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Article | May 31, 1999
PLS speeds changeover, boosts reliability for Lever
A programmable limit switch speeds changeover and extends capabilities of new hot melt applicator and rebuilt filler.
At Lever Ponds Toronto an effort is underway to modernize the plant's aging fleet of filling equipment for Lever's Sunlight brand powdered laundry detergent. One goal: replace mechanical components such as timing cams with specialized high-speed electronic controls known as programmable limit switches.
The term 'programmable limit switch' is a bit misleading. It's actually a specialized controller that's far more sophisticated than what's implied by the word 'switch'--in functionality as well as price. By "reading" the position of the main drive shaft and translating that into carton position these devices provide extremely precise package positioning information--more accurate than a PLC and sensors can provide on their own.
The PLSs are contributing to faster more repeatable changeover plus more reliable operation according to company officials. On one line a PLS has trimmed 30 minutes from a 90-minute hot melt applicator changeover and another 30 minutes from a 5-hour filler changeover. It has also reduced post-changeover trial-and-error adjustments on the adhesive applicators from 30 minutes down to as little as five.
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Among the first machines retrofitted in the Toronto plant was a rotary-style volumetric filler on Line 2 which fills 1- 2- and 3-kg boxes of powdered detergent. The filler was brought in from another Lever plant and rebuilt. It includes a Gemco programmable limit switch from Patriot Sensors & Controls (Clawson MI). The PLS is actually a module that plugs into a Rockwell Automation/Allen-Bradley (Milwaukee WI) PLC 5/40 also new. The PLC/PLS combo controls the filler as well as two new hot melt adhesive applicators from Slautterback (Monterey CA). A local integrator Sterner Automation (Toronto Ontario Canada) installed and programmed the PLS.
Easier for operators
The PLS control speeds changeover of the two hot melt applicators by allowing operators to enter the actual measurements--in millimeters--of where to start and stop the glue pattern across the length of the bottom and top carton flaps. The previous applicators also used PLSs but they were more limiting. Operators had to enter numbers in "degrees"--actually the package pitch (distance from the center of one package to the next) divided by 360. That made it hard to judge precisely where the glue pattern should occur resulting in some trial-and-error ramp-ups after a size changeover.
Now the PLS PLC and touchscreen display--a QuickPanel unit from Total Control Products (Melrose Park IL)--all work together to graphically depict the adhesive pattern that will be applied. That provides visual confirmation of what the glue pattern will be before the packaging line is started up.
The benefit: as much as 75% less time spent on ramp-ups to ensure proper adhesive application according to Dave Pearce Lever's reliability coordinator. "We're very much improved over what we had before" he says. "It's not hit or miss like in the past." If minor adjustments do need to be made an operator can make them on the fly. Before a mechanic would have to make such adjustments usually with the machine stopped.
Because this positioning information is stored and can be recalled electronically right from the touchscreen display hot melt applicator changeover has been reduced by a third from 90 minutes down to an hour according to Pearce.
Another benefit is tight integration of the control of the filler and adhesive applicator. Because the PLS is actually a module that plugs into the PLC the real-time position tracking capabilities of the PLS can be leveraged across both the filler and the gluer. That means the PLS is not only used for hot melt applicator control it's also used to enhance timing functions on the filler related to scoop dropping package rejects no-carton/no-scoop and no-carton/no-fill. "We've got a lot more capability with the filler with the Patriot unit communicating with our PLC" says Pearce.
The PLS has also permitted Lever to eliminate several mechanical components such as cams. Instead most of the timing settings are now stored and recalled electronically via the PLS.
"Operators don't have to unscrew this part and move that lever. They can actually change the timing on the touchscreen" explains Pearce.
That has trimmed about 30 minutes from the filler's previous five-hour changeover time. The nature of the PLS's accurate position tracking capabilities has also increased the accuracy of these functions compared to before according to Pearce.
And because the PLS is tightly integrated into the PLC itself connected to the operator interface both filler and hot melt applicator are changed from the same touchscreen operator interface. Previously operators had to make changes at a separate filler control panel and adhesive applicator control panel.
Such integration makes programming easier too according to Christian Sterner of Sterner Automation. "We actually write a PLC program and that program in turn programs the PLS" he says. This eliminates the need to learn another programming language or to write separate programs that must communicate with one another. It also reduces engineering time.
The previous hot melt applicators also had a programmable limit switch. However that PLS was an external unit dedicated solely to the applicators. "The PLS had no communication with the main PLC that controlled the fillers so we couldn't use the PLS to do anything else [aside from hot melt control]" says Pearce. Although many glue settings could be recalled electronically it was a more cumbersome process according to Pearce and wasn't as streamlined as it is now.
The hot melt adhesive applicators have also reduced glue consumption though Pearce attributes that to the fact that the applicators are electrically fired via solenoids which are more precise than the older pneumatic applicators.
How the PLS works
The PLS works in concert with a resolver (also from Patriot) a mechanism that "reads" the rotary position of the filler's main drive shaft. The shaft position is then used by the PLC to compute the location of a given box of detergent as it moves through the filler.
Because a PLS scans the resolver constantly--every 250 microseconds--the PLS can compute the exact location of a given carton. A typical PLC by contrast might scan sensors and prox switches every 10 to 30 milliseconds--which makes the PLS anywhere from 40 to 120 times faster and thus more accurate.
Economics are justified
The complete PLS package which includes the unit itself the resolver cables and I/O board cost C$9 (about US$6). The cost to install and program the PLS couldn't be broken out separately for Packaging World because the integrator performed other work as part of the modernization project.
While that amount is far less than a new machine it's still a significant sum. Sterner Automation's Christian Sterner admits it wasn't possible to recommend a PLS to Lever just for the hot melt applicator alone.
"I had the idea a long time ago to unify the controls by using one PLS for sealer control and filler control" he says. "But there was never a significant-enough project to justify doing the PLS. When the new applicator project came along that lent itself to a PLS and those two things together justified the PLS."
Pearce confirms that Lever didn't start out with the idea of buying a PLS.
"The major consideration for us was purchasing a new box sealing system" he says. "Then we had to decide how to control that hot melt application. By using the Patriot's resolver and PLS to do that we opened the door to a lot more functionality on both the filler and hot melt applicator in the future."
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