Braking !!!

Hi have this application where there if a string which is being pulled from a coil and I want to set a tension to it.

2 Motors, 2 Inverters and a Mitsu PLC are involved.

Motor 1 turns a X speed and pulls the string.

Motor 2 turn slower (of course). But since the coil being unwinded get smaller all the time (2mm every turns), I want to keep the tension close to constant.

My question is:

Can I adjust a negative constant Tork on an Inverter)

Click to expand...

... or something like it?

My pops once (years ago) did a tensioning device on a spooler using a small 1/2 or 3/4 hp Shunt wound 230 volt DC motor. Basicly the fields were used as a brake of sorts with no power to the armature.

Mike..

I believe that the application isnt so trivial as one might think.
It has been discussed here before. Search for "unwinder" or "unwinding" or similar.

Wouldn't it be simpler to have the unwinder mechanically braked/retarded (no drive)and monitor the current of the first drive?? Or am I missing something??

kennyb said:

Wouldn't it be simpler to have the unwinder mechanically braked/retarded (no drive)and monitor the current of the first drive?? Or am I missing something??

Click to expand...

In many cases No. The problem is, often that the winder needs to keep a constant variable tension on the rope/string/cable/wire etc... This requires a slight constant pullback on the feeding reel. A motor is best suited for this application, as it can be used to start, stop, provide tension and reverse the feeding reel as necessary.

A mechanical brake would be a maintenance nightmare and would wear, and cause exessive heating etc..

JesperMP,

I dont remember this aspect of the winder issue covered in the foum before this thread. Besides that I'm not in love with the idea of trying to calculate the speed/rpm of one drum or another. IMO the speed of the material should be obtained from the line in between drums with a pinch/traction idler if possable. However in the case of flat steel roll this would likely require a reel speed reference and dia calculation. But in the case of a wire or cable you really should be getting the speed, and tension from the material it's self, and then adjusting the speed and pullback of the two reals accordingly.

Mike

Its very application specific when it comes to unwinding material. I have seen coils 1/2 inch and larger steel wire that was pulled from the spool by the process. I have seen sheet metal that used motorized unwinds. I have seen paper that used a variety of methods most not employing a motor to unwind (This company specializes in wind/unwind apps: http://www.martinauto.com/ using pneumatics in many cases).

I have made twine, extruder process, that used the system line speed as a reference for the spooling. Then it went to a twisting department that would entwine (hence the term twine) several spools onto another spool, usually the unwind in this process didnt need anything but to be fed vertically just like the steel wire.

If there is no "line speed" reference to use then one way is to setup a master/slave system where the rewind controls the unwinds speed. The math for developing the speeds to match the motors for the correct tension has been given here in several threads. This may help with the design: http://www.warnernet.com/smartcat/warnerweb/PDF/DesignConsids_Selection.pdf

http://www.dfe.com/app-tens101.html

It is very common to use clutch/brake systems on the unwind, Warner and other brake/clutch companies offer an abundance of info on this.

You need to specify a drive that includes a regenerative unit. The braking resistors supplied with most VFDs are only rated for a few stops per hour. A tensioning application means that the drive is continuously stopping. Furthermore, you need maximum torque at minimum RPM, so you may need additional cooling for the motor.

It can be done, but you should get the drive manufacturer's engineering department involved. Don't accept the drive salesman's claims that his drives can do anything. Ask to see application notes on applying the drive for tensioning applications. Better still, ask for references where it has been successfully applied.

JesperMP-i believe that the application isnt so trivial as one might think

Click to expand...

Thats what I tought!

Let me show you the real McCoy!

We are going to build a Roll Wrapper

Today this client packages his Textile Rolls in Long Bags. Then heat shrinks the ends, then insert some device to hold them hard.

Its not that nice and resistant.

We have played a bit with his stuff and with one guy who hold hard on a roll of poly, we can make a very descent package.



The test where on a seperat Roll-off mechanism but it will really be in-line and fully automated.

It stretch wrap material so it will not be that important to have the very best control.

We usually would use a densor arm but I wonder if I could do it just by setting the brake tork or DC injection braking on that drive... It will be a Mitsubishi FRE-540-0.75K

Going DC would be way to expansive because we don't need anything more than what we already have.

We have exact speed of the Roll-off AND of the stretch film coil shaft.

Hey! Why not do it in style

I wouldn't use a motor/drive in this application. I think its overkill, IMHO.

Pierre, you said...

We have played a bit with his stuff and with one guy who hold hard on a roll of poly, we can make a very descent package.

Click to expand...

Anything you can build successfully with this level of 'tension control' is pretty forgiving. I think a brake is more than suited to this application. If you go with a motor you have two options: velocity control or torque control. If you use velocity control you need a VERY good idea of roll diameter and master speed since you aren't using any real tension feedback. If you use torque control you need a drive that can actually control torque, which takes you out of the entry level inverters. You also want to stay away from high ratio geartrains. You end up losing so much torque backdriving the geartrain that you will have trouble 'feeling' the tension.
I would say go with a brake, an E/P transducer and an ultrasonic sensor to give you poly roll diameter. You should be able to keep your tension well within the 'guy holding hard on the roll' range with this.

Keith

What kind of drives are you using?? Eurothem (now SSD) have canned macros for this application. Open loop and closed loop winding and unwinding. I have used them and they work. Also under speed the unwind and control the torque signal to the drive. Sense the diameter change and reduce the speed of the unwind as the diameter reduces.

GO JETS

Pierre,

Let me get this right..

The project is to apply the shrink wrap only or to do it all (roll, cut, and shrink wrap)?

Is that a picture of the shrink wrapping process?

Are there already motors and drives in place that you want to re-use?

Mike..

elevmike said:

The project is to apply the shrink wrap only or to do it all (roll, cut, and shrink wrap)?

Click to expand...

Of course we do it all with no operator.

The pictures shows how nice the roll of textile material can become when wrapped with stretchfilm.

We already have everything, but for the way we will control it.

We have some math that tells us how fast to drive and it will work ok BECAUSE this film can stretch 300%.

I was just wondering if we could setup an inverter for tork control BUT on the negative side... i.e. braking?

This would bring some elegance in our design.

I dont know about using a drive to provide tension on the feeding roller...

How about a tension feedback??

The shrink seems to be a narrower roll, like 6" inches or so.. Right?
Anyway, how about placing an idler roll on the shrink wrap somewhere inbetween the two rolls? The idler will have a weight or spring pushing down on the shrink wrap. the idler position would be indicated by something like an absolute encoder mouned on the pivot of the idler arm, or even a few High-low micro switches. The vertical position of the idler roller would be indicitive of the tension on the shrink wrap. This would provide feedback for closed loop control for the tensioning device (braking motor drive)...Too much tension = increase speed; too little tension = decrease speed.

That's how I would attempt it... Mike.

The tension feedback device (load cell or dancer) is an analog input to the drive. The drive does its own diameter calc. I have done this with Eurotherm and ABB.

http://www.ssddrives.com/Portal_m1_en.php (Eurotherm site) Check out the manuals for 690+ AC and 590+ DC


GO JETS

Jim,

Good suggestion.. Check out this page: http://www.ssddrives.com/usa/Applications/textiles.php