Single-Axis Pointing of an Underactuated Spacecraft Equipped with Two Reaction Wheels

The paper proposes a novel approach for single-axis pointing of an underactuated spacecraft using only two reaction wheels (RWs), which is based on a simple yet effective wheel rate command. The control law can be used for aiming the line of sight of a sensor, a nozzle, or an antenna toward a target direction, or solar panels toward the sun, after failure of one wheel for a nonredundant control system hardware or, in the case of multiple failures, for redundant systems. The proposed control methodology represents the practical, dynamic implementation of a kinematic planning scheme proposed by the same authors, developed under the assumptions of zero overall angular momentum and triaxial inertia tensor. Under the zero angular momentum hypothesis, the control law also provides three-axis stabilization at zero angular speed. Only rest-to-rest maneuvers are thus dealt with, as a steady residual rotation rate around an arbitrary body-fixed axis cannot be attained in general unless the axis is the principal of inertia and aligned with the spin axis of one of the two active reaction wheels. The limited computational demand of the control law also makes it a practical solution in the case of small-size satellites, where the computational budget is severely limited by the available CPU processing capabilities. The effects of a nonzero residual angular momentum and control axes not aligned with the principal axes of inertia is also investigated, highlighting limitations on pointing precision and convergence performance.