Hi Chris,  I found this one, but maybe you know of other pictures or =
sites http://jclahr.com/science/psn/epics/reports/folded/
I find this real interesting, have you tried it? Anyone?
Thanks, Ted

  ----- Original Message -----=20
  From: ChrisAtUpw@..........
  To: psn-l@.................
  Sent: Sunday, June 24, 2007 3:46 PM
  Subject: Re: Lehman advantages


  In a message dated 2007/06/24, tchannel@.............. writes:


    Subj:Lehman advantages=20

    Hi Folks, I am thinking about building another Lehman style Horz =
Pendulum sensor.  I have some construction ideas I wanted to try.  =
Before I start, could you describe the benefits of these points.

    1 FIRMLY ATTACHED THE SENSOR THE EARTH.  I wish to make the contact =
between the earth, and the sensor as firm as possible, in this case the =
concrete slab setting on the earth and the sensor.  Presently the sensor =
has three feet which just set on the concrete slab. I know some people =
use adhesive to the concrete.  If I found a way to bolt all three feet =
into the concrete, and a new way to make the necessary adjustments, what =
benefits would be derived?
    I understand that even the concrete floor floats on the earth.  I am =
just talking about the benefits of a tighter connection between the =
sensor and the floor.


  Hi Ted,

      Bolting the seismometer mounts to the floor may give problems when =
the seismometer expands with temperature at a different rate or at a =
different time to the floor. They are very unlikely to match.=20

         I use three 2" square x 1/8" SS squares glued to the concrete =
floor. You can also use >5mm glass or even glazed tiles. The benefit is =
that you have a grit free, dead flat surface. Your level settings should =
not show drift either with temperature or over time, or be effected by =
large quakes. You can use pool cement to glue the plates.

         The mounting bolts need to be rigidly attached to the frame. To =
avoid thermal drift, I glue SS nuts to the underside of the arm with =
acrylic glue. On top of the arm I glue a 1/2" SS tube pillar and add a =
wavy washer. The SS set bolt has a SS ball bearing glued to a V in the =
end, to provide a central rotating contact with mounting plate. The set =
bolt also has a nut at the top end. After setting the correct height, I =
partly compress the wavy washer with the top nut. This keeps the thread =
in tension. The vertical alignment / side slop is controlled by the SS =
pillar and the tension.


    2 A RIGID VERTICAL SUPPORT FOR THE UPRIGHT.  I know that on a =
typical Lehman the vertical needs to be rigid and minimise the flex =
between the vertical and the horz members.  If I found a way to minimise =
this flex, what benefits might I see?  The one I have has no flex that I =
can see, but If I added addition braces so the vertical was at 90 to the =
horz with the minimum of flex, What benefit would there be?


         The vertical and horizontal arms need to be connected quite =
rigidly. This can be done conveniently with large triangular reinforcing =
plates at the T joint or ~ 45 deg bracing members to both the main beam =
and to the cross beam. This will minimise any cross alignment drift and =
tend to suppress arm oscillations, due to the vertical + arm + mass =
flexing. Unless you do this you are likely to pick up spurious resonant =
signals. The original Lehman design was inadequate in this respect.

         Thump the mass vertically and what do you see on the output? =
You need to eliminate any oscillations.


    3 USING A LONGER ARM.  I used a normal length arm, and I understand =
if space was not an issue a very long 100 meters arm would result in a =
longer period.  I am just asking if space was avail would a 5 foot arm =
result in any benefits, over a 3 foot arm?


         You can provide reasonable temperature and air motion control =
for a 2 to 3 ft arm, but not for anything much larger. A 1 m long =
pendulum has a period of about 2 sec. To get a 4 sec period you need a 4 =
m pendulum. A 20 sec period would require a 100m pendulum.=20
         The main factor you need to consider is the ratio between the =
natural period of an arm of length L and the desired seismometer period =
- the 1/sinA factor. If you try to get greater than x10 period =
extension, A becomes a very small angle. You may need fine thread =
adjustment screws or a slow motion drive. =20
         A folded pendulum design is likely to be more satisfactory / =
easier to construct for mechanical periods over about 30 sec.
         An alternative method is to provide position and velocity force =
feedback to stabilise the position of the arm, but the electronics gets =
more complicated. Using electronic feedback control can run into noise =
and stability problems, but you can turn a 20 sec pendulum into a 200 =
sec sensor. See http://www.keckec.com/seismo/

         What period do you want?  The Rayleigh and Love surface waves =
tend to have periods of about 20 seconds and few are over 40 sec. For =
very long extension periods you need to measure the position of the arm, =
not it's velocity, or you just see noise.


    The last question is, if I had a sensor which was firmly attached to =
the floor, with a very ridged vertical, and a longer arm.
    (with all the other important factors aside) What kind of =
improvements might I expect?  I think I could build a new and improved =
sensor, addressing these three issues.  But would these three issues =
make much different.  If I would, see improvements would they only be =
for teleseismic events, or would the improvements be evident in =
recording regional as well.


      You should see the true ground motion. There should be NO =
artefacts from the apparatus. You are more likely to be bothered by =
short period signals, but the P and S waves that you want to detect are =
above 0.5 Hz, often 1 to 5 Hz.

      Remember that IT IS THE EARTH WHICH MOVES ---> NOT THE SEISMOMETER =
ARM !!

      You have missed out some important considerations. You need to =
suppress, damp, or be insensitive to the natural oscillations / modes of =
the apparatus. Earthquakes are transient pulse type signals and can =
excite any natural oscillation modes.
      The arm and the suspension need to be rigid. The arm should be =
prevented from rotating around it's long axis. There will inevitably be =
some vertical bounce at the end of the arm, but the frequency should be =
above that of the low pass electronic filter and the sensor should be =
designed to have a low sensitivity to vertical motion. You also need to =
design the sensor to have a constant and linear sensor voltage output =
for mass position drifts of ~ +/-1/2". You ALWAYS get some position =
drift with a Lehman. They are very sensitive to tiny shifts in the local =
ground plane due to temperature, rain and seasonal changes. You need =
NdFeB bar magnets and rectangular coils to do this, or alternatively =
long cylindrical coils with many turns + magnets, similar to a =
loudspeaker, but with clearance gaps and coil lengths which allow for a =
1/2" mass drift.
      The damping force should act ~on the line between the centre of =
mass and the lower bearing, otherwise it will try to rotate the arm =
about it's long axis. Also, place the centre of the pickup coil close to =
this axis.=20
      Have a look at =
http://jclahr.com/science/psn/chapman/school/MKII/index.html The top =
wire suspension was changed to either a V cable or to a 1/2" tube. Both =
worked OK. Both hinges were altered to be crossed rods, although a ball =
on a plane works equally well. You mount the vertical rods or balls on =
the vertical support column, NOT on the arm.  The horizontal rods or the =
flats are mounted on the moving arm.
      Also have a look at the Sprengnether at =
http://www.geocities.com/meredithlamb/

         Have a look at 416 SS 'shoulder screws' 93985A205 or similar =
from www.mcmaster.com.=20
         Alternatively, buy solid tungsten carbide drills and use the =
shank. They are sold for drilling fibreglass circuit board and other =
hard materials. See www.DigiKey.com or www.smallparts.com. Smallparts =
also sell bearings. You can buy flat triangular Tungsten Carbide tips =
for lathe tools quite cheaply with ~ 0.3" sides. Alternatively, you can =
use a bit of a SS knife blade glued to the end of the arm.

      Regards,








Hi Chris,  I found this one, but =
maybe you=20
know of other pictures or sites http://jclah=
r.com/science/psn/epics/reports/folded/
I find this real interesting, have you =
tried it?=20
Anyone?
Thanks, Ted
 

  
----- Original Message ----- 
  From:=20
  ChrisAtUpw@....... 
  
To: psn-l@.............. 
  
Sent: Sunday, June 24, 2007 =
3:46 PM
  
Subject: Re: Lehman =
advantages
  

In a=20
  message dated 2007/06/24, tchannel@..............=20
  writes:


  Subj:Lehman advantages 

Hi Folks, I am =
thinking=20
    about building another Lehman style Horz Pendulum sensor.  I =
have some=20
    construction ideas I wanted to try.  Before I start, could you =
describe=20
    the benefits of these points.

1 FIRMLY =
ATTACHED THE SENSOR=20
    THE EARTH.  I wish to make the contact between the earth, and =
the=20
    sensor as firm as possible, in this case the concrete slab setting =
on the=20
    earth and the sensor.  Presently the sensor has three feet =
which just=20
    set on the concrete slab. I know some people use adhesive to the=20
    concrete.  If I found a way to bolt all three feet into the =
concrete,=20
    and a new way to make the necessary adjustments, what benefits would =
be=20
    derived?
I understand that even the concrete floor floats on =
the=20
    earth.  I am just talking about the benefits of a tighter =
connection=20
    between the sensor and the floor.

Hi Ted,

    Bolting the =
seismometer=20
  mounts to the floor may give problems when the seismometer expands =
with=20
  temperature at a different rate or at a different time to the floor. =
They are=20
  very unlikely to match. 

       I =
use=20
  three 2" square x 1/8" SS squares glued to the concrete floor. You can =
also=20
  use >5mm glass or even glazed tiles. The benefit is that you have a =
grit=20
  free, dead flat surface. Your level settings should not show drift =
either with=20
  temperature or over time, or be effected by large quakes. You can use =
pool=20
  cement to glue the plates.

       =
The=20
  mounting bolts need to be rigidly attached to the frame. To avoid =
thermal=20
  drift, I glue SS nuts to the underside of the arm with acrylic glue. =
On top of=20
  the arm I glue a 1/2" SS tube pillar and add a wavy washer. The SS set =
bolt=20
  has a SS ball bearing glued to a V in the end, to provide a central =
rotating=20
  contact with mounting plate. The set bolt also has a nut at the top =
end. After=20
  setting the correct height, I partly compress the wavy washer with the =
top=20
  nut. This keeps the thread in tension. The vertical alignment / side =
slop is=20
  controlled by the SS pillar and the tension.


  2 A RIGID=20
    VERTICAL SUPPORT FOR THE UPRIGHT.  I know that on a typical =
Lehman the=20
    vertical needs to be rigid and minimise the flex between the =
vertical and=20
    the horz members.  If I found a way to minimise this flex, what =

    benefits might I see?  The one I have has no flex that I can =
see, but=20
    If I added addition braces so the vertical was at 90 to the horz =
with the=20
    minimum of flex, What benefit would there be?

       The=20
  vertical and horizontal arms need to be connected quite rigidly. This =
can be=20
  done conveniently with large triangular reinforcing plates at the T =
joint or ~=20
  45 deg bracing members to both the main beam and to the cross beam. =
This will=20
  minimise any cross alignment drift and tend to suppress arm =
oscillations, due=20
  to the vertical + arm + mass flexing. Unless you do this you are =
likely to=20
  pick up spurious resonant signals. The original Lehman design was =
inadequate=20
  in this respect.

       Thump the =
mass=20
  vertically and what do you see on the output? You need to eliminate =
any=20
  oscillations.


  3 USING A LONGER ARM.  I used a normal length arm, =
and I=20
    understand if space was not an issue a very long 100 meters arm =
would result=20
    in a longer period.  I am just asking if space was avail would =
a 5 foot=20
    arm result in any benefits, over a 3 foot arm?

       You can=20
  provide reasonable temperature and air motion control for a 2 to 3 ft =
arm, but=20
  not for anything much larger. A 1 m long pendulum has a period of =
about 2 sec.=20
  To get a 4 sec period you need a 4 m pendulum. A 20 sec period would =
require a=20
  100m pendulum. 
       The main =
factor you=20
  need to consider is the ratio between the natural period of an arm of =
length L=20
  and the desired seismometer period - the 1/sinA factor. If you try to =
get=20
  greater than x10 period extension, A becomes a very small angle. You =
may need=20
  fine thread adjustment screws or a slow motion drive. =20
  
       A folded pendulum design is =
likely to=20
  be more satisfactory / easier to construct for mechanical periods over =
about=20
  30 sec.
       An alternative method =
is to=20
  provide position and velocity force feedback to stabilise the position =
of the=20
  arm, but the electronics gets more complicated. Using electronic =
feedback=20
  control can run into noise and stability problems, but you can turn a =
20 sec=20
  pendulum into a 200 sec sensor. See=20
  =
http://www.keckec.com/seismo/

      =
 What=20
  period do you want?  The Rayleigh and Love surface waves tend to =
have=20
  periods of about 20 seconds and few are over 40 sec. For very long =
extension=20
  periods you need to measure the position of the arm, not it's =
velocity, or you=20
  just see noise.


  The last question is, if I had a sensor which was firmly =

    attached to the floor, with a very ridged vertical, and a longer=20
    arm.
(with all the other important factors aside) What =
kind of=20
    improvements might I expect?  I think I could build a new and =
improved=20
    sensor, addressing these three issues.  But would these three =
issues=20
    make much different.  If I would, see improvements would they =
only be=20
    for teleseismic events, or would the improvements be evident in =
recording=20
    regional as well.

    =
You should see the true ground =
motion. There=20
  should be NO artefacts from the apparatus. You are more likely to be =
bothered=20
  by short period signals, but the P and S waves that you want to detect =
are=20
  above 0.5 Hz, often 1 to 5 Hz.

    Remember that =
IT IS=20
  THE EARTH WHICH MOVES ---> NOT THE SEISMOMETER ARM =
!!

    You have =
missed out some=20
  important considerations. You need to suppress, damp, or be =
insensitive to the=20
  natural oscillations / modes of the apparatus. Earthquakes are =
transient pulse=20
  type signals and can excite any natural oscillation=20
  modes.
    The arm and the suspension need to be =
rigid. The=20
  arm should be prevented from rotating around it's long axis. There =
will=20
  inevitably be some vertical bounce at the end of the arm, but the =
frequency=20
  should be above that of the low pass electronic filter and the sensor =
should=20
  be designed to have a low sensitivity to vertical motion. You also =
need to=20
  design the sensor to have a constant and linear sensor voltage =
output for=20
  mass position drifts of ~ +/-1/2". You ALWAYS get some position =
drift with=20
  a Lehman. They are very sensitive to tiny shifts in the local ground =
plane due=20
  to temperature, rain and seasonal changes. You need NdFeB bar magnets =
and=20
  rectangular coils to do this, or alternatively long cylindrical coils =
with=20
  many turns + magnets, similar to a loudspeaker, but with clearance =
gaps and=20
  coil lengths which allow for a 1/2" mass drift.
    =
The=20
  damping force should act ~on the line between the centre of mass and =
the lower=20
  bearing, otherwise it will try to rotate the arm about it's long axis. =
Also,=20
  place the centre of the pickup coil close to this axis. =

   =20
  Have a look at =
http://jclahr.com/science/psn/chapman/school/MKII/index.html=20
  The top wire suspension was changed to either a V cable or to a 1/2" =
tube.=20
  Both worked OK. Both hinges were altered to be crossed rods, although =
a ball=20
  on a plane works equally well. You mount the vertical rods or balls =
on the=20
  vertical support column, NOT on the arm.  The horizontal rods =
or the=20
  flats are mounted on the moving arm.
    Also have a =
look at=20
  the Sprengnether at =
http://www.geocities.com/meredithlamb/

       Have a=20
  look at 416 SS 'shoulder screws' 93985A205=20
  or similar from www.mcmaster.com. =

      =20
  Alternatively, buy solid tungsten carbide drills and use the shank. =
They are=20
  sold for drilling fibreglass circuit board and other hard materials. =
See=20
  www.DigiKey.com or www.smallparts.com. Smallparts also sell bearings. =
You can=20
  buy flat triangular Tungsten Carbide tips for lathe tools quite =
cheaply with ~=20
  0.3" sides. Alternatively, you can use a bit of a SS knife blade glued =
to the=20
  end of the arm.

    Regards,

    Chris=20
  Chapman