Ways to improve noise immunity of magnetometric search tools and their practical implementation..
SHCHERBAKOV Grigory Nikolaevich, professor, doctor of technical sciences,
ANTSELEVICH Mikhail Aleksandrovich, professor, doctor of technical sciences,
UDINTSEV Dmitry Nikolaevich, candidate of technical sciences,
FILIN Vladimir Grigorievich,
VOLOSHKO Vitaly Sergeevich.
WAYS TO INCREASE THE NOISE IMMUNITY OF MAGNETOMETRIC SEARCH DEVICES AND THEIR PRACTICAL IMPLEMENTATION
The article considers ways to practically increase the noise immunity and safety of magnetometric search devices. Their implementation is presented in the form of a prototype of a portable magnetometric search device for detecting covertly worn ferromagnetic objects (weapons, mines and explosive objects) in an unfavorable noise environment.
At present, in the context of criminal and radical groups, increased terrorist activity, the relevance of identifying such objects is determined by the need to solve the problem of ensuring the safety of the population, organizations and enterprises. More and more often we see a picture of manual inspection by security officers of suspicious persons and their belongings in order to search for objects that are a source of increased danger to the population.
The aim of the research in this area is to create technical means of combating terrorism, designed for the rapid and safe detection of firearms, grenades, bladed weapons, disguised under clothing or in luggage.
Currently, law enforcement agencies and private security organizations use various metal detectors that operate on the eddy current (induction) detection principle and are designed to search for metal objects made of both non-ferrous and ferrous metals [1 – 3]. The main disadvantage of such a metal detector is the large number of false alarms due to the reaction to any metal objects (watches, jewelry, confectionery foil, etc.), which significantly reduces the search speed. The ability to distinguish non-ferrous and ferrous metals by phase does not always give a positive effect and depends on the shape and geometric dimensions of the disturbing body.
When it is necessary to inspect a large number of people, for example, during mass events, the low search speed makes it impossible to conduct quality control.
In addition, induction metal detectors are active, i.e., have their own probing fields, search devices, and can lead to the detonation of an explosive device, such as a suicide belt [4].
Modern portable metal detectors designed to detect firearms, grenades, bladed weapons, disguised under clothing or in luggage, must meet the following requirements:
- detect items made of ferrous metals (ferromagnetic materials) only;
- not have their own probing fields.
To detect local field inhomogeneities in non-ferromagnetic covering environments (earth, water, snow, etc.), caused by ferromagnetic objects of artificial origin, the most widely used magnetically sensitive devices are ferroprobe gradiometers or magnetometers [1, 2, 5 – 8]. A similar task arises when searching for steel oil and gas pipelines, sunken equipment, small arms, firearms and bladed weapons, unexploded aerial bombs and artillery shells, most anti-tank, anti-landing and anti-personnel mines. These objects either have their own magnetic field or distort the uniform field of the Earth, and in both cases the magnitude of the magnetic field in the zone of the sensitive element – the ferroprobe changes its magnitude and direction. This is a sign of the presence of a ferromagnetic object. In relation to the sought object, these devices are passive, that is, they do not have any effect on the object.
The emf at the output of the sensitive system of the gradiometer [6] is proportional to the difference in the values of the magnetic field strength at two points in space (Fig. 1) located at a distance l from each other (the gradiometer base). The main parameter of a magnetometer is its sensitivity. Sensitivity is measured by the value of magnetic induction or magnetic field strength that the device is capable of registering. Another, no less important characteristic is the resolution, which determines the minimum difference in the magnetic field parameters that can be registered by the device. Modern magnetometers have a resolution from 0.01 to 1 nT, depending on the operating principle and the class of problems being solved.
The disadvantages of such devices are, firstly, the presence of their own magnetic fields, although significantly smaller than those of induction devices, induced by ferroprobe sensitive elements, which reduce the safety of work near explosive objects; secondly, unstable operation near artificial structures and other objects that distort the Earth's magnetic field. In our case, the measuring system must record local inhomogeneities without reacting to a relatively smooth change in the magnetic field.
Fig. 1. Detection of small-sized ferromagnetic inhomogeneities
in non-ferromagnetic covering media using ferroprobe gradiometers
The first drawback is eliminated by using sensitive elements that do not have their own fields. One of the passive magnetically sensitive elements that best meets the requirements for elements included in portable devices are Hall sensors [10]. The studies conducted have shown that their use made it possible to get rid of their own probing fields and thereby increase the safety of the search.
The second drawback, which significantly limits the scope of application of magnetometric ferroprobe portable search devices, is eliminated by choosing the optimal base length (distance between magnetically sensitive elements).
In [9], a dependence is given that connects the main characteristics of a flux-gate gradiometer with the characteristics of the search object approximated by a ferromagnetic sphere and its detection range:
, (1)
where 
– the difference between the strengths of the external disturbed magnetic field at the centers of the magnetometer ferroprobes, A/m;
H0 – the strength of the Earth's constant magnetic field, A/m;
Rsf – the radius of the ferromagnetic sphere, m;
msf – the relative magnetic permeability of the search object;
mvn – the relative magnetic permeability of the external sheltering medium;
l – the distance between the ferroprobes (base length), m;
rобн – the maximum detection distance by the ferroprobe gradiometer of the search object, approximated by the ferromagnetic sphere, m.
In practice, the most accurate characteristic of a ferromagnetic search object is not its reduced radius, but the mass of the ferromagnetic material. In this case, dependence (1) will take the following form:
, (2)
mоп – mass of the ferromagnetic material of the search object, kg;
rоп – density of the material of the search object, kg/m3.
Fig. 2 shows the dependence of the detection range rобн of the search object (mсф = 100) on the mass of its ferromagnetic material (steel) and the distance between the ferroprobes l of a differential magnetometer with a magnetic field strength resolution of 0.1 A/m.
Fig. 2. Dependence of the detection range rобн of the search object (mсф = 100) depending on the mass of its ferromagnetic material (steel) and the distance between the ferroprobes l of a differential magnetometer with a resolution of 0.1 A/m for magnetic field strength
Analysis of dependencies (1, 2) and the graph (Fig. 2) showed that:
- reducing the length of the base increases the value of the minimum mass of the detected ferromagnetic material and reduces the maximum detection distance of the search object;
- for each base length there is a maximum distance at which the device practically does not detect any object that is an interference;
- the appropriate base length depends on and should be consistent with the expected ferromagnetic mass of the search object and its geometric dimensions;
- the appropriate base length is one that exceeds the reduced radius of the search object by 1.5 — 2 times.
The correct choice of the length of the base of the magnetometric search device allows for the search of local objects in conditions of electromagnetic interference that exist in practice, leading to a smooth change in the external magnetic field.
For example, with a base length of 0.1 m and a magnetic field strength resolution of 0.1 A/m, objects with a ferromagnetic mass (steel) of more than 46 g are detected at a distance of 0.1 m, and more than 165 kg at a distance of 1 m. Accordingly, at the inspection site at a distance of more than 1 m, objects weighing up to 165 kg do not affect the quality of the inspection equipment with the specified characteristics. At a distance of more than 2 m, foreign objects made of steel weighing up to 2410 kg can already be neglected.
In some cases, for example, work in shielded structures, in places where there are a large number of foreign ferromagnetic objects, it is extremely difficult to use the Earth's magnetic field to detect search objects. In this case, it is possible to use your own magnetic field source [11], the distortion of the magnetic field lines of which is recorded by the search device. Experimental studies have shown that to ensure stable operation of the device, regardless of the presence and degree of distortion of the Earth's magnetic field, the intensity of its magnetic field in the search zone must be at least an order of magnitude greater than the total intensity of the external magnetic field.
The theoretical and experimental studies allow us to draw the following conclusions
- It is advisable to use magnetometric search devices as technical means of combating terrorism, designed for the rapid and safe detection of firearms, grenades, bladed weapons, disguised under clothing or in luggage.
- To ensure the safe operation of a magnetometric search device, it is necessary to use sensitive elements that do not have their own electromagnetic fields, or have fields that are not capable of detonating an explosive device due to their characteristics.
- The noise immunity of these search tools is achieved by optimizing the length of the base and introducing its own source of magnetic field.
These ideas are patented [11, 12].
At present, the presented ideas are implemented in the detector of ferromagnetic objects “Zond-F” (photo 1), designed to search for ferromagnetic objects (weapons, mines and explosive objects) under human clothing.
Photo 1. External appearance of the prototype of the detector of ferromagnetic objects designed to search for weapons, mines and explosive objects under human clothing
The main advantages of this magnetometric device over existing induction metal detectors:
- Higher detection speed due to searching only for ferromagnetic products and electronic devices that are in an active state. The device does not react to products made of non-ferrous metals (foil from confectionery and tobacco products, jewelry, coins, etc.).
- High security of searching for explosive devices due to the absence of its own probing fields that cause accidental activation of explosive devices.
- Small dimensions, allowing you to carry the product in a shirt pocket.
The tests conducted showed the potential of using this tool for the accelerated and safe detection of explosive devices, firearms, grenades, and bladed weapons disguised under clothing or in luggage.
The authors would like to thank Stanislav Ivanovich Mironov and Boris Pavlovich Glushchak for their assistance in creating the experimental setup and prototype.
Literature
1. Shcherbakov G.N. Detection of objects in concealing environments. For forensic science, archeology, construction and the fight against terrorism. Moscow: Arbat-Inform, 1998.
2. Shcherbakov G.N. Detection of hidden objects for humanitarian demining, forensic science, archeology, construction and the fight against terrorism. Moscow: Arbat-Inform, 2004.
3. Saulov A.Yu. Metal detectors for amateurs and professionals. St. Petersburg: Science and Technology, 2004, 224 p.: ill.
4. Shikin A.S. How to protect yourself from an explosion. Moscow: World of Security, 1999, 79 p.
5. Arbuzov S.O. Magnetically sensitive search devices./Special equipment, 2000, No. 6.
6. Lyubimov V.V. Diagnostic magnetometers for electromagnetic monitoring in urban conditions and modern methods and means of individual-mass visualization of its results. Review. Preprint No.6 (1116). Moscow: IZMIRAN, 1998.
7. Magnetic exploration. Geophysics handbook./Ed. V.E. Nikitsky, Yu.S. Glabovsky. Moscow: Nedra, 1980.
8. Afanasyev Yu.V. Ferrozones. Leningrad: Energy, 1969.
9. Shcherbakov G.N., Antselevich M.A., Udintsev D.N. Estimation of the maximum detection depth of artificial ferromagnetic objects in the thickness of a semiconducting medium./Special equipment, 2004, 2.
10. Baranochnikov M.L. Micromagnetoelectronics. T. 1. Moscow: DMK Press, 2001.
11. Detector of ferromagnetic objects./Shcherbakov G.N., Antselevich M.A., Udintsev D.N., Mironov S.I. Patent No. 38962 dated 06.02.2004.
12. Detector of ferromagnetic objects./Shcherbakov G.N., Antselevich M.A., Udintsev D.N., Mironov S.I., Glushchak B.P., Filin V.G. Patent No. 42329 dated 08.09.2004.