ZHANG et al.: SVR LEARNING-BASED SPATIOTEMPORAL FUZZY LOGIC CONTROLLER 1637
the form of 3-D fuzzy rules. As the research on the 3-D FLC
is just at the beginning stage, control rule extraction from a
spatiotemporal dataset is still a challenging and open problem
for spatially distributed dynamic systems.
Data-driven design methods for the traditional FLC were
developed in past three decades. Generally speaking, the data-
driven FLC design consists of two parts: 1) rule generation;
and 2) system optimization (including structure optimization
and parameter optimization) [15]. For instance, clustering
techniques [16] can be used to generate rules automatically;
fusing similar fuzzy sets [17] can be applied to reduce the rule
number and realize the structure optimization; gradient decent
approaches [13] can be adopted for fine-tuning the member-
ship functions and realizing the parameter optimization. In
addition, an FLC can be designed directly through support
vector machine (SVM) learning [18], [19], i.e., the optimal
rule generation and parameter design of an FLC come from
the learning results of a SVM. For instance, a λ-fuzzy rule-
based system was constructed from a trained SVM in [20] and
additive fuzzy systems can be directly derived from the SVM
in [21] and [22].
SVM, as a new learning algorithm with the superior
learning power and the good generalization ability, brings
forward a promising solution to the data-driven-based design
of the traditional fuzzy system. However, the current SVM
has no inherent ability to express and cope with spatial
information. In particular, the SVM may not work well for
a spatiotemporal input–output dataset from a spatiotemporal
system.
In this paper, we will expand the learning ability of the SVM
to spatiotemporal datasets and develop a new data-driven 3-D
FLC design method based on the SVM learning. As SVM can
be classified into SVM classification [23] and SVM regression
(also called support vector regression (SVR)] [24]–[26], we
herein focus on SVR with an ε-insensitive loss function,
i.e., ε-SVR. Initially, the spatial information expression and
processing as well as the fuzzy linguistic expression and rule
inference of a 3-D FLC are integrated into spatial fuzzy basis
functions (SFBFs), and then the 3-D FLC can be described
by a three-layer network structure. Through relating SFBFs
of the 3-D FLC directly to spatial kernel functions (SKFs)
of an SVR, an equivalence relationship of the 3-D FLC and
the SVR is established. With the SKFs, the learning ability
of the SVR can be enhanced for spatiotemporal datasets; the
3-D FLC can be designed with the help of the SVR learning.
For an easy implementation, a systematic SVR learning-based
3-D FLC design scheme is formulated in four steps as follows:
1) data collection; 2) spatial support-vector learning; 3) 3-D
fuzzy rule construction; and 4) 3-D fuzzy controller integra-
tion. In addition, the universal approximation capability of the
proposed 3-D FLC is presented.
This paper is organized as follows. In Section II, pre-
liminaries about 3-D FLC are addressed. In Section III, an
SVR learning-based 3-D FLC design methodology is pre-
sented, including the design theory and the systematic design
framework, and the universal approximation capability of
the proposed 3-D FLC is given. In Section IV, a catalytic
packed-bed reactor is presented as an example to illustrate the
proposed 3-D FLC and validate its effectiveness, and finally,
Section V summarizes the conclusion.
II. P
RELIMINARIES
A. 3-D FLC
A 3-D FLC is a controller based on a linguistic rule
base, which can be represented and computed using linguistic
words; on the other hand, it is a nonlinear mapping from
an input space to an output space, which can be expressed
in a precise mathematical formula. Here, we will present the
mathematical description of a 3-D FLC as a nonlinear mapping
and then describe its network structure based on SFBFs.
1) Nonlinear Mapping: The basic structure of a 3-D FLC is
composed of 3-D fuzzifier, 3-D rule inference, and defuzzifier.
Because of its unique 3-D nature, some detailed operations
of a 3-D FLC are different from a traditional one for spatial
information expression, processing, and compression. For their
detailed operations, we can refer to [9]. Once each component
of a 3-D FLC is set, the nonlinear mathematical description of
the 3-D FLC can be derived (see Appendix I for a brief deriva-
tion). Assume we employ 3-D singleton fuzzifier, Gaussian
type 3-D MF, 3-D fuzzy rules as shown in (A.1) of Appendix I,
product t-norm and weighted aggregation dimension reduction
[9] in the 3-D rule inference, singleton fuzzy sets for the
output variable, and linear defuzzifier [27], the 3-D FLC can
be mathematically expressed as follows:
u(x
z
) =
N
l=1
ζ
l
p
j=1
a
j
s
i=1
μ
¯
C
l
i
(x
i
(z
j
))
=
N
l=1
ζ
l
p
j=1
a
j
s
i=1
exp
−((x
i
(z
j
) − c
l
ij
)/σ
j
)
2
=
N
l=1
ζ
l
p
j=1
a
j
exp
−||x(z
j
) − c
l
(z
j
)||
2
/σ
2
j
(1)
where σ
j
is set as 10% of the length of the input interval of
x(z
j
) [28] denoted by
σ
j
= max
1≤i≤s
(x
max
i
(z
j
) − x
min
i
(z
j
))/10
(2)
where x
max
i
(z
j
) and x
min
i
(z
j
) are the maximum and the mini-
mum bound value of the ith spatial input variable, respectively,
at the jth sensing location.
Equation (1) shows that the 3-D FLC is a nonlinear mapping
from the input space x
z
∈ ⊂ R
p×s
to the output space
u(x
z
) ∈ U ⊂ R. It provides a way to understand and analyze
the 3-D FLC from the point of view of function approximation.
2) SFBFs and Three-Layer Network Structure: In (1), let
l
(x
z
) =
p
j=1
a
j
s
i=1
μ
¯
C
l
i
(x
i
(z
j
)) (3)
then (1) can be rewritten as follows:
u(x
z
) =
N
l=1
ζ
l
l
(x
z
).