Magnetic Shield Manufactured from Mu Metal
Most people are familiar with what a Magnetic Shield does, it simply stops or negates unwanted interference from magnetic fields. Not so many people are familiar with how it actually does it.
There are various types of ways and materials used in Magnetic Shielding depending on the frequency and strengths of the magnetic field.
In order to explain for this instance we will use passive low frequency Magnetic Shielding as an example. Shielding of this nature is commonly made from Mu Metal ® (a specialty, high nickel content alloy) and is fashioned into a sheet metal component or assembly before applying a heat treatment process to maximise the shielding factor of the material.
It is natural to assume that a Magnetic Shield does exactly what it say’s and acts as a shield deterring the magnetic field away from the protected area. In essence a Magnetic Shield does exactly the opposite, acting as a “sponge” drawing the magnetic field in. It is here that the clever bit happens – the magnetic field is now directed along the path of the shield, bypassing the protected area inside. This allows the field to circle around our protected area before continuing its journey once it leaves the opposing side of the shield.
Providing the magnetic field is lower than the saturation of mu-metal (0.76 Tesla) the protected area remains free of spurious magnetic field. Once this level of saturation is reached and exceeded, leakage of the field into the protected area will once again occur.
Action of a Magnetic Shield Diverting a Magnetic Field
Hello
I am currently working on a mumetal based protection for a sensor
I want to protect the sensor from an horizontal magnetic field so I designed some kind of bell which I put on my sensor.
However I still get teh perturbations. Can you explain me how the field lines will react to such a protection ?
Thank you in advance
Dear Sir,
Thank you for your question.
In order to help with your query I would need some further information;
1. What is the magnetic field strength that you are trying to shield out?
2. What is the proximity of the field strength to the sensor?
3. Do you have a reading from a Hall probe at the sensor? (not essential)
4. What thickness mumetal are you currently using?
Also a brief sketch of the current set up would be advantageous to obtain a further understanding of your current shield shape and position.
With Kind Regards,
I wind pickups for guitars and pedal steel guitars. I am having trouble with interference form the 60 cycle hum.
Would you recommend your mumetal ? If so, I would appreciate it if you tell me how I use it i.e. build a “cage” around the pickup for example.
The pickups are made out of 10 1/4″x 3/4″ pole magnets and 1000′s of copper windings around the magnets in order to achieve a resistance of up to 19000 OHM
Thanks,and looking forward to your reply.
Bent
Dear Sir,
Firstly, you are correct in assuming that mu metal is the ideal type of shielding at 60 Hz and low fields.
You could use foil to wrap around sides and back of the pick up as you suggest. The main problem with this is that mumetal is very magnetostrictive, and this means that it will lose significant amounts of its shielding properties when bent, so normally it would be best to fabricate a shield and then carry out the special high temperature heat treatment to develop the material properties fully. Obviously, from an aesthetics point of view, a fabricated shield would also be far more visually pleasing than a bent foil shield.
With Kind Regards