Saturday, February 7, 2009

How Metal Detectors Work

Knowing how metal detectors work will help one choose the right detector for your purposes and be able to use it more effectively.

Operating a metal detector is a simple process. Once you have turned the unit on, you simply move the coil back and forth slowly over the area you want to search. When the coil passes over a target object, say a small sliver or nugget of gold, for example, an audible signal is produced by the control unit to alert you that something has been found.

The more advanced the metal detectors the more it can pinpoint the type of metal it has located including the exact spot and depth.

A metal detector is usually fairly light-weight and is made up of the following parts:
A Stabilizer. Some of the more sophisticated detectors will have a stabilizer which is used to steady the unit as you sweep from side to side.

A Control box, of course which contains the circuitry, microprocessor, controls, speaker, and batteries.

The shaft. This connects the control box and the coil; Try to get a detector with an adjustable shaft so it can be set at a comfortable time for your height.

Search coil. This is the part that actually senses the object, also called the "search head" or "loop" and even, "antenna".
All metal detectors use one or more of the three following technologies:
Very low frequency (VLF)
VLF Phase Shifting
Pulse induction (PI)
Beat-frequency oscillation (BFO)
Very Low Frequency
VLF is also called induction balance, and is probably the most popular system used today. This is the system that uses two coils. A transmitter coil and a receiver coil.

In the Transmitter coil, an alternating current is sent along this wire, first in one direction and then in the other, thousands of times each second. The times the current changes direct each second is the frequency of the unit. The receiver coil is designed to act as an antenna and pick up to pick up frequency changes caused by metal objects entering the magnetic field produced by the first coil.

A metal detector can tell from the variations in the magnetic field detected by the detector coil how deep an object is and even, to a large extend, what sort of metal it is.

Different metals have different resistance levels to electricity. Also some conduct electricity better than others. Copper, for example, is an excellent conductor of electricity. Lead is a poor conductor. So the resistance to conducting electricity affects the magnetic field as does the depth of the article. The deeper it is the longer it takes to affect the magnetic field.

All these factors are picked up and analyzed by the metal detector and the information is displayed on the control panel.

Most metal detectors these days use integrated circuitry, like a mini computer, to process the information it received from the coil and displays for you to read.

VLF Phase Shifting
VLF Phase Shifting is the method by which a VLF Metal Detector can tell the difference between different metals. This uses what is called phase shifting. A phase shift is the difference in timing between the transmitters coil’s frequency and the frequency of the object.

This discrepancy can be the result of a couple of things.

Inductance – The is the speed at which a object conducts electricity. The easier it is to conduct the slower it is to react. 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 is reluctant to conduct electricity has a high resistance. Plastic and ceramic will not conduct it at all, hence is used as an insulator. Various metals all have different levels of resistance.

An object that has a high inductance, therefore, is going to have a larger phase shift because it will take longer to change its magnetic field. An object with a small resistance will have a smaller phase shift.

So your metal detector can distinguish between various metals by analyzing the phase shift of those objects you detect, and will display the results for you on the metal detectors display panel.

This means it can discriminate and this is useful when you are working an area that may have mineralized sand as you can filter out the results caused by the mineralized sand as well as single objects, junk such as bottle tops, pull tabs and so on.

Of course this is all done in fractions of a second and all you will see are the result displayed by the detector.,

PI Technology
Less common but still very useful is the metal detector based on PI technology. PI means Pulse Induction.

As you might have guessed, this uses a pulse to detect object rather than a magnetic field and so uses only one coil to perform the purpose of detection.

A PI detector sends out a pulse of current through the coil of wire. When it does a brief magnetic field is produced. When the pulse ends the magnetic field reverses polarity (remember all magnetic fields have a north and south pole?) and this collapses very fast and results in a sharp electrical spike. That spike then causes another current to run through the coil. This is very fast and lasts just 30 microseconds. Another pulse is sent and the process repeats. Usually about 100 or more times per second.

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.

This is rather like echoes. You remember the difference when you talk or sing in a room with a hard surface compared to one with soft surfaces? A bathroom is a good example. The sound in a room with hard surfaces lasts longer than in a room with soft surfaces. That’s why your voice sounds so wonderful in the bathroom when taking a shower but not quite so good in the living room with the carpet and soft drapes. In a PI metal detector, the magnetic fields from target objects add their "echo" to the reflected pulse, making it last just a fraction of a second longer than it would without them. Then a sampling circuit monitors the length of the reflected pulse sand compares that to the length expected. The circuit can tell the difference between the decay rates of the reflected pulse sand deduct if there is a metal object interfering with it.

BFO Technology
Another system to detect metal is by the use of a beat-frequency oscillator (BFO). In a BFO system, there are two coils of wire. A larger coil is in the search head, and a smaller coil is located inside the control box. Each coil is connected to an oscillator generating thousands of pulses of current per second the frequency of which is slightly offset between the two coils.

The coil generates radio waves as the pulses travel through each coil and a tiny receiver in the control box picks up the radio waves. From these it then generates and creates an audible series of tones (beats) based on the difference between the frequencies.

Simply put, when the coil in search head passes over a metal object, such as a gold nugget, it creates a magnetic field around the gold nugget. This magnetic field interferes with the frequency of the radio waves generated by the search-head coil and this causes a change in the audio tone.

The BFO system is very simple and so they can be manufactured at a very low cost.

Last Word on Metal Detectors
The metal detector you decide to use will depend primarily on your budget, the purpose you will use it for and how often, but having the above information and knowing how metal detectors work will go a long way to buying and using the right metal detector for you.

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