Robust Control of Industrial Boiler Using Quantitative Feedback Theory (QFT)
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Date
2019-01
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Addis Ababa University
Abstract
Most practical Boiler systems are characterized by high uncertainty which makes them difficult
to maintain good stability margins and performance properties for closed loop system. In case of
conventional control, if plant parameter changes we cannot assure about the system performance.
Hence it is necessary to design robust control for uncertain plant. Among the various strategies
proposed to tackle this problem, Quantitative Feedback Theory (QFT) has proved its superiority
especially in the face of significant parametric uncertainty. The feature of QFT is that it can take
care of large parametric uncertainty along with phase (degree) and magnitude (dB) information
at each working frequency. To achieve good performance of industrial boiler system which
reduce fuel consumption rate and improves efficiency, dynamic variables such as fuel flow, air
flow and pressure of boiler must be controlled. In the first step plant is identified by using
experimental data by the mean of converting into a group of linear time invariant (LTI) uncertain
plants. After representation of the uncertain dynamic system in general control configuration and
modeling the parametric uncertainties, nominal performance, robust stability and robust
performance against disturbances are analyzed by the concept of robust control.
Finally, Linear time invariant (LTI) has been carried out and two controllers are compared. As in
the PID controller case, the initial PID design balances reference tracking and disturbance
rejection. In this case as well, the controller yields some overshoot in the reference-tracking
response, and suppresses the input disturbance with a longer settling time. The output in response
to a unit step disturbance should remain within the range [−1, 1] at all times, and it should return
to 0 as quickly as possible y(t) should at least be less than 0.1 after 3 sec).
From simulation we observed settling time 20.9 seconds and overshoots 30.7%. But both
Controller and pre-filter controllers guarantee robust performance of the system against the
uncertainties and result in desired time responses of the output variables. By applying QFT
robust control, system tracks the desire reference inputs in a less time and with smoother time
responses. It is observed from the simulation results that the overshoot is 0%, rising time is
0.0199 seconds and the settling time is 0.0369 seconds and Peak time is 0.0939 seconds with
QFT controller. It is further observed that the proposed controller has robust stable and
Performance.
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Keywords
Boiler, Pressure, Air flow, Fuel flow, QFT, Stability Margin, robust, performance