Optomechanics – Using Ivory for Early Structural Concepts

Posted on April 16, 2014 by Al

Colleagues:

If spherical surfaces didn’t make pretty good images we’d live in a whole different world. It seems that optics is an art that was just meant to work. The mechanics? Well, that’s maybe a whole different story.

I recently helped to demonstrate that a proposed optical system could be made to work. It’s stability requirements were 2 1/2 times tighter than the earlier system on which the proposal had been based, and the earlier one had been a challenge in its time. That extrapolation was a risk that the contractor had to eliminate very early.

So, the CAD engineer and I shared a cubicle. He collected information on all the stuff that had to go into the system. I created a structural finite element model to analyze the image stability: I started with the CodeV prescription, which I read into AEH/Ivory and then imported the Ivory file into Patran; I also imported into Patran the step-files (and ray bundle) for all the optical elements; I attached the Ivory file to the elements (Any time I moved an element I could then read the resulting motion of the image on the detector); finally, I imported into Patran the proposed flat honeycomb plate to which the optical elements were to be mounted. The boring part was over.

And the real fun began. I used Patran as a design tool: I put cells around each of the optical elements; I tied the cells down to the flat plate; I ran a 6 DOF rigid body check and a 3 axis static gravity check in Nastran. Everything behaved well computationally. But in random vibration it was out of bed by ~3X. There was work to be done.

With the AEH/Ivory data imported to Excel I could identify which elements were the big drivers of the image motions. So, I started beefing up the bracing on those elements. The CAD engineer was checking my work while he started his own modeling effort. He guided me in positioning the optical mountings and I guided him in locating the other services (electronic, thermal, servo, mechanical) that had to work in proximity to the optics on the inner gimbal. The bracing and the services all had to fit. Ultimately, we (he and I) were able to reduce the image motions by over 3X and show safe margin on the stability requirements with everything on the gimbal.

All of this was done in the opening days of the project. In fact, if you cannot make the optical system work when the design spaces are malleable, you will be unlikely to make it work later. It only gets harder (and I’ve been there too). This early structural concept was itself malleable and would change over time as all of the disciplines agreed to the design. It might be months before all the CAD interfaces would be settled. Meanwhile, the project had a structural concept that promised to meet the stability requirements and could guide the detail mechanical design. And an engineering tool for occasional spot-checks and trade-off studies.

Joy and Happiness!

Ahhh… April.

Al H.
4-16-14

Optomechanics – Computational Zero is a Relative Term

Posted on March 17, 2014 by Al

Colleagues:

You more astute readers have made a good point about my call for completeness and congruence in the modeling data for Nastran optomechanical models: These properties are a necessary condition but, alas, not sufficient.

The goal is to achieve computational zeros in the sums of a few dozen terms of a finite element model that may itself contain a few million terms. But “computational zero” is a relative, not an absolute, term. The engineer needs to demonstrate that the actual value of the computational zeros are small enough to be ignored. Rigid body checks alone can’t do that.

And no technology is more sensitive to this issue than optics. For instance:

AEH was called to verify the predicted magnitude of dynamic jitter in a spectral imaging system. AEH assembled an all-up meshed solid finite element Nastran model of the system, put the optical prescription through the AEH/Ivory Optomechanical Modeling Tools to generate the complete and congruent data that control the motions of the image on the detector and ran the rigid body checks. Everything so far seemed reasonable.

Then AEH ran three-axis static gravity checks. Considering the frequencies, displacements and accelerations the instrument would see AEH judged that the computational zeros, which dominated the rigid body checks, were two orders of magnitude too large compared to the static gravity checks. Correcting this required three more significant figures in the optomechanical modeling data (both the geometry and the coefficients) for sufficient precision in the dynamic jitter analysis.

Fortunately, the software also produces both geometric and coefficient data with eighteen significant figures. AEH was able to edit the additional significant figures into the Nastran database. Finally, AEH ran the frequency response spectrum. The image jitter proved to be safely within the system’s specified maximum.

But, the engineer had to take the critical steps to demonstrate sufficient precision in the optomechanical modeling data.

AEH offers completeness, congruence and precision in those critical optomechanical analyses.

Well, the March Hare is here, “I’m late, I’m late, I’m late!”

And as Lewis Carol further wrote, “The time has come….”

See you all in Baltimore.

Al H.
3-17-14

Optomechanics – Save Time with Ivory

Posted on November 12, 2013 by Al

Colleagues:

Skeptics like to tease me about using my own software tools to create optomechanical models in finite element codes. I could simply use the coefficients provided by the optical designer, they suggest. In a way they’re right. But I have found that my software allows me to enjoy more of my evenings and weekends. Let me explain:

The task is to incorporate the optical image formation properties into a structural finite element model. Rigor is required because small modeling errors can create large misleading results in the subsequent analyses. A complete set of coefficients and congruent descriptionsof the geometries are essential for a properly formulated optomechanical model. This allows the optomechanical engineer to validate the integrity of the entire system model with what are called “rigid-body checks.”

But, how to satisfy the “complete” and “congruent” criteria? Well, I use Ivory.

Now, about the importance of those rigid-body checks:

First, structures: A rigid-body check exercises the otherwise unconstrained complete model in three translations (Tx, Ty and Tz) and three rotations (Rx, Ry and Rz). The check discloses malformed elements, erroneous constraints and other errors, which the engineer must correct to have confidence in subsequent analytical results. It’s a tried-and-true method for checkout of structural models.

Then, optics: In the optics domain the image motions on the detector during rigid-body checks should be either computational zeros or the effective focal length depending on the status of the object being imaged. If the model’s image motions contain anomalies (motions other than 0. or the efl) in any of the whole model’s rigid-body motions then the model is poorly formed and the optomechanical engineer must correct it before relying on any subsequent results. This is a tried-and-true method for checkout of optomechanical models.

Without a complete set of optomechanical coefficients and assured congruent geometries it is very difficult to tell whether any anomalies are artifacts of geometric differences or of inaccurate and/or missing coefficients. Small imaging anomalies can create large errors in the analyses. But even small (or perhaps “Especially small”) anomalies can be very time consuming to find and correct. There are many potential sources of small errors.

That’s where my software lets me enjoy evenings and weekends. I start with a complete set of Ivory’s coefficients and congruent geometries (from the optical designer’s prescription), check their validity in a simple finite element model (with rigid-body checks) before putting them into the structural engineer’s larger model of the system.

When the two are married, Voila! Bliss! Well, fewer surprises anyway, and more evenings and weekends for me.

So, don’t spend your Holidays in front of your work-station when you should be with your family and friends.

The Season’s Cheer to you all. I can almost hear the sleigh bells coming.

Al H.
11-12-13

Optomechanics – Samuel Colt’s Principle of Interchangeable Parts

Posted on September 23, 2013 by Al

Colleagues:

Well, San Diego’s history now. Whew!

Thanks to the OMTG Program Committee for an absolutely terrific two-days of papers (plus a poster session). Thanks to Phil Pressel for an awesome evening presentation on the Hexagon camera system. And thanks to Eugene Arthurs and the SPIE staff for keeping it (all the rest of the Symposium) together: What a herd of cats!

And if you missed the Exhibit Hall, well that’s your problem, understandable but still your problem.

Back to optomechanical engineering. One of the mechanical engineer’s duties on an optical project is to survey the available mechanical design space looking for problems. The mechanical design space includes dimensions, temperatures, stresses, deflections, tolerances, pressures, masses, damping, friction, durability, service life, stability, …. Oh, I shouldn’t leave cost out of the design space either.

In my practice of the mechanical engineering arts I’ve become a disciple of Samuel Colt. He’s the guy who introduced the principle of interchangeable parts to the manufacture of his infamous .44 caliber revolver in 1841. Up to that time firearms were assembled by a gunsmith who would grind, file and polish all the manufactured parts until they fit together and operated to his satisfaction. Their weapons were very expensive. On the other hand the Colt revolver’s price was so low that “The Great Equalizer” became available to almost everyone.

My tolerancing method applies Colt’s principle to optical products. Using influence coefficients from my AEH/Ivory Optomechanical Modeling Tools, I calculate the maximum worst-case assembly errors between the image and the detector in all seven registration variables: Tx, Ty, Tz, Rx, Ry, Rz and dM/M. I include the tolerances on the lens design variables (R1, R2, t and n) in addition to all the mechanical dimensional tolerances. Then I tweak all the tolerances (in a spreadsheet) so that the pain is equally shared between the mechanical suppliers, the optical suppliers and the assembly technicians. And, all the manufactured parts get used as-is.

When I describe this principle someone is usually perplexed at how I can do this without using the statistical distribution of each dimension. I point out that I can put the statistical distributions into the calculations if I choose but they won’t change the maximum worst-case assembly errors.

Scrap is another one of the problems that mechanical engineers work to avoid. Thank you Samuel!

Well, I bought some candy corn this morning. All Hollow’s Eve is on the way.

Boo!

Al H.
9-23-13

Optomechanics – Keep Your Tools Sharp

Posted on June 18, 2013 by Al

Colleagues:

If spherical surfaces didn’t make pretty good images our optical industry would be entirely different. As befits a technology that basically works as intended, cliches and rules-of-thumb perform a yeoman’s service. And they work! I’m glad that many of you enjoyed my parable about “kinematic” mounts. Well, that is, they work until they don’t, as in that misunderstanding between the laser physicists and the mechanical engineers. Thanks for all of your comments.

More recently I’ve been inspired by some of my students to publish the optomechanical influence coefficients of diffraction gratings (i.e., the ratios of a spectrum’s motions to the grating’s motions). Gratings are often simulated as mirrors. But the grating’s influence coefficients differ slightly from the mirror’s and there are more of them. I’ll present my results in Mark Kahn’s conference, “Optical Modeling and Performance Predictions VI,” at SPIE’s meeting in San Diego this August.

Imaging spectrometers (using gratings of course) are particularly challenging to the optomechanical engineer because the images of both the far-field object and the near-field slit (the spectrum) need to be stabilized simultaneously on the detector plane. The slit operates as a field stop and the two images behave somewhat (and sometimes importantly) differently. “Mining” the resulting “data cube” requires close registration between the spectrum and the far-field object’s image. The grating will work in my Ivory Optomechanical Modeling Tools software.

In San Diego I’ll also present a paper in my own conference, “Optomechanical Engineering 2013.” This presentation will describe the use of my Jade Optomechanical Modeling Tool. Jade models the subsurface cracks induced by grinding and polishing. I use it to engineer, for structural safety, components made of glasses, ceramics and other brittle materials. As an example I’ll show how I applied Jade to meter-class optical windows for a civilian transport-class aircraft. The windows have been in service for years.

Engineers develop tools to keep themselves out of trouble. In the public works domain these have developed into codes and standards that engineers are obligated (by their insurance companies) to follow. Elsewhere, engineers develop tools for themselves. In optomechanical engineering there are few rules of thumb to help. There are, however, a few cliches.

Summer is coming! Get out the sunscreen and water skis again!

Keep your tools sharp and your wits even sharper.

Hasta luego, caimán.

Al H.
6-18-13

Optomechanics – The Intersection of Heat Transfer, Structures and Optics

Posted on January 8, 2013 by Al

Dear Colleagues:

A Happy and Joyous New Year to you all!

What a year 2012 was: two structural window designs, three spectrometers, two cryogenic dewars and an extended-band imager. Each of these was pressing the limits of the mechanical arts in one way or another. Then there were the calls for help, or just to say hello, that filled the smaller spaces. A great time.

One recurring theme in my work has been the intersection of heat transfer, structures and optics. The common issue is stability, either static or dynamic, of the optics. This is a tricky combination because the three disciplines are, in most ways, mutually independent. The trickiest part seems to be getting the heat transfer results (temperature fields) into the structural model in a reasonable fashion. “Reasonable” is the key word here. The engineer faces some difficult choices to achieve optical accuracies.

An approach, for design purposes, has been to linearize the radiation heat transfer and run the whole problem in a finite element code such as MSC/Nastran. The temperature fields can be made to agree reasonably (there’s that word) well with the Sinda results, ~1% or so. Import an AEH/Ivory optomechanical model of the image into the MSC/Nastran model and you have the ultimate thermo-opto-elastic engineering design tool. Fiddle with the mechanics, either thermal or structural, and see the results in the image! All in one computer model.

An image from one of my instruments was featured on the cover of Aviation Week’s 75th anniversary issue. (Well, yes, I have been at this a while.)

Now, most important of all…

Don’t forget our conference in San Diego in August. Submit your paper abstracts at http://spie.org/OP303 and call me if you have questions.

Now is your best chance to get in on the fun.

Here comes 2013…. Hang on tight.

Al Hatheway
1-8-13

Optomechanics – Optomechanics and the Tolerancing of Instruments

Posted on May 7, 2012 by Al

Colleagues:

I will be presenting my tutorial “Optomechanics and the Tolerancing of Instruments” in San Diego on August 13th during SPIE’s Optics and Photonics Symposium. The course is primarily intended for mechanical engineers and designers in the optics industry and may be of interest to other optics professionals as well.

In the tutorial I develop the full theory of the Optomechanical Constraint Equations (OCE). They relate the position, orientation and size of an image to the position and orientation of each of the optical elements that form it. I work through numerical examples to assist the student’s understanding and illustrate the application of the OCE with examples, static and dynamic, from my engineering practice.

The OCE are the basis for the AEH/Ivory Optomechanical Modeling Tools (version 2.6 currently available) that are the mechanical engineer’s first choice for design and analysis of optical systems.

For more details on the course visit SPIE’s web site,

http://spie.org/app/program/index.cfm?fuseaction=COURSE&export_id=x30628&ID=x30533&redir=x30533.xml&course_id=E1036499&event_id=896190&programtrack_id=1038730

or send your questions to me.

I’ll also be hosting an evening meeting of the Optomechanical Technical Group, probably on Tuesday (but more on that later) as well as cruising the conference rooms, receptions and exhibit halls. It’ll be a big week!

Hope to see you all in San Diego from August 12th through the 16th.

Al H.
5-7-12

Optomechanics – Just what is it you do?

Posted on April 17, 2012 by Al

Dear Colleagues:

It’s been busy the last few months. Some of you have asked, “Just what is it you do?” Well, here’s just a sampling:

I’ve researched the intellectual property of mechanisms for focus and diaphragm control in camera lenses;

I’ve analyzed microradian-level image jitter in an off-axis imaging spectrometer (with AEH/Ivory and MSC/Nastran of course);

I’ve co-invented a remote sensing 10:1 zoom system with five moving lens groups, three of which are lenslet arrays.

Who could possibly have more fun than a mechanical engineer in this optics industry?

I hope you will join me in this paean to Springtime. Joy!

Al H.
4-17-12

Optomechanics – Engineered Design

Posted on September 26, 2011 by Al

Colleagues:

“To design or to analyze? Aye, that is the question.” My apologies to the Bard.

I spent the first half of my career in design. But even as a designer I was always a numbers guy. I wanted to know why some things worked and others did not. And I found that the numbers could actually help explain the workings of things. An example:

I designed the first IR missile active jammer to go into the Navy’s service (AN-ALQ-123). The IR source was fragile and had never been qualified for military use. I was frustrated by the reluctance of the structural engineering department to support me. They wouldn’t touch the quartz, Lucalox and niobium that the source was made from. Nor would they let me anywhere near their finite element code. So, I had to run the tests to determine its fragility and then analyze my mounting design to assure that the IR source could survive the “cats-n-traps” of carrier take-offs and landings. I even had to write my own source code for the analysis. It worked! The design was a grand success and hundreds of -123s entered the Navy’s inventory. It even made the cover of the Old Crows monthly magazine. I was a proud papa.

I still design. Here’s a view of a newer design job. It’s an active stand-off sensor

consisting of a zoom lens with five moving lens groups, all servo controlled, plus a sixth movable lens group to control the focus at a distant target. I gave my client all of my design files and they took it from there. The last I heard, the prototype was working just fine. There’s a really hard part to design consulting though: Letting someone else adopt the offspring.

Numbers are a lot less sentimental. I do a lot of analysis these days but I still think as a designer. Luckily, as an engineer I can work both sides of that street.

I’ll be talking-up my design tools to the analysis folks at the MSC Software Users’ Conference in Santa Ana on October 5th. (Just as I talked-up my analysis tools to the design folks at the SPIE Symposium in August.)

Will I see a few of your cheerful faces there on the 5th?

Al H.
9-26-11

AEH Inc

Optomechanical Solutions

Category Archives: Ivory