
Saunder January 22, 2014 10:43 K15053˙Book
32 Disturbance Observer-Based Control: Methods and Applications
where τ
p1
, τ
p2
, and β are positive real numbers, indicating a right-half-plane (RHP)
zero in the transfer function. Suppose that the filter in the disturbance observer is
chosen as a first-order low-pass form
Q(s ) =
1
λs +1
. (2.13)
If we use the previous method for the minimum phase case to construct the distur-
bance observer for system (2.12), it yields
G
−1
n
(s )Q(s ) =
(τ
p1
s +1)(τ
p2
s +1)
k
p
(−βs + 1)(λs + 1)
e
τ s
. (2.14)
Note that the zeros of the process model become the poles of disturbance observer.
This resultsin an unstable observer andthus