RE: [femm] Speed Sensor

["Earanky, Vijay (GEAE, Unison-JAX)" <Vijay.Earanky@xxxxxxxxxxxxxxxx>]

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@xxxxxxxx]
Sent:	Wednesday, May 28, 2003 10:33 AM
To:	femm@xxxxxxxxxxxxxxx
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@xxxxxxxx
www: http://femm.berlios.de/dmeeker


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