Dual-Acting Control; Manual Reset (Proportional Band Offset) - ABB AWT420 Operating Instruction

106 AW T4 2 0 | U N I V E R S A L 4 - W I R E , D U A L- I N P U T T R A N S M I T T E R | O I/A W T4 2 0 - E N R E V. B
...Appendix D PID control
...Control action

Dual-acting control

• Two control outputs (Base Output and Acid Output)
• P or P+I (Base Controller)
• Base Output increases as Process Value deviates below
Base Setpoint
• Base Output is zero if Process Value is greater than
Base Setpoint
• Base proportional band is positioned below Base setpoint
• P or P + I (Acid Controller)
• Acid Output increases as Process Value deviates above
Acid Setpoint
• Acid Output is zero if Process Value is less than Acid Setpoint
• Acid proportional band is positioned above Acid Setpoint
Output
Base controller
output
100 %
0 %
Proportional
band (Base)
Setpoint
(Base)
Figure 40 Dual-acting control
Acid controller
output
PV
Proportional
band (Acid)
Setpoint
(Acid)

Manual Reset (proportional band offset)

A Manual Reset value* is available on Reverse or Direct-acting
controllers when the integral term is disabled (i.e., Control Type
is configured for P, or P+D).
When the process variable is equal to the control setpoint, the
output value is equal to the Manual Reset value – this effectively
changes the position of the proportional band.
* By default the manual reset value is zero.
Manual Reset = 0 %
Output
100 %
0 %
Proportional
band
Manual Reset = 50 %
Output
100 %
50 %
0 %
Manual Reset = 100 %
Output
100 %
0 %
Figure 41 Manual Reset (proportional band offset)
Setpoint
Proportional
band
Setpoint
Proportional
band
Setpoint
PV
PV
PV