The Next Generation of Color SensorsJanuary 1, 2006 By: Bill Letterle, EMX Industries Inc. Sensors
A new sensor's keen eye for subtle color variations helps line workers correct problems with nearly beige, almost silver, and sort of gray.
Color sensors have long been used on assembly lines to detect specific components. The challenge has been to detect subtle differences among similar or highly reflective colors. For example, metallic paint used in the automotive industry makes it difficult to differentiate among shades of gray or gold. This is important when matching subassemblies, such as mirrors to bodies or bumpers to park-assist sensors. Furthermore, color sensors have been limited by the number of colors they can detect and by their limited abilities to quickly change setups or handle multiple colors.
Fortunately, advances in electronics, optics, and software have led to the development of color sensors that output the reading intensity and color value. This information allows the controller or operator to see not only what color is detected but also how much color is present. The technology makes for a more sensitive sensor that can ignore luster and discriminate among subtle shades. Now processes can be fine-tuned to facilitate flexible manufacturing and precise color correction.
Programmable Color Sensing
A typical color sensor has a high-intensity white LED that projects modulated light on the target. The reflection from the target is analyzed for the constituent red, green, and blue (RGB) values and intensities. This information is used to verify that the right parts are present and assembled correctly, and to control the color of the manufactured goods.
In a typical application, the machine operator holds a color sample in front of the sensor, programming it to match for that particular color. During and after the process, the operator may notice matching failures involving colors that are slightly dark or slightly light, but still within acceptable quality standards. The operator then reprograms the sensor with wider high/low set points and through a trial-and-error process establishes the ideal range.
If the sensor has multiple channels, it can be programmed to recognize multiple colors—one color on each channel (Figure 1). The signal for each channel is a discrete alarm output. This technology enables simple color identification or matching, such as sorting or part-identification functions, where a pass/fail criterion is sufficient.
Figure 1. The ColorMax-1000 color sensor from EMX Industries, Inc., outputs color intensity and displays the reading numerically as a percentage of the range.
Other processes, however, require in-depth monitoring and do not always fit the simple pass/fail scenario. Next-generation color sensors provide three additional outputs representing the RGB color values of each reading. The benefits include more intelligent control of manufacturing.
In practice, the sensor outputs raw RGB readings as analog signals. Analog signals are better suited for communications because digital readings for three channels every 150 µs would exceed the throughput limitations of typical serial protocols. A sensor that converts the raw RGB signals to analog with 10-bit resolution will output 5 mV for each of the 1023 steps.
No Longer a Blind Process
When a color match fails, the operator looks at the process and makes whatever changes are necessary to bring the color back into spec. The problem is that the engineer is working blind, realizing that a match failed, but not knowing by how much or why. Was the paint applied too lightly (nozzle clogged)? Is it the wrong hue (picked an outdated part)? Are the parameters set too conservatively (color was close enough)? These questions can be answered by seeing the intensity of the individual RGB readings (Figure 2). If the color-intensity data are fed to a control-panel display, the operator can see if the readings are drifting high or low and take action before an alarm occurs.
Figure 2. Color Matching Using Intensity of Individual RGB Readings
If the cause of color variation is not readily apparent or if no display is available, the analog RGB signals can be digitized and fed into a data acquisition system. This allows for comprehensive trending and analysis of sensor readings. Some sensors offer a data-dump mode in which the raw digital readings are communicated via an RS-232 connection.
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