J Control Theory Appl 2008 6 (1) 22–25
DOI 10.1007/s11768-008-7186-8
Stability analysis of networked control systems with
time-varying sampling periods
Fuchun LIU, Yu YAO, Fenghua HE, Songlin CHEN
(
Control & Simulation Center, Harbin Institute of Technology, Harbin Heilongjiang 150080, China)
Abstract: In this paper, we present an interval model of networked control systems with time-varying sampling
periods and time-varying network-induced delays and discuss the problem of stability of networked control systems using
Lyapunov stability theory. A sufficient stability condition is obtained by solving a set of linear matrix inequalities. In the
end, the illustrative example demonstrates the correctness and effectiveness of the proposed approach.
Keywords: Networked control systems (NCS); Time-varying sampling periods; Network-induced delay; Linear ma-
trix inequality (LMI)
1 Introduction
Networked control systems (NCS) are feedback control
systems whose feedback paths are implemented by a real-
time network. Recently, much attention has been paid to the
study of stability analysis and control design of networked
control systems, due to their low cost, reduced weight and
power requirements, simple installation and maintenance,
and high reliability [1].
A basic problem in an NCS is the stability of the sys-
tem. Network-induced delay, data packet dropout and time-
varying transmission periods are the main characteristic
in NCS. In real-time control systems, delay and packet
dropouts will degrade the performance of control systems
and even make systems unstable.
There are many research studies on network-induced de-
lay [1∼4], in most of which the delay is assumed to be con-
stant. In [5], a new method to obtain a maximum allowable
delay bound for a scheduling of NCS was proposed, and the
results are formulated in terms of LMI. In [3], the stabil-
ity of networked control systems was analyzed by a hybrid
system approach where the induced delay is deterministic
(constant or time-varying) and the controller gain is con-
stant. The input-output L
p
stability of networked control
systems under TOD scheduling protocols has been studied
by Nesic [6], where a unifying framework for generating
new scheduling protocols that preserve L
p
stability is pro-
vided. Very recently, the results related to the stabilization
problem for a class of NCS with random delays have been
reported in [7], in which sufficient conditions for the solv-
ability of this problem were obtained and an LMI approach
was proposed. In the case where the network-induced delay
at each sampling instant is random and less than one sam-
pling period, the problem of stability analysis and control
design for NCS was solved in the work by Nilsson et al. [8],
where a stochastic controller related to sensor-to-controller
delays was designed. The results in [8] have recently been
extended to the case with longer delays in [9].
On the other hand, robust control for uncertain systems
has been extensively investigated, leading to many practi-
cal applications. Especially, interval systems arise naturally
in many practical situations, such as airplane motion sys-
tem and electric machine control system. A useful trans-
formation approach from an interval system to an equiva-
lent norm-bounded uncertain description has been provided
in [10], where a sufficient condition for the existence of an
H
∞
robust controller is proposed in terms of an algebraic
Riccati inequality.
In this paper, motivated by the results in [10, 11], we
deal with the problem of robust stability analysis for net-
worked control systems with time-varying sampling periods
and time-varying network-induced delay using interval ma-
trix methods. The closed-loop NCS system is described as
an interval dynamic system. For a given state-feedback con-
troller, the sufficient condition of stability for this class of
NCS is obtained based on the Lyapunov stability theory. A
linear matrix inequality approach is developed to solve the
problem.
The rest of this paper is organized as follows. In Sec-
tion 2, we give the problem formulation of NCS with
time-varying sampling periods and time-varying network-
induced delay. Then in Section 3, we derive the stability
condition based on Lyapunov stability theory. Section 4
contains the numerical simulation results which show the
feasibility and efficiency of the proposed method. Finally,
conclusions are presented.
Received 5 September 2007.
This work was supported by the National Natural Science Foundation of China (No.60674043).