Check Application To Specify Sensors

With proximity sensors—proxes—and photoelectric sensors, the question is not only whether to use them, but when and how. For instance, proxes may provide robust solutions because they have no lenses that need cleaning and require little or no adjustment, observes Greg Knutson, senior applications engineer at Minneapolis-based vendor Banner Engineering Corp. (www.bannerengineering.com).

But in selecting inductive or capacitive proxes, the choice of material is the biggest question, suggests Christian Vitale, applications engineering manager for automation supplier Turck USA Inc. (www.turck.com), also in Minneapolis. “More importantly, is it ferrous or non-ferrous?” he says, in reference to the material to be sensed. If metallic, pick inductives, advises Wayne Meyer, product manager, industrial sensors group, at Sick Inc. (www.sickusa.com) another Minneapolis vendor. “That also assumes you can get extremely close, about 25 millimeters (mm) away [from objects to be sensed].” But target size has to be at least three times nominal sensing distance, recommends Thurston Horton, product and technology training manager for Balluff USA Inc. (www.balluff.com), a Florence, Ky, components vendor.

Typical applications for inductives include sensing bottles with metal caps, cans of food or liquids, or other metallic objects on conveyor belts, Vitale explains. But beyond metallics, Meyer says you must ask: “What am I trying to detect? Are there size, shape, color and/or surface irregularities?”

Answers may point to using capacitive proxes, which can see targets from 50 to 60 mm away. “But sensing range isn’t the deciding factor,” Vitale says. It’s capacitives’ flexibility to see metals and non-metals. That can backfire, though. “If the sensor is set up to see wood, and you put your hand there [in the light beam], it doesn’t know the difference between your hand and wood,” Horton explains. That could cause safety- or operations-related problems.

Capacitives uniquely detect liquid levels in containers, too. “They can see inside of cups, milk cartons, shampoo bottles, and so on,” Horton notes. Tuning them for optimum effectiveness means first tuning to the container and then setting the sensor to ignore that signal and see only the liquid. With oil tanks, for example, sight glasses have to be installed through the shell and the capacitive tuned to ignore the glass, he says.

Dusty or wet environments may require inductives or capacitives, depending on the application, Meyer adds. So where do photoelectric sensors, including fiber optic ones, fit? When the sensor needs to be farther away from the object and where proxes don’t work, such as inside machines or underneath a carriage, he explains. “Photoelectrics can see wood, metal or other materials,” notes Horton.

Retro look

But how they see differs. With thru-beam and retro-reflective sensors, the beam is interrupted; with diffuse sensors, the beam is reflected. “Thru-beam is best for conveyors. It only has to look once across them,” Horton says. Thru-beams are better for dusty environments, he also says, though retro-reflectives are cheaper and easier to install. With diffuse photoelectrics, the brighter the target, the greater the sensing distance, Horton notes. Regardless, “with photoelectrics, you have to pay more attention to color, surface and reflectivity, and also ambient lighting,” cautions Meyer.

Remote sensing or harsh environments dictate fiber optic sensors, a photoelectrics subset. Knutson notes that with these, “you can get 20 to 30 feet or more” away from targets. Inside ovens is one high-temperature environment in which these sensors are used, because they are rated to at least 600 to 1,000 degrees Fahrenheit. “Maybe I need to detect if the part is in the proper place,” Horton says. For example, end-users might set the light beam along the conveyor’s sides in a bakery’s continuous oven to determine pans’ lateral positions, explains Knutson.

So whether the technology is inductive or capacitive proxes, or photoelectrics such as fiber optics, discrete sensors can cover a wide range of applications for automation end-users. As Meyer puts it, “There’s something for everyone.” 

 

C. Kenna Amos,

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