What is an optimal configuration of (neodymium)magnets for induction. 1

[gowan]

I'm making a geophone, a seismic sensor.
My plan is to get a 3/4" diameter by 1" neodymium round bar magnet. Then surround it with a collar with a space sufficient to accept a thin plastic cylinder wound with speaker wire.
I notice that in the commercial versions, the wire is wound at the top 1/4 and the bottom 1/4 of the cylinder.
Also, it seems that a 3/4" by 1" cylinder is available, but I would have to buy the outer collar in sections.
I anticipate the movement of the cylindar with the coil(s)
will be typically 1 - 2 mm. (mixing my lenght units)
Should the induction coils be totally inside the the space between the two magnets, or should 1/3 or 1/2 of the coil extend out of the air space? I'm guessing it should be centered in the area of the greatest concentration of flux.
Upon trying this project, I realize that I know little of magnetic circuits,, Where to position the magnets to get a condensed area of flux into which to put the induction coil(s) All help and redirection is most appreciated!

 

[gowan]

My apologies for my previous post.
There is a lot of info I can get on the web which would make the post sound better. Now I see it is an incoherent mess.
I see that what I was trying to describe is a speaker driver, running in reverse.
I found the following software which should help me learn some.

http://www.femm.info/wiki/Documentation.

Thanks

 

[Skogsgurra]

"incoherent mess"! Exactly the words I would have used. But we are such a nice bunch that we don't. Good that you found means to build understanding.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[MagnetDave]

I assume there will be a more coherent version coming. To help that along, let me add a few things:

What's the outer collar made of? Which way is the magnet poled?

And you should generally make the coil overlong. Helps mechanically and magnetically.

 

[gowan]

In my ignorance, I thought the outer coller would also be a magnet with the poles opposit to those of the inner core.
I thought this would concentrate the flux through the coil, expecially if I could put a smaller coil at each end. It was intuitive to me that this would generate the largest voltage for the smallest movement. My intuitions are fallable I have found.
Thanks for the response Peter

 

[electricpete]

FEMM is a very good, powerful, easy-to-use electromagnetic software, with a little bit of a learning curve. The commands can be invoked either with menu's or from Matlab.
There is a very active and helpful user group:

http://tech.groups.yahoo.com/group/Femm/

It is limited to 2-D geometry.

=====================================
(2B)+(2B)' ?

 

[Warpspeed]

If you want the beast to be reasonably linear in operation over a reasonable range of travel, you need a uniform radial magnetic field over a suitable length.

And either the stationary field, or the moving coil, one needs to be very much longer than the other.