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Control Technique using Fuzzy Logic
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Introduction
Fuzzy control is a practical alternative
for a variety of challenging control applications since it provides a convenient
method for constructing non-linear controllers via the use of heuristic information.
Since heuristic information may come from an operator who has acted as "a
human in the loop" controller for a process. In the fuzzy control design
methodology, a set of rules on how to control the process is written down and
then it is incorporated into a fuzzy controller that emulates the decision making
process of the human. In other
cases, the heuristic information may come from a control engineer who has performed
extensive mathematical modelling, analysis and development of control algorithms
for a particular process. The rest of the process is the same as the earlier case.
The ultimate objective of using fuzzy control is to provide a user-friendly formalism
for representing and implementing the ideas we have about how to achieve high
performance control. Apart from being a heavily used technology these days, fuzzy
logic control is simple, effective and efficient. In this paper, the structure,
working and design of a fuzzy controller is discussed in detail through an in-depth
analysis of the development and functioning of a fuzzy logic pH controller. PH
Control To illustrate the application of
fuzzy logic, the remaining section of the paper is directed towards the design
and working of a pH control system using fuzzy logic. PH
is an important variable in the field of production especially in chemical plants,
sugar industries, etc. PH of a solution is defined as the negative of the logarithm
of the hydrogen ion concentration, to the base 10. I.e., PH= -log 10 [H+]
Let us consider the stages of operation of a sugar industry, where PH control
is required. The main area of concern is the clarification of raw juice of sugarcane.
The raw juice will be having a PH of 5.1 to 5.5. The clarified juice should ideally
be neutral. I.e., the set point should be a PH of 7. The process involves addition
of lime and SO2 gas for clarifying the raw juice. The addition of these two are
called liming and sulphitation respectively. Since the process involves continuous
addition of lime and SO2 ; lime has a property of increasing the PH of the clarified
juice. This is the principle used for PH control in sugar industries. The PH of
the raw juice is measured and this value is compared to the set point and this
is further used for changing the diameter of the lime flow pipe as per the requirement.
The whole process can be summarised as follows.
The PH sensor measures the PH. This reading is amplified and recorded. The output
of the amplifier is also fed to the PH indicator and interface. The output of
this block is fed to the fuzzy controller. The output of fuzzy controller is given
to the stepper motor drive. This inturn adjusts the diameter of lime flow pipe
as per the requirement. Thus, the input to the fuzzy controller is the PH reading
of the raw juice. The output of the fuzzy
controller is the diameter of the lime flow pipe valve or a quantity that controls
the diameter of the lime flow pipe valve like a DC current, voltage, etc. The
output obtained from the fuzzy controller is used to drive a stepper motor which
inturn controls the diameter of the value opening of the lime flow pipe. This
output tends to maintain the pH value of sugar juice to a target value. A detailed
description of the design and functioning of the fuzzy controller is given in
the following section.
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