In the past two decades, there has been a renewed and sustained effort devoted to modeling the dynamics of air-breathing hypersonic vehicles, for both simulation and control design purposes. The highly nonlinear characteristics of flight dynamics in hypersonic regimes and the consequent significance variability of the response with the operating conditions requires the development of innovative flight control solutions, hence the development of suitable model of the vehicle dynamics that are amenable to design, validation and rapid calibration of control algorithms. In this dissertation, a control-oriented and a simulation model of a generic hypersonic vehicle were derived to support the design and calibration of model-based flight controllers. A nonlinear robust adaptive controller was developed on the basis of the control-oriented model, that was shown to provide stable trajectory tracking in higher fidelity computer simulations.
The first stage of this research was the development of a control design model (CDM) for the 6-degree-of-freedom dynamics of an air-breathing hypersonic aircraft based on an available high-fidelity first principle model. A method that incorporates the theory of compressible fluid dynamics and system identification methods, was proposed and implemented. The development of the CDM is based on curve fit approximation of the forces and moments acting on the vehicle, making the model suitable for control design. Kriging and Least Squares methods were used to find the most appropriate curve-fitted model of the aerodynamic forces for both the control design and the control simulation models.
It was shown that the 6-DOF model can be both categorized as an under-actuated mechanical system, as well as an over-actuated system with respect to a chosen in- put/output pair of interest.
An important contribution of this work is the development of a nonlinear adaptive controller for the 6-DOF control design model. The controller was endowed with a modular structure, comprised of an adaptive inner-loop attitude controller and a robust nonlinear outer-loop controller of fixed structure. The purpose of the outer- loop controller is to avoid the typical complexity of solutions derived from adaptive backstepping methods. A noticeable feature of the outer-loop controller is the presence of an internal model unit that generates the reference for the angle-of-attack, in spite of parametric model uncertainty. Airspeed, lateral velocity, vehicle’s heading and altitude were considered as regulated outputs of the system. Simulation results on the control simulation model show the effectiveness of the developed controller in spite of significant variation in the flight parameters.