Energy-efficient motion of a space manipulator

In space robotics missions, either in the scenario of space servicing or debris capture, both the minimization of the energy required for the manipulator manoeuvre and the minimization of the reactions transferred to the base spacecraft are of crucial importance. Indeed, both of them are related to the possibility of having a longer system operative life in space: for the energy minimization this is straightforward, and for the reaction minimization this is due to the fact that the Attitude Control System is going to spend less fuel for attitude recovery and correct pointing of antennas after the manipulator manoeuvre if the reactions transferred to the base spacecraft have been minimized. In this paper, a minimum kinetic energy inverse kinematics solution for redundant manipulators has been introduced and validated by simulation. Its performance has been compared to the classical inverse kinematics solution which minimizes joint velocities and to the one that minimizes the reaction torque transferred to the base spacecraft for a 3-degrees-of-freeedom planar manipulator. Two different end-effector trajectories have been used for validation and then for the comparison of the different inverse kinematics solutions in terms of: total energy required for performing the task, maximum required power, maximum kinetic energy, maximum reaction torque, and maximum joint angles, velocities, and accelerations. Finally, the new concept of iso-energy curve has been introduced, and a set of them has been drawn over the robot workspaces, which have been computed using the kinetic energy minimization solution and the reaction torque minimization solution, considering or not the robot joint limits. This method shows that preferential minimum energy directions exist for the robot end-effector motion, and these are approximately the same for the different inverse kinematics solutions considered.