(583a) Root Cause Analyis of Closed Loop Oscillatory Systems

A process plant may have a few hundred control loops to several thousands depending on the complexity of the plant. Oscillations are a common type of plant-wide disturbance whose detection and diagnosis has generated considerable interest in recent years. The presence of oscillations in control loop measurements significantly affect control loop performance; oscillatory disturbances can propagate plant-wide and often force plants to back off from optimal operating conditions. In a linear closed loop system oscillations can occur due to one or more of the following reasons, namely, (i) presence of nonlinearity due to sticky valve or faulty instrument, (ii) aggressively tuned controller and (iii) linear external disturbances.

In recent years, significant research has been performed on control loop oscillation diagnosis. However, there are several gray areas and issues which remain unsolved in root cause analysis of oscillatory linear systems. several methods have been deeloped to address the diagnosis problem by focusing on only one of the causes for oscillation. In the current study, a unified approach for root cause analysis is developed using a combined non-parametric (Hilbert-Huang spectrum analysis) and parametric scheme (model based analysis). Unlike the existing techniques, this approach identifies and distinguishes between the three causes of oscillation in linear closed loop systems. In addition, multiple causes of oscillation are also identified using the current approach, which is not possible using the existing approaches in the literature. The proposed approach will be demonstrated on several simulation and industrial datasets