22
Fig. 7.2 Effect of Control Responses on Processes
…7.1
Introduction to Standard Control
…7
CONTROL LEVEL
Response
Contributions
Effect Of Response Settings
Too High
Too Low
On/Off Hysteresis
Helps to prevent
rapid switching of
output
Process swings
too far above
and below set
point
Output switches
too rapidly
Hysteresis too high
Hysteresis too Low
Proportional Band
Stable control with
the minimum offset
a n d m i n i m u m
period of oscillation
c o n s i s t e n t w i t h
stability.
• More stable
• Longer period
• Larger oofset
S t a b i l i t y
decreases
High Prop. Band
Low Prop. Band
Derivative
Increases stability,
permitting smaller
proportional band
and larger integral
action times to be
used.
Reduces height of
first peak.
Reduces period of
oscillation.
• Stability decreases
• Process noise is
amplified
M a x i m u m
contribution not
realized
Derivative Action
Time too High
Derivative Action
Time Correct
Derivative Action
Time too Low
Integral
Eliminates offset
between Process
a n d S e t P o i n t .
Time for variable to
return to set point
increases
• Stability
decreases
• Period of
oscillation
increases
Integral Action Time
too Low
Integral Action too
High
Correct Integral
Action Time
D e c a y
1
4
