Dynamics and robust control of a space manipulator system servicing a satellite with flexible appendages

Abstract

Space Manipulator Systems (SMS) are becoming pivotal in space exploitation and exploration, offering a versatile range of solutions from space debris capture to structure assembly. However, recent missions involving manipulators aboard satellites and space structures must contend with lightweight and large elements that exhibit flexible behaviors. Despite the challenges posed by flexible elements in their surroundings, enhancing the autonomy of SMS remains crucial to ensure their viability as solutions. For the pre-design of the SMS, path-planning applications, or controller design, there is a necessity for methods to assess the couplings between the manipulator, the SMS platform, and any flexible elements moved by a manipulator or attached to the platform. Moreover, recently proposed control strategies have demonstrated a keen interest in developing model-based controllers, which advantageously provide an efficient utilization of actuators and mitigation of internal disturbances within the system. This paper initially presents, for control purposes, the derivation of the kinematics and dynamics of a free-floating Space Manipulator System (SMS) with a flexible body at the end of a kinematic chain, employing a Lagrangian formalism. Subsequently, a robust joint-space control law is designed for the On-Orbit Servicing (OOS) of a satellite with flexible appendages. The control structure consists of Nonlinear Dynamic Inversion (NDI) for system linearization and decoupling. A structured $H_{\infty }$ controller synthesis is developed to provide robustness against flexible disturbances, model uncertainties, and sensor noise. To evaluate the effectiveness of this control strategy, it is implemented on a real-time simulation platform that ensures tight and high-fidelity space robot dynamics and flexible structures. This platform incorporates visual environment models and virtual sensors to provide an accurate representation of real-world conditions. The considered use-case involves the servicing of a satellite equipped with a flexible solar array using a free-floating dual-arm SMS with flexible appendages.