1056 IEEE TRANSACTIONS ON AUTOMATIC CONTROL, VOL. 57, NO. 4, APRIL 2012
V. C ONCLUSION
It is shown that two different types of pole placement constraints for
critical and noncritical poles can be formulated in terms of the state
feedback controller gains. Due to non-convexity of the region
C
s
cor-
responding to the region
, an LMI stability region is constructed inside
C
s
. This enables the formulation of an equivalent SDP over feedback
controllers. Similarly, other relevant controller and closed loop charac-
teristics, like closed loop sensitivity and controller
H
1
norm can be
likewise optimized, and are topics of current research. It would also be
interesting to extend the current technique to multi-input systems.
A
CKNOWLEDGMENT
The authors would like to acknowledge the helpful suggestions of
the anonymous reviewers for preparing this article.
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lehigh.edu 2010
Adaptive Asymptotic Rejection of Unmatched General
Periodic Disturbances in Output-Feedback
Nonlinear Systems
Pengnian Chen, Mingxuan Sun, Qiuzhen Yan, and Xueyi Fang
Abstract—This note deals with asymptotic rejection of periodic distur-
bances in a class of nonlinear systems via output feedback. The disturbance
undertaken is unmatched but the period of the disturbance is known. A
novel method for estimating periodic disturbances is proposed, which en-
sures convergence of the estimation error to zero. By applying such estima-
tion, a controller is constructed such that all the signals in the closed loop
are bounded and the output converges to zero. A numerical example is pre-
sented to show the utility of the results presented in the note.
Index Terms—Adaptive control, nonlinear systems, periodic distur-
bances.
I. I
NTRODUCTION
Disturbance rejection is one of the most important topics in con-
trol theory and its applications. Traditionally, the disturbances to
be rejected are mainly sinusoidal disturbances (see, for instance,
[1]–[5] and the references therein ). In recent years, the asymptotic
rejection for general periodic disturbances has been received much
attention. The asymptotic rejection of general periodic disturbances
for output-feedback nonlinear systems is considered in [6], [7] and
[8]. The periodic disturbances in [6] and [7] are matched, while
in [8] the periodic disturbances are unmatched. The general pe-
riodic disturbances considered in [6], [7] and [8] are of the form
w
(
t
)=
aw
b
(
t
+
)
, where
a
and
are unknown constants, and
w
b
(
t
)
,
called a ware pattern of the periodic disturbance, is a known function.
Under the condition that the ware pattern of a general periodic distur-
bance is known, the method in [6], [7] and [8] can reject the periodic
disturbance and guarantee that the output of the closed loop system
Manuscript received December 19, 2010; revised June 01, 2011; accepted
September 14, 2011. Date of publication October 25, 2011; date of current ver-
sion March 28, 2012. This work was supported by the National Nature Science
Foundation of China under Grants 21006086, 60674023, 60874041, 61174034.
Recommended by Associate Editor A. Astolfi.
P. Chen is with the College of Mechatronics Engineering, China Jil-
iang University, Hangzhou 310018, China (e-mail: pnchen@mail.hz.zj.cn,
pnchen@cjlu.edu.cn).
M. Sun and Q. Yan are with the College of Information Engineering, Zhe-
jiang University of Technology, Hangzhou 310023, China (e-mail: mxsun@zjut.
edu.cn, mxsun@e.ntu.edu.sg; zjhzyqz@gmail.com).
X. Fang is with the State Key Laboratory of Industrial Control Technology,
Zhejiang University, Hangzhou 310027, China (e-mail: xyfang@iipc.zju.edu.
cn).
Digital Object Identifier 10.1109/TAC.2011.2173409
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