Model-Based Controller Design for Unstable, Non-Minimum-Phase, Nonlinear Processes

This paper presents a nonlinear control method that is applicable to 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 desired output setpoints. This state-tracking approach results in a nonlinear state feedback that tries to induce linear responses to the state variables. The controller system includes the nonlinear state feedback and a reduced-order, nonlinear state observer to reconstruct unmeasured state variables. The application and performance of the proposed control system are illustrated for a bioreactor and two chemical reactors with multiple steady states. The controller is used to operate these reactors at steady states that are unstable and nonminimum-phase. The simulation results show that the proposed control method can achieve closed-loop stability successfully.