PSN-L Email List Message

Subject: Fwd: questions on force balance instruments
From: Mauro Mariotti mariotti@.........
Date: Thu, 18 Aug 2011 11:07:21 +0200




Hi all,

i am not considering myself a true expert.
I am basicly an amataeur. I have made a lot of experiments (from 2008)
with force balance systems for both BB sensor and accelerometers.
I have built two BB sensors prototypes and several FB cells.
What we found is that all is useful and all contribute a little in the
instrument improvement. Amateurs's instruments at the same time can and
cannot perform like a commercial (better to say professional) instrument.
Can, because you may little by little experiment and improve one detail
at a time, step by step.
Cannot, because this require time and money an amateour often don't want
to invest in a super super instrument.
Cannot also because the best materials are not supplied (or not easy to
find) to private people.

What we found in our experiment is that the most important factor for
noise cancelling is the thermal stability of the sensor installation.
In the STS2 manual is written that you barely can go down to 30 seconds
in a "non protected" installation (in terms of themperature).
If you want to get the 120 seconds performance of the STS2 you have to
follow exactly their guidelines in installation, insulation, cable
deployent, basement design, etc.

We surprisingly found confirmed this (at the beginning we was a little
skeptical on this) but we found that (for example) only few degrees of
themperature variation on one BB leg (the adjustable leg over a BB
sensor usually stands for levelling) can dramatically impact on noise
performance.

I found VERY (and i repeat VERY) interesting Randall's consideration on
materials regarding the spring.
It is true that a spring has the only function to keep the mass in the
"rest" position. You can consider the spring like a mechanical reference
(or bias?) for the electronic FB loop but this is not completely true.
For example any themperature variation impacting on the springs will
intruduce a noise in the bias.
In the case of the STS2 (or ours BB prototypes) the hinge and the
springs contribute to keep the mass center of gravity at 54.7 degree
respect to the fulcrum because they are triaxial homogeneous seismometer
in this case both the picometric variations of the hinge flexibility and
spring stiffness contribute to the noise generation.

At 120 seconds a velocity of 1 nanometer per second correspond to a
displacement of about 10 nanometers; the INVAR alloy, in majority used
in BB technology, has a thermal coefficient of 1 ppm/C
In a bar of such material of 0.1 m wich represent the averaged dimension
of one of the component on an amateur design this coefficient
means a displacement of 100 nanometers per 1 (one) Celsius degrees.
And i am considering linear expansion, not volumetric expansion which is
much more.
So the contribution of the thermal noise is a factor greather 10 times
the displacement you are going to measure at 120 seconds with an high
performance alloy.

What is wondering me at momemnt is what kind of mechanical noise can
generate (due to internal frictions on materials) at higher frequency
this 100 nanometers strain.
Yes, strain because this 0.1 m bar is likely screwed and tighted with
other components.
This is one reason that lead BB manufacturers to bake in ovens all
mechanical parts and not only springs during the manufacturing process.

Another great noise contribution is the convective flows of air inside
the instrument case. STS-2 is air-tight and there is vacuum (don't know
how strong) inside in order to avoid air flows to deteriorate or disturb
the mass position.

best regards
Mauro








Il 17/08/2011 19:21, Randall Peters ha scritto:
> Gary,
>       You misunderstand my purpose in all that is being said.  I don't have to be convinced of the wonderful attributes of force balance seismometers. If I had the time and expertise to build one of the superb instruments several of you are using, I would have it sitting right here somewhere in (or close) to my house.
> What I really want to see happen, if possible, is for their performance to increase, based on things that I've learned the last twenty years.
>       The 'replacement to the STS' pictured in the link provided by Brett is stunningly beautiful.  Perhaps these pro's have figured out how to altogether remove any limitations of the spring.  But Brett indicated (?) that his low frequency noise level is not as good as what the professionals are accustomed to  Is it possible for us 'rank amateurs' to do a better job.  Maybe.  Certainly, from my perspective it is a goal to strive for (just on the basis of Kelvin's quote) that Brett recalled to our attention.
>       Randall
>

-- 
Mauro Mariotti
SARA electronic instruments s.r.l.
Via A.Mercuri 4 - 06129 - Perugia
Tel. +39 075 5051014 Fax +39 075 5006315

-- 
Mauro Mariotti
http://www.infoeq.it
http://www.sara.pg.it

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