Single axis pointing by means of two reaction wheels

The paper deals with attitude dynamics of an underactuated spacecraft, proposing a technique for aiming a body-fixed axis (e.g. sensor, antenna, nozzle, etc.) arbitrarily close to a prescribed direction in space by means of two reaction wheels only, when the total angular momentum of the spacecraft is zero. The technique is based on a simple kinematic planning scheme, where an eigenaxis rotation is performed around an admissible rotation axis, that is, an axis that lies on the plane of the two active reaction wheels. Proof of almost global stability is provided, together with a detailed discussion of a few theoretical shortcomings. The latter can be circumvented in the numerical implementation of the control law, that remains simple and computationally efficient. Numerical simulations of single-axis pointing maneuvers for a reference spacecraft prove the practical viability and validity of the approach. The effect of non-ideal conditions, such as a small residual angular momentum, command torque saturation and off-diagonal terms in the inertia tensor, are also investigated.