Optomechanics – Using Ivory for Early Structural Concepts

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

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