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A wide range of technologies are available today to protect hazardous locations and areas on machines and systems. One option is to physically separate the person at risk from the hazard, which can be achieved by using movable guards, such as safety light curtains or enclosures.
In order to carry out maintenance work on the system, or set up the machine while normal operation is in progress, these protective devices are fitted with doors or flaps. However, if these movable guards have to be used frequently, for example because the nature of the system requires regular manual access, this takes time and requires "free hands." In this case, fitting non-contact electro-sensitive protective equipment to machines and systems often affords more efficient and convenient protection.
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Non-contact electro-sensitive protective equipment (ESPE) detects any persons or parts of the body that enter a protective field, without needing physical contact. Various types of sensor are used for this purpose, such as capacitive sensors or systems that work with ultrasound, microwaves or infrared light. Nowadays, however, opto-electronic protective devices, such as light barriers, light curtains or laser scanners, have become most prevalent. Due to their flexibility, camera-based systems are also increasingly common.
Regardless of how they work, the sensors detect a body entering the protective field formed by the device. This triggers a switching operation that prevents or terminates a machine's dangerous movement.
The technical safety requirements for non-contact electro-sensitive protective equipment on machines and systems are defined in standard EN 61496-1. Particular requirements for opto-electronic protective devices are described in EN 61496-2.
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In comparison to movable guards, the use of a light curtain or other non-contact electro-sensitive protective equipment is of particular interest if the machine operator has to frequently engage with the machine or system.
The advantages include the following:
However, when using a safety light curtain, light barrier or other non-contact systems, the operator must ensure that, if the protective field is entered, the dangerous movement comes to a safe stop before the person or hand has reached the hazardous area. This requires a suitable safety distance to be established. If this minimum distance cannot be guaranteed because, for example, it would take too long to decelerate a movement, movable guards must be used. The minimum distances for non-contact electro-sensitive protective equipment are defined in safety standard EN ISO 13855.
Non-contact electro-sensitive protective equipment based on opto-electronic systems can be used, for example, to protect machine tools, in crane systems and continuous conveyors, and also on forklift trucks.
It can be used to protect the following hazardous locations:
It can also be used to protect larger hazardous areas or accessible areas where people can reach into the hazardous locations of power-driven machinery.
Protective devices that operate on an opto-electronic system consist of a transmitting element and a receiving element. A beam of light (e.g. from a light barrier) or a series of parallel light beams (light curtain) are sent from the transmitter to the receiver. They form a kind of optical separating grid. If one or more beams are interrupted by an object, the receiver signals this to its switching output and the dangerous machine state is stopped.
Systems that detect objects larger than 40 mm are called multi-beam safety light barriers. They are used to safeguard access to hazardous areas, for example. If opto-electronic devices can detect objects less than or equal to 40 mm, they are referred to as safety light curtains. These are used to directly protect hazardous locations (e.g. by detecting anybody reaching into a machine).
Thanks to microelectronics, beams can also be utilised individually. For example, specific areas, such as hose bushings, can be hidden or objects of a defined size can pass through the light curtain without triggering it. This additional function is called blanking.
Another additional function is muting: This allows non-contact electro-sensitive protective equipment to be temporarily bypassed or muted during operation. For example, material can be transported to a hazardous area without triggering the protective device. This function is controlled automatically by the machine via separate electromechanical switches or opto-electronic sensors.
The following example shows monitoring a hazardous area with a light curtain: Non-contact electro-sensitive protective equipment (ESPE) per IEC 61496-1 (or more precisely, an active opto-electronic protective device per IEC 61496-2) is deployed. It is intended to immediately shut down all dangerous movements within the defined safety distance of the ESPE.
After the supply voltage has been applied at safety relay K1 (connection A1-A2), the power LED indicates that the enabling paths are ready to be activated. If there is no object in the protective field of the light curtain B1, the enabling relays inside the ESR 5 are activated by the rising edge at input S34. The LEDs K1 and K2 indicate this state. The safety contactors Q1 and Q2 energise and open their auxiliary contacts. The OSSD outputs of the ESPE are deactivated if the protective field is breached. The enabling relays inside the ESR de-energise (13–14, 23–24 instantaneously; 37–38, 47–48 with adjustable delay) and thereby cancel the enable to safety contactors Q1 and Q2.
This circuit attains a safety integrity level of SIL 2 or Performance Level "d."
The following setup allows material to be moved through the protective field of the protective device without stopping the work processes (muting). In this setup, the muting sensors can differentiate between objects and people.
Further options for setting up non-contact electro-sensitive protective equipment are given in the Eaton Safety Manual. Not only does it explain all relevant standards, it also offers you the opportunity to get to grips with the extensive subject of functional safety technology, one step at a time.
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