Wound-roll diameter: How do you measure it?
How do you measure wound-roll diameter?
By David R. Roisum, Technical Editor -- Converting Magazine, 2/1/2009 12:00:00 AM
Roll diameter is vital in commerce and in operation. For example, unwind-roll diameter is zero, which is needed to avoid runoff at the core when the operator is busy or inattentive. Here, we automatically shut down or go into a roll transfer on a preset diameter, usually a part of an inch above the core to avoid damaged material there. On the winder we have analogous diameter needs to automatically stop or transfer the winder at a preset shipping diameter or to avoid breaking the winder if the operator falls asleep. These simple systems will usually pay for themselves in a few months due to savings in waste, delay and labor.
Continuous diameter measurement is also essential for tension-control purposes on unwinds and windups. It's not possible to do fine tension control without what is called inertia compensation. Most people think that closing the tension loop on a dancer or load-cell trim signal with a proper PID (Proportional-Integral-Derivative) adjustment is all one needs. Nothing can be further from the truth. Good tension control is 98-99 percent calculation and only 1-2 percent trim. Calculating how much brake/more torque is needed during speed changes requires knowing how much inertia must be accelerated. This, then, requires knowing roll density, roll width and, you guessed it, roll diameter. Inertia compensation or gain scheduling is, surprisingly, also required at a steady speed because the winder/unwind has to change speeds ever so slightly to change tensions.
To these general needs, we add another need for the winder known as taper or roll structure. Here, the TNTs of winding are automatically varied as the roll builds. TNTs (Tension, Nip, Torque and possibly speed) of the winder are programmed as a function of roll diameter. In converting, this is often a simple linear function, but we shouldn't be so limited. We know, for example, that roll weight and swinging-arm compensation requires a curve. We know that the best "curve" to avoid the most common types of wound-roll telescoping defects is something like a slanted S or Z shape. In any case, roll-diameter measurement is essential.
You could measure roll diameter on some winders with a mechanical pointer on a scale. Just barely more evolved is a sensor arm that rides on the winding roll where the sensor may be a rotary pot. This is not very accurate—perhaps +/-0.1 in. at best. However, the big sin here is that the arm is in the operator's way. An improvement would be a non-contacting (ultrasonic) sensor. Here we get improved accuracy—perhaps +/-0.01 in., and the sensor is out of the way.
The gold standard of wound-roll measurement is, however, the two-tachometer method. First, we need to know web speed on one tach, pretty much a given these days. To this we add wound-roll rotation on another tach or even just a precise target. These methods can continuously measure roll diameter to the accuracy of a single wrap in commercial use and a fraction of a wrap in research. Here the operator can pick the specific unwind wrap on which to make an automated transfer. On the winder, the operator can make the transfer so that the unwind splice ends up on the outermost wrap facing him when the winding roll comes to a stop. This is the web-handling equivalent of NASA spacecraft threading the needle to reach a distant planet's orbit.
