Photoelectric Sensors and Oil--They Do Mix
I recently have come across some false fears of those in our industry convinced that photoelectric sensors do not work in an oily die. Nothing can be further from the truth. Photoelectric sensors do perform very well in oily environments if they are properly selected and tested beforehand.
As you may recall from high-school physics, a beam of light comprises tiny particles called photons. These photons in and of themselves do not have any color, rather they vibrate. When a beam of light hits a surface, most of these vibrating photons are absorbed by the item being illuminated. Except, that is, for those photons vibrating at a certain frequency that cause them to bounce off of the item. To the viewer, the vibration pattern from these photons enters your eye, where sensors in the retina identify the frequency, then send this information to your brain where the sensation of color is generated. Green comes from a different photon frequency than does blue or red.
Just outside of our eyes’ color-sensitivity range is a color called infrared. It’s made up of photons that vibrate at a frequency not detectable by our eyes. Some infrared light can penetrate oils. However, infrared comes in many different shades. Before metalformers implement infrared sensors in their dies, they should test the sensors using the exact type and viscosity of oil or lubricant that will be used in the dies. This is best done using a spray bottle filled with the oil, and the sensor mounted on a fixture. Tests of several sensor types may be required, from different manufacturers, before finding the one that best works with a given lubricant or oil. Note: Only spray the sensor with lubricant if it has a rating by the manufacturer allowing you to do so.
Another important factor when selecting a sensor for your oily application is the brightness or intensity of the infrared beam. That beam may be reflected back by the target into the same sensor housing (reflective), or shot through space to a separate receiver (through-beam) where the target interrupts the beam. The strength of the beam and the ability for the receiving circuitry to detect it is crucial to proper sensor selection. You may wind up using a sensor with an enormous range—several feet, for example—in an application where the target will be located only inches from the sensor. This excessive distance capacity by the sensor, called “excess gain,” may be just what the doctor ordered for an oily die, particularly when combined with the proper shade of infrared light.
Sensors that pass a metalformer’s tests then can be used for applications such as feed detection, part ejection and cam return without having to worry that the sensor will detect the oil and not the intended target. There is no short cut to the above tests. You cannot simply call a sensor manufacturer, tell them the type of oil you will be using and expect them to provide an infrared sensor guaranteed to work in that specific application. Periodically such guess work may produce a lucky guess, but it is better by far to perform the tests yourself to ensure that the sensor will not generate nuisance stops.Many stamping shops have attempted to use infrared photoelectric sensors with only an assurance from the sensor catalog that the unit will work within oily environments, without generating their own test data. They install the untested sensor in the die and shortly thereafter the stream of nuisance stops arises. Not too long afterwards they rip the sensor out of the die, leaving behind a sour taste in everyone’s mouth for photoelectric sensors. Instead, follow the advice described above and discover the photoelectric sensors that will work with your oils and coolants, making die protection a universally applicable technology for all of your dies and tooling. MF
Related Enterprise Zones: Sensing/Electronics
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