Nonlinear control of electrodynamic tether in equatorial or somewhat inclined orbits

This paper applies different control design methods to a tethered satellite system (TSS) to investigate essential control properties of this under-actuated and nonlinear system. When the tether position in the orbit plane is controlled by the tether current, out of orbit plane motions occur as an unwanted side effect, due to nonlinear interaction with the Earth´s magnetic field. This paper focus on the uncontrollable out-of-plane motions and the robustness against B-field uncertainty associated with each of three popular controller design methodologies for nonlinear systems: linear quadratic feedback designed for the controllable subspace of the system, a feedback linearization design and a sliding mode control. The controllers are evaluated by their ability to suppress variations in the B-field and their robustness with respect to the internal dynamics.