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.
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).
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?