DESIGN OF A ROBUST DYNAMIC INVERSION LATERAL/DIRECTIONAL FLIGHT CONTROLLER Sadok Hougui* Gary J. Balast William L. Garrardt Department of Aerospace Engineering and Mechanics University of Minnesota, Minneapolis MN 55455 Abstract In this paper dynamic inversion and structured sin- gular value (p) techniques are applied to the de- sign of lateral/directional controllers for a high per- formance aircraft. The controllers are designed to achieve specified handling qualities with robustness to modeling errors. Introduction This paper describes the application of dynamic inversion and structured singular value to the la- teral/directional control of a high performance aircraft. The aircraft considered is the NASA High Alpha Research Vehicle (HARV), a typical future fighter aircraft. It has the ability to fly at high angles of attack, and longitudinal and lateral thrust vecto- ring capability as well as conventional aerodynamic control surfaces. Since the aircraft must operate at a wide range of flight conditions, design and implementation of a flight control system using standard gain schedu- ling techniques require a great deal of engineering effort. Recently there have been a number of pa- pers in which dynamic inversion has been applied to the design of aircraft control systems to reduce gain scheduling and improve performance [I]-[6]. In dy- namic inversion, feedback control is used to "cancel" the system dynamics and replace them with desired 'Research Assistant, student member AIAA +~ssistant Professor, member AIAA t~rofessor, associate fellow AIAA response characteristics. The original system is re- duced to a simple system of integrators, and control laws for the simplified system can then be designed using a variety of approaches. Standard dynamic inversion techniques assume that the system dynamics can be canceled exactly. In fact the system dynamics are never known exactly; the- refore, exact cancellation can never be achieved. In order to use dynamic inversion successfully, it is ne- cessary to design control laws which are robust to er- rors resulting from inexact cancellation of the system dynamics. The approach used in this paper is to use inner loop dynamic inversion to cancel the system dynamics and then design outer loop linear control laws to provide desired dynamic response and r e bustness to errors. The outer loop control laws are designed using p synthesis. This paper follows an a p proach developed previously for longitudinal design using dynamic inversion and p synthesis [7]. Mathe- matical models of the errors resulting from inexact cancellztion of lateral/directional system dynamics are developed. These error models are incorporated in the p synthesis design model. The p synthesis control laws are designed to minimize the weighted error between the aircraft response and the response of desired handling quality models for lateral and directional response. Measurement errors and limits on commanded angular accelerations are also inclu- ded in the control design. Copyright O 1995 by the American Institute of Aeronautics 738 and Astronautics, Inc. All rights reserved. The aircraft response is evaluated using a high fide- lity digital nonlinear simulation of the HARV which includes nonlinear aerodynamics, digital implemen- tation of control laws, sensor and actuator dynamics, and limits on actuator deflections and rates. The