PSN-L Email List Message

Subject: Re: Equipment: SG vs. Lehman...Hybrid
From: Thomas W Leiper twleiper@........
Date: Fri, 25 May 2001 12:45:37 -0400


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: 

It seems like the thing to do then is build Lehman with a displacement 
sensor similar to the one found on a Shackleford-Gundersen seismometer in

place of the magnet and coil on the boom of the Lehman. 

Hi there Mark, 

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


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






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<= /DIV>
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=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 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=20 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 a<= /DIV>
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=20 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 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 = biggest=20 problem. Lehmans are very sensitive
to tilt effects. The longer the=20 period, the greater the tilt sensitivity.
[edited]
 
The normal coil assembly= used on=20 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 = Website.=20

      Regards, Chris Chapman=20

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Larry Cochrane <cochrane@..............>