FUZZY SYSTEMS - A TUTORIAL
by
James F. Brule'
(c) Copyright James F. Brule' 1985. Permission to copy without fee all or
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INTRODUCTION
Fuzzy sytems is an alternative to traditional notions of set membership and
logic that has its origins in ancient Greek philosophy, and applications at
the leading edge of Artificial Intelligence. Yet, despite its long-standing
origins, it is a relatively new field, and as such leaves much room for
development. This paper will present the foundations of fuzzy systems, along
with some of the more noteworthy objections to its use, with examples drawn
from current research in the field of Artificial Intelligence. Ultimately, it
will be demonstrated that the use of fuzzy systems makes a viable addition to
the field of Artificial Intelligence, and perhaps more generally to formal
mathematics as a whole.
THE PROBLEM: REAL-WORLD VAGUENESS
Natural language abounds with vague and imprecise concepts, such as "Sally
is tall," or "It is very hot today." Such statements are difficult to
translate into more precise language without losing some of their semantic
value: for example, the statement "Sally's height is 152 cm." does not
explicitly state that she is tall, and the statement "Sally's height is 1.2
standard deviations about the mean height for women of her age in her culture"
is fraught with difficulties: would a woman 1.1999999 standard deviations
above the mean be tall? Which culture does Sally belong to, and how is
membership in it defined?
While it might be argued that such vagueness is an obstacle to clarity of
meaning, only the most staunch tradtionalists would hold that there is no loss
of richness of meaning when statements such as "Sally is tall" are discarded
from a language. Yet this is just what happens when one tries to translate
human language into classic logic. Such a loss is not noticed in the
development of a payroll program, perhaps, but when one wants to allow for
Šnatural language queries, or "knowledge representation" in expert systems, the
meanings lost are often those being searched for.
For example, when one is designing an expert sstem to mimic the diagnostic
powers of a physician, one of the major tasks i to codify the physician's
decision-making process. The designer soon learns that the physician's view of
the world, despite her dependence upon precise, scientific tests and
measurements, incorporates evaluations of symptoms, and relationships between
them, in a "fuzzy," intutive manner: deciding how much of a particular
medication to administer will have as much to do with the physician's sense of
the relative "strength" of the patient's symptoms as it will their
height/weight ratio. While some of the decisions and calculations could be
done using traditional logic, we will see how fuzzy systems affords a broader,
richer field of data and the manipulation of that data than do more
traditional methods.
HISTORIC FUZZINESS
The precision of mathematics owes its success in large part to the efforts
of Aristotle and the philosophers who preceded him. In their efforts to devise
a concise theory of logic, and later mathematics, the so-called "Laws of
Thought" were posited [7]. One of these, the "Law of the Excluded Middle,"
states that every proposition must either be True or False. Even when
Parminedes proposed the first version of this law (around 400 B.C.) there were
strong and immediate objections: for example, Heraclitus proposed that things
could be simultaneously True and not True.
It was Plato who laid the foundation for what would become fuzzy logic,
indicating that there was a third region (beyond True and False) where these
opposites "tumbled about." Other, more modern philosophers echoed his
sentiments, notably Hegel, Marx, and Engels. But it was Lukasiewicz who first
proposed a systematic alternative to the bi-valued logic of Aristotle [8].
In the early 1900's, Lukasiewicz described a three-valued logic, along with
the mathematics to accompany it. The third value he proposed can best be
translated as the term "possible," and he assigned it a numeric value between
True and False. Eventually, he proposed an entire notation and axiomatic
system from which he hoped to derive modern mathematics.
Later, he explored four-valued logics, five-valued logics, and then
declared that in principle there was nothing to prevent the derivation of an
infinite-valued logic. Lukasiewicz felt that three- and infinite-valued logics
were the most intriguing, but he ultimately settled on a four-valued logic
because it seemed to be the most easily adaptable to Aristotlean logic.
Knuth proposed a three-valued logic similar to Lukasiewicz's, from which he
speculated that mathematics would become even more elegant than in traditional
Šbi-valued logic. His insight, apparently missed by Lukasiewicz, was to use the
integral range [-1, 0 +1] rather than [0, 1, 2]. Nonetheless, this alternative
failed to gain acceptance, and has passed into relative obscurity.
It was not until relatively recently that the notion of an infinite-valued
logic took hold. In 1965 Lotfi A. Zadeh published his seminal work "Fuzzy
Sets" ([12], [13]) which described the mathematics of fuzzy set theory, and by
extension fuzzy logic. This theory proposed making the membership function (or
the values False and True) operate over the range of real numbers [0.0, 1.0].
New operations for the calculus of logic were proposed, and showed to be in
principle at least a generalization of classic logic. It is this theory which
we will now discuss.
BASIC CONCEPTS
The notion central to fuzzy systems is that truth values (in fuzzy logic)
or membership values (in fuzzy sets) are indicated by a value on the range
[0.0, 1.0], with 0.0 representing absolute Falseness and 1.0 representing
absolute Truth. For example, let us take the statement:
"Jane is old."
If Jane's age was 75, we might assign the statement the truth value of
0.80. The statement could be translated into set terminology as follows:
"Jane is a member of the set of old people."
This statement would be rendered symbolically with fuzzy sets as:
mOLD(Jane) = 0.80
where m is the membership function, operating in this case on the fuzzy set of
old people, which returns a value between 0.0 and 1.0.
At this juncture it is important to point out the distinction between fuzzy
systems and probability. Both operate over the same numberic range, and at
first glance both have similar values: 0.0 representing False (or non-
membership), and 1.0 representing True (or membership). However, there is a
distinction to be made between the two statements: The probabilistic approach
yields the natural-language statement, "There is an 80% chance that Jane is
old," while the fuzzy terminology corresponds to "Jane's degree of membership
within the set of old people is 0.80." The semantic difference is significant:
the first view supposes that Jane is or is not old (still caught in the Law of
the Excluded Middle); it is just that we only have an 80% chance of knowing
Šwhich set she is in. By contrast, fuzzy terminology supposes that jane is
"more or less" oild, or some other term correspondig to the value of 0.80.
Further distinctions arising out of the operations will be noted below.
The next step in establishing a complete system of fuzzy logic is to define
the operations of EMPTY, EQUAL, COMPLEMENT (NOT), CONTAINMENT, UNION (OR), and
INTERSECTION (AND). Before we can do this rigorously, we must state some
formal definitions:
Definition 1: Let X be some set of objects, with elements noted as x. Thus,
X = {x}.
Definition 2: A fuzzy set A in X is characterized by a membership function
mA(x) which maps each point in X onto the real interval [0.0, 1.0]. As
mA(x) approaches 1.0, the "grade of membership" of x in A increases.
Definition 3: A is EMPTY iff for all x, mA(x) = 0.0.
Definition 4: A = B iff for all x: mA(x) = mB(x) [or, mA = mB].
Definition 5: mA' = 1 - mA.
Definition 6: A is CONTAINED in B iff mA <= mB.
Definition 7: C = A UNION B, where: mC(x) = MAX(mA(x), mB(x)).
Definition 8: C = A INTERSECTION B where: mC(x) = MIN(mA(x), mB(x)).
It is important to note the last two operations, UNION (OR) and
INTERSECTION (AND), which represent the clearest point of departure from a
probabilistic theory for sets to fuzzy sets. Operationally, the differences
are as follows:
For independent events, the probabilistic operation for AND is
multiplication, which (it can be argued) is counterintuitive for fuzzy
systems. For example, let us presume that x = Bob, S is the fuzzy set of smart
people, and T is the fuzzy set of tall people. Then, if mS(x) = 0.90 and
uT(x) = 0.90, the probabilistic result would be:
mS(x) * mT(x) = 0.81
whereas the fuzzy result would be:
Š
MIN(uS(x), uT(x)) = 0.90
The probabilistic calculation yields a result that is lower than either of the
two initial values, which when viewed as "the chance of knowing" makes good
sense.
However, in fuzzy terms the two membership functions would read something
like "Bob is very smart" and "Bob is very tall." If we presume for the sake of
argument that "very" is a stronger term than "quite," and that we would
correlate "quite" with the value 0.81, then the semantic difference becomes
obvious. The probabilistic calculation would yield the statement
If Bob is very smart, and Bob is very tall, then Bob is a quite tall,
smart person.
The fuzzy calculation, however, would yield
If Bob is very smart, and Bob is very tall, then Bob is a very tall,
smart person.
Another problem arises as we incorporate more factors into our equations
(such as the fuzzy set of heavy people, etc.). We find that the ultimate
result of a series of AND's approaches 0.0, even if all factors are initially
high. Fuzzy theorists argue that this is wrong: that five factors of the value
0.90 (let us say, "very") AND'ed together, should yield a value of 0.90
(again, "very"), not 0.59 (perhaps equivalent to "somewhat").
Similarly, the probabilistic version of A OR B is (A+B - A*B), which
approaches 1.0 as additional factors are considered. Fuzzy theorists argue
that a sting of low membership grades should not produce a high membership
gradel instead, the limit of the resulting membership grade should be the
strongest membership value in the collection.
Other values have been established by other authors, as have other
operations. Baldwin [1] proposes a set of truth value restrictions, such as
"unrestricted" (mX = 1.0), "impossible" (mX = 0.0), etc.
The skeptical observer will note that the assignement of values to
linguistic meanings (such as 0.90 to "very") and vice versa, is a most
imprecise operation. Fuzzy systems, it should be noted, kay no claim to
establishing a formal procedure for assignments at this level; in fact, the
only argument for a particular assignment is its intuitive strength. What
fuzzy logic does propose is to establish a formal method of operating on these
values, once the primitives have been established.
Š
HEDGES
Another important feature of fuzzy systems is the ability to define
"hedges," or modifier of fuzzy values. These operations are provided in an
effort to maintain close ties to natural language, and to allow for the
generation of fuzzy statements through mathematical calculations. As such, the
initial definition of hedges and operations upon them will be quite a
subjective process and may vary from one project to another. Nonetheless, the
system ultimately derived operates with the same formality as classic logic.
The simplest example is in which one transforms the statement "Jane is old"
to "Jane is very old." The hedge "very" is usually defined as follows:
m"very"A(x) = mA(x)^2
Thus, if mOLD(Jane) = 0.8, then mVERYOLD(Jane) = 0.64.
Other common hedges are "more or less" [typically SQRT(mA(x))], "somewhat,"
"rather," "sort of," and so on. Again, their definition is entirely
subjective, but their operation is consistent: they serve to transform
membership/truth values in a systematic manner according to standard
mathematical functions.
A more involved approach to hedges is best shown through the work of
Wenstop [11] in his attempt to model organizational behavior. For his study,
he constructed arrays of values for various terms, either as vectors or
matrices. Each term and hedge was represented as a 7-element vector or 7x7
matrix. He ten intuitively assigned each element of every vector and matrix a
value between 0.0 and 1.0, inclusive, in what he hoped was intuitively a
consistent manner. For example, the term "high" was assigned the vector
0.0 0.0 0.1 0.3 0.7 1.0 1.0
and "low" was set equal to the reverse of "high," or
1.0 1.0 0.7 0.3 0.1 0.0 0.0
Wenstop was then able to combne groupings of fuzzy statements to create new
fuzzy statements, using the APL function of Max-Min matrix multiplication.
These values were then translated back into natural language statements, so
Šas to allow fuzzy statements as both input to and output from his simulator.
For example, when the program was asked to generate a label "lower than sortof
low," it returned "very low;" "(slightly higher) than low" yielded "rather
low," etc.
The point of this example is to note that algorithmic procedures can be
devised which translate "fuzzy" terminology into numeric values, perform
reliable operations upon those values, and then return natural language
statements in a reliable manner.
Similar techniques have been adopted by others, primarily in the study of
fuzzy systems as applicable to linguistic approximation (e.g. [2], [3], [4]).
APL appears to be the language of choice, owing to its flexibility and power
in matrix operations.
OBJECTIONS
It would be remarkable if a theory as far-reaching as fuzzy systems did not
arouse some objections in the professional community. While there have been
generic complaints about the "fuzziness" of the process of assigning values to
linguistic terms, perhaps the most cogent criticisms come from Haack [6]. A
formal logician, Haack argues that there are only two ares in which fuzzy
logic could possibly be semonstrated to be "needed," and then maintains that
in each case it can be shown that fuzzy logic is not necessary.
The first area Haack defines is that of the nature of Truth and Falsity: if
it could be shown, she maintains, that these are fuzzy values and not discrete
ones, then a need for fuzzy logic would have been demonstrated. The other area
she identifies is that of fuzzy systems' utility: if it could be demonstrated
that generalizing classic logic to encompass fuzzy logic would aid in
calculations of a given sort, then again a need for fuzzy logic would exist.
In regards to the first statement, Haack argues that True and False are
discrete terms. For example, "The sky is blue" is either true or false; any
fuzziness to the statement arises from an imprecise definition of terms, not
out of the nature of Truth. As far as fuzzy systems' utility is concerned, she
maintains that no area of data manipulation is made easier through the
introduction of fuzzy calculus; if anything, she says, the calculations become
more complex. Therefore, she asserts, fuzzy logic is unnecessary.
Fox [5] has responded to her objetions, indicating that there are three
areas in which fuzzy logic can be of benefit: as a "requisite" apparatus (to
describe real-world relationships which are inherently fuzzy); as a
"prescriptive" apparatus (because some data is fuzzy, and therefore requires a
fuzzy calculus); and as a "descriptive" apparatus (because some inferencing
systems are inherently fuzzy).
Š
His most powerful arguments come, however, from the notion that fuzzy and
classic logics need not be seen as competitive, but complementary. He argues
that many of Haack's objections stem from a lack of semantic clarity, and that
ultimately fuzzy statements may be translatable into phrases which classical
logicians would find palatable.
Lastly, Fox argues that despite the objections of classical logicians,
fuzzy logic has found its way into the world of practical applications, and
has proved very successful there. He maintains, pragmatically, that this is
sufficient reason for continuing to develop the field.
APPLICATIONS
Areas in which fuzzy logic has been successfully applied are often quite
concrete. The first major commercial application was in the area of cement
kiln control, an operation which requires that an operator monitor four
internal states of the kiln, control four sets of operations, and dynamically
manage 40 or 50 "rules of thumb" about their interrelationships, all with the
goal of controlling a highly complex set of chemical interactions. One such
rule is "If the oxygen percentage is rather high and the free-lime and kiln-
drive torque rate is normal, decrease the flow of gas and slightly reduce the
fuel rate" (see Zadeh [14]). A complete accounting of this very successful
system can be found in Umbers and King [10].
The objection has been raised that utilizing fuzzy systems in a dynamic
control environment raises the likelihood of encountering difficult stability
problems: since in control conditions the use of fuzzy systems can roughly
correspond to using threshholds, there must be significant care taken to
insure that oscillations do not develop in the "dead spaces" between
threshhold triggers. This seems to be an important area for future research.
Other applications which have benefited through the use of fuzzy systems
theory have been information retrieval systems, a navigation system for
automatic cars, a predictive fuzzy-logic controller for automatic operation of
trains, laboratory water level controllers, controllers for robot arc-welders,
feature-definition controllers for robot vision, graphics controllers for
automated police sketchers, and more.
Expert systems have been the most obvious recipients of the benefits of
fuzzy logic, since their domain is often inherently fuzzy. Examples of expert
systems with fuzzy logic central to their control are decision-support
systems, financial planners, diagnostic systems for determining soybean
pathology, and a meterological expert system in China for determining areas in
which to establish rubber tree orchards [14]. Another area of application,
akin to expert systems, is that of information retrieval [9].
Š
CONCLUSIONS
Fuzzy systems, including fuzzy logic and fuzzy set theory, provide a rich
and meaningful addition to standard logic. The mathematics generated by these
theories is consistent, and fuzzy logic may be a generalization of classic
logic. The applications which may be generated from or adapted to fuzzy logic
are wide-ranging, and provide the opportunity for modeling of conditions which
are inherently imprecisely defined, despite the concerns of classical
logicians. Many systems may be modeled, simulated, and even replicated with
the help of fuzzy systems, not the least of which is human reasoning itself.
REFERENCES
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and Its Applications, E.H. Mamdani and B.R. Gaines (eds.), London: Academic
Press, 1981.
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fuzzy relational products," in Fuzzy Reasoning and Its Applications, E.H.
Mamdani and B.R. Gaines (eds.), London: Academic Press, 1981.
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influence," paper presented at the International Fuzzy Systems Association
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[13] L.A. Zadeh, "Fuzzy algorithms," Info. & Ctl., Vol. 12, 1968, pp. 94-
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[14] L.A. Zadeh, "Making computers think like people," I.E.E.E. Spectrum,
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