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Rosemount Model 8800C Vortex Flowmeter with FOUNDATION
TM
Fieldbus
C-14
Solution If the flow is controlled, steam pressure variations will be compensated
before they significantly affect the heat exchanger temperature. The
output from the master temperature loop is used as the setpoint for the
slave steam flow loop. The BKCAL_IN and BKCAL_OUT connections
on the PID blocks are used to prevent controller windup on the master
loop when the slave loop is in Manual or Automatic mode, or it has
reached an output constraint. Figure C-8 illustrates the correct
function block configuration.
Figure C-8. PID Function Block Diagram for Cascade Control Example.
Application Example:
Cascade Control with Override
You can use the PID function block with other function blocks for
complex control strategies. Figure C-9 illustrates the function block
diagram for cascade control with override.
When configured for cascade control with override, if one of the PID
function blocks connected to the selector inputs is deselected, that PID
block filters the integral value to the selected value (the value at its
BKCAL_IN). The selected PID block behaves normally and the
deselected controller never winds up. At steady state, the deselected
PID block offsets its OUT value from the selected value by the
proportional term. When the selected block becomes output-limited, it
prevents the integral term from winding further into the limited region.
When the cascade between the slave PID block and the Control Selector
block is open, the open cascade status is passed to the Control Selector
block and through to the PID blocks supplying input to it. The Control
Selector block and the upstream (master) PID blocks have an actual
mode of IMan.
Outlet
Temperature
Input
AI
Function
Block
PID
Function
Block
AI
Function
Block
Steam
Flow
Input
PID
Function
Block
AO
Module
Block
OUT
IN
OUT
IN
OUT
IN
CAS_IN
BKCAL_OUT
BKCAL_IN
BKCAL_OUT
BKCAL_IN
OUT
FT 101
FC 101
TCV 101
TT 101
TC 101
FIELDBUS-FBUS_19A