Comparation of linear and nonlinear adaptive control of pH-process

Physical inspection leads to the modelling of a pH-reactor by a linear, dynamic flow and mixing process followed by a static nonlinearity. A pH-process very difficult to control is simulated for testing different kinds of adaptive algorithms. In one algorithm, the flow and mixing model is considered known. The nonlinear adaptive controller is constructed based on a combination of a pole placement design method with a piecewise-polynomial approximation of a titration function, the coefficients of which are estimated from pH measurements. The other algorithm uses an inverse overall process model obtained by combination of the flow and mixing model with a piecewise-polynomial approximation of the titration curve. This model is applied to the development of a nonlinear controller based on the model reference adaptive technique. Both methods are applied for tracking a given pH-variable and for regulation. The effectiveness of linear and nonlinear adaptive controllers, obtained using the linear or nonlinear approximation of the titration curve, respectively, are examined and compared for different process solutions and different applications.