Light Curtains Information
Light curtains are presence sensing devices that provide an infrared perimeter around perilous machinery or processes. When an infrared beam is interrupted between the transmitter and receiver, a killswitch arrests the hazard until the optical boundary has been reestablished.
A light curtain's main advantage is that it does not impose a physical barrier on personnel. As such, an operator is able to access guarded components without mechanical or observational limitations. Maintenance processes are also eased by their use. Light curtains can also be used to monitor cargo in or out of a restricted access area, as well as provide strictly one-way access.
A transmitter is established and an array of parallel infrared beams provide a boundary across the restricted access point. The transmitter can emit its rays in virtually any configuration provided the infrared beams conclude at a receiver. Mirrors are frequently used to relay beams around corners or obstacles which reduces the need for multiple transmitters, but for every mirror the beam range is reduced by 25%. An alignment laser or LED indicator is frequently integrated in the transmitter for accurate positioning. Manufacturers may elect to incorporate light curtains in standalone machinery and these devices will have increased sensitivity. Light curtains have variable and convenient installation patterns.
RobotWorx provides a good illustration of a light curtain operating in a manufacturing setting.
Video credit: RobotWorx / CC BY 3.0
The infrared light beams are sequenced and modulated and the receiver is designed to recognize a specific infrared beam, decreasing the chance for issues from ambient infrared light. When a beam is obstructed by an opaque object, a safety relay halts the machinery or process within milliseconds. Some light curtains allow the process to resume once the obstruction is removed, while others require an operator to manually restart the process. A PLC (integral or external) is required for the light curtain to interface with the machinery it encloses. A timed mute feature allows operators to control the flow of materials through the perimeter without stopping the machine. Some muting scenarios allow the passage of humans, while if other muting sensors register a human or irregularity then the process will cease.
While the most prominent use of light curtains is to convey a stop command, they can also be used to initiate a process by registering when an operator is an appropriate distance away.
Light curtains can be tailored to individual applications and are very versatile in their deployment. The following specifications should be considered to ensure functionality.
Detection zone - A light curtain can be perpendicular, parallel, or diagonal to the operator's angle-of-approach, but must be positioned so once crossed it halts the machine before the operator is placed at risk.
Spanning distance - The functional distance between the transmitter and receiver when accounting for range reductions due to mirrors. The term light curtains typically refers to many infrared beams typically less than 20 m apart, while light grids contain just a few infrared beams that have span distance of over 50 m.
Protection height - The vertical dimension of the curtain's sensing field. This measurement contributes to the number of beams in a particular array, and therefore sensitivity of the curtain. Generally speaking, the lowest infrared beam should be no higher than 30 cm above the floor and the highest beam should be no less than 90 cm above the floor to prevent circumvention around the light curtain.
Sensitivity - The smallest size object that arrays can detect, which is important based upon what items or body parts are likely to cross the curtain array. While a few beams are suitable to detect an employee's whole body, it will not be enough to detect hands or limbs.
Controller - A programmable logic controller may be integrated in some light curtains, while external controllers are linked to the curtain for programming and status. External controllers are often able to control more than one set of light curtains.
Floating Beam - Allows the disabling of beams at any location in the sensing field. The safety light curtain can be programmed to produce an intentionally disabled light beam that can move up and down ("float") in order to allow the feeding of an object through the curtain without causing a trip or latch condition.
Integral Alignment Status LEDs - The transmitter or receiver has integral LEDs for light beam alignment during initial equipment setup.
Multiple Openings Protection - Light curtain can be configured to protect more than one opening (whole cell protection). Light curtain used for guarding machinery with access to the hazardous zone from more than one side or for perimeter guarding.
Self-Checking Circuitry - The light curtain has the capability to run a self-check during startup, operation, or continuously to ensure no loss of function.
Blanking or Banking - Disables one or more fixed locations within the sensing field. The safety light curtain can be programmed so that a variable number of light beams are 'blanked' or 'masked' to allow an object to extend through the field without affecting the machinery's operation.
Integral Fault or Equipment Status LCD - The transmitter or receiver has an integral LCD to inform operators of faults or equipment status.
Latching Safety Relays - Once the sensing field of the curtain has been penetrated and a stop signal sent to the guarded machine, the light curtain will not remove the stop signal until the controls are manually reset outside the protected area.
Muting - Muting is the temporary, automatically controlled deactivation of a safeguarding device during the non-hazardous portion of the machine cycle. Allows complete bypass or disabling of the sensing field or stop signal to the machine during the non-hazardous portion of the cycle.
Light Curtain Safety
Light curtains are often categorized by type to represent the degree of protection the devices provides.
Type 2 - A lower-cost light curtain that is best-suited for lower-risk applications. Type 2 curtains have a large field of view and run a self-check during startup.
Type 3 - Type 3 devices are primarily safety laser scanners and are best used in applications where light curtains would be unsuitable. More than one component must fail before it loses safety functionality.
Type 4 - Type 4 devices guard personnel from points of operation, access, and perimeters. Type 4 curtains have the highest levels of fault tolerance and have a persistent self-checking mechanism to prevent any reduction in safety.
ASNI B11.19 - Performance criteria for safeguarding
IEC 61496 - Electro-sensitive protective equipment
IEC/TS 62046 - Presence detecting equipment applications
ISO 13855 - Machinery safeguards for personnel
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