ESA GNC Conference Papers Repository
Wave based control analysis and experimental validation for reliable space transportation applications
Space debris mitigation is a relevant goal that must be fulfilled in the next future. However, performing an active debris removal mission would be very challenging and complicated. In fact, for such kind of mission, it is hard to guarantee an enough level of reliability with a low level of risk. One of the possible active debris removal missions is the one in which a space tug exploits flexible systems, such as nets and tethers, to capture a debris. Then, a pulling phase is performed to decrease its energy and bring it to a ballistic reentry path through the atmosphere.Into this topic falls the SatLeash experiment, a microgravity experiment in parabolic flight environment. The SatLeash experiment was going to #75938343 the dynamics and control of tow-tethers, for space transportation. Guidance and control design of space tugs is a big challenge. Once the capture occurs, a passive orbiting target and the active chaser are connected through a flexible link. Then, a maneuver is performed by the chaser that excites the stack dynamics. The chaser has to robustly and reliably perform operations, while damping dangerous vibrations of flexible elements and connections, avoiding instability, collisions and tether entanglement. The bounce-back effect is one of the most critical effects of the flexible systems exploitation during towing operations. It means that, whenever the thrusting phase is over, the tether slackens and the residual tension accelerates the two objects towards each other, increasing the collision risk. The control recovery is then difficult and not always possible. The tether may entangle on the target or the chaser itself and, hence, break. It is mandatory to ensure a minimum distance between the chaser space tug and the target space debris throughout all the phases of the mission.This paper presents the control strategy that has been chosen for the experiment and its results. A wave-based controller has been selected to mitigate the negative effects of flexible systems dynamics. A stability analysis has been conducted to identify the stability region of the controller and its performances to control the velocity of the target, relative to the chaser. The control law has been implemented into a programmable logic controller, able to drive a linear actuator which simulates the chaser behavior. Robustness and reliability of the control strategy has been verified during the 65th ESA parabolic flight campaign occurred in fall 2016. Exploiting a low friction table it was possible to characterize the different kinds of tether and empirically identify the correct controller parameters to obtain the required control performances. The experiment results are presented to demonstrate the effectiveness of the control strategy. After a deep analysis of the sensors data, the reconstructed relative dynamics between the floating object and the slider of the linear actuator evidences the capability and strength of the proposed control strategy. Conclusively, the control theory is mathematically analyzed, in order to justify the obtained empirical parameter and robustness of the controller.