Sensor to measure the length of yarn used

Hello, all! Hopefully, I have posted this question in the right place.

I am trying to make an arduino controlled device that will hold a spool of yarn and dispense it. The part that I am having trouble with is how to get the device to keep track of how much yarn has been used (to alert the user when yarn is running low). I already have the user input the initial length of yarn as listed on the yarn packaging. I was wondering if there is any sensor that would keep track of the length of yarn used more accurately than (what I am currently using) a formula that converts the number of turns of the servo controlling dispensing into a length.

I looked at using an ultrasonic sensor, but the only thing I could think to do with it would be to make the path the string of yarn takes out of the device vary the string's proximity to the sensor cyclically every x inches. Then keep track of the number of cycles to get how much yarn has been used. This just seems less efficient than my aforementioned formula.

Thank you!!

Great question and relatively easy to do. Years ago at an estate auction I bought all the components to manufacture a machine to dispense heavy commercial fishing line and measure the length going on the spool being sold. I was never able to get the plans and documentation.

The line was wound one full turn around a "pulley" and then wound on the takeup spool. The "pulley" was attached to a mechanical counter. The "pulley" was sized so 10 turns equaled 1 yard of line.

You can do the same with the yarn if you can keep tension on your "pulley". Use a reflective spot on the pulley, or other means of allowing the Arduino to count the turns of the "pulley". You know the circumference of the pulley and that times the turns gives the length of the yarn that has been dispensed.

Paul

Or use a pinch wheel with an encoder.

Any progress on this issue? Did you find a workable solution to the problem? If yes please share. thanks

There are many threads around here on how to count rotations of a wheel, with various solutions to the problem. I'm sure there'll be something that's applicable to your situation.

Yes counting the revolutions or fractions thereof is an easy solution to the problem but not an accurate one since the diameter of the wheel/bobbin is constantly changing. I'm looking more towards Non-contact, laser-based encoder solutions.

That's why @Paul_KD7HB suggests to use a second wheel. No issue with changing diameter.

True, but then there is the slippage issue and other concerns..., ideally a sensor recognizing yarn presence, movement and speed would be a reasonable solution but, how? in my case I need to use vacuum to suck the yarn into a transparent tube which wouldn't be too difficult to sense it's entrance into the tube but how could one then sense it's movement and speed?

Cyquant: True, but then there is the slippage issue and other concerns..., ideally a sensor recognizing yarn presence, movement and speed would be a reasonable solution but, how?

The first is no problem: use some kind of optical sensor and you can sense the yarn presence.

The second is a problem: unless your yarn has markings at regular distance you could count (most don't), there is no way to see whether it's moving and how fast. Even if it's a braided thread it's difficult - you could optically count threads but that's about it.

I don't see the problem of using a second wheel where the fibre is wound around one time. Keep the wheel light weight, add a high-friction surface such as a rubber (high-friction for whatever yarn you use), keep the yarn taught, and you should have no issue with slippage of that wheel. The second wheel of course is also rotating freely, and rotation counts of the wheel can be done contactless. It's going to be tricky to sense very slow movements that way, if you need to do that, count more often than once every full rotation, but maybe every 45° or 30°.

The issues are exactly as you have noted and the yarn is mildly braided silk and wool with measurements sometimes as small as 3 cms. which only

The issues are exactly as you have noted and the yarn is mildly braided silk or wool with measurements sometimes as small as 3 cms. which only adds to the problem and makes it difficult to add the second wheel since the machine will be knotting different colors of yarn together with different lengths hence if not impossible it would be extremely complicated to wrap the yarn around the slave wheel etc. I,m wondering if by measuring the electrical resistance along the way could help? It sounds so simple that makes it difficult to believe today's' technology cannot solve this problem!

Cyquant: I,m wondering if by measuring the electrical resistance along the way could help?

Are your yarns conductive at all?

It sounds so simple that makes it difficult to believe today's' technology cannot solve this problem!

How would you solve the same problem by hand - i.e. without electronics? How is it measured now, if at all?

Agreed it's a simple problem. That doesn't make it easy.

I hope you realise that the reason you didn’t get any useful answers is because you didn’t give the full picture, and you actually appear to have given incorrect information.

First you asked how to measure the uptake of a single spool of yarn. You got several answers on exactly how to do that. Now it turns out to be multiple yarns. Also the measures are apparently small.

Still I don’t see why you can’t use the second wheel option - one for each yarn. Smaller wheels for those that have smaller use (so more rotation per length of yarn); extra measuring points along the wheel to get an even smaller resolution. No problem this way to get to <1 cm resolution.

right now the yarns are being measured by hand. There are say 256 bobbins each with a different color that need to measure in different sizes of each color and knot them together hence creating a space dyed effect. But this part is relevant as long as one could electronically measure the yarn then the rest of it could be solved(which is why I avoided complicating my question and only tried to find a way of measuring yarn length electronically).

for future ref. to help others, below more sophisticated ways and means by utilizing capacitance measurements and optical laser solutions: http://nptel.ac.in/courses/116102029/37

Cyquant: True, but then there is the slippage issue and other concerns..., ideally a sensor recognizing yarn presence, movement and speed would be a reasonable solution but, how?

I doubt any other speed sensor is going to match a pinch wheel for accuracy - so long as you have a reasonable large and light wheel with good rubber surface and enough constant tension you'll get very good accuracy (better that the variation in yarn length with changes in humidity, for instance, which is a large factor in some materials).

Its very easy to attach neoprene strip to a surface with superglue for a long-lasting chemically inert hard rubber coating.

And I can't think of any reliable non-contact way to measure speed of a strand of yarn.

Quite agree and will try it out next week. The pinch wheel system has proved itself for decades in the recording industry and it must be the only way for now.

(to alert the user when yarn is running low ).

It seems this is the crux of the matter. Why not a distance sensor calibrated to the distance of an almost empty spool?

 distance
      []------->(     -->(O  spool
  sensor      full      empty