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The SkeinTwister

Automating the skein-twisting process for indie dyers; eliminating repetitive stress injuries and helping their product look more consistent

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The SkeinTwister is a motorized hook that twists a skein of yarn. One end of the skein is placed in the hook, the other is held in the hand. A foot pedal starts the motor. The twister automatically shuts off when the proper "skein firmness" is detected, which the user sets with a knob. The user can also change the direction of the motor, according to their preferred direction of twist. There will eventually be a custom enclosure, which clamps the twister to a table or post, or can be wall-mounted.

This process is usually done by hand, by holding the skein between two hands and rotating the ends in opposite directions. It requires training, is easy for some people, difficult for others. The result is an inconsistent (and possibly ugly) final product, depending on the person performing the twisting, the humidity that day, and the original skein winding consistency. Dyers who produce hundreds of skeins per week are susceptible to repetitive stress injuries and elbow t

The SkeinTwister - Hackaday Prize 2016 Anything Goes Challenge

Being an indie dyer myself, I'm familiar with the challenges of making artisan products by hand, and trying to produce enough to make a sustainable living. The SkeinTwister is the logical next step to my previous project the SkeinMinder - the twister automates that last step of twisting hanks of yarn into tidy, pretty skeins.

It may sound funny that you can get a repetitive stress injury from yarn, but it's absolutely true. Dyers who produce hundreds of skeins per week are increasingly likely to get elbow tendonitis from the act of twisting, especially if they're already prone to arthritis. I hope to eliminate the pain of twisting for these fine, hard-working folks.

Twisting can also be surprisingly hard to teach others. It requires a specific set of concerted motions, and a third body part or outside tool to manage the twist when performing the final fold. Some people get it with a little practice, and some never do. Almost everyone has a slightly different technique, which leads to variation in the final product when skeins are twisted by different people. Humidity can also greatly affect skein twisting, as can the initial consistency of the wound hank.

Twisting is the final step of creating an artisan yarn. It determines the look of the final product, and greatly impacts the customer's reaction. A flooby twist with straggler strands looks sloppy and has less appeal than a neat and tidy skein of yarn. Automating the twisting process eliminates pain, makes it easier to train others, and produces a consistent final product.

SkeinTwister Schematic.pdf

A sketch of how the SkeinTwister hardware is hooked up.

Adobe Portable Document Format - 66.37 kB - 05/26/2016 at 21:18

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main.c

C code that runs the SkeinTwister.

C Source File - 3.61 kB - 05/26/2016 at 20:29

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HookDesign.pdf

Mechanical Sketch of the custom hook.

Adobe Portable Document Format - 31.79 kB - 05/25/2016 at 03:38

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  • 1 × DC Motor 12VDC motor
  • 1 × ATTiny85 Microprocessors, MicrocontMrollers, DSPs / ARM, RISC-Based MicrocontrollersMain
  • 1 × INA199 Power Management ICs / Power Supply Support
  • 1 × Foot pedal Control input
  • 1 × Trim Pot Threshold input for skein firmness

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  • Oh yeah, so it's a thing now

    Carrie Sundra08/21/2019 at 03:27 1 comment

    Long long ago, I took a break from this project for a while.  Then I picked it back up again and totally made it a thing and forgot I had even put it on Hackaday until I googled myself and remembered again.  :)  I've delivered 370 SkeinTwisters thus far, and after this next batch is made, there will be SkeinTwisters in 45 US States and on 6 continents.  If anyone stationed in Antarctica wants one, I'll just send it to you for free.  Actually, no wait.  I'll deliver it PERSONALLY.  I dig mountains, snow, cold weather, love wearing and sleeping in down, and don't complain in the face of hardship.  I'll be a freakin asset.  Call me.

    Anyway...it's totally a product, albeit a niche one.  I still don't know when I'll saturate the market and the demand will stop because there isn't really any demographic info available about the number of people dyeing yarn out of their houses, garage, and small workshops.  So I build them in batches, and I crowdfund/run pre-orders for each batch so that I don't build too many and get stuck with inventory.  As long as people keep wanting them, I'll keep making them!

    Official website is: www.alpenglowyarn.com/skeintwister

  • The Twisted Guts of It All

    Carrie Sundra05/26/2016 at 21:16 0 comments

    How does it all work, anyway? I've explained that generally, a knob sets the desired twist firmness. Press a foot pedal to turn the twister on, and it'll automatically turn off when it gets to the set firmness. But how do is firmness calculated and compared?

    Yarn is essentially a big spring. As the skein gets more and more twisted, the amount of torque required to keep it twisted goes up. I'm using a brushed DC motor, which means that current is directly proportional to torque.

    Torque is difficult to measure directly (from a pure electronics perspective), but current is easy. So instead of setting a torque limit, we can set a current limit. First, we'll need to measure the motor current. Here's the schematic, for reference (also in the "Files" section as a pdf).

    Starting on the GND side of the motor, there's a FET. Ignore it for now. Next is a 0.005 ohm current sense resistor (it's really just a calibrated thick bar), and the voltage across the resistor goes into an INA199 current sense amplifier. The motor can pull about 5A before it stalls. Using trusty:

    means that the voltage across the resistor at 5A is 0.025V. We're running the processor off 5V and would like to use that entire range to sense current, so we can really dial it in. 5V/0.025V = 200. It just so happens that the B3 model of the INA199 has a gain of 200V/V, so it's perfect. Add a little TVS clamp to prevent any accidental over-voltage from damaging the processor pin, and a little RC filter to smooth out the signal. That signal then goes into the AIN0, or the "positive" input to the analog comparator, and is a measurement of our motor current.

    Now we need to be able to set the current limit. Really, we don't even need to know what the exact current or torque is, users don't know this piece of information and will just turn the knob until the skein feels right. So a simple trim pot that varies the comparison voltage from GND to 5V works great. Moving the pot towards GND will be less torque and a "softer" skein, moving it towards 5V will be more torque and a "harder" skein. This voltage is fed into the AIN1 input, or the "negative" input to the comparator. When AIN0 > AIN1 (or motor current > the setpoint), the comparator generates an interrupt.

    Next, we need some method of telling the motor when to turn on and turn off. A momentary foot pedal is great for this. Feed this into a digital input that is internally pulled high. When the foot pedal is depressed, the pin is shorted to GND. When the foot pedal is in the normal position, the pin is pulled to 5V. This pin is also INT0, meaning it generates an interrupt. I've set this to falling edge detection, so when the pin goes from 5V to GND (the foot pedal is depressed), the interrupt fires.

    Finally, we need to be able to actually turn the motor on and off. The dirtiest, simplest method is to just use an N-channel FET on the GND side. When the gate is high, current flows through the FET and the motor is turned on. When the gate is low, no current flows through the FET and the motor is off. Important (but not shown on the above sketch) is that there's an additional diode across the motor, to prevent voltage spikes when the current is switched on/off.

    Putting it all together:

    • The foot pedal is pressed, generating an interrupt.
    • The interrupt routine toggles the FET from off to on. Motor turns on.
    • If the foot pedal is pressed again while the motor is on, the interrupt routine again toggles the FET. This time the motor turns off.
    • Otherwise, when the motor current reaches the setpoint, the comparator interrupt fires.
    • The interrupt routine sets the gate to GND and turns the motor off.

    That's it! I've included the C code in the files section. Nothing happens in the main loop, it's entirely interrupt-based. I may eventually add a soft start (instead of turning the FET fully on, send a PWM to the gate of the FET which slows ramps it up over ~ 0.5s). This will decrease the start-up current spike, and allow for lower knob settings (more softly...

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  • Aaaaarrrr, Me Hook!

    Carrie Sundra05/25/2016 at 03:26 0 comments

    The SkeinTwister's first major milestone has been achieved! The prototype unit is now with a fellow dyer, who is putting it through its paces even as I write this. I had the opportunity to demo it to a few more dyers at Vogue Knitting LIve than I anticipated, which was fantastic for getting varied feedback early on. It's always interesting to see how other people use it for the first time, the new motions definitely take a little practice to master, and dyers who have twisted thousands of skeins by hand have a lot of muscle memory to overcome. Despite that, the feedback was very positive. It's amazing how prevalent repetitive motion injuries are in this line of work - one dyer showed me scars on both hands from surgery due to stress injuries from skein twisting. The relief that several people expressed to me, and the difference it will make in their daily lives is hugely motivating. I know this is not curing cancer, but I'm happy I can make an improvement in a community I care about.

    Onto more mechanical thoughts - the hook! If you'd think you buy a hook in just about any shape & size, you'd be wrong. I found this out early on, as I scoured both the internet and my local hardware store. Screw hooks are about the closest thing, but they're meant to carry large loads. They don't have a large radius, and the bigger ones are made from pretty thick wire so they don't deform. This application sees smaller loads, and needs a larger radius to accommodate fat skeins of yarn. Here's a selection from the hardware store. As you can see, they're a little too small for the end of this fatty skein to easily fit in.

    Bicycle hooks are the next size up, and just too large. Plus, they tend to be plastic-coated, which is too grabby. The yarn should be able to slide off the hook easily. A shallower angle on the tip is also preferable, it should only be bent back enough to hold the yarn under tension. An "L" at first seems ideal, except that you don't want the yarn to rotate around the axis of rotation (and wobble), the yarn should rotate exactly on the axis. So a hook that only doubles-back slightly is best. This is a draft sketch of the hook and shaft coupler (more on the shaft coupler in a few.):

    At first, I thought I may need a small hook for mini-skeins and a large hook for fatty skeins, and make them interchangeable. Toollessly, of course. The more I thought about this, the more I realized that mini skeins didn't actually need smaller hooks. People are used to winding them around a finger, which is much larger than the hook size I'd use for fattie skeins. So really, the same hook can be used as long as it's big enough for fatty skeins to fit on. I'm currently thinking around 0.2" wire OD is about right to provide a stiff enough hook.

    The hook will be attached to the motor via a shaft coupler. Many motors come with a flat shaft designed to be used with a set screw, but most motor applications are also designed solely for torsional loads. This has both a torsion and axial load component. The dyer has to hold a fair bit of tension on the skein, to prevent it from doubling up on itself until _after_ the twisting is done. So there's a fair bit of load coming straight off of the shaft. A set screw only relies on friction to keep it in place axially, and there were a few mishaps early on where the hook did come flying off at me. I needed to devise a way to fix it permanently to the shaft in the presence of this axial load, so I decided to pin it. And since one hook will suffice, the shaft coupler can be permanently pinned to the shaft, and the hook can be permanently pinned to the shaft coupler. I'll use standard spring roll pins which are easy to install. I can have the motor shafts drilled by the manufacturer, have the shaft coupler made by a machine shop, and the hook made by a wire forming company. The next step will be to make more formal drawings, and send the hook and shaft coupler out to be quoted.

    The enclosure & clamping mechanism is the last big...

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  • Stick it in a Box and Give it to a Friend

    Carrie Sundra05/12/2016 at 00:20 0 comments

    So, with the initial spaghetti-breadboard proof-of-concept phase behind me, it's time for Phase 2. I always like to get my projects into another person's hands as soon as possible. I mean, I think the Twister is good and that I'm on the right path, but is this actually true or am I just being optimistic? I have the perfect opportunity to get it into a professional dyer and peer's hands this weekend, at the Vogue Knitting Live event in Pasadena. Brooke Sinnes of Sincere Sheep is one of those rare people that won't fixate on the current prototype-y form factor, but will give excellent feedback about the general function and her particular use case.

    There's nothing like an external deadline for motivation, so I've been busy taking the breadboard version, and soldering it into more stable & permanent perfboards. I love these little 1" square guys from Sparkfun, I try to always have a handful of both the single and double-sided ones on hand. Super useful.

    The SkeinTwister electronics have gone from the spaghetti mess you see in the background here:

    To the slightly less spaghetti-mess cable harness on the bench:

    To installed in an enclosure:

    The two connector inputs at the top are 12V from the power supply brick, and a 1/4" audio plug from the foot pedal. The foot pedal is momentary, currently configured so that one tap starts the motor, and another tap stops it. The green board in the upper right is the motor current feedback amplifier (the SOT-23-6 chip on the green adapter board), and the motor power control FET. In the lower left is the processor chip and a 12V to 5V linear regulator. I've kept the processor (an Atmel ATTiny85) in a carrier to make software updates easy. I can simply send Brooke a chip with new code if I have to, and she can easily replace it.

    The top of the enclosure shows the twist-firmness knob, and the ever-present-on-a-prototype blinking LED:

    The knob sets the automatic cutoff on twist torsion. The dyer can set it high for large skeins that need lots of torque to bundle up nicely, and lower for mini skeins that need less torque. When the foot pedal is pressed once (momentary action), the motor will turn on and the hook will start twisting the skein. When the proper amount of torque/current is sensed, the processor turns the motor off automatically. Or, if the setting is too high for the skein you have and your fingers are about to have the life squeezed out of them, you can press the foot pedal again and it'll stop. Ask me how I figured out the need for that feature.

    The wires exiting the right side go to the motor, which for the moment, is simply clamped directly to a table, like the first photo above. I've connectorized it, and I'll actually give Brooke a couple of different motors to try. Some are faster, some are slower, some are torquier, some are less torquey. I know which I like best, we'll see if she concurs. Mechanical design is next on the development list, for the hook, enclosure, and clamping mechanism.

    This is obviously a rough prototype, but I believe strongly in getting user feedback early, before I'm too far down one path. It makes changes easier and less expensive. I also believe in getting feedback on pre-production prototypes - what I define as my first attempt to make a real production unit. Those will have custom hook, enclosure, and clamping mechanisms, as well as actual PCBs. I'll make roughly a dozen of them, and have a beta test group run them through their paces. Then I'll have representative feedback from a variety of different use cases and conditions, and be able to make adjustments before committing to a production run.

    I'm excited for the first unit to fly the nest! I might even be able to demo it for a few more dyers at the show this weekend, the more feedback the better. Hopefully I'll get a few videos to post too!

  • Proof-of-Concept Models, Ahoy!

    Carrie Sundra04/27/2016 at 03:42 0 comments

    Hey! So I've built a very rough proof-of concept model of the SkeinTwister. This is the first version - just a foot pedal that connected and disconnected power to a motor. Here's a video of it in action, so those of you not familiar with the Wonderful World of Yarn have some idea of what I'm talking about:

    A few dyers I talked to were interested in the ability to twist a skein exactly the same number of times each time, automatically. This thought percolated in the back of my mind for a while. I certainly could instrument it with a rotation counter, and dyers could use it with a SkeinMinder. But I felt that this wasn't the right solution. Even though people were asking for the same number of twists, what they really meant is that they wanted each skein to look and feel the same.

    If each skein is exactly the same, this equates to number of twists. But the reality is that yarn is practically a living thing that hates to stay put. During dyeing, sometimes loops will get snagged, causing the skein to get a little out of whack, and need a different amount of twist to look tidy. Also, skeins will actually measure slightly different circumferences, depending on how far out on the winder arms they were initially wound. Not to even mention that it's difficult to keep perfectly consistent winding tension over different winding sessions. The upshot is that hanks will require different amounts of twist to look and feel consistent when in their final twisted put-up.

    So if there were just some magical way to measure the force it takes to wind a skein.....good thing DC motors are pretty magical in that force = current. The second proof-of-concept has a current sense amplifier fed into a comparator on an ATTiny85 processor. The other side of the comparator is hooked up to a trim pot (knob), so the user can adjust the "skein firmness" and wind consistently, no matter how the skein is prepared.

    A video demonstrating the automatic stopping capability:

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Discussions

nemoreally wrote 08/21/2019 at 22:37 point

Well written.  Even though I don't have any skeins to wind, I still feel tempted.  

I was just browsing ATtiny85 projects to get a feel for what they can do, and this is certainly an interesting example of the kind of "micro"controller project I had in mind for an ATtiny - a 'smart' op-amp/comparator replacement or messy-interface to I2C/SPI converter.  

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Ken Yap wrote 08/21/2019 at 04:50 point

👍 Great writeup, you're a good documenter. Admirable project even though I'll never have the aptitude to be a knitwit.

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Carrie Sundra wrote 08/21/2019 at 04:57 point

Thanks, Ken!  I should do it more, I enjoy sharing my (sometimes convoluted) thought process.  And I don't believe you don't have the aptitude to knit, I'm sure there's a style of learning & teaching that would work for you!  I can't give you more time in the day, but I can definitely encourage you to pick up some needles or hooks.

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Dave Lewis wrote 05/22/2019 at 01:22 point

This is cool.  The motor looks like a very solid one, what would you recommend if someone was to try and put one together ?

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Carrie Sundra wrote 08/21/2019 at 03:41 point

Hi, Dave.  I ended up going with a Tsiny TS-40GZ868-49.  It was the best combo of speed, size, and price that I could find.  See the reply I just posted to the below comment for more info!

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jonnypumfrey wrote 04/25/2017 at 12:15 point

Hi Carrie, wow I'm very impressed this looks great.  I don't really know how Hackaday works but I would love to have a go at making one of your skein winders.  Partly to save my hands when skeining of course but also because it just looks cool!  You have posted a pretty thorough set of drawings, does this mean you're happy for people to test your design?  If yes, then I would love to know what motor you have gone for.  I live in Ireland so I doubt if I would be able to get my hands on the exact same model but I could look up the spec and this would certainly save a load of back and forth testing.  I'd be delighted to send back any comments. Thanks  Jonny

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Carrie Sundra wrote 08/21/2019 at 03:40 point

Hey Jonny, I'm 2 years late on replying here, finally decided to take another look at Hackaday and post some more projects.  :)  I did eventually change a lot of things, and took the SkeinTwister into production.  So it's not totally open-source anymore, but I'm happy to chat about design and components with anyone interested.  I ended up using a Tsiny TS-40GZ868-49 motor.  It's no-load RPM is 160.  It was the best combination of speed (I wanted a higher speed than a lot of worm gear motors are built for) and size and price I could find.  I have had some quality issues - the bearings  & gear train don't always sound that great, so I triage them before shipping.  I have a whole pile of motors to "check out someday", maybe see if they'll quiet down with better bearings, or maybe just sell for cheap on eBay.  However, despite all of that, I now have 370 SkeinTwisters in the field, most have over a year of use on them and have twisted thousands or tens of thousands of skeins, and I've had very few issues.  They will eventually wear out, but I think the cost of replacing a cheaper motor every so often is better than doubling or tripling the price of the product up-front.

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Carrie Sundra wrote 05/25/2016 at 17:49 point

Good question, I should have touched on that!  I bought a selection of foot pedals, and I'm still evaluating them to see which I like best.  The one that's currently on the prototype is a Philmore 30-17090.  It's momentary-style terminated in a 1/4" audio plug.  

I'm not using the pedal itself to do any controls.  A single press & release turns the motor on, and the motor automatically turns off when it gets to the firmness setting/current feedback determined by the trim pot.  While the motor is on, a single press & release turns it off.  A manual way of shutting it off is needed in case the knob is set too high.  Don't want to break any fingers!  The momentary press & release is nice, so you don't have to stand there holding the foot pedal.  I thought it was a little more ergonomic and my testers have agreed so far.  So the pedal just provides an on/off input to the processor, which controls the motor.  

This means I'm more looking for pedals that are used for audio applications, not electric motor applications which pass 120VAC power through them.  The Philmore is OK, but the action is a little mushy.  I noticed when demo-ing the prototype that people were having a hard time determining if it actuated or not, which led to a few false start/stops.  Also, the cord is pretty wimpy.  This will need to stand up to being stepped on and tripped over for years, so I'd like a relatively nice power-style cord with thick jacketing.

I've actually just discovered that there's a decent selection of tattoo machine foot pedals that are the momentary style with 1/4" audio plug, so I'll be exploring them soon.  They also come in more fun colors and styles, which is nice when it's a creative application like this one!  I also have a uxcell a13102500ux0609 pedal, and it has much nicer action.  I just need to wire and test it.  I've looked at several Linemaster ones too, but they tend to be more expensive and maybe a little too burly.

Thanks for asking!

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Jarrett wrote 05/25/2016 at 16:42 point

What are you doing for footpedals?

Those are pretty much the Best user controls system, being hands-free and all. A little surprised they don't show up in more projects, actually, but it totally makes sense that a crafting person would be on that!

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Carrie Sundra wrote 05/25/2016 at 22:57 point

oops, replied in the wrong spot, see above!

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