Muscle Wire

[kE7JLM]

I was looking at these new (new to me at least) muscle wires. I just wanted to know how well they work.

Specs:
The largest size can pull 4.4 pounds.
Max Wattage: 60w
Can stretch up to 40% or there length

What's this I hear about the wire being from the Roswell crash!!!! j/k

Here's an awsome video
http://www.youtube.com/watch?v=iXUip...eature=related

http://www.youtube.com/watch?v=-zzFb...eature=related

http://www.youtube.com/watch?v=OozEY...eature=related


Thanks for any feedback.

[Al Skierkiewicz]

John,
These have been around for a while. A Motorola engineer showed me an idea about 7 years ago he had for using this design. There are a number of drawbacks but they are pretty cool none the less. 60 watts for the largest size is a tough specification. Hard to get rid of the heat or supply the current but I am sure there are some interesting uses. Fine motor control of actuators is something that jumps to mind.

[JesseK]

I'll start this off by saying that I'm not an expert, but here's my experience with 'muscle wire', aka 'memory wire', aka Nitinol. These have deterred me from considering its use in FRC robots, though you may derive different conclusions should you try it yourself. I agree with Al's comments on heat dissipation; the wire itself loves to get toasty if you use it in heavy loading situations...much like a motor near stall. Altogether, here are the major drawbacks I found:

1.) Heat dissipation to air via direct convection was not so great. My application was underwater robot fin control, so putting the wire in a dissipative gel in a tube that would be constantly cooled by water solved that problem for me. For a FIRST FRC robot, you're not supposed to have exposed wire, and insulating memory wire to satisfy that requirement will definitely bring about the issue of heat.

2.) After 130 or so repetitions at ~33% of the max calculated 'load', the wire started fraying. After ~160 it snapped. The wire diameter was the equivalent of 24AWG wire, though it is rated for much less current than normal wire. Perhaps thicker or better-cooled wire has a longer life?

3.) The expansion and contraction of the wire was not immediate. Especially as the wire became more worn, the delay was more noticable. These delays are usually in graph form in the spec sheet of the wire.

Also, keep in mind for your application that when the wire expands, you can't expect your mechanism to automatically return to it's default position -- doing so is literally like pushing on a string. In other words, the nitinol wire returned to it's normal length, but doing so didn't exert any usable force. That said, any mechanism used to return your device to it's 'default' position (such as a spring) must always be taken into account when calculating loading during contraction.

[Kevin Sevcik]

Originally Posted by Al Skierkiewicz

60 watts for the largest size is a tough specification. Hard to get rid of the heat or supply the current but I am sure there are some interesting uses.

The specification is 60W/m, so you're only dumping that much power if you're using a meter of the wire. The specs actually recommend a supply current of 2.75A for the largest wire, and 8Ω/m resistance. So on an FRC robot, you'd actually be limited to about .5m at most between terminals unless you were getting fancy with multiple sources or grounds on the wire. Which means you'd be looking at 30W dissipation, which is paltry compared to most of our motors. All the other flavors have other length limits, but they all work out to around .5m with a 12V supply.

I think the real trick to using this stuff would be properly controlling the current in the short segments you'd be likely to use on a robot. For a 10cm segment of the thick stuff, you'd want to hold 2.2V RMS to heat it properly. That works out to a 3.3% duty cycle, which is the minimum duty cycle on a Victor, and pretty darned small on a jaguar.

[Qbranch]

Maybe it has troubles... but still. This is the coolest thing I've seen since the giant crushing thing in the materials testing lab!

It looks like magic when you watch the videos... I think I may have to work a couple extra hours and drop some money on a spool of this stuff... it's just too darn cool not to play around with.

I take it it's actuation is entirely silent?

Amazing stuff. I'm in awe at the stuff material scientists come up with...

-q

[JesseK]

Originally Posted by Qbranch

Amazing stuff. I'm in awe at the stuff material scientists come up with...

-q

This one is even better! If I remember correctly, nitinol was an accidental discovery in a naval research program of the 60's.

[Al Skierkiewicz]

Kevin,
You are right but the power is concentrated in a very fine wire which for most purposes needs to be insulated. If the application calls for a sustained period of constriction then the heat needs to be dealt with. Add to that the possibility of using more than one to increase the pull and the heat sources add to each other.
But imagine the possibilities. A few short peices lifting a few pounds over a short distance. It would take a sizable motor and tranny to do the same. Think about mirror adjustment, lens alignment, vents, etc.

[Kevin Sevcik]

I would tend to think more in terms of my usual nemesis, quick release triggers, and various other places where I need short stroke linear action. Especially because these are enormously lighter than pneumatics. Multiple strands would actually help with the issue of driving them properly, as long as you wired them in series. It'd almost make more sense to use them in even numbers, so your source and drain could be on the same side of the actuator.