25
0
100
% Output
Set Point
1000
Reverse Acting
Output
0
Engineering Range (PV)
PB = 100.0%
PB = 200%
PB = 50%
PB = 50.0%
PB = 200.0%
PB = 100.0%
0
100
1000
Direct Acting
Output
0
% Output
Set Point
Engineering Range (PV)
0
200
400
600
800
1000
500
60%
20%
1500
–500
200%
PV
Control
Set Point
Engineering Range
Examples shown with a Proportional Band Offset of 50%
Proportional Band Settings
Proportional
Band
Set
Point
50%
50%
80%
20%
Manual Reset Settings
PV
50%
80%
20%
80%
20%
5
CONTROL OPERATION…
5.9
Introduction to Standard Control
Information.
Proportional control gives an output that
is proportional to the deviation of the process variable
from the set point. The range over which the output is
adjusted from 0 to 100% is called the proportional band
and this is expressed as a percentage of the
engineering range span.
Fig. 5.8 Proportional Control
Fig. 5.9 Manual Reset
Fig. 5.10 Control Action
Information.
•
Reverse Acting
– the output decreases as the process variable increases.
•
Direct Acting
– the output increases as the process variable increases.
Information.
With Proportional Control (P only or P
and D) the controller may stabilize the process at a
value which is offset from the set point. This offset can
be removed using the manual reset adjustment which
effectively shifts the Proportional Band.
