Color Sensing Needs Should Be Considered When Replacing a Machine Vision System

How Much Color Sensing Is Necessary?: When Should a Slow Machine Vision System Be Replaced by Color Sensors Without Implementing Overkill?

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The primary in-line inspection on our consumer products assembly equipment has to distinguish several colors, some of them only subtly different. The simplicity of color sensors looks attractive, compared to the outdated, too-slow machine vision system we have to replace. Some customers are looking for a simpler way. What specific needs should we add up to make sure we get the right color capabilities and resolution without lapsing into overkill and buying functions we don't need?

—from August '09 Control Design

ANSWERS

More Than Color
First, determine if color identification is the only requirement in the application. If the differentiation of one or more colors is the only purpose, then a color sensor invariably is the best solution to consider. Color sensors are typically faster, simpler and more cost-effective, and they can be as powerful as color vision technology in color identification capability.

If, on the other hand, the application also requires that a part's orientation, contour or a pattern be checked in addition to its color, then color vision sensors or color vision systems are required.

Advanced color sensors can identify minute differences in color similar to a color vision sensor, but sometimes an application can be easily and more cost effectively solved by a lower-level color sensor. So for color identification applications, determine the type of color sensor that is best suited for the application.

In general, lower-level potentiometer or push-button programmable color sensors are a solid and affordable solution for differentiating visibly different colors. These sensors typically have RGB (red, green and blue) LED transmitters, which often are used to distinguish colors such as red, yellow, brown, purple and others.

If the application requires distinguishing very similar shades of the same color, or if it requires more control or analysis, then a programmable advanced color sensor should be employed. Advanced color sensors use a white LED transmitter to better cover the color spectrum and use software to graphically display taught colors. Control functions could include checking not just color but the correct sequence of colors, such as the order of colored wires going into a wiring harness, the definition of acceptable color tolerances and even how colors are analyzed by the sensor.

Jeff Allison, product manager,
Pepperl+Fuchs,
www.pepperl-fuchs.com

Threshold of 10
Looking for the simplest color sensor solution that does the job is a good goal. There are several factors to be considered that might drive the selection decision to a product with more functionality than the simplest solutions, yet with much less complexity and cost compared to full-blown machine vision systems.

Color sensors that can be taught to distinguish between 10 or more different colored objects offer more versatility, especially for automotive applications, where there are often four or more color options.

Sensing colors on textured and other difficult surfaces and differentiating between subtle colors might require more sophisticated color sensors that have large sensing areas illuminated by diffuse off-axis lighting generated by multiple LEDs for each wavelength evaluated. Many inexpensive color sensors operate from on-axis lighting from a single white LED and are unreliable for distinguishing colors on textured surfaces.

Even though a higher performance color sensor might have more features than a particular operation needs, well-written setup software will have wizards to quickly guide production engineers to the configuration that fits their applications. And this time savings can add up to significant cost saving in applications where product changeovers occur often.

The stakes are high—shipping incorrect color products is not only expensive to correct but can lead to penalties imposed by the customer. The cost of a slip-up could be the value of an entire rejected production run. Hence, catching one incorrect color can pay for even the most expensive color sensor.

Steve Nylund, CEO,
Delta Computer Systems,
www.deltamotion.com

Subtle Differences
Many vendors make color sensors that measure average color in a small area of an object—your "consumer products." Most sensors output a triple of spectral values such as red, green, blue or L,a,b. Some sensors output more spectral values to help detect subtle color differences. You might need this hyper-spectral resolution if your colors are very subtly different.

If you need to measure color on more than one area of an object or multiple colors in an area, then a modern color machine vision system is a better choice. In this case, a camera, processor and software measure and distinguish colors. As with a color sensor, average color values can be returned, but now from multiple locations on the object. You can also measure multiple colors in an area of the object, perhaps produced by a color texture.

The measured color depends on the spectrum of the illumination, the angle between the illumination and the object, the angle of view of the sensor or camera and the spectral response of the sensor or camera. The illumination and sensor angles are fixed, and the sensor or camera response is known. You must control the lighting spectrum to reliably measure color. Some sensors include lighting. Most color machine vision systems don't include lighting, but the vendor can help you with lighting selection. Light sources change their spectrum over time or operating conditions, such as temperature or power level. If you include a neutral—white or gray—reference area in the camera's field of view, that can be used to dynamically compensate for lighting spectral changes.

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