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The trouble with triangles February 18, 2011

Posted by mareserinitatis in engineering, work.
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I’m doing electromagnetic modeling of a device that has some fine scale features.  In particular, there’s an extremely thin coating of metal that has to be taken into account.  I spent half a day developing a model of one part of the project, and I showed it to the mechanical engineer I’m working with.  (He’s looking at things like thermal and mechanical behavior.)  He asked how I made the model, and after I’d explained it, he said, “That’s how I did mine, too, and I think it needs to be changed.”

The problem is actually fairly simple and can be explained using a diagram of a right triangle:

If you imagine that the surface of coating follows c, we  are supposed to have a particular thickness that we would measure along f or at a right angle to the coating surface.  However, I created it such that the thickness was measured along b.  In retrospect, it wouldn’t have been difficult to make the change, but it would have been somewhat time-consuming.

Instead, I whipped out my calculator (the one on my computer…I sadly have gotten out of the habit of carrying my calculator with me) and determined how different those lengths would be.  It turned out that the worst case scenario, where the angle between b and c is the smallest, gives f at 98% of b.

Realistically, that difference will be swamped by any manufacturing tolerance or measurement precision issues.  Further, this is a tiny part of the overall structure, so it seems silly to spend a bunch of time trying to make it absolutely perfect.

Mike happened to pop in later and saw the drawings on my white board.  When I recounted the conversation to him, he just grinned and said, “You know, you really are sounding like an engineer.”

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