Friction compensation for a process control valve

A nonlinear locally intelligent actuator design is developed to control a v alve independently of the distributed control system. Nonlinear control is implemented through the direct synthesis of a sliding-stem valve model with in a nonlinear structure. Input-output linearization with discontinuity smo othing is used to cancel friction nonlinearities as well as to reduce contr ol action chattering. A closed-loop nonlinear Luenberger observer is used t o reconstruct an unmeasurable state as well as to provide robust control ac tion in the presence of parametric uncertainty, A model-based fault detecto r 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 app roaches 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 outper form linear control set-point tracking measured integral absolute error (IA E). In conclusion, the advantages of local nonlinear control are discussed. (C) 2000 Elsevier Science Ltd. All rights reserved.