Flight quality characteristics and observer-based anti-windup finite-time terminal sliding mode attitude control of aileron-free full-wing configuration UAV

Abstract

Abstract This paper evaluates the flight quality of an aileron-free full-wing configuration UAV and proposes a highly robust attitude controller considering the typical control problems (i.e., manipulation saturation, coupling, susceptibility to the disturbance, nonlinearity, and uncertainty) of the aileron-free full-wing configuration UAV. First, the flight quality characteristics are analyzed through the flight mode changes at different trim state points in the flight envelope. Then, the back-stepping method is introduced for decoupling and systematic control law design. Based on the Lagrange multiplier approach, the pseudoinverse control allocation is adopted to realize command decoupling and acquire the minimal 2-norm control output. In order to improve the robustness and control accuracy of the controller, a novel continuous finite-time terminal sliding mode control scheme with adaptive law is designed, which has the virtue of suppressing the chattering and avoiding singularity. In addition, an anti-windup compensator (AWC) is developed to ensure the stability of the flight state under manipulation saturation and make the manipulation output exit the saturation region quickly. To further solve the problem that the aileron-free full-wing configuration UAV is susceptible to disturbances, an adaptive super-twisting disturbance observer (ASTDO) is designed to estimate and compensate the uncertain compound disturbance including the uncertain time-varying disturbance and nonlinear dynamic term to improve control performance. The proposed observer can suppress chattering, adjust gain parameters adaptively and have high observation accuracy. Based on the Lyapunov stability theorem, the anti-windup compensation variable in the AWC and the disturbance estimation error in the ASTDO can gradually converge to neighborhoods of the equilibrium points in finite time. Moreover, the practical finite-time stability of the closed-loop control system is proved. According to the flight quality evaluation, the aileron-free full-wing configuration UAV has poor flight quality at the boundary of the flight envelope and exhibits inherent longitudinal high-frequency oscillation characteristics. The simulation results demonstrate that the designed ASTDO-AWFTTSMC guarantees strong robustness, high accuracy, and fast convergence speed for attitude tracking, solves the saturation problem and realizes state decoupling and manipulation decoupling. Moreover, the designed controller overcomes the adverse effects of disturbance, nonlinearity, and uncertainty, greatly suppresses the longitudinal high-frequency oscillation and still exhibits excellent control performance at the flight state points on the flight envelope boundary.