Pixel-counting sensor inspects bottles for labels

A full-service outsourcing company for prescription drugs, OTC drugs, and cosmetics, DPT Laboratories (San Antonio, Tex.) develops and manufactures creams, lotions, gels, ointments, suspensions, powders, and oral liquids. Recently, DPT conducted a quality improvement program to confirm that bottles processed at its plant had the proper labels on front and back.


A full-service outsourcing company for prescription drugs, OTC drugs, and cosmetics, DPT Laboratories (San Antonio, Tex.) develops and manufactures creams, lotions, gels, ointments, suspensions, powders, and oral liquids.

Recently, DPT conducted a quality improvement program to confirm that bottles processed at its plant had the proper labels on front and back. Previously, line workers visually checked each bottle for labels before packing. To automate and error-proof this process, DPT engineers began evaluating various machine vision systems and sensors.

The engineers found a quality vision system that repeatably detected label presence, area, center, and height of the bottle, but had a complex toolset not required for DPT's application. Besides its initial tens-of-thousands-of-dollars price tag, this system also had significant training and maintenance costs. The engineers decided the vision system was too expensive and had too many unneeded features.

DPT also evaluated a teachable, low-contrast fiber-optic sensor from Banner Engineering (Minneapolis, Minn.). It used a second photoelectric sensor as a "gate" sensor, which signaled the machine's PLC when it sensed the leading edge of each bottle. The PLC then triggered the fiber-optic sensor to inspect and determine whether it was receiving light reflected from a label—label present—or from a bottle—label missing. This sensor triggered a light and alarm when it did not sense a label. The initial cost of this system was less than the earlier vision system and its operation was much simpler.

If each of DPT's machines were used for only one type of bottle, each with the same label, the fiber-optic system would have been cost-effective and reliable. However, DPT packages more than 400 types of bottles of many shapes, sizes, colors, and materials. Also, the labels come in many shapes, sizes and colors. So, for each line change, the fiber-optic system would need to be physically repositioned to sense each new bottle size and shape, and the sensor amplifier would need to be recalibrated. Some bottle-and-label combinations would have required changing to another sensor amplifier to emit a different color beam—green, blue, red or white. These line-change complications made the fiber-optic system too time-consuming and costly.

Pixel counting

DPT began evaluating Banner's new PresencePlus pixel-counting sensor in March 2000 for its label-detection processes. Similar to the fiber-optic system, a photoelectric gate sensor detects the leading edge of bottles and triggers inspections. Because this sensor scans a 1.3- x 1.0-in. area, line changes could be enabled without mechanical repositioning of the sensor.

PresencePlus captures a 512 x 384-pixel grayscale image of the bottles and converts it to black and white pixels—depending on a user-adjusted threshold; counts the pixels; and renders a "pass" or "fail" judgement. When a bottle is missing one or more labels, the PresencePlus activates a warning light and sounds an alarm.

DPT now uses more than 20 of these sensors to inspect bottles for label presence. Production lines run continuously, three eight-hour shifts per day. As many as four times each shift, the line is changed over to another label-and-bottle combination. DPT has more than 40 technicians making these line changes. Part of their line-change routine is to run several bottles with labels and several bottles without labels to "teach" the PresencePlus sensor which bottles to pass or fail.

While available, hand-held teaching pendants aren't needed for sensor operation, DPT's frequent product changes and its many technicians allow the pendants to remain connected. A security code allows only authorized technicians to teach the sensors new run-time parameters. While the line is running, the teaching pendants are configured to continually display a "Run Judge" screen that dynamically shows current condensed images and numeric pixel counts with the pass or fail percentage.

DPT is so pleased with the practicality and reliability of the PresencePlus sensors that it is evaluating additional applications, including inspections of date code printing.

Jeff Schmitz, corporate business manager for vision systems, Banner Engineering Corp, Minneapolis, Minn.

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