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Rosemount Model 8800C Vortex Flowmeter with FOUNDATION
TM
Fieldbus
C-12
Solution The PID loop uses TT101 as an input and provides a signal to the
analog output TCV101. The BKCAL_OUT of the AO block and the
BKCAL_IN of the PID block communicate the status and quality of
information being passed between the blocks. The status indication
shows that communications is functioning and the I/O is working
properly. Figure C-4 illustrates the correct function block configuration.
Figure C-4. PID Function
Block Diagram for Steam Heater
Control Example.
Application Example:
Feedforward Control
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.
Figure C-5. PID Function Block
Feedforward Control Example.
Outlet
Temperature
Input
AI
Function
Block
PID
Function
Block
AO
Function
Block
TT101
TC101
TCV101
BKCAL_IN
BKCAL_OUT
OUT
IN
CAS_IN
OUT
OUT
FIELDBUS-FBUS_15A
Steam Supply
TCV
101
Steam Heater
Condensate
TC
101
TT
101
TT
100
FF
FIELDBUS-FBUS_16A