In a message dated 2008/05/08, jonfr@......... writes:

> My lehman type sensor is ready. I hope that it gives me up to 30 seconds
> peroid. But I won't know if that is the case of not until tomorrow when I
> put it online at home.

Hi Jon,

       Sorry, but you have made a couple of design mistakes with your Lehman.

       The coil as shown in the photograph WOULD be sensitive IF the magnet 
were able to move in the direction of the arm, but NOT at right angles to it! 
       Turn the magnet at right angles and put the coil out to one side. You 
get a true linear signal as the coil approaches or recedes from the open jaws 
of the magnet, NOT when the magnet jaws stay at ~the same distance from the 
coil, moving parallel to the sufaces of the jaws.
       The setup as shown will only have a very small and non linear 
sensitivity to small movements. It will give a signal of double the frequency. You 
definitely need to change it.

       Putting the magnet on the arm and the coil on the frame allows the arm 
to be sensitive to changes in the Earth's magnetic field, to surges in the 
electricity supply cables, to passing trucks and cars....  With the existing 
setup, you will get quite a noisy signal. It is VASTLY PREFERABLE to put the coil 
on the arm and the magnet on the base frame!

       Using a U magnet and a coil will only give about 1/10 the signal that 
you can get from quad NdFeB magnets on mild steel backing plates. 

       The arm will be sensitive to rocking motions about it's axis. I put a 
cross bar on the arm close to the mass and a V suspension up to the top hinge 
to avoid this problem. 

       I can't see from the photo how you have constructed the bottom 
bearing, or the top suspension. The top suspension is as important as the bottom 
bearing in getting a long period.

       Regards,
  
       Chris Chapman        
In a me=
ssage dated 2008/05/08, jonfr@......... writes:



My lehman type sensor is ready.=
 I hope that it gives me up to 30 seconds

peroid. But I won't know if that is the case of not until tomorrow when I
put it online at home.




Hi Jon,



       Sorry, but you have made a couple of de=
sign mistakes with your Lehman.



       The coil as shown in the photograph WOU=
LD be sensitive IF the magnet were able to move in the direction of the arm,=
 but NOT at right angles to it! 

       Turn the magnet at right angles and put=
 the coil out to one side. You get a true linear signal as the coil approach=
es or recedes from the open jaws of the magnet, NOT when the magnet jaws sta=
y at ~the same distance from the coil, moving parallel to the sufaces of the=
 jaws.

       The setup as shown will only have a ver=
y small and non linear sensitivity to small movements. It will give a signal=
 of double the frequency. You definitely need to change it.



       Putting the magnet on the arm and the c=
oil on the frame allows the arm to be sensitive to changes in the Earth's ma=
gnetic field, to surges in the electricity supply cables, to passing trucks=20=
and cars....  With the existing setup, you will get quite a noisy signa=
l. It is VASTLY PREFERABLE to put the coil on the arm and the magnet on the=20=
base frame!



       Using a U magnet and a coil will only g=
ive about 1/10 the signal that you can get from quad NdFeB magnets on mild s=
teel backing plates. 



       The arm will be sensitive to rocking mo=
tions about it's axis. I put a cross bar on the arm close to the mass and a=20=
V suspension up to the top hinge to avoid this problem. 



       I can't see from the photo how you have=
 constructed the bottom bearing, or the top suspension. The top suspension i=
s as important as the bottom bearing in getting a long period.



       Regards,

  

       Chris Chapman     <=
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