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 Laser Scanners
An alternative to light curtains and hard-guarding (fencing) for a work area, a Safety Laser Scanner can monitor a 2-dimensional plane and watch for intrusions by operators into a programmed “Safe Zone”. These are commonly used on Automatic Guided Vehicles (AGV’s) and in Robotic work-cells.
A Safety Mat operates when a user (or some heavy object) steps onto a rubberized mat consisting of two electrically charged plates in parallel. When the user steps onto the mat, the safety outputs activate and the safety function (for example, stopping a robot) occurs. Safety Mats are great for dirty environments, and can be a cost-effective way to guard an unusually-shaped work area around a machine.
Emergency Stop Devices
Emergency Stop Devices (“E-Stops”) are required by NFPA codes on all machinery, and should be placed within arm’s reach for any operator of a hazardous machine or process – these are intended to immediately stop a machine in the case that something has already gone wrong. E-Stops come in a variety of forms, such as the ubiquitous red-on-yellow mushroom pushbuttons and rope-pull switches. Frequently these contain dissimilar contacts, such as one normally-open and one normally-closed, to indicate situations like cut cables or stuck contacts.
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.