Control of wing rock phenomenon with a variable universe fuzzy controller

Wing rock is a highly nonlinear phenomenon in which aircraft undergo limit-cycle roll oscillations at a high angle of attack (AOA). It is a challenge to design an appropriate controller, especially with modeling errors and external disturbances. The methodology of fuzzy logic control (FLC) appears very useful when a process is too complex or when an available source of information is interpreted qualitatively, inexactly, or uncertainly, but we also note that the FLC of a process under disturbances usually exhibit a tracking error when the controlled system tends to steady state. In this paper, a variable universe fuzzy control design approach is utilized to improve both tracking precision and robustness of fuzzy PD control. A switching mechanism is developed to achieve this control scheme: when the tracking error is in a large range, fuzzy PD control is used to keep fast adjustments and to reduce the error; when the tracking error is in a small range, variable universe fuzzy control is then used as a fine controller to eliminate the error. Simulation studies for the nonlinear wing-rock control show that the new control scheme is a powerful tool to improve control system performances.