FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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A 3 by k array was constructed with twelve nominally identical
coils. It was made by potting the ferrite ends in holes which
were drilled carefully in a fiberglass plate. This assembly was
then potted and the leads connected to a mechanical switch. See
F igure 2.
Mechanical Scanner ^R j g
In order to assess the effect of coil location on the defect
response, a number of scans were carried out. A computer
controlled ultrasonic immersion X-Y scanner provided accurate
encoding of the X,Y position of the coil, as well as an isometric
display. A number of repeatable scans with variable scanning
patterns could then be made. A block diagram of the experimental
set-up is shown in Figure 3.
INFLUENCE OF NEIGHBOURS ON THE PERFORMANCE jDF CO ILS
A series of experiments was developed to assess the influence of
neighbours. The first test consisted of moving the coils with
respect to each other while recording the differences in signal
output of the eddy current instrument. Impedance of a coil
was also measured before and after it was placed into an array.
The principal reason for the change in impedance when neighbours
are present < Lnought to be the presence of ferrite cores. Due
to this higl- permeability material there is an increase in the
impedance uf the coil. Figure U shows the change in impedance
as a function of distance between two coils. It shows the
exponential drop off with distance, with the effect disappearing
at about *i mm. In our array,coils are placed within a distance of
less than two millimeters and are therefore strongly affected
by neighbours. The impedances and inductances were also measured
on twelve coils before and after they were arranged in an array
with distance between nearest neighbours equal to 1.7 mm. The
change in inductance increases with increasing Neighbouring
Factor. While the standard deviation was 3-6 JJH for individual
coils, it became 8.9 fiH when the coils were in an array. The
changes between individual coils entail that:
1. rebalancing of each coil in the array is
important for traditional eddy current
system approach;
2. the lift-off direction will be different
for each coi1 ;
3. calibration curves will be coil dependent.
These coils were wound within one turn of each other on
Identical ferrite slugs. It is unlikely that the
reproducibility of the coils can be improved with existing
techniques. Photo lithography was an alternative which was
exploited.