Digital control keeps this packaging line at Hershey Chocolate North America moving smoothly. It precisely spaces candy bars on a servo-driven infeed conveyor, and electronically synchronizes the conveyor with this flow wrapper. Six digital servo drives are controlled by a single multi-tasking control card, communicating over a Sercos interface.

A packaging line is only as productive as its weakest link – the one spot in the line where problems persist. To fix such hang-ups, stop the line, mechanically adjust the timing, start it up and watch the results. Not quite right? Stop the line again and tweak the timing a little more. By the time this trial-and-error calibration is good enough, you've scrapped products, wasted a lot of the operator's time, and logged some unimpressively low production numbers.

Losses are compounded when multiple axes come to a standstill, or if the peculiarities of a product – dairy goods that require refrigeration, or items that use fast-drying glue – don't allow long stoppages. What's worse, you'll have to tweak the line again as mechanical inaccuracies inevitably drift into another bottleneck.

Out of the bottle

Adjustable-speed drives, steppers, and resolver-based servos used to be good enough. But today's push for speed and accuracy demands tighter control of packaging lines. Digital controls with electronic gearing prevent the bottleneck burden by accurately executing packaging operations at high speeds.

For example, they can precisely synchronize complex multi-axis operations with varying motion profiles – as in flow wrappers and vertical form, fill, and seal (VFFS) machines and cartoners. Each axis has its own digital servo drive for precise velocity, position, or torque control. All drives are daisychained back to a master controller via a single fiber-optic cable.

Even in an accurately synchronized machine, wear in mechanical components eventually causes a change in axis position, with a servo axis not precisely following its command. In response, position feedback devices on all motors report such errors to the controller, which continuously signals all drives to compensate for the errors and to automatically re-synchronize. With changes executed in real time, there's no chance for bottlenecks.

More than a servo

An electronically geared packaging control system differs from a standard multi-axis servo system, which typically requires one controller for every three or four drives, plus extensive wiring and programming to integrate the multiple controllers. Today's electronic system controls 32 or more drives from a single device, such as a control card that plugs into the backplane of a PC or even into one of the digital drives.

Digital control systems provide electronic gearing for many high-speed packaging machines, including VFFS, random infeeders, cartoners, flow wrappers, case packers, and palletizers.

A digital control system consists of a motion controller that provides synchronization, a number of servo drives that control the position of their respective motors, and servomotors that provide machine motion. Such systems replace adjustable-speed motors, mechanical lineshafts, gears, chains, clutches, cams, and belts that synchronize motion in a mechanically driven machine. These mechanical components usually are the source of those costly bottlenecks.

A digital system may incorporate 8 to 15 servo axes into flow wrappers and cartoners. Each motor is independent of the others, but the motion controller synchronizes them all via a master clock signal.

The motion axes run from slow to very high speeds, such as 250 six-packs of beer or 1,000 candy bars per minute. Axes are synchronized to one common tolerance, as opposed to a mechanical design wherein errors due to tolerances are cumulative throughout the machine and tend to get worse with wear.

Some servomotors have a high-resolution feedback that provides 2,000,000 counts per revolution, and thus they can maintain synchronization within 1 arc minute, or 0.016 degrees, at any speed. The control system senses disturbances in the machine and instantly commands the appropriate response to maintain synchronization – ranging from speed adjustments on one or more axes to a controlled system shutdown.

Saving time

An electronically geared system provides anti-bottlenecking time and cost-saving features including speed and synchronization control, advanced registration control, electronic camming, and electronic gearing. These features make the system easier to install and provide more flexibility to accommodate product changes.

Continue on page 2