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Electromagnetic detection of metallic particles
... Metal balls were suggested as appropriate targets for metal detector testing some years ago [3], [4]. There are a number of advantages in using metal balls: · the orientation of a ball does not have to be considered, · metal balls in a wide range of diameters are easily and cheaply available, · analytical theoretical solutions for the effect of a metal ball on a metal detector coils exist Experiments are described below that have been used to support the CEN Workshop, helping the development of the tests within the CEN Workshop Agreement. ...
Plots of maximum detection distance as a function of target radius, for a set of metal balls, have been used as a measure of the sensitivity of metal detectors. This simple idea allows detection capability at a given distance to be quantified. One of the advantages of the sphere is that it is easily modelled mathematically. We compare results of detector measurements with theory for some of the metals tested. In particular, both theory and experiment support the use of permeable steel balls as standard test targets.
Eddy-current or permeability-contrast based metal detectors may be
characterized by static sensitivity maps, a new graphic representation
that maps detector response to a standardized infinitesimal object in a
static field. A sphere is shown to be a suitable standard object because
its behavior describes, to within multiplicative constants, objects of
arbitrary conductivity and permeability. Static sensitivity maps take
full account of both the excitation and detection coil shapes. They are
compact, easy to understand, closely related to industrial testing
procedures, and likely to prove a useful design tool. Their usefulness
is illustrated by examples based on a metal detector for which published
data exist