Spacecraft attitude maneuver control using two parallel mounted 3-DOF spherical actuators

A parallel configuration using two 3-degree-of-freedom(3-DOF) spherical electromagnetic momentum exchange actuators is investigated for large angle spacecraft attitude maneuvers.First, the full dynamic equations of motion for the spacecraft system are derived by the NewtonEuler method. To facilitate computation, virtual gimbal coordinate frames are established. Second,a nonlinear control law in terms of quaternions is developed via backstepping method. The proposed control law compensates the coupling torques arising from the spacecraft rotation, and is robust against the external disturbances. Then, the singularity problem is analyzed. To avoid singularities, a modified weighed Moore-Pseudo inverse velocity steering law based on null motion is proposed. The weighted matrices are carefully designed to switch the actuators and redistribute the control torques. The null motion is used to reorient the rotor away from the tilt angle saturation state. Finally, numerical simulations of rest-to-rest maneuvers are performed to validate the effectiveness of the proposed method.