A Robust Post-Grasping Control Design for Robotic Testbed Demonstration of Space Debris Disposal

In this article, a ground-based floating platform setup that emulates the zero-gravity spacecraft's motion on the ground is utilized for an active debris removal mission scenario. There are various phases in an active debris removal mission, which could be listed as close-range rendezvous, attitude coordination in case of tumbling target, grasping of the debris spacecraft, and the post grasping (debris removal phase). This article focuses on the post-grasping phase of the debris removal mission, with the assumption that the floating platform has grabbed (grasped) a debris of unknown mass and mass distribution, which adds to the modeling uncertainty of the newly unified platform (floating platform+debris). In sequel of the debris grasping, the floating platform needs to follow the desired path to move the debris to a desired secure location. This post-grasping scenario introduces multiple challenges rising from various parametric uncertainties in the robot's dynamics rising from nonlinearities, inaccuracy in estimating its inertia, discretization of thruster inputs using PWMs, and external disturbances. Also, since the robot maneuvers and operates in eminently constrained environments, the controller must ensure impeccable accuracy. The state-of-the-art controllers ensuring a constrained control of space robots with parametric uncertainties use a strict barrier Lyapunov function (BLF), which demands the initial conditions of the states to be within a specified bound, which is impractical in many scenarios. Hence, we propose a robust time-varying BLF controller for space robots, which tackles uncertainties and external disturbances while avoiding strict initial conditions for the constrained states. The controller's stability is validated using a Lyapunov-like method, and its performance is verified using a simulated Slider model.