Unwinders and winders are used in almost all web-handling industries including paper, plastics, converting, ferrous and non-ferrous metals, wire, printing, and textiles. The winder is often the limiting factor in machine throughput when continuous winders and unwinders are not used. Moreover, close winder control is essential to quality production.

This article deals with the most common type — dancer-controlled, speedregulated center winders — and examines the difficulties of winder control and the strategies used to wind and unwind various web materials.

Need for close control

In general, the problems associated with winder control include obvious physical deformities such as crushed cores, air gaps, starring (caused by tension fluctuations), telescoping, and necking, Figure 1. There are also less obvious winder-related problems, such as scratches produced by relative movement within the roll (cinching, Figure 1) and web breaks. These are frequently caused by compression forces in a wound roll, which can crush some materials causing fractures. Web breaks become apparent on the machine that uses the roll, not on the machine that produces the roll.

Types of winders

Winders predominately fall into two broad categories: center winders and surface winders. In addition, there are winders that combine the characteristics of these two types. An example of a combination winder is a speed-regulated surface winder with a torque-controlled centerwind used to wind slippery materials.

Control considerations for a center unwinder are similar to those for a centerdriven winder. Therefore, almost all of the winder information in this article is also applicable to unwinders.

In a center winder, a motor drives the core or shaft of the roll being wound. Although the motor drive can use any technology, ac drives and dc drives are most commonly used on large winders. On smaller winders, servo drives can be used.

Center winders are usually controlled either in a speed mode or in a torque mode. When controlled in a speed mode, a speed reference is provided to the drive by the winder control. In such speed-regulated systems, the speed regulator commands torque in the motor.

In torque-regulated systems, a speed regulator is not used. The motor will run at the highest speed it can reach with the available torque.

Winder control system challenges

Winders are often the most difficult part of a machine to control, because both the diameter and inertia of the wound roll are continually changing.

Roll diameter. Consider two different times during the winding of a single roll, Figure 2. Assume, at both times, the motor is turning at 100 rpm. While the roll has a 10-in. diameter, the machine is winding at 261.8 fpm. However, the 50-in. roll is winding at 1,309 fpm. If you increase motor speed by one rpm, the 10-in. roll winds 2.6 fpm faster, but material on the 50-in. roll is wound 13.09 fpm faster. Thus, the same speed-reference command change causes different web-speed changes depending on roll diameter. This difference— a system gain change—can complicate the control strategy in speed-controlled winder applications.

Roll inertia. A second complicating factor is roll inertia. For example, a 3.5- in. aluminum shaft weighing 60 lb has a moment of inertia of 2.5 lb-ft2. If 50 in. of paper is wound on this shaft and the roll weighs 2,500 lb, its inertia is over 21,000 lb-ft2. This is an inertia change of over 8,000 to 1. Such inertia changes affect the tuning of speed-controlled winders, the inertia compensation of torque-controlled winders, and the web-tension regulation capability of brake-controlled unwinds.

The effect of inertia on speed-controlled winders can be considerable. For example, an empty spindle on a winder, if the drive is tuned properly, will closely follow commanded speed changes. However, a large-diameter roll on the winder, tuned at empty core, will be unstable and will not closely follow commanded speed changes. In fact, the roll may be wildly unstable and may rotate in one direction, and then in the other, even if zero speed is commanded. This instability results because the optimal integral gain at the core is too fast for the high-inertia load to follow.

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