- Open Access
On stability analysis of discrete-time uncertain switched nonlinear time-delay systems
© Kermani and Sakly; licensee Springer. 2014
Received: 13 March 2014
Accepted: 4 August 2014
Published: 9 September 2014
This paper addresses the stability analysis problem for a class of discrete-time switched nonlinear time-delay systems with polytopic uncertainties. These considered systems are characterized by delayed difference nonlinear equations which are given in the state form representation. Then, a transformation under the arrow form is employed. Indeed, by constructing an appropriated common Lyapunov function, and also by resorting to the Kotelyanski lemma and the M-matrix proprieties, new delay-independent stability conditions under arbitrary switching law are deduced. Compared with the existing results of switched systems, those obtained results are formulated in terms of the unknown polytopic uncertain parameters, explicit and easy to apply. Moreover, this method allows us to avoid the search for a common Lyapunov function which is a difficult matter. Finally, a numerical example is presented to illustrate the effectiveness of the proposed approach.
Switched systems are an important class of hybrid systems. Generally speaking, a switched system is composed of a family of subsystems described by differential or difference equations and a switching rule orchestrating the switching between the subsystems that have attracted much attention in control theory and practice during recent decades. Switched systems can be efficiently used to model many practical systems which are inherently multi-model in the sense that several dynamical systems are required to describe their behavior. For example, many physical processes exhibit switched and hybrid nature. Switched systems have strong engineering background in various areas and are often used as a unified modeling tool for a great number of real-world systems, such as power electronics, chemical processes, mechanical systems, automotive industry, aircraft and air traffic control and many other fields [1–6].
Undoubtedly, stability is the first requirement for a system to work properly; thus, stability analysis of switched systems presents a theoretical challenge, which has attracted growing attention in the literature [1–37]. However, stability under arbitrary switching is a fundamental issue and an important topic in the design and analysis of these systems. To solve this matter, many effective methods have been developed [11, 15, 16, 24]. Within this framework, we are required to find conditions that guarantee asymptotic stability under arbitrary switching rules. Indeed, it is well known that the existence of a common Lyapunov function [7–9] for all subsystems is sufficient to guarantee the stability under arbitrary switching law. However, finding such a function is often difficult even for discrete-time switched linear systems . Consequently, this problem becomes more complicated for discrete-time switched nonlinear systems, and relatively fewer results have been reported in this context.
On the other hand, to avoid the problem of the existence of a common Lyapunov function, some attention has been widely given to seeking conditions that guarantee the stability of the switched systems under restricted switching. Although many efficient approaches and important results have been proposed for this alternative, such as the multiple Lyapunov function approach  and average dwell time method [11, 12], stability under arbitrary switching, which is considered in this work, remains most preferable for practical systems. Indeed, it offers great flexibility and it allows us to achieve other performances for designing a control law along stability maintained.
As is well known, time-delay phenomena are usually confronted in many engineering systems [1, 16–24, 28–30, 38, 39], such as chemical engineering systems, hydraulic systems, inferred grinding model, neural network, nuclear reactor, population dynamic model and rolling mill. Recently, stability analysis for discrete-time switched time-delay systems has been investigated [1, 16–24, 28–30].
It is noteworthy that there are two divisions in the recent literature addressing the stability analysis of time-delay systems, namely delay-independent criteria and delay-dependent criteria. Therefore, in view of delay-independent criteria, this paper will try to aid the stability analysis under arbitrary switching law.
On the other hand, when practical systems are modelled, uncertainties of system parameters are often included. Therefore, most of the systems refer to uncertainties in real applications. Indeed, basically, two kinds of uncertainties are simultaneously encountered in the open literature, widely polytopic uncertainties and norm-bounded. From the practical viewpoint, it is important to investigate switched systems with uncertain parameters [19, 20, 24, 28–30, 33, 36]. Thus, polytopic uncertainties exist in many real systems, and most of the uncertain systems can be approximated by systems with polytopic uncertainties . On the other hand, the polytopic uncertain systems are less conservative than systems with norm-bounded uncertainties .
Up to now, discrete-time uncertain switched time-delay systems have received more and more attention. Although many interested and significant results on stability problems for those systems have been established [19, 20, 26, 28–30], those previous works were mainly focused on several hot topics of Lyapunov stability theory and most of them were interested in the linear case [19, 22–26, 28–30]. Thus, due to the complexity of switched nonlinear systems, unfortunately, the available results on the stability of uncertain discrete-time switched nonlinear time-delay systems are limited .
Motivated by these mentioned shortcomings for the existing results as well as in the sense of various methods that can be employed, in this paper we aim to establish new stability analysis for a class of discrete-time switched time-delay systems with polytopic uncertainties. Indeed, based on transforming, the representation of these systems are studied under consideration into the arrow form matrix [31–33, 40–51]. Then, by constructing an appropriated common Lyapunov function and by resorting to the Kotelyanski lemma  and the M-matrix proprieties [53, 54], new delay-independent stability conditions under arbitrary switching law are deduced.
Within the frame of studying the stability analysis, the said approach has already been introduced in [40, 41] for continuous-time delay systems and in our previous work  for discrete-time switched linear time-delay systems in a field related to the study of convergence.
To the best of the authors’ knowledge, no results have been reported in the literature on the stability of discrete-time switched time-delay systems with polytopic uncertainties by employing the Kotelyanski stability conditions that can be very effective in dealing with a class of more general nonlinear systems.
The contributions in our work mainly include two aspects. First, due to the conservatism of the methods for stability analysis which are based on the common Lyapunov function, this proposed method can guarantee stability under arbitrary switching and allows us to avoid searching for a common Lyapunov function. Second, these obtained results are formulated in terms of the unknown polytopic uncertain parameters, explicit and easy to apply. Furthermore, this proposed approach could be further used as a constructive solution to the problems of state and static output feedback stabilization.
The layout of the paper is as follows. Section 2 presents the problem formulation and some preliminaries. The main results of this paper are presented in Section 3. Section 4 is devoted to deriving new delay-independent conditions for asymptotic stability of switched nonlinear systems defined by difference equations. Some remarks and a numerical example are presented in Section 5 to illustrate the theoretical results. Finally, Section 6 concludes this paper.
Notation The notation used throughout this paper is as follows. For a matrix A, we denote the transpose by . Let ℜ denote the field of real numbers, denote an n-dimensional linear vector space over the reals with the norm . For any , we define the scalar product of the vectors u and v as ; is the space of matrices with real entries. is a set of positive real numbers. is a set of integers and is an identity matrix with appropriate dimension.
2 Problem formulation and preliminaries
2.1 Problem formulation
where , and , are matrices that have nonlinear elements with appropriate dimensions.
It is obvious that .
where , and , are respectively the vertex matrices denoting the extreme points of the polytope , and , , is the number of the vertex matrices , is the number of the vertex matrices and the weighting factors , , , are unknown polytopic uncertain parameters for each belong to , and , .
Now, the following definition, theorem and lemma are preliminarily presented for further development.
For system (2.1) or (2.6) we can give the following definition.
Definition 1 System (2.6) is said to be uniformly robust asymptotically stable if for any , there is such that implies , . For arbitrary switching law and all admissible uncertainties (2.4) and (2.5), there is also such that implies as for arbitrary switching signal (2.3).
The following lemma and theorem will play an important role in our later development.
Kotelyanski lemma 
Real parts of the eigenvalues of the matrix , with nonnegative off-diagonal elements, are less than a real number μ if and only if all those of the matrix , where , are positive with being the n identity matrix.
In this case, all the principal minors of matrix are positive. Then, the Kotelyanski lemma permits to deduce the stability properties of the system given by .
Theorem 1 The matrix is said to be an M-matrix if the following properties are verified:
All the eigenvalues ofhave a positive real part;
The real eigenvalues are positive;
The principal minors ofare positive:(2.7)
For any positive real numbers, the algebraic equationshave a positive solution.
Remark 1 A discrete-time system given by a matrix is stable if the matrix verifies the Kotelyanski conditions, in this case is an M-matrix.
3 Main results
This section will present the main results on the robust stability conditions for the discrete-time nonlinear switched time-delay system with polytopic uncertainties (2.6).
Such sufficient conditions, which are the main theoretical contribution of the paper, are stated in the theorem below.
Proof Let us consider system (2.6) of any switching law (2.3) and all admissible uncertainties (2.4) and (2.5), let with components (, ) and be the state vector.
where the matrix is defined in (3.1).
Therefore, switched system (2.6) is robust asymptotically stable under switching law (2.3) and all admissible uncertainties (2.4) and (2.5). This completes the proof. □
4 Application to discrete-time uncertain switched nonlinear time-delay systems defined by difference equations
where , are the components of the switching function given in (2.3), , and , are unknown polytopic uncertain parameters given in (2.4) and (2.5) for each .
Therefore, the presence of time-delay terms, the nonlinearities of coefficients, and unknown polytopic uncertain parameters makes the stability analysis for system (4.1) very difficult.
where is the state vector of components , . , are the components of the switching function given in (2.3), , and , are unknown polytopic uncertain parameters given in (2.4) and (2.5) for each .
To simplify the application to the Kotelyanski lemma, we consider a coordinate transformation into the arrow matrix form.
where (), , , are free real parameters that can be chosen arbitrarily.
Now, after the above formulation, based on the Kotelyanski lemma, we are in a position to give sufficient stability conditions for system (4.5) that are presented in the following theorem.
where the elements are defined in (4.22).
From (4.23), it is clear that all the elements of are positive. Therefore, the application of the Kotelyanski lemma to the matrix enables us to conclude the stability of system (4.5).
It is clear that for , condition (4.20) is verified as follows: .
It follows that .
This ends the proof of Theorem 3. □
To simplify the use of the stability conditions, Theorem 3 can be reduced to the corollary below.
Now, by considering relations (4.26) and (4.28) in Corollary 1 and substituting (4.15), (4.16) into (4.31), we obtain (4.27).
which indicates that condition (4.27) is satisfied.
Robust asymptotical stability of system (4.5) under any arbitrary switching law (2.3) and all admissible uncertainties (2.4) and (2.5) follows. This completes the proof. □
5 An illustrative example
where τ is the time-delay, and are nonlinear coefficients, and are unknown polytopic uncertain parameters for each , and . is the arbitrary switching rule given in (2.3).
where and are general nonlinear functions.
Let us introduce unknown uncertainty parameters satisfying the following conditions: , , and with being a general nonlinearity such that .
+ < 0,
+ < 0,
This example was used to illustrate the effectiveness of our developed approach with different values of delay and different switched time. Besides, this example shows that the obtained stability conditions are sufficient and very close to be necessary.
It is should be noted that this proposed approach is less conservative that searching for a common Lyapunov function. In fact, in  the authors introduced a simple linear example without time-delay for which a common Lyapunov function does not exist. Therefore, we cannot guarantee stability under arbitrary switching.
This paper has investigated new robust delay-independent stability conditions for a class of discrete-time switched nonlinear time-delay systems with polytopic uncertainties. These stability conditions were deduced with the help of the construction of an appropriated common Lyapunov function, and also by the resort to the Kotelyanski lemma and the M-matrix proprieties.
Compared with the existing results of switched systems, these obtained results are formulated in terms of the uncertain parameters, explicit and easy to apply. Moreover, this method allows us to avoid searching for a common Lyapunov function. A numerical example is given to show the effectiveness of our proposed approach.
This proposed approach could be further used as a constructive solution to the problems of state and static output feedback stabilization.
The limits of this paper are that it has been confined to the boundaries of numerical examples. It would be beneficial to extend the research further so as to include real systems and to conduct a comparative study with an example for which a common Lyapunov function exists.
The authors would like to thank the referees and the editor for helpful suggestions incorporated into this paper.
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