Freyja

High-level flight stack for precise multirotor control, designed and used extensively in the Nimbus Lab.

Freyja bundles ROS packages that implement fast and accurate state estimators, an optimal feedback controller (LQR/LQG), and generalized interfaces for common autopilots that make it suitable for several precise and aggressive flight manuevers. The implementation uses standard control-systems style and terminology, and follows a conventional ROS architecture, thereby making it easy to substitute custom controllers, observers and communication interfaces. Freyja can remain oblivious to the specific type of multirotor as long as the onboard autopilot can stabilize its attitude accurately, and can exploit differentially-flat trajectory planning methods to support feed-forward elements in the controller. Agile and precise flights can be performed outdoors, even under high wind conditions, when bias compensation and RTK GPS integration is enabled.

Overview

Freyja is a collection of three primary ROS packages:

• state_manager : interfaces several input data sources (motion capture, gps, camera estimate) to produce one state_vector [denoted x in typical control schemes]. Some parts of state_vector are calculated (such as velocity), and some are merged from different callbacks. Also implements a collection of commonly used filters (Gaussian, LWMA, Median ..)
• lqg_control : the core controller node that takes x from state_manager, a reference state x_r, and calculates the optimal control required to regulate x to x_r. Implements a standard LQR controller and a full state observer (Kalman) to function as a Linear Quadratic Gaussian (LQG) controller.
• autopilot_handler : contains communication interfaces to autopilots. Currently supported autopilots are Pixhawk (with ArduCopter stack, px4 experimental) and Ascending Technologies.

The trajectory reference, x_r, can be smooth parametric curves, piecewise continuous paths, or be produced by more advanced trajectory planners. The optional waypoint_manager node provides a convenience handler for providing discrete waypoints to the system, which are automatically converted to smooth trajectory references (using linear or time-optimal paths). It is possible to directly publish discrete x_r references; however, the behavior then is not guaranteed to be smooth.

Build

Clone the repository into the src/ directory of your ROS workspace. Other packages in your project (such as camera processing, machine learning, trajectory generators) will sit adjacent to Freyja/. A recommended directory structure would look like so:

- my_awesome_flight_project/
  | -- build/
  | -- devel/
  | -- src/
       | -- Freyja/
       | -- my_trajectory_provider/
       | -- my_image_classifier/

Run catkin_make or catkin build inside the project level directory (my_awesome_flight_project/ in the example above). Freyja will be compiled alongside your other packages, and the build products are located within the common build/ and devel/ directories.

The default communication interface (for ArduPilot/px4) has a dependency on mavros (and packages it depends on), all of which you must install (see Wiki for how-to). If you'd like to circumvent this dependency (a custom interface, different autopilot etc), you can pass -DNO_PIXHAWK as an argument to catkin_make. Note that this needs you to provide some means of communicating the Freyja-generated [roll,pitch,yaw,thrust] commands (in respective physical SI units) to an autopilot.

Run

Freyja includes a freyja_controller.launch file that spawns the constituent nodes, and accepts commonly used parameters as launch arguments. The simplest way to launch the full system is:

$> roslaunch src/Freyja/freyja_controller.launch total_mass:=1.2 start_rosbag:=true

Here, total_mass and start_rosbag are two of the launch arguments; others may be provided as needed. When flying indoors, vicon_topic might be a commonly used and required argument.

For bigger projects that involve more packages, prepare your own launch file, and include Freyja's launch file in it.

<launch>
  <!-- launch Freyja with custom arguments -->
  <include file="src/Freyja/freyja_controller.launch" >
    <arg name="total_mass"                value="1.2" />
    <arg name="vicon_topic"               value="/vicon/FLH1/FLH1" />
    <arg name="start_rosbag"              value="false" />
    <arg name="thrust_scaler"             value="100" />
    <arg name="use_waypoint_handler"      value="true" />
    <!-- see launch file for more available arguments -->
  </include>
  
  <node name="my_traj" pkg="my_trajectory_provider" type="my_glamorous_trajectory" />
  :
  :
</launch>  

In the example above, the launch argument use_waypoint_handler is set to true for providing discrete waypoints (and not reference states). See the launch file for more available arguments.

Wiki

Check out Freyja's wiki to get more details on everything, from a basic multirotor build, to advanced configuration and set up, all the way to how trajectories can be written.

License, Credits and Usage

Freyja is developed in the Nimbus Lab, with rigorous testing under a multitude of operational scenarios and is used internally for various projects. Generous thanks to members who have tested and reported bugs, issues and feature limitations. The software is public under the GNU GPLv3 license. Please use wisely, and recommend improvements!

If you use Freyja for your work, please cite the ICRA 2021 paper that describes it in full detail:

inproceedings{shankar2021freyja,
  title={Freyja: A full multirotor system for agile \& precise outdoor flights},
  author={Shankar, Ajay and Elbaum, Sebastian and Detweiler, Carrick},
  booktitle={2021 IEEE International Conference on Robotics and Automation (ICRA)},
  pages={217--223},
  year={2021},
  organization={IEEE}
}

"Freyja: A Full Multirotor System for Agile & Precise Outdoor Flights",
A. Shankar, S. Elbaum, C. Detweiler;
IEEE International Conference on Robotics and Automation (ICRA), 2021.