326 IEEE TRANSACTIONS ON EVOLUTIONARY COMPUTATION, VOL. 11, NO. 3, JUNE 2007
Clustering-Based Adaptive Crossover and Mutation
Probabilities for Genetic Algorithms
Jun Zhang, Member, IEEE, Henry Shu-Hung Chung, Senior Member, IEEE, and Wai-Lun Lo, Member, IEEE
Abstract—Research into adjusting the probabilities of crossover
and mutation
p
m
in genetic algorithms (GAs) is one of the most sig-
nificant and promising areas in evolutionary computation.
p
x
and
p
m
greatly determine whether the algorithm will find a near-op-
timum solution or whether it will find a solution efficiently. Instead
of using fixed values of
p
x
and
p
m
, this paper presents the use of
fuzzy logic to adaptively adjust the values of
p
x
and
p
m
in GA. By
applying the
K
-means algorithm, distribution of the population in
the search space is clustered in each generation. A fuzzy system is
used to adjust the values of
p
x
and
p
m
. It is based on considering
the relative size of the cluster containing the best chromosome and
the one containing the worst chromosome. The proposed method
has been applied to optimize a buck regulator that requires sat-
isfying several static and dynamic operational requirements. The
optimized circuit component values, the regulator’s performance,
and the convergence rate in the training are favorably compared
with the GA using fixed values of
p
x
and
p
m
. The effectiveness of
the fuzzy-controlled crossover and mutation probabilities is also
demonstrated by optimizing eight multidimensional mathematical
functions.
Index Terms—Evolutionary computation, fuzzy logics, genetic
algorithms (GA), power electronics.
I. INTRODUCTION
T
HE CONVENTIONAL approach to circuit optimization is
to develop a formal model that can resemble actual circuit
responses closely, but is solvable by means of available mathe-
matical methods, such as linear and nonlinear programming. In
the area of power electronics, state-space averaging and the vari-
ants [1]–[3] have been the dominant modeling techniques since
1970. By recognizing that power electronic circuits (PECs) typ-
ically have output filter cutoff frequency that is much lower
than the switching frequency, linear time-invariant models, such
as the control-to-output or input-to-output transfer functions,
can be formulated to approximate the time-variant and piece-
wise-linear properties of the circuits. Although this approach
has been proven to be very successful in many applications, it
has the drawbacks of oversimplifying the circuit behaviors and
of having limitations on particular operating mode and control
Manuscript received June 15, 2005; revised January 11, 2006, April 4, 2006,
and May 4, 2006.
J. Zhang is with the Department of Computer Science, Sun Yat-sen Univer-
sity, Guangzhou, China (e-mail: junzhang@ieee.org).
H. S.-H. Chung is with the Department of Electronic Engineering, City Uni-
versity of Hong Kong, Kowloon Tong, Hong Kong (e-mail: eeshc@cityu.edu.
hk).
W.-L. Lo is with the Department of Computer Science, Chu Hai College of
Higher Education, Tsuen Wan, Hong Kong.
Digital Object Identifier 10.1109/TEVC.2006.880727
schemes. As a circuit has been converted into a mathematical
model and its state variables have been averaged, no detailed in-
formation about the exact waveforms and the response profiles
can be obtained. Circuit designers would sometimes find it diffi-
cult to predict precisely the circuit responses under large-signal
variations [3].
As power electronics technology continues to develop, a large
number of combinatorial issues, including circuit complexity,
static and dynamic responses, thermal problems, electromag-
netic compatibility, control schemes, costing, etc., are associ-
ated. A plethora of such multimodal functions exist in a PEC.
In particular, there is a growing need for automated synthesis
that starts with high-level statements of the desired behaviors
and optimizes the circuit component values for meeting required
specifications.
Optimization strategies that are based on satisfying con-
strained equations might be subject to becoming trapped into
local minima, leading to suboptimal parameter values, and
thus, having a limitation on operating in large, multimodal,
and noisy spaces. Since 1950, other strategies that employ
Darwin’s evolution theory have been proposed [4]–[6]. The
most significant advantage of using this evolutionary search
lies in the gain of flexibility and adaptability to the task at
hand and the global search characteristics. Among various
evolutionary computation methods (ECM), genetic algorithms
(GA), which have been applied to many optimization problems
[7], [8], employ a random, yet directed, search for locating the
global optimal solution. They are superior to gradient descent
techniques, as the search is not biased towards the local optimal
solution. They differ from random sampling algorithms, as they
can direct the search towards relatively prospective regions in
the search space [9]. However, the usage of GA was progressed
slowly in real applications. Apart from the shortcomings of
early approaches, it was also largely due to the lack of powerful
computer platforms at that time [10], [11].
Due to the recent advancements in computer technology,
much research effort has been emphasized on developing new
GA-based optimization methods. There are many new design
schemes for analog circuits, like voltage reference circuit [12],
transconductance amplifier [13], and analog circuit synthesis
[14], [15]. Recently, GA have been applied to PEC optimiza-
tion [16]–[18]. The circuit behaviors [16], [17] and controller
functions [18] are described by well-defined mathematical
functions with unknown optimal component values.
The parameters of the search space in GA are encoded in the
form of a chromosome-like structure. A group of these chro-
mosomes constitutes a population. An index of merit (fitness
value) is assigned to each individual chromosome, according
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