Chris,
Thank you for the fine information. A couple questions:
1. What is the period of your Lehman.
2. What D/A converter board are you using.
Thank you,
Gary
_____
From: psn-l-request@.............. [mailto:psn-l-request@............... On
Behalf Of ChrisAtUpw@.......
Sent: Saturday, March 22, 2008 6:09 PM
To: psn-l@..............
Subject: Re: Anything New in Building the Lehman Seismometer
In a message dated 22/03/2008, gel@................. writes:
I'm looking to finally build my Lehman seismometer and I was wondering what
are the latest recommendations in the construction. I've downloaded many of
the changes and recommendations that people have submitted. I have one
question, is the pickup coil and magnet still the best method to detect
motion. I thought I saw some mention of using Hall-Affect devices as
sensors. Any hints are much appreciated.
Hi Gary,
I don't know what counts as 'new'?
Hall effect devices are fine for 1 to 2 second pendulums / SG systems,
but not too good for Lehmans. They only have a measurement range of +/- 0.5
to +/- 1 mm and Lehmans suffer from tilt drift with time of several mm.
You would be much better using a rectangular coil + quad NdFeB magnet
system. See drawings at
http://jclahr.com/science/psn/chapman/lehman/index.html The quad magnet
system gives about 10x the output of older Alnico U magnet + coil systems
with much smaller coils. It is also ~cheaper. Put the coil on the arm and
the damping and sensor magnets on the frame. Brass is heavy, non magnetic,
easy to machine and makes a good mass of 1 to 2 lbs. 3/4" square or round Al
tube, about 2 ft long, makes a good boom - definitely keep it rigid, but
light in comparison to the mass.
You can use SS ball bearing on a flat SS blade suspension or crossed SS
shoulder bolts or crossed tungsten carbide drill shanks. You can also use
flex wire and crossed 8 thou music wire systems. The all work well. Avoid
point in a cup or knife edge suspensions.
I make a T frame out of 3" x 1" x 1/8" U channel Al with 1/8" thick
corner plates, bolted together with 1/4" SS marine bolts. Ordinary steel and
brass will corrode Al quite rapidly. This is 'easy' to do and works well.
You mount all the components on the one frame, which you then tilt to adjust
the balance and set the period. Avoid the older systems which use separately
mounted damping and sensor components. Use a cross bar near the mass and a V
top wire suspension. I use 40 lb fishing trace. This prevents the arm from
rotating during a quake. Try to mount the damping blade ~level with the C of
G of the mass - underneath the arm.
Use magnetic damping; definitely avoid oil damping. Oil is only good for
about a +/-2 C Deg temperature range, less than average daily room
temperature variations and it is messy. Magnetic damping is easy to set up
and adjust and it is not effected by temperature. It is also clean.
Also have a look at
http://www.bgs.ac.uk/education/school_seismology/seismometer.html This is
the UK school seismometer system and uses top and bottom crossed tungsten
carbide roller suspensions. The mass is on the end of the arm followed by
the sensor magnet and the mounted coil, the suspension block and the damper
blade + sliding damper magnet block system. To set it up, you adjust the
cross balance first, then set the period to ~25 seconds by adjusting the
frame tilt and then slide the damper magnet over the damping blade till you
get 0.7 critical damping.
Hope that is helps.
Regards,
Chris Chapman
Chris,
Thank you for the fine information. =
A
couple questions:
- What
is the period of your Lehman.
- What
D/A converter board are you using.
Thank =
you,
Gary
From:
psn-l-request@.............. [mailto:psn-l-request@............... =
On Behalf Of =
ChrisAtUpw@.......
Sent: Saturday, March 22, =
2008
6:09 PM
To: =
psn-l@..............
Subject: Re: Anything New =
in
Building the Lehman Seismometer
In a message dated 22/03/2008, =
gel@................. =
writes:
I’m looking to finally build my Lehman seismometer =
and I was
wondering what are the latest recommendations in the construction. =
I’ve
downloaded many of the changes and recommendations that people have =
submitted.
I have one question, is the pickup coil and magnet still the best method =
to
detect motion. I thought I saw some mention of using Hall-Affect devices =
as
sensors. Any hints are much appreciated.
Hi =
Gary,
=
I don't =
know what
counts as 'new'?
=
Hall =
effect
devices are fine for 1 to 2 second pendulums / SG systems, but not too =
good for
Lehmans. They only have a measurement range of +/- 0.5 to +/- 1 mm and =
Lehmans
suffer from tilt drift with time of several =
mm.
=
You would =
be much
better using a rectangular coil + quad NdFeB magnet system. See =
drawings
at http://j=
clahr.com/science/psn/chapman/lehman/index.html The
quad magnet system gives about 10x the output of older Alnico U magnet + =
coil
systems with much smaller coils. It is also ~cheaper. Put the coil on =
the arm
and the damping and sensor magnets on the frame. Brass is heavy, non =
magnetic,
easy to machine and makes a good mass of 1 to 2 lbs. 3/4" =
square or
round Al tube, about 2 ft long, makes a good boom - definitely keep =
it
rigid, but light in comparison to the mass.
You can =
use SS
ball bearing on a flat SS blade suspension or crossed SS shoulder bolts =
or
crossed tungsten carbide drill shanks. You can also use flex wire and =
crossed 8
thou music wire systems. The all work well. Avoid point in a cup or =
knife edge
suspensions.
I make a T =
frame
out of 3" x 1" x 1/8" U channel Al with 1/8" thick =
corner
plates, bolted together with 1/4" SS marine bolts. Ordinary steel =
and
brass will corrode Al quite rapidly. This is 'easy' to do and works =
well. You
mount all the components on the one frame, which you then tilt to adjust =
the
balance and set the period. Avoid the older systems which use separately
mounted damping and sensor components. Use a cross bar near the mass and =
a V
top wire suspension. I use 40 lb fishing trace. This prevents the arm =
from
rotating during a quake. Try to mount the damping blade ~level with the =
C of G
of the mass - underneath the arm.
Use =
magnetic
damping; definitely avoid oil damping. Oil is only good for about a +/-2 =
C Deg
temperature range, less than average daily room temperature =
variations and
it is messy. Magnetic damping is easy to set up and adjust and it is not
effected by temperature. It is also clean.
=
Also =
have a
look at http://www.bgs.ac.uk/education/school_seismology/seismometer.html&n=
bsp;This
is the UK
=
Hope that =
is
helps.
=
Regards,
=
Chris =
Chapman