A Method of Controlling Unstable, Non-Minimum-Phase, Nonlinear Processes

Abstract

This paper presents a nonlinear control system that is applicable tc stable and unstable processes, whether non-minimum- or minimum-phase. The closed-loop stability is ensured by forcing all process state variables to follow their corresponding reference trajectories. These trajectories are linear and lead the process state variables to their steady state values corresponding to the desired output set-points. This approach results in a nonlinear state feedback that induces approximately linear responses to the state variables. The control system includes the nonlinear state feedback and a reduced-order nonlinear state observer. The application and performance of the control system are shown by implementing it on a chemical reactor with multiple steady states. The control system is used to operate the reactor at one of the steady states, which is unstable and non-minimum-phase. The simulation results show that the closed-loop system is asymptotically stable for all physically-meaningful initial conditions.