Red Lion PCU - Manual (Page 67)

PID CONTROL
PROPORTIONAL BAND
Proportional band is defined as the “band” of units the process changes to
cause the percent output power to change from 0% to 100%. The band may or
may not be centered about the setpoint value depending upon the steady state
requirements of the process. The band is shifted by manual offset or integral
action (automatic reset) to maintain zero error. Proportional band is
expressed as percent of scaled display range.
The proportional band should be set to obtain the best response to a
disturbance while minimizing overshoot. Low proportional band settings (high
gain) result in quick controller response at expense of stability and increased
overshoot. Settings that are excessively low produce continuous oscillations at
setpoint. High proportional band settings (low gain) result in a sluggish
response with long periods of process “droop”. A proportional band of 0.0%
forces the controller into ON/OFF control mode with its characteristic cycling
at setpoint (See
ON/OFF Control, page 62, for more information).
INTEGRAL TIME
Integral time is defined as the time, in seconds, in which the output due to
integral action alone equals the output due to proportional action with a
constant process error. As long as a constant error exists, integral action
repeats the proportional action every integral time. Integral action shifts the
center point position of the proportional band to eliminate error in the steady
state. The units of integral time are seconds per repeat.
Integral action (also known as “automatic reset”) changes the output power
to bring the process to setpoint. Integral times that are too fast (small times)
do not allow the process to respond to the new output value. This causes over
compensation and leads to an unstable process with excessive overshoot.
Integral times that are too slow (large times) cause a slow response to steady
state errors. Integral action may be disabled by setting the time to zero. If time
is set to zero, the previous integral output power value is maintained.
If integral action is disabled, manual reset is available by modifying the
output power offset (“OPOF” initially set to zero) to eliminate steady state
errors. This parameter appears in unprotected parameter mode when integral
time is set to zero. The controller has this feature to prevent integral action
when operating outside the proportional band. This prevents “reset wind-up”.
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Figure 28, Proportional Band
Figure 29, Integral Time
Note: The Proportional band shift due to integral action may itself be
“reset” by temporarily setting the controller into the ON/OFF control
mode (proportional band=0)