The entire purpose of a “safety” light curtain is to protect humans from dangerous machinery. “Dangerous machinery” is considered any machine or piece of equipment that moves in a way that could cause injury or kill you. So what do light curtains have to do with safety? What are they and what do they even do?
The best way to describe a light curtain is to first label is as a photoelectric type device, very similar to a opposed beam photocell but arranged in a vertical array with either 2 or more beams in various spacing configuration that create a “curtain”.
In even simpler terms, the light curtain uses presence detection between a transmitter and receiver. The transmitter sends infrared light to the receiver’s photoelectric cells and when these cells stop receiving their input they trigger that something is indeed in the way.
The primary function of a light curtain is to be on constantly, scanning for potential interference of the infrared beam by a human or anything else. An output signal would then be created and typically fed to a safety relay device providing redundant protection; the relay then would signal to turn off or shutdown the machine to prevent an accident.
Once the curtain and relay have been tripped a lock out procedure would then take place. This will shut down the designated equipment and a reset of the machine would be required to clear the emergency shutdown.
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What are the primary concerns in sizing and specifying a light curtain?
- What is the area that one is trying to protect, two corners or four corners?
- What is the required height of the protection field? Most are measured from 150-3000mm
- What is the require distance between the corners, ie; the distance between the emitter and receiver, also known as the operating range? (normally listed in meters)
- Resolution, a.k.a. the spacing between the beams: the tighter the spacing is for finger or hand type detection, usually measured in millimeters, IE: Type 2 and Type 4 requirements
- Mount requirements: What would one need to mount each sensor (brackets available)
- Cables: If the units are not cabled then one would need standard available cables to plug into the Q.D. for each sensor.
- What Safety levels are needed? One should consult with a plant safety engineer, as a distributor would not make that specification and safety levels are product specific.
- Other design requirements: “AIDA” compliant, Extra Shock Resistance needed?, IP67/69K needed?, Smart Processing Gating?, Test Signal Input?
Let’s look at an example; a great product featured is the LEUZE MLC series with a 30mm (Resolution).
Each MLC 500 series light curtain comes with two separate part numbers, one for the transmitter and one for the receiver. A single set would cover one side of a machine.
|Transmitter: MLC500T30-2700||Receiver: MLC510R30-2700|
So let’s breakdown the part numbers…
The “T” in the code is for transmitter, the “R” code stands for receiver. The “30” code stands for the vertical distance in millimeters between beams in the emitter/receiver. The “2700” will indicate the vertical height in millimeters for the entire safety curtain. This particular part number group will provide spacing performance between the emitter/receiver of up to 10 meters. The design uses M12 type connectors with 5 pin configuration for power and output signal. Please note that these items are usually paired with a safety relay, like a MSI-SR4B (example).
Please note that there are seven SKU’s for cables, 10 different SKU’s for different mounting brackets, 1 SKU for test rod, and 1 SKU for device column for protection and mounting and adjustable in 3 directions.
The accessories are always sold separately, check out the associated products for more information.