Reactionless camera inspection with a free-flying space robot under reaction null-space motion control

The possibility of implementing reactionless motion control w.r.t. base orientation of a free-flying space robot in practical tasks is addressed. It is shown that such possibility depends strongly on the kinematic/dynamic design parameters as well as on the mission task. A successful implementation of a camera inspection task is reported. The presence of kinematic redundancy and the manipulator attachment position are shown to play important roles. More specifically, for a manipulator arm with a typical seven degree-of-freedom (DoF) kinematic structure, it is shown that two motion patterns, wrist reorientation and folding/unfolding of the arm, result in almost reactionless motion. The orientation pattern is adopted as the main task for camera inspection, while the remaining four DoFs are used to ensure complete reactionless motion and to minimize the position errors. Since the composition of these tasks introduces the so-called algorithmic singularities, two methods are suggested to alleviate the problem. Furthermore, it is shown that other types of singularities may also be introduced in case of an inappropriate choice of the manipulator attachment position. At the end, numerical analysis is performed to show that reactionless motion provides an advantage in terms of kinetic energy as well.