Optimal Nonlinear Dynamic Inversion-Based Flight Control System Design for an Aerospace Vehicle

Abstract

Design of flight control systems for an aerospace vehicle is a significant challenge to the control designer. Since the aerospace vehicle is highly nonlinear system, the linear control design may not be effective as well as does lead to large operational domains. Once the number of operating points increases, gain scheduling process in linear control technique becomes cumbersome. To overcome the demerits of linear control design techniques, the nonlinear control strategy techniques can be applied. This paper first discusses mathematical model of the aerospace vehicle. In the present work, the nonlinear control methods are designed to achieve effective performance of an aerospace vehicle without linearizing the plant dynamics. The vehicle dynamics are separated using timescale separation principle, and control synthesis is designed using nonlinear dynamic inversion (NDI) methodology as well as optimal dynamic inversion (ODI). Nonlinear dynamic inversion is the control synthesis technique in which the inherent dynamics of a dynamical system are cancelled out and replaced by desired performance parameters which are selected by the designer. Two-loop control structure is adopted based on vehicle dynamics. In this approach, the outer loop control law generates yaw rate, pitch rate and roll rate commands for the inner loop, based on the side slip angle, angle of attack and bank angle commands. These body rate commands are further tracked by inner loop control law by generating necessary control surface deflections. Since the number of control surfaces is more than the tracking objectives, the inner loop control law is designed by using optimal dynamic inversion (ODI) technique. Nonlinear dynamic inversion and optimal dynamic inversion methods are popular in the application of control design. The detailed generic formulation for the NDI and ODI is discussed. Different perturbation studies on aerodynamic parameters are completed, and robustness of the design is validated. The second-order actuator model for the aerodynamic control surface (fins) is considered in this design. The nominal control law is designed and compared the simulation results for a step input of guidance profiles. Later nominal control design results are compared with respect to off-nominal cases like upper, lower bounds. All the design specification parameters show the satisfactory performance, and design is validated using different guidance profiles. To study the robustness of the control design against parameter uncertainty or model inaccuracies, the design is validated using different guidance profiles.