Journal of Applied Mathematics

Volume 2012 (2012), Article ID 237430, 13 pages

http://dx.doi.org/10.1155/2012/237430

## Robust Exponential Stability for LPD Discrete-Time System with Interval Time-Varying Delay

^{1}Department of Mathematics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand^{2}Centre of Excellence in Mathematics, CHE, Si Ayuthaya Road, Bangkok 10400, Thailand

Received 5 April 2012; Accepted 11 July 2012

Academic Editor: B. V. Rathish Kumar

Copyright © 2012 Kanit Mukdasai. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This paper investigates the problem of robust exponential stability for uncertain linear-parameter dependent (LPD) discrete-time system with delay. The delay is of an interval type, which means that both lower and upper bounds for the time-varying delay are available. The uncertainty under consideration is norm-bounded uncertainty. Based on combination of the linear matrix inequality (LMI) technique and the use of suitable Lyapunov-Krasovskii functional, new sufficient conditions for the robust exponential stability are obtained in terms of LMI. Numerical examples are given to demonstrate the effectiveness and less conservativeness of the proposed methods.

#### 1. Introduction

Over the past decades, the problem of stability analysis of delay discrete-time systems has been widely investigated by many researchers. Because the existence of time delay is frequent, a source of oscillation instability performances degradation of systems. Stability criteria for discrete-time systems with time delay is generally divided into two classes: delay-independent ones and delay-dependent ones. Delay-independent stability criteria tend to be more conservative, especially for small-size delay; such criteria do not give any information on the size of the delay. On the other hand, delay-dependent stability criteria is concerned with the size of the delay and usually provide a maximal delay size. Moreover, robust stability of linear continuous-time and discrete-time systems subject to time-invariant parametric uncertainty has received considerable attention. An important class of linear time-invariant parametric uncertain system is linear parameter-dependent (LPD) system in which the uncertain state matrices are in the polytope consisting of all convex combination of known matrices. To address this problem, several results have been obtained in terms of sufficient (or necessary and sufficient) conditions; see [1–15] and references cited therein. Most of these conditions have been obtained via the Lyapunov theory approaches in which parameter dependent Lyapunov functions have been employed. These conditions are always expressed in terms of linear matrix inequalities (LMIs) which can be solved numerically by using available tools such as LMI toolbox in MATLAB. The results have been obtained for robust stability for LPD systems in which time delay occur in state variable such as [6, 11, 14] present sufficient conditions for robust stability of LPD continuous-time system with delays. However, a few results have been obtained for robust stability for LPD discrete-time systems with delay.

In this paper, we deal with the problem of robust exponential stability for uncertain LPD discrete-time system with interval time-varying delay. Combined with the linear matrix inequality technique and the use of suitable Lyapunov-Krasovskii functional, new sufficient conditions for the robust exponential stability are obtained in terms of LMI. Finally, numerical examples have demonstrated the effectiveness of the criteria.

#### 2. Problem Formulation and Preliminaries

We introduce some notations and definitions that will be used throughout the paper. denotes the set of non negative integer numbers; denotes the -dimensional space with the vector norm ; denotes the Euclidean vector norm of ; that is, ; denotes the space of all matrices of -dimensions; denotes transpose of the Matrix ; is symmetric if ; denotes the identity matrix; denotes the set of all eigenvalues of ; ; Matrix is called semi positive definite () if , for all is positive definite () if for all ; Matrix is called semi-negative definite () if , for all is negative definite () if for all ; means ; means ; represents the elements below the main diagonal of a symmetric matrix.

Consider the following uncertain LPD discrete-time system with interval time-varying delay in the state where , is the system state and is a initial value at . , are uncertain matrices belonging to the polytope of the form and are unknown matrices representing time-varying parameter uncertainties, we assumed to be of the form The class of parametric uncertainties , which satisfies is said to be admissible where is a known matrix satisfying and is uncertain matrix satisfying In addition, we assume that the time-varying delay is upper and lower bounded. It satisfies the following assumption of the form where and are known positive integers.

*Definition 2.1. * The uncertain LPD discrete-time-delayed system in (2.1) is said to be robustly exponentially stable if there exist constant scalars and such that
for all admissible uncertainties.

Lemma 2.2 (see [5] (Schur complement lemma)). * Given constant matrices of appropriate dimensions with . Then if and only if
*

Lemma 2.3 (see [2]). *Given constant matrices , and of appropriate dimensions with . Then,
**
where , if and only if
**
for some scalar .*

#### 3. Main Results

In this section, we present our main results on the robust exponential stability criteria for uncertain LPD discrete-time system with interval time-varying delays. We introduce the following notation for later use:

Lemma 3.1. *For any in (3.1), and given by
**
are parameter-dependent positive definite Lyapunov matrices such that
**
if and only if
*

*Proof. *Consider
We assume that
Using Lemma 2.2, we obtain
We rewrite the latter inequality as
Using Lemma 2.3, inequality (3.8) holds if and only if there exists such that
If we apply to (3.9), then we obtain
Premultiplying (3.10) by diag and postmultiplying by diag, we get that (3.4) and the lemma is proved.

Lemma 3.2. * If there exist positive definite symmetric matrices , , , and positive real numbers such that
**
then, for any in (3.1), and are parameter-dependent positive definite Lyapunov matrices in Lemma 3.1 such that (3.4) holds. *

* Proof. * Consider
Using the fact that , we obtain the following identities:
Then, it follows from (3.11), (3.12), and (3.13) that (3.4) holds. The proof of the lemma is complete.

Theorem 3.3. *The system (2.1) is robustly exponentially stable if the LMI conditions (3.11) are feasible. *

* Proof. *Consider the following Lyapunov-Krasovskii function for system (2.1) of the form
where
A Lyapunov-Krasovskii difference for the system (2.1) is defined as
Taking the difference of and , the increments of and are
Form , the two last terms of the right-hand side of the latter equality yield
Thus, we obtain
The increment of is easily computed as
It is easy to see that
for simplicity, we let . Since, , we obtain that
Therefore, we conclude that
It follows form (3.24) that
where and . By (3.11), (3.25), and Lemma 3.1, and 3.2, we obtain
where . By (3.14), it is easy to see that
where
It can be shown that there always exists a scalar satisfying
For any scalar , it follows from (3.26) and (3.27) that
where
Therefore, for any integer , summing up both sides of (3.30) from to gives
For ,
From (3.32) and (3.33), we obtain
Observe
Then, it follows from (3.29), (3.33), and (3.35) that
By Definition 2.1, this means that the system (2.1) is robustly exponentially stable. The proof of the theorem is complete.

#### 4. Numerical Example

*Example 4.1. *Consider the following uncertain LPD discrete-time system with time-varying delays (2.1) where , that is, and
and . By using LMI Toolbox in MATLAB, we use condition (3.11) in Theorem 3.3 for this example. The solutions of LMI verify as follows of the form , , and (see Figure 1).

*Example 4.2. *Consider the following the LPD discrete-time system with time-varying delays (2.1) where, with
Table 1 lists the comparison of the upper-bound delay for asymptotic stability of system (2.1) where by different method. We apply Theorem 3.3 and see from Table 1 that our result is superior to those in [7, Theorem 3.2].

#### Acknowledgments

This work was supported by the Thailand Research Fund (TRF), the Office of the Higher Education Commission (OHEC), and the Khon Kaen University (Grant number MRG5580006), and the author would like to thank his advisor Associate Professor Dr. Piyapong Niamsup from the Department of Mathematics, Faculty of Science, Chiang Mai University, Chiang Mai, Thailand.

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