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Q. Jia et al.: Fault Reconstruction for Continuous-time Systems via Learning Observers 551
Further, fault-reconstructing error e
f
(t) has a new form
e
f
(t)=f
a
(t)−K
1
f
a
(t − 𝜏)−K
2
Ce
x
(t)
− K
1
f
a
(t − 𝜏)+K
1
f
a
(t − 𝜏)
= K
1
e
f
(t − 𝜏)−K
2
Ce
x
(t)+
f
a
(t)
(6)
where
f
a
(t)∶=f
a
(t)−K
1
f
a
(t − 𝜏).
To derive the stability and convergence of the LO
such that accurate reconstruction of time-varying faults
can be guaranteed, the following assumption is needed.
Assumption 1. Assume that
f
a
(t)
∞
≤ k
f
,wherek
f
is a
small positive constant.
Note that Assumption 1 has a restriction on actu-
ator faults f
a
(t).GivenK
1
= I
m
and constant actuator
faults occur in the system (1), we have
f
a
(t) ≡ 0. There-
fore, it follows from (6) that
e
f
(t)=e
f
(t − 𝜏)−K
2
Ce
x
(t). (7)
Remark 1. In [8,9], fast AOs are proposed to reconstruct
time-varying faults under the assumption that the deriva-
tive of f
a
(t) is norm-bounded, i.e.
f
a
(t) ≤ f
1
where
0 ≤ f
1
≤ ∞. Comparatively, the fault-constraining
condition described in Assumption 1 is less restrictive
than that in [8,9] because the norm-bounded condition
on derivative of actuator fault vector is not required in
this paper. Under Assumption 1, the proposed LO-based
fault-reconstructing strategy can be applied to recon-
struct time-varying faults, especially fast-varying faults.
In addition, by selecting a gain matrix K
1
, which is close
to I
m
, and a sufciently small learning interval 𝜏, the pro-
posed LO can achieve accurate reconstruction of actua-
tor faults; no matter they are constant or time-varying.
3.2 Stability analysis of the learning observer
In this subsection, the stability and convergence
of the proposed LO will be proved and reconstruction
of actuator faults will be guaranteed. The following
theorem serves this purpose.
Theorem 1. Suppose that Assumption 1 holds, if there
exist positive-denite symmetric matrices P ∈ R
n×n
, Q ∈
R
m×m
and K
1
∈ R
m×m
, and matrices K
2
∈ R
m×p
and
Y ∈ R
n×p
such that the following relations hold
PA + A
T
P − YC − C
T
Y
T
< 0, (8)
E
T
P = 𝛼
1
K
2
C, (9)
and
𝛽
1
K
T
1
K
1
− Q ≤ 0, (10)
where 𝛼
1
=(1 + 𝜖
0
)𝜆
max
(Q),and𝛽
1
= 𝛼
1
(1 + 𝜖
1
) with
𝜖
0
≥ 0and𝜖
1
> 0, then the LO (4) can guarantee that
state-estimating and fault-reconstructing errors are uni-
formly ultimately bounded, and observer gain matrix L
can be determined by L = P
−1
Y.
Proof. Consider the following Lyapunov function candi-
date:
V(t)=e
T
x
(t)Pe
x
(t)+
∫
t
t−𝜏
e
T
f
(s)Qe
f
(s)ds (11)
where P and Q are positive-denite symmetric matrices.
The derivative of the Laypunov candidate with
respect to time can be derived as
V(t) ≤e
T
x
(t)
P(A − LC)+(A − LC)
T
P
e
x
(t)
+ 2e
T
x
(t)PEe
f
(t)+e
T
f
(t)Qe
f
(t)
− e
T
f
(t − 𝜏)Qe
f
(t − 𝜏)
≤e
T
x
(t)
P(A − LC)+(A − LC)
T
P
e
x
(t)
+ 2e
T
x
(t)PEe
f
(t)−𝜖
0
e
T
f
(t)Qe
f
(t)
+ 𝛼
1
e
T
f
(t)e
f
(t)−e
T
f
(t − 𝜏)Qe
f
(t − 𝜏)
(12)
where 𝛼
1
=(1 + 𝜖
0
)𝜆
max
(Q) with 𝜖
0
≥0.
Substituting (6) into (12) leads to
V(t) ≤e
T
x
(t)
P(A − LC)+(A − LC)
T
P
e
x
(t)
+ 2e
T
x
(t)
PE − 𝛼
1
(K
2
C)
T
K
1
e
f
(t − 𝜏)
− e
T
x
(t)
2PE − 𝛼
1
(K
2
C)
T
K
2
Ce
x
(t)
+ 2e
T
x
(t)
PE − 𝛼
1
(K
2
C)
T
f
a
(t)
+ 𝛼
1
e
T
f
(t − 𝜏)K
T
1
K
1
e
f
(t − 𝜏)
+ 2𝛼
1
e
T
f
(t − 𝜏)K
T
1
f
a
(t)+𝛼
1
f
T
a
(t)
f
a
(t)
− 𝜖
0
e
T
f
(t)Qe
f
(t)−e
T
f
(t − 𝜏)Qe
f
(t − 𝜏).
(13)
Further, with the aid of (9), the above inequality can
be simplied as
V(t) ≤e
T
x
(t)
P(A − LC)+(A − LC)
T
P
e
x
(t)
− 𝜖
0
e
T
f
(t)Qe
f
(t)+2𝛼
1
e
T
f
(t − 𝜏)K
T
1
f
a
(t)
+ 𝛼
1
f
T
a
(t)
f
a
(t)−e
T
f
(t − 𝜏)Qe
f
(t − 𝜏)
+ 𝛼
1
e
T
f
(t − 𝜏)K
T
1
K
1
e
f
(t − 𝜏).
(14)
© 2014 Chinese Automatic Control Society and John Wiley & Sons Australia, Ltd