Okay, I've been busy, but it's finally
time for another artificial muscle update! I've achieved some of my
best results yet, and I think I'm about ready to try using these in
an actual application.
Oven Calibration and Annealing
I wanted the ability to test different
annealing temperatures. However, the helpful KD5ZXG,
who has been leaving a lot of comments on the first post in this
series, warned me that temperatures in my toaster oven might not
match what's shown on the dial. To get an idea of the actual
temperature, I calibrated my oven with sugar and found out that it
runs about 50°F
hotter than the dial indicates when I'm trying to set the oven in the
300°F
neighborhood.
Once
I had a better idea of what the oven was actually doing, I baked a
bunch of small nylon loops at different temperatures and for
different lengths of time. These weren't muscles, just short pieces
of nylon wound into circles, with their ends pinned in place by an
alligator clip. Each time I turned up the heat, I allowed the oven
and the metal pan to pre-heat for at least ten minutes before
introducing any nylon samples. This is more time than it actually
seems to need to get up to temperature, but I wanted to be safe –
the oven can swing to even higher temperatures during its pre-heat
cycle, so it's best to let it gain stability before annealing
muscles. All the temperatures given in the conclusions below take
the 50°F
offset of the dial into account.
My
green 711 um line (Trilene Big Game) is fine with being annealed for
up to 30 minutes at 300°F,
but starts melting at 350°F.
The other two brands I'm working with (Cousin Clear Monofilament 381
um and Zebco Omniflex 533 um) do just fine at 350°F
and below. I previously saw some of the Cousin stuff melt at 350°F,
but I wonder if that was because I didn't pre-heat the oven properly
before putting it in.
At
200°F,
10 minutes in the oven won't do much to set the loops of nylon …
they want to unwind again after being unclipped. However, 20 minutes
or more at this temperature will do the trick. At 250°F,
10 minutes still isn't enough, though the loops set better than at
200°F.
At 300°F,
10 minutes is sufficient to set a simple loop, though for tightly
coiled muscles you might want to go longer. This is the recommended
annealing temperature, and since I've confirmed that it doesn't melt
any of the lines (assuming I set my oven dial so I actually
get 300°F!)
it's the temperature I'll be using in the future. I've been
annealing my most recent muscles for 20 minutes at 300°F.
Heating
Elements
In
my last post, I mentioned that tinsel wire seemed like the best
heating element out of the ones I tried. My supply was very limited,
though, since I pulled it out of an old pair of headphones – and
try as I might, I could not find any place to buy it! The only
tinsel wire that I found for sale in small quantities was intended
for repairing speakers, and its diameter was far too large. So
instead, I bought some 10/46 litz wire from this Ebay seller. Litz
wire is made from many small copper strands coated with a film of
insulation, much like tinsel wire, but it lacks the fiber core. It's
so flexible that I suspect you could use it like conductive thread as
well. I also bought some 30 AWG nichrome wire from this seller.
Conclusion:
the litz wire is my favorite. I might buy a slightly thicker version
next time, since it seems a little delicate; it's not nearly as bad
as the single-strand magnet wire, but I've had it snap on me a couple
of times. As part of a muscle, it heats fast, cools fast, and
generally provides great performance.
As
for the nichrome, it's reasonably good, but due to its high
resistivity, it doesn't heat up nearly as fast as the litz wire. If
your muscle is very short, your system has enough voltage to drive a
reasonable current through the nichrome, or you just don't need your
muscle to respond very fast, it's probably fine – but I would say
that it's not as versatile as the litz wire. (You can always reduce
voltage if you want less current through your muscle. Stepping it up
is harder.) It's also not insulated, so if adjacent coils short
together when the muscle is fully contracted, interesting things
might happen. The best thing about it is its strength. I never had
problems with the nichrome wanting to break during coiling.
But
nichrome is MADE for heating! Why are you saying it doesn't work as
well as copper?!
Nichrome
is used in places like your toaster mainly because it can get very hot without melting or oxidizing. But I don't need or
want the heating elements in my artificial muscles to be red-hot. I
would
like to be able to use them in low-voltage systems, say 5-6V. The
amount of resistive heat you can get out of a wire is proportional to
the amount of power you pump into it. And since P = V2/R,
increasing resistance when voltage is fixed will decrease your power.
All else being equal (same power supply, same length and diameter of
wire, same environment), a copper wire will produce more heat,
faster, than a nichrome wire. Copper also has better thermal
conductivity than nichrome, meaning it will cool more quickly after
the power is turned off.
Demonstration
Here's
a video of my latest muscle. This one has two litz wires wrapped
around the nylon in opposite directions to provide better coverage,
for faster and more even heating. It is lifting a bag of pennies
weighing 35g (slightly more than one ounce). You might notice that
the coils near the top are going flat. I got greedy and
overheated it. I'm powering it with a 3V wall wart, and it is drawing 1.17 A.
Until the next cycle,
Jenny
