Structured H 1 Command and Control-Loop Design for Unmanned Helicopters J. Gadewadikar * Alcorn State University, Lorman Mississippi 39096 F. L. Lewis † and Kamesh Subbarao ‡ University of Texas at Arlington, Fort Worth, Texas 76118 and Ben M. Chen § National University of Singapore, Singapore 117576, Republic of Singapore DOI: 10.2514/1.31377 The aim of this paper is to present rigorous and efficient methods for designing flight controllers for unmanned helicopters that have guaranteed performance, intuitive appeal for the flight control engineer, and prescribed multivariable loop structures. Helicopter dynamics do not decouple as they do for the fixed-wing aircraft case, and so the design of helicopter flight controllers with a desirable and intuitive structure is not straightforward. We use an H 1 output-feedback design procedure that is simplified in the sense that rigorous controller designs are obtained by solving only two coupled-matrix design equations. An efficient algorithm is given for solving these that does not require initial stabilizing gains. An output-feedback approach is given that allows one to selectively close prescribed multivariable feedback loops using a reduced set of the states at each step. At each step, shaping filters may be added that improve performance and yield guaranteed robustness and speed of response. The net result yields an H 1 design with a control structure that has been historically accepted in the flight control community. As an example, a design for stationkeeping and hover of an unmanned helicopter is presented. The result is a stationkeeping hover controller with robust performance in the presence of disturbances (including wind gusts), excellent decoupling, and good speed of response. Nomenclature A = system or plant matrix a s = longitudinal blade angle B = control-input matrix b s = lateral blade angle C = output or measurement matrix D = disturbance matrix D in = inner-loop disturbance matrix D o = outer-loop disturbance matrix dt = disturbance G = nominal plant G s = loop-shaped plant K = static output-feedback gain matrix p = roll rate in the body-frame components Q = state weighting matrix q = pitch rate in the body-frame components R = control weighting matrix r = yaw rate in the body-frame components r fb = yaw-rate feedback U = velocity along the body-frame x axis ut = control input V = velocity along the body-frame y axis W = velocity along the body-frame z axis X = inertial position x axis x in t = inner-loop state vector x o t = outer-loop state vector Y = inertial position y axis y in t = inner-loop output vector y o t = outer-loop output vector Z = inertial position z axis zt = performance output  = system L 2 gain  in = L 2 gain inner loop  o = L 2 gain outer loop  = pitch angle  = roll angle  = yaw angle I. Introduction O VER the past few years, there has been significant interest in using unmanned aerial vehicles for applications such as search and rescue, surveillance, and remote inspection. Rotorcraft (especially helicopters) have several significant advantages over conventional fixed-wing platforms in conducting several of these tasks. The advantages are exemplified by certain unique capabilities of rotorcraft; for example, they can hover and they can take off and land in very limited spaces. The ability to reliably follow prescribed 3-D position and yaw commands in the presence of disturbances is a requirement common to rotary-wing unmanned aerial vehicles (UAVs). Position-tracking control system design for a UAV is challenging because strong coupling among all states is present in the rotorcraft, which must be confronted in any design technique. Moreover, the rotor flexibility dynamics must generally be included in any design to guarantee stability robustness [1]. In fixed-wing aircraft control, by contrast, the dynamics conveniently decouple in Received 3 April 2007; revision received 23 October 2007; accepted for publication 2 November 2007. Copyright © 2007 by Jyotirmay Gadewadikar. Published by the American Institute of Aeronautics and Astro- nautics, Inc., with permission. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0731-5090/08 $10.00 in correspondence with the CCC. * Assistant Professor, Robotics and Automation, Department of Advanced Technologies, 1000 ASU Drive, No. 360; jyo@alcorn.edu. † Associate Director of Research, Automation and Robotics Research Institute, 7300 Jack Newell Boulevard South; Professor, Department of Electrical Engineering. ‡ Assistant Professor, Department of Mechanical and Aerospace Engineering, 500 West First Street, Box 19018, 211 Woolf Hall. Life Member AIAA. § Professor, Department of Electrical and Computer Engineering. JOURNAL OF GUIDANCE,CONTROL, AND DYNAMICS Vol. 31, No. 4, July–August 2008 1093