Metal detectors. Issues of standardization of requirements.
Berezansky David Pavlovich, Candidate of Technical Sciences
Metal detectors — inspection devices.
Issues of standardization of requirements.
The article is reprinted from the journal «Special Equipment» No. 2, 1998.
For a wide range of readers of the magazine, a number of problems on standardization of requirements for metal detectors, as one of the important types of inspection equipment, are proposed for discussion. In conditions when in the Russian Federation there is still no State standard regulating regulatory requirements for metal detectors as inspection devices, the issues set out in the article are relevant.
Metal detectors — detectors of items prohibited for carrying to these facilities — play an important role in organizing the protection of buildings, structures, and monitoring and regulating the movement of people.
Metal detectors, according to the classification accepted today [1], are classified as inspection devices that are part of integrated security systems or used autonomously. Structurally, such metal detectors can be built into barrier devices of access control and management systems, autonomous stationary or autonomous portable.
Items prohibited for visitors to bring into protected facilities and detected by a metal detector (hereinafter OP — search objects) primarily include:
— firearms;
— hand grenades;
— knives,
The functional purpose of metal detectors is generally quite broad: from searching for non-ferrous metals weighing ~ 1 g to detecting objects made of ferromagnetic metals that significantly exceed the OP by weight. The tasks that metal detectors solve in protection and security systems are narrower and have their own characteristics that determine specific requirements for such devices. These include:
— reliable detection of OP;
— ensuring selectivity with respect to metal objects permitted to be brought into the protected facility;
— ensuring noise immunity in working conditions at the protected facility;
— ensuring special security.
To date, there is no state standard in Russia that regulates most of the requirements discussed above. As a result, consumers of metal detectors suffer. Having purchased a product, the documentation for which provides only qualitative assessments of its main characteristics (reliably detects weapons, does not react to small metal objects, is resistant to interference, etc.), the consumer, having discovered that the metal detector does not suit him in terms of actual capabilities, cannot make a claim to the manufacturer or seller. A situation often arises in which the purchased universal metal detector has various operating programs. However, the operating documents do not indicate the characteristics provided by the product when using the programs implemented in it. And even references to the installation of the security level according to the international standard “NILECJ-SCD 0601-00 Security level 1…5” [2] do not allow the domestic consumer to decide on the choice of mode.
From the above it follows that there is a need to develop a Russian standard for metal detectors — inspection devices. Let us consider in more detail each of the requirements for such devices.
Detection characteristics.
The OPs considered above have different sizes, weights, shapes, electrical conductivity, and magnetic permeability. In a metal detector, the signal at the output of the receiving antenna depends on both these characteristics and on the location of the OP relative to the antennas. Accordingly, the detection characteristics of the metal detector should be assessed taking into account the factors listed above.
The conducted studies allowed us to develop a number of test samples with design parameters generalized for OP [3]. The use of such test samples or real OPs allows us to estimate the probability of their detection by a metal detector with different spatial orientations of the OP in the most probable locations on a person.
Selective characteristics.
Selective characteristics, as applied to metal detectors — access devices, are considered the probabilities of passing objects of smaller size and weight than the OP, permitted to be carried into the protected facility (hereinafter PLE — personal use items), or the probability of a false alarm of the product.
The specified characteristics in a metal detector are directly related to the probability of detecting an OP. The electromagnetic field is significantly non-uniform across the width of the metal detector passage. Even the use of a special coil configuration and special signal processing from the receiving antenna does not significantly improve this non-uniformity. Therefore, when carrying the same object at the same angle in the immediate vicinity of the coils and in the middle of the passage, the specified signals can differ by 2 … 4 times. Thus, the signal from the OP in the middle of the passage is comparable with the signal from the PLP carried near the coils. In addition, most of the metal detectors under consideration have flat antenna systems and are sufficiently sensitive in one or, at best, in two directions. Therefore, to ensure confident detection at any orientation of such OPs as a pistol or a knife, which have significantly different sizes in different directions, a low sensitivity threshold is required, which reduces the selective characteristics of the metal detector.
Selectivity also has a purely psychological significance for security personnel at facilities equipped with metal detectors. From the author's practical observations, it follows that frequent false alarms from PLPs reduce the security personnel's attention to any alarm signal, including when triggered by OPs. The use of metal detectors with low selectivity is most often just a pretext for security personnel to conduct a complete search of visitors.
Noise immunity.
A metal detector installed in a protected area is exposed to a number of external conditions (interference) that make it difficult or impossible for it to perform its functions. Interference is divided into electromagnetic and that caused by the presence of large masses of metal, closed circuits, moving or stationary, near the antennas. The most common sources of electromagnetic interference are:
— power grids and their switching equipment;
— operating power electrical devices (electric motors, electric generators, transformers);
— fluorescent lamps;
— monitors;
— televisions, etc.
The presence of electromagnetic interference can lead to false alarms, which in some cases become continuous and make it practically impossible to use the metal detector. In addition, electromagnetic interference negatively affects selectivity.
False alarms are also caused by structures and closed circuits located near the metal detector, moving according to their functional purpose (doors, elevator cabins, etc.) or moving due to the non-rigidity of structures, vibrations.
If the passage through the metal detector is not properly organized, false alarms may be caused by moving metal objects (carry-on luggage, umbrellas, etc.) of visitors located near the antennas.
The noise immunity of a metal detector is determined by its ability to maintain its characteristics under the influence of the above-mentioned interference. To ensure the noise immunity of metal detectors, a number of both design and organizational measures are used:
— special circuit solutions for electronic units;
— special processing of signals from receiving antennas;
— various types of synchronization with interference;
— removal of metal detectors from moving metal objects;
— exclusion of carrying hand luggage through the area controlled by the metal detector, and near it.
X-ray machines or metal detectors for hand luggage, located in close proximity to the metal detector intended for screening people, may be used to control hand luggage. The listed devices are sources of additional electromagnetic interference, and the measures discussed above must be taken to reduce their impact.
Special security.
Metal detectors generate an electromagnetic field that a person crosses when passing through and which security personnel remain near for a long time. Therefore, in addition to meeting the usual safety requirements for devices with power supply, they must ensure:
— safety in relation to the human body;
— an acceptable level of influence on implantable pacemakers;
— an acceptable level of influence on magnetic storage media.
Thus, special requirements are imposed on metal detectors — inspection devices. The analysis of the requirements for such devices, carried out in this article, allows us to formulate the main approaches to the development of the standard.
Proposals for standardization of requirements
The main characteristic of a metal detector — inspection device can be the probability of detecting an object. The requirements for the value of this characteristic may vary depending on the required level of protection of the object. The author suggests dividing these requirements into four groups: reduced, normal, increased and high protection, with the following numerical values of the probability of detection Robn.:
— reduced with Robn. >= 0.95;
— normal with Robn. >= 0.97;
— increased with Robn. >= 0.98;
— high with Robn. >= 0.99.
The proposed values are selected based on the need to ensure a sufficient level of selectivity and noise immunity, as well as taking into account the technical feasibility of these values in existing metal detectors. In addition, such a division will allow certifying the entire variety of Russian and imported products and having a certain reserve for promising metal detectors.
The most important point in assessing Robn. is the measurement technique. To create it, it is necessary:
— select the nomenclature of OP or their imitators with specific design characteristics;
— select and justify the measurement points in the controlled opening of the metal detector;
— select and justify the number of measurements based on the required confidence probability;
— develop and justify measurement processing algorithms.
The second important characteristic of specific screening devices is selectivity. It can also be assessed in four groups depending on the value of the probability of detection of the PLP (the probability of false alarm Rlt.):
— reduced with Rlt. <= 0.1;
— normal with Rlt. <= 0.05;
— increased with Rlt. <= 0.02;
— high with Rlt. <= 0.01.
The approach to the methods of measuring RLT can be similar to that discussed above.
The proposed Robn. standards are closely related to the time the OP is in the area controlled by the metal detector. Therefore, the maximum speed of carrying objects, which ensures certain probabilistic characteristics, should also be standardized. Usually, the speed of a calmly walking person is 0.5-1.0 m/s. However, the relative speeds of individual parts of his body (arms, legs), and accordingly the OP, can reach 1.5-2 m/s. The methods for measuring the maximum speed of carrying the OP (or the throughput capacity resulting from it) are quite obvious.
The requirements for the noise immunity of metal detectors in relation to external electromagnetic interference are formulated in the standard being developed [1]. Standardization of requirements for noise immunity from moving metal masses is generally impossible.
The safety requirements for the electromagnetic field of a metal detector in relation to the human body are determined by: SanPiN 2.2.4/2.1.8.055-96 “Electromagnetic radiation of the radio frequency range”, MSanPiN 001-96 “Sanitary standards for permissible levels of physical factors when using consumer goods in household conditions”.
With regard to implantable pacemakers, according to the information available to the author, neither in Russia nor in other countries have standardized requirements been established for the permissible characteristics of surrounding electromagnetic fields. Therefore, the safety requirements for the electromagnetic field of a metal detector
can be formulated in relation to a specific (fixed) pacemaker, for example, the most sensitive to the effect in question. In particular, a number of imported metal detectors available on the Russian market have been tested for safety in relation to specific models of pacemakers.
When developing a standard, it is necessary to define the requirements for the safety of the electromagnetic field of a metal detector in relation to magnetic storage media (magnetic tape, disks, etc.), which is most likely important for metal detectors for screening hand luggage.
Measurement methods for most areas of safety of the electromagnetic field emitted by a metal detector consist of taking measurements with conventional measuring devices of field strength levels in certain frequency ranges and comparing them with permissible standards.
General requirements for the safety of metal detectors are set out in GOST 12.1.004, GOST 12.1006, GOST 12.2.003, GOST 12.2.004, GOST 12.2.006.
International standards
The two most well-known international standards define the requirements for metal detectors used to detect weapons on a person [2,3].
Let us consider the relationship between the main requirements proposed in this article for standardization and the corresponding requirements of the US Federal Aviation Administration standard [3]. The US Federal Aviation Administration standard regulates the lower limit of Robn. for three specific samples of firearms. The samples are made of different materials (steel, stainless steel, special alloys), have fairly small weight and dimensions. Tests are carried out at three fixed points on the human body. The standard does not contain requirements for the probabilities of detecting grenades and knives. In addition, there is no gradation by Robn. values, which does not allow comparing metal detectors with each other. In the author's opinion, a number of provisions set out in [3] can be used in the Russian standard being developed, for example, the selection of similar or the same OPs, the use of similar locations of weapons on the human body and in the same spatial orientation. However, the nomenclature of OPs used in testing, the number of points at which the probabilities of their detection are estimated, the volume of tests (confidence probabilities) in the Russian standard should be higher. This will allow for a more reliable assessment of the parameters of metal detectors in relation to the tasks considered above.
The author was unable to analyze the requirements of the standard [2] within the framework of this article, since the text of the standard was not available to him at the time of writing. Therefore, issues of comparison with the accepted classification and permissible values of the characteristics of metal detectors according to this international standard require additional consideration.
1 — Draft GOST R ХХХХХ-98, “System of standards for technical means of protection and security, Access control and management means and systems. Barrier devices, General technical requirements and test methods”.
2 — NILECJ-SCD 0601-00 Security level 1…5 — international standard, security levels 1…5.
3 — Test samples according to the standards of the Federal Aviation Administration of the USA 3-GUN-TEST, 3-GUN-TEST ENHANCED.