Metal detectors — weapon detectors.
David Pavlovich Berezansky, Candidate of Technical Sciences
Source: magazine «Special Equipment» ;
Currently, devices that use magnetic methods to solve problems of detecting conductive objects in a non-conductive environment have become widespread in various areas of human activity.
Metal detectors (metal detectors, metal detectors) are used today in flaw detection (searching for metal inclusions in various materials), ore electrical exploration, in access control systems, theft prevention, etc.
The varieties of magnetic methods are induction eddy current with different types of magnetizing field and magnetoelectric using the natural geomagnetic field of the earth or an artificial magnetic field.
The article discusses the operating principles and features of metal detectors designed to detect weapons and explosive devices on people visiting protected facilities. Eddy current methods are most widely used in such devices today.
A metal detector must solve the problem of selectively detecting certain metal or metal-containing search objects (SO) against the background of metal personal items (MPI) usually found on visitors.
Selective detection is the ability to establish the fact of the presence of an OP against the background of the simultaneous presence of a PLP and not to give false alarms from the PLP in the absence of search objects. Selective detection can only be carried out if the OP has characteristic features.
These features are understood as any of their constant properties, revealed in one or another physical method implemented in the metal detector, according to which there are the greatest differences between the OP and the main part of the set of PLP.
Let's consider the eddy current method in more detail. It is based on the presence of the OP's main features inherent in metals: electrical conductivity and magnetic permeability.
Eddy currents are closed currents flowing in a conductive medium and induced in it by a changing magnetic field. Eddy currents are excited by an alternating electric field created by a special coil through which an alternating electric current flows. Electromagnetic energy penetrating a metal object is partially converted into heat and partially re-radiated.
Depending on the type of magnetizing field being formed, a distinction is made between the harmonic field method and the pulsed field method (transient process method).
From the fundamentals of harmonic analysis it follows that with the same harmonic composition of the magnetizing field, it is possible to obtain the same amount of information about the electromagnetic characteristics of a magnetized OP both in the frequency domain, by measuring the amplitudes and phases of the harmonics of its re-radiation field, and in the time domain, by studying the time course of this field.
When using the harmonic method, the OP is magnetized by the sum of harmonic fields of no more than three (usually two) frequencies.
When using the transient process method, magnetization is performed by complex-shaped pulses, which are theoretically the sum of an unlimited number of harmonic fields with frequencies that are multiples of the fundamental pulse repetition frequency.
HARMONIC MAGNETIZATION
A metal object placed in a harmonic magnetic field itself becomes a source of an alternating magnetic field changing with the same frequency.
The characteristic features of OPs are the features of their amplitude-frequency (AFC) and phase-frequency (PF) characteristics.
That is, the electrophysical properties of the materials of the search object, as well as the geometric dimensions of its elements, lead to the fact that at a certain value of the frequency of the magnetizing field, the amplitude and phase shift of the signal re-emitted by the OP will, at a specific orientation, differ from the set of PLPs.
Let's consider this using the following example.
The phase shift of the field re-emitted by a metal object is greater for a massive object, to which the OP is closer, than for a thin-walled one, which is more typical for PLPs.
This is due to the effect of the eddy current reaction on the magnetizing field, flowing closer to the metal surface. With depth, the electromagnetic field strength decreases due to surface eddy currents.
These currents have a shielding effect on the field penetration, which simultaneously causes their weakening and an increasing phase shift with depth relative to the magnetizing field.
The penetration depth of electromagnetic fields and eddy currents into the metal depends on the frequency:
where: f is the frequency, s is the electrical conductivity, m is the magnetic permeability.
The formula shows that the depth of penetration of eddy currents into the metal decreases with increasing frequency.
Therefore, at high frequencies, a massive metal object and a thin-walled one (of the same area and shape, made of the same material) will be sources of the same re-radiated fields.
That is, at high frequencies it is impossible to distinguish a massive object from a non-massive one.
The theory of the eddy current method makes it possible to determine the change in the active and reactive components of the complex resistance of the coil at different frequencies of the magnetizing field depending on the electrical conductivity, size and shape of the object placed in the coil.
The theory is based on Maxwell's equation.
From the solution of this equation, a number of formulas follow, on the basis of which it is possible to obtain a family of dependencies of the complex resistance of the coil on the electrical conductivity, magnetic permeability of the material and the dimensions of the object placed in it.
These dependencies show that there is a maximum reactive component of the complex resistance of the coil, corresponding to certain parameters (dimensions, material) of the object located in it.
Let us consider the influence on the characteristics of the magnetic moment induced in a conductive object, the type of material from which it is made.
Let a round, thin, flat non-ferromagnetic disk with radius r, thickness l , having electrical conductivity g be magnetized by a uniform sinusoidal electromagnetic field in time.
The field is directed perpendicular to the plane of the disk and has the parameters: amplitude Hm, circular frequency w .
Then, at certain frequencies (w < w res. x 0.5) of the magnetizing field, the eddy current causes the appearance in the disk of an induced magnetic moment P with an amplitude of:
lagging in phase by approximately 90° from the magnetizing field.
At some frequencies (large w res.) the eddy current lags in phase by 180° behind the magnetizing field, and therefore the magnetic flux it creates through the plane of the disk is directed towards the induction flux of the magnetizing field and almost compensates for it (complete compensation at w = ? ).
In this case, the magnetic moment lags by 180° behind the magnetizing field, and its amplitude is determined by the expression Рm = 6 x r3 x Hm.
Both given dependencies for the amplitude of the magnetic moment are preserved for a square with a side of a = 2r.
For a thin flat rectangular plate of thickness d with side dimensions a x an, where n is an arbitrary integer, the amplitude of the magnetic moment will be n times greater.
With certain assumptions, these dependencies are also preserved for objects of more complex shape.
Consequently, the induced magnetic moment of a non-ferromagnetic conducting object is mainly determined by the third and fourth powers of its smaller size in the plane perpendicular to the magnetizing field and depends to a much lesser extent on its other geometric characteristics.
Let us consider the features of magnetization of a ferromagnetic conductive object.
With a change in the frequency of the magnetizing field, the total vector of the magnetic moment of the object may first increase somewhat in magnitude, and then with an increase in the frequency of the magnetizing field it will decrease.
And will become significantly less than the value corresponding to zero frequency.
It follows from the above that the component of the induced magnetic moment, in phase with the magnetizing field, changes sign for a ferromagnetic object with increasing frequency, while it does not change for a non-ferromagnetic object.
The quadrature component of the magnetic moment always has the same sign for both a ferromagnetic and a non-ferromagnetic object. This makes it possible to distinguish between these objects.
In addition to the type of material, the location of the object relative to the coil has a different effect on the value of the complex resistance of the coil.
The dependence of this coil characteristic on the listed parameters accordingly affects differently the amplitude and phase of the EMF induced in it under the action of the field re-radiated by the object.
When such a coil is included in the corresponding measuring circuit, it becomes possible to isolate signals and evaluate their parameters, which are most characteristic of the detected OPs.
The conducted studies in the harmonic magnetic field of the characteristics of such an OP as the Makarov pistol showed that:
- the phase of the signal weakly depends on its orientation in the electromagnetic field (changes of no more than 5 — 7°),
- the amplitude of the signal changes depending on the orientation up to 10 — 12 times.
The presented data show that selection in a one-dimensional field only by amplitude does not provide acceptable detuning from the PLP.
In addition, the amplitude of the signal from the re-radiated field depends significantly on the distance between the object being examined and the coils.
The maximum signal corresponds to the object being located near the receiving or emitting coil, and the minimum to a position in the middle between them.
Special measures are used to equalize the sensitivity of the receiving coil across the width of the metal detector passage:
- they create designs for the emitting and receiving coils that provide cross electromagnetic fields in the controlled area;
- place the transmitting and receiving coils on both sides of the OP and process the data from the pair of receiving coils using special algorithms.
However, it is not possible to completely eliminate the unevenness of the sensitivity topography.
Figure 1 shows the functional diagram of a metal detector using harmonic magnetization.
Fig. 1. Functional diagram of a metal detector with harmonic magnetization.
With the harmonic method, the OP re-radiation field is measured against the background of a magnetizing field that exceeds it in amplitude by thousands and millions of times.
Therefore, a compensator is used in the metal detector, eliminating the signal induced in the receiving coil by the magnetizing field.
The threshold device evaluates the amplitude and phase shift of the re-radiation field of the OP, recorded by the receiving coil.
PULSE MAGNETIZATION
The characteristic features of the OP when using this method are the duration and type of the attenuation process of eddy currents in the object being examined, transferred to the signal induced in the receiving coil by the re-radiated field.
Instantaneous values of the transient response for different moments in time, as well as the result of their joint processing using special algorithms selected for recognizing the OP, can be used as selection criteria.
Theoretically, it is possible to obtain an unlimited amount of information about the electromagnetic characteristics of the OP by strobing the transient response of re-radiation in as much detail as desired.
In addition, at the moment of measurement, the magnetizing field is switched off and does not interfere with the assessment of the re-radiation field. However, the possibilities of technical implementation of the transient process method significantly reduce its detection and selective parameters.
When using this method, the ideal magnetizing field is one that changes according to a rectangular law.
However, in practice, this is currently impossible to achieve. The emitting coil has self-induction, which in devices designed for human inspection, where it is necessary to create a magnetic field in a significant space, can be tens of millihenries.
And to obtain the maximum intensity of the magnetizing field with limited dimensions (mass) of the coil and energy consumption, the active resistance of the coil is minimized (no more than units or tens of ohms).
The current in the coil connected to the rectangular pulse generator will increase exponentially with a time constant
t = L /R
With the limitations discussed above L and R (L » 0.01 H, R » 5 ohms) the time constant will be no less than units of ms. Consequently, the duration of the leading edge of the magnetizing field pulse will also be units of ms.
The trailing edge of the magnetizing current pulse depends on the response speed of the power keys that break the circuit of this current, and to an even greater extent on the conditions of the absence of damped oscillations of the magnetizing field after the current is turned off.
Under such conditions, the duration of the trailing edge of the magnetizing field wave can actually be no less than 10-4 sec.
Therefore, with pulse magnetization in a real metal detector, the maximum frequency of harmonic components will not exceed 10 kHz.
Currently, pulse magnetization with a field waveform in the form of half-sine segments (or a combination of such segments) has become widespread.
In this case, the time from the moment the magnetizing field is switched off until the moment of measurements should be no less than 10-4 sec.
In addition to the time constant of the magnetizing circuit (in the de-energized state), it is also necessary to take into account the time constant of the receiving coil that perceives the re-radiation field of the OP.
To prevent the occurrence of damped oscillations, this constant should also be no less than a certain value.
Based on this, the upper limit of the frequency range of the OP re-radiation field when using the transient process method, as well as for the magnetizing field, does not exceed 10 kHz.
Figure 2 shows the functional diagram of a metal detector that uses pulse magnetization.
Fig. 2. Functional diagram of a metal detector with pulse magnetization.
The delay unit ensures that measurements are taken after the excitation field pulse has ceased to act.
COMPARISON OF HARMONIC AND PULSE MAGNETIZATION METHODS
The frequency response and phase response of a significant number of OPs and PLPs have been studied in sufficient detail at present.
Figures 3–5 show the dependences of the induced magnetic moment P and its phase shift j relative to the magnetizing field on the frequency.
Fig. 3 Frequency dependence of the magnetic moment component P, in-phase with the magnetizing field
In the figure: PM1, PLP1 — for the PM pistol, a bunch of keys when the magnetizing field is oriented along the longest side of the objects; PM2, PLP2 — for the same objects when the magnetizing field is oriented across the plane of the objects.
Fig. 4 Frequency dependence of the quadrature component of the magnetic moment P.
The designations are the same as in Figure 3.
Fig. 5 Frequency dependence of the phase shift of the magnetic moment P relative to the magnetizing field.
The designations are the same as in Figure 3.
From these characteristics it is evident that the amplitude and phase shift of the induced magnetic moment change noticeably enough only when the observation frequency changes several times.
Therefore, detailed discretization of the transient characteristics of the OP and PLP does not provide additional information about their electromagnetic properties.
That is, the transient process method does not provide a noticeable advantage over the harmonic method in terms of selection in the case of using two — three rationally selected frequencies in the latter.
Based on the above, we can list the main advantages and disadvantages of the considered magnetization methods.
For the harmonic method:
Advantage — high noise immunity, due to the possibility of effective filtering in frequency ranges other than operating ones;
Disadvantage— the need for significant rigidity of the coil structures and protection from vibrations and contact with visitors.
Examples of metal detectors that use the harmonic method are the following models: 773 LF (Rens Manufacturing Co, USA), MP 1783 (Valon GmbH, Germany), Intelliscan 12000 (RANGER, USA).
For the transient process method:
Advantage— absence of high requirements for the rigidity of the coil structure and relative independence from small movements and vibrations.
Disadvantage — lesser ability to combat interference than the harmonic method. However, the use of pulsed magnetization with a field waveform in the form of half-sine segments significantly reduces this disadvantage.
Examples of metal detectors that use the transient process method are the following models: Metor-200 (Metorex International Oy, Finland), PMD 2 (C.E.I.A, Italy), Poisk-3 (Russia), Rubezh-2 (Russia).
Currently, serially produced models of metal detectors that use the transient process method significantly exceed the models with harmonic magnetization in quantity.
This is largely due to the above-mentioned relative independence of the number of false alarms from shaking and movement of the coil systems of the products.
Continue reading: Metal detectors — weapon detectors. Review of operating principles.