Terminal sliding mode control of payload re-entry using momentum exchange tether system

The Tethered Satellite System (TSS) are evolving rapidly, since its inception in 1960s. It has wide range of promising applications such as payload transfer, space debris mitigation, rendezvous maneuver and satellite formation among many. In 2007, Young Engineers Satellite (YES2) project was initiated by the ESA and Russia aimed to develop a momentum exchange tether system; a small re-entry capsule to transfer payload from space station to the earth. However, there was an obvious error in the deployment due to the problems faced in the designed control system. The tether was successfully opened but control system proved inefficient to recover the capsule that did not fall in expected areas. Therefore, robust and precision control of momentum exchange tether system is still a challenging task and an important issue in space tether experiments. This paper mainly focuses on payload re-entry of YES2 mission. In this work, an open loop nominal trajectory is designed by optimal oscillation damp method with the Lagrangian rigid model for safe separation of re-entry capsule from spacecraft. A closed-loop terminal sliding mode variable structure controller is designed considering the initial ejection error and tension control is applied to ensure the closed-loop tracking. This paper establishes the TSS dynamic model with Newtonian mechanics theory in earth-centered mertial reference, considering inertia of the control mechanism and tether under the elastic deformation condition. The re-entry process is analyzed in the existence of error conditions to verify the task execution results. The simulation results show that the closed-loop terminal sliding mode controller successfully restrains the initial error disturbance, achieves tracking of the nominal trajectory, and ensures safe tether deployment along the local vertical considering the tether under the elastic deformation condition. The re-entry analysis shows that the payload can accurately deploy into the atmosphere with existing error conditions. Thus simulation results exhibit a successful controller design with satisfying robustness and precision of TSS.