Thanks for the comments.  I hadn't thought about the temp coefficient
but these things usually end up in controlled temperature housings
anyway.  I built a unit once that used a copper plate in a HD magnet for
damping-the Lorenz force was truly amazing.  Dynamic range is a thought
I hadn't considered.   Our area is seismically so quiet tha it probably
won't be much of a problem.  I was thinking that this might be a good
student project as it wouldn't require much in the way of construction
and certainly could produce a demonstrable result.  I'm always looking
for odd-ball ways of doing things and not necessarily trying to produce
a scientific grade instrument for serious study.
=20
jn

	-----Original Message-----
	From: ChrisAtUpw@....... [mailto:ChrisAtUpw@...........
	Sent: Tuesday, June 24, 2003 3:15 PM
	To: psn-l@..............
	Subject: Re: novel idea, maybe.
=09
=09
	In a message dated 24/06/03, jnelson@................. writes:=20
=09
=09

		I was disassembling a defective half height hard drive
over the weekend and had an interesting idea. I was removing the
platters and was down to the magnet, coil and arm. I thought, the only
thing missing here to make a seismometer is a weight at the end of that
arm.  I put a car wheel weight (about 100 gm) and sure enough, looking
at a sound card input, it is about the same sensitivity as my big
vertical-at least from 1 hz to 40 hz.  It could just as easily be a
vertical with a different mounting and the addition of a spring.



	Hi John,=20
=09
	      The magnet / coil setup in a hard drive can give good
feedback or movement sensitivity, but the temperature coefficient of the
magnets is quite high. This four pole magnet setup will give very
effective damping if you put a Cu or Al damping plate in the magnet gap.

	      The existing bearings might or might not work. Stick /
slip is the problem. Adding a mass and a spring in such a way as to get
a natural extended period is likely to be more difficult.=20
	      Otherwise you need a good distance transducer and a
feedback circuit. Using the magnet / coil on a 1 sec pendulum of the SG
type should work.  =20
=09
=09

		This may prove to be an interesting project, I think it
may be at least as sensitive as a geophone. I still need to do some work
on the amplifier section, it's only half done. Incidentally, I'm using
LTC1063 digital 5 pole low pass filters for filtration-anybody else ever
use these?=20



	      Most use analogue filters. Digital filters can be useful,
but they have a relatively high noise floor, which limits the dynamic
range that they can handle. You might check if this is a limitation in
your application.=20
=09
	      Regards,=20
=09
	      Chris Chapman=20






Thanks=20
for the comments.  I hadn't thought about the temp coefficient but =
these=20
things usually end up in controlled temperature housings anyway.  I =
built a=20
unit once that used a copper plate in a HD magnet for damping-the Lorenz =
force=20
was truly amazing.  Dynamic range is a thought I hadn't=20
considered.   Our area is seismically so quiet =
tha it=20
probably won't be much of a problem.  I was thinking that this =
might be a=20
good student project as it wouldn't require much in the way of =
construction and=20
certainly could produce a demonstrable result.  I'm always looking =
for=20
odd-ball ways of doing things and not necessarily trying to produce a =
scientific=20
grade instrument for serious study.
 
jn

  

  
-----Original Message-----
From:=20
  ChrisAtUpw@....... [mailto:ChrisAtUpw@........ 
Sent: =
Tuesday, June=20
  24, 2003 3:15 PM
To: psn-l@..............
Subject: =
Re:=20
  novel idea, maybe.

In a message dated 24/06/03, jnelson@................. =
writes: 


  I was disassembling a defective half height hard drive =
over the=20
    weekend and had an interesting idea. I was removing the platters and =
was=20
    down to the magnet, coil and arm. I thought, the only thing missing =
here to=20
    make a seismometer is a weight at the end of that arm.  I put a =
car=20
    wheel weight (about 100 gm) and sure enough, looking at a sound card =
input,=20
    it is about the same sensitivity as my big vertical-at least from 1 =
hz to 40=20
    hz.  It could just as easily be a vertical with a different =
mounting=20
    and the addition of a spring.


Hi John,=20
  

      The magnet / coil setup in =
a hard=20
  drive can give good feedback or movement sensitivity, but the =
temperature=20
  coefficient of the magnets is quite high. This four pole magnet setup =
will=20
  give very effective damping if you put a Cu or Al damping plate in the =
magnet=20
  gap. 
      The existing bearings =
might or=20
  might not work. Stick / slip is the problem. Adding a mass and a =
spring in=20
  such a way as to get a natural extended period is likely to be more =
difficult.=20
  
      Otherwise you need a good =
distance=20
  transducer and a feedback circuit. Using the magnet / coil on a 1 sec =
pendulum=20
  of the SG type should work.   


  This may prove to be an interesting project, I think it =
may be=20
    at least as sensitive as a geophone. I still need to do some work on =
the=20
    amplifier section, it's only half done. Incidentally, I'm using =
LTC1063=20
    digital 5 pole low pass filters for filtration-anybody else ever use =
these?=20
  

      Most use =
analogue=20
  filters. Digital filters can be useful, but they have a relatively =
high noise=20
  floor, which limits the dynamic range that they can handle. You might =
check if=20
  this is a limitation in your application.=20
  

      Regards,=20
  

      Chris Chapman=20