The grand friction of control valves that are operated under the conditions of high pressure and temperature is much larger than ordinary valves. On the other hand, digital valve-positioners have been developed widely as digital field communication protocols are introduced to the industry. When digital positioners are attached to the control valves with large grand friction, it often invites hunting behaviors due to the integrator in the control algorithm. Thus, the authors constructed the simulator for industrial application in order to analyze the behaviors of the system and examine the control scheme. In consequence of examination, we confirmed that introducing the PI-D control with a gap in integrator and the minor feedback control of the diaphragm pressure to the system is the effective way to improve the stability and dynamic response of the system.
