Multiple root cause analysis of linear oscillatory closed-loop single-input single-output (SISO) systems

In general, oscillatory variables indicate poor performance of control loops. Therefore, diagnosis of the causes for oscillations in control loops is vital for maintaining the product quality within desired limits. In a linear closed-loop SISO system, oscillations can occur due to one or more of the following reasons: (i) poor controller tuning, (ii) control valve stiction and, (iii) external oscillatory disturbances. Several offline data-driven methods have been developed to address the diagnosis problem by focusing on only one of the causes for oscillations. In this work, an algorithm for identification of multiple root causes for oscillations in closed-loop systems is presented. The proposed approach comprises of: (i) Hammerstein based stiction detection algorithm, (ii) amplitude based discrimination algorithm using Hilbert Huang (HH) spectrum for identification of controller and disturbance caused oscillations and, (iii) an algorithm for analyzing the model obtained from Hammerstein approach. A decision algorithm based on the information obtained from the above three components is used for determination of multiple causes for oscillations in linear SISO systems.