How can you make two magnets next to each other not feel each other's presence? What material needs to be placed between them so that the power lines magnetic field from one magnet would not reach the second magnet?

This question is not as trivial as it might seem at first glance. We need to truly isolate the two magnets. That is, so that these two magnets can be rotated differently and moved differently relative to each other and yet, so that each of these magnets behaves as if there was no other magnet nearby. Therefore, any tricks involving placing a third magnet or ferromagnet nearby to create some special configuration of magnetic fields with compensation of all magnetic fields at any one particular point do not work in principle.

Diamagnetic???

Sometimes they mistakenly think that such a magnetic field insulator can serve diamagnetic. But this is not true. A diamagnetic material actually weakens the magnetic field. But it weakens the magnetic field only in the thickness of the diamagnetic itself, inside the diamagnetic. Because of this, many people mistakenly think that if one or both magnets are immured in a piece of diamagnetic material, then their attraction or repulsion will weaken.

But this is not a solution to the problem. Firstly, the field lines of one magnet will still reach another magnet, that is, the magnetic field only decreases in the thickness of the diamagnetic, but does not disappear completely. Secondly, if the magnets are immured in the thickness of the diamagnetic material, then we cannot move or rotate them relative to each other.

And if you just make a flat screen out of a diamagnetic material, then this screen will transmit a magnetic field through itself. Moreover, behind this screen the magnetic field will be exactly the same as if this diamagnetic screen did not exist at all.

This suggests that even magnets embedded in a diamagnetic material will not experience a weakening of each other’s magnetic field. In fact, where the walled magnet is located, there is simply no diamagnetic material directly in the volume of this magnet. And since there is no diamagnetic material where the walled magnet is located, it means that both walled magnets actually interact with each other in exactly the same way as if they were not walled up in the diamagnetic material. The diamagnetic material around these magnets is as useless as the flat diamagnetic shield between the magnets.

Ideal diamagnetic

We need a material that would not allow magnetic field lines to pass through itself at all. It is necessary that the magnetic field lines be pushed out of such a material. If magnetic field lines pass through a material, then, behind a screen made of such material, they completely restore all their strength. This follows from the law of conservation of magnetic flux.

In a diamagnetic material, the weakening of the external magnetic field occurs due to the induced internal magnetic field. This induced magnetic field is created by circular currents of electrons inside the atoms. When an external magnetic field is turned on, the electrons in the atoms should begin to move around the lines of force of the external magnetic field. This induced circular motion of electrons in atoms creates an additional magnetic field, which is always directed against the external magnetic field. Therefore, the total magnetic field inside the diamagnetic becomes less than outside.

But complete compensation of the external field due to the induced internal field does not occur. There is not enough circular current strength in the diamagnetic atoms to create exactly the same magnetic field as the external magnetic field. Therefore, the lines of force of the external magnetic field remain in the thickness of the diamagnetic material. The external magnetic field, as it were, “pierces” the diamagnetic material through and through.

The only material that pushes magnetic field lines out of itself is a superconductor. In a superconductor, an external magnetic field induces circular currents around the external field lines that create an oppositely directed magnetic field exactly equal to the external magnetic field. In this sense, a superconductor is an ideal diamagnetic.

On the surface of a superconductor, the magnetic field strength vector is always directed along this surface, tangential to the surface of the superconducting body. On the surface of a superconductor, the magnetic field vector does not have a component directed perpendicular to the surface of the superconductor. Therefore, magnetic field lines always bend around a superconducting body of any shape.

Bending of a superconductor by magnetic field lines

But this does not mean at all that if a superconducting screen is placed between two magnets, it will solve the problem. The fact is that the magnetic field lines of the magnet will go to another magnet, bypassing the superconductor screen. Therefore, a flat superconducting screen will only weaken the influence of magnets on each other.

This weakening of the interaction between the two magnets will depend on how much the length of the field line that connects the two magnets to each other has increased. The greater the length of the connecting field lines, the less interaction between two magnets with each other.

This is exactly the same effect as if you increase the distance between the magnets without any superconducting screen. If you increase the distance between magnets, then the lengths of the magnetic field lines also increase.

This means that in order to increase the lengths of the power lines that connect two magnets bypassing the superconducting screen, it is necessary to increase the dimensions of this flat screen both in length and width. This will lead to an increase in the lengths of bypass power lines. And the larger the dimensions of the flat screen compared to the distance between the magnets, the less interaction between the magnets becomes.

The interaction between the magnets completely disappears only when both dimensions of the flat superconducting screen become infinite. This is an analogue of the situation when magnets were separated to an infinitely large distance, and therefore the length of the magnetic field lines connecting them became infinite.

Theoretically, this, of course, completely solves the problem. But in practice we cannot make a superconducting flat screen of infinite dimensions. I would like to have such a solution that can be implemented in practice in the laboratory or in production. (We are no longer talking about everyday conditions, since it is impossible to make a superconductor in everyday life.)

Space division by superconductor

Alternatively, a flat screen of infinitely large dimensions can be interpreted as dividing the entire three-dimensional space into two parts that are not connected to each other. But it’s not just a flat screen of infinite size that can divide space into two parts. Any closed surface also divides space into two parts, the volume inside the closed surface and the volume outside the closed surface.

For example, any sphere divides space into two parts: the ball inside the sphere and everything outside.

Therefore, a superconducting sphere is an ideal insulator of a magnetic field. If you place a magnet in such a superconducting sphere, then no instrument can ever detect whether there is a magnet inside this sphere or not.

And, conversely, if you are placed inside such a sphere, then external magnetic fields will not act on you. For example, the Earth's magnetic field cannot be detected inside such a superconducting sphere by any instruments. Inside such a superconducting sphere, it will be possible to detect only the magnetic field from those magnets that will also be located inside this sphere.

Thus, in order for two magnets not to interact with each other, one of these magnets must be placed inside the superconducting sphere, and the second one must be left outside. Then the magnetic field of the first magnet will be completely concentrated inside the sphere and will not go beyond the boundaries of this sphere. Therefore, the second magnet will not feel the presence of the first. Likewise, the magnetic field of the second magnet will not be able to penetrate inside the superconducting sphere. And therefore the first magnet will not sense the close presence of the second magnet. Finally, we can rotate and move both magnets relative to each other as we please. True, the first magnet is limited in its movements by the radius of the superconducting sphere. But that's just how it seems. In fact, the interaction of two magnets depends only on their relative position and their rotations around the center of gravity of the corresponding magnet. Therefore, it is enough to place the center of gravity of the first magnet in the center of the sphere and place the origin of coordinates there at the center of the sphere. All possible options for the location of magnets will be determined only by all

Of course, instead of a sphere, you can take any other surface shape, for example, an ellipsoid or a box-shaped surface, etc. If only it divided the space into two parts. That is, there should not be a hole in this surface through which a power line can penetrate, which will connect the internal and external magnets.

The operating principle of most measuring instrument converters is based on the conversion of electrical and magnetic energies; therefore, electric and magnetic fields induced inside measuring instruments by nearby sources distort the nature of the conversion of electrical and magnetic energy in the measuring device. To protect sensitive elements of devices from the influence of internal and external electric and magnetic fields, shielding is used.

By magnetic shielding of any region of space we mean the weakening of the magnetic field inside this region by limiting it with a shell made of soft magnetic materials. In practice, another shielding method is also used, when a source of magnetic field is placed in the shell, thereby limiting the spread of the latter into the environment.

The basics of shielding are based on the theory of propagation of electric and magnetic fields. The emitted energy is transmitted by an electromagnetic field. When a field changes over time, its electric and magnetic components exist simultaneously, and one of them may be greater than the other. If the electrical component is greater, then the field is considered electric; if the magnetic component is greater, then the field is considered magnetic. Typically the field has a pronounced character near its source at a wavelength distance. In free space, at a large distance from the energy source (compared to the wavelength), both components of the field have an equal amount of energy. In addition, any conductor located in an electromagnetic field necessarily absorbs and emits energy again, therefore, even at small distances from such a conductor, the relative distribution of energy differs from the distribution of energy in free space.

The electric (electrostatic) component of the field corresponds to the voltage on the conductor, and the magnetic (electromagnetic) component corresponds to the current. Determining the need for one or another degree of shielding of a given electrical circuit, as well as determining the sufficiency of one or another type of shield, is almost beyond technical calculation, because theoretical solutions to individual simple problems turn out to be unacceptable for complex electrical circuits consisting of arbitrarily located space of elements emitting electromagnetic energy in a wide variety of directions. To calculate the screen, one would have to take into account the influence of all these individual radiations, which is impossible. Therefore, a designer working in this area is required to have a clear understanding of the physical action of each shielding part, its relative importance in the complex of screen parts, and the ability to perform approximate calculations of the screen’s effectiveness.

Based on the principle of operation, electrostatic, magnetostatic and electromagnetic screens are distinguished.

The shielding effect of a metal screen is determined by two reasons: the reflection of the field from the screen and the attenuation of the field when passing through the metal. Each of these phenomena is independent of one another and must be considered separately, although the overall shielding effect is the result of both.

Electrostatic shielding consists of closing an electric field on the surface of the metal mass of the screen and transferring electrical charges to the device body (Fig. 1.).

If between structural element A, which creates an electric field, and element B, for which the influence of this field is harmful, a screen B is placed, connected to the body (ground) of the product, then it will intercept electric power lines, protecting element B from harmful influence element A. Consequently, the electric field can be reliably shielded even by a very thin layer of metal.

The induced charges are located on the outer surface of the screen so that the electric field inside the screen is zero.

Magnetostatic shielding is based on the closure of the magnetic field in the thickness of the screen, which has increased magnetic permeability. The screen material must have a magnetic permeability significantly greater than the magnetic permeability environment. The principle of operation of the magnetostatic screen is shown in Fig. 2.

The magnetic flux created by a structural element (in this case, a wire) is closed in the walls of the magnetic shield due to its low magnetic resistance. The greater the magnetic permeability and thickness of such a screen, the greater the effectiveness of such a screen.

A magnetostatic screen is used only with a constant field or in the range of low frequencies of change in the field.

Electro magnetic shielding is based on the interaction of an alternating magnetic field with eddy currents induced by it in the thickness and on the surface of the conductive material of the screen. The principle of electromagnetic shielding is illustrated in Fig. 3. If a copper cylinder (screen) is placed in the path of a uniform magnetic flux, then alternating E.M.F. will be excited in it, which, in turn, will create alternating induced eddy currents. The magnetic field of these currents will be closed (Figure 3b); inside the cylinder it will be directed towards the exciting field, and outside it - in the same direction as the exciting field. The resulting field turns out to be weakened (Fig. 3c) inside the cylinder and strengthened outside it, i.e. displacement occurs from the space occupied by the cylinder, which is its shielding effect.

The efficiency of electromagnetic shielding increases with increasing reverse field, which will be greater the greater the eddy currents flowing through the cylinder, i.e. the greater the electrical conductivity of the cylinder.

The attenuation of a magnetic field by a metal can be calculated. It is proportional to the thickness of the screen, the eddy current coefficient and the square root of the product of the field frequency, magnetic permeability and conductivity of the screen material.

When shielding product elements with magnetostatic and electromagnetic shields, it should be taken into account that they will also be effective as electrostatic shields if they are securely connected to the device body.

Equipment, instruments and tools

When performing the work, the following are used: installation for creating electromagnetic field; special form signal generator G6-26; measuring coil for estimating the electromagnetic field strength; oscilloscope S1-64; voltmeter; a set of screens made from various materials.

The sine wave signal is supplied from the installation's signal generator through a step-down transformer. To connect the measuring coil 5 to the oscilloscope and the electromagnetic field excitation coil 1 to the signal generators, terminal sockets 6 and 7 are fixed on the base 3 of the installation. The installation is turned on by toggle switch 8.

To characterize the shielding material, two more values of penetration depth are used: x 0.1, x 0.01, which characterize the drop in field strength density (hole) by 10 and 100 times from the value on its surface

which are given in reference tables for various materials. Table 2 shows the values of x0, x0.1, x0.01, for copper, aluminum, steel and permalloy.

When choosing a shield material, it is convenient to use the shielding efficiency curves shown in the graphs in Fig. 4.

Characteristics of alloys for magnetic shields

Alloys with high magnetic permeability are used as materials for magnetic screens in weak fields. Permalloys, which belong to the group of malleable alloys with high magnetic permeability, are well processed by cutting and stamping. Based on their composition, permalloys are usually divided into low-nickel (40-50% Ni) and high-nickel (72-80% Ni). To improve electromagnetic and technological properties, permalloys are often alloyed with molybdenum, chromium, silicon, cobalt, copper and other elements. The main indicators of the electromagnetic quality of these alloys are the values of the initial µ initial and maximum µ max magnetic permeability. The coercive force H c of permalloys should be as low as possible, and the electrical resistivity ρ and saturation magnetization M s should be as high as possible. The dependence of these parameters for the Fe-Ni binary alloy on the percentage of nickel is shown in Fig. 5.

The characteristic µ initial (Fig. 5) has two maxima, relative (1) and absolute (2). The region of relative minimum, limited by a nickel content of 40-50%, corresponds to low-nickel permalloy, and the region of absolute maximum, limited by a nickel content of 72-80%, corresponds to high-nickel permalloy. The latter also has the largest value µ max. The current characteristics µ 0 M s and ρ (Fig. 5) indicate that the magnetic saturation and electrical resistivity of low-nickel permalloy are significantly higher than those of high-nickel permalloy. These circumstances differentiate the areas of application of low-nickel and high-nickel permalloys

Low-nickel permalloy is used for the manufacture of magnetic screens operating in weak constant magnetic fields. Alloyed with silicon and chromium, low-nickel permalloy is used at higher frequencies.

Alloys 79НМ, 80НХС, 81НМА, 83НФ with the highest magnetic permeability in weak magnetic fields and saturation induction of 0.5 -0.75 Tesla for magnetic screens, magnetic amplifier cores and contactless relays. Alloys 27KH, 49KH, 49K2F and 49K2FA, which have a high technical saturation induction (2.1 - 2.25 T), are used for magnetic shields that protect equipment from the effects of strong magnetic fields

Safety requirements

Before starting work

Understand the location and purpose of laboratory controls and measuring equipment.
Prepare workplace For safe work: Remove unnecessary items from the table and installation.
Check: the presence and serviceability of the grounding system, the integrity of the installation housing, power cords, plug connectors. Do not start work if the protective panels of the laboratory installation (stand) are removed.

During work

Work can only be carried out on working equipment.
It is not allowed to block the ventilation openings (louvres) in the buildings of laboratory installations with foreign objects.
Do not leave the unit turned on when you are away even for a short time.
In the event of a power outage, the installation must be turned off.

In emergency situations

The laboratory unit must be switched off immediately in the following cases:

accident or threat to human health;
the appearance of a smell characteristic of burning insulation, plastics, paint;
the appearance of crackling, clicking, sparking;
damage to the plug connection or electrical cable supplying the installation.

After finishing work

Turn off the laboratory unit and measuring instruments.
Disconnect the installation and measuring instruments from the network. Tidy up your workspace.
Remove foreign objects and clear any possible debris (unnecessary paper).

Task and research methodology

Determine areas experimentally effective use various materials for electrical magnetic materials when the frequencies of the electromagnetic field change from 102 to 104 Hz.

Connect the installation for creating an electromagnetic field to the signal generator. Connect the measuring coil to the input of the oscilloscope and to the voltmeter. Measure the amplitude U of the signal, proportional to the strength of the electromagnetic field inside the cylindrical frame of the field excitation coil. Cover the measuring coil with a screen

Measure the amplitude U' of the signal from the measuring coil. Determine shielding effectiveness

at a given frequency and write it down in the table (see appendix).

Take measurements according to clause 5.1.1. for frequencies 100, 500, 1000, 5000, 104 Hz. Determine the shielding effectiveness at each frequency.

Tested screen samples. An experimental study of the properties of materials for magnetic screens is carried out using samples in

in the form of cylindrical glasses 9 (Fig. 6), the main parameters of which are given in Table 3.

Screens can be either single-layer or multi-layer with an air gap between them, cylindrical or with a rectangular cross-section. Calculation of the number of shield layers can be carried out using rather cumbersome formulas, so it is recommended to select the number of layers according to the shielding efficiency curves given in reference books.

1 - electromagnetic field excitation coil;

2 - non-magnetic frame;

3 - non-magnetic base;

4 - step-down transformer;

5 - measuring coil;

6 and 7 - terminal sockets;

8 - toggle switch;

9 - magnetic screen;

10 - signal generator;

11 - oscilloscope;

12 - voltmeter.

Carry out measurements for screens made of ordinary quality steel, permalloy, aluminum, copper, brass.

Based on the measurement results, construct shielding efficiency curves for various materials similar to Fig. 4. Analyze the results of the experiment. Compare the results of the experiment with reference data and draw conclusions.

To experimentally determine the influence of the thickness of the screen wall (glass) on the shielding efficiency.

For materials with high magnetic permeability (steel, permalloy), conduct the experiment in an electromagnetic field at frequencies of 100 Hz, 500 Hz, 1000 Hz, 5000 Hz, 10000 Hz according to the method outlined for screens with different wall thicknesses.

For materials with electrical conductivity (copper, aluminum), conduct the experiment at frequencies of 100 Hz, 500 Hz, 1000 Hz, 5000 Hz, 10000 Hz according to the described method.

Analyze the results of the experiment. Compare the results of the experiment with the data given in Table 1. Draw conclusions

LITERATURE

1. Grodnev I. I. Electromagnetic shielding in a wide range of frequencies. M.: Communication. 1972. - 275 p.

2. Design of devices. In 2 books. / Ed. V. Krause; Per. with him. V.N. Palyanova; Ed. O.F. Tishchenko. - Book 1-M.: Mechanical Engineering, 1987.

3. Materials in instrument making and automation: Directory / pod. ed. Yu.M. Pyatina. - 2nd ed. Reworked And additional - M.: Mechanical Engineering, 1982.

4. Obergan A.N. Design and technology of measuring instruments. Tutorial. - Tomsk, Rotaprint TPI. 1987. - 95 p.

5. Govorkov V.A. Electric and magnetic fields. - M. Svyazizdat, 1968.

6. Sinusoidal signal generator G6-26. Technical description and operating instructions. 1980 - 88s.

7. Oscilloscope S1-64. Technical description and operating instructions.

Educational and methodological manual

Compiled by: Gormakov A. N., Martemyanov V. M.

Computer typing and layout by V. S. Ivanova

MAGNETIC SHIELDING

(magnetic) - protection of an object from magnetic influences. fields (constant and variable). Modern Research in a number of fields of science (physics, geology, paleontology, biomagnetism) and technology (space research, nuclear energy, materials science) is often associated with measurements of very weak magnetic fields. fields ~10 -14 -10 -9 T in a wide frequency range. External magnetic fields (for example, Earth Tl with noise Tl, magnets from electrical networks and urban transport) create strong interference with the operation of highly sensitive devices. magnetometric equipment. Reducing the influence of magnetic fields strongly determines the possibility of conducting magnetic fields. measurements (see, for example, Magnetic fields of biological objects). Among the methods of M. e. the most common are the following.

Shielding hollow cylinder made of ferromagnetic substance with ( 1 - ext. cylinder, 2 -internal surface). Residual magnetic field inside the cylinder

Ferromagnetic screen- sheet, cylinder, sphere (or any other shape) made of material with high magnetic permeability m low residual induction In r and small coercive force N s. The principle of operation of such a screen can be illustrated using the example of a hollow cylinder placed in a homogeneous magnetic field. field (fig.). External induction lines mag. fields B when passing from the medium to the screen material, the external fields become noticeably denser, and in the cavity of the cylinder the density of the induction lines decreases, i.e., the field inside the cylinder turns out to be weakened. Field weakening is described by f-loy

Where D- cylinder diameter, d- the thickness of its wall is mag. permeability of the wall material. To calculate the effectiveness of M. e. volumes decom. configurations often use file

where is the radius of the equivalent sphere (almost the average value of the screen dimensions in three mutually perpendicular directions, since the shape of the screen has little effect on the efficiency of the magnetoelectric system).

From formulas (1) and (2) it follows that the use of materials with high magnetic field. permeability [such as permalloy (36-85% Ni, rest Fe and alloying additives) or mu-metal (72-76% Ni, 5% Cu, 2% Cr, 1% Mn, rest Fe)] significantly improves the quality of screens (at iron). The seemingly obvious method of improving shielding by thickening the wall is not optimal. Multilayer screens with gaps between layers work more efficiently, for which the coefficients are shielding equal to the product coefficient for dept. layers. It is multilayer screens (outer layers made of magnetic materials that are saturated at high values IN, internal ones - made of permalloy or mu-metal) form the basis of the designs of magnetically protected rooms for biomagnetic, paleomagnetic, etc. studies. It should be noted that the use of protective materials such as permalloy is associated with a number of difficulties, in particular with the fact that their magnesium. properties under deformation and that means. heat deteriorate, they practically do not allow welding, which means. bends and other mechanical loads In modern mag. Ferromagnets are widely used in screens. metal glasses(metglasses), close in magnetic. properties to permalloy, but not so sensitive to mechanical influences. The fabric, woven from metglass strips, allows the production of soft magnets. screens free form, and multilayer shielding with this material is much simpler and cheaper.

Screens made of material with high electrical conductivity(Cu, A1, etc.) serve to protect against alternating magnetic fields. fields. When changing external mag. fields in the walls of the screen arise inductively. currents that cover the shielded volume. Magn. the field of these currents is directed opposite to the external one. indignation and partially compensates for it. For frequencies above 1 Hz coefficient. shielding TO increases in proportion to frequency:

Where - magnetic constant, - electrical conductivity of the wall material, L- screen size, - wall thickness, f- circular frequency.

Magn. screens made of Cu and A1 are less effective than ferromagnetic ones, especially in the case of low-frequency electromagnetic. fields, but ease of manufacture and low cost often make them more preferable for use.

Superconducting screens. The action of this type of screens is based on Meissner effect - complete displacement of magnet. fields from a superconductor. With any change in external mag. flow in superconductors, currents arise, which, in accordance with Lenz's rule compensate for these changes. Unlike ordinary conductors, inductive superconductors. the currents do not fade and therefore compensate for the change in flux during the entire period of existence of the external current. fields. The fact that superconducting screens can operate at very low temps and fields not exceeding critical. values (see Critical magnetic field), leads to significant difficulties in the design of large magnetically protected “warm” volumes. However, the discovery oxide high temperature superconductors(OBC), made by J. Bednorz and K. Müller (J. G. Bednorz, K. A. Miiller, 1986), creates new opportunities in the use of superconducting magnets. screens. Apparently, after overcoming the technological difficulties in the manufacture of SBCs, superconducting screens will be used from materials that become superconductors at the boiling point of nitrogen (and in the future, possibly at room temperatures).

It should be noted that inside the volume magnetically protected by the superconductor, the residual field that existed in it at the moment of the transition of the screen material to the superconducting state is preserved. To reduce this residual field it is necessary to take a special . For example, transfer the screen to a superconducting state at a low magnetic field compared to the earth's. field in the protected volume or use the “inflating screens” method, in which the folded shell of the screen is transferred to a superconducting state and then expanded. Such measures make it possible, for now, to reduce residual fields to a value of T in small volumes limited by superconducting screens.

Active interference protection carried out using compensating coils that create a magnetic field. a field equal in magnitude and opposite in direction to the interference field. When added algebraically, these fields cancel each other out. Naib. Helmholtz coils are known, which are two identical coaxial circular coils with current, separated by a distance equal to the radius of the coils. Fairly homogeneous mag. the field is created in the center between them. To compensate for three spaces. components require a minimum of three pairs of coils. There are many options for such systems, and their choice is determined by specific requirements.

An active protection system is typically used to suppress low-frequency interference (in the frequency range 0-50 Hz). One of its purposes is post compensation. mag. Earth's fields, which require highly stable and powerful current sources; the second is compensation for magnetic variations. fields, for which weaker current sources controlled by magnetic sensors can be used. fields, e.g. magnetometers high sensitivity - squids or fluxgates. To a large extent, the completeness of compensation is determined by these sensors.

There is an important difference between active magnetic protection. screens. Magn. screens eliminate noise throughout the entire volume limited by the screen, while active protection eliminates interference only in a local area.

All magnetic suppression systems interference need anti-vibration. protection. Vibration of screens and magnetic sensors. The field itself can become a source of additions. interference

Lit.: Rose-Ince A., Roderick E., Introduction to the physics of superconductivity, trans. from English, M., 1972; Stamberger G. A., Devices for creating weak constant magnetic fields, Novosibirsk, 1972; Vvedensky V.L., Ozhogin V.I., Ultrasensitive magnetometry and biomagnetism, M., 1986; Bednorz J. G., Muller K. A., Possible high Tc superconductivity in the Ba-La-Cr-O system, "Z. Phys.", 1986, Bd 64, S. 189. S. P. Naurzakov.

Physical encyclopedia. In 5 volumes. - M.: Soviet Encyclopedia. Editor-in-Chief A. M. Prokhorov. 1988 .

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Two methods are used to shield the magnetic field:

Bypass method;

Screen magnetic field method.

Let's take a closer look at each of these methods.

Method of shunting a magnetic field with a screen.

The method of shunting a magnetic field with a screen is used to protect against a constant and slowly changing alternating magnetic field. Screens are made of ferromagnetic materials with high relative magnetic penetration (steel, permalloy). If there is a screen, the lines of magnetic induction pass mainly along its walls (Figure 8.15), which have low magnetic resistance compared to the air space inside the screen. The quality of shielding depends on the magnetic permeability of the shield and the resistance of the magnetic circuit, i.e. The thicker the screen and the fewer seams and joints running across the direction of the magnetic induction lines, the shielding efficiency will be higher.

Method of displacement of a magnetic field by a screen.

The method of displacement of a magnetic field by a screen is used to screen alternating high-frequency magnetic fields. In this case, screens made of non-magnetic metals are used. Shielding is based on the phenomenon of induction. Here the phenomenon of induction is useful.

Let's place a copper cylinder in the path of a uniform alternating magnetic field (Figure 8.16a). Variable EDs will be excited in it, which, in turn, will create alternating inductive eddy currents (Foucault currents). The magnetic field of these currents (Figure 8.16b) will be closed; inside the cylinder it will be directed towards the exciting field, and outside it - in the same direction as the exciting field. The resulting field (Figure 8.16, c) turns out to be weakened near the cylinder and strengthened outside it, i.e. the field is displaced from the space occupied by the cylinder, which is its shielding effect, which will be more effective the lower the electrical resistance of the cylinder, i.e. the greater the eddy currents flowing through it.

Thanks to the surface effect (“skin effect”), the density of eddy currents and the intensity of the alternating magnetic field decreases exponentially as one goes deeper into the metal

, (8.5)

Where (8.6)

– indicator of the decrease in field and current, which is called equivalent penetration depth.

Here is the relative magnetic permeability of the material;

– magnetic permeability of vacuum, equal to 1.25*10 8 g*cm -1;

– resistivity of the material, Ohm*cm;

- frequency Hz.

The value of the equivalent penetration depth is convenient to characterize the shielding effect of eddy currents. The smaller x0, the greater the magnetic field they create, which displaces the external field of the pickup source from the space occupied by the screen.

For a non-magnetic material in formula (8.6) =1, the shielding effect is determined only by and . What if the screen is made of ferromagnetic material?

If they are equal, the effect will be better, since >1 (50..100) and x 0 will be less.

So, x 0 is a criterion for the shielding effect of eddy currents. It is of interest to estimate how many times the current density and magnetic field strength become lower at depth x 0 compared to what they are at the surface. To do this, we substitute x = x 0 into formula (8.5), then

from which it can be seen that at a depth x 0 the current density and magnetic field strength drop by e times, i.e. to a value of 1/2.72, which is 0.37 of the density and tension on the surface. Since the field weakening is only 2.72 times at depth x 0 not enough to characterize the shielding material, then use two more values of penetration depth x 0.1 and x 0.01, which characterize the drop in current density and field voltage by 10 and 100 times from their values on the surface.

Let's express the values x 0.1 and x 0.01 through the value x 0; for this, based on expression (8.5), we create the equation

AND ,

having decided which we get

x 0.1 =x 0 ln10=2.3x 0 ; (8.7)

x 0.01 = x 0 ln100 = 4.6x 0

Based on formulas (8.6) and (8.7) for various shielding materials, the values of penetration depths are given in the literature. For clarity purposes, we present the same data in the form of table 8.1.

The table shows that for all high frequencies, starting from the medium wave range, a screen made of any metal with a thickness of 0.5..1.5 mm is very effective. When choosing the thickness and material of the screen, you should not proceed from the electrical properties of the material, but be guided by considerations of mechanical strength, rigidity, resistance to corrosion, ease of joining individual parts and making transition contacts with low resistance between them, ease of soldering, welding, etc.

From the table data it follows that for frequencies greater than 10 MHz, a film of copper, and even more so of silver, with a thickness of less than 0.1 mm gives a significant shielding effect. Therefore, at frequencies above 10 MHz, it is quite acceptable to use screens made of foil getinax or other insulating material coated with copper or silver.

Steel can be used as screens, but you just need to remember that due to the high resistivity and hysteresis phenomenon, a steel screen can introduce significant losses into the shielding circuits.

Principles of magnetic field shielding

Two methods are used to shield the magnetic field:

Bypass method;

Screen magnetic field method.

Let's take a closer look at each of these methods.

Method of shunting a magnetic field with a screen.

Method of displacement of a magnetic field by a screen.

Thanks to the surface effect (“skin effect”), the density of eddy currents and the intensity of the alternating magnetic field decreases exponentially as one goes deeper into the metal

, (8.5)

Where (8.6)

– indicator of the decrease in field and current, which is called equivalent penetration depth.

Here is the relative magnetic permeability of the material;

– magnetic permeability of vacuum, equal to 1.25*10 8 g*cm -1;

– resistivity of the material, Ohm*cm;

- frequency Hz.

For a non-magnetic material in formula (8.6) =1, the shielding effect is determined only by and . What if the screen is made of ferromagnetic material?

If they are equal, the effect will be better, since >1 (50..100) and x 0 will be less.

Let's express the values x 0.1 and x 0.01 through the value x 0; for this, based on expression (8.5), we create the equation

AND ,

having decided which we get

x 0.1 =x 0 ln10=2.3x 0 ; (8.7)

x 0.01 = x 0 ln100 = 4.6x 0

Steel can be used as screens, but you just need to remember that due to the high resistivity and hysteresis phenomenon, a steel screen can introduce significant losses into the shielding circuits.

Filtration

Filtering is the primary means of reducing structural noise generated in DC power and switching circuits. alternating current ES. Interference filters designed for this purpose make it possible to reduce conducted interference, both from external and from internal sources. Filtration efficiency is determined by the attenuation introduced by the filter:

dB,

The following basic requirements are imposed on the filter:

Ensuring the specified efficiency S in the required frequency range (taking into account the internal resistance and load of the electrical circuit);

Limitation of the permissible drop in direct or alternating voltage across the filter at maximum load current;

Ensuring acceptable nonlinear distortions of the supply voltage, which determine the requirements for filter linearity;

Design requirements - shielding efficiency, minimum overall dimensions and weight, ensuring normal thermal conditions, resistance to mechanical and climatic influences, manufacturability of the design, etc.;

Filter elements must be selected taking into account the rated currents and voltages of the electrical circuit, as well as the voltage and current surges caused by them caused by electrical instability and transient processes.

Capacitors. They are used as independent noise suppression elements and as parallel filter units. Structurally, noise suppression capacitors are divided into:

Two-pole type K50-6, K52-1B, ETO, K53-1A;

Support type KO, KO-E, KDO;

Feed-through non-coaxial type K73-21;

Feedthrough coaxial type KTP-44, K10-44, K73-18, K53-17;

Capacitor units;

The main characteristic of a noise suppression capacitor is the dependence of its impedance on frequency. To reduce interference in the frequency range up to approximately 10 MHz, bipolar capacitors can be used, taking into account the short length of their leads. Reference noise suppression capacitors are used up to frequencies of 30-50 MHz. Symmetrical pass capacitors are used in a two-wire circuit up to frequencies of about 100 MHz. Pass capacitors operate over a wide frequency range up to approximately 1000 MHz.

Inductive elements. They are used as independent noise suppression elements and as sequential links of noise suppression filters. Structurally, the most common types of chokes are:

Turning on a ferromagnetic core;

Turn-free.

The main characteristic of a noise suppression choke is the dependence of its impedance on frequency. At low frequencies, it is recommended to use magnetodielectric cores of the PP90 and PP250 brands, made on the basis of m-permalloy. To suppress interference in equipment circuits with currents up to 3A, it is recommended to use HF chokes of the DM type, and for higher rated currents - chokes of the D200 series.

Filters. Ceramic pass-through filters of type B7, B14, B23 are designed to suppress interference in circuits of direct, pulsating and alternating currents in the frequency range from 10 MHz to 10 GHz. The designs of such filters are shown in Figure 8.17

The attenuation introduced by filters B7, B14, B23 in the frequency range 10..100 MHz increases from approximately 20..30 to 50..60 dB and in the frequency range above 100 MHz exceeds 50 dB.

Ceramic feed-through filters of the B23B type are built on the basis of ceramic disk capacitors and turn-free ferromagnetic chokes (Figure 8.18).

Turn-free chokes are a tubular ferromagnetic core made of grade 50 VCh-2 ferrite, mounted on a feed-through terminal. The inductance of the inductor is 0.08…0.13 μH. The filter housing is made of UV-61 ceramic material, which has high mechanical strength. The housing is metalized with a layer of silver to ensure low contact resistance between the outer lining of the capacitor and the grounding threaded bushing, which is used to secure the filter. The capacitor is soldered along the outer perimeter to the filter housing, and along the inner perimeter to the feed-through terminal. Sealing of the filter is ensured by filling the ends of the housing with a compound.

For B23B filters:

nominal filter capacitances – from 0.01 to 6.8 µF,

rated voltage 50 and 250V,

rated current up to 20A,

Overall dimensions of the filter:

L=25mm, D= 12mm

The attenuation introduced by B23B filters in the frequency range from 10 kHz to 10 MHz increases from approximately 30..50 to 60..70 dB and in the frequency range above 10 MHz exceeds 70 dB.

For on-board ES, the use of special noise-suppressing wires with ferrofillers having high magnetic permeability and high specific losses is promising. So, for PPE brand wires, the insertion attenuation in the frequency range 1...1000 MHz increases from 6 to 128 dB/m.

The design of multi-pin connectors is known, in which one U-shaped noise suppression filter is installed on each contact.

Overall dimensions of the built-in filter:

length 9.5 mm,

diameter 3.2 mm.

The attenuation introduced by the filter in a 50-ohm circuit is 20 dB at a frequency of 10 MHz and up to 80 dB at a frequency of 100 MHz.

Filtering of power supply circuits of digital electronic devices.

Pulse noise in power buses that occurs during the switching of digital integrated circuits (DIC), as well as penetrating externally, can lead to malfunctions in the operation of devices digital processing information.

To reduce the level of noise in power buses, circuit design methods are used:

Reducing the inductance of the “power” buses, taking into account the mutual magnetic coupling of the forward and reverse conductors;

Reducing the lengths of sections of “power” buses, which are common for currents for various digital information systems;

Slowing down the edges of pulse currents in the “power” buses using noise-suppressing capacitors;

Rational topology of power circuits on a printed circuit board.

Increase in size cross section conductors leads to a decrease in the intrinsic inductance of the buses, and also reduces their active resistance. The latter is especially important in the case of the ground bus, which is the return conductor for signal circuits. Therefore, in multilayer printed circuit boards, it is desirable to make “power” buses in the form of conducting planes located in adjacent layers (Figure 8.19).

The overhead power buses used in printed circuit assemblies on digital ICs have larger transverse dimensions compared to busbars made in the form of printed conductors, and therefore have lower inductance and resistance. Additional advantages of mounted power buses are:

Simplified routing of signal circuits;

Increasing the rigidity of the PP by creating additional ribs that act as limiters that protect the IC with mounted ERE from mechanical damage during installation and configuration of the product (Figure 8.20).

Highly manufacturable are the power buses, manufactured by printing and mounted vertically on the PCB (Figure 6.12c).

There are known designs of mounted busbars installed under the IC body, which are located on the board in rows (Figure 8.22).

The considered designs of the “supply” buses also provide a large linear capacitance, which leads to a decrease in the wave impedance of the “supply” line and, consequently, a decrease in the level of impulse noise.

The IC power distribution to the PP should not be carried out in series (Figure 8.23a), but in parallel (Figure 8.23b)

It is necessary to use power distribution in the form of closed circuits (Fig. 8.23c). This design is close in its electrical parameters to solid power planes. To protect against the influence of an external interference-carrying magnetic field, an external closed loop should be provided along the perimeter of the PP.

Grounding

The grounding system is an electrical circuit that has the property of maintaining a minimum potential, which is the reference level in a particular product. The grounding system in the power supply must provide signal and power return circuits, protect people and equipment from faults in power source circuits, and remove static charges.

The following basic requirements apply to grounding systems:

1) minimizing the overall impedance of the ground bus;

2) the absence of closed grounding loops that are sensitive to magnetic fields.

The ES requires at least three separate grounding circuits:

For signal circuits with low currents and voltages;

For power circuits with high power consumption (power supplies, ES output stages, etc.)

For body circuits (chassis, panels, screens and metallization).

Electrical circuits in the ES are grounded in the following ways: at one point and at several points closest to the grounding reference point (Figure 8.24)

Accordingly, grounding systems can be called single-point and multi-point.

The highest level of interference occurs in a single-point grounding system with a common series-connected ground bus (Figure 8.24 a).

The further away the grounding point is, the higher its potential. It should not be used for circuits with a large spread of power consumption, since high-power FUs create large return ground currents that can affect small-signal FUs. If necessary, the most critical FU should be connected as close as possible to the reference grounding point.

A multipoint grounding system (Figure 8.24 c) should be used for high-frequency circuits (f≥10 MHz), connecting the RES FU at the points closest to the reference grounding point.

For sensitive circuits, a floating ground circuit is used (Figure 8.25). Such a grounding system requires complete isolation of the circuit from the chassis (high resistance and low capacitance), otherwise it is ineffective. The circuits can be powered by solar cells or batteries, and signals must enter and leave the circuit through transformers or optocouplers.

An example of the implementation of the considered grounding principles for a nine-track digital tape drive is shown in Figure 8.26.

There are the following ground buses: three signal, one power and one body. The analog FUs most susceptible to interference (nine sense amplifiers) are grounded using two separated ground buses. Nine write amplifiers, operating at higher signal levels than the read amplifiers, as well as control ICs and interface circuits with data products are connected to the third signal bus “ground”. The three DC motors and their control circuits, relays and solenoids are connected to the power bus ground. The most sensitive driveshaft motor control circuit is connected closest to the ground reference point. The chassis ground bus is used to connect the chassis and casing. The signal, power, and chassis ground buses are connected together at one point in the secondary power supply. It should be noted that it is advisable to draw up structural wiring diagrams when designing RES.