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April 25, 2005
Is Motor Torque Control EVER The Best Choice?
I've been at a couple of client operations lately that have winding tension control in either open-loop torque control or dancer feedback to torque control using motors in low speed processes (low speed is < 300 fpm, semi-low speed is motor rpm is >2x core or roller fpm).
The main problem with torque control is "what torque gets to the web"? Sure, you can get a good torque proportional to amps out of the motor, but to deliver this to the web and create tension, you usually have to go through a gearbox and other sources of torque losses before you get to the web. I've learned (though I'm open to being taught by new technology, etc.) that better closed-loop tension control is found by using your tranducer or dancer roller to trim motor speed.
Every motor in a multi-drive web line should get a line speed reference signal and trim off of that +/- 5% of the base speed. For driven winders and unwinds, the motor rpm needs to be calculated after roll diameter changes.
This is not to say motor torque should be ignored. Even better tension control, especially at winders and unwinds, will have inertia compensation, allowing a control loop to know when an extra "ooomph" is needed for accelerating the high rotational inertia unwinding roll or decelerating a big winding roll.
As always... let me know your thoughts. +tjw
Posted by Tim Walker at April 25, 2005 08:23 PM
