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Of all electro-optical presence-sensing safeguarding

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Safeguarding
Light curtains and other optical safeguarding devices protect operators against machinery and manufacturingarea hazards without sacrificing production efficiency. Here’s how they work, and how metalformers can optimize their protective features.
BY ROGER HARRISON

On mechanical power presses, light-curtain use is limited to machines capable of stopping anywhere during their cycle, typically part-revolution airclutch machines such as this one. Light curtains cannot be used on full-revolution clutch presses because of their inability to stop before the cycle completes.

f all electro-optical presence-sensing safeguarding devices used in metalforming environments, infrared light curtains are most common. Light curtains first appeared in the mid-1950s and boast a variety of applications. A light curtain’s purpose: prevent and stop machine motion when its infrared beams are interrupted—usually by an operator’s hand or body. These devices can only be used on machines that stop quickly and consistently in mid-cycle, without damaging the machine or creating another hazard. On mechanical power presses, the use of light curtains is limited to presses capable of stopping anywhere during their cycle, typically partrevolution air-clutch machines. Light curtains cannot be used on full-revolution clutch presses because of their inability to stop before the cycle completes.

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Types of Light Curtains
A light curtain consists of two long, thin, vertically mounted light bars. Its transmitter projects a row of closely spaced infrared light beams to the receiver. The light beams rapidly sequence—turn on and off—one LED or light-emitting diode after another at a specific pulse and frequency. When an object interrupts one or more beams, the light curtain’s control logic sends a stop signal to the machine. Roger Harrison is director of training for Rockford Systems Inc., Rockford, IL. Tel. 800/922-7533; www.rockfordsystems.com.

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METALFORMING / MAY 2007

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Light curtains provide two types of protection: point-of-operation and perimeter. Point-of-operation light curtains protect hands and fingers, and require closely spaced light beams. The distance between light beams, or Minimum Object Sensitivity (MOS), for point-of-operation light curtains typically falls between 0.5 and 1.0 in. Perimeter light curtains detect a person walking into the perimeter of a hazardous area. Thus their light beams can be spaced much further apart and still provide adequate protection. These devices may be mounted horizontally providing that no one can reach over them into hazardous areas. Though useful for their intended purpose, perimeter light curtains are not a safe substitute for point-of-operation devices. Single-beam devices, often employed for perimeter protection, normally function in pairs to discourage personnel from crawling underneath or stepping over them.

Calculating Safety Distances
OSHA regulations and ANSI B11 safety standards provide formulas to calculate the minimum safety distance between the light curtain and the hazard area. The distance intended intended to prevent personnel from reaching through the light curtain sensing field rapidly enough to “beat the machine,” or reach moving parts before motion ceases. Stop-time measurement devices, often built into solid-state press-control systems, calculate safety distance. These devices measure how long a machine takes to stop when interrupted mid-cycle. That time is multiplied by an average reaching speed. The result is the number of inches that a light curtain must be mounted from the nearest hazard. Since the early1970s, OSHA has used this formula for determining safety distance on mechanical power presses.
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Laser-based devices that project sensing fields down the tip of the dies serve as alternatives to light curtains for the unique bending challenges inherent in press brakes, where operators’ hands often must be in close proximity to moving dies. Such devices mount with zero safety distance and are designed for twospeed hydraulic press brakes, quickly stopping die movement during their slower bending speed.

Safety Distance Formulas
Here’s the OSHA Safety Distance Formula in 29 CFR 1910.217 for mechanical power presses (results are considered to be an absolute minimum safety distance): Ds x Ts = Safety Distance, where Ds = 63 in./sec., the average reaching speed, also referred to as approach speed Ts = stopping time of the press when interrupted at the 3 o’clock position of crankshaft rotation because this usually produces the longest stopping time. The result provides the minimum

distance in inches from the light curtain to the nearest point-of-operation hazard, usually the front edge of the largest die used in the machine. More recent ANSI B11 safety standards refine the basic OSHA safety distance formula. These newer formulas account for all elements that might affect machine stopping time. As a result, the safety distance in ANSI formulas requires that a light curtain be located significantly further away than the safety distance arrived at using the OSHA formula described above. Using updated ANSI Standard formulas typically produces safety distances at least 40 to 50 percent further away than the minimum arrived at with the OSHA formula. Of the 24 ANSI B11 safety standards, those that provide an updated safety-distance formula include: B11.1 Mechanical Power Presses2001 B11.2 Hydraulic Power Presses-1995 (R2002) B11.16 Powder Metal Compacting Presses-2003 B11.19 Performance Criteria for Safeguarding-2003 ANSI/RIA R15.06-1999 Industrial Robot Safeguarding. In addition, six other ANSI B11 safety standards cross-reference ANSI B11.19 for the requirements of presence-sensing safeguarding devices, including safety distance. Here’s an updated safety distance formula for light curtains, found in ANSI B11.1-2001 for Mechanical Power Presses, clause E8.5.3.16: Ds = K (Ts + Tc + Tr + Tbm) + Dpf, where Ds = safety distance from light curtain to nearest point-of-operation hazard K = hand-speed constant (average reaching speed of 63 in./sec.) Ts = stopping time of the press from the final de-energized control element
METALFORMING / MAY 2007 35

Safegaurding

part to extend through the light curtain’s sensing field without interrupting the cycle. In some cases, the operator’s hand or even an entire arm also could extend through the sensing field along the side edge of the part, providing no safety protection whatsoever. To address these two challenges, light-curtain suppliers have devised laser-based devices that project sensing fields down the tip of the dies. Such devices mount with zero safety distance and are designed only for two-speed press brakes, capable of stopping quickly during the slower portion of the die movement. Though more expensive than light curtains, they provide answers to some of the more troublesome hydraulic-press-brake safety concerns.
A proximity laser scanner, or PLS, can be programmed on the shop floor with a laptop computer to provide area protection as an alternative to pressure-sensitive mats. Programmable protection zones can be of any shape, with warning zones reaches to 40 ft. and fault zones to 15 ft. The warning zone ties to an audible or visual awareness device while the fault zone ties to the emergency-stop circuit.

An Alternative to Safety Mats
Traditionally, perimeter guards and pressure-sensitive mats have been used for area safeguarding. Pressure-sensitive mats are subject to damage or chemicals on the shop floor. Optical scanning devices, which scan a dangerous envelope or area, can replace the mats. Known as proximity laser scanners, PLS devices or area scanners, industry originally employed them in the early-1990s as laser-bumpers for automatically guided vehicles. Their use has spread to other machines and equipment. A PLS device looks like a drip coffeemaker. A black polycarbonate lens covers a laser transmitter that projects a laser beam through a perimeter space to stop hazardous machine motion when anyone or anything enters that space. A PLS device can be programmed on the shop floor with a laptop computer. Programmable protection zones can of any shape, from simple semi-circles or rectangles to custom-defined shapes. These shapes are programmed into the PLS device via a test sweep to outline the specific perimeter. The device’s warning zone reaches to about 40 ft. with a fault zone reaching to about 15 ft. The warning zone ties to an audible or visual awareness device while the fault zone ties to the emergency-stop circuit of the machine. MF
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Tc = response time of the press-control system Tr = response time of the presencesensing device and its interface Tbm = additional stopping time of stopping performance monitor Dpf = added distance for penetration factor (MOS and any blanked channels). The active area of a light curtain’s sensing field does not reach from tip to tip on the light bars, often leaving an area near the end of the transmitter and receiver that contains no LED channels and, therefore, provides no protection. When mounting a light curtain, make sure that a small hand cannot reach through this area, especially at located at the bottom of the sensing field. This is of greatest concern on mechanical power presses when an operator manually feeds parts using a foot-switch for cycle actuation. Most hydraulic presses can effectively use a vertically mounted light curtain. Although still requiring a stoptime measurement test, the safety distance for hydraulic presses often is much less than that of mechanical presses.
36 METALFORMING / MAY 2007

Press-Brake Protection
Press brakes—hydraulic and mechanical—commonly employ light curtains. But excessive safety distance poses a problem with mechanical press brakes, sometimes requiring a brake replacement to improve stopping time. Two types of bending bring special challenges when attempting to use light curtains on press brakes. The first involves bending of small parts that must be held in place by hand, where an operator’s fingers reach relatively close to the front edge of the dies. The operator’s hands often block the light curtain’s sensing field, preventing the press brake from cycling. Some parts can be fixtured in place—mechanically, magnetically or with vacuum pressure—so that hands remain outside of the sensing field during the cycle. Unfortunately, fixturing is not practical for all types of parts. The second challenge involves large parts that require forming on adjoining sides, as with box bending. When using a light curtain for that type of work, a significant area of the sensing field must be blanked, or turned off, allowing the


				
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