6
IEEE
TRANSACTIONS ON INDUSTRIAL El.ECTRONICS,
VOL.
40,
NO.
I,
FEBRUARY
1993
general linear systems was more firmly established; the
general system is of the form
x =Ax
+
Bu
where dim-x
=
n
and dim-u
=
m. an m-dimensional lin-
ear vector switching function 4x1 was postulated for the
VSC. The control structure for each of the m inputs was
described as
(23
1
u,(x)
=
@,+(x) when
s,(x)
>
0
(24)
=
@,-(x) when
s,(x)
<
0
i
=
1,2;..,m
and every scalar switching function s,(x) was linear in the
state variables rather than quadratic. However, the estab-
lished VSC theory did not attract much attention for
many practical applications. The reason seems to be
twofold. First, VSC theory was overshadowed by the popu-
lar linear control system design techniques. Second, the
important robustness properties of the VSC system were
not
yet
fully recognized or appreciated.
(b) The Present Stage
of
Adilancing Der>elopment
(1980-Present): Since
1980,
two
developments have greatly
enhanced the attention given to VSC systems. The first is
the existence of a genral VSC design method for complex
systems. The second is a full recognition of the property
of
perfect robustness of a VSC system with respect to
system perturbation and disturbances.
As
a result, re-
search and development of VSC methods have been
greatly accelerated, both in theory and in applications.
The
R
&
D work may be classified into
five
categories:
1)
Development for different system models. This in-
cludes the development of VSC theory for nonlinear
systems, discrete-time systems, systems with time
delay, stochastic systems, large-scale systems and
infinite-dimensional systems.
2)
Extension of the objectives of control. The functions
of VSC have been extended beyond system stabiliza-
tion to include motion following or tracking, model
following, model reaching, adaptive and optimal con-
trol, and state observation.
3)
Exploration of additional properties of VSC. Such
properties include invariance of the sliding mode to
system perturbations, robustness of the reaching or
nonsliding mode, and the elimination or reduction of
control chatter.
4)
Establishment of VSC laws that possess certain
characteristics.
5)
Applications in various engineering problems.
The purpose of this tutorial paper is to present the
fundamental theory and main results for the design of
VSC systems. The basic notions and a brief history of
VSC development have already been presented. More
precise definitions and deeper concepts are presented
next by considering VSC for linear plants. Concepts dis-
cussed are the design of switching surfaces, characteriza-
tion of the sliding mode and the reaching or nonsliding
mode, control law design, and basic properties of a vari-
able structure system response. The extensions of these
concepts to nonlinear systems are then presented. Further
extensions of variable structure concepts to other control
objectives besides stabilization and regulation are briefly
described. Known studies of variable structure control for
some special systems are noted. Since application of VSC
has been widespread in recent years, some notable contri-
butions are listed. Finally, a very extensive list of refer-
ences is included. Other survey papers by Utkin
[1551-
11571, DeCarlo er al. [35], and a monograph by Gao [601
are also recommended.
11.
VSC
FOR
LINEAR
SYSTEMS
The discussion in this section concerns the general
linear time-invariant system represented by the state
equation
X=Ax+Bu
(25)
The state vector x is n-dimensional and the input
U
is
m-dimensional. The m column vectors of the
B
matrix
are designated as
b,,
for
i
=
1
to m.
A. Basic Definitions
Basic terminology of variable structure systems with
sliding mode are more carefully defined in this section.
Definition
1:
The structure in a VSC system is governed
by the sign of a vector-valued function s(x), which is
defined to be the switching function. A switching function
is generally assumed to be
m
dimensional and linear, i.e.,
s(x)
=
cx
(26)
where
S(X)
=
[s,(x)
s?(x)
...
S,,,(X)]''
(27)
and
c
=
[.;
.;
...
4
I
7-
(28)
Thus
s,(x)
=
c7'x.
(29)
Each scalar switching function
s,(x)
describes a linear
surface s,(x)
=
0,
which is defined to
be
ii
switching
sur-
face. The term switching manifold is often used.
In
addi-
tion, the surface can be called a switching hyperplane
because the switching function is linear. Notice in the
introductory exampled, however, that the set
s(x,
y)
=
0
consisted of
two
intersecting lines. Such a set is not a
manifold in the mathematical sense. Hence, the terms
manifold and hyperplane are avoided in the remainder of
this paper.
Let
x,)
be the initial state of the system at the initial
time to, x(t) be the state at any time t, and
S
be a
switching surface that includes the origin x
=
0.
Definition
2:
If, for any xo in
S,
we have x(t) in
S
for all
t
>
to, then
x(t)
is a sliding motion or sliding mode of the
system.
Definition
3:
If
every point in
S
is an end point, that is,
for every point in
S
there are trajectories reaching it from