In a message dated 2007/06/30, tchannel1@............ writes:

> Hi Chris,  If I understand, you would use the Arm for the Mass or the Arm 
> also acts as the mass.
> With the mass already centered, you would only need a small amount of mass 
> on a threaded rod to tweak the period?
> Thank You, Ted

Hi Ted, 

       That is correct. Making the horizontal arm fairly heavy to act as the 
mass should give it a very high position stability and symmetry. There should 
be minimal thermal expansion problems. You would then just move the period 
trim mass, which might be only a few % of the total mass. A small error in the 
position of the trim mass should be tiny in comparison to the total mass. 

       The lateral forces from the two pendulums subtract, hence you need 
very high stability. I suggest that you first make the frame, the pendulums and 
the arm and then slide a trim block along the arm. You might start with about 
5% and see if this is adequate or if it could be reduced. You will likely need 
to fit a magnetic damper from the start. Remember that the damping required 
decreases as the period increases.

       If you make the main mass, the trim mass, the mounting fittings and 
the threaded adjuster all out of brass, all the thermal expansions should match. 
The Australians chemically machined a complete square form seismometer, 
including the flexures, out of a solid block of bronze to get very high dimensional 
stability. 

       There are several interesting papers on seismometers and flexures in 
the publications list at http://www.gravity.uwa.edu.au/ There used to be some 
on line both there and a LIGO in the USA.

       Regards,

       Chris Chapman   
In a me=
ssage dated 2007/06/30, tchannel1@............ writes:



Hi Chris,  If I understand=
, you would use the Arm for the Mass or the Arm also acts as the mass.

With the mass already centered, you would only need a small amount of mass=20=
on a threaded rod to tweak the period?

Thank You, Ted




Hi Ted, 



       That is correct. Making the horizontal=20=
arm fairly heavy to act as the mass should give it a very high position stab=
ility and symmetry. There should be minimal thermal expansion problems. You=20=
would then just move the period trim mass, which might be only a few % of th=
e total mass. A small error in the position of the trim mass should be tiny=20=
in comparison to the total mass. 



       The lateral forces from the two pendulu=
ms subtract, hence you need very high stability. I suggest that you first ma=
ke the frame, the pendulums and the arm and then slide a trim block along th=
e arm. You might start with about 5% and see if this is adequate or if it co=
uld be reduced. You will likely need to fit a magnetic damper from the start=
.. Remember that the damping required decreases as the period increases.



       If you make the main mass, the trim mas=
s, the mounting fittings and the threaded adjuster all out of brass, all the=
 thermal expansions should match. The Australians chemically machined a comp=
lete square form seismometer, including the flexures, out of a solid block o=
f bronze to get very high dimensional stability. 



       There are several interesting papers on=
 seismometers and flexures in the publications list at http://www.gravity.uw=
a.edu.au/ There used to be some on line both there and a LIGO in the USA.


       Regards,



       Chris Chapman