PSN-L Email List Message
Subject: Re: Seismometer Pivot Studies - email
From: Charles Patton charles.r.patton@........
Date: Thu, 16 Oct 2008 23:22:35 -0700
(Bob sent the following email and said it would be OK to share it and my
reply with the PSN list.)
Bobhelenmcclure@....... wrote:
************************************
You advise the following:
Do NOT use either a 'point in a cup' or a genuine metal knife edge on a
flat for a suspension system. Both apply stresses over or near the
maximum strength of the metal and even if they work initially, they will
not last long. If the edges do not either roll over or shatter, they
will dig into the counterface. These systems are a common cause of poor
performance in amateur seismometers.
I have violated this advice for years, and have never experienced any
problems. I use pivots and sapphire cups salvaged from milliammeters. My
horizontal pendulums weigh only 100 grams, but this is still a much
higher load than the original meter coil assemblies imposed. My
horizontal sensors are described at
http://bobmcclure90.googlepages.com/horiz
Please feel free to comment.
Cheers,
Bob
************************************
Hi Bob,
I hadn't seen your Lehman before. Nice. I certainly won't argue with
success, but I would make a couple of quick comments.
Certainly a great contributor to your success with jewel pivots was the
sparing use of weight, but I wonder if that doesn't make you a bit more
susceptible to drafts? Also there have been several questions to the PSN
list from beginners in the vein of, "..how many pounds should my bob
weigh?" So there is a compromise somewhere there -- and I honestly
don't know where it should be. There is a minimum mass number that is
set by thermal motion noise. See:
http://www.iris.washington.edu/stations/seisWorkshop04/PDF/Wielandt-Design3.pdf
and an excerpt of the paper by Erhard Wielandt, Institute of Geophysics,
Stuttgart University is as follows:
“Brownian (thermal) noise
The resolution of an inertial seismometer is ultimately limited by the
Brownian motion of its seismic mass. The energy of this motion can be
concentrated near the mechanical eigenfrequency by minimizing the
damping (maximizing the quality factor). Since this enhances at the same
time the response to ground motion near the eigenfrequency, a high-Q
suspension in fact has a reduced noise level at all frequencies when the
noise is expressed as an equivalent ground motion. Treating the
mechanical pendulum as a simple damped harmonic oscillator with mass M
[kg], period T[s] and quality factor Q, one finds that the coefficients
involved cancel approximately, and the condition for resolving minimum
ground noise in the free-mode band is simply MTQ > 1 kg s. So
theoretically, even a pendulum with a small mass or a small free period
could be used for very-long period recording when the quality factor is
high enough. However, this is not generally true. The
harmonic-oscillator model neglects the fact that the mechanical damping
(especially of thermally self compensated spring materials) is not
viscous. For a given pendulum with an adjustable period such as a
LaCoste suspension, the quality factor decreases more rapidly at long
eigenperiods than it should do for viscous damping. Therefore, the
Brownian noise of mechanical suspensions increases at long periods,
comparable to the 1/f-noise of semiconductors. Unfortunately, very few
data are available on this phenomenon.”
I was recently reading a patent(?) on the invention of the taut-band
panel meter movement for use in long term industrial service (such as
power stations, etc.) They knew that the meter life was limited by the
jewel movement, and were touting the lifetime of the taut-band removing
that limitation. Your use of jewels with heavier loading than the
original movement certainly brings into question operating life.
Additionally, what kind of damping do you experience without the
resistor loading and magnet assembly in place? This would equate back
to the pivot friction plus the air eddy loss. If there is intent to
use force feedback at some point, then the Q of the setup becomes of
interest (as does the thermal noise for that matter.)
Chris remarked, “…What stiction levels do you get? Static friction is
always greater than dynamic friction and it is always present in a
mechanical sliding system. I can remember having to change meters which
had 'sticky' suspensions.”
I can certainly identify with the copper magnetic properties. Some 50+
years ago I made a scale with soda straws and straight pin on razor
blade edges. I attached a coil on the end to make a meter and noticed
the attraction when I added the magnets. But with regard to your
implementation of the Lehman, you made the argument yourself for
increased bob weight in order to get around the diamagnetic, and
paramagnetic effects of your coil. This is useful information to argue
for the use of capacitance sensing with aluminum vanes? (Although even
there and with the use of varible reluctance, too, there are direct
attraction effects from the voltages (or magnetics) at the levels we're
interested in.
Chris remarked, “…Another point which Bob has not mentioned is that
perspex is diamagnetic. I am well aware of some copper wire having Fe
traces. I have had severe problems with aluminum vanes on Lehman
seismometers giving instability when the sensor has drifted off centre.
Pure Cu seems to be OK. You need to keep the outer magnet high field
edges inside the damping plate at all times.
But back to the original point of the paper. I don't pretend it's the
end all of the discussion, but rather a rough guide to help center the
builder's approach. There are always other ways to accomplish an end
goal, but some paths generally have more leeway while on the way to
success. We're just trying to give some rough rules-of-thumb that might
help guide the unwary. Remember the experiments of Robt. Baker in Sci
Am? He used glass slides, RTV and razor blade edges with optical (LED)
sensors. Fascinating approach. Never have heard about whether it had
long term stability problems, though, which would be my concern.
Chris had a bit stronger comment, “Vividly. A really rubbish
construction which was sensitive to everything and did not enable you to
separate out any single effect.”
If it is OK with you, I would really like to post this email to the PSN
list, I think it's beneficial that #1, people see other approaches are
possible, but #2 what I think some of the trade-offs are. I certainly
am not a Lehman authority, I just started this discussion/paper on
pivots because of that very fact. I wanted more insight into the
possibilities. Discussions with Brett and Chris certainly helped guide
me. The paper was a distillation of those discussions so the PSN
community can share in some of the conclusions -- and de-bunk them if
necessary.
Regards,
Charles R Patton
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