FIS-BA-e-0512
36
There are several ways of increasing the accuracy in piece counting.
•
When determining the reference weight, use, for example, 10 rather than one
reference piece (weigh 10 pieces and divide the obtained weight value by 10).
- This yields a number of advantages. The first is that the ratio between the
number of pieces to be counted and the number of reference pieces
reduces (figures from the above example), the ratio now being 1000/10 =
100 instead of 1000/1 = 1000 as before. Let us assume that the weight of
the 10 pieces has been determined with a balance error of 1%. This error
now only results in an uncertainty of
±
1 piece when counting
approximately 1000 pieces.
- The second advantage of using several reference pieces is that the error
caused by the differing weights of the individual pieces is covered up by
the use of several pieces as reference size. In accordance with the rules
of mathematics, it is to be expected that this error is reduced by the factor
of the square root of the number of reference pieces, in this case by a
factor of approximately 3 (the square root of 10 is about 3).
When weighing reference pieces, external error sources should be eliminated as
far as possible. Such error sources might be:
- Wind pressure acting on the load plate as a result of air movements
- Vibrations and shocks
- Electrical disturbances
The first error source in particular may involve considerable errors when
determining the reference weight of very light parts.
When using several reference pieces, (particularly when these pieces are very
light), place them on the load plate together and
not one by one
.
The reason for this lies in certain equipment functions, which are normally not
noticed by the user and which serve to optimize the accuracy of the unit
(automatic zero plotting). These functions attempt to detect whether small weight
changes are ‘real’ changes or faults caused by vibrations, great temperature
changes or wind pressure.
If the reference pieces are very light and if the pieces are placed on the load plate
slowly one after another, the functions working in the background may interpret
the associated, small weight changes as interference and eliminate them.
