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10. Appendix A
10.2. Standard Function Blocks
R400
Experion PKS Series A Fieldbus Interface Module User's Guide
413
July 2010
Honeywell
Description
The Ratio function block set point is the ratio of its output to
its input. A ratio set point of 0.5 produces an output that is
one half of its input. The input (IN_1) is either a wild flow or
the output of a blend-pacing controller. The output can be
used as the set point for a secondary flow controller. An
input (IN) from the secondary measurement is used to
calculate the actual ratio, which is displayed as the PV. A
functional schematic of the block is shown in the previous
illustration for reference.
Function Notes
Supports Out of Service (OOS), Initialization Manual
(IMan), Local Override (LO), Manual (Man), Automatic
(Auto), Cascade (Cas), and Remote Cascade (RCas)
modes.
The input 1 (IN_1) value to be ratioed passes through
a filter with a time constant of RA_FTIME. The filtered
value is multiplied by the Set point (SP) and GAIN to
become the target output. The GAIN controls the
number of zeros in the SP display.
The input (IN) value is the actual value of the ratioed
variable and it passes through a filter with a time
constant of PV_FTIME. The filtered IN value is divided
by the filtered IN_1 value and the GAIN to become the
PV. The units of IN are
not
PV, but OUT. The units of
IN_1 are OUT units divided by PV units.
The full cascade SP sub-function is used, with rate
and absolute limits. Additional control options are
available to have the SP value track the PV value,
when the block's actual mode is IMan, LO, Man, or
ROut. Limits do not cause SP-PV tracking.
Use the Act on IR CONTROL_OPTS to select
whether to pass initialization requests or act on them
locally by changing the SP value. If this option is OFF
or to pass, a status of Not Invited (NI) or Initialization
Request (IR) at BKCAL_IN will be passed to
BKCAL_OUT. The BKCAL_OUT value will be
calculated from the value of BKCALC_IN divided by
GAIN and IN_1. When the upstream block sends
Initialization Acknowledge (IA) status, the block sends
the IA status, since its output will now be nearly
identical to the value BKCAL_IN. If this option is ON or
to act, a status of NI or IR at BKCAL_IN adjusts the SP
to balance the output to the value of BKCAL_IN. The
IA status is sent as soon as IR is detected. The
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