Symmetry-Preserving Combined Controller for a Space Free-Flying Robot

Abstract

Future on–orbit servicing, inspection, and debris–removal missions rely on free–flying robotic systems capable of executing precise manipulation tasks while subject to the actuation and sensing constraints of spacecraft. In these missions, the servicer must coordinate base and manipulator motion while relying on onboard navigation systems and sensors, which typically provide inertial attitude and velocity estimates for the spacecraft as well as relative pose measurements of the target. At the same time, the spacecraft must rely on reaction control thrusters that operate with low bandwidth and discontinuous activation constraints. The presence of deadband regions, minimum–impulse–bit constraints, and the resulting intermittent actuation make it challenging to coordinate the spacecraft and its manipulator without degrading the quality of the manipulation task. This thesis addresses these limitations by developing a unified control framework that expresses the end-effector tracking problem directly in the moving base frame and coordinates base and arm motion through a whole–body dynamic model. The formulation explicitly preserves the symmetry structure of free-floating systems, arising from the invariance of the Lagrangian under rigid-body transformations and the associated conservation of momentum. Within this structure, the controller exploits the system’s center-of-mass position to retain the conceptual elegance of centroidal–based formulations inside a unified task-space representation. To reflect realistic operational conditions, the method is integrated with a representative thrusters model incorporating both translational and rotational deadbands, allowing a systematic assessment of how these actuation constraints impact manipulation performance. Theoretical stability guarantees are established for the proposed control law, and its practical effectiveness is demonstrated through numerical simulations of representative on–orbit manipulation scenarios. The results show that the framework achieves accurate and resilient end-effector tracking, remains compatible with fuel-aware spacecraft actuation, and provides a promising foundation for future orbital servicing missions requiring coordinated base-manipulator control.