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Possibly easy/concise question:
Is droop in PID control alway against the setpoint? (as the name implies?)
Background:
I have a humidifier that is consistently controlling high. The control point is room dewpoint, the room has outside air supply (dry in winter) and the humidifier uses plant steam to generate clean steam. The valve position fluctuates with the weather (OA Dewpoint), indicating it is under control and responsive, but *always* ends up with about the same offset above setpoint.
I see no physical reason why this should happen. I'm not controlling to a proxy like duct dewpoint, I'm controlling to the point I want to control to, so it should eventually get there, right?
I thought droop was a function of proportional gain - essentially a certain amount below setpoint causes a certain amount the valve is opened. Since the OA is always trying to pull the humidity in the room down, I would expect the error to be low.
Or, when the weather changes, the humidity in the room should creep up, reversing the push/pull direction? Or only if it changes fast enough to get you on the other side of the setpoint?
Is this a simple tuning issue, needing more integral control to bring the room conditions in-line with the setpoint or am I missing something in the control theory?
As primarily a design engineer, I design systems and assume the controls can make it happen. But on this project (study), I'm working with the commissioning department because the client isn't sure if it is a control problem or a design problem. So this is a little more down in the weeds than I usually get.
Is droop in PID control alway against the setpoint? (as the name implies?)
Background:
I have a humidifier that is consistently controlling high. The control point is room dewpoint, the room has outside air supply (dry in winter) and the humidifier uses plant steam to generate clean steam. The valve position fluctuates with the weather (OA Dewpoint), indicating it is under control and responsive, but *always* ends up with about the same offset above setpoint.
I see no physical reason why this should happen. I'm not controlling to a proxy like duct dewpoint, I'm controlling to the point I want to control to, so it should eventually get there, right?
I thought droop was a function of proportional gain - essentially a certain amount below setpoint causes a certain amount the valve is opened. Since the OA is always trying to pull the humidity in the room down, I would expect the error to be low.
Or, when the weather changes, the humidity in the room should creep up, reversing the push/pull direction? Or only if it changes fast enough to get you on the other side of the setpoint?
Is this a simple tuning issue, needing more integral control to bring the room conditions in-line with the setpoint or am I missing something in the control theory?
As primarily a design engineer, I design systems and assume the controls can make it happen. But on this project (study), I'm working with the commissioning department because the client isn't sure if it is a control problem or a design problem. So this is a little more down in the weeds than I usually get.