All,
The simple fact remains -- Feedback seismometers are the world standard. =
No other technology can come anywhere near their performance and =
operational flexibility.
Their noise levels and bandwidth of the typical off the shelf instrument =
from several sources are such that the only area where significant =
improvement is desired relates to extremely long period performance for =
the study of whole earth modes at 1000 seconds and longer.
Most of the development activity is related to making smaller and less =
expensive instruments and greater flexibility in installation. One =
significant exception is Metrozet where a new instrument to replace the =
STS-1 is under development.
Non feedback instruments are a relic of the past or short period =
geophone or geophone-like instruments which have their niche in local =
event monitoring and the amateur community.=20
The optical open loop instrument is unlikely to have any success (in =
my opinion) simply because it will never be stable. When I fist read =
the paper I concluded the project had no chance of becoming an =
operational instrument but it was an interesting a academic exercise. =
Others with the best credentials in the business have shared that =
sentiment.
Regarding creep effects -- When a spring is first installed in an =
instrument there will be "pops" related to what is probably dislocation =
effects in the spring material . Their frequency will gradually reduce =
in time. The solution is too bake the assembly, with the spring at its =
operational stress, at ~ 160 C for several hours. This will essentially =
eliminate the effect.
After the bake the spring can be removed and reinstalled as long as the =
stress is applied in the same direction upon reinstallation. The =
springs in my instruments are typically at a stress of 150,000 to =
190,000 psi with a yield strength of ~300,000 psi. (17-7 stainless =
treated to CH900 condition )
Dave Nelson
Rolling Hills Estates , California=20
From: Randall Peters=20
Sent: Saturday, August 13, 2011 7:39 PM
To: 'psnlist@..................
Subject: nonlinearities
Also to All:
Nonlinearity (better called mechanical complexity) is what =
ultimately limits, at low frequencies, the performance of every =
seismograph; but it is not the kind of nonlinearity that feedback =
overcomes in remarkable fashion, as is commonly well known. The kind =
that is not accommodated is related to the very reason materials =
creep-because of defect structures. At the mesoscopic level, these =
defect structures cause the potential energy well to be other than =
smooth. In other words, the force required to accomplish feedback =
(standard electrical engineering) is not able, at very low levels, to =
operate on an error signal that is consistent with simple-minded =
theoretical expectations. As one of my astute students years ago said, =
"physics is easy if you don't dig too deep". =20
If the force feedback approach were as perfect as some want to =
believe, then there would have been no reason to hold the IRIS sponsored =
"broadband conference" several years ago, which I attended. A poster =
session that I presented at that conference is online at =
http://www.iris.edu/stations/seisWorkshop04/PDF/tahoeI1.pdf
Anyone with practical experience in materials science (well versed =
in the foundations of the discipline) will recognize that internal =
friction of the spring in a seismometer has got to have consequences. =
One of the first occasions for the reality of this fact to be noted was =
when Gunar Streckeisen measured the damping as a function of period of a =
vertical instrument operating with a LaCoste zero-length spring. What =
he found as a grad student doing this experiment (as I was told by =
Erhard Wielandt, the world's best known expert concerning force feedback =
instruments) - was that the quality factor was not proportional to the =
frequency as required by a viscous damped simple harmonic oscillator =
theory. Rather it was measured to be quadratic in the frequency, which =
is described by the nonlinear damping theory that I developed years ago. =
If you want to Google 'nonlinear damping' and also 'linear damping' =
(without the tick marks of a literal search) you will see that I have =
devoted many years of intense research to this subject. It is indeed =
complex, to the point of bewildering. But it is high time that some =
capable people begin to look at whether my claims have merit or not. =
Some reputable individuals have slowly come around to thinking I'm not =
quite as crazy as they once thought. =20
If you Google "optical seismograph ucsd", you will find a paper =
concerned with a latest generation (bonafide) instrument that is not of =
force feedback type. The author list includes Prof. Wielandt. Much of =
the work presented in that paper was performed by PhD student Otero. I =
encourage folks to take a look at this article, since it describes an =
instrument that could once and for all settle the debate that has come =
now to Larry Cochrane's list-serve. =20
By the way, five years ago Dr. Wielandt wrote a paper that you might =
also want to look at; it is online at
http://www.docstoc.com/docs/48159410/Hysteresis-Creep-Internal-Friction-a=
nd-Damping-of-mechanical--dislocation
and is titled "Are hysteresis, creep, and damping of mechanical =
oscillators consequences of the same mechanism of internal friction"
If you have trouble with the link I pasted here, just Google =
'damping creep Wielandt'. He quotes me in this article, and it has been =
in place for the last five years. His thinking at the time was not =
'settled', but my guess is that this 'preliminary' manuscript would have =
by now vanished from the web if he had changed his mind about it.
Randall
=20
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=20
to making smaller and less expensive instruments and=20
greater flexibility in installation. One significant =
exception=20
is Metrozet where a new instrument to replace the STS-1 is under=20
development.
Non feedback instruments are a =
relic of=20
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 optical open loop =
instrument is=20
unlikely to have any success (in my opinion) simply because it will =
never be=20
stable. When I fist 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.
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=20
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.
After the bake the spring can be =
removed and=20
reinstalled as long as the stress is applied in the same direction upon=20
reinstallation. The springs in my instruments are typically at a =
stress of=20
150,000 to 190,000 psi with a yield strength of ~300,000 psi. (17-7 =
stainless treated to CH900 condition )
Dave Nelson
Rolling Hills Estates , California =
From: Randall Peters
Sent: Saturday, August 13, 2011 7:39 PM
To: 'psnlist@............... =
Subject: nonlinearities
Also to All:
Nonlinearity (better =
called mechanical complexity) is what ultimately limits, at low=20
frequencies, the performance of every seismograph; but it is not the =
kind of=20
nonlinearity that feedback overcomes in remarkable fashion, as is =
commonly well=20
known. The kind that is not accommodated is related to the =
very=20
reason materials creep=97because of defect structures. At the =
mesoscopic=20
level, these defect structures cause the potential energy well to be =
other than=20
smooth. In other words, the force required to accomplish feedback=20
(standard electrical engineering) is not able, at very low levels, to =
operate on=20
an error signal that is consistent with simple-minded theoretical=20
expectations. As one of my astute students years ago said, =
=93physics=20
is easy if you don=92t dig too deep=94.
If the =
force=20
feedback approach were as perfect as some want to believe, then there =
would have=20
been no reason to hold the IRIS sponsored =93broadband conference=94 =
several years=20
ago, which I attended. A poster session that I presented at =
that=20
conference is online at http=
://www.iris.edu/stations/seisWorkshop04/PDF/tahoeI1.pdf Anyone with practical =
experience in=20
materials science (well versed in the foundations of the discipline) =
will=20
recognize that internal friction of the spring in a seismometer has got =
to have=20
consequences. One of the first occasions for the reality of this =
fact to=20
be noted was when Gunar Streckeisen measured the damping as a function =
of period=20
of a vertical instrument operating with a LaCoste zero-length =
spring. What=20
he found as a grad student doing this experiment (as I was told by =
Erhard=20
Wielandt, the world=92s best known expert concerning force feedback =
instruments) =96=20
was that the quality factor was not proportional to the frequency as =
required by=20
a viscous damped simple harmonic oscillator theory. Rather it was =
measured=20
to be quadratic in the frequency, which is described by the nonlinear =
damping=20
theory that I developed years ago. If you want to Google =
=91nonlinear=20
damping=92 and also =91linear damping=92 (without the tick marks of a =
literal search)=20
you will see that I have devoted many years of intense research to this=20
subject. It is indeed complex, to the point of bewildering. =
But it=20
is high time that some capable people begin to look at whether my claims =
have=20
merit or not. Some reputable individuals have slowly come around =
to=20
thinking I=92m not quite as crazy as they once thought. =
If you Google =
=93optical=20
seismograph ucsd=94, you will find a paper concerned with a latest =
generation=20
(bonafide) instrument that is not of force feedback type. The =
author list=20
includes Prof. Wielandt. Much of the work presented in that paper =
was=20
performed by PhD student Otero. I encourage folks to take a look =
at this=20
article, since it describes an instrument that could once and for all =
settle the=20
debate that has come now to Larry Cochrane=92s list-serve.=20
By the way, five years ago =
Dr.=20
Wielandt wrote a paper that you might also want to look at; it is online =
at
http://www.docstoc.com/doc=
s/48159410/Hysteresis-Creep-Internal-Friction-and-Damping-of-mechanical--=
dislocation
and is titled =93Are hysteresis, creep, and damping =
of=20
mechanical oscillators consequences of the same mechanism of internal=20
friction=94
If you have trouble with the =
link I pasted=20
here, just Google =91damping creep Wielandt=92. He quotes me in =
this article,=20
and it has been in place for the last five years. His thinking at =
the time=20
was not =91settled=92, but my guess is that this =91preliminary=92 =
manuscript would have=20
by now vanished from the web if he had changed his mind about =
it.
Randall