ABB COMMANDER 1911 Operating Manual page 28

...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 variable-set point)
prevents 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
Proportional Band Settings
Set Point
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
0.5
1.0
2.0
Set Point
+ Hysteresis
Set Point
– Hysteresis
Reverse
Acting
Direct
Acting