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# Re: [femm] Convergence problem with pure iron?

- To: femm@yahoogroups.com
- Subject: Re: [femm] Convergence problem with pure iron?
- From: David Meeker <dmeeker@foster-miller.com>
- Date: Tue, 15 May 2001 09:54:12 -0400
- Organization: Foster-Miller, Inc.
- References: <9dmnch+rdgg@eGroups.com>

    uwenh@gmx.de 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.

Attachment: `zip00011.zip`
Description: Zip compressed data

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    org:Foster-Miller, Inc.;Electrical and Electronic Systems Group
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- Follow-Ups:
  - Re: [femm] Convergence problem with pure iron?
    - From: Dave Squires <djsquires@plix.com>
  - Re: Convergence problem with pure iron?
    - From: uwenh@gmx.de
  - Re: Convergence problem with pure iron?
    - From: uwenh@gmx.de

- References:
  - Convergence problem with pure iron?
    - From: uwenh@gmx.de

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