IT'S A NEW MUSCLE POST, EVERYONE!
Sorry, I got distracted by other projects and life in general. One
note before I get into my most recent work: I am now collecting other
blogs/sites that deal with nylon muscles in the right-hand sidebar.
If you run or know of a site that isn't listed, please tip me off so
I can include it.
There were several things I got very
tired of while experimenting on these muscles, and one of them was
trying to measure deflections on the order of millimeters by
squinting at a ruler mounted beside the muscle. Dangling everything
from the edge of the dining table or ottoman was a bit awkward too.
So I built myself a muscle test rig out of scrap wood. It provides a
place to suspend an actuator and a weight and converts the linear
motion of the actuator into the angular motion of a long needle.
Small movements at one end of the needle are amplified at the other
end, making them easier to see and measure.
The first version of the test rig just
had a needle which pivoted on a piece of stiff wire driven through
the board behind it. The pivot point was located very close to one
end of the needle, and on the short end there were too loops of wire
attached to the needle: one to connect the muscle, and the other to
connect the weight. This arrangement left some things to be desired.
For one thing, I could never get the wire perfectly straight, or
constrain the needle so that it would lie flush with the graduated
backdrop. That meant the needle's rotation was not planar, or it
would stick as it turned, etc. On top of that, the motion of the
short end of the needle didn't leave the muscle free to move straight
up and down. Near the needle's zero point, the motion of the muscle
is approximately vertical … but as the needle continues to rotate,
its end begins moving more and more in a horizontal direction,
changing the mechanical advantage the muscle has and introducing
complications that I would rather not deal with.
Left: Muscle suspending weight on deflection test rig, version 1. Right: close-up of the reel on version 2.
Wanting something better, I replaced
this lever system with a reel. A screw through the center of the
reel provides it with a rotary axle. Two strings are tied to holes
in the reel and wrap around it so that unwinding one string winds the
other. When both strings are put under tension, they remain
perpendicular to the side of the reel, regardless of the reel's
angular position. I connect one string to the muscle and hang the
weight from the other string, and the muscle is forced to lift the
weight when it contracts. The needle is a piece of thin aluminum
tubing inserted into a hole in one side of the reel; I can “zero”
it by adjusting the length of string between the muscle and the reel.
(I knotted multiple loops in the string, and a piece of flexible
wire between the muscle end and one of the loops is helpful for
getting things just right.)
For testing muscles that only contract
over a short distance, this thing is amazing. No more staring
at the muscle and thinking, Huh, is it doing something? I'm not
quite sure. When the muscle starts moving, I get an obvious
deflection out of the needle. The biggest remaining issue is that
there's enough friction and/or elasticity in the system that there
isn't a well-defined zero point for the needle. For a given
muscle-and-weight setup hanging passively (muscle is turned off),
there's a fairly wide angular range within which I can position the
needle and have it remain stable. When I take a muscle through a
heat-cool cycle, the needle generally doesn't return to its original
position at the end of the cool cycle. How much of this is due to
the muscle stretching out and how much is just the equipment, I
unfortunately can't say.
The other little quality-of-life
improvement I attempted for this round of muscle experiments has to
do with ease of manufacturing. I was sick of going through the
effort of making a muscle, only to have the fragile heating wire snap
at the last minute. When that happens, the nylon can't be returned
to its pristine state, and now the wire is too short – so often I
would be forced to throw everything away and start over. Putting
extra slack in the wire could result in bunching and loose wire
coils, promoting uneven and inefficient heating of the muscle. So I
tried a couple of different methods to relieve strain on the wire and
keep it unified with the nylon.
Coiling a muscle with tape tags. |
For Method 1, I tried attaching the
wire to the nylon at intervals of a couple inches, using little tags
of adhesive tape. These can be removed after annealing by sliding a
straight pin in next to the nylon and pulling outward to separate the
two sides of the tape tag. Method 2 was a little more wild: I tacked
the wire to the nylon by coating both with a thin layer of silicone
caulk. Messy as it sounds, I found that the best way to apply this
was to stroke it on with my fingers. It cleans up just fine with
some mineral spirits (paint thinner).
In the end, I'm not sure if
either trick helped a lot. For each method, I made four muscles and
lost one out of the four (due to snapped wire). That's not horrible,
but certainly not great either. I did seem to get nice even coiling
of the wire around the nylon.
Besides trying out these
manufacturing tricks, the principal experiment for this month
involved rod-coiled muscles with spread coils. I had previously
noted that the muscles with a smaller coil diameter could lift more
weight, but had difficulty achieving a good contraction distance
because their coils were already so tightly packed. I thought that
coiling them around the rod with some spacing between the coils might
improve that situation.
Actually doing this turned
out to be harder than I expected. Homochiral muscles naturally form
close-packed secondary windings. You have to fight the muscle to get
it to lie on the rod any other way – and the small-diameter ones
fight pretty hard. What you see in the photo is the best I could do.
The mandrel diameter used for all of these is ~1 mm (large size
paperclip wire). One of each type has a silicone coating, and one
doesn't. The close-packed “controls” are on the top, and the
ones with spread coils are on the bottom.
Yeeccch. Those look
terrible. But I decided to see if they would work anyway. I
used a current of ~220 mA and ran a bunch of tests with different
weights. All the muscles were allowed to heat for at least 4 minutes
and cool for at least 9 minutes, with the idea that this would be
sufficient time for them to reach “steady state.” Results are
given in terms of the needle displacement in degrees, and represent
the maximum distance the needle moved from whatever its initial
position was. An entry of “failure” in the table means that the
muscle started to stretch under the load when heated, i.e. the needle
displacement was negative. None of these muscles went flat or limp
and became permanently unusable. For all the muscles, the
lightest-weight test was the last one performed.
Muscle lifting @ 220 mA
|
71.6 g
|
60.0 g
|
50.0 g
|
25.0 g
|
Muscle 1: Packed, no silicone
|
6.5º
|
10º
|
13º
|
26º
|
Muscle 2: Spread, no silicone
|
Failure
|
Failure
|
2º
|
18º
|
Muscle 3: Packed, silicone
|
Failure
|
Failure
|
1º
|
16º
|
Muscle 4: Spread, silicone
|
Failure
|
2º
|
4º
|
12º
|
Thanks to the new test rig,
I think these are more reliable than results I've posted previously –
but you should still take them with a grain of salt, because running
the tests spanned a hot summer afternoon, and I can't hold ambient
temperature in the house constant. I wish I could repeat all of
these many times and take an average, but I really don't have the
time right now. So I'm putting up what results I have.
With that disclaimer out of
the way – the plain old close-packed muscle without silicone is the
best performer by a good margin. I wanted to see if the silicone
coating would have any detrimental effects on the properties of the
muscle, and it appears that it did … so even if it does help cut
down on manufacturing failures, it's probably not a good choice. My
awkward attempt to spread the coils on the annealing rod doesn't
appear to have panned out well either. But that doesn't mean
spread-coil muscles are entirely out of the question.
Perhaps one could wrap the
muscle around the rod with close-packed coils – as it is naturally
inclined to configure itself – and anneal it that way, achieving
nice, even coils. Then the muscle could be removed from the rod, put
under load and stretched a fixed distance, and annealed again
by running an especially high current through the heating element. I
suspect this would achieve much nicer results.
Lots of failure in this
post, in that none of my little “improvements” really worked out
for the better – but maybe someone else can avoid the same dead
ends.
Until the next cycle,
Jenny
I thought you gave up. Been a year, where's my roguelike?
ReplyDeleteStill got old coiling and annealing pics with nowhere to upload.
Techniques you need to see because they give much more
uniform coils, very difficult to overstretch.
Nothing new on my end since distracted by LittleBoxProject.
Learned a swimming headful bout magnetics and zero voltage
switching (which should be called -0.7V switching). But blew
the deadline to have a fully documented working demo.
Maybe will give muscle coils another try...
Interesting series of posts! Have you ever tried silver-painting?
ReplyDeletehttp://hdl.handle.net/1721.1/97478
Can you do a more detailed post (maybe with pictures) of the annealing process?
ReplyDeletePeople! DIY artificial muscles are possible. But there is many theoretical questions to use it in practice. I'm trying to create the DIY thechnology this type muscles.You can see video of my muscles working process here: https://youtu.be/GOvpSyYvGDk
ReplyDeleteAnd here contracting with cooling: http://www.youtube.com/watch?v=B9YvCy-WjZk
ReplyDelete