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# Re: Convergence problem with pure iron?

- To: femm@yahoogroups.com
- Subject: Re: Convergence problem with pure iron?
- From: uwenh@gmx.de
- Date: Tue, 15 May 2001 20:08:59 -0000
- In-reply-to: <3B013504.6D77C3C3@foster-miller.com>
- User-agent: eGroups-EW/0.82

    Hi Dave!

    thanks a lot for the support. The calc. really got faster as well -
    nice "spin-off" ;))

    Uwe.

    --- In femm@y..., David Meeker <dmeeker@f...> wrote:
    > uwenh@g... wrote:
    >
    > > Hello Everyone, Hello Dave!
    > >
    > > well, since I'm new to the group let me introduce myself. My name
    is
    > > Uwe, I'm living in Frankfurt, Germany.
    > > First of all I wish to express my gratitude to Dave for supplying
    us
    > > with such a good piece of free software. Thanks.
    > > I'm using FEMM now for some months, first of all to get into the
    > > field of electro-magnetics. I'm a mechanical engineer primarily
    > > dealing with electro-mechanical issues who wants to expand his
    > > understanding. Currently I'm working on a solenoid actuator for a
    > > pressure valve. The model consists of a plunger and some
    stationary
    > > parts (let's call these bobbin) carrying the coil. Plunger and
    bobbin
    > > are made of VACOFER S1, that is a high purity iron (more info on
    this
    > > web-site http://www.vacuumschmelze.de/100p_fra.htm). The coil is
    > > modelled by a rectangle to which a current density is applied
    > > amounting to a current per turn of 0.5A, 1A, 1.5A and so on. For
    my
    > > inital trials with the FE model I just used M19 steel. These
    > > calcualtions always ran straight. But of course the forces I got
    were
    > > way off of what to expect. Then I got the data of VACOFER in Excel
    > > and I created my own material entry. But soon I realized that the
    > > computing time rises drastically with the number of data points
    > > used. Sometimes the calculation even seemed to be stuck in an
    > > endless loop. All right I thought, there's to be some trick in
    making
    > > a suitable non-linear BH curve for the computation and started
    using
    > > the "pure iron" material entry of the FEMM material database. Now
    my
    > > model seemed to run reasonable well at least for a current
    density of
    > > 0.8A/m2 (that is 0.5A per turn). But as soon as I switch to higher
    > > currents the calculation seems to stall during the Conjugate
    Gradient
    > > Solver (the cpu load stays 100% for the fkern process, but memory
    > > usage does not change anymore, the .ans file is not yet created).
    I
    > > also tried different meshes etc. But at some point the calculation
    > > seems to get stuck. Now, am I rather impatient (my PC is
    PII300Mhz,
    > > 256MB RAM) or do I have a bad model, or is it possible that the
    > > solver gets numerically ill-conditioned during the calculation
    for a
    > > material like pure iron?
    >
    > The program fits the BH data points with a cubic spline. However,
    it doesn't
    > currently do any checking to make sure that the resulting curve is
    > single-valued. That is, the spline goes through the specified data
    points,
    > but it does funny things in between the data points. If the BH
    curve is not
    > single-valued, the solver has problems converging (and the problem
    is in some
    > sense not physical).
    >
    > Often, you can just plot the BH curve and "eyeball" it to see if it
    looks
    > ok. Unfortunately, because of the scaling of the plot, it is
    difficult to
    > tell whether or not the BH curve is ok. This is actually what is
    going on
    > with the "Pure Iron" curve--the curve fit is messed up in the steep
    initial
    > section, but it's hard to see in the plot. I looked through the
    library, and
    > it appears that the "430 Stainless" entry is also mucked up in a
    similar
    > way--the rest of them appear to be OK.
    >
    > Anyhow, I've attached a modified version of your geometry. I've
    fixed the BH
    > curve in the "Pure Iron" material defined in this problem by adding
    some more
    > points into the BH curve to ensure the the program gets a single-
    valued fit
    > to the BH curve. You can replace the definition of this material
    in the
    > Materials Library with the one in the file by going into the
    library by
    > selecting Properties | Materials Library off of the main menu,
    deleting the
    > Pure Iron entry out of the library, then adding to the library the
    Pure Iron
    > entry from the model.
    >
    > I've also been looking into ways to fix the sensitivity in the BH
    curves. It
    > is easy to programatically check if the curves are OK, but I
    haven't decided
    > whether or not I like the fix that I have implemented in my
    development
    > version--I just repeatedly smooth the BH points using a 3-point
    moving
    > average filter until the resulting fit is single valued. Now, this
    has the
    > advantage that it always works in the sense of providing a smooth,
    > single-valued BH curve that won't hang the solver. However, it is
    possible
    > for a repeatedly smoothed curve not to match up that well with the
    original
    > BH data points...any suggestions are welcome.
    >
    > Dave.

- References:
  - Re: [femm] Convergence problem with pure iron?
    - From: David Meeker <dmeeker@foster-miller.com>

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