Research On Tethered Towing Removal Of Dysfunctional Satellites

The unceasing explorations to unknown space bring us myriad rewards,during which it is also inevitable for satellites to be dysfunctional.The dysfunctional satellites occupy the valuable orbits,threatening the space environment by possible collisions with other spacecraft.How to efficiently remove them is therefore being a challenging task with increasing interests.Given many Active Debris Removal schemes,the use of space tether proves promising due to its merits of flexibility and safety.During the towing removal using tether,the avoidance of collision between the two end bodies and the avoidance of tether entanglement around the satellites as well as the suppression of tether sway are three main challenges to be tackled.For these reasons,the main contributions of this thesis are as follows.(1)To study the influence of the platform thrust on system's dynamics,a tethered towing system model is established,governing the platform orbit,attitude of the two end bodies,the relative motion,tether elasticity and flexibility.The simulation results and Fast Fourier Transformation show that the switching of thrust orientation and magnitude excites the ‘Sway' and ‘Bounce' effect of the tether and the ‘Tail Wagging' of the satellites,which are coupled together and should be suppressed.(2)To avoid the ‘Collision' caused by the ‘Bounce' effect,the tether tension control is proposed using the reel mechanism.Kalman Filter and Recursive Least-Squares Algorithm are used to estimate the system states and the unknown satellite mass.The bounded rate of reeling in/off is optimized by Nominal Robust Model Predictive Control(NRMPC).This control method enables the tension to follow a step signal and contributes to the distance stabilization.(3)To avoid the ‘Entanglement' caused by the ‘Tail-Wagging' effect,given the stable tension conducive to ‘Collision' avoidance,the attitude take-over control of the satellites is proposed by using the tethered manipulator(TM)to change the tension arm.For this underactuated system subject to constraints and disturbances,the attitude control torque with anti-windup module is designed based on the Hierarchical Sliding Model Control(HSMC)theory,and a Super-Twisting-Algorithm(STA)based observer is employed to estimate the flexible appendages vibration.By this method,the satellite can orient to the equilibrium with steady oscillations,however,the TM motions cause the tether to sway.(4)To simplify the TM structure and avoid the aforementioned ‘Sway',the tethered linear actuator(TLA)is proposed to fulfill the attitude take-over control.The limited TLA length change adds a state constraint into the underactuated system with input constraints.NRMPC and novel aggregated HSMC are therefore combined to deal with this problem,together with a network based observer.Under the control of the optimal length change rate,the satellite with an uncertain capture can orient to the equilibrium.(5)To suppress the ‘Sway' and track the desired platform orbit,due to the fact that the tension has been used for the ‘Collision' and ‘Entanglement' avoidance,the tether sway suppression method is proposed using the platform attitude maneuver.The nonaffine and underactuated system necessitates a dual-loop control structure.In the outer-loop,the higherorder HSMC and STA based observer are used to design the desired angular velocity.Then,in the inner-loop,the attitude control torque is optimized by NRMPC for the angular velocity tracking.Simulation results show that the sway angle can reach the equilibrium at the cost of degenerating the orbit-tracking performance,meanwhile,the platform has to rock around the desired attitude in order to maintain the suppression.