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Redesigns, Redesigns, and More Redesigns

Posted by Tom Benedict on 26/02/2013

That’s been the story of my life recently. So here are three redesigns I’m going through right now, plus a new design I’m working on:

Redesign #1 comes from work. So does redesign #2. Since they’re related, I’ll tell them together. The cameras we’re making for this new instrument need a charcoal getter, and a way to attach the cold strap to the getter. I’d designed some really pretty two-sided getters, complete with sub-micron filtration to keep the charcoal dust from getting on the camera electronics. Even better, I’d designed a new way to attach the cold strap to the getter using magnets. Yay! New designs are cool! Unfortunately the new getter took up space we needed for the electronics, so I had to machine one side of it off. And the magnetic cold strap just didn’t give us the positive attachment we needed.

My new getter design isn’t as pretty as the old one, but it’s dead nuts easy to make, and can be made using a conventional lathe and mill. No CNC required. Also no bolts. Its various parts are attached using the same Stycast potting resin we’re using on our feedthroughs. Our fastest cool-down to date only pulls three Kelvin per minute, so we don’t even face thermal shock issues. Even better, it has more volume than the original getter, and should be able to pull harder and last longer.

The cold strap attachment moved to a single bolt that pushes a wedge into a groove. Its bizarre nature is only made more strange by how you actually get to that bolt: you have to remove the vacuum gauge, pull the gauge baffle, and stick a 2.5mm ball head hex wrench in through the vacuum port. Not my favorite idea. But I really had no other way to pull it off. The camera had initially been designed around a spring and cone arrangement that was shot down during the preliminary design review. The magnet and bolt-wedge ideas were hacks. But the bolt-wedge one worked.

I start machining the final parts on each of these tomorrow morning.

Another project I’m working on on the side had its own redesign. But this one appears to be working first try. It’s a 6WD platform for moving a camera around via remote control. The problem is the platform has a wonderfully soft suspension with an almost unbelievable amount of hysteresis built in. So it never leaves the camera in a level position, even when it’s on flat level pavement. Rather than the pan/tilt head that was originally planned, it needed a pan/tilt/roll head. Making everything fit has been… interesting. But I think I finally got it to work. I’m finishing the wiring now. Once it’s done, I should be able to hand it over for testing.

The problem with any redesign is that it is essentially a hack job at that point. If it wasn’t, it would’ve been designed right in the first place. The trick is to make it look and behave as if you’d designed it that way on purpose.


Design it to behave like Legos or a Meccano set. That way when you build something that works, it looks like you did it on purpose from the get-go, regardless of how much redesign work you had to do. This is why I like the KAP bits from Brooks Leffler so much. Each part has a ton of holes in it for attaching all manner of things. And if the right hole isn’t there, it’s trivial to drill a new one. It’ll fit right in with the others! This is the approach I’m taking for a new project.


The New Project – KAP Pendulum Suspension

This one has been a re-re-re-redesign project from the start. I’ve been drawing a KAP pendulum for years now – more years than I’ve been doing KAP. But I think I understand enough of the issues involved to finally make what I really wanted from the beginning: a KAP pendulum Lego set. Here’s the basic idea:

So far I’ve used Picavet suspensions on my rigs. They’re great: they’re lightweight, portable, self-damping, and not that hard to use once you get the trick of storing your lines afterward. But they don’t constrain the rig’s pan rotation enough for my taste, they suffer from “nodding” if a kite starts pumping the line, and when the kite line approaches vertical, they just plain stop working.

So I want to move toward a pendulum. But I’m dissatisfied with most of the pendulum suspensions I’ve seen. The pivot axles tend to be wobbly so the pan axis is still under-constrained, they tip the rig when the wind blows on them – similar to a Picavet, and they have oscillation issues of their own. But I still think they’re the right approach to take. What’s needed is some serious experimentation time in the sky.

The pendulum rig I want is collapsible for storage and transport, light(ish) weight, has highly constrained pivots (zero side-play if I can pull it off), and includes the capabilities to have damped pivots, a parallelogram pendulum, a double pendulum, counterweighting, and anything else that strikes my fancy as I go along. I want all this while still making it look like I made the thing on purpose so that by the time I finish, it all looks intentional.

Too much to ask? Surprisingly, no. With the possible exception of the light(ish) weight, I’ve managed to design in all of this in a set of fairly easy to manufacture parts. Pivot points are all double-row precision ball bearings. There is provision for single pendulum, double pendulum, a parallelogram arrangement, and tunable damping on the major pivot joints. I’m finalizing the design for a damped but still positively locating pan joint as well, and have started drawings for a counterweight system I’d like to test for damping rig roll as well. If this set of tools doesn’t let me explore pendulum suspension, I don’t know what will.

To put the icing on the cake, there’s one last bit I want to play with once the pendulum experiments are done. A number of people are offering flight stabilizers for fixed wing RC aircraft. One in particular, the i86AP flight stabilizer board from Hobby King, has been tested as a camera gyro controller. Each axis has tunable gain, so you should be able to adapt it to most KAP rigs. And at 17.99USD apiece, it’s an inexpensive bit of gear to test.

So I might finally be getting closer to my holy grail of KAP: nighttime city skyline panoramas from a kite. Only time will tell.

– Tom

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