Pulse metal detector, theory. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Pulse 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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In the types of electronic metal detectors considered earlier, the reflected signal is separated from the emitted signal either geometrically - due to the relative position of the receiving and emitting coils, or using special compensation circuits. Obviously, there may be a temporal way to separate the emitted and reflected signals. This method is widely used, for example, in pulse echo and radar. When locating, the delay mechanism of the reflected signal is due to the significant time of signal propagation to the object and back.

With regard to metal detectors, such a mechanism can be the phenomenon of self-induction in a conductive object. How to use it in practice? After exposure to a magnetic induction pulse, a damped current pulse appears and is maintained for some time (due to the phenomenon of self-induction) in a conducting object, causing a reflected signal delayed in time. It carries useful information, and it must be registered.

Thus, another scheme for constructing a metal detector can be proposed, which is fundamentally different from those considered earlier in terms of the method of separating signals. Such a metal detector is called a pulse. It consists of a current pulse generator, receiving and emitting coils, which can be combined into one, a switching device and a signal processing unit.

The current pulse generator generates short millisecond current pulses that enter the radiating coil, where they are converted into magnetic induction pulses. Since the radiating coil - the load of the pulse generator - has a pronounced inductive character, overloads in the form of voltage surges occur at the pulse fronts of the generator. Such surges can reach tens-hundreds (!) Volts in amplitude, however, the use of protective limiters is unacceptable, since it would lead to a tightening of the front of the current pulse and magnetic induction and, ultimately, to complicating the separation of the reflected signal.

The receiving and emitting coils can be positioned relative to each other quite arbitrarily, since the direct penetration of the emitted signal into the receiving coil and the action of the reflected signal on it are separated in time. In principle, one coil can play the role of both receiving and emitting, but in this case it will be much more difficult to decouple the high-voltage output circuits of the current pulse generator and the sensitive input circuits.

The switching device is designed to produce the above-mentioned separation of the emitted and reflected signals. It blocks the input circuits of the device for a certain time, which is determined by the duration of the current pulse in the radiating coil, the time the coil is discharged, and the time during which short device responses from massive, weakly conductive objects such as soil are possible. After this time has elapsed, the switching device must ensure the transmission of a signal from the receiving coil to the signal processing unit.

The signal processing unit is designed to convert the input electrical signal into a form convenient for human perception. It can be designed based on solutions used in other types of metal detectors.

The disadvantages of pulsed metal detectors include the difficulty of implementing in practice the discrimination of objects by type of metal, the complexity of the equipment for generating and switching large-amplitude current and voltage pulses, and a high level of radio interference.

Author: Shchedrin A.I.

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