DESCRIBING FUNCTION BASED CONTROLLER FOR STICTION MINIMIZATION

Philip Skariah, Lina Rose

Abstract: In industry almost all the control valves are nonlinear. There are lot of nonlinearities such as stiction, deadzone, backlash, hysteresis etc. If a pressure is applied to the control valve, there will be an oscillation in the movement of control valve stem which is unsatisfactory in process control. This is termed as static friction or stiction, present in control valve cause oscillations which will cause loose of quality and expense of raw materials. Stiction can be diagnosed and eliminated by proper valve maintainances. It can done only during plant shut down which will take place only in 3 to 6 months. This project aims at compensating stiction while the plant is running. . Early used methods for compensations are dithering and impulsive control for electromechanical systems. Applications of dithering and impulsive control in pneumatic control valve do not produce good results, like filter all the high frequency signals. The other proposed method was based on input-output linearization called knocker. This method will reduce the variability of process variable with increasing the stem position. Describing function method mainly focuses on give retuning signals to the valve to attain the steady state position.

Keywords: stiction, deadzone, backlash, hysteresis, knocker.

DOI: https://doi.org/10.15623/ijret.2014.0303060