Pulse metal detectors PI (Pulse Induction), theory. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors
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Like radar metal detectors, pulse metal detectors are classified as TD (Time Domain) devices that use a pulsed signal (Fig. 1.6). In this case, the repetition rate of pulses generated in these devices ranges from several tens to several hundreds of hertz.
Rice. 1.6. Simplified block diagram of a pulse metal detector
In pulse metal detectors of the PI (Pulse Induction) type, to assess the presence of metal objects in the search area, the phenomenon of the occurrence of eddy surface currents in a metal object under the influence of an external electromagnetic field is used. However, in contrast to the TR-IB type devices considered earlier, pulsed metal detectors analyze the signal formed in the metal after exposure to a pulsed rather than a continuous signal.
The pulse signal generated by the pulse generator is amplified and fed to the transmitting coil, in which an alternating electromagnetic field is initiated accordingly. When a metal object appears in the zone of action of this field, eddy currents periodically arise on its surface under the influence of a pulsed signal. These currents are the source of the secondary signal, which is received by the receiving coil, amplified and fed to the analyzer. It should be noted that due to the phenomenon of self-induction, the duration of the secondary signal will be greater than the duration of the pulse emitted by the transmitting coil. In this case, the parameters of the trailing edge of the secondary pulse signal and are used for analysis with subsequent generation of data for the display unit.
It is easy to assume that if there is a special decoupling device or a commutator in pulsed metal detectors, instead of the transmitting and receiving coils, only one coil could be used, which would be alternately used to transmit and receive signals (Fig. 1.7).
Rice. 1.7. Simplified block diagram of a single coil pulse metal detector
The main advantages of pulse metal detectors are relatively high sensitivity, as well as the simplicity of the design of the coils. However, circuit solutions for individual blocks (for example, a pulse generator, switch, analyzer) are still quite complex. Among other things, such devices use microprocessors with appropriate software. Microprocessor programming also requires appropriate equipment and skills. Therefore, only trained radio amateurs will be able to assemble such a design.
Author: Adamenko M.V.
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