This paper presents a composite controller that combines nonlinear disturbance observer and second order sliding mode controller for attitude tracking of flexible spacecraft. First, a new nonsingular sliding surface is introduced. Then, a second order sliding mode attitude controller is designed to achieve high-precision tracking performance. An extended state observer is also developed to estimate the total disturbance torque consisting of environmental disturbances, system uncertainties and flexible vibrations. The estimated result is used as feed-forward compensation. Although unknown bounded disturbances, inertia uncertainties and the coupling effect of flexible modes are taken into account, the resulting control method offers robustness and finite time convergence of attitude maneuver errors. Finite-time stability for the closed-loop system is rigorously proved using the Lyapunov stability theory. Simulation results are presented to demonstrate the effectiveness and robustness of the proposed control scheme.
This paper presents a novel robust optimal control approach for attitude stabilization of a flexible spacecraft in the presence of external disturbances. An optimal control law is formulated by using concepts of inverse optimal control, proportional-integral-derivative control and a control Lyapunov function. A modified extended state observer is used to compensate for the total disturbances. High-gain and second order sliding mode algorithms are merged to obtain the proposed modified extended state observer. The second method of Lyapunov is used to demonstrate its properties including the convergence rate and ultimate boundedness of the estimation error. The proposed controller can stabilize the attitude control system and minimize a cost functional. Moreover, this controller achieves robustness against bounded external disturbances and the disturbances caused by the elastic vibration of flexible appendages. Numerical simulations are provided to demonstrate the performance of the developed controller.