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ROBOTIC RESOURCES

Safety Light Curtains and Robotic Work Cells

by Russ Wood, Application Engineering Manager
Robotic Industries Association

If they are to provide adequate personnel and machine safeguarding, robotic work cells require traditional safety considerations, as well as additional safety light curtain adaptations.

Robotics deliver flexible and tireless operation in many roles disagreeable or dangerous to humans.  But along with these benefits comes the responsibility of developing a safe working environment for humans and robots alike.  Safeguarding the border or perimeter between robotic work cells and the remainder of the plant is a fairly typical area of employ for safety light curtains.

Using multi-beam or single-beam safety light curtains in robotics applications requires use of the same basic safety principles used to safeguard any machine. These include a safety review, developing corrective procedures and appropriate training, as wells as using appropriate safety equipment.
 
Utilizing safety light curtains in a robotic work cell helps provide flexible operation when used in concert with mechanical means of perimeter and access guarding.

Greater range of motion
Robotic work cells provide flexibility and a range of motion during operation that requires added perimeter guarding equipment. Basic considerations start with the selection of a robot that provides suitable motion and lifting capacity. Mechanical guards and light curtains must be mounted outside the range of the robot's movement. A robot arm that travels outside the perimeter envelope creates another hazard. If space is critical, mechanical stops can be placed on the robot to limit its movement.
 
A safety light curtain "trips" when a light beam is momentarily broken as someone crosses into a hazardous work cell area. As a result, it is necessary to use a "guard mode" controller which latches in the de-energized position until manually reset. The operation stops work cell activity, or causes equipment to return to a safe operating position.
 
Safety light curtains are control reliable and provide redundant outputs.  Using both and wiring them in series minimizes problems with single component failures.  For added safety, it is preferable to include the output used to trigger the status, or alarm relay, as a third safety circuit if available. Contacting the protected machine's manufacturer can help determine the best way to interlock the light curtain's control switches to the machine control circuit.

Re-energizing the work cell
Many accidents occur when workers attend to operating problems, or during scheduled maintenance when the equipment was originally de-energized but inadvertently re-energized. Restoring energy to the robotic work cell should be initiated from a key controlled switch to eliminate the possibility of inadvertent activation. This switch should be positioned outside the hazardous area so that the operator may view all possible points of entry.
 
It may be desirable to combine the keyed switch with a four-digit combination or trapped key interlock to further minimize accidental re-energizing of the work cell. In certain situations, such as very large capacity robots, the addition of safety mats is a safety option that should be explored.

Visual indicators and added safety
Safety light curtains work with infrared beams that are invisible to the human eye. It is advisable to combine the light curtain with visual indicators or awareness barriers, such as a low railing or suspended chain, or by painting the guarded area yellow-orange to indicate areas of hazard.
 
Additional indicators include warning or informational signs placed in appropriate locations. Pilot lights on the light curtain and robot indicate their operational status and also add to the safety equation. As always, safety light curtains should only be utilized in applications when dust or smoke will not degrade visibility and safety light curtain capabilities.

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