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Inductive vs. Photoelectric Sensors

By: George Keremedjiev

Wednesday, March 21, 2018
 

Sometimes choosing the correct sensor boils down to the basics. Should one opt for an inductive-proximity or a photoelectric sensor? Let’s explore the pluses and minuses of both. Spoiler alert: You won’t find a one-size-fits-all answer here.

Inductive-Proximity Sensors

In a previous column, I described the inductive or radio field that emanates from an inductive-proximity sensor. In nearly every case, this field assumes a fish-like shape. The dimensions of this fish depend on the alloy of the target and its geometry. Determine the exact shape of the fish with a given target by carefully testing it on a sensor test bench. Then, the main challenge involves mounting the inductive-proximity sensor close enough to the target for the sensor to be activated, either with a lateral penetration into the fish body (if the target is traveling sideways into the fish), or via contact with both edges of the tail, if the target travels head-on into the sensor. Immune to water, oils and coolants, an inductive field also ignores nonmetallic materials, making it easy to fully or partially enclose the top of the sensor within a block of hard plastic. However, note that fluids heavily contaminated by metallic shavings may deposit themselves on the top of the sensor, causing the sensor to lock into an ON or OFF output–a rare, but possible, occurrence.

A conventional inductive-proximity sensor detects all types of metal alloys, though less-ferrous alloys have smaller fish-shaped detection fields. Thus, when detecting pure iron, for example, the field functions as its maximum size, as stated in a sensor catalog. Nonferrous targets such as copper, brass, etc. exhibit a substantially smaller field than what may be expected. Some varieties of inductive-proximity sensors have a constant inductive field, irrespective of the type of material detected, but be very careful in understanding the exact shape of that detection area. It likely will not be fish-shaped. Instead, its shape may be highly complex, making detection of a precise penetrating-target edge difficult.

Photoelectric Sensors

Photoelectric sensors offer much greater detection distances than inductive-proximity sensors, but at a price. For example, unless a photoelectric-sensor test uses the same lubricant to be used in production, the sensor may turn out to be an excellent detector of oil and coolant droplets that pass through its photoelectric detection field. This , however, can be catastrophic for the pressroom where a photoelectric sensor is used for, say, part-out detection. Here, press tooling may experience a die crash when the photoelectric sensor detects oil instead of the part itself.

Maintaining constant and accurate aim of the photoelectric sensor’s beam upon the target represents a significant challenge. Over time, photoelectric sensors mounted on flimsy sheetmetal brackets will move, rattle, vibrate, be hit or simply bend out of their original positions. To avoid these situations, mount these sensors within rigid small blocks of steel or aluminum. Photoelectric sensors provide excellent detection capabilities but must be tested carefully to ensure that they only detect specific items of interest. Better to test the photoelectric sensor on a test bench than in a running die.

Choose Based on Specific Applications and Testing

So, which sensor is better, inductive or photoelectric? Unfortunately, no clear answer exists. Both perform flawlessly within their respective constraints. You, the user, must decide—after arriving at a clear understanding of the shape of the inductive and photoelectric detection fields, their exact geometries, and their susceptibilities to the immediate in-die environment. MF

MetalForming magazine's online-exclusive Metalforming Electronics column is aimed at understanding electronic sensors as they are applied to tool and die protection, in-tool part-quality monitoring and plant-wide value-added operations.

For more than 30 years, George Keremedjiev had authored the Metalforming Electronics column in MetalForming magazine and continues his efforts here. He regularly consults with metalforming companies worldwide and provides metalformers with training on the application and implementation of sensors for die protection. For more information on his seminars and consultancies, contact: Tecknow Education Services, Inc. P.O. Box 6448 Bozeman, MT 59771, phone: 406/587-4751 fax: 406/587-9620 www.mfg-advice.com E-mail: gk@mfg-advice.com.

 

See also: Tecknow Education Services, Inc.

Related Enterprise Zones: Sensing/Electronics


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