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Old July 13th, 2005, 07:32 PM   #16
Peter Nachtwey
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Our product is used to do this sort of thing all the time.
ftp://ftp.deltacompsys.com/public/p...r%20figures.ppt
This is a good example of speed and torque limiting. The top and bottom dog grab the log gently and move the log through the saws. If too much torque is applied, the motors will shear the pins that hold the dogs on the chain. The force that the top dog applies much be enough to hold the log but not so much that it keeps the bottom dog from pushing the log through the saws.
The speed at which the log is moved through the saw varies depending on how much of the saw surface area is in contact with the wood. The holding force must be roughly proportional to the weight of the log. Since each log is different, all these parameter change from log to log. I know this is a compression instead of a tension application but the same principles apply.

If your motion controller can not adjust torque on the fly then I know one that is. It is done all the time. This is often done for fatigue testing. We can control the torgue in nice sineusoidal wave forms if necessary.

Yes you can regulate speed and torque but not to follow two different setpoints. I have this argument all the time with people that think you can control position to a position set point and pressure to a pressure set point at the same time. Normally torque is regulated to achieve the desired speed by controlling acceleration and deceleration. If you adjust torque without regards to speed then you soon will be going at the wrong speed. The same goes for adjusting speed without regards to torque. There can be only one. I like to use a minimum function for speed and torque that way I can limit both.

One can calculate the torque for the hydraulic motor by measuring the differential pressure between the motors A and B port. The hydraulic motors have a PSID to torque constant that allows one to calculate torque.
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Old July 14th, 2005, 08:36 AM   #17
msinclair
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Quote:
Originally Posted by Peter Nachtwey
Yes you can regulate speed and torque but not to follow two different setpoints.
I agree. What I recommended back in post 4 was to set a speed reference (for both drives), and trim that (on one drive) using loadcell feedback to maintain a tension setpoint.
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Old July 14th, 2005, 12:12 PM   #18
BITS N BYTES
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Why so complicated guys.

The method I suggest in post 15 would work great for his application.
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Old July 14th, 2005, 03:16 PM   #19
msinclair
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Quote:
Originally Posted by BITS N BYTES
Why so complicated guys.

The method I suggest in post 15 would work great for his application.
I don't know enough about his application, but this quote from your first link is one thing that I would worry about:

Quote:
Motors that are coupled together by contact with a web or strand of material can not be connected to a common controller. Such applications require an individual controller for each motor so that the individual section speeds and the web tension can be closely regulated.
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Old July 14th, 2005, 04:25 PM   #20
DickDV
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Well folks, I've said my piece and I hate arguing so, best wishes Mr. B. Please advise what method you chose and how it works out.
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Old July 14th, 2005, 04:43 PM   #21
rsdoran
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Y'all done screwed up

DickDV is the VFD man especially when it comes to web applications.

Bits, aint sure why your post in 15 would work...How do you get one VFD to provide a different speed, based on tension, to one of the motors using one VFD?

There are many ways to do this based on the material but most of those I have seen have a "pulling motor" that bases its speed on tension and the other motor runs at whatever you set it for. Technically this type application should be a standard these days.

I hope Dick comes back and y'all listen...no offense meant Peter (or anyone) but this round you are playing in Dick's yard.

MrB (sometimes I think you are playing games) why dont you state the specifics of the application...ie what material is involved, type machinery etc etc.
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Old July 14th, 2005, 05:22 PM   #22
Leadfoot
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Everybody seems to keep overlooking the fact Mr. B is using J7 drives. J7 drives are purely V/F basic inverters. No whistles, no bells just variable speed.

B motor's drive would need to be replaced with a V7 to be able to do torque control.

DickDV has definately given the best solution for web handling using 2 seperate motors.

The use of 1 VFD and multiple motors on a long belt has definate benefits. I know of a case in Dallas, TX where 20 motors, 1 HP each are on a conveyor. There is a single 30 HP drive with 20 individual fuse and overloads for each motors. Smooth starts and no problems moving material. Multiple motors on 1 VFD does not work so well on a web.
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Old July 14th, 2005, 06:37 PM   #23
BITS N BYTES
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Quote:
Originally Posted by rsdoran
DickDV is the VFD man especially when it comes to web applications.

Bits, aint sure why your post in 15 would work...How do you get one VFD to provide a different speed, based on tension, to one of the motors using one VFD?

There are many ways to do this based on the material but most of those I have seen have a "pulling motor" that bases its speed on tension and the other motor runs at whatever you set it for. Technically this type application should be a standard these days.

I hope Dick comes back and y'all listen...no offense meant Peter (or anyone) but this round you are playing in Dick's yard.

MrB (sometimes I think you are playing games) why dont you state the specifics of the application...ie what material is involved, type machinery etc etc.
Believe me it works - used it in many applications.
The natural slip factor of a AC motors provides the load/torque sharing he needs.
Remember he is using a real basic VFD!
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Old July 14th, 2005, 08:59 PM   #24
DickDV
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I haven't gone away in a huff and, no, I'm not pouting!! It's just that, at some point, as RSDoran points out, constructive dialog becomes impossible without some hard data. Let me illustrate.

If Mr. B's web is elastic and can be stretched, then pretty much any of the solutions offered will work to some degree since the elasticity of the sheet determines the tension. Mr. B has only told us that he wants "a little tension". I take that to mean that he doesn't want a lot of tension and that what little there is must be controlled with some precision.

That's where most of the above solutions become unacceptable. For example, running two motors from the same drive with some mechanical method for getting some offset speed will work but is extremely crude. You are getting torque control only by varying the slip of the motors. What happens if the load on roll A suddenly increases(a tight bearing, for example)? Motor A loads more and slips more. Motor B is seeing the same Hz so it tries to hold speed but, due to Motor A slipping more, the speed difference between the two motors increases which pulls Motor B down into increased slip and increased web tension. See the problem?

Now, let's look at two drives and two motors but both running as speed regulators. In order to prevent the loading problems in the above paragraph, you have to set the A drive as a tight speed regulator. That way the load changes in A and ahead of A will not be seen by B. You could then set B as a "loose" speed regulator or simply use open loop V/Hz operation. Since most modern motors run 2% or less slip from no load to full load, you have to control motor slip within a few 1/10's of a % to get meaningful control of tension in a rigid sheet. Mr. B hasn't told us anything about the sheet elasticity so whether this is good enough or not is pure speculation. And what basic drive do you know of that will give you control in 1/10's of a %?

It was proposed that the B drive be set up as a speed regulator with torque or current limit set to control the tension. This is probably ok too if the process doesn't vary or the machine isn't expected to run more than one product. What about operation when cold, or hot. Unfortunately, Mr. B hasn't told us that either so we have more speculation. I tend to bow out when that happens.

Also mentioned was the fact that Mr. B has chosen two basic V/Hz drives. Why? Maybe he shouldn't have!!! Maybe the best thing we can do for him is suggest he buy the right stuff for the job, if that is what is needed. Again, we don't know if Mr. B has any flexibility there either.

Mr. B did, however, tell us that a broken web is highly unlikely. In view of that, speed run-away of the B drive also becomes highly unlikely which makes running that drive as a torque regulator the simplest solution which offers flexibility and direct control of tension. To do this, Mr. B needs two reasonably good sensorless vector drives, A set as a tight speed regulator to isolate the tension section from external disturbances and B set as a torque regulator for direct tension control. No razzle-dazzle, no encoders, just two properly spec'ed drives to cover the possibilities. Unfortunately, there are way to many of those but you already know how I feel about that!!!

There! Now I'm done!!
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Old July 15th, 2005, 08:38 AM   #25
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Thanx Dick. Will see wot I'll do. Gonna b on a AB course next week so will only try sum solutions after that.

tx again to all for u're info.
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Old July 15th, 2005, 08:47 AM   #26
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Quote:
Originally Posted by rsdoran

MrB (sometimes I think you are playing games) why dont you state the specifics of the application...ie what material is involved, type machinery etc etc.
Nah dude.... I'm not playin games. I just wanted sum suggestions. I didn't want u guys 2 do my work for me. Just needed sum ideas hence me not posting a whole book explaining every detail in specifications... how'd u xpect me 2 learn like that huh????


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Old July 17th, 2005, 09:50 PM   #27
Jasonc24
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Although this seems to be "splitting hairs", due to the nature of the drives being used and the apparent sophistication of Mr B's current set-up, I personally agree with msinclair.

By using a speed matching in combination with a torque limit on B, the solution appears to have the answer to what Mr B is looking for. However, what is not taken into account are all other external factors like frictional drag and acceleration that may vary from A to B. Because of this, the regulation of the tension on the web will only be as good as the external factors allow. This is probably more than adequate for Mr B, but for other applications this could present a problem.

Therefore, the better way of doing this would be to incorporate a load cell (or dancer)to regulate the torque on B.
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Old July 17th, 2005, 10:19 PM   #28
BITS N BYTES
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Wink

The only analogue input I know of on the J7 drive is for frequency control, Your are not going to be able to regulate the torque using this. Unless there is a special add on option card available from Yaskawa, I stand by my original post suggestion, ONE drive, TWO motors.
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Old July 18th, 2005, 08:16 AM   #29
Jiri Toman
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Bits N Bytes,

This is from the link you have provided:

Motors that are coupled together by contact with a web or strand of material can not be connected to a common controller. Such applications require an individual controller for each motor so that the individual section speeds and the web tension can be closely regulated.

It seems that this would disqualify your idea if I understand the application correctly.

Mr_B, menier
I must say that Cape Town is a most beautiful city I have ever lived in. Where are you at?
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Old July 19th, 2005, 10:24 AM   #30
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Regularly use torque following with a speed master on web stretching applications , have to be carefull when use multi torque followers that in the event of one roll slipping because of the web properties , that other following rolls do not stop because of the torque following function - I always peg the follower speed so that it can only droop so much in the event of a wrap .
Operator cant change it ? they can and do with what we do , they are provided the facility to change all torque following setpoints in a octet roll following application together with overall stretch between to sets of octets .
How do you post a screen shot ?
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