Engineering Notes Fractional-Order Tension Control Law for Deployment of Space Tether System

O NE of the critical issues for space tether missions is the stable deployment of the tether [1,2]. Many effective control strategies and devices have been proposed for the deployment [3–7]. Among them, the tension feedback control has been proven the most effective because of its simplicity, implementability, real-time capability, and effectiveness [8]. Although effective, the existing tension control laws either take a relatively long time to deploy the tetherwith an overshoot in tether length or employ complex nonlinear feedback control strategies that require precise dynamic models of the space tether system. Recently, the fractional-order control theory has emerged as an effective generalization of the classic integer-order control theory [9– 11], such as Commande Robusted' Ordre Non Entier (CRONE) strategy, fractional PIλDμ controller, and fractional-order lead–lag compensators [12]. Compared with its integer-order counterpart, the fractional-order control has three unique advantages that specifically appealing to the space tether deployment. First, it is more suitable for modeling flexible structures because of its unique historical memory effect. Second, it provides an effective and clear-cut robust control strategy, especially for distributed parameter systems. Third, it responds to control input faster, with smaller overshoots than its integer counterpart. To our best knowledge, no attempt has been made to control the deployment of space tethers with a fractionalorder control law. In this Note, a new fractional-order tension control law is developed for the fast and stable deployment of space tethers.