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Published in IET Control Theory and Applications
Received on 13th March 2011
Revised on 20th October 2011
doi: 10.1049/iet-cta.2011.0133
ISSN 1751-8644
Exponential stability analysis for positive systems
with delays
S. Zhu
1
Z. Li
2
C. Zhang
3
1
School of Mathematics, Shandong University, Jinan 250100, People’s Republic of China
2
School of Statistics, Shandong University of Finance and Economics, Jinan 250014, People’s Republic of China
3
School of Control Science and Engineering, Shandong University, Jinan 250061, People’s Republic of China
E-mail: sqzhu@sdu.edu.cn
Abstract: This study investigates the exponential stability analysis problem for continuous-time and discrete-time positive
linear systems with delays. For given decay rate, necessary conditions for exponential stability of such systems are presented
based on the asymptotical stability results of positive linear time-delay systems, and sufficient conditions are provided by
using Lyapunov–Krasovskii functional method. It is shown that, the exponential stability with given decay rate of positive
linear time-delay systems has to do with the delays. Some illustrative examples are given to show the correctness of the
obtained theoretical results.
1 Introduction
Positive systems are dynamic systems whose states or
outputs are non-negative whenever the initial conditions
are non-negative. Such systems abound in almost all fields,
for instance, engineering, ecology, economics, biomedicine,
social science and so on. Since the states of positive systems
are confined within a ‘cone’ located in the positive orthant
rather than in linear spaces, many well-established results for
general linear systems cannot be readily applied to positive
systems. This feature makes the analysis and synthesis of
positive systems a challenging and interesting job. The
mathematical theory of positive systems is based on the
theory of non-negative matrices founded by Perron and
Frobenius [1–3] and many results about stability and control
have been obtained [4–8].
The reaction of real-world systems to exogenous signals
is never instantaneous and always infected by certain time
delays. For general linear systems, even nominal stable
systems when are affected by delays, may inherit very
complex behaviours such as oscillations, instability and bad
performances [9]. In addition, it is well known that small
constant delays may destabilise some systems, whereas
large constant delays may stabilise others. In contrast, for
continuous-time and discrete-time positive linear systems
with constant delays or bounded time-varying delays, it has
been shown that the asymptotic stability has nothing to do
with magnitudes of the delays and is only dependent of the
system matrices [10–16]. Such systems are asymptotically
stable if and only if the sum of the system matrices is a
Hurwitz matrix (for continuous-time case) or a Schur matrix
(for discrete-time case).
All the aforementioned references are concerned with the
asymptotical stability of positive linear systems with delays.
However, in practice, it is desirable that the system can
converge quickly (that is, has a certain decay rate). Hence,
it is necessary to investigate the exponential stability of
such systems. Obtaining exponential stability of a control
system is sometimes of great importance, such as in adaptive
control, it is known that an exponentially stable adaptive
system can tolerate a certain amount of disturbances
and unmodelled dynamics. In [17, 18], the exponential
stability of positive linear time-delay differential systems
was investigated and it was demonstrated that such systems
are exponentially stable if and only if the corresponding
systems without delay are exponentially stable, that is,
whether positive linear time-delay differential systems are
exponentially stable is also independent of the delays.
However, it can be seen that the decay rate was not
further considered in [17, 18]. For positive linear time-delay
systems, whether and how the delays affect the decay rate?
In other words, whether and how the exponential stability
changes with changing values of the delays. This paper will
discuss this problem for both continuous-time and discrete-
time positive time-delay systems and give the answer.
The remainder of this paper is organised as follows. For
given decay rate, the exponential stability for continuous-
time positive linear time-delay systems is considered in
Section 2. Necessary conditions and sufficient conditions
for α-exponential stability are presented, respectively. In
Section 3, the corresponding problem is treated for discrete-
time positive linear time-delay systems with necessary
conditions and sufficient conditions being provided as well.
It is shown that, the exponential stability with given decay
rate of positive linear time-delay systems is dependent of
the delays. Four numerical examples are given in Section 4
to show the correctness of the obtained theoretical results.
Finally, Section 5 presents some concluding remarks.
IET Control Theory Appl., 2012, Vol. 6, Iss. 6, pp. 761–767 761
doi: 10.1049/iet-cta.2011.0133 © The Institution of Engineering and Technology 2012