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
boom)
WU period 12 seconds
WA period 12 seconds
manual eyeball displacement check
displacement 5/10-inch (this is the manual pull back by
hand)
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
hand)
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
directions.
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
<< File: ATT00000.html >>
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