Presence Sensing: The Basics

How Machine Builders Determine if an Object Is at a Particular Location at a Given Time

By Hank Hogan

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When it comes to presence sensing, some things are constant. Machine builders are always trying to determine if an object is at a particular location at a given time. Those basics drive fundamental specifications and guidelines.

Consider the first two elements of the equation. The detection distance is important, as is the target.

"If it's metal and it's very close — close being less than 15 mm — then you're going to use an inductive sensor, typically," says Jack Moermond, marketing manager for object detection at Balluff. "If you start getting farther away, then you're going to start looking at different technologies."

However, trying to use an inductive presence sensor up close doesn't always work. Getting near a hot sheet of steel, for example, is one such instance. Thus, the environment the sensor is in also has to be taken into account, as well as the input voltage needed and the output required for interfacing with the rest of the system.

Environmental ratings should be carefully considered, says John Murphy, sensor product manager at Turck. Many presence sensors are IP67-rated and thus able to handle some moisture. A car wash or something similar, on the other hand, could require an IP68 or IP69K rating, as well as a one-piece, 303 stainless steel housing.

Inductive and capacitive presence sensors are either embeddable or non-embeddable, with the former mounted flush while the latter sticks out as much as 10 mm. Non-embeddable sensors typically have a wider sensing field and are better for detecting objects located to the side. For both versions, a bigger size usually equates to a longer sensing distance, an issue in some situations.

"Some applications want a very tiny sensor, but they want 2 in. of sensing distance," Murphy says. "Not going to happen."

Inductive sensors, of course, don't work in all settings. The object might be non-metallic or too far away. For those cases, a light-based approach could be best. Here, the need is to generate contrast to highlight when a part is present. The ideal is to have the object be bright white while the background is jet black. As a result, for optical presence sensing, it's all about lighting, lighting and lighting.

"If we can get a thing correctly lit, either using the correct type of light but more likely with the correct placement of the light with respect to the part and the camera, that usually is the name of the game," says Brent Evanger, senior vision products applications engineer at Banner Engineering.

For optical presence sensors, the importance of lighting influences what specs are critical, says Jeff Allison, product marketing manager for photoelectric sensing at Pepperl+Fuchs. Often a machine will have more than one presence sensor operating, with each possibly emitting light that can bounce around and potentially interfere with another. Thus, robust cross-talk protection is vital.

Equally important is how the sensor handles background or ambient light. Potential intermittent bright sources include welding arcs. Even high-intensity fluorescent lights can be a problem if a sensor is mounted beneath a conveyor and is looking up. In such situations, a presence sensor might trigger, but that can be avoided through careful attention to device details, Allison says.

"Usually you'll have a specification that will say how much ambient light it can disregard," he says. "Is it 20,000 lux or 50,000 lux or 10,000 lux? You can gauge how much light is in the environment by using a lux meter."

What's more, the color of an LED or other illuminating light source can be important. A different hue might be needed when detecting blue boxes, orange straws or other colored objects. Clear objects present their own challenges.

In a dust-filled sawmill, getting any optical method to work is difficult, says Tim Kelley, vice president of marketing for Tri-Tronics. In that case, ultrasonic presence detection might be the answer. However, the object has to be almost perpendicular to the ultrasonic source. Also, detecting lines or other fine features isn't feasible, Kelley cautions.

When discussing basic presence sensor specs, one thing is clear: The existence of inductive, capacitive, optical and ultrasonic technologies proves that there is no one approach that works for all of the people all of the time.

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