REDUCED EFFORT CONTROL LAWS FOR UNDERACTUATED RIGID SPACECRAFT

A nonsmooth, time-invariant feedback control law can be used to rotate an axisymmetric rigid spacecraft to the zero equilibrium using only t wo control torques, This method, however, may require a significant am ount of control effort, especially for initial conditions close to an equilibrium manifold corresponding to rotations about the unactuated p rincipal axis, A control law is proposed in this work that reduces the control effort required to perform rest to-rest maneuvers for initial conditions close to this equilibrium manifold, Specifically, the phas e space of the system is divided into two parts, one corresponding to initial conditions producing large control effort (the ''bad'' region) and the other corresponding to initial conditions producing small con trol signals (the ''good'' region). The proposed control law then rend ers this undesirable equilibrium manifold unstable, driving the trajec tories of the closed-loop system into the good region, where the origi nal control law is subsequently used. Numerical simulations indicate r eduction of the control magnitude on the order of 80-90% for initial c onditions close to the equilibrium manifold.