Method of detecting small metal objects and device for realising said method

FIELD: physics.

SUBSTANCE: detection zone in which the search object is located is exposed to low-frequency electromagnetic and microwave fields. The value of the low-frequency electromagnetic field sufficient for onset of mechanical vibrations of the search object in the detection zone is selected. The microwave signal reflected from the detection zone and modulated by mechanical vibration parametres of the search object is received. The useful signal with frequency Ω is isolated from the received modulated microwave signal, where Ω is frequency of mechanical vibration of the search object. The device has a low-frequency generator connected to an inductive radiator, an autodyne microwave transceiver with an antenna, a phase detector, a low-pass filter and a display.

EFFECT: increased sensitivity and reliability of detection.

2 cl, 2 dwg

 

The invention relates to the field of radiography and can be used for the detection of small objects made of precious metals, when control passes airports, sea ports and flow of industrial enterprises.

Known inductive apparatus for detecting metal objects in the detection zone [patent FR No. 2145679, MKI G01V 3/10, G08B 13/24, publ. 23.02.1973], containing at least one unit of inductance, the AC power source and the unit of processing of the received signal. The unit of inductance consists of a radiating coil, which is connected via adjustable coil connection with the detector coil. In the absence of a constant magnetic field produced by the radiating coil, a metal object by minimizing the mutual induction between the radiating and the detector coils on the output of the detector coil voltage is zero. An AC voltage source generating the signal in the frequency range from 200 to 4000 Hz, is connected with the radiating coil and the detector coil is connected to the processing unit of the received signal.

The closest technical solution is a device for the detection of metal objects in the search area [patent GB No. 1334295, MKI G01V 3/10, publ. 17.10.1973], comprising a transmitting circuit, a receiving circuit and an output unit. The transmitting circuit includes a master oscillator,which, through the pre-amplifier unit is connected to the transmitting coil. The master oscillator provides the sensing zone search of an electromagnetic signal with a certain level of energy per unit volume of the search area. Reception circuit consists of a receiving coil, the adder of the received signal, a summation transformer connected through one of the pre-amplifier of the transmitting circuit with zero-circuit, and the lock-in amplifier. The output of the selection circuit in the absence of a metal object in the search area offset voltage is zero. The output circuit includes a sensor unit phase, in parallel, connected to a post amplifier, and a block of phasers, United with one of the pre-amplifier of the transmitting circuit. The number of phasers equal to the number of phase sensors. Upon detection of a metal object in the search area there is a change in the amplitude and polarity of the output signal, and the polarity of the signal at the output of each sensor phase is judged on the type of metal (black or color).

The basis of the invention is to increase the sensitivity and reliability of detection of small metal objects that are made in the detection zone.

The problem is solved in that in the method of detection of small metal objects, based on excitation of the detection area of the low-frequency electromagnetic field and receive reflected from the zone detected by the mode signal, according to the invention additionally the detection zone is irradiated with the microwave field, select the amount of low-frequency electromagnetic field sufficient for the occurrence of mechanical vibration subject search in the detection zone, shall receive reflected from the detection zone of the microwave signal, modulated by the mechanical vibration of the object of the search, extracted from the received modulated microwave signal, the useful signal frequency Ω, where Ω is the frequency of mechanical vibration of the object of the search.

The problem is solved also by the fact that in the known device, containing a low-frequency generator connected to the induction radiator, added automatic UHF transceiver with an antenna, the first and second outputs of which are connected to first and second inputs of the phase detector, respectively, and the output of the phase detector through a low pass filter connected to the detection indicator.

Figure 1 presents the scheme of realization of the proposed method.

The subject 1 is placed in a low-frequency electromagnetic field

detection zone, the perimeter of which coincides with the average radius RGinduction emitter 3, fed by low-frequency oscillator 2. Under the action of low-frequency electromagnetic fieldszone about which Eugenia in the subject search 1 induced secondary eddy current I 2, the amplitude of which is equal to:

where- electromotive force induced harmonic current I1in the subject search 1;

µ=4π×10-7GN/m is the magnetic permeability of air;

R is the average radius of the subject 1, m;

RG- the average radius of induction emitter 3, m;

- resistance conductive ring with a cross-section of ∆ h, is equivalent to the subject of the search;

σ is the conductivity of the material of the subject 1, Cm/m;

h is the thickness of the object of search, m;

- the depth of the skin layer in the material of the subject search 1.

The Lorentz force arising from the interaction of the currents I1and I2and causing mechanical vibration of the subject 1 with frequency Ω=2πF and the deflection angle α from the initial horizontal position is determined by the expression:

We substitute the relation (1) in expression (2) and obtain the following equation for the determination of the Lorentz force acting on the subject of search 1:

Moreover, to ensure the vibrational displacement of the subject search 1 must comply with the following condition:

where P=ρπR2h - the weight of the item search 1;

ρ is the specific weight of the material of the object on the ska 1, kg/m3.

For the occurrence of mechanical vibration of the object search 1 frequency Ω=2πF pick up the value of the current amplitudeflowing in the induction emitter 3. From expressions (3) and (4) we obtain the desired value of the amplitude of the current I1to ensure the mechanical vibration of the object search 1:

As can be seen from expression (5), the value of current I1to ensure the mechanical vibration of the object search 1 depends on the frequency Ω and the mean radius of the induction emitter RΓ.

Additionally, the detection zone is irradiated with the microwave fieldfrequency ω=2πf with an antenna connected to the transceiver 4. Then admit reflected from the detection zone of the microwave signalthat is phase modulated by the frequency of vibration of searching Ω:

where Δφ(Ω) is the phase of the reflected microwave signal;

Eand Hthe initial value of the amplitude of electric and magnetic components of the microwave fieldrespectively;

The phase of the reflected electromagnetic signal is determined by the following expression:

where Ω=2πF is the frequency of mechanical vibration of the subject 1;

Α(Ω)=K×P Lthe amplitude of the mechanical vibration of the object of the search;

K - coefficient of proportionality;

λ is the wavelength of the reflected electromagnetic field.

Substituting the value of α=90° in the expression (7), we find the maximum amplitude of the mechanical vibration of the object search 1:

As can be seen from expression (7), the phase of the reflected electromagnetic signal Δφ(Ω) depends on the parameters of the mechanical vibration of the object of the search and, therefore, from the dimensions and electrical parameters of the subject of search 1.

Subsequent phase demodulation of the received microwave signalallows you to extract useful signal containing information about the dimensions and electrical parameters of the subject 1. Changing the frequency Ω, it is possible to identify the subject of search 1 classes. Thus, the proposed method allows to detect with greater reliability small metal objects in the detection zone than in the known method.

Will give a quantitative estimate of the amplitude of mechanical vibration of the object of search 1.

Let the subject of the search is the disk of radius R=5×l0-3m and thickness h=1×10-3feet Material subject search - gold with conductivity σ=5×107Cm/m and a specific gravity ρ≈20×103kg/m Radius of induction emitter R =0.5 m, and the frequency of the primary magnetic field F=1 kHz. We substitute the above data into the expression (5) and get the I1>103Α. If induction emitter to perform in the form of a spiral with n=10 turns of wire, the required amount of current I1will be I1/n=100 A. the Phase shift Δφ(Ω)=1/57 glad≅1° at the wavelength of the primary electromagnetic microwave field λ=10 cm were obtained when the amplitude of the mechanical vibration of the object of the search is equal to

Figure 2 presents a functional diagram of a device that implements the inventive method.

Device detection of small metal objects contains low-frequency oscillator 2 with inductive emitter 3 and automatic microwave transceiver 4 with the antenna. The first output automatic microwave transceiver 4 (figure 2 denoted as ω±Ω with an arrow) connected to the first input of the phase detector 5, and the second output (figure 2 denoted as ω with an arrow) with a second input of the phase detector 5. The phase detector 5 through a low-pass filter 6 is connected to the detection indicator 7. Induction emitter 3 is made in the form of a framework in which current flows. Figure 2 also shows the microwave field, irradiating the detection zone, and the microwave signalreflected from the subject 1, in the cat the rum under the action of low-frequency electromagnetic fields the detection area of the induced current.

The device operates as follows.

Low-frequency oscillator 3 generates a currentfrequency Ω=2πF, flowing in the induction emitter 2, which in the detection zone is induced low-frequency electromagnetic field. At the same time automatic microwave transceiver 4 produces a microwave signalfrequency ω=2πf, through which the antenna irradiates the detection zone. When injected into the detection area of the subject 1, which under the action of low-frequency electromagnetic fieldsinduced eddy currentmechanically vibrates with a frequency Ω=2πF. The amplitude of the mechanical vibration of the subject 1, which occurs under the action of the Lorentz force caused by the interaction of currentsandincreases with increasing current amplitude. Automatic microwave transceiver 4 using antenna receives reflected from the detection zone of the microwave signal

phase modulated frequency mechanical vibration searching Ω. Output automatic microwave transceiver 4 received microwave signalpostopia is at the first input of phase detector 5, to the second input of which is applied the probing microwave signalfrequency ω=2πf. The phase detector 5 connected to the low pass filter 6, selects from the received microwave signaluseful signal frequency Ω is input to the indicator 7.

Thus, the proposed method and device for its implementation can detect with greater reliability small metal objects carried through the detection zone than in the known method.

1. The method of detection of small metal objects, based on excitation of the detection area of the low-frequency electromagnetic field and receive reflected from the detection zone signal, characterized in that additionally the detection zone is irradiated with the microwave field, select the amount of low-frequency electromagnetic field sufficient for the occurrence of mechanical vibration subject search in the detection zone, shall receive reflected from the detection zone of the microwave signal, modulated by the mechanical vibration of the object of the search, extracted from the received modulated microwave signal, the useful signal frequency Ω, where Ω is the frequency of mechanical vibration of the object of the search.

2. Device for the detection of small metal objects that contain low-frequency generator connected to the induction and what suchtelen, characterized in that it further comprises automatic UHF transceiver with an antenna, the first and second outputs of which are connected to first and second inputs of the phase detector, respectively, and the output of the phase detector through a low pass filter connected to the detection indicator.



 

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