A Simple Harmonics Based Stiction Detection Method

Oscillation or large variability of the control loops results decreased economical advantage to process industries due to inferior quality products, larger rate of rejections, reduced average throughputs and overall increased energy requirements. Stiction is one of the root cause of oscillation. Many studies suggest that 30% of the control loops are oscillatory because of control valve problems such as stiction, deadband and hysteresis. In this study, a simple method has been developed to detect stiction. It has been shown that stiction in control valves produces signals containing odd harmonics. The proposed method estimates frequencies and amplitudes of control error signal using least square method. Then, harmonic relation among the frequencies are examined. The presence of odd harmonics indicates valve stiction. The method has been evaluated successfully using simulated, experimental and industrial data sets.