Appendix c cascade and remote cascade setups, Cascade information, Parameter connections for cascade initialization – National Instruments NI-FBUS User Manual
Page 128: Cascade and remote cascade setups, Cascade initalization
© National Instruments Corporation
C-1
NI-FBUS Configurator User Manual
C
Cascade and Remote Cascade
Setups
Cascade Initalization
Foundation Fieldbus provides a protocol called Cascade Initialization that
allows a control function block to smoothly transition from
Man
to
Auto
mode. Cascade Initialization allows the PID algorithm to know the current
setpoint of the AO block to balance the actual setpoint with the control
setpoint over time. Cascade Initialization is also used to prevent windup in
the PID.
Parameter Connections for Cascade Initialization
Cascade initialization takes place between two blocks: an upstream
controlling block, and a downstream controlled block. In a PID loop,
the upstream block is the PID block, and the downstream block is the
AO block. In the case of cascaded PID blocks, the upstream PID feeds a
setpoint into a second PID that is acting as the downstream block. In both
cases, the parameter connections are the same. The output (
OUT
) parameter
of the upstream block is connected to the cascade input (
CAS_IN
)
parameter of the downstream block. This connection controls the setpoint
of the downstream block. To allow the upstream block to determine the
current setpoint of the downstream block, you must also connect the
backward calculation output (
BKCAL_OUT
) parameter of the downstream
block with the backward calculation input (
BKCAL_IN
) of the upstream
block. The connections are shown in the following figure.