Randal Bauer
Senior Application Engineer
Danaher Motion
Wood Dale, Ill.

In virtually every industry, in the process of manufacturing products, there is some type of conversion required to transform raw or unfinished materials into end product. It may be converting bulk rolls of paper into flat sheets, for example, or cutting and stacking sheet stock into smaller 81/2 x 11-in. reams.

Where there's converting, not surprisingly, there's also motion control. Motors and drives, under the watchful eye of sensors and controllers, play the lead role in converting operations,including web winding, die cutting, laminating, gluing, printing, embossing, perforating, and scoring. Better converting, as a result, begins with better motion control, and today's technology makes it easier than ever.

Prime example

Consider the motion involved in a web-processing machine that converts large rolls of plastic film into individual bags. A typical system of this type would include an unwind section to dispense the web, a web dancer to control tension, one or more pull and nip-roll sections to draw the web, a web-forming section to fold material to the desired shape, a heat bar, and a cutoff or knife.

The unwind section is a motion system in and of itself. At minimum, it includes servomotors, servo amplifiers, bearings, couplings, gearboxes, a motion controller, and an array of feedback devices such as encoders, resolvers, tension transducers, and linear potentiometers. The controller must be equipped to handle the required I/O signals and fast enough to calculate motion algorithms between sampling periods.

Because of the nature of the process, all servo drives in the unwinder must operate in synchrony, in a master/slave relationship, with slave axes electronically geared to a master encoder. In general, any axis that contacts the web must be synchronized to maintain proper web tension, and coordinated with other operations on the web. This calls for a centralized controller with networking capabilities and real-time processing speed.

Managing tension

Tension control is an essential part of any conversion process, especially web handling. Here, tension requirements depend on what sort of operation is taking place, and they usual vary from one section of the web to another. Reflecting that fact, most web-processing systems are divided into sections, or tension zones, each designed to maintain a constant tension.

Electronic gearing is a control technique that synchronizes the speed and position of two or more motion axes. As in mechanical gearing, one axis follows the other at a given ratio. electronic gear ratios, however, are not limited to constant or fixed values. They can just as easily be variables that adjust, for example, to changing roll diameters in web processing applications.

In the case of the unwinder on the plastic-bag making machine, the first tension zone includes the unwind section, dancer, and the first pull roll. Bulk rolls of plastic, typically weighing hundreds of pounds, are placed on the unwind spindle. A servomotor drives the heavy roll through a chucking mechanism, providing closed-loop control. As the roll unwinds and dispenses, the web snakes through the machine through a series of idler rollers and dancer arms and into the pull roll.

Servomotors on the pull roll are electronically geared to those on the unwind spindle, maintaining constant web tension throughout the zone. When tension must be adjusted — to suit the material — it is done by slightly over or underspeeding the unwind motor relative to the pull roll. Speed, however, is relative. As the web unwinds, the roll diameter decreases, dispensing less and less material to the pull roll, which, in turn, increases web tension. This variable geometry complicates the math and requires additional flexibility in the controller.