Monday, August 31, 2015

DIY Nylon Muscles VII

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
Muscle 2: Spread, no silicone
Muscle 3: Packed, silicone
Muscle 4: Spread, silicone

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,


Sunday, January 4, 2015

On Dragon Age and Hard Choices

Woo, long blog hiatus! I was busy moving to a new house, and then along came the holidays, so I haven't had much time for robotics or the blog lately. But somehow, I found time to play Dragon Age in the middle of all that. No, I don't mean DA: Inquisition, I mean DA: Origins. I am VERY late to the party. But I suppose with the new game out, this is as good a time as any to talk about where the series started. (This article has mild spoilers.  If you haven't played, I think you can read it without ruining anything.)

DA:O did an excellent job of creating a living fantasy world that I actually felt part of. I could believe myself a Grey Warden, a powerful figure sworn to protect the world from an ancient evil. But the part that has really stuck with me from this story is the amount of free will I had as an actor in it, its propensity for pushing me to make choices in nuanced situations, and its mature take on the results. I'm used to wish-fulfillment from this genre -- games that let me craft the perfect hero story, and if they include much tragedy at all, make it inevitable and not my fault. DA:O is a bit different.

I was kind of naive at this point.
Example: this world has golems in it. Awesome. As I dug deeper, I found that the way they are made in this setting is ... not awesome. Eventually, I was faced with a choice: preserve this dread technology and hope against past experience that someone would use it wisely, or discard it, possibly eliminating golems from the world forever. I opted to destroy the means of golem creation. As much as I love golems for reasons of both personal fancy and practicality, there were more important things to preserve here. And this was not the most painful decision the story required me to make, by far.

Thanks to a combination of numerous options and different ways for players to interpret the story, the endgame of DA:O is going to be unique for every person. For me, the path I had followed through earlier parts made it especially pivotal, and having put emotional hooks in me, the plot seemed intent on exploiting them. I was offered the perfect fairy-tale ending ... the one I had been hoping for ... at the price of a couple of sketchy choices. Alternately, I could do what I thought was truly best for the world I was trying to save, but only by giving up what I wanted most in that world. I chose the latter, and the consequences landed. There was no miraculous deliverance, no last-minute "power of true love" or "good karma" or surprise rescue to fix everything. It was miserable and incredible and different from any other game experience I've had. Dragon Age actually made me pay to be a hero, and the payment was my choice.

Whether literally or figuratively, real heroes bleed.  A lot.
Some part of me wants to replay the game as a slightly less scrupulous character. I could throw self-sacrifice to the winds and get that ending I really wanted. "It's just a video game, you know," says this part of me. "You can do that. You won't actually hurt anything." But whenever I start thinking this way, a second part of me warns that doing this would cheapen the whole experience. As soon as it becomes "just a video game" rather than a world that I embrace on its own terms, I will have lost what really makes it meaningful. As soon as I stop thinking of it as a place where I make morally relevant choices, I won't be a Grey Warden any more. I'll be nothing but a petty escapist, playing at being a hero without caring what's actually involved.

And at the end of the day ... that's not what I want. What I want is to wear my heraldic griffon shirt and feel that in some tiny way, I earned it.

I haven't forgotten about the artificial muscles and other robotics projects, I swear.  Thanks for your patience.

Tuesday, September 30, 2014

Atronach's Eye: Case Build

I took a break from muscles to give the old mechanical eyeball a much-needed upgrade. The changes made in this round of development mostly affected the case. For more information about how the internals of Atronach's eye work, click the “Project Atronach” tag in the right-hand sidebar to see all previous posts.

A view of the internals mounted to the new case, just before final assembly.
The original case was a quick prototype, thrown together with cardboard and ice pop sticks. It was kind of embarrassing, but it did provide a good platform to see how all the moving parts worked and what improvements might be needed. The new version is mostly particle board and linoleum, with some other wooden bits (there are still ice pop sticks in there, actually, you just can't see them now).

Cutting the case.
The particle board became the front and back faces of the case, which I cut out by hand using a rotary tool with a cylindrical engraving cutter bit. This was rather tedious, and the results were imperfect. I wish, in particular, that the gear teeth were more symmetrical (I also wish I had a CNC machine). At least it's not functioning as a gear. It's structural art. Ssssshhh. I carved the lettering into the front face using a spherical engraving cutter, and I'm very happy with how that turned out.

Speaking of that lettering, it's derived from the legends of golems, which Atronach's name obliquely references[1]. These might be some of the earliest stories about artificial life, and there are additional reasons why I find them meaningful that probably deserve their own blog post. The original golems of Jewish folklore often had something written on their foreheads, such as the word emet (truth) used here. Not to mention that Hebrew script is gorgeous.

For the outer wall of the case, I wanted something that I could form into a curve, so I bought a single linoleum tile. It came with adhesive on the back, which I dissolved with vegetable oil and washed off. The wall is made from three pieces of linoleum, joined together at the seams with 24-gauge beading wire. Initially, the wall wanted to “peak” at the seams and go flat in between them, instead of assuming a nice round shape. I added a piece of stiff plastic (cut from a large zip tie) at each seam to brace it into roundness. I'm quite happy with how this construction technique worked out, as well. However, one does have to be careful when handling this type of linoleum, because the stiff plastic on the back is rather brittle. Bend it gently and don't try to bend it back once the desired shape is reached, or this rear layer will crack. Always drill holes starting from the back side, and leave plenty of margin between your holes and edges or corners. To support the sidewall, I drove wire nails up through the back face of the case. There is an inner ring and an outer ring of nails, and the sidewall slips snugly between them.

A closeup of the finished sidewall.
All of the motors, the ball cradle, etc. are mounted to the back plate of the case with screws now, which gives the structure more rigidity and provides for easy disassembly for repairs or modifications (gasp!). I added some wire hoops inside to provide additional guidance for the "tendons" and prevent them from catching on other parts.  The tendons themselves are now nylon monofilament instead of thread, which I found could fray through rather easily.  I also replaced the battery pack with a wall wart of comparable voltage.

Here's a video of the eye during the final post-assembly test:

The next step will be getting the computer into the loop … either by adding the camera and some video processing algorithms, or by establishing computer control over the motor movements.

Until the next cycle,

[1] “Atronach” is used in The Elder Scrolls games as a designator for either elemental creatures or certain types of golems. These are the wizard-built golems typical of fantasy contexts, but they owe their literary origins to these old folk stories.

Sunday, August 24, 2014

Nylon Fishing Line Artificial Muscles VI

I've finally done some characterization of these muscles. My primary goal was to investigate the relationship between the diameter of the secondary coils and various muscle properties/behaviors.

The Test Muscles

I made three rod-coiled test muscles, of different internal diameters determined by the size of the rods on which they were coiled. I attempted to keep other variables constant, including the following:

  • Materials: All muscles were made from Trilene Big Game “Solar Collector” fishing line (50 lb. test, 711 um in diameter), with a single heating element of 10/46 litz wire twisted in.
  • Secondary Coils: I tried to cut the filament to a length that would produce the same number of secondary coils in each muscle when finished (ensuring all muscles would have the same length when at rest). Due to a lack of preciseness in my manufacturing methods, I could not get them quite the same, but corrected for this by reporting relative contraction distance instead of absolute contraction distance in the weight lifting results.
  • Primary Coils: I measured the coiling time required, going at the speed of the drill I was using, before a length of filament would begin to spontaneously form secondary coils. I then used this time as a guide for coiling subsequent muscles, scaling it by the length of filament used. Ideally, this would ensure that each muscle had almost enough tension to begin self-coiling, but not quite. In practice, again, the results were less than precise; for instance, two of the muscles developed a few secondary coils at the top. (I treated these as part of the muscle's “tail”, so they did not affect measurements. However, they show that these muscles ended up with a higher primary coiling tension than the remaining one.)
  • Tension: I used the same 2 lb. weight as a load for each muscle during the coiling process. Unfortunately this does not entirely guarantee a consistent tension, because some of the nylon filaments were too long to suspend the weight for all or part of the coiling process – meaning it was acting as more of an anchor, and the force on the line would depend on the weight's friction with the floor as well.
  • Secondary Coil Spacing: When coiling each muscle around its rod, I packed all of the coils as tightly as possible, so that the muscle would be fully contracted when at rest.
  • Annealing: All muscles were annealed at a temperature of 300°F for 20 minutes.
  • Chirality: All muscles were homochiral.

Experimental Setup

The passive spring constant test involved suspending various weights from a muscle and measuring the amount of deformation (stretching) produced. The muscles were not powered for this test.

The weight lifting test was intended to measure each muscle's ability to lift weight under power. One muscle was tested at a time. It received its current through a power MOSFET switched by an ATTiny85 microcontroller. A multimeter was included in the circuit to measure current. I had programmed the ATTiny to read an analog voltage, which I could adjust by turning the dial on a potentiometer. The ATTiny sent a PWM signal to the gate of the transistor, varying the duty cycle based on the value of the analog voltage. I hand-adjusted the value of the potentiometer to get the same (effective) current value for each muscle. The distance through which the weight was lifted was measured at steady-state, after the load had risen as far as was possible for that current.


Passive Spring Constant Tests

Each data set has been fitted with a cubic curve.  A spring constant can be obtained for each muscle by taking the slope of the linear portion of the curve.

Weight Lifting Tests

Each data set has been fitted with a parabolic curve.  The (0,0) data point is assumed.


Coil Diameter
Spring Constant
Optimum Load*
Max. Possible Contraction**
4.763 mm
8.63 N/m
32.4 g
3.175 mm
40.12 N/m
60.3 g
1 mm
237.62 N/m
82.8 g
*Averaged over all currents tested on this muscle
**At highest current tested on this muscle, computed from curve


I am going to emphasize again that I'm not working with the greatest equipment here. Distances moved by all of these muscles were on the order of millimeters, and difficult to measure. Muscles can be reshaped by the heat, making the repeatability of the measurements imperfect. Couple that with the possibility of varying ambient temperatures in the house between test sessions, and you have a recipe for a lot of error. Nonetheless, I hope the data is useful in a rough, qualitative way. I'll posit the following:

  • All else being equal, muscles with a smaller secondary coil diameter have a larger spring constant.
  • Every muscle has an optimum load, a “sweet spot” on the curve, which allows for maximum lift distance when the muscle is powered. Too little weight doesn't stretch the coils far enough apart to provide a working distance, and too much begins to exceed the muscle's strength. I'm sure the optimum loads found for these muscles are dependent on the fact that they all have close-packed secondary coils. It would be interesting to re-try the experiment using muscles that are not fully contracted at rest, and thus have less need to be stretched by the weight.
  • The size of the optimum load increases as the secondary coil diameter decreases. I don't know whether the optimum load is related to the amount of current as well, or is the same regardless of current; the data is too irregular to tell.
  • At a given current, muscles with a large secondary coil diameter, lifting their optimum load, will lift higher than muscles with a small secondary coil diameter, lifting their optimum load. However, note that large-diameter muscles may have access to a smaller range of currents due to the risk of “going flat” (see yesterday's post).

I now have better answers to a couple of questions that have been asked previously:

Q: Once you've made a muscle contract, can you continue to run current through it and hold it in position?
A: Yes! Just be careful not to overheat the muscle. Current values that are tolerated in brief bursts may be enough to make the muscle flatten or go limp if they are applied for too long.

Q: What is the advantage of rod-coiled muscles over self-coiled muscles, or vice versa? What is the best secondary coil diameter?

A: The best diameter depends on your application. As I suspected, muscles with smaller diameters (including the self-coiled ones, which have the smallest diameter possible) can manage heavier weights, but can't lift them as far as large-diameter muscles can lift their optimal lighter weights (though I do wonder how the more tightly coiled muscles would perform if they were annealed with their coils spread out). The new revelation here is that diameter plays in to current requirements as well – large muscles working at their optimum point need less current than small muscles working at theirs.

Until the next cycle,

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,

Friday, June 13, 2014

Transistor: A Dream of the End

*** Warning *** This article contains major spoilers that may blunt the impact of the game. If you have not played Transistor and have the slightest shred of interest in doing so, you are not allowed to read this. Go get the game, finish it, and come back later. *** Warning ***

I'm usually a “wait for the sales” type of person, but Transistor is one of the few games I've bought within a couple of weeks of its release. I sprang for it on the strength of its predecessor Bastion, which is excellent (though it hasn't furnished me with any deep lessons that would make fodder for a blog post). And hoo boy, I was not disappointed. Transistor is one of THOSE games. The ones that stay with you for a long time.

Hey, God?  Can the New Jerusalem look a little like Cloudbank?  Maybe?
Apropos of the fact that its protagonist is a singer, Transistor makes skillful use of music and voice to add impact to its story. I was drawn in by the game's premise and beautiful visuals, but I wasn't really sold on it until the first boss fight. As Red confronts one of her nemeses – Sybil, a prominent socialite, now infected by the computerized menace known as the Process – a song begins. It isn't the standard battle fare that advertises excitement or danger. Instead it's slow, quietly intense, and – if you listen to the words – disturbing. As the battle progresses, the quality of the song deteriorates, symbolizing Sybil's descent into inhumanity. In the third round I start to realize that I'm on edge … and it isn't just because Red's health bar is sliding dangerously low, or because Sybil's wall-piercing parasol attack is kind of terrifying. It's because I've been listening to an eerie Borg-like voice wailing “I won't save you” in the background, reminding me exactly what kind of Serious Business I've gotten into. By the time the fight is over, the song is burned into my memory. Sybil had to be dealt with, but her death is not trivial.

One can tell, by this point if not sooner, that Transistor is not a light-hearted game. Maybe I should have taken more warning by that song than I did. Another warning note comes when Red stops at her apartment long enough to eat a meal, then proceeds to lock herself out … somehow, she knows she's not coming back. In the story's last act, the surreal, quiet, wistful beauty of a Fairbank district overrun by the Process hints at a journey toward the end. Nonetheless, I wasn't quite expecting what actually happened.

Red seldom met another living character, but I was able to get acquainted with the city's cast of influential citizens by means of their traces: digital remnants of each individual's personality and skills, safely stored in the memory of the eponymous Transistor. They're a colorful bunch, sometimes flawed, but all important. As Red hauled this precious cargo around the city, I looked forward to finishing the game and restoring them somehow. Even the members of the Camerata – full of hubris, morally gray, and at times personally unpleasant – caught my sympathy. There were little things I could identify with, like Asher's refusal to be seen without his cat, or the childlike glee of discovery that came through in Royce's study notes on the Process. I wanted to save them all.

In the end, I didn't save any of them.

Transistor evokes digital technology with its aesthetics, but it's subtle about it.  This isn't a Tron lookalike; the city ends up looking sort of vintage and futuristic at the same time.  The effect is unique and delicious.
Red finishes her story with the full power of the Transistor in her hands. The engineer's dream: think what you want to create, and it appears. But though the Transistor can repair the buildings and machines of Cloudbank with ease, it can't bring back the people. And without citizens, all the rest becomes worthless. There's nothing left for Red in the city, no reason to rebuild. Cloudbank is done for, destroyed by the machinations of a few people who thought they would forcibly change it for the better. And where does that leave us?

Some things can't be fixed. Renewed, yes (as one sees after waiting out the game's final song), but they can never go back to the way they were before tragedy struck. You won't … can't … save everyone. That's the theme this story leaves me with. It's an interesting contrast to Bastion's ending, which allows the player to literally rewind time and clean up the mess. It's also an unconventional message for me to like. I deal with enough of this particular type of powerlessness in real life, so why would I want it in my video games? Yet I appreciate it here, because … maybe it's a lesson I need to learn. I don't get the impression that Red failed, but rather, that she did everything she possibly could to save the city, and was ready to move on. Hers is a different kind of hero's journey in which the attempt is the important element, not so much the results. And this is something helpful for me to remember whenever I'm tempted to beat myself up because I haven't yet solved all the world's problems or created multiple works of genius. Transistor broke my heart for an afternoon, but oddly enough, it's left me with a greater feeling of peace about life in general.

I'm also full of inspiration and ready to get back into robotics work. Expect some updates about the artificial muscles, and maybe some other things, in the near future.

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.


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,