Nullspace-Based Control Allocation of Overactuated UAV Platforms

Abstract

Propeller failure is one major reason for the falling and crashing of multirotor Unmanned Aerial Vehicles (UAVs). While conventional multirotors can barely handle this issue due to underactuation, over-actuated platforms can still pursue the flight with proper fault-tolerant control (FTC). This paper investigates such a controller for one such over-actuated multirotor aerial platform composing quadcopters mounted on passive joints with input redundancy in both the high-level vehicle control and the low-level quadcopter control of vectored thrusts. To fully utilize the input redundancies of the whole platform under propeller failure, our proposed FTC controller has a hierarchical control architecture with three main components: (i) a low-level adjustment strategy to avoid propeller-level thrust saturation; (ii) a compensation loop to attenuate introduced disturbance; (iii) a nullspace-based control allocation framework to avoid quadcopter-level thrust saturation. Through reallocating actuator inputs in both the low-level and high-level control loops, the low-level quadcopter control can be maintained with at most two failed propellers and the whole platform can be stabilized without crashing. The proposed controller is extensively studied in both simulation and real-world experiments to demonstrate its superior performance.

Unmanned Aerial Vehicles: Control Methods and Future Challenges

“…Lyapunov guidance vector fields are developed for autonomous coordination of multiple unmanned aircraft [105], which yields feasible and globally stable paths with guaranteed target stand-off distance bounds. The loitering tracking strategy is further improved and extended in [106] and [107] by speeding the convergence process and imposing a curvature limitation, respectively.…”

A smooth control allocation method for a distributed electric propulsion VTOL aircraft test platform

“…The hierarchical control allocator can be used in all the flight mode, which is clearly different from the iterative allocation method such as in [11, 13, 15, 27]. CGI is subtly adopted to the FD‐based method to deal with the input constraints issue, which is clearly different from the optimization‐based methods as in [9, 17, 28]. A novel smooth strategy using Sucher Compliment is proposed so that the weight of the constraints on the forward force can be gradually reduced in transition to avoid signal jump for actuators’ input, which never has been developed in other works related to DEP‐VTOL aircraft.…”

“…The method could provide a fast and accurate solution to the non-linear allocation problem in arbitrary tilt angles, but the actuator constraints were not considered. Quadratic programming (QP)-based allocation can impose linear equality and inequality constraints [17]. However, it uses a linear approximation of the non-linear control effective model, resulting in errors for the solutions.…”

Full-pose Trajectory Tracking of Overactuated Multi-Rotor Aerial Vehicles with Limited Actuation Abilities

“…On the other hand, the authors in [5] show that for the fixed propeller overactuated heptarotor they designed, the minimum energy solution is guaranteed to break at least one of the actuation constraints. To avoid actuation constraints, the authors in [6] suggest a null space-based approach to find feasible control inputs to achieve a desired force and moment. Their approach optimizes the incremental control input added to the currently applied controls.…”

“…The optimization is relaxed by adding an error term, assumed to be in the null space of the allocation matrix. Similar to [6], the authors in [7] suggest to allocate incremental control inputs added to the currently applied controls; however, their proposed overactuated platform had only thrust constraints, and thus it was possible to operate away from actuation limits.…”