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...5.9 Introduction to Standard Control
Response
Helps to prevent rapid
On/Off Hysteresis
switching of output
Stable control with the
m i n i m u m o ff s e t a n d
m i n i m u m
Proportional Band
oscillation consistent with
stability.
Eliminates offset between
Integral
Process and Set Point.
Increases stability,
permitting smaller
proportional band and
larger integral action times
to be used.
Derivative
Reduces height of first
peak.
Reduces period of
oscillation.
Contributions
Too High
Process swings
too far above and
below set point
• More stable
• Longer period
p e r i o d
o f
• Larger offset
Time for variable to
return to set point
increases
• Stability decreases
• Process noise is
amplified
Table 5.2 Effect of Control Responses on Processes
Effect Of Response Settings
Too Low
Output switches
too rapidly
Stability decreases
• Stability decreases
• Period of oscillation
increases
Maximum contribution
not realized
5 CONTROL OPERATION...
Hysteresis too high
Hysteresis too Low
High Prop. Band
1
Decay
4
Low Prop. Band
Integral Action too High
Correct Integral Action
Time
Integral Action Time too
Low
Derivative Action Time
too Low
Derivative Action
Time Correct
Derivative Action
Time too High
29
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