Safety Light Curtains Guard Automated Systems

Safety Light Curtains Guard Automated Systems

Modern machinery can be hazardous in the best of cases. When the equipment in question operates in an automated mode, the potential hazards increase. In order to prevent injury, an effective protection system is needed to automatically interrupt operations and prevent accidents.

IBT recommends Safety Light Curtains made by Scientific Technologies Inc. (STI). They offer seven safety light curtain ranges that provide optical guarding for automated equipment.

STI light curtains are offered in compact models designed for machine locations where space is tight. At the top end, they have large, robust light curtains that are powerful enough to guard large perimeters. There are also a number of types available “in between.” In this type of installation, one size does not fit all.

It is possible to select a system of the size and with the specifications required. Systems can be tailored for the lowest possible system cost needed to do the job at hand. STI’s wide product line means you don’t have to force your application to fit their light curtain. From tight spaces to perimeter guarding, IBT and STI will supply the best product for your job.

Safety light curtains – also called light screens, optical guards, and presence sensing devices – offer freedom, flexibility and reduced operator fatigue when compared with traditional guarding methods (mechanical barriers, sliding gates and pullback restraints). Where they reduce the need for solid guards, safety light curtains simplify routine tasks like machine setup, maintenance and repair.

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

Safety light curtains are easy to understand. 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 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 are both sequenced – one LED is energized after another – and modulated – pulsed at a specific frequency.

The corresponding phototransistor and supporting circuitry in the receiving units are designed to detect only the specific pulse and frequency designated for each of them. 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.

One significant difference between safety light curtains and standard photoelectric sensors is a design concept known as Control Reliability. It is required by the Occupational Safety and Health Act (OSHA) and the American National Standards Institute (ANSI) for safety related machine control systems.

In meeting this requirement, STI 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. STI light curtains provide one of two possible types of safety outputs. The first are relays with force-guided contacts (also called captive contacts). In this type of relay, the two sets of contacts are mechanically connected and move together. This design permits monitoring of the relay contacts and guards against the danger caused by welded contacts.

Redundant relays are used for additional safeguarding. Should one relay fail, the second is used to send a stop signal to the protected machine. The second type of safety outputs are redundant, solid state devices. These devices are electronically cross-monitored and self checking. Should one output fail, the second is used to send a stop signal to the protected machine.

Light curtain applications are often categorized by the type of guarding required. 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, the 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 arm and body detection. For more information, contact IBT.


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