Different types of metal detectors and their principle of operation. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / metal detectors

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To successfully search for metal objects in the ground, it is not necessary to understand the scientific principles of a metal detector. However, it is useful to know in general terms how a metal detector works.

Metal detector is an electronic device that detects the presence of metal and informs us about it. A metal object, say a coin, located in the ground, by itself does not radiate anything and does not betray its presence. To detect it, it is necessary to irradiate it with radio waves and catch a secondary signal. All metal detectors are based on this principle. The difference between cheap and expensive models lies in the methods of emitting these radio waves, in the methods of capturing secondary signals, as well as in the ways of informing you about the presence of metal.

Rice. 12. Electromagnetic field of the search coil

Rice. 13. The occurrence of eddy currents on the surface of metal objects caught in the electromagnetic field of the search coil

When you turn on the metal detector, an alternating current flows in the search coil, creating an electromagnetic field around the search coil. This field passes into the environment, be it air, soil, water, stone, wood, etc. If a metal object is in the path of this field, then so-called eddy currents appear on its surface. These currents form their own electromagnetic field, which weakens the field of the transmitting coil. The instrument's electronic circuitry uses a coil to sense this field weakening caused by the presence of metal under the coil and informs you of this in one way or another. More complex electronic circuits provide better capture of weaker secondary signals, process them more accurately. Therefore, such devices are laborious to manufacture and more expensive. However, they are usually able to find objects at greater depths.

Rice. 14. Influence of soil mineralization on the detection depth

Eddy currents are formed on the surface of any electrically conductive materials - metals, minerals, etc. Non-ferrous metals are more electrically conductive than ferrous metals and minerals. Therefore, eddy currents on them decay longer. The metal detector feels in which case the eddy currents decay faster, and on this basis it can "tell" you which of the metals - black or non-ferrous - is under the coil.

Unfortunately, in some places the soil contains a large amount of electrically conductive minerals (magnetite, sodium and potassium salts), which are highly undesirable, since they mask the presence of the metal, reducing the depth of its detection. Iron and salt minerals are a big problem for manufacturers and users of metal detectors. By applying various filters, you can significantly reduce the influence of the pound. Some instruments have automatic ground balancing, others have it manually set by the operator, which is more accurate if done correctly.

In the literature, the following main approaches to the construction of circuitry for metal detectors:

1. Beat method - BFO (Bcat Frequency Oscillation).

2. Method of induction balance - IB/TR (Induction Balance / Transmitter-Receiver).

3. Induction balance method using very low operating frequencies - VLF/TR (Very Low Frequency/ Transmitter-Receiver).

4. Method of induction balance with spaced coils - RF (Radio Frequency).

5. Pulse method - PI (Pulse Induction).

6. Resonance disruption method - OR (OfTResonance).

Beat method - BFO

The measured parameter is the frequency of the LC oscillator, which includes the search head coil. The frequency is compared with the reference one, and the resulting difference beat frequency is displayed on a sound indication. The circuitry of the devices is quite simple, the coil does not require precision execution. Operating frequency 40-500 kHz. The sensitivity of BFO-devices is low with low stability of operation and a weak ability to tune out from wet and mineralized soil. The BFO method was used in mine detectors and serial foreign devices in the 60-70s. last century. Currently, this method is popular with radio amateurs and is found in inexpensive devices from Russian manufacturers. This also includes devices with direct frequency measurement, which are well implemented on microprocessors.

Induction balance method - IB/TR

The search head is formed by two coils located in the same 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 transmitter coil is often included in the LC oscillator circuit. 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 relatively deep depths (30-35 cm), but they are useless when searching on heavily mineralized soils and sea beaches.

Induction balance method using very low operating frequencies - VLF/TR

It was found that when the operating frequency is lowered below 20 kHz, it is possible to detune from the influence of the pound, the depth of operation of the device is somewhat reduced, but the stability of operation increases sharply and false signals disappear. Such devices are called VLF / TR, which stands for a transmitter-receiver type metal detector operating at very low frequencies.

VLF - method allows you to build highly sensitive devices with good discrimination of metals due to the analysis of phase characteristics. The circuitry of the devices is quite complex, the coils require precision balancing. Most serial devices, including computerized ones, are now being built on the basis of this method. Discrimination of objects and detuning from the ground in such devices is carried out relatively simply using phase-shifting circuits.

The TR principle (or its variation VLF / TR) provides for the analysis of the phase characteristics of the signal, so these devices easily distinguish between ferrous and non-ferrous metals, and are tuned out from debris and soil. They have high sensitivity and resolution, which depends on the diameter of the search coil - the larger it is, the deeper the detection, but the more difficult it is to search for small objects.

The disadvantage of such devices was that the ground balance could not be performed simultaneously with discrimination, and the operator must select either one or the other mode using a switch. Such devices were produced in the USA and England for 10 years until 1980, when they were replaced by the so-called dynamic metal detectors.

At the end of the 70s. XNUMXth century American J. Payne developed a scheme that allows both discrimination and ground detuning to be carried out simultaneously.

The first devices of this type had to be moved very quickly to achieve an acceptable depth of action, which was very tiring for the operator. Later models (due to the complexity of the circuit) made it possible to work with lower coil speeds without loss of depth.

In the early 80s. metal detectors have become heavy and difficult to set up. In essence, one device included four different types of metal detectors. The American company Fisher Research Laboratory promptly responded to the requests of treasure hunters to make a simpler, but no less sensitive device and, based on the latest achievements in microelectronics, developed the 1260s metal detector with automatic threshold adjustment, operating at a very low frequency. It had only a few controls and did not require any manual adjustment. It is lightweight, easy to use and sensitive to small objects, a device that successfully operates on poor mineralized soils. Its modification 1266 was produced until 2003.

This metal detector came to be called "dynamic", although, in essence, it belongs to the VLF / TR type. The previous static VLF/TR type metal detectors practically ceased to be produced, and all the leading companies 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.

Coil Spaced Induction Balance Method - RF

This is a high-frequency version of TR, where the transmitting and receiving coils do not form a flat transformer, but are separated in space and located perpendicular to each other. The receiving coil receives the signal reflected from the metal surface and emitted by the transmitting coil. This method is used in depth instruments and is characterized by insensitivity to small objects and the inability to distinguish between ferrous and non-ferrous metals.

Pulse method - IP

First developed in the United States for archaeologists, these devices were most widely used among amateurs in England in the late 60s. As in devices based on the principle of induction balance, impulse devices create an electromagnetic field that acts on an object, but this field does not act all the time, but periodically - it turns on and off (pulses) repeatedly within one second.

When the field is turned on, eddy currents are induced on the surface of the object. When the field is turned off, the eddy currents gradually decay, albeit for a very short period of time. At this point, the coil acts as a receiving antenna picking up this fading signal. At the same time, the threshold background of the device increases, indicating the presence of metal in the soil. Since the eddy currents of the soil decay much faster and are not captured by the device, pulsed metal detectors work effectively on poor mineralized soils and especially on wet salty soils of sea coasts.

The disadvantage of pulsed metal detectors is their high sensitivity to ferrous metals and difficulties with discrimination. However, in some cases (for example, when searching for metal at the bottom of the sea), they are superior to all other types of metal detectors.

Resonance Break Method - OR

The analyzed parameter is the amplitude of the signal on the coil of the oscillatory circuit, tuned close to resonance with the signal supplied to it from the generator. The appearance of metal in the field of the coil causes either the achievement of resonance or the departure 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, like BFO, was developed by radio amateurs.

Author: Bulgak L.V.

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