Linear Parameter-Varying Control of an F-16 Aircraft at High Angle of Attack

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Title: Linear Parameter-Varying Control of an F-16 Aircraft at High Angle of Attack
Author: Lu, Bei
Advisors: Dr. Ashok Gopalarathnam, Committee Member
Dr. Fen Wu, Committee Chair
Dr. Paul I. Ro, Committee Member
Dr. Mo-Yuen Chow, Committee Member
Abstract: To improve the aircraft capability at high angle of attack and expand the flight envelope, advanced linear parameter-varying (LPV) control methodologies are studied in this thesis with particular applications of actuator saturation control and switching control. A standard two-step LPV antiwindup control scheme and a systematic switching LPV control approach are derived, and the advantages of LPV control techniques are demonstrated through nonlinear simulations of an F-16 longitudinal autopilot control system. The aerodynamic surface saturation is one of the major issues of flight control in the high angle of attack region. The incorporated unconventional actuators such as thrust vectoring can provide additional control power, but may have a potentially significant pay-off. The proposed LPV antiwindup control scheme is advantageous from the implementation standpoint because it can be thought of as an augmented control algorithm to the existing control system. Moreover, the synthesis condition for an antiwindup compensator is formulated as a linear matrix inequality (LMI) optimization problem and can be solved efficiently. By treating the input saturation as a sector bounded nonlinearity with a tight sector bound, the synthesized antiwindup compensator can stabilize the open-loop exponentially unstable systems. The LPV antiwindup control scheme is applied to the nonlinear F-16 longitudinal model, and compared with the thrust vectoring control approach. The simulation results show that the LPV antiwindup compensator improves the flight quality, and offers advantages over thrust vectoring in a high angle of attack region. For a thrust vectoring augmented aircraft, the actuator sets may be different at low and high angles of attack. Also due to different control objectives, a single controller may not exist over a wide angle of attack region. The proposed switching LPV control approach based on multiple parameter-dependent Lyapunov functions provides a flexible design method with improved performance. A family of LPV controllers are designed, each suitable for a specific angle of attack region. They are switched according to the trajectory of angle of attack so that the closed-loop system remains stable and its performance is optimized. Two switching logics, hysteresis switching and switching with average dwell time, are examined. The control synthesis conditions for both switching logics are formulated as generally non-convex matrix optimization problems. To make the switching LPV control approach more applicable, two convexified methods are given according to the state of the controller is reset or not at switching time. The thrust vectoring augmented F-16 longitudinal model with different control objectives at low and high angles of attack is used to validate the results of the switching control scheme.
Date: 2005-02-02
Degree: PhD
Discipline: Mechanical Engineering

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