To reiterate and expand on what @mylespetro wrote, a direct-acting loop increases its Controlled Variable (CV; a.k.a. output) in response to an increase in the Process Variable (PV).
So in the cooling (direct-acting loop) example, if the temperature increases, the PID loop increases the amount (rate) of cooling (CV e.g. refrigeration, or flow rate of cold water into a heat exchanger) applied to the thermal system where the temperature (PV) is measured, in order to counter the increase in temperature (PV). Again, direct-acting loop means increased PV results in increased CV.
And in the heating (reverse-acting loop) example, if the temperature increases, the PID loop decreases the amount (rate) of heating (CV e.g. fuel to a furnace, or flow rate of hot water into a heat exchanger) applied to the thermal system where the temperature (PV) is measured, in order to counter the increase in temperature (PV). Again, reverse-acting loop means increased PV results in decreased CV.
How direct- vs. reverse-acting loops are implemented varies from vendor to vendor; I expect a look at the SoftPLC manual could resolve how you need to do it.
- One way is to always define Error as (PV - SP), where SP is SetPoint, and vary the sign of the gains: positive for a direct-acting loop (ΔPV>0 leads to ΔCV>0); negative for a reverse acting loop (ΔPV>0 leads to ΔCV<0).
- Another way is is always define the gains as positive values, and to vary the definition of the Error: Error = (PV - SP) for direct-acting loop; Error = (SP - PV) for a reverse-acting loop.
- A third way is to always define the gains as positive, and to have a switch that says "direct" or "reverse;" but read the vendor's description of this option and consider the sidebar below for the meaning of direct and reverse.
Sidebar: if you ask The Google about Reverse-acting vs. Direct-acting behavior, you may find some apparent inconsistencies, but usually that is because some writers refer to the loop action, and some refer to process response: a direct-responding process (e.g. furnace where increasing fuel flow rate increases temperature) is controlled by reverse-acting loop; and vice versa. Strictly speaking, to my mind at least, a process has a response, and a loop has an action, but that does not keep anyone from using the terms more loosely, and I am fairly certain we could find articles discussing a direct-acting process controlled by a reverse-acting loop.
Another process, similar to the heating/cooling example conceptually, is level control of a tank. If the loop's CV is controlling the rate of flow
into the tank and the flow out of the tank in uncontrolled (by the loop; obviously something is controlling the outlet flow), then that is analogous to a heating loop and the loop will be reverse-acting. If the loop's CV if controlling the rate of flow
out of the tank and the flow into the tank is uncontrolled (by the loop), then that is analogous to a cooling loop and the loop will be direct-acting.
OP mentioned their PV is a vacuum gauge, and CV is something to do with MFC. I don't know what the latter is (Mass Flow Controller?), but if it is something like the speed of a vacuum pump, then that would be a direct-responding process and require a reverse-acting loop (analogous to the heating process where specific heat, and its proxy temperature, is being added by the CV, or the level control process where the CV controls the inlet flow). But if the PV vacuum gauge measured absolute pressure, so a higher vacuum corresponds to a lower PV, then it would be a reverse-acting process and require a direct-acting loop.
