Free-flying manipulator systems are envisioned to perform servicing, inspection and assembling operations in orbit. The control of such systems is a challenging task, since the equations that govern their motion are highly nonlinear. Furthermore, unlike fixedbase manipulators a free-floating robot exhibits nonholonomic behavior as a result of the nonintegrability of the angular momentum conservation law.
Much effort has already been dedicated to free-flying and free-floating systems from the viewpoint of inertia coupling effects between the manipulator and base motion. In many cases such coupling effects are beneficial (base vibration suppression control using the manipulator system), in others they impose great difficulties for the control algorithms (applications related to reactionless motion planning). Extensive analysis of this phenomenon is necessary since it can extend the capabilities of space manipulators. In this thesis, different problems typically appearing as a result of the above mentioned dynamic coupling effects are discussed.
The work is divided into six parts. Chapter 1 is introductory and outlines some of the typically appearing difficulties during the utilization of free-flying and free-floating systems. It is organized as a short literature survey that makes an overview of some of the dynamic modeling, planning and control strategies introduced up to now.
Chapter 2 develops the dynamic equations governing the motion of a general manipulator system with open or closed-loop structure that is mounted on a free-floating base. The formulation presented is used as a framework for the remaining chapters of this thesis.
Chapter 3 makes an outline of some of the fundamental concepts and strategies used for the control of free-floating systems. It is intended to be a review of some of the existing methods, closely related to the problems studied in this thesis.
The main topic addressed in this study is the capture of a tumbling satellite using a robotic manipulator. In resent years, such operation has been recognized to be a priority task, since its solution is expected to be applied to a variety of space missions, involving servicing, inspection, and repairing operations. The approaching motion of a manipulator arm to a target satellite and the resulting post-impact motion of the system are discussed in Chapters 4 and 5, respectively. The aims of the analysis made can be outlined as follows;
The main contributions of Chapter 4 are: (i) introduction of the Holonomic Distribution Control, which can be utilized for reactionless path planning to a stationary target satellite; (ii) the introduction of the Bias Momentum Approach, methods for its application and discussion on its influence on the post-impact motion of the system.
In Chapter 5, analysis of manipulator motions that result in maintaining the stationary state of the spacecraft’s base in the presence of external wrenches is made. The Distributed Momentum Control is introduced and compared with existing post-impact control strategies.
The final chapter consists of conclusions and remarks for possible future work.
The utilization of the three new concepts introduced in this thesis;
can be beneficial for the solution of variety of practical problems. In each section, notes on the practical implementation of those concepts are made. In addition, numerical simulations are performed in order to verify and demonstrate their usefulness.