654 IEEE TRANSACTIONS ON FUZZY SYSTEMS, VOL. 20, NO. 4, AUGUST 2012
The control objective under normal conditions is to design a
proper state feedback controller u(t) such that the system (2) is
stable.
However, in practical application, actuators may become
faulty. Bias faults and gain faults are two kinds of actuator
faults that are commonly occurring in practice. An actuator bias
fault can be described as
u
f
i
(t)=u
i
(t)+f
i
(t),i=1,...,m, (6)
where f
i
(t) denotes a bounded signal, and an actuator gain fault
can be described as
u
f
i
(t)=(1−ρ
i
(t))u
i
(t),i=1,...,m, (7)
where 0 ≤ ρ
i
(t) ≤ 1, which is supposed to be unknown, de-
notes the remaining control rate. Therefore, the two previously
mentioned kinds of actuator faults can be uniformly described
as
u
f
i
(t)=(1−ρ
i
(t))u
i
(t)+f
i
(t). (8)
Furthermore, a more general fault model can be given as
u
f
i
(t)=(1−ρ
i
(t))u
i
(t)+
p
i
j=1
g
i.j
f
i,j
(t) (9)
where f
i,j
(t),i=1,...,m, j =1,...,p
i
, denotes a
bounded signal, p
i
is a known positive constant, and g
i,j
denotes an unknown constant. With no restriction, let us sup-
pose p
1
= p
2
= ···= p
m
= p, with p being a known positive
constant. Consider the following notation: a
i,j
(t)=g
i.j
f
i,j
(t).
Then, (9) can be rewritten as follows:
u
f
i
(t)=(1−ρ
i
(t))u
i
(t)+
p
j=1
a
i,j
(t). (10)
Denote
Γ(t) = diag(ρ
1
(t),...,ρ
m
(t)) (11)
F (t)=[f
1
,f
2
,...,f
m
]
T
,f
i
=
p
j=1
a
i,j
(t). (12)
Then, we have
u
f
(t)=(I − Γ(t))(u(t)+F (t)),t≥ t
j
(13)
where the failure time instant t
j
is unknown, and I denotes the
identity matrix with appropriate dimensions. In this paper, both
bias and gain faults are handled by considering the general fault
model (13).
Notice that, in the following, just for the sake of notational
simplicity, we will use h
i
,ρ
i
, and a
i,j
to denote h
i
(z(t)),ρ
i
(t),
and a
i,j
(t), respectively.
Now, the control objective is redefined as follows. An active
FTC approach is proposed to make the system (2) stable under
normal and faulty conditions. Under normal condition (no fault),
a state feedback control input u(t) is designed such that the
system (2) is stable. Meanwhile, the FDI algorithm is working.
As soon as an actuator fault is detected and isolated, the fault-
estimation algorithm is activated. The obtained fault estimation
is used to design a proper control input u(t) such that the system
(2) is still maintained stable under faulty case.
Remark 1: In the literature, many papers consider actuator
faults. However, most of them only considered bias faults. Gain
faults have not attracted enough attention. In [11], a class of bias
fault was studied, where the fault was assumed to be an unknown
constant. However, in practicalapplication, thefault may betime
varying. Equation (10) is a deterministic but uncertain actuator
model which represents a class of practical actuator faults such
as actuator gain variations and measurement errors. In fact, the
fault model in [9]–[11] can be described by (10). If ρ
i
(t)=0,
model (10) then becomes the bias fault model that is considered
in [9] and [10]. If ρ
i
(t) is an unknown constant and f
i
(t)=0,
then model (10) denotes the constant bias faults model as in [11].
Hence, the proposed actuator fault model (10) is more general
and has wider practical use than the classical ones.
III. F
AULT DIAGNOSIS AND ACCOMMODATION
In this section, the main technical results of this paper are
given. We will first formulate the fault-diagnosis and accommo-
dation problem of the aforementioned T–S fuzzy system. We
will then design a bank of sliding-mode observers (SMOs) to
generate residuals, investigate the FDI algorithm that is based
on the SMOs, and propose an FTC scheme to tolerate the fault
using estimated fault information.
A. Preliminary Description
Consider the T–S fuzzy faulty system that is described in (2).
We assume that only actuator faults occur, and no sensor fault
is involved. For simplicity, we consider the case that only one
single actuator is faulty at one time. The actuator fault-diagnosis
problem is formulated as follows: With the available output y,
we propose an observer-based scheme to identify the faulty
actuator, and then estimate the fault.
To solve the problem, we will design a bank of SMOs with
desired actuator fault-detection and fault-estimation properties.
Thus, the following assumptions are made in this paper.
Assumption 1: Matrix B
i
is of full column rank, and the pair
(A
i
,C
i
) is observable.
Assumption 2: There exist known positive constants ¯ρ
i
,
¯
¯ρ
i
,
¯ρ
1
, ¯ρ
2
such that |ρ
i
(t)|≤¯ρ
i
, |˙ρ
i
(t)|≤
¯
¯ρ
i
, ¯ρ
1
= max{¯ρ
1
,
¯ρ
2
,...,¯ρ
m
}, ¯ρ
2
= max{
¯
¯ρ
1
,
¯
¯ρ
2
,...,
¯
¯ρ
m
}, i =1,...,m.
Assumption 3: There exist known positive constants
¯a
1
, ¯a
2
, ¯a
i,j
,
¯
¯a
i,j
such that |a
i,j
(t)|≤¯a
i,j
, |˙a
i,j
(t)|≤
¯
¯a
i,j
, ¯a
1
=
max{¯a
1,1
,...,¯a
i,p
,...,¯a
m,1
,...,¯a
m,p
}, ¯a
2
=max{
¯
¯a
1,1
,...,
¯
¯a
i,p
,...,
¯
¯a
m,1
,...,
¯
¯a
m,p
}, i =1,...,m, j =1,...,p.
Our actuator fault-diagnosis and accommodation scheme
consists of FDI and FTC. We first design the fault-diagnosis ob-
server utilizing SMOs to detect, isolate, and estimate the fault,
and then propose an FTC method to compensate the fault.
B. Fault Detection
In order to detect the actuator faults, we design a fuzzy state-
space observer for the system (8), which is described as follows.