1452 IEEE TRANSACTIONS ON AUTOMATIC CONTROL, VOL. 56, NO. 6, JUNE 2011
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Upper Saddle River, NJ: Prentice-Hall, 1996.
Design of Observer-Based Robust
Repetitive-Control System
Min Wu, Senior Member, IEEE, Lan Zhou, and
Jinhua She, Senior Member, IEEE
Abstract—This technical note deals with the problem of designing a ro-
bust observer-based repetitive-control system that provides a given
H
dis-
turbance attenuation performance for a class of plants with time-varying
structured uncertainties. A continuous-discrete two-dimensional model is
built that accurately describes the features of repetitive control, thereby en-
abling the control and learning actions to be preferentially adjusted. A suf-
ficient condition for the repetitive-control system to have a disturbance-at-
tenuation bound in the
H
setting is given in terms of a linear matrix in-
equality (LMI). It yields the parameters of the repetitive controller and
the state observer. Finally, a numerical example demonstrates the effec-
tiveness of the method, whose main advantage is the easy, preferential ad-
justment of control and learning through the tuning of two parameters in
the LMI-based condition.
Index Terms—Disturbance attenuation, linear matrix inequality (LMI),
repetitive control, robust control, state observer, two-dimensional (2-D)
system.
I. INTRODUCTION
Repetitive control has a learning capability. For a given periodic ref-
erence input, a repetitive controller gradually reduces the tracking error
through repeated learning actions, resulting in the tracking of the ref-
erence input without steady-state error.
The key feature of a repetitive controller is that it contains an
internal model of a periodic signal, which theoretically guarantees
asymptotic tracking [1]. It contains a pure-time-delay positive-feed-
back loop, which adds the tracking error of the previous period to the
present error to produce a control signal. This action simulates human
learning. From the standpoint of system theory, a repetitive-control
system is a neutral-type delay system. A repetitive controller contains
an infinite number of poles on the imaginary axis. [2] pointed out that
this type of system can be stabilized only when the relative degree
Manuscript received March 20, 2009; revised September 26, 2009, September
29, 2009, June 16, 2010, and June 22, 2010; accepted January 19, 2011. Date of
publication February 07, 2011; date of current version June 08, 2011. This work
was supported in part by the National Science Foundation of China under Grants
60974045 and 60674016. Recommended by Associate Editor M. Egerstedt.
M. Wu and L. Zhou are with the School of Information Science and Engi-
neering, Central South University, Changsha 410083, China.
J. She is with the School of Computer Science, Tokyo University of Tech-
nology, Tokyo 192-0982, Japan and also with the School of Information Science
and Engineering, Central south University, Changsha 410083, China (e-mail:
she@cs.teu.ac.jp).
Digital Object Identifier 10.1109/TAC.2011.2112473
of the plant is zero. When the relative degree is larger than that, the
repetitive controller has to be modified by the insertion of a low-pass
filter into the time-delay feedback line. This modification means that
the controller now contains only an approximate internal model of a
periodic signal. As a result, tracking performance is not guaranteed for
periodic signals in the high-frequency band. Since the best tracking
performance is obtainable only when the plant has a relative degree of
zero, the design of a repetitive-control system for this limiting case is
theoretically significant.
Analysis of a repetitive-control system reveals two types of actions:
continuous control within each repetition period and discrete learning
between periods. Due to the difficulty of guaranteeing stability, almost
all methods of designing repetitive-control systems consider only the
overall results in the time domain. Consequently, they are incapable
of making fundamental improvements in control performance. For ex-
ample, [3] discussed the stability and robustness provided by a struc-
tured-singular-value method; but they used a trial-and-error technique
to find approximate upper and lower bounds on a structured singular
value. [4] presented a sufficient stabilization condition in the form of a
linear matrix inequality (LMI); but the tracking performance depends
on the iterative adjustment of the parameters of a low-pass filter and
the repetitive controller.
[5] presented a method of designing a robust, static, output-feed-
back repetitive-control system that is based on two-dimensional (2-D)
system theory [6], [7]; but it only considers the robust stability of the
system. To enable that method to handle a larger class of systems, this
technical note extends the static output feedback to dynamic output
feedback and presents the configuration of an observer-based repeti-
tive-control system. It focuses especially on the problem of designing
a robust repetitive-control system with a prescribed bound on distur-
bance attenuation for a class of linear systems with a relative degree
of zero and time-varying, structured, periodic uncertainties. First, we
build a continuous-discrete 2-D model to describe the system. Next, to
obtain satisfactory disturbance-attenuation performance, we formulate
the design problem as an
H
1
robust-stabilization problem for a contin-
uous-discrete 2-D system. Then, we derive a sufficient robust-stability
condition in the form of an LMI by using 2-D system stability theory
and the singular-value decomposition (SVD) of the output matrix. The
advantage of this method over others, including the one in [5], is that
it allows control and learning to be preferentially adjusted by means of
two parameters in the LMI. Finally, a numerical example demonstrates
the validity of the method.
Throughout this technical note,
+
is the set of non-negative real
numbers;
p
is the
n
-dimensional vector space over complex numbers;
+
is the set of non-negative integers;
@
is the linear space of all the
functions from
[0
;T
]
to
p
.
L
2
(
+
;
p
)
, or just
L
2
, is the linear space
of square integrable functions from
+
to
p
; and
`
2
(
+
;
@
)
, or just
`
2
, is the linear space of all the functions from
+
to
@
(discrete-time
signal).
II. P
ROBLEM DESCRIPTION
Consider the repetitive-control system in Fig. 1.
r
(
t
)
is a given pe-
riodic reference signal with a period of
T
. The compensated single-
input, single-output (SISO) plant has a relative degree of zero and time-
varying structured uncertainties
_
x
p
(
t
)=[
A
+
A
(
t
)]
x
p
(
t
)+[
B
+
B
(
t
)]
u
(
t
)+
B
w
w
(
t
)
y
(
t
)=
Cx
p
(
t
)+
Du
(
t
)
(1)
where
x
p
(
t
)
2
n
is the state of the plant;
u
(
t
)
;y
(
t
)
2
are the
control input and output, respectively; and
w
(
t
)
2
L
2
[0
;
+
1
)
is the
disturbance input. Setting
B
w
6
=0
adds the disturbance to the system,
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