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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Pulse metal detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors Creating sufficiently sensitive metal detectors is a rather difficult and thankless task. Radio amateurs periodically take up its decision, present exhibits for the exhibition, but rare of them meet the required parameters. So, for a long time, metal detectors were designed on the basis of two high-frequency generators tuned to close frequencies, one of which was stable in frequency (usually stabilized by a quartz resonator), and the other, the working one, was connected to the receiving frame and changed its frequency when approaching metals. . The signals of the two generators were summed, a low-frequency beat signal was isolated, and it was used to judge the presence of metal. After the appearance of a new element base, instead of reference signal generators, they began to design a metal detector with a voltage-frequency converter, analog-to-digital converters, frequency synthesizers and other possible novelties. Archaeologists and criminologists could be advised another measurement scheme - geophysical. In the area where metal inclusions are searched, a loop of wire with a diameter of 5 ... 25 m or more should be laid out, powered from an autonomous generator with a frequency of 500 Hz (the higher the frequency, the less depth). It is very convenient to use aircraft DC-to-AC converters with a frequency of 400 Hz (umformers). They have sufficient power. You can also use DC-to-AC converters made on powerful transistors. They can be made at several frequencies, and thereby carry out "frequency sounding", i.e., determine the depth of the alleged metal object. To conduct searches, in addition to the generator, it is necessary to have a receiver, which can be a selective amplifier tuned to the frequency (frequencies) of the generator and have a receiving magnetic antenna at the input, also tuned to the frequency (frequencies) of the generator. The idea of this search method is that in the area of the electromagnetic field of the wire loop, any metal bodies of continuous conductivity begin to radiate their field, which is ideally 90 ° shifted in phase relative to the primary one. The receiving frame is usually oriented relative to the primary field so that in the absence of metallic inclusions, the signal at the receiver output would be minimal or completely absent, and in the presence of metallic inclusions it would reach a maximum. Having carried out measurements at several frequencies, it is possible to determine the approximate depth of occurrence, and using receiving frames differently oriented in space, and the location of objects. The main advantage of this measurement method is that the desired metal object itself becomes a source of radiation. Equipment of this kind can be used for tracing underground pipes, laying cables, tracing hidden wiring, and other purposes. To do this, the generator is connected at one end to a traceable metal system, and the other end is grounded (if the search is carried out on the street, in the field) or connected to the pipes of the heating network, water supply (if the trace is carried out in the building). The loop induction method was widely presented at VRV as an application to contactless induction methods for turning on household electrical appliances (contactless headphones for listening to radio, television programs, etc., contactless telephones that are not connected by wires to the telephone network, which can be freely carried in hands while moving around room). It would seem that the task is different, but the principle of the solution is the same: an inductive connection between the loop in which the signal is generated and the receiver that picks up this signal. Pulse metal detector (Fig. 27). The author of the design is radio amateur V. S. Gorchakov. At 33 ER, the exhibit was awarded the Third Prize of the exhibition.
The device is designed to find metal objects in the ground. Its tests have shown that it can detect a 100 x100 x 2 mm aluminum plate at a depth of 75 cm, the same 200 x 200 x 2 mm plate at a depth of 100 cm, a long length steel pipe with a diameter of 300 mm at a depth of 200 cm, a sewer manhole well at a depth of 200 cm, a long steel pipe with a diameter of 50 mm at a depth of 120 cm, a copper washer with a diameter of 25 mm at a depth of 35 cm. The device (Fig. 27, a) consists of a master oscillator 1 at a frequency of 100 Hz, a pulse current amplifier 2, a radiating frame 3, a delay generator 4 for 100 μs, a gating pulse generator 5, a matching amplifier 6, an electronic switch 7, a receiving frame 8 , bilateral limiter 9, signal amplifier 10, integrator 11, DC amplifier 12, indicator 13, voltage stabilizer 14. The metal detector works as follows. The master oscillator emits a pulse of duration Ti (Fig. 27, b), the decay of which triggers the delay generator. The master oscillator pulse is amplified in power by a current amplifier and fed to the radiating loop. The delay generator produces a pulse with a duration of 100 μs, the decay of which triggers the gating pulse generator. This generator generates a strobe pulse with a duration of 30 μs, which controls the operation of the electronic switch through a matching amplifier. The switch opens the signal amplifier for the duration of the gate pulse and passes the signal from the amplifier 10 to the integrator. The signal from the output of the integrator through the DC amplifier is fed to the pointer indicator. On fig. 27b shows the time distribution of signals on the transmitting (radiating) frame (curve 1), on the receiving frame in the absence (curve 2) and in the presence of metal (curve 5). As a result of the experiments, it was found that in the absence of metal, the received pulse decreases rather sharply in amplitude over a time of 100 μs. In the presence of metal inclusions in the control zone, the duration of the decrease in amplitude of the received pulse is significantly delayed, mainly due to the action of Foucault currents. The property of deformation of the shape of the received signal due to the impact of metallic inclusions is the basis for the design of this device. The design of the sensor of the device is shown in fig. 27, c. The emitting and receiving frames are wound on a dielectric frame with an outer diameter of 300 mm. The receiving frame is wound inside the emitting frame. Its inner diameter is 260 mm. The transmitting frame contains 300 turns of PEV-2 0,44 wire, and the receiving frame contains 60 turns of PEV-2 0,14 wire. The fastening of the handle 1 is arbitrary and does not require special explanations. On fig. 28 shows a schematic diagram of the device.
The master oscillator is made on microcircuits DD1.1 and DD1.2. The signal from the output of the generator through the resistor R9 is fed to the input of the pulse current amplifier - transistors VT3-VT5, the load of which is the radiating frame L1.1. Through the capacitor C3, the pulse from the master oscillator is fed to the input of the delay generator, made on the elements DD1.3, DD1.4 according to the Schmidt trigger circuit. The decay of the delay pulse starts the strobe pulse generator, made on the elements DD2.1-DD2.3. The strobe pulse through the matching amplifier (transistors VT1, VT2) is fed to the electronic switch DA1, which controls the operation of the signal amplifier (DA1.1 and DA1.2) and the integrator (C12, R30), passing the DC signal to the DC amplifier (DA2) during the duration of the strobe pulse. The load of the DC amplifier is the pointer device RA1. To increase the stability of measurements, the power supply of the amplifying stages is additionally stabilized. Electronic stabilizers are made on transistors VT6, VT7.
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