3. Set that port's baud rate, for example 921600.

The PX4 offboard mode docs cover the failsafes PX4 applies when offboard setpoints stop.

Launch

Start the agent on the serial port PX4 is wired to, then the platform:

MicroXRCEAgent serial --dev /dev/ttyUSB0 -b 921600

ros2 launch as2_platform_pixhawk pixhawk_launch.py namespace:=drone0

The namespace defaults to the AEROSTACK2_SIMULATION_DRONE_ID environment variable, so pass it when that isn't set. Every parameter in the platform configuration file is also a launch argument.

Set max_thrust to your drone's real maximum thrust. Attitude and body rates commands are sent in newtons and scaled by it.

Control modes

PX4 has no hover mode here, so the motion controller holds the drone in place itself. Position and trajectory commands aren't accepted. Speed fits the PID speed controller, and body rates fit the differential flatness controller.

Topics

With the rest of the stack

PX4 already estimates the drone's state, so use the raw_odometry estimator plugin on its odometry. It reads sensor_measurements/odom by default:

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

When Aerostack2 knows more than PX4, for example from motion capture or a lidar odometry, set external_odom to true. The platform then sends the state estimate back to PX4 at 100 Hz as visual odometry, and PX4 fuses it once its estimator is configured for external vision.

Things to know

• Taking off and landing are done by Aerostack2's behaviors, not by PX4's own take-off and land modes.
• The platform can switch PX4 into offboard but never out of it. Leave offboard from the remote control. After that, the platform treats the pilot as in control.
• Arming and offboard are confirmed by PX4, so check platform/info rather than the service reply.
• The kill switch sends PX4 a manual kill switch command. Make sure the remote control can also kill the motors, since that's the one that always works.

The project_px4_vision repository has a complete setup for the PX4 Vision, both in PX4's simulator and on the real drone.