Foreign arched metal detectors. Informed choice.

#metal detectors

Foreign arched metal detectors — a conscious choice.

Sinelytsykov Georgy Alexandrovich

Metal detectors for searching weapons during personal inspections have become an integral part of most security systems.

When building such a system, the consumer faces the task of choosing from 30 models and modifications of domestic and foreign metal detectors currently available on the market.

The lack of state and industry standards for metal detectors and methods for testing and evaluating them significantly complicates the ability to correctly select a metal detector for specific purposes and inspection conditions.

However, each consumer can select a metal detector for their own purposes, having certain knowledge in the field of metal detection and the time needed to independently conduct simple tests.

To select a specific model of a metal detector, the user must first determine the objects of the search.

The ability of a metal detector to solve various problems when detecting metal objects allows classifying metal detectors by their application. Defining the search object allows selecting the class of metal detector.

Metal detectors (MD) for personal inspection can be classified as follows:

— general-purpose arched MD (designed to detect metal weapons);
— high-sensitivity arched MD (designed to detect fairly small metal objects);
— arched MDs of very high sensitivity (designed to detect objects made of non-ferrous metals weighing from units of grams);
— hand-held MDs (used in all the above cases as an independent device, as well as an additional localizing device when used together with arched MDs).

Hand-held metal detectors are practically no different from each other in their operating principle, and their main difference is in their consumer and operational characteristics.

When choosing a hand-held metal detector, you should pay attention to the compliance of their declared characteristics with the actual and special requirements of the user. Arch metal detectors of very high sensitivity have a limited scope of application.

They are used as an access and inspection system to prevent theft from enterprises associated with the extraction and processing of precious metals.

The operating principles of such metal detectors are based on the assessment of the difference between the signal measured during the controlled passage of a specific person and the signal from the same person, but “metally pure”, stored in the computer database.

These metal detectors use an integrated access system, are difficult to set up and have a fairly high price ($60-100 thousand).

Arch metal detectors of increased sensitivity and general purpose are similar in their operating principle and the technologies used.

The main quality indicator of a high-sensitivity metal detector is the ability to detect a fairly small search object (this could be a razor blade, a penknife, microchips, some jewelry, metal foil packaging, etc.), and since it is comparable or even much smaller than any personal items, the selectivity indicator in such metal detectors is not assessed.

Organizational measures applied when using such metal detectors include an open inspection point with visual inspection of presented metal objects, providing personnel with clothing and footwear that do not contain or contain the minimum possible amount of metal inclusions, in a satisfactory electromagnetic environment.

High-sensitivity arched metal detectors are most often used at enterprises to prevent theft of products, to inspect visitors to prisons, and nuclear magnetic resonance imaging rooms in hospitals.

Examples of high-sensitivity walk-through metal detectors include METOR124 by Metorex (Finland) and SMD600 by C.E.I.A (Italy).

Most of the metal detectors offered on the market are general-purpose metal detectors.

When choosing a metal detector, it should be remembered that especially at the present time, when the market is going through rather difficult times, the difference in price very clearly tracks the difference in technology.

Of course, the price is influenced by the marketing approaches of companies, but in general, analyzing both the Russian and foreign markets, it becomes obvious that expensive products mean the use of high technology and high detection characteristics.

These are products, when used, high security requirements become a reality.

On the contrary, inexpensive metal detectors are characterized by low characteristics, the use of cheap technology and low-skilled labor. Such products can only be used to ensure a low level of security with the adoption of additional strict organizational measures.

Analysis of the characteristics of different models of metal detectors can be carried out using only the same search objects and a set of personal items.

Very often, when purchasing a metal detector, Russian consumers define the Makarov pistol (PM) as the only search object.

However, as experience shows, after some time of using the metal detector, in 70% of cases, the actual search objects turn out to be smaller than the PM, and requirements for detecting objects made of non-magnetic steel and non-ferrous metals appear.

Therefore, it is necessary to choose a metal detector with a reserve for the future.

Of course, only the consumer can determine the search object necessary for him, but for approximate reference, we can provide a set of search objects, the so-called 3-gun test (see Table 1), defined by the FAA (US Federal Aviation Administration) to evaluate the detection characteristics of metal detectors intended for use in US airports.

By the way, the FAA evaluates metal detectors on a case-by-case basis, rather than certifying them generally for use at U.S. airports.

Table 1. FAA Test Items (3-Gun Test).

These items must be detectable by an arched metal detector with a sufficiently high probability in accordance with the methods of specific test procedures or standards.

NILECJ and ASTM methods are widely used in the world to evaluate the characteristics of metal detectors. NILECJ is the first and quite old standard (developed in 1974), which can be found references in all manufacturers of metal detectors.

It deals with objects that artificially simulate certain types of weapons.

According to many experienced users, this approach to analyzing the characteristics of metal detectors very often distorts the real detection characteristics of metal detectors.

The set of personal items may vary from country to country, from facility to facility, depending on habits, customs, and specific safety requirements.

Usually the set includes metal objects found in the average adult.

The location of these objects during testing should be standard and unchanged from test to test.

One example is the set of personal items defined by the FAA and used in the United States, which is also adopted as the original in the ASTM (American Society for Testing and Materials) standard:

1. Shoes with steel insoles;
2. Keys for the “English” lock (seven);
3. A watch in a metal case and on a metal bracelet;
4. Two ballpoint pens with metal cases;
5. Glasses in a metal frame;
6. Coins (5 pcs. — 1 cent, 3 pcs. — 10 cents, 3 pcs. — a quarter, 4 pcs. — 5 cents);
7. A belt buckle made of non-magnetic material;
8. A pack of cigarettes;
9. A gold wedding ring;
10. Lighter (Zippo, optional)

Each consumer defines their own set of search objects and personal items, since detection tasks may be different.

However, without preliminary determination of the composition of these sets, it is impossible to conduct a full analysis of the metal detector's performance.

The assessment of metal detector detection characteristics is usually based on compliance with the following requirements:

— high throughput;
— 100% or close to it probability of identifying search objects;*
— high selectivity (ability to distinguish between search objects and personal items);
— very low probability of false alarms;**- stability of parameters over time.

Notes:
*The value depends on the specific tests used, methods and depth of statistical sampling for some tests.

**The concept of “false alarms” has different interpretations. In some cases, it is triggered by any objects that are not search objects, in others — by personal items, in others — by external interference, as well as any combination of the above reasons.

Therefore, when assessing, it is necessary to choose one of the interpretations of the concept of “false alarms”

Satisfaction of each of the requirements is influenced by many fundamental and technological features incorporated and implemented in the designs of various metal detectors. Let us consider in more detail the various operating principles of metal detectors and the technological solutions used.

Two principles are used to create an electromagnetic field inside a metal detector:

the principle of a closed elementary circuit and the principle of a matched antenna system.

In the first case, the metal detector coil together with the capacitors of the electronics unit forms the simplest radio circuit, in which sinusoidal oscillations with a steady amplitude are excited.

The introduction of metal objects into such a circuit degrades its quality factor and the amplitude of oscillations decreases, which is recorded in the electronics unit.

Analysis of the oscillation amplitude alone does not allow achieving satisfactory selectivity. Limited dynamic range results in low noise immunity, and the design of the coil of such a circuit (limited ability to stabilize parameters) does not allow achieving uniform field distribution, which generally leads to low system characteristics.

A similar principle of field creation is found in metal detectors manufactured by SAFLEC (South Africa), for example, in the Slimline, PF450 (550), Popular2000 models.

The second principle is used by almost all manufacturers of metal detectors and consists of generating a field by a transmitter coil located in one panel of the metal detector and receiving a field signal by a receiver coil located in the opposite panel of the metal detector.

With this type of field generation, there are more possibilities for different coil designs, and, therefore, better uniformity of field distribution and, as a result, better selectivity and reduction of false alarms.

Another advantage of this type of field generation is the ability to analyze the field in time as it propagates from one panel to another, as well as the field attenuation parameters, which provides a lot of additional information for analyzing signals from metal objects.

All metal detectors considered below use the principle of field generation by means of a matched Transmitter-Receiver pair.

As noted earlier, the design of the transmitting and receiving coil configuration essentially determines the field homogeneity and is one of the main factors for ensuring the requirements for metal detector selectivity, detection probabilities and false alarms.

The choice of one or another coil configuration is made as a result of careful research and prototyping.

The design of the coils determines the uniformity of the field distribution inside the metal detector, as well as the ability to not react to radiation sources located outside it (suppression of “parasitic lobes” of the antenna system).

The effect of homogeneity on the characteristics of metal detectors can be illustrated as follows.

With a uniform field, the field strength is the same at each point of the metal detector, which means that when a reference object is carried, the signal from the object will be the same at each point in the metal detector space.

If the field is non-uniform, which means that there are points (areas) of space with a strength that differs from the neighboring ones, then the signal from the reference object will be different in each area.

In real use, this means the following: setting the metal detector to detect a search object, for example, on the right side of the waist and at chest level, where the field strength can be maximum, can lead to missing the same object at the left side of the waist or knee level with the same settings.

An attempt to set up a metal detector for reliable detection of a search object in all areas leads to false alarms for personal items.

To improve the homogeneity of the field in metal detectors, some special measures are used:

• the use of dual coils in the transmitter and receiver with independent processing of each channel (for example, METOR120, METOR160 manufactured by Metorex (Finland), SCDM2 manufactured by E.T. (Italy), 02PN8 and 02PN10 manufactured by C.E.I..A. (Italy), CHECKGATE 8000 manufactured by Control Screening (USA)). Moreover, the coils in the METOR120, METOR160 metal detectors create an electromagnetic field in mutually perpendicular planes, which significantly improves the detection of linearly extended objects, such as knives, reinforcement bars, etc.
• the use of pairs of transmitting and receiving coils connected in opposite directions. Thus, it becomes possible to determine the location of the search object in relation to the panels (used in metal detectors MT5500 manufactured by GARRETT (USA), CHECKGATE 9000 manufactured by Control Screening (USA), Intelliscan 6000, 9000, 12000 manufactured by Ranger Security (USA)). It should be noted that this method of compensating for heterogeneity does not provide a significant improvement in the results in the presented models, since it is initially based on an insufficiently good design of coils and is used as an inexpensive way of “patching holes”.
• the use of multi-coil systems with spatial separation of search areas. Independent sensitivity adjustments in such systems allow compensation for field inhomogeneities in metal detectors that arise due to interference, and spatial signal processing allows increasing selectivity for distributed personal items. The multi-zone principle is partially applied in the PMD2 metal detector manufactured by C.E.I.A. (Italy) (three transmitting coils and five receiving coils with coordinated switching and special processing are used), and completely — in metal detectors METOR200 (eight physical detection zones) manufactured by Metorex and Intelliscan 12000 (six zones) manufactured by Ranger Security.

The uniformity of the field is affected by the quality of the coil assembly, since deviations in the laying of the turns lead to a change in the total field strength of the coil and, naturally, the entire metal detector.

The use of robots in winding metal detector coils by C.E.I.A., Metorex, ET allows achieving absolute repeatability of the coil design in each panel copy.

Differences in uniformity are determined only by the original design.

An analysis of the coil winding of the Ranger Security Intelliscan 12000 metal detector yields depressing results: hand-wound coils secured to a sheet of plywood with adhesive tape not only do not provide repeatability, but also force the manufacturer to select pairs of coils for a specific specimen only to achieve acceptable field uniformity.

All the discussions about the design of the coils and the quality of their assembly are true for both the panel and column versions.

Column-shaped coils are produced by Italian manufacturers along with the usual panels.

Columns are good only from an aesthetic point of view, they are light, do not block the inspection space, save installation space.

This is where all their advantages are exhausted. You have to forget about the homogeneity of the field, selectivity drops sharply, the number of responses from items allowed to be carried increases.

The columns can be used in installation locations with low security requirements and a small flow of visitors; for installations with higher security requirements, manufacturers recommend the same metal detector models, but with panel-shaped coils.

The type of magnetizing field also influences the selectivity and probability of detection and false alarms.

Metal detectors use two methods of forming a magnetizing field — the harmonic field method and the pulsed field method.

(For more details, see ST No. 4-5, 1998).

In the harmonic type, the field is excited by a sinusoid of one frequency (harmonic).

In the pulse type, excitation is carried out by a pulse close to a rectangular shape. Each of these types has its own disadvantages and advantages.

First of all, these methods have different information content.

Harmonic oscillation has only two parameters for subsequent analysis: the amplitude and phase of the re-radiated field.

When the field is pulsed, the transient characteristics of the entire spectrum of the Fourier components of the rectangular pulse are used for analysis. This leads to a significant increase in the collected information about the characteristics and dimensions of the metal object being examined.

You can find statements from some metal detector manufacturers that the harmonic principle is tens of thousands of times more informative than the pulse principle due to the fact that the generated field is continuous.

A high school physics course shows that these statements are completely meaningless, since in a harmonic signal, i.e. a sine wave, the phase change can be measured no more than twice per signal period, and the peak amplitude values ​​when carrying a metal object cannot change more often than the signal frequency (the speed of the object is much less than the frequency of the electromagnetic field).

So, if the frequencies for different types of field generation do not differ by a thousand times, and they are within 100 Hz-3 kHz for all manufacturers, then there is no gain due to the “continuity of the sine wave”.

On the contrary, the information content is much higher in pulse systems, since they can analyze an infinite number of frequencies.

Pulse systems are much more resistant to vibrations, since when the components of the re-radiated field signal are processed together, it is possible to compensate for changes caused by the vibrational movement of the coils.

Due to their instability to vibrations, metal detectors using the harmonic principle are usually 30-50% heavier than pulse metal detectors, which on the one hand makes them rigid, but on the other hand further worsens their resistance to impacts.

The advantage of harmonic metal detectors is good noise immunity, since it is possible to effectively filter out noise with frequencies different from the excitation frequency. However, in practice, this advantage is practically not realized.

The noise immunity of pulse and harmonic metal detectors is approximately the same.

A practical gain has been achieved in the 02PN8, 02PN10 and PMD2 metal detectors manufactured by C.E.I.A..

When these metal detector models are installed in a row, they can be placed at a distance of 5 cm from each other, naturally spreading out the field generation frequencies.

If harmonic metal detectors are used to excite the field simultaneously with several specially selected frequencies, then it may be possible to bring their detection characteristics somewhat closer to pulse systems.

However, today all manufacturers of harmonic metal detectors use the monofrequency excitation method.

Only a few models of metal detectors using the harmonic field generation method are presented on the Russian market.

These are 02PN8, 02PN10 and PMD2 manufactured by C.E.I.A. and Intelliscan 12000 manufactured by Ranger Security.

All other manufacturers in the world use the pulse field generation method.

The pulse field generation method of MD seems to be the most promising in development.

When using transient analysis methods (assessment of the attenuation of the electromagnetic field in a metal detector after the excitation pulse has ended), there is a theoretical possibility of drawing a conclusion about the exact shape and electromagnetic characteristics of the object being studied.

This is achieved by measuring the amplitudes of transient characteristics at different frequencies with different delays relative to the trailing edge of the excitation pulse.

By accumulating the measurement results for several pulse cycles when the search object passes through the metal detector, and applying the necessary processing, it is possible to obtain an electromagnetic image of the object.

By comparing it with images from the existing library, it is possible to quite accurately identify objects that are search objects.

In a small volume, as far as technical capabilities and final cost allow, this measurement technology is used in METOR metal detectors manufactured by Metorex.

For ease of comparison of technologies, the metal detectors mentioned in the article, as well as other models most widely known in the Russian security market, are listed in Table 2.

It should be noted that the table does not list all MD models or all metal detector manufacturers whose products can be found installed at some sites in Russia.

The article reflects the most well-known models or those offered by well-known MD suppliers in Russia.

When choosing a metal detector, its cost is an important factor. However, the relatively low price of an MD is not an obvious and clear advantage.

If saving even twice as much on cost leads to the impossibility of solving newly emerging inspection tasks, and as a result, possible threats to life, additional costs for repairs and salaries of security personnel duplicating the work of such a metal detector, then the preference for a low price was wrong.

Human life, reliable functioning of public and private structures, protection of material assets are much more important than apparent momentary savings.