Science for metal detectors - how does a metal detectors work?

SCIENCE
Where does the power of the Nexus come from?

Please note that the term BEST in this article is
related to the following criteria;

Depth penetration capabilities.
Accuracy in discrimination analysis.
Any other aspect regarding practical
qualities of any metal detector will be set
aside, considering that it is not covered by
any claim made by Nexus Metal Detectors.

All information in this article is meant only for
basic educational purpose and not to define
which metal detectors trade mark or unit offered
on the market is best. That conclusion will be left
for all readers of this article to decide for them
selves after carefully considering all information.

How does a metal detector work?

The most common of detectors used in present
days are Induction Balance (IB) type. In the
basic version their search head consist of two
loops (coils). One is for transmitting signals (TX)
and the other is for receiving signals (RX).n

Another very popular metal detector type is the
Pulse Induction (PI). This type of detector utilises
in most designs search coil consisting of only
one loop used for both transmitting and receiving
signals. There is some PI designs utilising search
coils with more than one loop.

This article does not comment on other basic
topologies upon which metal detectors are build
for a reason that the IB and PI detectors are the
most efficient and powerful commonly used metal
detectors to date.

There is one most common believe that the
receiving coils in the search head of all metal
detectors are actually receiving signals reflected
by the metal targets buried under the ground.
The pure scientific fact how ever is this:

IB Detectors (Induction Balance)

In IB detectors the two loops forming the search
head are supposedly in perfect induction
balance, meaning that the residual signal (offset
voltage) in the receiving coil is as close as
possible to 0.
When a piece of metal comes across the electro-
magnetic field transmitted by the TX loop some
amount of Eddy currents will occur in the moment
of crossing the transmitted field. The Eddy
currents in the metal piece will cause an electro-
magnetic field to form around the metal piece.
That electro- magnetic field then will cause the
TX and the RX loops to get out of balance within
a certain amount. Out of balance will mean that
some amount of offset voltage signal will occur in
the RX loop. This offset voltage signal is not a
signal reflected or returned by the metal piece,
but a signal transmitted by the TX loop. The
presence of a metal piece in front of the search
head will result in the new offset voltage signal in
the RX exhibiting a bit different characteristics
than the original offset voltage signal remaining
after the factory balance procedures on the
search head. The difference of the new offset
signal to the original can be measured in both
phase and amplitude.
Exactly on this changes of the phase and the
amplitude of the residual offset voltage signal all
IB metal detectors are reacting, not on reflected
or returned signals.

Another way of explaining this process is to say
that the presence of any, external to the search
head, metal targets will alter to some degree the
electro-magnetic properties of the TX and the RX
loops, which will result in a different balance
situation between the TX and the RX loops.
The electronic circuits of all IB metal detectors
are actually measuring this very change in the
balance between the two loops in the search
head.
From this point of view it will be safe to state that
the sensitivity of any IB metal detector will
depend mostly on the electro-magnetic
properties of the search head loops. The most
important of those properties is the Q, quality
factor.
This article will not get any further in to the
academic aspect of the Q ,but wish to tell that the
Q is the main of all factors which could influence
the performance of any search head or single
loop.
The Q is of equal importance when PI metal
detectors are concerned.

PI Detectors (Pulse Induction)

However the PI works on a different principal
than the IB detectors.

This article explains the general differences
between the most common and popular type of
metal detectors used in present days.
The article is based on scientific facts, but it is
intentionally not written in academic manner, to
enable as many metal detector users as possible
to understand some of the basic principals on
which the functionality of all metal detectors are
based.

So how can any common detector user make
realistic assessment about the real capabilities of
his/her metal detector vs all others?

What are the most important factors defining
which metal detector is best?
Firstly, BEST is a very relative term when talking
about metal detectors. It is almost impossible to
define which detector is really the best, unless
some criteria is set first and then the particular
unit is judged according to that criteria.

A PI detector utilising search head
consisting of one loop, works (in a few
words) in the following way;
The loop in the search head is used to
transmit high voltage (few hundred volts)
pulses. After each transmitted pulse the
electronic circuit measures the residual
signal left in the loop after the pulse. That
residual signal will decay in certain manner.
The most important characteristic of the
residual signal is the time constant of the
decay.

If a metal target is placed in front of the loop
the time constant of the decay will change
as a result of the same Eddy currents
described in the case with IB detectors. In
this case the

Eddy currents will have pulse characteristic
as the transmitted signals. However the
basic principal stands, meaning that the
metal target placed in some proximity to the
loop will alter to some degree the
electro-magnetic properties of the loop,
which will cause different decay in the
residual signal in the loop.
Again, the most important of all
characteristics of the mentioned loop is the
Q.

The Q factor will define the capability of any
loop (coil) to transmit or receive any signals.

The Q of any loop can be maximized in only
one way; by using a capacitor with a
calculated value, which will form with the
loop a resonance circuit at the desired
operating frequency.
A loop tuned in resonance will always have
the highest Q and best performance
characteristics, compared with any other
loop, which is not tuned.
A loop can also be partially tuned, meaning
that the capacitor value in the resonance
circuit will be different than the calculated
for total resonance. In such a case the Q
will still be better than a loop not tuned at
all, but not as good as a loop tuned in total
resonance.

There is one other important aspect related
to the type of search heads consisting of at
least two loops or more, which has to be
mention before proceeding further:
This is the actual balance between the TX
and RX loops.
If two loops, one TX and one RX, with
constant Q for each one of them, are
balanced to a minimum offset voltage in the
RX at a certain frequency, they will become
a part of a constant electro-mechanical
system.
If the RX loop is tuned completely to the
frequency transmitted by the TX then the
two loops will get in full electro-magnetic
resonance. In this situation the Q of the
whole system will be at maximum, which
means that the search coil will be with best
detection capabilities.
If the same system is tuned later to work at
different frequency that will lead to loss of Q
regardless if it is still tuned to a total
resonance or not. The reason for this
phenomenon is quite simple. Ones a system
of two loops is balanced and dipped in
resin, the loops can not be moved to a
different mechanical position. In order to
achieve maximum Q at various frequency
settings every two loop system will need
some allowance for mechanical
repositioning of the loops. This requirement
comes as a result of the fact that for every
different frequency, at which the loops in
any system are tuned to work they will
exhibit different Q. From the well
established electro-technical laws it is
known that the Q of a loop (coil) is factor of
external and internal diameter of the loop,
cross section of the windings, number of
turns and operating frequency. Since the
diameter, the
cross section and the number of turns in a
loop are mechanical constant, then the only
factor which can seriously influence the Q is
the operating frequency. Another factor
which can lead to change of the Q is
temperature, but providing that metal
detectors are not often used in conditions
with excessive thermal changes, we can
accept that the temperature is a relatively
small problem for any loop.

How is all of the mentioned above related to
comparison between different types of

metal The graphic image in this article
will give explanation about Multi
Frequency Technology (MFT) type of
detectors.
Firstly, it is important to mention that
amongst all detectors based on IB
principal, the MFT has the lowest
efficiency, concerning depth
penetration capabilities and
discrimination accuracy.
The reasons:
Setting a search head to work on
various frequencies will define low Q at
all of the frequency setting. That will
result in average to poor depth
penetration and unreliable
discrimination.
TX loop is possible, but that kind of
transmission will make the work of any
IB detector impossible. Also
simultaneous frequency transmission
will most certainly result in a very low Q
for the whole system. MFT can not offer
any real practical advantage to the IB
type of metal detectors as far as depth
penetration and discrimination accuracy
is concerned.

Another popular type of IB metal
detectors are those with dual frequency
settings.
All of the mentioned above conditions
apply to this type of detectors as well as
to the MFT.

The difference in this case is that
having the search head system to work
only on two frequency setting (one at a
time) will provide an opportunity to have
maximum Q on at least one of the two
frequency setting. That will give a good
chance for better depth penetration
and discrimination accuracy.

The most common type of IB detectors
are the single frequency metal
detectors.
Metal detector set to work at only one
frequency, have the best chances to
achieve good depth penetration and
discrimination accuracy.

The next popular type of metal
detectors, capable to compete for
better depth, are the PI metal detectors.
Relative to the transmitted in the
ground energy, the PI detectors are
with lower efficiency than the IB.
However the fact that average PI metal
detector can detect targets deeper
under ground than average IB detector,
is due to the very high level of the
transmitted signals (few hundred volts
pulses). With that allowance in their
basic principal the PI metal detectors
can win almost any competition for
depth.
However, the greatest disadvantage for
the PI detectors is the lack of reliable
discrimination. Regardless of all claims
on the PI market, the PI principal can
not allow good discrimination to be
achieved.

The best metal detectors regarding
depth penetration and discrimination
accuracy are the IB detectors tuned in
total or as close as possible to total
electro-magnetic resonance.
The reasons:
It is not possible for any kind of loop to
achieve higher Q than the Q as a result
of electro-magnetic resonance.
In comparison to a standard, not tuned
in resonance search head system, one
tuned in total resonance can exhibit up
to 100 times higher sensitivity to any
desired target. That fact alone lead to
use of lower electronic amplification,
lower electronic instability and much
better resistance to thermal changes
for the electronic circuit.
A RX loop tuned in total resonance will
act also as 10 th order band pass filter
against any external interference, which
will
almost eliminate the need of any
interference prevention filters.
In comparison to off resonance RX
loop, one tuned in total resonance is
much more sensitive to phase and
amplitude changes, which fact can
guarantee best depth penetration and
discrimination accuracy as can be
achieved with any metal detector.