Canadian Occupational Safety

LIGHT SOURCES Light curtains, perhaps the most common type of presence-sensing device, consist of a transmitter and receiver. The transmitter unit contains light emitting diodes (LEDs), which send out parallel infrared light beams to a photoelectric receiver. When an opaque object breaks one or more of these beams, the device controller sends a stop signal to the machine. The set of beams are spaced according to what they need to detect. "If they are supposed to see a fi nger, like a fi nger reaching in, they need to be closer together. If they need to see a person, they can be a little farther apart," Gordon says. Light curtains can be used to control access at the point of operation of a machine, such as a brake press. They may also be used to protect the perimeter around a hazardous machine or access to a speci- fi ed area, such as the doorway into an enclosed area protecting a robotic cell. Programmable features such as "muting" and "blanking" provide a means of adapting the light curtain to production needs. Muting refers to the temporary, complete disabling of a light curtain's sensing fi eld or stop signal to the machine during the non-hazardous portion of a cycle. This is often done, for example, during the entry or exit of pallets on a palletizing machine. Blanking occurs when one or more sections of the light curtain's sensing fi eld are disabled. The rest of the light curtain remains active. In one common use, a small gap in a light curtain is blanked to allow the feeding of material through it to a punch press. Another safety device that uses a light source is the laser scanner, generally used to detect presence across larger areas. A scanner sends a laser beam across a fl oor area and when the beam is broken, it realizes something is in the area and can shut down a machine. Scanners can be customized and can have two programmable zones (warning and safety). "Light curtains are like an electronic fence," says Dan Plant, senior technologist at Cornwall, Ont.- based EFI, an engineering consulting fi rm. "Crossing that fence will trigger something. The laser, however, puts out a blanket across a whole area. If you jump 26 Canadian Occupational Safety | www.cos-mag.com anywhere onto the fl oor, it will still detect that you're standing in the area." Laser scanners are more expensive than light curtains but are more accurate and travel farther than infrared, says Owen Tucker, technical support manager at Barrie, Ont.-based Pilz Automation Safety Canada. Because laser and infrared devices are mounted at fl oor level or on machinery, they are prone to such problems as forklifts backing into them or machinery vibration. "Light curtain beams have to be aligned between two sensing devices, so if they go out of alignment, they will turn the equipment off. Being mounted at fl oor level, they are prone to maintenance. You have to be careful you don't knock them out of align- ment," Tucker says. PRESSURE-SENSITIVE Safety mats are placed in an area where it is unsafe to operate a piece of equipment, such as a stamping machine. The mat detects when someone steps on it in a hazard zone and shuts down the machine. Some people use mats to detect that an operator is standing in the spot where they are supposed to be — for example, at the operator's console — and moving off the mat causes the machine to stop. But this is an improper way to use them, Gordon says. One disadvantage of safety mats is that a person can deceive them into thinking an operator is stand- ing on the mat by putting a heavy weight, such as a tool box, on the pressure sensor. Moreover, due to cost, they are not a realistic solution where the goal is to protect a large area. Gordon says mats can also present diffi culties in manufacturing companies. "My industry is manufacturing and the struggle people have with safety mats is that there are a lot of forklifts and traffi c and people run over them and wreck them. Or there's a lot of dust and debris, like welding or wood dust, and it gets into them. They need to be installed correctly to make sure there's a tight seal." Pressure-sensitive safety edges are touch-sensitive strips attached to crushing and shearing edges of moving parts, such as machine tables or powered doors. When the edge comes into contact with an By Linda Johnson A n operator was bending metal parts using a 32-tonne, part-revolution power press brake. The machine, which was operated by foot control, was protected with a light curtain. However, about 3 to 4 inches of the light curtain had been "blanked out," or bypassed, during an earlier part run. Paus- ing work to adjust a part at the point of operation, the operator inadvertently activated the foot pedal, starting the machine and cutting off three fi nger tips. Presence-sensing safety devices are often chosen over physical guards because they allow easier access for loading parts or for routine maintenance, thus offering increased productivity. Yet, because they do not provide a physical, fi xed barrier, safety managers should always remember presence-sensing devices may create a false sense of security and actually increase the possibility of injury. Managers and work- ers need to know how the various technologies work and, equally important, what their limitations are. "There are two types of controls," says Mike Gordon, co-founder of Winnipeg-based Workplace Engineering Solutions. "There are controls, like fences and gates, that prevent access to hazards and there are ones that stop hazardous motion when you enter the danger area. Those are presence-sensing devices. They're meant both to detect a worker, a part of that worker and to stop hazardous motion when that worker encounters them." In general, presence-sensing devices are used to prevent access to equipment in three ways: at the point of operation; around the perimeter of machine; or across an area. Using presence-sensing devices requires knowing what they can do – and what they can't A

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