Single Waypoint ZEM/ZEV Based Closed-Loop Controller Design for Rendezvous with a Rotating Target
Alvarez Rolins, Armando
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The research presented in this thesis addresses a very specific kind of autonomous rendezvous procedure between two spacecraft in close proximity, namely that in which the chaser object is attempting to dock at a location on a rotating target which is tracing a circle perpendicular to the axis of rotation. The approaches that have been mentioned in literature which focus on this specific condition make use of offline optimization algorithms to plot the trajectory of the chaser and use closed-loop control to follow that trajectory. This method is computationally demanding and does not allow for feedback in the desired boundary conditions based off of new observations on the target’s motion. This research proposes to use a fully closed-loop control to perform this kind of rendezvous and compares the cost attained by three different control laws given the same problem parameters. The proposed solution makes use of a single waypoint to avoid a trajectory which causes a collision before meeting its boundary conditions. It was found that the three control laws proposed are comparable in their cost under the same problem parameters and waypoint selection, but it was observed that the waypoint selection relates to the cost in a nonlinear fashion. The choice of waypoint based on position, velocity and time of flight to the waypoint was discussed and a search for a global minimum was done for each of the three control laws. It was found that the problem of choosing the waypoint behaves similarly between the three control laws, however no method of systematically picking the minimum cost waypoint was found.