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...5 CONTROL OPERATION
...5.9 Introduction to Standard Control
30
Seconds
Example shows an Integral Action Time of 30 seconds
Information. The integral action time is the time in
which the output signal due to the integral action
increases by an amount equal to the part of the output
signal due to the proportional action when a constant
deviation is present.
Fig. 5.11 Integral Action Time
PV
Proportional + Derivative
Derivative Action Time
x%
Information. With the process variable changing at a
constant rate, the derivative action produces a change
in output proportional to this rate of change. The
derivative time constant, is the time interval in which the
part of the output signal due to proportional action
increases by an amount (y%) equal to the part of the
output signal due to derivative action (x%). The
derivative acting on the process variable instead of the
deviation
(process
unwanted derivative action when the set point is
changed.
Fig. 5.12 Derivative Action
26
Integral Step
= 100
Proportional
Step = 100
Time
Time
x%
y%
Proportional only
Time
variable-set
point)
prevents
Proportional Band Settings
Set Point
0.5
1.0
2.0
Approach Band Settings
Derivative Action Time Bandwidth =
Approach Band x Proportional Band
Information. The approach band can be used to
introduce the derivative term before the proportional
band is reached, i.e. using settings above 1.0. This has
the effect of slowing down the rate of rise. However, if
the rate of rise is very slow, the introduction of the
derivative term can be delayed, i.e. using settings
below 1.0.
Fig. 5.13 Approach Band
Set
Point
Information. Hysteresis is used with on/off control to
give acceptable control without causing the output to
switch too rapidly.
Fig. 5.14 On/Off Hysteresis
Set Point
+ Hysteresis
Set Point
– Hysteresis
Reverse
Acting
Direct
Acting
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