Watlow CLS200 - Manual (Page 171)

CLS200 Series User’s Guide Chapter 8: Tuning and Control
Doc.# 0600-3050-2000 Watlow Anafaze 155
Figure 8.2 Proportional Control
Proportional and Integral Control
With proportional and integral control, the integral term
corrects for offset by repeating the proportional band’s er-
ror correction until there is no error. For example, if a pro-
cess tends to settle about 5˚ F below the setpoint,
appropriate integral control brings it to the desired setting
by gradually increasing the output until there is no devia-
tion.
Figure 8.3 Proportional and Integral Control
Proportional and integral action working together can
bring a process to setpoint and stabilize it. However, with
some processes the user may be faced with choosing be-
tween parameters that make the process very slow to reach
setpoint and parameters that make the controller respond
quickly, but introduce some transient oscillations when the
setpoint or load changes. The extent to which these oscilla-
tions of the process variable exceed the setpoint is called
overshoot.
Proportional, Integral and Derivative Control
Derivative control corrects for overshoot by anticipating
the behavior of the process variable and adjusting the out-
put appropriately. For example, if the process variable is
Offset
Proportional
Setpoint
Process Variable
Band
Proportional
Setpoint
Process Variable
Band
Over-
shoot