Case Studies: Backlit Conveyor Speeds Visual Gaging of Parts
January 2, 2008
A custom integrator of turnkey automated inspection machines for quality control, Cincinnati Automation (Erlanger, KY) “saw the light,” adopting a new LED backlit conveyor to convert automotive part gaging from a step-by-step to continuous process, more than tripling throughput.
The AccuVision conveyor from Conveyor Technologies Ltd. (Milford, OH) has a grid of high-intensity LED lights beaming up through a translucent urethane conveyor belt. “The backlit conveyor let us change gaging to a continuous operation for faster processing, and with less complex systems engineering,” says Phillip Smith, Cincinnati Automation president.
Cincinnati Automation specializes in custom vision inspection and precision laser measurement solutions for automated quality control. The company gets approximately 50% of its work from the automotive industry, typically Tier 2 and 3. After automotive, its key customer industries come from medical device manufacturing, also strong regionally, followed by packaging and label inspection.
“For gaging operations, backlighting is positively preferred,” Smith notes. “You get a silhouette of the part with crisp edges, sharp contrast and no shadows for precise dimensional inspection against the programmed master.” In the past Cincinnati Automation processed the parts on a stationary backlight table, he says. “To gage overall geometry and edge geometry, which we’re doing here, we’d use a pick-and-place system for part transfer and a walking beam to advance parts to the first station for top inspection and on to the second station for side inspection,” he explains. “We’d have to move parts to the inspection station, then move them off, so doing backlighting on the conveyor itself is a great advantage in speed and simplicity.”
Pick-and-place backlit gaging was limited to about 10 parts per minute, compared to 36 a minute for the new continuous conveyor system. And, that higher processing speed is governed by the speed of the part loading system, Smith notes. He believes a backlit system could easily handle parts at 60 to 120 a minute, depending on part size and configuration and faster loading capabilities. “A high-speed vision system can process a part in less than 100 milliseconds, so theoretically a system could process up to 600 parts a minute, provided you get the parts on the belt that fast,” he adds.
The continuous gaging system is configured to run 40 different part numbers for the automotive supplier, ranging from 5 inches up to 8 inches outside diameter. Robot grippers load the parts three at a time on the 66 inch by 12 inch translucent belt. Parts advance along the belt to the LED light field, where each is visually gaged by an overhead Keyence digital camera with an 8.5-inch field of view. At the end of the conveyor, a second Keyence camera with a 0.5 inch field of view gages the side geometry, catching the lead edge just as it extends beyond the conveyor. Parts then pass off the conveyor to an incline slide with a diverter flap. Failed parts drop through a slot in the incline. The flap lowers for good parts, covering the slot and allowing parts to pass down to the next station.
Delivering 2 mega-pixel imaging, the Keyence cameras in the Cincinnati Automation system provide 0.006-inch pixel resolution for overall part gaging and 0.0003-inch pixel resolution for edge gaging, based on the respective fields of view. The machine control shows images of each part being gaged, with both views tiled on the screen at the same time along with read-out results. Gaging data can be recorded as a text file to the system’s PC for SPC quality control.
Integrating backlighting with the conveyor enabled a highly compact design, with the entire machine measuring just 96 inches long by 36 inches wide by 70 inches tall. “This is a very space-efficient, lean solution for plants,” Smith stresses.
The conveyor is mounted 55 inches off the floor, giving plenty of space for stacking parts for conveyor loading and for a scrap bin inside the machine to catch rejects. The bed of the Conveyor Technologies conveyor is 2.28 inches deep, even with the LED light module mounted inside, for space savings and application versatility.
“Backlit conveyors are still relatively new, but we were fortunate in finding a source almost in our backyard,” says Smith. “Like us, Conveyor Tech is located in the greater Cincinnati area. The AccuVision conveyor design was well thought out; the fit and finish are everything you could expect and the price was attractive-particularly considering the use of LEDs for the backlighting. On top of everything, they were flexible in making the conveyor to our specifications. Flexibility in where we could locate the light along the conveyor gave us more options in how the form factor of the machine all came together.”
SpecificationsCincinnati Automation specified a 10 inch by 12 inch light field. AccuVision light fields are available in six widths from 2 to 12 inches and five lengths from 6 to 30 inches. The backlit conveyors can be ordered in belt widths from 2.5 to 24 inches.
Made of translucent urethane, the conveyor belt “has really good diffusion properties, a good even light for the camera,” says Smith. “The light is bright enough that it washes out imperfections from dirt build-up or smudges on the belt.”
Cincinnati Automation selected white LED light for the conveyor so that dirt on the belt would be more obvious, to aid in routine cleaning, Smith explains. Conveyor Technologies also offers red LEDs, which would work just as well for the Keyence cameras, he notes, but have less contrast for the human eye in monitoring against dirt build-up.
The conveyor’s array of LEDs provides a more even light than fluorescents, notes Smith. “With fluorescents you typically wind up with hot spots where the bulbs are. To minimize that effect, we would locate the bulbs further away and install diffuse panels, maybe even a couple of panels. It’s more difficult to get uniformity across the light panel.” Conveyor Technologies’ units have rows of high-intensity LED light distributed evenly under the belt and the belt itself is the diffuse panel.
Freedom from frequency oscillation makes LEDs ideal for high-speed digital cameras. Fluorescent “flicker”-oscillating cycles of bright/dim- usually are not visible to the naked eye, but will be captured by the camera and are readily apparent on the control’s monitor screen. “The LEDs may cost more than fluorescents at the component level, but are more effective, operate far longer and are just easier to work with when you consider everything involved,” says Smith.
- Conveyor Technologies Ltd.