OK -- I have made the boom changes suggested ( see new pictures)
http://24.116.175.108/Over%20all%20pictures.htm I know the coil magnet
needs to come up a little and I am still not getting good damping. I am
going to change my copper plate to horizontal and pass it between by magnets
and see if that helps.
I am having a blast learning!!
Thanks for EVERY comment
Jan Marshall
jandmarshall@............
www.cableone.net/jandmarshall
Nampa, ID
-----Original Message-----
From: psn-l-request@.............. [mailto:psn-l-request@.................
Behalf Of Randall Pratt
Sent: Tuesday, October 01, 2002 9:14 PM
To: psn-l@..............
Subject: Re: New Lehman on line (almost)
Allen,
Have you used the method of calibration you referenced? It is very easy
to set up but my system does not behave quite as advertised. With the boom
blocked I don't get a step function as in fig 4.5.1a but rather an
exponential decay. I find that a bit confusing since I put a steady battery
voltage across the coil but I have attempted to determine the curve and
adjust subsequent readings by the correct factor over time. I'm also not
clear about para 9 where a0 is computed. What does that formula really
mean? How would it be adjusted for swings later in the wave train and what
is the ' on the end? Why would later pairs of values work when there is a
log decay in the swings?
Randy
----- Original Message -----
From: ACole65464@.......
To: psn-l@..............
Sent: Monday, September 30, 2002 9:04 PM
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
OK --=20
I have made the boom changes suggested ( see new pictures) http://24.116.17=
5.108/Over%20all%20pictures.htm =20
I know the coil magnet needs to come up a little and I am still not =
getting good=20
damping. I am going to change my copper plate to horizontal and =
pass it=20
between by magnets and see if that helps.
I am=20
having a blast learning!!
Thanks=20
for EVERY comment
Jan=20
Marshall
jandmarshall@............
www.cableone.net/jandmarshallNampa,=20
ID
-----Original Message-----
From: =
psn-l-request@.................
[mailto:psn-l-request@...............On Behalf Of Randall=20
Pratt
Sent: Tuesday, October 01, 2002 9:14 PM
To:=20
psn-l@..............
Subject: Re: New Lehman on line=20
(almost)
Allen,
Have you used the method of =
calibration you=20
referenced? It is very easy to set up but my system does not =
behave=20
quite as advertised. With the boom blocked I don't get a step =
function=20
as in fig 4.5.1a but rather an exponential decay. I find that a =
bit=20
confusing since I put a steady battery voltage across the coil but I =
have=20
attempted to determine the curve and adjust subsequent readings by the =
correct=20
factor over time. I'm also not clear about para 9 where a0 is=20
computed. What does that formula really mean? How =
would it be=20
adjusted for swings later in the wave train and what is the ' on the=20
end? Why would later pairs of values work when there is a log =
decay in=20
the swings?
Randy
----- Original Message -----
From:=20
ACole65464@.......
Sent: Monday, September 30, =
2002 9:04=20
PM
Subject: Re: New Lehman on =
line=20
(almost)
In a =
message dated=20
10/01/2002 12:14:06 AM !!!First Boot!!!, ChrisAtUpw@....... =
writes:
In a message dated 30/09/02, shammon1@............. =
writes:=20
The standard rule is to pull the boom back a few =
inches and=20
let it go. The boom
should loose 30% of its motion on each =
swing=20
past center and come to rest
in 3 1/2 swings.
Hi Steve, =
I=20
am puzzled as to where this *standard rule* is supposed to come =
from? But=20
using it will give you a quite seriously underdamped =
system! A=20
critically damped system experiences no oscillation at =
all.=20
This is inherent in the maths.
=
This is=20
important if you apply post processing to the recorded signal with =
the=20
assumption that it was critically damped to start with. It will =
also give=20
problems with the amplitudes and frequencies calculated in FFT =
displays=20
and may 'smear' P and S wave recordings.=20
A procedure to get critical =
damping=20
could involve deflecting the beam a very small amount =
(microns)=20
and recording the amplifier output. You progressively =
increase the=20
damping until the arm just returns to the balance position without =
having=20
crossed the zero line. If you increase the damping further, the =
arm will=20
simply take longer to get back to zero. If you use huge =
deflections like a=20
few inches, you are likely to encounter non linear effects which =
do not=20
apply to the tiny (hopefully!) signals that we normally=20
record.
It is =
helpful if=20
the recording displays just what the earth is doing. It is really =
not=20
helpful if the system adds an oscillating tail to every transient. =
Regards,=20
Chris Chapman
Steve,
In support of what Chris has =
stated,=20
please go to: =20
http://www.seismo.com/msop/msop79/inst/inst4.html#aa250 Go to =
section=20
4.5 for a text description, and then click on figure 4.5.1a to see =
how=20
pendulums are supposed to be damped. About Critical is the response =
you=20
should obtain. I hope this helps a little, the diagrams may not make =
much=20
sense at first but it shows how professional instruments =
(electromagnetic,=20
aka Lehman designs) are adjusted.
Regards,
Allan =
Coleman=20