FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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but not reseted, so the comparator, and therefore the sorting device, stays
in the same state.
One major constraint is the signal-to-noise ratio. Sources of acoustic
noise, especially those occurring at the moment of impact and in the
frequency range of cracked ball resonances, have to minimized. Of
particular importance in this matter is the anvil. It has to be designed so
that the frequency of its lowest mode of resonance is much higher than
cracked ball resonant frequencies. The resonant frequencies of the
prototype's anvil, a 2" steel cube, were measured and found to be all higher
than 25 kHz. The anvil has to be mounted on some sort of damped suspension
(foam rubber in our case) so that when it is struck by a grinder ball, the
high frequency part of the impact force is not transmitted to the supporting
structure and therefore does not ring high frequency resonances in it. For
this reason and also to maximize the portion of the impact energy
transmitted to the grinder ball, one would tend to make it as massive as
possible. The limiting factor is that the resonant frequencies diminish
with increasing size. It is possible to increase the mass of the anvil
without decreasing its resonant frequency by optimizing its shape. In this
respect, the sphere has the highest mass . frequency product followed by the
square cylinder (length equal to diameter), followed by the cube etc... The
cylinder is probably the most convenient compromise for automated systems
but, in theory, the best anvil to test a particular size of grinder ball is
another uncracked one of the same size.
Another design constraint is that the rate at which grinder balls can
be tested is limited by the travel time of a ball from the moment of impact
to the moment it clears the mechanical sorting device; the electronics are
not the limiting factor, as the good or cracked diagnosis is done before the
ball actually leaves the anvil. This travel time was more than half a
second for the rudimentary sorting tube of the laboratory prototype but
could easily be improved for an industrial machine.
Field Test Results
Sorting tests were conducted mainly on 3" diameter balls- A sample of
approximately 100 3" diameter balls was carefully hand sorted so as to form
a "good" pile and a "cracked" pile. Each time a ball was put through the
machine, the integrator output level was recorded. The data is presented in
the form of histograms (Figure 8). A normal curve has been drawn through
the good ball distribution and the 2.5 V. threshold is indicated.
Ideally, the distributions of good balls and of cracked balls would be
two delta functions and no matter how close to one another they would be,
one would always be able to position the threshold between them and achieve
100% success rate. However, in reality, a number of factors widen the
distributions causing overlapping and, thus, diminishing the success rate.
In the case of cracked balls, a number of those factors are uncontrollable.
They are: severity of the crack, geometry of the crack, direction of impact
with regards to crack and microphone orientation with regards to crack. The