Robust Nonlinear Control Of Stall And Flutter In Aeroengines

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 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.

The research completed has established connections between systematic methods of nonlinear stabilization and nonlinear optimal control. The redesigns of adaptive and robust nonlinear controllers developed under this grant are inverse optimal, and thus use less control effort and possess a certain margin of robustness to some uncertainties. This grant has also produced the first methods for stabilization for stochastic nonlinear systems. The most significant among the results is disturbance attenuation and adaptive stabilization for systems with noise of unknown covariance. Finally, the grant has revived a whole research area called extremum seeking control which deals with non-model-based on-line optimization. The PI and the group obtained the first stability guarantees for extremum seeking schemes and proposed techniques for their application to much more general classes of systems than previously possible. The ultimate goal of the theoretical research was an application to nonlinear instabilities arising in aeroengines. The PI has advanced the state of the art in this area in three directions. First he developed the first nonlinear control laws with guaranteed regions of attraction and robustness to some modeling errors. Second, he pioneered the methods for seeking of the maximum of the compressor pressure rise characteristic. Third, he designed the first model-based adaptive controllers for thermoacoustic instabilities in combustion chambers. The results of the work have been published (or submitted for publication) in 1 book, 22 journal papers, a number of conference papers, and four book chapters.

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...

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This project addresses the development of both control synthesis and analysis methods for affordable, highly integrated nonlinear aircraft control systems through basic research and development of control science and dynamical systems theory. The technical approach is to explore the following three primary research areas: (1) stability analysis, (2) robust nonlinear control, and (3) robust reconfigurable control. The stability analysis task assesses robustness of a baseline flight control system. The analysis results are used to develop on-board model and control allocation requirements for the other two tasks. The robust nonlinear control task includes development of compact on-board model and dynamic control allocation synthesis methods. The reconfigurable control design task includes development of an on-board model update synthesis method. The main accomplishment for the stability analysis task is using structured singular value analysis to formalize control allocation stability implications and to specify stability and control derivative accuracy requirements for on-board model synthesis. The main accomplishments for the robust nonlinear control design task are development of a dynamic control allocation formulation and development of a compact on-board model synthesis method. The main accomplishment for the robust reconfigurable control design task is the development of an on-line learning method for on-board model updates to enhance the robustness of indirect adaptive flight control systems.

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The objective of this research was to develop tools leading to effective control strategies for tailless fighter aircraft. Emphasis was placed on the development of control algorithms that yield robust performance in the presence of actuator magnitude and rate limits and that account for the interaction of a pilot with the airframe and control system. The results of this research are documented in the 22 papers that are listed in this report. These papers present the development of numerous new analysis tools for nonlinear control systems with an emphasis on disturbance attenuation and sampled data control. These algorithms were applied to the manual flight control problem for open loop unstable fighter aircraft.

"Air flow velocity field control is of crucial importance in aerospace applications to prevent the potentially destabilizing effects of phenomena such as cavity ow oscillations, flow separation, flow-induced limit cycle oscillations (LCO) (flutter), vorticity, and acoustic instabilities. Flow control is also important in aircraft applications to reduce drag in aircraft wings for improved flight performance. Although passive flow control approaches are often utilized due to their simplicity, active flow control (AFC) methods can achieve improved flight performance over a wider range of time-varying operating conditions by automatically adjusting their level of control actuation in response to real-time sensor measurements. Although several methods for AFC have been presented in recent literature, there remain numerous challenges to be overcome in closed-loop nonlinear AFC design. Additional challenges arise in control design for practical systems with limited onboard sensor measurements and uncertain actuator dynamics. In this thesis, robust nonlinear control methods are developed, which are rigorously proven to achieve reliable control of fluid flow systems under uncertain, time-varying operating conditions and actuator model uncertainty. Further, to address the practical control design challenges resulting from sensor limitations, this thesis research will investigate and develop new methods of sliding mode estimation, which are shown to achieve finite-time state estimation for systems with limited onboard sensing capabilities. The specific contributions presented in this thesis include: 1) the application of proper orthogonal decomposition (POD)-based model order reduction techniques to develop simplified, control-oriented mathematical models of actuated fluid flow dynamic systems; 2) the rigorous development of nonlinear closed-loop active flow control techniques to achieve asymptotic regulation of fluid flow velocity fields; 3) the design of novel sliding mode estimation and control methods to regulate fluid flow velocity fields in the presence of actuator uncertainty; 4) the design of a nonlinear control method that achieves simultaneous fluid flow velocity control and LCO suppression in a flexible airfoil; and 5) the analysis of a discontinuous hierarchical sliding mode estimation method using a differential inclusions-based technique."--Abstract.