5860 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 62, NO. 9, SEPTEMBER 2015
function forms of g
i
, while the observer gain l
i
is set as
constant. The novelty of AESO lies in that g
i
is still chosen as
g
i
(e)=e, kept in a simple linear form as that in LESO, while
the observer gain is set as time varying, that is, l
i
= l
i
(t),to
obtain more design flexibilities. In this seminal idea, AESO is
designed in the form of
⎧
⎪
⎪
⎪
⎪
⎪
⎪
⎨
⎪
⎪
⎪
⎪
⎪
⎪
⎩
˙
ˆx
1
=ˆx
2
− l
1
(t)(ˆx
1
− x
1
)
.
.
.
˙
ˆx
n−1
=ˆx
n
− l
n−1
(t)(ˆx
1
− x
1
)
˙
ˆx
n
=ˆx
n+1
+ b
0
u − l
n
(t)(ˆx
1
− x
1
)
˙
ˆx
n+1
= −l
n+1
(t)(ˆx
1
− x
1
).
(5)
Here, we first transform the estimate error dynamics of
AESO (5) into a canonical form that is convenient for theoret-
ical analysis. Then, we give a brief introduction of DAST and,
based on that, develop an adaptive method to tune the time-
varying observer gain l
i
(t). Finally, we conduct a comprehen-
sive analysis of the stability and estimate error bounds of the
proposed AESO (5).
A. Transformation of AESO Error Dynamics
In view of (2) and (5), AESO estimate error dynamics can be
obtained as
⎧
⎪
⎪
⎪
⎪
⎪
⎪
⎨
⎪
⎪
⎪
⎪
⎪
⎪
⎩
˙e
1
= e
2
− l
1
(t)e
1
.
.
.
˙e
n−1
= e
n
− l
n−1
(t)e
1
˙e
n
= e
n+1
− l
n
(t)e
1
˙e
n+1
= −l
n+1
(t)e
1
− h(x, w)
(6)
or written in a vector differential equation form as
˙e = A(t)e + b (−h(x, w)) (7)
where
A(t)=
⎡
⎢
⎢
⎢
⎢
⎢
⎣
−l
1
(t)10··· 0
−l
2
(t)01··· 0
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
−l
n
(t)0··· 01
−l
n+1
(t)0··· 00
⎤
⎥
⎥
⎥
⎥
⎥
⎦
,b=
⎡
⎢
⎢
⎢
⎢
⎢
⎣
0
0
.
.
.
0
1
⎤
⎥
⎥
⎥
⎥
⎥
⎦
and e =[e
1
,...,e
n+1
]
T
, e
i
=ˆx
i
− x
i
denoting the estimate
erroroftheith state, i =1, 2,...,n+1. Obviously, (7) is an
LTV system with an unknown but bounded input −h(x, w).In
fact, (7) is a canonical form of LTV systems that are uniformly
controllable [29]. Next, we need to design proper gain l
i
(t) to
guarantee the stability of (6) and (7) and, additionally, to obtain
good estimate performance. The starting point is to transform
(7) into an equivalent canonical (phase-variable) form [30]
given below, and the AESO properties will be discussed in the
new canonical form. Thus
˙z = A
c
(t)z + b
c
(−h(x, w)) (8)
where
A
c
(t)=
⎡
⎢
⎢
⎢
⎢
⎢
⎣
010··· 0
001··· 0
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
00··· 01
−a
1
(t) −a
2
(t) ··· −a
n
(t) −a
n+1
(t)
⎤
⎥
⎥
⎥
⎥
⎥
⎦
b
c
=
⎡
⎢
⎢
⎢
⎢
⎢
⎣
0
0
.
.
.
0
1
⎤
⎥
⎥
⎥
⎥
⎥
⎦
.
Here, the element a
i
(t) is assumed to be smooth (it owns con-
tinuous derivatives at any order, if needed in the later derivation)
and bounded. Clearly, the canonical form (8) is a realization of
the scalar linear differential system, i.e.,
ξ
(n+1)
+ a
n+1
(t)ξ
(n)
+ ···+a
2
(t)
˙
ξ+a
1
(t)ξ = b
c
(−h) (9)
where the elements a
i
(t) in A
c
(t) are directly related to the
coefficients of (9). These representations (8) and (9) are central
to investigating LTV systems, which will be addressed later.
Here, we first discuss how to transform (7) into (8).
The controllability matrix of LTV system (7) is defined as
M =[p
1
p
2
··· p
n+1
] (10)
where M and p
i
, i =1,...,n+1, are, respectively, a square
matrix and a column vector of dimension n +1, and
p
k+1
= −A(t)p
k
+
d
dt
p
k
,p
1
= b, k =1,...,n. (11)
The controllability matrix M
c
of LTV system (8) is similarly
defined. With the definitions (10) and (11), it can be verified by
direct construction that
M =
⎡
⎢
⎢
⎢
⎢
⎢
⎣
00··· 0(−1)
n
00··· (−1)
n−1
0
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
0(−1)
1
··· 00
(−1)
0
0 ··· 00
⎤
⎥
⎥
⎥
⎥
⎥
⎦
(12)
M
c
=
⎡
⎢
⎢
⎢
⎢
⎢
⎣
00··· 0(−1)
n
00··· (−1)
n−1
q
n,n
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
0(−1)
1
··· q
n−1,2
q
n,2
(−1)
0
q
1,1
··· q
n−1,1
q
n,1
⎤
⎥
⎥
⎥
⎥
⎥
⎦
(13)
where q
i,k
is defined as in the equation
q
i,k
=
⎧
⎪
⎪
⎪
⎪
⎪
⎨
⎪
⎪
⎪
⎪
⎪
⎩
−q
i−1,k−1
+˙q
i−1,k
, 1 <k<i≤ n
(−1)
i+1
a
n−i+2
+
i−2
j=0
a
n−j+1
q
i−1,j+1
+˙q
i−1,1
,k=1<i≤ n
(−1)
i+1
a
n+1
, 1 ≤ k = i ≤ n.
It is seen that the controllability matrix M of the LTV system
(7) is constant, whereas the controllability matrix M
c
of the