ABB 2600T Series Instruction Manual page 94

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Arithmetic Function Block (AR)
Overview
This block is designed to permit simple use of popular measurement math functions. The user does not have to know how to write
equations. The math algorithm is selected by name, chosen by the user for the function to be done.
The following algorithms are available selectable from ARTH_TYPE:
1 Flow compensation, linear.
2 Flow compensation, square root.
3 Flow compensation, approximate.
4 BTU flow.
5 Traditional Multiply Divide.
6 Average.
7 Traditional Summer.
8 Fourth order polynomial.
9 Simple HTG compensated level.
Block Diagram
Description
The AR block is intended for use in calculating measurements from combinations of signals from sensors. It is not intended to be
used in a control path, so it does not support control status propagation or back calculation. It has no process alarms.
The block has 5 inputs. The first two are dedicated to a range extension function that results in a PV, with status reflecting the
input in use.
The remaining three inputs are combined with the PV in a selection of four term math functions that have been found useful in a
variety of measurements. The inputs used to form the PV should come from devices with the desired engineering units, so that the
PV enters the equation with the right units. Each of the additional inputs has a bias and gain constant. The bias can be used to
correct for absolute temperature or pressure. The gain can be used to normalize terms within a square root function. The output
also has gain and bias constants for any further adjustment required. The range extension function has a graduated transfer,
controlled by two constants referenced to IN. An internal value, g, is zero for IN less than RANGE_LO. It is one when IN is greater
than RANGE_HI. It is interpolated from zero to one over the range of RANGE_LO to RANGE_HI. The equation for PV follows:
PV = g * IN + (1-g) * IN_LO.