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Old August 16th, 2019, 12:20 PM   #1
bodoo23
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A Question about Winder Application

We have a project including unwinder-rewinder process. We control motors via torque mode. Me and my collegue have different opinion about a thing.
Let's think line is going with fixed speed and tension. When we increase tension(increase torque command sent to drive),

My opinion is that before increasing torque, the rewinder motor was going with fixed speed in balanced way. As soon as we increase torque (to increase tension) this balance is broken and a positive torque is created. This positive torque makes motor speed up and this increase in speed causes tension to increase. Then motor finds a new balance point which is at higher speed and higher tension.

But my collegue doesn't agree with me and says as soon as we increase tension , torque increases immediately(independently from speed of winder) and this will cause tension to increase without making any change in speed. But it doesn't seem logical to me. For me, to create this additional tension rewinder speed should increase more or less. (The material is aluminium)

Is there anyone experience this situation or having opinion about this ?
Thanks in advance.

Last edited by bodoo23; August 16th, 2019 at 12:27 PM.
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Old August 16th, 2019, 01:57 PM   #2
chantecler
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Hi,

If the material were elastic like some plastic films you were right but since the material is not elastic that increment in speed that you talk about will be tiny (you won't notice) and as soon the new tension be reached the speed of the winder will be the same as the line speed

I would like to mention that in every winder application I have worked in which the winder is torque controlled the speed reference to the winder is between 5 to 10% higher than the line speed so that the motor wants to go faster than the line but is limited by the torque reference.

Hope this helps
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Old August 16th, 2019, 05:21 PM   #3
kamenges
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Quote:
Originally posted by chantecler:

If the material were elastic like some plastic films you were right but since the material is not elastic that increment in speed that you talk about will be tiny (you won't notice) and as soon the new tension be reached the speed of the winder will be the same as the line speed
I personally know of no material with an infinite modulus of elasticity. ALL materials are elastic to some degree. Good thing, too, or everything around us would break as soon as a spec of dust were to hit it. F=ma. When two objects hit each other the only thing that can control "a" at the instant of contact is elasticity.

Technically, you are correct, bodoo23. Tension (stress) in any given tension zone is the result of material elongation (strain) in that tension zone. New material entering a given tension zone may needs to change its length to achieve the tension in that zone if the tension is different than the preceeding zone. Continual change in length is differential speed.

So, yes, increasing torque will result in additional continuous elongation of the material entering the tension zone, resulting in an increase in the speed of the axis relative to the speed prior to the increase in torque. Whether or not you will be able to measure that increase in speed is another matter. But whether you can measure it or not, it is there.

Keith
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Old August 17th, 2019, 01:10 AM   #4
bodoo23
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Quote:
Originally Posted by kamenges View Post
I personally know of no material with an infinite modulus of elasticity. ALL materials are elastic to some degree. Good thing, too, or everything around us would break as soon as a spec of dust were to hit it. F=ma. When two objects hit each other the only thing that can control "a" at the instant of contact is elasticity.

Technically, you are correct, bodoo23. Tension (stress) in any given tension zone is the result of material elongation (strain) in that tension zone. New material entering a given tension zone may needs to change its length to achieve the tension in that zone if the tension is different than the preceeding zone. Continual change in length is differential speed.

So, yes, increasing torque will result in additional continuous elongation of the material entering the tension zone, resulting in an increase in the speed of the axis relative to the speed prior to the increase in torque. Whether or not you will be able to measure that increase in speed is another matter. But whether you can measure it or not, it is there.

Keith
Thanks for answer Keith. It helps a lot.

What do you think about this part (This positive torque makes motor speed up and this increase in speed causes tension to increase) ? Is that true increase in speed creates tension ?
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Old August 17th, 2019, 12:33 PM   #5
AutomationTechBrian
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I did my first wind/unwind project earlier this year. The line speed is set as a constant, based on the RPMs of the winder and the dimension of the roll. (The winder roll slows down as the roll gets bigger.) Independent of the speed is the tension. The tension is measured by load cells, and that was the basis of a PID loop. If the tension setpoint is increased, the negative torque increases on the unwinders, and the speed never changes.
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Old August 17th, 2019, 02:15 PM   #6
kamenges
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Quote:
Originally posted by bodoo23:

What do you think about this part (This positive torque makes motor speed up and this increase in speed causes tension to increase) ? Is that true increase in speed creates tension ?
That is something of a chicken and egg question. It depends on your perspective and what you are intending to control. But both will happen.

If you are controlling torque, the increase in torque will result in an increase in stress on the material in and entering the tension zone, which increases the material strain. This strain increase will result in an increase in speed in the driver of that tension zone.
If you are controlling speed, the increase in speed command will attempt to create in an increase in elongation of the material in and entering the tension zone. This elongation (strain) will require an increase in stress (tension) to accomplish and will require more torque from the driver in the zone.

So, as you see, both will happen. Which is the cause and which is the result is based largely on what you are controlling.

Quote:
Originally posted by AutomationTechBrian:

If the tension setpoint is increased, the negative torque increases on the unwinders, and the speed never changes.
As I said before, you may not have commanded a speed change, you may not have measured a speed change, but a speed change did occur. The material kind of requires that it did.

As an aside, this is the fundamental physical basis for draw control systems; web path control systems that are based on fractional percentage changes in speed between driven sections. In the more critical/accurate applications, the comparison isn't instantaneous speed but the ratio of accumulated encoder counts over a given span. That tends to be a more accurate/more forgiving control method in critical draw applications. But with drives getting better every year this probably isn't as big a deal anymore.

Keith
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Old August 17th, 2019, 03:33 PM   #7
Peter Nachtwey
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Quote:
Originally Posted by kamenges View Post
That is something of a chicken and egg question. It depends on your perspective and what you are intending to control. But both will happen.
Yes, both will happen but the speed up will only occur for a very short time.
I get analogous questions in hydraulic control or motion control with force all the time.

Here is what happens.
Increasing the motor torque creates a torque imbalance. The motor will rotationally accelerate until the sum of torques acting on it is 0. If the material being wound up has a high modulus then the speed up may only take milliseconds for the material to stretch enough to neutral the added torque.

During the time the new torque set point is being reached, the motor will accelerate faster at first then slower as the material stretching applies more and more resistance. Hopefully this interaction is not springy or a derivative for the torque control will be necessary.

Newton's 3 laws of motion will apply only in this case it is torque ( force at a distance ) instead of just torque.
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"Living is easy with eyes closed, misunderstanding all you see...." Strawberry Fields Forever, John Lennon
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Old August 17th, 2019, 03:55 PM   #8
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Quote:
Originally posted by Peter Nachtwey:

Yes, both will happen but the speed up will only occur for a very short time.
That would be true in a static system with a fixed distance between controlling points. But a continuously running web system always has web entering the controlled span that is (likely) at a different tension (stress due to strain) than the tension in the controlled zone. This entering web will also need its strain changed to match the strain required by the stress setpoint, just like the web entering behind it a millisecond later and the web that had entered a millisecond before. This is a continuous process that requires a continuous speed differential between the entry and exit points of the tension zone to maintain.

Keith

PS:
I may have misinterpreted what Peter wrote. It is correct that acceleration phase will likely be very short, especially if it is a high modulus material. However, once the acceleration phase is over the axis with the increased torque will continue on at the new, higher speed.

Last edited by kamenges; August 17th, 2019 at 04:07 PM. Reason: Clarification
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Old August 17th, 2019, 04:41 PM   #9
bodoo23
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Quote:
Originally Posted by Peter Nachtwey View Post
Yes, both will happen but the speed up will only occur for a very short time.
I get analogous questions in hydraulic control or motion control with force all the time.

Here is what happens.
Increasing the motor torque creates a torque imbalance. The motor will rotationally accelerate until the sum of torques acting on it is 0. If the material being wound up has a high modulus then the speed up may only take milliseconds for the material to stretch enough to neutral the added torque.

During the time the new torque set point is being reached, the motor will accelerate faster at first then slower as the material stretching applies more and more resistance. Hopefully this interaction is not springy or a derivative for the torque control will be necessary.

Newton's 3 laws of motion will apply only in this case it is torque ( force at a distance ) instead of just torque.
Quote:
Originally Posted by kamenges View Post
That would be true in a static system with a fixed distance between controlling points. But a continuously running web system always has web entering the controlled span that is (likely) at a different tension (stress due to strain) than the tension in the controlled zone. This entering web will also need its strain changed to match the strain required by the stress setpoint, just like the web entering behind it a millisecond later and the web that had entered a millisecond before. This is a continuous process that requires a continuous speed differential between the entry and exit points of the tension zone to maintain.

Keith

PS:
I may have misinterpreted what Peter wrote. It is correct that acceleration phase will likely be very short, especially if it is a high modulus material. However, once the acceleration phase is over the axis with the increased torque will continue on at the new, higher speed.
Peter and Keith, Thanks for appreciated answers. I feel I started to understand concept under processes. I totally understand our motor will continue on at new higher speed when we increase tension but I have last question. When I examined another unwinder-rewinder line which is doing the same work. I saw that when unwinder and master drive(mill) was stopped, operator pushed the tension button for rewinder. As soon as he did it rewinder accelerated(for a very short time), tension occured and rewinder stopped again with tension. Now I am curious about why rewinder stopped again ? which extra force made it stop because while unwinder and master is turning when we increase tension, speed gets higher but in the situation above motor gets again to zero speed. why ?

Thanks a lot.
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Old August 17th, 2019, 05:21 PM   #10
kamenges
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What you describe with the rewind case when tension is first turned on WHEN THE MASTER DRIVE IS STOPPED is correct.

In that case the rewind will turn to take up any slack that might exist between the master roll and the rewind and will then turn further until the web is stretched the appropriate amount to achieve the commanded tension in the static web located between the master roll and the rewind. Once this amount of strain is achieved the stress created by this strain is equal to the torque produced by the roll acting through the roll radius. These two forces are in balance so the rewind roll will stop moving.

If you increase the tension in this state (THE MASTER DRIVE IS STOPPED) the rewind would rotate as the web is stretched slightly more due to the increased stress and then will again stop due to the force balance situation.

The only time the differential speed situation occurs is when the web entering the tension zone is at a different tension than the tension in the tension zone. If no new web is entering the tension zone, the web already located in the tension zone is stretched a given amount based on the rewind torque and the system will become static.

Keith
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Old August 21st, 2019, 07:39 AM   #11
chantecler
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Quote:
Originally Posted by kamenges View Post
I personally know of no material with an infinite modulus of elasticity. ALL materials are elastic to some degree. Good thing, too, or everything around us would break as soon as a spec of dust were to hit it. F=ma. When two objects hit each other the only thing that can control "a" at the instant of contact is elasticity.

Technically, you are correct, bodoo23. Tension (stress) in any given tension zone is the result of material elongation (strain) in that tension zone. New material entering a given tension zone may needs to change its length to achieve the tension in that zone if the tension is different than the preceeding zone. Continual change in length is differential speed.

So, yes, increasing torque will result in additional continuous elongation of the material entering the tension zone, resulting in an increase in the speed of the axis relative to the speed prior to the increase in torque. Whether or not you will be able to measure that increase in speed is another matter. But whether you can measure it or not, it is there.

Keith
Hi,

Thank you Keith for your explanation and my apologies for my not-accurate intervention. I didn't know about the modulus of elasticity before.

With no spirit to argue or justify my first comment at all I would like to point out that "approximation" is a good thing too. Thanks to "approximation" we have A/D conversion, mp3 and countless things.

I had a professor in the university that tough me that what makes the difference between scientists (like physicists and mathematicians) and engineers is that engineers do approximate. He tough me criteria to discard terms which contribution is much smaller compared to other terms. That made a radical change in the way I think and face my work.

In this particular case: in practice (and only in practice) I treat considerably elastic materials like polyethylene differently to "not-so-elastic" materials like polypropylene, paper and aluminum foil.

Thanks again for making the tension thing much clearer.

Bye

P.S.: Excuse me for my English too, hope it is understandable enough.
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Old August 21st, 2019, 12:55 PM   #12
chantecler
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Pointer

typed tough instead of taught. The edit option didn't show

Last edited by chantecler; August 21st, 2019 at 12:58 PM.
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