wrote:
> FYI,, Synergy Systems has a surplus of Motorola 6 ch and 8 ch VP Oncore
> GPS
> OEM boards, similar to the GT+ boards some PSNers bought a while back.
> Their web site is:
>
> http://www.synergy-gps.com/
>
> They can be found under Excess Inventory Sale which is a PDF file.
>
> They are $25 to $36 The 8 ch units are used. I ordered 2 and because I
> am
> planning to play with DGPS and post processing I had them add the $15 carrier
> phase firmware to each, which puts a Z in the part number.
>
> The list shows about 900 of the 8 ch units (with 2 different plug types) as
> of
> Aug 12 2002.
>
> Stephen
> PSN Station #55
> 38.828N 120.979W
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Subject: Re: New Lehman on line (almost)
From: John & Jan Lahr johnjan@........
Date: Tue, 01 Oct 2002 00:54:14 -0600
Actually, the USGS short period 1Hz systems are adjusted for a bit less
than
critical damping. 0.8 is, I believe, the damping factor.
Damping issues are discussed
in this message from Sean Morrissey. I suppose one advantage to
slight underdamping
in an amateur system would be to avoid overdamping. It may be
easier to see a small
overshot and return to zero, whereas both critically damped and
overdamped
systems will both return to zero without crossing zero
eventually.
One can see a graph of the displacement from a damped harmonic
oscillator
on this page:
http://lectureonline.cl.msu.edu/~mmp/applist/damped/d.htm
The equations to keep in mind are:
Omega (2 pi frequency) = [sqrt(4mk - b**2)]/2m
The damping factor is b/ [2 sqrt(mk)]
If the damping factor is zero (b = 0) then omega = sqrt(k/m)
If the damping factor is 1 (b = 2 sqrt(mk) ) damping is critical
and
a displacement will return to zero exponentially.
If the damping factor is greater than 1, displacement will return
to zero at a slower exponential rate.
To see what a damping factor is 0.8 would look like, in the
applet above set m = k = 1 and b = 1.6. There is a small
overshoot
and then a return to zero.
Cheers,
John
At 08:13 PM 9/30/2002 -0400, you wrote:
In a
message dated 30/09/02, shammon1@............. writes:
The standard rule is to pull the
boom back a few inches and let it go. The boom
should loose 30% of its motion on each swing past center and come to rest
in 3 1/2 swings.
Hi Steve,
I am puzzled as to where this *standard
rule* is supposed to come from? But using it will give you a quite
seriously underdamped system! A critically damped system
experiences no oscillation at all. This is inherent in the maths.
This is important if you apply post
processing to the recorded signal with the assumption that it was
critically damped to start with. It will also give problems with the
amplitudes and frequencies calculated in FFT displays and may 'smear' P
and S wave recordings.
A procedure to get critical damping could
involve deflecting the beam a very small amount (microns) and
recording the amplifier output. You progressively increase the
damping until the arm just returns to the balance position without having
crossed the zero line. If you increase the damping further, the arm will
simply take longer to get back to zero. If you use huge deflections like
a few inches, you are likely to encounter non linear effects which do not
apply to the tiny (hopefully!) signals that we normally
record.
It is helpful if the recording displays
just what the earth is doing. It is really not helpful if the system adds
an oscillating tail to every transient.
Regards,
Chris Chapman
Subject: RE: New Lehman on line (almost)
From: Jack Ivey ivey@..........
Date: Tue, 1 Oct 2002 08:08:44 -0400
In defense of the underdamped proposition, a slight amount of underdamping
(that produces a second peak about, say, 10% the size of the first peak)
will
produce only a mild peak in the frequency response, and in fact will extend
the
low frequency response of the system slightly. The main advantage of this
arrangement is that it is very easy to tell exactly how much damping you
have.
If you adjust for critical or overdamped, you can only guess, and lots of
people
will end up with a massively overdamped system, reducing their low frequency
response unnecessarily.
It is important to use realistic deflections when measuring damping, I have
found
that if you use large (1/8") deflections, you sometimes get very different
(greater)
damping than you do with micrometer deflections. Bob Barns' calibrator (see
PSN site) is an excellent way to produce these small test signals.
Jack
-----Original Message-----
From: ACole65464@....... [mailto:ACole65464@........
Sent: Monday, September 30, 2002 10:05 PM
To: psn-l@..............
Subject: Re: New Lehman on line (almost)
In a message dated 10/01/2002 12:14:06 AM !!!First Boot!!!,
ChrisAtUpw@....... writes:
In a message dated 30/09/02, shammon1@............. writes:
The standard rule is to pull the boom back a few inches and let it go. The
boom
should loose 30% of its motion on each swing past center and come to rest
in 3 1/2 swings.
Hi Steve,
I am puzzled as to where this *standard rule* is supposed to come
from? But using it will give you a quite seriously underdamped system! A
critically damped system experiences no oscillation at all. This is inherent
in the maths.
This is important if you apply post processing to the recorded signal
with the assumption that it was critically damped to start with. It will
also give problems with the amplitudes and frequencies calculated in FFT
displays and may 'smear' P and S wave recordings.
A procedure to get critical damping could involve deflecting the beam
a very small amount (microns) and recording the amplifier output. You
progressively increase the damping until the arm just returns to the balance
position without having crossed the zero line. If you increase the damping
further, the arm will simply take longer to get back to zero. If you use
huge deflections like a few inches, you are likely to encounter non linear
effects which do not apply to the tiny (hopefully!) signals that we normally
record.
It is helpful if the recording displays just what the earth is doing.
It is really not helpful if the system adds an oscillating tail to every
transient.
Regards,
Chris Chapman
Steve,
In support of what Chris has stated, please go to:
http://www.seismo.com/msop/msop79/inst/inst4.html#aa250 Go to section 4.5
for a text description, and then click on figure 4.5.1a to see how pendulums
are supposed to be damped. About Critical is the response you should obtain.
I hope this helps a little, the diagrams may not make much sense at first
but it shows how professional instruments (electromagnetic, aka Lehman
designs) are adjusted.
Regards,
Allan Coleman
In
defense of the underdamped proposition, a slight amount of
underdamping
(that
produces a second peak about, say, 10% the size of the first peak)
will
produce only a mild peak in the frequency response, and in fact will
extend the
low frequency response of the system
slightly. The main advantage of this
arrangement is
that it is very easy to tell exactly how much damping you have.
If you adjust for critical or
overdamped, you can only guess, and lots of people
will
end up with a massively overdamped system, reducing their low frequency
response unnecessarily.
It is
important to use realistic deflections when measuring damping, I have
found
that
if you use large (1/8") deflections, you sometimes get very different (greater)
damping than you do with micrometer deflections.
Bob Barns' calibrator (see
PSN
site) is an excellent way to produce
these small test signals.
Jack
In a
message dated 10/01/2002 12:14:06 AM !!!First Boot!!!, ChrisAtUpw@.......
writes:
In a message dated 30/09/02, shammon1@............. writes:
The standard rule is to pull the boom back a few inches and
let it go. The boom
should loose 30% of its motion on each swing past
center and come to rest
in 3 1/2 swings.