Robust Fractional Order Attitude Controller for Launch Vehicle with Flexible Dynamics

Abstract

The importance of Fractional Order Proportional Integral Differential (FOPID) controller is increased due to the presence of two design parameters more than the conventional PID controller. This paper uses of these additional degrees of freedom in designing the attitude controller for launch vehicle system with unstable rigid body dynamics and flexible dynamics. The parameters of the controller are designed to meet time domain specifications by optimizing the performance indices using genetic algorithms. The proposed tracking controller exhibits the robustness property to the parameter variations during highly unstable atmospheric phase of the attitude controller. The tracking performance of the controller is analysed with flexible vehicle dynamics in presence of two bending modes. The proposed controller can reduce the steady state error by 30% and the settling time is reduced by 50%. The time domain analysis confirms the effectiveness of the proposed controller even in the presence of unmodelled dynamics and wind disturbance without losing stability of launch vehicle. In order to assure the robustness property of the proposed controller, the plant parameters such as control moment coefficient and aerodynamic coefficients are varied by 25%. The tracking performance with these off nominal parameters shows only 1% deviation from the response of plant with nominal parameters.