I am operating the process in the image below, where I am using a recirculating heated/cooled bath (RHCB) (Polystat 12122-56) that externally circulates hot water through a heat exchanger (HX) and back to the bath. enter image description here

The RHCB has a refrigeration coil (800 W) and a heating element (2 kW) immersed in the bath to control temperature, with the ability to edit “heat PID” settings and “cool PID” settings. I have the RHCB hooked up to LabVIEW, which I use to change the setpoint temperature every 5 minutes according to the variable setpoint temperature profile below. I tried using the RHCB to track the variable setpoint temperature profile using the factory PID settings of P=1, I=0.6, and D=0 for heat PID and cool PID but the RHCB had trouble achieving the higher setpoint temperatures (see the "Actual" temperature profile in the image below). Temperature Profile

I think the refrigeration cycle is preventing the heater from reaching the higher setpoint temperatures, and that I can stop that from happening by choosing the appropriate PID settings. So, my question is:

What is the best way to tune two separate PID settings (heat PID and cool PID in this example) when both of those are controlling the same variable and the setpoint changes frequently?

  • \$\begingroup\$ This question is far to big and wide to suit this site. Please consider simplifying to make it more accessible. \$\endgroup\$ – Andy aka Feb 10 '17 at 11:35
  • \$\begingroup\$ @Andyaka Good point, I edited down the text significantly and simplified the process diagram to the relevant portion. Thanks for the input. \$\endgroup\$ – ryanDavid Feb 10 '17 at 16:01
  • \$\begingroup\$ This is a bad design. Maybe put in a switch which flips between the two based on the sign of the fb error. \$\endgroup\$ – JMJ Feb 11 '17 at 6:23

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