Re: [ATM] PLOP and the mirror support.

[Jeff Anderson-Lee <jonah@eecs.berkeley.edu>]

Jerry Reddell wrote:

>While
>experimenting with different support rings, number of supports, and radius
>of support rings I found several surprises. The most notable being the
>dramatic negative affect on the error by adding supports in the area where
>the most droop ( i want to say sag) occurs for a near ideal support. The
>intuitive thought is that the added support would further reduce the error,
>but instead the error increase by a factor of 10 or more.....
>
True.  And not necessarily what you would expect.

The best way to run PLOP is typically to leave some variables for PLOP 
to optimize the cell.  Otherwise you are simply guessing, and PLOP tells 
you the result of your guess.

>The analysis obviously uses 'point' support for calculations. The size of
>these PLOP support points is very small compared to the size of the actual
>built support area...Since the as build support area is in affect many PLOP
>support points arranged in a group, and considering the above mentioned
>unexpected increase in error by adding support points in obvious locations,
>the question comes to mind that the as built cell and supports may adversely
>perform quite differently by increasing the error above what one expects
>using PLOP...
>  
>
The biggest problem seems to be not building or positioning the parts 
accurately enough.  On some designs (e.g. a 3 point cell) it seemingly 
matters far less than others.  In general, the more points you have, the 
more critical it seems to be that the parts are well constructed and 
precisely placed.  You can seemingly loose a factor of 2 or more due to 
such construction errors.

>Does anyone know of a way to get PLOP to simulate the actual as built
>support area?  
>  
>
Some folks have tried to model support pads as multiple closely spaced 
points.  The results don't seem to change much from single point 
supports, so it's generally not considered as worth the effort.

>Also when one builds a floatation type cell, is the mirror siliconed to each
>of  the support points or is the mirror allowed to 'slip and slide' a bit
>with the restraints being elsewhere.? 
>
Depends on who you ask.   Some folks swear by silicone, while others 
swear by minimizing friction.  Both work if done well and fail if done 
poorly.

Jeff Anderson-Lee

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