# A magnetic dilemma

#### cmartinez

Joined Jan 17, 2007
6,786
I designed a nifty little mechanism that works by actuating a bi-directional solenoid that uses a cylindrical neodymium magnet (4mm diam, 23 mm long) as its plunger. The solenoid is activated depending on on the RPMs being read from a small turbine that has a couple of small magnets (2x2 mm) at its tips, which are then sensed by a magnetoresistive electronic component.

Everything works wonderfully, except that the solenoid and the turbine are so close together, that the neodymium plunger's magnetic field interacts with the turbine's magnets and prevents the turbine from easily spinning freely.

The obvious solution would be to physically separate both the turbine and the solenoid, but that is simply not possible, considering the space restrictions that the mechanism has at this moment.

So, after some googling around, I found that there is indeed a material called mu-metal that is used to help shield magnetic fields. My intention is to somehow place this material (about 0.010" thick) as a barrier between the solenoid and the turbine, and see if that helps prevent the plunger's magnet from interacting with the turbine's magnets.

My question is, is there anyone here with experience on this sort of thing? I'm about to spend about $50 bucks on a sheet of mu-metal to experiment with, and it would be real sad for me to learn that it doesn't help a darn thing with what I'm trying to do. @nsaspook, I don't know why, but I'm under the impression that you might know something about this subject. #### nsaspook Joined Aug 27, 2009 6,945 I designed a nifty little mechanism that works by actuating a bi-directional solenoid that uses a cylindrical neodymium magnet (4mm diam, 23 mm long) as its plunger. The solenoid is activated depending on on the RPMs being read from a small turbine that has a couple of small magnets (2x2 mm) at its tips, which are then sensed by a magnetoresistive electronic component. Everything works wonderfully, except that the solenoid and the turbine are so close together, that the neodymium plunger's magnetic field interacts with the turbine's magnets and prevents the turbine from easily spinning freely. The obvious solution would be to physically separate both the turbine and the solenoid, but that is simply not possible, considering the space restrictions that the mechanism has at this moment. So, after some googling around, I found that there is indeed a material called mu-metal that is used to help shield magnetic fields. My intention is to somehow place this material (about 0.010" thick) as a barrier between the solenoid and the turbine, and see if that helps prevent the plunger's magnet from interacting with the turbine's magnets. My question is, is there anyone here with experience on this sort of thing? I'm about to spend about$50 bucks on a sheet of mu-metal to experiment with, and it would be real sad for me to learn that it doesn't help a darn thing with what I'm trying to do. @nsaspook, I don't know why, but I'm under the impression that you might know something about this subject.
Mu-metal is a good magnetic field shield that works like a short circuit (redirected) for flux. This means the magnetic field from the small magnets from the turbine tips will also be redirected to the mu-metal to some extent. If the relative field strength from the turbine magnets still activates the sensors with the shielding in place it might work but orientation of the magnet poles might be important.
https://www.kjmagnetics.com/blog.asp?p=shielding-materials

#### SteveSh

Joined Nov 5, 2019
105
Plain old iron or steel should work also, depending on how much shielding you need and what your cost constraints are. The advantage of mu-metal IIRC, is that it weighs significantly less than iron for the same shielding capability. But on the downside it costs a lot more.

#### cmartinez

Joined Jan 17, 2007
6,786
Mu-metal is a good magnetic field shield that works like a short circuit (redirected) for flux. This means the magnetic field from the small magnets from the turbine tips will also be redirected to the mu-metal to some extent. If the relative field strength from the turbine magnets still activates the sensors with the shielding in place it might work but orientation of the magnet poles might be important.
https://www.kjmagnetics.com/blog.asp?p=shielding-materials
So, if I understand correctly, mu-metal is magnetic itself, and the magnets will be attracted to it.... that's a bummer, because that would prevent the turbine from spinning freely too ... gonna give this thing more thought ... see if there's other way around it.

Many thanks for your help, btw. You too, Steve.

#### nsaspook

Joined Aug 27, 2009
6,945
So, if I understand correctly, mu-metal is magnetic itself, and the magnets will be attracted to it.... that's a bummer, because that would prevent the turbine from spinning freely too ... gonna give this thing more thought ... see if there's other way around it.

Many thanks for your help, btw. You too, Steve.
It does not retain a field after the removal of an external magnetizing force but it is attractive to magnets.
That's a necessary property for magnetic shielding so you must design around it.

#### Yaakov

Joined Jan 27, 2019
1,612
So, if I understand correctly, mu-metal is magnetic itself, and the magnets will be attracted to it.... that's a bummer, because that would prevent the turbine from spinning freely too ... gonna give this thing more thought ... see if there's other way around it.

Many thanks for your help, btw. You too, Steve.
In a very limited way, but useful to start thinking, you can analogize the mu-metal to a good electrical conductor and the magnetic field as high voltage electricity.

While you can create a magnetic circuit with the mu-metal, influencing where the lines of force go (shielding), just as with high voltage the surrounding air is not going to be an insulator, and the field will follow the path of least resistance as current does, so the other magnets will still be influenced.

Perhaps you can switch to optical sensing and/or using the shaft rather than the impeller?

#### Alec_t

Joined Sep 17, 2013
10,913
IMO, if the turbine were totally surrounded by a high-permeability material (steel or u-metal) the material would act as the magnetic analogue of a Faraday cage. If that cage were symmetrically positioned relative to the tubine magnets I'm guessing any forces on the magnets would 'cancel out'.
Got an empty baked bean can handy to test the theory?

#### Danko

Joined Nov 22, 2017
872
No side force, only increased axial force and shielded.