In defense of the underdamped proposition, a slight amount of underdamping
(that produces a second peak about, say, 10% the size of the first peak)
will
produce only a mild peak in the frequency response, and in fact will extend
the
low frequency response of the system slightly.  The main advantage of this 
arrangement is that it is very easy to tell exactly how much damping you
have.  
If you adjust for critical or overdamped, you can only guess, and lots of
people 
will end up with a massively overdamped system, reducing their low frequency

response unnecessarily.
 
It is important to use realistic deflections when measuring damping, I have
found
that if you use large (1/8") deflections, you sometimes get very different
(greater) 
damping than you do with micrometer deflections.  Bob Barns' calibrator (see

PSN site) is an excellent way to produce these small test signals.
 
Jack

-----Original Message-----
From: ACole65464@....... [mailto:ACole65464@........
Sent: Monday, September 30, 2002 10:05 PM
To: psn-l@..............
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 








In 
defense of the underdamped proposition, a slight amount of 
underdamping
(that 
produces a second peak about, say, 10% the size of the first peak) 
will
produce only a mild peak in the frequency response, and in fact will 
extend the
low frequency response of the system 
slightly.  The main advantage of this 

arrangement is 
that it is very easy to tell exactly how much damping you have.  

If you adjust for critical or 
overdamped, you can only guess, and lots of people 
will 
end up with a massively overdamped system, reducing their low frequency 

response unnecessarily.
 
It is 
important to use realistic deflections when measuring damping, I have 
found
that 
if you use large (1/8") deflections, you sometimes get very different (greater) 

damping than you do with micrometer deflections.  
Bob Barns' calibrator (see 
PSN 
site) is an excellent way to produce 
these small test signals.
 
Jack

  
-----Original Message-----
From: ACole65464@....... 
  [mailto:ACole65464@........
Sent: Monday, September 30, 2002 10:05 
  PM
To: psn-l@..............
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