Safety light curtains are most simply described as photoelectric presence sensors specifically designed to protect plant personnel from injuries related to hazardous machine motion. Also known as AOPDs (Active Opto-electronic Protective Devices), light curtains offer optimal safety, yet they allow for greater productivity and are the more ergonomically sound solution when compared to mechanical guards. They are ideally suited for applications where personnel need frequent and easy access to a point of operation hazard.

Safety light curtains consist of an emitter and receiver pair that creates a multi-beam barrier of infrared light in front of, or around, a hazardous area. When any of the beams are blocked by intrusion in the sensing field, the light curtain control circuit sends a signal to the machine’s e-stop. The emitter and receiver can be interfaced to a control unit that provides the necessary logic, outputs, system diagnostics and additional functions (muting, blanking, PSDI) to suit the application. When installed alone, the light curtain pair will operate as a control reliable switch.

Light curtains are also called light screens, optical guards, and presence sensing devices. Light curtains emit a "curtain" of infrared light beams in front of the hazardous area being protected. A stop signal is sent to the machine being guarded when any of the beams are blocked. Light curtains guard areas many meters wide, and can be diverted around corners using mirrors.

Application areas include perimeter guarding for industrial robots and machinery, and point of access guarding for automated machine assemblies. 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 light curtain controller sends a stop signal to the guarded machine. The transmitter unit contains light emitting diodes (LEDs) that emit pulses of invisible infrared light when energized by the light curtains timing and logic circuitry. Light pulses are both sequenced (one LED after another) and modulated (pulsed at a specific frequency). The receiver unit contains phototransistors and supporting circuitry to detect only the specific pulse and the frequency designated for it.

There are several important parameters to consider when specifying light curtains. These include spanning distance, protection height, sensitivity or resolution, response time, and operating temperature. The spanning distance is how far the transmitter and receiver can be separated and properly function. The transmitter and receiver must be able to function reliably within this minimum to maximum range of separation.

Using mirrors reduces the operating range up to 25% per mirror. (Example: 6 inches to18 feet.) Protection height is the dimension of the active sensing field (correlates to the number of beams required). Sensitivity or resolution is the smallest sized object that will be positively detected by the light curtain. The response time of the light curtain is the maximum time between actuation of sensing function and the output relays. Operating temperature is the full-required range of ambient operating temperature.

Light curtains have a basic object detection size, finger, hand or body. Finger and hand detection is necessary when the operator is a short distance away from the hazardous zone. Body (or arm detection) is suitable for perimeter guarding. Safety categories for light curtains can be type 2, type 3 or type 4.

• A type 2 light curtain runs a self-check when it is turned on or reset. The safety function shall be checked at machine start up and periodically by the machine control system. If a fault is detected, safe status shall be initiated or if that is not possible, a warning shall be given. The occurrence of a fault between checking functions can lead to a loss of the safety function between checking intervals.
• A type 3 light curtain requires that more than one component must fail before it loses functionality. A single fault in any of the system parts does not lead to a loss of the safety function. Some but not all faults will be detected. An accumulation of undetected faults can lead to a loss of the safety function.
• A type 4 light curtain continuously self-checks and will detect any loss of function immediately. A single fault does not lead to a loss of the safety function. The faults will be detected in time to prevent the loss of the safety function.

Most light curtains require an external controller. They may either come equipped with an integral controller, in which case they will not require the external controller, or they come equipped with the external controller. Multi-tasking controllers that monitor and control more than one set of light curtains are also available. Light curtains may be mounted on brackets, DIN rails, or the floor.

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