Disturbance Observer-Based Robust Saturated Control for Spacecraft Proximity Maneuvers

The problem of six-degrees-of-freedom (6-DOF) relative motion control is investigated for spacecraft close proximity maneuvers with input saturation and model uncertainties. A nonlinear disturbance observer is developed to estimate the lumped disturbance that comprises the effects of parametric uncertainties, measurement uncertainties, kinematic couplings, and external environment disturbances, while a linear compensator system is incorporated into the controller design to deal with the control input saturation. A 6-DOF robust state feedback saturated controller is designed for relative position tracking and attitude synchronization simultaneously, and the same is augmented by the linear saturation compensator and nonlinear disturbance observer to compensate the control input saturation effect and model uncertainties, respectively. Rigorous stability of the closed-loop system with the proposed control method is established under mild assumptions, and it is proven that the relative position and the relative attitude converge to a small neighborhood of zero. Numerical simulation result demonstrates the effectiveness of the proposed controller designing method.