A pulsing current is applied to the coil, which then induces a magnetic field shown in blue. When the magnetic field of the coil moves across metal, such as the coin in this illustration, the field induces electric currents (called eddy currents) in the coin. The eddy currents induce their own magnetic field, shown in red, which generates an opposite current in the coil, which induces a signal indicating the presence of metal.
MORE ID-DEPTH
INFORMATION ON HOW A METAL DETECTOR WORKS
Metal detectors are fascination machines. Many of the people who use
them are just as enthusiastic about extolling the virtues of their
favorite metal detector as they are about setting off in search of
buried treasure. Those of us who design and build these instruments for
a living listen carefully when one of our customers talks about his or
her experience in the field, because this is the primary means by which
we determine how well we are doing our jobs, and what sort of things we
need to do better. Sometimes though, communication is difficult. Almost
as though we and our customers speak different languages. Which in a
sense, we do. The purpose of this page is to try to narrow that
communication gap a little. And, to resolve some of that "typical
curiosity" metal detector operators have regarding what is going on
inside their instruments.
Is it necessary to know how a metal detector works in order to use it
effectively? Absolutely not. Will knowing how it works help someone to
use it more effectively in the future? Quite possibly yes, but only with
persistence and practice. The best metal detector available is still
only as good as the person using it.
VLF (Very Low Frequency) Transmitter & Receiver
Transmitter
Inside the metal detector's loop (sometimes called a search head,
coil, antenna, etc.) is a coil of wire called the transmit coil.
Electronic current is driven through the coil to create an
electromagnetic field. The direction of the current flow is reversed
several thousand times every second; the transmit frequency
"operating frequency" refers to the number of times per second
that the current flow goes from clockwise to counterclockwise and back
to clockwise again.
When the current flows in a given direction, a magnetic field is
produced whose polarity (like the north and south poles of a magnet)
points into the ground; when the current flow is reversed, the field's
polarity points out of the ground. Any metallic (or other electrically
conductive) object which happens to be nearby will have a flow of
current induced inside of it by the influence of the changing magnetic
field, in much the same way that an electric generator produces
electricity by moving a coil of wire inside a fixed magnetic field. This
current flow inside a metal object in turn produces its own magnetic
field, with a polarity that tends to be pointed opposite to the transmit
field.
Receiver
A second coil of wire inside the loop, the receive coil, is arranged
(by a variety of methods) so that nearly all of the current that would
ordinarily flow in it due to the influence of the transmitted field is
cancelled out. Therefore, the field produced by the currents flowing in
the nearby metal object will cause currents to flow in the receive coil
which may be amplified and processed by the metal detector's electronics
without being swamped by currents resulting from the much stronger
transmitted field.
The resulting received signal will usually appear delayed when
compared to the transmitted signal. This delay is due to the tendency of
conductors to impede the flow of current (resistance) and to impede
changes in the flow of current (inductance). We call this apparent delay
"phase shift". The largest phase shift will occur for metal
objects which are primarily inductive; large, thick objects made from
excellent conductors like gold, silver, and copper. Smaller phase shifts
are typical for objects which are primarily resistive; smaller, thinner
objects, or those composed of less conductive materials.
Some materials which conduct poorly or not at all can also cause a
strong signal to be picked up by the receiver. We call these materials
"ferromagnetic". Ferromagnetic substances tend to become
magnetized when placed in a field like a paper clip which becomes
temporarily magnetized when picked up with a bar magnet. The received
signal shows little if any phase shift. Most soils and sands contain
small grains of iron-bearing minerals which causes them to appear
largely ferromagnetic to the metal detector. Cast iron (square nails)
and steel objects (bottle caps) exhibit both electrical and
ferromagnetic properties.
It should be pointed out that this discussion describes an
"Induction Balance" metal detector, sometimes referred to as
"VLF" Very Low Frequency (below 30kHz). This is the most
popular technology at the present time, and includes the "LF"
Low Frequency (30 to 300kHz) instruments made for prospecting.
Discrimination
Since the signal received from any given metal object exhibits its
own characteristic phase shift, it is possible to classify different
types of objects and distinguish between them. For example, a silver
dime causes a much larger phase shift than an aluminum pull-tab does, so
a metal detector can be set to sound off on a dime yet remain quiet on
the pull-tab, and/or show the identification of the target on a display
or meter. This process of distinguishing between metal targets is called
"discrimination". The simplest form of discrimination allows a
metal detector to respond with an audio output when passed over a target
whose phase shift exceeds a certain (usually adjustable) amount.
Unfortunately, with this type of discriminator the instrument will not
respond to some coins and most jewelry if the discrimination is adjusted
high enough to reject common aluminum trash for example pull-tabs and
screw-caps.
A more useful scheme is what is called "Notch
Discrimination". With this type of system, a notch in the
discriminate response allows the metal detector to respond to targets
within a certain range (such as the nickel/ring range) while still
rejecting targets above that range (pull-tabs, screw-caps) as well as
below it (iron, foil). The more sophisticated notch metal detectors
allow for each of several ranges to be set for either accept or reject
responses. White's Spectrum XLT for example, provides 191 individually
programmable notches.
A metal detector may provide a numeric readout, meter indication, or
other display mechanism which shows the target's likely identity. We
refer to this feature as a Visual Discrimination Indicator, or V.D.I.
Metal Detectors with this capability have the advantage of allowing the
operator to make informed decisions about which targets they choose to
dig rather than relying solely on the instruments audio discriminator to
do all the work. Most, if not all, V.D.I. metal detectors are also
equipped with audio discriminators.
Metal detectors can distinguish metal objects from each other based
on the ratio of their inductance to their resistivity. This ratio gives
rise to a predictable delay in the receive signal at a given frequency.
An electronic circuit called a phase demodulator can measure this delay.
In order to separate two signals, such as the ground component and the
target component of the receive signal, as well as to determine the
likely identity of the target, we use two such phase demodulators whose
peak response is separated from each other by one fourth of the
transmitter period, or ninety degrees. We call these two channels
"X" and "Y". A third demodulated signal, we call
"G", can be adjusted so that its response to any signal with a
fixed phase relationship to the transmitter (such as the ground) can be
reduced to zero regardless of the strength of the signal.
Some metal detectors use a microprocessor to monitor these three
channels, determine the targets's likely identity, and assigning it a
number based on the ratio of the "X" and "Y"
readings, whenever the "G" reading exceeds a predetermined
value. We can find this ratio with a resolution of better than 500 to 1
over the full range from ferrite to pure silver. Iron targets are
orientation sensitive; therefore as the loop is moved above them, the
calculated numerical value may change dramatically. A graphic display
showing this numerical value on the horizontal axis and the strength of
the signal on the vertical axis is extremely useful in distinguishing
trash from more valuable objects. We call this display the "SignaGraph"
(TM).
Ground Balance
As previously mentioned, most sands and soils contain some amount of
iron. They may also have conductive properties due to the presence of
salts dissolved in the ground water. The result is that a signal is
received by the metal detector due to the ground itself which may be
thousands of times stronger than the signal resulting from small metal
objects buried at modest depths. Fortunately, the phase shift caused by
the ground tends to remain fairly constant over a limited area. It is
possible to arrange things inside the metal detector so that even if the
strength of the ground signal changes dramatically--such as when the
loop is raised and lowered, or when it passes over a mound or hole--the
metal detector's output remains constant. Such a metal detector is said
to be "ground balanced". Accurate ground balance makes it
possible to "pinpoint" the location of the targets with a good
deal of precision as well as to estimate the depth of the targets in the
ground. If you choose to search in a non-discriminate, or
"all-metal" mode, accurate ground balance is essential.
The simplest form of ground balance consists of a control knob which
the operator adjusts while raising and lowering the loop until good
balance is achieved. Although this method can be quite effective, it can
also be tedious, and some people find it to be difficult or confusing.
More advanced metal detectors will perform ground balance automatically,
typically by a two-step sequence in which the metal detector is balanced
with the loop raised, then balanced once more with the loop lowered to
the ground. The most sophisticated ground balance metal detectors will
gradually adjust themselves as changes in the composition of the ground
occur. We refer to this as "Tracking Ground Balance". A good
tracking metal detector allows you to balance once, then hunt for the
rest of the day without having to balance again. A word to the wise -
many metal detectors which are advertised as having
"automatic" or "Tracking" ground balance are
actually factory preset to a fixed balance point. Its a little like
welding your car's accelerator halfway to the floor and calling it
"cruise control".
Motion/Non-Motion Modes
Athough the ground signal may be much stronger than the target
signal, the ground signal tends to remain the same, or change very
slowly, as the loop is moved. The signal from the target, on the other
hand, will rise quickly to a peak and then subside when the loop is
swept over it. This opens up the possibility of using techniques to
separate ground from target signals by looking at the rate of change of
the receive signal rather than looking at the receive signal itself.
Metal detector modes of operation which rely on this principle are
called, not surprisingly, "Motion" modes. The most important
example is a mode called "Motion Discrimination". If we wish
to isolate the target signal well enough to determine the target's
identity, the ground balance alone is not enough. We need to look at the
target from a couple of different perspectives, sort of like the way
distances can by measured by triangulation if you have more than one
viewpoint. We can only be ground balanced from one particular
"viewpoint"; the other will contain some combination of target
and ground signal. Fortunately, we can use the motion technique to
minimize the effect of the remaining ground signal. At the present time,
all discriminating and V.D.I. metal detectors require loop motion to be
effective. This turns out not to be much of a penalty in practice since
you have to move the loop anyway in order to cover any ground.
Once you have located a target in the motion discrimination mode, you
will probably want to more precisely locate it for easy recovery. If
your metal detector is equipped with a depth meter, you will also want
to measure the target's depth. "Pinpoint" locating and depth
measurement are done in what is called the "All Metal" mode.
Since discrimination is not required to perform these functions, loop
motion is not usually required -- except for that motion required to get
the loop over the center of the target. More precisely, the speed at
which you move the loop is not important. The All Metal mode (also
sometimes called the "Normal" mode, or "D.C." mode)
is therefore called a "Non Motion" mode.
There are a few potential points of confusion here. Some metal
detectors are equipped with a feature called "Self Adjusting
Threshold", or S.A.T., which gradually increases or decreases the
audio output in an attempt to maintain a quiet but audible
"threshold" sound. This helps to smooth out audio changes
caused by the ground or inadequate ground balance. S.A.T. may be very
rapid or very slow depending on the metal detector and how it's
adjusted, but strictly speaking, S.A.T. implies a motion mode of
operation. This is why you will hear certain metal detectors referred to
as having a "True Non Motion" mode; meaning, of course, an All
Metal mode without S.A.T. Another sometimes confusing thing is that some
discriminators allow for adjustment down to the point that the
discriminator responds to all metals -- in other words, it's a
discriminator that doesn't discriminate. This is something very
different, however, than the All Metal mode previously described. For
this reason we often refer to it as a "Zero Disc" mode.
Microprocessor Control
The microprocessor is a complex electronic circuit which can perform
all of the logic, arithmetic, and control functions necessary to build a
computer. A sequence of stored instructions called a "Program"
is performed by the microprocessor, one at a time, at a speed which can
be as high as several million times every second.
The use of microprocessors in modern metal detectors has opened up
possibilities which were undreamed of just a few years ago. In the past,
adding new and useful features to a metal detector meant additional
control knobs and switches. There were obvious limits to this approach;
at some point size, cost, and operator confusion got out of hand. With a
microprocessor, a liquid crystal display, and a simple keypad the
problem is solved. A virtually unlimited number of features can be added
without adding any additional hardware. These features can be arranged
by a system of "Menus", so that anybody who can follow the
prompts on the display can easily find the control they're after and
adjust it to their liking. In this way, a single metal detector can be
set up for just about any application, or to suit anyone's personal
preference.
You might think that this sounds a little complicated -- what if you
don't want to be bothered with making all of those adjustments? Here's
the real beauty of microprocessor control; you don't have to. Each
control can be set to a typically useful position by the microprocessor
each time you turn the machine on so the beginner or casual user never
has to know that all those advanced features are there. Or better yet,
you can select your preference from the menu -- coin hunting,
prospecting, relic hunting, etc. -- and the microprocessor will make all
of the adjustments for you choosing settings that have been proven in
actual use by seasoned veterans.
In addition to these advantages, powerful software routines can be
used to enhance the metal detector's audio discrimination capabilities
and to display information in a variety of formats on an L.C.D. making
the operator's job of interpreting target responses faster and easier.
VLF Summary
Although the modern high performance VLF metal detector has been
several decades in the making, new advances will continue to be made.
Better, smarter, easier-to-use machines will eventually be introduced.
Today's very best metal detectors will not be easy to improve on but as
long as there is treasure to be found, you can be sure that research is
underway to take metal detecting technology to the next level.
P.I. (Pulse Induction)
Transmitter
The search coil or loop of a Pulse Induction metal detector is very
simple when compared to a VLF instrument. A single coil of wire is
commonly used for both the transmit and receive functions.
The transmitter circuitry consists of a simple electronic switch
which briefly connects this coil across the battery in the metal
detector. The resistance of the coil is very low, which allows a current
of several amperes to flow in the coil. Even though the current is high,
the actual time it flows is very brief. Pulse Induction metal detectors
switch on a pulse of transmit current, then shut off, then switch on
another transmit pulse. The duty cycle, the time the transmit current is
on with reference to the time it is off, is typically about 4%. This
prevents the transmitter and coil from overheating and reduces the drain
on the battery.
The pulse repetition rate (transmit frequency) of a typical PI is
about 100 pulses per second. Models have been produced from a low of 22
pulses per second to a high of several thousand pulses per second. Lower
frequencies usually mean greater transmit power. The transmit current
flows for a much longer time per pulse however, there are fewer pulses
per second. Higher frequencies usually mean a shorter transmit pulse and
less power however, there are more transmit pulses per second.
Lower frequencies tend to achieve greater depth and greater
sensitivity to items made from silver however, less sensitive to nickel,
and gold alloys. They typically have a very slow target response which
requires a very slow coil sweep speed.
Higher frequencies are more sensitive to nickel and gold alloys
however, less sensitive to silver. They may not penetrate quite as deep
as the lower frequency models regarding silver however, can be used with
a faster coil sweep speed. Higher frequency models are generally more
productive for treasure hunting because the faster sweep speed allows
more area to be searched in a given time, and they are more sensitive to
the ultimate beach find, gold jewelry.
As previously mentioned a typical PI search loop contains a single
coil of wire which serves as both the transmit and receive coil. The
transmitter operates in a manner similar to an automobile ignition
system. Each time a pulse of current is switched into the transmit coil
it generates a magnetic field. As the current pulse shuts off, the
magnetic field around the coil suddenly collapses. When this happens, a
voltage spike of a high intensity and opposite polarity appears across
the coil. This voltage spike is called a counter electromotive force, or
counter emf. In an automobile it is the high voltage that fires the
spark plug. The spike is much lower in intensity in a PI metal detector,
usually about 100 to 130 volts in peak amplitude. It is very narrow in
duration, usually less than 30 millionths of a second. In a PI metal
detector it is called the reflected pulse.
Receiver
Resistance is placed across the search coil to control the time it
takes the reflected pulse todecay to zero. If no resistance, or very
high resistance is used, it will cause the reflected pulse to
"ring". The result is similar to dropping a rubber ball onto a
hard surface, it will bounce several times before returning to rest. If
a low resistance is used the decay time will increase and cause the
reflected pulse to widen. It is similar to dropping a rubber ball onto a
pillow. Since we are interested in having it bounce once critical
damping for a rubber ball might be like dropping it onto carpet. A PI
coil is said to be critically damped when the reflected pulse decays
quickly to zero without ringing. An over or under dampened coil will
cause instability and or mask the fast conducting metals such as gold as
well as reduce detection depth.
When a metal object nears the loop it will store some of the energy
from the reflected pulse and will increase the time it takes for the
pulse to decay to zero. The change in the width of the reflected pulse
is measured to signal the presents of a metal target.
In order to detect a metal object we need to concern ourselves with
the portion of the reflected pulse where it decays to zero. The transmit
coil is coupled to the receiver through a resister and a diode clipping
circuit. The diodes limit the amount of transmit coil voltage reaching
the receiver to less than one volt so as not to overload it. The signal
from the receiver contains both the transmit pulse and the reflected
pulse. The receiver has a typical gain of 60 decibels. This means the
area where the reflected pulse reaches zero is amplified 1,000 times.
Sampling Circuit
The amplified signal coming from the receiver is connected to a
switching circuit which samples the reflected portion of the pulse as it
reaches zero. The reflected pulse up to this point references in
actuality a series of pulses at the transmit frequency. When a metal
object nears the coil the transmit portion of the signal will remain
unchanged while the reflected portion of the pulse will become wider.
The metal object stores some of the electrical energy from the transmit
pulse and increases the time it takes for the reflected pulse to reach
zero. An increase in duration of a few millionths of a second is enough
to allow the detection of a metal target. The reflected pulse is sampled
with an electronic switch controlled by a series of pulses which are
synchronized with the transmitter.
The most sensitive sampling point on the reflected pulse is as near
as possible to the point where it reaches zero. This is typically about
20 millionths of a second after the transmitter shuts off and the
reflected pulse begins. Unfortunately, this is also the area where a PI
can become unstable. For this reason most PI models sample the reflected
pulse at a decay of 30 or 40 millionths of a second, well after it
decays to zero.
Integrator
In order for an object to be detected the sample signals must be
converted to a DC voltage. This task is performed by a circuit called an
integrator. It averages the sampled pulses over time to provide a
reference voltage. This DC reference voltage increases when metal nears
the coil, then decreases as the object moves away. The DC voltage is
amplified and controls the audio output circuitry which increases in
pitch and/or volume to signal the presents of metal.
The time constant of the integrator determines how quickly the metal
detector will respond to a metal object. A long time constant (in the
range of seconds) has the advantage of reducing noise and making the
metal detector easier to tune. Long time constants require a very slow
sweep of the coil because a target might be missed if it passes quickly
by the search coil. Short time constants (in the range of tenths of a
second) respond more quickly to targets. This allows a quicker sweep of
the loop however, it also allows more noise and instability.
Discrimination
PI metal detectors are not capable of the same degree of
discrimination as VLF metal detectors.
By increasing the time period between transmitter shut-off and the
sampling point (pulse delay), certain metal items can be rejected.
Aluminum foil will be the first to be rejected followed by nickel, pull
tabs and gold. Some coins can be rejected at very long sample delays
however, iron cannot be rejected.
There have been many attempts to design a PI that can reject iron
however these attempts have had limited results. Iron is detectable at
very long time delays however, silver and copper have similar
characteristics. Such long time delays also have a negative affect on
detection depth. Ground mineralization will cause some widening of the
reflected pulse as well, changing the point at which a target responds
or rejects. If the time delay is adjusted so that a gold ring doesn't
respond in an air test, that same ring may respond in mineralized
ground. Mineralized ground thus changes everything regarding the time
delays and discrimination of PI metal detectors.
Ground Balance
Ground balancing, while very critical on VLF metal detectors, is not
necessary with PI circuits. Average ground mineralization will not store
any appreciable amount of energy from the search coil and will not
usually produce a signal. Such ground will not mask the signal from a
buried object. On the contrary, ground mineralization will add slightly
to the duration of the reflected pulse increasing the depth of
detection. The term "automatic ground balance" is often
applied to PI instruments because it will normally not react to
mineralization and there are no external adjustments for any specific
ground conditions.
Heavy black sand is an exception. It will cause a VLF coil to
overload, making metal detector penetration poor at best. A PI detector
will work in black sand however, some false signals may result if the
coil is held very close to the ground. Ground responses can be minimized
by using a longer time delay between the shut-off and sample point
(pulse delay). Advancing the time delay slightly will help to smooth out
the noises caused by most mineralization.
Automatic vs. Manual Tuning
Most PI detectors are manually tuned. This means the operator has to
adjust a control until a clicking or buzzing sound is heard in the
headphones. If the search conditions change, such as when moving from
black sand to neutral sand or from dry land to salt water, the tuning
must be re-adjusted. Failure to do so can result in reduced detection
depth and missed targets. Manual tuning is very difficult with short
integration time constants, so most manually tuned models use long time
constants and the search coil must be swept slowly.
This is not a problem when a PI is used for scuba diving because the
coil cannot be swept quickly underwater. When used at the surf line,
where the coil will be in and out of salt water, a manually tuned metal
detector can be very frustrating to use. The tuner must be adjusted
continually to maintain a threshold. Some operators elect to set it
slightly below the threshold however, that can result in a reduction in
depth as the ground conditions change.
Automatic tuning, or S.A.T. (Self Adjusting Threshold) offers a
significant advantage when searching in and out of salt water or over
mineralized ground. S.A.T. helps keep the metal detector operating at
maximum sensitivity without requiring constant adjustments by the
operator. It improves the stability, reduces noise, and allows higher
gain settings to be used. PI metal detectors do not emit strong,
negative signals like a VLF. As such they do not "overshoot"
on pockets of mineralization. With S.A.T. the coil must be kept in
motion while detecting a target. Stopping over a target will cause the
S.A.T. to tune it out or cease responding.
Audio Circuits
PI audio circuits generally fall into two categories: variable pitch
and variable volume. Variable pitch or V.C.O. (Voltage Controlled
Oscillator) audio has the advantage for faint targets because the change
in pitch is easier to hear than a change in volume at lower aud io
levels. This is primarily true for manually tuned models. The "fire
siren" sounds can become annoying and many have trouble hearing the
higher tones. A variant of this is the mechanical vibrator device
primarily used for deep water. It emits a slow clicking sound and
vibration that increases to a buzz to signal a find. The mechanical
device is easier to hear and feel over the sound of an underwater air
supply.
Many people prefer a more conventional audio tone that increases in
volume rather than pitch to signal a find. This audio system works best
with a PI metal detector that has a fast target response and automatic
tuning (S.A.T.). Automatic tuning makes the PI sound and respond similar
to a typical VLF metal detector.
PI Summary Pulse Induction metal detectors are specialized
instruments. They are generally not suitable for coin hunting urban
areas because they do not have the ability to identify or reject ferrous
(iron) trash. They can be used for relic hunting in rural areas where
iron trash is not present in large quantities, or is desired. They are
intended for maximum depth under extreme search conditions such as salt
water beaches and highly mineralized ground. In such conditions PI type
metal detectors produce superior results when compared to VLF models,
particularly in the ability to ignore such extreme ground and penetrate
it for maximum depth.
