Design magnetic shield is not a difficult science, sometimes just experience, carefully paying attention to details, some tests and error tests can be used to generate desired results.
There are many factors that affect shielding performance. Of course, the permeability and saturation sensing of the shield material itself must be carefully selected ... and the type of annealing. But this is just a start. Remember that magnetic shield is not true "shield" in the traditional sense. The magnetic field line cannot be blocked or blocked. They will move from the source north to the South Pole. What we can do is to change the path of these magnetic fields in their transmission. The magnetic shielding material "conduction" magnetic field line is better than air (and most other materials). In a sense,
They create a "minimum resistance path", where the magnetic field line can travel. However, if the magnetic field path is a lower energy path, it will travel in this alternating path. Not just because we want it.
The following is a list of incomplete factors must consider, as well as each comment:
Shielded structure shape
In general, the spherical and cylindrical shielding effects are best. This is because the magnetic field line resists sharp turns. When the spherical and cylindrical shapes are unlikely, it is best to be bent with a curve instead of using a crease. Note, but the cylindrical shield around the line does not prevent the magnetic field emission. However, it protects the wire from the impact of the magnetic field from the shield. You see chart.
Pay attention to curved materials. Since the magnetic energy is highly dependent on the size / shape of the metal crystals, any changes to the crystal size / shape, such as bending or high temperature heating, will adversely affect the ability of the magnetic field line through the region. Regular bending is absolutely necessary. A material that can be decreased by some performance, or renewal, to achieve peak performance.
Shielded size
Usually, the shield is larger, the more magnetic field lines "attracted". However, the magnetic field line that travels at a distance from the shielding position will not have motivation through the shield. See the right figure. On the other hand, larger shields will also "conduct" more Earth's magnetic fields. At approximately 400 mg, Earth fields can be saturated with high permeability shields if they are large enough.
For general recommendations for typical applications in home environments, people shielded circuit boxes with planar shields are regions where the extension exceeds the source of about 2-3 feet. It is therefore recommended to use approximately 6 feet x 6 feet of protective cover. If the existence of the side wall, the floor or ceiling limits the size of the shield, the shield material can continue at the corner, but the metal must be continuous or appropriately connected.
The number of layers (or thickness)
Shielding performance increases as the number of layers increases. This relationship is non-linear. In the end, the thickness required will depend on how much attenuation and cost is required.
General recommendations of typical applications in home environments are to shield a circuit cartridge with flat shield using at least 0.020 inches of highly magnetic magnetic material.
General recommendations for typical speaker magnet applications are 3 concentric cup-shaped masks for high saturation alloys. A cardboard gasket is used between layers. Place the stacked cup on the back of the speaker magnet.
Spacing between the shield
The spacing between the shield increases the shielding performance. The larger space, the better performance. Almost all materials can be used as a gasket: wood, plastic, dry wall, glass, air, cardboard, other metals, etc. Other factors must be considered, such as cost, strength, flammability, weight, and the like.
Shielded distance from the ground
As the distance source is increased, the concentration of the magnetic field line is lowered. Saturation may not be a problem unless you must increase the shield size.
In the direction of the magnetic field when blocking
Aligning the shield surface or almost parallel to the direction of the magnetic field will produce the best shield. Vertically shielded magnetic field lines do not change their routes when passing through materials. See the figure below.
This can be used to benefit. For example, a flat mask is perpendicular to the geostal magnetic field orientation to minimize the saturation problem caused by the Earth field.
Also note how the magnetic field is concentrated on the shielded edge shown on the right. The Gaussian reading at these edges will be higher than the case where there is no shield. Shield should be considered in the position of these edges.
Magnetic field intensity encountered
The magnetic field intensity at the source is independent of the shield design. The field strength at the proposed location of the shield is related. The stronger the position of the shield position, the thicker the shield should be.
Magnetic field frequency
Magnetic field shielding performance decreases as the magnetic field frequency increases. Attenuation is proportional to the penetration rate, all other factors are the same. Please note that the magnetic permeability declines in 1kHz
Earth magnetic field (and other environmental magnetic fields).
When the shield is very large, the ubiquitous Earth's magnetic field is ubiquitous to become an important factor in shield design. Saturation is the transient and reversible process, wherein the shield material "conduction" it can have all magnetic fields. The presence of shields does not affect additional magnetic fields. The higher the permeability of the material, the lower the saturation point.
There are several strategies that overcome saturation:
1] Increase the material thickness / layer number. This will be useful if the magnetic field is slightly more than the initial shield design.
2] Use a multi-material method: use a high saturation material on one side closest to the field, using a high saturation material on the other side.
3: If possible, position the shield to avoid saturate (as discussed above with the above part of the direction)
Handling seams and seams
When making a larger shroud using multiple shielding materials, you should pay attention to the seam / seam. The magnetic field line will "jump" the air gap between the two adjacent shields, but since the introduction such a gap will increase the overall "resistance" of the shield design, it will impair the shielding performance. The good contact of the metal and metal at the seam / seam is very important. This goal can be achieved in several ways:
1] overlap. The two edges of the shield can overlap, usually from 1 to 2 inches. Compression, such as the compression between the dry wall or the plywood layer will help achieve good metallic and metal contacts.
2] tape. The two edges can be docked, and then the adhesive is bonded using a high permeable tape (for example, Joint-ShieldTM). Of course, the shield should be appropriately fixed in place by the mechanical device to avoid partial separation.
Magnetic shields and foils can be fixed with nails, u-shaped nails, screws, glue, rivets, or any other suitable mechanical device in place, which will support weight and prevent annoying vibration or displacement. The small perforation caused by a small number of nails or screw holes does not significantly affect the shielding performance (different from the radio frequency shield). The edges of exposure should be avoided to prevent injury from contact.
Other factors that need to be considered, for example:
Temperature, vibration, sharp edge, corrosion resistance, structural strength, weight, fixation method, ventilation, perforation, cleaning ability, scratch / dent / treatment, etc., of course Also: cost. Finally, the magnet is always attracted by the magnetic shield alloy.
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