Optomechanics – Unified Approach to Thermal Structural Optical Analysis

Posted on October 23, 2017 by Al

Colleagues:

Optics is a crazy industry. We’re so dedicated to the digital computer that we often overlook underlying realities. That’s especially true in the mechanical engineering art of heat transfer. Management structures have insisted on each discipline using its own software which has Balkanized the disciplines of structures and heat transfer: i.e., the temperatures must be analyzed in a finite difference (FD) code and the results imported to a finite element (FE) code. The importation involves extensive extrapolation and interpolation of the FD data to the much larger (often by two or three orders of magnitude) of the FE data-set and it can lead to some peculiar results.

AEH has long preferred a more Unified approach: Select a code that can do them both, usually an FE code, and use engineering judgement to adapt the other discipline (usually a few boundary temperatures). It usually requires several runs of the problem to assure that the underlying assumptions of the adaptation were appropriate but they go much faster than the Balkanized approach (and, often, more accurately for the optics behavior).

It’s the thermal-structural-optical method I used on the LACE Ultra-Violet Plume Instrument, which made Aviation Week’s75th Anniversary Issue. Check out the UV plume image from AW’s cover, above!

More recently, a colleague was directed to thermally analyze an optical system in CFD and I was to apply his temperatures and gradients to calculate the boresight errors among the optical instruments. Well, what he was handing me made no sense at all. So, I added heat transfer terms to my opto-structural FE model and ran the operational transient of concern to the project. The thermo-structural-optical results were spot-on.

There are a lot of ways that an engineer needs to keep his tools sharp. And they require maintaining confidence in the analytical methods as well.

More later. Happy Holloween!!!!!

Al H.
10-23-17

Optomechanics – The Industry Needs to Know the Strength of Optical Glasses

Posted on September 7, 2017 by Al

Colleagues:

Fall is coming in two weeks and the Great Pumpkin is right down the road. It’s time to kick-back and just look around a bit at this Glorious Summertime…

I’ve had about four weeks on the road lately, touching base with all of you (or most of you, I hope). Then I cruised around SoCal in my spare time chinning and jawboning with the local industries’ participants. So, what are the “hot items” for optomechanical engineers today? Well, how ’bout…

precision dimensioning and tolerancing of optical structures
repeatability and stability of instrument performance
broken optical glass

I’d have trouble identifying the most important of these, it all depends upon what’s happening NOW. All three of these have kept me pretty busy since Valentine’s Day. The greatest uncertainty for the engineer, however, is in the strength of optical glasses. Its not just mirrors and windows.

I’ve participated in two projects in which the optical design had to be revised (compromising performance) to replace glasses that could not pass the environmental tests. Perhaps, someday, the glass houses will start to provide strength information on their data sheets near the Young’s modulus and Poison’s ratio. Today the engineer can’t be sure until the tests are passed.

And we, all of us, managed to squeeze-in a terrific conference with SPIE in San Diego last month (32 papers published). How we were able to pull that together, I just don’t know. But, it would never have happened without you and the terrific staff at SPIE who keep us on the right track.

Here comes Guy Fawkes Day. Get your bonfires, and marshmallows, ready! All our prayers this day are with those who may be in harm’s way.

Al H.
9-7-17

Optomechanics – MatLab Models and Ivory

Posted on July 10, 2017 by Al

Colleagues:

Hey, Control Systems Engineers, this one’s for you! I’m sure you remember my dear friend who likes to declare, “You have to know the answer before you do the analysis!” And his wicked “eye-twinkle” was part of that message too. I used him then as a vehicle to highlight the importance of engineers making estimates and developing a “sense of smell” about the quality of their decisions.

So, my question for you is: How do you incorporate the line-of-sight into the MatLab model of your “stabilized” optical system?

Well, AEH knows of three ways: 1) You can calculate it yourself, from the optical prescription, and insert it in your MatLab file, with some luck, or 2) you can copy the Optomechanical Constraint Equations (OCE) from Ivory and patch them into your Matlab file, with a little better luck, or 3) you can speak nicely enough to the structural engineer for him or her to import the OCE (from Ivory) into his FE model and the resulting eigenvectors, BINGO and…

If you want to learn more here’s an opportunity: On August 7th, all day, I’ll be teaching my course, Optomechanical Analysis, for SPIE’s Symposium Optics+Photonics 2017 at San Diego’s Convention Center and Marriott Marina Hotel. The first half of the course is all about the OCE, how you generate them and how they’re used. Then you might stick around for our Conference, Optomechanics 2017, on the 8th and 9th to find out what everyone else is doing. On Tuesday evening, the 8th, I’ll be hosting a meeting of the Optomechanical Technical Group between 8 and 10. Dan Vukobratovich will be our principal speaker followed by an open discussion.

Yeah, you guessed it. My dear friend is a control systems engineer. One of the things I did for him was to assure that the structural engineer incorporated Ivory’s OCE into the FE model that produced the eigenvectors he used in MatLab to design the control system. Later system tests on the shaker-table confirmed the quality “smell” of this decision. Ivory nailed it, dead-on!

I’ll see all of you in San Diego!

Al H.
7-10-17

Optomechanics – Using CodeV Prescription in Ivory and Jade to Find Structural and Thermal Weaknesses

Posted on June 5, 2017 by Al

Colleagues:

I’ve been known to lecture my students and colleagues on the need to keep their tools sharp. Some time ago AEH was invited to a design review as an observer and since I had no direct participation I sat at the back of the room, behind John, the systems engineer who was controlling the projector. The technical sessions went well but about half-way through the schedule and budget sessions he suddenly blackened the screen and turned on the overhead lights. He slowly turned and surveyed those of us sitting behind him. His gaze settled on me! “What, John?” I asked. He stared at my hands which were holding my pen knife and its sharpening steel. “Just keeping my tools sharp,” I declared sheepishly.

One of AEH’s sharpest tools, other than a pen knife, is Ivory’s Optomechanical Modeling Tools. It’s been under continuous development incorporating many of my personal insights working as a mechanical engineer in the optics industry. I recently put together an updated version and released it to all users of Version 3. That’s another way I keep AEH’s tools sharp (and protect AEH’s Ivory subscribers, too). Ivory is AEH’s prime tool for engineering thermally and structurally reliable optical systems. It’s designed to work in both Excel and Nastran and its application early in the design process prevents much embarrassment and saves many labor-hours from preventable failures that may occur later in qualification tests and service.

Somewhat more recently AEH was invited to participate in a “Tiger-Team” review of a sub-contractor. The initial issue was broken glass. The first thing I did was get a copy of the physical optical prescription (CodeV) and read it into Ivory (for the structure) and Jade (for the broken glass). I could then quantitatively infer where the principal structural and thermal weaknesses might be. With that insight I was able to form an independent assessment of the completeness of the design team’s engineering effort, which undergirded my report to the prime contractor.

I hope to see all of you at SPIE’s Optics+Photonics in San Diego come August. I’ll be teaching (Optomechanical Analysis), chairing (The Optomechanical Engineering Technical Group and Optomechanics 2017), presenting and publishing (on a new diffraction grating capability in Ivory) and begin planning our next SPIE Conference (Optomechanics 2019).

That also keeps AEH’s tools sharp.

Hasta luego, caiman.

Al H.
6-5-17

Optomechanics – Recommended Reading

Posted on March 10, 2017 by Al

Colleagues:

Joy to all, and thank you for your awesome support of our Optomechanical Engineering 2017 Conference in August. Just wait ’till you see the program.

Now, back to business. Fractures in glass optics seem ubiquitous.

So… the optomechanical engineer has a problem. He’s nominally responsible for protecting the glass elements in the optical systems he designs. But there’s very little (if any) information available on the structural properties of the glasses that optical designers specify. And the guidance for the engineer on using the available data is all-over-the-map, from the incomprehensible to the impossible. No wonder most engineers use simple “rules-of-thumb.”

That’s where I started, “Keep the tensile stresses under 2,000 psi.” But then the glass broke anyway! So I started testing the glass objects to 4,000 psi. I broke a few in testing but those that went into service are still in service, as far as I know. I didn’t get to make many, they were too big and heavy. A colleague who specialized in space-based ISR systems confided to me that he kept the stresses under 500 psi! That’s when another colleague gave me a copy of “Reconnaissance and Surveillance Window Design Handbook” (AFAL-TR-75-200).

Voila! Section 7.3.1 is the perfect introduction for the engineer to “Allowable Stresses in Glass.” It covers fabrication process controls, slow crack growth through stress corrosion (from moisture) and estimating the service life by integrating the stress corrosion equations for eight glasses. The Wizard’s green curtain is drawn back disclosing all of his secrets and Dorothy dances down the yellow brick road and back to safety in Kansas.

Every optomechanical engineer needs a copy of that Handbook to help him protect the optical glass that has been entrusted to him by the optical designer. It guides the design, analysis and fabrication of glass optical elements. Perhaps the engineer should enter the required structural properties, including the fracture toughness and stress corrosion constants, on the lens drawings (think the yellow brick road). The Handbook’s drawback is that only eight glasses are treated and some of those have since been re-formulated to remove toxic elements.

The glass suppliers also need copies of the Handbook so they’ll know what the engineer is requesting and, maybe someday, put the information in their glass catalogs and data sheets.

Joy! Spring is just around the corner. Ahh… Kansas in Springtime!

Al H.
3-10-17

Optomechanics – Poor Optical Performance is a Mechanical Problem

Posted on December 28, 2016 by Al

Colleagues:

Hey! It’s been one Heck-of-a-Year…

…horrible images, tricky windows, cracked laser rods, soft adhesive, savvy clients, a challenge from the nanoradian and writing a book-and-a-half. What’ll it be next?

Our industry is terrific fun. Optics is an art that was just meant to work, until it doesn’t. Then the solution is almost always mechanical: a longer stroke on the focus mechanism; find a way to install a “corrector” lens; fold the system here or there; find a new adhesive; brace the objective doublet; stiffen the cold finger, control-grind and polish the edges, etc.

And that’s all as it should be. Once the optical designer finishes his design it’s virtually impossible to improve it. If that were not true the optical designer would probably not stay employed at the firm. All deviations from the prescription are the mechanical engineer’s contribution. Poor performance is therefore a mechanical problem; something is improperly located, dimensioned or deformed and the solution is also mechanical, correcting the mechanical engineer’s positional or dimensional specifications or stiffen the structure.

The errors are almost always detected after the system is assembled and that’s what makes it so much fun. The mechanical engineer has to think ahead and analyze ahead to avoid the problems in that first assembly.

And that’s why we have a conference in San Diego this August with SPIE, to share with each other and our colleagues the discoveries and methods for making optical systems work in spite of it all!

Attached is the Conference Announcement. Just click on “submit an abstract” on the bottom of the second page to reserve your position in the Program.

Look out 2017, here we come!

Al H.
12-28-16

Optomechanics – Align the Image Plane over the Entire Detector Array

Posted on November 29, 2016 by Al

Colleagues:

I mentioned earlier that there is a great deal to optomechanics. Even full time practitioners can lose track of the current scope of the optomechanical arts. And with a well corrected optical design the onus falls to the optomechanical engineer “to make it all play.”

I was working with another dear friend shortly ago. He was contemplating a compound hyper-spectral imaging system and wondered if I knew anyone who could help stabilize the instrument. Of course, I immediately raised my own hand. He said that I didn’t understand: The entire image plane had to stay in strict alignment over the entire detector array all the time. He behaved incredulously but, generously, heard me out.

I explained: AEH’s Ivory Optomechanical Modeling Tools operate over the entire exit window (field-of-view on the image plane, if you prefer). They tell the optomechanical engineer what’s going on at the edges, the corners and the center or anywhere else in the image. The Tools can do this because their Optomechanical Constraint Equations determine the alignment of the entire image plane (position, orientation and size) over the entire detector array. He said, “Show me.”

So, I used the Ivory OMT to model the image’s corners, edges and center in shock, vibration and thermal environments. Initially the image’s stability over the entire detector array was barely marginal. But with a little tweaking of the structural design, informed by the influence coefficients from Ivory, we got it within specification with a comfortable margin. This system flies today with confidence.

Just Bridging the Chasm one more time.

I hope you all enjoyed a Great Thanksgiving. It’s down-hill from here to 12th-night!

Cheers.

Al H.
11-29-16

Optomechanics – Bridging the Chasm

Posted on October 24, 2016 by Al

Colleagues:

Ah yes, there is a great deal to optomechanics!

And so much of it is on the hairy edges of, slightly beyond, or in-between the academic disciplines that we study at the university. The engineering challenge is to make it work anyway. In my practice I refer to it as “Bridging the Chasm” or B/C. It involves surveying both sides of the gorge between disciplines before building on them. That’s just good engineering practice. It almost falls into Systems Engineering… almost.

One example of B/C is a dialog I had a while ago with a dear friend who was concerned about the fracture of glass lenses mounted in metal compression rings. Structural engineers were not interested in glasses. The opticians were disinterested in elastic analysis. I had to sit down and solve the differential equations of the problem myself (Proc. SPIE, 7424-09. 2009).

They showed that the actual tensile stresses were four orders of magnitude smaller than those predicted by the traditional method (Delgado and Hallinan, Opt. Eng., 1975). That “bridge” between the disciplines for metals and glasses has saved my clients’ buttons more than once.

Another B/C example was a modification to an unstable EO weapon sight… but that’s enough about me.

“The time has come,” the Walrus said, “to talk of many things…”

Yes, the great Optics+Photonics 2017 Roadshow (Symposium) is returning to San Diego this coming August and the Optomechanical Technical Group is organizing an exciting two-or-more day conference to highlight your accomplishments this past year.

You all have your own experiences and stories. Bring them to your Optomechanical Engineering 2017 Conference. It’ll be a great show!

“…of shoes and ships and sealing wax, of cabbages and Kings.”

Here’s hoping I’ll see all of you in San Diego come August, at least those who survive the Great Pumpkin.

Al H.
10-24-16

AEH Inc

Optomechanical Solutions

Author Archives: Al

Optomechanics – Unified Approach to Thermal Structural Optical Analysis

Optomechanics – MatLab Models and Ivory

Optomechanics – Using CodeV Prescription in Ivory and Jade to Find Structural and Thermal Weaknesses

Optomechanics – Recommended Reading

Optomechanics – Bridging the Chasm