A Task-Space Control Law for Free-Floating Space Robots

This paper examines the task-space control of a space robot in a free-floating mode of operation. The controller is developed by transforming the space robot's equations of motion into the task-space. It is then assumed that the space robot's translational thruster forces are zero and that its manipulator has three or more degrees of freedom. This presents an over-determined system of equations relating the space robot's actuator torques to the net task-space forces and torques. A computed-torque inner-feedback loop is then developed utilizing the pseudo-inverse solution that minimizes the norm of the actuator torques. The kinematics of the space robot's end-effector are then stabilized about a commanded pose with an error-quaternion based attitude control and a proportional translational position control. The computed-torque control is then further designed so that it drives the space robot at the velocities that stabilize the kinematics. This control technique is simulated on a space robot possessing a six degree of freedom manipulator.