PSN-L Email List Message

Subject: Re: Burning Questions
From: Brett Nordgren Brett3mr@.............
Date: Sat, 05 Jan 2008 10:12:06 -0500


re: 2)  Almost all modern commercial sensors use some form of an astatic 
leaf-spring suspension.
See fig. 10 of

With careful design and very careful adjustment, even quite a small 
pendulum (2") can be made to have a long free period.  However, such a 
setup is very prone to drift from temperature and other changes and is only 
useful when it is combined with feedback such as Chris describes, to 
broaden its frequency response curve and stabilize its tendency to 'wander'.

In such an instrument the real benefit of the long free period is to make 
it easier for the feedback to accurately control its motion.  Long free 
period = low restoring force = less feedback force needed to control the 


At 09:22 AM 1/4/2008 -0600, you wrote:
>While everyone has their "Thinking Caps" on from Ted's excellent 
>questions, I have a couple that have been smoldering for some time:
>1)    Months ago I posted an event and I received an email commenting on 
>it.  He said, "It was very good, but I might try improving my 
>P-wave."  HOW does one "improve" one phase over another?  It seems that 
>the P-wave is always less stronger.
>2)    Much has been written about the length of a pendulum needing to be 
>long to be effective for teleseismic detection.  However, the commercial 
>devices are quite compact and obviously have short pendulums.  Can someone 
>explain how they accomplish what they do with short pendulums?
>Thank you for "thinking for me."

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