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[Nagarajan et. al., Vol.6 (Iss.2): February 2019] ISSN: 2454-1907
DOI: 10.5281/zenodo.2585498
Http://www.ijetmr.com©International Journal of Engineering Technologies and Management Research [47]
PERFORMANCE IMPROVEMENTS OF SEPARATELY EXCITED DC
MOTOR USING FUZZY LOGIC CONTROL
R. Nagarajan 1, M. Gokulkannan 2, T. Dinesh 3, S. Murugesan 4, M. Naveenprasanth 5
1 Professor, Dept. of Electrical and Electronics Engineering, Gnanamani College of Technology,
Namakkal, India
2, 3, 4, 5 U.G. Students, Dept. of Electrical and Electronics Engineering, Gnanamani College of
Technology, Namakkal, India
Abstract:
This paper demonstrates the importance of a fuzzy logic controller over conventional method.
The performance of the separately excited DC motor is analyzed by using fuzzy logic controller
(FLC) in MATLAB/SIMULINK environment. The FLC speed controller is designed based on
the expert knowledge of the fuzzy rules system. The proposed DC motor speed control fuzzy rules
are designed for fuzzy logic controller. The output response of the system is obtained by using
fuzzy logic controller. The designed fuzzy controller for speed control performance is
investigated. Significantly reducing the overshoot and shortening the settling time of the speed
response of the motor. They validate different control of approaches, the simulation results show
improvement in motor efficiency and speed performance.
Keywords: Fuzzy Logic Controller; PID Controller; Fuzzy Inference System; Membership
Function.
Cite This Article: R. Nagarajan, M. Gokulkannan, T. Dinesh, S. Murugesan, and M.
Naveenprasanth. (2019). “PERFORMANCE IMPROVEMENTS OF SEPARATELY EXCITED
DC MOTOR USING FUZZY LOGIC CONTROL.” International Journal of Engineering
Technologies and Management Research, 6(2), 47-58. DOI: 10.5281/zenodo.2585498.
1. Introduction
An accurate control is critical to every process that leads to various types of controllers, which are
being widely used in process industries. The tuning methods for these controllers are very
important for process industries. The aim of this paper is to design a fuzzy logic controller for
speed control of a DC motor. Because of their high reliabilities, flexibilities and low costs [1]. The
DC motors are widely used in industrial applications, robot manipulators and home appliances
where speed and position control of motor are required [2]. All control systems suffer from
problems related to undesirable overshoot, longer settling times and vibrations and stability while
going from one state to another state. Real world systems are nonlinear, accurate modeling is
difficult, costly and even impossible in most cases conventional PID controllers generally do not
work well for non-linear systems. Therefore, more advanced control techniques need to be used
which will minimize the noise effects. To overcome these difficulties, there are three basic
approaches to intelligent control: knowledge based expert systems, fuzzy logic, and neural
networks. All three approaches are interesting and very promising areas of research and
[Nagarajan et. al., Vol.6 (Iss.2): February 2019] ISSN: 2454-1907
DOI: 10.5281/zenodo.2585498
Http://www.ijetmr.com©International Journal of Engineering Technologies and Management Research [48]
development. In this paper, we present only the fuzzy logic approach. The fuzzy logic, unlike
conventional logic system, is able to model inaccurate or imprecise models. The fuzzy logic
approach offers a simpler, quicker and more reliable solution that is clear advantages over
conventional techniques [3], [4].
The Fuzzy Logic has been successfully applied to a large number of control applications. The most
commonly used controller is the PID controller, which requires a mathematical model of the
system. A fuzzy logic controller provides an alternative to the PID controller. The control action
in fuzzy logic controllers can be expressed with simple “if-then” rules. Fuzzy controllers are more
sufficient than classical controllers because they can cover a much wider range of operating
conditions than classical controllers and can operate with noise and disturbances of a different
nature [5].
2. Modeling of Separately Excited Dc Motor
The term speed control stand for intentional speed variation carried out manually or automatically.
The DC motors are most suitable for wide range speed control and there are for many adjustable
speed drives [6], [7]. The Fig.1 shows block diagram of separately excited DC motor model
Figure 1: Block Diagram of Separately Excited DC Motor Model
The armature voltage equation is given by:
Now the torque balance equation will be given by:
Friction in rotor of motor is very small (can be neglected), so Bm = 0 Therefore, new torque
balance equation will be given by
[Nagarajan et. al., Vol.6 (Iss.2): February 2019] ISSN: 2454-1907
DOI: 10.5281/zenodo.2585498
Http://www.ijetmr.com©International Journal of Engineering Technologies and Management Research [49]
Taking the field flux as Φ and back emf Constant as K, the equation for back emf of motor will
be:
Also
Taking Laplace transform of the motor’s armature voltage equation we get
Now, taking equation (4) into consideration, we have:
Armature Time Constant
After simplifying the above motor model, the overall transfer function will be
For the DC motor with parameters the overall transfer function of the system is given as:
3. Fuzzy Logic Controller
The Fuzzy logic has rapidly become one of the most successful of today's technologies for
developing sophisticated control systems. The fuzzy logic control technology has been widely and
successfully utilized in industrial applications. Fuzzy Logic is a multi-valued logic, that allows
intermediate values to be defined between conventional evaluations like true/false, yes/no,
[Nagarajan et. al., Vol.6 (Iss.2): February 2019] ISSN: 2454-1907
DOI: 10.5281/zenodo.2585498
Http://www.ijetmr.com©International Journal of Engineering Technologies and Management Research [50]
high/low and emerged as a tool to deal with uncertain, imprecise, or qualitative decision making
problems. The fuzzy logic is a way to make machines more intelligent to reason in a fuzzy manner
like humans [8], [9]. Fig. 2 shows the fuzzy logic controller model.
Figure 2: Fuzzy Logic Model
A fuzzy logic model is a logical-mathematical procedure based on an “IF-THEN” rule system that
mimics the human way if thinking in computational form. Generally, a fuzzy rule system has four
modules such as,
• Fuzzification
• Fuzzy Inference
• Rule Base
• Defuzzification
Fuzzification
The Fuzzy number is called Fuzzification. In others words, means the assigning of linguistic value,
defined by relative small number of membership functions to variable.
Fuzzy Inference
Under inference, the truth value for the premise of each rule is computed, and applied to the
conclusion part of each rule. This results in one fuzzy subset to be assigned to each output variable
for each rule. Mostly MIN or PRODUCT is used as inference rules. In MIN inference, the output
membership function is clipped off at a height corresponding to the rule premise's computed degree
of truth (fuzzy logic AND). In PRODUCT inference, the output membership function is scaled by
the rule premise's computed degree of truth.
Rule Base
For the rule bases a classic interpretation of Mandani was used. Under rule base, rules are
constructed for outputs. The rules are in “If Then” format and formally the If side is called the
conditions and the Then side is called the conclusion. A rule base controller is easy to understand
and easy to maintain for a non-specialist end user and an equivalent controller could be
implemented using conventional techniques
[Nagarajan et. al., Vol.6 (Iss.2): February 2019] ISSN: 2454-1907
DOI: 10.5281/zenodo.2585498
Http://www.ijetmr.com©International Journal of Engineering Technologies and Management Research [51]
Defuzzification
The Defuzzification is a process in which crisp output is obtained by the fuzzy output. In other
words, process of converting fuzzy output to crisp number. There is more Defuzzification methods
in which two of the more common techniques are the CENTROID and MAXIMUM methods. In
the CENTROID method, the crisp value of the output variable is computed by finding the variable
value of the centre of gravity of the membership function for the fuzzy value. In the maximum
method, one of the variable values at which the fuzzy subset has its maximum truth value is chosen
as crisp value for the output variable
Designing Procedure
This paper presents a methodology for rule base fuzzy logic controller applied to a system. Before
running the simulation in MATLAB/Simulink, the fuzzy logic controller is to be designed. This is
done using the FIS editor [10]-[13]. The FIS file is created using the fuzzy logic toolbox. The
design of a fuzzy logic controller requires the choice of membership functions. After the
appropriate membership functions are chosen, a rule base is created. The set of linguistic rules is
the essential part of a fuzzy controller. The various linguistic variables to design rule base for
output of the fuzzy logic controller are shown in Fig.3. The response of the fuzzy logic controller
is obtained using in MATLAB/Simulink. A two input which is speed error (e) and change in error
(ec) and one-output change in control, fuzzy controller is created and the membership functions
and fuzzy rules are determined. The membership functions (MF) for inputs are shown in Fig. 3(a),
3(b) and the MF for output is shown in Fig. 3(c).
Membership Functions for Inputs and Output Variables
Figure 3(a): Fuzzy Input Variables “Error” Figure 3(b): Fuzzy Input Variables “Change Error”
Figure 3(c): Fuzzy Output Variable “Control”
[Nagarajan et. al., Vol.6 (Iss.2): February 2019] ISSN: 2454-1907
DOI: 10.5281/zenodo.2585498
Http://www.ijetmr.com©International Journal of Engineering Technologies and Management Research [52]
Here max-min type decomposition is used and the final output for system is calculated by using
center of area gravity method. i is an index of every membership function of fuzzy set, m is the
number of rules and is the inference result
Construction of Rules and Rule Viewer
In Figure 4(a) fuzzy if-then rules are shown and in Fig. 4(b) analysis of the two inputs (error and
change in error) and output are shown. There are total 9 rules output variable In Fuzzy
membership function there are two input variable and each input variable have seven linguist
values.
Figure 4(a) Fuzzy If-Then Rule Figure 4(b): Analysis of Both the Inputs and Outputs
4. MATLAB Implementation
The results of the system with using different type of controllers are shown here. The responses of
the system with several controllers such as PID, Fuzzy Logic Controller are being applied. In this
section transfer of the separately excited DC motor is used as a system and find out the response
of the system applying the step function as an input [14]-[18] .
PID Controller
Fig. 5 shows the PID control system designed in MATLAB/Simulink where controller parameters
are adjusted using (Z-N) method. The simulation output of the PID Controller for 2nd order system
is represented in Fig 6.
Figure 5: MATLAB/Simulink Model of PID Controller
[Nagarajan et. al., Vol.6 (Iss.2): February 2019] ISSN: 2454-1907
DOI: 10.5281/zenodo.2585498
Http://www.ijetmr.com©International Journal of Engineering Technologies and Management Research [53]
Figure 6: Step Response of the System with PID Controller
As can be seen from the Fig.6, the PID controlled response of the system has considerably high
overshoot and larger settling time values. Hence, an attempt is made to further improve the
response of the system using fuzzy logic controller [19]-[21].
Fuzzy Logic Controller
A fuzzy logic model is a logical-mathematical procedure based on an “IF-THEN” rule system that
mimics the human way if thinking in computational form. Generally, a fuzzy rule system has four
modules. There are specific components characteristic of a fuzzy controller to support a design
procedure between the pre-processing block and post processing block [22]-[24]. Fig. 7 shows the
MATLAB/Simulink model of system using fuzzy logic controller.
Figure 7: MATLAB/Simulink Model of Fuzzy Logic Controller
Figure 8: Step response Using FLC Figure 9: Step responses Using PID and FLC
[Nagarajan et. al., Vol.6 (Iss.2): February 2019] ISSN: 2454-1907
DOI: 10.5281/zenodo.2585498
Http://www.ijetmr.com©International Journal of Engineering Technologies and Management Research [54]
The simulation output of the Fuzzy Logic Controller for System is represented in Fig. 8. From
above figure, it can be easily seen that the overshoot has been considerably reduced with fuzzy
logic controller as compared to the PID using classic ZN method. Comparative step response for
PID regulated system and FLC controlled system is shown in Fig. 9. The Fig.9 shows that the
response of the system has greatly improved on application of fuzzy logic controller. The
overshoot of the system using FLC has been reduced, settling time; peak time of the system also
shows appreciable reduction as analyzed in Table I.
Table 1: Comparison between PID and FLC Controllers
SL. No.
Parameters
PID
FLC
1
Rise time (sec)
0.0286
0.1240
2
Peak time (sec)
1.0763
0.2662
3
Settling time(sec)
1.3950
0.3405
4
% Overshoot
39.519
4.1997
The triangular membership functions for input variable speed error and control output i.e. change
in PWM input are shown in Fig. 10. The general considerations in the design of the controller are:
1. If both error is zero, then maintain the present control setting i.e. output=0. 2. If the error is not
zero but is approaching this value at a satisfactory rate, then maintain the present control setting.
3. If the error is growing then change the control signal output depending on the magnitude and
sign of error to force the error towards zero [25]-[28].
Figure 10: Fuzzy Rules
These results confirmed that the Fuzzy PID controller demonstrates robustness under various
operating conditions and shows a very satisfactory performance. It can be easily seen that the
overshoot has been considerably reduced with fuzzy logic controller as compared to the PID using
classic ZN method. Comparative step response for PID regulated system and FLC controlled
system.The fuzzy inference diagram is the composite of all the smaller diagrams presented so far
in this section. It simultaneously displays all parts of the fuzzy inference process we have examined
[29]-[35]. It’s solve problem rise from some plant are defaulted to represent in mathematical
model, also in some plant to design the approaches mathematical model you must neglect some
parameter.
[Nagarajan et. al., Vol.6 (Iss.2): February 2019] ISSN: 2454-1907
DOI: 10.5281/zenodo.2585498
Http://www.ijetmr.com©International Journal of Engineering Technologies and Management Research [55]
Figure 11: Simulation Model of Separately Excited DC Motor
In order to validate the FLC control strategies as described above, the digital simulation were
carried out on a 5 HP, 240V and 1750 rpm separately excited DC drive system whose parameters
are mentioned in the MATLAB/Simulink model of system under study with fuzzy controller is
shown in Fig.11. The speed of separately excited DC motor has been successfully controlled by
using fuzzy logic controller technique.
Figure 12: Simulated Waveform of Speeds, Torque and Armature Current
The Fig. 12 shows the speeds, torque and armature current are the simulated waveform of
separately excited DC motor. The proportional integral controller is very much useful as it improve
steady state response of the system, and it time of response is high but there are some
disadvantageous of PI controller is tuning of PI controller, it give high ripple in torque and current
, and high peak overshoot and when sudden load is applied over it, the speed is drop very much,
these disadvantageous are very much improve by using rule base fuzzy logic controller, as tuning
is not required in fuzzy logic controller it reduces the ripple in torque and current very much and
there is no peak over shoot in the system, by comparing both the controller fuzzy logic controller
is proved to be much superior then PI controller.
[Nagarajan et. al., Vol.6 (Iss.2): February 2019] ISSN: 2454-1907
DOI: 10.5281/zenodo.2585498
Http://www.ijetmr.com©International Journal of Engineering Technologies and Management Research [56]
5. Conclusion
In this paper the speed of a separately excited DC motor is controlled by using fuzzy logic and
PID controller. The simulation results are obtained using MATLAB/Simulink environment. The
fuzzy logic response is compared with that of conventional PID controller. The results show that
the overshoot, settling time, peak time and control performance has been improved greatly by
using fuzzy logic controller. The proposed fuzzy logic controller has more advantages, such as
higher flexibility, control, better dynamic and static performance compared with conventional
controller. Hence, fuzzy logic controller design was proposed and implemented.
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*Corresponding author.
E-mail address: krnaga71@ yahoo.com
