Beating metal detector, theory. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Beating metal detector, 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 name "beating metal detector" is an echo of the terminology adopted in radio engineering since the time of the first superheterodyne receivers. A beat is a phenomenon that manifests itself most noticeably when two periodic signals with close frequencies and approximately the same amplitudes are added, and consists in the ripple of the amplitude of the total signal. The pulsation frequency is equal to the frequency difference of the two added signals. By passing such a pulsating signal through a rectifier (detector), a difference frequency signal can be isolated. Such circuitry has been traditional for a long time, but at present it is no longer used either in radio engineering or in metal detectors. Both there and there - amplitude detectors were replaced by synchronous detectors, but the term "on beats" has remained to this day.

The principle of operation of a beat detector is very simple and consists in registering the frequency difference from two generators, one of which is stable in frequency, and the other contains a sensor - an inductor in its frequency setting circuit. The device is adjusted in such a way that in the absence of metal near the sensor, the frequencies of the two generators coincide or are very close in value. The presence of metal near the sensor leads to a change in its parameters and, as a result, to a change in the frequency of the corresponding generator. This change is usually very small, but the change in the frequency difference between the two oscillators is already significant and can be easily registered.

The frequency difference can be recorded in a variety of ways, ranging from the simplest, when the difference frequency signal is heard on headphones or through a loudspeaker, and ending with digital frequency measurement methods.

The sensitivity of the metal detector to beats depends, among other things, on the parameters for converting the change in the impedance of the sensor into frequency.

Usually, the conversion consists in obtaining the difference frequency of a stable generator and a generator with a sensor coil in a frequency setting circuit. Therefore, the higher the frequencies of these generators, the greater the frequency difference in response to the appearance of a metal target near the sensor. Registration of small frequency deviations is a certain difficulty. So, by ear, you can confidently register a frequency drift of a tone signal of at least 10 Hz. Visually, by blinking the LED, you can register a frequency drift of at least 1 Hz. In other ways it is possible to achieve registration and a smaller frequency difference, however, this registration will require a significant time, which is unacceptable for metal detectors, which always work in real time.

The way to allocate a small frequency difference between two generators generates a significant technical problem - phase capture. The problem is that two oscillators tuned to very close frequencies tend to spuriously synchronize with each other. This synchronization is manifested in the fact that when you try to bring the difference frequency of two generators closer to zero in any way, when the difference frequency reaches a certain threshold, there is an abrupt transition to the state of the generators when their frequencies coincide. Generators become synchronized. Physically, the phase lock phenomenon is explained by the nonlinearities that are inevitably present in any generator, and the parasitic penetration of the signal from one generator into another (through power circuits, through parasitic capacitances, etc.). As practice shows, if you do not resort to special tricks such as optoelectronic decoupling of generators, then it is realistic to obtain for the difference frequency the threshold for the onset of parasitic synchronization of the order of 10"4 relative to the generator frequency. maximum sensitivity 10 ... 100 kHz and higher.

Selectivity for metals at such frequencies, which are very far from optimal, is very weak. In addition, it is practically impossible to determine the phase of the reflected signal from the oscillator frequency shift. Therefore, the metal detector has no selectivity on beats.

The response of the instrument to a metal object is inversely proportional to the sixth power of the distance. It is practically the same as that of metal detectors according to the "transmit-receive" principle. However, the detection range of this type of instrument is usually much worse due to the spurious synchronization effect.

Author: Shchedrin A.I.

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