Emerson Oxymitter 5000 Instruction Manual page 154

Model 4081FG Oxygen Analyzer with F
Application Example:
Feedforward Control
FIGURE C-5. PID Function Block
Feedforward Control Example.
FIGURE C-6. Function Block Diagram
for Feedforward Control.
Outlet
Temperature
Input
Inlet
Temperature
Input
C-12
OUNDATION
Situation
In the previous example, control problems can arise because of a time
delay caused by thermal inertia between the two flow streams (TT100
and TT101). Variations in the inlet temperature (TT100) take an
excessive amount of time to be sensed in the outlet (TT101). This delay
causes the product to be out of the desired temperature range.
Solution
Feedforward control is added to improve the response time of the basic
PID control. The temperature of the inlet process fluid (TT100) is input
to an AI function block and is connected to the FF_VAL connector on
the PID block. Feedforward control is then enabled (FF_ENABLE), the
feedforward value is scaled (FF_SCALE), and a gain (FF_GAIN) is
determined. Figure C-5 illustrates the process instrumentation
diagram, and Figure C-6 illustrates the correct function block
configuration.
Steam Supply
AI
OUT
Function
Block
FF_VAL
TT101
AI
Function
OUT
Block
TT100
fieldbus Communications
TCV
101
TT
100
Steam Heater
BKCAL_IN
PID
IN
OUT
Function
Block
TC101
FF
TC
101
TT
101
Condensate
BKCAL_OUT
AO
CAS_IN
Function
Block
TCV101
OUT