PI controlling of RF Power

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I am trying to maintain a constant output current in a 25 KW 300 khz RF generator with a PID controller (Eurotherm 818). The input signal is from a pick-up coil. This signal is rectified, filtered (60 sec time constant), and scaled to 0-5 V. The output of the controller adjusts the magnitude of 480 V 3-phase line voltage via a sturable reactor. This system is prone to severe instability. We have maximized the proprtional band, Minimized the integral time (0.1 sec) and turned the derivative action off. High and Low cutbacks are off. Any suggestions?

-- Larry Darken (ldarken@canberra.com), February 15, 2002


Dear Sir You have told that you have adjusted P , I and D component. But have you used any tuning method for this or simply by trial and error you have adjusted P , I and D components . If you have not used any tuning method, use some tuning method and get ultimate gain and using this as guideline find P , I and D. Also whether your system has any delay or nonlinearity . Then while tuning these should be considered. Your controller is digital or analog? Does it has autotuning feature ? If possible get model ( transfer function ) for the system and analyse the system for stability

yours kharadkar

-- r.kharadkar (r_kharadkar@hotmail.com), February 25, 2002.

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