In a message dated 02/01/2003, ivey@.......... writes:

> Mauro,
> I'm not sure it will be _very_ sensitive to axial movements.  Horizontal 
> movements of the magnet will not cause as much change in magnetic flux 
> linkage through the coil as vertical movements because of the geometry of 
> the coil and magnet.  For movement in one horizontal direction, the magnet 
> is moving parallel to the wires at the top of the coil.  For the other 
> horizontal direction, the magnet is moving parallel to the flux between 
> it's poles.  In either case any flux change through the coil is a second 
> order effect due to curvature of the coil and curvature of the magnet path. 
>  For vertical movement the flux change is a first order effect, 
> proportional to displacement.  As a guess I would expect the horizontal 
> sensitivity to be 5x to 10x down from the vertical.  
> Jack

Hi Jack,

     I am in agreement with your analysis so far as it goes, but it seems to 
be incomplete and the conclusion may not be correct. The magnetic damping is 
presumably adjusted to be near critical for vertical motion. This should also 
give reasonably good damping for horizontal motion in the plane of the plate. 
However, the plate will be only lightly damped for horizontal oscillations 
perpendicular to it's plane and it will show a resonant characteristic with 
enhanced swing amplitudes. The magnetic field is highly inhomogeneous and 
movements of the magnet in this plane can be expected to give significant 
induced signals. The sensitivity could be decreased by mounting the magnet 
and sensor coil at right angles to the original orientation shown. 

     An arrangement which is less sensitive to horizontal motion and can give 
a larger signal, may be made using a horizontally mounted flat coil with a 
vertical cylindrical NdBFe magnet in the centre. A 1/2" dia by 1/2" long 
magnet Type #13 from www.wondermagnet.com at $4 would be suitable. The end of 
the polepiece should be about the centre plane of the coil. A suitable coil 
would be about 3/8" long and 5/8" internal diameter, wound with 3,000 to 
5,000 turns.

     Neodymium magnets type #31 or #39 are capable of giving very high 
damping forces, probably at considerably less cost than the 100 lb pull 
damping magnets in the original design. I use two opposed #31s with a small 
1/8" thick Al plate to critically damp a 1.1 kg pendulum. With very strong 
cheap magnets readily available, we might as well make use of them.

     Regards,

     Chris Chapman 

> >> From: Mauro Mariotti [mailto:mariotti@..........
>> Sent: Monday, December 30, 2002 2:01 PM
>> Subject: Re: Notes to UK--Nick
>> Hi all,
>> I suggest a different approach for homemade vertical sensors. The sensor 
>> in the link you propose is VERY sensitive to axial movements
>> (horizontal movements) if you give a look to 
>>  http://www.infoeq.it/doc02v_e.htm  
>> and scroll the page until you reach:
>> Lacoste suspension sensor 
>> you can see a good approach to a sensitive long period seismometer.
>> ....... 
>> Regards Mauro
>> 
>> >>> From: John & Jan Lahr 
>>> I've scanned Jim's Short Period design plans and put a copy on my web 
>>> site here: http://jjlahr.com/science/psn/lehman/
>>> This will save on postage and make the plans available to more people.  
>>> They
>>> are a good example of how to document a seismic sensor design.

In a message dated 02/01/2003, ivey@.......... writes:



Mauro,

I'm not sure it will be _very_ sensitive to axial movements.  Horizontal movements of the magnet will not cause as much change in magnetic flux linkage through the coil as vertical movements because of the geometry of the coil and magnet.  For movement in one horizontal direction, the magnet is moving parallel to the wires at the top of the coil.  For the other horizontal direction, the magnet is moving parallel to the flux between it's poles.  In either case any flux change through the coil is a second order effect due to curvature of the coil and curvature of the magnet path.  For vertical movement the flux change is a first order effect, proportional to displacement.  As a guess I would expect the horizontal sensitivity to be 5x to 10x down from the vertical.  

Jack




Hi Jack,



     I am in agreement with your analysis so far as it goes, but it seems to be incomplete and the conclusion may not be correct. The magnetic damping is presumably adjusted to be near critical for vertical motion. This should also give reasonably good damping for horizontal motion in the plane of the plate. However, the plate will be only lightly damped for horizontal oscillations perpendicular to it's plane and it will show a resonant characteristic with enhanced swing amplitudes. The magnetic field is highly inhomogeneous and movements of the magnet in this plane can be expected to give significant induced signals. The sensitivity could be decreased by mounting the magnet and sensor coil at right angles to the original orientation shown. 



     An arrangement which is less sensitive to horizontal motion and can give a larger signal, may be made using a horizontally mounted flat coil with a vertical cylindrical NdBFe magnet in the centre. A 1/2" dia by 1/2" long magnet Type #13 from www.wondermagnet.com at $4 would be suitable. The end of the polepiece should be about the centre plane of the coil. A suitable coil would be about 3/8" long and 5/8" internal diameter, wound with 3,000 to 5,000 turns.



     Neodymium magnets type #31 or #39 are capable of giving very high damping forces, probably at considerably less cost than the 100 lb pull damping magnets in the original design. I use two opposed #31s with a small 1/8" thick Al plate to critically damp a 1.1 kg pendulum. With very strong cheap magnets readily available, we might as well make use of them.



     Regards,



     Chris Chapman 



From: Mauro Mariotti [mailto:mariotti@..........

Sent: Monday, December 30, 2002 2:01 PM

Subject: Re: Notes to UK--Nick

Hi all,

I suggest a different approach for homemade vertical sensors. The sensor in the link you propose is VERY sensitive to axial movements

(horizontal movements) if you give a look to 

 http://www.infoeq.it/doc02v_e.htm  

and scroll the page until you reach:

Lacoste suspension sensor 

you can see a good approach to a sensitive long period seismometer.

........ 

Regards Mauro



From: John & Jan Lahr 

I've scanned Jim's Short Period design plans and put a copy on my web site here: http://jjlahr.com/science/psn/lehman/

This will save on postage and make the plans available to more people.  They

are a good example of how to document a seismic sensor design.