A novel control design methodology for maintaining position and attitude of a power-sail
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This paper deals with control of the motion of a Power Sail relative to a chief satellite. This motion is critical for the mission as the relative motion drives the length of the umbilical tether attachment and control requirements. The desired configuration is for the tether to remain tension free. However the fact that this will not occur exactly and extreme cases where the tether may become taut are considered. This is more so because when the dynamics of the system changes abruptly, both the satellite and the power sail may experience translational and rotational impulses. A potential design wherein the umbilical is attached with a softly coiled tether is investigated. The feedback control on the Power Sail is designed to minimize tether tension variations, take up unwanted slack in the tether and accommodate a failsafe emergency brake to arrest relative separation velocity smoothly to zero (in the unlikely event that large along orbit separation occurs for any unanticipated reason). The attitude control law is designed so that the Power sail body axes tracks a desired target frame constructed using the relative position vector and the sun-pointing axis. The control strategies are evaluated against initial condition errors and disturbances due to drag and solar radiation pressure. The design philosophy seeks to minimize the fuel consumption in achieving the tracking objectives.