PSN-L Email List Message

Subject: Re: Damping
From: tchannel1@............
Date: Sat, 4 Oct 2008 07:25:14 -0600

Hi Folks,  As usually many good answers to my questions.   I am still trying 
different configurations on the Torsion Spring machine.  In my shop I made a 
24", a 36" and a 60".
I could get 12 seconds on the 24", and 21 seconds on the 60" and an 
oscillation of 75 mins.   These were crude setups, so I still need to build 
one with a better and proper three legged adj. base.
For these I just wanted to gather ideas for the next version.

  I have one version finished and waiting for an earthquake, it has a 
magnetic damper.   When or if it records its first earthquake, I will remove 
the Damper and try it again.   As others pointed out the background noises 
will most likely be so large, the sensor will be in motion almost 
constantly.  Having no damper, and being sensitive, it might just rock and 
roll responding to the noises, 24/7.
  I might leave it undampered for a few days, in hopes that if we have a 
quiet night, it might respond to a small event.   Presently I can just make 
out a 4.1m in California, on most of my sensors. I will see if undampered, 
if would pickup a 4.0

Again thanks for explaining how the properties of a Damper is different than 
the friction of the hinges.


----- Original Message ----- 
From: "Brett Nordgren" 
Sent: Friday, October 03, 2008 9:23 PM
Subject: Re: Damping

> Ted,
> The kind of damping you need will generate a retarding force which is 
> approximately proportional to velocity.  For very slow motions, it 
> generates almost no damping force.  Most types of sliding friction are 
> large until the joint 'breaks away' and starts moving, and then are 
> relatively constant with velocity.  This is definitely not what you want 
> for seismo damping.  Chris' magnets are a very good way of getting what 
> you need--no static 'break away' and zero damping force at DC, but having 
> the damping force linearly increasing with frequency.
> As far as hinges go, flexures are pretty good, and some rolling designs 
> are also.  Both have essentially no static, break away, friction.
> An undamped sensor is actually more sensitive than a damped one.  It's 
> just that an undamped sensor's output won't look at all like the actual 
> earth motion you are trying to record.  With damping, you trade 
> sensitivity for accuracy.
> Regards,
> Brett
> At 12:21 PM 10/3/2008 -0600, you wrote:
>>Hi Folks,
>>I understand the necessity of using a Damper to keep the arm or boom from 
>>oscillate or belling after a signal begins.   I would guess, after an 
>>earthquake signal arrives, an undamper arm would continue to oscillate for 
>>many minutes, overwriting most of the incoming signature.
>>I also understand the importance of minimizing the friction of the 
>>sensor's hinges.   The signals are very small and they must overcome the 
>>friction before any movement of the arm can take place.
>>My question is:  Does a Damper, oil or magnetic, not act the same as hinge 
>>friction?, in that, the signal must, first, overcome the resistance of the 
>>Is an undamper sensor  more sensitive than a dampered sensor?    I have 
>>never tried this.   Again, I know the signature would be of little value, 
>>but I am curious about the friction of the dampers.
>>Thanks, Ted
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