Discrete-Time Adaptive Posicast Controller for Uncertain Time-Delay Systems Khalid Abidi * Bahcesehir University, Dept. of Mechatronics, Besiktas 34353, Istanbul, Turkey Yildiray Yildiz † U. C. Santa Cruz, NASA Ames Research Center, Moffett Field, CA 94035, USA This paper presents the discrete-time Adaptive Posicast Controller (APC). The APC is an adaptive controller with explicit delay compensation built upon the ideas of the classical Smith Predictor, finite spectrum assignment (FSA) and adaptive control. A known result in the computer implementation of FSA is that as the approximation of the finite integral term gets finer, the stability margins get smaller. It can be shown that this approximation can even result in instability. Since it is based on FSA, APC also has the same issue. Although in earlier automotive (experimental) and flight control (simulation) implementations dramatic performance improvements were obtained, the full benefit of APC could not be realized due to this sensitivity to the integral approximation. In this paper, this problem is solved by developing discrete-time APC and thus eliminating the need for approximation. Simulation results are reported where APC is applied to control the longitudinal dynamics of a jet transport aircraft. These results demonstrate improved performance compared to conventional model reference adaptive controller (MRAC) and compared to the discrete-time approximation of the continuos APC. With the development of this discrete version, APC represents one of the most powerful candidates to handle uncertainties and time-delays in system dynamics. I. Introduction Time-delays are common in dynamical systems as computational delays, input delays, measurement delays and transportation/convection lags to name a few. A wide range of applications involve time-delay systems including chemical, biological, mechanical, physiological and electrical systems. 1–3 Detailed surveys of time-delay systems can be found in the literature. 4, 5 Time delays pose a problem for control engineers since they may induce instability or bad performance for the closed loop system due to the introduction of considerable phase lag. In many controller designs, the delay is neglected and stability and robustness margins are given with respect to delay. The same approach can also be found in some adaptive control designs. 6 However, in general, these approaches may produce small delay margins. The Smith Predictor (SP), proposed by Otto Smith in the 1950s, 7 is one of the most popular methods for controlling systems with time-delays. The main idea in this approach is predicting the future output of the plant, using a plant model, and using this prediction to cancel the effect of the time-delay. 8–12 The SP method, however, is not suitable for unstable systems due to the possibility of unstable pole- zero cancellations. A method based on finite spectrum assignment (FSA) was introduced by Manitius and * Corresponding Author. † Research scientist, AIAA Member. 1 of 11 American Institute of Aeronautics and Astronautics AIAA Guidance, Navigation, and Control Conference 08 - 11 August 2011, Portland, Oregon AIAA 2011-6442 Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.