It's time for some updates on the
artificial muscles made from nylon fishing line. In case you missed
the last blog post, I'm trying to replicate the research results
described in this news article:
http://io9.com/scientists-just-created-some-of-the-most-powerful-muscl-1526957560
I've successfully demonstrated Phase II of the project: activating a
muscle with heat produced by electricity running through a wire.
First, though, there's some unfinished business from Phase I to talk
about.
Some More Thoughts on Coiling
There are a couple of basic ways to
coil the nylon filaments. The first is to twist the filament as
tightly as possible without causing it to form secondary coils, then
wind it around a rod. I'm going to refer to the results of this
technique as “rod-coiled” muscles in the future. The second is
to keep twisting the filament until secondary coils form all along
its length, without any need for a rod in the center; I'll call these
“self-coiled” muscles. I was having trouble making self-coiled
muscles as of my last post. Rather than forming secondary coils, the
filament wanted to bunch sideways, forming whiskers that extended
horizontally away from the line.
Self-coiled filament close-up. Left to right: 711 um filament with wire, 533 um filament, 381 um filament. |
I blamed my cheap nylon line at first,
but since getting some better types, I've discovered that they all
have a tendency to form whiskers. A commenter on the previous blog
post tipped me off that the researchers loaded their lines with 17
MPa of tension during coiling. I decided to try adjusting the
weights on my lines to provide that level of tension, and got much
better results. The filaments do not form whiskers when kept
sufficiently tense during the coiling process. The following are the
three types of nylon monofilament I am working with now:
Cousin Clear Monofilament, test
strength 8 lb., estimated diameter 0.015 in. (381 um). I had to
guess the diameter, since it's not advertised on the package.
Zebco Omniflex, test strength 25 lb.,
diameter .021 in. (533 um).
Trilene Big Game, test strength 50 lb.,
diameter .028 in. (711 um). This stuff claims to have “Extreme
Fighting PowerTM,” and it's bright green, lest you
forget how Extreme it is.
Filament Weighting
Calculations
Test (lb)
|
Diameter (um)
|
Cross-Sectional Area (mm2)
|
Force (N)
|
Mass (g)
|
Weight
|
8
|
381
|
.114
|
1.94
|
198
|
6.99 oz
|
25
|
533
|
.223
|
3.79
|
387
|
13.6 oz
|
50
|
711
|
.397
|
6.75
|
689
|
1.52 lb
|
You might think that it's
safe to exceed the weight needed for 17 MPa of tension, so long as
you don't go over the line's test rating. But don't do that. The
coiling process puts additional stress on the line, and I've found
that it can snap, even when loaded with much less than its test
weight. The thinner the line, the more tightly you need to adhere to
the calculated ideal weight. When using the 711 um line, I had good
success with weights from 1.5 up to 2 lb., and did not try anything
heavier. For the 533 um line, 8 oz. is too little (resulting in
whisker formation), but 1 lb. is too much (causing the line to snap).
A weight of about 12 oz. works well, however. It was difficult to
find the right weight for the 381 um line – I don't know if this is
because of the smaller margin of error for this very thin filament,
or because this particular line is intended only for stringing beads
and is not of very high quality. It might also be thinner than I
thought it to be. The weight I used when I finally coiled a good
muscle with this filament was probably close to 6 oz. (I don't have
a scale yet. Sorry. I am also stubbornly using imperial units
because most of my items with known weights are labeled with those.)
The self-coiled muscles again, with a penny and a cat for scale. |
The ability to make the
self-coiled muscles opens up new possibilities. They have a larger
spring constant than the rod-coiled muscles, and can therefore
support and lift heavier loads. However, I suspect that there is a
trade-off between the maximum contraction force and the maximum
contraction length. The loops of the self-coiled muscles are already
very close together, so there just isn't anywhere they can go. The
loops of the rod-coiled muscles spread farther apart under load, and
contract dramatically when heat is applied. For this reason, I
recommend making a rod-coiled muscle for your very first tests …
it's just easier to see whether it's doing something when you heat
it. Also, you can only get heterochiral muscles that expand when
heated by using the rod-coiling method, as far as I know.
Electrical Heating of Muscles
Perhaps the most convenient
way to supply the temperature change needed to activate the muscles
is by electrically heating them. I made several muscles that were
supplied with a resistive heating element, in the form of a piece of
magnet wire twisted into the muscle. “Magnet wire” is very thin
copper wire with a coating of insulating enamel. I salvaged mine
from the coil of an old electromagnet, so I don't know its exact
thickness. For my most successful experiment, I made a homochiral
rod-coiled muscle from the 711 um line, with a single magnet wire
wrapped around the line. (I tied the nylon line and the wire
together at the ends, so that wire and line were twisted together
during the coiling process. Then I wrapped the two around the rod
and annealed the muscle in my toaster oven.) I loaded the muscle
with a pair of ferrite cores, weighing (I estimate) about 30 g. I
was able to see it contract and lift the cores a millimeter or two when
I connected the wire to a 6 V battery pack, and relax again when the
electrical current was removed. Not much, but it proves the concept.
Ideally, the muscles would
be run by a controller which would first provide a burst of high
current for quick heating, then back it off to avoid exceeding the
maximum temperature of the muscle. A small amount of current could
then be maintained to hold the muscle in its contracted position, if
desired. My “quick and dirty” version was to simply hook the
battery pack to the wire and disconnect it as soon as I saw the
muscle start to move. Given the length of wire I was using (about 60
cm) and a voltage drop of ~6 V, the amount of heat produced is enough
to permanently deform the muscles and eventually melt them in half,
if the current is left connected too long. So one must be very
careful and have quick reaction time if working with this much
voltage. My muscle would heat up enough to show a response within a
few seconds, and relax within maybe 15 s or so.
Besides the ease of
overheating, the other major annoyance was working with the magnet
wire itself. It breaks very easily when it experiences strain as a
result of being coiled up with the muscle. I don't think I made a
single 711 um muscle without having the wire break – for my
successful tests, I either manually wrapped some wire around the
muscle after it was coiled, or pulled one of the broken ends out of
the middle of the muscle and stripped it so I could get a connection
point.
To Do List (Stay Tuned)
Come up with a way to make
muscles reliably without constant wire breakage. I plan to
investigate some other possible resistive heating elements, besides
the single strand of magnet wire.
Calculate the heat
dissipated by the wire, and find maximum “safe” voltages for
muscles as a function of their parameters.
Use a microcontroller and
driver circuit to control a muscle.
Experiment with a variety of
muscles and get more details on lifting power, maximum compression or
extension, time to contract/relax, etc.
Until the next cycle,
Jenny
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