Date of Award

1988

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

First Advisor

Armando B. Corripio

Abstract

The correlation method of finding a system's impulse response weights, based on the Wiener-Hopf integral, was studied for its application to the tuning of feedback, feedforward, and decoupling control elements in closed loop operation using only ordinary operating data. Also included in the work is a thorough study of actual data for eight typical stochastic disturbances taken from a refinery light ends unit. A disturbance signal typical of these actual signals was used throughout the tuning studies to make results obtained more practically meaningful. These actual signals were also used to demonstrate the desirability of differencing input and output data prior to use of the correlation method. The correlation method was applied to the tuning of feedback and decoupling controllers by using it to find the closed loop impulse response of a system's output to set point changes. Given knowledge of the control structure and control elements, equations are derived which use this closed loop response to obtain the open loop impulse response. Many existing methods are available to design feedback controllers or decouplers given open loop responses. The correlation method was applied to the tuning of feedforward controllers by finding the closed loop impulse response of the system's output to disturbance changes. Equations are derived which convert this closed loop response directly into an improved feedforward controller given the control structure and control elements. An approximation of the manipulated variable transfer function is also required. The equations derived for both problems are applicable to a 2 x 2 interactive process such as a distillation column with or without feedback elements, feedforward controllers, and partial or simplified decouplers. Less complex processes and/or control schemes allow the equations to be significantly simplified. A criterion called the Impulse Confidence Ratio (ICR) is proposed which when interpreted properly will allow the determination of the value of a test result. The correlation method of impulse response determination and the tuning techniques were subjected to numerous tests of robustness in the face of various non-ideal situations which might be expected to arise in real application. The tuning techniques are applied successfully to a nonlinear distillation column model.

Pages

313

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