PSN-L Message - Subject: RE: Round external coils and stacked internal magnets

Meredith,

I can’t visualize the magnet layout you speak of = with 3 magnets. Do you have a magnet placement diagram so that I could map out the = magnetic fields.

Gary Lindgren

From:= psn-l-request@.............. [mailto:psn-l-request@............... On = Behalf Of meredith lamb
Sent: Friday, May 16, 2008 9:22 PM
To: psn-l@..............
Subject: Re: Round external coils and stacked internal = magnets

Hi Chris and = all,

Thanks Chris for the extensive reply below. Actually my impression = with the stacked magnets came from
trying to utilize a old surplus Texas Instruments rectangular coil form, = and at the time, the stacked
magnets approach seemed quite impressive at the time via a table top and voltmeter experiment.

The problem with the coil is it has relatively thin walls, and I'am not = so sure that a quad (4) magnet
setup is the best or only approach. Its possible to use 3 magnets = and essentially get a similar "quad" (4 pole)
arrangement, where the fields go through the sides of the coil = walls.

The coil form shape is quite similar to transformer coil forms, and in a = sense there maybe 2 ways to
use magnets with them. Esterline-Angus also made similar coils = back in the 1950's, of which
I also have some of those.

I put up a quick web page with 2 photos on the TI coil; along with my "notes/ramblings" related to such:

h= ttp://seismometer.googlepages.com/tirectiwritercoilconversion

Take care, Meredith Lamb

On Fri, May 16, 2008 at 2:56 PM, <ChrisAtUpw@.......> = wrote:

In a message dated 2008/05/16, paleoartifact@......... writes:

Am getting to the (shall we say a unqualified) opinion that with a choice = of round or square / rectangular coils with a magnet, that it seems like the = round coil approach is the more efficient route to take.

Hi Meredith,

       If you want an accurately linear = output from a Lehman, while allowing for the large sensor drifts that you = always get, you definitely need to use a quad arrangement of square NdFeB magnets = and a rectangular coil.

       Round coils will DEFINITELY NOT = give you a constant sensitivity over the drift range of a Lehman sensor.
       If you are using a vertical = pendulum, you can use a quad arrangement of rectangular magnets with the width of a = magnet ~ the ID of the coil. Alternatively, you can make up a magnet block and = attach it to the pendulum to form part or all of the mass. But you will then still = get a small sensitivity to environmental magnetic fields.

       You make the ID of the actual = windings about 3/4 of the width of a magnet and the internal length about 1/5 = greater. This allows for some position error either as the coil swings in an arc = or for any setup position error. The flux change for a given magnet = movement is then CONSTANT over maybe 3/4" drift with 1" square = magnets.

       Mount the magnets on 1/4" = thick mild steel backing plates held apart with zinc plated mild steel bolts. You = need to give the mild steel some etch primer and a coat of anti rust paint. I = use Hermetite. You paint the whole surface of the plates, then leave them a = couple of days for the paint to harden. I make up a cardboard cut-out, clamp it = to the plate and slide the magnets into position over the cardboard. This is = 'the easy way to do it'!

       One advantage of this construction = is that you only need maybe 2,500 turns on you coil. The voltage output is maybe = 10x what you get with a U Alnico magnet.

       This arrangement is shown on the = drawings at http://www.jclahr.com/science/psn/chapman/lehman/index.= html

       It is quite easy to make a = rectangular coil former out of thin glass circuit board / sheet glass fibre. DigiKey = sell both. You can either use the construction shown, or you can make a wood = / plastic inner coil former and screw / glue it between two sheets to form = the ends of the coil. I suggest winding it using a bolt through the centre = gripped by a hand drill, held in a vice. Use some fine plastic tube to guide the = wire onto the coil. Spray cans usually have suitable tube if you open one up. = I use two part Acrylic glue - Devcon Plastic Welder or similar.

       I defiitely avoid using 'coil dope' = - I find that I get too many shorted turns. You can use coil dope OK if you = very carefully wind a single layer of wire at a time and then use tissue = paper to interleave the winding layers.

       With luck, you are only going to = ever make one coil per seismometer, so it REALL IS worth a bit of extra effort to = get the best out of your instrument.

       Regards,

       Chris Chapman

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