Instrumentation & Measurement Magazine 23-3 - 40

current circulation is equal in x-z and y-z planes and therefore
λ11 = λ22 = λ and λ33 ≈ 0.
As seen in Fig. 3, if the type of metal the target is made of is
unknown, then there is a problem in determining the target's
shape due to inherent ambiguity related to the mutual relationship of tensor eigenvalues obtained at a single frequency.
Consequently, tensor eigenvalues obtained at multiple frequencies are required to reconstruct the target shape and metal
type unambiguously, leading naturally to the application of
tensor spectroscopy [3]. Similar conclusions apply to the case
of pulsed detectors, where the decay curves of tensor eigenvalues are observed instead.

Example: Metal Detector Upgraded
with Object Discrimination Abilities
Finally, we provide an illustrative example of the concepts
presented so far, leading to the implementation of a discrimination-enabled MD. The experimental laboratory set-up
is comprised of a pulsed MD featuring a circular mono-coil
search head, a magnetic system for tracking the position and
orientation of the search head, and a dipole inversion algorithm implemented on a PC (Fig. 4). The search head is moved
by hand over a wooden testing platform, under which different metallic targets are mounted. Inversion results are shown
in Fig. 5.
In case of a spherical target, all three eigenvalues seem to be
approximately equal during the whole time window. At later
time instances (t > 50 μs), the estimates become too noisy due
to low signal to noise ratio.
Inverted tensor eigenvalues corresponding to a coin clearly
indicate two equally strong responses in the transverse direction (λ11 and λ22) and a weaker response in the axial direction
(λ33). Different slopes of decay curves suggest that the responses come from a magnetic target, which is true since the
coin is made of copper plated steel. Consequently, the results
lead to a conclusion that the target features a plate-like shape.
At the early-time portion of the curve (i.e., for t < 50 μs), the
copper-plated coin behaves like a nonmagnetic plate, since the
eddy currents induced in the copper overshadow the magnetization effects due to lower skin depth. At later time instances,
which correspond to lower frequencies in the excitation spectrum, magnetic features become dominant.
For the case of a PMA-2 landmine detonator simulant,
which is in fact a hollow aluminum cylinder with a relatively
small aspect ratio, the eigenvalues corresponding to target's
transverse response (λ11 and λ22) are approximately equal and
somewhat larger compared to the eigenvalue corresponding
to target's axial response (λ33). Furthermore, decay curves of all
three eigenvalues are of similar shape (for t < 50 μs), indicating
the nonmagnetic nature of the target [9].
A small ferromagnetic cylinder exhibits the behavior opposite to that of an aluminium cylinder, i.e., the response is the
strongest in the axial direction (λ33), while the transverse responses (λ11 and λ22) are significantly lower and approximately
equal. Different slopes of axial and transverse eigenvalues indicate clearly that the target is magnetic [9].
40

Conclusions
In this paper, we have shown how innovative, next-generation I&M systems can be devised by taking advantage of new
scientific and technological advancements, while retaining a
strong foundation in fundamental knowledge and industry's
accumulated know-how. This is illustrated on a practical case
in humanitarian demining, where analytical modeling, stemming from first principles electromagnetism, is applied to a
rather simple metal detection device in order to upgrade it
with advanced metal discrimination capabilities.

Acknowledgement
The authors would like to thank the Sir Bobby Charlton Foundation for supporting the research reported in this paper.

References
[1] "Landmine Monitor 2018," International Campaign to Ban
Landmines (accessed Oct. 2019). [Online]. Available: http://themonitor.org/media/2918780/Landmine-Monitor-2018_final.pdf.
[2] D. Guelle, A. Smith, A. Lewis, and T. Bloodworth, Metal Detector
Handbook for Humanitarian Demining, Luxembourg: Office
for Official Publications of the European Communities, 2003.
[Online]. Available: https://www.nolandmines.com/PDF_files/
MetalDetectorHandbook.pdf.
[3] A. J. Peyton and D. Daniels, "Detecting landmines for a safer
world," Ingenia Online, issue 75, Jun. 2018. [Online], available at:
https://www.ingenia.org.uk/Ingenia/Articles/2f67b8a4-4fee4fc2-88d7-f0c535b0dc89.
[4] M. A. Reed and W. R. Scott, "Optimization and analysis of wirewound coil heads for EMI systems," IEEE Sens. J., vol. 19, no. 5,
pp. 1672-1682, Mar. 2019.
[5] D. Ambruš, D. Vasic´, and V. Bilas, "Comparative study of planar
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IEEE Sens. J., vol 20, no. 2, pp. 968-979, Sep. 2019.
[6] T. H. Bell, B. J. Barrow, and J. T. Miller, "Subsurface discrimination
using electromagnetic induction sensors," IEEE Trans. Geosci.
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[7] C. E. Baum, Ed., Detection and Identification of Visually Obscured
Targets. New York, NY, USA: Taylor & Francis, 1999.
[8] D. Ambruš, "Detection of low-metallic content landmines based
on electromagnetic induction model," Thesis for Doctoral degree,
University of Zagreb, Faculty of Electrical Engineering and
Computing, Zagreb, Croatia, 2019. Summary [Online]. Available:
https://urn.nsk.hr/urn:nbn:hr:168:453778.
[9] F. Shubitidze, K. O'Neil, K. Sun, and K. D. Paulsen, "Investigation
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conductive, permeable, arbitrarily shaped 3-D objects," IEEE
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Davorin Ambruš (Davorin.Ambrus@fer.hr) is a Postdoctoral
Researcher at the University of Zagreb, Faculty of Electrical
Engineering and Computing. He received his Dipl.Ing., M.Sc.
and Ph.D. degrees in electrical engineering from the University of Zagreb, Croatia in 1999, 2005, and 2019, respectively. His
research interests are in the fields of computational electromagnetic sensing, sensor electronic systems and signal processing.

IEEE Instrumentation & Measurement Magazine

May 2020

http://the-monitor.org/media/2918780/Landmine-Monitor-2018_final.pdf http://the-monitor.org/media/2918780/Landmine-Monitor-2018_final.pdf https://www.nolandmines.com/PDF_files/MetalDetectorHandbook.pdf https://www.nolandmines.com/PDF_files/MetalDetectorHandbook.pdf https://www.ingenia.org.uk/Ingenia/Articles/2f67b8a4-4fee-4fc2-88d7-f0c535b0dc89 https://www.ingenia.org.uk/Ingenia/Articles/2f67b8a4-4fee-4fc2-88d7-f0c535b0dc89 https://urn.nsk.hr/urn:nbn:hr:168:453778

Instrumentation & Measurement Magazine 23-3

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