28-11-2012, 02:53 PM
Design of a Hybrid PID Plus Fuzzy Controller for Speed Control of Induction Motors
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INTRODUCTION
In recent years, field-oriented induction machine (FOIM)
drives [1] have been increasingly utilized in motion control
applications due to easy implementation and low cost. Besides,
they have the advantage of decoupling the torque and flux
control, which makes high servo quality achievable. However,
the decoupling control feature can be adversely affected by load
disturbances and parameter variations in the motor so that the
variable-speed tracking performance of an IM is degraded. In
general, both conventional PI and PID controllers have the
difficulty in making the motor closely follow a reference speed
trajectory under torque disturbances. In this regard, an effective
and robust speed controller design is needed.
In [2]-[8], fuzzy-logic-based intelligent controllers have been
proposed for speed control of FOIM drives. Those intelligent
controllers are associated with adaptive gains due to fuzzy
inference and knowledge base. As a result, they can improve
torque disturbance rejections in comparison with best
trial-and-error PI or PID controllers. Nonetheless, no
performance advantages of intelligent controllers in combination
with a PI or PID controller are investigated in [2]-[8].
Motivated by the successful development and application
in [2]-[8], we propose a hybrid PID+fuzzy controller consisting
of a PID controller and a fuzzy logic controller (FLC) in a serial
arrangement for speed control of FOIM drives, more specifically,
direct field-oriented IM (DFOIM) drives. The Ziegler-Nichols
(Z-N)) method in [9] is adopted for designing a PID controller
(denoted as “the Z-N PID”) because its design rule is simple and
systematic. We next design a FLC carrying out fuzzy tuning of
the output of the Z-N PID controller to issue adequate torque
commands.
EXPERIMENTAL RESULTS
Based on the simulation model shown in Fig. 1, the
experimental setup is shown in Fig. 5. The objective of the
experiments is to investigate the effectiveness of the proposed
speed controller for the Nikki Denso NA21-3F 0.14Hp induction
motor. The control system is implemented in real time using the
MRC-6810 AD/DA servo control card as the interface between
software and hardware.
In Figs. 6-7, we examine the performance of the proposed
controller compared to other intelligent controllers in [8],[14].
We set b = 9 and select the best trial-and-error parameter
values K1 = 1.5 , K2 = 0.05 and K3 = 3 for the FLC under no
torque disturbance.
In Fig. 6, the command speed is increased from 0 r.p.m.
and reaches 900 r.p.m at 4.25 sec, and then starts decreasing
from 900r.p.m at 8.25 sec. In addition, no torque disturbance is
applied to the shaft. It shows that the proposed controller
outperforms both the NFC[8] and the FLC[14]. The speed
tracking response of the FLC yields large fluctuations when the
speed command is 900 r.p.m..
CONCLUSION
In this paper, a novel hybrid modified Z-N PID+FLC-based
speed control of a DFOIM has been presented. The proposed
controller has exhibited the combined advantages of a PID
controller and a FLC. Specifically, it can improve the stability,
the transient response and load disturbance rejection of speed
control of a DFOIM. The complete DFOIM drive incorporating
the proposed controller has been implemented in real time using
a MRC-6810 AD/DA servo control card for the Nikki Denso
NA21-3F 0.14Hp induction motor. The fuzzy logic and only
with three membership functions are used for each input and
output for low computational burden, which can achieve
satisfactory results. Simulation and experiment results have
illustrated that the proposed controller scheme has a good and
robust tracking performance. As suggested in [15] that a
modified Z-N PID can perform better than a Z-N PID, our future
effort will focus on how to further improve the performance of
the proposed controller herein by incorporating a modified Z-N
PID.