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Noob Q about inverter for an ornithopter Answered

I was looking in the back of my Jameco catalog the other day and they listed a product called "Muscle wire". It's apparently a nickel-titanium alloy that shrinks as it heats up.

It occurred to me that I could flap ornithopter wings with it (a really small ornithopter). I was trying to figure out how I could switch voltage from one pair of wires (the flap UP wires) to the other pair of wires (the flap DOWN wires) without a microchip.

I came up with something I'm sure wouldn't work involving a couple transistors, a diode and a cap. Today on wikipedia I figured out that what I was doing was something close to an inverter.

So here's the question: Anyone know of a simple switching circuit that runs on a/c power and does not have square-wave switching characteristics?

I'll keep researching this myself, but sometimes 'tis quicker to ask the big-brained and kind hearted (you know who you are).

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Kiteman (author)2007-11-07
Muscle wire is a bit slow to move, and requires an opposing force to return it to its original length.

For instance, in the Stiquito robot, the legs are made of springy piano wire:


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royalestel (author)Kiteman2007-11-07

Well, I was thinking of opposing pairs of muscle wire. Just thinking about it, that might be enough to double the 33 cycles/min. contraction rate. And it follows that I think that would be fast enough to keep a four-winged ornithopter afloat.

Guess I'll find out! :)

Say, do you know of a simple D/C switching circuit?

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Patrik (author)royalestel2007-11-07

Just because you have two opposing wires doesn't mean you can run them twice as fast. The whole cycle of contraction and expansion takes two seconds. If you use two opposing wires, one will be expanding while the other one is contracting, but the "flap" of your wings will still be ones every two seconds - two slow for anything more than an ultralight glider.

You can probably rig up a clever set of levers to turn each contraction of the wire into two or more wingstrokes, but it didn't sound like that's what you were proposing...

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royalestel (author)Patrik2007-11-07

You're right, I wasn't thinking of a levered deal. But after a little more thought, perhaps two pairs of offset ganged wires and temporally offset activation voltages would speed up the flapping cycle, as long as the wires have a sliding connection that only allows each wire to pull on the wings and exert no push. What think you?

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Patrik (author)royalestel2007-11-08

Nope. Although the contraction of the wire is very fast, you still need a long enough off-period for the wire to cool down. If you try to switch it faster, I assume you will only get a fraction of the full contraction length the wires are capable of.

If you want to flap faster, your best bet would be to for the thinner and higher temperature wires. For example, although the 100 micron LT (low temp) wires work at 33 cycles/min, the 110 micron HT wires can do 50 cycles/min, and the 37 micron Ht wires can do up to 68 cycles/min (based on http://www.robotstore.com/site/mwfaq.asp ). That's getting into a much more interesting range, for a flapping wing...

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royalestel (author)Patrik2007-11-08

Yeah. . . dang. Don't think the thinner wires will stand up to crash landings very well. Guess I'll just have to wait until those magnetically activated wires come out. . . .

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guyfrom7up (author)2007-11-07

yeah, in a make podcast it was a compotition between the stiquito and another robot and the stiquito barely moved at all.

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whatsisface (author)2007-11-07

I was going to suggest a 555 until I read the "No microchips" rule.

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Goodhart (author)whatsisface2007-11-07

a circuit that replicates the action of a 555 can be created with a few transistors though and maybe a comparator or two.

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royalestel (author)Goodhart2007-11-07

Well, wouldn't the comparator be bigger (physically) than the 555 chip and a board? Gotta learn more about electronics posthaste . . .

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Goodhart (author)royalestel2007-11-07

Um, you can get 8 pin comparators I believe (been too long away from fooling around with this stuff and I don't have time at the moment to look up the parts), but there are a few ways to use just a few transistors too.....but I can't remember, at the moment, just how.

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NachoMahma (author)2007-11-06

. Wild guess: Could you pair up a "regular" transistor and a (J-)FET, with each driving the complementary wire? No off state, one of the wires would always be hot. . What's the problem with a square wave? The wire should react to any current flow, whether it be triangle, saw, sine, square, ....

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royalestel (author)NachoMahma2007-11-06

Wild guess: Could you pair up a "regular" transistor and a (J-)FET, with each driving the complementary wire? No off state, one of the wires would always be hot. I think this is basically what I was thinking of--two transistors one normally open and one normally closed linked together. The cap was to try and avoid square wave switching, and give a smoother flapping motion. Now that you mention it, though, the wire itself probably has a ramped response to voltage change, so no smoothing would be required. I think I'm going to have to just buy some of this stuff and see what it does. Want some?

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NachoMahma (author)royalestel2007-11-07

. I've worked on chart recorders with Nitinol drives and it is very slow to respond. If the stuff you are getting doesn't respond much faster, I don't see how it will flap a wing fast enough. But the stuff I was working with was designed to be slow.
. Thanks for the offer, but I can't imagine what I would use it for.

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royalestel (author)NachoMahma2007-11-07

This has an advertised cycle of 33 contractions/min. But I was thinking if I offset-ganged a couple and staggered their timing I could increase that rate. Just don't want to pay $17 for a meter of the stuff. . .

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royalestel (author)2007-11-07

*Doh* I meant "a simple switching circuit that runs on d/c" Sonofa.

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