Fisher FIELDVUE DVC6200f Instruction Manual page 168

Detailed Setup—OS Function Block
July 2013
CAS_IN of the Splitter receives the OUT of PID1. BKCAL_IN of PID1 receives BKCAL_OUT of the Splitter. CAS_IN of the
AO receives OUT_1 of the Splitter and PID2 receives OUT_2 of the Splitter. BKCAL_IN_1 of the Splitter receives
BKCAL_OUT of the AO and BKCAL_IN_2 of the Splitter receives BKCAL_OUT of PID2.
The discussion in this section defines the behavior which is used to handle the initial value calculation and status which
can in turn be sent to PID1. This behavior is defined in such a way that no "bumps" are generated by changing modes,
and that PID1 does not wind up.
The splitter utilizes special handling for cascade initialization because it has two independent outputs. When a
downstream block indicates to the splitter that it wants to initialize, by asserting IR (initialization request) on its
BKCAL_OUT, one of two things happens. Under some circumstances, it is possible to pass an initialization request from
a downstream block back up to the block upstream of the splitter, so that all three blocks balance for bumpless
transfer to cascade mode. Otherwise, the requested splitter output goes to the requested value by placing an internal
offset between that output and the output of the curve, and then ramping that offset to zero in BAL_TIME seconds
after the cascade is made up.
The splitter normally runs with both outputs connected to blocks in cascade mode. If one or both of the blocks is not in
cascade mode, special limiting action is taken. Specifically, if one block indicates that it is not in cascade by NI (not
invited) status on its BKCAL_OUT, then the BKCAL_OUT of the splitter asserts limits at the range extremes of the block
that is still in cascade mode. Even if the upstream controller does not want to operate in that range, there will be no
reset windup when it can move into the range. If both downstream blocks show NI, then the splitter can only wait until
one of them requests cascade initialization. BKCAL_OUT of the splitter can hold the upstream block at the value of the
SP. The actual mode is IMan.
When cascade initialization is requested, by IR substatus on a BKCAL_IN, it is first necessary to determine if the other
BKCAL_IN has NI substatus. If so, the value at the BKCAL_IN asserting IR is taken as the Y value for its curve, and the
resulting X value is sent on BKCAL_OUT to PID1. If the other substatus is OK, then the internal offset and BAL_TIME is
used. If both blocks have IR substatus, then one output is processed until its cascade is closed. The choice is based on
the presence of limit status in BKCAL_IN. If BKCAL_IN_1 is limited, then if BKCAL_IN_2 is not limited then OUT_2 is
processed first, else OUT_1 is processed first.
Cascade initialization is also required when the block transitions from Auto to Cas mode. This action is identical to that
described for the PID block.
The required actions are summarized in table 4‐47.
Table 4‐47. OUT_ARRAY Coordinates
BKCAL_IN_1 BKCAL_IN_2
NI
NI
OK
IR
IR
NI
OK
Note 1: OK means the cascade is closed.
Note 2: Recommend using the average of BKCAL_IN_1 and BKCAL_IN_2, or just SP.
The BKCAL_OUT status shows limited high if an increase in SP cannot be effectively passed on to either output because
the BKCAL_IN_n of both outputs indicates that a move in the needed direction is limited. Similarly, limited low is set if
a decrease in SP cannot be effectively passed on to either output. The slope of the limited line(s) affects the limit
direction. BKCAL_OUT also shows limit status at the X extremes X11 and X22.
Initialization is not automatic under all circumstance because some situations have conflicting needs. In general, when
a control region is in trouble, the upstream controller is limited so it will not drive further into that region. When you
want to move to the other active region, you can put the upstream controller into Man mode and move it, or you can
166
BKCAL_OUT
NI NI
OK OK
NI OK
NI IR
OK OK
IR IR
IR OK
Action
Not Specified
BKCAL_OUT limited to X21 low and X22 high
BKCAL_OUT limited to X11 and X12 high
Initialize cascade to value given by curve X1 vs Y1
Initialize OUT_1 using internal offset from Y1
Initialize cascade to value given by curve X2 vs Y2
Initialize OUT_2 using internal offset from Y2
Instruction Manual
D103412X012