Path-Following Control for Coordinated Turn Aircraft Maneuvers David Cabecinhas * and Carlos Silvestre † and Paulo Rosa ‡ and Rita Cunha § Instituto Superior T´ ecnico, Institute for Systems and Robotics, 1049-001 Lisbon, Portugal This paper addresses the path-following problem of steering an autonomous airplane along a predefined 3-D path, while performing a coordinated turn maneuver. The presented solution relies on the definition of a path-dependent error space to express the dynamic model of the vehicle, and of an output tracking error that guarantees both path-following and coordinated turn compliance. For controller design purposes, the error dynamics are approximated by a polytopic Linear Parameter Varying (LPV) system representation with piecewise affine dependency on the parameters. The synthesis problem is stated as an H2 minimization problem with pole placement contraints, and solved using Linear Matrix Inequalities (LMIs). The nonlinear controller is implemented within the scope of gain- scheduling control theory, using the D-methodology. The performance of the designed controller is assessed in simulation, using the full nonlinear model of a small scale airplane. I. Introduction In recent years, the advent of enabling technology in sensing, computation, and communications has empowered Unmanned Aerial Vehicles (UAVs) to become cost-effective, reliable, and safe alternatives to conventional piloted aircraft in a wide variety of applications. For example, in forest fire surveillance, a small unmanned autonomous aircraft equipped with a camera and other sensing devices can be used to automatically detect fires and alert the nearest fire department. Other types of missions, where UAVs already play a key role, include surveillance and reconnaissance operations. These assignments are typically conducted in hostile and unexplored territory, where the risk of losing the aircraft is high. With UAVs, these missions can be accomplished without endangering human lives. Autonomous vehicles have great potential to perform high precision and repetitive tasks, since a whole new range of sensing devices can provide them with information that is not perceptible by a pilot. With this added information, autonomous flight control systems can be designed to provide enhanced performance and efficiency. Several different approaches to the problem of flight control for autonomous aircraft can be found in the literature. For example, in References 1 and 2, nonlinear control strategies are used to address the trajectory-tracking problem, whereas Shue et al. 3 present a mixed H 2 /H ∞ gain scheduling technique for aircraft control. A robust control solution for coordinated turn maneuvers is proposed in Reference 4. This paper addresses the problem of aircraft control for coordinated turn maneuvers, using a path- following approach. By lifting the strict temporal restrictions that are dictated by a time-parameterized reference, the path-following approach provides an alternative to trajectory-tracking that typically results in smoother convergence to the path and less demand on the control effort. The coordinated turn maneuver improves the aircraft’s flying qualities and performance, by constraining the vehicle’s orientation to be such that there is no component of aerodynamic force along the y-axis of a body-fixed coordinate frame. 5 In this paper, the path-following problem is addressed along the lines of the work reported in References 6–8. It relies on applying a nonlinear transformation to aircraft dynamic model to obtain a conveniently defined error model. The resulting error vector comprises velocity errors, orientation errors, and the distance to the * PhD Student, Department of Electrical Engineering and Computer Science (DEEC), Av. Rovisco Pais 1; dcabecinhas@isr.ist.utl.pt. † Assistant Professor, DEEC, Av. Rovisco Pais 1; cjs@isr.ist.utl.pt. Member AIAA. ‡ PhD Student, DEEC, Av. Rovisco Pais 1; prosa@isr.ist.utl.pt. § PhD Student, DEEC, Av. Rovisco Pais 1; rita@isr.ist.utl.pt. 1 of 19 American Institute of Aeronautics and Astronautics AIAA Guidance, Navigation and Control Conference and Exhibit 20 - 23 August 2007, Hilton Head, South Carolina AIAA 2007-6656 Copyright © 2007 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.