Safety Light Curtains
Safety Light Curtains are linear arrays of photoelectric emitters and receivers in a through-beam configuration, and are used to detect an operator entering a dangerous area – at which point the safety outputs are meant to turn off the dangerous machinery and protect the user from injury. The resolution of beams (for “finger”, “hand”, or “torso” detection) and distance from the hazard must be taken into account, along with the stopping time for the hazard and the OSHA hand-speed constant of 63 inches/second, such that the machine has enough time to stop before the user can get to the hazard. Safety Light Curtains are commonly used with loading/unloading stations that require frequent operator interaction.
Safety Monitoring Relays
A Safety Monitoring Relay is a panel-mounted electronic device that takes inputs from safety devices (such as Door Switches, E-Stops, and Mats) and provides the force-guided redundant relay contacts meant to switch power from the hazardous device (a saw-blade motor, for example). Additionally, Safety Monitoring Relays have redundant checking processes that monitor operation and condition of the safety devices, to check for things like simultaneous operation, wire breaks, or stuck contacts.
Think of a Safety Controller (or “Safety PLC”) as a Monitoring Relay with additional programmable control functions. A user wires in all their safety devices (light curtains, mats, scanners, door sensors, etc.) and uses simple logic programming to dictate machine safety behavior. Frequently, if a machine requires 3 or more Monitoring Relays, a Safety Controller may be a more cost-effective solution, especially in cases that require muting or safety zone-control.