Torque control calculation for unwinder application
Posted 22 January 2008 - 11:10 AM
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?
Posted 22 January 2008 - 08:13 PM
In web handling I try to use loadcells with sumation instead of a dancer bar.
"Nothing counts as much as blood. The rest are just strangers"
Posted 24 January 2008 - 12:30 AM
Posted 06 February 2008 - 12:42 AM
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.
Posted 06 February 2008 - 02:21 PM
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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