Machine Design

Sensor Sense: Automation light grids

Automation light grids take many forms and cover applications from simple to complex. Sizes range from a few inches to several feet, and resolutions from 2 to 100 mm. Nearly every sensor manufacturer offers some form of light grid.

A light-grid transmitter stacks multiple infrared LED light emitters in parallel within a single housing. Likewise, the receiver combines a number of single-point phototransistor light detectors into another separate housing. The end result is a special variant of the single beam sensor: a thru-beam sensor containing a grid of light beams.

The operating principle of a thru-beam light grid is similar to that of a single-beam sensor. But output options, parameterization methods, and interface types vary greatly among sensor models complicating the selection process. Fortunately, most automation light grids can be divided into two general types: switching and measuring.

Switching light grids are the most basic. They are well suited for applications where single-beam sensors prove unreliable or where there’s a need for a defined coverage area. Like single-beam sensors, they offer only one or two discrete outputs that indicate the presence or absence of a target. Common applications include parts ejection, object counting, edge guiding, elevator and doorway monitoring, leading-edge detection, and overhang detection. Major benefits of using switching light grids are low cost, simple installation, fast operation, and easy configuration.

Measuring light grids differ from switching light grids in that they can return the discrete size of an object by the number of beams the object breaks. For example, a typical application might sort shipping containers on a conveyor by height. Multiple outputs can be programmed for different sizes or heights. Alternatively, more advanced height data may be transmitted by communication interfaces such as IO-Link, Ethernet, Profibus, or DeviceNet.

Intelligent light-grid systems can identify objects through shape, even if the object contains gaps and openings. For example, distinguishing a car door hanging from a conveyor hook versus an empty hook.

Measuring light grids tend to need PC software for configuration, making installation more complex. Common applications include object sizing and profiling, product guiding and positioning, height measurement, and dimensional inspection.

Pepperl+Fuchs supplied information for this column.

© 2012 Penton Media, Inc.

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