Whole-Body Control of Orbital Robots

Abstract

Arm-equipped orbital robots are complex systems merging the nonlinear dynamics behavior of classical terrestrial robots with the complexities of the control architecture of space systems. From the one hand, their dynamics is characterized by nonlinearity and by the microgravity behavior; from the other hand, their control architecture relies on highly heterogeneous actuation and limited sensing capability. Tremendous impetus has characterized the research of the last decades towards the realization of such systems; many solutions have been investigated both theoretically and experimentally. Still, the solutions were investigated in a somewhat decoupled fashion stemming from the different points of view of the robotics and the space communities. Centralized control systems taking into account both aspects altogether, and exploiting their features, need to be developed for a successful deployment of the technology. The main contribution of this thesis is to define a unifying framework for designing controllers that better exploit the dynamics and actuation properties of the orbital robots. The framework leverages on the whole-body quantities of the system, and on their conservation laws. The robot requirements are enforced by introducing the innovative tasks of momentum dumping and control of the center-of-mass of the whole robot, which are more natural and thus more fuel efficient than the classical spacecraft positioning tasks. Thrusters-decoupled actuators allocation structures are further defined and used for the design of coordinated controllers that limit the allocation of thrusters. Within this framework, whole-body controllers are derived, which merge the advantages of common free-floating and free-flying strategies, while solving their main limitations. The derived controllers allow reducing the thrusters use and the fuel consumption of robotic operations, thereby allowing an extended life and reduced cost of orbital robotics missions. Beyond the conceptual innovation, attention is posed also to the theoretical and practical stability of the controllers. The actuators allocation is formulated in a way that main structural properties of the system are preserved, and thus can be exploited in the stability analysis. The theoretical stability is proven for some of the classical controller and for the whole-body controllers analyzed in the thesis. The practical stability and effectiveness of the controllers considering a realistic set of thrusters is analyzed via extensive numerical simulation. The practical stability and effectiveness of the controllers considering a real arm is validated with hardware experiments on a 3D robotic motion simulator of on-orbit servicing scenarios.