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Re: [ATM] Polishing / Figuring Simulator
Tuesday, May 9, 2006, 2:47:37 AM, you wrote:
m> Hi James,
m> Instead of calculating the pressure distribution , why not measure it ?
m> A modest 8-16 channel data acquisition system could do that , and it
m> wouldn't cost much .
m> The pros use not only computer controlled polishing but also computer
m> controlled stressed lap polishing.
m> This changes exactly the lap pressure distribution in the lap/mirror contact
m> area.
m> The actuators that stress the lap could be integrated with the pressure
m> sensors for (servo PID) closed loop operation .
m> Even if this solution appears more complex, at least it has the potential of
m> precisely solving the problem you are describing (determining the lap/mirror
m> contact area pressure distribution ).
m> I don't think there's even a remote chance of accurately calculating this
m> pressure distribution without actual value measurements with multiple
m> pressure sensors , and without knowing the real pressure distribution , all
m> your calculations become increasingly complex and the errors increasingly
m> large to the point I doubt they could be useful at all.
m> best regards,
m> matt tudor
m> -----Original Message-----
m> From: James Lerch <jlerch1@tampabay.rr.com>
m> To: ATM <atm@atmlist.net>
m> Date: Monday, May 08, 2006 8:55 AM
m> Subject: Re: [ATM] Polishing / Figuring Simulator
>>Hi Martin,
>>
>>Great start! I've been traveling a parallel path, similar to yours, for a
m> few
>>years now and have yet to crack this nut, I hope you'll have better luck
m> than I
>>have.
>>
>>First off, I'll start with a word of encouragement, the Pro's have been
m> using
>>Computer Controlled Polishing / figuring simulation since the early 1970's
m> (at
>>least according to Wilson's Reflecting Telescope Optics II, pages 3-5).
m> Using
>>Preston's equation (Rate of removal = Pressure * Velocity * a Constant)
m> sure
>>makes this simulation seem easy, but it is not. (YMMV).
>>
>>Here's where I'm stuck, solving for the pressure differential for a
m> cantilevered
>>disk on top of another disk. I'm pretty certain that is my remaining
m> 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
m> arm
>>motion) as well as relative angular velocity between the mirror and tool.
m> My
>>polishing machine also measures, records, and controls the linear AND
m> 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
m> in the
>>contact patch between mirror and lap. As the size of the contact area
m> changes
>>(lap overhang) I scale the relative pressure per unit area Up / Down as
m> needed.
>>However, I am confident this is NOT TRUE to the real world situation, as
m> 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,
m> 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
m> surface.
>>The result wouldn't be the graph you see at the bottom of your program.
m> 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
m> constant
>>(sphere, parabola, or best fit)
>>
>> In my case, I've experimented with exporting an error profile to a
m> surface
>>profile, modifying the surface profile according to the simulation, then
>>reducing the surface profile back to an error profile using a modified
m> version
>>of FigureXP. To some extent this worked. One interesting result was the
>>simulation and observation of the "Left Behind Edge" phenomenon, AKA
m> "Turned
>>Down Edge" to some, where insufficient tool overhang produces the
m> 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
m> luxury
>>of taking the guess work out of figuring optics.
>>
>>#2 I'm pretty certain the only unconstrained (and substantial) variable
m> left to
>>solve for is the cantilevered disk pressure distribution problem. The
m> 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
m> 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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>>ATM mailing list http://www.atmlist.net/
m> _______________________________________________
m> ATM mailing list http://www.atmlist.net/
Sorry I lost the link where I can download this software. Can you
share it again!
Thank's!
--
Clear skies!
Attila Schné
mailto:yolo@chello.hu
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