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Version: 1.5.0

Reference trackers within the MRS

Purpose of a tracker within the MRS control pipeline​

  • Receiving high-level references from a navigation software, those are generally
    • a single position + heading reference,
    • a trajectory, consisting of several position + heading references separated by a fixed time constant.
  • Generating all-state and feasible reference for the controllers, which consists of
    • desired position, velocity, acceleration, heading, and heading rate.
  • The trackers are provided with a set of state constraints that should be satisfied at all times.

Available trackers​

  • "MPC tracker"
    • the main workhorse of the MRS UAV System, it is used for most of the regular flying
    • based on a unique realtime simulated Model Predictive Control approach
    • originally published in: Baca, et al., "Model Predictive Trajectory Tracking and Collision Avoidance for Reliable Outdoor Deployment of Unmanned Aerial Vehicles", IROS 2018, link
    • produces feasible reference which is smooth up to snap and satisfies given state constraints.
    • can smoothly track trajectories
    • can efficiently stop a UAV from any previous motion
    • can be activated in mid-flight while in motion
  • "Landoff tracker"
    • particular variant of Line Tracker used only for landing and takeoff
    • can not fly to the desired reference nor follow a trajectory
    • can efficiently stop UAV from the previous motion
    • has special admittance properties while landing and taking off to provide robustness
    • is also used for emergency landings and estops
  • "Joy tracker"
    • provides control using a ROS-compatible joysticks
    • subscribes to /joy topic
    • tracks z and heading, the desired tilt is provided directly by a joystick
  • "Line tracker"
    • very simple tracker used mostly for debugging and testing
    • can fly to reference points
    • cannot track trajectories
    • the resulting reference is not smooth
    • can be used on a real UAV with caution, will induce spikes in tilt control
    • can stop a UAV from the previous motion
    • can be activated in mid-flight while in motion
    • don't use on a real UAV, kept around for debugging purposes

Tracker interface​

The trackers are compiled as ROS plugins (http://wiki.ros.org/pluginlib) with the interface defined by the control manager. A tracker from any ROS package can be loaded dynamically by the control manager without it being present during control manager's compile time. Loaded trackers can be switched by the control manager in mid-flight, which allows safe testing of new trackers and adds flexibility the MRS UAV system.

Loading trackers to control manager​

An example tracker plugin is provided here.