CONTINUOUS FIELD
- Condition of un-saturation
- To determine the induction inside the material (Bin), we use the following formula: Bin = k x Ho x D / t
(where k is chosen between 1 and 1.5 depending on the other Ho)
Then we verify that Bin <2/3 Bsat
- Calculation of attenuation (Simplified)
- For an infinite cylinder whose field is perpendicular to the axis: At = μ x t / D
- For a sphere:
A = 4/3 x μ x t / D
- More generally:
A = k x μ x t / D
k is a factor depending on the shape of the shield, its
orientation relative to the field and its degree of opening.
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ALTERNATING FIELD
- Alternating fields will create Eddy currents generating a field which opposes the interfering field.
- The result will be determined by a parameter called the skin effect: delta = [2,54.105 x ro / (µ x f)]1/2
(ro = resistivity of the material / f = frequency
- This skin effect will then be used in a chart giving coefficients as: Aac = p x Adc
(Aac: alternating field Attenuation)
(Adc: constant field Attenuation)
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Ho: Outfield
At or A: transverse attenuation or attenuation
μ: permeability
t: thickness of the material
D: diameter of the shield
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CAUTION
- All formulas are based on:
- The conservation of flux (field lines in the air are concentrated in the thickness of the shielding material)
- The principle that the ferromagnetic material deflects the magnetic field lines in a ratio of a given area (in a perpendicular field). However, this ratio depends on the permeability of the material and the size of the screen (a small mumetal cylinder acts differently on its environment than a pure iron screen 2 meters in diameter.
- Permeability is not the same:
- Depending on the field (B-H curve)
- According to the thickness
- According to the frequency
- Although this is only one approach, these formulas have solved thousands of designs.
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MULTILAYER
- The formula for an infinite cylinder with an axis perpendicular to the field with 2 layers of diameters D1 and D2 (D1 A = 1 + A1 + A2 + A1 x A2 x [1 - (D1 / D2) exp2]
- The principle is the same when adding other layers.
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