Types of metal detectors and criteria for their selection. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / metal detectors

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Depending on the level of service, price and purpose, metal detectors can be divided into devices for beginners, middle-class devices, devices with a microprocessor, deep devices, underwater devices and specialized devices for searching for native gold.

Devices for beginners are intended mainly for solving simple search problems. These are, as a rule, dynamic variants of VLF, i.e. devices that require continuous movement of the search coil. The signal occurs only when the coil moves over the object. If the coil is stopped over metal, the signal will disappear.

Rice. 15. Silver metal detector control panel μMAX by Tesoro

The dynamic principle has its advantages:

• automatic detuning from the "average" ground;
• an extremely simple setting, carried out according to the formula "turned on and went", leading to a reduction in cost and high reliability of the device.

The disadvantages include:

• decrease in sensitivity in comparison with static devices;
• inaccurate localization of the object;
• inconvenience when working indoors or heavily littered places.

Medium appliances class have both static and dynamic modes, although the devices of the 70-80s. more often were purely static.

These devices are characterized by high sensitivity and good resolution. They can be equipped with several interchangeable search coils of various diameters (sometimes oval). The number of controls reaches 8, the procedure for setting them up requires a certain skill and lasts several minutes.

Rice. 16. Tejon metal detector control panel by Tesoro

So, for example, manual detuning from the ground consists in turning the corresponding regulator to achieve the same initial signal with the coil raised and lowered to the ground. Discrimination in such devices is usually multi-stage with the ability to search for all metals (All Metal). The control is complemented by a quick adjustment button in the handle or on the body. Microprocessors are not yet used in these devices - hence the abundance of controls.

Computerized Instruments allow with a high degree of probability to judge the nature of the find. This is possible with the help of the microprocessor used in the device circuit.

Rice. 17. White`s Spectrum XLT computerized instrument

Distinctive features of computerized devices are the presence of a liquid crystal screen (or pointer indicator) and a minimum number of touch keys for entering programs. A program is understood as a set of states of virtual (not displayed on the front panel) controls. These controls are configured using a screen and a few buttons. The memory of the device most often contains several programs for searching in various conditions: on the beach, in the park, in buildings, etc. When searching with such a device, amplitude-phase characteristics are measured for each find, which makes it possible to classify the find. Often, the devices themselves carry out the classification, displaying the result on the screen or on the dial indicator. The number of gradations of the measured parameter varies from 8 to 190, which allows you to select and enter into the device's memory your own program that meets your needs quite accurately when searching in specific conditions. Thus, at your request, you can set any scheme for discriminating unwanted finds.

Depth instruments for searching for large objects (an object with a surface area of ​​​​more than 400 sq. cm is considered large) are produced by almost all major American firms and a number of German firms. There are two types of downhole instruments - RF and IP.

The increase in the depth of RF devices is carried out by spacing the transmitting and receiving coils (flat) in space. Both coils are rigidly fixed to the rod. In this case, one coil in the working position of the device is located parallel to the ground surface, the other is perpendicular. The dimensions of the coils are approximately 25x40 cm. One of the coils emits electromagnetic oscillations with a frequency of 12-82 kHz, the other receives the reflected signal. Pulse depth gauges (IP) have a search coil usually in the form of a square frame measuring 1x1 meter or 1,5x1,5 meters. Depth instruments can only detect objects larger than 10 cm.

Rice. 18. Fisher Geminy-3 deep metal detector

Underwater instruments - these are those that can withstand diving to great depths (up to 80 m), and are also able to exclude the influence of salt or fresh water as a search environment. It should be noted that almost all of the above devices have sealed search coils, which allows them to be immersed in water to a depth of the length of the rod and search in shallow waters, but this does not allow classifying the device as underwater.

Rice. 19. Tesoro Tiger Shark underwater metal detector control panel

Most underwater devices use the pulsed induction principle, which significantly reduces the impact on the search for water, wet and wet soils as an electrically conductive medium. Devices of this class automatically adjust to changing environmental conditions (soil mineralization, water salinity, etc.). However, they do not always accurately distinguish between metals. Of course, all underwater devices can also be used for searching in the air.

Specialized instruments for the search for native gold is a task worthy of its own technical solution. The fact is that when gold is detected by universal VLF/TR devices, small particles of this metal can be mistaken by the device for grains of ferrous metals and not be detected.

Rice. 20. Minelab GPX-4000 metal detector, the main purpose of which is the search for native gold

In addition, gold outcrops are often accompanied by "black sand" containing magnetite, as well as strong mineralization of the soil. To overcome these phenomena, the operating frequency is increased from 2-10 kHz (for universal devices) to 18-70 kHz (for specialized devices for gold prospecting) and the use of elliptical search coils.

Author: Bulgak L.V.

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