Yokogawa digitalYEWFLO (DY-FF) - Manual (Page 109)

<APPENDIX 6. Enhanced ARITHMETIC (AR) BLOCK>
A6-2
IM 01F06F00-01EN
A6.2 Input Section
There are fi ve inputs: IN and IN_LO main inputs
and IN_1, IN_2, and IN_3 auxiliary inputs.
IN and IN_LO are intended to connect devices with
different measurement ranges and allow the use
of switching a measurement range by selecting
the measuring device. However, because there
are slight differences between IN and IN_LO
values even when the same item is measured,
instantaneous switching causes abrupt changes in
the output.
To prevent this phenomenon, the Arithmetic block
uses a function known as range extension to
compensate the IN and IN_LO values between
RANGE_HI and RANGE_LO. This enables the
input to be switched smoothly. The result of the
range extension function is substituted into PV to be
used for calculations.
A6.2.1 Main Inputs
The range extension function determines the PV
value in the following order:
1. If IN
≥ RANGE_HI  PV = IN
2. If IN ≤ RANGE_LO  PV = IN_LO
3. If RANGE_HI > IN > RANGE_LO → PV = g × IN
+ (1- g) × IN_LO
g = (IN - RANGE_LO) / (RANGE_HI - RANGE_LO)
RANGE_HI and RANGE_LO are threshold values
for switching two main inputs seamlessly.
FA0602.ai
RANGE_LO
PV = IN_LO PV=g 3 IN+(1-g) 3 IN_LO PV =IN
RANGE_HI IN
Formula based on
(1) and (2)
(2): Range for IN
(1): Range
for IN_LO
Figure A6.2 Range Extension Function and PV
PV is a parameter with status information, and PV
status is determined by the value of “g.”
If “g” < 0.5  The status of IN_LO is used.
If “g” ≥ 0.5  The status of IN is used.
Determination of the status is made with a
hysteresis of 10% provided for 0.5.
If RANGE_LO > RANGE_HI, the statuses of PV
and OUT are “Bad. Confi guration Error.” Then
“Confi guration Error” is output to BLOCK_ERR.
If there is only one main input, the input is
incorporated into the computation section as is, not
taking into account RANGE_HI and RANGE_LO.
Example:
Assuming
that
RANGE_LO 20
RANGE_HI
300
the following are established:
IN = 310, IN_LO = 20 → PV = 310
IN = 230, IN_LO = 20 → g = (230 - 20) / (300 - 20)
= 0.75
PV = 0.75 Ч 230 + (1 -
0.75) Ч 20 = 177.5
IN = 90, IN_LO = 20 → g = (90 - 20) / (300 - 20)
= 0.25
PV = 0.25 Ч 230 + (1 +
0.25) Ч 20 = 37.5
IN = 19, IN_LO = 10 → PV = 10
A6.2.2 Auxiliary Inputs
There are bias and gain parameters for the IN_1,
IN_2, and IN_3 auxiliary inputs. The following
shows the equation using them.
t_i = (IN_i + BIAS_IN_i) × GAIN_IN_i
The bias parameter is used for calculating absolute
temperature or absolute pressure, while the gain
parameter is used for normalization of square root
extraction.