Metal detectors category FD (Frequency Domain), theory. Scheme, description

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Metal detectors category FD (Frequency Domain), theory. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / metal detectors

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Most of the designs of metal detectors known to the author belong to devices of the FD (Frequency Domain) category and use the principle of assessing the change in the electric field under the influence of a metal object. A common feature of such devices is an active coil that generates an electric field. However, there is an exception to this rule: metal object detectors that work on the "transmit-receive" principle. They use two coils: transmitting and receiving.

Separate designs of metal detectors of the FD group differ in the way of analyzing changes in field parameters under the influence of closely spaced metal objects, as well as in the criteria for evaluating these changes.

Among the devices of category FD, the most common, due to the simplicity of circuit solutions, are detectors of metal objects, which are based on the principle of measuring the frequency of beats that occur when two signals close in frequency are added. In specialized literature, such devices are often called BFO (Beat Frequency Oscillator) metal detectors. It must be recognized that when searching for metals with weak ferromagnetic properties, such as copper, tin or silver, BFO metal detectors are less sensitive than devices whose operation is based on other principles.

Metal detectors that use the principle of measuring the frequency deviation of the reference oscillator under the influence of metal objects that have fallen into the search coil coverage area can be divided into a group of devices called frequency meter-based metal detectors. In specialized literature, such devices are sometimes referred to by the abbreviation FM (Frequency Meter). It can be argued that at present, in connection with the development of the element base, devices of this type are experiencing a rebirth.

In the specialized literature, schemes of detectors of metal objects are sometimes given, which are based on the so-called off-resonance or near-resonance principle. In these devices, the change in the frequency and amplitude of the measuring oscillator is analyzed using a filter tuned to a near-resonant frequency, i.e., at the decline in its characteristics. Such devices are often referred to as OR (Off Resonance).

A separate group consists of bridge detectors of metal objects. A feature of the circuit of such devices is that the measuring (search) coil is included in one of the arms of the measuring bridge (at a resonant or near-resonant frequency). In this case, the change in voltage on the reactance diagonal is estimated.

Recently, there has been a significant increase in interest in metal object detectors, the operation of which is based on the so-called "transmit-receive" principle. It should be noted that, in a broad sense, metal detectors using the "transmit-receive" principle include not only devices operating with a continuous signal (FD category), but also devices using a pulsed signal (TD category). The main difference between these two groups of metal detectors lies not only in the form of the signal used. Devices that work with a sinusoidal signal are equipped with two coils - transmitting and receiving. In this case, the coil system is balanced to zero mutual induction. Therefore, such devices are often called balanced metal detectors. In foreign literature, these metal detectors are usually referred to as TR-IB (Transmitter Receiver - Induction Balance) or simply TR. In devices of the TR-IB type, during the search process, the receiving coil receives a signal initiated by eddy currents that occur in a metal object under the influence of a transmitting signal. Analysis of the parameters of the received signal (for example, amplitude and phase shift) is the source of information about the presence and features of metal objects found in the operating area of the device.

Author: Adamenko M.V.

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