How to link two Controllers to have a cascade control

Hello!

I need help to make a cascade control in WEST. The control of aeration is performed using ammonium and DO sensors. Ammonium is set between 1.5-3 mg N-NH4/l. Ammonium concentration of 3 mg/l coincides with 1.5 mg/l of DO and ammonium concentration of 1.5 mg/l coincides with 0 mg/l of DO. So the air flow is regulated accordingly. In the model instead of using the air flow I would use directly the Kla to the tanks. I try to use two OnOffBand controlled linked together, with the second one which receives information from DO sensor and the first controller, but it doesn't work. Somebody can explane step by step how I can do?

Thanks a lot for your attention and your time.

Iacopo Totaro

I believe the problem may be the OnOff controller for the DO. For a OnOff controller to be effectively controlled, both y_Min and y_Max need to be controlled (y_S is irrelevant ...). But, with the standard OnOff controller, you only have 1 outgoing signal: so from downstream (NH controller) to upstream (DO controller), you can only control 1 interface variable.
This is why, you should rather use a P(ID) controller for the DO - in which only y_S needs to be controlled.
I would therefore suggest the following:
NH ctl [OnOff]: y_Min = 1.5, y_Max = 3, u_Off = 0, u_On = 1.5
DO ctl [P(ID)]: can start off with all default
Interface link: From = u, To = y_S