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Seeing is Believing
Modern industrial sensors pack the intelligence to simplify high-speed packaging
By Terrence Lynch, Contributing Editor -- Design News, April 30, 2000
Here's a look at two interesting new sensor technologies, targeted at in-line packaging machines.
Introspective photoelectric sensor
Engineers at Telemechanique have developed a software-based system to continuously adjust for accumulating dust on the lens of its new XUKT retroreflective photosensors. Self-contained retro-reflective sensors emit a light beam at a prismatic target and continuously measure the strength of the signal produced by the returning light. The accumulation of dust or vapor on the lenses of the emitter or receiver reduces the apparent photodetector signal and poses an eternal maintenance problem.
Telemechanique alleviates the problem by monitoring signal-strength decay over time, and automatically adjusting the switching value for the current conditions. The sensor looks only for the differential between signal strength in the "object present" and the "no object present" conditions, regardless (within limits) of the actual signal strength. If signal strength falls below minimum detectable values, the controller triggers an alarm, indicating the need for cleaning.
The XUKT's detector is optimized for the relatively small signal differentials encountered with clear PVC, PET, or glass containers. A controller enables a one-button self-teach feature: place the installed unit (aligned with its reflector) in learn mode with no target object present, and press the button; install a target in the beam path, and press again. The sensor memorizes returned signal strength in both conditions and establishes the optimum setpoint trigger level. Operational mode settings (e.g. light/dark on) are controlled by DIP switches.
XUKT sensors operate on 10-30 Vdc with a 100-mA standard output and 50-mA alarm output as either PNP (source) or NPN (sink). Housed in IP-66-rated, 50 X 50-mm packages, the sensors offer a variety of cabling and bracketing options.
Clear object detector
Setting the switching levels of most photodetectors is done by adjusting potentiometers ("pots") with a screwdriver. Not only do pots provide an excellent means of entry for dust and moisture, but getting an accurate setpoint with a manual pot adjustment requires a calm, technically trained operator.
The Mini-Beam Expert Series Clear Object Detector from Banner Engineering (Minneapolis) eliminates such problems with a simple, one-button teaching system with a patented setpoint optimization algorithm. Select the teach mode on the retroreflective sensor, and a yellow LED lights up. At the same time, a bipolar red/green LED blinks red at a rate proportional to incoming signal strength. Once the sensor and reflector are properly aligned, place the target object in the beam path and press the teach button to set the "on" condition. Remove the target object, press the teach button again, and the sensor records the "off" signal-strength condition. The integrated microcontroller within the Mini-Beam then sets the switching hysteresis at a midpoint between the two levels for optimum object detection.
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| The Mini-Beam Expert Series Clear Object Detector from Banner Engineering eliminates such problems with a simple, one-button teaching system with a patented setpoint optimization algorithm. |
Back in run mode, the red/green LED glows green to show power on and blinks when the returning maximum signal strength approaches the threshold switching value-an indication of debris accumulating on the lens. The yellow LED lights whenever the output is triggered.
For detecting clear objects the Mini-Beam is often operated with a polarizing filter over the sensor's emitter and receiving lens. This technique greatly reduces "proxing," or the false triggering that sometimes occurs when light reflected off shiny object surfaces is perceived as a return from the retroreflector. Although this cuts down on the intensity of the light returning from the retroreflector, it is not an issue over short sensing distances.
Mini-Beam Clear Object Detectors operate at 10-30 Vdc with PNP or NPN outputs of 150 mA max. Response is set to signals of 500 msec or longer duration, 1 kHz max.
