Reply To: Start a cooling loop before temperature hits the set point

AdminFred Graham

Hi Jim,

What you’re describing is a directing acting process (versus the reverse acting a did in my YouTube tutorials on ControlLogix PIDE). So then as the temperature increase your output increases to add more chilled water. The problem you’re going to face is that chilled water (like steam heating in my example) is inefficient and there will be significant deadtime in the system as a result which you will need to manage.

What you could do is use the “Prog” parameters instead of the “Oper” parameters to control the loop. Programmatically run the loop in open-loop control mode until you reach your desired (and manageable) temperature of 68 F and then programmatically flip it into closed-loop mode from there to maintain.

I’m not entirely clear on your issue but this may be a way to programmatically control the loop to get the results you want.

Use the “ProgProgReq” to tell the controller to use Program Control, then you can use the “ProgAutoReq” and “ProgManReq” to programmatically flip the controller from Hand mode to Auto mode.