Atlas Copco PowerMACS 4000, User Manual

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SPC and Statistics
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The SPC and TDA function will store and calculate data on No. of subgroups to save subgroups. It will
not display any values until at least No. of subgroups to calculate has been collected. However, from
that point it will use all subgroups stored in memory in the calculations.
The short Term Trend function is used to give a detailed view of the latest recorded samples. It uses a
buffer where the last ( Subgroup size * No. of subgroups to save) samples are stored. Here are values
from both OK and NOK cycles collected.
When the Short Term Trend is to be displayed then average and the range or deviation values are
calculated for subgroups built from the data in the short Term trend buffer, starting with most resent value.
Which variables to collect are set up uniquely for each bolt/spindle and program.
7.2.2 Calculations for subgroups
Data in a subgroup is calculated as:
Average X
X
n
i n
i
  

; .. 1
Range R X X i n
i i
   
max( ) min( ) ; .. 1
S dard deviation S
X X
n
i n
i
tan
( )
; ..
 




2
1
1
S is approximated with
X
X
n
n
i n
i
i






2
2
1
1
( )
; ..
where n = size of subgroup
7.2.3 Calculations and checks
When specified number of subgroups to use for calculations has been collected, the SPC function will
start calculations to determine the statistical stability. This will be repeated every time a new subgroup is
ready.
Calculations are performed in following steps: (m = number of subgroups)
Calculation of Average of Average values:
X
X
m
i m
i
 

; .. 1