Color Sensors Gain on Vision Systems

Many Recent Advances in Color Sensors Have Been Aided by the Emergence of White-Light LEDs Strong Enough for RGB Elements to Detect

Jim MontagueBy Jim Montague, Executive Editor

Ironically, the line between color sensors and machine vision systems now is a little blurry. The very innovations and advances that allow many once-distinct technologies to flourish and grow can cause them to get their fingers into each other's pies. Many lower-cost photoelectric and color sensors are being updated with new electronics and data-processing functions, enabling users to gain some capabilities once reserved for higher-cost vision systems. Also, some color sensors are gradually increasing the distance at which they can operate, as well as expanding the array or field of view. 

"Simple, low-end, red-green-blue (RGB) color sensors with pushbuttons and potentiometers have been around for more than 10 years," says Jeff Allison, photoelectric sensing product manager at Pepperl+Fuchs (www.pepperl-fuchs.us). "However, users demand easier setup and plug-and-play, so they can commission them quickly, not have to relearn colors every time, and not repeatedly re-teach them. Users want simple and powerful color sensors with software that can learn colors, and then accept variances and narrower individual tolerances. For example, this means they can identify blue bottle caps, but also reject ones that might be a bit too lightly colored. A lot of people aren't aware of this flexibility." Besides having easier point-to-teach and tolerance functions, Allison adds that some color sensors also have remote teach functions.

"Color sensors are being designed and installed in smaller housings, so they can fit in smaller footprints," says Rick Bondy, product manager for Sick's (www.sick.com) advanced sensors group. "Their ASIC chips are much smaller, even though they have a lot more functions. Color and contrast sensors use higher-intensity, longer-life LEDs that run cooler and need less power, and achieve very high switching frequencies and have faster response times in the 25 µs range. This means quicker output cycles and better detection, which makes color sensors more cost-effective and usable in many more market segments and types of applications, such as handling previously hard-to-match automobile assemblies and color-coded semiconductor components on circuit boards."

In addition to having more-adjustable tolerance levels for different color shades, some sensors can be taught up to four completely different colors, and now have switching functions that let them recognize color changes and switch to different outputs as needed, Bondy adds. "In the future, I think we'll see more hybrid solutions like ours that recognize colors, but use vision system tools to work over a much larger area, such as 100x100 mm, and do it from 200 mm away."

Many recent advances in color sensors have been aided by emergence of white-light LEDs strong enough for RGB elements to detect, says Mark Sippel, marketing manager, photoelectric sensors at Balluff (www.balluff.com). "We still see a distinct line between color sensors and vision systems, but the sensors are handling a wider range of shades, and they've become more prolific in the automobile industry to verify interior color schemes during assembly."

Similarly, Brian Schriver, product manager for Rockwell Automation's (www.rockwellautomation.com) advanced optical sensors, acknowledges that vision systems were needed for inspection and sorting tasks, but adds they often were too costly to be widely used. "The new true-color or advanced-contrast sensors are less expensive, and they have advanced electronics, color-sensitive diodes, better white-light LEDs, and microcontrollers and software, which make them easier to teach and easier for users to program," he says. "So, as color sensors move closer to photoelectric sensors, users can solve application problems for just $300 to $500, with a return on investment that's a lot easier to justify. We recently worked on a dairy application which had photoelectric sensors that couldn't see if caps were present on the milk jugs. A true-color sensor was able to learn the color of the caps, which also helped with sorting."

The sheer variety of color sensors and their capabilities make it important for users to find equipment that best meets the needs of their specific application. "An RGB sensor is a simpler, lower-cost way to sense color, so some users may want to start with it, and see if it does the job they need," says Jeff Schmitz, marketing director for PPT Vision (www.pptvision.com).

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