Saturday, August 23, 2014

Homemade Nylon Artificial Muscles V

It's high time to talk about artificial muscles again. I have the long-awaited data from the Weight Lifting Test, but to keep this blog post from getting monstrous, I think I'm going to save that for tomorrow. Today's post will be devoted to a couple of side issues.

It's Not Just about Fishing Line Any More

So far, I've only heard of these muscles being made out of two kinds of nylon monofilament: 1) fishing line and 2) conductive thread with a nylon core. Trimmer line – for weed-eaters – is made of nylon too. And it's thick nylon. Special lines for deep-sea fishing are the only other filaments I've seen that are this stout. So I decided to try it. Behold the super-muscle:

Nylon artificial muscle: the giant edition.
It's made of RINO-TUFF Universal Trimmer Line, manufactured by Jarden Applied Materials. The cross-section of this trimmer line is round (there are no ridges around the outside, as some trimmer lines have).  The diameter is 2.0 mm, which puts the appropriate coiling load at about 5.45 kg (12.0 lb.). After initial twisting, I wound it around a 6.35 mm (0.25 in.) rod and baked it in the toaster oven as usual, though I let it go almost half an hour to make sure it was fully set, since the line is so thick. The heating element is a single enameled copper wire, I would guess 22 gauge.

During my first quick test, the super-muscle lifted 0.454 kg (1 lb.) of dry beans about 1.5 mm. That may not sound like a very impressive distance given the size of the muscle, but it's more than I would expect my other muscles to do with that much weight. It's also possible that the heating element I used was a hindrance; single-stranded copper wire at that thickness has a noticeable springiness of its own that gives the muscle an additional resistance to motion. A more flexible multi-stranded heating element would be preferable, but I haven't tried to make better versions yet; I mainly wanted to see if the trimmer line would respond. Since it seems to be a viable muscle material, I hope to make more of these big guys, and will of course let you all know how that goes.

The Flattening Problem

When rod-coiled muscles are overheated, well before the line melts through they will usually “go flat.” The coils tilt into the plane along which tension is being applied, ruining the muscle's effectiveness as a spring. Something I have been noticing during recent tests is that certain muscles seem to be more prone to going flat than others. Take as an example the one on the right below, which was originally going to be the largest diameter muscle in the Weight Lifting Test … until it spontaneously went flat, while hanging vertically without any load on it, when my apartment got particularly hot one summer afternoon. The other muscles hanging up beside it were not similarly affected.

A healthy rod-coiled muscle (left) compared to a ruined flat one (right).
The muscle that went flat was the largest member of the set, so I thought that perhaps rod-coiled muscles with a larger diameter were more prone to flattening. However, when I started working with the remaining muscles in the Weight Lifting Test, it turned out that things weren't that simple. Of the three muscles, the 1/8” muscle seemed the most prone to flattening (which is why, in the data coming tomorrow, you'll see me using  no currents larger than 40 mA for this muscle … it could barely take that much). It was at least as sensitive to high temperatures as the larger 3/16” muscle, if not more so.

Muscles that “go flat” can be re-annealed to get them back to their correct shape. While this restores their effectiveness (at least temporarily), it does not seem to take away their inherent propensity for going flat. Therefore I wonder if there is some variable in the manufacturing method that makes some muscles more likely to flatten than others. I made efforts to keep the muscles that I used for the Weight Lifting Test identical in all but diameter, but my methods aren't exactly precise, so perhaps some differences crept in. Here are my best thoughts of some things to look at:

1) Amount of tension applied via primary twisting before the muscle is coiled on the rod. I tried to keep this the same between muscles by using the same load and twisting each one for an amount of time proportional to the length of the line. Nonetheless, it's quite likely that they have small differences. I sometimes had to stop early because the line started forming secondary coils before time was up, or appeared about to.
2) Ambient temperature in the environment during primary twisting. I didn't make all the muscles on the same day, so there could be some differences here depending on what the weather was doing.
3) Amount of tension applied during secondary coiling, i.e. initial tightness of coils around the annealing rod.

Until the next cycle,

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