How a Metal Detector Works

If it is not necessary to know how a metal detector works to use it effectively, it can nevertheless be useful to improve its efficiency! Here is a brief introduction to the principle of operation of the most common types of detectors (VLF detectors), to the extent of my knowledge and what to read. ..

But remember that the best metal detector in the world can never be better than the one that uses it

Principle of operation of a metal detector:

A metal detector works by exploiting a physical phenomenon known to all science students (and discovered by Michael Faraday in 1831): electromagnetic induction. This phenomenon could be presented in the following way: A variation of the magnetic field (change of intensity and direction), is seen to be traversed within its volume by induced electric currents, called eddy currents. The characteristics of these flows (strength and guidance) depend on the features of the variation of the magnetic field and the nature of the object (shape, conductivity, etc ...). The eddy currents which appear within the conductive object in question, in turn, generate a magnetic field radiating around the object. By induction phenomenon, The variations of this magnetic field can, in turn, induce a current flow in a near conductive mass. In a metal detector, it will, therefore, be sufficient to know how to "collect" this current and to analyze it to be able to identify the presence of a Metal object. We will see how metal detectors exploit this phenomenon to fulfill their role.

VLF (Very Low Frequency)

Under this name, we will talk about detectors at very low frequency (<30 kHz) and low frequency (between 30 kHz and 300 kHz). This type of detector is by far the most widespread (and cheapest). In fact, 95% of the devices on the market is part of it.

In the detection head (disk) there are two coils of wire:

1) The transmitting coil:

This coil, traversed by an alternating sinusoidal electric current (reversed several thousand times per second), generates a magnetic field around it (the polarization of which is changed at the same frequency as that of the current). The frequency at which this inversion takes place is none other than the frequency of operation of your device of which you speak the advertisements (operating frequency). We will call this frequency "F." A current curve is passing through the transmitter coil. When a metal object passes through this magnetic field in constant polarity change (inverted polarity one / F times per second), eddy currents appear within it. These currents, in turn, generate a fluctuating magnetic field, which tends to compensate for the magnetic field produced by the emitter coil.

2) The take-up reel:

Another coil of wire located in the detection head will react with the magnetic field emitted by our metallic object (this coil is arranged in such a way that the current that should induce the magnetic field of the emitter is virtually canceled). An induced current then appears at its terminals. This current, amplified and processed by the detector electronics, provides the prospector with information (sounds, a meter, a digital display, etc.) in the presence of a metal object under the detection head. The signal received through the receiving coil appears to be delayed on the signal emitted by the transmitting coil. This delay is the result of two characteristics that all the conductive materials possess:

This delay is called the "phase shift." The largest phase shifts are observed for metal objects with the best inductance: large, thick objects made of perfect conductive metals (such as silver, gold or copper). Lower phase shifts appear for "resistant" objects: smaller objects, finer or less conductive objects. A non-contractual example of a phase shift between transmitted signal and received signal.

Some "materials," bad or even terrible drivers are likely to generate a strong signal. These are the "ferromagnetic." A ferromagnetic substance will tend to become magnetized when it is immersed in a magnetic field (much like a paperclip temporarily transforms into a magnet when in contact with a magnet). The phase shift generated by these objects is very small or even non-existent. Most land and sand contain small mineral grains (known as soil generalization). These grains have a high ferromagnetic behavior facing the detector. Draws and other steel caps have both ferromagnetic and electrical properties.

Discrimination :

Since for each type of metal and object, the signal received by the receiver coil shows a particular phase shift on the transmitted signal, it is possible to differentiate one metal from another, just by analyzing their offsets Respectively. For example, a silver coin will have a much higher phase shift than an aluminum zipper.

Starting from this principle, a detector can be designed to ring only on the silver piece and ignore the pull tab. This ability to distinguish metallic targets from one another is called "discrimination."

Conventional Discrimination:


The simplest form of discrimination is to allow the metal detector to emit a sound when it passes on a target whose phase shift exceeds a certain value (most often adjustable by a potentiometer). Unfortunately, with this type of discrimination, the detector may lose small gold objects (among others) or even parts, if the discrimination is set high enough to reject the aluminum (aluminum paper, pulls, capsules, etc ...).

Discrimination Notch:


A more efficient system is to accept (or reject for the most advanced devices) only a certain range of phase shift. This is Notch discrimination. With this system, you can for example only accept the band corresponding to the gold alliances and reject everything else (in theory). You can also (with the best-designed appliances) deny the strip that corresponds to the aluminum pulls without taking the risk of losing what is underneath (regarding phase shift).

Visual Discrimination:


Some metal detectors provide information about the identity of the target via a meter or LCD. This system of visual discrimination, coupled with noise discrimination, gives the prospector additional information enabling him to choose between digging and not digging. Some metal detectors can "identify" an object based on a resistance inductance table. These ratios are used to predict a phase shift for a given object type at a given frequency. Using several fixed or variable frequencies, the detector is able, depending on the reaction of the object detected at each of these frequencies, to predict its "identity".

The difficulty of discrimination lies in the fact that a given object with a given phase shift at a certain depth and with a particular orientation (flat for a part for example) may have a dramatically different phase shift at an Another depth and with a different direction (piece on edge in our example). Moreover, the shape of the object also makes a lot of it! The electromagnetic induction is high enough for a circular object (ring for example), where the current circulates in a closed circuit. When the object is "open" (metal rod for example), the induction is considerably less.

For a gold alliance, for instance, this may produce varying results depending on whether the ring is intact or broken. In all cases, Discrimination should be used with caution. If it is of certain help in many situations, it is never 100% safe. Even with a Notch system, for example, when you reject the strip of aluminum zippers, you still have a significant risk of losing a gold jewel, which according to its shape, position and depth will be interpreted As belonging to the rejected band.

Soil Effects Compensation:

As seen above, most of the sands and land contain a certain amount of minerals, dissolved in the water contained in the soil, as well as particles of iron or other metals. As a result, the detector receives a signal from the ground itself, which sometimes can be much greater in strength than the signal of a small medium-depth metal object. Fortunately, the phase shift of the signal due to ground effects remains more or less constant over a given area of ​terrain. With appropriate electronic processing on this phase shift, it is possible in the detector to cancel the effects of this signal, even if the signal changes in intensity (when raising or lowering the detection head, for example, We pass on a hole or a bump). The simplest form of ground effect compensation consists of a potentiometer that is adjusted by raising and lowering the sensing head by putting its detector in the appropriate mode.

While this method is effective, it may seem daunting and confusing for most people. The more elaborate detectors will allow an automatic adjustment of the compensation of the effects of soil. Only two adjustments are necessary: ​​one with the head raised and the other with the head lowered. In even more sophisticated models, the change will be made automatically as the prospection proceeds, taking into account the changes in the ground effect of the terrain. An excellent field effect compensation system is supposed to be set once and for all at the beginning of the session and so function all day without having to touch it (for a given soil).

Warning: some detectors, which are windy in pubs as having an automatic floor effect compensation, have in fact only a "preset" mode: fixed compensation value, adjusted at the factory and supposed to be adapted to most courses.

Sweep mode:

Although the signal due to the ground effect may be much more intense than the signal of a target, it remains relatively constant while the head passes over the field. On the other hand, the signal strength from a target will grow very rapidly as the head passes over the object and diminishes as the head moves away. Using this phenomenon, we can separate the signals Of soil effect of those of a target, analyzing the rate of change of the signal, rather than the signal itself. The operating mode of the metal detectors operating on this principle is called "Motion mode" because of course, they can only indicate a target if the head sweeps the ground. This explains that some detectors give various results according to The speed at which you sweep the ground!

At present, all detectors using discrimination do so in scan mode. This is not a disadvantage, since in any case, to cover the ground, we must carry out sweeps!

Static (non-motion), Pinpoint and Threshold:

Once the target is located in scanning mode, it is preferable to be able to determine it more precisely than by sweeping the ground always! This precise location ( Pinpoint ) is performed using the "all metals" mode. As by definition, discrimination is not required for this function, scanning is useless, except to center the head as best as possible on the target (and there, the scanning speed is utterly unimportant). This is why the "all metals" mode is also called "static mode" (non-motion). Associated with the static method and pinpoint, we find the threshold setting ( Threshold adjust ). This setting is often represented on the detector by a pushbutton (sometimes marked "pinpoint", since it is directly associated with it). This setting makes it possible to set the level sensed at the moment when the button is pressed for the threshold (signal level from which the detector sounds). Basically, the detector will adjust to subtract from the received signal the value of the signal picked up by pressing the button. This will eliminate the "ground effect" component of the signal (at a given distance from the ground) And thus to hear only the target when passing over it (always at the same given distance). If the sensor head is lowered, the signal due to the ground effect will increase and cause the detector to sound. It will then be necessary to press the button again to readjust the threshold by decreasing it.Idem if raising the detection head over the target: the detector will pick up a signal whose power will be below the threshold. It will not ring anymore. It will therefore be necessary to press the button again to increase the threshold accordingly. On certain models, the adjustment of the threshold is done automatically. The threshold is then increased and decreased gradually so as to obtain a slight sound at the threshold limit. The advantage of this system is that it makes it possible to compensate for small variations in the ground effect as well as an incorrect adjustment of the compensation. The major disadvantage is that it requires a scan to function. This is why you will hear about detectors with "True Non-Motion Mode", which does not have automatic threshold adjustment. The threshold is then increased and decreased gradually so as to obtain a slight sound at the threshold limit. The advantage of this system is that it makes it possible to compensate for small variations in the ground effect as well as an incorrect adjustment of the compensation. The major disadvantage is that it requires a scan to function. This is why you will hear about detectors with "True Non-Motion Mode", which does not have automatic threshold adjustment. The threshold is then increased and decreased gradually so as to obtain a slight sound at the threshold limit. The advantage of this system is that it makes it possible to compensate for small variations in the ground effect as well as an incorrect adjustment of the compensation. The major disadvantage is that it requires a scan to function. This is why you will hear about detectors with "True Non-Motion Mode", which does not have automatic threshold adjustment.

Another source of confusion is that some detectors allow discrimination adjustment at a position 0, which will let all metals pass through. The mode of operation remaining that of the discrimination (scanning mode), one can not speak of mode "all metals". The "0 Disc" mode will then often be heard.

Microprocessor control:

I would not be an insult to the initiates to give a detailed definition of the microprocessor. For short, one could say that the microprocessor is a complex electronic integrated circuit that allows to execute the logical, arithmetic and control operations that a computer needs to operate. It is the heart or rather the brain of a computer. It is capable of executing sequences of basic instructions called "programs" at a high speed (several million instructions per second). The use of microprocessors in metal detectors to open up possibilities unimaginable only a few years back! With the microprocessor, all (or almost all) adjustments are made via a single liquid crystal display and by the manipulation of a keyboard. From the main menu, a menu system allows access to all parameters of the machine and can be modified as required (each configuration of settings can be saved independently in memory, which makes it possible to recall them to anytime if need be, without having to adjust everything again). On a "classical" detector, adding new functions means adding new electronic circuits, knobs, potentiometers, etc. With the microprocessor, adding a service can be as simple as modifying the internal program of the machine (without adding circuit). And is it safe to bet that one day (near?), We can update the system of a detector like the Spectrum, simply by connecting it to his PC and going to search the update on the Internet! This may seem a bit complicated, but if you do not want to be cluttered with complex adjustments and manipulations, you can always use pre-set factory settings. Such a program will provide you with a prospecting without an aluminum capsule, another will be optimized for the detection of parts, etc. A desirable advantage is also that the capacities of the microprocessor can be used to significantly improve the analysis of the signal in the sound discrimination. And nothing prevents the use of this power to refine the rendering of the sound (modulation in amplitude and height potentially very beautiful). The liquid crystal display also allows a lot of improvements, such as visual analysis of the signal using bar graphs or response curves.

Beware. However, the microprocessor is not a miracle tool! For example, if you reject the aluminum paper and a small gold chain comes to have a signal very close to the said paper, you will lose it as well as with a conventional detector! In addition, these devices require a long experience for those who really want to use it and make the most of it!

Conclusion:

Although VLF detectors have several decades of existence behind them, innovations will continue to emerge. More powerful machines, more powerful, easier to use are brought to appear. As long as there are prospectors and "juicy" markets (USA, Australia, England, etc.) and there is something to be found, it is a safe bet that research will continue to The detectors of the future!

Click Here to Leave a Comment Below 4 comments
MichelineX - July 1, 2017

This review will help me to buy my Metal Detector. Thank You 🙂

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Adam barton - July 3, 2017

Great! this article was awesome. Lots of great information. This really helps to select a good metal detector

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