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Operating a metal detector is simple. Once you turn the unit on, you move slowly
over the area you wish to search. In most cases, you sweep the coil (search head)
back and forth over the ground in front of you. When you pass it over a target object,
an audible signal occurs. More advanced metal detectors provide displays that pinpoint
the type of metal it has detected and how deep in the ground the target object is
located.
Metal detectors use one of three technologies:
- Very low frequency (VLF)
- Pulse induction (PI)
- Beat-frequency oscillation (BFO)
VLF Technology
Very low frequency (VLF), also known as induction balance, is probably the most
popular detector technology in use today. In a VLF metal detector, there are two
distinct coils:
- Transmitter coil - This is the outer coil loop. Within it is a coil of wire. Electricity
is sent along this wire, first in one direction and then in the other, thousands
of times each second. The number of times that the current's direction switches
each second establishes the frequency of the unit.
- Receiver coil - This inner coil loop contains another coil of wire. This wire acts
as an antenna to pick up and amplify frequencies coming from target objects in the
ground.
The current moving through the transmitter coil creates an electromagnetic field,
which is like what happens in an electric motor. The polarity of the magnetic field
is perpendicular to the coil of wire. Each time the current changes direction, the
polarity of the magnetic field changes. This means that if the coil of wire is parallel
to the ground, the magnetic field is constantly pushing down into the ground and
then pulling back out of it.
As the magnetic field pulses back and forth into the ground, it interacts with any
conductive objects it encounters, causing them to generate weak magnetic fields
of their own. The polarity of the object's magnetic field is directly opposite the
transmitter coil's magnetic field. If the transmitter coil's field is pulsing downward,
the object's field is pulsing upward.
The receiver coil is completely shielded from the magnetic field generated by the
transmitter coil. However, it is not shielded from magnetic fields coming from objects
in the ground. Therefore, when the receiver coil passes over an object giving off
a magnetic field, a small electric current travels through the coil. This current
oscillates at the same frequency as the object's magnetic field. The coil amplifies
the frequency and sends it to the control box of the metal detector, where sensors
analyze the signal.
The metal detector can determine approximately how deep the object is buried based
on the strength of the magnetic field it generates. The closer to the surface an
object is, the stronger the magnetic field picked up by the receiver coil and the
stronger the electric current generated. The farther below the surface is, the weaker
the field is. Beyond a certain depth, the object's field is so weak at the surface
that it is undetectable by the receiver coil.
How does a VLF metal detector distinguish between different metals? It relies on
a phenomenon known as phase shifting. Phase shift is the difference in timing between
the transmitter coil's frequency and the frequency of the target object. This discrepancy
can result from a couple of things:
- Inductance - An object that conducts electricity easily (is inductive) is slow to
react to changes in the current. You can think of inductance as a deep river: Change
the amount of water flowing into the river and it takes some time before you see
a difference.
- Resistance - An object that does not conduct electricity easily (is resistive) is
quick to react to changes in the current. Using our water analogy, resistance would
be a small, shallow stream: Change the amount of water flowing into the stream and
you notice a drop in the water level very quickly.
Basically, this means that an object with high inductance is going to have a larger
phase shift, because it takes longer to alter its magnetic field. An object with
high resistance is going to have a smaller phase shift.
Phase shift provides VLF-based metal detectors with a capability called discrimination.
Since most metals vary in both inductance and resistance, a VLF metal detector examines
the amount of phase shift, using a pair of electronic circuits called phase demodulators,
and compares it with the average for a particular type of metal. The detector then
notifies you with an audible tone or visual indicator as to what range of metals
the object is likely to be in.
Many metal detectors even allow you to filter out (discriminate) objects above a
certain phase- shift level. Usually, you can set the level of phase shift that is
filtered, generally by adjusting a knob that increases or decreases the threshold.
Another discrimination feature of VLF detectors is called notching. Essentially,
a notch is a discrimination filter for a particular segment of phase shift. The
detector will not only alert you to objects above this segment, as normal discrimination
would, but also to objects below it.
Advanced detectors even allow you to program multiple notches. For example, you
could set the detector to disregard objects that have a phase shift comparable to
a soda-can tab or a small nail. The disadvantage of discrimination and notching
is that many valuable items might be filtered out because their phase shift is similar
to that of "junk." But, if you know that you are looking for a specific type of
object, these features can be extremely useful.
PI Technology
A less common form of metal detector is based on pulse induction (PI). Unlike VLF,
PI systems may use a single coil as both transmitter and receiver, or they may have
two or even three coils working together. This technology sends powerful, short
bursts (pulses) of current through a coil of wire. Each pulse generates a brief
magnetic field. When the pulse ends, the magnetic field reverses polarity and collapses
very suddenly, resulting in a sharp electrical spike. This spike lasts a few microseconds
(millionths of a second) and causes another current to run through the coil. This
current is called the reflected pulse and is extremely short, lasting only about
30 microseconds. Another pulse is then sent and the process repeats. A typical PI-based
metal detector sends about 100 pulses per second, but the number can vary greatly
based on the manufacturer and model, ranging from a couple of dozen pulses per second
to over a thousand.
If the metal detector is over a metal object, the pulse creates an opposite magnetic
field in the object. When the pulse's magnetic field collapses, causing the reflected
pulse, the magnetic field of the object makes it take longer for the reflected pulse
to completely disappear. This process works something like echoes: If you yell in
a room with only a few hard surfaces, you probably hear only a very brief echo,
or you may not hear one at all; but if you yell in a room with a lot of hard surfaces,
the echo lasts longer. In a PI metal detector, the magnetic fields from target objects
add their "echo" to the reflected pulse, making it last a fraction longer than it
would without them.
A sampling circuit in the metal detector is set to monitor the length of
the reflected pulse. By comparing it to the expected length, the circuit can determine
if another magnetic field has caused the reflected pulse to take longer to decay.
If the decay of the reflected pulse takes more than a few microseconds longer than
normal, there is probably a metal object interfering with it.
The sampling circuit sends the tiny, weak signals that it monitors to a device call
an integrator. The integrator reads the signals from the sampling circuit, amplifying
and converting them to direct current (DC). The direct current's voltage is connected
to an audio circuit, where it is changed into a tone that the metal detector uses
to indicate that a target object has been found.
PI-based detectors are not very good at discrimination because the reflected pulse
lengths of various metals are not easily separated. However, they are useful in
many situations in which VLF-based metal detectors would have difficulty, such as
in areas that have highly conductive material in the soil or general environment.
A good example of such a situation is salt-water exploration. Also, PI-based systems
can often detect metal much deeper in the ground than other systems.
BFO Technology
The most basic way to detect metal uses a technology called beat-frequency oscillator
(BFO). In a BFO system, there are two coils of wire. One large coil is in the search
head, and a smaller coil is located inside the control box. Each coil is connected
to an oscillator that generates thousands of pulses of current per second. The frequency
of these pulses is slightly offset between the two coils.
As the pulses travel through each coil, the coil generates radio waves. A tiny receiver
within the control box picks up the radio waves and creates an audible series of
tones (beats) based on the difference between the frequencies.
If the coil in the search head passes over a metal object, the magnetic field caused
by the current flowing through the coil creates a magnetic field around the object.
The object's magnetic field interferes with the frequency of the radio waves generated
by the search-head coil. As the frequency deviates from the frequency of the coil
in the control box, the audible beats change in duration and tone.
The simplicity of BFO-based systems allows them to be manufactured and sold for
a very low cost. But these detectors do not provide the level of control and accuracy
provided by VLF or PI systems.
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