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

> 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,

       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. 
       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.  
 
> 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 
> 

       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. 

       Regards,

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



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



Hi John,



       The magnet / coil setup in a hard d=
rive can give good feedback or movement sensitivity, but the temperature coe=
fficient 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 ga=
p.=20

       The existing bearings might or migh=
t not work. Stick / slip is the problem. Adding a mass and a spring in such=20=
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 pendul=
um of the SG type should work.  

 

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



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



       Regards,



       Chris Chapman