Hierarchical Nonlinear Integral Control of a Quad-Rotorcraft: Design and Stability Analysis

Abstract

A nonlinear hierarchical integral control framework is proposed in this paper for control of a quad-rotorcraft. A nonlinear position controller commands the required thrust to move the vehicle along the reference trajectory while a nonlinear attitude controller realizes the computed torque to achieve the desired orientation. The underlying principle is same as that of classical nonlinear hierarchical control with a difference of integral filtered-tracking error added in position and attitude control design. The integral action tracks the desired pose with lower control and filter gains and brings down the control effort. The effect reduces the steady-state error and can avoid actuator bandwidth saturation. A rigorous stability analysis proves that the overall closed-loop system is exponentially stable and all the signals are bounded in the cascaded control structure. Unlike Newton-Euler (N-E) dynamics where desired attitude is reached by treating vehicle angular velocity as an intermediate control input, the design exploits the full state space Euler-Lagrange (E-L) underactuated dynamics to track a command attitude using computed torque control. Simulation results show the performance of the proposed controller.