ABB ControlMaster Series Manual page 20

ControlMaster CM30
Universal process controller,
Cascade
The cascade template connects two PID loops together to enhance the control of a master variable (master loop) by manipulation
of a slave loop.
In this example, the temperature of a furnace is regulated (master loop) by monitoring and controlling the gas flow rate to the
burner (slave loop). By performing slave loop control on the gas flow rate, variations in gas supply can be accounted for and
better temperature control performance achieved. This template is available with or without a remote setpoint.
Furnace
Load
Burner
Fig. 19: Cascade
Cascade with feed-forward
This template combines the benefits of the feed-forward and cascade templates.
In this example, the effect of steam demand change on the level within the boiler drum is predicted and accounted for via the
feed-forward signal. Feed water supply is then controlled accurately using a 'slave' flow loop that accounts for any variations in
water supply.
Steam
Water Steam Interface
Water with Steam Bubbles
Boiler Drum
Fig. 20: Cascade with feed-forward
12 DS/CM30–EN Rev. L | ControlMaster CM30 | Universal process controller,
/ DIN
1
4
TT
FT
Drum Level
FT
Transmitter
LT
FT
I/P
Feedwater Control Valve
1 / DIN
4
Master PV
O/P
Slave PV
Gas
Feed
Steam
Feedforward
Disturbance
Master PV
Slave PV
O/P
Feedwater