What are the drawbacks to draw control?
David Roisum, Ph.D., Consulting Technical Editor -- Converting Magazine, 7/1/2008 2:00:00 AM
Speed or draw control is quite simple for electrical engineers. They merely need to make a motor turn at a target speed to an accuracy of a few parts per thousand. Maintenance is simply a matter of knowing roller diameters accurately to 4 or 5 digits. Speed control is exceptionally complex for everyone else, including those ranging from operators to process-control people.
If we use speed to control tension, what is the tension applied to the web? This simple, yet vital question cannot be answered easily. At best, we can say the tension in the web between any two rollers is partly controlled by the tension coming into that section in addition to the speed-control ratio. This isn't very satisfying to the operators who find the web drooping or breaking.
If we have a tension variation coming into a speed-controlled section, that tension variation will be passed downstream. Thus, draw control is not even a control in the traditional sense of the word, which means limiting variation. This makes the isolation nips seen just after many unwinds seem kind of silly.
If we have a change in moisture or temperature, the web tension will be affected. If it's paper and the moisture rises, the tension goes down. If it's paper and the temperature rises, the tension goes up. If it's film and the temperature rises, the tension goes down, except that some materials in a certain temperature range will heat-shrink, and the tension will rise. How confusing! If you make changes to just about anything, you may need to adjust the draws to compensate for the change in tension. It's undesirable to have one knob (say, dryer temperature) do two things: change temperature and change tension.
Speed control is closed-loop control, where the operator's hand or eyeball is the load cell. It does not matter that you have centerlines. It begs the question where the centerlines came from. They came from an operator who found a setting that worked, and the engineer copied it onto an SOP. Speed control cannot get started and cannot be maintained without the operator's interpretation of the web's tension. How does it make you feel to have your operator as a load cell? Do you think the process will be consistent or responsive?
Speed control is twitchy on stiff materials. Consider how close you need to get things in order for them to go into speed control and hold a steady tension for something like paper. Paper can break at a 1 percent strain. This means that a lightly tensioned web may be strained at something like a 0.1 percent strain. If you want to do even a passable job of control, you would want to hold variations to no more than 10 percent of a 0.1 percent strain. This means you would need to control speed and diameters to 1 part in 10,000. Not in our lifetime! So, how do we make draw control work on stiff materials? We use droop. Here, the motor is asked to follow a setting but will not do so if the web starts tugging on it too hard. In this case, we don't have speed control, nor do we have torque control but rather a hybrid. While we don't know where we are, if we ever did, the motor adds just enough forgiveness to make things run.
920/312-8466, drroisum@aol.com, www.roisum.com
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