Machine Design

Safety Behind Light Curtains

Removing barriers to worker productivity.

Physical barriers were once the only way to safeguard workers from potentially hazardous machinery. No more. Safety light curtains instead form a virtual barrier around equipment with infrared light beams and photosensors. When breached, the machine or process stops. Obviously, light curtains are appropriate only for equipment that’s able to halt immediately when signaled, even in midcycle. Not to mention light curtains do nothing to stop flying debris.

Those limitations aside, however, light curtains offer some advantages over traditional guards and safety switches. For one, they can help boost production because operators don’t have to open and close safety doors each cycle. Moreover, light curtains provide greater design flexibility than traditional hard-wired switches because light is steerable with optics.

For example, the perimeter of a tabletop machine can be monitored with a compact light curtain and corner mirrors. And guarding odd-shaped areas and multiple access planes is made possible by use of multiple transmitter (light source) and receiver (photosensor) pairs.

But geometric flexibility is just part of the picture. Modern light curtain systems such as those from Scientific Technologies Inc., Fremont, Calif., also have sophisticated controls to further tailor the sensing field. For example, when a machine segment or workpiece permanently blocks part of the sensing field, the system can be programmed to ignore that area. This is referred to as exact-channel select. In another application, a workpiece extends through the sensing field and moves up and down, sequentially blocking multiple beams. Here, the controller uses a technique called floating blanking to ignore one or two blocked beams and signal stops on the third (for instance). In yet another application an operator must enter the guarded area to manually feed and remove workpieces. The controller in this case orders stops only when the machine is in a potentially hazardous cycle portion (muting).

In practice, the controller may send these signals to a machine primary control element (MPCE), the last link in a time-sequenced, start/stop circuit. The controller checks that the MPCE operates within a reasonable time period. If not, the controller prevents restart until the problem clears. For redundancy and safety, such circuits often have two MPCEs.

To assure proper light curtain operation some controllers include a self-test function for simulating blocked beams. This helps maintenance locate problems and lets operators check system health before running a machine. Another diagnostic consists of indicator lights built into the photosensors. These are used to verify beam alignment and intensity. The lights can also flag optics for cleaning, important because exposure to smoke, dust, mists, corrosive vapors, and other contaminants may degrade performance.

However, in the proper environment, light curtains can outshine physical guards. With no moving parts and virtually no geometric constraints light curtains can provide simple, elegant solutions to otherwise complex safety applications.

Helping to define safe
Consider both the physical and regulatory environments before deploying light curtains on the factory floor. A good first step is a risk analysis. A machine may have numerous hazards associated with its operation including pinch points, shearing, hot surfaces, blades, protrusions, and electrical shock. Each hazard and possible injury from that hazard must be reviewed and scored based on probability of occurrence and severity.

© 2010 Penton Media, Inc.

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