Iasson Karafyllis

Stability and stabilization of nonlinear systems

Introduction to Control Systems.- Internal Stability Notions and Characterizations.- Converse Lyapunov Results.- External Stability Notions and Characterization.- Advanced Methods.- The Robust Output Feedback Stabilization Problem.- Applications.

Nonlinear Stabilization Under Sampled and Delayed Measurements, and With Inputs Subject to Delay and Zero-Order Hold

Sampling arises simultaneously with input and output delays in networked control systems. When the delay is left uncompensated, the sampling period is generally required to be sufficiently small, the delay sufficiently short, and, for nonlinear systems, only semiglobal practical stability is generally achieved. For example, global stabilization of strict-feedforward systems under sampled measurements, sampled-data stabilization of the nonholonomic unicycle with arbitrarily sparse sampling, and sampled-data stabilization of LTI systems over networks with long delays, are open problems. In this paper, we present two general results that address these example problems as special cases. First, we present global asymptotic stabilizers for forward complete systems under arbitrarily long input and output delays, with arbitrarily long sampling periods, and with continuous application of the control input. Second, we consider systems with sampled measurements and with control applied through a zero-order hold, under the assumption that the system is stabilizable under sampled-data feedback for some sampling period, and then construct sampled-data feedback laws that achieve global asymptotic stabilization under arbitrarily long input and measurement delays. All the results employ “nominal” feedback laws designed for the continuous-time systems in the absence of delays, combined with “predictor-based” compensation of delays and the effect of sampling.

From Continuous-Time Design to Sampled-Data Design of Observers

In this work, a sampled-data nonlinear observer is designed using a continuous-time design coupled with an inter-sample output predictor. The proposed sampled-data observer is a hybrid system. It is shown that under certain conditions, the robustness properties of the continuous-time design are inherited by the sampled-data design, as long as the sampling period is not too large. The approach is applied to linear systems and to triangular globally Lipschitz systems.

Stabilization by Means of Approximate Predictors for Systems with Delayed Input

Sufficient conditions for global stabilization of nonlinear systems with delayed input by means of approximate predictors are presented. An approximate predictor is a mapping which approximates the exact values of the stabilizing input for the corresponding system with no delay. A systematic procedure for the construction of approximate predictors is provided for globally Lipschitz systems. The resulting stabilizing feedback can be implemented by means of a dynamic distributed delay feedback law. Illustrative examples show the efficiency of the proposed control strategy.

Small-gain theorem for a wide class of feedback systems with control applications

A Small-Gain Theorem, which can be applied to a wide class of systems that includes systems satisfying the weak semigroup property, is presented in the present work. The result generalizes all existing results in the literature and exploits notions of weighted, uniform and non-uniform Input-to-Output Stability (IOS) property. Applications to partial state feedback stabilization problems with sampled-data feedback applied with zero order hold and positive sampling rate, are also presented.

On the Liapunov-Krasovskii methodology for the ISS of systems described by coupled delay differential and difference equations

The input-to-state stability of time-invariant systems described by coupled differential and difference equations with multiple noncommensurate and distributed time delays is investigated in this paper. Such equations include neutral functional differential equations in Hales form (which model, for instance, partial element equivalent circuits) and describe lossless propagation phenomena occurring in thermal, hydraulic and electrical engineering. A general methodology for systematically studying the input-to-state stability, by means of Liapunov-Krasovskii functionals, with respect to measurable and locally essentially bounded inputs, is provided. The technical problem concerning the absolute continuity of the functional evaluated at the solution has been studied and solved by introducing the hypothesis that the functional is locally Lipschitz. Computationally checkable LMI conditions are provided for the linear case. It is proved that a linear neutral system in Hales form with stable difference operator is input-to-state stable if and only if the trivial solution in the unforced case is asymptotically stable. A nonlinear example taken from the literature, concerning an electrical device, is reported, showing the effectiveness of the proposed methodology.

Global exponential sampled-data observers for nonlinear systems with delayed measurements

Abstract This paper presents new results concerning the observer design for certain classes of nonlinear systems with both sampled and delayed measurements. By using a small gain approach we provide sufficient conditions, which involve both the delay and the sampling period, ensuring exponential convergence of the observer system error. The proposed observer is robust with respect to measurement errors and perturbations of the sampling schedule. Moreover, new results on the robust global exponential state predictor design problem are provided, for wide classes of nonlinear systems.

A vector Small-Gain Theorem for general nonlinear control systems

A new Small-Gain Theorem is presented for general nonlinear control systems described either by ordinary differential equations or by retarded functional differential equations. The novelty of this research work is that vector Lyapunov functions and functionals are utilized to derive various input-to-output stability results. It is shown that the proposed approach recovers several recent results as special instances and is extendible to other important classes of control systems.

The Non-uniform in Time Small-Gain Theorem for a Wide Class of Control Systems with Outputs

The notions of non-uniform in time robust global asymptotic output stability and non-uniform in time input-tooutput stability (IOS) are extended to cover a wide class of control systemswith outputs that includes (finite or infinitedimensional) discrete-time and continuous-time control systems. A small-gain theorem,which makes use of the notion of non-uniform in time IOS property,is presented.

A converse Lyapunov theorem for nonuniform in time global asymptotic stability and its application to feedback stabilization

Lyapunov-like characterizations for the concepts of nonuniform in time robust global asymptotic stability and input-to-state stability for time-varying systems are established. The main result of our work enables us to derive (1) necessary and sufficient conditions for feedback stabilization for affine in the control systems and (2) sufficient conditions for the propagation of the input-to-state stability property through integrators.