The objective of this research has been twofold: (1) to develop nonlinear control design techniques that can systematically accommodate nonlinearities and have optimality properties, and (2) develop control methods for robust stabilization and optimization of performance of aeroengine compressor systems.

Controls research under this program has concentrated on the development of linear and nonlinear robust fixed structure control for aerospace systems. Specifically, a unified robust nonlinear design framework was developed that provides significant extensions of state of the art recursive backstepping nonlinear control methods to include notions of optimality, robustness, disturbance rejection, actuator constraints, and adaptation. Furthermore, a general nonlinear control design framework predicated on a hierarchical switching controller architecture was developed. The proposed framework provides a rigorous alternative to designing gain scheduled feedback controllers that guarantee global closed loop system stability for nonlinear systems. The aforementioned nonlinear control design frameworks were applied to the control of thermoacoustic combustion instabilities and compressor aerodynamic instabilities involving rotating stall and surge in aeroengines.

This book aims to develop systematic design methodologies to model-based nonlinear control of aeroengines, focusing on (1) modelling of aeroengine systems—both component-level and identification-based models will be extensively studied and compared; and (2) advanced nonlinear control designs—set-point control, transient control and limit-protection control approaches will all be investigated. The model-based design has been one of the pivotal technologies to advanced control and health management of propulsion systems. It can fulfil advanced designs such as fault-tolerant control, engine modes control and direct thrust control. As a consequence, model-based design has become an important research area in the field of aeroengines due to its theoretical interests and engineering significance. One of the central issues in model-based controls is the tackling of nonlinearities. There are publications concerning with either nonlinear modelling or nonlinear controls; yet, they are scattered throughout the literature. It is time to provide a comprehensive summary of model-based nonlinear controls. Consequently, a series of important results are obtained and a systematic design methodology is developed which provides consistently enhanced performance over a large flight/operational envelope, and it is thus expected to provide useful guidance to practical engineering in aeroengine industry and research.

This five year project has brought together researchers in nonlinear control theory (University of California and Caltech) and aeroengine dynamics and control (MIT) to develop new techniques for robust nonlinear Control of aeroengines. Technology transfer to the commercial sector has been achieved through strong and synergistic coupling with Pratt & : Whitney (P & :W) via United Technologies Research Center (UTRC). The specific application areas addressed were active control of. first, compression system rotating stall and surge, and second, blade flutter and forced vibration. These complex physical phenomena, which bear heavily on engine safety and performance, are arguably the most critical considerations in modern aeroengine design. The outcomes of this project are new tools and methods promising to improve both operability and reliability, of military and commercial aeroengines through robust nonlinear control. A highly integrated multi-disciplinary effort has successfully interlaced ideas and results from fluid dynamics, structural dynamics, experiments, and nonlinear control theory. For the highly nonlinear behavior associated with rotating stall and large disturbances new analytical, numerical, and experimental nonlinear techniques have been developed, transitioned to the industrial partner and widely disseminated to the engineering community. The results have appeared in over 40 journal papers, 50 conference proceedings as well as in 2 patents 1.2. Among the important spin-offs of this project are the new AFOSR flow control and mixing projects by M. Krsitc, I. Mezic, and B. Bamieh. They are now better prepared to broaden the scope of control theory as a vital enabling technology.

This paper examines the use of adaptive backstepping for multi-axis control of a high performance aircraft. The control law is demonstrated on a 6 Degree-of-Freedom simulation with nonlinear aerodynamic and engine models, actuator models with saturation, and turbulence. Simulation results are demonstrated for large pitch-roll maneuvers, and for maneuvers with failure of the right stabilator. There are substantial differences between the control law design and simulation models, which are used to demonstrate some robustness aspects of this control law. Actuator saturation is shown to be a considerable problem for this type of controller. However, the flexibility of the backstepping design provides opportunities for improvement. In particular, the Lyapunov function is modified so that the growth of integrated error and the rate of change of parameter growth are both reduced when the surface commands are growing at a rate that will likely saturate the actuators. In addition, the deadzone technique from robust linear adaptive control is applied to improve robustness to turbulence.

An interdisciplinary program in robust control for nonlinear systems with applications to a variety of engineering problems is outlined. Major emphasis will be placed on flight control, with both experimental and analytical studies. This program builds on recent new results in control theory for stability, stabilization, robust stability, robust performance, synthesis, and model reduction in a unified framework using Linear Fractional Transformations (LFT's), Linear Matrix Inequalities (LMI's), and the structured singular value micron. Most of these new advances have been accomplished by the Caltech controls group independently or in collaboration with researchers in other institutions. These recent results offer a new and remarkably unified framework for all aspects of robust control, but what is particularly important for this program is that they also have important implications for system identification and control of nonlinear systems. This combines well with Caltech's expertise in nonlinear control theory, both in geometric methods and methods for systems with constraints and saturations. Doyle, John C. and Murray, Richard and Morris, John Unspecified Center NAG2-792...