A Canadian manufacturer of composites used in construction materials, tires and other products moves work-in-process around its facility in carriers on a power and free conveyor system. Each carrier on the conveyor system has a tag with a 1-D barcode used to track which operations have been performed on its contents and direct it accordingly to the next station.
Tags can be challenging to read because the carriers go through ovens where they pick up soot, are sprayed with water and often dented in collisions. The laser scanners that read these tags had read rates averaging 86%. Operators had to stop the conveyor system about 20 times per shift for an average of 10 minutes to fix problems by cleaning the tags or clearing collisions.
Critical Material Handling
The carriers pick up composite filament, called balls of yarn, in the forming department, and move to a washing station where the filament is sprayed with water. Then the carrier goes through a drying oven. The next stop is the fabrication area where the filament is unloaded by operators for further processing. Finally, the carriers are directed back to the fabrication area to pick up more filament. During operation, the carriers reach each of these stations through spurs off a main conveyor loop. The barcode tags on the carriers are read at the entrance to each spur to determine whether the carrier should be directed to the spur or continue on the main line. If the read fails, then by default the carriers continue on the main line until an operator cleans the tags or manually directs them to the correct station.
"At the time, opto-mechanical laser scanners were used to read the tags because of their simplicity, low cost and ease of use," says Mike Marges, application engineer for Shelley Automation, a distributor of automation and control products and services based in Toronto. "However, this application presented particular challenges for laser scanners, such as the potential for codes to be in a degraded condition (Figure 1), and at varying angles and in different positions. Because they see only a single scan line at a time, laser scanners have a very-limited ability to read poorly printed, degraded, damaged or improperly presented codes." Consequently, says Marges, the manufacturer reported that the number of misreads for the system as a whole averaged about 60,000 per month.
If the scanner can't read the tag, then the carrier continues looping around the conveyor system, not entering any of the stations. "The conveyor tracks were largely filled with carriers with unreadable tags, generating wear and tear, and consuming power without adding any value," Marges states. "The conveyor system is powered by many chains that needed to be replaced about every two years at a cost of about $25,000 each. The high traffic volume also increased the frequency of events, such as carrier collisions that made it necessary to shut down and manually clear the conveyor system. To minimize these problems, operators were required to regularly clean off the tags so they could be read or manually direct the carriers to the correct station."
A Move to Image-Based Code Readers
Three years ago, the composites manufacturer began to transition from laser scanners to image-based barcode readers seeking higher reader rates, lower maintenance costs and process improvements. The manufacturer worked with Shelley Automation on this project.
"Image-based barcode readers can significantly improve read rates on damaged or degraded barcodes," Marges explains. "Instead of relying on a photocell to monitor the reflections of a single laser beam as it passes across the barcode to generate an electrical signal, image-based readers capture an image, and then use a series of algorithms to make it easier to read. A typical algorithm searches the entire image for the code and identifies the position and orientation of the code for easy reading. Other algorithms handle degradations in code quality, such as damage, reflections and quiet zone violations. A quiet zone is a zone around a code that separates the code from the background. Ideally you want this to be clear of any features that would reduce contrast and impede locating the code."
In contrast to laser scanners, which can demand frequent repair and replacement because they have moving parts to actuate the laser beam across the bar code, Marges says image-based readers are solid-state and don't wear out. As a result, image-based barcode readers provide two to three times the life of laser scanners because they don't rely on a motor-driven oscillating mirror that's subject to failure.
Imaged-based readers make it possible to save "no-read" images for later review. The ability to analyze "no-reads" through image-archiving provides the opportunity for continuous improvement. "Reviewing saved images of parts whose barcodes were not read will generally reveal if the problem resides within the reader setup or the process," Marges explains. "For example, was the tag missing? Was the tag dirty? Was the tag tilted? With visualization, the root issue can be addressed in a continuous process improvement program. Visualization also makes setup easier." Marges says an operator can use a monitor to view live images of what the reader is pointed at. The operator can make sure that the barcode is visible in the camera's field of view, see how crisp the image is, and know immediately how well the reader is performing.