Friction compensation for a process control valve

Abstract

A nonlinear locally intelligent actuator design is developed to control a valve independently of the distributed control system. Nonlinear control is implemented through the direct synthesis of a sliding-stem valve model within a nonlinear structure. Input–output linearization with discontinuity smoothing is used to cancel friction nonlinearities as well as to reduce control action chattering. A closed-loop nonlinear Luenberger observer is used to reconstruct an unmeasurable state as well as to provide robust control action in the presence of parametric uncertainty. A model-based fault detector is developed to monitor serious faults such that a warning may be sent to an operator describing the exact nature of the fault. Fault diagnostic approaches are also provided in the form of threshold detection and fault tree analysis. Setpoint tracking results are provided to compare against linear proportional–integral control. The nonlinear controller is shown to outperform linear control set-point tracking measured by integral absolute error (IAE). In conclusion, the advantages of local nonlinear control are discussed.