458936 Elucidating and Handling Valve-Induced Nonlinearity in Industrial Feedback Control Loops
In this work, we clarify the effects that static and dynamic nonlinearities can have on a closed-loop system under classical control methods and under model predictive control (MPC), both of which are control methods widely used in process industries. Using the insight of the results obtained, we discuss methods for improving industrial control-loop performance when such negative effects occur. For example, when control loop oscillations occur in a loop containing a controller with an integral term (e.g., proportional-integral control) and a sticky valve, we propose an anti-windup-inspired method for reducing the windup-like effect caused by stiction on the integrator. To improve set-point tracking when MPC is used in the presence of valve stiction, we propose a set of modifications of the MPC ranging from the inclusion of suitable, computationally-efficient models of the valve dynamics within the MPC to the incorporation of constraints on the variation of the control action within two successive MPC sampling times. We provide theoretical results establishing conditions under which our methods are guaranteed to work and demonstrate their applicability and performance using several chemical process examples.
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