What is a safety light curtain? How does it work?

In machine safety applications you can see different kinds of safety devices. Safety light curtain is one of the most unique solutions for safety applications such as:

• Automatic machinery.
• Packaging, handling, and storage machinery.
• Textile processing, woodworking, and ceramic processing machinery.
• Automatic or semi-automatic assembly lines.
• Automated high-bay racking.

Before describing the operating principle lets look at its definition:

A safety light curtain is used to safeguard access areas, danger points, or danger zones. It is designed for accident prevention following applicable international standards. The safety light curtain consists of a transmitter and a receiver, which are optically synchronized.

The infrared beams are controlled and monitored via a microprocessor. As soon as at least one of the beams emitted by the transmitter is interrupted, the light curtain outputs switch to low and the hazardous movement is stopped.

Safety light curtain working principle

A light curtain comprises two units, namely a transmitter and a receiver between which coded infrared beams are sequentially exchanged. The protective field is the area enclosed by these two components, the emitted light beams forming a permanent, though invisible, shield between the two units. The receiver unit is connected to a safety relay. The safety relay transmits the signal to the machine control unit.

Synchronization between the sender and receiver devices is performed optically, i.e. wired connection between the two units is not necessary. When properly installed, the protective device detects any relevant entry into the hazardous area. As soon as such an entry is detected, the protective device immediately triggers the safety relay, which in turn causes the machine control system to bring the machine to a safe status and/or complete stop, thus eliminating the hazard. Any internal fault is detected by the device’s permanent self-control function and has the same result as an intrusion into the protective field.

The size of the protective field depends on the dimension of the active optoelectronic protective device and the distance between the sender and receiver units. Active optoelectronic protective devices are also commonly used as sensors to automate industrial operations where no critical human safety issue is involved. However, when directly linked to the safety of persons, their design and installation are strictly regulated.

The unique operating principle of this device presents several advantages over mechanical safeguarding devices.

• Access time to the machine is reduced, thereby increasing productivity.
• Workplace ergonomics are greatly improved and less space is required.
• The invisible infrared beams allow better visibility of the machine and operating process.
• Protection applies to an approaching person.

1-Finger and hand detection

Detects a human finger when workers operate very close to the hazardous point of the machine.

2-Hand and person detection

Detects hands and prevents start-up while a person is in a hazardous area.

3-Single-sided access protection

Approaching persons are detected by identifying a person’s body.

4-Multi-sided access protection

Approaching persons are detected by identifying a person’s body.

5-With differentiation between person and material

Approaching persons are detected by identifying a human body. A distinction is made between humans and material.

6-Person detection

Approaching and present persons are detected by identifying legs in the scanning range.

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Positioning the safety light curtains

The level of safety depends on the way the device is positioned. The risk assessment conclusions will help decide what position is best suited for preventing foreseeable hazards. To ensure proper safeguarding, special care must be taken to find the position that will not allow the protective device to be bypassed and such that any hazardous machine movement is safely stopped before potential harm occurs.

There are different classical ways to position safety light curtains:

– Vertically (“perpendicular approach”)

– Horizontally (“parallel approach”)

– In an L shape (vertically and horizontally combined)

– Inclined (“angular approach”).

It must not be possible to pass over, below, around, or go behind the protective field. When positioning light curtains it must not be possible to pass over the highest beam, below the lowest beam, or between two beams. If this cannot be guaranteed, then additional protective devices must be used.