In a message dated 2006/09/30, tchannel@.............. writes:

> Hi Folks,  I have a question about the coil and magnet portion of a sensor. 
> I have seen very small coils and magnets used and very large ones. I guess 
> the common one is about a 25# pull magnet and a coil of about 500 ohms? I am 
> going to rebuild my assembly for two reasons, first I have been told my cow 
> magnet and small coil is just too small.  
> The second reason is I just found a great cheap powerful magnet.  It is the 
> size of a deck of cards, about 50# pull and about $8.00 at Harbor Freight. 
>  
>  Using the same (everything) but three different magnet/coils, undersized, 
> just right, and oversized, what effects would they have, on the gain, final 
> image, etc?
>  
>  I would create a "pancake" coil about the same size as the magnet, square 
> in shape and thin.  
>  
>  Your thought or suggestions please.  If this is a good magnet to use in a 
> coil assembly or using two in a damper assembly, I can give you the part 
> number etc.  They are welding assembly magnets, two in a package.


Hi Ted,

       You don't need this size or bulk. 

       You need quite a few thousand turns on a coil with the ID clear of the 
OD of the magnet, if you use a cow magnet. 

       You do need a coil of 350 to 1000 Ohms to match your opamp input 
impedance.

       The greater the number of turns and the greater the change of flux (= 
area x field) through the coil, the greater the output.

       If you put an unshielded magnet on the arm of a seismometer, you are 
likely to detect every pulse on the house utility power wiring, fridges 
switching on and off, passing cars and trucks and changes in the Earth's field.... 
The signal is likely to be very noisy. The sensitivity of a seismic detection 
system needs to be very high.

       The usual coil / magnet system involves mounting the coil on the arm, 
sitting your magnet on the baseplate and using a non magnetic mass - brass is 
easy to use, but you can also use lead or copper. You need a linear high 
sensitivity response. You have to allow for tilt drift over the week of about 3/8" 
with a Lehman and the sensitivity needs to stay constant and linear within 
this range. This can be provided with a rectangular coil of maybe 2000 turns and 
a quad NdFeB magnet system on mild steel backing plates. 

       I have described my detector system using four 1" square x 1/8" thick 
NdFeB magnets on 1/4" thick mild steel backing plates in the second half of  
http://jclahr.com/science/psn/chapman/lehman/index.html

       You may be able to extract the primary winding from a small low power 
mains transformer and use that? This is the sort which has side by side, not 
overlapping, P & S windings and a square / rectangular former. I don't know 
what is available to you? Larry sells round relay coils, but I prefer a larger 
rectangular form. You may also be able to buy 'spare' mains coils for timers and 
water flow valves on washing machines. Some have a suitable pancake form.

       The coil needs to have an internal rectangular measurement in the 
direction of motion of maybe 1/2" to 3/4" so that its output will be constant, 
allowing for the tilt drift on the seismometer.
 
       I also use an identical mild steel adjustable frame for the damping, 
but it uses a 1/16" copper plate and 1" x 1/2" x 1/4" thick NdFeB bar magnets. 
The damping tongue is mounted horizontally and the damping magnet block is 
simply slid further over it until you get ~0.7 critical damping. If you deflect 
the arm 10 mm and release it, it should swing 0.5 mm beyond the balance point 
before returning to balance. The damping force that you need decreases as the 
period set is increased, and increases with the seismic mass, so it needs to be 
easily adjustable. This construction is easy to set up and use and it is NOT 
temperature sensitive. This magnet + tongue damping arrangement is very 
effective.

       You can buy suitable NdFeB magnets from www.kjmagnetics.com. 

       Regards,

       Chris Chapman
In a me=
ssage dated 2006/09/30, tchannel@.............. writes:



Hi Folks,  I have a questi=
on about the coil and magnet portion of a sensor. I have seen very small coi=
ls and magnets used and very large ones. I guess the common one is about a 2=
5# pull magnet and a coil of about 500 ohms? I am going to rebuild my assemb=
ly for two reasons, first I have been told my cow magnet and small coil is j=
ust too small.  

The second reason is I just found a great cheap powerful magnet.  It i=
s the size of a deck of cards, about 50# pull and about $8.00 at Harbor Frei=
ght. 

 

 Using the same (everything) but three different magnet/coils, undersized,=20=
just right, and oversized, what effects would they have, on the gain, final=20=
image, etc?

 

 I would create a "pancake" coil about the same size as the magnet, square=20=
in shape and thin.  

 

 Your thought or suggestions please.  If this is a good magnet to use=20=
in a coil assembly or using two in a damper assembly, I can give you the par=
t number etc.  They are welding assembly magnets, two in a package.





Hi Ted,



       You don't need this size or bulk. 



       You need quite a few thousand turns on=20=
a coil with the ID clear of the OD of the magnet, if you use a cow magnet. <=
BR>


       You do need a coil of 350 to 1000 Ohms=20=
to match your opamp input impedance.



       The greater the number of turns and the=
 greater the change of flux (=3D area x field) through the coil, the greater=
 the output.



       If you put an unshielded magnet on the=20=
arm of a seismometer, you are likely to detect every pulse on the house util=
ity power wiring, fridges switching on and off, passing cars and trucks and=20=
changes in the Earth's field.... The signal is likely to be very noisy. The=20=
sensitivity of a seismic detection system needs to be very high.



       The usual coil / magnet system involves=
 mounting the coil on the arm, sitting your magnet on the baseplate and usin=
g a non magnetic mass - brass is easy to use, but you can also use lead or c=
opper. You need a linear high sensitivity response. You have to allow for ti=
lt drift over the week of about 3/8" with a Lehman and the sensitivity needs=
 to stay constant and linear within this range. This can be provided with a=20=
rectangular coil of maybe 2000 turns and a quad NdFeB magnet system on mild=20=
steel backing plates. 



       I have described my detector system usi=
ng four 1" square x 1/8" thick NdFeB magnets on 1/4" thick mild steel backin=
g plates in the second half of  http://jclahr.com/science/psn/chapman/l=
ehman/index.html



       You may be able to extract the primary=20=
winding from a small low power mains transformer and use that? This is the s=
ort which has side by side, not overlapping, P & S windings and a square=
 / rectangular former. I don't know what is available to you? Larry sells ro=
und relay coils, but I prefer a larger rectangular form. You may also be abl=
e to buy 'spare' mains coils for timers and water flow valves on washing mac=
hines. Some have a suitable pancake form.



       The coil needs to have an internal rect=
angular measurement in the direction of motion of maybe 1/2" to 3/4" so that=
 its output will be constant, allowing for the tilt drift on the seismometer=
..

 

       I also use an identical mild steel adju=
stable frame for the damping, but it uses a 1/16" copper plate and 1" x 1/2"=
 x 1/4" thick NdFeB bar magnets. The damping tongue is mounted horizontally=20=
and the damping magnet block is simply slid further over it until you get ~0=
..7 critical damping. If you deflect the arm 10 mm and release it, it should=20=
swing 0.5 mm beyond the balance point before returning to balance. The dampi=
ng force that you need decreases as the period set is increased, and increas=
es with the seismic mass, so it needs to be easily adjustable. This construc=
tion is easy to set up and use and it is NOT temperature sensitive. This mag=
net + tongue damping arrangement is very effective.



       You can buy suitable NdFeB magnets from=
 www.kjmagnetics.com. 



       Regards,



       Chris Chapman