Tuesday, December 29, 2015

Artificial Muscles Actuating Things

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.

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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