They're still not terribly fast, and
they don't move terribly far, but I've arrived at the crucial step of
getting artificial muscles to actuate something more than just a
weight hanging from a string. First up, we have a heterochiral
muscle (the type that expand when heated) flexing a piece of paper.
Each coil of the muscle is sewn to the paper on one side with a loop
of thread, so the coils expand on one side and are constrained on the
other, causing the paper to bend. This is almost a no-load movement,
and strikes me as being most useful for something decorative, such as
an artificial plant. You might notice that the cooling cycle is
almost as quick as the heating cycle. I attribute that to chilly
ambient temperatures in the upstairs laboratory.
Next we have a homochiral (contracting)
muscle, rotating a piece of cardboard on a hinge. The opposing force
of the rubber band on the opposite side pulls the cardboard back into
its original position during the cooling cycle. The homochiral
muscle featured in the video has been annealed with some space
between the coils when at rest, so it doesn't have to be put under
tension in order to have working room.
Both of these muscles are drawing about
1 A of current. They are made of Trilene Big Game fishing line, test
strength 50 lb., diameter 711 um, with a heating element of 10/46
copper litz wire. The homochiral muscle has two strands wired in
parallel. The heterochiral muscle was coiled on a rod of 3/16”
diameter, while the homochiral muscle was coiled on a 1/8” rod.
Annealing muscles with built-in coil
spacing
I determined in some previousexperiments that trying to spread the coils of a homochiral muscle
when one first coils it on the rod is a bad idea. At this point
there is a great deal of tension on the line – it wants to uncoil
itself – and the coils don't cooperate very well. Coiling the
muscle on a threaded rod is a possibility that I haven't tried yet;
the spacing of the threads would limit the coil spacings one could
achieve.
I started looking at ways to adjust the
coil spacing after the initial annealing. First, I tried putting the
muscle under tension (with no rod in the center) and running a high
current through the heating wire, hoping to anneal it into its new
shape. This method gives the most even coil spacing one could ask
for, but the amount of heat applied to the muscle was only sufficient
to “soft-set” it. I noticed that as it sat around for a few
days, the coils slowly returned to their original close-packed
configuration. When I tried annealing a muscle under tension at full
heat in the oven, without a supporting rod in the center, the coils just went flat.
In the end, the best method I found was
to put the muscle through its first annealing phase, manually spread
the coils on the rod, then anneal it a second time. It's a little
tedious – friction holds the coils against the rod, so you have to
slide each one into the right position with your fingernail to get
the spacing even – but it seems to work.
 |
| A close-up photo of the homochiral muscle with spread coils |
An aside about plastic springs
In addition to artificial muscles, you
can make simple passive springs by coiling nylon monofilament
around a rod and annealing it (without primary twisting or a heating
wire). The spring constant is determined by the thickness of the
filament (larger diameters yield larger constants) and the size of
the rod (smaller diameters yield larger constants). I got rather
excited about this a few months ago, thinking I'd never need to buy a spring again. The problem is, these plastic springs don't
necessarily hold up well.
If you follow any of my social media
feeds, you might remember when I posted this spider leg video.
There's a plastic spring at each joint, made from the 533 um Zebcom
Omniflex line, and I'm actuating them by pulling the tendons with my
fingers:
I took that video the day I finished
building the leg. A few days later, the leg was in sorry shape,
merely because I had allowed my house to heat up in the afternoons.
This was sufficient to make the springs relax a good deal, so that I
had to shorten them to get the same degree of tension I had before.
Naturally, this leaves me in some
concern about the muscles as well. How might they be affected
by high ambient temperatures? I haven't done any tests in which I
compared a muscle's performance across many sessions of operation,
with temperature spikes in between.
Blog news
Comments now require moderator
approval, because spammers have been really junking up the place. Sorry.
In the new year, I think I'm going to
try to build an “artificial muscle summary” page featuring
everything I've learned, for the benefit of others who want to
experiment. Once that's done, I may set muscles aside for a while.
There are soooo many other things I want to work on.
Have a most excellent New Year!
-- Jenny
Have a most excellent New Year!
-- Jenny
