Diagnosis of oscillatory process control loops

Among many control loops in industrial process plant, quite a few loops have oscillatory behavior. Root causes of these oscillations are poor controller tuning, inappropriate control structure, external disturbance and valve stiction. Depending on the root cause and the type of controlled variable, time series of several related variables have specific patterns. This paper provides a methodology to diagnose the oscillation using statistical analysis of process measurements. It is not very difficult to diagnose the root cause if the control loop has the measurement of the valve position. But the valve position is not often measured. The proposed method does not use the valve position but use only the signals that are always available. The method is successfully demonstrated on simulation and industrial data