Browsing by Author "Morais, Cecilia de Freitas"

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    Digital redesign of analogue dynamic output-feedback controllers for polytopic systems.
    (2017) Morais, Cecilia de Freitas; Braga, Marcio Feliciano; Tognetti, Eduardo Stockler; Oliveira, Ricardo Coração de Leão Fontoura de; Peres, Pedro Luis Dias
    This paper is devoted to the problem known as digital redesign, i.e. given a previously designed stabilising continuous-time controller for a continuous-time plant, synthesise a digital controller that provides the hybrid closed-loop system with output trajectories as similar as possible to the continuous-time ones. To accomplish this goal, two distinct optimisation criteria are investigated: (i) the Euclidean norm of the difference between the dynamic matrix of the discretised closed-loop continuous-time system and the dynamic matrix that represents the discretised open-loop system fed back by the designed digital controller; (ii) the H∞ norm of the transfer function from the noise input to the error between the outputs of the two systems. As main novelties with respect to the existing results on digital redesign, the proposed conditions can deal with polytopic systems, and can synthesise reduced-order dynamic output-feedback digital controllers as well.
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    Discretization and event triggered digital output feedback control of LPV systems.
    (2015) Braga, Marcio Feliciano; Morais, Cecilia de Freitas; Tognetti, Eduardo Stockler; Oliveira, Ricardo Coração de Leão Fontoura de; Peres, Pedro Luis Dias
    This paper investigates the problem of discretization and digital output feedback control design for continuous-time linear parameter-varying (LPV) systems subject to a time-varying networked-induced delay. The proposed discretization procedure converts a continuous-time LPV system into an equivalent discrete-time LPV system based on an extension of the Taylor series expansion and using an event-based sampling. The scheduling parameters are continuously measured and modeled as piecewise constant. A new transmission of the measured output to the controller is triggered by significant changes in the parameters, yielding time-varying transmission intervals. The obtained discretized model has matrices with polynomial dependence on the time-varying parameters and an additive norm-bounded term representing the discretization residual error. A two step strategy based on linear matrix inequality conditions is then proposed to synthesize a digital static scheduled output feedback control law that stabilizes both the discretized and the LPV model. The conditions can also be used to provide robust (i.e., independent of the scheduling parameter) static output feedback controllers. The viability of the proposed design method is illustrated through numerical examples.
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    H1 and H2 control design for polytopic continuous-time Markov jump linear systems with uncertain transition rates.
    (2015) Morais, Cecilia de Freitas; Braga, Marcio Feliciano; Oliveira, Ricardo Coração de Leão Fontoura de; Peres, Pedro Luis Dias
    This paper investigates the problems of H1 and H2 state feedback control design for continuous-time Markov jump linear systems. The matrices of each operation mode are supposed to be uncertain, belonging to a polytope, and the transition rate matrix is considered partly known. By appropriately modeling all the uncertain parameters in terms of a multi-simplex domain, new design conditions are proposed, whose main advantage with respect to the existing ones is to allow the use of polynomially parameter-dependent Lyapunov matrices to certify the mean square closed-loop stability. Synthesis conditions are derived in terms of matrix inequalities with a scalar parameter. The conditions, which become LMIs for fixed values of the scalar, can cope with H1 and H2 state feedback control in both mode-independent and modedependent cases. Using polynomial Lyapunov matrices of larger degrees and performing a search for the scalar parameter, less conservative results in terms of guaranteed costs can be obtained through LMI relaxations. Numerical examples illustrate the advantages of the proposed conditions when compared with other techniques from the literature.
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    Linear quadratic networked control of uncertain polytopic systems.
    (2016) Braga, Marcio Feliciano; Morais, Cecilia de Freitas; Tognetti, Eduardo Stockler; Oliveira, Ricardo Coração de Leão Fontoura de; Peres, Pedro Luis Dias
    This paper investigates the problem of designing robust linear quadratic regulators for uncertain polytopiccontinuous-time systems over networks subject to delays. The main contribution is to provide a procedureto determine a discrete-time representation of the weighting matrices associated to the quadratic criterionand an accurate discretized model, in such a way that a robust state feedback gain computed in the discrete-time domain assures a guaranteed quadratic cost to the closed-loop continuous-time system. The obtaineddiscretized model has matrices with polynomial dependence on the uncertain parameters and an additivenorm-bounded term representing the approximation residual error. A strategy based on linear matrix inequal-ity relaxations is proposed to synthesize, in the discrete-time domain, a digital robust state feedback controllaw that stabilizes the original continuous-time system assuring an upper bound to the quadratic cost ofthe closed-loop system. The applicability of the proposed design method is illustrated through a numericalexperiment. Copyright © 2015 John Wiley & Sons, Ltd.
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    Reduced order dynamic output feedback control of uncertain discrete-time markov jump linear systems.
    (2017) Morais, Cecilia de Freitas; Braga, Marcio Feliciano; Oliveira, Ricardo Coração de Leão Fontoura de; Peres, Pedro Luis Dias
    This paper deals with the problem of designing reduced order robust dynamic output feedback controllers for discretetime Markov jump linear systems (MJLS) with polytopic state space matrices and uncertain transition probabilities. Starting from a full order, mode-dependent and polynomially parameter-dependent dynamic output feedback controller, sufficient linear matrix inequality based conditions are provided for the existence of a robust reduced order dynamic output feedback stabilizing controller assuring an upper bound to the H∞ or the H2 norm of the closedloop system. The main advantage of the proposed method when compared to the existing approaches is the fact that the the decision variables of the problem. In other words, the matrices that define the controller realization do not depend explicitly on the state space matrices associated to the modes of the MJLS. As a consequence, the method is specially suitable to handle order reduction or cluster availability constraints in the context ofH∞ orH2 dynamic output feedback control of discrete-time MJLS. Additionally, as illustrated by means of numerical examples, the proposed approach can provide less conservative results than other conditions in the literature.
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    Robust stability analysis of grid-connected converters based on parameter-dependent Lyapunov functions.
    (2017) Braga, Marcio Feliciano; Morais, Cecilia de Freitas; Maccari Júnior, Luiz Antonio; Tognetti, Eduardo Stockler; Montagner, Vinicius Foletto; Oliveira, Ricardo Coração de Leão Fontoura de; Peres, Pedro Luis Dias
    This paper deals with the problem of robust stability analysis of grid-connected converters with LCL filters controlled through a digital signal processor and subject to uncertain grid inductance. To model the uncertain continuous-time plant and the digital control gain, a discretization procedure, described in terms of a Taylor series expansion, is employed to determine an accurate discrete-timemodel. Then, a linear matrix inequality-based condition is proposed to assess the robust stability of the polynomial discrete-time augmented system that includes the filter state variables, the states of resonant controllers and the delay from the digital control implementation. By means of a parameterdependent Lyapunov function, the proposed strategy has as main advantage to provide theoretical certification of stability of the uncertain continuous-time closed-loop system, circumventing the main disadvantages of previous approaches that employ approximate discretized models, neglecting the errors. Numerical simulations illustrate the benefits of the discretization technique and experimental results validate the proposed approach.