FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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Finally, resonances of saw-cut balls were practically undamped compared
to those of naturally cracked balls. Evidently the latter are damped by
friction occuring along the crack interface. Consequently, one can also
assume that the shear motion depicted in Fig.5 for the "shear" mode is
unlikely to occur in naturally cracked balls.
2. DEVELOPMENT OF AN ACODSTIC CRACK DETECTOR FOR SORTING GRINDER BALLS
Principle of operation
We have seen that noticeable differences exist between the sound
produced by the impact of uncracked grinder balls and the sound produced by
the impact of cracked ones. From this, it was concluded that a device that
would use these differences to acoustically detect and sort out the cracked
grinder balls from the good ones was technically feasible and a laboratory
scale prototype was developed. Figure 6 is a photograph of the device. The
mechanical assembly allows the balls to drop from a constant height on a
fixed anvil. It is composed mainly of 4" PVC tubing supported by a bolted
"Dexion" frame. The anvil is a 2" steel cube mounted in a foam filled box.
A 6" long aluminium rod mounted on a hinge inside the horizontal sorting
tube and activated by a solenoid deflects the cracked balls toward the first
opening, while the others exit by the end opening.
The impact sound is picked up by a nearby microphone and electronically
analyzed. The ball finally exits by one of two ports depending on whether
or not a crack has been detected. The electronic circuit "listens" for
those mid-range resonances typical of cracked grinder ball impacts that
immediately follow the initial pulse. It measures the "amount" of such
resonances and uses it as a criterion to decide whether the ball is good or
bad. Figure 7 is a simplified diagram of the circuit. The microphone is a
Bruel & Kjaer i" microphone and power supply flat to 30 kHz. It is located
in the target area, as near as possible to the trajectory of the grinder
balls, positioned downwards to prevent any buildup of dust or other
pollutants on its membrane. The detector consists of a band pass filter
(10-20 kHz), a rectifier and a gated integrator. The output voltage thus
represents the amount of acoustic energy between 10 and 20 kHz integrated
over the time window defined by a gate signal. It is compared to a
reference voltage to determine whether the ball is cracked or not. The
logic block carries out the measurement sequence. It uses the "initial
pulse" as a trigger event, resets the integrator and holds it on reset for
about 1 ms. This adjustable delay determines the start of the time window
and allows the blanking out of the initial pulse. It then frees the
integrator for about 10 ms and then interrupts the integration. This
defines the width of the time window, the end of which is positionned just
before the occurence of a second collision due to the recoil of the anvil
bouncing back on the grinder ball. While the time window is open, the
output of the integrator is compared to an adjustable reference and the
output of the comparator commands the operation of the sorting device. As
the time window closes, the trigger of the logic block is disabled for about
100 ms to prevent retriggering on the second collision mentionned above or
any other spurious noises. After that delay, it is ready to be retriggered
when an other ball hits the anvil. Until then, the integrator is on hold,