Controlling wire tension in a coil winder

Thread Starter

cmartinez

Joined Jan 17, 2007
7,411
I'm at my wit's end. I'm currently designer a miniature coil winder, and I need a way to maintain tension on the wire while it is doing its work. The first thing that came to mind was to buy an ordinary BLDC cooling fan and adapt it to the machine, mounting it on the spool and then make it gently counteract the motion generated by the step motor that I placed on the winder.

Then things got complicated. After breaking the fan apart, I found that it's using a deeply integrated special purpose control chip (U1) that uses an external hall sensor (U2) to monitor position (as it should) and that cuts power to the wingdings if it senses that no motion is taking place, which defeats the whole plan that I had in mind. And I can't think of a way to bypass that condition.

295badac-7a85-49d0-b1a2-f3d985eeb296.jpg

So I ordered a dc motor from sparkfun, and it should be delivered in the next few days. But I doubt that it's going to work the way I want it to, mainly because its reduction gearbox will maybe add too much torque resistance for what I need. I tried to order a motor with a 1:10 gearbox, but ended buying one with a 1:18 because they ran out of stock of the former. The other motors listed in their site have output shafts of 3mm dia and look to flimsy for my application. The motor I ordered has a 6mm dia shaft, which feels about right for my purpose. My main concerns are that maybe the motor will produce too much torque (even though I plan to PWM it) due to its gearbox, or that it might overheat because it will be slowly rotating backwards while I apply power for it to run forward.

Plan B would be to use a passive system by simply adding a friction clutch that could be adjusted with a spring and a screw, but I don't like it because it would leave too many variables out of my control; such as losing tension when the winder stops, or wide swings in tension when it changes speed.

The coils will be 10mm id x 20mm od x 5mm thick using 0.010" (I think it's #30 gauge) magnet wire. And I'll be using a couple of small step motors to control the winder and the wire guide. I've already designed and tested the main mechanism and the motion controller and it's working as it should be. The only thing left for me to do is find a way to control the tension in the wire.

I really need to make this thing work ASAP, or I'll smash my face into a deadline ... any suggestions?
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,411
Can you put a drag on the feeder spool?
That's what I meant when I said

Plan B would be to use a passive system by simply adding a friction clutch that could be adjusted with a spring and a screw, but I don't like it because it would leave too many variables out of my control; such as losing tension when the winder stops, or wide swings in tension when it changes speed.
 

BR-549

Joined Sep 22, 2013
4,938
Maybe the shaft that the feeder is on....could wind another coil while the feeder is un-wound.

A closed winding loop.
 

Hymie

Joined Mar 30, 2018
991
Would controlling the required motor speed/torque be achieved by applying a constant current to the DC motor – with the circuit allowing you to increase/decrease the set current, as required?
 

MrAl

Joined Jun 17, 2014
8,479
Hello,

When you say "controlling" do you really mean you have to design the whole thing?

The control part would come from an electrical system made to measure the tension and then apply a current somewhere and that would apply some braking or friction.

The actual braking or friction action would come from a mechanical mechanism designed to do that.

The sensor traditionally would be a spring loaded lever and pulley. The wire goes around the pulley. The lever is kept at an angle somewhat angled away from the spool, so that when the lever rotates the angle changes and thus the tension must be changing. Of course it also has to be far enough away from the spool so that the accumulation of turns on the spool do not change the angle much or you compensate for that.

Another idea would be to have both the take up reel and supply reel both mounted on motor shafts. Then you can control everything. You'd have to keep track of the diameters though as the reels accumulate and lose turns. It would be a little tricky.
 

Danko

Joined Nov 22, 2017
1,134
I used for it small induction motor from blower, which I connect in series with capacitor.
Value of capacitor was selected for necessary torque.
On shaft of wire spool was mounted spring brake, which prevents reverse rotation.
Wire.png
 

Reloadron

Joined Jan 15, 2015
6,064
While they were large coils we started out with a simple mechanical friction type brake. The problem was the tension was changing, we had the same problem with a banding machine where on an unusual motor design we were adding a fiberglass tape. One day I had an epiphany and we added an electric brake where the tension was based on a 4 to 20 mA loop where 4 to 20 mA was equal to 0 to X amount of tension. I monitored the coil dimension and as the diameter increased so went the current. :) I can't remember who made the brake but they came in assorted sizes using a 4 to 20 mA input. Less any feedback you could likely just use an electric brake and a current source to set the tension. Maybe something to consider.

Ron
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,411
I used for it small induction motor from blower, which I connect in series with capacitor.
Value of capacitor was selected for necessary torque.
On shaft of wire spool was mounted spring brake, which prevents reverse rotation.
View attachment 151065
When you say blower, you mean as in a hair dryer? And why a capacitor in series? What value would you recommend? Down here hair dryers run at 110vac
 

MaxHeadRoom

Joined Jul 18, 2013
23,634
What about using a DC brushed motor with shorted terminals driven by the take off drum, when the take off spool is full the motor rotation is low but the spool rpm will be lowest and require the lowest damping due to the leverage at low rpm, as the spool take off reduces in diameter the spool/motor speed will increase as will also the braking effect, but that may be OK as the tension may need to increase as the diameter reduces, if back pressure needs to be tuned/adjusted it could be done by a pot across the terminals.
Just thinking out loud and have never tried it.
Similar to Ron #11.
Max.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,411
What about using a DC brushed motor with shorted terminals driven by the take off drum, when the take off spool is full the motor rotation is low but the spool rpm will be lowest and require the lowest damping due to the leverage at low rpm, as the spool take off reduces in diameter the spool/motor speed will increase as will also the braking effect, but that may be OK as the tension may need to increase as the diameter reduces, if back pressure needs to be tuned/adjusted it could be done by a pot across the terminals.
Just thinking out loud and have never tried it.
Similar to Ron #11.
Max.
Thanks, Max. I've already located a suitable motor at Amazon, of all places. I plan to mount the spool directly on its axis, and pwm it to control the tension.

But Danko's suggestion is intriguing, mainly because a hair dryer motor is brushless (inductive shaded pole, if I remember correctly), and hence is more or less protected from overload. It's also easy to replace, and could be controlled (more or less) through a phase triggered triac. It's his suggestion of a capacitor in series that I don't fully understand.
 

MaxHeadRoom

Joined Jul 18, 2013
23,634
It's his suggestion of a capacitor in series that I don't fully understand.
I am assuming this is used as a means of lowering the current to the motor, it would not be wise to apply full power and brake the motor at the same time without reducing the stall current somehow.
At least the way I see it.
Max.
 

Tonyr1084

Joined Sep 24, 2015
6,267
What about using a DC brushed motor with shorted terminals driven by the take off drum, when the take off spool is full the motor rotation is low but the spool rpm will be lowest and require the lowest damping due to the leverage at low rpm, as the spool take off reduces in diameter the spool/motor speed will increase as will also the braking effect, but that may be OK as the tension may need to increase as the diameter reduces, if back pressure needs to be tuned/adjusted it could be done by a pot across the terminals.
Just thinking out loud and have never tried it.
Similar to Ron #11.
Max.
That's what I was thinking. Using a small motor with its leads shorted either directly or via some variable resistance to control tension.

My kids had battery powered ride on toys. The design was such that when two pedals were pressed the motors would drive forward. When one pedal was released the motors would go into free wheeling, but when you released both pedals then dynamic braking would take over and stop the car (toy). You could use an autotransformer to act as a variable load on the motor providing a varying degree of resistance to the tensioning motor.

For a moment I thought I was going to bring to the table a new idea (not new in that it's never been done before). But Max beat me to it. The NICE thing about this method is that from what I gathered, you wouldn't need to power this motor at all. Just a simple DC motor with a permanent magnet should do just fine. Perhaps dead shorting the motor would present too much drag. If you don't want to go with a large variable resistor you could add or subtract incandescent lamps to gain the tension you want. Of course, their resistance changes with heat, but maybe that could be a good thing. I wouldn't imagine you're going to be winding these coils at some high RPM's so low and slow may be the way to go.
 

Danko

Joined Nov 22, 2017
1,134
When you say blower, you mean as in a hair dryer? And why a capacitor in series? What value would you recommend? Down here hair dryers run at 110vac
In hair dryers you can find universal motor, permanent magnet motor and induction motor.
We need Permanent-Split Capacitor Induction Motor which have good start torque ( see http://people.ucalgary.ca/~aknigh/electrical_machines/other/split_phase.html ).
Universal and permanent magnet motors have torque pulsating because of armature teeth. Capacitor in series works as current-limiting device. Without it stopped motor will baked in some minutes. Also you can control torque by changing of capacitance value. I do not remember exact value of capacitance, because I used DIY capacitor bank, where I was able select any capacitance from 0.5uF to 15.5uF with pitch = 0.5uF.
capacitor-bank.png
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,411
In hair dryers you can find universal motor, permanent magnet motor and induction motor.
We need Permanent-Split Capacitor Induction Motor which have good start torque ( see http://people.ucalgary.ca/~aknigh/electrical_machines/other/split_phase.html ).
Universal and permanent magnet motors have torque pulsating because of armature teeth. Capacitor in series works as current-limiting device. Without it stopped motor will baked in some minutes. Also you can control torque by changing of capacitance value. I do not remember exact value of capacitance, because I used DIY capacitor bank, where I was able select any capacitance from 0.5uF to 15.5uF with pitch = 0.5uF.
View attachment 151083
Excellent explanation, thank you. I'll start looking for a hair drier that works with that kind of motor... and if I can't find it, then I'll just have to use a brushed dc motor, although I'll try my best to avoid that as a last resource
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,411
That's what I was thinking. Using a small motor with its leads shorted either directly or via some variable resistance to control tension.

My kids had battery powered ride on toys. The design was such that when two pedals were pressed the motors would drive forward. When one pedal was released the motors would go into free wheeling, but when you released both pedals then dynamic braking would take over and stop the car (toy). You could use an autotransformer to act as a variable load on the motor providing a varying degree of resistance to the tensioning motor.

For a moment I thought I was going to bring to the table a new idea (not new in that it's never been done before). But Max beat me to it. The NICE thing about this method is that from what I gathered, you wouldn't need to power this motor at all. Just a simple DC motor with a permanent magnet should do just fine. Perhaps dead shorting the motor would present too much drag. If you don't want to go with a large variable resistor you could add or subtract incandescent lamps to gain the tension you want. Of course, their resistance changes with heat, but maybe that could be a good thing. I wouldn't imagine you're going to be winding these coils at some high RPM's so low and slow may be the way to go.
Thanks, Tony. But that idea is what I call a "passive" tensioner, and what I want is an active one. That is, if the wire sags because the winder stalled a bit, or backtracked (which is something that can happen in my design, because of certain features that I incorporated in it), I need the mechanism controlling the spool to be able to re-wind a little bit, so as to maintain tension.
 

Danko

Joined Nov 22, 2017
1,134
Using brushed dc motor you still need current-limiting device, some rheostat. Then you will have hot rheostat, but only warm motor.
EDIT: If you will allow motor to rewind, then you may have two not good situations.
1. If wire was loosen and motor trying to rewind it, you will have chance to break wire, because of big inertia of system (motor + spool).
2. If your wire will broken, or not anchored yet, spool can take run to some thousand RPM.
It is why I used brake for reverse rotation.
 
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