Introduction To Standard Control - ABB COMMANDER 1911 Operating Manual

5.9 Introduction to Standard Control

Engineering Range
–500 0 200 400
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
0
Set Point
0
Engineering Range
Reverse Acting
500
600 800 1000 1500
20%
60%
200%
PB = 50%
PB = 200%
1000
Information.
• Reverse Acting – the output decreases as the process variable increases.
• Direct Acting – the output increases as the process variable increases.
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
5 CONTROL OPERATION...
80%
20%
20%
80%
80% 20%
Fig. 5.9 Manual Reset
PB = 100.0%
Set Point
1000
Engineering Range
Direct Acting
Proportional
Band
PB = 200.0%
25