1978 H. Wang et al.
interest in FD observers’ design for T–S fuzzy mod-
els [12–18].
In some cases, fault accommodation strategies are
needed, i.e., the control algorithm must be adapted
based on fault detection for controlling the faulty sys-
tem. Aircraft flight control systems [19] are good exam-
ples for applications of fault accommodation/active
reliable control. In such cases, it is important to carry
out fault estimation (FE) in addition to detection. In the
last decade, there have been fruitful results on adaptive
or robust FE, which can be found in [20–26]. In [24],
the authors studied the problem of robust FE observer
design for discrete-time T–S fuzzy systems via piece-
wise Lyapunov functions. In [25], the authors have
designed a bank of sliding mode observers to detect
and isolate the fault, and proposed a novel adaptive FE
observer to estimate actuator faults.
It should be pointed out that the membership func-
tions in all the above mentioned studies on fuzzy sys-
tems are required to be known. If the membership
functions are unknown, for example, they may con-
tain immeasurable premise variables or uncertain para-
meters, then the existing parallel distributed compen-
sator (PDC) strategy based on fault-detection and/or FE
results in the above studies cannot be used. In [27], a
linear fault-detection filter with fixed gains was pro-
posed for the T–S fuzzy Ito stochastic system. It is
noted that the linear fault-detection filter design may
be conservative to some degree because it does not use
any membership function information, especially for
highly nonlinear complex systems. In [28], a switching-
type fault-detection filter was used to detect the fault,
which had a promising feature by means of which the
membership function information can be employed to
construct. Although the comparison results have illus-
trated the merits of the proposed switching-type fault-
detection filter in [28], the computational burdens are
heavy, especially for the multiple faults. To the best
of the authors’ knowledge, up to now, the FD problem
for fuzzy systems with uncertain grades of membership
has not been fully investigated and remains as important
and challenging one, which has motivated the current
study.
The main objective of this article is to investigate
the FD problem of uncertain nonlinear systems against
actuator faults. The considered nonlinear systems are
described by T–S fuzzy models with local nonlinear
parts and uncertain grades of membership. The type
of actuator faults under consideration contains bias
faults and gain faults. First, by introducing a switching
technique, a fault-detection observer is constructed to
detect the fault. Furthermore, an adaptive FE observer
design method combined with the proposed switching
technique is developed. It is noted that the proposed
FD scheme utilizes the lower and upper bounds infor-
mation of the unknown membership functions, exter-
nal disturbances, faults, and their coupling, and the
obtained fault errors converge exponentially to zero.
Finally, an example of NSV reentry dynamic model is
given to illustrate the effectiveness and merits of the
proposed method.
The structure of this article is as follows: following
the introduction, the s ystem description and the prob-
lem under consideration are given in Sect. 2. In Sect.
3, the FD problem is addressed. An example is given
in Sect. 4 to show the superiority and effectiveness of
proposed method. Finally, conclusions are drawn in
Sect. 5.
2 System description and problem statement
2.1 System description
The following continuous-time T–S fuzzy dynamic
model with local nonlinear parts can be used to rep-
resent a class of complex nonlinear systems subject
to parameter uncertainties, in the form described as
follows:
⎧
⎪
⎪
⎪
⎪
⎪
⎨
⎪
⎪
⎪
⎪
⎪
⎩
˙x(t) =
N
i=1
α
i
(θ(t))( A
i
x(t) + B
i
u(t) + N
i
φ(x(t))
+G
i
w(t))
y(t) =
N
i=1
α
i
(θ(t))(C
i
x(t) + N
i1
φ(x(t))),
(1)
where N is the number of inference rules; x(t) ∈ R
n
is
the system state vector, and assumed to be measurable;
u(t) ∈ R
m
is the control input vector; y(t) ∈ R
s
is the
output vector; w(t) =
w
1
(t) ···w
k
(t) ···w
p
(t)
T
∈
R
p
is the disturbance input; θ(t) is the premise variable
that contains the system states and unknown parame-
ters; and A
i
, B
i
, G
i
, C
i
, N
i
, and N
i1
are the known
constant matrices with appropriate dimensions. As dis-
cussed in [29], φ(x(t)) is a known nonlinear function
and reserved as the nonlinear part of local models. The
membership functions α
i
(θ(t)) (i = 1,...,N) are
123