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Tension Control System

Tension Control System

(OP)
Hi,

This is my first post as eng-tips.com so I hope i have posted in the correct category,

Anyway, I am looking to control the tension in a steel wire that is being unwound from a coil, passes over two pulleys and delivered into a machine. The power for the unwind of the coil is coming from a pull-thru unit in the machine and not driven from the coil, however, the coil does have a built in pneumatic brake.

I would like to control the tension in this wire by using both a pulley that is spring-loaded or pneumatic, that will provide a tension reading to the brake and thus, controlling the tension of the unwinding wire.

So far i am using a pulley that is mounted onto a horizontal gas spring, and this allows some self-adjusted for the tension. I would like to set up a small rig using a pneumatic cylinder that will provide some kind of reading for the tension, Are these available to buy as one unit? or will i have to mount some kind of positional sensor alongside this? Also, not having much experience with controls systems, i would like to create some kind of feedback loop to the coil brake, but im not sure the best (and most simple way) to achieve this.

Is there somebody in this forum who has come across a similar set-up? or can advise me on some part no.s that would do the trick and get me started?

Cheers!

Matt

RE: Tension Control System

Tension in a strand can only come from a "tug of war", something is pulling and something is dragging (resisting). I don't understand what's doing the pulling here. Also, you mention cylinders or springs to apply tension, but they don't provide feedback to know what the tension actually is. You will need some other device, like a load cell, for that.

RE: Tension Control System

I am guessing you want some kind of electrical feedback? As Jboggs suggested, perhaps a load cell combined with a pulley?

Using a pneumatic cylinder with pulley mounted on the rod, the pressure will be proportional to the wire tension (assuming the cylinder isn't bottomed out). But I am not sure if this is the best way to monitor the tension as I am guessing pneumatic cylinders leak quite a bit, so accuracy could be affected?


Tom

RE: Tension Control System

(OP)
Hi Jboggs,

Thank you for your reply. We have a tug of war in play here, one end of the strand is being pulled through the machine while the other end is wound many times around a core that is feeding the machine. This is a raw wire material we have delivered. So when we have a fresh coil of wire feeding the machine, there is high tension in the wire as it requires a much greater torque to rotate the heavy mass, as it builds up momentum less force is required, as opposed to when we have used half the coil the mass is much lighter, and when there are machine stops this process repeats...etc...Our coils are feeding wire into the machine for a few hours so this variable tension is very difficult to manage and causes deformation in the wire.

I have attached a diagram to explain this...

RE: Tension Control System

(OP)
Hi kiwitom235

Thanks for your reply, yes the proportionality between the tension and the extension/compression of the cylinder rod was something i was going to take advantage of to feedback to the brake on the coil. As you both say, maybe a load cell is something that would perform better in this application.

Is this link something you have in mind when you suggest a load cell and pulley....
http://www.montalvo.com/pulley-load-cells-wire-fil...

RE: Tension Control System

If you pull the wire axially, not tangentially, the load on the wire will be lower. Then a tensioning system can be placed to provide controlled tension into the rest of the machine. If you look at certain kinds of fishing reels you can see how a guide can be added to prevent the coil from running away and unspooling.

RE: Tension Control System

In the old days they used "dancing rolls" where the product was wrapped around a roller hanging by gravity to tension the product. There should be some old films of these.

RE: Tension Control System

I suggest staying away from electronic load cells. They are the most sophisticated way to control tension, but they are also very expensive, and so sensitive that the mechanics and controls have to be very carefully designed to avoid control oscillation.

Steel wire has almost no stretch so using a dancer arm system to smooth out tension perturbations is a good idea. You may need to actually drive the unwind roll so that changes in roll weight do not affect the tension.

You cannot use standard air cylinders for tension control due to the high piston friction. Bellofram rolling diaphragm air cylinders work well, or Airpel air bearing type.

In a typical system your machine would start to draw-in wire, causing the dancer arm to move up. A position sensor detects the dancer arm movement and the unwind reel is sped-up to feed more wire into the tension loop until the dancer arm is back at center position.

Tension is controlled by the force applied to the dancer arm by an air cylinder or a weight.

RE: Tension Control System

It appears the ultimate "source" of your problem is the inability of your process to deal with the variable tension levels required to fully unwind the feed spool. You need an isolating mechanism between the feed spool and your processing function that is designed to pull wire from the feed spool at a given rate regardless of the tension required and then feed that wire out at that same rate. Your take-up on the other end of the process should operate in a tension mode so it is always pulling at the same tension. That isolating device will probably include what is commonly referred to as a "bridle" arrangement, wherein the strand is wrapped around two or more driven pulleys (similar to those shown in your diagram) with wrap angles large enough to create a significant amount of friction between the strand and the pulley. That friction is the "isolator" that separates the variable tension level upstream at your pay-out from the constant tension level downstream in your process.

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