In this paper, a nonlinear direct adaptive output control methodology is developed to address the problem of spacecraft attitude control under large dynamics uncertainties. The main advantage of the proposed approach over other adaptive approaches for spacecraft attitude control is that it can handle significantly large inertia uncertainties, without requiring any on-line estimation of the unknown moments of inertia. Furthermore, the implementation of the controller does not require a mathematical model of the plant as the control gain adaptation mechanism relies on feedforward signals from an ideal model designed to provide a satisfactory response to the desired attitude commands, as well as on output tracking errors between the uncertain spacecraft and the ideal model. By modeling the spacecraft as a square nonlinear state-space plant through the use of the modified Rodrigues parameters allows the system to satisfy the almost striclty passive conditions, which are required to establish the formal proof of stability. The performance of the new adaptive attitude control approach is illustrated in numerical simulations for both a simple rigid-body rest-to-rest maneuver and a high-fidelity ISS free-flyer robotic capture maneuver.
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