20-07-2012, 02:14 PM
Comparison between Conventional and Fuzzy Logic PID Controllers for Controlling DC Motors
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Abstract
Fuzzy logic and proportional-integral-derivative (PID)
controllers are compared for use in direct current (DC) motors
positioning system. A simulation study of the PID position
controller for the armature-controlled with fixed field and fieldcontrolled
with fixed armature current DC motors is performed.
Fuzzy rules and the inferencing mechanism of the fuzzy logic
controller (FLC) are evaluated by using conventional rule-lookup
tables that encode the control knowledge in a rules form. The
performance assessment of the studied position controllers is
based on transient response and error integral criteria. The results
obtained from the FLC are not only superior in the rise time,
speed fluctuations, and percent overshoot but also much better in
the controller output signal structure, which is much remarkable
in terms of the hardware implementation.
Introduction
Lotfi Zadeh, the father of fuzzy logic, claimed that many
sets in the world that surrounds us are defined by a nondistinct
boundary. Zadeh decided to extend two-valued
logic, defined by the binary pair {0, 1}, to the whole
continuous interval [0, 1], thereby introducing a gradual
transition from falsehood to truth [1].
Fuzzy control is a control method based on fuzzy logic.
Just as fuzzy logic can be described simply as "computing
with words rather than numbers"; fuzzy control can be
described simply as "control with sentences rather than
equations". A fuzzy controller can include empirical rules,
and that is especially useful in operator controlled plants.
A comprehensive review of the classical design and
implementation of the fuzzy logic controller can be found
in the literature [2], [3], [4]. A fuzzy IF-THEN rule-based
system consists of the following modules [5], [6].
Designing the PID-Like FLC
In the following section, we propose a design method for a
PID-like FLC. A simple mean for designating membership
functions for the PID-like FLC is presented. This method
allows a novice to construct a set of membership functions
for a specific linguistic variable systematically.
Optimizing the Membership Functions
In real applications of FLC, the membership functions are
constructed by assembling knowledge from the experts
and then modified by laboriously surveying the control
response of the process. In most control cases, the FLC
cannot be effective without carefully arranging the
membership functions.
In the theoretical analysis of the FLC, the selection of
membership functions does not get much attention from
the majority of researchers. Most use isosceles triangular
functions with equal spans throughout the whole universe
of discourse as membership functions for their FLCs [11],
[12], [17]. The main advantage of choosing this type of
membership function is that it eases the difficulties in
analyzing the structure of the FLC. However, almost all
the applications of FLC adopt non-equal span membership
functions to cope with the real control problems.
Instinctively, the closer the control response to the setpoint
(or normal condition), the narrow the membership
function range should be. For some highly nonlinear
processes a FLC with equal-span triangular membership
function is not adequate to achieve a good control result.
Conclusions
The design and implementation of armature-controlled and
field-controlled DC motor system using both conventional
PID and PID-like FLC have been presented. Comparisons
of experimental results of the conventional PID controller
and PID-like FLC show that the PID-like FLC is able to
perform better than the conventional PID controller.
Results indicate that even without knowing the detail of
the control plants, we were able to construct a well
performed fuzzy logic controller based on the experience
about the position controller.