Identification of time-varying pH processes using sinusoidal signals

Authors:
Alex D. Kalafatis;Liuping Wang;William R. Cluett
Affiliations:
Aspen Technology Inc., 1 Yonge Street, Suite 1801, Toronto, Ontario, Canada M5E 1W7;School of Electrical and Computer Engineering, RMIT University, Melbourne VIC 3000, Australia;Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3E5
Venue:
Automatica (Journal of IFAC)
Year:
2005

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Abstract

This paper presents an approach to the identification of time-varying, nonlinear pH processes based on the Wiener model structure. The algorithm produces an on-line estimate of the titration curve, where the shape of this static nonlinearity changes as a result of changes in the weak-species concentration and/or composition of the process feed stream. The identification method is based on the recursive least-squares algorithm, a frequency sampling filter model of the linear dynamics and a polynomial representation of the inverse static nonlinearity. A sinusoidal signal for the control reagent flow rate is used to generate the input-output data along with a method for automatically adjusting the input mean level to ensure that the titration curve is identified in the pH operating region of interest. Experimental results obtained from a pH process are presented to illustrate the performance of the proposed approach. An application of these results to a pH control problem is outlined.