EDDY CURENT METAL DETECTORS – PULSE VS. CW

Journal of ELECTRICAL ENGINEERING, VOL 57. NO 8/S, 2006, 175-177
EDDY CURENT METAL DETECTORS – PULSE VS. CW
Pavel Ripka* – Adam Lewis**
Most of the metal detectors are based on eddy-currents caused by the excitation field. Continuous wave (CW) detectors work in the
frequency range of typically 2 kHz to 30 kHz. Pulse detectors usually use bipolar field pulses 100 μs to 500 μs. Pulse detectors are
in principle more suitable for simple situations: they require only one coil, the associated electronics is quite simple, the instrument
is resistant to soils which contain small ferromagnetic particles and the instruments have low power consumption. However, these
instruments have limited capability to challenge the two most serious problems of modern metal detectors: background signal from
soil with magnetic viscosity and false alarms arising from metal scrap. CW detectors may work on multiple frequencies and allow
more sophisticated signal processing before and after A/D conversion.
Keywords: fluxgate sensor, printed circuit board, pulse excitation, gated integrators, signal extraction, magnetometer
1 INTRODUCTION
Metal detectors are used to search for dangerous or
valuable metal objects. These devices are mostly based on
eddy-currents caused by excitation field in the frequency
range of typically 2 kHz to 30 kHz. They are made both as
fixed frames (installed e.g. at the airports) and portable
devices. Portable detectors serve for humanitarian and
military demining and also for archeological investigation
and gold hunting.
Detectors for humanitarian demining are optimized to
find low-metal content anti-personnel mines at small
depths. There are two main problems of mine detectors:
high false alarm rate and signals from magnetic soils [1].
In this paper we compare the performance of pulse-mode
and continuous-wave (CW) metal detectors for humanitarian
demining in magnetic soils.
Time-domain metal detectors always have pulse excitation
and most pulse-mode detectors use time-domain
signal processing, so the names pulsed-mode and timedomain
are often used interchangeably. Typical pulse
length is 370 µs with 225 Hz repetition frequency (data
for Vallon VMH2). Bipolar pulses may be used in order to
reduce the risk of triggering magnetically-activated mines
and booby-traps.The detection coil reads the response to
the squarewave or ramp-shaped field pulses. After the
effect of the primary (excitation) field decays, the secondary
field from eddy currents in conducting objects can be
measured for 100 µs or more. In some devices (such as
Ebinger 420GC) the same coil is used both for the excitation
and detection. Schiebel AN 19 uses two concentric
coils – one for field excitation, the other for detection. In
most cases the detection circuits are switched-off during
the excitation pulse duration.
Fig. 1 shows the voltage induced by secondary field of
ferromagnetic object. The measured response is close to
the 1/t 2 , which is an ideal time dependence caused by
eddy currents. It can be shown that, for a ferromagnetic
⎯⎯⎯⎯⎯
* Czech Technical University, Faculty of Electrical Engineering, Technická 2, 166 27 Prague 6, Czech Republic, E-mail: ripka@feld.cvut.cz
** European Commission Joint Research Centre, Via Enrico Fermi 1, Ispra (VA) 21020 Italy, adam.lewis@jrc.it
ISSN 1335-3632 © 2006 FEI STU
material such as iron, only the surface layer contributes
significantly to the response.
0.01
0.001
0.0001
0.00001
voltage (V)
Iron ball 3.2 mm
1/x
1/x 2
10 100
time (µs)
Fig. 1. Time response of iron object (from [2])
Simple detectors of pulse type are preferable in soils
which contain salt water or large multidomain magnetic
particles. In such cases the false signal caused by the soil
is usually negligible after 20 microseconds. Further
ground compensation can be simply made by measuring
of the decay time and adjustment of the pulse length.
More sophisticated devices perform such soil compensation
automatically in self-learning mode when subjected
to the local soil type.
Frequency domain detectors mostly use continuous excitation:
the excitation field created by ac current into the
excitation coil is sinewave. Some advanced devices use
sinewave with variable frequency (“chirp-mode”) or a
mixture of sinewaves. For example Foerster Minex 2FD
uses a large field of 2400 Hz with a superimposed small
field of 19 200 Hz. Signal processing unit evaluates both
amplitude and phase of the sensed field. More complex
excitation and signal processing allows CW detectors to

176 P. Ripka et al: EDDY CURENT METAL DETECTORS – PULSE VS. CW
compensate for the soil effect also. It should also be mentioned
that signal processing in the frequency domain can
be used also for pulse detectors.
Some devices use differential receiving coil (e.g. double-D
coil) which allows measurement in the vicinity of
metal objects, e.g. rails. A similar effect is achieved by
using Dynamic mode, in which any stable signal is suppressed
by filters. These devices can detect a metal object
only when the detector is moving. This mode is used e.g.
by Vallon.
2 DIFFICULT SOILS
Some soils contain superparamagnetic nanoparticles,
which exhibit magnetic viscosity, i.e. frequency dependence
of susceptibility or non-instant response to the field
step [3]. The ideal time response according to Neel theory
and supposing uniform volume distribution of these particles
is 1/t. During our soil time-domain measurements we
found responses from 1/t 1.1 to 1/t 1.3 . An example of a
measured response of this type of soil is in Fig. 2.
0,01
0,001
0,0001
0,00001
1 10 time (µs) 100
V o lta g e (V )
DEP3A
1/t 1.1
Fig. 2. Time response of soil containing magnetic nanoparticles
We have also measured frequency response of soil
samples using insertion method. An example of the measured
frequency response is in Fig. 3. The frequency
characteristics is linear in loglin scale, which corresponds
to even volume distribution of superparamagnetic
particles. The target of our study is to show that in some
cases measurements on soil samples can replace field testing
of metal detectors when fast decision should be made
which type of available detector would fit the best the
local environmental conditions. Multifrequency metal
detectors allow to compensate for the influence of all soils
and they can be also used for automatic identification of
objects: initial results were demonstrated in [4].
For deep objects, large-loop detectors such as Geonix
EM 61 are used. Some of these devices (such as Ebinger
UPEX 748 M) also use pulse-mode excitation. These devices
can detect deeply burried large objects with detection
range up to 3 m for 1 m large coil.
1/t
3 OTHER DETECTION METHODS
There are several alternatives to eddy-current detectors:
Ground penetrating radar (GPR), DC magnetic gradiometer
and explosive detectors. Latest models of military
mine detectors such as AN/PSS-14 (CyTerra Corporation)
combine eddy current method with GPR [5]. While
eddy current sensor detects tiny metallic parts of the mine,
GPR allows to observe the shape of non-metallic part of
the mine using its contrast in susceptibility. This allows
to reduce the number of false alarms caused by metal
scrap. Due to the high susceptibility of water GPR fails in
wet soil.
k [1 0 e -5 S I]
600
500
400
300
200
100
0
TR
HP 4284A
Bartington
0,1 1 10 100 1000
f [kHz]
Fig. 3. Magnetic susceptibility of soil sample containing magnetic
nanoparticles is decreasing with frequency
Magnetic gradiometers use deformation of the Earth’s
field by ferromagnetic object. Both vectorial (fluxgate)
and scalar (proton, Overhauser, Cesium of Potassium vapour)
gradiometers are being used. These devices are able
to detect unexploded bomb up to 6 meters under the surface,
but they are obviously limited to large ferromagnetic
objects [6].
Explosive detectors include sensors based on
chemiluminiscence, ion mobility or other principles and
also trained dogs and other animals. At present, dogs are
the only explosive detection method used for humanitarian
purposes.
4 CONCLUSIONS
We have shown that it is possible to suppress the effect
of soil magnetic viscosity on time-domain metal detectors.
However, the practical application of this laboratory
method has limitations caused by noise. In the time domain
the possibilities of the signal processing before ADC
are limited. The induced voltage is fast decreasing and
soon reaches the noise level.
In the frequency domain the narrow-band filter can
suppress the noise very effectively. This is achieved by
synchronous detector followed by lowpass filter. Two
orthogonal detectors can measure amplitude and phase of
the induced voltage. Multifrequency metal detectors are
being used for discrimination experiments.

Journal of ELECTRICAL ENGINEERING, VOL 57. NO 8/S, 2006 177
REFERENCES
[1] GUELLE, D., SMITH, A., LEWIS, A., AND T. BLOODWORTH:
Metal Detector Handbook for Humanitarian Demining. European Communities
2003, ISBN 92-894-6236-1 Fulltext available at
http://serac.jrc.it/publications/pdf/metal_detector_handbook.pdf
[2] P. RIPKA, A. M. LEWIS, J. KUBIK: Mine Detection in Magnetic
Soils Book of Abstracts, EMSA 2006, AP239-MO5. Full paper subm.
for publication in Sensor Letters
[3] DABAS, M., JOLIVET, J., TABBAGH, A., 1992. Geophysical
Journal International 108, 101– 109.
[4] H. HUANG AND I.J. WON, 2003, Automated identification of
buried landmines using normalized electromagnetic induction spectroscopy,
IEEE Tran. Geosci v. 41, pp. 640-65
[5] CyTerra Corporation http://www.cyterra.com
[6] P. RIPKA: Magnetic sensors and magnetometers, Artech house,
2001
Received 21 November 2006
Pavel Ripka (Prof., Ing., CSc.), for biography see page 83 of
this issue.
Adam Lewis, biography not supplied.