Yokogawa AXFA14G/C - Manual (Page 77)

IM 01E20F02-01E A-18
APPENDIX 3. ARITHMETIC (AR) BLOCK
A3.2 Input Section
There are five 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.
A3.2.1 Main Inputs
The range extension function determines the PV value
in the following order:
1. If IN
м RANGE_HI → PV = IN
2. If IN
Ϲ RANGE_LO → PV = IN_LO
3. If RANGE_HI > IN > RANGE_LO
→ PV = g
ϫ IN + (1- g) ϫ IN_LO
g = (IN - RANGE_LO) / (RANGE_HI -
RANGE_LO)
RANGE_HI and RANGE_LO are threshold values
for switching two main inputs seamlessly.
FA0302.EPS
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 A3.2 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”
м 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. Configuration Error.” Then “Configu-
ration Error” is output to BLOCK_ERR.
If there is only one main input, the input is incorpo-
rated into the computation section as is, not taking into
account RANGE_HI and RANGE_LO.
Example:
Assuming that
RANGE_LO 20
RANGE_HI 300
TA0301.EPS
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
ϫ 230 + (1 - 0.75)
ϫ 20 = 177.5
IN = 90, IN_LO = 20
→ g = (90 - 20) / (300 - 20) = 0.25
PV = 0.25
ϫ 230 + (1 + 0.25)
ϫ 20 = 37.5
IN = 19, IN_LO = 10
→ PV = 10
A3.2.2 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) ϫ 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.