Instructables

Circular Knit Stretch Sensor

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Picture of Circular Knit Stretch Sensor
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Use a circular knitting machine to knit a stretch sensor with regular and conductive yarns in five minutes! The values of the sensor range from roughly 2.5 Mega Ohm when relaxed, to 1 Kilo Ohm when fully stretched.

The stretch sensing is actually due to the structure of the conductive yarn which is made up of lots of short steel fibers mixed with polyester. Even without knitting the yarn into a structure you can use it as a stretch sensor by simply pulling it taught or relaxing it. But the yarn is not very strong and easy to tear. The knit structure allows you to accumulate more yarn and thus more resistance in less length and also by combining the conductive yarn with regular yarn you can gauge the sensitivity of the sensor by choosing a thicker or thinner yarn - thicker yarn gets more in the way of the conductive yarn making extra contact through the knitted loop structure. Plus knitting creates the stretchy structure giving you some natural tangible feedback.

Circular knitting machines
Circular knitting machines range from 20-250$ in price and differ in diameter, number of needles and spacing of needles. Spool or wire knitters have only four needles and resemble manual knitting dolls, while the knitting machines resemble manual knitting wheels or looms. See these posts on HOW TO GET WHAT YOU WANT for a more detailed summaries of circular knitting machines:
>> http://www.kobakant.at/DIY/?p=1144
and circular knitting looms:
>> http://www.kobakant.at/DIY/?p=2067

Conductive yarn

Conductive yarn is hard to come by, but hopefully soon online shops like www.plugandwear.com will offer such yarns in various thicknesses, conductivity and at a good price. The conductive yarn used in this tutorial is called Nm 50/2 and is made up of 80% Polyurethane and 20% Inox steel fiber and is manufactured by the Austrian company Schoeller (www.schoeller-wool.com) but they only produce and distribute it in industrial quantities of 300KG. See the following post for more information:
>> http://www.kobakant.at/DIY/?p=1978

Circular knit stretch sensor post:

>> http://www.kobakant.at/DIY/?p=1996

Buy a circular knit stretch sensor on Etsy:
narrow >> http://www.etsy.com/view_listing.php?listing_id=37778885
wide >> http://www.etsy.com/view_listing.php?listing_id=37805729

The following Instructable will cover the knitting of stretch sensors using conductive and regular yarn with the following models of circular and spool knitting machines:
- Play Go Knit Knit (17 Euro from Amazon Germany)
- Singer Spool Knitter (19$ from Amazon USA)

Video of the full process of knitting and testing a circular knit stretch sensor using the Play Go Knit Knit circular knitting machine (6 minutes)


Video of the full process of knitting and testing a circular knit stretch sensor using the Singer Spool Knitter (5 minutes)



Video demonstration of both wide (circular knitting machine) and narrow (spool knitter) stretch sensors


 
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willemb2 years ago
hi plusea

thanks for this instructable.
I have been trying to make one myself. I bought conductive yarn Mn 10/3 with plug and wear to do so.
But instead of using a circular knitting machine i used a normal knitting machine (by brother) to make straight pieces of knitting.
This in order to make a sort of swaeter to wear so with the stretch sensor you could monitor breathing.
But now i made two small test patches, one with a rather loose knit, one with a much tighter knit. (see image)
but when i connect them to a circuit, there is absolutely no resistance, and therefore nothing happens when you stretch it.
Is the knitting not suitable for this sensor?

thanks willem
photo.JPG
Plusea (author)  willemb2 years ago
hi wilhelm. your project sounds great and i'd love to hear more.
i have tried knitting the conductive yarn with a flatbed knitting machine and found that the resistance was less because the stitches were tighter. also, because of the tighter knit the sensor would more easily "stretch out" after stretching it just a few times. what are you using to measure the resistance of your sensor?

a few things to try:

make sure you are measuring resistance across the warp of the knit (top to bottom, not left to right) as you will have more resistance here. if you are knitting stripes, then measure from the bottom of one side to the top of the other.

try knitting with only every other needle to make it even more loose.

if possible, try adding another non-conductive to your knit, so that you are knitting with two yarns at once.

if you are connecting your sensor to the analog pin of an arduino, try using a lower value (hundreds of ohms) external pull-up resistor instead of the internal 10K ohm pull-ups.

i hope some of this helps. let me know how it goes!

here is a photo of a machine knit sensor i made, i didn't take notes on the resistance range i got, but i can do this tomorrow and report back.
>> http://www.flickr.com/photos/plusea/5117772273/in/set-72157623009009197
willemb Plusea2 years ago
HI plusea,

thanks for your reply!

I of course myself have been trying some things out as well.
I already noticed that the resistance across the warp of the knit was higher, and therefore i noticed that there was some resistance in the end.

I have been sewing the knitting to a t shirt and attached it to an arduino, and i can use it as a respiation sensor as i originally planned.

I have been trying to add another non-conductive yarn, but that messed up the machine, so i gave up that option.

I will try to use a lower external resistor, i had tried higher ones, but the lower ones might work better.

So, in the end, i have the thing work properly.
I am adding a photo of the sensor on a friend of mine, and a graph of the results through arduino and processing.

thanks agian for your help.
I can write a more elaborate message about my plans wth the whole thing, and the project that i will be doing.

willem
maarten met sensor.JPGmaarten met sensor achterkant.JPG04.jpg
Plusea (author)  willemb2 years ago
looks great! thanks so much for sharing the pictures.

you should pick a pull-up resistor that is of a similar value to the range of your sensor. so if your sensor is very conductive, then you will need a lower value resistors:-)
willemb Plusea2 years ago
thanks,

just a bit of logical thinking of course, about the resistors.

I just made a new shirt, with one sensor around my stomach and one around my breast. It works ok, but not really better than with one sensor around the stomach.
i plugged them both in one port, so it counts up, sometimes ending in too much resistance.

I will write a new code, splitting both reisitors up, so i can later filter the resistance measured from the breast (mostly showing body-movements in stead of respiration), from the one taken from the sensor around the stomach.

i can keep ou posted if you like, also maybe through email?

w
Plusea. I'm indra from indonesia. I tried to make some like this but I got some problem. I couldn't find conductive yarn material. When I tried to find it in internet, it was so expensive and we should buy it over 1 kg. I just need some yarn, under 2 metres to make some like that. If u can give me some of your yarn, I would be so appreciate. I need that for my experience in my university. Thx for your information about that sensor.
Plusea (author)  indraandriana3 years ago
hi indra. i sell knit stretch sensor kits and if you buy one then i include a sample of the yarn as well.
>> http://www.etsy.com/listing/63665445/knit-stretch-sensor
hbusca Plusea2 years ago
@plusea,

hi plusea, im steve from the philippines i would like to ask about knit stretch sensor because this device probably we want to use in our design project at school.

Here are my questions:

-Do you have a knit rectangle stretch sensor?
-Do you have a specification sheet for knit rectangle sketch sensor?
-What are the disadvantage and advantage of this device?
-What are the limitation of this device?
-How much it cost and also for the shipping?
-What are the standard sizes for you device?
-What are the website to buy this device?

sir plusea, much better we want to have your active email address so that we can direct buy on you. if you like sir plusea you can send me directly on my email address.. this is my email address.. hubertsteve1986@gmail.com..
we will wait for you reply sir plusea.. sorry for bad english ^_^ thank you and god bless sir.
Plusea (author)  hbusca2 years ago
i sell kits on etsy: http://www.etsy.com/shop/Plusea
elmarco384 years ago
Totally awesome instructable!!!! I can think of tons of applications for this. Really cool!!!! 
PS1184 years ago
First I must say, thanks for sharing all your research online!  Great stuff!!!
As far as uses, there are probably tons! But here's what I'm thinking.

How about knitting an anti-obesity sweater?  Then it could tweet whenever the wearer gains or loses every 1/nth stomach inches.  Talk about keeping the diet on track!

Also, I don't know what weights this kind of yarn might come in, but you could also crochet an anti-clutter doily that yells at you when you put unauthorized items on it.
troykyo4 years ago
 Totally great!  Compliments!
Another application for this would be to put one of these on an air muscle which you can easilly find how to build in various other instructables, this way your microcontroller will be able to know just how much force is being applied. It might also be helpful to use some kind of elastic string with it.
Gerde4 years ago
Hej Plusea,
nice instructable.

Let me share my ideas for this sensor.
In principle it's a way sensor, which gives you a value for the stretching from start to finish, right?

I would suggest to pull the "sleeve" over a plastic tube, add a tension spring in the center of the tube, and that way you have a defined force-resistance-relationship for a force sensor, or a scale.

Another nice application could be two balloons in a "hour glass like" construction. Each balloon is cover with a sensor-sleeve and one is filled with time-sand (be careful to order original, calibrated time sand).
as soon as the lower one has expanded enough, the hour glass turns around, and fills up the lower balloon.
This can  be used as hour glass, or as kind of new lava lamp.

so far
bye Gerhard
wlai4 years ago
cool, how does the knitted structure/network work? i'm not sure i understand why the resistance change when you stretch it.

i have high hopes that knitted electronics and wearable gear will bring in more women into the fold.

Plusea (author)  wlai4 years ago
The stretch sensing is actually due to the structure of the conductive yarn which is made up of lots of short steel fibers mixed with polyester. Even without knitting the yarn into a structure you can use it as a stretch sensor by simply pulling it taught or relaxing it. But the yarn is not very strong and easy to tear. The knit structure allows you to accumulate more yarn and thus more resistance in less length and also by combining the conductive yarn with regular yarn you can gauge the sensitivity of the sensor by choosing a thicker or thinner yarn - thicker yarn gets more in the way of the conductive yarn making extra contact through the knitted loop structure. Plus knitting creates the stretchy structure giving you some natural tangible feedback.

Thanks for asking this question! I totally forgot to include this important information in the actual Instructable and have now done so.
Grathio4 years ago
The videos aren't showing for me, but I don't really need  them, this is a great Instructable.  The inflation sensor has me thinking of all sorts of fun applications. Great work!
Jayefuu4 years ago
Awesome ible! Really interesting. Thanks 5* :D