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Fuzzy control and its application to a pH process
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Huang, Kuo-En (1997) Fuzzy control and its application to a pH process. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b1345915~S15
Abstract
In the chemical industry, the control of pH is a well-known problem that presents
difficulties due to the large variations in its process dynamics and the static nonlinearity
between pH and concentration. pH control requires the application of advanced control
techniques such as linear or nonlinear adaptive control methods. Unfortunately, adaptive
controllers rely on a mathematical model of the process being controlled, the parameters
being determined or modified in real time. Because of its characteristics, the pH control
process is extremely difficult to model accurately.
Fuzzy logic, which is derived from Zadeh's theory of fuzzy sets and algorithms,
provides an effective means of capturing the approximate, inexact nature of the physical
world. It can be used to convert a linguistic control strategy based on expert knowledge,
into an automatic control strategy to control a system in the absence of an exact
mathematical model. The work described in this thesis sets out to investigate the
suitability of fuzzy techniques for the control of pH within a continuous flow titration
process.
Initially, a simple fuzzy development system was designed and used to produce an
experimental fuzzy control program. A detailed study was then performed on the
relationship between fuzzy decision table scaling factors and the control constants of a
digital PI controller. Equation derived from this study were then confirmed
experimentally using an analogue simulation of a first order plant. As a result of this
work a novel method of tuning a fuzzy controller by adjusting its scaling factors, was
derived. This technique was then used for the remainder of the work described in this
thesis.
The findings of the simulation studies were confirmed by an extensive series of
experiments using a pH process pilot plant. The performance of the tunable fuzzy
controller was compared with that of a conventional PI controller in response to step
change in the set-point, at a number of pH levels. The results showed not only that the
fuzzy controller could be easily adjusted to provided a wide range of operating characteristics, but also that the fuzzy controller was much better at controlling
the highly non-linear pH process, than a conventional digital PI controller. The fuzzy
controller achieved a shorter settling time, produced less over-shoot, and was less
affected by contamination than the digital PI controller.
One of the most important characteristics of the tunable fuzzy controller is its ability
to implement a wide variety of control mechanisms simply by modifying one or two
control variables. Thus the controller can be made to behave in a manner similar to that
of a conventional PI controller, or with different parameter values, can imitate other
forms of controller. One such mode of operation uses sliding mode control, with the
fuzzy decision table main diagonal being used as the variable structure system (VSS)
switching line. A theoretical explanation of this behavior, and its boundary conditions,
are given within the text.
While the work described within this thesis has concentrated on the use of fuzzy
techniques in the control of continuous flow pH plants, the flexibility of the fuzzy
control strategy described here, make it of interest in other areas. It is likely to be
particularly useful in situations where high degrees of non-linearity make more
conventional control methods ineffective.
Item Type: | Thesis (PhD) | ||||
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Subjects: | Q Science > QA Mathematics Q Science > QD Chemistry |
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Library of Congress Subject Headings (LCSH): | Hydrogen-ion concentration, Fuzzy logic, Chemical process control -- Mathematical models | ||||
Official Date: | July 1997 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | School of Engineering | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Storey, Neil ; Jones, R. Peter (Richard Peter) | ||||
Sponsors: | Gaoxiong gong shang zhuan xiao [National Kaohsiung Institute of Technology] | ||||
Extent: | v, 186 p. | ||||
Language: | eng |
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