PSN-L Email List Message

Subject: Re: mesoanelastic complexity
From: Brett Nordgren brett3nt@.............
Date: Tue, 16 Aug 2011 22:42:43 -0400


At 08:21 PM 8/16/2011, you wrote:
>As I follow the discussion I have to ask for clarification of some points.
>What is the magnitude of the nonlinearities in relation to other 
>instrument noise sources?

That was what I was hoping to evaluate, but I need to know their 
magnitudes in absolute terms in order to make a meaningful comparison.

>The commercial instruments seem to be well below the low noise model 
>without taking this into account.

Quite true.

>  We are not operating in silence looking for a single long period 
> signal to rise from nothing so would not normal noise dither the instrument?

I think it should, quite effectively.  The 6-second microseisms are 
relatively quite large.

>Next I can see 2 results for a feedback instrument.  My instinct 
>would say that feedback should lower the stress gradients in the 
>spring and thus lower the occurrence of 'pops' proportionately.  On 
>the other hand less motion of the mass would create greater 
>opportunity for any sticky behavior to manifest.

In practice, the motions will be rather small even without feedback, 
and perhaps 100-10000 times smaller with feedback.  In the 
steady-state the spring is quite highly stressed as it supports the 
mass against gravity--as much as 60% of its yield strength.

We are quite familiar with the symptoms of "sticky" behavior which is 
most often caused by a single lint fiber having become lodged between 
the sensor plates or in the magnet assembly.  When that happens it 
causes a characteristic type of 'pop' in the output which clearly 
indicates that someone needs to open up their instrument and give it 
a good cleaning.  I would think that granularity in spring motion, if 
large enough, might have a similar signature.

>  Final thoughts for now.  Because a leaf spring has length, would a 
> defect near the fixed end create more effect than a defect near the 
> mass end by the fact a small angular change would be magnified by the length?

In practice, probably not, but I'll give it some more thought.

>Does this make a case for shorter springs? Does the non metallic 
>hinge mount absorb some of the defect energy?

The spring length is intimately connected with the mass value and 
location and the geometry of the boom, and tends to be more of a 
dependent variable.  Some more recent designs have used significantly 
shorter springs than the first model but they are no quieter.
My impression is that the Kapton flexures don't act much differently 
from the metal ones, except that they are considerably less stiff 
though somewhat tougher.

I would really like to be able to observe and measure this 
effect.  And in that, I am reminded of the words of Lord Kelvin when 
he writes: "I often say that when you can measure what you are 
speaking about, and express it in numbers, you know something about 
it; but when you cannot measure it, when you cannot express it in 
numbers, your knowledge is of a meagre and unsatisfactory kind; it 
may be the beginning of knowledge, but you have scarcely in your 
thoughts advanced to the state of Science, whatever the matter may be."
I'd prefer to know something about it.



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