Cascade Control With Override - Emerson Oxymitter 5000 Instruction Manual

Oxymitter 5000
Figure E-8. PID Function Block
Diagram for Cascade Control
Example
Outlet
Temperature
Input
AI
Function
Block
TT 101
Steam
Flow
Input
AI
Function
Block
FT 101
Cascade Control with
Override
E-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 E-8
illustrates the correct function block configuration.
BKCAL_IN
Function
Block
OUT IN
TC 101
BKCAL_IN
CAS_IN
Function
Block
OUT IN
FC 101
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.
BKCAL_OUT
PID
OUT
PID
OUT
Instruction Manual
IM-106-350, Rev 2.2
BKCAL_OUT
AO
IN
Module
Block
TCV 101
July 2008