loop become so unstable

David,

I understand this might be unrelated, but this is my first time here and I am facing commissioning deadline.

I am using CONT_C  in a function block. I call the FB in a Cyclic Program (OB32).  when the PID is in manual I can change the Block CV  by entering a manual value of 0-100%.  The problem is starts when I put the loop in Automatic.  It is a simple Flow control loop. As soon as I put the loop in Automatic, the loop becomes very unstable. Proportional gain is 0.1, Ti is 0 ms, PV_FAC=1.  Derivative is also 0.  SP=1100, PV (0-1800).

My Question:  Why is this loop become so unstable with such a very small gain?  Is this because my SP and PV can only be between (-100 to 100 %)?

Thank you very much for helping me.

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Split from FB41 CONT_C : PID-BLOK.

Hello AutoGen_1560996,

please don't post twice for the same problem.

Closed cause question posted twice.

The second thread with the subject Siemens, CONT_C, PID, TIA 15 is published within the conference SIMATIC Technology.

Best regards
Jen_Moderator