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Re: [femm] transient analysis

William,

Can I make a few comments without putting you off experimenting....

If I was building a capacitor discharge induction launcher, i.e. one that produces current in the projectile by induction (and I have built a few) I wouldn't have iron anywhere near it. The performance of an induction launcher is affected dramatically by the magnitude of the projectile current, that current is dependent on the magnitude of the emfs induced in the projectile by the drive coil field and the emfs are dependant on the rate at which the field changes. Any iron (or any parasitic conducting media in the region of the drive coil) will slow down that rate of change. In addition, for good performance the flux densities in the region between the drive coil and projectile must be, at least transiently, very high; probably at least tens of tesla. This is far bigger than iron can sustain and it will just act like air.

I would suggest that you just use FEMM to work out the system inductances and their variation with projectile position and then use those characteristics in a MatLab simulation to work out the forces. The simulation would be able to make a reasonable estimate of the projectile current too. It might be possible to determine from AC FEMM simulations an effective "shorted-turn" representation of the iron so that eddy-current effects could be included in a MatLab simulation.

I have found in the past that static AC simulations - where the projectile is assumed not to move - made at the approximate ringing frequency of the system, allow pretty good comparisons of different coil designs to be made. They don't work in absolute terms but comparisons seem to be OK.

Have fun.

Keith.

At 16:59 13/06/2003 +0930, you wrote:

/me is yet another coilgun idiot, something that FEMM appears to be plagued
with :) I'm the one slashdotted a few months ago for collecting flash
capacitors from disposable cameras; my simulation efforts are resuming now
that I've got hold of some suitable SCRs.

I'm trying to do transient analysis of a thompson coilgun (coil induces
current in conductive ring, currents are opposite due to Lenz's law and
therefore repel). By careful placement of iron, I hope to maintain force on
the armature over non-trivial distances, more than "the length of the
solenoid" that is available from dragging a ferromagnetic armature into a
coil. Therefore I can have longer pulse lengths for a given muzzle velocity,
bigger Cs, lower currents, cheaper switching and more efficiency. I hope :)

I wrote a lua script which measured lorentz force between the coils for
different armature position, size, current, etc, but what I haven't figured
out is how to determine the coupling between the coils and therefore the
actual current that would be induced in the armature. The currents are
currently hardwired in and give me a {F, L} cf position cf I profile for the
structure. Then I plug those values into matlab to simulate an actual
discharge.

So, is there a handy block integral in FEMM that can tell me something like
voltage gradient induced in the ring based on dI/dt (therefore dB/dt) from the
driver coil? I'm using an axis-symmetric model.


for those playing with solenoids,
http://users.on.net/gbt/coilgun/solenoid-9-6-03.tgz
contains yet another solenoid sim. Included is .FEM models, lua scripts,
femm/lua output data and matlab source for simulating a discharge (output is
I, F & velocity graphs). Consider it to be in the public domain.


William Brodie-Tyrrell

--
"There is no God and Dirac is his prophet"
-- Wolfgang Pauli

<wfbrodie@smug.adelaide.edu.au>
http://www.cs.adelaide.edu.au/~william






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