Automation Direct PID Cooling Loops

Has anyone used the automation direct plc to do water cooling of an extruder barrel? Heating is no problem but setting up a heat & cool zone is giving me fits. Thanks

You can do somthing like the attached JPG below. Take the PID and spit its output in the middle, use the upper range to rescale as an output to the cooling valve, rescale the lower range to be the output to the heating valve. Throw in some deadband/hysteresis around the middle so you valves won't fight each other.



If you want to make the PID use the 16-bit mode for the output, but your input may be 12 bit you can use the independent mode to set the PV and the CV to different scales. At least on the D4-450 I used earlier this week.

Also, you may have to change the PID gains as you move from one valve to the other, since the fluid temperature differences, valve responses, coil/heat exchanger differences can cause the need for different gains when heating vs. cooling. What I do not know is how well the A-D handles dynamically changing the gains. This can cause oscillations. You may need to reset the loop when you modify the gains. Do not know if the A-D documention addresses this potential issue.

Darren

By the way, I forgot to mention the graph above the PID loop was set to direct acting (E=PV-SP). It would be reversed if you chose to use the PID as reverse acting.

Darren