## PSN-L Email List Message

Subject: Re: Verticals
From: Brett Nordgren brett3nt@.............
Date: Sun, 27 Dec 2009 12:01:39 -0500

```Hi Ted,

At 06:59 AM 12/27/2009 -0700, you wrote:
>SpringCalc.exe.    I have put in some numbers and see it calculates very
>long periods under certain conditions, periods of >20.
>
>My question is can long periods like these really be achieved?

In a word, no.  The problem is that the support spring strength changes
with temperature, enough to mess up any long-period setup.  A coil-magnet
sensor would be more tolerant of position drift, but maintaining a stable
long period will be difficult enough that I believe most would call it
impossible.

LaCoste & Romberg use a double oven to very precisely control the spring
temperature in their gravity meters, and some commercial instruments use
exotic low temperature coefficient of elasticity alloys for their springs
to reduce the temperature effects (for other reasons).  But, in my opinion,
the only way to get a stable long period design will be with feedback.

Though I will freely admit that I am biased on the subject, if anyone can
demonstrate a successful long natural-period (>=20 second) vertical design
that doesn't use feedback, I'd very much like to learn how it works.

Regards,
Brett

>I have build several models and some working sensors using this approach,
>but I have never gotten more than about 3x the springs natural
>frequency.   A spring of .5 second, just hanging vertically, resulted in a
>sensor of 1.5 seconds.   I guess the AS1 is another example.   The design
>of the AS1 is widely used and am I correct that it is operating at about
>1.5 seconds?   In theory, is this shape, capable >10 seconds?
>
>I hope to use Bob's program to construct another vertical, but I am
>confused, about my results.    I know there are methods for extending the
>period, but here I am just asking about the physical results of
>using     a spring-supported pendulum constrained to move in a vertical
>plane.   Perhaps these calculated long periods would only be possible
>under the best of conditions, where all factors were perfect.    I seem to
>get a period of  around x3  that of the spring hanging vertical with a
>mass.    I have tried many different springs and the result were very
>similar in achieving only x3.
>
>Thanks, Ted

Watch our wiggles
http://bnordgren.org/seismo/gif_images.htm

or watch some very very good wiggles
http://aslwww.cr.usgs.gov/Seismic_Data/telemetry_data/ANMO_24hr.html

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