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Controls system simplifies automatic case sortation

In the central packaging area of the two-year-old snack plant built by Pretzels, Inc. in Bluffton, In, corrugated cases of bagged snacks are conveyed from packaging to palletizing on a single powered roller conveyor.
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FILED IN:  Controls  > Strategy
     

Once cases reach the palletizing area they must be channeled down one of 10 conveyor spurs so that operators building pallet loads can put each case on the correct pallet. The company executes this task of case sortation automatically with a system of 10 bar-code scanners in direct communication with a programmable logic controller across an industrial network.

Many of the components in the sortation system including those used in the DeviceNet network were supplied by Allen-Bradley (Milwaukee WI). The powered roller conveyors--one central conveyor about 35' long and 10 short spurs leading from it--came from Rapistan (Grand Rapids MI).

An Allen-Bradley industrial grade bar-code scanner is positioned at each conveyor spur. As a case nears the scanner its bar-code label is scanned and the information it contains is sent along the DeviceNet network to an Allen-Bradley 5/30 PLC. If the bar code identifies the case as one that should be diverted from the main conveyor and sent down that spur to a palletizing station the PLC sends an output--again through the network--that signals a pneumatic kickout device to push the case down the spur. If the case has the wrong bar code for that spur the case continues down the main conveyor.

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Assisting Pretzels Inc. in designing and installing the sortation system was V-Tech Eng. (Bluffton IN). According to Bo Alstoft V-Tech president getting the bar-code scanners to communicate with a PLC as they do at Pretzels Inc. is a fairly recent accomplishment in the world of packaging line controls.

"The communications protocol had always been a problem because bar-code scanners typically send messages in the form of ASCII characters" says Alstoft. Historically that has meant that somebody has to write a protocol translation so the data would be in a format that a PLC can use. Because the PLC and bar-code scanners communicate via a common network protocol no translation is necessary. The PLC then determines if the bar code that has been scanned requires a kickoff or not.

Because Pretzels Inc. is primarily a private-label co-packer serving an always-changing roster of customers new SKUs and bar codes to identify them are a constant fact of life. But according to maintenance manager Steve Kellogg "We can add a new bar code to the system any time we want right at the touchscreen panel." The panel he refers to is the Allen-Bradley RSView32 which serves as the human-machine interface for the entire sortation system.

Cases are palletized by hand and the pallets are then stretch wrapped by a Lantech (Louisville KY) unit. Operators use a Zebra (Vernon Hills IL) thermal-transfer printer to print a bar-code label for each pallet and manually apply the pressure-sensitive label to the pallet wrap. The forklift operator responsible for taking the pallet to the warehouse scans the bar code so that a computerized inventory control system can tell him to which warehouse slot the pallet should go. "It's a big improvement over having to hunt visually for a place to put the pallet" says Kellogg.

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