The accessibility property of the class of driftless single input nonlinear time-delay systems is characterized. This result is obtained within a newly introduced geometric approach. In this context all those possible autonomous elements, which can depend on the variables with time-delay, are also characterized when the system is not accessible.
This paper addresses the problem of the estimation of unknown time-varying delays of discrete-time systems. The problem is reformulated as a mode detection for hybrid systems with a switching law depending on the unknown delay. Some existing data-based residual methods used for the estimation of the mode of switching linear discrete-time systems are briefly reviewed and presented in a unified way. Furthermore, it is shown that the usual existence conditions of some detectors can be relaxed. Next, results concerning the so-called discernibility property are particularized to the hybrid formulation of the estimation problem. Finally, it is shown that the issue is especially interesting in the context of chaotic secure communications both for information recovery and cryptanalysis purposes.
In this paper, we analyze the stability properties of a class of systems governed by linear difference equations. Lyapunov stability and asymptotic stability are analyzed using two different approaches, namely Lyapunov-Krasovskii techniques and spectral techniques. In the case of commensurable delays, we carry out an analytical form of the solution, allowing us to determine necessary and sufficient conditions for stability. A link with discrete-time systems is made, and some comments on time-varying delays are proposed.***Copyright©2013 IFAC
This paper addresses some properties of simple characteristic roots of quasipolynomials including commensurate delays. Although such a problem seems easy and was largely treated in the literature, to the best of the authors' knowledge, the invariance and the ultimate stability properties have not been fully investigated. In this paper, we propose a new frequency-sweeping framework, which simultaneously considers the Taylor series of the critical imaginary roots as well as the Puiseux series of singular points of the frequency-sweeping curves. Through analyzing the algebraic properties of the corresponding frequency-sweeping curves, we are able to completely characterize the invariance property of the simple imaginary roots with respect to the corresponding critical delay values. As a consequence of the invariance property, the ultimate stability property can be easily derived. Finally, as a byproduct of the approach proposed in the paper, the complete stability for time-delay systems with only simple imaginary roots can be systematically derived. Some illustrative examples complete the presentation.
In the huge literature dedicated to stability of time-delay systems, the most popular approach remains the use of Lyapunov-Krasovskii functionals. This framework allows to study a large class of time-delay systems including constant or time-varying delays. Since several years, the main challenge is to propose new functionals and techniques for deriving less and less conservative stability conditions. Nevertheless, all these approaches usually adopt the same procedure which is based on the well-known Jensen's inequality which generally induces some conservatism difficult to overcome. This paper analyses firstly the conservatism induced by the Jensen's inequality and secondly proposes a wide class of new parametrized inequalities. All these are based on an extensive use of Wirtinger inequality which has been recently introduced in Liu and Fridman [2012] and Seuret and Gouaisbaut [2012b] for stability analysis.
