Hello, everyone, I am doing my final project. Now I have a very urgent question. I am designing a PI controller on superheat by mass flow of cooling water, when I make the step response in mass flow of cooling water, the superheat will change.

But when I make the close loop controller, the mass flow of cooling water(controller output) will change into the negative, actually it is not possible.

I want to ask for help, if I want to design the PI controller, but I could not use the Zielger-Nichols method, this system is a nonlinear and 11 order system.What could I derive from the parameters of PI controller?

-- Hao Wang (ff_wh@hotmail.com), April 08, 2003