Pepperl+Fuchs Blog

Know Your Sensing Utensils: Using Slot Sensors

Posted by John Appleson on Thu, May 09, 2013

A slot sensor is a thru-beam photoelectric sensor Fork-type. U-shaped. Slotted. Vane-style. Slot housing. Whatever you call it, the housing is distinct among all other photoelectric sensor housings. And just as a fork is better suited for certain eating tasks than a spoon or knife, the fork sensor, or as it is more commonly called, the slot sensor, is a utensil that has its own strengths.

The photoelectric slot sensor has a light emitter on the inside end of one leg of the slot and a receiver on the inside end of the other leg. Both emitter and receiver face each other, as the slot sensor operates in thru-beam mode. When the light beam from the emitter to the receiver is unobstructed, the receiver detects the light and the sensor’s output is in one state. And when something obstructs the beam from the emitter to the receiver, then the receiver doesn’t see the light and the sensor’s output switches to the other state to indicate presence detection of the object. Slot sensors are available in a variety of slot widths, from a few millimeters to 200+ mm (8+ inches), as depicted below.

Slot sensors in various widths

Among the strengths of the photoelectric slot sensor is its high efficiency. Thru-beam mode is more efficient than diffuse or retroreflective modes, because light makes only one trip—from the emitter to the receiver. In diffuse and retroreflective modes, the light travels from the sensor’s emitter to an object or reflector and makes a second trip back when it is reflected to the sensor’s receiver. These modes rely on something to reflect light back to the sensor, but some of the light’s signal strength is lost during that reflection. This makes the slot sensor ideal for dirty, dusty, misty, or oily areas, such as in paper mills or tooling areas, since the higher optical signal is more resistant to dust and fibers.

Photoelectric slot sensors have the easiest setup among thru-beam mode sensors. Mounting, wiring, and alignment are faster and simpler, as slot sensors consist of a single housing rather than the two housings of more conventional thru-beam mode sensors. In fact, using a slot sensor cuts setup time by 50%, because there are half the housings to mount and wire. Also, with a conventional two-housing thru-beam, the emitter and receiver must be optically aligned to each other to ensure optimal signal strength. A slot sensor is already pre-aligned, so no alignment is needed.

Slot sensors can be among the most cost-effective methods of thru-beam sensing. For basic presence sensing applications of very thin targets, such as verifying the presence of a piece of paper, a family of nano photoelectric slot sensors including GL2, GL3, and GL5 series (depicted below) offer an economical sensing method. These sensors contain only the basic components to detect presence with a variety of mounting configurations.

Slot sensors for presence sensing applications
Another reason that the slot sensor is a valuable component in any parts crib is that it offers high repeatability and precision. Some slot sensors, like the GL and GLP series, can detect targets as small as 0.3 mm. And some offer repeat accuracy of 0.05 mm (2/1000 inch), meaning a particular sensor will see the same target at the same position to within that distance. For some applications, positioning is key because less error in positioning means less waste. Slot sensors detect objects to a very small size and at a repeatable position.

Certain applications are best-suited for photoelectric slot sensors. For example, edge guidance of a web requires high gain to burn through paper dust, short sensing ranges because the paper is thin, and simple alignment in case the housing is moved. (See image below.)

Slot sensors for edge guidance of a web

Counting a web of self-adhesive labels on backing is another application that can be solved by slot sensors. In this application, the light beam intensity is weaker when passing through the label and backing, and the light beam intensity is stronger when passing through the backing only. The sensor can be taught these two levels of received light so that as the stream of labels passes through the slot senor, it can count each individual label. (See image below.)

Slot sensors for label detection
Slot sensors can also detect end of travel of an object and positioning. Special types of slot sensors include fiber optic slot heads, especially suited for very tight spaces or harsh environmental conditions, and slot light grids, to detect parts of random size or in random positions, such as in parts ejection.

A slot sensor packs the benefits of thru-beam sensing into a single housing. Before forking over a requisition for your next sensor, consider slot sensors as a sensing utensil.

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Topics: Applications, Photoelectric Sensors

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