All,
The simple fact remains -- Feedback =
seismometers=20
are the world standard. No other technology can come anywhere near =
their=20
performance and operational flexibility.
Their noise levels and =
bandwidth of the=20
typical off the shelf instrument from several sources are such that =
the=20
only area where significant improvement is desired relates to =
extremely=20
long period performance for the study of whole earth modes at =
1000 seconds and longer.
Most of the development activity is =
related to=20
making smaller and less expensive instruments and greater flexibility in =
installation. One significant exception is Metrozet where a new =
instrument to=20
replace the STS-1 is under development.
$$$$ Is that the only area where =
significant=20
improvements are desired ? WHO sell these LESS expensive instruments, =
please ?=20
And less expensive than what ? A new car maybe ?
The lowest cost professional =
quality 3 axis=20
force balance broadband costs about $8000 ( Nanometrics Trillium =
Compact). =20
My vertical force balance vertical can be built by a skilled amateur for =
a few=20
hundred dollars. It is very low noise and has a flat frequency response =
from 30=20
Hz to 50 seconds and will operate over a ~50 Kelvin temperature range =
without=20
re-centering. It does require a very good site and proper =
installation in=20
order to realize its low noise potential. This is true for all low =
noise=20
instruments.
The scientific community seems to =
choose the=20
most expensive instruments when a lower cost instrument might do the =
job. It=20
seems the higher the cost of your equipment the greater the credibility =
of your=20
work. Human nature I guess. However ,for =
research=20
into the structure of the earth ,excellent very long period performance =
is=20
essential . Clearly lower cost should be a goal. However, for =
the=20
professional installation the instrument is not usually the major =
cost=20
item.
Non feedback =
instruments are a relic=20
of the past or short period geophone or geophone-like instruments =
which have=20
their niche in local event monitoring and the amateur=20
community.
$$$$ The UK is covered by an=20
official network of Willmore vertical seismometers. The sensor band =
is=20
either variable 1 to 3 seconds or sometimes 20 seconds to 20 Hz, =
depending=20
on the Model. But we have over 400 horizontal school =
seismometers sensing=20
from 5 Hz to 25 seconds producing signals comparable to=20
the "professional" seismometers. You simply require reasonably =
good=20
design and construction. See http://www.mindsetsonline.co.uk/images/Seismometer.pdf Long period vertical seismometers are more difficult to=20
make.
You make my point. =20
The optical open loop instrument =
is unlikely=20
to have any success (in my opinion) simply because it will never be=20
stable. When I first read the paper I concluded the project had no =
chance=20
of becoming an operational instrument but it was an interesting a =
academic=20
exercise. Others with the best credentials in the business have shared =
that=20
sentiment.
$$$$ If you add "in it's present =
form", I=20
would agree with you. But go back a few centuries and "Others with the =
best=20
credentials in the business" said that the world was flat =
!
Yes -- In its present form. =
However, I still=20
have serious reservations' regarding the fundamental concept .The =
objective of=20
zero power dissipation in the instrument case can probably be met =
with a=20
feedback design with much lower complexity and vastly greater =
stability. =20
The optical readout of the mass position is extremely complex. It =
requires=20
measuring the instantaneous phase of the optical signal to 1 =
degree=20
or better.
Regarding creep effects -- =
When a=20
spring is first installed in an instrument there will be "pops" =
related to=20
what is probably dislocation effects in the spring material . Their =
frequency will gradually reduce in time. The solution is too bake =
the=20
assembly, with the spring at its operational stress, at ~ 160 C for =
several=20
hours. This will essentially eliminate the effect.
$$$$ I =
wish it were=20
that simple. Heat treating the spring under tension does greatly =
reduce the=20
noise, but it doesn't eliminate it. You also have noise from the =
feedback=20
circuit. Differentiating a signal is a noisy process. It could be a =
distinct advantage to combine active damping with passive damping, which =
is=20
quieter.
Brett's analytical work and the actual =
performance=20
of the instruments say otherwise. Passive =
damping=20
will increase noise Not reduce noise. I try to minimize =
passive=20
damping and keep the open loop Q as high as possible. The ONLY =
SIGNIFICANT=20
source of spring related noise is due to the temperature coefficient =
of =20
Young's modulus. It results in a signal proportional to =
the=20
rate of change of temperature.
I will let Brett comment=20
further.
My Best Regards to you Chris --It is =
good to hear=20
from you.
Dave Nelson
Rolling Hills Estates , California=20