The research of electromagnetic influences on technical systems
Khaled Akvan
Student of Royal Institute of Technology, Stockholm, Sweden
Abstract
The recommendations about support of electromagnetic compatibility of technical devices and the cable equipment of the system processing the secure information are offered in the article. Such recommendation essentially raises its immunity to electromagnetic hindrances and by that promotes protection of the information against inadvertent influence.
Key words:technical devices, electromagnetic field, electromagnetic compatibility, electromagnetic hindrances.
In modern conditions the possession of the information in various forms of its display is important advantage. For this reason the problem of its protection against losses by various ways is rather actual. In mass media and the published literature the basic attention to illumination of questions of protection of the information, mainly, from leak and unapproved influence is paid. Thus unfairly aside there is a complex of problems of protection of the information from inadvertent influence. Really, if first two directions reflect, basically, problems of direct protection of the information from deliberate actions of interested or simply curious persons last direction assumes such organization of using the information and her means processing that her not to deform and the more so not to lose. Differently the complex of actions of protection of the information from inadvertent influence assumes the internal organization of process of processing of the protected information the proprietor (or from his permission the user) so that on ignorance or other reasons the actions not to promote her distortion or loss. In the list of the problems solved within the limits of this direction of protection of the information, the leading position is occupied with a problem of maintenance of electromagnetic compatibility of technical devices.
Until recently maintenance electromagnetic compatibility was considered only concerning radio electronic devices, owing to radiation of an electromagnetic field obviously expressed in the course of work. It defines and today necessity of procedure of the coordination of their sharing. However in the modern world rapid development of microelectronics and wide introduction of her products in structure of almost all devices (hardware), including processing the protected information (further as a mean it is understood, means involved (or in common functioning) for processing of the protected information) is observed. Presence as a part of such means of elements of microelectronics, as a rule, carrying out operating functions, or storing the information directly, essentially raises their susceptibility to influence of electromagnetic fields or electromagnetic hindrances. The concept “a susceptibility to hindrances” defines ability of the hardware processing the information, at influence of electromagnetic hindrances to deform the maintenance or irrevocably to lose the information, to stop or break managerial process by her processing, to change structure and sequence of functions of means, etc., and also physical destruction of microcells. It obliges at the organization of protection of the information to solve problems of maintenance Ems of her means processing.
Article has the purpose to show the maintenance, problems Ems modern means in problems of protection of the information, a direction and the ways, allowing raising stability of system of processing of the protected information to influence of electromagnetic hindrances.
In a broad sense the solution of a problem Ems a separate mean consists in creation of conditions at which it is ideally compatible to the surrounding his environment or, in other words, is unreceptive to external hindrances and does not create a hindrance for other means. In all cases the electromagnetic hindrance arises at presence three factors: the hardware-source of a hindrance, environment of her distribution and a mean possessing a susceptibility to this hindrance (also known as receptor).
Undesirable influence on a receptor can be direct or indirect. At indirect influence there is no direct transfer of electromagnetic energy to a receptor. In this case hindrance influence consists in change of the environment of functioning, parameters of elements, devices of a mean or modes of their work. Direct influence is caused by transmission of energy of a hindrance from a source to a receptor or her radiation in space surrounding it, or on conductors (to grounding and power supplies chains, connecting and switching lines, the chassis and to casings of means). Often in practice there are cases of the combined way when the hindrance influences a receptor, extending in space as an electromagnetic field, and by means of an electric current induced by him in conductors. Having presented the separate technical device as a black box (fig. 1) it is possible to track possible ways both his susceptibilities to hindrances, and ways of distribution by him of hindrances to other devices.
The noise transferred on explorers, it is possible to classify by the form communications: conductive, capacity (electrical) and inductive (magnetic). Conductive coupling grows out of the contact between two means (elements of their circuits or explorers connecting them, etc.). It can arise because of the galvanic communication at imperfection of insulation or presence of the general circuits of grounding, etc. The capacitive coupling grows out of stray capacitance, and inductive coupling are result of mutual inductance between the hardware-source of a noise and the hardware-receptor. The capacitive coupling is caused by influence, in the core, electric field when it in a short-range zone is prevailing. It concerns the explorers having the big resistance concerning "earth". As an example of it can serve a multi wire cable. Inductive coupling arises between the low-impedance explorers having small resistance concerning "earth" and forming under the form a loop (frame), that is being radiators of a magnetic field.
Fig. 1. Possible ways of a susceptibility of the technical devices to noises and propagations of noises by it: SS – a susceptibility space; CS – conduction susceptibility; RS – radiation space; CR – conductive radiation (on wires)
It is necessary to mark some conditionality of the resulted classification as capacity and inductive noises are transferred to a receptor actually on an explorer in which they are excited by an appropriate field.
The space noise is formed practically by operation of any mean, thus created by them in environmental space electromagnetic fields can be subdivided on:
– Functional – radiated for the purpose of helpful information transmission by the radio-electronic means intended for it through the antenna-feeder device; their level aspire to strengthen in a direction on the correspondent in enough necessary frequency band and thus whenever possible to occupy it as it is possible a smaller interval of time;
– Accompanying (straying) – accompanying operation of a mean and being its effect technical The imperfections having hindering influence to operation of the adjacent technical device; their level aspire to eliminate or lower to tolerable limits application of designer and circuit decisions, as a rule, at a design stage and the subsequent manufacture of the given means.
Anyway the electromagnetic field round an operating mean exclusively occupies any space in certain frequency band for the operation. And irrespective of the fact which the field for its source, for technical devices processing the securable information is, it is a noise. Space parameters of an electromagnetic noise are characterized by formation of "a mixing zone”. As a mixing zone understand space area in which limits level of energy and the frequency spectrum of the emitted technical devices of an electromagnetic field doesn't allow using simultaneously other technical devices without lowering of quality of their functioning. The sizes of a zone of mixing depend on frequency band in which the field, its energy level, and also a method of its radiation and environmental propagation conditions is generated.
From here the problem of support of electromagnetic compatibility of the technical devices just also consists in the coordinated application of these means. If needed it is necessary for the organization of teamwork of several means:
– To allocate them in space so that “zones of their mixing” weren't intersected;
– If it can't be fulfilled, at the fixed distance to space apart radiation of source EMH on frequency possibility settles up with a band of a susceptibility of a receptor with that registration that the protective interval in between admitted mean functioning, without lowering of quality of its operation;
– If also the second direction is impossible to realize (for example, there is a control of frequency of an emitted field), at the fixed distance and to frequency band of a generated noise possibility of diversity of the periods of operation of means in time is considered.
The enumerated problem resolutions of electromagnetic compatibility of the technical devices can be defined as organizational actions for support of electromagnetic compatibility of potentially conflict means (potentially conflict means are understood as such means which radiate in environmental space (electrical circuits) an electromagnetic field forming “a mixing zone”, and its spectrum coincides (partially coincides) with a band of frequency of a susceptibility to electromagnetic hindrances other mean). They are implemented, as a rule, at a stage of setting (mounting) of a mean and system of protection of the information as a whole, by preparation and during maintenance. Their main directivity is the organization of efforts on:
– To lowering of level accompanying (undesirable for radio electronic devices) radiations at the expense of distance magnification between potentially conflict means;
– To a spacing of conterminous hindering radiations of the technical devices (partially coinciding) on a spectrum and receptor susceptibility on the frequency interval admitting their teamwork without lowering of quality of functioning last;
– To a spacing in time of intervals of operation of potentially conflict means – usage of the general radio resource under the temporal schedule.
However one organizational action the task of support of electromagnetic hindrances of the technical devices to solve it is problematic enough, therefore as means should be ready initially to maintenance in rigid conditions of the modern electromagnetic conditions.
With a view of support of electromagnetic hindrances of the technical devices of the government of the countries install strictly regulated order of selection of the frequency bands for operation radio electronic devices of different function, special conditions of development, designing, Buildings, acquisitions, maintenance and import from other country and high-frequency devices, and also the series of measures on protection of a radio reception against industrial (accompanying) noises defines. In the international community are developed and the international standards and other normative and technical documents regulating the characteristics of means and methods of their measurement operate. They define requirements both to abilities of the technical devices to resist and radiate a noise, and to conditions of their maintenance (for example, electric power quality requirements). The device should possess certain stability to influence of an electromagnetic noise, on the one hand, and to provide demanded restriction of levels accompanying radiations. Thus everyone let out in the country (imported from abroad) the mean should transit certification in the field of support of electromagnetic hindrances of the technical devices in the subdividing having the license for the right of carrying out of these operations. By results of researches such subdividing produces to the vendor of the technical devices its certificate of electromagnetic hindrances and its sign.
Before technical devices representation on certification by the vendor at a design stage are developed and by its manufacture the technical measures are implemented, allowing to finish characteristics of electromagnetic hindrances of this type of means to the values demanded by the state, international normative and technical documents. Technical measures are applied on purpose:
– Lowering of level of accompanying radiation and to it achieve lowering of the sizes of "a mixing zone” the noise hardware-source;
– Decreases of a spectrum of accompanying radiations and by that promote more economical usage by the given technical devices of the frequency resource.
Among ways of the problem resolution of electromagnetic compatibility of the technical devices it is possible to select with technical measures:
• screening – the surrounding or a source hindering of electromagnetic hindrances, or that meets in practice, a receptor is more often, a casing protection is under construction of the metal alloy which composition depends that is defined to protect and from what type of a field;
• a filtration – creation on a propagation path of the stray currents causing accompanying (undesirable for radio electronic devices) radiations, the filters eliminating (reducing to allowed level) appearance of hindering noises;
• grounding – the action, having for an object to provide a refluxing formed on screens, the case and others multi circuits connections of a mean of stray currents in the earth, thereby, excepting accumulation of potential to dangerous (including for the person) limits.
Tasks of manufacture and equipment of the technical devices by screens are difficult enough in calculations and technology of execution and consequently in the course of technical device’s applications, as a rule, don’t dare. However the user needs to know a role and value of these adaptations, at a choice and mounting of the cable equipment and the devices during creation (upgrade) of processing system of the securable information, and also at local repair not to neglect their setting. Screening in tasks of electromagnetic compatibility and information protection assumes, mainly, protection of information lines and the technical devices from directions, caused exterior casual on time electrical and magnetic fields. At the same time screening of cable lines promotes also to lowering of level of electromagnetic hindrances field gradient, created by communication lines and power supplies in environmental space, but this function is less significant, as values of pressure and currents lines are insignificant, and cross impact is eliminated at their setting at a stage of mounting of all system. For achievement of this purpose it is necessary to adhere to following recommendations:
At an equipment choice:
1) In a choice of the cable equipment and the technical devices it is necessary to give preference:
• to the means having smaller levels radiated in environmental space or transferred in exterior cable lines (including power supplies network);
• to the cable means having screening jacket (coaxial cable), twisted pair cable (bifilar) or three twisted wires (trifilar), one of which it is used as the screen, a triax cable (the coaxial cable placed in an additional screening jacket) or a screened plane cable;
• to optical fiber cables which don't radiate electromagnetic hindrances and are unreceptive to them.
2) At mounting (upgrade) of processing system of the securable information:
• the most economic is group screening of information lines by a cable screening box;
• the intersection of cable lines should be carried out at right angle without physical contact of their screening jackets;
• power cables are recommended to be allocated more close to elements of systems of heating, water supply and cooling (life-support) and a construction of a building which "absorb" a part radiated electromagnetic hindrances by them;
• information lines are necessary for deleting at a spacer from elements of life-support and a construction of a building, if needed their intersections the corner should be to straight lines, and a screening jacket of a cable shouldn't have with it physical contact.
3) At equipment repair:
• the screen at setting should have dense contact to the bus of the case which should be in turn grounded;
• the screen casing shouldn't have slots arbitrarily formed at repair, holes, etc. violations of integrity except what are provided by its manufacture;
• appearance of inadmissible interference level by operation of device after repair underlines not careful enough performance of screening of a receptor (or a noise source) after repair.
Screening practically isn't fulfilled without support of a filtration of entering (quitting) explorers. On fig. 2 the example of application of the screen of electromagnetic hindrances’ source as a part of the technical device in a combination to the power line filter is shown.
Fig. 2. Screening of a source of an electromagnetic noise with power line filter application:
a) – a source of a noise without the screen;
b) – a noise source in the screen
Tasks of support of a sufficient filtration in means can be implemented and separately from screens. They also are difficult in calculations and technologies of execution and in the course of technical device’s application don’t dare. A filtration in a mean carry out for an exception of electromagnetic hindrances influence exterior on a receptor on all connections and inputs, and also for protection of cable lines against the noises created by means. Besides filters are provided for an exception of noises on circuits of power supplies, control, control and switching. As an example of this fact protection of the computer against any noises on circuits of power supplies by means of the network filter can serve. The filter normally represents G - T - or square-topped LC - the links which are included in rupture of a phase and zero wires of a network of a supply. Anyway competent usage of filtering devices at installation of equipment of the system processing the securable information will allow preventing its distortion or loss at electromagnetic hindrances influence.
Its user in setting and while in service is engaged in the decision of the task of creation of the correct grounding of a mean. Under the international standard a home network of power supplies except normal for the Russian network of wires "phase" and "zero" contains the third wire "earth" which for the home technical devices is grounding. Absence in a network of power supplies of our country of a wire "earth" defines necessity of the independent organization of competent grounding for the user of the technical devices.
In the technician of grounding conductors which are understood as metal electrodes of any form (a pipe, the rod, sheet, etc.), being immediately in an earthen ground and having with it electrical contact of certain resistance (the it is less, the grounding is more effective) are used. Quality of grounding depends on an amount of grounding conductors, the area of their contact with a ground, the finite resistance equal to the total of electrical resistance of wires bringing from means, the transitive contact between grounding conductors and a ground, and also resistance to spreading of currents in adjoining layers of earth. Last depends on conductivity of soil, a construction of grounding conductors and their layout (efficiency of grounding grows at Soil humidifying by a hydrochloric solution). Often meeting lacks leading to appearance of hindering noises in circuits of grounding, cases, when are:
- Different means are grounded by the general explorer to the grounding bus;
- In grounding circuit’s closed circuits, the general for the different connected means and others are formed.
The general recommendations at the organization of grounding of device for the purpose of lowering of noises can be:
- The active and inductive resistance of connective explorers should be minimum for what their length should be as less as possible;
- Electrical connections in all points of contact should provide its minimum resistance, considering climatic and other destabilizing factors (presence of nondense contact at essential values of a current can lead to appearance of contact noises);
- At grounding creation it is necessary to reduce to a minimum number of the general explorers for means and circuits in system;
- At electric field screening on low frequencies all metal elements of a construction of the technical devices should be connected to its general case (earth).
Grounding diagrams put into practice can be divided into three groups:
- Serial in one point (fig. 2A);
- Parallel in one point (fig. 2B);
- Multipoint (fig. 2C).
Fig. 3. Variants of grounding diagrams of groups of devices
The first variant of grounding the most simple, but corresponds to it the greatest interference level caused by course of currents on the general sections of the grounding circuit.
The second circuit has no this lack, but demands a great number of stretched explorers because of which length it is difficult to provide small grounding impedance.
The multipoint circuit eliminates limitation of first two variants, however, at its application there can be difficulties with the advent of resonance noises in circuit circuits. Normally at the grounding organizations apply hybrid circuits: on low frequencies prefer single-point, and on more high frequencies – the multipoint circuit.
In summary it is necessary to mark that performance stated above recommendations about support of electromagnetic compatibility of technical devices and the cable equipment of the system processing the securable information, essentially raises its immunity to electromagnetic hindrances and by that promotes protection of the information against inadvertent influence. At the same time the problem resolution of support of electromagnetic compatibility of technical devices is the multiedged collective task which dares jointly all interested users of the devices in the region.