Spacecraft Attitude Fractional Feedback Control Using Rotation Matrices and Exponential Coordinates

Abstract

Two fractional proportional–integral–derivative controllers are proposed for rigid spacecraft rotational dynamics. In the first strategy, the controller is developed on the tangent bundle of SO(3), which is the Lie group of rigid-body rotational motion, using states consisting of a rotation matrix and an angular velocity vector in the fractional-order derivative and integral feedback terms. In the second strategy, the exponential coordinates are used to design the controller. The fractional derivative and integral feedback terms have adjustable orders that can be used to tune the closed-loop response. The performances of the controllers are studied numerically for different values of the derivative and integral fractional orders in terms of control effort, steady-state response characteristics, and robustness to unmodeled disturbances; and the results are compared with those of an integer-order controller. It is shown that the inclusion of fractional orders in the controllers allows for a desired shape and spectrum of the controlled response to be obtained by tuning the fractional derivative and integral orders. For instance, the settling time and control effort may be simultaneously reduced, in contrast to the case of integer feedback control in which a tradeoff exists between these competing design specifications. The proposed fractional feedback attitude control strategies thus allow for a greater degree of flexibility over their integer-order analogs.