Disturbance accommodating sliding mode controller for spacecraft attitude maneuvers

In the absence of an external disturbance and uncertainty, sliding mode (variable structure) control is guaranteed to asymptotically stabilize a system, which is provided by using a correction control input calculated using a Lyapunov-type condition i.e., sliding mode existence condition. When bounded unmodeled external torques are added, the closed-loop system is no longer globally asymptotically stable since steady-state errors are present. The error can be minimized by increasing the correction control gain or decreasing the thickness of boundary layer of sliding mode control. But for limited actuator capability the maximum control gain and the minimum thickness of boundary layer being allowed may be restricted. Disturbance accommodating control is a signal synthesis adaptive control. For a short time interval the disturbance is assumed to be modeled by a linear combination of previously selected basis functions. A disturbance accommodating observer can be used to identify unmeasurable internal and external disturbances. In this paper, sliding mode control is combined with disturbance accommodating control (i.e., Disturbance Accommodating Sliding Mode Control) in terms of modified Rodrigues parameters for a spacecraft attitude regulation and tracking maneuvers. The presented disturbance accommodating sliding mode control has the following advantages: 1) the design procedure is more effective than the traditional sliding surface stabilizing problem since steady-state errors are reduced, 2) the designed disturbance accommodating observer is linear, and 3) the robustness of sliding mode is guaranteed in the range of actuator capability. Simulation results are shown that use the disturbance accommodating sliding mode control to reduce steady-state errors in the case of applied external disturbances.