Sense a Presence

June 30, 2008
Sensors Are Used In Applications Where Automated Machinery or Processes Need to Be Synchronized or Controlled
By Philip Burgert

Presence sensors assist in many processes and offer a multitude of options how they should be applied and what benefits they will provide.

Gary Frigyes, product marketing manager for photoelectric sensors at Pepperl+Fuchs, notes that sensors are used for staging, sequencing and double feed detection on the front end of the process. During the process they can monitor the amount of raw material, position the product for the next stage and verify the process has been completed. At the end of the process, sensors can check if the product is labeled, ensure it has been placed in the box and verify it has left the machine.

“Sensors are used when automated machinery or processes need to be synchronized or controlled,” says Craig Brockman, marketing manager, presence sensing with Rockwell Automation. This might require verification of part presence, part orientation, the presence of the correct part or the presence of a desired product feature, he says.

Presence sensors also should be used when absence or presence detection is critical to prevent damage to machinery or the product being manufactured, adds Greg Nelson, business director of the Schneider Electric Electric Sensor Competency Center.

To accomplish these tasks a sensor could provide either a discrete or an analog output, says Brockman. A discrete output is a simple on/off signal that typically is used to detect the presence or absence of a target. “An analog output sensor provides a continuous output to measure or precisely position a product,” adds Brockman.

The biggest benefits of sensor use are seen on highly automated machines, especially when operations require precise object positioning, says Wayne Meyer, product manager for industrial sensors at Sick. Presence sensors often eliminate contact-style sensors like rocker arms, whisker switches and physical stops, helping protect the product from damage, he says.

Photoelectric sensors are used to sense presence of non-transparent objects such as boxes or product across a conveyor or over long spans regardless of material, says Nelson.

“They can be affected by varying target color and are more susceptible to contamination,” says Brockman. “Thus, photoelectric sensors tend to require more maintenance to be sure the lenses are clear.”

Inductive sensors are a rugged, reliable, economical solution to sense metallic objects, says Nelson. They generally are able to detect metal targets only at short distances—typically 40 mm or less, adds Brockman. Capacitive sensors work well in detecting liquids and bulk materials, and, while the target is not limited to metal, the sensing range of capacitive sensors are similar to inductive sensors, he says.

Ultrasonic sensors are good for all objects, regardless of material, with precise distance measurement capability, says Nelson.  They tend to be significantly more expensive, and performance can be influenced by humidity, says Brockman.

Limit switch sensors can detect any material target at short ranges and have very repeatable switch points, notes Brockman, adding, however, that the target must contact the mechanical switch and is more susceptible to damage.

In general, inductive sensors are used in metallic applications. Photoelectric sensors fit a range of packaging and conveying applications, while ultrasonic sensors are used in specialty applications where inductive and photoelectric sensors fail. “There is never one unique sensor for any application,” says Gregg Farnsworth, product manager for photoelectric sensors at Omron Electronics. “Each application can be solved by a number of sensors.”

Garrett Place, product manager for position sensors and object evaluation for ifm efector says currently the two primary sensing technologies in the machine building market are inductive and photoelectric presence sensors. “Any application that is working with metal tools or parts is a great area for the inductive sensors,” he says. “Most of the other applications can be solved with photoelectric sensors.”

PHIL BURGERT is an independent writer, specializing in the technical trade media.

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