A nonlinear variable structure control strategy is combined with the reaction invariant to control pH in a mixing tank with process delay. A nonlinear predictor is utilized to estimate the actual pH from its measured value. System response is significantly improved by overlaying a variable structure on the controller gains. Uncertainty in the process delay, titration curve, and influent disturbance is incorporated into the controller design. Simulation studies show that Robust controller design is obtained by incorporating a procedure based on the binary classification of the system response to produce system behavior which is insensitive to the given system parameter uncertainty. This procedure provides a practical design of a sliding-mode controller regulating a system with significant process delay where the system state is not directly available.

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