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Optical Assembly Improves Photoelectric Sensors
Plastic injection-molded mirrors gather more light
By Charles J. Murray, Senior Regional Editor -- Design News, May 8, 1995
Eden Prairie, MN-Building a better photoelectric switch isn't easy. To enhance its light-gathering capacity, the switch's optics need longer focal distances. That requirement means larger size and higher cost.
Now, however, engineers have a way to increase light-gathering capacity without making a photoelectric switch bigger or costlier. Developed by engineers at Erwin Sick GmbH, Waldkirch, Germany, the new technique reduces required focal distances by a factor of about ten. As a result, photoelectric packages using the technique are significantly smaller than comparable conventional switches.
Greater effectiveness results from an optical arrangement that includes a pair of injection-molded, plastic parabolic mirrors with gold deposited on them. The two mirrors include a larger unit and a "capture mirror." Designers attach the capture mirror to the larger mirror by means of a plastic arm, or "stand-off." The stand-off fits into an opening in the larger mirror.
During operation, an LED is placed in the opening of the larger mirror. Light from the LED travels along the optical axis of both mirrors, striking the convex side of the capture mirror and bouncing to the concave side of the larger mirror. By designing the collimator in this way, instead of passing the light through successive transparent components, engineers from SICK dramatically reduced the focal distance needed between the optics. At the same time, they enhanced the device's light-gathering ability.
SICK engineers also effectively controlled the cost of the new technology by employing plastic injection-molded lenses and depositing gold on them. "Because of our experience in deposition, we were able to produce this as a plastic lens at a competitive price," notes Joe Dolinsky, photoelectric division manager for SICK Optic-Electronic, Inc.
In a new photoelectric sensor design known as the W-24 series, engineers further reduced unit size by employing double-sided PC boards with surface-mount components on both sides. As a result, the company's photoelectric sensor has been reduced to the size of a cigarette pack. Previously, the switch was about as big as a pair of large floppy-disk boxes.
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Used in through-beam, retro-reflective, and proximity-type switches, the technology provides dramatic performance improvements. In through-beam applications, the cigarette-pack-sized switch can offer the same sensing range as much larger devices. In retro-reflective designs, previous sensing distances of 16 feet increased to 26 feet. And in prox switches, engineers no longer must specify performance ratings by testing devices on cards with 90% reflectivity. Instead, SICK engineers say, they have tested the device on 6% reflectivity cards.
Engineers also used the reflector technology to produce intrinsically safe, high-power switches for explosive atmospheres.
Additional details...Contact Michael Hudson, SICK Optic-Electronic Inc., 7694 Golden Triangle Drive, Box 444-240, Eden Prairie, MN 55344, 612-941-6780.
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