Emerson Fisher FIELDVUE DVC6000f, Instruction Manual

Page: 179 / 384

OS Function Block
December 2009 4-119
that there is no difference between the calculated
output and the back-calculation input. This provides
bumpless transfer for the first downstream block.
Limit Handling
The splitter function block is designed to combine the
limit information from the two downstream blocks into
limits for the upstream block. The general principle is
to allow the upstream block to continue control for as
long as possible.
The upstream block is high-limited (BKCAL_OUT of
the Splitter block has high-limited status) when:
Both downstream blocks are high-limited (both
BKCAL_INS of the Splitter block have high-limited
status)
or
One downstream block is high-limited, the
associated output slope is positive, and the other block
has Bad status or is not in Cas mode
or
One downstream block is low-limited, the
associated output slope is negative, and the other
block has Bad status or is not in Cas mode
or
SP is greater than or equal to X22
The upstream block is low-limited when:
Both downstream blocks are low-limited
or
One downstream block is low-limited, the
associated output slope is positive, and the other block
has Bad status or is not in Cas mode
or
One downstream block is high-limited, the
associated output slope is negative, and the other
block has Bad status or is not in Cas mode
or
SP is less than or equal to X11
Input to Output Mapping
The relationship of each output to the input may be
defined by a line. Each line may be defined by its
endpoints. Examples of graphical representations of
OUT_1 and OUT_2 vs. SP are shown in figure 4-20
for a split range and a sequencing application.
Table 4-44. IN_ARRAY Coordinates
Index Coordinate
0
X
11
−
Start value of SP for the OUT_1 line.
(X
11
< X
12
)
1
X
12
−
End value of SP for the OUT_1 line.
(X
11
< X
12
)
2
X
21
−
Start value of SP for the OUT_2 line.
(X
21
< X
22
)
3
X
22
−
End value of SP for the OUT_2 line.
(X
21
< X
22
)
Table 4-45. OUT_ARRAY Coordinates
Index Coordinate
0 Y
11
−
Value of OUT_1 at X
11
1 Y
12
−
Value of OUT_1 at X
12
2 Y
21
−
Value of OUT_2 at X
21
3 Y
22
−
Value of OUT_2 at X
22
The block has the same SP structure as the PID
block, except that there is no limiting applied to the
SP. The SP may be used in Auto mode for testing.
The operator would use the output of the PID to
accomplish the same purpose. Each downstream
block can be taken out of cascade if it becomes
necessary to gain control of them.
The examples shown in figure 4-20 do not show the
full range of possibilities. The lines could overlap like
an X, or both start from the origin but have different
slopes. The endpoints do not have to lie within
0
−
100%. Limits in the external blocks may effect the
useful range of a line. Units of percent are used in the
examples because the common application of this
block is to valves, but any units may be used to suit
the application.
The following parameters as paired coordinates are
used to specify the output splitter operation:
X
11
, Y
11
, X
12
, Y
12
X
21
, Y
21
, X
22
, Y
22
Where X
nJ
is the value of SP associated with OUT_n
and X
n1
and X
n2
refer to the 1st and 2nd coordinates
of the nth curve respectively. Y
nJ
is the value of
OUT_n and Y
n1
and Y
n2
refer to the 1st and 2nd
coordinates of the nth curve respectively.
By specifying the coordinates as shown in tables 4-44
and 4-45, the endpoints of the lines are defined. The
contents of the respective X’s are held in the
IN_ARRAY parameter and the contents of the
respective Y’s are held in the OUT_ARRAY
parameter. If a set of points are specified such that a
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