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Integral Time
With the proportional band alone, the process tends to reach equilibri-
um at some point away from the main setpoint. This offset is due to the
difference between the output needed to maintain setpoint and the out-
put of the proportional band at setpoint. In the case of the controller
where the proportional band is equidistant from the main setpoint, the
output is around 50%. If anything more or less than 50% output is
required to maintain setpoint, an offset error will occur. Integral action
eliminates this offset. See the diagrams below.
Integral action eliminates offset by adding to or subtracting from the
output of the proportional action alone. This increase or decrease in
output corrects for offset error within the proportional band in estab-
lishing steady-state performance at setpoint. It is not intended to cor-
rect for process disturbances. See the following diagram.
100%
Main Setpoint
Output
0%
PV