A look inside light curtains

Safety light curtains are an advanced method of safeguarding personnel around many hazardous machines. They 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, for solid guards where applicable, safety light curtains simplify routine tasks like machin...

By Russ Wood June 1, 2006

Safety light curtains are an advanced method of safeguarding personnel around many hazardous machines. They 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, for solid guards where applicable, safety light curtains simplify routine tasks like machine setup, maintenance, and repair.

How they work

In a light curtain, 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.

Safety light curtains must only be used on machinery that can consistently and immediately stop anywhere in its cycle or stroke. They should never be used on a full revolution clutched power press or 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. Light pulses are 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.

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

A difference between safety light curtains and standard photoelectric sensors is a design concept called “control reliability.” Control reliability is required by the U.S. Occupational Safety and Health Administration (OSHA) and American National Standards Institute (ANSI) for safety related applications. 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.

Where to use them

Point of operation guarding protects an operator from the hazards associated with material positioning or from the area where a process is performed. Point of operation is often called the zone of hazardous operation, or pinch point. This type of guarding is associated with mechanical and hydraulic power presses, molding presses, and 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.

Author Information
Russ Wood is application engineering manager at Scientific Technologies Inc.,