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Abstract

Content

1. Purpose, basic characteristics and range of tasks to be solved by a metal detector

A metal detector is an inductive device, which makes it possible to detect objects from metal in a weakly conducting medium. It can work in water, determine the valuable contents of the wall, soil, wood, clothing, food and luggage. The rapid development of microelectronics has provided real seekers of valuable items with a modern compact and reliable metal detector [1].

Let's consider the basic technical characteristic [2]:

The tasks of the metal detector depend on its type and working direction [3]:

  • Ground metal detector – is designed to search for treasures, coins and jewelry, scrap metal. As a rule, it is built by induction technology. Has many settings, DSP-processor, metal discriminator – a special function for determining the metal from which the object is supposedly composed in the ground. Depth of detection of objects from 20 cm to 1 meter;
  • Military metal detector (mine detector) – designed to search predominantly min. As a rule, it is built on the principle of reception-transmission. Has a minimum of settings. The depth of detection of a mine is from 20 cm (Soviet Mine Detector IMP) to 1 meter (modern military mine detectors IMP-2);
  • Inspection metal detector – a manual metal detector designed for security services. Serves to detect metal objects on the human body (pistol, knife). The detection range of the Makarov pistol is up to 25 cm;
  • An arched (frame) metal detector is a metal detector that is used to control large flows of people, for example, in the metro, at stations. They represent a frame through which a person passes;
  • Deep metal detector – is designed to search for large deep targets, such as a chest with gold. It has two spools spaced apart from each other, or one large frame with a coil. It is based on the principle of reception-transfer. A distinctive feature of this type of metal detectors is that it reacts not only to metals, but also to any changes in the depth of the soil (transitions from one soil to another, old foundations of buildings, etc.). Depth of detection of objects from 50 cm to 3 meters;
  • Magnetometer – designed to search for ferromagnetic objects (for example, iron). This type of metal detector is the most compact and most sensitive, since the search head can fit in the palm of your hand. Magnetometers can also be used to search for gold, copper, aluminum. But for this we need an additional causative agent, which will be made of non-ferromagnetic metals, figuratively speaking, electromagnets.

    • 2. Overview of methods and means of detecting metal objects

    Each metal detector is individually individual. One of the differences and expressions of the individuality of a metal detector is the choice of the method on which the construction of a metal detector will be based. There are many methods of implementation.

    2.1 Method of induction balance – IB/TR

    The search head is formed by two coils arranged in one plane and balanced so that when a signal is applied to the transmitting coil, a minimum signal is present at the outputs of the receiving coil. The transmitting coil is often included in the loop of the LC generator. The measured parameter is the amplitude of the signal on the receiving coil and the phase shift between the transmitted and received sinusoidal signals. Such metal detectors have an operating frequency of 80-100 kHz. They can detect small objects at a relatively large depth (30-35 cm), but they are useless when searching for highly mineralized pounds and sea beaches [4].

    Method of induction balance using very low operating frequencies – VLF/TR

    It was found that when the operating frequency is lower than 20 kHz, it is possible to rebuild from the influence of the pound, the depth of action of the device is somewhat reduced, but the stability of operation increases sharply, and false signals disappear. Such devices are called VLF/TR, which is deciphered as a metal detector of the transmitter-receiver type operating at very low frequencies [5].

    This metal detector has come to be called dynamic, although, in essence, it is of the VLF/TR type. Previous static metal detectors such as VLF/TR almost ceased to be produced, and all leading firms quickly switched to the production of devices using this dynamic principle. Numerous small companies that did not have time to do this were forced to cease to exist. Since then, only about a dozen companies producing metal detectors have remained in the world.

    Induction balance method with spaced coils – RF

    This is a high-frequency version of TR, where the transmitting and receiving coils form not a flat transformer, but are spaced apart and arranged perpendicular to each other. The receiving coil receives the signal reflected from the metal surface, radiated by the transmitting coil. This method is used in deep instruments and is characterized by insensitivity to small objects and the inability to distinguish between ferrous and non-ferrous metals [6].

    2.2 Method of resonance failure – OR

    The parameter being analyzed is the amplitude of the signal on the coil of an oscillating circuit tuned close to resonance with the signal fed from it to the generator. The appearance of metal in the field of the coil causes or the achievement of resonance or escape from it, depending on the type of metal, which leads to an increase or decrease in the amplitude of oscillations on the coil. This method, as well as BFO, was developed by radio amateurs.

    2.3 Autogenerator metal detector

    Known self-generator metal detectors for detecting metallic inclusions in the mass of a non-conductive material are made in the form of a tube generator with a remote oscillating circuit, an inductor that serves as a parametric metal sensor. The disadvantages of the well-known auto-generator metal detectors include their low sensitivity and poor noise immunity [7].

    2.4 Pulsed metal detectors

    The principle of operation of pulsed metal detectors, is based on the time separation of the radiation signals and the reception of the reflected signal. A pulse of a large amplitude (tens or hundreds of volts) is pumped into the coil for a short time, when the metal object is exposed to such impulses, vortex currents appear on the surface of the metal. These currents are the source of signals for the metal detector. Their registration the metal detector produces in intervals between pulses. Therefore, the metal detector coil serves both receiving and transmitting [8].

    Pulsed metal detectors are the most sensitive, among all types of metal detectors, and have the best depth of search.

    2.5 Single-coil metal detector of induction type

    The device of this type has in the sensor one coil of any convenient shape, excited by an alternating signal. The appearance of a metallic object near the sensor causes the appearance of a reflected (re-emitted signal), which induces an additional electrical signal in the coil. It remains only to single out this additional signal [9].

    Conclusion

    Based on the material studied, it is possible to isolate a pulsed metal detector. Its advantages: simplicity in development, they are the most sensitive, the principle of transfer-reception makes it possible to find metals at fairly large depths. Cons: you cannot set up a search for metal discrimination. This type of metal detector was chosen for master's work.

    At the time of writing this essay, the master's work does not yet have a completed status. The final completion is planned for the summer of 2018. The full text of the work and materials on the topic can be obtained from the author or his supervisor after the indicated date. You can contact via e-mail: rozhkov.alex@inbox.ru.

    References

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    9. А. Щедрин Новые металлоискатели для поиска кладов и реликвий. [Электронный ресурс]; Режим доступа: http://izmer-ls.ru/met/sche1-1.html.