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The most famous author of designs of metal detectors belong to devices category FD (Frequency Domain) and use the principle of estimating the change the electric field under the influence of a metal object. A common feature of such device is active coil forming an electric field. However, from to this rule there is an exception: metal detectors working on the principle of "transmit-receive". They use two coils: transmission and reception.

The individual design of the detectors of the group FD differ in the method of analysis changes in the parameters field under the influence of closely spaced metal items and criteria for evaluation of these changes.

Among the appliances category FD is the most common due to the simplicity circuit solutions are metal detectors, based which is based on the principle of measuring the beat frequency arising from the addition of two close in frequency signals. In the specialized literature such devices often called the metal detector BFO (Beat Frequency Oscillator). Need to acknowledge that when searching for metals with a weak ferromagnetic properties, such as copper, tin or silver, BFO metal detectors have a lower sensitivity compared with the devices, which are based on other principles.

Metal detectors that use the principle of measuring the frequency deviation reference oscillator under the influence of metal objects, caught in the zone the validity of the search coil, it is possible to allocate to a device group called metal detectors on the basis of frequency. In the specialized literature such devices sometimes referred to by the acronym FM (Frequency Meter). It can be argued that at the present time, in connection with development of element base, devices of this type are experiencing a rebirth.

In the literature sometimes contains diagrams metal detectors items, which are based on the so-called unresonably or near-resonance principle. In these devices, change frequency and amplitude measurement of the generator is analyzed using a filter that is configured on near-resonance frequency, i.e., at the decline of its characteristics. Such devices often referred to by the acronym OR (Off Resonance).

A separate group of bridged metal detectors. Feature diagram of these devices is that measuring (search) the coil is turned in one arm of the measuring bridge (resonant or near-resonance frequency). When this estimated change in the voltage on diagonal reactance.

In recent years significantly increased the interest in metal detectors subjects whose functioning is based on the so-called principle "transmit-receive". Note that in a broad sense to the detectors, using the principle "transmit-receive" include not only the device working with a continuous signal (FD), but also devices that use pulse signal (TD). The main difference between these two groups metal detector is not only used in the form of a signal. Devices operating with a sinusoidal signal, is equipped with two coils - transmitting and receiving. The system of coils is balanced to zero mutual induction. Therefore, such devices often referred to as balanced metal detectors. In foreign literature, these detectors usually referred to by the acronym TR-IB (Transmitter Receiver - Induction Balance) or just TR. In devices of the type TR-IB in the process of finding the receiving coil a signal initiated by the eddy currents arising in metal the subject under the influence of the transmitting signal. Analysis of the parameters received signal (such as amplitude and phase shift) and is a source of information about the presence and characteristics of metal objects discovered in the work area device.

Author: M. V. Adamenko