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INTRODUCTION
Metal detectors use electromagnetic induction to detect metal.
THE FIRST DETECTORS
Towards the end of the 19th century, many scientists and engineers used their growing knowledge of electrical theory in an attempt to devise a machine which would pinpoint metal.
The use of such a device to find ore-bearing rocks would give a huge advantage to any miner who employed it.
The German physicist Heinrich Wilhelm Dove invented the induction balance system, which was incorporated into metal detectors a hundred years later.
Early machines were crude, used a lot of battery power, and worked only to a very limited degree.
Alexander Graham Bell used such a device to attempt to locate a bullet lodged in the chest of American President James Garfield in 1881; the attempt was unsuccessful because the metal bed Garfield was lying on confused the detector.
MODERN DEVELOPMENTS
The modern development of the metal detector began in the 1930s. Gerhard Fisher had developed a system of radio direction-finding, which was to be used for accurate navigation.
The system worked extremely well, but Fisher noticed that there were anomalies in areas where the terrain contained ore-bearing rocks. He reasoned that if a radio beam could be distorted by metal, then it should be possible to design a machine which would detect metal using a search coil resonating at a radio frequency.
In 1937 he applied for, and was granted, the first patent for a metal detector. However, it was one Lieutenant Josef Stanislaw Kosacki, a Polish officer attached to a unit stationed in St Andrews, Fife, Scotland during the early years of World War II, that refined the design into a practical detector. They were heavy, ran on vacuum tubes, and needed separate battery packs. The design invented by Stanislaw was used extensively during the clearance of the German mine fields.
As it was a wartime research operation to create and refine the design of the detector, the knowledge that Stanislaw created the first practical metal detector was kept secret for over 50 years.
After the war, there were plenty of surplus mine detectors on the market; they were bought up by relic hunters who used them for fun and profit. This helped to form metal detecting into a hobby. Metal detector can be used for robotics.


ANATOMY OF METAL DETECTOR

A typical metal detector is light-weight and consists of just a few parts:
Stabilizer (optional) - used to keep the unit steady as you sweep it back and forth.
Control box - contains the circuitry, controls, speaker, batteries and the microprocessor.
Shaft - connects the control box and the coil; often adjustable so you can set it at a comfortable level for your height.
Search coil - the part that actually senses the metal; also known as the "search head," "loop" or "antenna".

Garrett GTI 1500 metal detector
Most systems also have a jack for connecting headphones, and some have the control box below the shaft and a small display unit above.
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 DETECTOR TECHNOLOGIES

o Very low frequency (VLF)
o Pulse induction (PI)
o 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.

This Land Ranger metal detector from Bounty Hunter uses VLF.
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.



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.
Resistance - An object that does not conduct electricity easily (is resistive) is quick to react to changes in the current.
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.
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 thousa.nd.

This Garrett metal detector uses pulse induction.
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.