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Re: [ATM] Polishing / Figuring Simulator
Hi James,
Instead of calculating the pressure distribution , why not measure it ?
A modest 8-16 channel data acquisition system could do that , and it
wouldn't cost much .
The pros use not only computer controlled polishing but also computer
controlled stressed lap polishing.
This changes exactly the lap pressure distribution in the lap/mirror contact
area.
The actuators that stress the lap could be integrated with the pressure
sensors for (servo PID) closed loop operation .
Even if this solution appears more complex, at least it has the potential of
precisely solving the problem you are describing (determining the lap/mirror
contact area pressure distribution ).
I don't think there's even a remote chance of accurately calculating this
pressure distribution without actual value measurements with multiple
pressure sensors , and without knowing the real pressure distribution , all
your calculations become increasingly complex and the errors increasingly
large to the point I doubt they could be useful at all.
best regards,
matt tudor
-----Original Message-----
From: James Lerch <jlerch1@tampabay.rr.com>
To: ATM <atm@atmlist.net>
Date: Monday, May 08, 2006 8:55 AM
Subject: Re: [ATM] Polishing / Figuring Simulator
>Hi Martin,
>
>Great start! I've been traveling a parallel path, similar to yours, for a
few
>years now and have yet to crack this nut, I hope you'll have better luck
than I
>have.
>
>First off, I'll start with a word of encouragement, the Pro's have been
using
>Computer Controlled Polishing / figuring simulation since the early 1970's
(at
>least according to Wilson's Reflecting Telescope Optics II, pages 3-5).
Using
>Preston's equation (Rate of removal = Pressure * Velocity * a Constant)
sure
>makes this simulation seem easy, but it is not. (YMMV).
>
>Here's where I'm stuck, solving for the pressure differential for a
cantilevered
>disk on top of another disk. I'm pretty certain that is my remaining
constraint
>yet to be modeled, and the key to cracking this nut (I think...).
>
>Here's what I have done so far.
>
>#1 Relative Velocity
> My version of the simulation accounts for linear velocity (tool over
arm
>motion) as well as relative angular velocity between the mirror and tool.
My
>polishing machine also measures, records, and controls the linear AND
angular
>velocity of both the mirror and pitch lap. (driven pitch lap)
>
>#2 Pressure per unit area
> I'm not doing well here, the best I can do is assume uniform pressure
in the
>contact patch between mirror and lap. As the size of the contact area
changes
>(lap overhang) I scale the relative pressure per unit area Up / Down as
needed.
>However, I am confident this is NOT TRUE to the real world situation, as
Donald
>Good recently wrote.
>
>#3 Surface Profiling.
> At the bottom of your program, you have a chart showing the cumulative
>amount of glass removal. As a first approximation, this is an OK start,
but it
>needs a little more work to provide an accurate representation as to what
>happens to the optical surface.
>
> For instance, assume you started with a spherical surface with a known
>Radius of Curvature and manifested the simulation onto that optical
surface.
>The result wouldn't be the graph you see at the bottom of your program.
What
>you would get (after measurement via some method) is two fold
> A) A change in the Radius of Curvature
> B) A change in the surface error profile relative to some conic
constant
>(sphere, parabola, or best fit)
>
> In my case, I've experimented with exporting an error profile to a
surface
>profile, modifying the surface profile according to the simulation, then
>reducing the surface profile back to an error profile using a modified
version
>of FigureXP. To some extent this worked. One interesting result was the
>simulation and observation of the "Left Behind Edge" phenomenon, AKA
"Turned
>Down Edge" to some, where insufficient tool overhang produces the
appearance of
>turning down the edge of the mirror.
>
>In summary.
>
>#1 The Pro's have been doing Computer Controlled Optical Surfacing for 30+
>years, and have achieved a level of sophistication that allows them the
luxury
>of taking the guess work out of figuring optics.
>
>#2 I'm pretty certain the only unconstrained (and substantial) variable
left to
>solve for is the cantilevered disk pressure distribution problem. The
closest
>equation I can find is based on cantilevered beams, but cantilevered disks
>appear to be substantially different than cantilevered beams.
>
>Take Care,
>James Lerch
>http://lerch.no-ip.com/atm (My telescope construction,testing, and coating
site)
>http://lerch.no-ip.com/ChangFa_Gen (My 15KW generator project)
>
>"Anything that can happen, will happen" -Stephen Pollock from:
>"Particle Physics for Non-Physicists: A Tour of the Microcosmos"
>
>" Press on: nothing in the world can take the place of perseverance.
>Talent will not; nothing is more common than unsuccessful men with talent.
>Genius will not; unrewarded genius is almost a proverb.
>Education will not; the world is full of educated derelicts.
>Persistence and determination alone are omnipotent. "
> Calvin Coolidge
>
>
>
>
>
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