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C-13
PID Function Block
Figure C-6. Function Block Diagram for Feedforward Control.
Application Example: Cascade
Control with Master
and Slave Loops
Situation A slave loop is added to a basic PID control configuration to measure
and control steam flow to the steam heater. Variations in the steam
pressure cause the temperature in the heat exchanger to change. The
temperature variation will later be sensed by TT101. The temperature
controller will modify the valve position to compensate for the steam
pressure change. The process is slow and causes variations in the
product temperature. Figure C-7 illustrates the process
instrumentation diagram.
Figure C-7. PID Function Block
Cascade Control Example.
Outlet
Temperature
Input
Inlet
Temperature
Input
BKCAL_IN
BKCAL_OUT
OUT
OUT
OUT OUT
IN
CAS_IN
FF_VAL
AI
Function
Block
AI
Function
Block
PID
Function
Block
AO
Function
Block
TT101
TC101
TCV101
TT100
FIELDBUS-FBUS_17A
Steam
Supply
FC
101
Steam Heater
Condensate
TC
101
TT
101
FT
101
TT
100
TCV
101
FIELDBUS-FBUS_18A