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PERMANENT MAGNETS LIMITED Phone no: +91 22 28454164 Fascimile: +91 22 29452128 Email: sales@pmlindia.com |
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Magnetic Shields
Introduction of
Shields
We are
surrounded by magnetic fields (both AC and DC) from the earth’s magnetic
field to man-made sources such as magnets, 
motors and transformers. To avoid
effect of this field on sensitive equipments, magnetic shielding is required.
e.g. cathode ray tubes, photomultiplier tubes, audio transformers, scanning
electron microscopes, position sensors.
The
shield is made of soft ferromagnetic material with a high Permeability
(μ). 36%, 48% &80% NiFe are most commonly used soft magnetic alloys.
In order to have a low Hysteresis, the shield is annealed after shaping. Any
stress after annealing will deteriorate the performance and hence should be
avoided.
How does a magnetic
shield work?
There is no known
material that can block magnetic fields without itself being attracted to the
magnetic force. A magnetic shield acts as a kind of sponge redirecting the magnetic
field around the shield instead of passing through the sensitive instrument
which is being shielded. A good magnetic shielding material must have high
permeability which means that the magnetic field lines are strongly attracted
to the shielding material. If the magnetic field is too high for the material
chosen it will saturate and become ineffective. In this case, a multi layer
shield with a combination of the above alloys can be used.
Shielding Factor:
The
Shielding Factor is defined as the ratio between the external and the internal
field when the shield is placed in an area with a homogenous magnetic field.
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The
Shielding Factor is defined as the ratio between the external and the internal
field when the shield is Shielding Factor = External Field / Internal Field.
Saturation:
The
magnetic saturation of the shield is given by the magnetic properties of the
shield material.
Ferromagnetic
material saturates between 0.5 and 2 Tesla. Once the shield is saturated, its
apparent Permeability will decline to the value 1, meaning that the shield acts
like air. A saturated shield does not guide nor attract magnetic flux lines.
Therefore, the shield design has to be chosen such that the field generated by
the current and the field generated by external sources do not saturate the
shield material.
General Principles of shield Designing:
In
the following sections we will describe how the different geometries influence
the Shielding Factor, Magnetic gain and Saturation level.
Influence of Width W:
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Influence |
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W
has an influence on the magnetic gain and also on the Shielding Factor. We
recommend choosing W as small as possible. The magnetic gain and the Shielding Factor are reciprocally proportional to W. |
Influence of Length L:
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Influence |
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L
has an influence on the Shielding Factor. We recommend choosing L as big as
possible. The absolute minimum of L is the length of the part (visible in
blue). The
maximum is open, but 8mm more than L is sufficient. The
Shielding Factor increases with the length of the shield L. Once the shield
is long enough, approximately the length of the part ( l +8mm), a longer
shield will not improve the Shielding Factor significantly. |
Influence of Height H:
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Influence |
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H
has an influence on the magnetic gain and on the Shielding Factor. We
recommend making H’ (from the top of the PCB surface to the end of the
shield) about 10mm. The absolute minimum of H’ is about 6mm (visible in
blue). The maximum is not relevant, but the shielding effect will not
increase significantly for a shield with H longer than 15mm. It is important
that both sides of the shield have the same height H. The Shielding Factor increases with the height of the shield H. Once the shield is high enough, approximately 15mm, a higher shield will not improve the Shielding Factor significantly. |
Influence of the Thickness t :
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Influence |
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t
has an influence on the saturation level of the shield. We recommend making t
minimum 0.8mm. Once the shield is thick enough, the increase of the thickness
will not improve the saturation level anymore. In the most of the case a
shield thicker than 2mm is not needed. The
saturation level increases with the thickness t of the shield. Once the
shield is thick enough, increase of t will not improve the saturation level. |
Magnetic Shielding Definitions
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Property |
Unit |
Formula |
Definition |
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1 |
Attenuation (g) |
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g= Ho/ Hin |
Ho : Field intensity outside in Oersteds
(Oe), Hi : Field intensity inside in Oersteds (Oe) |
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2 |
Shielding Efficiency (SE) |
dB |
20log o. g |
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3 |
% shielding |
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(1-1/g) x 100 |
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4 |
Attenuation in Uniform DC field ( g DC) |
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(µ/4) x (1-a²/b²) |
µ -
Permeability of Material in Gauss |
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a
- Inner radius of shield |
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b
- Outer radius of shield |
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t
- thickness of shield |
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(µ/4) x (T/b) (2-T/b)≈µT/2b |
Shielding effectiveness depends only on the
permeability of the material and the ratio of wall thickness to outer radius.
This holds true for cylinders with a length to diameter ratio of 4 or more |
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5 |
Attenuation in AC field g (AC) |
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Designing for a DC field provides a
maximized shield in AC fields of equal density ( AC Peak) |
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Field Strength (H) |
Oersteds(Oe) |
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lines/cm² in Air |
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Flux Density(B) |
Gauss (G) |
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lines/cm² in Air |
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Permeability (µ) |
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µ=B/H |
measure of material's capability to conduct
magnetic lines of force or flux |
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Magnetic saturation level |
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The flux level at which the material can no
longer conduct any additional lines of force. |
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Reluctance ( R ) |
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R = l/µA |
The measure of material's resistance to the
passage to magnetic flux. l = flux path length (cm), A = cross sectional area
(cm²) |
What PML
can Offer:
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Shields which are punched
& bent or punched and deep drawn for various applications like
electricity meters, electronic control protections in automobiles, avionics,
medical devices, audio devices.
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