I am searching for unwinding system
and i found Perma-Tork product which is approprite to use my system and can constantly give constant force.
but it is too expensive to use my system
so i wanna know product similar to Perma-tork
if you know that, can you tell me?
I think by controlling the current of the standard DC motor possible to control the torque so that the brake, feed and return, behave like a bobbin with a spring action.
thank you, is it possible to constantly give constant torque?
Unwinding string? I thought this was something to do with Arduino strings.
Do you mean unwinding twisted structure string like cords and twisted pairs?
My mother used to do a lot of knitting and the wool came as a ball of wool. It needed two people to unwind it, one with arms outstretched, the other looping the wool yarn from ball to arms.
It would be nice to know why you want to unwind the string in the first place, then you might get some useful answers.
I can think of a couple of things that use winding and unwinding. Sewing machines have a bobbin winding accessory and spool type fishing reels have a great winding mechanism as well.
You can find cheap electromagnetic clutches on eBay. I have a couple around here that I think I paid around $10 each.
Constant current in a DC motor gives constant torque to a good approximation. However there's a risk of frying it if the current is too large and the motor isn't moving, since there's then no airflow over the windings. Google will find you various constant current circuit ideas, programmable or fixed.
Thanks for sharing, good information. I'm new here but I keep learning new things!
Me too.
Have a look at how cable and string is sold - often you’ll find a box containing the string with a small hole for the end to poke through - this allows the string to be pulled out without tangling , providing this doesn’t happen, the pulling torque is largely Irrelevant it’s more about sudden starts and stops . Do some experiments .
But the problem is that since it is pulled from the inside of a roll, it is twisted. 
Blockquote
My mother used to do a lot of knitting and the wool came as a ball of wool. It needed two people to unwind it, one with arms outstretched, the other looping the wool yarn from ball to arms.
Are you sure?
I think it is the other way round winding from a skein into a ball (which is much easier to handle)
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