May 2008

Light curtains

Hazardous machines mash and mangle

When an operator puts his fingers, hands, or arms too close to the whopper chopper, the operator ends up with mangled or missing body parts.

In days past, factory proprietors used physical barriers as safety devices that precluded one from putting ones self in danger. For various reasons, workers would override, ignore, or disable such safety mechanisms.

Safety light curtains are an advanced method of safeguarding personnel around such hazardous machines. We also call them light screens, optical guards, and presence sensing devices. Safety light curtains offer freedom, flexibility, and reduced operator fatigue when compared with traditional guarding methods such as mechanical barriers, sliding gates, and pullback restraints. By reducing the need, where applicable, for solid guards, safety light curtains simplify routine tasks like machine setup, maintenance, and repair.

How they work

Safety light curtains use a photoelectric transmitter that projects an array of synchronized, parallel infrared light beams to a receiver unit.

When an opaque object interrupts one or more beams, the control logic of the light curtain sends a stop signal to the guarded machine.

The transmitter unit contains light emitting diodes (LEDs), which emit pulses of invisible infrared light when energized by the light curtain's timing and logic circuitry.

The light pulses work in sequence-one LED energizes after another-and modulates or pulses at a specific frequency.

Corresponding phototransistors and supporting circuitry in the receiving unit detect only the specific pulse and frequency designated for it. These techniques offer enhanced safety and rejection of external light sources.

The control logic, user controls, and diagnostic indicators may be contained in a separate enclosure or reside in the same housing as the receiver electronics.


Where they work

We often categorize light curtain applications by the type of required guarding.

Protecting an operator from the hazards associated with material positioning or where a process happens is "point of operation" guarding.

The point of operation is the "zone of hazardous operation," or the "pinch point."

This type of guarding goes with mechanical and hydraulic power presses, molding presses, stamping, forming, riveting, eyelet, and automated assembly machinery.

Light curtains used in these applications are for finger and hand protection.

Perimeter guards protect the perimeter or boundary defined by a machine, robot, or other equipment. In these applications, the light curtains detect the presence of personnel and signal the machine controller to prevent hazardous conditions while personnel are present within the protected area.

In addition, the light curtain reset switch must be located outside and within view of the protected area to prevent inadvertent resumption of machine motion. Light curtains for perimeter guarding applications are for arm and body detection.

Nicholas Sheble ( writes and edits Automation Basics. Content for this article comes from OMRON Scientific Technologies Inc. (