Hybrid variable structure control of flexible spacecraft under input saturation

This paper presents a control system design method for the three-axis-rotational maneuver of a flexible spacecraft. The design of attitude controller was based on variable structure control (VSC) theory leading to a discontinuous control law. A switching mechanism is employed to design the attitude controller such that outside the sliding region VSC law with a time-varying sliding surface is implemented and inside the region the VSC law with a linear sliding surface is 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 is implemented to offset the disadvantages of existing saturation-respecting controller and chattering. Numerical simulations are performed to show that rotational maneuver and vibration suppression are accomplished in spite of the presence of disturbance torque, parameter uncertainty and control saturation nonlinearity.