By Dan Hebert, PE, Senior Technical Editor
Checking color on machined parts used to be done by visual inspection, but this method ultimately proved to be too expensive and prone to human error.
"Using human vision for color checking is unreliable, as color perception by eyesight isn't consistent among people," says Jeff Allison, product manager of photoelectric sensing at Pepperl+Fuchs (www.pepperl-fuchs.us). "Changes in ambient lighting can affect how color is seen, and people can fatigue and identify colors as identical when they are not."
For many machine builders, the next steps beyond human inspection were expensive and sophisticated online vision systems or offline color analysis instruments. "Using color vision systems, colorimeters or spectrophotometers for color sensing results in a very hefty price tag, as well as substantial setup time," Allison says.
Fortunately, modern color sensors provide an economical and reliable online solution that avoids human frailties.
"A color sensor can verify that an installed component matches the desired color scheme," says Victor Caneff, automotive business manager for Banner Engineering (www.bannerengineering.com). "With a white LED light source, the color sensor analyzes reflected light from the surface of the component requiring inspection. By comparing the red, green and blue content present in the reflected light, the sensor can determine if the part matches a stored value from a known good sample previously programmed into its memory."
Caneff adds that intensity thresholds also can be established via a color sensor in order to detect whether or not the part is the correct shade of the appropriate color.
These color sensors not only can analyze and compare one color to another, but can detect varying intensities of a single color. Sensing time is very fast—on the order a few milliseconds—making it possible to use color sensors in online applications, for inspection or even for real-time control.
A color sensor typically is connected to a discrete input of a machine controller. The controller can use the on-off status of the color sensor for two purposes. The most common is inspection, by which a color sensor tells a machine to accept or reject a part.
A more sophisticated industrial application combines inspection with real-time control. In this instance, two color sensors—or a single multi-channel sensor—are connected to the machine controller. The first sensor is set to a very tight color tolerance level. A part activating this first sensor might still be within the established specification limits, but signals an impending problem. A second sensor is set to the go/no go level, and its activation indicates an unacceptable part.
Activation by the first sensor could trigger offline inspection, and results from this inspection could be used to adjust machine parameters to bring part production back into tight color tolerances. Using two color sensors in this manner allows for continuous production without machine shutdown.
A typical industrial application for these sensors is checking the color of parts produced for vehicle interiors.
"Color sensing is frequently used to error-check plastic molded parts used in the interior of a car such as a rearview mirror assembly, a console or a door interior," Allison explains. "When verifying the correct color of a molded plastic piece, it's necessary to fixture the part in place for inspection. If the part wobbles or vibrates, readings can be inaccurate because of reflected light, which can cause unstable detection of the color. Also, if there's texture or variation to the plastic molded part, then it might be necessary to use a color sensor that allows variable color tolerances to be taught."
Allison adds that his company's color sensors are available in one-channel or multi-channel versions. "With our 10-channel sensor, each channel senses a different color level, and local LEDs light up if the programmed color is detected," he explains. "Three other LEDs are used as adjustment accessories to teach and program the sensor via the membrane keypad."