Introduction To Standard Control - ABB C1900RC Operating Manual

5.9 Introduction to Standard Control

Engineering Range
–500 0 200 400
200%
Proportional Band Settings
Control
Set Point
Examples shown with a Proportional Band Offset of 50%
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
Output
PB = 100.0%
100
% Output
Set Point
0
0
Engineering Range (PV)
Reverse Acting
Information.
• Reverse Acting – the output decreases as the process variable increases.
• Direct Acting – the output increases as the process variable increases.
500
600 800 1000 1500
20%
60%
PB = 50%
PB = 200%
1000
Fig. 5.10 Control Action
PV
PV
50%
Set
Point
50%
50%
Manual Reset Settings
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.
Output
100
PB = 50.0%
% Output
0
0
Engineering Range (PV)
5 CONTROL OPERATION...
80%
20%
20%
80%
80% 20%
Fig. 5.9 Manual Reset
PB = 100.0%
Set Point
1000
Direct Acting
Proportional
Band
PB = 200.0%
25