Switch Functions - ABB SM2000 User Manual Supplement

...2 MATH CONFIGURATION
2.9.6 Switch Functions – Figs. 2.9 and 2.10
Switch functions are used to select between the highest, lowest
and median of three analog values.
°C
80
x
60
y
40
z
20
Fig. 2.9 High, Median and Low Select
°C
80
x
60
y
40
s
20
Output for Mux(x, y, s) when s is true
Fig. 2.10 Multiplexer Operation
2.9.7 Absolute Value Function – Fig. 2.11
The Absolute Value Function converts any negative value to it's
positive equivalent – see Fig. 2.12.
x
1
0
-1
abs(x)
1
0
Fig. 2.11 Absolute Value Function
Therefore, if the Absolute Value Function is used to monitor the
difference between two flows A and B, where Flow A could be
greater than or less than Flow B, the function will always return a
positive value, which represents the unsigned magnitude of the
difference between the flow rates.
12
Output for Hs(x, y, z)
y
Output for Ms(x, y, z)
z
Output for Ls(x, y, z)
x
True
False
t
F
2.10 Application Examples
2.10.1 Liquid Flow – Fig. 2.12
Liquid Flow is measured in two ways:
a. Using a linear flow device such as Vortex, Swirl, Ultrasonic,
Turbine and Magnetic Flowmeters
b. Using a differential pressure transmitter across an orifice
plate or wedge.
Corrections can be applied to compensate for variations in
temperature and density – see Fig. 2.12.
Square Root Extraction and Scaling
Normally, square root extraction and scaling ( Q = K
achieved in the DP transmitter or using an input linearizer within
the instrument.
If this is not possible, a math block can be used as follows:
Math1
Math2 Math3 Math4 Math5 Math6
Equation
999.9
999.9*
*
999.9*Sqr(
999.9*Sqr(a1)
Temperature Compensation
Assuming linearization and scaling has been achieved on either
the DP transmitter or linearizer input, temperature compensation
can be calculated as follows:
Qc =
This is implemented in the instrument (assuming the linearized
flow is on input a1) as follows:
Qc =
This requires two math blocks:
m2 = t – t x a+1, and m3=a1/m2, created as follows:
r b
--->
Q
1+(t – t )a
r b
a1
t – t x a+1
r b
h ) is