Shape change maneuvers for attitude control of underactuated satellites

An asymptotically stable set point control law is introduced for attitude control of the underactuated spacecraft of a satellite system through its appendage shape changes. It is proposed that rotational maneuvers of the spacecraft are possible while simultaneously achieving a desired shape if the total angular momentum is conserved and the system is initially motionless. In addition, it is also proposed that shape changes can be used to stabilize a tumbling satellite system with only one reaction wheel. The approach assumes that appendage shape changes can result in effective changes in inertia properties. A robust control algorithm based on sliding mode approach was proposed for stabilization and tracking control of underactuated multibody mechanical systems in an earlier work. It was shown that the construction of first order sliding surfaces leads only to marginally stable control law when angular momentum is conserved and equals zero. Here, we propose that the marginally stable control laws can be put together to achieve an asymptotically stable discontinuous control law. The control law essentially uses shape changes leading to changes in effective moments of inertia of the system about the axis of rotation. It is proposed that repetitive application of such maneuvers will lead to asymptotic convergence of the shape to the desired configuration. The controller is applied to the model of an existing complex satellite system and the relevant maneuvers are discussed.