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# RE: [femm] Speed Sensor

- To: "'femm@yahoogroups.com'" <femm@yahoogroups.com>
- Subject: RE: [femm] Speed Sensor
- From: "Earanky, Vijay (GEAE, Unison-JAX)" <Vijay.Earanky@unison.ae.ge.com>
- Date: Wed, 28 May 2003 11:07:26 -0400

    David,
    Thanks for prompt advice. Like you suggested I want to develop the model.
    Are there any tutorials or available models that can be modified for this
    problem that I can get some guidance? I appreciate if you can help me
    finding some working models to go about. Thanks.

    Vijay

    -----Original Message-----
    From:	David Meeker [SMTP:dmeeker@ieee.org]
    Sent:	Wednesday, May 28, 2003 10:33 AM
    To:	femm@yahoogroups.com
    Subject:	Re: [femm] Speed Sensor

    Earanky, Vijay (GEAE, Unison-JAX) wrote:

    > David,
    >
    > I want to simulate a speed sensor using FEMM. I am new to this
    program and
    > its capability. I will describe the problem briefly here. I have a

    > permanent
    > magnet and attached to it is a core (1010 steel). There are 5000
    turns of
    > wire on it. At the end of the core/coil assembly, there is a gear
    wheel of
    > ferrous material. As the wheel rotates it creates flux in the coil
    and
    > hence
    > an emf generated of ac voltage. Now I want to develop a model to
    see the
    > relationship between no of turns, core material, the air gap
    (between gear
    > tooth and core) and magnet strength. The idea is to get a linear
    curve
    > between the speed and the induced voltage. Can the FEMM model this
    and
    > if so
    > can someone show a direction. Thanks in advance.
    >
    > Vijay
    >
    FEMM can model this sort of problem (as long as 2D is a good
    approximation of your geometry), but you'll get a lot more mileage
    as
    far as getting a feel for number of turns, core material, and air
    gap,
    by first making a relatively simple analytical model of your
    device--the
    effects of all of these parameters would be clearly in evidence in
    such
    a model.

    One interesting thing a simple model would show that if eddy
    currents
    (and possibly mechanical vibrations that would create "significant"
    variations in the nominal air gap or interacting pole area) are
    neglected, the induced voltage waveform ought to to have the same
    profile regardless of speed (just compressed over a shorter time),
    and
    the voltage amplitude ought to scale linearly with speed. This
    means
    that if you are seeing big nonlinearities in the output amplitude,
    I'd
    guess that you should be either looking at motion-induced eddy
    current
    effects in your assembly (FEMM can be used as /part/ of such an
    analysis, but not trivially--again, it would be best to start by
    including eddy current effects in the simple model as parasitic
    "shorted
    turns" around various parts of the teeth, core, and magnet), or
    artifacts of the way that you are measuring and/or filtering the
    output
    voltage. Then again, this could be the kind of situation where a
    little
    Horowitz and Hill would go a long way, e.g. make a circuit that
    looks
    for zero crossings in the induced voltage and ignore second-order
    effects in your sensor.

    Dave.
    --
    David Meeker
    email: dmeeker@ieee.org
    www: http://femm.berlios.de/dmeeker

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