OFFBOARD Mode & Open Loop Control

At the beginning of this class we flew our drones in MANUAL mode, piloting it using the remote control. This mode was difficult to control and required constant input from the pilot in order to prevent it from drifting off and crashing into the walls. This constant input from the pilot to maintain control was a required because the drone had very little knowledge of the world; i.e. it had no way of localizing itself in the world which meant that it had no way of knowing that it’s position was drifting toward a wall.

After implementing off-the-shelf optical flow and ARTag localization libraries, we were able to fly the drone in POSITION mode (sometimes refered to as POSITION CONTROL or POSCTL). This mode still took inputs from the pilot via the remote control, but it was much easier to fly since the drone could effectively hover in place without drifting. In this mode, the input from the pilot commanded the drone to make slow movements in body-frame directions.

Our next step is to utilize the OFFBOARD mode, which is removes the need for pilot input and is our first venture into truly autonomous flight! This practical will walk us through sending short duration velocity commands that the drone will execute. We call it “open loop” because the commands are pre-scripted and don’t adapt or update based on new information (i.e. sensor/camera data).

For more information on PX’4 various flight modes, see here


OFFBOARD mode is potentially one of the more dangerous modes because you are handing over control of your quadrotor to computer program that may not be perfect. We will implement as many software-based safety checks as possible, but the last line of defence is still the Pilot in Command. Therefore, for safety purposes, the pilot we will always have the remote control in hand in order to regain control when necessary.

The learning objectives for this practical include:

  1. Understand how to safely enable and operate PX4’s OFFBOARD mode

  2. Develop the code necessary to send velocity commands

  3. Gain intuition about the various coordinate frames in which we can command velocities (e.g. world reference frames vs. body reference frames)

Step 1: Complete Control Script

In the aero_control repository/ROS package, we have given you a incomplete script for running a simple open-loop translation control demonstration. The purpose of this script is to publish low-speed velocity commands in the form of ROS Twist messages to the topic /mavros/setpoint_velocity/cmd_vel_unstamped for a short duration (e.g. 3 seconds) and then send “hover” messages (i.e. Twist message of all zeros) causing the drone to safely hover in place. The script should also be able to take various coordinate frames in which to control the drone (e.g body-up, body-down, local NED, local ENU, downward camera).

To access the partially complete script, git pull upstream master in you drone and laptops aero_control repository that is part of your catkin workspace. You should now find the file aero_control/open_loop_control/src/ Complete portions of code marked as TODO.

Step 2: Setup for Test Flight

Once has been completed and pulled onto the drone, run the flight test by following these procedures

  • Power on remote control

  • Power on the Intel RTF

  • Connect to the UAV via QGroundControl

  • SSH into the UAV (you will need to have at least four terminals SSHed into the UAV). use X-forwarding in order to pass graphics (i.e. stream the camera feed from the drone to your computer). Example replacing <team-drone-name> with the name of your drone:

    ssh -X uav@<team-drone-name>
  • In terminal 1, start distance sensor

    sudo systemctl start aero-teraranger.service
  • In terminal 1, start ROS

  • In terminal 2, start patched optical flow + down-camera streaming. Note: this is different from standard optical flow service that prevents streaming downward-facing camera. Needed for line detection. See here for more info on setting up

    cd ~/bwsi-uav/catkin_ws/src/aero-optical-flow/build
    sudo -E ./aero-optical-flow
  • Without arming drone, switch to POSITION CONTROL mode to ensure previous steps worked as expect. If QGroundControl declares that that position control is rejected, restart process.

  • In terminal 3, launch mavros and translation control

    cd ~/bwsi-uav/catkin_ws
    source devel/setup.bash
    roslaunch aero_control translation_control.launch

Step 3: Test Flight


Pilot-in-Command must always be ready to regain control by switching back to POSCTL mode. Never take your hands off the remote control!


  1. Arm the quadrotor

  2. Takeoff

  3. Position in a safe location in the air

  4. Switch to OFFBOARD mode to run test

  5. Regain control with POSITION mode

  6. Land quadrotor

  7. Disarm

  8. Collect flight log

Step 4: Reference Frame Intuition

Now we can start to explore the different reference frames that may be of use for quadrotor flight such as the body-up, body-down, local NED, local ENU, forward camera, and downward camera reference frames.

In, try using different frames and different velocity vectors for the variables maneuver_reference_frame and maneuver_velocity_setpoint. For each of the following reference frames, your task is to:

  1. select a velocity vector to be flown (making sure it is sufficiently slow so as not to cause a crash)

  2. used a fixed, short duration such as 2 seconds

  3. before flying, based on your understanding for the various reference frames, record your prediction of the direction in which the drone will fly. For example, I might say: “we oriented the drone pointing away from us and gave it a velocity setpoint of [0.3, 0.0, 0.0] m/s expressed in the fc (forward camera) reference frame. Since the fc x-axis points in to the right-side of the quadrotor, we expect the quadrotor to fly right at 0.3 m/s for 2 seconds

  4. Execute the flight and record your observations. Did the drone do what you expected? If not, can you explain why? Did you have an incorrect understanding of the reference frames involved?

These steps should be done for each of the following reference frames:

  • dc = downward-facing camera (body-fixed, non-inertial frame. Origin: downward camera focal plane. Alignment with respect to drone airframe: x-forward, y-right, z-down)

  • fc = forward-facing camera (body-fixed, non-inertial frame. Origin: forward camera focal plane. Alignment with respect to drone airframe: x-right, y-down, z-forward)

  • bu = body-up frame (body-fixed, non-inertial frame. Origin: drone center of mass. Alignment with respect to drone airframe: x-forward, y-left, z-up)

  • bd = body-down frame (body-fixed, non-inertial frame. Origin: drone center of mass. Alignment with respect to drone airframe: x-forward, y-right, z-down)

  • lenu = local East-North-Up world frame (world-fixed, inertial frame. Origin: apprx at take-off point, but not guaranteed. Alignment with respect to world: x-East, y-North, z-up)

  • lned = local North-East-Down world frame (world-fixed, inertial frame. Origin: apprx at take-off point, but not guaranteed. Alignment with respect to world: x-North, y-East, z-down)

  • m = marker frame (inertial or non-inertial, depending on motion of marker. Origin: center of marker. Alignment when looking at marker: x-right, y-up, z-out of plane toward you)

The team’s Research Specialist is to document these flights (preferably in a .md file) and push it to the team’s documents repository

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