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Torque control calculation for unwinder application

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#1 wsy_nte

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Posted 22 January 2008 - 11:10 AM

Hi...
I have winder application using controllogix and GV-3000. I want to control the torque of unwinder. The line speed 120 mpm, torque reference to GV-3000 using analog signal (0-10 VDC). How to calculate the amount of torque?
Thanks.
James

#2 JeffKiper

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Posted 22 January 2008 - 08:13 PM

James What kind of product are you unwinding Paper, Plastic, Steel? That will make a huge difference in how you handle the torque control on a system. If you have a web or sheet system it could be unwound with very little drag on the system. On the other hand if you are unwinding 1/4" half hard Stainless steel off a reel without a holddown arm you will be using the brake more than the motor.

In web handling I try to use loadcells with sumation instead of a dancer bar.
JAK

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#3 wsy_nte

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Posted 24 January 2008 - 12:30 AM

We have dry laminator machine with running speed is 130 MPM. The web matirial is plastic with 12 to 70 micron thickness. The full roll diameter is 750mm and core is 92 mm> My calculation is run the gv3000 on torque mode and the torque will reduce on diameter decrease. The core speed will crease as diameter decrease.

#4 Keith Menges

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Posted 06 February 2008 - 12:42 AM

You need to be a little careful using true torque control, that is providing the drive a torque command. If the web breaks and there is nothing for the motor to pull against the motor will accelerate to very high speeds. You may be better off operating in velocity mode with a torque limit. At the very least monitor the speed feedback and if the feedback is outside of some expected range shut down the drive.

How well torque control works on an unwind largely depends on how much of the tension the drive system 'feels'. If you have significant drivetrain torque losses relative to the torque needed for tension torque control becomes very problematic. The torque command is simply:



Desired_Tension(lb) * RollRadius(in) / GearRatio / MotorTorqueAt10VDC * BitsAt10VDC

You may want to also offset this by the known drivetrain torque losses.

Keith

#5 kaiser_will

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Posted 06 February 2008 - 02:21 PM

I would side with Keith here. Running coil-to-coil unwind/wind applications in torque mode can have strange consequences (tension lag or strip break) due to slight variations in strip properties (coil flat spots, coil head/tail tapering). If you have to run in torque mode, then put limitations around the torque command. You might find that the the response rate of the limitation bandwidth is not ideal, meaning you run into oscillation of strip tension due to the output varying as a function of itself.

If your line has a belly roll for maintaining web tension, you might be able to fit it with an encoder and now you are set for web speed control (which is your ideal output anyway). Maintain linear web speed increase or decrease as your coil winds/unwinds, or maintain constant web speed as the payoff reel speed is increased/windup reel speed is decreased.

Similar to the formula pointed out earlier, you want to either keep strip speed constant (i.e., payoff reel will be sped up as diameter its gets smaller) or keep payoff reel speed constant (i.e., web speed will be slowed as diameter gets smaller). Figure out your desired constant and develop a formula for your payoff strip tangential speed (taking into account gearbox ratios, etc.).

You can trim off tension by adding a small percentage formula tied to a fluctuation in strip speed/effective unwinder torque or an operator-controlled potentiometer. Some operators love having the ability to trim off the strip tension based on their visual queues as the web is flying by, but give them access to only enough to not crash the line.

On a related note, perform a FMEA (failure mode analysis) to determine the critical control variables, desired constants (such as strip speed or tension or unwinder torque), then brainstorm how to monitor or control or sense those critical control variables. If you work through the process thoroughly, the means to solve the scenario becomes apparent.




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