MRS UAV Controllers

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Purpose of a controller within the MRS control pipeline

• receiving an all-state reference from reference trackers
• controlling the states of the UAV by outputting a desired angular rate (or desired orientation) and desired thrust

Available controllers

• "SE(3) controller"
  • geometric state feedback in SE(3) capable of precise reference tracking and fast maneuvers
  • pros: precise control, fast response, fast convergence
  • cons: sensitive to measurement noise, requires feasible and smooth reference, needs to be tuned
  • originally published in: Lee, et al., "Geometric tracking control of a quadrotor UAV on SE(3)", CDC 2010, link
• "MPC controller"
  • SO(3) force tracking + Linear MPC for acceleration feedforward
  • pros: robust control, immune to measurement noise and reference infeasibilities
  • cons: slow convergence, only for slow speeds (< 2 m/s), may have large control errors while tracking motion
  • briefly described in: Petrlik, et al., "A Robust UAV System for Operations in a Constrained Environment", RA-L 2020, link
• "Failsafe controller"
  • feedforward controller for landing without a state estimator
  • relies on the Pixhawk's attitude controller for leveling
  • is triggered in case of emergency
  • gradually decreases thrust while keeping the UAV leveled

Controller interface

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

Loading controllers to control manager

Controllers are defined in controller.yaml (example). Each entry such as

Se3Controller:
  address: "mrs_uav_controllers/Se3Controller"
  namespace: "se3_controller"
  eland_threshold: 1.5 # [m], position error triggering eland
  failsafe_threshold: 2.5 # [m], position error triggering failsafe land
  odometry_innovation_threshold: 1.5 # [m], position odometry innovation threshold

creates an instance of a controller, in this case mrs_uav_controllers/Se3Controller is loaded under the alias Se3Controller. Multiple instances are allowed and are used to introduce the same controller with various configurations that can be switched in mid-flight. Once the controller alias is defined within controllers.yaml, it needs to be part of the controllers list within control_manager.yaml (example) config:

# - list of names of dynamically loaded controllers
controllers : [
  "Se3Controller",
  "MpcController",
  "FailsafeController",
  "EmergencyController",
]

Only the controllers within this list are actually loaded. Switching to a controller with the alias Se3Controller is done by calling a service:

rosservice call /uav1/control_manager/switch_controller Se3Controller