Thomas, Do you have any pictures of your sensor. the link you sent is =
broke.
  ----- Original Message -----=20
  From: Thomas W Leiper=20
  To: psn-l@.................
  Sent: Friday, May 25, 2001 11:45 AM
  Subject: Re: Equipment: SG vs. Lehman...Hybrid


  I haven't posted here in a while, but thought I might chime in here.

  I built an extremely sensitive displacement sensor to use with one of
  my Lehmans. Now I could see all kinds of drift artifact that I could
  not see before. [ By the way, the beech tree that I speculated (on =
this
  reflector) was causing tilt artifact when fully foliated...confirmed. =
Half
  of it collapsed under sticky spring snow storm in '00, so I removed =
the
  whole tree. No change last fall or this spring.] Anyway, since I had
  already gone to all the (excessive) effort I was willing in order to =
make
  the system mechanically stable, I decided to use a jack screw and =
stepper
  motor to move a chunk of lead back and forth on the slab to =
automatically
  cancel out drift. This worked OK, but seemed a bit crude, so I finally
  decided to use the force balance approach, which simply establishes a
  feedback loop to hold the boom in place, and takes the acceleration
  component from the feedback loop. It also allowed me to go with a pure
  digital phase locked loop detection method of extraordinary precision,
  since you now are only concerned with "any displacement at all" verses
  "accurate representation of displacement"in your primary detection =
loop.
  The result is a real time stream of digital acceleration words which =
you
  COULD clock right into the parallel port on your PC, but I, who prefer
  analog recording on cash register paper, instead run them through a
  D/A converter and integrate it in my recorder...JUST LIKE THE MAGNET
  AND COIL DID ORIGINALLY.

  So it proves that you CAN, in fact, through ample application of =
ingenuity
  and fanaticism, keep improving a system until it works almost as well =
as it
  did before...

  Tom

  On Fri, 25 May 2001 10:09:26 EDT ChrisAtUpw@....... writes:
    In a message dated 25/05/01, mja6042@............. writes:=20

    It seems like the thing to do then is build Lehman with a =
displacement=20
    sensor similar to the one found on a Shackleford-Gundersen =
seismometer in=20
    place of the magnet and coil on the boom of the Lehman.=20

    Hi there Mark,=20

          Drift is likely to be the biggest problem. Lehmans are very =
sensitive=20
    to tilt effects. The longer the period, the greater the tilt =
sensitivity.=20

    [edited]

    The normal coil assembly used on a Lehman is sensitive to the rate =
of
    change of position and isn't worried by small drifts.=20
          Have a look at:- A Force-Balance Seismometer by Karl =
Cunningham=20
          on the PSN Website.=20

          Regards, Chris Chapman=20







Thomas, Do you have any =
pictures of=20
your sensor. the link you sent is broke.

  
----- Original Message ----- 
  From:=20
  Thomas =
W Leiper=20
  
  
To: psn-l@.............. 
  
Sent: Friday, May 25, 2001 =
11:45 AM
  
Subject: Re: Equipment: SG vs.=20
  Lehman...Hybrid
  


  
I haven't posted here in a while, but thought I might chime in=20
here.
  
 
  
I built an extremely sensitive displacement sensor to use with =
one=20
  of
  
my Lehmans. Now I could see all kinds of drift artifact that I=20
could
  
not see before. [ By the way, the beech tree that I speculated =
(on=20
  this
  
reflector) was causing tilt artifact when fully =
foliated...confirmed.=20
  Half
  
of it collapsed under sticky spring snow storm in '00, so I =
removed=20
  the
  
whole tree. No change last fall or this spring.] Anyway, since I=20
had
  
already gone to all the (excessive) effort I was willing in order =
to=20
  make
  
the system mechanically stable, I decided to use a jack screw and =

  stepper
  
motor to move a chunk of lead back and forth on the slab to=20
  automatically
  
cancel out drift. This worked OK, but seemed a bit crude, so I=20
  finally
  
decided to use the force balance approach, which simply =
establishes=20
  a
  
feedback loop to hold the boom in place, and takes the =
acceleration
  
component from the feedback loop. It also allowed me to go with a =

  pure
  
digital phase locked loop detection method of extraordinary=20
  precision,
  
since you now are only concerned with "any displacement at all"=20
  verses
  
"accurate representation of displacement"in your primary =
detection=20
  loop.
  
The result is a real time stream of digital acceleration words =
which=20
  you
  
COULD clock right into the parallel port on your PC, but I, =
who=20
  prefer
  
analog recording on cash register paper, instead run them through =
a
  
D/A converter and integrate it in my recorder...JUST LIKE THE=20
MAGNET
  
AND COIL DID ORIGINALLY.
  
 
  
So it proves that you CAN, in fact, through ample =
application=20
  of ingenuity
  
and fanaticism, keep improving a system until it works almost as =
well as=20
  it
  
did before...
  
 
  
Tom
  
 
  
On Fri, 25 May 2001 10:09:26 EDT ChrisAtUpw@....... =
writes:
  

    
In a message dated =
25/05/01,=20
    mja6042@............. writes: 

It seems like the thing to do =
then is=20
    build Lehman with a displacement 
sensor similar to the one found =
on a=20
    Shackleford-Gundersen seismometer in 
place of the magnet and =
coil on the=20
    boom of the Lehman. 

    
Hi there Mark,=20
    

      Drift is likely to be =
the=20
    biggest problem. Lehmans are very sensitive 
to tilt effects. The =
longer=20
    the period, the greater the tilt sensitivity. =


    
[edited]
    
 
    
The normal coil =
assembly used=20
    on a Lehman is sensitive to the rate of
    
change of position =
and isn't=20
    worried by small drifts. =

      Have a look=20
    at:- A Force-Balance =
Seismometer=20
    by Karl Cunningham 
      on the =
PSN=20
    Website. 

      Regards, Chris =
Chapman=20