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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