Rockwell Automation 1772-L8_LW_LWP_LX_LXP,D17726.5.8 User Manual - Manual (Page 255)

EAF
Process Control Instructions
Chapter 16
16-44
Standard Deviation
The Standard Deviation instruction determines the standard deviation of a
group of six digit values. The maximum number of values the instruction
can handle is 999 or is limited by the data table area available.
Standard deviation instruction uses the formula:
y
+
NJ
ȍ
(x
i
* x
avg
) 2
n
Nj
1/2
where:
x
i
= the group of values whose standard deviation you want to
calculate
x
avg
= the arithmetic average of the group of values
n = the number of values in the group
Entry and Display of Input and Result Values
Figure 16.13 shows one method you can use to enter values and display
results of the 6-digit standard deviation. Although there are several
techniques for accomplishing this, we chose Get instructions.
Figure 16.13
6ĆDigit Standard Deviation Ladder Diagram
EXECUTE AUX
FUNCTION NUMBER:
DATA ADDR:
RESULT ADDR:
07
0040
0160
00
0030
005
G
0031
005
G
FUNCTION
150
EN
FILE TO FILE MOVE
COUNTER ADDR:
POSITION:
FILE LENGTH:
0033
001
021
FILE A:
FILE R:
RATE PER SCAN
0040-0064
0160-0204
021
DN
0033
17
0033
15
000
0171
001
G
0172
414
G
0170 0173
210
G
010
00
G
The first eight words after the result address are used for intermediate
calculation. The standard deviation ladder diagram is the same as the
ladder diagram for averaging with the exception already discussed. The
result address stores intermediate calculations in the first 8 words
(160-167) and the final result in the next 4 words (170-173, the implied
decimal point is after word 171). The done bit is 17015.