Power-Optimal Steering of a Space Robotic System Driven by Control-Moment Gyroscopes

Control-moment gyroscopes (CMGs) are power-efficient attitude-control actuators that produce high torques on spacecraft. In this study, a method is investigated for maneuvering robotic arm segments in a spacecraft-mounted, CMG-actuated agile imaging payload with minimal power usage. A real-time optimization method is presented that includes null motion in a closed-loop end-effector tracking problem. The proposed approach involves first establishing kinematic relationships between the end-effector attitude coordinates and joint coordinates. These transformations are used to track a two-coordinate end-effector attitude command by simultaneously controlling three joint degrees of freedom. With the redundant degree of freedom, there are multiple joint-angle solutions corresponding to a given boresight attitude command. While tracking this command, null motion is added to the commanded joint angles. The augmented joint-angle command causes a change in the body and gimbal motion that ultimately reduces power consumption. The null-angle component is determined by computing the null projection of the cost gradient with respect to the joint angles. A quadratic cost function is defined, which is the sum of the squares of power for each CMG gimbal. Simulation results demonstrate that the power consumption of the system, when measured by the integral of non-recoverable power, is reduced by up to 42% when a certain amount of null motion is included in the feedback loop.