Variable Structure Control of Decoupleable Systems and Attitude Control of Spacecraft in Prescence of Uncertainty

This paper considers control of a class of uncertain nonlinear systems which can be decoupled by state variable feedback. A variable structure control (VSC) law is derived such that in the closed-loop system the output variable asymptotically tracks a given output trajectory in spite of the uncertainty in the system. Based on this result, a control law is derived for the attitude control of an orbiting spacecraft in the presence of uncertainty using reaction jets. The controlled outputs are the three Euler angles which describe the orientation of the spacecraft relative to an orbital frame. Simulation results are presented to show that in the closed-loop system precise attitude control is accomplished in spite of the uncertainty in the system.