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Valve hysteresis compensation considering flow conditions for digital valve positioner

This paper analyzes the behavior of a pneumatic control valve/digital positioner system that has a large hysteresis due to a tightened grand packing and examines its control scheme in an effort to improve the system performance. The authors have confirmed that introducing a PID control action with a gap in the integrator of the position control and a minor feedback control of the diaphragm pressure is an effective way to ensure the stability and dynamic response of the system. In order to further improve the behavior under this cascading control against small changes of the position set point, this paper proposes a valve hysteresis compensation algorithm considering flow conditions. In this compensation, the diaphragm pressure is controlled with feedforward so that the stem immediately starts to move against the large valve hysteresis including the fluid resistance. Through experimental investigations, the effectiveness of this compensation algorithm is confirmed.