A new class of control laws for spacecraft attitude tracking using switching and trajectory rescaling

We present some new results in controller design for spacecraft attitude trajectory tracking. A model-based nonlinear controller and a controller gain switching strategy are presented. The nonlinear controller together with the switching strategy result in exponential convergence and also provide a near method to overcome problems associated with closed loop behavior near an unstable equilibrium that are inherent to controllers with quaternion feedback. Bounds of convergence rates and “turn around time” to the stable equilibrium are explicitly calculated. A “time-scaling” approach for partial reference trajectory redesign is then presented to achieve tracking when constraints on the magnitude of the available input torques are present