Owner's manual

A-15
Applications
Application Example:
Cascade Control with Override
You can use the PID function block with other function blocks for
complex control strategies. Figure A-18 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.
If the instrument connected to the AI block fails, you can place the AI
block in Manual mode and set the output to some nominal value for use
in the Integrator function block. In this case, IN at the slave PID block
is constant and prevents the integral term from increasing
or decreasing.
Figure A-18. Function Block Diagram
for Cascade Control with Override
PID
Function
Block
AO
Function
Block
PID
Function
Block
Control
Selector
Function
Block
PID
Function
Block
PID
Function
Block
AI
Function
Block
Configured for High Selection
Master Controller
Slave Controller
Master Controller
OUT
OUT
OUT
OUT
OUT
IN_1
CAS_IN
CAS_IN
IN
SEL_1
SEL_2
BKCAL_SEL_2
BKCAL_SEL_1
BKCAL_OUT
BKCAL_IN
FIELDBUS-FBUS_20A