Sensor sense: Safety Light Curtains

Sept. 8, 2009
Safety light-curtains shutdown dangerous equipment when an object blocks any single light beam of several dozen that span an opening.

Safety light curtains play an important role in many machine-safety strategies. Light curtains consist of two linear arrays of through-beam optical sensors commonly referred to as light bars. Only one bar actually holds a light source. Its LEDs send a number of infrared beams across the gap between the two bars creating a curtain of light. The other bar holds an array of optical sensors tuned to detect the light. Light bars can be up to 6-ft long and can span an opening from a few inches to a dozen feet or more, depending upon the model. An object placed within the gap blocks at least one beam, triggering the sensor.

This “blocked” condition turns off the system’s redundant, safety-rated outputs, providing a control-reliable stop signal to arrest hazardous motion before it can do harm to life or limb. The historical challenge for machine designers has been to integrate these needed presence-sensing safety devices into machines. They should not restrict desired operator access, such as in a load and unload area, or create an ergonomic obstacle or hazard themselves.

Not long ago, the optics and electronics needed to construct a self-contained light curtain could only fit in an enclosure with a cross-sectional footprint exceeding 12 sq. in. Taking up much of this space was the bulky 120-Vac power supply and socket-mounted, customer-replaceable electromechanical output relays. Today’s safety-control architectures use 24-Vdc power and safety-rated redundant solid-state outputs, along with significant advances in electrooptic and integrated-circuit technology, that deliver heightened safety and performance in a package less than 1-in. square.

This dramatic reduction in footprint size gives machine designers greater flexibility and freedom in locating and mounting safety devices. In many situations, the safety devices can set inside aluminum channel or hard-guard framing, making the machine more accessible while providing impact protection for the safety device.

The ultraslim form factor has another important side benefit: less weight. For example, an ultraslim 1,200-mm protected-height light bar weighs only 760 gm or less than 1.75 lb. This eliminates the need for special mounting posts, frames, and structures to securely support the weight of earlier safety light curtains. Without these mounting structures, the installed footprint of the safety device shrinks and saves material costs as well.

Pepperl+Fuchs (www.am.pepperl-fuchs.com) supplied information for this column.

Edited by Robert Repas

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