Automation technology has progressed exponentially in a short time as packaging machine builders adopt digital motion control technology. Digital electronic accuracy is true accuracy, unlike conventional machine calibration which often came down to how much error an operator was willing to tolerate. Here, trial-and-error adjustments frequently led to compromise mid-range operating parameters that inevitably drifted as the machine picked up speed. Digital control, though, achieves perfect axis coordination and repeatable performance, even while operating at the fastest available drive speed.
Knowing that accuracy won’t be sacrificed, production can run machines at their maximum possible speed. This benefit is most dramatic when old machines are retrofit with electronic control. A Hershey Chocolate North America plant, for example, had the gears and chains gutted from one packaging line, then retrofit the frames and mechanics with digital ac servo drives linked through a Sercos network to a multi-axis control card. Now the line wraps Reese’s Crunchy Cookie Cup candy bars at more than double the speed with greater uptime and fast changeover. “When refurbishing older machines, it’s not uncommon for a mechanical-to-digital conversion to increase flow wrapping production from 350 to 750 candy bars per minute,” says Wade Latz, manager of packaging systems engineering at Hershey.
Digital-based devices are easy to change and upgrade. Often, installing a new chip or new software code is all that’s needed. And it doesn’t come at the expense of axis count, wasted hardware, speed, accuracy, or uptime. In short, digital control can optimize packaging production for every level of machine complexity — from single-axis applications, to simple sequenced multi-axis packaging, to highly complex lines with multiple coordinated motion functions.
Because early digital controls and drives provided high capability at high cost, single-axis packaging systems usually stayed loyal to the economy of analog control. When industry first offered less expensive digital drives, proprietary interfaces limited users’ options to upgrade or use the drives in a larger system.
The latest group of open digital servo drive systems, however, integrate multiple functions — such as motion control, amplifier, power supply, and axis inputoutput (IO) tasks — into a single control unit that’s adaptable to new applications. These drives are becoming common on simple point-to-point applications like auger fillers, single-axis sweeps, and random infeeders.
Sequenced multi-axis motion
The demand for accuracy at high speeds inspired a new generation of digital controls that need minimial integration to precisely sequence multiple motions. Such controls are so flexible that it no longer makes sense to purchase packaging machines for immediate needs without serious consideration of future needs.
The most flexible packaging controls are PC-based with an open-architecture. Various cards or printed-circuit boards for multiple axes of motion control, logic, and IO plug into the control backplane, integrating all into one unit. Such tightly integrated controls frequently use intelligent digital drives with internal digital signal processors. The drives handle much of the master control’s traditional functions — closing the position, velocity, and torque loops. In many cases, the integrated PC-based control can execute motion and logic functions without separate PLC hardware and the associated costs.
Consider the benefits of a motion controller and intelligent digital drives for a typical 10-axis high-speed cartoner. Previous systems might consist of three proprietary 4-axis controls, or 10 single-axis controls, sequenced by an external logic control. This scheme requires extensive wiring and programming to smoothly sequence multiple motion controls with separate processors, and to coordinate them with a separate logic program.
With digital technology, that same 10- axis cartoner is controlled by one motion control processor with integrated logic. It communicates in real time along a fiber-optic ring, in a daisy-chained network between all 10 axis drives. The packager purchases only one control, significantly reducing the time and cost of integration and wiring. In addition, such a system is flexible enough for new programs and process changes. Accuracy is high because all axes work on the same processor clock, and the fiber-optic network is noise immune. One packaging machine builder, Klockner-Bartelt, Sarasota, Fla., says that start-up for such a cartoner usually takes just two days.
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Complex, synchronized multi-axis packaging
Another advantage of digital technology — packaging operations that require synchronization of multiple axes with varying motion profiles can now use “shaftless” machines. These machines consist of modular standalone sections, each powered by its own digital drive. The stiffness of the drive means it won’t respond to mechanical disturbances and vibrations in the modular sections. In addition, replacing the mechanical drive train with electronics eliminates transmission of mechanically-coupled disturbances from one section of the machine to another.
Connected to a master control by a fiber-optic network, the sections of such machines are independently controlled for velocity, position, or torque. Electronic line shafting (ELS) software controls each motion axis, electronically synchronizing all simultaneous operations.
Because ELS-controlled packaging machines are designed, assembled, and tested as individual modules, machine upgrades are easily incorporated into a line. Modular layouts let repair personnel easily access and isolate any section that requires maintenance. Using open communication buses, software problems can be corrected remotely — from the master control, another area of the plant, or a different part of the world. ELS re-synchronizes all operations when the machine is restarted.
This type of software permits a high order of programmed machine control for:
Advanced registration control. Triggered from registration marks or the material edge, electronic registration simulates the functions of a backlash-free gearbox, and continuously adjusts positioning between the master axis and the digital drives to compensate for even minute position errors. For example, this capability precisely gaps products on a spacing conveyor, or registers material to a rotary knife, while eliminating the need for costly precision gear sets.
Electronic camming. This function is a necessity for high-speed packaging machines — synchronizing rotary knives or pusher axes, and slashing setup time and product changeovers to minutes. Electronic cams mathematically define nonlinear motion in a software cam table. Resolution can be as high as 1,024 points per revolution. Systems with cam tables in the digital drives, rather than in the control, can execute non-linear motion even faster because there’s less processing delay. This arrangement also frees the control for other activities.
Kinematic positioning. Modular kinematic robots are replacing pick-andplace robots on high-speed cartoners, case packers, and palletizers. Some digital controls incorporate mathbased kinematic formulas for complex mechanisms, such as 4-bar linkages. These robots simutaneously coordinate the movement of multiple moving axes in perfect, smooth synchronization. Even delicate products, such as cookies and crackers, can be quickly inserted into trays without breaking.
Absolute feedback. As packaging machines handle more complex tasks, axis interaction can require more homing routines. Absolute feedback built into some controls frees the user from this task because it provides automatically directed electronic homing memory. This feature also minimizes scrap during a jam — as the jam clears, the drive need not be re-referenced to home position.
Sercos networks let packaging machines take advantage of high-performance digital drives. They are a noiseimmune alternative to the ±10-V lines and sensitive position feedback cables used with analog drives.
Sercos communicates in real time along a fiber-optic cable, relaying operating parameters, diagnostics, status reports and other data to and from the host control. Operators can acquire instant diagnostic information for every axis motor and drive at every section of the packaging line. Troubleshooting procedures recommended by the controller speed repairs and minimize downtime.
Hershey Chocolate North America was one of the first to recognize the potential of Sercos. Some years ago, Hershey’s Latz and Keith Campbell, manager of automation and integration, led their company to standardize on it, and then urged packaging industry suppliers to develop compliant products. Hershey now has nearly 60 Sercos-based packaging applications throughout its 20 plus plants.
“We have long objected to the limitations imposed by proprietary, black-box control technology,” says Campbell. “With the open architecture, standard Sercos network as our key to enterprise-wide packaging flexibility, we can focus on open-system control technology, rather than on companies. And because the standard reaches across all applications, highspeed wrappers, baggers, and multi-axis cartoners all use the same technology, reducing training and maintenance in the plant while holding down costs.”
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Standards-based digital control lets users easily and quickly change packaging parameters. Using controls and drives from various manufacturers, Hershey, for example, can change a line from wrapping single-cup Reese’s packages to producing multi-cup packages in only 5 minutes.
In a typical open-architecture environment, an operator calls up a Windowsbased program and changes the motion profile by clicking on icons to indicate new end-product length, style, pattern, and size.
This versatility lets manufacturers routinely run small quantities to meet just-in-time delivery requirements. And the operator doesn’t need to shut down the machine to decouple some mechanics, connect others, change gears and manually set ratios to set up packaging for the next product.
Digital technology provides the flexibility to allow a multi-axis machine to handle frequent changes in product packaging — sometimes while the machine is still running — without any physical rearranging of machinery. Not only does this increase machine utilization, it saves floor space and costs by eliminating the need for other, more dedicated machines or offline processes.
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Robert P. Brennan is vice president of industrial automation technology for the Indramat Div. of The Rexroth Corp., Hoffman Estates, Ill.