Control of nonlinear chemical processes using adaptive proportional-integral algorithms

It is believed that a fixed-parameter proportional-integral derivative (PID ) may not do well for nonlinear, time-variant, or coupled processes. It nee ds to be re-tuned adequately to retain robust control performance over a wi de range of operating conditions. Alternatively, nonlinear control algorith ms can be employed. To avoid complexity introduced by such nonlinear contro llers, modified PID algorithms that have the ability to adapt their tuning parameters on-line can be used instead to perform as well. An automatic on- line tuning strategy for PI controllers is proposed and compared with other existing adaptive PI algorithms such as fuzzy gain scheduling, model-based gain scheduling, a nonlinear version of PI, internal model control, and se lf-tuning adaptive control. The proposed tuning methodology adapts the PI s ettings by direct utilization of explicit expressions for the gradients of the closed-loop response with respect to the PI settings. The adapted param eters are determined such that the resulting closed-loop response lies insi de predefined time-domain constraints. Application of the proposed techniqu e as well as the other aforementioned systems to two nonlinear simulated co ntinuously stirred tank reactor examples is demonstrated. These examples pr esent challenging control. problems because of their interesting dynamics s uch as time-varying gain and gain with changing sign character. Simulation results indicated that the proposed tuning algorithm can provide comparable ,: if not superior, performance to those obtained by the other tested algor ithms.