Friday, April 8, 2011
【 Weak current College 】 shielding technology profile 】
Shielding is on two space area of isolation between the metal to control electric, magnetic and electromagnetic waves from one zone to another zone of induction and radiation. Specifically, it is a shield to components, circuits, components cable, or an entire system of sources of interference, avoid interference surrounded by electromagnetic field to diffusion; Shield will receive circuits, equipment or system surrounded by external electromagnetic fields prevent them. Because the shield from the wires, cables, components, circuit or system, and other external interference electromagnetic waves and internal electromagnetic waves are absorbed energy play (Eddy current loss), reflective energy (electromagnetic wave shield interface reflection on) and offset the energy (Electromagnetic induction in the shield on the reverse of electromagnetic field, electromagnetic interference may be offset by some), so the shield with reduced interference function.
(1) when the interference of the higher frequency electromagnetic field, the use of low resistivity of metallic materials in the Vortex, formation of external electromagnetic radiation neutralize effect, so as to achieve the shielding effect.
(2) when the interference electromagnetic waves of frequencies lower, to adopt high permeability rate of material, so that the lines limited to shield itself, prevent the spread to the shield of space.
(3) in some instances, if the requirements for high-frequency and low frequency electromagnetic fields has a good shielding effect, often using different metallic materials composition of multi-layered shield.
Many people don't understand the principle of electromagnetic shielding, think as long as a metal box, and then drag the box grounding, is able to play the role of electromagnetic shielding. In this concept under the guidance of the result is fail. Because, electromagnetic shielding and shield ground or not and there is no relationship. Realimpact shield shielding effectiveness of only two factors: one is the shield is conductive surface must be contiguous, and one cannot be directly penetrate the shield of the conductor. Shield on a lot of conductive points, the main one is the shield combination of different parts of the form of non-conductive crevice. These non-conductive crevice produces electromagnetic leaks, as containers of fluid from the slit on the leak. This leak of a method is to fill in the gaps and eliminate the conductive elastic material is not conductive. This is like in fluid container gaps filled rubber of truth. This flexible conductive fill material is magnetic gasket.
In many documents in electromagnetic shield compared to a liquid seal container only when using conductive elastic material will crack sealing to leak levels to prevent electromagnetic leaks. In fact this is inaccurate. Because gaps or voids will leak the electromagnetic wave, depends on the gap or opening relative to the size of electromagnetic wavelength. When the wave of long term greater than open size, and no obvious leaks. Therefore, when a disturbance of the frequency is high, then the shorter wavelength, it is necessary to use magnetic gasket. Specifically, when a disturbance frequency over time, we should consider the 10MHz uses electromagnetic sealing gasket. Where there is flexibility and conductive good material can be used as a magnetic gasket. In accordance with the principle of the manufacture of magnetic gasket:
Conductive rubber: silicone rubber is filled with a share of 70 to 80 per cent of the weight of metal particles, such as silver, copper, aluminum, silver-coated copper powder, silver-coated aluminium, silver-coated glass balls, etc. This kind of material preserved part of the characteristics of silicone rubber good elasticity, while with good electrical conductivity.
Metal braid: beryllium copper wire, monel wire or stainless steel wire braided into tubular long bar, muchlike shielded cable shielding layer. But it's woven method and the cable shield, the cable shield is compiled with multiple wires, and shield liner is made of a thread. Make a figuratively, just like the sweater sleeve. In order to increase the elasticity of metal mesh, sometimes in network management include rubber core.
Finger Reed: beryllium copper made of Reed, has good flexibility and high electrical conductivity. Conductivity and elastic.
Multiple conductive rubber: consists of two layers of rubber, and strata is ordinary silicone rubber, rubber outer is electrically conductive. This material to overcome the traditional poor conductive rubber elastic shortcomings rubber elasticity to embody. It works somewhat like with rubber-metal wire mesh.
Select use what kind of electromagnetic gasket to consider four factors: shielding effectiveness requirements, there are no environmental sealing requirements, installation requirements, cost structures. Shielded by mechanism can be divided into electric shield, magnetic shielding and electromagnetic shielding.
1 electric shielding
【 Shield mechanism 】: electric field induction as distributed capacitance coupled between.
【 Design 】:
A, shielding plate to near protected as well, and the grounding shield plate must be good!!!
B, shielding plate shape on the shielding effectiveness of significant impact. Fully enclosed metal box of the best, but works very hard to do!
C, shielding plate materials to good conductor, but no requirements for thickness, as long as you have enough strength to.
2 magnetic field shielding
Magnetic shielding typically refers to DC or low frequency magnetic field shield, its effect than electric shielding and electromagnetic field shielding to the poor.
【 Shield mechanism 】: mainly rely on high-TC superconducting magnetic materials with low resistance, magnetic flux plays a role of tapped, makes the internal magnetic field shield is greatly diminished.
【 Design 】:
A, adopt high magnetic materials such as Permalloy;
B, increase the thickness of the shield;
None of the above is to reduce the shielding of magnetic resistance;
C, masked objects do not cause immediately shield location to minimize by masked objects in flux;
D, note the shield structure design, where the seams, ventilation air may increase the shield of reluctance, thereby reducing the shielding effect.
E, for strong magnetic field shielding can be doubleMagnetic shield structure.
On to the external magnetic field shield, shield hard outer Select saturation of materials such as silicon steel; internal can be selected easily reached saturation of high permeability material, such as Permalloy, etc. Conversely, if you want to shield the internal magnetic field, then the material order to over. During the installation of a two-layer shield inside and outside, it is necessary to pay attention to their insulation. When no grounding requirements, supporting insulation materials made available. If you need ground, can choose non-ferromagnetic materials (such as copper and aluminum) do support pieces.
3 electromagnetic field shielding
Electromagnetic shielding is the use of shield prevent electromagnetic fields in the space-borne a measure.
【 Electromagnetic field shielding mechanism 】:
A, when electromagnetic waves reach the shield surface, due to the air and metal interface on impedance discontinuity, the incident wave reflex. This reflection does not require shielding material must have a certain thickness, it only requires interface on the discontinuity;
B, no surface reflection off into the shield of energy in the body and forward transmission process, masked material attenuation. It is the so-called absorption;
C, not yet in the shield attenuation off surplus energy to the material of another surface, encounter metal-air resistance is not continuous interface, will form a reflection, and again returnedto shield the body. This reflection on the two metal interface may have multiple reflection.
In short, electromagnetic shield attenuation of electromagnetic is based on the reflection of electromagnetic waves and electromagnetic wave absorption. 【 Absorption loss 】 different materials, different material thickness for electromagnetic wave absorption effect is different. According to the material absorption loss column chart.
【 Reflection loss 】 is divided into three categories: low-impedance electric field, magnetic field, high impedance, plane wave field.
Which low-impedance magnetic field and high-impedance electric field reflection loss column chart calculation methods are the same, and the frequency of metal materials, and the emitter to the shield of distance.
For plane-wave, wave impedance is a constant, and the emitter to shield regardless of the distance, in nomogram of simply connect metal material and the interest rate you can find out at this time of reflection loss values.
4 practical electromagnetic shield
【 Structure material 】
A, apply to the floor and the enclosure of the materials most are good conductors, such as copper, aluminium, etc, you can shield shielding electric field, the main mechanism is reflected signals instead of absorption.
B, on magnetic field shielding need ferromagnetic material, such as high-permeability alloys and iron. The main absorption mechanism of shielding is instead of reflection.
C, in strong electromagnetic environment, requested materials can shield electric and magnetic fields two constituents, therefore need to structure the ferromagnetic material on intact. Shielding efficiency directly affected by material thickness as well as lap and grounding method is good or bad.
D, for plastic shell, is in its wall spraying shield, or in the steam plastic mixed with metal fiber.
Must minimize structure of electrical discontinuity in order to control the plate and shell access to the leakage radiation. Increase gap shielding effectiveness of structural measures including increased the gap depth, reducing the gap length, add in the combination of surface conductive gasket, seams coloured conductive coatings, shorten the distance between the screws.
【 Lap 】
A, in the case of each floor and a crack and discontinuity to best possible lap. The worst power lap on the shell of shielding effectiveness plays a decisive role.
B, guarantee seams metal to metal contact, to prevent the leakage of electromagnetic energy and radiation.
C, where possible, joint should be welded. In restricted circumstances, available spot welding, riveting and Xiao spacing of screws to fixed.
D, without conductive gasket, screw spacing should normally be less than the maximum operating frequency of 1%, at least not greater than 1/20 wavelength.
E, with screw or rivet for lap time, first in the middle of a good lap joints, and then gradually to extend the ends to prevent bending metal surface.
F, guarantee fastening methods have sufficient pressure to deformation stress, shock, vibration, maintain contact with the surface.
G, the unevenness of the place, or on a removable Panel and other places, you must use conductive gasket or finger spring material.
H, select high conductivity and elastic cushioning. Select the pad combined premises to consider when using frequency.
I, select hard pad made of ductile material to cut any metal surface.
J, guaranteed with cushioning material with metal surface does not have any non-conductive layer of protection.
K, when you need an active contact with finger pressure spring, and it is necessary to pay attention to maintain flexibility means spring pressure.
L, conductive rubber gaskets used in aluminum metal surface, it is necessary to pay attention to the role, electro-chemical corrosion. Sterling Silver fillings of rubber or monel linear pad will appear the most serious of electrochemical corrosion. Silver plated aluminium packing of conductive rubber is salt environment for aluminium metal with the best cushioning material surface.
The following are ranked according to priority range of liners.
1 metal RF padding nets easily deformation, pressure 1.4kg/cm, attenuation as 54db.
Data show that the frequency is low attenuation Max. For permanent seal better, not suitable for open and close panels.
2 copper plating alloy has high conductivity and good corrosion resistance.
Good elasticity, the most suitable for and active panel. Can be made into a bar, spiral and jagged surface. Attenuation as 100db.
3 conductive rubber applies to just name a connection and a small amount of screws.
Realization of water vapor sealing and electrical seal by the 48-hour 1500 ° c, the body after agingResistivity is 10 ~/cm 20m ω (max). Deformation of limit values for 25 per cent. Data show that the frequency is high attenuation as a maximum.
4 conductive inner fabric in foam on a piece of silver plated braid, forming a soft cushion, occupies most of the loose space, mainly civilian, suitable for foam gasket Cabinet and door panel.
【 Penetrating and opening 】
A, it is necessary to pay attention to as the cable through the chassis so that the overall reduction of shielding effectiveness. Typical wire is not filtering through the shield, the shielding effectiveness over lowering 30db.
B, power cables into the enclosure, all through the filter cartridge. Filter's input should be put out to the outer shielding casing; if the structure is not suitable for wearing out the filter shell, power cables into the enclosure out specifically for the filter settings of a compartment.
C, signal line, control line into/in and out of the enclosure, through the appropriate filter. With filter insert the needle of multi-core connectors suitable for such use.
D, through shield metal control axis, we should use metal touch tablet,ground nut or RF liner grounded. You can also do not have a grounded metal shaft, and other axis through the waveguide cutoff frequency ratio frequency high Park tube to do control the shaft.
E, attention must be paid within the due wave guided hole through metal shaft or wire shielding effectiveness is seriously degraded.
F, when asked to use the insulation of the metal control axis, the available short invisible control axis, not regulation, nut or metal gasket elasticity install cardboard cover.
G, fuse, Jack, etc. with metal caps.
H, conductive gaskets and washers, nuts, etc. implement button child switch leakage installed.
I, the shield, ventilation and strength requirements of high quality not demanding, with honeycomb plate shield vents, the best way to keep the lines of welded connections, preventing leakage.
J, as in indicator, monitor followed by shielding, and on all lead through capacitor filter used.
K, not from the rear shield indicator/display and pairs leader filtering use and continuous connection of metal casing or conductive glass shield indicator/in front of the monitor. Wafer wire shielding glass, while maintaining reasonable transmittance, for 30 ~ 1000m of shielding effectiveness of up to 50 ~ 110db. In transparent plastic or glass coating of transparent conductive film, its shielding effect is usually not greater than 20db. But the latter eliminates observation Windows on electrostatic accumulation, commonly used in the instrument.