Metal detector on the principle of Transmission-Reception, theory. Scheme, description

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Metal detector on the principle of Transmission-Reception, 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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The terms "transmit-receive" and "reflected signal" in various search instruments are usually associated with methods such as pulse echo and radar, which is a source of confusion when it comes to metal detectors. Unlike various types of locators, in metal detectors of this type, both transmitted (radiated) and received (reflected) signals are continuous, they exist simultaneously and coincide in frequency.

The principle of operation of metal detectors of the "transmission-reception" type is to register a signal reflected (or, as they say, re-emitted) by a metal object (target). The reflected signal occurs due to the impact on the target of an alternating magnetic field of the transmitting (radiating) coil of the metal detector. Thus, a device of this type implies the presence of at least two coils, one of which is a transmitter and the other is a receiver.

The main fundamental problem that is solved in metal detectors of this type is the choice of the mutual arrangement of the coils, in which the magnetic field of the emitting coil, in the absence of foreign metal objects, induces a zero signal in the receiving coil (or in the system of receiving coils). Thus, it is necessary to prevent the direct impact of the emitting coil on the receiving coil. The appearance of a metal target near the coils will lead to the appearance of a signal in the form of a variable electromotive force (emf) in the receiving coil.

At first it may seem that in nature there are only two options for the mutual arrangement of coils, in which there is no direct signal transmission from one coil to another (see Fig. 1, a and b) - coils with perpendicular and with crossing axes.

Metal detector on the principle of Transmission-Reception, theory
Rice. 1. Options for the mutual arrangement of the metal detector sensor coils according to the "transmission-reception" principle

A more thorough study of the problem shows that there can be an arbitrarily large number of such different systems of metal detector sensors. But these are more complex systems with more than two coils, suitably connected electrically. For example, in fig. 1c shows a system of one radiating (in the center) and two receiving coils connected in opposite directions according to the signal induced by the radiating coil. Thus, the signal at the output of the receiving coil system is ideally equal to zero, since the emfs induced in the coils are mutually compensated.

Of particular interest are sensor systems with coplanar coils (i.e. located in the same plane). This is explained by the fact that metal detectors are usually used to search for objects in the ground, and it is possible to bring the sensor as close as possible to the ground surface only if its coils are coplanar. In addition, such sensors are usually compact and fit well into protective "pancake" or "flying saucer" housings.

The main options for the mutual arrangement of coplanar coils are shown in Fig. 2, a and b. In the diagram in fig. 2, and the mutual arrangement of the coils is chosen so that the total flux of the magnetic induction vector through the surface bounded by the receiving coil is equal to zero. In the scheme of Fig. 2, b, one of the coils (receiving) is twisted in the form of a "figure eight", so that the total emf, induced on the halves of the turns of the receiving coil, located in one wing of the "figure eight", compensates for a similar total emf. , induced in the other wing of the "eight". Other various designs of sensors with coplanar coils are possible, for example, Fig. 2, e.

Metal detector on the principle of Transmission-Reception, theory
Rice. 2. Coplanar options for the mutual arrangement of the metal detector coils according to the "transmission-reception" principle

The receiving coil is located inside the emitting coil. Emf induced in the receiving coil is compensated by a special transformer device that selects part of the signal from the radiating coil.

Author: Shchedrin A.I.

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