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 8½ × 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.
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.
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.
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 under-speeding 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.
To maintain proper tension as web diameters change, motion controllers on converting equipment continually update the electronic gear ratio between the unwind spindle and pull roll. This calls for a controller that can dynamically change electronic gear ratios on-the-fly, adaptively increasing (or decreasing) servomotor speed. The better the controller does this, the better the converting machine will work.
Encoders and web dancers provide much of the feedback on converting equipment, but a few more strategically placed sensors can make the controller's job easier and its actions more effective. Adding a dancer assembly with an analog output, for example, lets machines run faster and more accurately because it provides direct tension feedback. Here, the feedback from the dancer roll is brought into the controller where it factors into the dynamic electronic gearing algorithm.
Another technique is to monitor the current drawn by the servomotors that drive the unwind and pull rolls. As web tension varies, so does the torque (and current) applied to the servomotor. Like direct tension measurements, current feedback signals can be brought into the controller, where they serve as an additional input in the electronic gearing algorithm. Current monitoring may be used alone or together with direct tension feedback for better results.
The second tension zone on the plastic-bag making machine lies between the two sets of pull rolls. Tension is typically maintained across this zone using a combination of electronic gearing, a web dancer, and servodrive current monitoring.
Usually, one pull-roll axis or the other is designated as the master, and all other axes within the zone adjust relative to the master to maintain the desired tension. Designating a single master prevents the various axes from trying to dominate each other, which usually results in erratic tensions around the zone.
Converting operations that occur in zone two include folding the film with mechanical plows, heat sealing the seams and, depending on the application, perforating the film by scoring the plastic with perforation cylinders or wheels. Perforation imparts operational qualities to the finished bag such as mounting tabs and breakouts. The success of all these mechanical operations hinges on maintaining proper servo-axis coordination and correct web tension.
Printing is yet another operation performed in zone two, between the two sets of pull rolls. Naturally, it requires additional components — print cylinders and servomotors — though they're controlled in a manner similar to tension-control motors, using electronic gearing techniques. One difference, however, is that the controller must apply position offsets to the print cylinders to properly phase individual colors in multi-color images. Subsequent web operations, such as embossing or die cutting, may also require offsetting.
Another difference where printing is involved, weather the web is pre-printed or printed on the machine, is the need for registration to account for offset variations or web compliance issues such as stretch or slippage at the pull rolls. Registration corrects for such errors as well as most minor disturbances, accurately re-aligning print areas with the machine.
High-speed product registration requires an additional sensor, a photo eye, to detect print registration marks on the web. When the marks are detected, the slave's axis position is latched or captured in the motion controller. The difference in position between current and previous marks is used to calculate the distance between registration marks.
In the same sampling period, this measured distance is also compared to a theoretical distance. The resulting error signal is used to generate a correction factor that's applied to the slave axis to shift the web to the desired machine location. The intense math behind the correction process calls for a high-performance motion controller.
Cut and run
After the web passes through the second set of pull rolls, the conversion process enters its final stage, where the web is cut to the defined bag length. The cutting tool might be an electronically geared rotary knife or sequenced shear. Both offer programmable control over cut length.
Variable-length cutting makes it possible to run different products without hardware changeovers. The controller need only adjust the electronic gear ratio (for machines using knives) or the initiating sequence (on machines using shears). In either case, the operation requires correct web tension as well as precise product registration to align the web with the desired cut point.
With a little imagination, the concepts discussed here can be expanded to other web operations such as unwind, rewind, lamination, embossing, cut to length, and so on. What's more, many of the techniques described also apply to non-web machinery, with less emphasis (of course) on web tension and more on product positioning and registration.
For more information contact the author, Randal Bauer, at (866) 993-2624 or via e-mail at [email protected].