Machinedesign 2137 Digital Control1198 0 0


Nov. 1, 2000
Chronic trouble spots in a packaging line used to be a real drag on efficiency and speed, but not anymore. Electronic gearing makes it easier to unplug these clogs and get the line moving at a faster pace.

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.

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.

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Speed and synchronization

Conventional packaging machines are often operated slowly to prevent the inevitable bottlenecks at higher speeds. Digital controls on the other hand, precisely synchronize all drives (at various speeds) on multi-axis equipment, speeding up the weak-link portion and the entire packaging line.

For example, products such as candy bars have a low per-unit value, making high production volume paramount to profitability. De-bottlenecking the line kicks it into high gear and pushes down the unit cost. One prominent confectionery manufacturer removed the gears and chains from a packaging line and replaced them with electronic gearing. The equipment now runs at more than twice the previous speed (from 350 to 750 packages per minute), with more rapid changeovers.

Advanced registration control

Digital controllers allow high-speed registration, triggered from printed registration marks or the edge of the product. This data is used to automatically correct even minute position errors, thereby precisely positioning products as they feed into a machine.

In a bottle labeler, for example, an improperly aligned label on a bottle wastes time and money. With digital control, a photocell detects the position of a label's printed registration mark and reports this information to the servo drive, which adjusts motion on the fly. The system captures the position in 1 msec and calculates a correction in 2 msec. There is no need to slow the machine, let alone stop it. If the process is so far out of tolerance that a correction can't be made, the system issues a signal to reject that product downstream.

Some controllers use absolute feedback devices on the servomotors to automatically track all axis positions. If the machine is stopped to clear a jam, absolute feedback eliminates the need to re-home and re-synchronize the system.

Electronic camming

A mechanical cam converts rotation into a non-circular motion profile, adjusting the speed over time for a specific movement. This type of cam is subject to errors due to mechanical wear, and it must be cut to a particular shape, locking it in to a single motion profile. Changing the profile means replacing the cam.

With a digital system, a software cam table defines non-linear motion mathematically. Cam tables of up to 1,024 points can be set up on a computer and downloaded into a servo drive. And they are easily adjusted via programming changes. Digital control systems that incorporate these tables in the drive, rather than in the master controller, execute motion profiles faster because there is less processing delay.

Software camming handles multiple product sizes economically on a single packaging line. For instance, when you select a shorter package length, a servo drives a rotary knife at higher speed to bring it into position faster, then matches its speed with the registration mark for a cut. This allows a single cutting cylinder to handle various cut lengths without replacing mechanical parts or sensors, eliminating yet another bottleneck.

Electronic gearing

Machines often incorporate mechanical drive trains to run axes at given speed ratios with respect to the main drive. Inexpensive drive trains are generally less accurate; whereas more accurate versions drive up the cost. And both types are subject to wear-related inaccuracies – a potential source of bottlenecks.

Electronic gearing lets a motor run at a given ratio with respect to the machine speed, maintaining synchronization and eliminating the inaccuracies and backlash inherent in mechanical drive trains.

One common bottleneck in cartoners occurs in trying to time rotary feeders to carton transfer chains. Two axes may not be correctly synchronized, with position off a fraction of an inch. This causes frequent carton misfeeds, creating stoppage on packaging lines with conventional controls. With electronic gearing, the motor's high-resolution feedback device (encoder) and the drive's high-speed position control loop are used to automatically correct even minute position errors, maintaining proper timing between material handling sections of the cartoner.

More pluses

Electronically geared control systems make machine operation easier in other ways, offering improved upgradeability, flexibility, easier installation, and reduced spares inventory.

Upgradeability. Packaging machines can be designed and assembled as individual modules, making it easier to upgrade the line. For example, you can add a spacing or gapping conveyor for improved product spacing next year when the capital budget allows. The additional servo axes easily plug into the fiber optic communications ring, without the rewiring involved with other control methods.

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Flexibility. You can easily and quickly reprogram a multi-axis machine to handle frequent changes in product packaging without physically rearranging any hardware. One prominent manufacturer reports that it can switch a line from wrapping individual products to multi-unit packages in only 5 minutes. This boosts machine output, and in some cases could even eliminate the cost of an additional dedicated machine.

Installation. Flexible packaging controls have PC-based open-architecture. Such an open system can incorporate an industrial PC; a standard I/O bus such as Interbus, Profibus or DeviceNet; a motion bus such as Sercos; and packaging software with the look and feel of Windows applications. PC cards for motion control, logic, and I/O plug into the system backplane, all integrated into a single unit.

Intelligent digital drives with builtin signal processors can handle many of the master controller's traditional functions – closing position, velocity, and torque loops. In many cases, the controller executes motion and logic functions, eliminating separate PLCs.

For example, a traditional servo control package with electronic gearing for a 10-axis high-speed cartoner might consist of three rack-mounted 4-axis controllers or 10 single-axis controllers, all synchronized by a logic controller. Extensive wiring and programming would be necessary to smoothly sequence and coordinate all of the controllers.

With intelligent drives operating over digital networks, one control processor with integrated logic could control the 10-axis cartoner. The processor communicates through a fiber-optic ring, daisy chained to the drives for all 10 axes. This reduces the time and cost of integration and wiring. Accuracy is high because the same processor clock synchronizes all axes, and the fiber-optic network is immune to electrical noise. One packaging machine builder says the start-up for such a machine usually takes just 2 days, compared to 1 or 1½ weeks for a conventional cartoner.

Reduced spares inventory. An electronically geared system minimizes the required inventory of controller and drive system components. For example, some servomotors have a chip that tells the servo drive what type of motor it's talking to, so that any drive from the same company can run any type or size of their motors. This lets you replace a failed servomotor with a different size unit from stock, and still operate within safe parameters of the drive to maintain production.

Communication networks

In many cases, an electronically geared system relies on fiber-optic communication using a Sercos network interface (SErial Realtime COmmunications System). Such networks are a noise-immune digital alternative to the ±10 V drive command lines and position feedback cables used with analog drives.

Sercos communicates over a fiberoptic cable, relaying operating parameters, status reports, and other data to and from the host controller to achieve accurate high-speed registration. Operators can acquire instant diagnostic information for every axis of the packaging line, as well as troubleshooting procedures recommended by the controller to speed repairs and minimize downtime.

The Sercos interface is an international standard (IEC 61491 & EN 61491) for communications between digital controls and drives. As an open standard, it ensures that drive components from different manufacturers, even those from different countries, work together harmoniously.

In a recent study, Servo Drive Outlook for North America, Automation Research Corp. says it is the most widely used digital interface standard for communications between drives and controllers for both CNC and general- purpose motion-control applications. The company predicts that its use will be more widespread in the next two to five years.

David Fisher is a packaging machinery engineer for the Indramat Div. of Mannesmann Rexroth Corp., Hoffman Estates, Ill.

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