PSN-L Email List Message

Subject: Re: Fw: Long Period Pendulum
From: "Geoff" gmvoeth@...........
Date: Sun, 24 Feb 2008 13:21:28 -0700

WHAT WOULD be wrong with building a
device that is fixed in a level
fashion and measure only tilt since
distortions stress and strain in the earth
will manifest itself as a tilt ?

It may be easier and cheaper to do this ?


----- Original Message ----- 
From: "Brett Nordgren" 
Sent: Sunday, February 24, 2008 1:15 PM
Subject: Re: Fw: Long Period Pendulum

> Ted,
> I think you are finding that simply having long period does not necessarily 
> make a good sensor.  What seems to be happening is that you are largely 
> lengthening the period by reducing the restoring force on a distributed 
> pendulum.  In the design you describe, that is also having the effect of 
> reducing its horizontal motion sensitivity, pretty much in proportion.
> Another design which would act in the same way would be a heavy disk with 
> an axle drilled just slightly off-center.  It would rotate heavy side down 
> and would oscillate quite slowly, but its sensitivity to horizontal motion 
> would be very small.  It might still be fairly sensitive to tilt 
> oscillation, though.
> Brett
> At 12:01 PM 2/24/2008 -0700, you wrote:
>>Hi John and All,   This is clear to me, because its right in front of me, 
>>so I hope you can picture the mock up from my description.
>>My latest attempt go like this;
>>A 48" dowel with a pivot, at 24".  Equal mass (two machine nuts) one fixed 
>>on the bottom of the propeller, and the top nut adjustable up or down.
>>The goal being to adjust the top nut in the up direction until it near 
>>reached the top of the dowel, which would equal the mass and position of 
>>the fixed bottom nut.   Stopping just short of the end of the dowel should 
>>result in a longer period, and still always come to rest at vertical.
>>Earlier all this did happen, but the propeller tended to come to rest at 
>>the 2:oo o'clock position, and as the nut moved up the resting position 
>>wanted to become horizontal.
>>I though, my pivot hole was off center, just a bit, and that could cause 
>>the propeller to become other than vertical.
>>So I added this:    I drilled and installed a piece of wire about 12" long 
>>into the bottom of the propeller.  Now it looks like an inverted "T" and 
>>by moving the wire to the left or right, I can bring the propeller back to 
>>vertical.   The wire shift the weight of the prop. until it hang vertical.
>>After this correction, I continued to move the top nut up to increase the 
>>period;   I was able to get 45 seconds maximum, but it was unstable.   I 
>>lowered the top nut a bit, and got 20 seconds, still a bit unstable.   I 
>>lowered more and got 13 seconds stable and hanging vertical.
>>I have no idea what to do with it now, except to confirm it works, but it 
>>is touch-y.   A 24" pendulum was extended from 1.5 seconds to 45 seconds max.
>>Thanks, Ted
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