The loop will heat and cool on its way to the setpoint. I would like it to heat only when the setpoint is above the process variable and cool only, when the setpoint is below the process varialbe.
What do you mean by cool? Turn the heater off or is there truly a cooling system?
In the case where there is just a on/off heater and the plant has two or more time constants, the heater must NOT be on the whole time while the PV is approaching SP otherwise the PV will overshoot.
An ideal temperature system would just have one time constant (real pole). There would be no lag in the temperature measurement. In this case you don't need a PID at all. Just a simple compare SP to PV and turn off or on.
Yes, there is a truly a cooler. The system is compised of a large cast aluminium heater with cooling loops built in to it. I have tried to use two PID loops (one for the heater/one for the cooler) but the system goes into oscilation.
The TSX premium does have a dual output loop for heating and cooling but it oscilates worse then two seperate PIDs.
