Pliant Corp is a producer of value-added films and flexible packaging products. Engineers at its Macedon N.Y. plant asked Omron IDM Controls (Houston, Tex.) to retrofit the drives on one of the plant's older six-color flexographic printing presses. Pliant uses the older press to develop new printed substrates made from different poly film constructions of varying thicknesses.

The existing dc drive system was unable to maintain tight control of web tension after each material change, which is the precision needed to produce high-performance, cost-effective packaging. Quality had to be maintained at minimal cost, so Pliant chose to retrofit the existing machine with ac drives rather than purchase a new press.

Design plans called for installation of an ac drive system at each of nine machine sections in which Pliant engineering installed high-performance Omron IDM G5 Series ac vector drives. Each drive is connected via a Profibus network to an existing Allen-Bradley SLC-5/04 PLC. An additional I/O chassis was included to accommodate the drive system.

Because the engineering team was working with existing automation hardware, software, and infrastructure, several integration challenges arose. Since the ac drives came with a Profibus interface, a Profibus-DP Master Card from SST (Waterloo, Ontario) had to be integrated with the Allen-Bradley PLC for communications to the drives. While this is not a difficult task from a programming standpoint, coordinating the Allen-Bradley PLC, Profibus DP Master Card, and Profibus parameters within the drives to work together in a seamless, efficient manner was labor intensive.

In the new configuration, Pliant's engineers integrated the drive control logic into the existing PLC program, redirected the old I/O points into the new logic, and connected the new drives over the network, greatly simplifying the wiring scheme and increasing network efficiency. In addition, tight clearances required that all new hardware and wiring be incorporated into existing cabinet space.

The technology applied in this system is most evident on the rewind end of the machine. As the printed film rewinds onto the spindle, the roll must turn slower as it increases in diameter to maintain the same surface speed as the line. The new winder control software calculates the diameter and regulates tension during this process. This function is critical to the quality of the final product. The ac drive's CASE (Custom Application Software Environment) capability was used to perform this function internal to the drive, thus saving implementation time and costs that Omron IDM Controls was able to pass along to Pliant. Additional time, money, and space savings were achieved using a common dc bus system to power the drives. This configuration uses a single dc power supply and a braking unit for all the drives, reducing the parts count and system wiring, and allowing the drives to share power. Benefits of the retrofit include a dramatic reduction in press downtime and a decrease in scrap due to precise web tensioning.

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