Re: [femm] Problem in force calculations

[David Meeker <dmeeker@xxxxxxxxxxxxxxxxx>]

Dave Squires wrote:

> David,
> I have a problem with force calculations in a simple
> permanent magnet simulation. The setup is to have
> two permanent magnets set up in a opposed arrangement
> with + to + (N to N) and an intervening layer of
> soft magnetic material like supermalloy or pure iron.
> This material would be placed against one end of one
> magnet. The other magnet in repulsion is brought
> closer and closer to the iron. There should be a point
> where the force switches from positive to zero and then
> to negative. It will reverse direction from repulsion
> to attraction at some distance. I don't see this with
> FEMM no matter how close I get.
> Any idea why this would be the case?
>
> You can do this test on the bench with a couple of
> NdFeB magnets to prove it to yourself. Just use an
> equal thickness of magnets and iron or steel.
> So in this case FEMM does not seem to reflect reality.
> How come? Is there any way to fix it so it gets the
> right answer? Or am I doing something wrong?

It's sort of hard to say exactly what is going on without seeing the .fem
file for your geometry. In the past, I've tested the force produced on
arrays of permanent magnets against some analytical solutions, and against
some of the examples from Quickfield. The results seem to be sane (although
this is no guarantee, I suppose).

Anyhow, there are several things that you might look at that could be
influencing things. A lot of these issues are summarized in section 4.10 of
the manual. The first is the line over which you perform the integration.
Generally, it's a lot more accurate to calculate forces along a contour
through the air that surrounds a body, rather than on a line at the surface
of the body. The reason is that there can be discontinuities in B and H at
the surface of a ferromagnetic or PM body. These sudden changes aren't
resolved that well by a first-order triangle formulation. In theory, the
integration path doesn't matter so long as it encloses of interest, so more
accurate results are obtained by drawing a path through the air, where
things are better behaved. The second tip is to always use a relatively
fine mesh in the area where perform the integration. Lastly, be careful
that you have located your boundaries adequately far away from the magnets.
For example, the "default" boundary condition (normal derivative of A=0)
functionally behaves like the surface of an iron object. Permanent magnets
are attracted to the boundary in just the same way as if you'd put a big
metal plate there. As long as the boundaries are "far enough" away, there's
no problem--the forces from the boundary as small enough that they don't
muck things up too much. If the boundaries are too close, they influence
the forces of the object of interest in a significant way. (See appendix A3
for more stuff about approximating "open" domains).

Dave.


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