How Safety Light Curtains Work

Summary

Safety light curtains are an advanced method of safeguarding personnel around many hazardous machines. Safety light curtains offer freedom, flexibility and reduced operator fatigue when compared with traditional guarding methods such as mechanical barriers, sliding gates and pull-back 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
A photoelectric transmitter projects an array of synchronized, parallel infrared light beams to a receiver unit. When an opaque object interrupts one or more beams in the sensing field, the control logic of the light curtain sends a stop signal to the guarded machine.

The transmitter unit contains light emitting diodes (LEDs) that emit pulses of invisible infrared light when energized by the light curtain’s timing and logic circuitry. The light pulses are both sequenced – one LED is energized after another – and modulated – pulsed at a specific frequency. Corresponding phototransistors and supporting circuitry in the receiving unit are designed to 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 be enclosed in the same housing as the receiver electronics.

Safety light curtains are Control Reliable. Required by the Occupational Safety and Health Act (OSHA) and the American National Standards Institute (ANSI) for safety related ANSI B11.19-2003, subclause 3.14 defines Control Reliability as “the capability of the machine control system, the safeguarding, other control components and related interfacing to achieve a safe state in the event of a failure within their safety related functions.”

Unlike photoelectric sensors, safety light curtains use self-checking circuitry to monitor the curtain for internal faults. If an internal fault is detected, the safety light curtain immediately sends a stop signal to the guarded machine. The light curtain then enters a lockout condition. Only after replacement of the failed component and an appropriate reset will the light curtain be restored to operating condition. Redundant safety outputs are another example of safety monitoring.

Typical Applications
Protecting an operator from the hazards associated with material positioning or where a process is performed is called point of operation guarding. The point of operation is often called the zone of hazardous operation, or the pinch point. This type of guarding is associated with mechanical and hydraulic power presses, molding presses, stamping, forming, riveting, eyelet and automated assembly machinery. Light curtains used in these applications are typically selected for finger and hand protection.

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

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