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Subject: compact rare-earth magnet assembly
From: sean@...........
Date: Wed, 9 May 2001 00:56:24 -0500 (CDT)

About making a suitable magnet for a seismometer moving-coil transducer:

(This was first presented in February 2000)

A useful annular or "pot" magnet for seismometer transducers can be 
made from off the shelf or "bought" hardware. There are many possible 
permutations in configuring a magnet to work with a coil. The problem 
is finding the soft iron parts of the magnet assembly without resorting 
to a machine shop. A companion find is the form for the coil to fit the gap.
A singular find will readily work in a one-of-a-kind application, but this
doesn't work for a multiplicity of instruments.

For a prototype of an new broadband instrument, I have been able to
assemble an annular magnet (similar to a speaker magnet) that has an
effective field strength across the gap of about  3600 gauss, and 
when used with the compact formless coil of 5/8" ID x 3/4" OD by 5/8" L
with 12 layers with a total of about 1200 turns, has a constant of
10.6 Newtons/Ampere (Volts/meter/second). (I will describe the coil
in a separate email).

The clearances of the 12-layer coil are still large enough to allow 6 mm 
of movement at an arc length of only 100 mm. So this also makes it
a candidate for a 1/2 sized "STM-8" leaf spring vertical. (The long boom
of the original STM-8 is because of the minimal clearance of the coil in
the 10" speaker magnet, which is designed to move in a straight line.)
It is also useable for a compact (6" boom) horizontal seis of the Lehman

A cross section pictorial of the magnet would look like:

     c     c
     c     c

Where MMM is the magnet, WW is the thick pole washer, PP,BB, and T
are the shoulder bolt, with PP being its head, BB the shaft, and TT 
the threads; www are the base washers. The coil is represented by cc.

THe parts are purchased from McMaster-Carr supply: (630-833-0300; they
take charge cards, no minimum; ask for ~3000 page catalogue)

The magnet is a  rare-earth bonded ring magnet 1.38"OD x 0.827"ID x 3/8" thick.
MCM # 5901K76 for $28.40.

The soft iron upper pole is an oversize washer 1.5" OD x 13/16"ID x 1/4" thick.
MCM # 98099A036 for $1.34
(THe center hole is stamped = tapered by about 0.030; file it square).

The lower pole consists of FOUR washers 1.5" OD x 9/32ID x 3/64" thick
MCM # 91090A113 ; box of 100 for $5.83
(select the thicker ones and drill and de-burr the center hole to 5/16")

THe center pole is a shoulder bolt, with a 3/8" stem 3/8" long, 
a 5/16" thread 1/2" long, , and a 9/16" diameter socket head that is the 
center pole of the magnet gap.
MCM # 91259A617; $1.00 ea.

The thread of the shoulder bolt is somewhat short, so I tap (thread)
the mounting hole in the aluminum bracket. Or less than 4 washers can
be used with a somewhat reduced field (about 5% less with 2 washers)
to allow fastening with a nut.

The magnet will hold itself together for assembly. A touch of super-glue
will permanently position the parts. The magnet will also pinch fingers
and grab anything handy. (The VRDT in the BBT needs to be shielded from it).

The effective field could be at least twice the observed value if the
soft iron parts were custom machined to more effectively concentrate
the field in the annular gap.  A smaller annular diameter might help,
but it requires a different temporary form for the coil. Another trade-off
is the ID/OD possibilities of the thick pole washer. And the shoulder bolt.
(The next smaller washer ( 21/32"ID x 1 3/8" OD) is only 3/16" thick)
Using two magnets and a longer shoulder bolt increases the field by 
about 10%.

The force of the 5/8" long coil is measurably constant over a range of
+ or - 1/16" (2 mm). A shorter coil (3/8") with about 80% of the output has
measurable non-linearities. It seems that the coil should be at least
twice the length of the soft iron poles. Adding a second 1/4" pole washer
makes no appreciable improvement.

Drawings of the design will be posted on my web site when I get a chance
to bring it up to date.  For a few stamps and an address I can mail a copy.


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Larry Cochrane <cochrane@..............>