Photoelectric sensors are good at picking up opaque objects because they block a lot of light, causing the output to switch. Clear objects, however, do not block or scatter enough light to be reliably detected.
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Package designers, though they probably won’t admit it, take great pleasure in throwing curves to system engineers. Impossible demands run down hill, so they say, and there’s a long list to prove it. Today’s equipment must handle a wide range of package sizes and materials, it must be retoolable in software, it has to be fast and precise, and it can’t cost much more than the sum of the parts.

Now consumer goods manufacturers want machines that can handle nearly invisible, or clear, objects. Recent product development and marketing campaigns — primarily in cosmetics, pharmaceutical, and beverage industries — are hammering on the message that clear means clean and pure. This strategy has paved the way for a parade of translucent and clear soaps, gels, soft drinks, and foods, and is largely responsible for the recent surge in the use of clear plastic and glass packaging.

Clear objects may look nice on store shelves and in homes, but they are a real challenge to system engineers because they are difficult to sense. Ordinary photoelectric sensors (transmitted- beam or retroreflective types) are made for opaque objects. They are typically set for maximum sensitivity and light variation (high margin). In a photoelectric sensor, the unblocked light signal is many times stronger than that required to energize the output. This translates to higher detection reliability even if dirt and dust accumulate on the sensor or reflector. But there is a down side.

If a clear object enters such a sensing field, it probably won’t be detected. The reason is that clear materials typically don’t change the light intensity enough to trigger the sensor. Even though there are several losses in the optical path — light must pass through the object, bounce off a reflector, then return through the object to the sensor — the high margin on the sensor all but prevents the output from changing. Basically, the object is ignored.

This isn’t to say ordinary photoelectric sensors are totally immune to clear objects. If the light beam is polarized, the direction and extent of polarization will be altered to a greater or lesser degree depending on the optical properties of the material. In some cases, the variation could depolarize the light sufficiently to reflect a detectable signal — but if it happens, it would merely be by chance. In fact, many unsuspecting system engineers have confused irregular sensor activity stemming from random polarization for intermittent operation, wasting hours, if not days, barking up the wrong tree.

Materials that greatly change polarized light are called “optically active.” Most plastics used for containers and packaging are optically active. For this reason, photoelectric sensors often incorporate polarizing filters to block depolarized reflections from shiny surfaces on bottles, plastic wrap, and other objects.

Too focused

Something else about standard photoelectric sensors that makes them less than optimal for detecting clear objects is that they typically have a large (20 to 40%) hysteresis. Hysteresis, the difference between turn on and turn off points, prevents the output from “chattering” due to noise generated by electromagnetic interference (EMI), radio frequency interference (RFI), and ambient light. It sensitivity is tuned to energize the output with the reflector in the field-of-view, a passing object must reduce the signal by at least 20 to 40% to trigger the sensor. This is too much to ask of a clear object.

As clear plastic bottles move laterally through a sensor’s field of view, light loss due to reflection is greater at the edges than near the center. This explains the off-on-off sensor “chatter” often associated with clear objects.
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When light enters perpendicular to the surface of a clear object, it loses approximately 5% of its energy due to reflection. The same amount is lost on the way out. Two passes through an empty bottle, therefore, involves eight surfaces for a total loss of about 40%. A single glass plate, on the other hand, constitutes two surfaces, through which two passes add up to a 20% loss.

It follows then that a sensor with 20% hysteresis would just barely turn off and back on again for a single glass plate, and only if perfectly adjusted. In reality, however, one would not seriously attempt such an application because it doesn’t allow for any sensitivity drift or misadjustment.

Clear-object detection applications, thus, call for a special type of low-hysteresis sensor, having a hysteresis range of 8 to 10%. This ensures a stronger “off” condition. Granted, a sensor with 20% hysteresis would work in an application that sees a 40% light loss, but it only allows for a gain margin buffer of 12.5% between the “on” and “off” thresholds.

In the special case of bottles, 40% is the theoretical minimum light loss that occurs at the center of the bottle, where the surfaces are perpendicular to the light beam. Away from the center, where the surfaces tend to be more parallel with the light, the loss due to reflection is greater. This discrepancy (large variations in reflected light as bottles move through the field-ofview) explains the undesirable sensor chatter, or “off-on-off” response, frequently observed in bottle handling applications.

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