Large Envelope Flight Control System Robust Design Study

Robust control theories and their application on flight control system design are mainly studied in this dissertation to design large envelope flight control system. Robust control methods such as H~,, control theory, structural singular value synthesis and self- scheduled control of linear parameter-varying system are respectively studied and applied to the design of the large envelope flight control law. The main two contributions are summarized as follows: Part one: Considering the characteristics of large envelope aircraft, the description of plant uncertainty, division of flight regions and the selection of nominal plant in each region are investigated and new methods of flight region division and nominal plant selection are proposed. (1) One reason for the conservatism of robust control is that the uncertainty range taken into account during design is too wide, which restricts the performance of the closed-loop system. Accordingly, several methods of uncertainty description are discussed, and a new method with less conservatism is put forward during the design of H0. controller, which turned out to be effective in maintaining the robust stability and performance of the final system simultaneously. (2) Further analysis showed that the previous method given in (1) hadn抰 completely overcome the conservatism of uncertainty description. Then another method using linear fraction transformation is raised. Applying ii synthesis, a time domain and a frequency domain description of plant uncertainty are put forward separately based on the linear fraction transformation concept. The results proved that these two description methods could greatly reduce the conservatism in the design of robust controller. (3) Having studied the methods of dividing flight regions and the selection of nominal plant in corresponding region, a new way of division and selection is put forward which proved to be effective in maintaining the robustness and performance of the aircraft. A software on the division of flight region is also presented. Part two: Based on the description of large envelope aircraft, robust H0. control theory, structural singular value synthesis and linear parameter-varying system抯 self- scheduled control methods are respectively used to design the large envelope flight control system. (I) H0. control for parameter perturbation system is studied and this technique is used to design large envelope augmented stability flight control system. Robust mixed III -7 sensitivity H~, control is studied and this technique is used to design large envelope pitch angle control system. (2) Based on the time domain and frequency domain descriptions of plant uncertainty, lateral direction flight control system is designed applying structural value synthesis method. (3) Self-scheduled control of linear parameter-varying system is studied and the problems of current method are discussed. A lot of restrictive conditions that baffling the real applications are overcame. Finally a large envelope self-scheduled pitch rate control system is designed using single Lyapunov function method.