ROBUST-CONTROL OF FREE-FLOATING SPACE ROBOT SYSTEMS

In this paper, we propose a robust control scheme for a free-floating space robot system where a robot arm is mounted on a free-floating bas e-a satellite, spacecraft or space station. The base is not controlled by external moments or forces. We model the system as an extended rob ot, including a pseudo-arm representing the base motion produced by si x hypothetical passive joints, and a real robot arm. This model allows us to categorize the free-floating space robot system as a specific t ype of under-actuated system with mixed passive and active joints. We then discuss some fundamental properties of such a system. By means of an input-output linearization technique, we demonstrate that the inte rnal dynamics of the system are nonlinear parametric and therefore, th e control of the system using conventional robot control schemes is no t feasible. To overcome the difficulty in controlling the internal dyn amics subject to parameter uncertainty, and to avoid the measurement o f the base acceleration, we develop a robust control scheme based on t he second method of Lyapunov. We illustrate an example of a six-degree s-of-freedom space robot system to show the computational procedure an d simulation results.