Saturday, May 3, 2014

Homemade Artificial Muscles IV

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

13 comments:

  1. hello jenny i'm interested in these muscles too, it have a lots of applications .. in fact i have made many of it at my home and the main problem was the nichrome i tried to heat the muscles using a normal supply of 12 v and i noticed it wasn't enough to heat, i was very disappointed about it but if you say that tinsel wire is better ..let's try it
    the nylon muscles

    ReplyDelete
    Replies
    1. 12 V is considerably more voltage than I've been using, so even with nichrome, I'm surprised you didn't get a good response. Here are a few things to make sure you've thought about before you buy more wire:

      How much current can your supply provide? If it has an adjustable current limit, are you setting it high enough?

      Is there something else that might be hurting the performance of your muscles? For instance, I've noticed that if I'm using a very tightly coiled muscle and the heating wire is too thick, it can actually prevent the coils from contracting by taking up all the space between them.

      How long is your wire? Try making a shorter muscle with a shorter heating wire (less resistance) and see if that works better. You could also try two wires in parallel.

      Good luck!

      Delete
  2. It seems really slow to react maybe could you try to put the nylon wire in a tube with hot/cold water or air in it, it would be faster I guess.

    ReplyDelete
  3. This video demonstrates a faster response after weaving what I assume is conductive thread(?) through a series of coiled lines and heating the lines through the thread.. I want to try this! https://www.youtube.com/watch?v=fNS0pxnQfdY

    ReplyDelete
  4. Internal expansion of the line drives the uncoiling of the helical bias.
    Uncoiling the helical bias of the line tightens the coil of the spring.

    So, we just need the inside filler of the line to expand. Don't matter
    the outside helical sleeve expands, might even be counterproductive...

    Could imagine a hollow line pumped with a liquid or gas and no temp
    change whatsoever... Or perhaps a conductive liquid could serve an
    an internal heating and expanding, or external heating only element...

    No way to make hollow leak-proof fishing line, therefore pointless.

    -------------

    Anyways, my idea for a machine stands at two rear bicycle wheels,
    with 20% different fixed gears (chained together) and no tires. The
    muscles would form a belt around both wheel rims. We dip the lower
    quarter of belt (between wheels) in a pan of boiling water to expand.
    The top quarter (if still wet I would assume) to cool by evaporation.

    I am slowly coiling and baking my way toward a plausible belt.

    KD5ZXG

    ReplyDelete
  5. Completely off-topic, but you mention Litz Wire...

    I had been thinking of using Litz to make a high frequency shorting ring.
    But how to join the ends into a ring without ruining the Litz-ness of it???

    Occurs to me that we need only one really long loop and fold it figure 8
    and double it over. and Figure 8 again and again. Mobious Litz anyone???

    Can anything know this shorting ring has 64 or 128 turns rather than 1?
    Or only knows the aggregate Ampre turns, hopefully using the entire
    Litz bundle to greater effect than one solid turn just skin deep.

    KD5ZXG

    ReplyDelete
    Replies
    1. Unfortunately, I don't know much about the use of Litz wire for that sort of application. Sorry!

      Delete
  6. Dumb question, but are you powering your Litz-wire-powered muscle with DC or AC?

    ReplyDelete
    Replies
    1. It's been DC only so far. I know that Litz wire is specifically designed to carry AC, but I'm really using it because it is a super-flexible multi-stranded wire, not for its more special properties.

      Delete
  7. Ferrite powder gets hot in a microwave.
    Maybe not the most sensible thought...

    KD5ZXG

    ReplyDelete
  8. FYI,
    If you get tired of messing with Litz wire, try searching Amazon for "Conductive Sewing Thread Size 92" from Jameco. It's nylon. It's conductive. It coils up nicely with a 200g weight hanging from it, and best of all, it's self-annealing! The operating voltage depends, of course on the length of your muscle, but as a rule of thumb, I don't take my muscles above about 350mA (500mA will melt the nylon)

    Good luck, have fun, and keep up the good work.

    ReplyDelete
  9. https://www.sparkfun.com/products/12806
    Blend of 20% stainless steel. Havn't tried it, no idea...

    ReplyDelete
  10. I stumbled across your blog while searching multiple places about the fishing line / conductive sewing thrread muscles. I will be experimenting with this in the near future. I wonder what the people at JoAnn fabrics will think when I carry my multimeter in to measure the resistance of their metalic coated thread?? ;) I saw a demo video of a thread muscle immersed in water and they were cycling it at like 5 Hz ! It looked impressive to me. One of my thoughts about a possible use of this is to use it as the actuators for an animatronic snake. Just put a series of batteries (liPo's ?) down the centerline of the snake body inside some sort of flexible tubing. You could probably just embed the muscles into a silicone skin, prewired of course, and get a fair amount of bending motion.

    Just some thoughts.

    Steve

    ReplyDelete