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Leader-Follower task

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Leader-follower operation without communication

In this task, we will explore a state-of-the-art system for coordinated motion of multiple robots without direct communication. The system consists of two parts - blinking UV markers, which are fitted to each robot, and onboard cameras capable of UV-only sensing. Using this unique approach, the robots are capable of determining the relative position of their neighbors without the need to exchange any information via a communication link. This allows the UAVs to operate in environments with dense electromagnetic interference or coordinate motion of multiple robots with mutually incompatible localization systems (e.g. GPS and SLAM).

Installation

  1. The installation requires the open-source MRS UAV System. Please follow the installation instructions and install the stable full version. The full version also includes a Gazebo simulation environment which you will use. We highly recommend the native installation.

  2. Install uvdar_core and uvdar_gazebo_plugin. With the native installation, it can be added from CTU-MRS PPA:

sudo apt install ros-noetic-uvdar-core ros-noetic-uvdar-gazebo-plugin

otherwise the repos can be cloned and built in a catkin workspace.

  1. After installing the system, clone this repository and its submodules:
cd ~/git
git clone https://github.com/ctu-mrs/leader_follower_task.git
  1. [FOR NATIVE ROS INSTALLATION]: Setup a local catkin workspace (e.g. in a folder named workspace in your home)
cd ~/
mkdir -p workspace/src
cd workspace/src
ln -sf ~/git/leader_follower_task .
cd ~/workspace
catkin init
catkin config --extend /opt/ros/noetic
  1. Build the catkin workspace
cd ~/workspace
catkin build

Task overview

You are given a system consisting of two autonomous unmanned aerial vehicles (UAVs). Both vehicles are equipped with the MRS control pipeline and the UVDAR mutual localization system.

One of the UAVs is designated as a formation leader. It will follow a smooth, pre-defined trajectory. This UAV is out of your control and you should not interfere with the onboard control software.

The second UAV is designated as a follower. This is the vehicle, that you will be directly interacting with.

Your task is to develop a control routine for the follower such that it keeps up with the leader for as long as possible. We have prepared some C++ code to get you started. Your entry point to the system is the src/follower.cpp file. It creates a library, which is periodically used to generate new control commands for the follower UAV. The file already contains the following methods:

  • initialize - called only once on startup. This method is suitable for loading parameters from a configuration file. This way, you can tweak the parameters without needing to compile the package with every change. The default configuration file is config/follower.yaml.
  • dynamicReconfigureCallback - use is optional, recommended for fine tuning of system parameters on the fly. Allows you to tweak system parameters while the program is running using the rqt_reconfigure. The default dynamic configuration file is cfg/Follower.cfg
  • receiveOdometry - called every time a new odometry message is received. This will provide you with the latest information on the state of the follower UAV (position, orientation, velocity)
  • receiveTrackerOutput - called every time a new message is received. This will provide you with the actual state of the UAV, which already compensates for external disturbances. Use this instead of odometry, if you want to control the UAV relative to its actual position.
  • receiveUvdar - called every time a new UVDAR message is received. This will provide you the estimate of position of the leader UAV, based on the markers visible by the onboard cameras.
  • createReferencePoint - called periodically by the supervisor node. You are expected to provide the onboard controller a new reference point through this method.
  • createReferenceTrajectory - called periodically by the supervisor node. You are expected to provide the onboard controller a series of reference points through this method.
  • createVelocityReference - called periodically by the supervisor node. Alternative to the reference point commands. Allows swarm-like control by using velocity commands instead of position references.
  • getCurrentEstimate - uses a simple Kalman filter to estimate the position and velocity of the leader UAV. Filtering the raw UVDAR data will allow you to smooth out the control commands. Aggressive manoeuvres generate tilt, which may result in loss of visual contact with the leader.

These methods are periodically called by the supervisor. This is a ROS node running onboard the follower UAV. Do not modify the supervisor node! Your supervisor node will not be used for evaluation!

Simulation and a Reference solution

The provided code also includes a very crude solution. This solution will take the latest known leader pose, add a desired offset (loaded from a config file), and set it as the reference point for the follower. You may try running this code by opening the tmux_scripts/two_drones_uvdar folder and running the script:

./start.sh

Upon running the script, you should see 3 windows opening up:

  • Gazebo - realistic simulator with 3D graphics visualization
  • RVIZ - lightweight visualization tool for graphic representation of ROS messages
  • UVDAR visualization - view of the two cameras onboard the follower UAV

How to confirm everything works

  • In the Gazebo GUI, check that the time at the bottom of the window is running.
  • After a few seconds, you should see two drones. The drones will take off automatically.
  • After the takeoff, you should see two windows with the view of the UVDAR cameras. The view will be mostly black, but you may see small markers in one of the windows which indicate the UV markers on the leader drone.

Starting the task

You may notice, that your terminal opened multiple tabs. Consult the first page of the MRS Cheatsheet if you need help navigating the tabs and panes. After the UAVs take off, your input has to be provided manually.

  • Navigate to the goto_start tab, hit up arrow key and then enter. There is a pre-filled series of commands, which will load the Leader trajectory and take both UAVs to their starting position.
  • Switch to the follower tab, hit the up arrow and then enter. The pre-filled command will launch the supervisor. This is the program, which will periodically call your code.
  • Finally, switch to the start_challenge tab, hit the up arrow and pres enter. The leader will begin tracking the trajectory, and the score counter will start.
  • Now, you can navigate back to the follower tab, and observe the simulation. Once the visual contact is broken, the process in the follower tab will print out the score.

How to close the simulation

  • Do not close the terminal window, the processes may remain running in the background.
  • Use the following key shorcut: press ctrl + a, then release and then press k. A popup window will ask you to confirm. Navigate to it and press Enter or hit 9 to confirm. This will close all the terminal tabs, which have been opened by the simulator
  • We strongly recommend using the key shortcut to kill every simulation, to prevent zombie processes ruining your future simulations.

Implementation tips

You may notice that the reference solution does not produce a smooth control input for the follower UAV. Also, the following may break after a while. The jumps in follower motion are caused by multiple contributing factors:

  • The camera as well as the target do not stay completely still, even during hovering.
  • Aggressive control manoeuvres will also move the camera more, and make it harder to estimate the leader position accurately.
  • Setting a single reference point to the controller will cause the UAV to decelerate and stop moving once the destination has been reached.

Let's improve the follower code

There are a few steps that may help you. It is not necessary to follow them. You may skip this section completely and craft a solution on your own.

  • Experiment with the desired offset. You may get better results just by adjusting the distance between the leader and the follower. You can use the config/follower.yaml if you want to change the settings without the need for compilation. (You will still need to restart the node.)
  • Experiment with the control action rate, also adjustable in the config file.
  • Estimate the leader's velocity. An estimator based on the Kalman filter is already provided in the code, but in the default solution it is unused. Filter parameters may be loaded from the config file as well.
  • You can give the follower three types of control commands: ReferencePoint (this is done in the default solution), ReferenceTrajectory and VelocityReference. Inside each command, there is a boolean variable called use_for_control. If you set this value to true, the supervisor node will attempt to use this command for control.
  • If multiple commands are available at the same time, only the highest ranking command in the following hierarchy will be used: VelocityReference > ReferenceTrajectory > ReferencePoint.
  • In computer vision, distance to an object tends to be more difficult to estimate than its bearing. Therefore, the position tracking works much better if the line of sight from camera is directly perpendicular to the direction of leader's motion. If the leader moves along the line of sight from the camera, retrieving the position becomes much more difficult.
  • You are not required to perfectly copy the leader's trajectory. You only have to track the leader for as long as possible.

Things to avoid

  • Planned collisions. The UAVs are configured to automatically avoid collisions. The avoidance is done by moving the follower into a higher altitude. This mechanism will be triggered once the distance between the vehicles is less than 3 m. During this time, all of your commands will be ignored. Triggering the collision avoidance will not result in a direct score penalty, however, the leader may evade you in the meantime.
  • Height changes. The leader will always fly at a height of 3 m. Control commands with height below 2 m and above 4 m will be discarded by the follower.
  • Erratic position changes. Position reference, which is over 15 m apart from the current UAV position will be discarded.
  • Pushing physical limits of the UAV. Velocity command larger than 5 m/s will be discarded.
  • Violating these restrictions 10 times in a row will result in challenge termination.

References

Walter V., Staub N., Franchi A., and Saska M., "UVDAR System for Visual Relative Localization With Application to Leader–Follower Formations of Multirotor UAVs", IEEE Robotics and Automation Letters 4(3):2637-2644, July 2019. 
@article{uvdar_dirfol,
  author = "V. {Walter} and N. {Staub} and A. {Franchi} and M. {Saska}",
  journal = "IEEE Robotics and Automation Letters",
  title = "UVDAR System for Visual Relative Localization With Application to Leader–Follower Formations of Multirotor UAVs",
  year = 2019,
  volume = 4,
  number = 3,
  pages = "2637-2644",
  doi = "10.1109/LRA.2019.2901683",
  issn = "2377-3766",
  month = "July",
  pdf = "data/papers/walterRAL2019.pdf"
}
Baca, T., Petrlik, M., Vrba, M., Spurny, V., Penicka, R., Hert, D., and Saska, M., "The MRS UAV System: Pushing the Frontiers of Reproducible Research, Real-world Deployment, and Education with Autonomous Unmanned Aerial Vehicles", Journal of Intelligent & Robotic Systems 102, 26 (2021). 
@article{baca2021mrs,
  author = "Baca, Tomas and Petrlik, Matej and Vrba, Matous and Spurny, Vojtech and Penicka, Robert and Hert, Daniel and Saska, Martin",
  doi = "10.1007/s10846-021-01383-5",
  issue = 1,
  journal = "Journal of Intelligent {\&} Robotic Systems",
  month = "May",
  number = 26,
  pages = "1--28",
  pdf = "https://arxiv.org/pdf/2008.08050",
  publisher = "Springer",
  title = "{The MRS UAV System: Pushing the Frontiers of Reproducible Research, Real-world Deployment, and Education with Autonomous Unmanned Aerial Vehicles}",
  type = "article",
  url = "https://link.springer.com/article/10.1007/s10846-021-01383-5",
  volume = 102,
  year = 2021
}