PSN-L Email List Message

Subject: RE: Damping/general
From: steve hammond shammon1@.............
Date: Thu, 3 Oct 2002 16:38:31 -0700

Jim, I read your post with interest and got a chance today to pull the 
covers on the boxes and check the damping using your walk-up-walk-away 
(WUWA) suggestion. That is a very cool test and I have used it many times 
without thinking about the trace being in the opposite direction.
I took the following reading without making any changes to the device:

ATE (this device has a 40cm boom with a 5 LB mass mounted at the end of the 
   WU period 12 seconds
   WA period 12 seconds
      manual eyeball displacement check
   displacement          5/10-inch    (this is the manual pull back by 
   return overshoot      2/10-inch    (this is the release)

ATN (this device has a 40cm boom with the 5 LB mass mounted at 28cm behind 
the coil with the damping flag mounted at the 40 cm point)
   WU period 18 seconds
   WA period 19 seconds
      manual eyeball displacement check
    displacement        5/10-inches   (this is the initial pull back by 
    return overshoot   1.5/10-inches  (this is the release)

I was wondering if there was any rule about the boom coming to complete 
rest as decried by Richter, Elementary Seismology (step 12a), h = 1 in 
terms of critical damping. I guess what I'm asking is, should there be any 
extremely small oscillation following the return overshoot or should it be 
flat line back to zero? I'm seeing a slight overshoot less than .5/10th 
-inches. I would be interested in your comments. I also just posted two 
event files on Larry's site AT1 and AT2    Aptos, CA for the 6.4 event 
today in the Gulf of California if you are interested in seeing actual 
event data from the devices. Thanks for your input.

Regards, Steve Hammond  PSN San Jose  Aptos, CA

-----Original Message-----
From:	Connie and Jim Lehman [SMTP:lehmancj@............
Sent:	Wednesday, October 02, 2002 1:20 PM
To:	psn-l@..............
Subject:	Damping/general

The notes on damping show the variety of how to get the job done.  A swing 
of 3.5 times past the equilibrium position sounds a lot "looser" than 
optimum.  In setting up a damping system, I would eyeball the action by 
displacing--say one cm, and watch the return to overshoot two mm, and 
settle to equilibrium again--or a 5 to 1 ratio was in the ballpark.  The 
standard check one can make is the "walk-up" test.  Walk up to the base 
perpendicular to the boom, stand for 20 seconds or so, and then back 
away----If all is well, there will be two traces--alike, but in opposite 
    As you walk up, the sensor moves slightly to a new equilibrium 
position.  The damping ratio or situation will show nicely---bearing in 
mind this is a velosity readout rather than a displacement--but close 
enough approximation to give us the damping condition quick & easy!!!
     I usually went for a 8 to 1 ratio.  I know with no damping, the boom 
swings forever from the energy of microseisms.  With critical damping, one 
kills most or all of the action,  The objective of damping is to remove the 
natural period or swing of the pendulum--but not kill it---and there is 
some leeway--------
                                                           Good damping--- 
               Jim Lehman
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