Variable structure attitude control with time-varying sliding surface for flexible spacecraft under control input constraint

A control system design method is presented for the three-axis-rotational attitude stabilization of a spacecraft with flexible appendages. The design of attitude controller is based on the variable structure control (VSC) theory leading to a discontinuous control law. To accomplish asymptotic attitude maneuvering and be insensitive to the interaction of elastic modes in the presence of unknown disturbances/uncertainties and input saturation as well, a switching mechanism was employed to design the sliding surface, such that outside the sliding region, the VSC law with a time-varying sliding surface was implemented, and inside the region, the VSC law with a linear sliding surface was activated. Furthermore, a hyperbolic tangent function in conjunction with a sharpness function permitted to vary with time according to a set of user-defined parameters was implemented to offset the disadvantages of existing saturation-respecting controller and chattering. Numerical simulations were performed to show that the rotational maneuver and vibration suppression can be accomplished in spite of the presence of disturbance torque, parameter uncertainty and control saturation nonlinearity.