Diffused lighting gives even illumination
Vision-inspection module uses diffuse lighting to fill in dark areas during defect inspection of plastic bottles.
Air Logic Power Systems Inc. (ALPS; Milwaukee, WI), a systems integrator, has designed and built an integrated leak-detection and vision-based seal-inspection system. Tony Proctor, ALPS machine-vision engineer, says the system inspects multilayer polypropylene bottles that contain two layers—one dark and one light. The system camera looks down on the necks of the bottles from above. The ALPS' customer required the inspection system to evaluate the seal-width and ovality (the degree to which the bottle opening deviates from a perfect circle) parameters of both layers.
"To meet system requirements, we needed lighting that could ultimately 'fill in' the dark areas without saturating the entire field of view," Proctor says. "The illuminator had to deliver even illumination across the inspection area by providing continuous diffuse lighting in the same optical path as the camera," he adds.
US Automation Inc. (Brookfield, WI), a distributor partner of RVSI Northeast Robotics (RVSI/NER; Weare, NH), supplies the inspection-system lenses and lighting elements. RVSI/NER manufactures the Nerlite SCDI-75-FO illuminator, which the company says is well suited for "moderately faceted and undulating specular surfaces," such as those presented by the plastic soft-drink bottles that the ALPS system inspects. The vision system looks for defects in the seal surface, threads, and support ring at the top of two-liter bottles (see Fig. 1). "It's difficult to illuminate all of these items with the same intensity," says Dean Mouradian, US Automation president.
The ALPS vision-based inspection system is integrated with the company's Model 7385 high-speed, rotary leak-inspection machine, which can be installed on virtually any style of conveyor. This vision system can inspect as many as 600 bottles per minute, although the customer's specifications in this application do not exceed 200 per minute. The basic rotary inspection machine offers bottle-specific handling, as well as provisions for vision cameras and lighting.
Various camera/lighting configurations can be mounted in four locations on the system: over the in-feed and exit-star wheels serving the conveyor and two locations on the rear of the system's turntable. The vision system uses a stand-alone control station, which can house as many as four PCs, a main computer, a color monitor, and an industrial keyboard (see Fig. 2).
The bottles are routed off the conveyor onto the in-feed star wheel by a programmable logic controller (PLC) that directs the bottle movements. They index around the wheel to the vision-inspection station, where a strobe triggers the SCDI illuminator and the overhead control camera to capture an image of the bottle's seal surface, threads, and support ring.
The camera looks down on the illuminator, which is mounted above the bottles being inspected. Light rays from a fiberoptic light source reflect off a beamsplitter in the SCDI unit directly off the bottle top at nearly 90° (see Fig. 3). In this manner, specular surfaces perpendicular to the camera appear to be illuminated. Surfaces at an angle to the camera appear to be dark because nonspecular surfaces absorb light rather than reflect it.
The camera, made by Sony Electronics Inc. Visual Imaging Products Div. (Park Ridge, NJ), is an XCH-R300 1/2-in., progressive-scan CCD unit that provides a resolution of 782 × 582 pixels. The camera-enclosure/vision module also includes the NER illuminator and a 25-mm lens from Tamron USA Inc. (Commack, NY).
The Nerlite unit and the X-Strobe 400 light from PerkinElmer Optoelectronics (Fremont, CA) are connected by a quad-fiberoptic bundle supplied by Schott-Fostec (Auburn, NY).
The input/output rack serving the vision module houses the processors, frame grabber, and image-processing software. The elements in this rack communicate with the vision module, the overall ALPS 7385 rotary leak-inspection machine, and the PLC over a PCI bus connection (see Fig. 4).
An Intel Pentium 4-based rack-mounted PC running Windows 2000 is dedicated to image acquisition and processing. Images captured by the camera are fed to a Meteor-II frame grabber from Matrox Imaging (Dorval, QC, Canada). The image-processing software is Matrox Imaging Library version 7.0.
ALPS' Proctor says the company's second-generation vision system includes a main computer for graphical-user-interface control and for communication to the ALPS 7385 rotary leak-inspection machine and at least one PC dedicated strictly to image-acquisition and processing. "This setup results in faster throughput and enables the customer to make changes to product files on-the-fly [in the event a different bottle is to be inspected] because this is a continuous-motion inspection," he adds.
"We went with PCs and frame grabbers instead of PCs and vision processors for two reasons: the cost of vision processors is higher than frame grabbers, and advances in processing speed are usually reflected in PCs sooner than in vision processors," Proctor explains.
Seal inspection, consisting of looking at the bottle's seal surface, threads, and support ring, takes place at the in-feed star wheel of the rotary leak-inspection system, which is located in-line after the blow-molding or injection-molding equipment that forms the bottles. Based on pass/fail results, bottles are either rejected at the out-feed star wheel, where the bottle's base is also inspected, or sent down the line, where they are either stored and shipped for filling later or sent directly to the another part of the facility for immediate filling.
Indexing of the star wheels is driven by the Model 505 PLC from Rockwell Automation/Allen Bradley (Milwaukee, WI), which controls the overall rotary leak-inspection system. An FX301 digital fiber sensor from SunX Sensors USA (Des Moines, IA) triggers the Nerlite illuminator and camera functions on-the-fly in conjunction with the strobe's operation.
Pass/fail decisions are based on a comparison of a bottle's seal width and ovality, with the tolerances for those criteria established by the ALPS customer for that bottle; these parameters are stored in the seal-inspection PC. "By the time bottles reach the out-feed star wheel, the pass/fail results have been delivered to the PLC, and failed bottles are automatically rejected by an air jet," Proctor says.
He comments that because the vision-inspection module was added to the existing rotary leak-inspection machine, there were no additional mechanical-handling costs. Only some extra floor space was required; the vision stand measures 2 × 3 × 5.5 ft. ALPS has been granted a patent for its integrated leak-detection and vision-based inspection system.
Proctor stresses that the integrated system offers more than seal width and ovality inspections. Its graphical user interface displays screen shots that show parameters such as outside and inside diameter, width, and inside and outside nicks (see Fig. 5). The system also save images for later evaluation, fine-tuning, or troubleshooting.